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Ligand-Aided Glycolysis of PET Using Functionalized Silica-Supported Fe2O3 Nanoparticles
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Ligand-Aided Glycolysis of PET Using Functionalized Silica-Supported Fe2O3 Nanoparticles
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ACS Sustainable Chemistry & Engineering

Cite this: ACS Sustainable Chem. Eng. 2023, 11, 43, 15544–15555
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https://doi.org/10.1021/acssuschemeng.3c03585
Published October 18, 2023

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Abstract

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The development of efficient catalysts for the chemical recycling of poly(ethylene terephthalate) (PET) is essential to tackling the global issue of plastic waste. There has been intense interest in heterogeneous catalysts as a sustainable catalyst system for PET depolymerization, having the advantage of easy separation and reuse after the reaction. In this work, we explore heterogeneous catalyst design by comparing metal-ion (Fe3+) and metal-oxide nanoparticle (Fe2O3 NP) catalysts immobilized on mesoporous silica (SiO2) functionalized with different N-containing amine ligands. Quantitative solid-state nuclear magnetic resonance (NMR) spectroscopy confirms successful grafting and elucidates the bonding mode of the organic ligands on the SiO2 surface. The surface amine ligands act as organocatalysts, enhancing the catalytic activity of the active metal species. The Fe2O3 NP catalysts in the presence of organic ligands outperform bare Fe2O3 NPs, Fe3+-ion-immobilized catalysts and homogeneous FeCl3 salts, with equivalent Fe loading. X-ray photoelectron spectroscopy analysis indicates charge transfer between the amine ligands and Fe2O3 NPs and the electron-donating ability of the N groups and hydrogen bonding may also play a role in the higher performance of the amine-ligand-assisted Fe2O3 NP catalysts. Density functional theory (DFT) calculations also reveal that the reactivity of the ion-immobilized catalysts is strongly correlated to the ligand–metal binding energy and that the products in the glycolysis reaction catalyzed by the NP catalysts are stabilized, showing a significant exergonic character compared to single ion-immobilized Fe3+ ions.

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Low metal loading catalysts for PET glycolysis using surface-bound organic amines to enhance the activity of Fe2O3 NPs on SiO2 support.

1. Introduction

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In recent years, research in developing pathways to convert waste polymers into its constituent monomers or value-added products has grown exponentially with the key goal of achieving a circular economy in the polymer life cycle. (1) Polyethylene terephthalate (PET) is a very useful polymer in our everyday life which includes textiles, packaging, beverages among many others. (2,3) However, it is also one of the largest components of postconsumer plastic waste in landfills, highlighting the need for effective recycling strategies for this polymer. This has gained huge interest for researchers to find low cost, and environmental friendly ways of degrading PET to useful monomers with the aim of achieving a circular economy in the polymer life cycle. (2,4) There are a variety of chemical recycling pathways for PET including, hydrolysis, (5) alcoholysis, (6) and glycolysis. (3,7) Glycolysis is particularly attractive due to its low cost and mild reaction conditions compared to other depolymerization pathways. (8) The process involves transesterification reaction of PET with excess glycol, usually ethylene glycol, to obtain bis(hydroxyethyl) terephthalate (BHET). (9) BHET can be repolymerized to PET and it is also used in the synthesis of unsaturated polyesters, rigid or flexible polyurethanes, and other fine chemicals. (10)
The glycolysis of PET is typically performed in the presence of a catalyst as the reaction is slow and requires elevated temperatures in the absence of a catalyst. (11) Homogeneous catalysts such as metal salts, (12) metal oxides, (13) and ionic liquids (2,14) are the most commonly used; however, this leads to issues regarding catalyst recovery and contamination of the BHET product.
Numerous heterogeneous catalysts have been developed for PET glycolysis, which are advantageous compared to homogeneous catalysts because they can be easily recovered and reused. Biomass-derived heterogeneous catalysts have been prepared from waste orange peel ash, (15) waste bamboo leaf ash, and calcium-oxide-based catalysts made from eggshells and seafood shells. (16) Cobalt nanoparticles (3 nm) stabilized by tannic acid ligands showed high activity for PET glycolysis with 96% conversion and 77% BHET yield. (17) Various metal oxides nanostructures such as ZnO (18) and Fe3O4 (19) nanodispersions have been reported. Son et al. (20) reported the use of exfoliated manganese oxide nanosheets for PET glycolysis, leading to full conversion of PET at 0.01 wt % catalysts, at 200 °C after 30 min. Molybdenum-doped ultrathin ZnO nanosheets display far superior yields of BHET (94.5%) than standard undoped ZnO catalysts (54.7%) which have been previously reported in literature. (21) The Mo atoms replace Zn atoms at defect sites, forming Mo–Zn bonds, influencing the electronic structure of the catalyst, and promoting electron transfer of the glycolysis reaction. Mesoporous SBA-15 catalysts doped with ZnO gave a 91% BHET yield, and the catalyst displayed good stability and high catalytic activity when recycled. Ion-based heterogeneous catalysts such as metal organic frameworks (MOFs) (22,23) and zeolites (24) have also been reported as good catalysts for the depolymerization of PET due to their highly ordered structures, high surface areas, and porosity. (23) Yang et al. (25) reported the use of metal azolate framework-6 catalyst (a sub-class of MOFs) with a high density of zinc ion species immobilized on the surface achieving 92.4% conversion of PET and 81.7% yield of BHET. Wang et al. (26) investigated the use of metal ions immobilized on a polymer ionic liquid, (BVim)NTf2-Zn2+, which gave 95.4% PET conversion and 77.8% BHET yield.
It is evident from the literature that a wide variety of heterogeneous catalysts are effective for PET Glycolysis, ranging from ion- immobilized catalysts to nanoparticles, and Table S1 summarizes reports of heterogeneous catalysts and conditions in the literature. The aim of this work is to gain insight into the optimum heterogeneous catalyst design for PET glycolysis with a particular focus on the immobilization of metal-ion and nanoparticle-based catalysts. Surface modification of a catalyst support, such as SiO2, with organic linkers is a convenient strategy to immobilize a high density of transition metal ions dispersed on a support material. The nature of the organic linker used to tether the ion can influence the steric and the electronic environment with the metal center, which in turn can influence catalytic activity. (27) In this work, Fe ions were immobilized onto porous silica functionalized with different organic linkers, which are illustrated in Scheme 1. Solid-state NMR is used for quantitative analysis of the modified surfaces. The ion-based catalysts were then converted to NP catalysts either by calcination, which removes the organic ligands, or by chemical reduction, which preserves the ligand. The heterogeneous catalysts were evaluated for PET glycolysis to gain insight into how the immobilization of the metal species and the organic ligands influenced catalytic performance. Finally, DFT calculations were carried out to unveil the reaction mechanism, and by this way correlating experimental observations with an atomistic interpretation.

Scheme 1

Scheme 1. (a) Synthesis Pathways to Prepare Mesoporous SiO2 with Five Different Organic Capping Ligands SiO2–NH2 (1); SiO2–NH2–SB (2); SiO2–NH–NH2 (3), SiO2–NH–NH2–SB (4) and SiO2–Pincer (5), and (b) the Catalysts Preparation Scheme
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2. Results and Discussion

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2.1. Catalyst Design and Synthesis

Silica is an excellent catalytic support material that allows the surface chemistry to be easily altered through surface functionalization using silylation chemistry. (28) Mesoporous cellular foam (MCF) SiO2 is a three-dimensional (3D)-connected pore structure that was chosen in this study because of its relatively large pore diameter (100 nm) in comparison to those of other porous silicas. Scheme 1a shows the synthesis pathways to prepare mesoporous SiO2 with five different capping ligands: SiO2–NH2 (1), SiO2–NH2–SB (2), SiO2–NH–NH2 (3), SiO2–NH–NH2–SB (4), and SiO2–pincer (5) (see the Supporting Information for Experimental Section and reaction details).
X-ray photoelectron spectroscopy (XPS) was employed to verify the successful functionalization of the SiO2 support with each of the five ligands. After functionalization, a N signal appeared in all the N 1s core levels as shown in Figure 1a. All catalysts displayed a dominant peak at a binding energy (BE) of 399 eV attributed to the amine functional group and a shoulder peak at ∼400 to 402 eV attributed to protonated amines typically observed on modified SiO2. (29) No N signal was detected in bare SiO2. The O 1s core level, shown in Figure 1b, also confirms functionalization with the BE of the bare silica centered at 533.75 eV downshifted to 532.5 eV, associated with the loss of Si–OH groups after surface modification. (30) The Si 2p core level of bare SiO2, shown in the Supporting Information Figure S2, shows a single peak at 104.25 eV and after functionalization, the Si 2p peak can be deconvoluted in several peaks associated with different Si environments.

Figure 1

Figure 1. XPS analysis of the (a) N 1s spectra and (b) O 1s core level for bare silica and the functionalized SiO2 with –NH2 (1), –NH2 SB (2), –NHNH2 (3), –NHNH2 SB (4), and pincer (5), respectively.

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It is well recognized that silylation chemistry can result in significantly more complicated surface attachment chemistry than is often illustrated across the literature. (29) Solid-state NMR is a powerful tool to provide insight into the grafting chemistry. (31,32) The 1D 29Si and 13C NMR spectra of the bare and five different functionalized SiO2 ions are presented in Figure 2. For the 29Si NMR spectra, spectral deconvolution was carried out using ssNake (33) (using Gaussian/Lorentzian line-shapes for fitting) to identify the individual species present. The 29Si NMR spectrum of bare SiO2 (Figure 2a) exhibit resonances at ca. −91.8, −100.9, and −110.1 ppm, corresponding to Q2, Q3, and Q4 sites, respectively. Detailed fitting of these spectra is shown in Figure 2a,b. The spectra of all six samples in Figure 2c, show the presence of Si(SiO)n(OH)4–n sites (denoted Qn with n = 2, 3 and 4), characteristic of silica-type materials. (34)

Figure 2

Figure 2. 1D (a) 29Si MAS NMR spectra together with their deconvolution of bare silica and (b) functionalized SiO2–NH2 (1). (c) Stacked plot of the 29Si MAS NMR spectra of all samples; bare silica, SiO2–NH2 (1), SiO2–NH2–SB (2), SiO2–NH–NH2 (3), SiO2–NH–NH2–SB (4), and SiO2–pincer (5); (d) 1H → 13C CPMAS NMR spectra of the functionalized silica with ligands (1) to (5).

