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“Activated Borane”: A Porous Borane Cluster Polymer as an Efficient Lewis Acid-Based Catalyst

  • Martin Lamač*
    Martin Lamač
    Department of Molecular Electrochemistry and Catalysis, J. Heyrovsky Institute of Physical Chemistry of the Czech Academy of Sciences Dolejškova 2155, 182 00 Prague 8, Czech Republic
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-9499-9526
  • , 
  • Béla Urbán
    Béla Urbán
    Department of Molecular Electrochemistry and Catalysis, J. Heyrovsky Institute of Physical Chemistry of the Czech Academy of Sciences Dolejškova 2155, 182 00 Prague 8, Czech Republic
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  • , 
  • Michal Horáček
    Michal Horáček
    Department of Molecular Electrochemistry and Catalysis, J. Heyrovsky Institute of Physical Chemistry of the Czech Academy of Sciences Dolejškova 2155, 182 00 Prague 8, Czech Republic
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    Orcidhttps://orcid.org/0000-0002-3203-3080
  • , 
  • Daniel Bůžek
    Daniel Bůžek
    Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic
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    Orcidhttps://orcid.org/0000-0002-7387-9461
  • , 
  • Lucie Leonová
    Lucie Leonová
    Department of Chemistry, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic
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    Orcidhttps://orcid.org/0000-0002-8002-8307
  • , 
  • Aleš Stýskalík
    Aleš Stýskalík
    Department of Chemistry, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic
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  • , 
  • Anna Vykydalová
    Anna Vykydalová
    Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic
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  • , 
  • Karel Škoch
    Karel Škoch
    Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic
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  • , 
  • Matouš Kloda
    Matouš Kloda
    Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic
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  • , 
  • Andrii Mahun
    Andrii Mahun
    Department of Structural Analysis, Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Prague 6, Czech Republic
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    Orcidhttps://orcid.org/0000-0003-1057-2910
  • , 
  • Libor Kobera
    Libor Kobera
    Department of Structural Analysis, Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Prague 6, Czech Republic
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    Orcidhttps://orcid.org/0000-0002-8826-948X
  • , 
  • Kamil Lang
    Kamil Lang
    Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic
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    Orcidhttps://orcid.org/0000-0002-4151-8805
  • , 
  • Michael G. S. Londesborough
    Michael G. S. Londesborough
    Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic
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    Orcidhttps://orcid.org/0000-0002-6161-6592
  • , and 
  • Jan Demel*
    Jan Demel
    Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0001-7796-6338
Cite this: ACS Catal. 2023, 13, 22, 14614–14626
Publication Date (Web):October 30, 2023
https://doi.org/10.1021/acscatal.3c04011

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Borane cluster-based porous covalent networks, named activated borane (ActB), were prepared by cothermolysis of decaborane(14) (nido-B10H14) and selected hydrocarbons (toluene, ActB-Tol; cyclohexane, ActB-cyHx; and n-hexane, ActB-nHx) under anaerobic conditions. These amorphous solid powders exhibit different textural and Lewis acid (LA) properties that vary depending on the nature of the constituent organic linker. For ActB-Tol, its LA strength even approaches that of the commonly used molecular LA, B(C6F5)3. Most notably, ActBs can act as heterogeneous LA catalysts in hydrosilylation/deoxygenation reactions with various carbonyl substrates as well as in the gas-phase dehydration of ethanol. These studies reveal the potential of ActBs in catalytic applications, showing (a) the possibility for tuning catalytic reaction outcomes (selectivity) in hydrosilylation/deoxygenation reactions by changing the material’s composition and (b) the very high activity toward ethanol dehydration that exceeds the commonly used γ-Al2O3 by achieving a stable conversion of ∼93% with a selectivity for ethylene production of ∼78% during a 17 h continuous period on stream at 240 °C.

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Introduction

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Lewis acid-catalyzed reactions are numerous and of considerable significance. (1) In chemical and petrochemical industries, it is well established that heterogeneous catalysts offer an important advantage over their homogeneous counterparts that they allow the design of continuous production processes. For this and other advantageous reasons, significant resources have, in recent decades, been allocated to the development of novel acid-based heterogeneous catalysts.
Traditionally, heterogeneous catalysts such as inorganic oxides, aluminosilicates, sulfonated zirconia, zeolites, and alumina halides have all contained metals as the Lewis acid centers. Alternatively, the incorporation of boron into zeolites and amorphous silica has been thoroughly studied and found to promote the oxidative dehydrogenation of alkanes to alkenes (ODH). (2) Interestingly, however, isolated tricoordinated BO3 sites in MCM-22 have been shown to be inactive. (3) Instead, an amorphous oligomeric boron oxide/hydroxide layer was shown to be active in ODH, the activity and selectivity being beneficially influenced by the ability of the catalyst to form radicals. (4) Mixed boria–alumina oxides represent another family of boron-containing inorganic catalysts. These materials can be described as mildly acidic solids and are used in alcohol dehydration, (5) Beckmann rearrangement, (6) and phenyl glyoxal conversion. (7)
The traditional inorganic catalysts described above are very robust and effective even at very high temperatures. However, the set of reactions catalyzed by these materials is different from those catalyzed by molecular tricoordinated boron catalysts, which can, e.g., utilize “frustrated Lewis pair” (FLP) reactivity. (8) The first examples of such tricoordinated boron molecular catalysts were based on boron halides BX3 and perfluoroalkyl boranes; however, such compounds are difficult to handle and/or are thermolabile. Since the 1980s, B(C6F5)3 (commonly referred to as “BCF”) has become the homogeneous catalyst of choice for many applications because of the strong electron-withdrawing effect, bulkiness of the pentafluorophenyl substituents, and the general ease and convenience of its use. (9) Indeed, the versatility of BCF provided the impetus to its heterogenization by linking it to nonreactive solid supports. Thus, silica, (10) alumina, (11) graphene oxide, (12) organic polymers, (13) and other substrates were modified with -B(C6F5)2 acid centers. However, these heterogenized catalysts are laborious to synthesize, and their catalytic activity is in most cases significantly lower than that of the homogeneous BCF counterpart.
Recently, borane clusters (usually represented by derivatives of icosahedral carborane C2B10H12) have been proposed as a viable alternative to the commonly used polyfluorinated aryl substituents at the tricoordinated boron atom sites of BCF. (14) Kondo and co-workers have recently reported the catalytic dehydration of ethanol to ethylene by hydrogenated borophene, a 2D material with an empirical formula H1B1. (15) These HB sheets likely possess Brønsted sites represented by protons on the edges of the 2D structure, but the precise nature of the catalytically active sites in these materials is still unknown. This development prompted us to cast a wider net in the borane cluster field and test the boron acid centers found in activated borane (abbreviated hereafter as ActB) (16) for heterogeneous catalytic activity. ActB is a porous polymer that is formed by cothermolysis of decaborane(14) (nido-B10H14) and organic solvents such as toluene or hexane. We have shown that the structure of ActB is most probably composed of closo borane clusters linked together by organic linking moieties. (17)
Here, in this paper, we present an improved strategy for the synthesis of ActB and present evidence for the catalytic activity of its Lewis acidic boron sites as well as a comparison of its performance compared with contemporary catalysts. The nature of the Lewis acid sites in ActB was characterized by the Gutmann–Beckett method (31P solid-state NMR after the adsorption of Et3PO) and by temperature-programmed desorption of ammonia. Furthermore, we showcase the utility of ActB for hydrosilylation/deoxygenation reactions of various carbonyl compounds and also in the dehydration of ethanol into ethylene. In the latter reaction, ActB exerted a higher catalytic activity than commonly used alumina and excellent catalyst stability even after 17 h on stream.

Results and Discussion

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Synthesis of Activated Borane

Building on our previous findings, (16) we have improved the synthesis of ActB and, by using an Ar-filled glovebox, a stainless steel autoclave (Berghof BR-300), and a Schlenk line, we have prepared pristine samples of ActB devoid of B–OH groups (see below IR and ssNMR). In order to compare the effect of aromatic, cyclic aliphatic, and acyclic aliphatic linking molecules, we have prepared ActBs by heating nido-B10H14 in toluene, cyclohexane, and n-hexane to 250 °C in a high-pressure reactor for 24 h (Scheme 1). The crude dark solids were Soxhlet-extracted with the same solvent as that used for the synthesis to remove all unreacted and partially unreacted impurities. The residual solvent was removed by heating to 100 °C under vacuum for 4 h. For details on synthesis, see the SI. The samples are denoted as ActB-Tol (toluene), ActB-cyHx (cyclohexane), and ActB-nHx (n-hexane).

Scheme 1

Scheme 1. Synthesis of ActB Materials
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Characterization

All prepared ActBs are solids lacking long-range order. ActB-Tol and ActB-cyHx are black powders, whereas ActB-nHx is brown-colored. This crude observation is consistent with the materials’ UV–vis spectra, in which all ActBs absorb strongly below 1000 nm, with the absorption edge for ActB-nHx being shifted to lower wavelengths in comparison with ActB-Tol and ActB-cyHx. This translates to optical band gaps of 1.19, 1.20, and 1.34 eV for ActB-Tol, ActB-cyHx, and ActB-nHx, respectively, as derived from Tauc plots using the Kubelka–Munk-transformed reflectance spectra (for details, see the SI). The elemental analysis of ActBs was done by the standard CHN analysis, and the content of boron was determined by ICP-MS (see the SI for details). ICP-MS was also used to check for the presence of metal traces; from the comparison with blank samples, we did not observe elevated amounts of metals, which could be responsible for catalytic activity (such as Pd, Ni, Cu, Cr, Fe, Zn, Cr, etc.). Scanning electron microscopy (SEM) (Figure 1) shows larger particles of irregular shape for ActB-Tol and ActB-cyHx, while ActB-nHx forms spherical particles approximately 1 μm in diameter.

Figure 1

Figure 1. SEM images of ActB-Tol (left), ActB-cyHx (middle), and ActB-nHx (right). For additional images, see the SI.

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The Ar adsorption isotherms for all ActBs correspond well to those typical for microporous materials (Figure 2). ActB-Tol and ActB-cyHx contain pore size maxima between 0.9 and 1.6 nm, while ActB-nHx contains pore size maxima between 1.0 and 2.0 nm (Figure S4). The extent of porosity and specific surface area, however, varies greatly between the derivatives, with the clear trend ActB-Tol > ActB-cyHx > ActB-nHx (see Table 1). The adsorption of CO2 follows a similar pattern, with the highest adsorption achieved for ActB-Tol and a lower adsorption achieved for ActB-cyHx and ActB-nHx. In this case, however, no significant difference between the latter two was observed. To confirm the batch-to-batch reproducibility of the synthesis of ActB-Tol, we compared the adsorption isotherms of Ar and pore size distribution for individual batches (see Figures S5 and S6 in the SI).

Figure 2

Figure 2. Adsorption isotherms of Ar at 87 K (top) and CO2 at 195 K (bottom) for ActB-Tol, ActB-cyHx, and ActB-nHx.

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Table 1. Specific Surface Areas, Pore Diameters, and Pore Volumes for ActB-Tol, ActB-cyHx, and ActB-nHx
sampleSBET (m2g–1)apore size (nm)bVpore (cm3g–1)c
ActB-Tol7840.9–1.60.47
ActB-cyHx6080.9–1.50.30
ActB-nHx1601.0–2.00.10
a

BET specific surface area.

b

Pore size maxima calculated by the MDFT method.

c

Total pore volume at p/p0 = 0.99.

The FTIR spectra (Figure 3) for these species contain peaks corresponding to both constituent moieties─borane cluster and organic linker. Most notably, the peaks at approximately 2900 and 2550 cm–1 correspond to C–H and B–H stretchings, respectively. In all cases, the absence of any peak at around 1900 cm–1 suggests the absence of μ-H (B–H–B bridges) in the structure. As expected, the peak at approximately 1600 cm–1, which can be attributed to the aromatic stretching vibration, is present only in ActB-Tol. A significant difference from the original ActB spectrum published earlier (16) is the absence of the BO–H stretching peak over 3000 cm–1, which can be explained by the different preparations and measurement procedures used that avoid oxygen contamination. To study the effect of humid air, we exposed ActBs for 1 min to air and remeasured FTIR, which in all cases revealed a quick water adsorption onto the material, manifested by the appearance of the O–H stretching band at around 3200 cm–1.

Figure 3

Figure 3. FTIR (ATR-Si) spectra of ActB-Tol (top), ActB-cyHx (middle), and ActB-nHx (bottom); red lines correspond to samples exposed to air for 1 min.

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The thermal stabilities of ActBs were studied by TGA coupled with MS (Figures S10–S19). ActB-Tol showed the lowest stability with the onset of its main degradation step (Tonset) at around 245 °C, compared to the higher limits measured for ActB-cyHx and ActB-nHx at 319 and 322 °C, respectively. For a detailed discussion, see the SI.
The 1H MAS NMR spectrum of ActB-Tol (Figure 4, left column) reveals two broad signals at 1.64 and 6.5 ppm, which can be attributed to the methyl group and aromatic protons of toluene, respectively, overlapped with signals from borane clusters. (16) The 1H MAS NMR spectra of ActB-cyHx and ActB-nHx samples show a broad and asymmetric dominant peak at 1.14 and 0.65 ppm, respectively, assigned to aliphatic protons overlapped with signals from borane clusters. Interestingly, both spectra contain a broad shoulder at about 6.5 and 6.2 ppm, respectively, which indicate the presence of unsaturated (−CH═) species, suggesting that partial dehydrogenation of cyclohexane and n-hexane occurs during the synthesis (further confirmed by 13C CP/MAS NMR; see below). In all cases, signals from open-face “bridging” μ-hydrogens of nido-B10H14 are not detected (the expected range of signals between −4 and 0 ppm), (18) confirming the transformation of open nido-borane clusters into closed cluster derivatives and/or full substitution of the μ-hydrogens (16) indicated by FTIR. The 13C CP/MAS NMR spectrum of ActB-Tol (Figure 4, middle column) exhibits three major peaks at 19.3, 126.6, and 134.6 ppm and minor peaks at 0.5 and 29.4 ppm. The major peaks are assigned, respectively, to the methyl carbon and the nonequivalent groups of aromatic carbons from immobilized toluene. The minor peak at 0.5 ppm was attributed to carbon atoms bonded to the borane cluster forming nodal points, similar to the bonding system that was observed for boron carbide systems. (19) The second minor broad peak at 29.4 ppm, ranging from about −10 to 70 ppm, can be assigned to methyl groups directly connected to borane clusters and/or carbons incorporated into these clusters possibly forming a substituted carborane cluster. (20) The 13C CP/MAS NMR spectra of ActB-cyHx and ActB-nHx display a series of partially overlapping signals ranging from about 0 to 40 ppm that correspond well with aliphatic carbons (Figure 4, middle column). The presence of signals around 133 ppm confirms the formation of unsaturated (−CH═) species. Furthermore, the distribution of chemical shifts points to a wide range of chemical environments typical of solids lacking a long-range order. In addition, the signal at around 0.7 ppm on the respective spectra of ActB-cyHx and ActB-nHx indicates carbon atoms forming nodal points similarly to what was observed for ActB-Tol.

Figure 4

Figure 4. 1H MAS NMR (left column), 13C CP/MAS NMR (middle column), and 11B MAS NMR (right column) spectra of ActB-Tol, ActB-cyHx, and ActB-nHx samples.

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All 11B MAS NMR spectra (Figure 4, right column) possess two broad composite signals with maxima at around −7.3 and 1.5 ppm (3.5 ppm in the case of ActB-Tol) corresponding to the borane cluster molecules substituted at different positions forming a variety of chemical environments. (16) Even 2D 11B 3Q/MAS NMR spectra did not reveal additional signals and clearly confirmed the structure lacking a long-range order (Figure S8).

Lewis Acid Sites

The Gutmann–Beckett method is commonly used to assess the relative acidity of molecular Lewis acid-based catalysts. It uses the sensitivity of the 31P NMR chemical shift (δP) in Et3PO (TEPO) to interactions with Lewis acids. (21) As a result, the δP of TEPO can be shifted from 48 to over 90 ppm depending on the strength of the acid–base interaction. In recent years, the Gutmann–Beckett method was also used for porous solid acids using ssNMR. However, in this case, the NMR lines are broadened not only because of the measurement in solid state but also because of the varying chemical environments within the pores. (22)
From a comparison of all 31P MAS NMR spectra (Figure 5) of the investigated samples, it is evident that TEPO interacts only with ActB-Tol and ActB-cyHx and forms LA-TEPO adducts. Interestingly, the signals are very narrow (half-width of dominant signals is ca. 1.3 ± 0.2 kHz), displaying similar sharpness as zeolite-Y-TEPO adduct and much higher sharpness than γ-alumina or micromesoporous zirconosilicates. (22) Therefore, the LA strength can be compared: BCF > ActB-Tol > ActB-cyHx. Given that ActBs do not contain any perfluorinated electron-withdrawing substituents, it points to a remarkable Lewis acidity of ActB-Tol. On the other hand, only one sharp signal, with almost unchanged position in comparison with pristine TEPO, was detected in the 31P MAS NMR spectrum of ActB-nHx. This indicates the presence of TEPO molecules in the investigated system but without any detectable chemical interaction.

Figure 5

Figure 5. 31P MAS ssNMR spectra from top: BCF+TEPO adduct, TEPO, ActB-Tol+TEPO, ActB-cyHx+TEPO, and ActB-nHx+TEPO.

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11B 3Q/MAS NMR spectra after the adsorption of TEPO (Figure 6) display one specific, relatively sharp signal with 11B δiso = 1 ppm (δiso=17F1+10F210, where F1 = 1.3 ± 0.5 ppm and F2 = 0.5 ± 0.5 ppm) in ActB-Tol+TEPO and ActB-cyHx+TEPO systems. The shape of the signal and 11B NMR shift suggest an almost ideal tetracoordinated geometry of affected boron atoms. (23) This clearly confirms the Lewis acidity of ActB-Tol and ActB-cyHx by the formation of tetracoordinated borons upon reaction with the Lewis base (TEPO). It should be noted that only a small fraction of borons in ActBs are LA sites, and therefore, the peaks are relatively small in comparison with the rest of the spectra. In 13C CP/MAS NMR spectra, ActB-TEPO samples contain additional signals corresponding to TEPO and toluene, which come from the TEPO treatment; otherwise, the spectra are unchanged in comparison with as-prepared ActBs, thus confirming the preserved disordered structure (see Figure S9).

Figure 6

Figure 6. 11B 3Q/MAS NMR spectra of ActB-Tol+TEPO (a), ActB-cyHx+TEPO (b), and ActB-nHx+TEPO (c). The newly detected signal is highlighted by gray boxes.

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To quantify the number of acid sites, we employed temperature-programmed desorption of ammonia (TPD). All three samples provided a similar pattern in which ammonia desorbs first at ∼120 °C (weakly adsorbed NH3) and then at ∼420 °C (strongly chemisorbed NH3; Figure 7 and Table 2). The fraction of the weakly adsorbed NH3 out of the total number of NH3 molecules adsorbed is very uniform for all three samples and ranges from 7 to 8%. Of note are the high ammonia desorption temperatures (varying from 403 °C for ActB-cyHx to 438 °C for ActB-Tol) that lie within the range of the strong Brønsted acid sites present in HZSM-5 zeolite (400–470 °C) (24) and much higher than for the acid sites in amorphous aluminosilicates (250–300 °C). (25) Importantly, ActB-Tol possesses the strongest acid sites according to ammonia TPD, which is in line with 31P MAS NMR after TEPO adsorption; see above.

Figure 7

Figure 7. TPD curves for ActB-Tol (left), ActB-cyHx (middle), and ActB-nHx (right).

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Table 2. Summary of Ammonia TPDa
sampleweakly adsorbed NH3 (mmol g–1)chemisorbed NH3 (mmol g–1)Tdes of chemisorbed NH3 (°C)
ActB-Tol0.303.47438
ActB-cyHx0.608.10403
ActB-nHx0.667.31415
a

The amounts of adsorbed NH3 were estimated from the areas under the TPD curves.

The most significant difference between the ActBs samples is the total number of NH3 molecules adsorbed (Table 2). While ActB-Tol exhibited 3.77 mmol g–1 adsorbed NH3 in total, ActB-nHx and ActB-cyHx provided more than twice as much, with the maximum observed for ActB-cyHx (8.70 mmol g–1). These results indicate that ActB-cyHx and ActB-nHx contain a significantly higher number of acid sites in comparison to ActB-Tol. Interestingly, ActB-nHx contains a high number of acid sites according to NH3 TPD but virtually none according to 31P MAS NMR after TEPO adsorption. We assume that these differences originate in the different size and basicity of the probe molecules. Nevertheless, all obtained values are very high, suggesting that there might be multiple NH3 molecules adsorbed to each available LA site and/or strongly bound NH3 in other areas within the framework not only to the LA centers.
To analyze the ability of ActBs to adsorb medium-sized molecules, pyridine and benzene were preadsorbed onto each sample at RT followed by TGA measurement. The difference between the adsorption of neutral molecules (benzene) and basic molecules (pyridine) can be used as a proxy for the estimation of the number of accessible acid sites to medium-sized molecules. Thus, the amount of adsorbed pyridine followed the trend ActB-Tol > ActB-cyHx > ActB-nHx. Based on these data, we can assume that the number of LA sites accessible to medium-sized molecules is between 1 and 1.5 mmol g–1 for ActB-Tol and ActB-cyHx and yet probably only around 0.5 mmol g–1 for ActB-nHx (see Table 3). This correlates well with the specific surface area of the ActBs materials as determined by Ar adsorption and the Gutmann–Beckett method utilizing TEPO adsorption but not complementary with the ammonia TPD (for a detailed discussion, see the SI).
Table 3. Summary of Quantification of Probe Molecular Adsorption
sampleadsorbed benzene (mmol g–1)adsorbed pyridine (mmol g–1)chemisorbed NH3 (mmol g–1)Δpyridine–benzene (mmol g–1)a
ActB-Tol0.82.13.471.3
ActB-cyHx0.31.38.101.0
ActB-nHx0.30.77.310.4
a

The difference between absorbed benzene and absorbed pyridine.

The origin of the LA boron atoms probably lies in the solvothermal thermolysis of open clusters. It has been observed earlier (e.g., see the work of J. Kennedy (26)) that open borane clusters subject to high energy (e.g., thermolysis, photolysis, and electron bombardment) form clusters with different number of vertices. We made similar observations during the thermal treatment of nido-B10H14 in benzene at 200 °C where we detected substituted B18H22 clusters. (17) This change in the number of boron vertices is probably accompanied by the formation of highly reactive lower boranes that are the seeds for LA centers.

Catalytic Activity

Hydrosilylation and Deoxygenation Reactions

Lewis acidic boron molecules, e.g., BCF, are known to activate the Si–H bond. (27) This can be utilized in hydrosilylation or reductive deoxygenation of carbonyl compounds in mild conditions. (28) Such reactions cannot be catalyzed by conventional heterogeneous inorganic LA catalysts, e.g., zeolites, which are, on the other hand, known to promote hydrodeoxygenation (HDO) processes during biomass processing. (29) These reactions, however, typically require high temperatures and pressures of hydrogen gas, as well as the presence of metal dopants, which are responsible for the hydrogenation reactivity. In addition, hydrosilylation reactions, related to those described herein, were only reported using solid-supported metal-based catalysts in several literature reports. (30) For this reason, we endeavored to test the new ActBs described here for their ability to catalyze the reduction of selected substrates with silanes. Initial testing was carried out in batch reactions with an exclusion of air and moisture, typically employing 1 mmol of the corresponding substrate, 20 mg of the ActB catalyst, 2 mL of solvent, and an excess of Et3SiH (or other silanes).
Benzophenone (1) as a model substrate was cleanly converted over both ActB-Tol and ActB-cyHx materials to the corresponding deoxygenated product, diphenylmethane (2), by Et3SiH in toluene (Scheme 2), whereas the silyl ether intermediate 3 was only observed when using the ActB-nHx material at the conditions applied. Using 1.5 equiv of Et3SiH with ActB-Tol gave incomplete conversion, while 3 equiv of Et3SiH afforded full conversion, and the product was subsequently isolated by column chromatography in an 88% yield. Expectedly, the siloxane (Et3Si)2O was identified as the only other product of deoxygenation. The kinetic profiles at 60 and 100 °C show that ActB-Tol is slightly more active than ActB-cyHx, while ActB-nHx exhibits only poor activity in this deoxygenation reaction.

Scheme 2

Scheme 2. Deoxygenation of Benzophenone (1) Performed Using Different ActB Catalysts, Yields Determined by GC (Isolated Yield in Parentheses)a
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a1 mol % B(C6F5)3 (BCF) used as a catalyst (reaction time 2 h). kinetic profiles (bottom) of the deoxygenation of 1 catalyzed by ActBs at 60 or 100 °C. Conversion of 1 determined by GC.

These differences in performance are consistent with the observed relative LA strengths of the ActB materials, where ActB-Tol contained the most strongly acidic sites and ActB-nHx contained the weakest or least accessible LA sites. To assess the relevance of these results, we performed a comparison test with the BCF catalyst. Thus, the exact same deoxygenation reaction was done but in the presence of 1 mol % of BCF as a catalyst. Here, the BCF converted 1 in 2 h, but in contrast to the ActB-Tol and ActB-cyHx catalysts, BCF gave predominantly the silylated product 3 (84%) and only a small amount of 2. We also tested the pristine nido-B10H14 as well as a sample prepared by thermolysis of nido-B10H14 without solvent at the same conditions as applied for the synthesis of ActB materials (dark brown powder without detectable porosity measured by Ar adsorption at 87 K). Neither of these, however, exhibited any catalytic activity in the above transformation (for details, see the SI).
Next, we employed acetophenone (4) as a substrate for ActB catalytic assessment. In general, the reaction proceeded more slowly than for benzophenone, and the selectivity was greatly affected by conditions; see Scheme 3. ActB-Tol displayed a clear preference for deoxygenation under all tested conditions, with no detectable amounts of the silyl ether product 5. Significant formation of styrene (6) was observed, accompanied by ethylbenzene (7). The relative amount of 7 was maximized at higher reaction temperatures (100 °C being the optimum) when an excess of silane was used (Scheme 3b), which also ensured full conversion of the starting material. ActB-cyHx, in contrast, gave predominantly hydrosilylation product 5 irrespective of the amount of silane used. Prolonged reaction times with less available silane generally increased the amount of styrene formed. In contrast, ActB-nHx was much less active but gave almost exclusively the hydrosilylation product 5.

Scheme 3

Scheme 3. Hydrosilylation/deoxygenation of Acetophenone (4): (a) Screening of Different ActB Materials Using 1.5 or 3 equiv of Silane and Toluene as the Solvent; (b) Reaction Temperature Optimization Using ActB-Tol in Toluene; and (c) Variation of Reaction Conditions in THF Solvent. Yields Determined by GC
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The styrene formation has been previously noted as a minor product in reactions of 4 catalyzed by BCF; (31) its formation was significantly affected when 60 bar of H2 was applied combined with the presence of molecular sieves. (32) In our hands, a control reaction employing BCF as a catalyst yielded only a mixture of ethylbenzene 7 (70%) and silyl ether 5 (25%), while no styrene was detected. Contrarily, the ActB-Tol catalyst under adjusted conditions (THF solvent, 60 °C; see Scheme 3c) afforded styrene (6) as the major product with a 65% selectivity (at an 89% conversion of 4). It should be noted that the formation of olefins from ketones on acidic zeolite catalysts at temperatures above 250 °C yielding mixture of hydrocarbons is well-recognized. (33)
Reaction conditions were optimized for various silane hydrides, solvents, reaction times, temperatures, and type of ActB catalyst. The results can be summarized as follows: (a) best conversions were achieved with secondary silanes (Et2SiH2 or Ph2SiH2), yet it was still comparable with cheaper Et3SiH; (b) by means of conversion of acetophenone 4, toluene appears to be the solvent of choice followed by THF; (c) selectivity is significantly affected by solvent and catalyst type: ActB-Tol showed increased selectivity toward the formation of styrene, whereas ActB-cyHx yields predominantly silyl ether 5; (d) excess of silane favors the formation of symmetrical ether 8; and (e) moderate temperature favors the formation of ethers 5 and 8, whereas increased temperature deoxygenation products 6 and 7. For a reaction overview, see Scheme 3, and for complete results, see the SI.
The effect of exposure of the ActB-Tol catalyst to air for a limited time (1 h) was studied. Interestingly, the activity does not seem to be affected much, which highlights the relative robustness of the ActB catalysts. Additionally, we tested the variability of catalytic properties of different batches of ActB-Tol in the hydrosilylation/deoxygenation of 1 and 4. Results summarized in Table S16 confirmed the good reproducibility of activity as well as selectivity.
In order to uncover the reaction mechanism of acetophenone transformation, we performed additional experiments employing products 5 and 6 and the related rac-1-phenylethanol (9) (Scheme 4). Silyl ether 5 is a plausible intermediate in the formation of both 6 and 7, while 6 is the preferred product in the absence of silane as shown in (reaction b) in Scheme 4. When 3 equiv of Et3SiH was applied (reaction a) in (Scheme 4), 7 becomes the dominant product, as expected. Styrene (6), when exposed to the standard reaction conditions as depicted in (reaction c), is not converted to any other molecular products even in the presence of the silane. Alcohol 9 is also completely converted to a mixture of products 5–7 under the same conditions (reaction d). The formation of styrene under these conditions, and especially without the presence of silane (reaction e), caught our attention as this is formally a dehydration reaction. (34) This transformation was further studied in the context of the technologically important dehydration of ethanol to ethylene; see below.

