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Ethylene Polymerization over Metal–Organic Framework Crystallites and the Influence of Linkers on Their Fracturing Process

  • Miguel Rivera-Torrente
    Miguel Rivera-Torrente
    Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
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  • , 
  • Paul D. Pletcher
    Paul D. Pletcher
    Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
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  • , 
  • Maarten K. Jongkind
    Maarten K. Jongkind
    Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
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  • , 
  • Nikolaos Nikolopoulos
    Nikolaos Nikolopoulos
    Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
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  • , and 
  • Bert M. Weckhuysen*
    Bert M. Weckhuysen
    Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
    *E-mail for B.M.W.: [email protected]
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    Orcidhttp://orcid.org/0000-0001-5245-1426
Cite this: ACS Catal. 2019, 9, 4, 3059–3069
Publication Date (Web):March 11, 2019
https://doi.org/10.1021/acscatal.9b00150

Copyright © 2022 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

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Abstract

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The physical properties and morphologies of polymers are pivotal for their manufacturing and processing at the industrial scale. Here, we present the formation of either fibers or micrometer-sized polyethylene beads by using the MIL-100(Cr) and MIL-101(Cr) zeotypes. The MOF structures have been used for ethylene polymerization with diethylaluminum chloride (DEA) as a cocatalyst, resulting in very different activities and morphologies. In situ DR UV–vis–NIR and CO-probe FT-IR spectroscopy revealed the formation of different types of Cr species for each catalyst material, suggesting that the linker (for the same metal and topological structure) plays a crucial role in the formation of Cr olefin polymerization sites. Activity in ethylene polymerization in toluene at 10 bar and 298 K was related to the observed spectra, corroborating the presence of different types of active sites, by their different activities for high-density polyethylene (HDPE) formation. SEM micrographs revealed that although MIL-100 and MIL-101 exhibit identical zeolitic MTN topology, only the latter is able to collapse upon addition of DEA and subsequent ethylene insertion and to fracture forming polymer beads, thus showing noticeable activity in HDPE formation. We ascribed this effect to the higher pore volume and, thus, fragility of MIL-101, which allowed for polymer formation within its larger cages. MOFs were compared to the nonporous chromium(III) benzoate [Cr3O(O2CPh)6(H2O)2](NO3)·nH2O complex (1), in order to study the effect of the embodiment in the porous framework. The properties of the polymer obtained under identical reaction conditions were comparable to that of MIL-101(Cr) but very different morphologies were observed, indicating that the MIL-101(Cr) structure is necessary to impart a certain architecture at the microscale. This work clearly shows that MOFs can be used as catalytically active morphology regulators for ethylene polymerization. Moreover, even for an identical topology and metal in a MOF structure, the linker and the pore structure play crucial roles and have to be carefully considered in the design microporous coordination polymers for catalytic purposes.

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1. Introduction

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Polyethylene (PE) production has been estimated to reach 100 million tons per year by 2020 and will continue to be an ubiquitous material for our modern society in the decades to come. A wide variety of reactor configurations (i.e. liquid, slurry, and gas phase) and catalyst materials (metallocene, Ziegler–Natta, and Phillips) are used for its production depending on the required physicochemical properties, such as density, melting point, and viscosity as well as morphology, (1) that need to be conferred to the polymer for further processing. (2−4) In order to alter the architecture of the polymer at the macroscale, different systems in which the support plays the role of a “cast” have been used in the past. Examples of such efforts include the so-called extrusion polymerization in which the active olefin polymerization sites have been incorporated in the family of MCM-41 and SBA-15 mesoporous materials, yielding polyethylene microcrystalline fibers. (5−10)
Metal–organic frameworks (MOFs) are another interesting class of porous materials that have been recently used for polymer templating. (11−17) MOFs consist of coordination networks with organic ligands connected to metal nodes or cations, containing potential voids upon guest molecule removal. (18) Their well-defined metal sites, together with their porosity, have fostered their use as single-site porous catalysts. (19−21) Indeed, a number of MOFs have been studied as solid catalysts for the polymerization and oligomerization of short olefins, due to their industrial relevance and relatively mild conditions required for operation, together with the necessity of fine-tuning selectivity. (22−26) Ethylene oligomerization (in both the liquid (27) and the gas phase (28−33)) and polymerization, (34−37) as well as propylene dimerization in the gas phase, (38−40) or even isoprene polymerization, (41) can be catalyzed by a number of MOFs, postsynthetic modification being necessary in most cases. However, despite its paramount importance in later applications, polymer morphology has been often overlooked and most of the MOF-based catalyst materials described often show either poorly or nonshaped materials. Recently, Liu et al. (42) reported on the selective oligomerization of ethylene by the chromium nodes of MIL-100(Cr) after activation with organoaluminum species, wherein the activity and selectivity can be tuned by simple thermal treatments without the need for additional modifications on the MOF framework. Their work has inspired our efforts in investigating the possibilities of MIL-100(Cr) materials for olefin polymerization. MIL-100(Cr) (43) and MIL-101(Cr) (44) frameworks (where MIL stands for Matériau de l’Institut Lavoisier) are among the most studied MOFs due to their high porosity (SBET ≈ 1500–3000 m2/g), tunability, and chemical and hydrothermal stability.
In this work, we have taken advantage of the ability of MIL-101(Cr), in contrast to the more solid MIL-100(Cr) and the homogeneous Cr complex [Cr3O(O2CPh)6(H2O)3](NO3)·nH2O (1), to fracture upon addition of a strongly oxophilic organoaluminum activator and subsequent ethylene polymerization in the liquid phase. Moreover, we sought to understand the effects of the cocatalysts when they are used in combination with these MOF materials, as well as the reaction conditions, by means of in situ spectroscopy together with a detailed analysis of the polymer products formed. We will show that even for an identical pore structure and metal in a MOF, the organic linker plays a crucial role and has to be chosen specifically to obtain targeted polyolefin polymer products.

2. Experimental Section

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2.1. Catalyst Preparation

The synthesis of the coordination complex 1, MIL-100(Cr), and MIL-101(Cr) was carried out according to previously published procedures. (43−45) Details on the preparation and routine characterization of the materials can be found in the Supporting Information.

2.2. Material Characterization

X-ray diffraction (XRD) patterns were obtained with a Bruker-AXS D2 Phaser powder X-ray diffractometer in Bragg–Brentano geometry, using Co Kα1,2 = 1.79026 Å, operated at 30 kV. The measurements were carried out between 2 and 30° using a step size of 0.05° and a scan speed of 1 s, with a 0.1 mm slit for the source. Simulated XRD patterns were obtained by processing the corresponding .cif files with VESTA (λ = 1.79026 Å, fwhm = 0.2) from refs (43and46).
N2 adsorption isotherms were measured at 77 K on a Micromeritics TriStar 3000 instrument. Prior to all measurements, samples were dried at 423 K under dynamic vacuum. Specific surface areas (SSAs) were calculated using the multipoint BET method (0.05 < p/p0 < 0.25). Pore volumes (Vp) were calculated by the t-plot method; pore size distributions (PSDs) were obtained by DFT using N2 and spherical pores in the package MicroActive 4.06 (Micromeritics).
Fourier transform infrared (FT-IR) spectroscopy measurements at 85 K were recorded with a PerkinElmer 2000 instrument, in a specially designed cell fitted with CaF2 windows. The degassed (at 423 K) MOF materials studied were pressed into 3–5 mg wafers inside of a glovebox and fitted into the FTIR cell. The cell was sealed, connected to the gas/vacuum system, and carefully evacuated to ∼10–5 mbar at 298 K. A 10% CO/90% He (v/v) mixture was slowly introduced into the cell until the pressure reached 250 mbar and allowed to equilibrate. The sample was then cooled using liquid nitrogen (to 85 K) to allow the CO to saturate and equilibrate on the surface. The cell gas pressure was decreased, and spectral measurements were taken after the pressure stabilized. A final measurement was taken after outgassing the cell for 20 min. To measure the CO FT-IR spectra of the sample activated at 623 K, the cell was slowly warmed to 298 K under vacuum and then heated to 623 K at a rate of 5 K min–1 and maintained at 623 K for 1 h. The sample was then cooled to room temperature, and the previously described CO FT-IR measurement protocol was used to probe the surface acidity. For the Et2AlCl (DEA, Sigma-Aldrich, 97%) activated samples, ∼20 mg of MOF material was mixed with 100 equiv of DEA for 20 min in the glovebox. The solvent was decanted and the remaining solid dried under vacuum. Then, ∼5 mg of material was then pressed into a pellet and the CO FT-IR spectroscopy measurements were carried out by following the previously described measurement protocol.
UV–vis–NIR diffuse reflectance spectroscopy (UV–vis–NIR DRS) measurements were performed under in situ conditions by making use of a Varian Cary 500 spectrophotometer with a DRS accessory. The measurements were performed in the spectral range of 4000–45000 cm–1 with 33 ms data point scan time and spectral resolutions of 17 and 7 cm–1, in the 12500–45000 and 4000–12500 cm–1 spectral ranges, respectively. Two artifacts in the measured spectra were corrected for the detector/grating and light source changeovers at 12500 and 28570 cm–1, while the spectral feature appearing at 11250 cm–1 is due to an instrument artifact. For every measurement, the cell was loaded in a N2-filled glovebox (<2 ppm of H2O, O2), thereby keeping the samples from contact with atmospheric oxygen and water. The samples were measured against a Teflon-white measured in the same cell loaded with the same volume of 30 μm beads of Teflon powder. For measurement of the materials, 100 mg of catalyst material was loaded in a homemade cell. Subsequently, the required amounts of DEA were injected under a N2 flow of 10 mL min–1. After injection, the spectra were recorded until no further spectral changes were observed. Subsequently, the gas feed was switched to 10 mL min–1 C2H4 for gas-phase polymerization (Linde AG, C2H4, 99.9%). Spectra were recorded until no further changes occurred.
Scanning electron microscopy (SEM) micrographs of the MOF crystallites and the MIL-100(Cr) after reaction were recorded on a FEI Helios nanolab 600 Dual Beam with an Oxford Instruments Silicon Drift Detector X-Max energy-dispersive spectroscope. After the sample was deposited onto Al stabs with carbon tape (Electron Microscopy Sciences, Hartfield, PA, USA), the samples were sputtered with 22 nm of Pt(-Pd). Thereafter, imaging and EDX analysis were carried out at a beam of 15 kV and 0.1 nA. Micrographs of the polymer samples were recorded on a PhenomPro X microscope (FEI Company, USA), equipped with a CsB detector for backscattered electrons (BSE), operated at 10 kV. The samples were mounted on holey carbon tape supported on Al stubs that were not coated prior to measurements.
Differential scanning calorimetry (DSC) analyses were done with ca. 3–4 mg of polymer; each run was analyzed using a Texas Instrument Discovery DSC featuring an automatic sampler and TRIOS software. Each sample was heated from 298 to 473 K at a rate of 10 K min–1, held at 473 K for 2 min to erase the thermal history, and then cooled to 313 K at 1 K min–1 to ensure a slow recrystallization of the polymer. A second heating ramp was used to determine the melting temperature (Tm) and enthalpy (ΔHm), in which the sample was heated at 10 K min–1 to 473 K, before being cooled back to 298 K. The crystallinity of the polymer samples was determined assuming ΔHm0 = 293 J/g, for 100% crystalline ultrahigh-molecular-weight polyethylene (UHMWPE).
Gel permeation chromatography (GPC) was carried out on a Polymer Laboratories PL-GPC220 instrument, equipped with a PL BV-400 refractive index detector. The column set consisted of three Polymer Laboratories 13 μm PLgel Olexis 300 × 7.5 mm columns, and the calibration was performed with linear polyethylene (PE) and polypropylene (PP) standards. PP molar mass calibration was obtained after conversion from PE to PP using the Mark–Houwink constants.

2.3. Catalytic Testing

The degassed MOFs and complex 1 were tested for the polymerization of ethylene at 10 bar in the liquid phase. For the runs, 5 × 10–2 mmol of catalyst was introduced into a stainless-steel Parr reactor, together with Al:Cr ratio molar ratios of 100, 500, or 1000 of DEA in toluene (Sigma-Aldrich, anhydrous, 99.8%) inside an Ar-filled glovebox (O2, H2O <2 ppm). The autoclave was connected to an ethylene (Linde AG, 99.9%) line, the system was evacuated and flushed with ethylene three times before the cell was pressurized to 10 bar at 298 K, and the mixture was mechanically stirred at 1000 rpm. The cell was depressurized of ethylene after 1 h of reaction, the autoclave was cooled to 195 K using an acetone/dry ice bath, and the pyrophoric materials were quenched using acidified (37 wt % HCl(aq), Merck KGaA) methanol. The solid polyethylene was filtered, rinsed with MeOH (98%, VWR International), and dried under high vacuum overnight. The catalyst activity was based on the weighed polymer product.

3. Results and Discussion

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MIL-100(Cr) and MIL-101(Cr) frameworks are built up with chromium cations in an octahedral coordination forming the so-called chromium oxo-bridged trimer units, i.e. [(Cr3(μ3-O)], bonded to trimesic acid (H3BTC) or terephthalic acid (H2BDC) for MIL-100(Cr) or MIL-101(Cr), respectively. Coordinatively unsaturated positions (CUS) on the Cr3+ cations can be generated by removing the ligand in the axial position (typically nitrate, halide, hydroxy, or carboxylate anions arising from the synthesis) upon thermal treatment of the solid under vacuum. (47) In order to be able to solely compare the effects of MOF topology and exclude the presence of residual potential poisons, we carried out an anion exchange with Cl– ions. Although the structure exhibits well-defined Cr3+ ions, the nature of the metal active sites upon addition of alkylating agents, such as diethylaluminum chloride (DEA), is not well understood. In order to verify that, isoreticular MIL-100(Cr) and MIL-100(Cr) were prepared following standard protocols and their crystallinity, purity, porosity, and morphology confirmed by means of X-ray diffraction (XRD), N2 adsorption at 77 K, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) (Sections 1–5 in the Supporting Information), respectively. The materials were subsequently impregnated with DEA and studied by XRD.

3.1. Structural Stability and Active Sites

XRD patterns of the materials after reaction with DEA, contact with ethylene at 298 K, and subsequent gentle quenching in air were obtained in order to study the stability of the MOFs upon contact with the cocatalyst. For this purpose, ca. 10 mg of activated MIL-100(Cr) and MIL-101(Cr) was mixed with DEA in an Ar-filled cell. Then, a 2 mL min–1 stream of air was flowed for 30 min at 298 K through the cell to quench any unreacted organoaluminum species. In Figure 1a, it can be seen that although the material is diffracting X-rays less intensely, the main XRD peaks at 2.36, 3.97, 4.64, and 4.89°, corresponding to the (220), (311), (222), and (400) reflections, respectively, are still present, indicating that the MOF structure is partially retained. Even after ethylene was flowed through the cell, the material did not show significant changes or the presence of a polymeric material. In contrast, the XRD patterns in Figure 1b show that the reaction of MIL-101(Cr) with DEA resulted in a complete loss in crystallinity, as none of the main reflections are present and only an amorphous pattern remains.

Figure 1

Figure 1. XRD patterns of (a) MIL-100(Cr) and (b) MIL-101(Cr) as synthesized (red), after contact with 125 equiv of DEA (Al:Cr = 100) (blue), and after ethylene was flowed at 298 K for 15 min (pink). The asterisk (*) indicates the formation of some high-density polyethylene (HDPE). Insets show the area with the main XRD reflections of the crystalline MOF.

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This observation can be explained by a cleavage of the Cr–O bonds by the cocatalyst, resulting in the formation of defects, reduced and alkylated Cr, and linker sites as well as alumina or aluminum oxide like species. When ethylene was passed through the cell, a white solid, which was confirmed to be traces of HDPE (Supporting Information, Section S6 and Figure S11) appeared. This demonstrated that some of the Cr sites are able to form a polymer in the gas phase, unlike the case for the MIL-100(Cr) structure.
In order to understand if different Cr sites were present in the MOF frameworks after degassing and upon addition of DEA, in situ FT-IR spectroscopy experiments with CO as a probe molecule were carried out. In line with the literature, different types of sites were observed when the material was dried at 423 or 623 K. (47) First of all, in Figure 2a–c, spectra of MIL-100(Cr) dosed with CO show the presence of very few coordinatively unsaturated sites [Cr3+···CO] when degassing was done at 423 K (at 2188 cm–1). A very intense band at 2154 cm–1 (next to that at 2136 cm–1 of physisorbed CO) indicates that most sites were still hydroxylated. Interestingly, a small feature at 2106 cm–1 was observed, which may correspond to the formation of extraframework or defectively coordinated Cr cations during synthesis or degassing. We tentatively ascribe this feature to partially reduced Cr3+/2+(CO)xXy (X = Cl–, NO3, OH–, or H2BTC–) species, as they are typically present at such low wavenumber regions. (48−52)

Figure 2

Figure 2. FT-IR spectra with CO as a probe molecule measured at 85 K of (a–c) MIL-100(Cr) and (d–f) MIL-101(Cr) after activation under vacuum (10–5 mbar) for 16 h at 423 K, at 623 K, and after impregnation with diethylaluminum chloride (DEA) (with Al:Cr molar ratio 100) and CO dosage (0–1 mbar) at 85 K. Degassed MIL-100(Cr) was subtracted as a reference in (b) and (c). MIL-101(Cr) was subtracted as a reference spectrum in (e) and (f).