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Figure 2c compares the NMR spectra of unfunctionalized and functionalized SiO2. The spectra of the functionalized SiO2 show the appearance of two peaks centered at −67.3 and −60 ppm, which arise from T2 [(≡SiO−)2SiR(−OH)], with R being the alkyl chain)) and T3 [(≡SiO−)3SiR] sites, respectively, (32,35) confirming the successful grafting of the amine groups onto SiO2.
Quantitative 29Si NMR spectra allowed the determination of the relative surface coverage of each species present (Qn and Tn sites) (see Table 1). The relative areas of Q4 sites remains almost constant in all the samples, and the degree of condensation of the Si atoms as calculated in Table 1 remain almost constant for all the ligands, confirming that the SiO2 structure remains intact after functionalization. In addition, the low content of Q2 sites indicates the preference of the ligands to occupy the Q2 sites. The relative areas of Qn sites can be used to determine the degree of surface functionalization, giving surface coverages between 40 and 73%, as shown in Table 1. However, no clear correlation between the surface coverage and structure of the ligand was found, but the analysis demonstrates that the attachment of the ligand, and therefore coverage, is strongly influenced by the primary linker molecule. When the Schiff base (SB) ligand is reacted with SiO2–NH2 to convert ligand 1 to ligand 2 there is a decrease in Q2 + Q3 sites (with increasing T3 + T2), indicating that the SB precursor (2-pyridinecarboxaldehyde) is attaching to surface –OH groups of the SiO2 particles in addition to the terminal –NH2 groups. Consequently, the SB ligand has a significantly higher surface coverage (73%) compared to those of the other ligands. This statement is supported by the appearance of an additional peak in the 13C CPMAS spectra of the SiO2–NH2 SB and SiO2–NH–NH2 SB catalysts (in the region of 50–100 ppm) (Figure S3). This signal could be attributed to the presence of an aldehyde group (36) with the surface silanol groups, which would explain the differences observed in 29Si NMR. In contrast, using the SiO2–NH–NH2 ligand (ligand 3 → 4) to attach the SB resulted in an increase in the relative areas for the Q2 + Q3. This observation may indicate that the NH2 groups in the diamino ligands interact with SiO2 surface likely due to the longer ligand length and, therefore, silanol groups can be introduced by rehydroxylation. (37) Consequently, the surface coverage of the SiO2–NH–NH2–SB catalyst was the lowest of those of the ligands.
Table 1. 29Si NMR Quantitative Data for Each of Five Different Capping Ligands SiO2–NH2 (Ligand 1); SiO2–NH2–SB (Ligand 2); SiO2–NH–NH2 (Ligand 3), SiO2–NH–NH2–SB (Ligand 4), and SiO2–Pincer (Ligand 5)
  % Si sites    
 sampleQ4Q3Q2T3T2Q3 + Q2T 3 + T 2relative surface coverageadegree condens. Si atomsb
 SiO268.227.84.1  31.8   
Ligand 1SiO2 + NH258.515.11.515.79.116.624.847.792.4
Ligand 2SiO2 + NH2SB61.180.412.118.48.430.573.691.7
Ligand 3SiO2 + NHNH268.515.21.48.26.816.615.048.093.3
Ligand 4SiO2 + NHNH2SB65.818.51.74.99.120.213.936.591.5
Ligand 5SiO2 + Pincer61.718.40.69.99.419.019.240.392.0
a

The relative surface coverage was determined using the equation .

b

The degree of condensation of Si atoms was determined using (see the Supporting Information for details).

Figure 2 shows the stacked 13C CPMAS spectra of the functionalized Si–NH2 catalysts with their principal functional groups. The presence of the alkyl groups appearing in SiO2–NH2 (1) and SiO2–NH–NH2 (3) catalysts (from 60 to 10 ppm) further confirms successful grafting of both amine ligands onto SiO2. (38,39) The SiO2–NH2 SB and SiO2–NH–NH2 SB catalysts, show the presence of aromatic peaks between 175 and 100 ppm confirming successful functionalization with the Schiff base ligands. (39) The alkyl groups and aromatic peaks were also observed in SiO2-pincer when the spectrum was recorded at −40 °C, confirming successful functionalization of the pincer ligand.
After organic functionalization, Fe ions were immobilized onto the SiO2 (see the Experimental Section for details). We chose iron as the metal catalyst as iron salts are effective for glycolysis of PET and have lower environmental impact compared with other effective metal-ion catalysts such as Zn2+. Inductively coupled plasma-mass spectroscopy (ICP-MS) was used to determine the Fe loading in each of the catalysts. Table S2 (see Supporting Information) displays the quantitative results with the Fe loadings for each ligand, with the highest loading for the simple amine ligand SiO2–NH2 (4172.8 ppm) and lowest for the SiO2–pincer ligand (3904 ppm), which can be expected given the bulky nature of the ligand. The Fe-ion-immobilized catalysts were then converted to iron oxide nanoparticle (NP) catalysts by calcination at 450 °C, which removed the organic ligands on the SiO2 surface, or by treatment with aqueous NaBH4 as a rapid reducing agent, which preserved the organic ligands, as represented in Scheme 1b.
Figure 3a–c shows the TEM analysis of the SiO2 before and after NP formation by calcination and chemical reduction, respectively and associated SEM images are shown in the Supporting Information Figure S4. Catalysts prepared by calcination produced larger NPs compared to catalysts produced by chemical reduction, which is to be expected as rapid reduction by NaBH4 is known to produce small diameter NPs. (40) XRD analysis of the calcined catalysts, shown in Figure 3d, shows the characteristic peaks for α-Fe2O3 in excellent agreement with JCPDS card no. 33-0664. Diffraction peaks were not observed for the catalyst produced by chemical reduction. Although challenging to visual due to their small size and low atomic mass contrast in TEM, the presence of nanoparticles embedded in the SiO2 can be seen in TEM Figure 3c. EDX mapping of the catalyst prepared by chemical reduction is shown in Figure 3e–h and confirms the uniform presence of Fe across the SiO2 support.

Figure 3

Figure 3. TEM images of SiO2 support (a) before Fe-ion immobilized, (b) after calcinations, and (c) after chemical reduction. (d) XRD of SiO2 catalysts prepared by calcination and chemical reduction, (e) high angle angular dark field image of SiO2 catalyst after chemical reduction, and associated (f) Si, (g) Fe, and (h) O EDX maps. Scale bars in (e,f) are 100 nm.

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XPS analysis of the NP catalysts was carried out to determine the chemical states of the iron oxide NP and assess the interaction of the ligand and the NPs. Figure 4a shows the Fe 2p spectrum for the SiO2–NH2–SB NP prepared by calcination (C) and a reducing agent (R), which are very similar. The Fe 2p3/2 is located at a BE of 711 eV and in good agreement with that reported for Fe2O3. (41) The Fe 2p3/2 also has a clear shake up satellite peak at 718 eV, a characteristic of Fe3+, further indicating the presence of Fe2O3. Figure 4 (b) shows the O 1s core level before Fe adsorption with a dominant peak at 532.6 eV associated with the SiO2. After NP formation, an additional peak at a BE of 530.2 eV appears in the O 1s for catalysts prepared by calcination and chemical reduction, which is in excellent agreement with the BE for Fe–O–Fe, consistent with the formation of NPs. (42) The O 1s core level of the other NP catalysts, shown in the the Supporting Information, Figure S5, displays the same trend with the appearance of the Fe–O–Fe peak in catalyst be annealing and chemical reduction, which is attributed to NP formation. The Fe 3p shown in the Supporting Information (Figure S6) is fit to a single peak with a BE of 55 eV, again indicating the presence of Fe2O3. Figure 4c shows the N 1s core level of the SiO2–NH2–SB catalyst, with a dominant peak at a BE of 399 eV, due to the amine functional group and a shoulder peak at 401 eV, attributed to protonated amines, as aminosilane protonation occurs from the support Si–OH. (29) The peak intensity of the N 1s in the calcined catalyst decreases substantially, as excepted, due to loss of the ligand, but there are residual ligands present. The Si/N ratio estimates that ∼85% of the ligands are removed. The N 1s peak of both the calcined and NaBH4-treated catalysts is upshifted to a BE of 339.8 eV, indicating charge transfer from the organic ligand to the Fe2O3 NPs. XPS analysis of the other catalysts also shows the same trend with the N 1s core level shifting upward (see the Supporting Information Figure S7).

Figure 4

Figure 4. XPS core level spectra of (a) Fe 2p (b) O 1s and (c) N 1s for the SiO2–NH-SB and the corresponding Fe2O3 NP catalysts prepared via calcination (C) and chemical reduction (R).

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2.2. Catalytic Evaluation in PET Glycolysis

The catalytic performance of all 20 catalysts for the glycolysis of PET was investigated under the same reaction conditions. Figure 5a–d compares the PET conversion and isolated yields of BHET for (a) SiO2 modified with the organic ligands, that is, before metal loading, (b) the Fe-ion-based catalysts (c), the Fe2O3 NPs prepared by calcination (d), and the ligand-Fe2O3 NP catalysts prepared by chemical reduction. It is worth noting that the analysis of the supernatant showed residual BHET remaining in the solution left after recrystallization, but no BHET dimer was detected by NMR. The activity of the functionalized SiO2 was tested as reference before the immobilizing the iron ion precursor. Interestingly, the SiO2 consisting only of the organic ligand attached to the surface displayed moderately good PET conversion as shown in Figure 4a. Good catalyst activity corresponded to organic ligands with terminal primary amines, that is, the NH2 and NH–NH2 ligands, having a conversion of 61 and 75%, respectively. This activity is associated with the ligands being organic bases and depolymerization occurred by based-catalyzed glycolysis of PET. Strong organic bases such as guanidines are popular for homogeneous catalyzed PET glycolysis and are attractive as they offer a metal-free depolymerization route but organocatalysts can lead to contamination in the monomer product. While we did not further investigate the catalytic activity of the organically functionalized, that is, metal-free SiO2 in this study, the results in Figure 4a nevertheless illustrate the potential of using heterogeneous organocatalysts for PET glycolysis. The bare SiO2 support did not catalyze the reaction to any extent.

Figure 5

Figure 5. Percentage conversion of PET (blue) and isolated yield of BHET (green) obtained from glycolysis reaction catalyzed by (a) organic ligand modified SiO2 (before metal loading), (b) iron–ion-immobilized SiO2, (c) Fe2O3–NP SiO2 via calcination, and (d) Fe2O3–NP SiO2 prepared by chemical reduction.