Scheme 4

Scheme 4. Reactions Involved in the Mechanism of 4-Hydrosilylation/Deoxygenation by ActB-Tol: (a) Deoxygenation of Silyl Ether 5 in the Presence of Silane, (b) without Silane, (c) Attempted Reaction of Styrene (6), (d) Reaction of Alcohol 9 in the Presence of Silane, and (e) without Silane. Conditions: 60 °C, 22 h, Toluene Solvent (1 mmol Substrate, 20 mg of ActB). GC Yields Given in Parentheses
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ActB-Tol and ActB-cyHx catalysts were further tested in the hydrosilylation/deoxygenation reaction of various other carbonyl-containing substrates (Scheme 5), namely, benzaldehyde (10), trans-chalcone (13), benzil (16), cyclohexanone (22), and 2-heptanone (27). It should be noted that in all cases, the reaction conversion was higher than 90%, often yielding a mixture of products, the composition of which is strongly dependent on the conditions used.

Scheme 5

Scheme 5. Hydrosilylation/Deoxygenation of (a) Benzaldehyde (10), (b) Trans-chalcone (13), (c) Benzil (16), (d) Cyclohexanone (22), and (e) 2-Heptanone (27), using ActB-Tol or ActB-cyHx as the Catalysts at Various Conditions (Silane Stoichiometry, Temperature, Time) as Indicated. All Reactions Performed in Toluene. Yields Determined by GC
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While each substrate behaves uniquely, it is evident that it follows similar trends as in the case of acetophenone: with ActB-Tol, higher temperature and excess of Et3SiH favoring deoxygenation products or the formation of ethers, while ActB-cyHx affording predominantly hydrosilylated products. The formation of hydrosilylated products was obtained in much higher selectivities, which could be because of the milder conditions used. For full details of the optimization procedures and discussion of each substrate, see the SI.

Flow Conditions

On the basis of the batch experiments, ActB-Tol and ActB-cyHx catalysts were chosen for further investigation under flow conditions using a microfluidics-based flow reactor (X-Cube) in continuous flow mode. CatCart cartridges were loaded with the catalysts, and the catalyst bed was washed continuously with toluene solutions of the reactants and the resulting reaction mixtures were analyzed by GC. Based on the results from the preparative batch experiments described above, two reactions with high selectivity were chosen to test using flow conditions─the benzophenone (1) deoxygenation catalyzed by ActB-Tol and trans-chalcone (13) 1,4-hydrosilylation catalyzed by ActB-cyHx. In both cases, the catalysts were able to operate in the flow mode, giving initially full conversion of the substrate to the corresponding product when running with a flow rate of 0.1 mL min–1. Gradual loss of activity was observed for the ActB-Tol catalyst, while the ActB-cyHx also started losing its activity after ca. 2 h on stream (Figure 8). The same behavior was observed even when the flow rate was increased to 0.2 mL min–1. The leaching of boron moieties into the reaction mixture during the flow experiments was monitored by ICP-MS. The boron concentration in samples taken between 30 min and 2 h on stream was identical to the starting reaction mixture. For this reason, we conclude that the structure of ActBs is stable but its catalytic centers are gradually deactivated under hydrosilylation/deoxygenation reaction conditions.

Figure 8

Figure 8. Time-dependence of the catalytic activity of ActB-Tol in benzophenone (1) deoxygenation and ActB-cyHx in trans-chalcone (13) 1,4-hydrosilylation under flow conditions (X-Cube, 100 °C, flow rate: 0.1 mL min–1─solid lines─or 0.2 mL min–1─segmented line, concentration of substrates 0.5 mM mL–1).

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Dehydration of Ethanol

As noted above, the reaction of 1-phenylethanol in the absence of silane produced mostly the products of dehydration (styrene and ether (8)). These results gave us reason to test ActBs for the industrially relevant gas-phase dehydration of ethanol and compare catalytic properties with traditional solid acid catalysts, namely, zeolite HZSM-5 (24bc25a35) and γ-alumina. (25b36) To validate the need for strong LA sites present in ActBs, we performed a blank experiment: impregnated activated charcoal with boric acid in B: C molar ratio is similar to ActBs.
Ethanol dehydration was performed within the temperature range of 170 and 240 °C; see Figure 9. The conversion decreases in the following order: HZSM-5>ActB-cyHx>ActB-Tol ≈ ActB-nHx ≈ γ-Al2O3, with ethylene yields following a similar pattern. Thus, ActB-cyHx achieved conversions over 90% and a ca. 80% yield of ethylene, while the control-activated charcoal–boric acid material was inactive throughout the whole temperature range.

Figure 9

Figure 9. Ethanol conversion (top) and ethylene yield (bottom) during ethanol dehydration at 170, 190, 210, and 240 °C. Weight hour space velocity (WHSV) was kept for all measurements at 2.2 g g–1 h–1.

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The major products of the dehydration reactions were ethylene and diethyl ether, the ratio of which is dependent on the temperature. In this regard, ActBs behave similarly to many conventional catalysts. Only in the case of HZSM-5 were butenes observed as products of ethylene dimerization, which is in line with the literature. (24b) In contrast to γ-Al2O3 and HZSM-5, all three ActBs produced only small amounts of ethane (ActB-cyHx provided up to 3.6% ethane at 240 °C). We assume that ethane is formed via ethylene hydrogenation, with the hydrogen needed for this step coming from the B–H bonds in ActBs. This hypothesis is in agreement with the following observations: (i) ethane yield progressively decreased with time in all cases, (ii) addition of H2 gas to the reactant flow did not change the amount of formed ethane, and (iii) significant decrease in the intensity of B–H absorption bands in IR spectra (∼2550 cm–1) after the catalytic experiments; see Figure S33.
From all of the ActBs, ActB-cyHx showed the most promising catalytic performance in ethanol dehydration. This behavior can be related to the high number of acidic sites as evidenced by ammonia TPD analysis and the materials’ porosity. ActB-Tol exhibited a similar porosity but a significantly lower number of acidic sites.
In an effort to probe the limits of the stability of ActBs, a 17 h time-on-stream experiment at 240 °C was conducted (Figure 10). Importantly, all ActBs proved to be stable, with ActB-cyHx exerting a stable conversion of ∼93% and an ethylene selectivity of ∼78%. HZSM-5, in particular, has been shown to suffer from ethylene oligomerization, (35b) coking, and deactivation. (37) In line with the literature, HZSM-5 proved to be unstable with ethanol conversion decreasing from 98 to 76% and ethylene yield dropping from 99 to 62%. For a detailed discussion, all ethanol conversions, selectivities, and carbon balances, see the SI.

Figure 10

Figure 10. Ethanol conversion (top) and ethylene yield (bottom) during ethanol dehydration at 240 °C overnight (stability test). WHSV was kept at 4.4 g g–1 h–1 except for HZSM-5 due to its high activity. WHSV was set to 17.6 g g–1 h–1; for details, see the SI.

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Conclusions

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We have demonstrated that controlled cothermolyses of decaborane with aromatic or aliphatic hydrocarbons in strictly anaerobic conditions give highly porous and halide-free solid materials, ActBs, containing high numbers of Lewis acidic (LA) sites in their structures. These sites give these new materials a level of catalytic activity that approaches that of the well-established molecular borane LA B(C6F5)3 but with the added benefit of being a heterogeneous catalyst, allowing the use in flow reactors. The strength and number of LA sites in ActBs seem to be controllable by the nature of the hydrocarbon linkers incorporated into the material. We proved ActBs to be valuable heterogeneous catalysts in batch as well as flow conditions for hydrosilylation/deoxygenation reactions of various carbonyl substrates using silanes as the reductants. More importantly, not only can ActBs be used for analogous reactions as catalyzed by B(C6F5)3 but they also show some unique reactivity, e.g., the formation of styrene from acetophenone. In general, the toluene-based ActB-Tol preferred deoxygenation pathways in comparison to the other two catalysts with aliphatic linkers, which were, in most cases, more selective toward the silylation products.
The potential for dehydration reactions was studied in detail in ethanol dehydration in the gas phase, which revealed the capability of ActB catalysts to operate efficiently under flow conditions at 240 °C with excellent stability over 17 h on stream. The activity and selectivity for ethylene exceed those of commonly used γ-alumina catalysts. Further investigation of the effects of the catalytic material composition on its LA and consequently catalytic properties as well as utilization in other types of catalytic reactions is currently under way.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.3c04011.

  • Full experimental details of syntheses and catalytic experiments, additional characterization data for the ActB materials, detailed results of catalytic investigations (product distribution, characterization data) including an extended discussion. The authors have cited additional references within the Supporting Information. (38) (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Béla Urbán - Department of Molecular Electrochemistry and Catalysis, J. Heyrovsky Institute of Physical Chemistry of the Czech Academy of Sciences Dolejškova 2155, 182 00 Prague 8, Czech Republic
    • Michal Horáček - Department of Molecular Electrochemistry and Catalysis, J. Heyrovsky Institute of Physical Chemistry of the Czech Academy of Sciences Dolejškova 2155, 182 00 Prague 8, Czech Republic;  Orcidhttps://orcid.org/0000-0002-3203-3080
    • Daniel Bůžek - Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic;  Orcidhttps://orcid.org/0000-0002-7387-9461
    • Lucie Leonová - Department of Chemistry, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic;  Orcidhttps://orcid.org/0000-0002-8002-8307
    • Aleš Stýskalík - Department of Chemistry, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic
    • Anna Vykydalová - Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic
    • Karel Škoch - Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic
    • Matouš Kloda - Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic
    • Andrii Mahun - Department of Structural Analysis, Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Prague 6, Czech Republic;  Orcidhttps://orcid.org/0000-0003-1057-2910
    • Libor Kobera - Department of Structural Analysis, Institute of Macromolecular Chemistry of the Czech Academy of Sciences, Heyrovského nám. 2, 162 06 Prague 6, Czech Republic;  Orcidhttps://orcid.org/0000-0002-8826-948X
    • Kamil Lang - Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic;  Orcidhttps://orcid.org/0000-0002-4151-8805
    • Michael G. S. Londesborough - Department of Materials Chemistry, Institute of Inorganic Chemistry of the Czech Academy of Sciences, Husinec-Řež 1001, 250 68 Řež, Czech Republic;  Orcidhttps://orcid.org/0000-0002-6161-6592
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the Czech Science Foundation (No. GA 23-05818S), Research Infrastructure NanoEnviCz, the Ministry of Education, Youth and Sports of the Czech Republic under Project no. LM2023066, and the Ministry of Education, Youth and Sports of the Czech Republic and The European Union─European Structural and Investments Funds within the project Pro-NanoEnviCz II (Project No. CZ.02.1.01/0.0/0.0/18_046/0015586); the ssNMR measurement was supported by No. GA 20-01233S; and the catalytic testing of dehydration of ethanol was supported by the Grant Agency of Masaryk University (GAMU) No. MUNI/J/0007/2021. The authors are grateful to Dr. Jan Šubrt for SEM measurements, Slavomír Adamec for ICP-MS measurements, and Dr. Francesco Walenszus (3P Instruments GmbH) for TPD measurements.