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The presence of the main interaction OC···Cr3+(MOF) bands at 2200–2180 cm–1 makes it impossible to distinguish the presence of Cr2+ sites by using this region. This feature is observed at 2090 cm–1 when the sample is degassed at 623 K, indicating that indeed the treatment has a certain effect on the coordination environment of the Cr cations. Moreover, the band at 2188 cm–1 is slightly shifted toward 2191 cm–1 and is much more intense, indicating that a much larger number of Lewis acid Cr3+ sites appear upon degassing at higher temperature. Addition of the organo-aluminum compound results in a strong modification of the baseline as well as in the formation of a shoulder at 2200 cm–1, which indicates an abstraction of additional counterions from the Cr sites. No further indications of the presence of Cr2+ sites are present, suggesting that they are alkylated and are not available for binding with CO as probe molecule. When an identical treatment was carried out on MIL-101(Cr), a number of differences were observed. First of all, when the material was degassed at 423 K, a higher fraction of Lewis Cr3+ sites was observed as a band at 2190 cm–1, and a very small amount of hydroxylated Cr sites (seen as a broad shoulder at ca. 2167 cm–1) was probed. Again, a small feature (inset in Figure 2d) appeared at lower wavenumbers (2090 cm–1) together with another feature at 2062 cm–1 after dosing CO, indicating Cr2/3+(CO)x species. It is important to remark that, in this case, two types of Cr carbonyl sites are formed, indicating that not one but multiple reduced Cr sites appear. When the material was degassed at 623 K, a trend similar to that for MIL-100(Cr) was seen, meaning more (and more acidic) Lewis acid sites and hydroxylated Cr3+ sites are present. The small spectral feature at 2090 cm–1 remained unchanged, while that at 2062 cm–1 became less intense and was shifted to 2057 cm–1, indicating that the thermal treatment results in a higher degree of reduction (i.e., a red shift indicates an enhanced π back-donation of the Cr sites). (53)
Addition of DEA to MIL-101(Cr) has a dramatic effect on the spectral features (Figure 2f). The band corresponding to Lewis Cr3+ sites broadens significantly, indicating the presence of a wide variety of different highly acidic Cr3+ sites (e.g., extraframework cations, embedded in the lattice), as they are quickly saturated at low CO pressures. Similarly, the feature at 2065 cm–1, corresponding to the Cr2+(CO)x species, broadens and increases in intensity dramatically. Again, a large number of differently coordinated Cr2+ sites are formed upon reaction with the alkylating cocatalyst. It is unclear whether any of the carbonyl adducts are binding atop or bridging at this moment. Decomposition of the MIL-101(Cr) framework by DEA results in another unassigned strong spectroscopic feature at 2261 cm–1. Our interpretation of the Cr reduction is further supported by the in situ UV–vis–NIR DR spectra recorded after addition of DEA (Figure 3).

Figure 3

Figure 3. (a) Diffuse reflectance (DRS) spectra in the vis–NIR region for degassed MIL-100(Cr) (red), after injection of 125 equiv of diethylaluminum chloride (DEA) (blue) and after 15 min (pink). (b) Deconvolution of the UV–vis–NIR DRS spectrum after injection of DEA showing different Cr species. The inset in (b) shows the presence of small amounts of Cr2+ species (both Oh and Th geometries). Green lines show the residual values of the fitting procedure. (c) DRS spectra in the UV–vis–NIR region after flowing ethylene (10 mL min–1) at 298 K and 1 bar for 1 h into the cell (from dark blue (t = 0) to red (t = 60 min)). The inset shows bands where polyethylene C–H combination bands should appear if polymerization occurred. (d) DRS spectra in the vis–NIR range of activated MIL-101(Cr) (red), after injection of 125 equiv of DEA (blue) and after 15 min (pink). (e) Deconvolution of the spectrum after injection of DEA showing the different Cr species. The inset in (e) shows the presence of small amounts of Cr2+ species (only Oh geometry). Green lines show the residual of the fitting procedure. (f) DRS spectra in the NIR region after flowing ethylene (10 mL min–1) at 298 K and 1 bar for 1 h into the cell (from dark blue (t = 0) to red (t = 60 min)), showing the bands of crystalline PE forming over time. The inset shows additional combination bands of the polymer CH2 groups, indicating polymer formation in contrast to the case of (c).

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The bands observed in the spectrum of activated MIL-100(Cr) (red spectrum, Figure 3a) correspond to characteristic bands for octahedral (Oh) Cr3+ species with an oxide-like ligand environment at 24000 and 16000 cm–1 of the Cr trimers. (54,55) The t2g → t2geg1 transition, split into the 4T1g ← A2g and the 4T2g ← 4A2g transitions, relate to the higher and lower energy bands, respectively. After addition of DEA (Figure 3a, blue spectrum), the DRS spectrum (Figure 3b) is characterized by two absorption bands at ∼11000–10000 cm–1, indicative of the presence of coordinatively unsaturated Cr2+ species, suggesting the formation of a few reduced chromium sites. This observation is in line with our previous CO-probe FT-IR spectroscopy results and previously mentioned literature. (42) Other relevant studies show that metal terephthalate MOFs containing exclusively Cr3+ show no absorption below 13000 cm–1 (even for CrTh geometries), supported by both time-dependent density functional theory (TD-DFT) calculations and UV–vis experiments, suggesting that indeed the DEA acts as a reducing agent of Cr cations. (56) The presence of some [Cr3+···OH2] species even after thermal treatment is verified by the absorption band at ∼5220 cm–1 (see Figure S12b), which corresponds to the O–H combination band vOH + δOH, confirming the presence of a few remaining nondehydroxylated Cr sites (as previously shown with FT-IR spectroscopy). (47) Moreover, we observed a decrease in the intensity of the ligand to metal charge transfer (LMCT) bands of π* → d transitions at 45000–38000 cm–1 (Figure S12a). This can be ascribed to a partial degradation of the MOF framework caused by a cleavage of the Cr–O bonds, which react with the organo-aluminum species. Flowing ethylene over the activated MOF results in a decrease in the intensity of the characteristic Cr2+ absorption bands (located at ca. 12000–10000 cm–1) shortly after, although no significant changes are seen in the C–H region, indicating no polyethylene formation. Again, different species were observed for MIL-101(Cr) after both addition of the cocatalyst and addition of ethylene. A much more pronounced decrease in the intensities of the LMCT bands occurred (Figure S13a), corroborating the structural collapse observed in XRD patterns. Deconvolution of the DRS spectrum, as shown in Figure 3e, indicates the presence of reduced Cr2+ sites, although only octahedral species are formed in this case. Furthermore, the main contribution of the low-energy Cr3+Oh is shifted toward ca. 15000 cm–1, indicating changes in the nature of the Cr3+ sites as well, in agreement with the broad band observed in the related FT-IR data. The peaks at 4180, 4250, and 4325 cm–1, which correspond to C–H combination bands, indicate alkylation of the MOF. When ethylene was introduced in the cell, the formation of HDPE was steadily detected by the sharpening of the C–H combination bands (Figure 3e).
An increase in the bands at ca. 8400, 5900, and 4500–4000 cm–1 with increasing time corroborates the formation of a solid phase. Indeed, flowing ethylene at slightly higher temperature (313 K) for 1 h into the cell, after activation with 100 equiv of DEA, resulted in the formation of significant amounts of HDPE from the gas phase, as confirmed by XRD (Figure S11). In brief, the DRS data show that, in spite of having the same structural motifs (i.e. carboxylates and Cr3(μ-O) trimers), each MOF forms different Cr3+ and Cr2+ species with similar coordination environments after addition of DEA, and only MIL-101(Cr) is capable of producing some polymer.
In order to compare both MOFs as active catalyst materials, the catalytic polymerization of ethylene in a liquid-phase reactor (in toluene) at 10 bar was carried out. Three different amounts of cocatalyst (i.e., Al:Cr = 100, 500, 1000) were tested, in order to understand the influence of the cocatalyst on the activity and final materials produced. As we hypothesized, Table 1 shows that, despite both MOFs possessing identical topology, MIL-100(Cr) is practically unable to catalyze the formation of polyethylene, regardless of the Al:Cr ratio used. In contrast, MIL-101(Cr) showed high activity (i.e., 15.4 kg PE (mol Cr)−1 h–1 bar–1) for a Al:Cr ratio of 100. Higher amounts of cocatalyst were detrimental for the performance under the same reaction conditions (i.e., ∼ 4–5 kg PE (mol Cr)−1 h–1 bar–1), as previously reported for organoaluminum alkylated Phillips systems. (57) In order to further understand if pore confinement and anchoring of the Cr trimer sites have an influence on the active sites, the coordination complex 1 (see Section S1 for details on the synthesis) was used as a homogeneous counterpart. A parallel trend can be observed in activity (Table 1), indicating that the active sites present after addition of DEA are rather similar to those of MIL-101(Cr). The physical properties (melting temperature, Tm; heat of fusion, ΔHm) of crystalline HDPE are governed by its molecular structure, which is very much determined by the catalytic sites. DSC analyses (Table 1 and Section S8 and Figure S14) were carried out to compare the properties of the obtained materials. As can be concluded from Table 1, both MIL-101(Cr) and complex 1 show the typical %X, Tm and ΔHm ranges for HDPE with low branching. To elucidate if the cocatalyst led to Cr leaching of MIL-101, the activity of the solution obtained after mixing DEA with MIL-101(Cr) catalyst was evaluated, showing that indeed some HDPE was formed, with properties similar to those of complex 1. This indicates the same active sites for both catalyst materials: i.e., trimers dissolved in toluene. However, to our surprise, MIL-100(Cr) was inactive under the same conditions.
Table 1. Catalytic Activity of the Different Materials under Study, Including the Physicochemical Properties of the High-Density Polyethylene Products Obtained after Performing the Ethylene Polymerization Experiments
catalystAl:CrbactivitycTm (K)ΔHm (J/g)%XdMw (103 kDa)PDIe
1a10013.0408.3182.162.21.88.3
 5006.15407.7169.157.71.510.6
 10004.12408.2181.962.11.215.7
MIL-100(Cr)a,f1001.3 × 10–3<0.03    
 5000.4 × 10–3<0.03    
 10002.1 × 10–3<0.03    
MIL-101(Cr)a10015.4408.5189.764.71.127.0
 5004.71410193.165.90.922.6
 10005.41410.6182.962.40.8518.0
MIL-101(Cr)a,g5000.63409.3187.664.01.411.2
a

Reaction conditions unless specified otherwise: 1 h at 10 bar of C2H4 in 20 mL of toluene at 298 K in a 35 mL Parr-autoclave reactor.

b

Al:Cr molar ratio.

c

Activity: kg PE (mol Cr)−1 h–1 bar–1.

d

In comparison to 293 J/g for 100% crystalline UHMWPE.

e

Determined by GPC calibrated with PE and PP standards.

f

Not enough polymer product for analysis. Experiments at 10, 20, 30, and 40 bar showed no activity.

g

MOF/AlEt2Cl was stirred together in toluene for 30 min and filtered into reactor before reaction.

Accordingly, GPC analysis (Table 1 and Section S9) shows that Mw ≈ (0.85–1.8) × 103 kDa for the polymers produced, i.e. lower-end to mid-sized HDPE chains, in line with the DSC data. Nevertheless, average molecular weights (Mw) were slightly higher when nonporous complex 1 was used instead of the zeotypic MIL-101(Cr), following the activity trend. Different correlations between catalyst porosity and Mw have been observed in the past. (58,59) We envision a process in which Cr trimers that leach and remain entrapped in the pores lead to different (probably shorter) polymer chains. This is not the case when complex 1 is used: chains can grow with no physical constraints, only limited by chain transfer reactions or catalyst deactivation processes. In our case, the high-MW shoulder is more prominent for low Al loading and complex 1 catalyst. GPC of the HDPE obtained with complex 1 shows longer tails (log Mw ≈ 6.5 in Figures S15–S17), which may be associated with enhanced chain transfer reactions, as demonstrated by similar trends with increasing cocatalyst amounts (Section S9 in the Supporting Information) in the experiments in which MIL-101(Cr) was used. Thus, we hypothesize that the variety of chain lengths is related not only to different active sites, MOF-embodiment, or solubilization but also to the presence of increasing cocatalyst amounts. A very high Al:Cr may result in an overalkylation, chain termination promotion, or simply alumina coverage of the growing chains. Again, molecular weight distributions (MWDs) and polydispersity index (PDI) values of the polymer obtained after filtration of the MIL-101(Cr) fully correlate to that of the material obtained with 1. This indicates that the active sites consist of alkylated Cr sites derived from the trimers in solution. Recent studies have shown multiple possibilities (e.g. metallacycle, Green–Rooney, Cossee–Arman) for the initiation mechanism of ethylene insertion in similar Cr systems. (60,61) However, the agreement is that chain growth in Cr-based Phillips catalysts operates mostly via the Cossee–Arlman mechanism. (2,62) In the case of MIL-100(Cr) Liu et al. described a mechanism involving multiple metal centers acting via a metallacycle mechanism, yielding short (C6–C12) α-olefins. (42) With the results previously described in this work, we propose that the active species for MIL-101(Cr) and 1 consist of Cr-alkyl sites formed by partial degradation of the lattice that lead to the formation of active sites that can insert ethylene, as described in Scheme 1. These Cr sites arise from the Cr trimers being most likely Cr3+ or Cr2+ sites (according to the spectroscopy study), although the oxidation state and coordination geometry still remain elusive. The inability of MIL-100(Cr) to fracture would explain the high activity in the formation of α-olefins.

Scheme 1

Scheme 1. Tentative Activation of Cr Sites by Diethylaluminum Chloride (DEA) and Subsequent Ethylene Insertion and Polymerization in MIL-101(Cr) Materials
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3.2. Polymer Morphology

Control over the particle morphology is crucial for large-scale synthesis of polyethylene. (1,2,24,25) Regardless of the catalytic process employed, a number of issues such as reactor fouling, heat transfer, and product properties have to be taken into account. As already mentioned, Cr-based catalysts are typically supported onto solid oxides that crumble during ethylene polymerization, allowing the necessary chain growth. The above results led us to evaluate the ability of both MIL-100(Cr) and MIL-101(Cr) to generate structured polyethylene upon fragmentation of the MOF catalysts. Surprisingly, Figure 4a,b shows that, under identical reaction conditions, MIL-101(Cr) generates spherical beads of polyethylene, in contrast to MIL-100(Cr) (Figure 4c,d), which produces nanosized (<1 μm), ill-shaped material. These findings show that not only is MIL-101(Cr) active in ethylene polymerization but also that its use as a catalyst confers to the polymer a certain architecture at the macroscale. Experiments with different Al:Cr ratios show that the templating effect is retained in every case, with a minimal effect on the size distribution (Figure S17). Analogously, a similarly poorly shaped material was covering MIL-100(Cr) crystallites for all the Al:Cr ratios tested, indicating no significant chain growth under these conditions.

Figure 4

Figure 4. Scanning electron microscopy (SEM) micrographs of the polymer product obtained with (a, b) MIL-101(Cr), (c, d) MIL-100(Cr) (note that only nanosized crystallites are observed at this magnification), complex 1 (e, f), and the leached Cr from MIL-101(Cr) (g, h). Conditions: Al:Cr mol ratio of 100, diethylaluminum chloride (DEA), T = 298 K, p = 10 bar of C2H4, t = 1 h.

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We explain this difference on the basis of our previous XRD analysis, which indicated the total collapse of MIL-101(Cr). This enables the Cr active sites to grow polymer chains in the solvent medium, in contrast to MIL-100(Cr), which retains its crystalline structure, preventing the crystallites from fracturing upon ethylene insertion. Thus, the obtained SEM images indicate that MIL-101(Cr) acts as a sacrificial template, in which the catalytically active species reside and that is later covered on polyethylene during the reaction, leading to the observed polymer beads.
The SEM images of the polymer obtained with catalytic complex 1 show randomly oriented, intergrown polymer fibers of ca. 3–5 μm thickness (for the same Al:Cr ratios; Figure S17), indicating that, in addition to the Cr trimer units, a solid scaffold is required to control the chain growth. In order to confirm its role as a structural template and discriminate it from possible leached species that polymerize in solution, the SEM images of the polymer recovered from toluene filtered after suspending DEA-activated MIL-101(Cr) were recorded. Figure 4g,h shows fibers very similar to those obtained with complex 1, proving that the sole presence of Cr trimers in solution is not enough to obtain the morphologically controlled polymer. SEM images in Figure 5 and Figure S19 of the recovered MIL-100(Cr) solid after reaction with 500 equiv of cocatalyst corroborate our hypothesis. After filtration from the toluene solvent, most of the MOF crystallites appeared intact (compare to SEM in Figure S9), and only a few polymer fibers arising from the pores were observed. Some species that have leached into solution (the strong alkylating agent causes partial degradation of MIL-100, as shown by XRD) formed polyethylene layers that covered the MOF (Figure S19). This has been previously observed in other catalysts supported in porous polymers. (59)

Figure 5

Figure 5. Scanning electron microscopy (SEM) micrographs of MIL-100(Cr) after reaction (10 bar of C2H4, 298 K, 500 equiv of DEA, toluene, 1 h) at high magnification, showing MIL-100(Cr) crystallites still intact. The arrows in the inset image indicate polymer fibers of MOF crystallites that were not able to fracture. However, no evident signs of polymerization around the MOF crystallites, into shaped beads as is the case for MIL-101(Cr), were observed. In Figure 5, the inset shows a few polymer fibers that emerge from certain MOF crystallites. This is in stark contrast with MIL-101(Cr) and complex 1, and it evidences the inability of MIL-100(Cr) to crumble upon ethylene insertion.