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Figure 5 compares the catalytic results after Fe-ion immobilization. The PET conversion increased for Fe immobilized on the SiO2–NH2, and SiO2–NH2–SB, with conversions of 81 and 84%, respectively, which is expected due to the presence of Fe3+ ions. A notable conclusion from Figure 5b is the considerable difference in activity observed between the Fe ions immobilized on SiO2 using the two Schiff base ligands. High conversion was observed for Fe-ion immobilized using the SiO2–NH2–SB ligand (84%), however only a 10% conversion was observed when using a SiO2–NH–NH2–SB ligand. While the Fe loading used in the reaction was equivalent, the organic ligand coverage on the SiO2–NH2–SB catalyst (74% surface coverage) is much greater than that of the SiO2–NH–NH2–SB (37% surface coverage) as previously described by the solid-state NMR analysis, indicating that the organic ligand and coordination chemistry of the iron on the surface-modified SiO2 plays a crucial role in the observed catalytic behavior. The Fe3+ ions immobilized using the pincer ligand did not catalyze the reaction to any extent, which was attributed to the tight binding of the Fe ions to the ligand.
Figure 5c,d shows the PET conversion and isolated yields obtained by using the NP catalysts obtained by calcination and chemical reduction, respectively. Generally, the NP catalysts displayed similar or higher PET conversions and BHET yields compared to their ion-immobilized version, regardless of whether they were prepared by calcination or reduction, with exception of one catalyst: SiO2–NH2. The conversion of PET decreased from 81% in the ion SiO2–Fe3+–NH2 catalyst to 45% when the catalyst was converted to NPs by calcination. However, when the catalyst was prepared by a chemical reduction, a conversion of 90% was obtained. This trend is attributed to the dual catalytic depolymerization, as reported by Dove and coworkers, (43) due to the preservation of the amine ligands, which are catalytically active in the reaction, as also seen in Figure 5a. All the NP catalysts prepared by chemical reduction were superior to catalysts prepared by calcination, attributed to the formation of smaller NPs, and also the presence of the amine ligands that remain on the surface. In addition to the based catalysis glycolysis, synergistic effects have been shown to occur between metal salts and organic additives for PET glycolysis under homogeneous conditions. A recent report showed the addition of anisole to homogeneous PET glycolysis using alkali metal salts facilitated a lower reaction temperature. (44) DFT calculations suggested that the electron-donating methoxy group in anisole transfers electron density to the carbonyl O atom making it more nucleophilic and thereby susceptible to attack by metal-ion species. The electron-donating ability of N containing groups in the ligands, as evidenced by XPS analysis, may have a similar effect in aiding glycolysis. Additionally, the role of H-bonding using N-containing organocatalysts for PET glycolysis has been shown to be important in activating the carbonyl group of PET and the −OH of EG. (45)
The catalytic evaluation highlights some important features of catalyst design for PET glycolysis: (i) the organic ligand used to bind the metal-ion can significantly impact the catalytic performance as illustrated by the different reactivity behavior, (ii) the metal oxide NP catalysts are in general superior to metal-ion-immobilized catalysts, and (iii) the presence of both the organic ligand and metal oxide NP (i.e., catalysts prepared by chemical reducing agent) gave the best performance. We used DFT calculations to rationalize the observed catalytic trends and gain further insights into the reaction.
First, we analyze the strength of the interaction between the Fe3+ metal cation and the functionalized surface, by calculating the interaction energies (ΔEint, see the Experimental Section in the Supporting Information for details). We performed optimizations of the functionalized SiO2/Fe3+ surfaces considering three possible electronic spin states, translated to three different electronic spin multiplicities (M = 2, 4, 6), showing that the high-spin M = 6 is the ground state (see the Supporting Information, Table S3). The nature of the surface under study allows the SiO2 surface to be functionalized with up to three organic linkers in the same unit cell. The calculated ΔEint values represented in Table 2 are for the different organic linkers containing one, two, or three ligands (1L, 2L, and 3L, respectively).
Table 2. Calculated Interaction Energies (ΔEint, in kcal·mol–1) for the Fe+3 Ion Coordination with the Organic Ligands Supported on the SiO2 Surfaces in the Solvent Phase (Water)
structure1L2L3L
SiO2–NH2–1.66–80.73–143.30
SiO2–NH2–SB–40.90–172.47–200.58
SiO2–NH–NH2–123.83–173.80–258.27
SiO2–NH–NH2–SB–84.48–227.11–303.82
SiO2–pincer–132.54–273.49–290.01
The organic ligands present a similar trend with ΔEint dramatically decreasing as fewer ligands are involved, meaning that the iron ion is trapped within a net of coordinating bonds between the organic ligands (see Figure 6, where the SiO2-ligand structures interacting with Fe3+ are shown). The two poorest performing catalysts, Fe3+ immobilized on SiO2–NH–NH2–SB and SiO2-pincer, have the most favorable ΔEint values as shown in Table 2, which effectively traps the ion in between the three sterically bulky organic ligands. For the SiO2-pincer catalyst, Fe3+ is coordinated with six N groups of the three pyridine rings. For the SiO2–NH–NH2–SB catalyst, the Fe3+ is coordinated to the three imine and the three pyridinic N groups. For these ligands, Fe3+ presents the most favorable interaction energies, the metal being coordinated in an octahedral geometry and so sterically blocked from reacting, giving poor performance as observed. In the SiO2–NH2–SB catalyst Fe3+ is coordinated between three pyridinic Ns and two imine groups but, unlike the SiO2–NH–NH2–SB catalyst, the metal-ion is not octahedrally coordinated, resulting in a slightly lower ΔEint value. This less ΔEint and lower coordination geometry results in a less sterically hindered Fe3+ center, which would contribute to a better catalytic performance of the SiO2–NH2–SB catalyst (84% conversion) compared to the SiO2–NH–NH2–SB catalyst (10% conversion). A highly favorable metal-ion ΔEint is also attributed to why the performance for the SiO2–NH–NH2 catalyst did not improve after Fe-ion immobilization. In this system, the ligand only catalyst demonstrated relatively good reactivity with 75% PET conversion. After Fe-ion immobilization (loading 4043.3 ppm), there was no change in the PET conversion (73%). In this catalyst, the Fe3+-ion is coordinated to six amine groups, combining three primary amines and three secondary amines, in an octahedral geometry. The high interaction energy limits the participation of the Fe3+ in the reaction and furthermore, the presence of Fe3+ also prevents the terminal amine groups from catalyzing the reaction. This is in contrast with the SiO2–NH2 catalyst where Fe3+ is coordinated only by three amine groups giving it a lower BE and so the PET conversion increased after Fe3+ immobilization.

Figure 6

Figure 6. Optimized geometries for the different SiO2/ligands coordinating the Fe3+ ion. Color scheme: gray (C), white (H), blue (N), orange (Fe), red (O), and beige (Si).

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The glycolysis reaction using the Fe3+ion and the NP catalysts were also modeled. The glycolysis of PET is the molecular degradation of the polymer by ethylene glycol (EG) through a transesterification reaction, where the ester linkage breaks and is transformed into hydroxyl groups (see the Supporting Information, Figure S7). The reaction process starts with the coordination of PET to the metal center by the oxygen of the carbonyl group of the ester. Since both ester groups are separated by only two –CH2 it is possible to face the coordination of both adjacent ester groups. The following step is the transesterification of the ester by the EG, where the carbonyl carbon is attacked by the free electron pair present on the hydroxyl group of EG. This is followed by the binding between the hydroxyl ethyl group of EG with the carbonyl carbon of PET, resulting in breaking the long polymer chain into two short oligomers. The subsequent glycolysis will break this oligomer forming the BHET product.
This mechanism for PET depolymerization was modeled here in the absence and in the presence of either a single Fe3+-ion-immobilized catalyst or the NP catalyst (see Figure 7). In these simulations, two repetitive units of the PET polymer (A) were accounted for. In modeling the Fe3+-ion-immobilized catalyst, it is important to consider the availability of the metal center as active site. Therefore, systems where the Fe3+ ion is fully coordinated or trapped between the most voluminous ligands will not perform well due to steric hindrance (e.g., SiO2–NHNH2–SB and SiO2–pincer), as experimentally demonstrated. Therefore, the system selected to represent the single Fe3+-ion catalysts was the SiO2–NH2 catalyst (see Figure 7a). Although this is the system presenting the less favorable ΔEint, it is the structure presenting the least steric impediments. The NP catalyst, to keep the simulations computationally affordable, has been modeled here as an iron oxide nanocluster using the bulk cut nanoparticle model (BCN-M), a computational tool that automatically generates Wulff-like nanoparticle and nanocluster models for binary materials with controlled stoichiometry. (46) This program, by introducing all the data relative to the Fe2O3 surfaces, generated as a minimal cluster a species of stoichiometry Fe6O8, in which four iron centers have a formal oxidation state of Fe3+ and the other two Fe2+. This composition makes our NC model have 28 total of unpaired electrons (five unpaired electrons for the four Fe3+ cations and four unpaired electrons for the two Fe2+ cations), rendering our NC to have a total electronic spin multiplicity of 29. Although the nanocluster (NC) does not perfectly match an Fe2O3 system (due to technical limitations of the software), since the structural modeling of nanostructures of binary ionic systems is a highly delicate task, we assume that the BCN-M-generated Fe6O8 NC is reasonable enough to reproduce the chemistry of a Fe2O3 supported on a SiO2 surface. Thus, we simulated the PET decomposition catalyzed by Fe2O3 NPs, using the adsorbed Fe6O8 NC on a silica surface as a catalyst model (see Figure 7b).

Figure 7

Figure 7. (a) Optimized geometry of the adsorbed PET molecular cluster over the Fe3+ ion. (b) Optimized geometry of the adsorbed PET over the iron oxide NC. Color scheme: gray (C), white (H), blue (N), orange (Fe), red (O, and beige (Si). (c) Relative Gibbs energy profiles at 190 °C (in kcal·mol–1) for PET glycolysis considering EG as a solvent, via uncatalyzed (blue color), catalyzed by a single atom Fe3+ (red color), and catalyzed by the iron oxide nanocluster (purple color).

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Figure 7 depicts the potential energy surfaces (PESs) for the uncatalyzed reaction and those catalyzed by single Fe3+ immobilized on SiO2–NH2 and by the NC catalyst. For the catalyzed reaction, the inset figure shows the PET fragment coordinates with the metal center through the carbonyl group (Fe–O═C). The transesterification reaction mechanism proposed here goes through a four-membered ring transition state (TSA-B) where the carbonyl carbon of the PET molecular cluster is attacked by the hydroxyl group of the EG subsequently forming the hydroxyl–carbonyl bond. Energetic values show that the uncatalyzed reaction not only goes through a high energy barrier but also presents an endergonic character. The activation energy is significantly reduced when performing the reaction with the Fe3+ and NC catalysts, which are similar but lower in the latter case (3.09 kcal mol–1 vs 1.20 kcal mol–1, respectively). The reactions with Fe3+ and the NC also revert the thermodynamics, becoming exoergic processes. Interestingly, reaction free energies are slightly more favorable than those catalyzed by Fe3+, −4.57 and −3.04 kcal mol–1, respectively), as the NC provides more anchoring points that confer stability to the products. Finally, for the sake of comparison, we also performed the same study using a larger NC with stoichiometry (Fe2O3)(FeO)11, that is, with a predominance of formally Fe2+ species (see Table S4). Interestingly, the energy barriers are similar for the two NC types (1.35 kcal mol–1 on the larger NC) but differ on their reaction energies, i.e., that associated with the larger NC is dramatically more favorable (−16.20 kcal mol–1), which also due to the presence of more interacting points that stabilize the newly formed products. Therefore, when performing the glycolysis reaction catalyzed by the NP based catalysts, it is enhanced not only kinetically as the activation energy is lower but also, thermodynamically, favoring the products formation. The increased exergonic character is the main difference between the single Fe3+-ion catalyst and the NP system, due to stabilization of the products and so therefore the metal oxide NP catalyst show superior reaction performance.
Finally, we used the best performing catalyst as identified by the evaluation study, that is, the Fe2O3 NP catalysts prepared by the reduction using the SiO2–NH2–SB and SiO2–pincer ligands, to further study the impact of reaction parameters such as temperature, time, catalyst loading, and PET/EG ratio. Figure 8 compares the effect of reaction temperature and catalyst concentration. The PET conversions decreased at lower catalyst loading reaching full conversion at 100 mg of SiO2 catalyst (0.4 wt % Fe). The optimal temperature of the reaction was found to be 190 °C, with PET conversions decreasing at lower temperatures. Figure 8b shows the ratio of PET/EG was critical for the reaction with a ratio of 1:5 being optimal for full PET conversion. On decreasing the ratio to 1:3, there was insufficient EG to cover the PET for the reaction to proceed smoothly. Figure 8c shows a reaction time series, carried out under the same conditions and iron loading, ranging from one to three h for the heterogeneous SiO2 catalysts compared with a homogeneous FeCl3 catalyst. Interestingly, while all catalysts could achieve full PET conversion, both the heterogeneous catalysts display higher PET conversions compared to the homogeneous catalyst. It is clear that under these conditions, the Fe2O3 NP catalyst outperformed the ion-immobilized catalyst but also a homogeneous catalyst of equivalent Fe loading. This is attributed to the small diameter and high dispersion of the NPs on the SiO2 support, as evidenced by TEM and the presence of the organic ligands, which have a promotional effect on the PET glycolysis due to being organic amines. Figure S9 shows the recyclability of the SiO2–NP pincer catalyst tested over five cycles. The catalyst achieved full PET conversion on two reaction recycles. The conversion does drop after the third cycle (82%) but still displays favorable recyclability given the low loading metal (0.4 wt %) of the catalyst. An additional benefit of using a heterogeneous catalyst was the very low metal contamination in the BHET product, which was analyzed by ICP-MS (see the Supporting Information Table S2). Metal contamination of the BHET monomer from the catalyst in PET glycolysis often necessitates additional purification steps. BHET obtained using FeCl3 as the catalyst contained 4.9 ppm of Fe and visually had a pale yellow-orange color, while the BHET obtained using the heterogeneous catalyst was colorless, with an Fe concentration measured to be 1.3 ppm. Finally, the use of additives and colorants are commonly found in postconsumer plastic, and their impact on catalyst performance is not always explored. The Fe2O3 NP catalyst was used to depolymerize black and green colored PET as shown in Figure 8d, giving high PET conversions of 94% for both black and green plastic showing that this catalyst can effectively be used for colored postconsumer PET.