References

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    Non-zeolitic supported boria-alumina catalysts where B is part of the catalyst lattice, were tested for the cyclohexanone-oxime vapor phase rearrangement to caprolactam. The catalysts were obtained by the sol-gel route using (NH4)2B4O7. The catalyst compn. and structure were characterized using solid state 27Al- and 11B-MAS-NMR and x-ray powder diffraction; the acidity was detd. by TPD [temp. programmed desorption] of ammonia. The effect of B/Al ratio and the role of different polar agents in the feed were also studied. Total cyclohexanone-oxime conversion was attained using catalysts with low no. of acid sites and the selectivity toward caprolactam is strongly dependent on the type and distribution of acid sites. The B2O3-Al2O3 sol-gel catalysts have long lifetime but low selectivity toward caprolactam and loss of B occurred during regeneration of spent catalyst by heating at 500° for 15 h.
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    (b) Kröcher, O.; Elsener, M. Hydrolysis and oxidation of gaseous HCN over heterogeneous catalysts. Appl. Catal., B 2009, 92, 75– 89,  DOI: 10.1016/j.apcatb.2009.07.021
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    (c) Curtin, T.; McMonagle, J. B.; Hodnett, B. K. Influence of boria loading on the acidity of B2O3/Al2O3 catalysts for the conversion of cyclohexanone oxime to caprolactam. Appl. Catal., A 1992, 93, 91– 101,  DOI: 10.1016/0926-860X(92)80296-O
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    6c
    Influence of boria loading on the acidity of boron oxide (B2O3)/alumina catalysts for the conversion of cyclohexanone oxime to caprolactam
    Curtin, T.; McMonagle, J. B.; Hodnett, B. K.
    Applied Catalysis, A: General (1992), 93 (1), 91-101CODEN: ACAGE4; ISSN:0926-860X.
    Boria catalysts supported on alumina were prepd. with boria loadings of 0-20 wt.%. These materials were characterized by BET surface area measurements, x-ray diffraction, and temp.-programmed desorption of NH3, and were used as catalysts for the Beckmann rearrangement of cyclohexanone oxime (I) to caprolactam (II) by passing the I vapor through a reactor contg. the catalysts at 300°. The surface areas of the samples decreased as the boria loading increased, and all catalysts tested for the rearrangement reaction diminished in activity with time-onstream. There was a direct relationship between the amt. of coke which formed on the surface and the loss in catalytic activity. This loss was least for the highest boria loading tested, and this catalyst also featured the least amt. of coke formation. Selectivity to II was assocd. with the presence of surface acidic sites of intermediate strength. A correlation was obsd. between the concn. of these surface sites and the selectivity to II.
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  7. 7
    Yang, W.; Kim, K. D.; O’Dell, L. A.; Wang, L.; Xu, H.; Ruan, M.; Wang, W.; Ryoo, R.; Jiang, Y.; Huang, J. Brønsted acid sites formation through penta-coordinated aluminum species on alumina-boria for phenylglyoxal conversion. J. Catal. 2022, 416, 375– 386,  DOI: 10.1016/j.jcat.2022.11.012
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    7
    Bronsted acid sites formation through penta-coordinated aluminum species on alumina-boria for phenylglyoxal conversion
    Yang, Wenjie; Kim, Kyung Duk; O'Dell, Luke A.; Wang, Lizhuo; Xu, Haimei; Ruan, Mengtong; Wang, Wei; Ryoo, Ryong; Jiang, Yijiao; Huang, Jun
    Journal of Catalysis (2022), 416 (), 375-386CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    Recently, Penta-coordinated Al (AlV) have attracted large attention and is promising for tailoring catalysts with the capability to form new BAS in specific mixed oxide. In this study, amorphous alumina-boria materials with enriched surface AlV were prepd. as platforms for the formation of potential new BAS. With the aid of solid-state NMR, interacting species of AlV with three-coordinated boron have been identified in the highly distorted local structure with BAS forming potential (AlV -OH...BIII). The catalytic activity of the AlV-BAS has been tested by the phenylglyoxal conversion, where the functionality of AlV-BAS has been further verified by the in situ 1H NMR. Herein, the ssNMR observation demonstrates the first exptl. visualization of a new type AlV-BAS in non-silica alumina materials present with predominance. The AlV-BAS is promising to serve as one new state-of-the-art strategy to improve acid catalytic performance in varied mixed oxides.
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  8. 8
    (a) Lam, J.; Szkop, K. M.; Mosaferi, E.; Stephan, D. W. FLP catalysis: main group hydrogenations of organic unsaturated substrates. Chem. Soc. Rev. 2019, 48, 3592– 3612,  DOI: 10.1039/C8CS00277K
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    8a
    FLP catalysis: main group hydrogenations of organic unsaturated substrates
    Lam, Jolie; Szkop, Kevin M.; Mosaferi, Eliar; Stephan, Douglas W.
    Chemical Society Reviews (2019), 48 (13), 3592-3612CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. This article is focused on recent developments in main group mediated hydrogenation chem. and catalysis using 'frustrated Lewis pairs' (FLPs). The broading range of substrates and catalyst systems is reviewed and the advances in catalytic redns. and the development of stereoselective, asym. redns. made since 2012 was considered.
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    (b) Stephan, D. W. Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen. J. Am. Chem. Soc. 2021, 143, 20002– 20014,  DOI: 10.1021/jacs.1c10845
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    8b
    Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen
    Stephan, Douglas W.
    Journal of the American Chemical Society (2021), 143 (48), 20002-20014CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A review. This perspective article focuses on the new directions and developments that are emerging from this frustrated Lewis pair (FLP) chem. involving hydrogen. Three areas are discussed including new applications and approaches to FLP redns., the redns. of small mols., and the advances in heterogeneous FLP systems. These foci serve to illustrate that despite having its roots in main group chem., this simple concept of FLPs is being applied across the discipline.
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  9. 9
    (a) Piers, W. E.; Chivers, T. Pentafluorophenylboranes: from obscurity to applications. Chem. Soc. Rev. 1997, 26, 345– 354,  DOI: 10.1039/cs9972600345
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    9a
    Pentafluorophenylboranes: from obscurity to applications
    Piers, Warren E.; Chivers, Tristram
    Chemical Society Reviews (1997), 26 (5), 345-354CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    Pentafluorophenyl substituted boranes and borates are important as co-catalysts in metallocene-based industrial processes for the homogeneous polymn. of olefins. Although 1st prepd. in the early 1960s, the remarkable properties of tris(pentafluorophenyl)borane have only recently been exploited for applications in catalysis. Spurred by these developments, the related compds. bis(pentafluorophenyl)borane and salts of the tetrakis(pentafluorophenyl)borate anion also found application in olefin polymn. and other fields. The rise of pentafluorophenyl B compds. from curiosities to important commodities is reviewed with 39 refs.
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    (b) Lawson, J. R.; Melen, R. L. Tris(pentafluorophenyl)borane and Beyond: Modern Advances in Borylation Chemistry. Inorg. Chem. 2017, 56, 8627– 8643,  DOI: 10.1021/acs.inorgchem.6b02911
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    9b
    Tris(pentafluorophenyl)borane and beyond: modern advances in borylation chemistry
    Lawson, James R.; Melen, Rebecca L.
    Inorganic Chemistry (2017), 56 (15), 8627-8643CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    A review. As main-group chem., in particular boron chem., has expanded and developed over the past 20 years, one reagent has risen to prominence as well. Tris(pentafluorophenyl)borane, B(C6F5)3 (commonly known as BCF), has demonstrated extensive applications in a wide variety of reactions, including borylation, hydrogenation, hydrosilylation, frustrated Lewis pair (FLP) chem., Lewis acid catalysis, and more. The high Lewis acidity of B(C6F5)3 is derived from the electronic effects of its three C6F5 rings, rendering it a versatile reagent for a great no. of reactions. In addn., the steric bulk of these rings also allows it to function as the Lewis acid in a FLP, granting this reagent yet another synthetically useful application. However, as main-group chem. continues to evolve as a field, new reagents are required that go beyond BCF, increasing not only the range of reactions available but also the breadth of compds. attainable. Great strides have already been made in order to accomplish this task, and this review will highlight modern advances in boron chem. relating to borylation reactions. Herein, we will show the recent uses of B(C6F5)3 in borylation reactions while also focusing on current advances in novel borane and borocation usage that eclipses that of the stalwart B(C6F5)3.
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  10. 10
    Wanglee, Y.-J.; Hu, J.; White, R. E.; Lee, M.-Y.; Stewart, S. M.; Perrotin, P.; Scott, S. L. Borane-Induced Dehydration of Silica and the Ensuing Water-Catalyzed Grafting of B(C6F5)3 To Give a Supported, Single-Site Lewis Acid, ≡SiOB(C6F5)2. J. Am. Chem. Soc. 2012, 134, 355– 366,  DOI: 10.1021/ja207838j
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    10
    Borane-induced dehydration of silica and the ensuing water-catalyzed grafting of B(C6F5)3 to give a supported, single-site Lewis acid, ≡SiOB(C6F5)2
    Wanglee, Ying-Jen; Hu, Jerry; White, Rosemary E.; Lee, Ming-Yung; Stewart, S. Michael; Perrotin, Philippe; Scott, Susannah L.
    Journal of the American Chemical Society (2012), 134 (1), 355-366CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Reaction of B(C6F5)3 with silica surface in the presence of water traces was used for immobilization of borane Lewis acid as a cocatalyst in zirconocene- and nickel-catalyzed alkene polymn. A supported, single-site Lewis acid, ≡SiOB(C6F5)2, was prepd. by water-catalyzed grafting of B(C6F5)3 onto the surface of amorphous silica, and its subsequent use as a cocatalyst for heterogeneous olefin polymn. was explored. Although B(C6F5)3 has been reported to be unreactive toward silica in the absence of a Bronsted base, we find that it can be grafted even at room temp., albeit slowly. The mechanism was investigated by 1H and 19F NMR, in both the soln. and solid states. In the presence of a trace amt. of H2O, either added intentionally or formed in situ by borane-induced dehydration of silanol pairs, the adduct (C6F5)3B·OH2 hydrolyzes to afford C6F5H and (C6F5)2BOH. The latter reacts with the surface hydroxyl groups of silica to yield ≡SiOB(C6F5)2 sites and regenerate H2O. When B(C6F5)3 is present in excess, the resulting grafted boranes appear to be completely dry, due to the eventual formation of [(C6F5)2B]2O. The immobilized, tri-coordinate Lewis acid sites were characterized by solid-state 11B and 19F NMR, IR, elemental anal., and C5H5N-time-programmed desorption (TPD). Their ability to activate two mol. C2H4 polymn. catalysts, Cp2ZrMe2 and an (α-iminocarboxamidato)nickel(II) complex, was explored.
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  11. 11
    Tian, J.; Wang, S.; Feng, Y.; Li, J.; Collins, S. Borane-functionalized oxide supports: development of active supported metallocene catalysts at low aluminoxane loading. J. Mol. Catal. A: Chem. 1999, 144, 137– 150,  DOI: 10.1016/S1381-1169(98)00341-0
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    11
    Borane-functionalized oxide supports: development of active supported metallocene catalysts at low aluminoxane loading
    Tian, Jun; Wang, Shaotian; Feng, Yuding; Li, Jieming; Collins, Scott
    Journal of Molecular Catalysis A: Chemical (1999), 144 (1), 137-150CODEN: JMCCF2; ISSN:1381-1169. (Elsevier Science B.V.)
    Treatment of hydroxylated silica or alumina with tris(perfluorophenyl)borane (1), bis(perfluorophenyl)borane (2) or bis(perfluorophenyl)boron chloride (3), provides borane-functionalized supports of variable compn., as revealed by in situ monitoring by 19F NMR spectroscopy and/or elemental anal. These chem. treated supports can be impregnated with Cp2ZrMe2 to provide supported catalysts for ethylene polymn. Although some of these supported catalysts are active for ethylene polymn. in the presence of alkylaluminum compds. (e.g., TMA, TIBAL), all of these catalysts are more efficiently activated in the presence of small quantities of Me aluminoxane, even at very low Al:Zr ratios of 10:1. The polymer properties are quite similar to those produced using the sol. catalyst Cp2ZrMe2/B(C6F5)3, again in the presence of MAO at low loading. A variety of expts. suggest that minimal leaching of the metallocene complex from the support occurs under the conditions studied; in particular, prodn. of polyethylene with high bulk densities (>0.2 g/cm3) and little reactor fouling, even at elevated temp. in toluene slurry, is obsd.
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  12. 12
    Correa, S. A.; Diaz-Droguett, D. E.; Galland, G. B.; Maraschin, T. G.; De Sousa Basso, N.; Dogan, F.; Rojas, R. S. Modification of rGO by B(C6F5)3 to generated single-site Lewis Acid rGO-O-B(C6F5)2 as co activator of nickel complex, to produce highly disperse rGO-PE nanocomposite. Appl. Catal., A 2019, 580, 149– 157,  DOI: 10.1016/j.apcata.2019.05.004
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    12
    Modification of rGO by B(C6F5)3 to generated single-site Lewis Acid rGO-O-B(C6F5)2 as co activator of nickel complex, to produce highly disperse rGO-PE nanocomposite
    Correa, Sebastian A.; Diaz-Droguett, D. E.; Galland, Griselda B.; Maraschin, Thuany G.; De Sousa Basso, Nara; Dogan, Fulya; Rojas, Rene S.
    Applied Catalysis, A: General (2019), 580 (), 149-157CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
    We investigated the incorporation of reduced graphene oxide (rGO) into branched polyethylene, achieving nanocomposites with different rGO contents. The strategy included the direct reaction between B(C6F5)3 (BCF) with rGO (reduced graphene oxide) and demonstrated by XPS, 11B, 19F -MAS NMR, FT-IR the efficient modification of the rGO. The resulting solid contains sites -O-B(C6F5)2 capable of acting as co-catalysts in the activation of an (α-iminocarboxamidate)nickel (II) complex. The results show the efficient activation of this nickel catalyst through an exocyclic Lewis acid-base interaction and the subsequent ethylene polymn. We achieved with this heterogeneous catalytic system activity greater than those previously reported for the BCF-Ni, homogenous system, while the characterization of the black colored polymer generated in situ, showed a complete delamination of the rGO. It became the first rGO-B(C6F5)2-Ni system fully characterized and capable of delaminate the rGO support by producing a nanocomposite rGO-LLDPE without loss of activity, compared to the homogeneous system. In addn., the polymer contains about 70% of Me branches and a m.p. higher than 125 °C.
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  13. 13
    (a) Horton, T. A. R.; Wang, M.; Shaver, M. P. Polymeric frustrated Lewis pairs in CO2/cyclic ether coupling catalysis. Chem. Sci. 2022, 13, 3845– 3850,  DOI: 10.1039/D2SC00894G
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    13a
    Polymeric frustrated Lewis pairs in CO2/cyclic ether coupling catalysis
    Horton, Thomas A. R.; Wang, Meng; Shaver, Michael P.
    Chemical Science (2022), 13 (13), 3845-3850CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Frustrated Lewis pairs (FLPs) are now ubiquitous as metal-free catalysts in an array of different chem. transformations. In this paper we show that this reactivity can be transferred to a polymeric system, offering advantageous opportunities at the interface between catalysis and stimuli-responsive materials. Formation of cyclic carbonates from cyclic ethers using CO2 as a C1 feedstock continues to be dominated by metal-based systems. When paired with a suitable nucleophile, discrete aryl or alkyl boranes have shown significant promise as metal-free Lewis acidic alternatives, although catalyst reuse remains illusive. Herein, we leverage the reactivity of FLPs in a polymeric system to promote CO2/cyclic ether coupling catalysis that can be tuned for the desired epoxide or oxetane substrate. Moreover, these macromol. FLPs can be reused across multiple reaction cycles, further increasing their appeal over analogus small mol. systems.
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    (b) Yolsal, U.; Horton, T. A. R.; Wang, M.; Shaver, M. P. Cyclic Ether Triggers for Polymeric Frustrated Lewis Pair Gels. J. Am. Chem. Soc. 2021, 143, 12980– 12984,  DOI: 10.1021/jacs.1c06408
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    13b
    Cyclic Ether Triggers for Polymeric Frustrated Lewis Pair Gels
    Yolsal, Utku; Horton, Thomas A. R.; Wang, Meng; Shaver, Michael P.
    Journal of the American Chemical Society (2021), 143 (33), 12980-12984CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Sterically hindered Lewis acid and base centers are unable to form Lewis adducts, instead forming frustrated Lewis pairs (FLPs), where latent reactivity can be utilized for the activation of small mols. Applying FLP chem. into polymeric frameworks transforms this chem. into responsive and functional materials. Here, we report a versatile synthesis strategy for the prepn. of macromol. FLPs and explore its potential with the ring-opening reactions of cyclic ethers. Addn. of the cyclic substrates triggered polymer network formation, where the extent of crosslinking, strength of network, and reactivity are tuned by the steric and electronic properties of the ethers. The resultant networks behave like covalently crosslinked polymers, demonstrating the versatility of FLPs to simultaneously tune both small-mol. capture and mech. properties of materials.
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    (c) Vidal, F.; McQuade, J.; Lalancette, R.; Jäkle, F. ROMP-Boranes as Moisture-Tolerant and Recyclable Lewis Acid Organocatalysts. J. Am. Chem. Soc. 2020, 142, 14427– 14431,  DOI: 10.1021/jacs.0c05454
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    13c
    ROMP-Boranes as Moisture-Tolerant and Recyclable Lewis Acid Organocatalysts
    Vidal, Fernando; McQuade, James; Lalancette, Roger; Jakle, Frieder
    Journal of the American Chemical Society (2020), 142 (34), 14427-14431CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Although widely used in catalysis, the multistep syntheses and high loadings typically employed are limiting broader implementation of highly active tailor-made arylborane Lewis acids and Lewis pairs. Attempts at developing recyclable systems have thus far met with limited success, as general and versatile platforms are yet to be developed. We demonstrate a novel approach that is based on the excellent control and functional group tolerance of ring-opening metathesis polymn. (ROMP). The ROMP of highly Lewis acidic borane-functionalized phenylnorbornenes afforded both a sol. linear copolymer and a cross-linked organogel. The polymers proved highly efficient as recyclable catalysts in the reductive N-alkylation of arylamines under mild conditions and at exceptionally low catalyst loadings. The modular design presented herein can be readily adapted to other finely tuned triarylboranes, enabling wide applications of ROMP-borane polymers as well-defined supported organocatalysts.
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    (d) Chen, L.; Liu, R.; Yan, Q. Polymer Meets Frustrated Lewis Pair: Second-Generation CO2-Responsive Nanosystem for Sustainable CO2 Conversion. Angew. Chem., Int. Ed. 2018, 57, 9336– 9340,  DOI: 10.1002/anie.201804034
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    13d
    Polymer Meets Frustrated Lewis Pair: Second-Generation CO2-Responsive Nanosystem for Sustainable CO2 Conversion
    Chen, Liang; Liu, Renjie; Yan, Qiang
    Angewandte Chemie, International Edition (2018), 57 (30), 9336-9340CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Frustrated Lewis pairs (FLP), a couple comprising a sterically encumbered Lewis acid and Lewis base, can offer latent reactivity for activating inert gas mols. However, their use as a platform for fabricating gas-responsive materials has not yet developed. Merging the FLP concept with polymers, we report a new generation CO2-responsive system, differing from the first-generation ones based on an acid-base equil. mechanism. Two complementary Lewis acidic and basic block copolymers, installing bulky borane- and phosphine-contg. blocks, were built as the macromol. FLP. They can bind CO2 to drive micellar formation, in which CO2 as a cross-linker bridges the block chains. This dative bonding endows the assembly with ultrafast response (<20 s), thermal reversibility, and excellent reproducibility. Moreover, such micelles bound highly active CO2 can function as nanocatalysts for recyclable C1 catalysis, opening a new direction of sustainable CO2 conversion.
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    (e) Yolsal, U.; Horton, T. A. R.; Wang, M.; Shaver, M. P. Polymer-supported Lewis acids and bases: Synthesis and applications. Prog. Polym. Sci. 2020, 111, 101313  DOI: 10.1016/j.progpolymsci.2020.101313
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    13e
    Polymer-supported Lewis acids and bases: Synthesis and applications
    Yolsal, Utku; Horton, Thomas A. R.; Wang, Meng; Shaver, Michael P.
    Progress in Polymer Science (2020), 111 (), 101313CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
    In the past decade the synthesis of novel stimuli-responsive materials has been driven by the pursuit of new applications and, more recently, sustainable and reusable systems. Of these materials, those which incorporate main group Lewis acids (LAs) and bases (LBs) into their polymer backbones have shown extraordinary utility as a result of their synthetic diversity, enabling fine tuning of reactivity and ensuing properties tailored to the desired application. Herein, the recent progress made in the synthesis and applications of macromol. LAs and LBs is being highlighted. Interactions between polymeric LAs and LBs can be exploited to build supramol. polymeric networks based on both conventional and frustrated Lewis pairs, while using either functionality individually enables the prepn. of sensors for anions, cations, explosives and biol. mols. The presence of polymer-supported LAs/LBs in organocatalysis has been extended to controlling polymer morphol., enabled improvements in activity through compartmentalization and the coexistence of classically incompatible functionalities. Finally, the versatility of this field is being demonstrated by highlighting some of the recent advances in CO2 chemisorption systems employing amine-based polymeric LBs for carbon capture and redn.
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  14. 14
    (a) Zhang, C.; Wang, J.; Su, W.; Lin, Z.; Ye, Q. Synthesis, Characterization, and Density Functional Theory Studies of Three-Dimensional Inorganic Analogues of 9,10-Diboraanthracene─A New Class of Lewis Superacids. J. Am. Chem. Soc. 2021, 143, 8552– 8558,  DOI: 10.1021/jacs.1c03057
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    14a
    Synthesis, Characterization, and Density Functional Theory Studies of Three-Dimensional Inorganic Analogs of 9,10-Diboraanthracene-A New Class of Lewis Superacids
    Zhang, Chonghe; Wang, Junyi; Su, Wei; Lin, Zhenyang; Ye, Qing
    Journal of the American Chemical Society (2021), 143 (23), 8552-8558CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The three-dimensional inorg. analogs of 9,10-diboraanthracene, B2X2(C2B10H10)2 (X = Cl, 1; X = Br, 2), were attained by salt elimination of Li2C2B10H10 and trihaloboranes. The Me- and phenyl-substituted compds. B2Me2(C2B10H10)2 (3) and B2Ph2(C2B10H10)2 (4) were obtained by treating 1 or 2 with the corresponding Grignard reagents. These compds. were fully characterized by NMR, cyclic voltammetry (CV), IR, and single-crystal x-ray diffraction analyses. Exptl. (CV and Gutmann-Beckett method) and computational (fluoride ion affinity, hydride ion affinity and LUMO energy) results suggest that the order of Lewis acidity is 2 > 1 > 4 > 3 > SbF5. Treatment of 1 or 2 with HSiEt3 gave a rare neutral borane-silane adduct, (Et3SiH)2B2H2(C2B10H10)2 (5). The equil. of 5 in soln. was thoroughly studied by spectroscopy and quantum calcns.
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    (b) Akram, M. O.; Tidwell, J. R.; Dutton, J. L.; Martin, C. D. Tris(ortho-carboranyl)borane: An Isolable, Halogen-Free, Lewis Superacid. Angew. Chem., Int. Ed. 2022, 61, e202212073  DOI: 10.1002/anie.202212073
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    14b
    Tris(ortho-carboranyl)borane: An Isolable, Halogen-Free, Lewis Superacid
    Akram, Manjur O.; Tidwell, John R.; Dutton, Jason L.; Martin, Caleb D.
    Angewandte Chemie, International Edition (2022), 61 (46), e202212073CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The synthesis of tris(ortho-carboranyl)borane (BoCb3), a single site neutral Lewis superacid, in one pot from com. available materials is achieved. The high fluoride ion affinity (FIA) confirms its classification as a Lewis superacid and the Gutmann-Beckett method as well as adducts with Lewis bases indicate stronger Lewis acidity over the widely used fluorinated aryl boranes. The electron withdrawing effect of ortho-carborane and lack of pi-delocalization of the LUMO rationalize the unusually high Lewis acidity. Catalytic studies indicate that BoCb3 is a superior catalyst for promoting C-F bond functionalization reactions than tris(pentafluorophenyl)borane [B(C6F5)3].
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    (c) Yruegas, S.; Axtell, J. C.; Kirlikovali, K. O.; Spokoyny, A. M.; Martin, C. D. Synthesis of 9-borafluorene analogues featuring a three-dimensional 1,1′-bis(o-carborane) backbone. Chem. Commun. 2019, 55, 2892– 2895,  DOI: 10.1039/C8CC10087J
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    14c
    Synthesis of 9-borafluorene analogues featuring a three-dimensional 1,1'-bis(o-carborane) backbone
    Yruegas, Sam; Axtell, Jonathan C.; Kirlikovali, Kent O.; Spokoyny, Alexander M.; Martin, Caleb D.
    Chemical Communications (Cambridge, United Kingdom) (2019), 55 (20), 2892-2895CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    The synthesis of [1,1'-bis(o-carboranyl)]boranes was achieved through the deprotonation of 1,1'-bis(o-carborane) reagents followed by salt metathesis with (iPr)2NBCl2. X-ray crystallog. confirms planar central BC4 rings and Gutmann-Beckett studies reveal an increase in Lewis acidity at the boron center in comparison to their biphenyl congener, 9-borafluorene.
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  15. 15
    (a) Fujino, A.; Ito, S.-i.; Goto, T.; Ishibiki, R.; Kondo, J. N.; Fujitani, T.; Nakamura, J.; Hosono, H.; Kondo, T. Hydrogenated Borophene Shows Catalytic Activity as Solid Acid. ACS Omega 2019, 4, 14100– 14104,  DOI: 10.1021/acsomega.9b02020
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    15a
    Hydrogenated Borophene Shows Catalytic Activity as Solid Acid
    Fujino, Asahi; Ito, Shin-ichi; Goto, Taiga; Ishibiki, Ryota; Kondo, Junko N.; Fujitani, Tadahiro; Nakamura, Junji; Hosono, Hideo; Kondo, Takahiro
    ACS Omega (2019), 4 (9), 14100-14104CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    The catalytic activity of hydrogenated borophene (HB) sheets in EtOH reforming is reported. HB sheets catalyze the conversion of EtOH to ethylene and H2O at >493 K with high selectivity, independent of the contact time, and with an apparent activation energy of 102.8 ± 5.5 kJ/mol. HB sheets act as solid-acid catalysts.
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    (b) Li, Q.; Kolluru, V. S. C.; Rahn, M. S.; Schwenker, E.; Li, S.; Hennig, R. G.; Darancet, P.; Chan, M. K. Y.; Hersam, M. C. Synthesis of borophane polymorphs through hydrogenation of borophene. Science 2021, 371, 1143– 1148,  DOI: 10.1126/science.abg1874
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    15b
    Synthesis of borophane polymorphs through hydrogenation of borophene
    Li, Qiucheng; Kolluru, Venkata Surya Chaitanya; Rahn, Matthew S.; Schwenker, Eric; Li, Shaowei; Hennig, Richard G.; Darancet, Pierre; Chan, Maria K. Y.; Hersam, Mark C.
    Science (Washington, DC, United States) (2021), 371 (6534), 1143-1148CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
    Synthetic two-dimensional polymorphs of B, or borophene, have attracted attention because of their anisotropic metallicity, correlated-electron phenomena, and diverse superlattice structures. Although borophene heterostructures were realized, ordered chem. modification of borophene has not yet been reported. Here, the authors synthesize borophane polymorphs by hydrogenating borophene with at. H in ultrahigh vacuum. Through at.-scale imaging, spectroscopy, and first-principles calcns., the most prevalent borophane polymorph possesses a combination of two-center-two-electron B-H and three-center-two-electron B-H-B bonds. Borophane polymorphs are metallic with modified local work functions and can be reversibly returned to pristine borophene through thermal desorption of H. Hydrogenation also provides chem. passivation because borophane reduces oxidn. rates by more than two orders of magnitude after ambient exposure.
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  16. 16
    Bůžek, D.; Škoch, K.; Ondrušová, S.; Kloda, M.; Bavol, D.; Mahun, A.; Kobera, L.; Lang, K.; Londesborough, M. G. S.; Demel, J. ″Activated Borane″ - A Porous Borane Cluster Network as an Effective Adsorbent for Removing Organic Pollutants. Chem.─Eur. J. 2022, 28, e202201885  DOI: 10.1002/chem.202201885
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    16
    "Activated Borane" - A Porous Borane Cluster Network as an Effective Adsorbent for Removing Organic Pollutants
    Buzek, Daniel; Skoch, Karel; Ondrusova, Sona; Kloda, Matous; Bavol, Dmytro; Mahun, Andrii; Kobera, Libor; Lang, Kamil; Londesborough, Michael G. S.; Demel, Jan
    Chemistry - A European Journal (2022), 28 (67), e202201885CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The unprecedented co-thermolysis of decaborane(14) (nido-B10H14) and toluene results in a novel porous material (that the authors have named "activated borane") contg. micropores between 1.0 and 1.5 nm in diam. and a sp. surface area of 774 m2 g-1 (Ar, 87 K) that is thermally stable up to 1000°C. Solid state 1H, 11B and 13C MAS NMR, UV-vis and IR spectroscopies suggest an amorphous structure of borane clusters interconnected by toluene moieties in a ratio of about three toluene mols. for every borane cluster. In addn., the structure contains Lewis-acidic tri-coordinated boron sites giving it some unique properties. Activated borane displays high sorption capacity for pollutants such as sulfamethoxazole, tramadol, diclofenac and bisphenol A that exceed the capacity of com.-available activated carbon. The consistency in properties for each batch made, and the ease of its synthesis, make activated borane a promising porous material worthy of broad attention.
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  17. 17
    Demel, J.; Kloda, M.; Lang, K.; Škoch, K.; Hynek, J.; Opravil, A.; Novotný, M.; Bould, J.; Ehn, M.; Londesborough, M. G. S. Direct Phenylation of nido-B10H14. J. Org. Chem. 2022, 87, 10034– 10043,  DOI: 10.1021/acs.joc.2c00997
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    17
    Direct Phenylation of nido-B10H14
    Demel, Jan; Kloda, Matous; Lang, Kamil; Skoch, Karel; Hynek, Jan; Opravil, Adam; Novotny, Matyas; Bould, Jonathan; Ehn, Marcel; Londesborough, Michael G. S.
    Journal of Organic Chemistry (2022), 87 (15), 10034-10043CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
    As a preliminary step toward its condensation into the porous polymer Activated Borane, the thermolysis of nido-B10H14 (1) in benzene at 200° gave a no. of phenylated borane mol. species. The principal product is the new monophenylated compd. 5-Ph-nido-B10H13 (2), isolated in 48% yield (based on consumption of 1) and structurally characterized by single-crystal x-ray diffraction anal., NMR, and mass spectrometry along with other minor products, such as 6-Ph-nido-B10H13 (3), for which the authors observe UV-light-driven conversion into 2 via a vertex-flip mechanism, and novel diphenylated 5,8-Ph2-nido-B10H12 (4). Together, the phenylated derivs. provide a valuable insight into the assembly of Activated Borane and ultimately inform on its structure. The new compds. also display strong blue fluorescence in both solid-state and in soln. and are the 1st examples of the direct phenylation of nido-B10H14, thus opening the door to the straight-forward synthesis of highly luminescent org.-borane hybrid systems.
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  18. 18
    Hermanek, S. Boron-11 NMR spectra of boranes, main-group heteroboranes, and substituted derivatives. Factors influencing chemical shifts of skeletal atoms. Chem. Rev. 1992, 92, 325– 362,  DOI: 10.1021/cr00010a007
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    18
    Boron-11 NMR spectra of boranes, main-group heteroboranes, and substituted derivatives. Factors influencing chemical shifts of skeletal atoms
    Hermanek, Stanislav
    Chemical Reviews (Washington, DC, United States) (1992), 92 (2), 325-62CODEN: CHREAY; ISSN:0009-2665.
    A review with 286 refs.
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  19. 19
    (a) Harazono, T.; Hiroyama, Y.; Watanabe, T. Solid State NMR of 11B and 13C in Boron Carbide, B12C3 and 11B Enriched B12C3. Bull. Chem. Soc. Jpn. 1996, 69, 2419– 2423,  DOI: 10.1246/bcsj.69.2419
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    19a
    Solid-state NMR of 11B and 13C in boron carbide, B12C3 and 11B-enriched B12C3
    Harazono, Toshie; Hiroyama, Yukiyo; Watanabe, Tokuko
    Bulletin of the Chemical Society of Japan (1996), 69 (9), 2419-2423CODEN: BCSJA8; ISSN:0009-2673. (Nippon Kagakkai)
    11B- and 13C-solid state NMR of boron carbide with different isotope ratios, B4C(11B/10B = 80.42/19.58; natural abundance isotope) and 11B4C(11B/10B = 99.5/0.5; 11B enriched sample), was studied. The linewidth of the icosahedral B (6h1 and 6h2) in 11B-static NMR signal in 11B4C was greater in 11B4C, which indicates that the B-B dipole-dipole interaction is the main mechanism for the broadening of the 11B-static NMR signal. The quadrupole coupling const. calcd. from the signal position of (±1/2↔±3/2) transition was ∼0.1 MHz. 11B-MAS spectra revealed at least two addnl. B sites (37 ppm and near -60 ppm) besides the icosahedral B sites (-6 ppm).
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    (b) Mauri, F.; Vast, N.; Pickard, C. J. Atomic Structure of Icosahedral B4C Boron Carbide from a First Principles Analysis of NMR Spectra. Phys. Rev. Lett. 2001, 87, 085506  DOI: 10.1103/PhysRevLett.87.085506
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    19b
    Atomic structure of icosahedral B4C boron carbide from a first principles analysis of NMR spectra
    Mauri, Francesco; Vast, Nathalie; Pickard, Chris J.
    Physical Review Letters (2001), 87 (8), 085506/1-085506/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    D. functional theory is demonstrated to reproduce the 13C and 11B NMR chem. shifts of icosahedral boron carbides with sufficient accuracy to ext. previously unresolved structural information from exptl. NMR spectra. B4C can be considered as an arrangement of 3-atom linear chains with C-B-C structure and 12-atom icosahedra. Most of the icosahedra have a B11C structure with the C atom placed in a polar site, and a few % have a B12 structure or a B10C2 structure with the two C atoms placed in two antipodal polar sites.
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  20. 20
    Diaz, M.; Jaballas, J.; Arias, J.; Lee, H.; Onak, T. 13C NMR Studies on Carboranes and Derivatives: Experimental/Calculational Correlations. J. Am. Chem. Soc. 1996, 118, 4405– 4410,  DOI: 10.1021/ja954089y
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    20
    13C NMR Studies on Carboranes and Derivatives: Experimental/Calculational Correlations
    Diaz, Martin; Jaballas, Jojo; Arias, Joachin; Lee, Hans; Onak, Thomas
    Journal of the American Chemical Society (1996), 118 (18), 4405-10CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The measured 13C chem. shifts of over forty carborane compds. correlate very well with ab-initio/IGLO/NMR calcd. values at both the DZ//3-21G and DZ//6-31G* (IGLO-NMR//Gaussian-geometry-optimized) levels of theory as well as with the ab-initio/GIAO/NMR values at the 6-31G*//6-31G* level of theory. For the carboranes the linear relations δ(13C-IGLO-DZ//6-31G*) = 0.941δ(13C-exp) - 1.897 (r2 = 0.990) and δ(13C-GIAO-6-31G*//6-31G*) = 0.893δ(13C-exp) - 2.554 (r2 = 0.991) are derived. Combined together with recently reported 11B NMR correlations between expt. and theory, a significant means is added to the arsenal of carborane NMR structure proof methods available to the experimentalist having access to only modest computational resources. And this procedure, of course, also has the addnl. feature of yielding reasonably good structural information (bond distances, angles, etc.).
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  21. 21
    Beckett, M. A.; Strickland, G. C.; Holland, J. R.; Sukumar Varma, K. A convenient n.m.r. method for the measurement of Lewis acidity at boron centres: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity. Polymer 1996, 37, 4629– 4631,  DOI: 10.1016/0032-3861(96)00323-0
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    21
    A convenient NMR method for the measurement of Lewis acidity at boron centers: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity
    Beckett, Michael A.; Strickland, Gary C.; Holland, John R.; Varma, K. Sukumar
    Polymer (1996), 37 (20), 4629-4631CODEN: POLMAG; ISSN:0032-3861. (Elsevier)
    The reaction rate detd. from viscosity measurements of Lewis acid catalyzed epoxide (oxirane) polymns. was correlated with the Gutmann's Acceptor No. (AN) scale derived from measurement of 31P NMR chem. shifts of Et3PO (TPO) dissolved in the boron-contg. Lewis acid. The 31P chem. shifts, acceptor no., and polymn. rate of phenylglycidyl ether are given for (Me2N)3B, (BuO)3B, (EtO)3B, (MeO)3B, (BuO)3B3O3, (MeO)3B3O3, BF3 etherate, BCl3, BBr3, and BI3. The method allows for accurate measurement of Lewis acidity of the boron center, which dets. the cure rate of epoxy resins.
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  22. 22
    (a) Lang, S.; Benz, M.; Obenaus, U.; Himmelmann, R.; Hunger, M. Novel Approach for the Characterization of Lewis Acidic Solid Catalysts by Solid-State NMR Spectroscopy. ChemCatChem. 2016, 8, 2031– 2036,  DOI: 10.1002/cctc.201600372
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    22a
    Novel Approach for the Characterization of Lewis Acidic Solid Catalysts by Solid-State NMR Spectroscopy
    Lang, Swen; Benz, Michael; Obenaus, Utz; Himmelmann, Robin; Hunger, Michael
    ChemCatChem (2016), 8 (12), 2031-2036CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Acetone-2-13C, trimethylphosphine oxide (TMPO), and ammonia were applied as probe mols. for solid-state NMR investigations of Lewis acid sites on γ-Al2O3, TiO2/anatase, and lithium-exchanged zeolite Na-Y. An indication for Lewis acid sites are the 31P MAS NMR signals at 48-51 ppm for TMPO-loaded catalysts. The detn. of the Lewis site d. through the evaluation of these 31P MAS NMR signals, however, is difficult owing to the demanding TMPO adsorption procedure. Upon ammonia adsorption, the formation of ammonium ions at Bronsted acid sites and the coordination of ammonia at Lewis acid sites cause 1H MAS NMR signals at 6-7 ppm and -0.5-3 ppm, resp. The integration of these signals results in the densities of Bronsted and Lewis acid sites. Future studies have to clarify whether the different 1H chem. shifts in the range of -0.5-3 ppm for ammonia coordinated at Lewis acid sites on solid catalysts are a hint at the different strengths of these surface sites.
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    (b) Wiper, P. V.; Amelse, J.; Mafra, L. Multinuclear solid-state NMR characterization of the Brønsted/Lewis acid properties in the BP HAMS-1B (H-[B]-ZSM-5) borosilicate molecular sieve using adsorbed TMPO and TBPO probe molecules. J. Catal. 2014, 316, 240– 250,  DOI: 10.1016/j.jcat.2014.05.017
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    22b
    Multinuclear solid-state NMR characterization of the Bronsted/Lewis acid properties in the BP HAMS-1B (H-[B]-ZSM-5) borosilicate molecular sieve using adsorbed TMPO and TBPO probe molecules
    Wiper, Paul V.; Amelse, Jeffrey; Mafra, Luis
    Journal of Catalysis (2014), 316 (), 240-250CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    The acid properties of a dehydrated borosilicate, HAMS-1B (H-[B]-ZSM-5), including the acid types, strengths, location, and quantities are investigated by means of trialkylphosphine oxides through multinuclear 1D/2D MAS NMR expts. 11B DQF-STMAS combined with 1H MAS NMR studies revealed B-OH and distinct Si-OH protons assocd. with trigonal boron. 31P NMR spectra of TMPO-treated HAMS-1B reveal three Bronsted and three Lewis acid sites. We have found a no. of limitations applying the TMPO/TBPO method to identify internal/external acidity. Therefore, we propose a new approach to unambiguously discriminate external/internal acid sites by treating a pore-free and pore-blocked HAMS-1B zeolite. This method provided unique structural insight regarding the identification of boron species/coordinations assocd. with Bronsted/Lewis acid sites. Addnl., ICP anal. in tandem with solid-state NMR enabled full assignment of the detected internal/external acid species and the study of their acid strength. Moreover, we identify the nature of TMPO complexes arising from Bronsted/Lewis interactions.
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    (c) Hradsky, D.; Machac, P.; Skoda, D.; Leonova, L.; Sazama, P.; Pastvova, J.; Kaucky, D.; Vsiansky, D.; Moravec, Z.; Styskalik, A. Catalytic performance of micro-mesoporous zirconosilicates prepared by non-hydrolytic sol-gel in ethanol-acetaldehyde conversion to butadiene and related reactions. Appl. Catal., A 2023, 652, 119037  DOI: 10.1016/j.apcata.2023.119037
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    22c
    Catalytic performance of micro-mesoporous zirconosilicates prepared by non-hydrolytic sol-gel in ethanol-acetaldehyde conversion to butadiene and related reactions
    Hradsky, Dalibor; Machac, Petr; Skoda, David; Leonova, Lucie; Sazama, Petr; Pastvova, Jana; Kaucky, Dalibor; Vsiansky, Dalibor; Moravec, Zdenek; Styskalik, Ales
    Applied Catalysis, A: General (2023), 652 (), 119037CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
    The open porosity and no. of Lewis acid sites in metal silicates (M = Zr, Ta) have been reported as key factors enabling reaching high butadiene (BD) productivity from ethanol. However, some microporous zeolites recently displayed very high BD yields. To gain a deeper insight, we have applied non-hydrolytic sol-gel (NHSG) in the prepn. of micro-mesoporous zirconosilicates. The porosity, structure, and acidity of these materials have been described and compared to a benchmark sample prepd. by dry impregnation. The detailed characterization proved that NHSG prepn. provided highly homogeneous Zr dispersion in silica leading to almost doubled Lewis acid site nos. and higher activity in ethanol-acetaldehyde conversion to BD, Meerwein-Ponndorf-Verley (MPV) reaction, and aldol condensation, in comparison to the catalyst prepd. by dry impregnation. The selectivity and stability were similar for catalysts prepd. by NHSG and dry impregnation.
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  23. 23
    (a) Britovsek, G. J. P.; Ugolotti, J.; White, A. J. P. From B(C6F5)3 to B(OC6F5)3: Synthesis of (C6F5)2BOC6F5 and C6F5B(OC6F5)2 and Their Relative Lewis Acidity. Organometallics 2005, 24, 1685– 1691,  DOI: 10.1021/om049091p
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    23a
    From B(C6F5)3 to B(OC6F5)3: Synthesis of (C6F5)2BOC6F5 and C6F5B(OC6F5)2 and Their Relative Lewis Acidity
    Britovsek, George J. P.; Ugolotti, Juri; White, Andrew J. P.
    Organometallics (2005), 24 (7), 1685-1691CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    The pentafluorophenyl esters of bis(pentafluorophenyl)borinic acid (C6F5)2BOC6F5 (2) and pentafluorophenylboronic acid C6F5B(OC6F5)2 (3) have been prepd. and characterized by multinuclear NMR and x-ray anal. VT NMR studies have shown that restricted rotation around the B-O bond in 2 occurs below 193 K, corresponding to ΔG⧧ = 35 kJ/mol for this process. This low barrier and the random torsion angles around the B-O bonds obsd. in the solid state structures of compds. 2, 3, and B(OC6F5)3 (4) suggest that these torsion angles are not related to pπ-pπ interactions between boron and oxygen, but more likely a consequence of the extensive intermol. F-π interactions seen in the solid state structures. The Lewis acidity of 2, 3, and 4 has been compared with B(C6F5)3 (1), using various Lewis bases. All compds. 1-4 appear to be strong Lewis acids, whereby 4 interacts more strongly with hard bases whereas 1 binds more strongly to softer bases.
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    (b) Beringhelli, T.; Donghi, D.; Maggioni, D.; D’Alfonso, G. Solution structure, dynamics and speciation of perfluoroaryl boranes through 1H, 11B and 19F NMR spectroscopy. Coord. Chem. Rev. 2008, 252, 2292– 2313,  DOI: 10.1016/j.ccr.2008.01.018
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    23b
    Solution structure, dynamics and speciation of perfluoroaryl boranes through 1H, 11B and 19F NMR spectroscopy
    Beringhelli, Tiziana; Donghi, Daniela; Maggioni, Daniela; D'Alfonso, Giuseppe
    Coordination Chemistry Reviews (2008), 252 (21+22), 2292-2313CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. Multinuclear NMR spectroscopy was used to study the soln. properties of several perfluoroaryl borane derivs. The information obtained from all the NMR active isotopes present in these mols. made it possible to establish not only chem. identity, structure and dynamics of their reaction products, but also the complexity of the soln. speciation. Multinuclear NMR helped in unraveling the real forms in which they are present in soln., that in some cases can change dramatically according to even slight changes of the soln. conditions. Examples will be presented related to the chem. of tris(pentafluorophenyl)borane, B(C6F5)3, and bis(pentafluorophenyl)borinic acid, B(C6F5)2OH.
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    (c) Lewiński, J.; Kubicki, D. NMR Spectroscopy, Heteronuclei, B, Al, Ga, In, Tl. In Encyclopedia of Spectroscopy and Spectrometry, 3rd ed.; Lindon, J. C.; Tranter, G. E.; Koppenaal, D. W., Eds.; Academic Press: Oxford, 2017; pp 318– 329.
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  24. 24
    (a) Zhang, X.; Wang, R.; Yang, X.; Zhang, F. Comparison of four catalysts in the catalytic dehydration of ethanol to ethylene. Microporous Mesoporous Mater. 2008, 116, 210– 215,  DOI: 10.1016/j.micromeso.2008.04.004
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    24a
    Comparison of four catalysts in the catalytic dehydration of ethanol to ethylene
    Zhang, Xian; Wang, Rijie; Yang, Xiaoxia; Zhang, Fengbao
    Microporous and Mesoporous Materials (2008), 116 (1-3), 210-215CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier)
    The aim of this study was to compare the activity and stability of γ-Al2O3, HZSM-5 (Si/Al = 25), silicoaluminophosphate (SAPO-34) and Ni-substituted SAPO-34 (NiAPSO-34) as catalysts in the dehydration of ethanol to ethylene. γ-Al2O3- and HZSM-5 were com. catalysts. SAPO-34 and NiAPSO-34 mol. sieves had been synthesized with hydrothermal method in the lab., characterized with x-ray powder diffraction (x-ray diffraction), IR Spectroscopy (FT-IR), H2 temp.-programmed redn. (H2-TPR) technique and NH3 temp.-programmed desorption (NH3-TPD) technique. The incorporation of Ni2+ into the SAPO-34 framework generated in NiAPSO-34 sample was proved by x-ray diffraction, FT-IR and H2-TPR techniques. NH3-TPD study had revealed that substitution of Ni2+ for Al3+ in the SAPO-34 framework led to increase the weak and moderately strong acid strength and give rise to weak acid sites. Dehydration of ethanol was carried out over four catalysts and the results showed that conversion of ethanol and selectivity to ethylene decreased in the order HZSM-5 > NiAPSO-34 > SAPO-34 > γ-Al2O3. As to the stability of catalyst, NiAPSO-34 and SAPO-34 were better than other two catalysts. Considering the activity and stability of the four catalysts comprehensively, NiAPSO-34 was the suitable catalyst in the dehydration of ethanol.
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    (b) Xin, H.; Li, X.; Fang, Y.; Yi, X.; Hu, W.; Chu, Y.; Zhang, F.; Zheng, A.; Zhang, H.; Li, X. Catalytic dehydration of ethanol over post-treated ZSM-5 zeolites. J. Catal. 2014, 312, 204– 215,  DOI: 10.1016/j.jcat.2014.02.003
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    24b
    Catalytic dehydration of ethanol over post-treated ZSM-5 zeolites
    Xin, Hongchuan; Li, Xiangping; Fang, Yuan; Yi, Xianfeng; Hu, Wenhui; Chu, Yueying; Zhang, Feng; Zheng, Anmin; Zhang, Hongpeng; Li, Xuebing