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Although Cr–O bond strength has been argued as a possible reason for differences in reactivity (as it may not allow for the alkylation of Cr), X-ray diffraction studies have shown that Cr–O distances are ca. 1.975 Å for all catalysts, indicating very similar bonding strengths. On the other hand, porosity has been shown to greatly affect activity for ethylene polymerization in Cr-based systems. Differences in activity are due to the porosity of each material. It is well-known that support and catalyst porosity have a tremendous effect on polymer production. (2)
It is generally accepted that at least Vp ≥ 1.0 cm3 g–1 is necessary in silica in order for the support to crumble upon chain growth. Despite the different chemical and physical nature in comparison to silica and its high porosity, MIL-100 showed a pore volume (Vp = 0.81 cm3 g–1) below that threshold (Figure S3). In contrast, MIL-101 (Vp = 1.27 cm3 g–1) was well above the threshold (Figure S4), pointing to a critical effect on polymerization activity. As pointed out by McDaniel, it is not the difference in pore volume but the fragility (which may arise from this difference in pore volume and seems to be the cause for this difference) that prevents higher polymerization activity. In fact, not only a difference in pore volume but also a difference in pore diameter and window exists. While MIL-100 has two types of cages, of 25 and 29 Å diameter (with pore windows 5 and 8.6 Å, respectively), (43) MIL-101 exhibits diameters of 27 and 34 Å (with 11.6 and 16 Å windows). (44) This difference in pore diameter, specifically the presence of the larger 34 Å cages, seems to be critical for ethylene polymerization. We argue that chain growth occurring in that specific cage is enough to tear apart MIL-101 crystallites during the process, giving rise to new active sites that further polymerize ethylene. DFT pore size distributions shown in Figure 6 clearly illustrate the difference in pore diameter cages between both MOFs.

Figure 6

Figure 6. Density functional theory (DFT) pore size distributions of both MOFs calculated from the experimental N2 adsorption isotherms at 77 K.

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An alternative explanation for these differences in reactivity may be the low temperature, i.e. 298 K, at which the polymerization was conducted, which would be insufficient to collapse the MIL-100(Cr) framework.
For coordination complex 1, it readily dissolves in the reaction medium (toluene), acting as a homogeneous catalyst with no need of fracturing, as observed in the SEM imaged randomly oriented polymer fibers.
In Figure 7, a summary of the different activities and morphologies that are obtained in ethylene polymerization under the studied conditions is presented. MIL-101(Cr) is a competent catalyst that, although it performs below commercial Philips benchmarks, is able to structure the polymer at the microscale, while producing high-molecular-weight PE with relatively narrow PDI (orange). In contrast, MIL-100(Cr) (green) is unable to fracture and produce polymer, while complex 1 results in a nonstructured polymer (purple).

Figure 7

Figure 7. (a) MIL-100(Cr) is unable to fracture upon polyethylene formation, resulting in low catalytic activity. (b) MIL-101(Cr) is degraded and partially leaches Cr clusters into the solution upon addition of the cocatalyst, leading to different morphologies. (c) Coordination complex 1 in solution generates polymer fibers, as the chain growth is not templated by any solid support.

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4. Conclusions

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It was found that the MOF material MIL-101(Cr) can be used as an active morphology template in the catalytic polymerization of ethylene. In contrast, the isoreticular MIL-100(Cr) material is unable to reach significant ethylene polymerization activities due to the high stability of the crystallites against fragmentation. XRD studies revealed that, while MIL-100(Cr) is still crystalline after addition of the organo-aluminum cocatalyst (i.e., diethylaluminum chloride, DEA), MIL-101(Cr) completely collapses, leading to an amorphous solid material. Furthermore, in situ FT-IR spectroscopy with CO as a probe molecule and UV–vis–NIR DRS spectroscopy show that different reduced and alkylated Cr sites are formed for each MOF, further indicating different stabilities toward the cocatalyst DEA. The exact nature of the active Cr species (as in the case of the long-debated Phillips system) is not yet fully understood, but experiments with the putative homogeneous coordination complex showed trends in reactivity in the production of HDPE similar to those of MIL-101(Cr), suggesting that alkylated Cr sites are the main ethylene polymerization sites. In contrast, MIL-100(Cr) shows very low polymerization activity, mostly producing gaseous and liquid olefins instead.
SEM studies revealed that, in addition to catalyzing chain growth, MIL-101(Cr) can also act as a structure modulator for the polymer material obtained, leading to the formation of spherical micrometer-sized polyethylene beads. Again, control experiments with the homogeneous complex 1 confirm this hypothesis, as the coordination complex catalyst 1 produces interwoven polymer fibers in a random orientation. In other words, Cr-based MOF materials have the ability to act as a self-sacrificial template for structuring polymer materials while they form in the internal pore structure over active ethylene polymerization sites. We foresee that our research will foster the design of new polyolefin architectures, as well as expand our understanding of highly stable MOFs (i.e. MIL-100 and 101) upon reaction with reactive activators for the selective production of short-chain olefins, such as 1-octene and 1-hexene.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b00150.

  • Data and experimental details for the synthesis and characterization of coordination complex 1, experimental details on XRD, N2 adsorption at 77 K, TGA-MS, and SEM of the MIL-100 and MIL-101 materials, characterization of polyethylene obtained from the gas phase by XRD with MIL-101(Cr), more detailed UV–vis–NIR DRS experiments upon addition of DEA, and details on the physicochemical properties of the polymer materials obtained by DSC, GPC, and additional SEM images (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
  • Authors
    • Miguel Rivera-Torrente - Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
    • Paul D. Pletcher - Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
    • Maarten K. Jongkind - Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
    • Nikolaos Nikolopoulos - Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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T. N. Ran (Utrecht University, UU) is thanked for the ESI-MS measurements, while C. Hernández-Mejía (Utrecht University, UU) is acknowledged for help during the in situ FT-IR spectroscopy experiments. We also thank N. Friederichs (SABIC) for the GPC measurements. N. Maaskant (UU), R. Pluijm (UU), G. de Reijer (UU), and P. Dolata (UU) are acknowledged for their help with sample preparation. This project has received funding from the European Union Horizon 2020 research and innovation program under the Marie-Sklodowska-Curie grant agreement 641887 (DEFNET) as well as funding from TKI Chemistry (The Netherlands).