Figure 8

Figure 8. (a) Optimization of catalyst loading (w.r.t the SiO2 mass) and reaction temperature, (b) PET/EG ratio optimization, (c) time series of FeCl3, SiO2–Fe2O3 pincer NP and SiO2–Fe2O3–NH2SB NP both prepared via a reducing agent. (d) Application of heterogeneous catalysts to colored postconsumer PET.

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3. Conclusions

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In conclusion, we investigated the catalyst design for the glycolysis of PET using heterogeneous mesoporous SiO2 catalysts. The SiO2 support was modified with a range of N containing organic ligands used for the immobilization of Fe ions. Solid-state NMR allowed for the investigation of the structures of the modified SiO2 supports, confirming the successful grafting of the ligands and revealing the distinct types of silicon environments. Surface coverages were found to be between 40 and 73% based on quantitative 29Si NMR experiments. The Fe-ion-immobilized catalysts were converted into Fe2O3 NP supported catalysts by calcination or treatment with aqueous NaBH4 as confirmed by TEM and XPS. In general, the NP-based catalysts performed better than their ion-immobilized catalysts and the presence of both the Fe2O3 NP and organic ligand displayed the best performance giving full PET conversion and highest BHET yields. The ligand-assisted Fe2O3 NP catalyst also outperformed a homogeneous FeCl3 catalyst. The superior performance of the ligand-stabilized Fe2O3 NP catalyst was attributed to cooperative effects between the organic ligand and the NPs. The N-containing amine ligands facilitated base-catalyzed glycolysis of PET, similar to homogeneous organocatalysts and also behaved as Lewis bases to facilitate electron transfer to the carbonyl group making it more susceptible to nucleophilic attack by the metal catalyst. This work shows the significant potential of nanoparticle-based heterogeneous catalysts for PET glycolysis and the use of organically functionalized support materials to further enhance catalytic activity.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acssuschemeng.3c03585.

  • Comparison of heterogeneous catalysts in the literature, catalyst synthesis and DFT computation details, SEM images of unfunctionalized, ion-immobilized SiO2 and NP-immobilized SiO2, Si 2p, Fe 3p, O 1s, and N 1s XPS of catalysts, NMR of BHET product, 13C solid-state NMR spectra of functionalized SiO2, ICP-MS analysis for Fe quantification, and calculated relative Gibbs energies for the modeled reaction systems, catalyst recyclability evaluation (PDF)

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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
    • Éadaoin Casey - School of Chemistry, University College Cork, Cork T12 YN60, Ireland;  AMBER Centre, Environmental Research Institute, University College Cork, Cork T23 XE10, Ireland;  Orcidhttps://orcid.org/0000-0002-1482-3599
    • Rachel Breen - School of Chemistry, University College Cork, Cork T12 YN60, Ireland;  AMBER Centre, Environmental Research Institute, University College Cork, Cork T23 XE10, Ireland
    • Jennifer S. Gómez - Institute for Molecules and Materials, Radboud University, Nijmegen 6525 AJ, The Netherlands
    • Arno P. M. Kentgens - Institute for Molecules and Materials, Radboud University, Nijmegen 6525 AJ, The Netherlands;  Orcidhttps://orcid.org/0000-0001-5893-4488
    • Gerard Pareras - Departament de Química, Universitat Autònoma de Barcelona, Bellaterra, Catalonia 08193, Spain;  Orcidhttps://orcid.org/0000-0002-8435-3297
    • Albert Rimola - Departament de Química, Universitat Autònoma de Barcelona, Bellaterra, Catalonia 08193, Spain;  Orcidhttps://orcid.org/0000-0002-9637-4554
    • Justin. D. Holmes - School of Chemistry, University College Cork, Cork T12 YN60, Ireland;  AMBER Centre, Environmental Research Institute, University College Cork, Cork T23 XE10, Ireland;  Orcidhttps://orcid.org/0000-0001-5087-8936
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This research was funded by Science Foundation Ireland (AMBER grant no: 12/RC2278_P2). We thank the Advanced Microscopy Laboratory at Trinity College Dublin and the Bernal Institute at University of Limerick for XPS. This project received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no 101008500 (PANACEA). The Dutch Science Council (NWO) is acknowledged for the support of the solid-state NMR facility for advanced materials science, which is part of the uNMR-NL ROADMAP facilities (NWO project no. 184.035.002). G.P. is indebted to the “Magarita Salas” program. This research was funded by MINECO (project PID2021-126427NB-I00). G.P. and A.R. gratefully acknowledge the computer resources and technical support provided by the Barcelona Supercomputing Centre (CNS-BSC) and the Consorci de Serveis Universitaris de Catalunya (CSUC).