    Journal of Catalysis (2014), 312 (), 204-215CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    Microporous ZSM-5 zeolite was post-treated by desilication with sodium hydroxide, dealumination with oxalic acid, or both of them in a sequential way to finely tune the zeolite catalysts with hierarchically porous structure and varying acidity. In the catalytic dehydration of ethanol, di-Et ether and ethylene were two main products competitively formed at 200 °C and atm. pressure. The post-treated ZSM-5 catalysts could display stable ethanol conversion and ethylene selectivity within time-onstream of around 12 h. The correlation between the steady-state ethylene selectivity and the amt. of weak acid sites from ammonia temp.-programmed desorption (NH3-TPD) indicated that the weak acid sites facilitated the ethylene prodn. during ethanol transformation under present reaction conditions. The reaction pathways for di-Et ether and ethylene formations from ethanol were investigated by theor. calcn. Both the activation energies and natural charges of the transition states strongly supported that the selectivity for the di-Et ether tended to deteriorate with decreasing catalytic Bronsted acidity.
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    (c) Bi, J.; Guo, X.; Liu, M.; Wang, X. High effective dehydration of bio-ethanol into ethylene over nanoscale HZSM-5 zeolite catalysts. Catal. Today 2010, 149, 143– 147,  DOI: 10.1016/j.cattod.2009.04.016
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    24c
    Highly effective dehydration of bio-ethanol into ethylene over nanoscale HZSM-5 zeolite catalysts
    Bi, Jiandong; Guo, Xinwen; Liu, Min; Wang, Xiangsheng
    Catalysis Today (2010), 149 (1-2), 143-147CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    Nanoscale and microscale HZSM-5 zeolite catalysts were prepd. and characterized by using SEM, XRD, IR, TPD and modified Hammett indicator method. Their performances in the dehydration of bio-ethanol into ethylene were compared in a fixed-bed reactor at 240 °C under atm. pressure. The results show that nanoscale HZSM-5 zeolite catalyst exhibits better stability than microscale HZSM-5 zeolite catalyst. When the 95(v) % bio-ethanol is used as the reactant, over nanoscale HZSM-5 catalyst, the conversion of bio-ethanol and the selectivity for ethylene almost keep const. during 630 h reaction, while over microscale HZSM-5 zeolite catalyst, the conversion of bio-ethanol decreases after 60 h reaction; in the case of the 45(v) % bio-ethanol employed as the feedstock, over nanoscale HZSM-5 catalyst, the conversion of bio-ethanol and the selectivity for ethylene almost keep const. during 320 h reaction, while over microscale HZSM-5 zeolite catalyst, both the conversion of bio-ethanol and the selectivity for ethylene decrease almost at the beginning of the reaction.
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  25. 25
    (a) Li, Y.; Yang, Q.; Yang, J.; Li, C. Mesoporous aluminosilicates synthesized with single molecular precursor (sec-BuO)2AlOSi(OEt)3 as aluminum source. Microporous Mesoporous Mater. 2006, 91, 85– 91,  DOI: 10.1016/j.micromeso.2005.11.021
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    25a
    Mesoporous aluminosilicates synthesized with single molecular precursor (sec-BuO)2AlOSi(OEt)3 as aluminum source
    Li, Ying; Yang, Qihua; Yang, Jie; Li, Can
    Microporous and Mesoporous Materials (2006), 91 (1-3), 85-91CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
    Single mol. precursor bis(sec-butoxy)aluminoxytriethoxysilane [(sec-BuO)2AlOSi(OEt)3] was used as Al source for the synthesis of mesoporous aluminosilicates with low Si/Al ratios from 1.0 to 10.0 via a template-assisted method. The samples with Si/Al ratios of 1.0 and 3.0 have wormhole-like pore structure, while the samples with Si/Al ratios of 5.0 and 10.0 have ordered hexagonal mesoporous structures with extensive void defects amid the nanochannels. IR spectra of pyridine adsorption and NH3-TPD characterizations suggest that the mesoporous aluminosilicates have both Bronsted and Lewis acid sites with medium strength. The amts. of acid sites of the mesoporous aluminosilicates do not run parallel with the concn. of the tetrahedrally coordinated Al in the mesoporous framework.
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    (b) Styskalik, A.; Kordoghli, I.; Poleunis, C.; Delcorte, A.; Moravec, Z.; Simonikova, L.; Kanicky, V.; Aprile, C.; Fusaro, L.; Debecker, D. P. Hybrid mesoporous aluminosilicate catalysts obtained by non-hydrolytic sol–gel for ethanol dehydration. J. Mater. Chem. A 2020, 8, 23526– 23542,  DOI: 10.1039/D0TA07016E
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    25b
    Hybrid mesoporous aluminosilicate catalysts obtained by non-hydrolytic sol-gel for ethanol dehydration
    Styskalik, Ales; Kordoghli, Imene; Poleunis, Claude; Delcorte, Arnaud; Moravec, Zdenek; Simonikova, Lucie; Kanicky, Viktor; Aprile, Carmela; Fusaro, Luca; Debecker, Damien P.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2020), 8 (44), 23526-23542CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Ethanol dehydration is effectively catalyzed by solid acids, such as HZSM-5, alumina, or silica-alumina. In these catalysts, the amt., nature, and strength of acid sites is believed to det. catalyst activity and stability. However, surface hydrophilicity or hydrophobicity can be suggested as another decisive materials property that can directly affect performance. For example, a more hydrophobic surface might be beneficial in repelling the co-product of the reaction, water. However, these aspects have been studied only scarcely in the context of alc. dehydration. Here, a series of mesoporous hybrid aluminosilicate materials contg. CH3Si groups was prepd. in one pot by non-hydrolytic sol-gel (NHSG). The presence of the Me groups was verified by IR, solid-state NMR, and ToF-SIMS. Aluminum is mostly incorporated in tetrahedral coordination in the hybrid silica matrix. Two parameters were varied: the aluminum loading (Si:Al ratio) and the degree of methylation (Si:MeSi ratio). On the one hand, changing the Si:Al ratio had a marked and expected impact on acidity. On the other hand, unexpectedly, the introduction of Me groups had no clear influence on sample hydrophobicity. Nevertheless, some of the methylated aluminosilicate catalysts markedly outperformed the purely inorg. catalysts and a com. silica-alumina benchmark. While a direct influence of surface hydrophilicity or hydrophobicity is unlikely, characterization of acidity (IR-pyridine) revealed that the improved performance for hybrid catalysts is correlated with a modification of the acidic properties (higher proportion of Lewis acid sites) caused by the introduction of Me groups during the sol-gel process. A decisive role of acidity in ethanol dehydration was confirmed by an expt. with delayed addn. of the Al precursor in the NHSG synthesis of the hybrid aluminosilicate. This led to a higher Al surface concn., marked acid sites no. increase, and better catalytic performance, even competing with the state of the art HZSM-5 in terms of ethylene yield.
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  26. 26
    Bould, J.; Clegg, W.; Teat, S. J.; Barton, L.; Rath, N. P.; Thornton-Pett, M.; Kennedy, J. D. An approach to megalo-boranes. Mixed and multiple cluster fusions involving iridaborane and platinaborane cluster compounds. Crystal structure determinations by conventional and synchrotron methods. Inorg. Chim. Acta 1999, 289, 95– 124,  DOI: 10.1016/S0020-1693(99)00071-7
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    26
    An approach to megalo-boranes. Mixed and multiple cluster fusions involving iridaborane and platinaborane cluster compounds. Crystal structure determinations by conventional and synchrotron methods
    Bould, Jonathan; Clegg, William; Teat, Simon J.; Barton, Lawrence; Rath, Nigam P.; Thornton-Pett, Mark; Kennedy, John D.
    Inorganica Chimica Acta (1999), 289 (1-2), 95-124CODEN: ICHAA3; ISSN:0020-1693. (Elsevier Science S.A.)
    Several new macropolyhedral metallaboranes were isolated from thermolytic mixed cluster fusion reactions involving metallaboranes and molten B10H14 as solvent. Co-thermolysis of B10H14 with nine-vertex [(CO)(PMe3)2HIrB8H12] (1) engenders 18-vertex [(CO)(PMe3)2IrB17H20] (3), via double cluster fusion; this has the 18-vertex configuration of syn-B18H22, but with a metal atom in the 10-position. From the same reaction, triple cluster fusion engenders 28-vertex [(PMe3)2IrB26H24Ir(CO)(PMe3)2] (4), which structurally is based on an intimate interfusion of closed 10-vertex and 12-vertex subclusters, to generate a tetrahedral tetraboron core that also has a more open commo 1-B linkage to a nido nine-vertex {IrB8} subcluster. Compd. 4 exhibits interesting consequences of cluster-crevice formation and introduces the concept of globular megalo-borane structures that have borons-only cores surrounded by B-hydride sheaths. Examn. for incipient megalo-borane globular behavior in another system, viz. [IrCl(PPh3)3] (7) with anti-B18H22, reveals a four-atom core feature in 19-vertex [(PPh3)HIrB18H18(PPh3)] (6), which has a closo-type {IrB10} 11-vertex subcluster fused to a nido 10-vertex {B10} subcluster to generate a four-atom {IrB3} tetrahedron. Examn. for mixed cluster fusion in other systems reveals the generation of [(PMe2Ph)2Pt-anti-B18H20] (8), from the co-thermolysis of [(PMe2Ph)2PtB8H12] (2) and B10H14, and examn. for multiple cluster fusion reveals the formation of 30-vertex [(PMe2Ph)2(PMe2C6H4)2Pt2B28H32] (10), 29-vertex [(PMe2Ph)2PtB28H32] (11) and 27-vertex [(PMe2Ph)2PtB26H26(PMe2Ph)] (12) from the same reaction. Structurally, compd. 10 is based on a 10-vertex arachno-{6,9-Pt2B8} unit linked, via one B-B two-electron two-center bond each, to two 10-vertex nido-{B10} units; it also exhibits mol. condensation as two P-phenylene ortho-cycloboronations. Compd. 11 is based on the 19-vertex [(PMe2Ph)2Pt-η4-anti-B18H22] configuration with an addnl. 10-vertex nido-{B10H13} moiety bound to the nonplatinated subcluster via one B-B two-electron two-center bond. Compd. 12 is based on two nido 11-vertex {PtB10} units joined by a single commo Pt vertex, with one of these units conjoined to an arachno eight-B unit via a two-B common edge and an open bridging {B-H(exo)-Pt-μ-B2} link. Thermolysis of [(PMe2Ph)2PtB8H12] (2) with the pre-formed double-cluster compd. anti-B18H22 generates triple-contiguity 27-vertex [(PMe2Ph)PtB26H26(PMe2Ph)] (13) which, structurally, consists of a nido 11-vertex {PtB10} unit that is fused to a 2nd 11-vertex nido {PtB10} unit with a triangular {PtB2} face in common, and also fused to a 10-vertex nido {B10} unit with a {B2} edge in common. The sequence 12 → 11 → 10 → 13 → 4 represents a progression of increasing intimacy of cluster fusion. Small crystals of compds. 3, 11 and 12 necessitated synchrotron x-radiation for sufficient diffraction intensity.
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    (a) Parks, D. J.; Blackwell, J. M.; Piers, W. E. Studies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilation of Carbonyl Functions. J. Org. Chem. 2000, 65, 3090– 3098,  DOI: 10.1021/jo991828a
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    27a
    Studies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilation of Carbonyl Functions
    Parks, Daniel J.; Blackwell, James M.; Piers, Warren E.
    Journal of Organic Chemistry (2000), 65 (10), 3090-3098CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
    The strong organoborane Lewis acid B(C6F5)3 catalyzes the hydrosilation (using R3SiH) of arom. and aliph. carbonyl functions at convenient rates with loadings of 1-4%. For aldehydes and ketones, the product silyl ethers were isolated in 75-96% yield; for esters, the aldehydes produced upon workup of the silyl acetal products can be obtained in 45-70% yield. Extensive mechanistic studies point to an unusual silane activation mechanism rather than one involving borane activation of the carbonyl function. Quant. kinetic studies show that the least basic substrates are hydrosilated at the fastest rates; furthermore, increased concns. of substrate have an inhibitory effect on the obsd. reaction rate. Paradoxically, the most basic substrates are reduced selectively, albeit at a slower rate, in competition expts. The borane thus must dissoc. from the carbonyl to activate the silane via hydride abstraction; the incipient silylium species then coordinates the most basic function, which is selectively reduced by [HB(C6F5)3]-. In addn. to the kinetic data, this mechanistic proposal is supported by a kinetic isotope effect of 1.4(5) for the hydrosilation of acetophenone, the observation that B(C6F5)3 catalyzes H/D and H/H scrambling in silanes in the absence of substrate, computational studies, the synthesis of models for proposed intermediates, and other isotope labeling and crossover expts.
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    (b) Piers, W. E.; Marwitz, A. J. V.; Mercier, L. G. Mechanistic Aspects of Bond Activation with Perfluoroarylboranes. Inorg. Chem. 2011, 50, 12252– 12262,  DOI: 10.1021/ic2006474
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    27b
    Mechanistic Aspects of Bond Activation with Perfluoroarylboranes
    Piers, Warren E.; Marwitz, Adam J. V.; Mercier, Lauren G.
    Inorganic Chemistry (2011), 50 (24), 12252-12262CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    A review. In the mid-1990s, it was discovered that tris(pentafluorophenyl)borane, B(C6F5)3, was an effective catalyst for hydrosilylation of a variety of carbonyl and imine functions. Mechanistic studies revealed a counterintuitive path in which the function of the borane was to activate the silane rather than the org. substrate. This was the first example of what has come to be known as "frustrated Lewis pair" chem. utilizing this remarkable class of electrophilic boranes. Subsequent discoveries by the groups of Stephan and Erker showed that this could be extended to the activation of dihydrogen, initiating an intense period of activity in this area in the past 5 years. This article describes the early hydrosilylation chem. and its subsequent applications to a variety of transformations of importance to org. and inorg. chemists, drawing parallels with the more recent hydrogen activation chem. Here, authors emphasize the current understanding of the mechanism of this process rather than focusing on the many and emerging applications of hydrogen activation by fluoroarylborane-based frustrated Lewis pair systems.
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    (c) Rendler, S.; Oestreich, M. Conclusive Evidence for an SN2-Si Mechanism in the B(C6F5)3-Catalyzed Hydrosilylation of Carbonyl Compounds: Implications for the Related Hydrogenation. Angew. Chem., Int. Ed. 2008, 47, 5997– 6000,  DOI: 10.1002/anie.200801675
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    27c
    Conclusive evidence for an SN2-Si mechanism in the B(C6F5)3-catalyzed hydrosilylation of carbonyl compounds: implications for the related hydrogenation
    Rendler, Sebastien; Oestreich, Martin
    Angewandte Chemie, International Edition (2008), 47 (32), 5997-6000, S5997/1-S5997/31CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Effective Walden-type anal. showcases the usefulness of silanes with a stereogenic Si center as stereochem. probes. The B(C6F5)3-catalyzed hydrosilylation of acetophenone with chiral (SiR)-tetrahydro(isopropyl)silanaphthalene and likely the related hydrogenation proceed through linear B-H-Si-O transition states, as verified by flawless inversion of the abs. configuration at Si to give chiral silyl ether (SiR,R)-I which subsequently undergoes DIBAL-redn. to a chiral (R)-1-phenylethanol.
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    (d) Oestreich, M.; Hermeke, J.; Mohr, J. A unified survey of Si–H and H–H bond activation catalysed by electron-deficient boranes. Chem. Soc. Rev. 2015, 44, 2202– 2220,  DOI: 10.1039/C4CS00451E
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    A unified survey of Si-H and H-H bond activation catalysed by electron-deficient boranes
    Oestreich, Martin; Hermeke, Julia; Mohr, Jens
    Chemical Society Reviews (2015), 44 (8), 2202-2220CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    The bond activation chem. of B(C6F5)3 and related electron-deficient boranes is currently experiencing a renaissance due to the fascinating development of frustrated Lewis pairs (FLPs). B(C6F5)3's ability to catalytically activate Si-H bonds through η1 coordination opened the door to several unique redn. processes. The ground-breaking finding that the same family of fully or partially fluorinated boron Lewis acids allows for the related H-H bond activation, either alone or as a component of an FLP, brought considerable momentum into the area of transition-metal-free hydrogenation and, likewise, hydrosilylation. This review comprehensively summarises synthetic methods involving borane-catalyzed Si-H and H-H bond activation. Systems corresponding to an FLP-type situation are not covered. Aside from the broad manifold of C=X bond redns. and C=X/C-X defunctionalisations, dehydrogenative (oxidative) Si-H couplings are also included.
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    (a) Parks, D. J.; Piers, W. E. Tris(pentafluorophenyl)boron-Catalyzed Hydrosilation of Aromatic Aldehydes, Ketones, and Esters. J. Am. Chem. Soc. 1996, 118, 9440– 9441,  DOI: 10.1021/ja961536g
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    28a
    Tris(pentafluorophenyl)boron-Catalyzed Hydrosilation of Aromatic Aldehydes, Ketones, and Esters
    Parks, Daniel J.; Piers, Warren E.
    Journal of the American Chemical Society (1996), 118 (39), 9440-9441CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Tris(pentafluorophenyl)borane (1-4 mol %) catalyzes the addn. of Ph3SiH to carbonyl functions of arom. substrates p-XC6H4COR (X = Me, H, Cl, NO2; R = H, Me, OEt). Turnover frequencies for X = H substrates are 19, 45 and 637 h for R = H, Me and OEt, resp. and rates of hydrosilation increase as X becomes more electron withdrawing. Mechanistic studies, including substrate/borane binding equil., structural studies on substrate/borane adducts and kinetic measurements support a hydrosilation mechanism characterized by borane activation of the silane reagent rather than the carbonyl substrate. Key observations in support of this unusual nucleophilic/electrophilic hydrosilation mechanism were the obsd. inhibition of the reaction by increased concns. of substrate and the obsd. order of reactivity which indicates that the weaker binding substrates are reduced faster.
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    28b
    Defunctionalisation catalysed by boron Lewis acids
    Fang, Huaquan; Oestreich, Martin
    Chemical Science (2020), 11 (47), 12604-12615CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    A review. This review summarized various defunctionalization reactions such as deoxygenation, decarbonylation, desulfurization, deamination and dehalogenation, all of which catalyzed by boron Lewis acids.
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    (a) Prabhudesai, V. S.; Gurrala, L.; Vinu, R. Catalytic Hydrodeoxygenation of Lignin-Derived Oxygenates: Catalysis, Mechanism, and Effect of Process Conditions. Energy Fuels 2022, 36, 1155– 1188,  DOI: 10.1021/acs.energyfuels.1c02640
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    29a
    Catalytic Hydrodeoxygenation of Lignin-Derived Oxygenates: Catalysis, Mechanism, and Effect of Process Conditions
    Prabhudesai, Vallabh S.; Gurrala, Lakshmiprasad; Vinu, Ravikrishnan
    Energy & Fuels (2022), 36 (3), 1155-1188CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
    A review. The high oxygen content of pyrolysis bio-oil with many org. functional groups in it limits its direct application as a blendstock. The upgradation of biomass-derived oxygenates into renewable fuels and value-added chems. via catalytic hydrodeoxygenation (HDO) has received considerable attention in recent years. This review focuses on HDO of key model compd. oxygenates, which sets the ground to propose the overall reaction mechanism of HDO of bio-oils. Catalysts play a vital role in HDO, and its design poses many challenges because of different reactions involved such as hydrogenolysis, hydrogenation, decarbonylation, and dehydration occurring simultaneously at different catalyst-active sites. The main objective here is to present a comprehensive introduction to the reaction mechanism involved in the HDO of bio-oil model oxygenates. For this, a thorough discussion of different reaction pathways taking place during the HDO of five model oxygenates, viz., anisole, guaiacol, eugenol, vanillin, and dibenzofuran, is presented. The model compds. are selected to provide a good description of the HDO of lignin-derived compds. present in bio-oils. Particular emphasis is placed on the effect of the catalyst, temp., hydrogen partial pressure, and solvent employed on the product distribution. This review will aid not just in understanding the interrelations between the nature of the catalyst, HDO mechanism, and product distribution but will also provide thoughtful directions for the applications of HDO in real bio-oil upgradation.
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    (b) Kumar, A.; Jindal, M.; Maharana, S.; Thallada, B. Lignin Biorefinery: New Horizons in Catalytic Hydrodeoxygenation for the Production of Chemicals. Energy Fuels 2021, 35, 16965– 16994,  DOI: 10.1021/acs.energyfuels.1c01651
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    29b
    Lignin Biorefinery: New Horizons in Catalytic Hydrodeoxygenation for the Production of Chemicals
    Kumar, Adarsh; Jindal, Meenu; Maharana, Sonita; Thallada, Bhaskar
    Energy & Fuels (2021), 35 (21), 16965-16994CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
    A review. Lignin is a byproduct of the paper mill and second-generation (2G) ethanol biorefinery and the largest naturally available source of arom. building blocks. It can serve as a feedstock to produce bulk, fine, and functionalized arom. compds. along with aliph. cyclic alcs., jet fuel, and aliph. hydrocarbons. Hydrodeoxygenation (HDO) is one of the promising strategies to produce these compds. from lignin-derived phenolics. Many reactions occur during the HDO; hence, it is very crucial to design active, selective, and long-life catalysts and choose the optimal reaction conditions to synthesize targeted end products. Herein, we first survey the conventional and new lignin fractionation techniques and det. the lignin structure, compn., and bonding with cellulose and hemicellulose. After this, we review the recent advances in the prodn. of alkanes, arenes, cyclic alcs., jet fuel range hydrocarbons, and HDO of raw lignin bio-oil. This review exclusively discusses the product distribution during HDO based on catalyst compn., solvent, and reaction parameters. Moreover, the review also underlines the selectivity of various catalytic systems to yield specific products such as BTX, important petrochem. materials, aliph. hydrocarbons, cyclic alcs., and jet fuel range hydrocarbons.
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    (c) Perego, C.; Bosetti, A.; Ricci, M.; Millini, R. Zeolite Materials for Biomass Conversion to Biofuel. Energy Fuels 2017, 31, 7721– 7733,  DOI: 10.1021/acs.energyfuels.7b01057
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    29c
    Zeolite Materials for Biomass Conversion to Biofuel
    Perego, Carlo; Bosetti, Aldo; Ricci, Marco; Millini, Roberto
    Energy & Fuels (2017), 31 (8), 7721-7733CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
    A review. The use of zeolite catalysts for the prodn. of biofuels from biomass is reviewed. Zeolites as such or modified by the addn. of other active phases are used in several processes for the transformation of the biomass and for the upgrading of the bio-oils deriving from its primary treatment. For each of the different processes, the most relevant results reported in the literature are provided together with some considerations on the effective or potential industrial applicability of the technologies.
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    (d) Shi, Y.; Xing, E.; Wu, K.; Wang, J.; Yang, M.; Wu, Y. Recent progress on upgrading of bio-oil to hydrocarbons over metal/zeolite bifunctional catalysts. Catal. Sci. Technol. 2017, 7, 2385– 2415,  DOI: 10.1039/C7CY00574A
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    29d
    Recent progress on upgrading of bio-oil to hydrocarbons over metal/zeolite bifunctional catalysts
    Shi, Yanchun; Xing, Enhui; Wu, Kejing; Wang, Jianlong; Yang, Mingde; Wu, Yulong
    Catalysis Science & Technology (2017), 7 (12), 2385-2415CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    Upgrading of bio-oil is of high necessity and popularity in converting biomass to high-quality hydrocarbons (transportation fuels and petrochems.) to reduce the overall CO2 emissions of fossil based materials. There are hundreds of different oxygenated compds. identified in bio-oil, resulting in a high oxygen content (30% to 50%). This review focuses on recent progress in the upgrading of bio-oil over metal/zeolite bifunctional catalysts, with model compds. and real bio-oil included. Firstly, typical model compds. and corresponding reaction routes are summarized, based upon the compn. of the bio-oil and a basic knowledge of chem. reactions. Secondly, careful analyses are conducted on the deoxygenation mechanisms over different metal active centers and acid-catalyzed reactions, such as isomerization and cracking, over zeolitic acid sites, resp. Moreover, detailed analyses have focused on the effect of metal loadings on zeolites, the effects of zeolitic porosity and acidity on the metal, and their overall effects on reaction activity, selectivity and stability. Thirdly, the fundamental understanding of the interaction between the metal centers and zeolite acid sites in bifunctional catalysts and their influences on complex reaction networks, including deoxygenation and acid-catalyzed reactions, is analyzed. The metal/acid balance may be the key in improving the catalytic activity and product selectivity in the upgrading of bio-oil, which needs further careful design. Finally, the potential challenges and opportunities for the upgrading of bio-oil over metal/zeolite bifunctional catalysts are outlined.
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    (e) Tang, X.; Ding, W.; Li, H. Improved hydrodeoxygenation of bio-oil model compounds with polymethylhydrosiloxane by Brønsted acidic zeolites. Fuel 2021, 290, 119883  DOI: 10.1016/j.fuel.2020.119883
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    29e
    Improved hydrodeoxygenation of bio-oil model compounds with polymethylhydrosiloxane by Bronsted acidic zeolites
    Tang, Xingfei; Ding, Wentao; Li, Hao
    Fuel (2021), 290 (), 119883CODEN: FUELAC; ISSN:0016-2361. (Elsevier Ltd.)
    Bio-oil, one of significant renewable energy, was blocked from its direct application by thermodn. instability due to the high oxygen content, so its deoxygenation needs urgent soln. Avoiding the disadvantages of the traditional catalytic method, such as harsh reaction conditions, potential explosion risk and environmental pollution, we utilize solid-acidic zeolites as low-load Pd-based catalyst supports instead of corrosive acid additives, with polymethylhydrosiloxane (PMHS) as the hydrogen-supplying agent, to achieve efficient hydrodeoxygenation of bio-oil models (carbonyl compds.) under mild conditions. The reaction conditions such as Si/Al ratio of HZSM-5 zeolite, temp., solvent, and the type of Pd salts precursor are optimized. In particular, we have found that polar protic solvents improve catalytic efficiency by promoting proton transfer in the reaction. In an open-to-air, 97.9% ethylbenzene yield can be obtained for acetophenone conversion under mild conditions (0.5 wt% Pd/HZSM-5(18), 65°C, 3 h, n-butanol as solvent), which is more efficient and environmental friendly than currently reports. Meanwhile, hydrogenation-dehydration mechanism was proposed, and the Bronsted acid in HZSM-5 promotes the dehydration of the alc. (rate-limiting step) by efficiently accelerating the removal of hydroxyl groups and the proton transfer of the reaction. Furthermore, the catalytic scheme exhibits the excellent stability (reusable seven times) and versatility. The potential of a green catalytic technol. using with PMHS opens attractive opportunities for bio-oil upgrading.
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    (f) Witsuthammakul, A.; Sooknoi, T. Selective hydrodeoxygenation of bio-oil derived products: ketones to olefins. Catal. Sci. Technol. 2015, 5, 3639– 3648,  DOI: 10.1039/C5CY00367A
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    29f
    Selective hydrodeoxygenation of bio-oil derived products: ketones to olefins
    Witsuthammakul, Ayut; Sooknoi, Tawan
    Catalysis Science & Technology (2015), 5 (7), 3639-3648CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    The hydrodeoxygenation (HDO) of various ketones (acetone, Me Et ketone and cyclohexanone) to olefins via hydrogenation-dehydration was conducted in a fixed bed reactor at 373-573 K under H2. A ketone can be hydrogenated over the metal function to an alc. intermediate that is subsequently dehydrated to an olefin over the acidic function. A preliminary study on hydrogenation of acetone to 2-propanol over metal/SiO2 catalysts (Cr, Fe, Co, Ni, Cu and Pd) shows that Ni and Cu are active at >373 K. Although Ni possesses an activity higher than that of Cu, it promotes olefin hydrogenation and alc. hydrogenolysis at >473 K. Hydrogenolysis of alc. intermediate is suppressed over the Ni-Cu alloy catalyst. An optimum conversion with 100% selectivity to alc., can be obtained at 448 and 473 K for Ni and Cu, resp. The dehydration of 2-propanol to propylene over proton zeolites (ZSM-5, Y, Mordenite and β) can be achieved at >398 K. The zeolites with three-dimensional pore structure (β and Y) provide relatively higher activity (>90% conversion). However, a bimol. dehydration to ether is also promoted. Only HZSM-5 shows excellent selectivity to propylene (98%). Hydrodeoxygenation of ketones was tested with (i) a double bed of 5%Ni/SiO2 and HZSM-5 (Si/Al ∼ 13), (ii) a phys. mixed bed of 5%Cu/SiO2 and HZSM-5 (Si/Al ∼ 13) and (iii) a bi-functional catalyst of 5%Cu/HZSM-5 (Si/Al ∼ 250). It was found that high alkene selectivity was readily obtained at 448 K. While, over the phys. mixed bed and bi-functional catalyst, the hydrogenation activity was enhanced as the alc. intermediate was removed from the system. The reactivity of the ketone depends on its adsorption on the metal surface and steric hindrance, i.e. acetone > cyclohexanone > Me Et ketone.
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  30. 30
    (a) Christensen, D. B.; Mortensen, R. L.; Kramer, S.; Kegnæs, S. Study of CoCu Alloy Nanoparticles Supported on MOF-Derived Carbon for Hydrosilylation of Ketones. Catal. Lett. 2020, 150, 1537– 1545,  DOI: 10.1007/s10562-019-03065-2
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    30a
    Study of CoCu Alloy Nanoparticles Supported on MOF-Derived Carbon for Hydrosilylation of Ketones
    Christensen, David B.; Mortensen, Rasmus L.; Kramer, Soeren; Kegnaes, Soeren
    Catalysis Letters (2020), 150 (6), 1537-1545CODEN: CALEER; ISSN:1011-372X. (Springer)
    Carbonized zeolitic imidazolate frameworks (ZIFs) show potential as mesoporous heterogeneous catalysts with high metalloadings. ZIF-67 and ZIF-8 were used to create mono- and bimetallic CoCu particles supported on nitrogen-doped carbonvia self-assembly in methanol at room temp., followed by carbonization at 675°C. A Cu precursor, Cu(NO3)2·2H2O, was impregnated into the ZIF-67 before carbonization to obtain bimetallic catalysts. Nanoalloy particles with different CoCu ratio were synthesized and characterized using XRD. The materials were further characterized using TEM, SEM, XRF andnitrogen physisorption. The different alloys were tested in conversion of cyclohexanone to the corresponding silyl ether.Complete conversion of cyclohexanone at 90°C for 24 h were obtained. The catalyst Co99Cu1@NC gave a 60% increase in yield over a pure Co analog.
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    (b) Le Roux, E.; De Mallmann, A.; Merle, N.; Taoufik, M.; Anwander, R. Immobilization of Heteroleptic Bis(oxazoline) Zinc Catalysts on SBA-15 for Asymmetric Hydrosilylation. Organometallics 2015, 34, 5146– 5154,  DOI: 10.1021/acs.organomet.5b00714
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    30b
    Immobilization of Heteroleptic Bis(oxazoline) Zinc Catalysts on SBA-15 for Asymmetric Hydrosilylation
    Le Roux, Erwan; De Mallmann, Aimery; Merle, Nicolas; Taoufik, Mostafa; Anwander, Reiner
    Organometallics (2015), 34 (20), 5146-5154CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    The C2-sym. bis(oxazoline)s (4S,4S)-R1-BOX, with R1 = tBu and Ph, react with ZnEt2 straightforwardly to the four-coordinate complexes ((4S,4S)-R1-BOX)ZnEt2 in high yield. Their immobilization on highly dehydroxylated (700°) mesoporous silica SBA-15 with enlarged pore diams. (dp = 15.3 nm) led to the well-defined surface species [(≡SiO)Zn(Et)((4S,4S)-R1-BOX)]. These Et zinc-terminated materials react readily with one equiv. of anhyd. methanol and acetic acid to give predominantly and selectively the corresponding surface ligand-exchanged species [(≡SiO)Zn(R)((4S,4S)-R1-BOX)] (R = OMe, OAc). All hybrid materials were characterized via N2 physisorption, elemental anal., DRIFT, 1H/13C MAS NMR, and EXAFS spectroscopies. The SBA-15 supported heteroleptic bis(oxazoline) zinc Et, methoxide, and acetate surface species were scrutinized as catalysts for the asym. hydrosilylation of acetophenone.
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    (c) Lázaro, G.; Fernández-Alvarez, F. J.; Iglesias, M.; Horna, C.; Vispe, E.; Sancho, R.; Lahoz, F. J.; Iglesias, M.; Pérez-Torrente, J. J.; Oro, L. A. Heterogeneous catalysts based on supported Rh–NHC complexes: synthesis of high molecular weight poly(silyl ether)s by catalytic hydrosilylation. Catal. Sci. Technol. 2014, 4, 62– 70,  DOI: 10.1039/C3CY00598D
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    30c
    Heterogeneous catalysts based on supported Rh-NHC complexes: synthesis of high molecular weight poly(silyl ether)s by catalytic hydrosilylation
    Lazaro, Guillermo; Fernandez-Alvarez, Francisco J.; Iglesias, Manuel; Horna, Cristina; Vispe, Eugenio; Sancho, Rodrigo; Lahoz, Fernando J.; Iglesias, Marta; Perez-Torrente, Jesus J.; Oro, Luis A.
    Catalysis Science & Technology (2014), 4 (1), 62-70CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    The new rhodium(i) complexes [Rh(Cl)(COD)(R-NHC-(CH2)3Si(OiPr3)3)] (R = 2,6-diisopropylphenyl (2a); Bu (2b)) were synthesized and fully characterized. The study of their application as ketone hydrosilylation catalysts showed a clear N-substituent effect, 2a being the most active catalyst precursor. Complex 2a was immobilized in the mesoporous materials MCM-41 and KIT-6. The new hybrid materials were fully characterized and used as catalyst precursors for the prepn. of poly(silyl ether)s by catalytic hydrosilylation. The heterogeneous catalytic systems based on the materials 2a-MCM-41 and 2a-KIT-6 afford polymers with high av. mol. wt. (Mw) Mw = 2.61 × 106 g mol-1 (2a-MCM-41) and Mw = 4.43 × 105 g mol-1 (2a-KIT-6).
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    Keess, S.; Simonneau, A.; Oestreich, M. Direct and Transfer Hydrosilylation Reactions Catalyzed by Fully or Partially Fluorinated Triarylboranes: A Systematic Study. Organometallics 2015, 34, 790– 799,  DOI: 10.1021/om501284a
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    Direct and Transfer Hydrosilylation Reactions Catalyzed by Fully or Partially Fluorinated Triarylboranes: A Systematic Study
    Keess, Sebastian; Simonneau, Antoine; Oestreich, Martin
    Organometallics (2015), 34 (4), 790-799CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    The present survey serves several purposes. Selected electron-deficient boron Lewis acids catalyze the release of hydrosilanes from cyclohexa-2,5-dien-1-yl-substituted silanes. The two-step process consists of a hydride abstraction to generate a silicon-stabilized Wheland complex and capture of the arene-stabilized silicon cation by the borohydride formed in the previous step. The same boron catalyst will then activate the Si-H bond for the reaction with representative π- and σ-donating substrates, alkenes/alkynes and ketones/ketimines, resp. The net transformation is a transfer hydrosilylation, and the effect that the substitution pattern of the cyclohexa-1,4-diene core and the substituents at the silicon atom exert on these hydrosilane surrogates is systematically investigated. The results are compared with those obtained employing the hydrosilane directly. Another part of this comprehensive anal. is dedicated to the comparison of literature-known fully or partially fluorinated triarylboranes in both the direct and the transfer hydrosilylation of the aforementioned substrates. The data are tabulated and color-coded, finally providing an overview of promising substrate/reductant/borane combinations. The often different reactivities of π- and σ-basic substrates are explained, and it is shown that the Lewis acidity of the boron atom, estd. by the Gutmann-Beckett method, is not the only decisive feature of these boron Lewis acids. Practical mechanistic models are presented to rationalize the interplay between the Lewis acidity and steric situation at the boron and, likewise, the silicon atom as well as the need for fluorination ortho to the boron atom in certain cases.
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    Mahdi, T.; Stephan, D. W. Facile Protocol for Catalytic Frustrated Lewis Pair Hydrogenation and Reductive Deoxygenation of Ketones and Aldehydes. Angew. Chem., Int. Ed. 2015, 54, 8511– 8514,  DOI: 10.1002/anie.201503087
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    Facile protocol for catalytic frustrated lewis pair hydrogenation and reductive deoxygenation of ketones and aldehydes
    Mahdi, Tayseer; Stephan, Douglas W.
    Angewandte Chemie, International Edition (2015), 54 (29), 8511-8514CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The catalytic hydrogenation of ketones and aldehydes, generating the corresponding alcs. in a nonpolar solvent was achieved using cyclodextrin or mol. sieves in the presence of a catalytic amt. of B(C6F5)3. This combination of sol. borane and insol. materials mediates the redns. thus avoiding protodeborylation and providing a facile protocol for such redns. Similar treatment of aryl ketones resulted in metal-free deoxygenation yielding arom. hydrocarbons.
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    Chang, C. D.; Silvestri, A. J. The conversion of methanol and other O-compounds to hydrocarbons over zeolite catalysts. J. Catal. 1977, 47, 249– 259,  DOI: 10.1016/0021-9517(77)90172-5
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    The conversion of methanol and other O-compounds to hydrocarbons over zeolite catalysts
    Chang, Clarence D.; Silvestri, Anthony J.
    Journal of Catalysis (1977), 47 (2), 249-59CODEN: JCTLA5; ISSN:0021-9517.
    The conversion of MeOH [67-56-1] and other O-compds. to C2-C10 hydrocarbons using a new class of shape-selective zeolites is reported. MeOH, Me2O [115-10-6], or an equil. mixt. thereof is converted in a 1st reaction sequence to olefins predominantly in the C2-C5 range. In the final steps of the reaction path, the C2-C5 olefins are converted to paraffins, aroms., cycloparaffins, and C6+ olefins. The final hydrocarbons are largely in the gasoline (C4-C10) boiling range. The thermochem. of the MeOH to hydrocarbon is described and possible reaction mechanisms are discussed.
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    (a) Korstanje, T. J.; Jastrzebski, J. T. B. H.; Klein Gebbink, R. J. M. Catalytic Dehydration of Benzylic Alcohols to Styrenes by Rhenium Complexes. ChemSusChem 2010, 3, 695– 697,  DOI: 10.1002/cssc.201000055
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    Catalytic Dehydration of Benzylic Alcohols to Styrenes by Rhenium Complexes
    Korstanje, Ties J.; Jastrzebski, Johann T. B. H.; Klein Gebbink, Robertus J. M.
    ChemSusChem (2010), 3 (6), 695-697CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Rhenium-based catalysts were found to be very effective in the dehydration of benzylic alcs. In all cases the TOFs of Re207 were superior to those of sulfuric acid, the current benchmark catalyst, without sacrificing selectivity for the olefin products. The rhenium-based protocol has shown an excellent activity and selectivity in the dehydration reaction of a methoxy-substituted phenylethyl alc. and holds great promise for the dehydration of other lignin-based alcs.
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    (b) Bertero, N. M.; Trasarti, A. F.; Apesteguía, C. R.; Marchi, A. J. Liquid-phase dehydration of 1-phenylethanol on solid acids: Influence of catalyst acidity and pore structure. Appl. Catal., A 2013, 458, 28– 38,  DOI: 10.1016/j.apcata.2013.03.018
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    Liquid-phase dehydration of 1-phenylethanol on solid acids: Influence of catalyst acidity and pore structure
    Bertero, Nicolas M.; Trasarti, Andres F.; Apesteguia, Carlos R.; Marchi, Alberto J.
    Applied Catalysis, A: General (2013), 458 (), 28-38CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
    The liq.-phase dehydration of 1-phenylethanol (PHE) over different solid acids was studied at 363 K using cyclohexane as solvent. It was found that the catalyst activity and selectivity strongly depended on: (1) the nature, strength and d. of acid sites and (2) the textural properties of the catalyst. Catalysts contg. mainly surface Bronsted acid sites of medium-high strength, such as Amberlyst 15, HPA/SiO2, and HMOR zeolite, showed low initial styrene (STY) selectivity because the PHE dehydrated to alpha-methylbenzyl ether (AME) at higher or similar rates than to STY. Both primary products, STY and AME, were consecutively transformed to other heavy products (HP). Catalysts contg. predominantly Lewis acid sites, such as ZnO/SiO2, Al-MCM-41 and SiO2-Al2O3, selectively transformed PHE to AME. The ether can be sequentially converted to STY and HP, depending on the solid acidity. Solid acids having strong surface Lewis sites, e.g. SiO2-Al2O3, showed high dehydration rate and HP prodn. Samples contg. exclusively weak Lewis acid sites, e.g. γ-Al2O3, were not active in the PHE dehydration at 363 K. Only zeolites HZSM-5, HBEA and HY, with similar surface d. of Bronsted and Lewis acid sites, converted selectively PHE into STY, giving initial STY selectivities between 83 and 96%. However, HY zeolite was rapidly deactivated due to the blockage of its microporous structure by bulky compds. formed inside the large cages of 13 Å. Instead, on HBEA, STY was converted to HP that can diffuse through the solid microporous structure. Then, the selectivity to STY was drastically reduced with time. A const. and high STY selectivity at total PHE conversion was only obtained with HZSM-5. The pore size of this zeolite is enough to allow the diffusion and conversion of PHE into STY, but it is not adequate to form the surface intermediates leading to AME and HP. Thus, it was proved that: (1) the surface Bronsted to Lewis ratio strongly influences the initial selectivity in the liq.-phase PHE dehydration; (2) a similar Bronsted to Lewis ratio on the solid acid surface is necessary to obtain high initial selectivity to STY; and (3) the right porous structure is crucial to avoid HP prodn. and keep const. the STY selectivity at high PHE conversion. From the exptl. results obtained in this work, a mechanistic approach was proposed in order to explain the influence of both acid site nature and pore size on the selective PHE dehydration in liq. phase.
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    (a) Styskalik, A.; Vykoukal, V.; Fusaro, L.; Aprile, C.; Debecker, D. P. Mildly acidic aluminosilicate catalysts for stable performance in ethanol dehydration. Appl. Catal., B 2020, 271, 118926  DOI: 10.1016/j.apcatb.2020.118926
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    Mildly acidic aluminosilicate catalysts for stable performance in ethanol dehydration
    Styskalik, Ales; Vykoukal, Vit; Fusaro, Luca; Aprile, Carmela; Debecker, Damien P.
    Applied Catalysis, B: Environmental (2020), 271 (), 118926CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    Ethanol dehydration is effectively catalyzed by strongly acidic zeolites which, however, exhibit poor time-onstream stability. For example, HZSM-5 features strong acid sites, catalyzes ethylene oligomerization, and is prone to coking. Alumina and silica-alumina on the other hand have lower acidity, are relatively stable, but reach only low activity. Here, a series of mesoporous aluminosilicate catalysts was prepd. by non-hydrolytic sol-gel (NHSG) and are shown to feature an intermediate level of acidity (both in strength and nature), resulting in intermediate catalytic performance. Importantly, the best NHSG-made samples were very stable with time on stream, did not produce any traces of ethylene oligomers, did not show any trace of coke formation, and their texture was unaffected. Characterization (ICP-OES, N2-physisorption, TEM, XPS, IR-pyridine, Raman and solid state NMR spectroscopies) revealed that this behavior must be correlated with the remarkable degree of homogeneity in the NHSG-made aluminosilicate (only tetrahedrally coordinated Al species).
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    (b) Wang, Z.; O’Dell, L. A.; Zeng, X.; Liu, C.; Zhao, S.; Zhang, W.; Gaborieau, M.; Jiang, Y.; Huang, J. Insight into Three-Coordinate Aluminum Species on Ethanol-to-Olefin Conversion over ZSM-5 Zeolites. Angew. Chem., Int. Ed. 2019, 58, 18061– 18068,  DOI: 10.1002/anie.201910987
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    Insight into Three-Coordinate Aluminum Species on Ethanol-to-Olefin Conversion over ZSM-5 Zeolites
    Wang, Zichun; O'Dell, Luke A.; Zeng, Xin; Liu, Can; Zhao, Shufang; Zhang, Wenwen; Gaborieau, Marianne; Jiang, Yijiao; Huang, Jun
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    Applied Catalysis, A: General (2015), 493 (), 77-89CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
    Ethanol dehydration was investigated over com. H-FER, H-MFI, H-MOR, H-BEA, H-Y and H-USY zeolite samples, and alumina and silica alumina for comparison. The catalysts were characterized using FT-IR spectroscopy of the surface OH groups and of adsorbed CO and pyridine. UV-vis, Raman and TG-DTA were applied to characterize coke, formed more on H-MOR and H-BEA. H-zeolites are definitely more active than silica alumina and alumina on catalyst wt. base. The H-MOR sample is the most active but the H-MFI samples with Si/Al2 ratios 280 and 50 show higher reaction rates per Al ion, H-FER and faujasites show highest ethylene yield (99.9% at 573 K). At lower temp. and higher space velocities, di-Et ether is formed with high yield (>70% at 453-473 K on H-BEA and H-MFI (50)). Overconversion of ethylene mainly to aroms. is obsd. on H-MFI (50). The different behavior of protonic zeolites can predominantly be explained by confinement effects on the different zeolite cavities.
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    Journal of Catalysis (2014), 311 (), 102-113CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    Conversion of ethanol was investigated over four com. aluminas prepd. by different industrial procedures and one com. silica-alumina. Characterization was performed by TEM, XRD, SBET and porosity measurements, and IR spectroscopy of the surface OH groups and of adsorbed CO and pyridine. Different features are attributed to different phases (γ-, δ-, θ-Al2O3) and different impurities (Na+, Cl-). Total conversion of ethanol with >99% selectivity to ethylene is achieved at 623 K over the purer Al2O3 catalyst (Na < 0.002 wt%). The most active sites are believed to be Lewis acidic Al3+ sites in a tetrahedral environment located on edges and corners of the nanocrystals. Ethanol adsorbs dissociatively on Lewis acid-base pair sites but may also displace water and/or hydroxyl groups from Lewis acidic Al3+ sites forming the active intermediate ethoxy species. Surface ethoxy groups are supposed to be intermediate species for both di-Et ether and ethylene prodn. Silica-alumina also works as a Lewis acid catalyst. The slightly lower activity on surface area basis of silica-alumina than aluminas attributed to the lower d. of Lewis acid sites and the absence of significant basicity.
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    Mesoporous nickel aluminosilicate, a solid acid catalyst prepd. by sol-gel technique was used as a heterogeneous catalyst for the synthesis of sym. ethers by dehydro-condensation of alcs. The prepd. catalysts were characterized by Fourier-transform infra red spectroscopy (FTIR), powder XRD, SEM, energy dispersive X-ray anal. (EDAX), N2 adsorption-desorption anal., temp. programmed desorption of ammonia (TPD) and X-ray photoelectron spectroscopic techniques. The presence of the catalyst assisted the etherification reaction in 30 min. Ethers formed in these reactions were quantified by gas chromatog. (GC) and the identities of few of them were confirmed by NMR spectral data (NMR).
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    Intense sample heating through high-speed magic-angle spinning (MAS; up to 58 K temp. difference) is demonstrated. The role of probehead and spinner design, as well as that of the temp. of the bearing air on the heating of a rotating sample, was examd. MAS-induced heating can affect the accurate detn. of the isotropic value of the chem. shift as well as the principal values, asymmetry and anisotropy parameters of the chem. shift tensor. In some cases, a very large temp. gradient (12 K) within the fast rotating sample was found, which may limit the resoln. of high-speed 1H MAS NMR spectra.
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  • Abstract