References

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    Radical polymn. of divinylbenzenes (DVBs) in the one-dimensional nanochannels of porous coordination polymers allows linearly extended topotactic polymn. without crosslinking. The main factors that influence this polymn. are the channel size and host framework flexibility.
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    Uemura, T.; Ono, Y.; Kitagawa, K.; Kitagawa, S. Radical Polymerization of Vinyl Monomers in Porous Coordination Polymers: Nanochannel Size Effects on Reactivity, Molecular Weight, and Stereostructure. Macromolecules 2008, 41, 87– 94,  DOI: 10.1021/ma7022217
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    Radical Polymerization of Vinyl Monomers in Porous Coordination Polymers: Nanochannel Size Effects on Reactivity, Molecular Weight, and Stereostructure
    Uemura, Takashi; Ono, Yukari; Kitagawa, Kana; Kitagawa, Susumu
    Macromolecules (Washington, DC, United States) (2008), 41 (1), 87-94CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    Radical polymn. of vinyl monomers (styrene, Me methacrylate, and vinyl acetate) was performed in various nanochannels of porous coordination polymers (PCPs), where relationships between the channel size and polymn. behaviors, such as monomer reactivity, mol. wt., and stereostructure, were studied. The capability for precise size tuning of nanospaces has afforded the first systematic study of radical polymn. in microporous channels based on PCPs. In this polymn. system, the polymer-growing radicals were remarkably stabilized by efficient suppression of termination reactions in the nanochannels, resulting in living radical polymns. with relatively narrow mol. wt. distributions. A significant nanochannel effect on the polymer stereoregularity was also seen, leading to a clear increase of isotacticity in the resulting polymers.
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    Wang, S.; Kitao, T.; Guillou, N.; Wahiduzzaman, M.; Martineau-Corcos, C.; Nouar, F.; Tissot, A.; Binet, L.; Ramsahye, N.; Devautour-Vinot, S. A.; Kitagawa, S.; Seki, S.; Tsutsui, Y.; Briois, V.; Steunou, N.; Maurin, G.; Uemura, T.; Serre, C. A phase transformable ultrastable titanium-carboxylate framework for photoconduction. Nat. Commun. 2018, 9, 1660,  DOI: 10.1038/s41467-018-04034-w
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    A phase transformable ultrastable titanium-carboxylate framework for photoconduction
    Wang Sujing; Nouar Farid; Tissot Antoine; Serre Christian; Wang Sujing; Guillou Nathalie; Martineau-Corcos Charlotte; Nouar Farid; Tissot Antoine; Steunou Nathalie; Serre Christian; Kitao Takashi; Kitagawa Susumu; Uemura Takashi; Kitao Takashi; Uemura Takashi; Kitao Takashi; Uemura Takashi; Kitao Takashi; Uemura Takashi; Wahiduzzaman Mohammad; Ramsahye Naseem; Devautour-Vinot Sabine; Maurin Guillaume; Martineau-Corcos Charlotte; Binet Laurent; Kitagawa Susumu; Seki Shu; Tsutsui Yusuke; Briois Valerie
    Nature communications (2018), 9 (1), 1660 ISSN:.
    Porous titanium oxide materials are attractive for energy-related applications. However, many suffer from poor stability and crystallinity. Here we present a robust nanoporous metal-organic framework (MOF), comprising a Ti12O15 oxocluster and a tetracarboxylate ligand, achieved through a scalable synthesis. This material undergoes an unusual irreversible thermally induced phase transformation that generates a highly crystalline porous product with an infinite inorganic moiety of a very high condensation degree. Preliminary photophysical experiments indicate that the product after phase transformation exhibits photoconductive behavior, highlighting the impact of inorganic unit dimensionality on the alteration of physical properties. Introduction of a conductive polymer into its pores leads to a significant increase of the charge separation lifetime under irradiation. Additionally, the inorganic unit of this Ti-MOF can be easily modified via doping with other metal elements. The combined advantages of this compound make it a promising functional scaffold for practical applications.
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    Takayanagi, M.; Pakhira, S.; Nagaoka, M. Control of Diffusion and Conformation Behavior of Methyl Methacrylate Monomer by Phenylene Fin in Porous Coordination Polymers. J. Phys. Chem. C 2015, 119, 27291– 27292,  DOI: 10.1021/acs.jpcc.5b09332
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    16
    Control of Diffusion and Conformation Behavior of Methyl Methacrylate Monomer by Phenylene Fin in Porous Coordination Polymers
    Takayanagi, Masayoshi; Pakhira, Srimanta; Nagaoka, Masataka
    Journal of Physical Chemistry C (2015), 119 (49), 27291-27297CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Radical polymn. in nanoscale channels of porous coordination polymers has been explored as a promising technique to control the properties of product polymers. In particular, tacticity of poly(Me methacrylate) was successfully controlled by utilizing one-dimensional channels of [M2(L)2TED]n type of porous coordination polymers by changing the dicarboxylate ligand L. Toward understanding the atomistic mechanism of this tacticity control, we computationally studied the behavior of Me methacrylate monomers in the one-dimensional channels by mol. dynamics simulations. Smooth monomer diffusion in the direction along the channel was shown as expected from the channel shape. In addn., we confirmed that the monomers can pass through the narrow apertures between the channels and can diffuse slowly in the direction perpendicular to the channel. In the channels, the ratio of the s-trans and s-cis conformations of the monomers is different from that in monomer liq. We found two factors affecting these behaviors: the host-guest electrostatic interactions and the anisotropic shape of the channels by the planar and nonplanar dicarboxylate ligands (L). These factors will contribute to understanding of the tacticity modification mechanism by different ligands from the atomistic point of view.
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    Kitao, T.; Zhang, Y.; Kitagawa, S.; Wang, B.; Uemura, T. Hybridization of MOFs and Polymers. Chem. Soc. Rev. 2017, 46, 3108– 3133,  DOI: 10.1039/C7CS00041C
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    Hybridization of MOFs and polymers
    Kitao, Takashi; Zhang, Yuanyuan; Kitagawa, Susumu; Wang, Bo; Uemura, Takashi
    Chemical Society Reviews (2017), 46 (11), 3108-3133CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Metal-org. frameworks (MOFs) have received much attention because of their attractive properties. They show great potential applications in many fields. An emerging trend in MOF research is hybridization with flexible materials, which is the subject of this review. Polymers possess a variety of unique attributes, such as softness, thermal and chem. stability, and optoelec. properties that can be integrated with MOFs to make hybrids with sophisticated architectures. Hybridization of MOFs and polymers is producing new and versatile materials that exhibit peculiar properties hard to realize with the individual components. This review article focuses on the methodol. for hybridization of MOFs and polymers, as well as the intriguing functions of hybrid materials.
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    Zhou, H.-C.; Long, J. R.; Yaghi, O. M. Introduction to Metal–Organic Frameworks. Chem. Rev. 2012, 112, 673– 674,  DOI: 10.1021/cr300014x
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    Introduction to Metal-Organic Frameworks
    Zhou, Hong-Cai; Long, Jeffrey R.; Yaghi, Omar M.
    Chemical Reviews (Washington, DC, United States) (2012), 112 (2), 673-674CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review is presented on prepn., structure and application of Metal-Org. Frameworks.
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    Corma, A.; García, H.; Llabrés i Xamena, F. X. Engineering Metal Organic Frameworks for Heterogeneous Catalysis. Chem. Rev. 2010, 110, 4606– 4655,  DOI: 10.1021/cr9003924
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    Engineering Metal Organic Frameworks for Heterogeneous Catalysis
    Corma, A.; Garcia, H.; Llabres i Xamena, F. X.
    Chemical Reviews (Washington, DC, United States) (2010), 110 (8), 4606-4655CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review, including the design of MOFs for catalysis and evaluation of its potential as catalyst as well as catalysis by MOFs with active metal sites, with reactive functional groups, and MOFs as host matrixes or nanometric reaction cavities.
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    Rogge, S. M. J.; Bavykina, A.; Hajek, J.; Garcia, H.; Olivos-Suarez, A. I.; Sepulveda-Escribano, A.; Vimont, A.; Clet, G.; Bazin, P.; Kapteijn, F.; Daturi, M.; Ramos-Fernandez, E. V.; Llabres i Xamena, F. X.; Van Speybroeck, V.; Gascon, J. Metal-organic and Covalent Organic Frameworks as Single-site Catalysts. Chem. Soc. Rev. 2017, 46, 3134– 3184,  DOI: 10.1039/C7CS00033B
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    Metal-organic and covalent organic frameworks as single-site catalysts
    Rogge, S. M. J.; Bavykina, A.; Hajek, J.; Garcia, H.; Olivos-Suarez, A. I.; Sepulveda-Escribano, A.; Vimont, A.; Clet, G.; Bazin, P.; Kapteijn, F.; Daturi, M.; Ramos-Fernandez, E. V.; Llabres i Xamena, F. X.; Van Speybroeck, V.; Gascon, J.
    Chemical Society Reviews (2017), 46 (11), 3134-3184CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    Heterogeneous single-site catalysts consist of isolated, well-defined, active sites that are spatially sepd. in a given solid and, ideally, structurally identical. In this review, the potential of metal-org. frameworks (MOFs) and covalent org. frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly. In the first part of this article, synthetic strategies and progress in the implementation of such sites in these two classes of materials are discussed. Because these solids are excellent playgrounds to allow a better understanding of catalytic functions, we highlight the most important recent advances in the modeling and spectroscopic characterization of single-site catalysts based on these materials. Finally, we discuss the potential of MOFs as materials in which several single-site catalytic functions can be combined within one framework along with their potential as powerful enzyme-mimicking materials. The review is wrapped up with our personal vision on future research directions.
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    Cohen, S. M.; Zhang, Z.; Boissonnault, J. A. Toward “metalloMOFzymes”: Metal–Organic Frameworks with Single-Site Metal Catalysts for Small-Molecule Transformations. Inorg. Chem. 2016, 55, 7281– 7290,  DOI: 10.1021/acs.inorgchem.6b00828
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    Toward "metalloMOFzymes": Metal-Organic Frameworks with Single-Site Metal Catalysts for Small-Molecule Transformations
    Cohen, Seth M.; Zhang, Zhenjie; Boissonnault, Jake A.
    Inorganic Chemistry (2016), 55 (15), 7281-7290CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Metal-org. frameworks (MOFs) are being increasingly studied as scaffolds and supports for catalysis. The solid-state structures of MOFs, combined with their high porosity, suggest that MOFs may possess advantages shared by both heterogeneous and homogeneous catalysts, with few of the shortcomings of either. Herein, efforts to create single-site catalytic metal centers appended to the org. ligand struts of MOFs are discussed. Reactions important for advanced energy applications, such as H2 prodn. and CO2 redn., will be reviewed. Examg. how these active sites can be introduced, their performance, and their existing limitations should provide direction for design of the next generation of MOF-based catalysts for energy-relevant, small-mol. transformations. Finally, the introduction of second-sphere interactions (e.g., hydrogen bonding via squaramide groups) as a possible route to enhancing the activity of these metal centers is reported.
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    Kissin, Y. V. Polyethylene, linear low density (LLDPE). In Kirk-Othmer Encyclopedia of Chemical Technology, 5th ed.; Wiley: New York, 2006; Vol. 20, pp 179– 211.
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    Lappin, G. R.; Nemec, L. H.; Sauer, J. D.; Wagner, J. D. Higher Olefins. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley: New York, 2000; Vol. 17, pp 709– 723.
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    Maraschin, N., Polyethylene, high density (HDPE). In Kirk-Othmer Encyclopedia of Chemical Technology, 5th ed.; Wiley: New York, 2006; Vol. 20, pp 211– 239.
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    Lieberman, R. B.; Dorini, M.; Mei, G.; Rinaldi, R.; Penzo, G.; Berge, G. T. Polypropylene. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley: New York, 2000; Vol. 17, pp 1– 15.
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    Canivet, J.; Aguado, S.; Schuurman, Y.; Farrusseng, D. MOF-Supported Selective Ethylene Dimerization Single-Site Catalysts through One-Pot Postsynthetic Modification. J. Am. Chem. Soc. 2013, 135, 4195– 4198,  DOI: 10.1021/ja312120x
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    MOF-Supported Selective Ethylene Dimerization Single-Site Catalysts through One-Pot Postsynthetic Modification
    Canivet, Jerome; Aguado, Sonia; Schuurman, Yves; Farrusseng, David
    Journal of the American Chemical Society (2013), 135 (11), 4195-4198CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The one-pot postfunctionalization allows anchoring a mol. nickel complex into a mesoporous metal-org. framework (Ni@(Fe)MIL-101). It is generating a very active and reusable catalyst for the liq.-phase ethylene dimerization to selectively form 1-butene. Higher selectivity for 1-butene is found using the Ni@(Fe)MIL-101 catalyst than reported for mol. nickel diimino complexes.
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    Madrahimov, S. T.; Gallagher, J. R.; Zhang, G.; Meinhart, Z.; Garibay, S. J.; Delferro, M.; Miller, J. T.; Farha, O. K.; Hupp, J. T.; Nguyen, S. T. Gas-Phase Dimerization of Ethylene under Mild Conditions Catalyzed by MOF Materials Containing (bpy)NiII Complexes. ACS Catal. 2015, 5, 6713– 6718,  DOI: 10.1021/acscatal.5b01604
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    28
    Gas-Phase Dimerization of Ethylene under Mild Conditions Catalyzed by MOF Materials Containing (bpy)NiII Complexes
    Madrahimov, Sherzod T.; Gallagher, James R.; Zhang, Guanghui; Meinhart, Zachary; Garibay, Sergio J.; Delferro, Massimiliano; Miller, Jeffrey T.; Farha, Omar K.; Hupp, Joseph T.; Nguyen, SonBinh T.
    ACS Catalysis (2015), 5 (11), 6713-6718CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    NU-1000-(bpy)NiII, a highly porous MOF material possessing well-defined (bpy)NiII moieties, was prepd. through solvent-assisted ligand incorporation (SALI). Treatment with Et2AlCl affords a single-site catalyst with excellent catalytic activity for ethylene dimerization (intrinsic activity for butenes that is up to an order of magnitude higher than the corresponding (bpy)NiCl2 homogeneous analog) and stability (can be reused at least three times). The high porosity of this catalyst results in outstanding levels of activity at ambient temp. in gas-phase ethylene dimerization reactions, both under batch and continuous flow conditions.
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    Metzger, E. D.; Brozek, C. K.; Comito, R. J.; Dinca, M. Selective Dimerization of Ethylene to 1-Butene with a Porous Catalyst. ACS Cent. Sci. 2016, 2, 148– 153,  DOI: 10.1021/acscentsci.6b00012
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    Selective Dimerization of Ethylene to 1-Butene with a Porous Catalyst
    Metzger, Eric D.; Brozek, Carl K.; Comito, Robert J.; Dinca, Mircea
    ACS Central Science (2016), 2 (3), 148-153CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
    Current heterogeneous catalysts lack the fine steric and electronic tuning required for catalyzing the selective dimerization of ethylene to 1-butene, which remains one of the largest industrial processes still catalyzed by homogeneous catalysts. Here, we report that a metal-org. framework catalyzes ethylene dimerization with a combination of activity and selectivity for 1-butene that is premier among heterogeneous catalysts. The capacity for mild cation exchange in the material MFU-4l (MFU-4l = Zn5Cl4(BTDD)3, H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin) was leveraged to create a well-defined and site-isolated Ni(II) active site bearing close structural homol. to mol. tris-pyrazolylborate complexes. In the presence of ethylene and methylaluminoxane, the material consumes ethylene at a rate of 41,500 mol per mol of Ni per h with a selectivity for 1-butene of up to 96.2%, exceeding the selectivity reported for the current industrial dimerization process.
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    Metzger, E. D.; Comito, R. J.; Hendon, C. H.; Dinca, M. Mechanism of Single-Site Molecule-Like Catalytic Ethylene Dimerization in Ni-MFU-4l. J. Am. Chem. Soc. 2017, 139, 757– 762,  DOI: 10.1021/jacs.6b10300
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    30
    Mechanism of Single-Site Molecule-Like Catalytic Ethylene Dimerization in Ni-MFU-4l
    Metzger, Eric D.; Comito, Robert J.; Hendon, Christopher H.; Dinca, Mircea
    Journal of the American Chemical Society (2017), 139 (2), 757-762CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A recently developed metal-org. framework (MOF) catalyst for the dimerization of ethylene has a combination of selectivity and activity that surpasses that of com. homogeneous catalysts, which have dominated this important industrial process for nearly 50 years. The uniform catalytic sites available in MOFs provide a unique opportunity to directly study reaction mechanisms in heterogeneous catalysts, a problem typically intractable due to the multiplicity of coordination environments found in many solid catalysts. In this work, we use a combination of isotopic labeling studies, mechanistic probes, and DFT calcns. to demonstrate that Ni-MFU-4l operates via the Cossee-Arlman mechanism, which has also been implicated in homogeneous late transition metal catalysts. These studies demonstrate that metal nodes in MOFs mimic homogeneous catalysts not just functionally, but also mechanistically. They provide a blueprint for the development of advanced heterogeneous catalysts with similar degrees of tunability to their homogeneous counterparts.
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    Pellizzeri, S.; Barona, M.; Bernales, V.; Miró, P.; Liao, P.; Gagliardi, L.; Snurr, R. Q.; Getman, R. B. Catalytic Descriptors and Electronic Properties of Single-site Catalysts for Ethene Dimerization to 1-butene. Catal. Today 2018, 312, 149– 157,  DOI: 10.1016/j.cattod.2018.02.024
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    31
    Catalytic descriptors and electronic properties of single-site catalysts for ethene dimerization to 1-butene
    Pellizzeri, Steven; Barona, Melissa; Bernales, Varinia; Miro, Pere; Liao, Peilin; Gagliardi, Laura; Snurr, Randall Q.; Getman, Rachel B.
    Catalysis Today (2018), 312 (), 149-157CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    Six first-row transition metal cations (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) were evaluated as catalysts for ethene dimerization to 1-butene. This is an important reaction in the chem. of C-C bond formation and in the conversion of natural gas to higher hydrocarbons. Two related classes of transition metal cation catalysts were investigated: (1) single transition metal cations supported on zirconium oxide nodes of the metal-org. framework NU-1000 and (2) small metal hydroxide clusters with two metal atoms (M2) that could be grown by at. layer deposition on a support exhibiting isolated hydroxyl groups. Using scaling relations, the free energies of co-adsorbed hydrogen and ethene (i.e., (H/C2H4)*) and adsorbed Et (i.e., C2H5*) were identified as descriptors for ethene dimerization catalysis. Using degree of rate control anal., it was detd. that the rate controlling steps are either ethene insertion (C-C bond forming) or β-hydride elimination (C-H bond breaking), depending on the metal. Using degree of catalyst control anal., it was detd. that activity on all the catalysts studied could be improved by tuning the free energy of C2H5*.
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    Ye, J.; Gagliardi, L.; Cramer, C. J.; Truhlar, D. G. Single Ni Atoms and Ni4 Clusters have Similar Catalytic Activity for Ethylene Dimerization. J. Catal. 2017, 354, 278– 286,  DOI: 10.1016/j.jcat.2017.08.011
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    Single Ni atoms and Ni4 clusters have similar catalytic activity for ethylene dimerization
    Ye, Jingyun; Gagliardi, Laura; Cramer, Christopher J.; Truhlar, Donald G.
    Journal of Catalysis (2017), 354 (), 278-286CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    Atomic layer deposition (ALD) of Ni on the metal-org. framework NU-1000 has been shown to generate a material that serves as a catalyst for ethylene dimerization. However, the precise nature of the active catalytic site or sites remains uncertain. Here the authors employ periodic d. functional calcns. to characterize the structure and reactivity of the deposited species. Optimized lattice consts. for a sequence of structures incorporating successively more Ni4-hydroxo clusters in the c pore of NU-1000 show good agreement with exptl. trends involving multiple ALD cycles; therefore the authors study the catalytic cycle for this cluster in detail, and the authors compare it to that for a site with only a single Ni atom. Both the at. Ni catalyst and the Ni4-hydroxo cluster have higher catalytic activity in the singlet state than in the triplet state. Also the two catalysts have very similar activity. Thus, precise size control of the active catalytic species is not essential for ethylene dimerization in this system.
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  33. 33
    Bernales, V.; League, A. B.; Li, Z.; Schweitzer, N. M.; Peters, A. W.; Carlson, R. K.; Hupp, J. T.; Cramer, C. J.; Farha, O. K.; Gagliardi, L. Computationally Guided Discovery of a Catalytic Cobalt-Decorated Metal–Organic Framework for Ethylene Dimerization. J. Phys. Chem. C 2016, 120, 23576– 23583,  DOI: 10.1021/acs.jpcc.6b07362
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    33
    Computationally Guided Discovery of a Catalytic Cobalt-Decorated Metal-Organic Framework for Ethylene Dimerization