References

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    Technical, Economic, and Environmental Comparison of Closed-Loop Recycling Technologies for Common Plastics
    Uekert, Taylor; Singh, Avantika; DesVeaux, Jason S.; Ghosh, Tapajyoti; Bhatt, Arpit; Yadav, Geetanjali; Afzal, Shaik; Walzberg, Julien; Knauer, Katrina M.; Nicholson, Scott R.; Beckham, Gregg T.; Carpenter, Alberta C.
    ACS Sustainable Chemistry & Engineering (2023), 11 (3), 965-978CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    Over 400 million metric tons of plastic waste are generated globally each year, resulting in pollution and lost resources. Recycling strategies can recapture this wasted material, but there is a lack of quant. and transparent data on the capabilities and impacts of these processes. Here, we develop a data set of material quality, material retention, circularity, contamination tolerance, min. selling price, greenhouse gas emissions, energy use, land use, toxicity, waste generation, and water use metrics for closed-loop polymer recycling technologies, including mech. recycling and solvent-based dissoln. of polyethylene, polyethylene terephthalate (PET), and polypropylene, as well as enzymic hydrolysis, glycolysis, and vapor methanolysis of PET. Mech. recycling and PET glycolysis display the best economic (9%-73% lower than competing technologies) and environmental (7%-88% lower) performances, while dissoln., enzymic hydrolysis, and methanolysis provide the best recyclate material qualities (2%-27% higher). We identify electricity, steam, and org. solvents as top process contributors to these metrics and apply sensitivity and multicriteria decision analyses to highlight key future research areas. The ests. derived in this work provide a quant. baseline for comparing and improving recycling technologies, can help reclaimers identify optimal end-of-life routes for given waste streams, and serve as a framework for assessing future innovations.
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  9. 9
    Chen, F.; Wang, G.; Shi, C.; Zhang, Y.; Zhang, L.; Li, W.; Yang, F. Kinetics of glycolysis of poly(ethylene terephthalate) under microwave irradiation. J. Appl. Polym. Sci. 2013, 127 (4), 2809– 2815,  DOI: 10.1002/app.37608
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    9
    Kinetics of glycolysis of poly(ethylene terephthalate) under microwave irradiation
    Chen, Feifei; Wang, Guanghui; Shi, Chuan; Zhang, Yichen; Zhang, Long; Li, Wei; Yang, Feng
    Journal of Applied Polymer Science (2013), 127 (4), 2809-2815CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
    The glycolysis of poly(ethylene terephthalate) (PET) was carried out using excess ethylene glycol (EG) in the presence of zinc acetate as catalysts under microwave irradn. The effects of particle size, microwave power, the wt. ratio of EG to PET, the wt. ratio of catalyst to PET, reaction temp. and stirring speed on the yield of bis(hydroxyethyl terephthalate)(BHET) were studied. The glycolysis rate was significantly influenced by stirring speed and initial particle size. The optimal parameters of glycolysis reactions were the wt. ratio of catalyst to PET of 1%, the wt. ratio of EG to PET of 5,500 W and 196 °C, the yield of BHET reached to 78% at only 35 min. The glycolysis products were analyzed and identified by FTIR, differential scanning calorimetry, and elemental anal. The kinetics of glycolysis of PET under microwave irradn. could be interpreted by the shrinking core model of the film diffusion control. The apparent activation energy was evaluated using the Arrhenius equation and it was 36.5 KJ/mol, which was lower compared to the same process using conventional heating. The reaction time was significantly decreased under microwave irradn. as compared with it by conventional heating. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2012.
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  10. 10
    Vollmer, I.; Jenks, M. J. F.; Roelands, M. C. P.; White, R. J.; van Harmelen, T.; de Wild, P.; van der Laan, G. P.; Meirer, F.; Keurentjes, J. T. F.; Weckhuysen, B. M. Beyond Mechanical Recycling: Giving New Life to Plastic Waste. Angew. Chem., Int. Ed. 2020, 59 (36), 15402– 15423,  DOI: 10.1002/anie.201915651
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    10
    Beyond Mechanical Recycling: Giving New Life to Plastic Waste
    Vollmer, Ina; Jenks, Michael J. F.; Roelands, Mark C. P.; White, Robin J.; van Harmelen, Toon; de Wild, Paul; van der Laan, Gerard P.; Meirer, Florian; Keurentjes, Jos T. F.; Weckhuysen, Bert M.
    Angewandte Chemie, International Edition (2020), 59 (36), 15402-15423CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Increasing the stream of recycled plastic necessitates an approach beyond the traditional recycling via melting and re-extrusion. Various chem. recycling processes have great potential to enhance recycling rates. In this Review, a summary of the various chem. recycling routes and assessment via life-cycle anal. is complemented by an extensive list of processes developed by companies active in chem. recycling. We show that each of the currently available processes is applicable for specific plastic waste streams. Thus, only a combination of different technologies can address the plastic waste problem. Research should focus on more realistic, more contaminated and mixed waste streams, while collection and sorting infrastructure will need to be improved, i.e., by stricter regulation. This Review aims to inspire both science and innovation for the prodn. of higher value and quality products from plastic recycling suitable for reuse or valorization to create the necessary economic and environmental push for a circular economy.
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  11. 11
    Duque-Ingunza, I.; López-Fonseca, R.; de Rivas, B.; Gutiérrez-Ortiz, J. I. Process optimization for catalytic glycolysis of post-consumer PET wastes. J. Chem. Technol. Biotechnol. 2014, 89 (1), 97– 103,  DOI: 10.1002/jctb.4101
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    11
    Process optimization for catalytic glycolysis of post-consumer PET wastes
    Duque-Ingunza, I.; Lopez-Fonseca, R.; Rivas, B.; Gutierrez-Ortiz, J. I.
    Journal of Chemical Technology and Biotechnology (2014), 89 (1), 97-103CODEN: JCTBED; ISSN:0268-2575. (John Wiley & Sons Ltd.)
    Glycolysis has been the subject of increased interest as a valuable feedstock recycling for poly(ethylene terephthalate) (PET). However, there are no reports in the tech. literature that deal with the design and optimization of the global process. Conversion into bis(2-hydroxyethyl) terephthalate (BHET) of the non-glycolyzed solid was feasible, carried out in the presence or absence of fresh PET. The yield varied between 63 and 80%. The monomer was recovered by extn. with hot water followed by crystn. at 4 °C. The optimized H2O:BHET ratio was 6.7 mL g-1. The EG(ethylene glycol)/H2O mixt. was effectively sepd. by vacuum distn. and EG of 99.6% purity was recovered, which could be used again. Finally, the highly active catalytic role of sodium carbonate salt for glycolysis was evident not only for transparent PET wastes but also for complex wastes. The recirculation of the solid residue from extn., the sepn. of EG/H2O mixt. from crystn. and the subsequent refeeding of the org. reactant into the reactor, the minimization of the amt. of water to efficiently recover the monomer and the suitability of the chem. recycling of complex PET wastes are operational aspects that significantly increased the global efficiency. © 2013 Society of Chem. Industry.
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  12. 12
    (a) Shukla, S. R.; Kulkarni, K. S. Depolymerization of poly(ethylene terephthalate) waste. J. Appl. Polym. Sci. 2002, 85 (8), 1765– 1770,  DOI: 10.1002/app.10714
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    12a
    Depolymerization of poly(ethylene terephthalate) waste
    Shukla, S. R.; Kulkarni, K. S.
    Journal of Applied Polymer Science (2002), 85 (8), 1765-1770CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
    Poly(ethylene terephthalate) waste was depolymd. with ethylene glycol in the presence of different catalysts, two conventional metal catalysts (zinc acetate and lead acetate) and two alkalies (sodium carbonate and sodium bicarbonate). The resulting monomer bis(2-hydroxyethyl) terephthalate was characterized by thin layer chromatog., m.p., IR spectroscopy, differential scanning calorimetry, and elemental anal. The results show that the qual. and quant. yields of the monomer obtained with alkalies as catalysts were most comparable with the conventional heavy metal catalysts, thus providing a further advantage for the recycling of polyester waste for the cause of environmental pollution abatement.
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    (b) López-Fonseca, R.; Duque-Ingunza, I.; de Rivas, B.; Flores-Giraldo, L.; Gutiérrez-Ortiz, J. I. Kinetics of catalytic glycolysis of PET wastes with sodium carbonate. Chem. Eng. J. 2011, 168 (1), 312– 320,  DOI: 10.1016/j.cej.2011.01.031
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    12b
    Kinetics of catalytic glycolysis of poly(ethylene terephthalate) wastes with sodium carbonate
    Lopez-Fonseca, Ruben; Duque-Ingunza, Itxaso; de Rivas, Beatriz; Flores-Giraldo, Laura; Gutierrez-Ortiz, Jose I.
    Chemical Engineering Journal (Amsterdam, Netherlands) (2011), 168 (1), 312-320CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
    The kinetics of glycolysis of poly(ethylene terephthalate) wastes with ethylene glycol to give highly pure bis(2-hydroxyethyl terephthalate) was examd. in a batch reactor. An excess of ethylene glycol (EG:PET molar ratio of 7.6:1) was used and the reaction was carried out in the presence of sodium carbonate as active catalyst. The influence of several operating conditions covering temp. (165-196°), mean particle size (0.14-3 mm), stirring rate (50-800 rpm), reaction time (0-10 h), and catalyst type and concn. was analyzed. The selected PET particle size and stirring rate for kinetic calcns. were 0.25 mm and 600 rpm, resp. Using a PET:catalyst molar ratio of 100:1 about 80% BHET yield was attained at 196° after 1 h. A simple theor. power-law model was developed to predict the time evolution of conversion. This kinetic model was found to be consistent with exptl. data. The rate consts. for both direct and reverse reactions were estd. Also the values of the activation energy (185 kJ mol-1) and enthalpy of reaction (12 kJ mol-1) were derived.
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    (c) Fang, P.; Liu, B.; Xu, J.; Zhou, Q.; Zhang, S.; Ma, J.; lu, X. High-efficiency glycolysis of poly(ethylene terephthalate) by sandwich-structure polyoxometalate catalyst with two active sites. Polym. Degrad. Stab. 2018, 156, 22– 31,  DOI: 10.1016/j.polymdegradstab.2018.07.004
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    12c
    High-efficiency glycolysis of poly(ethylene terephthalate) by sandwich-structure polyoxometalate catalyst with two active sites
    Fang, Pengtao; Liu, Bo; Xu, Junli; Zhou, Qing; Zhang, Suojiang; Ma, Junying; lu, Xingmei
    Polymer Degradation and Stability (2018), 156 (), 22-31CODEN: PDSTDW; ISSN:0141-3910. (Elsevier Ltd.)
    Catalyst in the process on glycolysis of poly (ethylene terephthalate) (PET) wastes is a significant crit. factor, which det. the efficiency and the cost of PET degrdn. In this study, a kind of transition-metal-substituted polyoxometalates (POMs) Na12 [WZnM2(H2O)2(ZnW9O34)2] (M = Zn2+, Mn2+, Co2+, Cu2+, Ni2+) which have a sandwich-structure and more than two transition metal active sites show excellent catalytic performance in the glycolysis of PET under mild conditions. We investigated the effects of temp., reaction time and catalyst amt. on PET degrdn. and obtained the glycolysis optimal conditions. The PET could be fast and completely degraded at 190 °C for 40 min with low catalyst/PET molar ratio (0.018%) and high PET/Ethylene Glycol (EG) wt. ratio (1:4), and the yield of bis(hydroxyethyl) terephthalate (BHET) is higher than 84.5%. After four-times recycling, the conversion of PET and the yield of BHET can still reach 100% and 84.5%. The exact structure of POMs Na12 [WZnM2(H2O)2(ZnW9O34)2] is confirmed by Single Crystal X-ray Diffraction (SC-XRD). Compared with traditional heteropolyacid catalysts, this catalyst possessed of at least two transition metal active sites, which lead to its an excellent catalytic capacity. The possible coordination activates mechanism for PET glycolysis is also proposed.