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    Scheme 1

    Scheme 1. Synthesis of ActB Materials
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    Figure 1

    Figure 1. SEM images of ActB-Tol (left), ActB-cyHx (middle), and ActB-nHx (right). For additional images, see the SI.

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    Figure 2

    Figure 2. Adsorption isotherms of Ar at 87 K (top) and CO2 at 195 K (bottom) for ActB-Tol, ActB-cyHx, and ActB-nHx.

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    Figure 3

    Figure 3. FTIR (ATR-Si) spectra of ActB-Tol (top), ActB-cyHx (middle), and ActB-nHx (bottom); red lines correspond to samples exposed to air for 1 min.

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    Figure 4

    Figure 4. 1H MAS NMR (left column), 13C CP/MAS NMR (middle column), and 11B MAS NMR (right column) spectra of ActB-Tol, ActB-cyHx, and ActB-nHx samples.

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    Figure 5

    Figure 5. 31P MAS ssNMR spectra from top: BCF+TEPO adduct, TEPO, ActB-Tol+TEPO, ActB-cyHx+TEPO, and ActB-nHx+TEPO.

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    Figure 6

    Figure 6. 11B 3Q/MAS NMR spectra of ActB-Tol+TEPO (a), ActB-cyHx+TEPO (b), and ActB-nHx+TEPO (c). The newly detected signal is highlighted by gray boxes.

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    Figure 7

    Figure 7. TPD curves for ActB-Tol (left), ActB-cyHx (middle), and ActB-nHx (right).

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    Scheme 2

    Scheme 2. Deoxygenation of Benzophenone (1) Performed Using Different ActB Catalysts, Yields Determined by GC (Isolated Yield in Parentheses)a
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    a1 mol % B(C6F5)3 (BCF) used as a catalyst (reaction time 2 h). kinetic profiles (bottom) of the deoxygenation of 1 catalyzed by ActBs at 60 or 100 °C. Conversion of 1 determined by GC.

    Scheme 3

    Scheme 3. Hydrosilylation/deoxygenation of Acetophenone (4): (a) Screening of Different ActB Materials Using 1.5 or 3 equiv of Silane and Toluene as the Solvent; (b) Reaction Temperature Optimization Using ActB-Tol in Toluene; and (c) Variation of Reaction Conditions in THF Solvent. Yields Determined by GC
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    Scheme 4

    Scheme 4. Reactions Involved in the Mechanism of 4-Hydrosilylation/Deoxygenation by ActB-Tol: (a) Deoxygenation of Silyl Ether 5 in the Presence of Silane, (b) without Silane, (c) Attempted Reaction of Styrene (6), (d) Reaction of Alcohol 9 in the Presence of Silane, and (e) without Silane. Conditions: 60 °C, 22 h, Toluene Solvent (1 mmol Substrate, 20 mg of ActB). GC Yields Given in Parentheses
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    Scheme 5

    Scheme 5. Hydrosilylation/Deoxygenation of (a) Benzaldehyde (10), (b) Trans-chalcone (13), (c) Benzil (16), (d) Cyclohexanone (22), and (e) 2-Heptanone (27), using ActB-Tol or ActB-cyHx as the Catalysts at Various Conditions (Silane Stoichiometry, Temperature, Time) as Indicated. All Reactions Performed in Toluene. Yields Determined by GC
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    Figure 8

    Figure 8. Time-dependence of the catalytic activity of ActB-Tol in benzophenone (1) deoxygenation and ActB-cyHx in trans-chalcone (13) 1,4-hydrosilylation under flow conditions (X-Cube, 100 °C, flow rate: 0.1 mL min–1─solid lines─or 0.2 mL min–1─segmented line, concentration of substrates 0.5 mM mL–1).

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    Figure 9

    Figure 9. Ethanol conversion (top) and ethylene yield (bottom) during ethanol dehydration at 170, 190, 210, and 240 °C. Weight hour space velocity (WHSV) was kept for all measurements at 2.2 g g–1 h–1.

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    Figure 10

    Figure 10. Ethanol conversion (top) and ethylene yield (bottom) during ethanol dehydration at 240 °C overnight (stability test). WHSV was kept at 4.4 g g–1 h–1 except for HZSM-5 due to its high activity. WHSV was set to 17.6 g g–1 h–1; for details, see the SI.

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  • References

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    This article references 38 other publications.