    Bernales, Varinia; League, Aaron B.; Li, Zhanyong; Schweitzer, Neil M.; Peters, Aaron W.; Carlson, Rebecca K.; Hupp, Joseph T.; Cramer, Christopher J.; Farha, Omar K.; Gagliardi, Laura
    Journal of Physical Chemistry C (2016), 120 (41), 23576-23583CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The catalytic performance of a cobalt(II) single-site catalyst supported on the zirconia-like nodes of the metal org.-framework (MOF) NU-1000 is herein characterized by quantum chem. methods and compared to an iso-structural analog incorporating nickel(II) as the active transition metal. The mechanisms of at. layer deposition in MOFs and of catalysis are examd. using d. functional theory. We compare the catalytic activity of Co and Ni installed on the zirconia-like nodes for ethylene dimerization, considering three plausible pathways. Multiconfigurational wave function theory methods are employed to further characterize the electronic structures of key transition states and intermediates. Finally, we report confirmation of Co catalytic activity for ethylene dimerization from expts. that were prompted by the computational prediction.
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    Liu, B.; Jie, S.; Bu, Z.; Li, B.-G. A MOF-supported Chromium Catalyst for Ethylene Polymerization through Post-synthetic Modification. J. Mol. Catal. A: Chem. 2014, 387, 63– 68,  DOI: 10.1016/j.molcata.2014.02.028
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    A MOF-supported chromium catalyst for ethylene polymerization through post-synthetic modification
    Liu, Bing; Jie, Suyun; Bu, Zhiyang; Li, Bo-Geng
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    Isoreticular metal-org. framework-3 (IRMOF-3) has been post-synthetically modified to generate a Cr(III)-based heterogeneous catalyst (IRMOF-3-SI-Cr) for ethylene polymn., which has been characterized by a variety of phys. methods. The x-ray diffraction anal. indicated that the structure integrity of the final solid was preserved after the functionalization with the imine and the subsequent coordination to chromium. The BET surface area of the final solid was slightly reduced as detd. by N2 adsorption-desorption expts. The material exhibited a unique behavior for ethylene polymn. upon activation with various alkylaluminium co-catalysts, and the polyethylenes formed featured high mol. wts. and broad mol. wt. distributions.
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    Ji, P.; Solomon, J. B.; Lin, Z.; Johnson, A.; Jordan, R. F.; Lin, W. Transformation of Metal–Organic Framework Secondary Building Units into Hexanuclear Zr-Alkyl Catalysts for Ethylene Polymerization. J. Am. Chem. Soc. 2017, 139, 11325– 11328,  DOI: 10.1021/jacs.7b05761
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    Transformation of Metal-Organic Framework Secondary Building Units into Hexanuclear Zr-Alkyl Catalysts for Ethylene Polymerization
    Ji, Pengfei; Solomon, Joseph B.; Lin, Zekai; Wilders, Alison M.; Jordan, Richard F.; Lin, Wenbin
    Journal of the American Chemical Society (2017), 139 (33), 11325-11328CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Authors report the stepwise and quant. transformation of the Zr6(μ3-O)4(μ3-OH)4(HCO2)6 nodes in Zr-BTC (MOF-808) to the [Zr6(μ3-O)4(μ3-OH)4Cl12]6- nodes in ZrCl2-BTC, and then to the organometallic [Zr6(μ3-O)4(μ3-OLi)4R12]6- nodes in ZrR2-BTC (R = CH2SiMe3 or Me). Activation of ZrCl2-BTC with MMAO-12 generates ZrMe-BTC, which is an efficient catalyst for ethylene polymn. ZrMe-BTC displays unusual electronic and steric properties compared to homogeneous Zr catalysts, possesses multimetallic active sites, and produces high-mol.-wt. linear polyethylene. Metal-org. framework nodes can thus be directly transformed into novel single-site solid organometallic catalysts without homogeneous analogs for polymn. reactions.
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    Comito, R. J.; Fritzsching, K. J.; Sundell, B. J.; Schmidt-Rohr, K.; Dinca, M. Single-Site Heterogeneous Catalysts for Olefin Polymerization Enabled by Cation Exchange in a Metal-Organic Framework. J. Am. Chem. Soc. 2016, 138, 10232– 10237,  DOI: 10.1021/jacs.6b05200
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    Single-Site Heterogeneous Catalysts for Olefin Polymerization Enabled by Cation Exchange in a Metal-Organic Framework
    Comito, Robert J.; Fritzsching, Keith J.; Sundell, Benjamin J.; Schmidt-Rohr, Klaus; Dinca, Mircea
    Journal of the American Chemical Society (2016), 138 (32), 10232-10237CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The manuf. of advanced polyolefins was critically enabled by the development of single-site heterogeneous catalysts. Metal-org. frameworks (MOFs) show great potential as heterogeneous catalysts that may be designed and tuned on the mol. level. Exchange of zinc ions in Zn5Cl4(BTDD)3, (H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin) (MFU-4l) with reactive metals serves to establish a general platform for selective olefin polymn. in a high surface area solid promising for industrial catalysis. Characterization of polyethylene produced by these materials demonstrates both mol. and morphol. control. Notably, reactivity approaches single-site catalysis, as evidenced by low polydispersity indexes, and good mol. wt. control. Further these new catalysts copolymerize ethylene and propylene. Uniform growth of the polymer around the catalyst particles provides a mechanism for controlling the polymer morphol., a relevant metric for continuous flow processes.
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    Klet, R. C.; Tussupbayev, S.; Borycz, J.; Gallagher, J. R.; Stalzer, M. M.; Miller, J. T.; Gagliardi, L.; Hupp, J. T.; Marks, T. J.; Cramer, C. J.; Delferro, M.; Farha, O. K. Single-Site Organozirconium Catalyst Embedded in a Metal–Organic Framework. J. Am. Chem. Soc. 2015, 137, 15680– 15683,  DOI: 10.1021/jacs.5b11350
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    Single-Site Organozirconium Catalyst Embedded in a Metal-Organic Framework
    Klet, Rachel C.; Tussupbayev, Samat; Borycz, Joshua; Gallagher, James R.; Stalzer, Madelyn M.; Miller, Jeffrey T.; Gagliardi, Laura; Hupp, Joseph T.; Marks, Tobin J.; Cramer, Christopher J.; Delferro, Massimiliano; Farha, Omar K.
    Journal of the American Chemical Society (2015), 137 (50), 15680-15683CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A structurally well-defined mesoporous Hf-based metal-org. framework (Hf-NU-1000) is employed as a well-defined scaffold for a highly electrophilic single-site d0 Zr-benzyl catalytic center. This new material Hf-NU-1000-ZrBn is fully characterized by a variety of spectroscopic techniques and DFT computation. Hf-NU-1000-ZrBn is a promising single-component catalyst (i.e., not requiring a catalyst/activator) for ethylene and stereoregular 1-hexene polymn.
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    Comito, R. J.; Metzger, E. D.; Wu, Z.; Zhang, G.; Hendon, C. H.; Miller, J. T.; Dinca, M. Selective Dimerization of Propylene with Ni-MFU-4l. Organometallics 2017, 36, 1681– 1683,  DOI: 10.1021/acs.organomet.7b00178
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    Selective Dimerization of Propylene with Ni-MFU-4l
    Comito, Robert J.; Metzger, Eric D.; Wu, Zhenwei; Zhang, Guanghui; Hendon, Christopher H.; Miller, Jeffrey T.; Dinca, Mircea
    Organometallics (2017), 36 (9), 1681-1683CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    We report the selective dimerization of propylene to branched hexenes using Ni-MFU-4l, a solid catalyst prepd. by cation exchange. Anal. of the resulting product distribution demonstrates that the selectivity arises from 2,1-insertion and slow product reinsertion, mechanistic features reproduced by a mol. nickel tris-pyrazolylborate catalyst. Characterization of Ni-MFU-4l by X-ray absorption spectroscopy provides evidence for discrete, tris-pyrazolylborate-like coordination of nickel, underscoring the small-mol. analogy that can be made at metal-org. framework nodes.
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    Mlinar, A. N.; Keitz, B. K.; Gygi, D.; Bloch, E. D.; Long, J. R.; Bell, A. T. Selective Propene Oligomerization with Nickel(II)-Based Metal–Organic Frameworks. ACS Catal. 2014, 4, 717– 721,  DOI: 10.1021/cs401189a
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    Selective Propene Oligomerization with Nickel(II)-Based Metal-Organic Frameworks
    Mlinar, Anton N.; Keitz, Benjamin K.; Gygi, David; Bloch, Eric D.; Long, Jeffrey R.; Bell, Alexis T.
    ACS Catalysis (2014), 4 (3), 717-721CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Two Ni2+-contg. metal-org. frameworks, Ni2(dobdc) and Ni2(dobpdc), are active for the oligomerization of propene in the gas phase. The metal-org. frameworks exhibit activity comparable to Ni2+-exchanged aluminosilicates but maintain high selectivity for linear oligomers. Thus, these frameworks should enable the high yielding synthesis of linear propene oligomers for use in detergent and diesel fuel applications.
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    Dubey, R. J. C.; Comito, R. J.; Wu, Z.; Zhang, G.; Rieth, A. J.; Hendon, C. H.; Miller, J. T.; Dinca, M. Highly Stereoselective Heterogeneous Diene Polymerization by Co-MFU-4l: A Single-Site Catalyst Prepared by Cation Exchange. J. Am. Chem. Soc. 2017, 139, 12664– 12669,  DOI: 10.1021/jacs.7b06841
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    Highly Stereoselective Heterogeneous Diene Polymerization by Co-MFU-4l: A Single-Site Catalyst Prepared by Cation Exchange
    Dubey, Romain J.-C.; Comito, Robert J.; Wu, Zhenwei; Zhang, Guanghui; Rieth, Adam J.; Hendon, Christopher H.; Miller, Jeffrey T.; Dinca, Mircea
    Journal of the American Chemical Society (2017), 139 (36), 12664-12669CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Mol. catalysts offer tremendous advantages for stereoselective polymn. because their activity and selectivity can be optimized and understood mechanistically using the familiar tools of organometallic chem. Yet, this exquisite control over selectivity comes at an operational price that is generally not justifiable for the large-scale manuf. of polyolefins. In this report we identify Co-MFU-4l, prepd. by cation exchange in a metal-org. framework, as a solid catalyst for the polymn. of 1,3-butadiene with high stereoselectivity (>99 % 1,4-cis). To our knowledge, this is the highest stereoselectivity achieved with a heterogeneous catalyst for this transformation. The polymer's low polydispersity (PDI ∼2) and the catalyst's ready recovery and low leaching indicate that our material is a structurally resilient single-site heterogeneous catalyst. Further characterization of Co-MFU-4l by X-ray absorption spectroscopy provided evidence for discrete, tris-pyrazolylborate-like coordination of Co(II). With this information, we identify a sol. cobalt complex that mimics the structure and reactivity of Co-MFU-4l, thus providing a well-defined platform for studying the catalytic mechanism in the soln. phase. This work underscores the capacity for small-mol. like tunability and mechanistic tractability available to transition metal catalysis in metal-org. frameworks.
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    Vitorino, M. J.; Devic, T.; Tromp, M.; Férey, G.; Visseaux, M. Lanthanide Metal Organic Frameworks as Ziegler–Natta Catalysts for the Selective Polymerization of Isoprene. Macromol. Chem. Phys. 2009, 210, 1923– 1932,  DOI: 10.1002/macp.200900354
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    Liu, S.; Zhang, Y.; Han, Y.; Feng, G.; Gao, F.; Wang, H.; Qiu, P. Selective Ethylene Oligomerization with Chromium-Based Metal–Organic Framework MIL-100 Evacuated under Different Temperatures. Organometallics 2017, 36, 632– 638,  DOI: 10.1021/acs.organomet.6b00834
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    Selective Ethylene Oligomerization with Chromium-Based Metal-Organic Framework MIL-100 Evacuated under Different Temperatures
    Liu, Suyan; Zhang, Ying; Han, Yang; Feng, Guangliang; Gao, Fei; Wang, Hui; Qiu, Ping
    Organometallics (2017), 36 (3), 632-638CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    MIL-100(Cr) was synthesized and evacuated under different temps. to generate a series of heterogeneous catalysts for ethylene oligomerization. These catalysts showed moderate catalytic activities for ethylene oligomerization but high selectivities to low carbon olefins C6, C8, and C10. Moreover, the oligomer distribution was different depending on the evacuation temp. The XPS results showed the redn. of some CrIII active sites in the MIL-100(Cr) structure to CrII active sites, which made the catalysts show polymn. activities. The MIL-100(Cr)-250 catalyst evacuated at 250° exhibited the highest oligomerization and polymn. activities up to 9.27 × 105 g/(molCr·h) and 0.99 × 105 g/(molCr·h), resp. The oligomerization selectivity to low carbon olefins C6, C8, and C10 was ∼99%. The byproduct polymer from MIL-100(Cr)-250 belonged to linear polyethylene with ultrahigh mol. wt. and broad mol. wt. distributions. MOFs contg. coordinatively unsatd. metal sites might be a promising selective catalyst for ethylene slurry oligomerization.
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    Férey, G.; Serre, C.; Mellot-Draznieks, C.; Millange, F.; Surblé, S.; Dutour, J.; Margiolaki, I. A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction. Angew. Chem., Int. Ed. 2004, 43, 6296– 6301,  DOI: 10.1002/anie.200460592
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    Molecular modeling: A hybrid solid with giant pores prepared by a combination of targeted chemistry, simulation, and powder diffraction
    Ferey, Gerard; Serre, Christian; Mellot-Draznieks, Caroline; Millange, Franck; Surble, Suzy; Dutour, Julien; Margiolaki, Irene
    Angewandte Chemie, International Edition (2004), 43 (46), 6296-6301CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The assocn. of a chromium(III) trimeric building unit and 1,3,5-benzenetricarboxylate led to the powd. solid MIL-100. Simulations provided a crystal structure soln., which matched the exptl. powder XRD pattern. This unique simulation/diffraction combination allowed the structure detn. of a giant-pore solid with a zeotype architecture, built up from hybrid supertetrahedra.
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    Férey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.; Surblé, S.; Margiolaki, I. A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area. Science 2005, 309, 2040– 2042,  DOI: 10.1126/science.1116275
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    A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area
    Ferey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.; Surble, S.; Margiolaki, I.
    Science (Washington, DC, United States) (2005), 309 (5743), 2040-2042CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    We combined targeted chem. and computational design to create a crystal structure for porous chromium terephthalate, MIL-101, with very large pore sizes and surface area. Its zeotype cubic structure has a giant cell vol. (∼702,000 cubic angstroms), a hierarchy of extra-large pore sizes (∼30 to 34 angstroms), and a Langmuir surface area for N2 of ∼5900 ± 300 square meters per g. Beside the usual properties of porous compds., this solid has potential as a nanomold for monodisperse nanomaterials, as illustrated here by the incorporation of Keggin polyanions within the cages.
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    Harton, A.; Nagi, M. K.; Glass, M. M.; Junk, P. C.; Atwood, J. L.; Vincent, J. B. Synthesis and Characterization of Symmetric and Unsymmetric Oxo-bridged Trinuclear Chromium Benzoate Complexes: Crystal and Molecular Structure of [Cr3O(O2CPh)6(py)3]ClO4. Inorg. Chim. Acta 1994, 217, 171– 179,  DOI: 10.1016/0020-1693(93)03766-4
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    Synthesis and characterization of symmetric and unsymmetric oxo-bridged trinuclear chromium benzoate complexes: crystal and molecular structure of [Cr3O(O2CPh)6(py)3]ClO4
    Harton, Anthony; Nagi, Maysa K.; Glass, Miriam M.; Junk, Peter C.; Atwood, Jerry L.; Vincent, John B.
    Inorganica Chimica Acta (1994), 217 (1-2), 171-9CODEN: ICHAA3; ISSN:0020-1693.
    Techniques were developed for the synthesis of [Cr3O(O2CR)6(L)3]+ (R = Ph or tolyl and L = H2O or py) in nonaq. solvents. Addnl., the synthesis of an unsym. trinuclear complex, [Cr3O(OBz)6(OH)(py)2], is reported. These complexes were characterized by a no. of spectroscopic and magnetic techniques including x-ray crystallog., electronic and IR spectrophotometry, NMR and EPR spectroscopy, mass spectrometry, and soln. and solid state susceptibility measurements. [Cr3O(OBz)6(py)3]ClO4 crystallizes in the hexagonal space group P‾6, a 13.387(4), c 19.186(6) Å, Z = 2, R = 7.6 and Rw = 10.1.
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    Lebedev, O. I.; Millange, F.; Serre, C.; Van Tendeloo, G.; Férey, G. First Direct Imaging of Giant Pores of the Metal–Organic Framework MIL-101. Chem. Mater. 2005, 17, 6525– 6527,  DOI: 10.1021/cm051870o
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    First Direct Imaging of Giant Pores of the Metal-Organic Framework MIL-101
    Lebedev, O. I.; Millange, F.; Serre, C.; Van Tendeloo, G.; Ferey, G.
    Chemistry of Materials (2005), 17 (26), 6525-6527CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Cubic type metal-org. framework MIL-101 exhibits unprecedented features: a mesoporous zeotype architecture, a giant cell vol., a hierarchy of extra-large pore size, and a record sorption capacity. This paper presents the first direct imaging of its giant pore.
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    Vimont, A.; Goupil, J. M.; Lavalley, J. C.; Daturi, M.; Surblé, S.; Serre, C.; Millange, F.; Férey, G.; Audebrand, N. Investigation of Acid Sites in a Zeotypic Giant Pores Chromium(III) Carboxylate. J. Am. Chem. Soc. 2006, 128, 3218– 3227,  DOI: 10.1021/ja056906s
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    Investigation of Acid Sites in a Zeotypic Giant Pores Chromium(III) Carboxylate
    Vimont, Alexandre; Goupil, Jean-Michel; Lavalley, Jean-Claude; Daturi, Marco; Surble, Suzy; Serre, Christian; Millange, Franck; Ferey, Gerard; Audebrand, Nathalie
    Journal of the American Chemical Society (2006), 128 (10), 3218-3227CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A study of the zeotypic giant pores chromium(III) tricarboxylate CrIII3OFx(OH)1-x(H2O)2·{C6H3-(CO2)3}2·nH2O (MIL-100) has been performed. First, its thermal behavior, studied by X-ray thermodiffractometry and IR spectroscopy, indicates that the departure of water occurs without any pore contraction and no loss in crystallinity, which confirms the robustness of the framework. In a second step, IR spectroscopy has shown the presence of three distinct types of hydroxy groups depending on the outgassing conditions; first, at high temps. (573 K), only Cr-OH groups with a medium Bronsted acidity are present; at lower temps., two types of Cr-H2O terminal groups are obsd.; and at room temp., their relatively high Bronsted acidity allows them to combine with H-bonded water mols. Finally, a CO sorption study has revealed that at least three Lewis acid sites are present in MIL-100 and that fluorine atoms are located on a terminal position on the trimers of octahedra. A first result of grafting of methanol mols. acting as basic org. mols. on the chromium sites has also been shown, opening the way for a postsynthesis functionalization of MIL-100.
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    Shufler, S. L.; Sternberg, H. W.; Friedel, R. A. Infrared Spectrum and Structure of Chromium Hexacarbonyl, Cr(CO)6. J. Am. Chem. Soc. 1956, 78, 2687– 2688,  DOI: 10.1021/ja01593a008
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    Infrared spectrum and structure of chromium hexacarbonyl, Cr(CO)6
    Shufler, S. Leonard; Sternberg, Heinz W.; Friedel, R. A.
    Journal of the American Chemical Society (1956), 78 (), 2687-8CODEN: JACSAT; ISSN:0002-7863.
    The infrared spectrum of Cr(CO)6 indicates only one intense C:O stretching frequency in the vapor phase. This constitutes strong evidence for the equivalence of all C:O bands and a regular octahedral structure for Cr(CO)6 and other metal hexacarbonyl. These results provide an exptl. means for detecting metal hexacarbonyl in metal carbonyl reaction.
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    Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A. The Structure of Active Centers and the Ethylene Polymerization Mechanism on the Cr/SiO2 Catalyst: A Frontier for the Characterization Methods. Chem. Rev. 2005, 105, 115– 154,  DOI: 10.1021/cr040083s
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    The Structure of Active Centers and the Ethylene Polymerization Mechanism on the Cr/SiO2 Catalyst: A Frontier for the Characterization Methods
    Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A.
    Chemical Reviews (Washington, DC, United States) (2005), 105 (1), 115-183CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review on progress in the understanding of structures active sites and strategies and techniques of study of the the catalyst under working conditions. Methods adopted for studying Cr/SiO2 catalyst cab be extended to other catalytic systems. Surface chem. of silica support was discussed. Literature on Phillips catalyst until 1985 was reviewed.
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    Groppo, E.; Lamberti, C.; Cesano, F.; Zecchina, A. On the Fraction of CrII Sites involved in the C2H4 Polymerization on the Cr/SiO2 Phillips Catalyst: A Quantification by FTIR Spectroscopy. Phys. Chem. Chem. Phys. 2006, 8, 2453– 2456,  DOI: 10.1039/b604515d
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    On the fraction of CrII sites involved in the C2H4 polymerization on the Cr/SiO2 Phillips catalyst: a quantification by FTIR spectroscopy
    Groppo, E.; Lamberti, C.; Cesano, F.; Zecchina, A.
    Physical Chemistry Chemical Physics (2006), 8 (21), 2453-2456CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    An estn. of the fraction of CrII sites involved in the C2H4 polymn. on a CrII/SiO2 Phillips catalyst has been obtained by means of in situ alternated CO adsorption and C2H4 polymn. FTIR expts.: about 28% of the total surface sites react fast with C2H4, while a lower fraction, which depends upon the temp. reaction conditions, is more slowly involved, in agreement with XANES results.
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    Kohler, S. D.; Ekerdt, J. G. Infrared Spectroscopic Characterization of Chromium Carbonyl Species Formed by Ultraviolet Photoreduction of Silica-Supported Chromium(VI) in Carbon Monoxide. J. Phys. Chem. 1994, 98, 4336– 4342,  DOI: 10.1021/j100067a021
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  • Abstract