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  13. 13
    (a) Chen, F.; Wang, G.; Li, W.; Yang, F. Glycolysis of Poly(ethylene terephthalate) over Mg-Al Mixed Oxides Catalysts Derived from Hydrotalcites. Ind. Eng. Chem. Res. 2013, 52 (2), 565– 571,  DOI: 10.1021/ie302091j
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    13a
    Glycolysis of Poly(ethylene terephthalate) over Mg-Al Mixed Oxides Catalysts Derived from Hydrotalcites
    Chen, Feifei; Wang, Guanghui; Li, Wei; Yang, Feng
    Industrial & Engineering Chemistry Research (2013), 52 (2), 565-571CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
    Poly(ethylene terephthalate) (PET) was depolymd. by ethylene glycol (EG) in the presence of Mg-Al hydrotalcites and their corresponding mixed oxides as solid base catalysts. Mg-Al hydrotalcites with different Mg/Al molar ratios were prepd.; it was confirmed by powder X-ray diffraction (XRD) that the materials had hydrotalcite structure. Compared with their corresponding precursors, Mg-Al mixed oxides obtained by the calcination of hydrotalcites exhibited higher catalytic activity for the glycolysis of PET. Furthermore, Mg-Al mixed oxides calcinated at 500 °C with Mg/Al molar ratio of 3 offered the highest catalytic activity for the glycolysis of PET. The exptl. results showed that the basicity of catalyst played an important role on the glycolysis activity. The solid catalyst could be easily sepd. and reused after calcination again. The influences of exptl. parameters on the yield of bis(hydroxyethyl terephthalate)(BHET) were investigated. The evolution of the glycolysis of PET was described by IR spectroscopy (IR), viscosity-av. mol. wt., and scanning electron microscope (SEM).
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    (b) Fuentes, C. A.; Gallegos, M. V.; García, J. R.; Sambeth, J.; Peluso, M. A. Catalytic Glycolysis of Poly(ethylene terephthalate) Using Zinc and Cobalt Oxides Recycled from Spent Batteries. Waste Biomass Valorization 2020, 11 (9), 4991– 5001,  DOI: 10.1007/s12649-019-00807-6
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    13b
    Catalytic Glycolysis of Poly(ethylene terephthalate) Using Zinc and Cobalt Oxides Recycled from Spent Batteries
    Fuentes, Cynthia A.; Gallegos, Maria V.; Garcia, Juan R.; Sambeth, Jorge; Peluso, Miguel A.
    Waste and Biomass Valorization (2020), 11 (9), 4991-5001CODEN: WBVAAG; ISSN:1877-2641. (Springer)
    The chem. recycling of polyethylene terephthalate (PET) to bis(2-hydroxyethyl) terephthalate (BHET) was studied using recycled metal oxides. Recovered zinc (RZnO) and cobalt (RCoO) oxides were obtained after a biohydrometallurgical process to recycle spent alk. and lithium-ion batteries (LIBs), resp. Besides, a mixed oxide (Co/RZnO) was prepd. by mech. milling of 2.5 wt% of RCoO on RZnO. The structural, textural, and acidity properties of the catalysts were analyzed by XRD, XANES, SEM, TEM, FT-IR, SBET and pyridine-TPD. The depolymn. of PET (from soft-drink bottles) was carried out with ethylene glycol (EG) at 196°C for 2 h, using PET/catalyst and PET/EG ratios of 100:1 and 1:8, resp. The yields of the BHET monomer in the presence of RZnO, RCoO and Co/RZnO as catalysts were 50%, 10% and 80%, resp. The highest catalytic activity of Co/RZnO could be attributed to the presence of weak and strong acid sites, its overall higher concn. of acid sites and a synergetic effect between Co3O4 and ZnO. The obtained BHET was characterized by DSC, FT-IR, 1H NMR and 13C NMR analyses, which confirmed the purity and structure of the monomer. Metal oxides obtained using spent alk. and lithium-ion batteries as raw materials could be used as catalysts for waste PET treatment and pure BHET monomer synthesis. Graphic Abstr.: [Figure not available: see fulltext.].
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  14. 14
    (a) Wang, Q.; Geng, Y.; Lu, X.; Zhang, S. First-Row Transition Metal-Containing Ionic Liquids as Highly Active Catalysts for the Glycolysis of Poly(ethylene terephthalate) (PET). ACS Sustainable Chem. Eng. 2015, 3 (2), 340– 348,  DOI: 10.1021/sc5007522
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    14a
    First-Row Transition Metal-Containing Ionic Liquids as Highly Active Catalysts for the Glycolysis of Poly(ethylene terephthalate) (PET)
    Wang, Qian; Geng, Yanrong; Lu, Xingmei; Zhang, Suojiang
    ACS Sustainable Chemistry & Engineering (2015), 3 (2), 340-348CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    First-row transition metal-contg. ionic liqs. (ILs) were synthesized and used to catalyze the degrdn. of poly(ethylene terephthalate) (PET) in ethylene glycol (EG). One important feature of these IL catalysts is that they have good thermal stability, and most of them, esp. [bmim]2[CoCl4] (bmim = 1-butyl-3-methyl-imidazolium) and [bmim]2[ZnCl4], exhibit higher catalytic activity, compared with traditional catalysts, conventional IL catalysts, and some functional ILs. For example, utilizing [bmim]2[CoCl4] as catalyst, the conversion of PET, selectivity of bis(hydroxyethyl) terephthalate (BHET), and mass fraction of BHET in products reach up to 100%, 81.1%, and 95.7%, resp., under atm. pressure at 175 °C for only 1.5 h. Another important feature is that BHET can be easily sepd. from these IL catalysts and has high purity. Moreover, recycling results show that [bmim]2[CoCl4] worked efficiently after being used six times. These all show that [bmim]2[CoCl4] is an excellent IL catalyst for the glycolysis of PET. Finally, based on in situ IR spectra and exptl. results, the possible mechanism of degrdn. with synthesized IL is proposed.
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    (b) Yue, Q. F.; Xiao, L. F.; Zhang, M. L.; Bai, X. F. The Glycolysis of Poly(ethylene terephthalate) Waste: Lewis Acidic Ionic Liquids as High Efficient Catalysts. Polymers 2013, 5 (4), 1258– 1271,  DOI: 10.3390/polym5041258
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    14b
    The glycolysis of poly(ethylene terephthalate) waste: Lewis acidic ionic liquids as high efficient catalysts
    Yue, Qun Feng; Xiao, Lin Fei; Zhang, Mi Lin; Bai, Xue Feng
    Polymers (Basel, Switzerland) (2013), 5 (4), 1258-1271, 14 pp.CODEN: POLYCK; ISSN:2073-4360. (MDPI AG)
    Poly(ethlyene terephthalate) waste from a local market was depolymd. by ethylene glycol (EG) in the presence of Lewis acidic ionic liqs. [Bmim]ZnCl3 and the qual. anal. showed that bis(hydroxyethyl) terephthalate was the main product. Compared with ionic liq. [Bmim]Cl, the Lewis acidic ionic liqs. showed highly catalytic activity in the glycolysis of poly(ethylene terephthalate) PET. Significantly, the conversion of PET and the yield of bis(hydroxyethyl) terephthalate were achieved at 100% and 83.8% with low catalyst ([Bmim]ZnCl3) loading (0.16 wt %). Investigation also showed that the catalytic activity of [Bmim]ZnCl3 was higher than that of [Bmim]MnCl3. Catalyst [Bmim]ZnCl3 can be reused up to five times and 1H-NMR results show that the recovered catalyst is similar to the fresh one. A mechanism of the glycolysis of PET catalyzed by [Bmim]ZnCl3 was proposed.
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  15. 15
    Lalhmangaihzuala, S.; Laldinpuii, Z.; Lalmuanpuia, C.; Vanlaldinpuia, K. Glycolysis of Poly(Ethylene Terephthalate) Using Biomass-Waste Derived Recyclable Heterogeneous Catalyst. Polymers 2020, 13 (1), 37,  DOI: 10.3390/polym13010037
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    (a) Yunita, I.; Putisompon, S.; Chumkaeo, P.; Poonsawat, T.; Somsook, E. Effective catalysts derived from waste ostrich eggshells for glycolysis of post-consumer PET bottles. Chem. Pap. 2019, 73 (6), 1547– 1560,  DOI: 10.1007/s11696-019-00710-3
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    16a
    Effective catalysts derived from waste ostrich eggshells for glycolysis of post-consumer PET bottles
    Yunita, Isti; Putisompon, Siraphat; Chumkaeo, Peerapong; Poonsawat, Thinnaphat; Somsook, Ekasith
    Chemical Papers (2019), 73 (6), 1547-1560CODEN: CHPAEG; ISSN:1336-9075. (Springer International Publishing AG)
    Herein, we report an effective chem. recycling of poly(ethylene terephthalate) (PET) using sustainable sources of catalysts, calcium oxide (CaO) derived from ostrich eggshells. The active catalysts were demonstrated in the chem. depolymn. of post-consumer PET bottles. Beverage bottles were proceeded with 1 wt% catalyst derived from ostrich eggshells in the presence of ethylene glycol at 192 °C under atm. pressure to give the major product as bis(2-hydroxyethyl terephthalate) (BHET) which was confirmed by m.p., IR spectroscopy, 1H-, 13C-NMR spectroscopy and mass spectrum. The catalyst could fully depolymerize PET within 2 h, producing a good yield of highly pure BHET monomer. The catalysts were successfully characterized by X-ray powder diffraction, XPS, field-emission SEM with energy dispersive X-ray spectroscopy anal., and thermo-gravimetric anal. Furthermore, catalysts derived from chicken eggshells, geloina, mussel, and oyster shells were run to compare the catalytic activities. For better understanding of catalytic parameters, effects of calcination temps. of catalyst, wt. ratio of catalyst, ratio of wt. of solvent, and time of depolymn. for the ostrich eggshells catalyst were also investigated.
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    (b) Laldinpuii, Z.; Lalhmangaihzuala, S.; Pachuau, Z.; Vanlaldinpuia, K. Depolymerization of poly(ethylene terephthalate) waste with biomass-waste derived recyclable heterogeneous catalyst. Waste Manage. 2021, 126, 1– 10,  DOI: 10.1016/j.wasman.2021.02.056
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    16b
    Depolymerization of poly(ethylene terephthalate) waste with biomass-waste derived recyclable heterogeneous catalyst
    Laldinpuii, Zathang; Lalhmangaihzuala, Samson; Pachuau, Zodinpuia; Vanlaldinpuia, Khiangte
    Waste Management (Oxford, United Kingdom) (2021), 126 (), 1-10CODEN: WAMAE2; ISSN:0956-053X. (Elsevier Ltd.)
    Poly(ethylene terephthalate) (PET) is one of the most widely used polymeric materials in chem. industry representing about 13% of the world's prodn. With the exponentially increasing consumption of plastics combined with its non-biodegradability, the accumulation of plastic waste in the environment rises steeply and its recycling has attracted enormous attention among researchers in recent years. In this present work, we describe bamboo leaf ash (BLA) as a bio-waste derived recyclable heterogeneous catalyst for the depolymn. of waste PET. The prepd. catalyst was characterized by FT-IR, XRD, SEM, TEM, EDX, TGA and BET analyses to assess its morphol. and compn. Postconsumer PET bottles were shredded and processed with 20 wt% BLA and 16 equiv of ethylene glycol (EG) at 190°C for 3.5 h under atm. pressure to give recrystd. bis(2-hydroxyethyl) terephthalate (BHET) monomer in 83% yield. The catalyst can be reused for four catalytic cycles and the residual EG was recovered for subsequent catalytic reactions. Excellent activity, cost-free, environmental-friendliness and ease of prepn., handling and reusability of the catalyst with simple work-up procedure are the notable advantages of this protocol.
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  17. 17
    Veregue, F. R.; Pereira da Silva, C. T.; Moisés, M. P.; Meneguin, J. G.; Guilherme, M. R.; Arroyo, P. A.; Favaro, S. L.; Radovanovic, E.; Girotto, E. M.; Rinaldi, A. W. Ultrasmall Cobalt Nanoparticles as a Catalyst for PET Glycolysis: A Green Protocol for Pure Hydroxyethyl Terephthalate Precipitation without Water. ACS Sustainable Chem. Eng. 2018, 6 (9), 12017– 12024,  DOI: 10.1021/acssuschemeng.8b02294
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    17
    Ultrasmall Cobalt Nanoparticles as a Catalyst for PET Glycolysis: A Green Protocol for Pure Hydroxyethyl Terephthalate Precipitation without Water