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      (a) Forni, L.; Fornasari, G.; Tosi, C.; Trifirò, F.; Vaccari, A.; Dumeignil, F.; Grimblot, J. Non-conventional sol–gel synthesis for the production of boron-alumina catalyst applied to the vapour phase Beckmann rearrangement. Appl. Catal., A 2003, 248, 47– 57,  DOI: 10.1016/S0926-860X(03)00147-9
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      6a
      Non-conventional sol-gel synthesis for the production of boron-alumina catalyst applied to the vapor phase Beckmann rearrangement
      Forni, L.; Fornasari, G.; Tosi, C.; Trifiro, F.; Vaccari, A.; Dumeignil, F.; Grimblot, J.
      Applied Catalysis, A: General (2003), 248 (1-2), 47-57CODEN: ACAGE4; ISSN:0926-860X. (Elsevier Science B.V.)
      Non-zeolitic supported boria-alumina catalysts where B is part of the catalyst lattice, were tested for the cyclohexanone-oxime vapor phase rearrangement to caprolactam. The catalysts were obtained by the sol-gel route using (NH4)2B4O7. The catalyst compn. and structure were characterized using solid state 27Al- and 11B-MAS-NMR and x-ray powder diffraction; the acidity was detd. by TPD [temp. programmed desorption] of ammonia. The effect of B/Al ratio and the role of different polar agents in the feed were also studied. Total cyclohexanone-oxime conversion was attained using catalysts with low no. of acid sites and the selectivity toward caprolactam is strongly dependent on the type and distribution of acid sites. The B2O3-Al2O3 sol-gel catalysts have long lifetime but low selectivity toward caprolactam and loss of B occurred during regeneration of spent catalyst by heating at 500° for 15 h.
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      (b) Kröcher, O.; Elsener, M. Hydrolysis and oxidation of gaseous HCN over heterogeneous catalysts. Appl. Catal., B 2009, 92, 75– 89,  DOI: 10.1016/j.apcatb.2009.07.021
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      (c) Curtin, T.; McMonagle, J. B.; Hodnett, B. K. Influence of boria loading on the acidity of B2O3/Al2O3 catalysts for the conversion of cyclohexanone oxime to caprolactam. Appl. Catal., A 1992, 93, 91– 101,  DOI: 10.1016/0926-860X(92)80296-O
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      6c
      Influence of boria loading on the acidity of boron oxide (B2O3)/alumina catalysts for the conversion of cyclohexanone oxime to caprolactam
      Curtin, T.; McMonagle, J. B.; Hodnett, B. K.
      Applied Catalysis, A: General (1992), 93 (1), 91-101CODEN: ACAGE4; ISSN:0926-860X.
      Boria catalysts supported on alumina were prepd. with boria loadings of 0-20 wt.%. These materials were characterized by BET surface area measurements, x-ray diffraction, and temp.-programmed desorption of NH3, and were used as catalysts for the Beckmann rearrangement of cyclohexanone oxime (I) to caprolactam (II) by passing the I vapor through a reactor contg. the catalysts at 300°. The surface areas of the samples decreased as the boria loading increased, and all catalysts tested for the rearrangement reaction diminished in activity with time-onstream. There was a direct relationship between the amt. of coke which formed on the surface and the loss in catalytic activity. This loss was least for the highest boria loading tested, and this catalyst also featured the least amt. of coke formation. Selectivity to II was assocd. with the presence of surface acidic sites of intermediate strength. A correlation was obsd. between the concn. of these surface sites and the selectivity to II.
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    7. 7
      Yang, W.; Kim, K. D.; O’Dell, L. A.; Wang, L.; Xu, H.; Ruan, M.; Wang, W.; Ryoo, R.; Jiang, Y.; Huang, J. Brønsted acid sites formation through penta-coordinated aluminum species on alumina-boria for phenylglyoxal conversion. J. Catal. 2022, 416, 375– 386,  DOI: 10.1016/j.jcat.2022.11.012
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      7
      Bronsted acid sites formation through penta-coordinated aluminum species on alumina-boria for phenylglyoxal conversion
      Yang, Wenjie; Kim, Kyung Duk; O'Dell, Luke A.; Wang, Lizhuo; Xu, Haimei; Ruan, Mengtong; Wang, Wei; Ryoo, Ryong; Jiang, Yijiao; Huang, Jun
      Journal of Catalysis (2022), 416 (), 375-386CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Recently, Penta-coordinated Al (AlV) have attracted large attention and is promising for tailoring catalysts with the capability to form new BAS in specific mixed oxide. In this study, amorphous alumina-boria materials with enriched surface AlV were prepd. as platforms for the formation of potential new BAS. With the aid of solid-state NMR, interacting species of AlV with three-coordinated boron have been identified in the highly distorted local structure with BAS forming potential (AlV -OH...BIII). The catalytic activity of the AlV-BAS has been tested by the phenylglyoxal conversion, where the functionality of AlV-BAS has been further verified by the in situ 1H NMR. Herein, the ssNMR observation demonstrates the first exptl. visualization of a new type AlV-BAS in non-silica alumina materials present with predominance. The AlV-BAS is promising to serve as one new state-of-the-art strategy to improve acid catalytic performance in varied mixed oxides.
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    8. 8
      (a) Lam, J.; Szkop, K. M.; Mosaferi, E.; Stephan, D. W. FLP catalysis: main group hydrogenations of organic unsaturated substrates. Chem. Soc. Rev. 2019, 48, 3592– 3612,  DOI: 10.1039/C8CS00277K
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      8a
      FLP catalysis: main group hydrogenations of organic unsaturated substrates
      Lam, Jolie; Szkop, Kevin M.; Mosaferi, Eliar; Stephan, Douglas W.
      Chemical Society Reviews (2019), 48 (13), 3592-3612CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. This article is focused on recent developments in main group mediated hydrogenation chem. and catalysis using 'frustrated Lewis pairs' (FLPs). The broading range of substrates and catalyst systems is reviewed and the advances in catalytic redns. and the development of stereoselective, asym. redns. made since 2012 was considered.
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      (b) Stephan, D. W. Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen. J. Am. Chem. Soc. 2021, 143, 20002– 20014,  DOI: 10.1021/jacs.1c10845
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      8b
      Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen
      Stephan, Douglas W.
      Journal of the American Chemical Society (2021), 143 (48), 20002-20014CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A review. This perspective article focuses on the new directions and developments that are emerging from this frustrated Lewis pair (FLP) chem. involving hydrogen. Three areas are discussed including new applications and approaches to FLP redns., the redns. of small mols., and the advances in heterogeneous FLP systems. These foci serve to illustrate that despite having its roots in main group chem., this simple concept of FLPs is being applied across the discipline.
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    9. 9
      (a) Piers, W. E.; Chivers, T. Pentafluorophenylboranes: from obscurity to applications. Chem. Soc. Rev. 1997, 26, 345– 354,  DOI: 10.1039/cs9972600345
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      9a
      Pentafluorophenylboranes: from obscurity to applications
      Piers, Warren E.; Chivers, Tristram
      Chemical Society Reviews (1997), 26 (5), 345-354CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      Pentafluorophenyl substituted boranes and borates are important as co-catalysts in metallocene-based industrial processes for the homogeneous polymn. of olefins. Although 1st prepd. in the early 1960s, the remarkable properties of tris(pentafluorophenyl)borane have only recently been exploited for applications in catalysis. Spurred by these developments, the related compds. bis(pentafluorophenyl)borane and salts of the tetrakis(pentafluorophenyl)borate anion also found application in olefin polymn. and other fields. The rise of pentafluorophenyl B compds. from curiosities to important commodities is reviewed with 39 refs.
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      (b) Lawson, J. R.; Melen, R. L. Tris(pentafluorophenyl)borane and Beyond: Modern Advances in Borylation Chemistry. Inorg. Chem. 2017, 56, 8627– 8643,  DOI: 10.1021/acs.inorgchem.6b02911
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      9b
      Tris(pentafluorophenyl)borane and beyond: modern advances in borylation chemistry
      Lawson, James R.; Melen, Rebecca L.
      Inorganic Chemistry (2017), 56 (15), 8627-8643CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A review. As main-group chem., in particular boron chem., has expanded and developed over the past 20 years, one reagent has risen to prominence as well. Tris(pentafluorophenyl)borane, B(C6F5)3 (commonly known as BCF), has demonstrated extensive applications in a wide variety of reactions, including borylation, hydrogenation, hydrosilylation, frustrated Lewis pair (FLP) chem., Lewis acid catalysis, and more. The high Lewis acidity of B(C6F5)3 is derived from the electronic effects of its three C6F5 rings, rendering it a versatile reagent for a great no. of reactions. In addn., the steric bulk of these rings also allows it to function as the Lewis acid in a FLP, granting this reagent yet another synthetically useful application. However, as main-group chem. continues to evolve as a field, new reagents are required that go beyond BCF, increasing not only the range of reactions available but also the breadth of compds. attainable. Great strides have already been made in order to accomplish this task, and this review will highlight modern advances in boron chem. relating to borylation reactions. Herein, we will show the recent uses of B(C6F5)3 in borylation reactions while also focusing on current advances in novel borane and borocation usage that eclipses that of the stalwart B(C6F5)3.
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    10. 10
      Wanglee, Y.-J.; Hu, J.; White, R. E.; Lee, M.-Y.; Stewart, S. M.; Perrotin, P.; Scott, S. L. Borane-Induced Dehydration of Silica and the Ensuing Water-Catalyzed Grafting of B(C6F5)3 To Give a Supported, Single-Site Lewis Acid, ≡SiOB(C6F5)2. J. Am. Chem. Soc. 2012, 134, 355– 366,  DOI: 10.1021/ja207838j
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      10
      Borane-induced dehydration of silica and the ensuing water-catalyzed grafting of B(C6F5)3 to give a supported, single-site Lewis acid, ≡SiOB(C6F5)2
      Wanglee, Ying-Jen; Hu, Jerry; White, Rosemary E.; Lee, Ming-Yung; Stewart, S. Michael; Perrotin, Philippe; Scott, Susannah L.
      Journal of the American Chemical Society (2012), 134 (1), 355-366CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Reaction of B(C6F5)3 with silica surface in the presence of water traces was used for immobilization of borane Lewis acid as a cocatalyst in zirconocene- and nickel-catalyzed alkene polymn. A supported, single-site Lewis acid, ≡SiOB(C6F5)2, was prepd. by water-catalyzed grafting of B(C6F5)3 onto the surface of amorphous silica, and its subsequent use as a cocatalyst for heterogeneous olefin polymn. was explored. Although B(C6F5)3 has been reported to be unreactive toward silica in the absence of a Bronsted base, we find that it can be grafted even at room temp., albeit slowly. The mechanism was investigated by 1H and 19F NMR, in both the soln. and solid states. In the presence of a trace amt. of H2O, either added intentionally or formed in situ by borane-induced dehydration of silanol pairs, the adduct (C6F5)3B·OH2 hydrolyzes to afford C6F5H and (C6F5)2BOH. The latter reacts with the surface hydroxyl groups of silica to yield ≡SiOB(C6F5)2 sites and regenerate H2O. When B(C6F5)3 is present in excess, the resulting grafted boranes appear to be completely dry, due to the eventual formation of [(C6F5)2B]2O. The immobilized, tri-coordinate Lewis acid sites were characterized by solid-state 11B and 19F NMR, IR, elemental anal., and C5H5N-time-programmed desorption (TPD). Their ability to activate two mol. C2H4 polymn. catalysts, Cp2ZrMe2 and an (α-iminocarboxamidato)nickel(II) complex, was explored.
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    11. 11
      Tian, J.; Wang, S.; Feng, Y.; Li, J.; Collins, S. Borane-functionalized oxide supports: development of active supported metallocene catalysts at low aluminoxane loading. J. Mol. Catal. A: Chem. 1999, 144, 137– 150,  DOI: 10.1016/S1381-1169(98)00341-0
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      11
      Borane-functionalized oxide supports: development of active supported metallocene catalysts at low aluminoxane loading
      Tian, Jun; Wang, Shaotian; Feng, Yuding; Li, Jieming; Collins, Scott
      Journal of Molecular Catalysis A: Chemical (1999), 144 (1), 137-150CODEN: JMCCF2; ISSN:1381-1169. (Elsevier Science B.V.)
      Treatment of hydroxylated silica or alumina with tris(perfluorophenyl)borane (1), bis(perfluorophenyl)borane (2) or bis(perfluorophenyl)boron chloride (3), provides borane-functionalized supports of variable compn., as revealed by in situ monitoring by 19F NMR spectroscopy and/or elemental anal. These chem. treated supports can be impregnated with Cp2ZrMe2 to provide supported catalysts for ethylene polymn. Although some of these supported catalysts are active for ethylene polymn. in the presence of alkylaluminum compds. (e.g., TMA, TIBAL), all of these catalysts are more efficiently activated in the presence of small quantities of Me aluminoxane, even at very low Al:Zr ratios of 10:1. The polymer properties are quite similar to those produced using the sol. catalyst Cp2ZrMe2/B(C6F5)3, again in the presence of MAO at low loading. A variety of expts. suggest that minimal leaching of the metallocene complex from the support occurs under the conditions studied; in particular, prodn. of polyethylene with high bulk densities (>0.2 g/cm3) and little reactor fouling, even at elevated temp. in toluene slurry, is obsd.
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    12. 12
      Correa, S. A.; Diaz-Droguett, D. E.; Galland, G. B.; Maraschin, T. G.; De Sousa Basso, N.; Dogan, F.; Rojas, R. S. Modification of rGO by B(C6F5)3 to generated single-site Lewis Acid rGO-O-B(C6F5)2 as co activator of nickel complex, to produce highly disperse rGO-PE nanocomposite. Appl. Catal., A 2019, 580, 149– 157,  DOI: 10.1016/j.apcata.2019.05.004
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      12
      Modification of rGO by B(C6F5)3 to generated single-site Lewis Acid rGO-O-B(C6F5)2 as co activator of nickel complex, to produce highly disperse rGO-PE nanocomposite
      Correa, Sebastian A.; Diaz-Droguett, D. E.; Galland, Griselda B.; Maraschin, Thuany G.; De Sousa Basso, Nara; Dogan, Fulya; Rojas, Rene S.
      Applied Catalysis, A: General (2019), 580 (), 149-157CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
      We investigated the incorporation of reduced graphene oxide (rGO) into branched polyethylene, achieving nanocomposites with different rGO contents. The strategy included the direct reaction between B(C6F5)3 (BCF) with rGO (reduced graphene oxide) and demonstrated by XPS, 11B, 19F -MAS NMR, FT-IR the efficient modification of the rGO. The resulting solid contains sites -O-B(C6F5)2 capable of acting as co-catalysts in the activation of an (α-iminocarboxamidate)nickel (II) complex. The results show the efficient activation of this nickel catalyst through an exocyclic Lewis acid-base interaction and the subsequent ethylene polymn. We achieved with this heterogeneous catalytic system activity greater than those previously reported for the BCF-Ni, homogenous system, while the characterization of the black colored polymer generated in situ, showed a complete delamination of the rGO. It became the first rGO-B(C6F5)2-Ni system fully characterized and capable of delaminate the rGO support by producing a nanocomposite rGO-LLDPE without loss of activity, compared to the homogeneous system. In addn., the polymer contains about 70% of Me branches and a m.p. higher than 125 °C.
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    13. 13
      (a) Horton, T. A. R.; Wang, M.; Shaver, M. P. Polymeric frustrated Lewis pairs in CO2/cyclic ether coupling catalysis. Chem. Sci. 2022, 13, 3845– 3850,  DOI: 10.1039/D2SC00894G
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      13a
      Polymeric frustrated Lewis pairs in CO2/cyclic ether coupling catalysis
      Horton, Thomas A. R.; Wang, Meng; Shaver, Michael P.
      Chemical Science (2022), 13 (13), 3845-3850CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Frustrated Lewis pairs (FLPs) are now ubiquitous as metal-free catalysts in an array of different chem. transformations. In this paper we show that this reactivity can be transferred to a polymeric system, offering advantageous opportunities at the interface between catalysis and stimuli-responsive materials. Formation of cyclic carbonates from cyclic ethers using CO2 as a C1 feedstock continues to be dominated by metal-based systems. When paired with a suitable nucleophile, discrete aryl or alkyl boranes have shown significant promise as metal-free Lewis acidic alternatives, although catalyst reuse remains illusive. Herein, we leverage the reactivity of FLPs in a polymeric system to promote CO2/cyclic ether coupling catalysis that can be tuned for the desired epoxide or oxetane substrate. Moreover, these macromol. FLPs can be reused across multiple reaction cycles, further increasing their appeal over analogus small mol. systems.
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      (b) Yolsal, U.; Horton, T. A. R.; Wang, M.; Shaver, M. P. Cyclic Ether Triggers for Polymeric Frustrated Lewis Pair Gels. J. Am. Chem. Soc. 2021, 143, 12980– 12984,  DOI: 10.1021/jacs.1c06408
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      13b
      Cyclic Ether Triggers for Polymeric Frustrated Lewis Pair Gels
      Yolsal, Utku; Horton, Thomas A. R.; Wang, Meng; Shaver, Michael P.
      Journal of the American Chemical Society (2021), 143 (33), 12980-12984CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Sterically hindered Lewis acid and base centers are unable to form Lewis adducts, instead forming frustrated Lewis pairs (FLPs), where latent reactivity can be utilized for the activation of small mols. Applying FLP chem. into polymeric frameworks transforms this chem. into responsive and functional materials. Here, we report a versatile synthesis strategy for the prepn. of macromol. FLPs and explore its potential with the ring-opening reactions of cyclic ethers. Addn. of the cyclic substrates triggered polymer network formation, where the extent of crosslinking, strength of network, and reactivity are tuned by the steric and electronic properties of the ethers. The resultant networks behave like covalently crosslinked polymers, demonstrating the versatility of FLPs to simultaneously tune both small-mol. capture and mech. properties of materials.
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      (c) Vidal, F.; McQuade, J.; Lalancette, R.; Jäkle, F. ROMP-Boranes as Moisture-Tolerant and Recyclable Lewis Acid Organocatalysts. J. Am. Chem. Soc. 2020, 142, 14427– 14431,  DOI: 10.1021/jacs.0c05454
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      13c
      ROMP-Boranes as Moisture-Tolerant and Recyclable Lewis Acid Organocatalysts
      Vidal, Fernando; McQuade, James; Lalancette, Roger; Jakle, Frieder
      Journal of the American Chemical Society (2020), 142 (34), 14427-14431CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Although widely used in catalysis, the multistep syntheses and high loadings typically employed are limiting broader implementation of highly active tailor-made arylborane Lewis acids and Lewis pairs. Attempts at developing recyclable systems have thus far met with limited success, as general and versatile platforms are yet to be developed. We demonstrate a novel approach that is based on the excellent control and functional group tolerance of ring-opening metathesis polymn. (ROMP). The ROMP of highly Lewis acidic borane-functionalized phenylnorbornenes afforded both a sol. linear copolymer and a cross-linked organogel. The polymers proved highly efficient as recyclable catalysts in the reductive N-alkylation of arylamines under mild conditions and at exceptionally low catalyst loadings. The modular design presented herein can be readily adapted to other finely tuned triarylboranes, enabling wide applications of ROMP-borane polymers as well-defined supported organocatalysts.
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      (d) Chen, L.; Liu, R.; Yan, Q. Polymer Meets Frustrated Lewis Pair: Second-Generation CO2-Responsive Nanosystem for Sustainable CO2 Conversion. Angew. Chem., Int. Ed. 2018, 57, 9336– 9340,  DOI: 10.1002/anie.201804034
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      13d
      Polymer Meets Frustrated Lewis Pair: Second-Generation CO2-Responsive Nanosystem for Sustainable CO2 Conversion
      Chen, Liang; Liu, Renjie; Yan, Qiang
      Angewandte Chemie, International Edition (2018), 57 (30), 9336-9340CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Frustrated Lewis pairs (FLP), a couple comprising a sterically encumbered Lewis acid and Lewis base, can offer latent reactivity for activating inert gas mols. However, their use as a platform for fabricating gas-responsive materials has not yet developed. Merging the FLP concept with polymers, we report a new generation CO2-responsive system, differing from the first-generation ones based on an acid-base equil. mechanism. Two complementary Lewis acidic and basic block copolymers, installing bulky borane- and phosphine-contg. blocks, were built as the macromol. FLP. They can bind CO2 to drive micellar formation, in which CO2 as a cross-linker bridges the block chains. This dative bonding endows the assembly with ultrafast response (<20 s), thermal reversibility, and excellent reproducibility. Moreover, such micelles bound highly active CO2 can function as nanocatalysts for recyclable C1 catalysis, opening a new direction of sustainable CO2 conversion.
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      (e) Yolsal, U.; Horton, T. A. R.; Wang, M.; Shaver, M. P. Polymer-supported Lewis acids and bases: Synthesis and applications. Prog. Polym. Sci. 2020, 111, 101313  DOI: 10.1016/j.progpolymsci.2020.101313
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      13e
      Polymer-supported Lewis acids and bases: Synthesis and applications
      Yolsal, Utku; Horton, Thomas A. R.; Wang, Meng; Shaver, Michael P.
      Progress in Polymer Science (2020), 111 (), 101313CODEN: PRPSB8; ISSN:0079-6700. (Elsevier Ltd.)
      In the past decade the synthesis of novel stimuli-responsive materials has been driven by the pursuit of new applications and, more recently, sustainable and reusable systems. Of these materials, those which incorporate main group Lewis acids (LAs) and bases (LBs) into their polymer backbones have shown extraordinary utility as a result of their synthetic diversity, enabling fine tuning of reactivity and ensuing properties tailored to the desired application. Herein, the recent progress made in the synthesis and applications of macromol. LAs and LBs is being highlighted. Interactions between polymeric LAs and LBs can be exploited to build supramol. polymeric networks based on both conventional and frustrated Lewis pairs, while using either functionality individually enables the prepn. of sensors for anions, cations, explosives and biol. mols. The presence of polymer-supported LAs/LBs in organocatalysis has been extended to controlling polymer morphol., enabled improvements in activity through compartmentalization and the coexistence of classically incompatible functionalities. Finally, the versatility of this field is being demonstrated by highlighting some of the recent advances in CO2 chemisorption systems employing amine-based polymeric LBs for carbon capture and redn.
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    14. 14
      (a) Zhang, C.; Wang, J.; Su, W.; Lin, Z.; Ye, Q. Synthesis, Characterization, and Density Functional Theory Studies of Three-Dimensional Inorganic Analogues of 9,10-Diboraanthracene─A New Class of Lewis Superacids. J. Am. Chem. Soc. 2021, 143, 8552– 8558,  DOI: 10.1021/jacs.1c03057
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      14a
      Synthesis, Characterization, and Density Functional Theory Studies of Three-Dimensional Inorganic Analogs of 9,10-Diboraanthracene-A New Class of Lewis Superacids
      Zhang, Chonghe; Wang, Junyi; Su, Wei; Lin, Zhenyang; Ye, Qing
      Journal of the American Chemical Society (2021), 143 (23), 8552-8558CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The three-dimensional inorg. analogs of 9,10-diboraanthracene, B2X2(C2B10H10)2 (X = Cl, 1; X = Br, 2), were attained by salt elimination of Li2C2B10H10 and trihaloboranes. The Me- and phenyl-substituted compds. B2Me2(C2B10H10)2 (3) and B2Ph2(C2B10H10)2 (4) were obtained by treating 1 or 2 with the corresponding Grignard reagents. These compds. were fully characterized by NMR, cyclic voltammetry (CV), IR, and single-crystal x-ray diffraction analyses. Exptl. (CV and Gutmann-Beckett method) and computational (fluoride ion affinity, hydride ion affinity and LUMO energy) results suggest that the order of Lewis acidity is 2 > 1 > 4 > 3 > SbF5. Treatment of 1 or 2 with HSiEt3 gave a rare neutral borane-silane adduct, (Et3SiH)2B2H2(C2B10H10)2 (5). The equil. of 5 in soln. was thoroughly studied by spectroscopy and quantum calcns.
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      (b) Akram, M. O.; Tidwell, J. R.; Dutton, J. L.; Martin, C. D. Tris(ortho-carboranyl)borane: An Isolable, Halogen-Free, Lewis Superacid. Angew. Chem., Int. Ed. 2022, 61, e202212073  DOI: 10.1002/anie.202212073
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      14b
      Tris(ortho-carboranyl)borane: An Isolable, Halogen-Free, Lewis Superacid
      Akram, Manjur O.; Tidwell, John R.; Dutton, Jason L.; Martin, Caleb D.
      Angewandte Chemie, International Edition (2022), 61 (46), e202212073CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The synthesis of tris(ortho-carboranyl)borane (BoCb3), a single site neutral Lewis superacid, in one pot from com. available materials is achieved. The high fluoride ion affinity (FIA) confirms its classification as a Lewis superacid and the Gutmann-Beckett method as well as adducts with Lewis bases indicate stronger Lewis acidity over the widely used fluorinated aryl boranes. The electron withdrawing effect of ortho-carborane and lack of pi-delocalization of the LUMO rationalize the unusually high Lewis acidity. Catalytic studies indicate that BoCb3 is a superior catalyst for promoting C-F bond functionalization reactions than tris(pentafluorophenyl)borane [B(C6F5)3].
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      (c) Yruegas, S.; Axtell, J. C.; Kirlikovali, K. O.; Spokoyny, A. M.; Martin, C. D. Synthesis of 9-borafluorene analogues featuring a three-dimensional 1,1′-bis(o-carborane) backbone. Chem. Commun. 2019, 55, 2892– 2895,  DOI: 10.1039/C8CC10087J
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      14c
      Synthesis of 9-borafluorene analogues featuring a three-dimensional 1,1'-bis(o-carborane) backbone
      Yruegas, Sam; Axtell, Jonathan C.; Kirlikovali, Kent O.; Spokoyny, Alexander M.; Martin, Caleb D.
      Chemical Communications (Cambridge, United Kingdom) (2019), 55 (20), 2892-2895CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      The synthesis of [1,1'-bis(o-carboranyl)]boranes was achieved through the deprotonation of 1,1'-bis(o-carborane) reagents followed by salt metathesis with (iPr)2NBCl2. X-ray crystallog. confirms planar central BC4 rings and Gutmann-Beckett studies reveal an increase in Lewis acidity at the boron center in comparison to their biphenyl congener, 9-borafluorene.
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    15. 15
      (a) Fujino, A.; Ito, S.-i.; Goto, T.; Ishibiki, R.; Kondo, J. N.; Fujitani, T.; Nakamura, J.; Hosono, H.; Kondo, T. Hydrogenated Borophene Shows Catalytic Activity as Solid Acid. ACS Omega 2019, 4, 14100– 14104,  DOI: 10.1021/acsomega.9b02020
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      15a
      Hydrogenated Borophene Shows Catalytic Activity as Solid Acid
      Fujino, Asahi; Ito, Shin-ichi; Goto, Taiga; Ishibiki, Ryota; Kondo, Junko N.; Fujitani, Tadahiro; Nakamura, Junji; Hosono, Hideo; Kondo, Takahiro
      ACS Omega (2019), 4 (9), 14100-14104CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      The catalytic activity of hydrogenated borophene (HB) sheets in EtOH reforming is reported. HB sheets catalyze the conversion of EtOH to ethylene and H2O at >493 K with high selectivity, independent of the contact time, and with an apparent activation energy of 102.8 ± 5.5 kJ/mol. HB sheets act as solid-acid catalysts.
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      (b) Li, Q.; Kolluru, V. S. C.; Rahn, M. S.; Schwenker, E.; Li, S.; Hennig, R. G.; Darancet, P.; Chan, M. K. Y.; Hersam, M. C. Synthesis of borophane polymorphs through hydrogenation of borophene. Science 2021, 371, 1143– 1148,  DOI: 10.1126/science.abg1874
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      15b
      Synthesis of borophane polymorphs through hydrogenation of borophene
      Li, Qiucheng; Kolluru, Venkata Surya Chaitanya; Rahn, Matthew S.; Schwenker, Eric; Li, Shaowei; Hennig, Richard G.; Darancet, Pierre; Chan, Maria K. Y.; Hersam, Mark C.
      Science (Washington, DC, United States) (2021), 371 (6534), 1143-1148CODEN: SCIEAS; ISSN:1095-9203. (American Association for the Advancement of Science)
      Synthetic two-dimensional polymorphs of B, or borophene, have attracted attention because of their anisotropic metallicity, correlated-electron phenomena, and diverse superlattice structures. Although borophene heterostructures were realized, ordered chem. modification of borophene has not yet been reported. Here, the authors synthesize borophane polymorphs by hydrogenating borophene with at. H in ultrahigh vacuum. Through at.-scale imaging, spectroscopy, and first-principles calcns., the most prevalent borophane polymorph possesses a combination of two-center-two-electron B-H and three-center-two-electron B-H-B bonds. Borophane polymorphs are metallic with modified local work functions and can be reversibly returned to pristine borophene through thermal desorption of H. Hydrogenation also provides chem. passivation because borophane reduces oxidn. rates by more than two orders of magnitude after ambient exposure.
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    16. 16
      Bůžek, D.; Škoch, K.; Ondrušová, S.; Kloda, M.; Bavol, D.; Mahun, A.; Kobera, L.; Lang, K.; Londesborough, M. G. S.; Demel, J. ″Activated Borane″ - A Porous Borane Cluster Network as an Effective Adsorbent for Removing Organic Pollutants. Chem.─Eur. J. 2022, 28, e202201885  DOI: 10.1002/chem.202201885
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      16
      "Activated Borane" - A Porous Borane Cluster Network as an Effective Adsorbent for Removing Organic Pollutants
      Buzek, Daniel; Skoch, Karel; Ondrusova, Sona; Kloda, Matous; Bavol, Dmytro; Mahun, Andrii; Kobera, Libor; Lang, Kamil; Londesborough, Michael G. S.; Demel, Jan
      Chemistry - A European Journal (2022), 28 (67), e202201885CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The unprecedented co-thermolysis of decaborane(14) (nido-B10H14) and toluene results in a novel porous material (that the authors have named "activated borane") contg. micropores between 1.0 and 1.5 nm in diam. and a sp. surface area of 774 m2 g-1 (Ar, 87 K) that is thermally stable up to 1000°C. Solid state 1H, 11B and 13C MAS NMR, UV-vis and IR spectroscopies suggest an amorphous structure of borane clusters interconnected by toluene moieties in a ratio of about three toluene mols. for every borane cluster. In addn., the structure contains Lewis-acidic tri-coordinated boron sites giving it some unique properties. Activated borane displays high sorption capacity for pollutants such as sulfamethoxazole, tramadol, diclofenac and bisphenol A that exceed the capacity of com.-available activated carbon. The consistency in properties for each batch made, and the ease of its synthesis, make activated borane a promising porous material worthy of broad attention.
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    17. 17
      Demel, J.; Kloda, M.; Lang, K.; Škoch, K.; Hynek, J.; Opravil, A.; Novotný, M.; Bould, J.; Ehn, M.; Londesborough, M. G. S. Direct Phenylation of nido-B10H14. J. Org. Chem. 2022, 87, 10034– 10043,  DOI: 10.1021/acs.joc.2c00997
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      17
      Direct Phenylation of nido-B10H14
      Demel, Jan; Kloda, Matous; Lang, Kamil; Skoch, Karel; Hynek, Jan; Opravil, Adam; Novotny, Matyas; Bould, Jonathan; Ehn, Marcel; Londesborough, Michael G. S.
      Journal of Organic Chemistry (2022), 87 (15), 10034-10043CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      As a preliminary step toward its condensation into the porous polymer Activated Borane, the thermolysis of nido-B10H14 (1) in benzene at 200° gave a no. of phenylated borane mol. species. The principal product is the new monophenylated compd. 5-Ph-nido-B10H13 (2), isolated in 48% yield (based on consumption of 1) and structurally characterized by single-crystal x-ray diffraction anal., NMR, and mass spectrometry along with other minor products, such as 6-Ph-nido-B10H13 (3), for which the authors observe UV-light-driven conversion into 2 via a vertex-flip mechanism, and novel diphenylated 5,8-Ph2-nido-B10H12 (4). Together, the phenylated derivs. provide a valuable insight into the assembly of Activated Borane and ultimately inform on its structure. The new compds. also display strong blue fluorescence in both solid-state and in soln. and are the 1st examples of the direct phenylation of nido-B10H14, thus opening the door to the straight-forward synthesis of highly luminescent org.-borane hybrid systems.
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    18. 18