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    Figure 1

    Figure 1. XRD patterns of (a) MIL-100(Cr) and (b) MIL-101(Cr) as synthesized (red), after contact with 125 equiv of DEA (Al:Cr = 100) (blue), and after ethylene was flowed at 298 K for 15 min (pink). The asterisk (*) indicates the formation of some high-density polyethylene (HDPE). Insets show the area with the main XRD reflections of the crystalline MOF.

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    Figure 2

    Figure 2. FT-IR spectra with CO as a probe molecule measured at 85 K of (a–c) MIL-100(Cr) and (d–f) MIL-101(Cr) after activation under vacuum (10–5 mbar) for 16 h at 423 K, at 623 K, and after impregnation with diethylaluminum chloride (DEA) (with Al:Cr molar ratio 100) and CO dosage (0–1 mbar) at 85 K. Degassed MIL-100(Cr) was subtracted as a reference in (b) and (c). MIL-101(Cr) was subtracted as a reference spectrum in (e) and (f).

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    Figure 3

    Figure 3. (a) Diffuse reflectance (DRS) spectra in the vis–NIR region for degassed MIL-100(Cr) (red), after injection of 125 equiv of diethylaluminum chloride (DEA) (blue) and after 15 min (pink). (b) Deconvolution of the UV–vis–NIR DRS spectrum after injection of DEA showing different Cr species. The inset in (b) shows the presence of small amounts of Cr2+ species (both Oh and Th geometries). Green lines show the residual values of the fitting procedure. (c) DRS spectra in the UV–vis–NIR region after flowing ethylene (10 mL min–1) at 298 K and 1 bar for 1 h into the cell (from dark blue (t = 0) to red (t = 60 min)). The inset shows bands where polyethylene C–H combination bands should appear if polymerization occurred. (d) DRS spectra in the vis–NIR range of activated MIL-101(Cr) (red), after injection of 125 equiv of DEA (blue) and after 15 min (pink). (e) Deconvolution of the spectrum after injection of DEA showing the different Cr species. The inset in (e) shows the presence of small amounts of Cr2+ species (only Oh geometry). Green lines show the residual of the fitting procedure. (f) DRS spectra in the NIR region after flowing ethylene (10 mL min–1) at 298 K and 1 bar for 1 h into the cell (from dark blue (t = 0) to red (t = 60 min)), showing the bands of crystalline PE forming over time. The inset shows additional combination bands of the polymer CH2 groups, indicating polymer formation in contrast to the case of (c).

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    Scheme 1

    Scheme 1. Tentative Activation of Cr Sites by Diethylaluminum Chloride (DEA) and Subsequent Ethylene Insertion and Polymerization in MIL-101(Cr) Materials
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    Figure 4

    Figure 4. Scanning electron microscopy (SEM) micrographs of the polymer product obtained with (a, b) MIL-101(Cr), (c, d) MIL-100(Cr) (note that only nanosized crystallites are observed at this magnification), complex 1 (e, f), and the leached Cr from MIL-101(Cr) (g, h). Conditions: Al:Cr mol ratio of 100, diethylaluminum chloride (DEA), T = 298 K, p = 10 bar of C2H4, t = 1 h.

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    Figure 5

    Figure 5. Scanning electron microscopy (SEM) micrographs of MIL-100(Cr) after reaction (10 bar of C2H4, 298 K, 500 equiv of DEA, toluene, 1 h) at high magnification, showing MIL-100(Cr) crystallites still intact. The arrows in the inset image indicate polymer fibers of MOF crystallites that were not able to fracture. However, no evident signs of polymerization around the MOF crystallites, into shaped beads as is the case for MIL-101(Cr), were observed. In Figure 5, the inset shows a few polymer fibers that emerge from certain MOF crystallites. This is in stark contrast with MIL-101(Cr) and complex 1, and it evidences the inability of MIL-100(Cr) to crumble upon ethylene insertion.

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    Figure 6

    Figure 6. Density functional theory (DFT) pore size distributions of both MOFs calculated from the experimental N2 adsorption isotherms at 77 K.

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    Figure 7

    Figure 7. (a) MIL-100(Cr) is unable to fracture upon polyethylene formation, resulting in low catalytic activity. (b) MIL-101(Cr) is degraded and partially leaches Cr clusters into the solution upon addition of the cocatalyst, leading to different morphologies. (c) Coordination complex 1 in solution generates polymer fibers, as the chain growth is not templated by any solid support.