    Veregue, Fernanda Reis; Pereira da Silva, Cleiser Thiago; Moises, Murilo Pereira; Meneguin, Joziane Gimenes; Guilherme, Marcos Rogerio; Arroyo, Pedro Augusto; Favaro, Silvia Luciana; Radovanovic, Eduardo; Girotto, Emerson Marcelo; Rinaldi, Andrelson Wellington
    ACS Sustainable Chemistry & Engineering (2018), 6 (9), 12017-12024CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    Polyethylene terephthalate (PET) is a very stable polymer widely used in the modern world. Due to its stability, this polymer can remain in the environment for several years before its complete degrdn. The glycolysis reaction of PET has emerged as a green approach to obtain the PET monomer, thus avoiding such environmental problems and adding value to this waste. PET waste was depolymd. by glycolysis using ultrasmall cobalt nanoparticles (1.5%) as the catalyst for the prodn. of bis-2-hydroxyethyl terephthalate (BHET). A capping agent (tannic acid, TA) and a borohydride redn. approach were used to obtain such ultrasmall cobalt nanoparticles (∼3 nm). A PET depolymn. yield of 96% was achieved within 3 h at 180°. The pptn. of 77% of pure BHET was achieved without the need for water. The remaining ethylene glycol soln. contg. the ultrasmall cobalt nanoparticle catalyst was reused five times for this glycolysis process, demonstrating the feasibility of solvent reuse without the need for any treatment. A reaction mechanism is proposed to explain the high BHET yield obtained by this ultrasmall cobalt nanoparticle catalyst stabilized with TA.
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  18. 18
    Du, J.-T.; Sun, Q.; Zeng, X.-F.; Wang, D.; Wang, J.-X.; Chen, J.-F. ZnO nanodispersion as pseudohomogeneous catalyst for alcoholysis of polyethylene terephthalate. Chem. Eng. Sci. 2020, 220, 115642,  DOI: 10.1016/j.ces.2020.115642
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    18
    ZnO nanodispersion as pseudohomogeneous catalyst for alcoholysis of polyethylene terephthalate
    Du, Jin-Tao; Sun, Qian; Zeng, Xiao-Fei; Wang, Dan; Wang, Jie-Xin; Chen, Jian-Feng
    Chemical Engineering Science (2020), 220 (), 115642CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
    Chem. depolymn. and recycling of polyethylene terephthalate (PET) is a sustainable way to preserve the resources and protect the environment. In this work, methanol and ethylene glycol dispersions of ultrasmall ZnO nanoparticles are firstly adopted as pseudohomogeneous catalysts for alcoholysis of PET. The as-prepd. ZnO nanoparticles have a uniform size of 4 nm and can be stable in dispersions for 6 mo. In the methanolysis process of PET, the effects of various parameters on the conversion of PET and the yield of di-Me terephthalate (DMT) were investigated. The results show that higher temp. (170°C) was beneficial to the conversion of PET and the yield of DMT, which can reach about 97% and 95% after 15 min, resp. The excellent activity of 553 g PET h-1 (g ZnO)-1 was achieved. Furthermore, the methanolysis of PET have shorter reaction time (1/4) and higher activity (4.7 times) than the glycolysis of PET.
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  19. 19
    Sun, Q.; Zheng, Y.-Y.; Yun, L.-X.; Wu, H.; Liu, R.-K.; Du, J.-T.; Gu, Y.-H.; Shen, Z.-G.; Wang, J.-X. Fe3O4 Nanodispersions as Efficient and Recoverable Magnetic Nanocatalysts for Sustainable PET Glycolysis. ACS Sustainable Chem. Eng. 2023, 11 (19), 7586– 7595,  DOI: 10.1021/acssuschemeng.3c01206
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    19
    Fe3O4 Nanodispersions as Efficient and Recoverable Magnetic Nanocatalysts for Sustainable PET Glycolysis
    Sun, Qian; Zheng, Yuan-Yuan; Yun, Ling-Xia; Wu, Hao; Liu, Rong-Kun; Du, Jin-Tao; Gu, Yu-Hang; Shen, Zhi-Gang; Wang, Jie-Xin
    ACS Sustainable Chemistry & Engineering (2023), 11 (19), 7586-7595CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    Polyethylene terephthalate (PET), as one of the most indispensable synthetic org. compds. with high strength and transparency properties, can be widely used for textile and food packaging. With the increasing demand for PET prodn., the recycling of discarded PET has attracted great interest. In this work, we first proposed ethylene glycol (EG) dispersions of highly dispersed Fe3O4 nanoparticles, which were prepd. through a co-pptn. route, as efficient and recoverable nanocatalysts for a PET chem. depolymn. achieved by a glycolysis reaction. The as-prepd. Fe3O4 nanoparticles have an av. size of 11 nm and can be stably dispersed in EG for up to 6 mo. This glycolysis process was optimized in terms of catalyst concns., EG dosages, degrdn. temp., and reaction time. Furthermore, the possible reaction mechanism of PET glycolysis using Fe3O4 as a catalyst was presented. More importantly, 100% PET conversion was achieved, and the bis(2-hydroxyethyl) terephthalate (BHET) yield reached more than 93% under optimal conditions (Fe3O4/PET = 2%, EG/PET = 13, 210°C 30 min) even after three cycles. The Fe3O4 nanocatalysts are relatively stable during recycling and have great application prospects in chemocatalysis for future research.
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  20. 20
    Son, S. G.; Jin, S. B.; Kim, S. J.; Park, H. J.; Shin, J.; Ryu, T.; Jeong, J.-M.; Choi, B. G. Exfoliated manganese oxide nanosheets as highly active catalysts for glycolysis of polyethylene terephthalate. FlatChem 2022, 36, 100430,  DOI: 10.1016/j.flatc.2022.100430
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    20
    Exfoliated manganese oxide nanosheets as highly active catalysts for glycolysis of polyethylene terephthalate
    Son, Seon Gyu; Jin, Se Bin; Kim, Seo Jin; Park, Hong Jun; Shin, Junho; Ryu, Taegong; Jeong, Jae-Min; Choi, Bong Gill
    FlatChem (2022), 36 (), 100430CODEN: FLATAL; ISSN:2452-2627. (Elsevier B.V.)
    Glycolysis of poly(ethylene terephthalate) (PET) is one of the main methods by which PET is recycled, which is in the need of an inexpensive and efficient catalyst. We report a glycolysis reaction of PET into bis(hydroxyethyl) terephthalate (BHET) catalyzed by ultrathin exfoliated MnO2 nanosheets (e-MON). The exfoliation of MnO2 was achieved using a shear flow system with a mixing behavior that facilitates the intercalation of potassium ions into the interlayer voids of MnO2, delaminating MnO2 into the mono- and few-layer e-MON. Glycolysis was optimized by varying the reaction temp., time, and loading of e-MON. The e-MON catalyst provided a BHET yield of 100% for a reaction lasting 30 min at 200°C, which was much higher than the 77.6% yield achieved with bare bulk MnO2. The high yield persisted over five cycles, demonstrating the reusability of the e-MON catalyst.
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  21. 21
    Cao, J.; Lin, Y.; Jiang, W.; Wang, W.; Li, X.; Zhou, T.; Sun, P.; Pan, B.; Li, A.; Zhang, Q. Mechanism of the Significant Acceleration of Polyethylene Terephthalate Glycolysis by Defective Ultrathin ZnO Nanosheets with Heteroatom Doping. ACS Sustainable Chem. Eng. 2022, 10 (17), 5476– 5488,  DOI: 10.1021/acssuschemeng.1c08656
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    21
    Mechanism of the Significant Acceleration of Polyethylene Terephthalate Glycolysis by Defective Ultrathin ZnO Nanosheets with Heteroatom Doping
    Cao, Jingjing; Lin, Yuheng; Jiang, Wei; Wang, Wei; Li, Xiaodong; Zhou, Tianpeng; Sun, Ping; Pan, Bingcai; Li, Aimin; Zhang, Quanxing
    ACS Sustainable Chemistry & Engineering (2022), 10 (17), 5476-5488CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    The current scale of polyethylene terephthalate (PET) prodn. and unreasonable handling have caused widespread environmental, economic, and health-related consequences. The catalytic glycolysis of PET waste is one effective method to solve this issue. Recently, improving yield and selectivity has become a major focus in the glycolysis of PET. Herein, we report a study on the synthesis of Co/ZnO and Mo/ZnO nanosheets that have been employed as efficient catalysts in the glycolysis of PET. Under optimized conditions, the bis(hydroxyethyl) terephthalate prodn. rate and the space time yield are about two times and six times higher for Mo/ZnO nanosheets compared to those of a conventional catalyst prepd. with Zn(OAc)2, resp. We demonstrated that Mo and Co were atomically dispersed over ZnO nanosheets via oxygen bridge bonds, thus forming Mo-O or Co-O bonds on the catalyst surface. The dynamically constructed Mo or Co species drive holes into oxygen ligands to facilitate intramol. oxygen coupling, thereby triggering lattice oxygen activation to form Mo-Zn or Co-Zn dual sites as ultimate catalytic centers with highly intrinsic activity. Besides, the cooperation between the Zn-O bond and Mo-Zn or Co-Zn dual sites resulted in an active interfacial structure for PET activation and conversion. This work paved the way for engineering efficient catalysts for PET conversion through tunable compns. and electronic structures.
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  22. 22
    Suo, Q.; Zi, J.; Bai, Z.; Qi, S. The Glycolysis of Poly(ethylene terephthalate) Promoted by Metal Organic Framework (MOF) Catalysts. Catal. Lett. 2017, 147 (1), 240– 252,  DOI: 10.1007/s10562-016-1897-0
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    22
    The Glycolysis of Poly(ethylene terephthalate) Promoted by Metal Organic Framework (MOF) Catalysts
    Suo, Qianqian; Zi, Jiangzhi; Bai, Zilong; Qi, Shoushan
    Catalysis Letters (2017), 147 (1), 240-252CODEN: CALEER; ISSN:1011-372X. (Springer)
    Three metal org. framework catalysts (ZIF-8, ZIF-67, MOF-5) were prepd. and used in the glycolysis of poly(ethylene terephthalate) (PET). All catalysts showed excellent catalytic activities, esp. ZIF-8. The raw material ratio in ZIF-8 catalysts was optimized. The effects of reaction conditions on the conversion of PET and yield of bis(hydroxyethyl) terephthalate (BHET) were examd.
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  23. 23
    Wu, Y.; Wang, X.; Kirlikovali, K. O.; Gong, X.; Atilgan, A.; Ma, K.; Schweitzer, N. M.; Gianneschi, N. C.; Li, Z.; Zhang, X. Catalytic Degradation of Polyethylene Terephthalate Using a Phase-Transitional Zirconium-Based Metal-Organic Framework. Angew. Chem., Int. Ed. 2022, 61 (24), e202117528  DOI: 10.1002/anie.202117528
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    Catalytic Degradation of Polyethylene Terephthalate Using a Phase-Transitional Zirconium-Based Metal-Organic Framework
    Wu, Yufang; Wang, Xingjie; Kirlikovali, Kent O.; Gong, Xinyi; Atilgan, Ahmet; Ma, Kaikai; Schweitzer, Neil M.; Gianneschi, Nathan C.; Li, Zhong; Zhang, Xuan; Farha, Omar K.
    Angewandte Chemie, International Edition (2022), 61 (24), e202117528CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Polyethylene terephthalate (PET) is utilized as one of the most popular consumer plastics worldwide, but difficulties assocd. with recycling PET have generated a severe environmental crisis with most PET ending its lifecycle in landfills. We report that zirconium-based metal-org. framework (Zr-MOF) UiO-66 deconstructs waste PET into the building blocks terephthalic acid (TA) and mono-Me terephthalate (MMT) within 24 h at 260°C (total yield of 98% under 1 atm H2 and 81% under 1 atm Ar). Extensive structural characterization studies reveal that during the degrdn. process, UiO-66 undergoes an intriguing transformation into MIL-140A, which is another Zr-MOF that shows good catalytic activity toward PET degrdn. under similar reaction conditions. These results illustrate the diversity of applications for Zr-MOFs and establish MOFs as a new class of polymer degrdn. catalysts with the potential to address long-standing challenges assocd. with plastic waste.
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  24. 24
    Shukla, S. R.; Palekar, V.; Pingale, N. Zeolite catalyzed glycolysis of poly(ethylene terephthalate) bottle waste. J. Appl. Polym. Sci. 2008, 110 (1), 501– 506,  DOI: 10.1002/app.28656
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    Zeolite catalyzed glycolysis of poly(ethylene terephthalate) bottle waste
    Shukla, S. R.; Palekar, Vikrant; Pingale, Navnath
    Journal of Applied Polymer Science (2008), 110 (1), 501-506CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