      Hermanek, S. Boron-11 NMR spectra of boranes, main-group heteroboranes, and substituted derivatives. Factors influencing chemical shifts of skeletal atoms. Chem. Rev. 1992, 92, 325– 362,  DOI: 10.1021/cr00010a007
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      18
      Boron-11 NMR spectra of boranes, main-group heteroboranes, and substituted derivatives. Factors influencing chemical shifts of skeletal atoms
      Hermanek, Stanislav
      Chemical Reviews (Washington, DC, United States) (1992), 92 (2), 325-62CODEN: CHREAY; ISSN:0009-2665.
      A review with 286 refs.
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    19. 19
      (a) Harazono, T.; Hiroyama, Y.; Watanabe, T. Solid State NMR of 11B and 13C in Boron Carbide, B12C3 and 11B Enriched B12C3. Bull. Chem. Soc. Jpn. 1996, 69, 2419– 2423,  DOI: 10.1246/bcsj.69.2419
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      19a
      Solid-state NMR of 11B and 13C in boron carbide, B12C3 and 11B-enriched B12C3
      Harazono, Toshie; Hiroyama, Yukiyo; Watanabe, Tokuko
      Bulletin of the Chemical Society of Japan (1996), 69 (9), 2419-2423CODEN: BCSJA8; ISSN:0009-2673. (Nippon Kagakkai)
      11B- and 13C-solid state NMR of boron carbide with different isotope ratios, B4C(11B/10B = 80.42/19.58; natural abundance isotope) and 11B4C(11B/10B = 99.5/0.5; 11B enriched sample), was studied. The linewidth of the icosahedral B (6h1 and 6h2) in 11B-static NMR signal in 11B4C was greater in 11B4C, which indicates that the B-B dipole-dipole interaction is the main mechanism for the broadening of the 11B-static NMR signal. The quadrupole coupling const. calcd. from the signal position of (±1/2↔±3/2) transition was ∼0.1 MHz. 11B-MAS spectra revealed at least two addnl. B sites (37 ppm and near -60 ppm) besides the icosahedral B sites (-6 ppm).
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      (b) Mauri, F.; Vast, N.; Pickard, C. J. Atomic Structure of Icosahedral B4C Boron Carbide from a First Principles Analysis of NMR Spectra. Phys. Rev. Lett. 2001, 87, 085506  DOI: 10.1103/PhysRevLett.87.085506
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      19b
      Atomic structure of icosahedral B4C boron carbide from a first principles analysis of NMR spectra
      Mauri, Francesco; Vast, Nathalie; Pickard, Chris J.
      Physical Review Letters (2001), 87 (8), 085506/1-085506/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      D. functional theory is demonstrated to reproduce the 13C and 11B NMR chem. shifts of icosahedral boron carbides with sufficient accuracy to ext. previously unresolved structural information from exptl. NMR spectra. B4C can be considered as an arrangement of 3-atom linear chains with C-B-C structure and 12-atom icosahedra. Most of the icosahedra have a B11C structure with the C atom placed in a polar site, and a few % have a B12 structure or a B10C2 structure with the two C atoms placed in two antipodal polar sites.
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    20. 20
      Diaz, M.; Jaballas, J.; Arias, J.; Lee, H.; Onak, T. 13C NMR Studies on Carboranes and Derivatives: Experimental/Calculational Correlations. J. Am. Chem. Soc. 1996, 118, 4405– 4410,  DOI: 10.1021/ja954089y
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      20
      13C NMR Studies on Carboranes and Derivatives: Experimental/Calculational Correlations
      Diaz, Martin; Jaballas, Jojo; Arias, Joachin; Lee, Hans; Onak, Thomas
      Journal of the American Chemical Society (1996), 118 (18), 4405-10CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The measured 13C chem. shifts of over forty carborane compds. correlate very well with ab-initio/IGLO/NMR calcd. values at both the DZ//3-21G and DZ//6-31G* (IGLO-NMR//Gaussian-geometry-optimized) levels of theory as well as with the ab-initio/GIAO/NMR values at the 6-31G*//6-31G* level of theory. For the carboranes the linear relations δ(13C-IGLO-DZ//6-31G*) = 0.941δ(13C-exp) - 1.897 (r2 = 0.990) and δ(13C-GIAO-6-31G*//6-31G*) = 0.893δ(13C-exp) - 2.554 (r2 = 0.991) are derived. Combined together with recently reported 11B NMR correlations between expt. and theory, a significant means is added to the arsenal of carborane NMR structure proof methods available to the experimentalist having access to only modest computational resources. And this procedure, of course, also has the addnl. feature of yielding reasonably good structural information (bond distances, angles, etc.).
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    21. 21
      Beckett, M. A.; Strickland, G. C.; Holland, J. R.; Sukumar Varma, K. A convenient n.m.r. method for the measurement of Lewis acidity at boron centres: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity. Polymer 1996, 37, 4629– 4631,  DOI: 10.1016/0032-3861(96)00323-0
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      21
      A convenient NMR method for the measurement of Lewis acidity at boron centers: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity
      Beckett, Michael A.; Strickland, Gary C.; Holland, John R.; Varma, K. Sukumar
      Polymer (1996), 37 (20), 4629-4631CODEN: POLMAG; ISSN:0032-3861. (Elsevier)
      The reaction rate detd. from viscosity measurements of Lewis acid catalyzed epoxide (oxirane) polymns. was correlated with the Gutmann's Acceptor No. (AN) scale derived from measurement of 31P NMR chem. shifts of Et3PO (TPO) dissolved in the boron-contg. Lewis acid. The 31P chem. shifts, acceptor no., and polymn. rate of phenylglycidyl ether are given for (Me2N)3B, (BuO)3B, (EtO)3B, (MeO)3B, (BuO)3B3O3, (MeO)3B3O3, BF3 etherate, BCl3, BBr3, and BI3. The method allows for accurate measurement of Lewis acidity of the boron center, which dets. the cure rate of epoxy resins.
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    22. 22
      (a) Lang, S.; Benz, M.; Obenaus, U.; Himmelmann, R.; Hunger, M. Novel Approach for the Characterization of Lewis Acidic Solid Catalysts by Solid-State NMR Spectroscopy. ChemCatChem. 2016, 8, 2031– 2036,  DOI: 10.1002/cctc.201600372
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      22a
      Novel Approach for the Characterization of Lewis Acidic Solid Catalysts by Solid-State NMR Spectroscopy
      Lang, Swen; Benz, Michael; Obenaus, Utz; Himmelmann, Robin; Hunger, Michael
      ChemCatChem (2016), 8 (12), 2031-2036CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Acetone-2-13C, trimethylphosphine oxide (TMPO), and ammonia were applied as probe mols. for solid-state NMR investigations of Lewis acid sites on γ-Al2O3, TiO2/anatase, and lithium-exchanged zeolite Na-Y. An indication for Lewis acid sites are the 31P MAS NMR signals at 48-51 ppm for TMPO-loaded catalysts. The detn. of the Lewis site d. through the evaluation of these 31P MAS NMR signals, however, is difficult owing to the demanding TMPO adsorption procedure. Upon ammonia adsorption, the formation of ammonium ions at Bronsted acid sites and the coordination of ammonia at Lewis acid sites cause 1H MAS NMR signals at 6-7 ppm and -0.5-3 ppm, resp. The integration of these signals results in the densities of Bronsted and Lewis acid sites. Future studies have to clarify whether the different 1H chem. shifts in the range of -0.5-3 ppm for ammonia coordinated at Lewis acid sites on solid catalysts are a hint at the different strengths of these surface sites.
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      (b) Wiper, P. V.; Amelse, J.; Mafra, L. Multinuclear solid-state NMR characterization of the Brønsted/Lewis acid properties in the BP HAMS-1B (H-[B]-ZSM-5) borosilicate molecular sieve using adsorbed TMPO and TBPO probe molecules. J. Catal. 2014, 316, 240– 250,  DOI: 10.1016/j.jcat.2014.05.017
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      22b
      Multinuclear solid-state NMR characterization of the Bronsted/Lewis acid properties in the BP HAMS-1B (H-[B]-ZSM-5) borosilicate molecular sieve using adsorbed TMPO and TBPO probe molecules
      Wiper, Paul V.; Amelse, Jeffrey; Mafra, Luis
      Journal of Catalysis (2014), 316 (), 240-250CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      The acid properties of a dehydrated borosilicate, HAMS-1B (H-[B]-ZSM-5), including the acid types, strengths, location, and quantities are investigated by means of trialkylphosphine oxides through multinuclear 1D/2D MAS NMR expts. 11B DQF-STMAS combined with 1H MAS NMR studies revealed B-OH and distinct Si-OH protons assocd. with trigonal boron. 31P NMR spectra of TMPO-treated HAMS-1B reveal three Bronsted and three Lewis acid sites. We have found a no. of limitations applying the TMPO/TBPO method to identify internal/external acidity. Therefore, we propose a new approach to unambiguously discriminate external/internal acid sites by treating a pore-free and pore-blocked HAMS-1B zeolite. This method provided unique structural insight regarding the identification of boron species/coordinations assocd. with Bronsted/Lewis acid sites. Addnl., ICP anal. in tandem with solid-state NMR enabled full assignment of the detected internal/external acid species and the study of their acid strength. Moreover, we identify the nature of TMPO complexes arising from Bronsted/Lewis interactions.
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      (c) Hradsky, D.; Machac, P.; Skoda, D.; Leonova, L.; Sazama, P.; Pastvova, J.; Kaucky, D.; Vsiansky, D.; Moravec, Z.; Styskalik, A. Catalytic performance of micro-mesoporous zirconosilicates prepared by non-hydrolytic sol-gel in ethanol-acetaldehyde conversion to butadiene and related reactions. Appl. Catal., A 2023, 652, 119037  DOI: 10.1016/j.apcata.2023.119037
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      22c
      Catalytic performance of micro-mesoporous zirconosilicates prepared by non-hydrolytic sol-gel in ethanol-acetaldehyde conversion to butadiene and related reactions
      Hradsky, Dalibor; Machac, Petr; Skoda, David; Leonova, Lucie; Sazama, Petr; Pastvova, Jana; Kaucky, Dalibor; Vsiansky, Dalibor; Moravec, Zdenek; Styskalik, Ales
      Applied Catalysis, A: General (2023), 652 (), 119037CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
      The open porosity and no. of Lewis acid sites in metal silicates (M = Zr, Ta) have been reported as key factors enabling reaching high butadiene (BD) productivity from ethanol. However, some microporous zeolites recently displayed very high BD yields. To gain a deeper insight, we have applied non-hydrolytic sol-gel (NHSG) in the prepn. of micro-mesoporous zirconosilicates. The porosity, structure, and acidity of these materials have been described and compared to a benchmark sample prepd. by dry impregnation. The detailed characterization proved that NHSG prepn. provided highly homogeneous Zr dispersion in silica leading to almost doubled Lewis acid site nos. and higher activity in ethanol-acetaldehyde conversion to BD, Meerwein-Ponndorf-Verley (MPV) reaction, and aldol condensation, in comparison to the catalyst prepd. by dry impregnation. The selectivity and stability were similar for catalysts prepd. by NHSG and dry impregnation.
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    23. 23
      (a) Britovsek, G. J. P.; Ugolotti, J.; White, A. J. P. From B(C6F5)3 to B(OC6F5)3: Synthesis of (C6F5)2BOC6F5 and C6F5B(OC6F5)2 and Their Relative Lewis Acidity. Organometallics 2005, 24, 1685– 1691,  DOI: 10.1021/om049091p
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      23a
      From B(C6F5)3 to B(OC6F5)3: Synthesis of (C6F5)2BOC6F5 and C6F5B(OC6F5)2 and Their Relative Lewis Acidity
      Britovsek, George J. P.; Ugolotti, Juri; White, Andrew J. P.
      Organometallics (2005), 24 (7), 1685-1691CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      The pentafluorophenyl esters of bis(pentafluorophenyl)borinic acid (C6F5)2BOC6F5 (2) and pentafluorophenylboronic acid C6F5B(OC6F5)2 (3) have been prepd. and characterized by multinuclear NMR and x-ray anal. VT NMR studies have shown that restricted rotation around the B-O bond in 2 occurs below 193 K, corresponding to ΔG⧧ = 35 kJ/mol for this process. This low barrier and the random torsion angles around the B-O bonds obsd. in the solid state structures of compds. 2, 3, and B(OC6F5)3 (4) suggest that these torsion angles are not related to pπ-pπ interactions between boron and oxygen, but more likely a consequence of the extensive intermol. F-π interactions seen in the solid state structures. The Lewis acidity of 2, 3, and 4 has been compared with B(C6F5)3 (1), using various Lewis bases. All compds. 1-4 appear to be strong Lewis acids, whereby 4 interacts more strongly with hard bases whereas 1 binds more strongly to softer bases.
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      (b) Beringhelli, T.; Donghi, D.; Maggioni, D.; D’Alfonso, G. Solution structure, dynamics and speciation of perfluoroaryl boranes through 1H, 11B and 19F NMR spectroscopy. Coord. Chem. Rev. 2008, 252, 2292– 2313,  DOI: 10.1016/j.ccr.2008.01.018
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      23b
      Solution structure, dynamics and speciation of perfluoroaryl boranes through 1H, 11B and 19F NMR spectroscopy
      Beringhelli, Tiziana; Donghi, Daniela; Maggioni, Daniela; D'Alfonso, Giuseppe
      Coordination Chemistry Reviews (2008), 252 (21+22), 2292-2313CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. Multinuclear NMR spectroscopy was used to study the soln. properties of several perfluoroaryl borane derivs. The information obtained from all the NMR active isotopes present in these mols. made it possible to establish not only chem. identity, structure and dynamics of their reaction products, but also the complexity of the soln. speciation. Multinuclear NMR helped in unraveling the real forms in which they are present in soln., that in some cases can change dramatically according to even slight changes of the soln. conditions. Examples will be presented related to the chem. of tris(pentafluorophenyl)borane, B(C6F5)3, and bis(pentafluorophenyl)borinic acid, B(C6F5)2OH.
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      (c) Lewiński, J.; Kubicki, D. NMR Spectroscopy, Heteronuclei, B, Al, Ga, In, Tl. In Encyclopedia of Spectroscopy and Spectrometry, 3rd ed.; Lindon, J. C.; Tranter, G. E.; Koppenaal, D. W., Eds.; Academic Press: Oxford, 2017; pp 318– 329.
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    24. 24
      (a) Zhang, X.; Wang, R.; Yang, X.; Zhang, F. Comparison of four catalysts in the catalytic dehydration of ethanol to ethylene. Microporous Mesoporous Mater. 2008, 116, 210– 215,  DOI: 10.1016/j.micromeso.2008.04.004
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      24a
      Comparison of four catalysts in the catalytic dehydration of ethanol to ethylene
      Zhang, Xian; Wang, Rijie; Yang, Xiaoxia; Zhang, Fengbao
      Microporous and Mesoporous Materials (2008), 116 (1-3), 210-215CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier)
      The aim of this study was to compare the activity and stability of γ-Al2O3, HZSM-5 (Si/Al = 25), silicoaluminophosphate (SAPO-34) and Ni-substituted SAPO-34 (NiAPSO-34) as catalysts in the dehydration of ethanol to ethylene. γ-Al2O3- and HZSM-5 were com. catalysts. SAPO-34 and NiAPSO-34 mol. sieves had been synthesized with hydrothermal method in the lab., characterized with x-ray powder diffraction (x-ray diffraction), IR Spectroscopy (FT-IR), H2 temp.-programmed redn. (H2-TPR) technique and NH3 temp.-programmed desorption (NH3-TPD) technique. The incorporation of Ni2+ into the SAPO-34 framework generated in NiAPSO-34 sample was proved by x-ray diffraction, FT-IR and H2-TPR techniques. NH3-TPD study had revealed that substitution of Ni2+ for Al3+ in the SAPO-34 framework led to increase the weak and moderately strong acid strength and give rise to weak acid sites. Dehydration of ethanol was carried out over four catalysts and the results showed that conversion of ethanol and selectivity to ethylene decreased in the order HZSM-5 > NiAPSO-34 > SAPO-34 > γ-Al2O3. As to the stability of catalyst, NiAPSO-34 and SAPO-34 were better than other two catalysts. Considering the activity and stability of the four catalysts comprehensively, NiAPSO-34 was the suitable catalyst in the dehydration of ethanol.
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      (b) Xin, H.; Li, X.; Fang, Y.; Yi, X.; Hu, W.; Chu, Y.; Zhang, F.; Zheng, A.; Zhang, H.; Li, X. Catalytic dehydration of ethanol over post-treated ZSM-5 zeolites. J. Catal. 2014, 312, 204– 215,  DOI: 10.1016/j.jcat.2014.02.003
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      24b
      Catalytic dehydration of ethanol over post-treated ZSM-5 zeolites
      Xin, Hongchuan; Li, Xiangping; Fang, Yuan; Yi, Xianfeng; Hu, Wenhui; Chu, Yueying; Zhang, Feng; Zheng, Anmin; Zhang, Hongpeng; Li, Xuebing
      Journal of Catalysis (2014), 312 (), 204-215CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Microporous ZSM-5 zeolite was post-treated by desilication with sodium hydroxide, dealumination with oxalic acid, or both of them in a sequential way to finely tune the zeolite catalysts with hierarchically porous structure and varying acidity. In the catalytic dehydration of ethanol, di-Et ether and ethylene were two main products competitively formed at 200 °C and atm. pressure. The post-treated ZSM-5 catalysts could display stable ethanol conversion and ethylene selectivity within time-onstream of around 12 h. The correlation between the steady-state ethylene selectivity and the amt. of weak acid sites from ammonia temp.-programmed desorption (NH3-TPD) indicated that the weak acid sites facilitated the ethylene prodn. during ethanol transformation under present reaction conditions. The reaction pathways for di-Et ether and ethylene formations from ethanol were investigated by theor. calcn. Both the activation energies and natural charges of the transition states strongly supported that the selectivity for the di-Et ether tended to deteriorate with decreasing catalytic Bronsted acidity.
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      (c) Bi, J.; Guo, X.; Liu, M.; Wang, X. High effective dehydration of bio-ethanol into ethylene over nanoscale HZSM-5 zeolite catalysts. Catal. Today 2010, 149, 143– 147,  DOI: 10.1016/j.cattod.2009.04.016
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      24c
      Highly effective dehydration of bio-ethanol into ethylene over nanoscale HZSM-5 zeolite catalysts
      Bi, Jiandong; Guo, Xinwen; Liu, Min; Wang, Xiangsheng
      Catalysis Today (2010), 149 (1-2), 143-147CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      Nanoscale and microscale HZSM-5 zeolite catalysts were prepd. and characterized by using SEM, XRD, IR, TPD and modified Hammett indicator method. Their performances in the dehydration of bio-ethanol into ethylene were compared in a fixed-bed reactor at 240 °C under atm. pressure. The results show that nanoscale HZSM-5 zeolite catalyst exhibits better stability than microscale HZSM-5 zeolite catalyst. When the 95(v) % bio-ethanol is used as the reactant, over nanoscale HZSM-5 catalyst, the conversion of bio-ethanol and the selectivity for ethylene almost keep const. during 630 h reaction, while over microscale HZSM-5 zeolite catalyst, the conversion of bio-ethanol decreases after 60 h reaction; in the case of the 45(v) % bio-ethanol employed as the feedstock, over nanoscale HZSM-5 catalyst, the conversion of bio-ethanol and the selectivity for ethylene almost keep const. during 320 h reaction, while over microscale HZSM-5 zeolite catalyst, both the conversion of bio-ethanol and the selectivity for ethylene decrease almost at the beginning of the reaction.
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    25. 25
      (a) Li, Y.; Yang, Q.; Yang, J.; Li, C. Mesoporous aluminosilicates synthesized with single molecular precursor (sec-BuO)2AlOSi(OEt)3 as aluminum source. Microporous Mesoporous Mater. 2006, 91, 85– 91,  DOI: 10.1016/j.micromeso.2005.11.021
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      25a
      Mesoporous aluminosilicates synthesized with single molecular precursor (sec-BuO)2AlOSi(OEt)3 as aluminum source
      Li, Ying; Yang, Qihua; Yang, Jie; Li, Can
      Microporous and Mesoporous Materials (2006), 91 (1-3), 85-91CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
      Single mol. precursor bis(sec-butoxy)aluminoxytriethoxysilane [(sec-BuO)2AlOSi(OEt)3] was used as Al source for the synthesis of mesoporous aluminosilicates with low Si/Al ratios from 1.0 to 10.0 via a template-assisted method. The samples with Si/Al ratios of 1.0 and 3.0 have wormhole-like pore structure, while the samples with Si/Al ratios of 5.0 and 10.0 have ordered hexagonal mesoporous structures with extensive void defects amid the nanochannels. IR spectra of pyridine adsorption and NH3-TPD characterizations suggest that the mesoporous aluminosilicates have both Bronsted and Lewis acid sites with medium strength. The amts. of acid sites of the mesoporous aluminosilicates do not run parallel with the concn. of the tetrahedrally coordinated Al in the mesoporous framework.
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      (b) Styskalik, A.; Kordoghli, I.; Poleunis, C.; Delcorte, A.; Moravec, Z.; Simonikova, L.; Kanicky, V.; Aprile, C.; Fusaro, L.; Debecker, D. P. Hybrid mesoporous aluminosilicate catalysts obtained by non-hydrolytic sol–gel for ethanol dehydration. J. Mater. Chem. A 2020, 8, 23526– 23542,  DOI: 10.1039/D0TA07016E
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      25b
      Hybrid mesoporous aluminosilicate catalysts obtained by non-hydrolytic sol-gel for ethanol dehydration
      Styskalik, Ales; Kordoghli, Imene; Poleunis, Claude; Delcorte, Arnaud; Moravec, Zdenek; Simonikova, Lucie; Kanicky, Viktor; Aprile, Carmela; Fusaro, Luca; Debecker, Damien P.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2020), 8 (44), 23526-23542CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Ethanol dehydration is effectively catalyzed by solid acids, such as HZSM-5, alumina, or silica-alumina. In these catalysts, the amt., nature, and strength of acid sites is believed to det. catalyst activity and stability. However, surface hydrophilicity or hydrophobicity can be suggested as another decisive materials property that can directly affect performance. For example, a more hydrophobic surface might be beneficial in repelling the co-product of the reaction, water. However, these aspects have been studied only scarcely in the context of alc. dehydration. Here, a series of mesoporous hybrid aluminosilicate materials contg. CH3Si groups was prepd. in one pot by non-hydrolytic sol-gel (NHSG). The presence of the Me groups was verified by IR, solid-state NMR, and ToF-SIMS. Aluminum is mostly incorporated in tetrahedral coordination in the hybrid silica matrix. Two parameters were varied: the aluminum loading (Si:Al ratio) and the degree of methylation (Si:MeSi ratio). On the one hand, changing the Si:Al ratio had a marked and expected impact on acidity. On the other hand, unexpectedly, the introduction of Me groups had no clear influence on sample hydrophobicity. Nevertheless, some of the methylated aluminosilicate catalysts markedly outperformed the purely inorg. catalysts and a com. silica-alumina benchmark. While a direct influence of surface hydrophilicity or hydrophobicity is unlikely, characterization of acidity (IR-pyridine) revealed that the improved performance for hybrid catalysts is correlated with a modification of the acidic properties (higher proportion of Lewis acid sites) caused by the introduction of Me groups during the sol-gel process. A decisive role of acidity in ethanol dehydration was confirmed by an expt. with delayed addn. of the Al precursor in the NHSG synthesis of the hybrid aluminosilicate. This led to a higher Al surface concn., marked acid sites no. increase, and better catalytic performance, even competing with the state of the art HZSM-5 in terms of ethylene yield.
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    26. 26
      Bould, J.; Clegg, W.; Teat, S. J.; Barton, L.; Rath, N. P.; Thornton-Pett, M.; Kennedy, J. D. An approach to megalo-boranes. Mixed and multiple cluster fusions involving iridaborane and platinaborane cluster compounds. Crystal structure determinations by conventional and synchrotron methods. Inorg. Chim. Acta 1999, 289, 95– 124,  DOI: 10.1016/S0020-1693(99)00071-7
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      26
      An approach to megalo-boranes. Mixed and multiple cluster fusions involving iridaborane and platinaborane cluster compounds. Crystal structure determinations by conventional and synchrotron methods
      Bould, Jonathan; Clegg, William; Teat, Simon J.; Barton, Lawrence; Rath, Nigam P.; Thornton-Pett, Mark; Kennedy, John D.
      Inorganica Chimica Acta (1999), 289 (1-2), 95-124CODEN: ICHAA3; ISSN:0020-1693. (Elsevier Science S.A.)
      Several new macropolyhedral metallaboranes were isolated from thermolytic mixed cluster fusion reactions involving metallaboranes and molten B10H14 as solvent. Co-thermolysis of B10H14 with nine-vertex [(CO)(PMe3)2HIrB8H12] (1) engenders 18-vertex [(CO)(PMe3)2IrB17H20] (3), via double cluster fusion; this has the 18-vertex configuration of syn-B18H22, but with a metal atom in the 10-position. From the same reaction, triple cluster fusion engenders 28-vertex [(PMe3)2IrB26H24Ir(CO)(PMe3)2] (4), which structurally is based on an intimate interfusion of closed 10-vertex and 12-vertex subclusters, to generate a tetrahedral tetraboron core that also has a more open commo 1-B linkage to a nido nine-vertex {IrB8} subcluster. Compd. 4 exhibits interesting consequences of cluster-crevice formation and introduces the concept of globular megalo-borane structures that have borons-only cores surrounded by B-hydride sheaths. Examn. for incipient megalo-borane globular behavior in another system, viz. [IrCl(PPh3)3] (7) with anti-B18H22, reveals a four-atom core feature in 19-vertex [(PPh3)HIrB18H18(PPh3)] (6), which has a closo-type {IrB10} 11-vertex subcluster fused to a nido 10-vertex {B10} subcluster to generate a four-atom {IrB3} tetrahedron. Examn. for mixed cluster fusion in other systems reveals the generation of [(PMe2Ph)2Pt-anti-B18H20] (8), from the co-thermolysis of [(PMe2Ph)2PtB8H12] (2) and B10H14, and examn. for multiple cluster fusion reveals the formation of 30-vertex [(PMe2Ph)2(PMe2C6H4)2Pt2B28H32] (10), 29-vertex [(PMe2Ph)2PtB28H32] (11) and 27-vertex [(PMe2Ph)2PtB26H26(PMe2Ph)] (12) from the same reaction. Structurally, compd. 10 is based on a 10-vertex arachno-{6,9-Pt2B8} unit linked, via one B-B two-electron two-center bond each, to two 10-vertex nido-{B10} units; it also exhibits mol. condensation as two P-phenylene ortho-cycloboronations. Compd. 11 is based on the 19-vertex [(PMe2Ph)2Pt-η4-anti-B18H22] configuration with an addnl. 10-vertex nido-{B10H13} moiety bound to the nonplatinated subcluster via one B-B two-electron two-center bond. Compd. 12 is based on two nido 11-vertex {PtB10} units joined by a single commo Pt vertex, with one of these units conjoined to an arachno eight-B unit via a two-B common edge and an open bridging {B-H(exo)-Pt-μ-B2} link. Thermolysis of [(PMe2Ph)2PtB8H12] (2) with the pre-formed double-cluster compd. anti-B18H22 generates triple-contiguity 27-vertex [(PMe2Ph)PtB26H26(PMe2Ph)] (13) which, structurally, consists of a nido 11-vertex {PtB10} unit that is fused to a 2nd 11-vertex nido {PtB10} unit with a triangular {PtB2} face in common, and also fused to a 10-vertex nido {B10} unit with a {B2} edge in common. The sequence 12 → 11 → 10 → 13 → 4 represents a progression of increasing intimacy of cluster fusion. Small crystals of compds. 3, 11 and 12 necessitated synchrotron x-radiation for sufficient diffraction intensity.
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      (a) Parks, D. J.; Blackwell, J. M.; Piers, W. E. Studies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilation of Carbonyl Functions. J. Org. Chem. 2000, 65, 3090– 3098,  DOI: 10.1021/jo991828a
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      27a
      Studies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilation of Carbonyl Functions
      Parks, Daniel J.; Blackwell, James M.; Piers, Warren E.
      Journal of Organic Chemistry (2000), 65 (10), 3090-3098CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      The strong organoborane Lewis acid B(C6F5)3 catalyzes the hydrosilation (using R3SiH) of arom. and aliph. carbonyl functions at convenient rates with loadings of 1-4%. For aldehydes and ketones, the product silyl ethers were isolated in 75-96% yield; for esters, the aldehydes produced upon workup of the silyl acetal products can be obtained in 45-70% yield. Extensive mechanistic studies point to an unusual silane activation mechanism rather than one involving borane activation of the carbonyl function. Quant. kinetic studies show that the least basic substrates are hydrosilated at the fastest rates; furthermore, increased concns. of substrate have an inhibitory effect on the obsd. reaction rate. Paradoxically, the most basic substrates are reduced selectively, albeit at a slower rate, in competition expts. The borane thus must dissoc. from the carbonyl to activate the silane via hydride abstraction; the incipient silylium species then coordinates the most basic function, which is selectively reduced by [HB(C6F5)3]-. In addn. to the kinetic data, this mechanistic proposal is supported by a kinetic isotope effect of 1.4(5) for the hydrosilation of acetophenone, the observation that B(C6F5)3 catalyzes H/D and H/H scrambling in silanes in the absence of substrate, computational studies, the synthesis of models for proposed intermediates, and other isotope labeling and crossover expts.
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      (b) Piers, W. E.; Marwitz, A. J. V.; Mercier, L. G. Mechanistic Aspects of Bond Activation with Perfluoroarylboranes. Inorg. Chem. 2011, 50, 12252– 12262,  DOI: 10.1021/ic2006474
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      27b
      Mechanistic Aspects of Bond Activation with Perfluoroarylboranes
      Piers, Warren E.; Marwitz, Adam J. V.; Mercier, Lauren G.
      Inorganic Chemistry (2011), 50 (24), 12252-12262CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A review. In the mid-1990s, it was discovered that tris(pentafluorophenyl)borane, B(C6F5)3, was an effective catalyst for hydrosilylation of a variety of carbonyl and imine functions. Mechanistic studies revealed a counterintuitive path in which the function of the borane was to activate the silane rather than the org. substrate. This was the first example of what has come to be known as "frustrated Lewis pair" chem. utilizing this remarkable class of electrophilic boranes. Subsequent discoveries by the groups of Stephan and Erker showed that this could be extended to the activation of dihydrogen, initiating an intense period of activity in this area in the past 5 years. This article describes the early hydrosilylation chem. and its subsequent applications to a variety of transformations of importance to org. and inorg. chemists, drawing parallels with the more recent hydrogen activation chem. Here, authors emphasize the current understanding of the mechanism of this process rather than focusing on the many and emerging applications of hydrogen activation by fluoroarylborane-based frustrated Lewis pair systems.
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      (c) Rendler, S.; Oestreich, M. Conclusive Evidence for an SN2-Si Mechanism in the B(C6F5)3-Catalyzed Hydrosilylation of Carbonyl Compounds: Implications for the Related Hydrogenation. Angew. Chem., Int. Ed. 2008, 47, 5997– 6000,  DOI: 10.1002/anie.200801675
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      Conclusive evidence for an SN2-Si mechanism in the B(C6F5)3-catalyzed hydrosilylation of carbonyl compounds: implications for the related hydrogenation
      Rendler, Sebastien; Oestreich, Martin
      Angewandte Chemie, International Edition (2008), 47 (32), 5997-6000, S5997/1-S5997/31CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Effective Walden-type anal. showcases the usefulness of silanes with a stereogenic Si center as stereochem. probes. The B(C6F5)3-catalyzed hydrosilylation of acetophenone with chiral (SiR)-tetrahydro(isopropyl)silanaphthalene and likely the related hydrogenation proceed through linear B-H-Si-O transition states, as verified by flawless inversion of the abs. configuration at Si to give chiral silyl ether (SiR,R)-I which subsequently undergoes DIBAL-redn. to a chiral (R)-1-phenylethanol.
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      (d) Oestreich, M.; Hermeke, J.; Mohr, J. A unified survey of Si–H and H–H bond activation catalysed by electron-deficient boranes. Chem. Soc. Rev. 2015, 44, 2202– 2220,  DOI: 10.1039/C4CS00451E
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      A unified survey of Si-H and H-H bond activation catalysed by electron-deficient boranes
      Oestreich, Martin; Hermeke, Julia; Mohr, Jens
      Chemical Society Reviews (2015), 44 (8), 2202-2220CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      The bond activation chem. of B(C6F5)3 and related electron-deficient boranes is currently experiencing a renaissance due to the fascinating development of frustrated Lewis pairs (FLPs). B(C6F5)3's ability to catalytically activate Si-H bonds through η1 coordination opened the door to several unique redn. processes. The ground-breaking finding that the same family of fully or partially fluorinated boron Lewis acids allows for the related H-H bond activation, either alone or as a component of an FLP, brought considerable momentum into the area of transition-metal-free hydrogenation and, likewise, hydrosilylation. This review comprehensively summarises synthetic methods involving borane-catalyzed Si-H and H-H bond activation. Systems corresponding to an FLP-type situation are not covered. Aside from the broad manifold of C=X bond redns. and C=X/C-X defunctionalisations, dehydrogenative (oxidative) Si-H couplings are also included.
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    28. 28
      (a) Parks, D. J.; Piers, W. E. Tris(pentafluorophenyl)boron-Catalyzed Hydrosilation of Aromatic Aldehydes, Ketones, and Esters. J. Am. Chem. Soc. 1996, 118, 9440– 9441,  DOI: 10.1021/ja961536g
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      Tris(pentafluorophenyl)boron-Catalyzed Hydrosilation of Aromatic Aldehydes, Ketones, and Esters