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      Heterogeneous single-site catalysts consist of isolated, well-defined, active sites that are spatially sepd. in a given solid and, ideally, structurally identical. In this review, the potential of metal-org. frameworks (MOFs) and covalent org. frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly. In the first part of this article, synthetic strategies and progress in the implementation of such sites in these two classes of materials are discussed. Because these solids are excellent playgrounds to allow a better understanding of catalytic functions, we highlight the most important recent advances in the modeling and spectroscopic characterization of single-site catalysts based on these materials. Finally, we discuss the potential of MOFs as materials in which several single-site catalytic functions can be combined within one framework along with their potential as powerful enzyme-mimicking materials. The review is wrapped up with our personal vision on future research directions.
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      Cohen, S. M.; Zhang, Z.; Boissonnault, J. A. Toward “metalloMOFzymes”: Metal–Organic Frameworks with Single-Site Metal Catalysts for Small-Molecule Transformations. Inorg. Chem. 2016, 55, 7281– 7290,  DOI: 10.1021/acs.inorgchem.6b00828
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      21
      Toward "metalloMOFzymes": Metal-Organic Frameworks with Single-Site Metal Catalysts for Small-Molecule Transformations
      Cohen, Seth M.; Zhang, Zhenjie; Boissonnault, Jake A.
      Inorganic Chemistry (2016), 55 (15), 7281-7290CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Metal-org. frameworks (MOFs) are being increasingly studied as scaffolds and supports for catalysis. The solid-state structures of MOFs, combined with their high porosity, suggest that MOFs may possess advantages shared by both heterogeneous and homogeneous catalysts, with few of the shortcomings of either. Herein, efforts to create single-site catalytic metal centers appended to the org. ligand struts of MOFs are discussed. Reactions important for advanced energy applications, such as H2 prodn. and CO2 redn., will be reviewed. Examg. how these active sites can be introduced, their performance, and their existing limitations should provide direction for design of the next generation of MOF-based catalysts for energy-relevant, small-mol. transformations. Finally, the introduction of second-sphere interactions (e.g., hydrogen bonding via squaramide groups) as a possible route to enhancing the activity of these metal centers is reported.
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      Kissin, Y. V. Polyethylene, linear low density (LLDPE). In Kirk-Othmer Encyclopedia of Chemical Technology, 5th ed.; Wiley: New York, 2006; Vol. 20, pp 179– 211.
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      Lappin, G. R.; Nemec, L. H.; Sauer, J. D.; Wagner, J. D. Higher Olefins. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley: New York, 2000; Vol. 17, pp 709– 723.
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      Maraschin, N., Polyethylene, high density (HDPE). In Kirk-Othmer Encyclopedia of Chemical Technology, 5th ed.; Wiley: New York, 2006; Vol. 20, pp 211– 239.
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      McDaniel, M. P.; DesLauriers, P. J. Ethylene Polymers, HDPE. In Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 2015; pp 1– 40.
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      Lieberman, R. B.; Dorini, M.; Mei, G.; Rinaldi, R.; Penzo, G.; Berge, G. T. Polypropylene. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley: New York, 2000; Vol. 17, pp 1– 15.
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      Canivet, J.; Aguado, S.; Schuurman, Y.; Farrusseng, D. MOF-Supported Selective Ethylene Dimerization Single-Site Catalysts through One-Pot Postsynthetic Modification. J. Am. Chem. Soc. 2013, 135, 4195– 4198,  DOI: 10.1021/ja312120x
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      MOF-Supported Selective Ethylene Dimerization Single-Site Catalysts through One-Pot Postsynthetic Modification
      Canivet, Jerome; Aguado, Sonia; Schuurman, Yves; Farrusseng, David
      Journal of the American Chemical Society (2013), 135 (11), 4195-4198CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The one-pot postfunctionalization allows anchoring a mol. nickel complex into a mesoporous metal-org. framework (Ni@(Fe)MIL-101). It is generating a very active and reusable catalyst for the liq.-phase ethylene dimerization to selectively form 1-butene. Higher selectivity for 1-butene is found using the Ni@(Fe)MIL-101 catalyst than reported for mol. nickel diimino complexes.
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    28. 28
      Madrahimov, S. T.; Gallagher, J. R.; Zhang, G.; Meinhart, Z.; Garibay, S. J.; Delferro, M.; Miller, J. T.; Farha, O. K.; Hupp, J. T.; Nguyen, S. T. Gas-Phase Dimerization of Ethylene under Mild Conditions Catalyzed by MOF Materials Containing (bpy)NiII Complexes. ACS Catal. 2015, 5, 6713– 6718,  DOI: 10.1021/acscatal.5b01604
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      Gas-Phase Dimerization of Ethylene under Mild Conditions Catalyzed by MOF Materials Containing (bpy)NiII Complexes
      Madrahimov, Sherzod T.; Gallagher, James R.; Zhang, Guanghui; Meinhart, Zachary; Garibay, Sergio J.; Delferro, Massimiliano; Miller, Jeffrey T.; Farha, Omar K.; Hupp, Joseph T.; Nguyen, SonBinh T.
      ACS Catalysis (2015), 5 (11), 6713-6718CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      NU-1000-(bpy)NiII, a highly porous MOF material possessing well-defined (bpy)NiII moieties, was prepd. through solvent-assisted ligand incorporation (SALI). Treatment with Et2AlCl affords a single-site catalyst with excellent catalytic activity for ethylene dimerization (intrinsic activity for butenes that is up to an order of magnitude higher than the corresponding (bpy)NiCl2 homogeneous analog) and stability (can be reused at least three times). The high porosity of this catalyst results in outstanding levels of activity at ambient temp. in gas-phase ethylene dimerization reactions, both under batch and continuous flow conditions.
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    29. 29
      Metzger, E. D.; Brozek, C. K.; Comito, R. J.; Dinca, M. Selective Dimerization of Ethylene to 1-Butene with a Porous Catalyst. ACS Cent. Sci. 2016, 2, 148– 153,  DOI: 10.1021/acscentsci.6b00012
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      29
      Selective Dimerization of Ethylene to 1-Butene with a Porous Catalyst
      Metzger, Eric D.; Brozek, Carl K.; Comito, Robert J.; Dinca, Mircea
      ACS Central Science (2016), 2 (3), 148-153CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
      Current heterogeneous catalysts lack the fine steric and electronic tuning required for catalyzing the selective dimerization of ethylene to 1-butene, which remains one of the largest industrial processes still catalyzed by homogeneous catalysts. Here, we report that a metal-org. framework catalyzes ethylene dimerization with a combination of activity and selectivity for 1-butene that is premier among heterogeneous catalysts. The capacity for mild cation exchange in the material MFU-4l (MFU-4l = Zn5Cl4(BTDD)3, H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin) was leveraged to create a well-defined and site-isolated Ni(II) active site bearing close structural homol. to mol. tris-pyrazolylborate complexes. In the presence of ethylene and methylaluminoxane, the material consumes ethylene at a rate of 41,500 mol per mol of Ni per h with a selectivity for 1-butene of up to 96.2%, exceeding the selectivity reported for the current industrial dimerization process.
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    30. 30
      Metzger, E. D.; Comito, R. J.; Hendon, C. H.; Dinca, M. Mechanism of Single-Site Molecule-Like Catalytic Ethylene Dimerization in Ni-MFU-4l. J. Am. Chem. Soc. 2017, 139, 757– 762,  DOI: 10.1021/jacs.6b10300
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      30
      Mechanism of Single-Site Molecule-Like Catalytic Ethylene Dimerization in Ni-MFU-4l
      Metzger, Eric D.; Comito, Robert J.; Hendon, Christopher H.; Dinca, Mircea
      Journal of the American Chemical Society (2017), 139 (2), 757-762CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A recently developed metal-org. framework (MOF) catalyst for the dimerization of ethylene has a combination of selectivity and activity that surpasses that of com. homogeneous catalysts, which have dominated this important industrial process for nearly 50 years. The uniform catalytic sites available in MOFs provide a unique opportunity to directly study reaction mechanisms in heterogeneous catalysts, a problem typically intractable due to the multiplicity of coordination environments found in many solid catalysts. In this work, we use a combination of isotopic labeling studies, mechanistic probes, and DFT calcns. to demonstrate that Ni-MFU-4l operates via the Cossee-Arlman mechanism, which has also been implicated in homogeneous late transition metal catalysts. These studies demonstrate that metal nodes in MOFs mimic homogeneous catalysts not just functionally, but also mechanistically. They provide a blueprint for the development of advanced heterogeneous catalysts with similar degrees of tunability to their homogeneous counterparts.
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    31. 31
      Pellizzeri, S.; Barona, M.; Bernales, V.; Miró, P.; Liao, P.; Gagliardi, L.; Snurr, R. Q.; Getman, R. B. Catalytic Descriptors and Electronic Properties of Single-site Catalysts for Ethene Dimerization to 1-butene. Catal. Today 2018, 312, 149– 157,  DOI: 10.1016/j.cattod.2018.02.024
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      Catalytic descriptors and electronic properties of single-site catalysts for ethene dimerization to 1-butene
      Pellizzeri, Steven; Barona, Melissa; Bernales, Varinia; Miro, Pere; Liao, Peilin; Gagliardi, Laura; Snurr, Randall Q.; Getman, Rachel B.
      Catalysis Today (2018), 312 (), 149-157CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      Six first-row transition metal cations (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) were evaluated as catalysts for ethene dimerization to 1-butene. This is an important reaction in the chem. of C-C bond formation and in the conversion of natural gas to higher hydrocarbons. Two related classes of transition metal cation catalysts were investigated: (1) single transition metal cations supported on zirconium oxide nodes of the metal-org. framework NU-1000 and (2) small metal hydroxide clusters with two metal atoms (M2) that could be grown by at. layer deposition on a support exhibiting isolated hydroxyl groups. Using scaling relations, the free energies of co-adsorbed hydrogen and ethene (i.e., (H/C2H4)*) and adsorbed Et (i.e., C2H5*) were identified as descriptors for ethene dimerization catalysis. Using degree of rate control anal., it was detd. that the rate controlling steps are either ethene insertion (C-C bond forming) or β-hydride elimination (C-H bond breaking), depending on the metal. Using degree of catalyst control anal., it was detd. that activity on all the catalysts studied could be improved by tuning the free energy of C2H5*.
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    32. 32
      Ye, J.; Gagliardi, L.; Cramer, C. J.; Truhlar, D. G. Single Ni Atoms and Ni4 Clusters have Similar Catalytic Activity for Ethylene Dimerization. J. Catal. 2017, 354, 278– 286,  DOI: 10.1016/j.jcat.2017.08.011
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      Single Ni atoms and Ni4 clusters have similar catalytic activity for ethylene dimerization
      Ye, Jingyun; Gagliardi, Laura; Cramer, Christopher J.; Truhlar, Donald G.
      Journal of Catalysis (2017), 354 (), 278-286CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Atomic layer deposition (ALD) of Ni on the metal-org. framework NU-1000 has been shown to generate a material that serves as a catalyst for ethylene dimerization. However, the precise nature of the active catalytic site or sites remains uncertain. Here the authors employ periodic d. functional calcns. to characterize the structure and reactivity of the deposited species. Optimized lattice consts. for a sequence of structures incorporating successively more Ni4-hydroxo clusters in the c pore of NU-1000 show good agreement with exptl. trends involving multiple ALD cycles; therefore the authors study the catalytic cycle for this cluster in detail, and the authors compare it to that for a site with only a single Ni atom. Both the at. Ni catalyst and the Ni4-hydroxo cluster have higher catalytic activity in the singlet state than in the triplet state. Also the two catalysts have very similar activity. Thus, precise size control of the active catalytic species is not essential for ethylene dimerization in this system.
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    33. 33
      Bernales, V.; League, A. B.; Li, Z.; Schweitzer, N. M.; Peters, A. W.; Carlson, R. K.; Hupp, J. T.; Cramer, C. J.; Farha, O. K.; Gagliardi, L. Computationally Guided Discovery of a Catalytic Cobalt-Decorated Metal–Organic Framework for Ethylene Dimerization. J. Phys. Chem. C 2016, 120, 23576– 23583,  DOI: 10.1021/acs.jpcc.6b07362
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      33
      Computationally Guided Discovery of a Catalytic Cobalt-Decorated Metal-Organic Framework for Ethylene Dimerization
      Bernales, Varinia; League, Aaron B.; Li, Zhanyong; Schweitzer, Neil M.; Peters, Aaron W.; Carlson, Rebecca K.; Hupp, Joseph T.; Cramer, Christopher J.; Farha, Omar K.; Gagliardi, Laura
      Journal of Physical Chemistry C (2016), 120 (41), 23576-23583CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The catalytic performance of a cobalt(II) single-site catalyst supported on the zirconia-like nodes of the metal org.-framework (MOF) NU-1000 is herein characterized by quantum chem. methods and compared to an iso-structural analog incorporating nickel(II) as the active transition metal. The mechanisms of at. layer deposition in MOFs and of catalysis are examd. using d. functional theory. We compare the catalytic activity of Co and Ni installed on the zirconia-like nodes for ethylene dimerization, considering three plausible pathways. Multiconfigurational wave function theory methods are employed to further characterize the electronic structures of key transition states and intermediates. Finally, we report confirmation of Co catalytic activity for ethylene dimerization from expts. that were prompted by the computational prediction.
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      Liu, B.; Jie, S.; Bu, Z.; Li, B.-G. A MOF-supported Chromium Catalyst for Ethylene Polymerization through Post-synthetic Modification. J. Mol. Catal. A: Chem. 2014, 387, 63– 68,  DOI: 10.1016/j.molcata.2014.02.028
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      A MOF-supported chromium catalyst for ethylene polymerization through post-synthetic modification
      Liu, Bing; Jie, Suyun; Bu, Zhiyang; Li, Bo-Geng
      Journal of Molecular Catalysis A: Chemical (2014), 387 (), 63-68CODEN: JMCCF2; ISSN:1381-1169. (Elsevier B.V.)
      Isoreticular metal-org. framework-3 (IRMOF-3) has been post-synthetically modified to generate a Cr(III)-based heterogeneous catalyst (IRMOF-3-SI-Cr) for ethylene polymn., which has been characterized by a variety of phys. methods. The x-ray diffraction anal. indicated that the structure integrity of the final solid was preserved after the functionalization with the imine and the subsequent coordination to chromium. The BET surface area of the final solid was slightly reduced as detd. by N2 adsorption-desorption expts. The material exhibited a unique behavior for ethylene polymn. upon activation with various alkylaluminium co-catalysts, and the polyethylenes formed featured high mol. wts. and broad mol. wt. distributions.
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    35. 35
      Ji, P.; Solomon, J. B.; Lin, Z.; Johnson, A.; Jordan, R. F.; Lin, W. Transformation of Metal–Organic Framework Secondary Building Units into Hexanuclear Zr-Alkyl Catalysts for Ethylene Polymerization. J. Am. Chem. Soc. 2017, 139, 11325– 11328,  DOI: 10.1021/jacs.7b05761
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      Transformation of Metal-Organic Framework Secondary Building Units into Hexanuclear Zr-Alkyl Catalysts for Ethylene Polymerization
      Ji, Pengfei; Solomon, Joseph B.; Lin, Zekai; Wilders, Alison M.; Jordan, Richard F.; Lin, Wenbin
      Journal of the American Chemical Society (2017), 139 (33), 11325-11328CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Authors report the stepwise and quant. transformation of the Zr6(μ3-O)4(μ3-OH)4(HCO2)6 nodes in Zr-BTC (MOF-808) to the [Zr6(μ3-O)4(μ3-OH)4Cl12]6- nodes in ZrCl2-BTC, and then to the organometallic [Zr6(μ3-O)4(μ3-OLi)4R12]6- nodes in ZrR2-BTC (R = CH2SiMe3 or Me). Activation of ZrCl2-BTC with MMAO-12 generates ZrMe-BTC, which is an efficient catalyst for ethylene polymn. ZrMe-BTC displays unusual electronic and steric properties compared to homogeneous Zr catalysts, possesses multimetallic active sites, and produces high-mol.-wt. linear polyethylene. Metal-org. framework nodes can thus be directly transformed into novel single-site solid organometallic catalysts without homogeneous analogs for polymn. reactions.
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    36. 36
      Comito, R. J.; Fritzsching, K. J.; Sundell, B. J.; Schmidt-Rohr, K.; Dinca, M. Single-Site Heterogeneous Catalysts for Olefin Polymerization Enabled by Cation Exchange in a Metal-Organic Framework. J. Am. Chem. Soc. 2016, 138, 10232– 10237,  DOI: 10.1021/jacs.6b05200
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      Single-Site Heterogeneous Catalysts for Olefin Polymerization Enabled by Cation Exchange in a Metal-Organic Framework
      Comito, Robert J.; Fritzsching, Keith J.; Sundell, Benjamin J.; Schmidt-Rohr, Klaus; Dinca, Mircea
      Journal of the American Chemical Society (2016), 138 (32), 10232-10237CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The manuf. of advanced polyolefins was critically enabled by the development of single-site heterogeneous catalysts. Metal-org. frameworks (MOFs) show great potential as heterogeneous catalysts that may be designed and tuned on the mol. level. Exchange of zinc ions in Zn5Cl4(BTDD)3, (H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin) (MFU-4l) with reactive metals serves to establish a general platform for selective olefin polymn. in a high surface area solid promising for industrial catalysis. Characterization of polyethylene produced by these materials demonstrates both mol. and morphol. control. Notably, reactivity approaches single-site catalysis, as evidenced by low polydispersity indexes, and good mol. wt. control. Further these new catalysts copolymerize ethylene and propylene. Uniform growth of the polymer around the catalyst particles provides a mechanism for controlling the polymer morphol., a relevant metric for continuous flow processes.
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    37. 37
      Klet, R. C.; Tussupbayev, S.; Borycz, J.; Gallagher, J. R.; Stalzer, M. M.; Miller, J. T.; Gagliardi, L.; Hupp, J. T.; Marks, T. J.; Cramer, C. J.; Delferro, M.; Farha, O. K. Single-Site Organozirconium Catalyst Embedded in a Metal–Organic Framework. J. Am. Chem. Soc. 2015, 137, 15680– 15683,  DOI: 10.1021/jacs.5b11350
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      Single-Site Organozirconium Catalyst Embedded in a Metal-Organic Framework
      Klet, Rachel C.; Tussupbayev, Samat; Borycz, Joshua; Gallagher, James R.; Stalzer, Madelyn M.; Miller, Jeffrey T.; Gagliardi, Laura; Hupp, Joseph T.; Marks, Tobin J.; Cramer, Christopher J.; Delferro, Massimiliano; Farha, Omar K.
      Journal of the American Chemical Society (2015), 137 (50), 15680-15683CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A structurally well-defined mesoporous Hf-based metal-org. framework (Hf-NU-1000) is employed as a well-defined scaffold for a highly electrophilic single-site d0 Zr-benzyl catalytic center. This new material Hf-NU-1000-ZrBn is fully characterized by a variety of spectroscopic techniques and DFT computation. Hf-NU-1000-ZrBn is a promising single-component catalyst (i.e., not requiring a catalyst/activator) for ethylene and stereoregular 1-hexene polymn.
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      Comito, R. J.; Metzger, E. D.; Wu, Z.; Zhang, G.; Hendon, C. H.; Miller, J. T.; Dinca, M. Selective Dimerization of Propylene with Ni-MFU-4l. Organometallics 2017, 36, 1681– 1683,  DOI: 10.1021/acs.organomet.7b00178
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      Selective Dimerization of Propylene with Ni-MFU-4l
      Comito, Robert J.; Metzger, Eric D.; Wu, Zhenwei; Zhang, Guanghui; Hendon, Christopher H.; Miller, Jeffrey T.; Dinca, Mircea
      Organometallics (2017), 36 (9), 1681-1683CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      We report the selective dimerization of propylene to branched hexenes using Ni-MFU-4l, a solid catalyst prepd. by cation exchange. Anal. of the resulting product distribution demonstrates that the selectivity arises from 2,1-insertion and slow product reinsertion, mechanistic features reproduced by a mol. nickel tris-pyrazolylborate catalyst. Characterization of Ni-MFU-4l by X-ray absorption spectroscopy provides evidence for discrete, tris-pyrazolylborate-like coordination of nickel, underscoring the small-mol. analogy that can be made at metal-org. framework nodes.
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    39. 39
      Mlinar, A. N.; Keitz, B. K.; Gygi, D.; Bloch, E. D.; Long, J. R.; Bell, A. T. Selective Propene Oligomerization with Nickel(II)-Based Metal–Organic Frameworks. ACS Catal. 2014, 4, 717– 721,  DOI: 10.1021/cs401189a
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      Selective Propene Oligomerization with Nickel(II)-Based Metal-Organic Frameworks
      Mlinar, Anton N.; Keitz, Benjamin K.; Gygi, David; Bloch, Eric D.; Long, Jeffrey R.; Bell, Alexis T.
      ACS Catalysis (2014), 4 (3), 717-721CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Two Ni2+-contg. metal-org. frameworks, Ni2(dobdc) and Ni2(dobpdc), are active for the oligomerization of propene in the gas phase. The metal-org. frameworks exhibit activity comparable to Ni2+-exchanged aluminosilicates but maintain high selectivity for linear oligomers. Thus, these frameworks should enable the high yielding synthesis of linear propene oligomers for use in detergent and diesel fuel applications.
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    40. 40
      Dubey, R. J. C.; Comito, R. J.; Wu, Z.; Zhang, G.; Rieth, A. J.; Hendon, C. H.; Miller, J. T.; Dinca, M. Highly Stereoselective Heterogeneous Diene Polymerization by Co-MFU-4l: A Single-Site Catalyst Prepared by Cation Exchange. J. Am. Chem. Soc. 2017, 139, 12664– 12669,  DOI: 10.1021/jacs.7b06841
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      40
      Highly Stereoselective Heterogeneous Diene Polymerization by Co-MFU-4l: A Single-Site Catalyst Prepared by Cation Exchange
      Dubey, Romain J.-C.; Comito, Robert J.; Wu, Zhenwei; Zhang, Guanghui; Rieth, Adam J.; Hendon, Christopher H.; Miller, Jeffrey T.; Dinca, Mircea
      Journal of the American Chemical Society (2017), 139 (36), 12664-12669CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Mol. catalysts offer tremendous advantages for stereoselective polymn. because their activity and selectivity can be optimized and understood mechanistically using the familiar tools of organometallic chem. Yet, this exquisite control over selectivity comes at an operational price that is generally not justifiable for the large-scale manuf. of polyolefins. In this report we identify Co-MFU-4l, prepd. by cation exchange in a metal-org. framework, as a solid catalyst for the polymn. of 1,3-butadiene with high stereoselectivity (>99 % 1,4-cis). To our knowledge, this is the highest stereoselectivity achieved with a heterogeneous catalyst for this transformation. The polymer's low polydispersity (PDI ∼2) and the catalyst's ready recovery and low leaching indicate that our material is a structurally resilient single-site heterogeneous catalyst. Further characterization of Co-MFU-4l by X-ray absorption spectroscopy provided evidence for discrete, tris-pyrazolylborate-like coordination of Co(II). With this information, we identify a sol. cobalt complex that mimics the structure and reactivity of Co-MFU-4l, thus providing a well-defined platform for studying the catalytic mechanism in the soln. phase. This work underscores the capacity for small-mol. like tunability and mechanistic tractability available to transition metal catalysis in metal-org. frameworks.
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    41. 41
      Vitorino, M. J.; Devic, T.; Tromp, M.; Férey, G.; Visseaux, M. Lanthanide Metal Organic Frameworks as Ziegler–Natta Catalysts for the Selective Polymerization of Isoprene. Macromol. Chem. Phys. 2009, 210, 1923– 1932,  DOI: 10.1002/macp.200900354
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      Liu, S.; Zhang, Y.; Han, Y.; Feng, G.; Gao, F.; Wang, H.; Qiu, P. Selective Ethylene Oligomerization with Chromium-Based Metal–Organic Framework MIL-100 Evacuated under Different Temperatures. Organometallics 2017, 36, 632– 638,  DOI: 10.1021/acs.organomet.6b00834
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      Selective Ethylene Oligomerization with Chromium-Based Metal-Organic Framework MIL-100 Evacuated under Different Temperatures
      Liu, Suyan; Zhang, Ying; Han, Yang; Feng, Guangliang; Gao, Fei; Wang, Hui; Qiu, Ping
      Organometallics (2017), 36 (3), 632-638CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      MIL-100(Cr) was synthesized and evacuated under different temps. to generate a series of heterogeneous catalysts for ethylene oligomerization. These catalysts showed moderate catalytic activities for ethylene oligomerization but high selectivities to low carbon olefins C6, C8, and C10. Moreover, the oligomer distribution was different depending on the evacuation temp. The XPS results showed the redn. of some CrIII active sites in the MIL-100(Cr) structure to CrII active sites, which made the catalysts show polymn. activities. The MIL-100(Cr)-250 catalyst evacuated at 250° exhibited the highest oligomerization and polymn. activities up to 9.27 × 105 g/(molCr·h) and 0.99 × 105 g/(molCr·h), resp. The oligomerization selectivity to low carbon olefins C6, C8, and C10 was ∼99%. The byproduct polymer from MIL-100(Cr)-250 belonged to linear polyethylene with ultrahigh mol. wt. and broad mol. wt. distributions. MOFs contg. coordinatively unsatd. metal sites might be a promising selective catalyst for ethylene slurry oligomerization.
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      Férey, G.; Serre, C.; Mellot-Draznieks, C.; Millange, F.; Surblé, S.; Dutour, J.; Margiolaki, I. A Hybrid Solid with Giant Pores Prepared by a Combination of Targeted Chemistry, Simulation, and Powder Diffraction. Angew. Chem., Int. Ed. 2004, 43, 6296– 6301,  DOI: 10.1002/anie.200460592
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      Molecular modeling: A hybrid solid with giant pores prepared by a combination of targeted chemistry, simulation, and powder diffraction
      Ferey, Gerard; Serre, Christian; Mellot-Draznieks, Caroline; Millange, Franck; Surble, Suzy; Dutour, Julien; Margiolaki, Irene
      Angewandte Chemie, International Edition (2004), 43 (46), 6296-6301CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The assocn. of a chromium(III) trimeric building unit and 1,3,5-benzenetricarboxylate led to the powd. solid MIL-100. Simulations provided a crystal structure soln., which matched the exptl. powder XRD pattern. This unique simulation/diffraction combination allowed the structure detn. of a giant-pore solid with a zeotype architecture, built up from hybrid supertetrahedra.
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      Férey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.; Surblé, S.; Margiolaki, I. A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area. Science 2005, 309, 2040– 2042,  DOI: 10.1126/science.1116275
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      A Chromium Terephthalate-Based Solid with Unusually Large Pore Volumes and Surface Area
      Ferey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.; Surble, S.; Margiolaki, I.
      Science (Washington, DC, United States) (2005), 309 (5743), 2040-2042CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      We combined targeted chem. and computational design to create a crystal structure for porous chromium terephthalate, MIL-101, with very large pore sizes and surface area. Its zeotype cubic structure has a giant cell vol. (∼702,000 cubic angstroms), a hierarchy of extra-large pore sizes (∼30 to 34 angstroms), and a Langmuir surface area for N2 of ∼5900 ± 300 square meters per g. Beside the usual properties of porous compds., this solid has potential as a nanomold for monodisperse nanomaterials, as illustrated here by the incorporation of Keggin polyanions within the cages.
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      Harton, A.; Nagi, M. K.; Glass, M. M.; Junk, P. C.; Atwood, J. L.; Vincent, J. B. Synthesis and Characterization of Symmetric and Unsymmetric Oxo-bridged Trinuclear Chromium Benzoate Complexes: Crystal and Molecular Structure of [Cr3O(O2CPh)6(py)3]ClO4. Inorg. Chim. Acta 1994, 217, 171– 179,  DOI: 10.1016/0020-1693(93)03766-4
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      Synthesis and characterization of symmetric and unsymmetric oxo-bridged trinuclear chromium benzoate complexes: crystal and molecular structure of [Cr3O(O2CPh)6(py)3]ClO4
      Harton, Anthony; Nagi, Maysa K.; Glass, Miriam M.; Junk, Peter C.; Atwood, Jerry L.; Vincent, John B.
      Inorganica Chimica Acta (1994), 217 (1-2), 171-9CODEN: ICHAA3; ISSN:0020-1693.
      Techniques were developed for the synthesis of [Cr3O(O2CR)6(L)3]+ (R = Ph or tolyl and L = H2O or py) in nonaq. solvents. Addnl., the synthesis of an unsym. trinuclear complex, [Cr3O(OBz)6(OH)(py)2], is reported. These complexes were characterized by a no. of spectroscopic and magnetic techniques including x-ray crystallog., electronic and IR spectrophotometry, NMR and EPR spectroscopy, mass spectrometry, and soln. and solid state susceptibility measurements. [Cr3O(OBz)6(py)3]ClO4 crystallizes in the hexagonal space group P‾6, a 13.387(4), c 19.186(6) Å, Z = 2, R = 7.6 and Rw = 10.1.
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      Lebedev, O. I.; Millange, F.; Serre, C.; Van Tendeloo, G.; Férey, G. First Direct Imaging of Giant Pores of the Metal–Organic Framework MIL-101. Chem. Mater. 2005, 17, 6525– 6527,  DOI: 10.1021/cm051870o
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      First Direct Imaging of Giant Pores of the Metal-Organic Framework MIL-101
      Lebedev, O. I.; Millange, F.; Serre, C.; Van Tendeloo, G.; Ferey, G.
      Chemistry of Materials (2005), 17 (26), 6525-6527CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Cubic type metal-org. framework MIL-101 exhibits unprecedented features: a mesoporous zeotype architecture, a giant cell vol., a hierarchy of extra-large pore size, and a record sorption capacity. This paper presents the first direct imaging of its giant pore.
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      Vimont, A.; Goupil, J. M.; Lavalley, J. C.; Daturi, M.; Surblé, S.; Serre, C.; Millange, F.; Férey, G.; Audebrand, N. Investigation of Acid Sites in a Zeotypic Giant Pores Chromium(III) Carboxylate. J. Am. Chem. Soc. 2006, 128, 3218– 3227,  DOI: 10.1021/ja056906s
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      Investigation of Acid Sites in a Zeotypic Giant Pores Chromium(III) Carboxylate
      Vimont, Alexandre; Goupil, Jean-Michel; Lavalley, Jean-Claude; Daturi, Marco; Surble, Suzy; Serre, Christian; Millange, Franck; Ferey, Gerard; Audebrand, Nathalie
      Journal of the American Chemical Society (2006), 128 (10), 3218-3227CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A study of the zeotypic giant pores chromium(III) tricarboxylate CrIII3OFx(OH)1-x(H2O)2·{C6H3-(CO2)3}2·nH2O (MIL-100) has been performed. First, its thermal behavior, studied by X-ray thermodiffractometry and IR spectroscopy, indicates that the departure of water occurs without any pore contraction and no loss in crystallinity, which confirms the robustness of the framework. In a second step, IR spectroscopy has shown the presence of three distinct types of hydroxy groups depending on the outgassing conditions; first, at high temps. (573 K), only Cr-OH groups with a medium Bronsted acidity are present; at lower temps., two types of Cr-H2O terminal groups are obsd.; and at room temp., their relatively high Bronsted acidity allows them to combine with H-bonded water mols. Finally, a CO sorption study has revealed that at least three Lewis acid sites are present in MIL-100 and that fluorine atoms are located on a terminal position on the trimers of octahedra. A first result of grafting of methanol mols. acting as basic org. mols. on the chromium sites has also been shown, opening the way for a postsynthesis functionalization of MIL-100.
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      Shufler, S. L.; Sternberg, H. W.; Friedel, R. A. Infrared Spectrum and Structure of Chromium Hexacarbonyl, Cr(CO)6. J. Am. Chem. Soc. 1956, 78, 2687– 2688,  DOI: 10.1021/ja01593a008
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      Infrared spectrum and structure of chromium hexacarbonyl, Cr(CO)6
      Shufler, S. Leonard; Sternberg, Heinz W.; Friedel, R. A.
      Journal of the American Chemical Society (1956), 78 (), 2687-8CODEN: JACSAT; ISSN:0002-7863.
      The infrared spectrum of Cr(CO)6 indicates only one intense C:O stretching frequency in the vapor phase. This constitutes strong evidence for the equivalence of all C:O bands and a regular octahedral structure for Cr(CO)6 and other metal hexacarbonyl. These results provide an exptl. means for detecting metal hexacarbonyl in metal carbonyl reaction.
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      Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A. The Structure of Active Centers and the Ethylene Polymerization Mechanism on the Cr/SiO2 Catalyst: A Frontier for the Characterization Methods. Chem. Rev. 2005, 105, 115– 154,  DOI: 10.1021/cr040083s
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      The Structure of Active Centers and the Ethylene Polymerization Mechanism on the Cr/SiO2 Catalyst: A Frontier for the Characterization Methods
      Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A.
      Chemical Reviews (Washington, DC, United States) (2005), 105 (1), 115-183CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review on progress in the understanding of structures active sites and strategies and techniques of study of the the catalyst under working conditions. Methods adopted for studying Cr/SiO2 catalyst cab be extended to other catalytic systems. Surface chem. of silica support was discussed. Literature on Phillips catalyst until 1985 was reviewed.
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      Groppo, E.; Lamberti, C.; Cesano, F.; Zecchina, A. On the Fraction of CrII Sites involved in the C2H4 Polymerization on the Cr/SiO2 Phillips Catalyst: A Quantification by FTIR Spectroscopy. Phys. Chem. Chem. Phys. 2006, 8, 2453– 2456,  DOI: 10.1039/b604515d
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      On the fraction of CrII sites involved in the C2H4 polymerization on the Cr/SiO2 Phillips catalyst: a quantification by FTIR spectroscopy
      Groppo, E.; Lamberti, C.; Cesano, F.; Zecchina, A.
      Physical Chemistry Chemical Physics (2006), 8 (21), 2453-2456CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      An estn. of the fraction of CrII sites involved in the C2H4 polymn. on a CrII/SiO2 Phillips catalyst has been obtained by means of in situ alternated CO adsorption and C2H4 polymn. FTIR expts.: about 28% of the total surface sites react fast with C2H4, while a lower fraction, which depends upon the temp. reaction conditions, is more slowly involved, in agreement with XANES results.
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      Nenu, C. N.; Groppo, E.; Lamberti, C.; Beale, A. M.; Visser, T.; Zecchina, A.; Weckhuysen, B. M. Dichloromethane as a Selective Modifying Agent To Create a Family of Highly Reactive Chromium Polymerization Sites. Angew. Chem., Int. Ed. 2007, 46, 1465– 1468,  DOI: 10.1002/anie.200602593
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      Hadjiivanov, K. I.; Vayssilov, G. N. Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule. Adv. Catal. 2002, 47, 307– 511,  DOI: 10.1016/S0360-0564(02)47008-3
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      Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule
      Hadjiivanov, Konstantin I.; Vayssilov, Georgi N.
      Advances in Catalysis (2002), 47 (), 307-511CODEN: ADCAAX; ISSN:0360-0564. (Elsevier Science)
      A review. The review is a summary and anal. of the data characterizing CO adsorption on surface cationic sites of oxides including supported materials and microporous and mesoporous materials. The contributions of various types of CO bonding to the IR frequency shifts of C-bonded mols. are analyzed, namely, the increase of the CO stretching frequency in cases of electrostatic and σ bonding and the decrease of the frequency with π bonding. Polycarbonyls, bridging CO, oxygen-bonded CO, and tilted CO are also considered. The main part of the review is a collection of the exptl. results characterizing carbonyls of individual metal ions. The spectral behavior of CO bonded to metal atoms is also assessed in the cases when the metal ions are easily reduced to metal (Cu, Ag, Au, Pd, or Pt) or cationic carbonyls are produced after CO adsorption on supported metals (Ru, Rh, Ir, and Os). The interaction of CO with surface OH groups is also considered. IR spectroscopy of adsorbed CO is an efficient methodol. to characterize cationic surface sites in terms of their nature, oxidn. states, coordination environment and coordinative unsatn., and location at faces, edges or corners of microcrystallites. When applied to materials with surface hydroxy groups CO undergoes hydrogen bonding and information can be collected on the proton acid strength.
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      Weckhuysen, B. M.; Verberckmoes, A. A.; Baets, A. R. D.; Schoonheydt, R. A. Diffuse Reflectance Spectroscopy of Supported Chromium Oxide Catalysts: A Self-Modeling Mixture Analysis. J. Catal. 1997, 166, 160– 171,  DOI: 10.1006/jcat.1997.1518
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      Diffuse reflectance spectroscopy of supported chromium oxide catalysts: a self-modeling mixture analysis
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      Journal of Catalysis (1997), 166 (2), 160-171CODEN: JCTLA5; ISSN:0021-9517. (Academic)
      Diffuse reflectance spectra of hydrated, calcined, and reduced chromia/silica-alumina (Cr/SiO2·Al2O3) catalysts with different SiO2 contents have been investigated by using an interactive self-modeling mixt. anal. Four pure components are revealed in the spectra of Cr-catalysts before and after calcination: these are component A with three characteristic bands at 225, 325, and 495 nm, component B with three bands at 220, 275, and 400 nm, component C absorbing at 565 nm, and component D which absorbs in the region 205-270-350 nm. Components A and B are due to chromate and dichromate, resp. and their relative ratio increases with decreasing SiO2-content of the support. Component c is assigned to pseudo-octahedral Cr3+ and is esp. present on SiO2 after calcination, while component D is a background due to the support. After CO-redn. three (E, F, and G) and four (E, F, G, and H) pure components were extd. from the spectra of Cr/Al2O3 and Cr/SiO2, resp. Components E and G have absorptions around 225, 355, and 475 nm and are due to Cr6+. They decrease with increasing redn. temp. Component F absorbs at 635 nm on Al2O3 and at 855 nm on SiO2. These bands are assigned to pseudo-octahedral Cr3+ and Cr2+, resp. Pure component H, only present on Cr/SiO2, absorbs at 305 and 540 nm and is possibly due to traces of Cr3+. All these findings are discussed in relation with previous results obtained by spectral deconvolution.
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      Sattler, J. J. H. B.; Gonzalez-Jimenez, I. D.; Mens, A. M.; Arias, M.; Visser, T.; Weckhuysen, B. M. Operando UV-Vis spectroscopy of a catalytic solid in a pilot-scale reactor: deactivation of a CrOx/Al2O3 propane dehydrogenation catalyst. Chem. Commun. 2013, 49, 1518– 1520,  DOI: 10.1039/c2cc38978a
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      Operando UV-Vis spectroscopy of a catalytic solid in a pilot-scale reactor: deactivation of a CrOx/Al2O3 propane dehydrogenation catalyst
      Sattler, J. J. H. B.; Gonzalez-Jimenez, I. D.; Mens, A. M.; Arias, M.; Visser, T.; Weckhuysen, B. M.
      Chemical Communications (Cambridge, United Kingdom) (2013), 49 (15), 1518-1520CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A novel operando UV-Vis spectroscopic set-up has been constructed and tested for the investigation of catalyst bodies loaded in a pilot-scale reactor under relevant reaction conditions. Spatiotemporal insight into the formation and burning of coke deposits on an industrial CrOx/Al2O3 catalyst during propane dehydrogenation has been obtained.
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      Brozek, C. K.; Dinca, M. Ti3+-, V2+/3+-, Cr2+/3+-, Mn2+-, and Fe2+-Substituted MOF-5 and Redox Reactivity in Cr- and Fe-MOF-5. J. Am. Chem. Soc. 2013, 135, 12886– 12891,  DOI: 10.1021/ja4064475
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      Ti3+-, V2+/3+-, Cr2+/3+-, Mn2+-, and Fe2+-Substituted MOF-5 and Redox Reactivity in Cr- and Fe-MOF-5
      Brozek, Carl K.; Dinca, Mircea
      Journal of the American Chemical Society (2013), 135 (34), 12886-12891CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The metal nodes in metal-org. frameworks (MOFs) are known to act as Lewis acid catalysts, but few reports have explored their ability to mediate reactions that require electron transfer. The unique chem. environments at the nodes should facilitate unusual redox chem., but the difficulty in synthesizing MOFs with metal ions in reduced oxidn. states has precluded such studies. Herein, we demonstrate that MZn3O(O2C-)6 clusters from Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) serve as hosts for V2+ and Ti3+ ions and enable the synthesis of the first MOFs contg. these reduced early metal ions, which can be accessed from MOF-5 by postsynthetic ion metathesis (PSIM). Addnl. MOF-5 analogs featuring Cr2+, Cr3+, Mn2+, and Fe2+ at the metal nodes can be obtained by similar postsynthetic methods and are reported here for the first time. The inserted metal ions are coordinated within an unusual all-oxygen trigonal ligand field and are accessible to both inner- and outer-sphere oxidants: Cr2+- converts into Cr3+-substituted MOF-5, while Fe2+-MOF-5 activates NO to produce an unusual Fe-nitrosyl complex.
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      Zeng, Y.; Chammingkwan, P.; Baba, R.; Taniike, T.; Terano, M. Activation and Deactivation of Phillips Catalyst for Ethylene Polymerization Using Various Activators. Macromol. React. Eng. 2017, 11, 1600046– 1600051,  DOI: 10.1002/mren.201600046
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      McDaniel, M. P. Influence of Catalyst Porosity on Ethylene Polymerization. ACS Catal. 2011, 1, 1394– 1407,  DOI: 10.1021/cs2003033
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      Influence of Catalyst Porosity on Ethylene Polymerization
      McDaniel, M. P.
      ACS Catalysis (2011), 1 (10), 1394-1407CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The structure and porosity of Cr/silica catalysts has a strong influence on its activity in ethylene polymn. and on the character of the polymer it produces. In this study, silicas of widely varying phys. structure were chosen so that the influence of surface area, pore vol., pore diam., and coalescence could be independently investigated by monitoring the surface activity, the polymer mol. wt. (MW), MW distribution, melt flow, and the amt. of long-chain branching (LCB). The results are discussed with respect to (1) fragmentation of the silica during polymn., and (2) egress of polymer from pores inside the resulting fragments. Pores of narrow diam. were found to inhibit polymer egress, resulting in lower surface participation, which in turn raised the mol. wt. Pores of wide diam. were found to produce relatively const. surface participation and polymer mol. wt., but increased the amt. of LCB in the polymer. Variations in MW are seen as a function of the amt. of "crowding" within the pore, whereas variations in LCB are seen as a function of the no. of active sites within the pore cavity. The phys. principles obsd. from Cr/silica catalysts were found to be independent and additive to other (chem.) influences from Cr/silica catalysts. Moreover, these phys. influences apply to other supports and to metallocene catalysts, as well.
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      Wang, X.; Li, Z.; Han, X.; Han, Z.; Bai, Y. Highly tunable porous organic polymer (POP) supports for metallocene-based ethylene polymerization. Appl. Surf. Sci. 2017, 420, 496,  DOI: 10.1016/j.apsusc.2017.05.157
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      McGuinness, D. S.; Davies, N. W.; Horne, J.; Ivanov, I. Unraveling the Mechanism of Polymerization with the Phillips Catalyst. Organometallics 2010, 29, 6111– 6116,  DOI: 10.1021/om100883n
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      Unraveling the Mechanism of Polymerization with the Phillips Catalyst
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      The mechanism of polymer chain growth at the Phillips catalyst was studied, with the three prevailing mechanistic proposals, the Cosee-Arlman, Green-Rooney, and metallacycle mechanisms, considered. Through anal. of low mol. wt. oligomers/polymers formed during ethylene/α-olefin copolymn. with labeled monomers, the isotopomer distribution is inconsistent with a metallacycle mechanism. Further anal. of polymer formed by copolymn. of labeled ethylene was used to rule out a Green-Rooney mechanism. The results support the notion of chain growth via a Cossee-Arlman process.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtlans7fN&md5=638253c7750fb79c604cb1527d4f4c9b
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      Venderbosch, B.; Oudsen, J. P. H.; Wolzak, L. A.; Martin, D. J.; Korstanje, T. H.; Tromp, M. Spectroscopic investigation of the activation of a chromium-pyrrolyl ethene trimerization catalyst. ACS Catal. 2019, 9, 1197– 1210,  DOI: 10.1021/acscatal.8b03414
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      ACS Catalysis (2019), 9 (2), 1197-1210CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      1-Hexene is an important α-olefin for polyethylene prodn. and is produced from ethene. Recent developments in the α-olefin industry have led to the successful commercialization of ethene trimerization catalysts. An important industrially applied ethene trimerization system uses a mixt. of chromium 2-ethylhexanoate, AlEt3, AlEt2Cl, and 2,5-dimethylpyrrole (DMP). Here, we have studied the activation of this system using catalytic and spectroscopic expts. (XAS, EPR, and UV-vis) under conditions employed in industry. First, chromium 2-ethylhexanoate was prepd. and characterized to be [Cr3O(RCO2)6(H2O)3]Cl. Next, the activation of chromium 2-ethylhexanoate with AlEt3, AlEt2Cl, and DMP was studied, showing immediate redn. ( < 5 ms) of the trinuclear Cr(III) carboxylate and formation of a neutral polynuclear Cr(II) carboxylate complex. Over time, this Cr(II) carboxylate complex is partially converted into a chloro-bridged dinuclear Cr(II) pyrrolyl complex. In cyclohexane, small quantities of an unknown Cr(I) complex (∼1% after 1 h) are obsd., while in toluene, the quantity of Cr(I) is much higher (∼23% after 1 h). This is due to the formation of cationic bis(tolyl)Cr(I) complexes, which likely leads to the obsd. inferior performance using toluene as the reaction solvent. Catalytic studies allow us to exclude some of the obsd. Cr(I) and Cr(II) complexes as the active species in this catalytic system. Using this combination of techniques, we have been able to structurally characterize complexes of this selective Cr-catalyzed trimerization system under conditions which are employed in industry.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXisFKitbjO&md5=a3254ac5dce82bfc5acda3fc721636a2
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    • Data and experimental details for the synthesis and characterization of coordination complex 1, experimental details on XRD, N2 adsorption at 77 K, TGA-MS, and SEM of the MIL-100 and MIL-101 materials, characterization of polyethylene obtained from the gas phase by XRD with MIL-101(Cr), more detailed UV–vis–NIR DRS experiments upon addition of DEA, and details on the physicochemical properties of the polymer materials obtained by DSC, GPC, and additional SEM images (PDF)


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