      Parks, Daniel J.; Piers, Warren E.
      Journal of the American Chemical Society (1996), 118 (39), 9440-9441CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Tris(pentafluorophenyl)borane (1-4 mol %) catalyzes the addn. of Ph3SiH to carbonyl functions of arom. substrates p-XC6H4COR (X = Me, H, Cl, NO2; R = H, Me, OEt). Turnover frequencies for X = H substrates are 19, 45 and 637 h for R = H, Me and OEt, resp. and rates of hydrosilation increase as X becomes more electron withdrawing. Mechanistic studies, including substrate/borane binding equil., structural studies on substrate/borane adducts and kinetic measurements support a hydrosilation mechanism characterized by borane activation of the silane reagent rather than the carbonyl substrate. Key observations in support of this unusual nucleophilic/electrophilic hydrosilation mechanism were the obsd. inhibition of the reaction by increased concns. of substrate and the obsd. order of reactivity which indicates that the weaker binding substrates are reduced faster.
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      Defunctionalisation catalysed by boron Lewis acids
      Fang, Huaquan; Oestreich, Martin
      Chemical Science (2020), 11 (47), 12604-12615CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      A review. This review summarized various defunctionalization reactions such as deoxygenation, decarbonylation, desulfurization, deamination and dehalogenation, all of which catalyzed by boron Lewis acids.
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      (a) Prabhudesai, V. S.; Gurrala, L.; Vinu, R. Catalytic Hydrodeoxygenation of Lignin-Derived Oxygenates: Catalysis, Mechanism, and Effect of Process Conditions. Energy Fuels 2022, 36, 1155– 1188,  DOI: 10.1021/acs.energyfuels.1c02640
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      Catalytic Hydrodeoxygenation of Lignin-Derived Oxygenates: Catalysis, Mechanism, and Effect of Process Conditions
      Prabhudesai, Vallabh S.; Gurrala, Lakshmiprasad; Vinu, Ravikrishnan
      Energy & Fuels (2022), 36 (3), 1155-1188CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
      A review. The high oxygen content of pyrolysis bio-oil with many org. functional groups in it limits its direct application as a blendstock. The upgradation of biomass-derived oxygenates into renewable fuels and value-added chems. via catalytic hydrodeoxygenation (HDO) has received considerable attention in recent years. This review focuses on HDO of key model compd. oxygenates, which sets the ground to propose the overall reaction mechanism of HDO of bio-oils. Catalysts play a vital role in HDO, and its design poses many challenges because of different reactions involved such as hydrogenolysis, hydrogenation, decarbonylation, and dehydration occurring simultaneously at different catalyst-active sites. The main objective here is to present a comprehensive introduction to the reaction mechanism involved in the HDO of bio-oil model oxygenates. For this, a thorough discussion of different reaction pathways taking place during the HDO of five model oxygenates, viz., anisole, guaiacol, eugenol, vanillin, and dibenzofuran, is presented. The model compds. are selected to provide a good description of the HDO of lignin-derived compds. present in bio-oils. Particular emphasis is placed on the effect of the catalyst, temp., hydrogen partial pressure, and solvent employed on the product distribution. This review will aid not just in understanding the interrelations between the nature of the catalyst, HDO mechanism, and product distribution but will also provide thoughtful directions for the applications of HDO in real bio-oil upgradation.
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      (b) Kumar, A.; Jindal, M.; Maharana, S.; Thallada, B. Lignin Biorefinery: New Horizons in Catalytic Hydrodeoxygenation for the Production of Chemicals. Energy Fuels 2021, 35, 16965– 16994,  DOI: 10.1021/acs.energyfuels.1c01651
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      29b
      Lignin Biorefinery: New Horizons in Catalytic Hydrodeoxygenation for the Production of Chemicals
      Kumar, Adarsh; Jindal, Meenu; Maharana, Sonita; Thallada, Bhaskar
      Energy & Fuels (2021), 35 (21), 16965-16994CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
      A review. Lignin is a byproduct of the paper mill and second-generation (2G) ethanol biorefinery and the largest naturally available source of arom. building blocks. It can serve as a feedstock to produce bulk, fine, and functionalized arom. compds. along with aliph. cyclic alcs., jet fuel, and aliph. hydrocarbons. Hydrodeoxygenation (HDO) is one of the promising strategies to produce these compds. from lignin-derived phenolics. Many reactions occur during the HDO; hence, it is very crucial to design active, selective, and long-life catalysts and choose the optimal reaction conditions to synthesize targeted end products. Herein, we first survey the conventional and new lignin fractionation techniques and det. the lignin structure, compn., and bonding with cellulose and hemicellulose. After this, we review the recent advances in the prodn. of alkanes, arenes, cyclic alcs., jet fuel range hydrocarbons, and HDO of raw lignin bio-oil. This review exclusively discusses the product distribution during HDO based on catalyst compn., solvent, and reaction parameters. Moreover, the review also underlines the selectivity of various catalytic systems to yield specific products such as BTX, important petrochem. materials, aliph. hydrocarbons, cyclic alcs., and jet fuel range hydrocarbons.
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      (c) Perego, C.; Bosetti, A.; Ricci, M.; Millini, R. Zeolite Materials for Biomass Conversion to Biofuel. Energy Fuels 2017, 31, 7721– 7733,  DOI: 10.1021/acs.energyfuels.7b01057
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      29c
      Zeolite Materials for Biomass Conversion to Biofuel
      Perego, Carlo; Bosetti, Aldo; Ricci, Marco; Millini, Roberto
      Energy & Fuels (2017), 31 (8), 7721-7733CODEN: ENFUEM; ISSN:0887-0624. (American Chemical Society)
      A review. The use of zeolite catalysts for the prodn. of biofuels from biomass is reviewed. Zeolites as such or modified by the addn. of other active phases are used in several processes for the transformation of the biomass and for the upgrading of the bio-oils deriving from its primary treatment. For each of the different processes, the most relevant results reported in the literature are provided together with some considerations on the effective or potential industrial applicability of the technologies.
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      (d) Shi, Y.; Xing, E.; Wu, K.; Wang, J.; Yang, M.; Wu, Y. Recent progress on upgrading of bio-oil to hydrocarbons over metal/zeolite bifunctional catalysts. Catal. Sci. Technol. 2017, 7, 2385– 2415,  DOI: 10.1039/C7CY00574A
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      29d
      Recent progress on upgrading of bio-oil to hydrocarbons over metal/zeolite bifunctional catalysts
      Shi, Yanchun; Xing, Enhui; Wu, Kejing; Wang, Jianlong; Yang, Mingde; Wu, Yulong
      Catalysis Science & Technology (2017), 7 (12), 2385-2415CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      Upgrading of bio-oil is of high necessity and popularity in converting biomass to high-quality hydrocarbons (transportation fuels and petrochems.) to reduce the overall CO2 emissions of fossil based materials. There are hundreds of different oxygenated compds. identified in bio-oil, resulting in a high oxygen content (30% to 50%). This review focuses on recent progress in the upgrading of bio-oil over metal/zeolite bifunctional catalysts, with model compds. and real bio-oil included. Firstly, typical model compds. and corresponding reaction routes are summarized, based upon the compn. of the bio-oil and a basic knowledge of chem. reactions. Secondly, careful analyses are conducted on the deoxygenation mechanisms over different metal active centers and acid-catalyzed reactions, such as isomerization and cracking, over zeolitic acid sites, resp. Moreover, detailed analyses have focused on the effect of metal loadings on zeolites, the effects of zeolitic porosity and acidity on the metal, and their overall effects on reaction activity, selectivity and stability. Thirdly, the fundamental understanding of the interaction between the metal centers and zeolite acid sites in bifunctional catalysts and their influences on complex reaction networks, including deoxygenation and acid-catalyzed reactions, is analyzed. The metal/acid balance may be the key in improving the catalytic activity and product selectivity in the upgrading of bio-oil, which needs further careful design. Finally, the potential challenges and opportunities for the upgrading of bio-oil over metal/zeolite bifunctional catalysts are outlined.
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      (e) Tang, X.; Ding, W.; Li, H. Improved hydrodeoxygenation of bio-oil model compounds with polymethylhydrosiloxane by Brønsted acidic zeolites. Fuel 2021, 290, 119883  DOI: 10.1016/j.fuel.2020.119883
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      29e
      Improved hydrodeoxygenation of bio-oil model compounds with polymethylhydrosiloxane by Bronsted acidic zeolites
      Tang, Xingfei; Ding, Wentao; Li, Hao
      Fuel (2021), 290 (), 119883CODEN: FUELAC; ISSN:0016-2361. (Elsevier Ltd.)
      Bio-oil, one of significant renewable energy, was blocked from its direct application by thermodn. instability due to the high oxygen content, so its deoxygenation needs urgent soln. Avoiding the disadvantages of the traditional catalytic method, such as harsh reaction conditions, potential explosion risk and environmental pollution, we utilize solid-acidic zeolites as low-load Pd-based catalyst supports instead of corrosive acid additives, with polymethylhydrosiloxane (PMHS) as the hydrogen-supplying agent, to achieve efficient hydrodeoxygenation of bio-oil models (carbonyl compds.) under mild conditions. The reaction conditions such as Si/Al ratio of HZSM-5 zeolite, temp., solvent, and the type of Pd salts precursor are optimized. In particular, we have found that polar protic solvents improve catalytic efficiency by promoting proton transfer in the reaction. In an open-to-air, 97.9% ethylbenzene yield can be obtained for acetophenone conversion under mild conditions (0.5 wt% Pd/HZSM-5(18), 65°C, 3 h, n-butanol as solvent), which is more efficient and environmental friendly than currently reports. Meanwhile, hydrogenation-dehydration mechanism was proposed, and the Bronsted acid in HZSM-5 promotes the dehydration of the alc. (rate-limiting step) by efficiently accelerating the removal of hydroxyl groups and the proton transfer of the reaction. Furthermore, the catalytic scheme exhibits the excellent stability (reusable seven times) and versatility. The potential of a green catalytic technol. using with PMHS opens attractive opportunities for bio-oil upgrading.
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      (f) Witsuthammakul, A.; Sooknoi, T. Selective hydrodeoxygenation of bio-oil derived products: ketones to olefins. Catal. Sci. Technol. 2015, 5, 3639– 3648,  DOI: 10.1039/C5CY00367A
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      29f
      Selective hydrodeoxygenation of bio-oil derived products: ketones to olefins
      Witsuthammakul, Ayut; Sooknoi, Tawan
      Catalysis Science & Technology (2015), 5 (7), 3639-3648CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      The hydrodeoxygenation (HDO) of various ketones (acetone, Me Et ketone and cyclohexanone) to olefins via hydrogenation-dehydration was conducted in a fixed bed reactor at 373-573 K under H2. A ketone can be hydrogenated over the metal function to an alc. intermediate that is subsequently dehydrated to an olefin over the acidic function. A preliminary study on hydrogenation of acetone to 2-propanol over metal/SiO2 catalysts (Cr, Fe, Co, Ni, Cu and Pd) shows that Ni and Cu are active at >373 K. Although Ni possesses an activity higher than that of Cu, it promotes olefin hydrogenation and alc. hydrogenolysis at >473 K. Hydrogenolysis of alc. intermediate is suppressed over the Ni-Cu alloy catalyst. An optimum conversion with 100% selectivity to alc., can be obtained at 448 and 473 K for Ni and Cu, resp. The dehydration of 2-propanol to propylene over proton zeolites (ZSM-5, Y, Mordenite and β) can be achieved at >398 K. The zeolites with three-dimensional pore structure (β and Y) provide relatively higher activity (>90% conversion). However, a bimol. dehydration to ether is also promoted. Only HZSM-5 shows excellent selectivity to propylene (98%). Hydrodeoxygenation of ketones was tested with (i) a double bed of 5%Ni/SiO2 and HZSM-5 (Si/Al ∼ 13), (ii) a phys. mixed bed of 5%Cu/SiO2 and HZSM-5 (Si/Al ∼ 13) and (iii) a bi-functional catalyst of 5%Cu/HZSM-5 (Si/Al ∼ 250). It was found that high alkene selectivity was readily obtained at 448 K. While, over the phys. mixed bed and bi-functional catalyst, the hydrogenation activity was enhanced as the alc. intermediate was removed from the system. The reactivity of the ketone depends on its adsorption on the metal surface and steric hindrance, i.e. acetone > cyclohexanone > Me Et ketone.
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    30. 30
      (a) Christensen, D. B.; Mortensen, R. L.; Kramer, S.; Kegnæs, S. Study of CoCu Alloy Nanoparticles Supported on MOF-Derived Carbon for Hydrosilylation of Ketones. Catal. Lett. 2020, 150, 1537– 1545,  DOI: 10.1007/s10562-019-03065-2
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      30a
      Study of CoCu Alloy Nanoparticles Supported on MOF-Derived Carbon for Hydrosilylation of Ketones
      Christensen, David B.; Mortensen, Rasmus L.; Kramer, Soeren; Kegnaes, Soeren
      Catalysis Letters (2020), 150 (6), 1537-1545CODEN: CALEER; ISSN:1011-372X. (Springer)
      Carbonized zeolitic imidazolate frameworks (ZIFs) show potential as mesoporous heterogeneous catalysts with high metalloadings. ZIF-67 and ZIF-8 were used to create mono- and bimetallic CoCu particles supported on nitrogen-doped carbonvia self-assembly in methanol at room temp., followed by carbonization at 675°C. A Cu precursor, Cu(NO3)2·2H2O, was impregnated into the ZIF-67 before carbonization to obtain bimetallic catalysts. Nanoalloy particles with different CoCu ratio were synthesized and characterized using XRD. The materials were further characterized using TEM, SEM, XRF andnitrogen physisorption. The different alloys were tested in conversion of cyclohexanone to the corresponding silyl ether.Complete conversion of cyclohexanone at 90°C for 24 h were obtained. The catalyst Co99Cu1@NC gave a 60% increase in yield over a pure Co analog.
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      (b) Le Roux, E.; De Mallmann, A.; Merle, N.; Taoufik, M.; Anwander, R. Immobilization of Heteroleptic Bis(oxazoline) Zinc Catalysts on SBA-15 for Asymmetric Hydrosilylation. Organometallics 2015, 34, 5146– 5154,  DOI: 10.1021/acs.organomet.5b00714
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      30b
      Immobilization of Heteroleptic Bis(oxazoline) Zinc Catalysts on SBA-15 for Asymmetric Hydrosilylation
      Le Roux, Erwan; De Mallmann, Aimery; Merle, Nicolas; Taoufik, Mostafa; Anwander, Reiner
      Organometallics (2015), 34 (20), 5146-5154CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      The C2-sym. bis(oxazoline)s (4S,4S)-R1-BOX, with R1 = tBu and Ph, react with ZnEt2 straightforwardly to the four-coordinate complexes ((4S,4S)-R1-BOX)ZnEt2 in high yield. Their immobilization on highly dehydroxylated (700°) mesoporous silica SBA-15 with enlarged pore diams. (dp = 15.3 nm) led to the well-defined surface species [(≡SiO)Zn(Et)((4S,4S)-R1-BOX)]. These Et zinc-terminated materials react readily with one equiv. of anhyd. methanol and acetic acid to give predominantly and selectively the corresponding surface ligand-exchanged species [(≡SiO)Zn(R)((4S,4S)-R1-BOX)] (R = OMe, OAc). All hybrid materials were characterized via N2 physisorption, elemental anal., DRIFT, 1H/13C MAS NMR, and EXAFS spectroscopies. The SBA-15 supported heteroleptic bis(oxazoline) zinc Et, methoxide, and acetate surface species were scrutinized as catalysts for the asym. hydrosilylation of acetophenone.
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      (c) Lázaro, G.; Fernández-Alvarez, F. J.; Iglesias, M.; Horna, C.; Vispe, E.; Sancho, R.; Lahoz, F. J.; Iglesias, M.; Pérez-Torrente, J. J.; Oro, L. A. Heterogeneous catalysts based on supported Rh–NHC complexes: synthesis of high molecular weight poly(silyl ether)s by catalytic hydrosilylation. Catal. Sci. Technol. 2014, 4, 62– 70,  DOI: 10.1039/C3CY00598D
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      30c
      Heterogeneous catalysts based on supported Rh-NHC complexes: synthesis of high molecular weight poly(silyl ether)s by catalytic hydrosilylation
      Lazaro, Guillermo; Fernandez-Alvarez, Francisco J.; Iglesias, Manuel; Horna, Cristina; Vispe, Eugenio; Sancho, Rodrigo; Lahoz, Fernando J.; Iglesias, Marta; Perez-Torrente, Jesus J.; Oro, Luis A.
      Catalysis Science & Technology (2014), 4 (1), 62-70CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      The new rhodium(i) complexes [Rh(Cl)(COD)(R-NHC-(CH2)3Si(OiPr3)3)] (R = 2,6-diisopropylphenyl (2a); Bu (2b)) were synthesized and fully characterized. The study of their application as ketone hydrosilylation catalysts showed a clear N-substituent effect, 2a being the most active catalyst precursor. Complex 2a was immobilized in the mesoporous materials MCM-41 and KIT-6. The new hybrid materials were fully characterized and used as catalyst precursors for the prepn. of poly(silyl ether)s by catalytic hydrosilylation. The heterogeneous catalytic systems based on the materials 2a-MCM-41 and 2a-KIT-6 afford polymers with high av. mol. wt. (Mw) Mw = 2.61 × 106 g mol-1 (2a-MCM-41) and Mw = 4.43 × 105 g mol-1 (2a-KIT-6).
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    31. 31
      Keess, S.; Simonneau, A.; Oestreich, M. Direct and Transfer Hydrosilylation Reactions Catalyzed by Fully or Partially Fluorinated Triarylboranes: A Systematic Study. Organometallics 2015, 34, 790– 799,  DOI: 10.1021/om501284a
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      Direct and Transfer Hydrosilylation Reactions Catalyzed by Fully or Partially Fluorinated Triarylboranes: A Systematic Study
      Keess, Sebastian; Simonneau, Antoine; Oestreich, Martin
      Organometallics (2015), 34 (4), 790-799CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      The present survey serves several purposes. Selected electron-deficient boron Lewis acids catalyze the release of hydrosilanes from cyclohexa-2,5-dien-1-yl-substituted silanes. The two-step process consists of a hydride abstraction to generate a silicon-stabilized Wheland complex and capture of the arene-stabilized silicon cation by the borohydride formed in the previous step. The same boron catalyst will then activate the Si-H bond for the reaction with representative π- and σ-donating substrates, alkenes/alkynes and ketones/ketimines, resp. The net transformation is a transfer hydrosilylation, and the effect that the substitution pattern of the cyclohexa-1,4-diene core and the substituents at the silicon atom exert on these hydrosilane surrogates is systematically investigated. The results are compared with those obtained employing the hydrosilane directly. Another part of this comprehensive anal. is dedicated to the comparison of literature-known fully or partially fluorinated triarylboranes in both the direct and the transfer hydrosilylation of the aforementioned substrates. The data are tabulated and color-coded, finally providing an overview of promising substrate/reductant/borane combinations. The often different reactivities of π- and σ-basic substrates are explained, and it is shown that the Lewis acidity of the boron atom, estd. by the Gutmann-Beckett method, is not the only decisive feature of these boron Lewis acids. Practical mechanistic models are presented to rationalize the interplay between the Lewis acidity and steric situation at the boron and, likewise, the silicon atom as well as the need for fluorination ortho to the boron atom in certain cases.
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      Mahdi, T.; Stephan, D. W. Facile Protocol for Catalytic Frustrated Lewis Pair Hydrogenation and Reductive Deoxygenation of Ketones and Aldehydes. Angew. Chem., Int. Ed. 2015, 54, 8511– 8514,  DOI: 10.1002/anie.201503087
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      Facile protocol for catalytic frustrated lewis pair hydrogenation and reductive deoxygenation of ketones and aldehydes
      Mahdi, Tayseer; Stephan, Douglas W.
      Angewandte Chemie, International Edition (2015), 54 (29), 8511-8514CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The catalytic hydrogenation of ketones and aldehydes, generating the corresponding alcs. in a nonpolar solvent was achieved using cyclodextrin or mol. sieves in the presence of a catalytic amt. of B(C6F5)3. This combination of sol. borane and insol. materials mediates the redns. thus avoiding protodeborylation and providing a facile protocol for such redns. Similar treatment of aryl ketones resulted in metal-free deoxygenation yielding arom. hydrocarbons.
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      Chang, C. D.; Silvestri, A. J. The conversion of methanol and other O-compounds to hydrocarbons over zeolite catalysts. J. Catal. 1977, 47, 249– 259,  DOI: 10.1016/0021-9517(77)90172-5
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      The conversion of methanol and other O-compounds to hydrocarbons over zeolite catalysts
      Chang, Clarence D.; Silvestri, Anthony J.
      Journal of Catalysis (1977), 47 (2), 249-59CODEN: JCTLA5; ISSN:0021-9517.
      The conversion of MeOH [67-56-1] and other O-compds. to C2-C10 hydrocarbons using a new class of shape-selective zeolites is reported. MeOH, Me2O [115-10-6], or an equil. mixt. thereof is converted in a 1st reaction sequence to olefins predominantly in the C2-C5 range. In the final steps of the reaction path, the C2-C5 olefins are converted to paraffins, aroms., cycloparaffins, and C6+ olefins. The final hydrocarbons are largely in the gasoline (C4-C10) boiling range. The thermochem. of the MeOH to hydrocarbon is described and possible reaction mechanisms are discussed.
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      (a) Korstanje, T. J.; Jastrzebski, J. T. B. H.; Klein Gebbink, R. J. M. Catalytic Dehydration of Benzylic Alcohols to Styrenes by Rhenium Complexes. ChemSusChem 2010, 3, 695– 697,  DOI: 10.1002/cssc.201000055
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      Catalytic Dehydration of Benzylic Alcohols to Styrenes by Rhenium Complexes
      Korstanje, Ties J.; Jastrzebski, Johann T. B. H.; Klein Gebbink, Robertus J. M.
      ChemSusChem (2010), 3 (6), 695-697CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Rhenium-based catalysts were found to be very effective in the dehydration of benzylic alcs. In all cases the TOFs of Re207 were superior to those of sulfuric acid, the current benchmark catalyst, without sacrificing selectivity for the olefin products. The rhenium-based protocol has shown an excellent activity and selectivity in the dehydration reaction of a methoxy-substituted phenylethyl alc. and holds great promise for the dehydration of other lignin-based alcs.
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      (b) Bertero, N. M.; Trasarti, A. F.; Apesteguía, C. R.; Marchi, A. J. Liquid-phase dehydration of 1-phenylethanol on solid acids: Influence of catalyst acidity and pore structure. Appl. Catal., A 2013, 458, 28– 38,  DOI: 10.1016/j.apcata.2013.03.018
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      Liquid-phase dehydration of 1-phenylethanol on solid acids: Influence of catalyst acidity and pore structure
      Bertero, Nicolas M.; Trasarti, Andres F.; Apesteguia, Carlos R.; Marchi, Alberto J.
      Applied Catalysis, A: General (2013), 458 (), 28-38CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
      The liq.-phase dehydration of 1-phenylethanol (PHE) over different solid acids was studied at 363 K using cyclohexane as solvent. It was found that the catalyst activity and selectivity strongly depended on: (1) the nature, strength and d. of acid sites and (2) the textural properties of the catalyst. Catalysts contg. mainly surface Bronsted acid sites of medium-high strength, such as Amberlyst 15, HPA/SiO2, and HMOR zeolite, showed low initial styrene (STY) selectivity because the PHE dehydrated to alpha-methylbenzyl ether (AME) at higher or similar rates than to STY. Both primary products, STY and AME, were consecutively transformed to other heavy products (HP). Catalysts contg. predominantly Lewis acid sites, such as ZnO/SiO2, Al-MCM-41 and SiO2-Al2O3, selectively transformed PHE to AME. The ether can be sequentially converted to STY and HP, depending on the solid acidity. Solid acids having strong surface Lewis sites, e.g. SiO2-Al2O3, showed high dehydration rate and HP prodn. Samples contg. exclusively weak Lewis acid sites, e.g. γ-Al2O3, were not active in the PHE dehydration at 363 K. Only zeolites HZSM-5, HBEA and HY, with similar surface d. of Bronsted and Lewis acid sites, converted selectively PHE into STY, giving initial STY selectivities between 83 and 96%. However, HY zeolite was rapidly deactivated due to the blockage of its microporous structure by bulky compds. formed inside the large cages of 13 Å. Instead, on HBEA, STY was converted to HP that can diffuse through the solid microporous structure. Then, the selectivity to STY was drastically reduced with time. A const. and high STY selectivity at total PHE conversion was only obtained with HZSM-5. The pore size of this zeolite is enough to allow the diffusion and conversion of PHE into STY, but it is not adequate to form the surface intermediates leading to AME and HP. Thus, it was proved that: (1) the surface Bronsted to Lewis ratio strongly influences the initial selectivity in the liq.-phase PHE dehydration; (2) a similar Bronsted to Lewis ratio on the solid acid surface is necessary to obtain high initial selectivity to STY; and (3) the right porous structure is crucial to avoid HP prodn. and keep const. the STY selectivity at high PHE conversion. From the exptl. results obtained in this work, a mechanistic approach was proposed in order to explain the influence of both acid site nature and pore size on the selective PHE dehydration in liq. phase.
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      (a) Styskalik, A.; Vykoukal, V.; Fusaro, L.; Aprile, C.; Debecker, D. P. Mildly acidic aluminosilicate catalysts for stable performance in ethanol dehydration. Appl. Catal., B 2020, 271, 118926  DOI: 10.1016/j.apcatb.2020.118926
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      Mildly acidic aluminosilicate catalysts for stable performance in ethanol dehydration
      Styskalik, Ales; Vykoukal, Vit; Fusaro, Luca; Aprile, Carmela; Debecker, Damien P.
      Applied Catalysis, B: Environmental (2020), 271 (), 118926CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Ethanol dehydration is effectively catalyzed by strongly acidic zeolites which, however, exhibit poor time-onstream stability. For example, HZSM-5 features strong acid sites, catalyzes ethylene oligomerization, and is prone to coking. Alumina and silica-alumina on the other hand have lower acidity, are relatively stable, but reach only low activity. Here, a series of mesoporous aluminosilicate catalysts was prepd. by non-hydrolytic sol-gel (NHSG) and are shown to feature an intermediate level of acidity (both in strength and nature), resulting in intermediate catalytic performance. Importantly, the best NHSG-made samples were very stable with time on stream, did not produce any traces of ethylene oligomers, did not show any trace of coke formation, and their texture was unaffected. Characterization (ICP-OES, N2-physisorption, TEM, XPS, IR-pyridine, Raman and solid state NMR spectroscopies) revealed that this behavior must be correlated with the remarkable degree of homogeneity in the NHSG-made aluminosilicate (only tetrahedrally coordinated Al species).
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      (b) Wang, Z.; O’Dell, L. A.; Zeng, X.; Liu, C.; Zhao, S.; Zhang, W.; Gaborieau, M.; Jiang, Y.; Huang, J. Insight into Three-Coordinate Aluminum Species on Ethanol-to-Olefin Conversion over ZSM-5 Zeolites. Angew. Chem., Int. Ed. 2019, 58, 18061– 18068,  DOI: 10.1002/anie.201910987
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      Insight into Three-Coordinate Aluminum Species on Ethanol-to-Olefin Conversion over ZSM-5 Zeolites
      Wang, Zichun; O'Dell, Luke A.; Zeng, Xin; Liu, Can; Zhao, Shufang; Zhang, Wenwen; Gaborieau, Marianne; Jiang, Yijiao; Huang, Jun
      Angewandte Chemie, International Edition (2019), 58 (50), 18061-18068CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Com. bioethanol can be readily converted into ethylene by a dehydration process using solid acids, such as Bronsted acidic H-ZSM-5 zeolites, and thus, it is an ideal candidate to replace petroleum and coal for the sustainable prodn. of ethylene. Now, strong Lewis acidic extra-framework three-coordinate Al3+ species were introduced into H-ZSM-5 zeolites to improve their catalytic activity. Remarkably, Al3+ species working with Bronsted acid sites can accelerate ethanol dehydration at a much lower reaction temp. and shorten the unsteady-state period within 1-2 h, compared to >9 h for those without Al3+ species, which can significantly enhance the ethanol dehydration efficiency and reduce the cost. The reaction mechanism, studied by solid-state NMR, shows that strong Lewis acidic EFAl-Al3+ species can collaborate with Bronsted acid sites and promote ethanol dehydration either directly or indirectly via an aroms.-based cycle to produce ethylene.
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      Applied Catalysis, A: General (2015), 493 (), 77-89CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
      Ethanol dehydration was investigated over com. H-FER, H-MFI, H-MOR, H-BEA, H-Y and H-USY zeolite samples, and alumina and silica alumina for comparison. The catalysts were characterized using FT-IR spectroscopy of the surface OH groups and of adsorbed CO and pyridine. UV-vis, Raman and TG-DTA were applied to characterize coke, formed more on H-MOR and H-BEA. H-zeolites are definitely more active than silica alumina and alumina on catalyst wt. base. The H-MOR sample is the most active but the H-MFI samples with Si/Al2 ratios 280 and 50 show higher reaction rates per Al ion, H-FER and faujasites show highest ethylene yield (99.9% at 573 K). At lower temp. and higher space velocities, di-Et ether is formed with high yield (>70% at 453-473 K on H-BEA and H-MFI (50)). Overconversion of ethylene mainly to aroms. is obsd. on H-MFI (50). The different behavior of protonic zeolites can predominantly be explained by confinement effects on the different zeolite cavities.
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      Journal of Catalysis (2014), 311 (), 102-113CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Conversion of ethanol was investigated over four com. aluminas prepd. by different industrial procedures and one com. silica-alumina. Characterization was performed by TEM, XRD, SBET and porosity measurements, and IR spectroscopy of the surface OH groups and of adsorbed CO and pyridine. Different features are attributed to different phases (γ-, δ-, θ-Al2O3) and different impurities (Na+, Cl-). Total conversion of ethanol with >99% selectivity to ethylene is achieved at 623 K over the purer Al2O3 catalyst (Na < 0.002 wt%). The most active sites are believed to be Lewis acidic Al3+ sites in a tetrahedral environment located on edges and corners of the nanocrystals. Ethanol adsorbs dissociatively on Lewis acid-base pair sites but may also displace water and/or hydroxyl groups from Lewis acidic Al3+ sites forming the active intermediate ethoxy species. Surface ethoxy groups are supposed to be intermediate species for both di-Et ether and ethylene prodn. Silica-alumina also works as a Lewis acid catalyst. The slightly lower activity on surface area basis of silica-alumina than aluminas attributed to the lower d. of Lewis acid sites and the absence of significant basicity.
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      Mesoporous nickel aluminosilicate, a solid acid catalyst prepd. by sol-gel technique was used as a heterogeneous catalyst for the synthesis of sym. ethers by dehydro-condensation of alcs. The prepd. catalysts were characterized by Fourier-transform infra red spectroscopy (FTIR), powder XRD, SEM, energy dispersive X-ray anal. (EDAX), N2 adsorption-desorption anal., temp. programmed desorption of ammonia (TPD) and X-ray photoelectron spectroscopic techniques. The presence of the catalyst assisted the etherification reaction in 30 min. Ethers formed in these reactions were quantified by gas chromatog. (GC) and the identities of few of them were confirmed by NMR spectral data (NMR).
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      Intense sample heating through high-speed magic-angle spinning (MAS; up to 58 K temp. difference) is demonstrated. The role of probehead and spinner design, as well as that of the temp. of the bearing air on the heating of a rotating sample, was examd. MAS-induced heating can affect the accurate detn. of the isotropic value of the chem. shift as well as the principal values, asymmetry and anisotropy parameters of the chem. shift tensor. In some cases, a very large temp. gradient (12 K) within the fast rotating sample was found, which may limit the resoln. of high-speed 1H MAS NMR spectra.
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    • Full experimental details of syntheses and catalytic experiments, additional characterization data for the ActB materials, detailed results of catalytic investigations (product distribution, characterization data) including an extended discussion. The authors have cited additional references within the Supporting Information. (38) (PDF)


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