    Polyethylene terephthalate (PET) bottle waste was depolymd. using excess of ethylene glycol (EG) in the presence of zeolites, β-zeolite and Y-zeolite as transesterification catalyst. The glycolysis reaction was carried out under reflux in excess of ethylene glycol up to 8 h. The product of glycolysis was mainly the virtual monomer, bis(2-hydroxyethyl) terephthalate (BHET) admixed with dimer as residue. The BHET was obtained in pure cryst. form. Influence of the reaction time, PET: EG ratio, type, and concn. of catalyst on the yield of the glycolysis products was investigated. The characterization of the purified monomer was carried out by elemental anal., m.p., IR spectroscopy, DSC, and NMR. The yield of BHET monomer was more than 60%, which is comparable with the conventionally used heavy metal catalysts such as zinc acetate and lead acetate. This process of glycolysis of PET is economically viable and the catalysts are environment friendly.
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  25. 25
    Yang, R.-X.; Bieh, Y.-T.; Chen, C. H.; Hsu, C.-Y.; Kato, Y.; Yamamoto, H.; Tsung, C.-K.; Wu, K. C. W. Heterogeneous Metal Azolate Framework-6 (MAF-6) Catalysts with High Zinc Density for Enhanced Polyethylene Terephthalate (PET) Conversion. ACS Sustainable Chem. Eng. 2021, 9 (19), 6541– 6550,  DOI: 10.1021/acssuschemeng.0c08012
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    25
    Heterogeneous Metal Azolate Framework-6 (MAF-6) Catalysts with High Zinc Density for Enhanced Polyethylene Terephthalate (PET) Conversion
    Yang, Ren-Xuan; Bieh, Yen-Tsz; Chen, Celine H.; Hsu, Chang-Yen; Kato, Yuki; Yamamoto, Hideki; Tsung, Chia-Kuang; Wu, Kevin C.-W.
    ACS Sustainable Chemistry & Engineering (2021), 9 (19), 6541-6550CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    Polyethylene terephthalate (PET) has been extensively used for the fabrication of various packaging materials, creating million tons of waste per yr. Degrading and recycling PET waste has been identified as a prominent issue. Herein, we demonstrate an effective process to chem. convert PET to bis(2-hydroxyethyl) terephthalate (BHET) through the use of metal azolate framework-6 (MAF-6) as a catalyst in the presence of ethylene glycol. MAFs are a subclass of metal-org. frameworks (MOFs), with MAF-6 comprised of the metal ion Zn2+ and the org. ligand 2-ethylimidazole. We have optimized the reaction temp., reaction time, and catalyst amt. to achieve up to a 92.4% conversion of PET and an 81.7% yield of BHET at 180°C for 4 h. MAF-6 was easily recovered and reused for at least five times. We have also hypothesized a mechanism for the high conversion and yield of the PET glycolysis reaction catalyzed by MAF-6. The use of MAF-6 as a catalyst opens a new route for the postconsumer recycling of PET with remarkable practicality.
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    Wang, T.; Shen, C.; Yu, G.; Chen, X. Metal ions immobilized on polymer ionic liquid as novel efficient and facile recycled catalyst for glycolysis of PET. Polym. Degrad. Stab. 2021, 194, 109751,  DOI: 10.1016/j.polymdegradstab.2021.109751
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    Metal ions immobilized on polymer ionic liquid as novel efficient and facile recycled catalyst for glycolysis of PET
    Wang, Tianlin; Shen, Chuanchao; Yu, Guangren; Chen, Xiaochun
    Polymer Degradation and Stability (2021), 194 (), 109751CODEN: PDSTDW; ISSN:0141-3910. (Elsevier Ltd.)
    Environmental pollution aroused by accumulation of waste PET has attracted worldwide attention. Metal chloride as Lewis acid catalyst has excellent catalytic performance in enormous org. reaction. However, the high soly. of the metal chloride in ethylene glycol and the residue of metal ions in the product will have an adverse impact on the subsequent process. In this work, polymer ionic liq. contg. [NTf2]- which is insol. in ethylene glycol at room temp. was synthesized and used to immobilize metal ions to catalyze glycolysis of PET. After reaction, the polymer ionic liq.-metal ion catalyst could be recovered at room temp. by simple filtration. PET conversion reached 95.4% and BHET yield reached 77.8% using PIL-Zn2+ as catalyst at 195°C for 120 min. Ionic liq. was characterized by FT-IR, NMR and ESI-MS. Interaction between ionic liq. and metal ions was characterized by FT-IR. The interaction energy of polymer ionic liq.-metal ion complex is higher than that of ethylene glycol-metal ion complex by DFT calcn. (D. functional calcn.). The both content of metal ion in polymer ionic liq.-metal ion complex and catalytic performance of as-synthesized catalysts did not obviously decrease after the catalyst was reused for five times. This work mainly provided a way of recovering catalyst and promote development of glycolysis of PET.
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    Kim, J.; Song, B.; Chung, I.; Park, J.; Yun, Y. High-performance Pt catalysts supported on amine-functionalized silica for enantioselective hydrogenation of α-keto ester. J. Catal. 2021, 396, 81– 91,  DOI: 10.1016/j.jcat.2021.02.001
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    High-performance Pt catalysts supported on amine-functionalized silica for enantioselective hydrogenation of α-keto ester
    Kim, Jeongmyeong; Song, Byeongju; Chung, Iljun; Park, Jisu; Yun, Yongju
    Journal of Catalysis (2021), 396 (), 81-91CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    Herein, mesocellular silica foams (MCFs) functionalized by primary, secondary, and tertiary amine groups were used as supports for Pt nanoparticles. The catalytic performances of the Pt/amine-functionalized MCFs were evaluated for the enantioselective hydrogenation of Me pyruvate in the presence of cinchonidine. Compared to Pt/MCF, the Pt/amine-functionalized MCFs exhibited enhanced activity and enantioselectivity. Particularly, 0.5 wt% Pt/NH2-MCF showed a superior performance than 5 wt% Pt/Al2O3, a highly efficient catalyst in the enantioselective hydrogenation of α-keto esters, despite a 10 times lower Pt loading. Furthermore, the Pt/NH2-MCF yielded 100% conversion and 96% ee at 0.1 MPa H2 pressure during nine successive cycles, thus showing high reusability. The excellent performance of the Pt/amine-functionalized MCFs is attributed to the formation of electron-deficient Pt species through strong interactions between the Pt nanoparticles and amine groups.
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    Li, H.; Chen, X.; Shen, D.; Wu, F.; Pleixats, R.; Pan, J. Functionalized silica nanoparticles: classification, synthetic approaches and recent advances in adsorption applications. Nanoscale 2021, 13 (38), 15998– 16016,  DOI: 10.1039/D1NR04048K
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    Functionalized silica nanoparticles: classification, synthetic approaches and recent advances in adsorption applications
    Li, Hao; Chen, Xueping; Shen, Danqing; Wu, Fan; Pleixats, Roser; Pan, Jianming
    Nanoscale (2021), 13 (38), 15998-16016CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    A review. Nanotechnol. is rapidly sweeping through all the vital fields of science and technol. such as electronics, aerospace, defense, medicine, and catalysis. It involves the design, synthesis, characterization, and applications of materials and devices on the nanometer scale. At the nanoscale, phys. and chem. properties differ from the properties of the individual atoms and mols. of bulk matter. In particular, the design and development of silica nanomaterials have captivated the attention of several researchers worldwide. The applications of hybrid silicas are still limited by the lack of control on the morphol. and particle size. The ability to control both the size and morphol. of the materials and to obtain nano-sized silica particles has broadened the spectrum of applications of mesoporous organosilicas and/or has improved their performances. On the other hand, adsorption is a widely used technique for the sepn. and removal of pollutants (metal ions, dyes, orgs.,...) from wastewater. Silica nanoparticles have specific advantages over other materials for adsorption applications due to their unique structural characteristics: a stable structure, a high sp. surface area, an adjustable pore structure, the presence of silanol groups on the surface which allow easy modification, less environmental harm, simple synthesis, low cost, etc. Silica nanoparticles are potential adsorbents for pollutants. We present herein an overview of the different types of silica nanoparticles going from the definitions to properties, synthetic approaches and the mention of potential applications. We focus mainly on the recent advances in the adsorption of different target substances (metal ions, dyes and other orgs.).
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    Graf, N.; Yegen, E.; Gross, T.; Lippitz, A.; Weigel, W.; Krakert, S.; Terfort, A.; Unger, W. E. S. XPS and NEXAFS studies of aliphatic and aromatic amine species on functionalized surfaces. Surf. Sci. 2009, 603 (18), 2849– 2860,  DOI: 10.1016/j.susc.2009.07.029
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    XPS and NEXAFS studies of aliphatic and aromatic amine species on functionalized surfaces
    Graf, Nora; Yegen, Eda; Gross, Thomas; Lippitz, Andreas; Weigel, Wilfried; Krakert, Simone; Terfort, Andreas; Unger, Wolfgang E. S.
    Surface Science (2009), 603 (18), 2849-2860CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    The chem. constitution of functionalized supports is an important parameter that dets. their performance in a broad range of applications, e.g. for immobilization of biomols. Supports with amino functionalized surfaces are also often used for DNA microarray expts. However, spectral data which were reported for surfaces with amino functionalities suffer from some inconsistencies. A detailed XPS and NEXAFS (Near edge x-ray absorption fine structure) database for amino functionalized surfaces is presented. Amino-terminated surfaces prepd. from aliph. and arom. aminosilanes or aminothiols and a field sample are considered. Effects of aging in air and damage by radiation are addressed as well.
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    Paengjun, N.; Vibulyaseak, K.; Ogawa, M. Heterostructural transformation of mesoporous silica-titania hybrids. Sci. Rep. 2021, 11 (1), 3210,  DOI: 10.1038/s41598-020-80584-8
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    Heterostructural transformation of mesoporous silica-titania hybrids
    Paengjun, Navarut; Vibulyaseak, Kasimanat; Ogawa, Makoto
    Scientific Reports (2021), 11 (1), 3210CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
    Mesoporous silica (SBA-15 with the BJH pore size of 8 nm) contg. anatase nanoparticles in the pore with two different titania contents (28 and 65 mass%), which were prepd. by the infiltration of the amorphous precursor derived from tetraisopropyl orthotitanate into the pore, were heat treated in air to investigate the structural changes (both mesostructure of the SBA-15 and the phase and size of the anatase in the pore). The mesostructure of the mesoporous silica and the particle size of anatase unchanged by the heat treatment up to 800°C. The heat treatment at the temp. higher than 1000°C resulted in the collapse of the mesostructure and the growth of anatase nanoparticles as well as the transformation to rutile, while the transformation of anatase to rutile was suppressed esp. for the sample with the lower titania content (28 mass%). The resulting mesoporous silica-anatase hybrids exhibited higher benzene adsorption capacity (adsorption from water) over those heated at lower temp., probably due to the dehydroxylation of the silanol group on the pore surface. The photocatalytic decompn. of benzene in water by the present hybrid heated at 1100°C was efficient as that by P25, a benchmark photocatalyst.
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    Millot, Y.; Hervier, A.; Ayari, J.; Hmili, N.; Blanchard, J.; Boujday, S. Revisiting Alkoxysilane Assembly on Silica Surfaces: Grafting versus Homo-Condensation in Solution. J. Am. Chem. Soc. 2023, 145 (12), 6671– 6681,  DOI: 10.1021/jacs.2c11390
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    Revisiting Alkoxysilane Assembly on Silica Surfaces: Grafting versus Homo-Condensation in Solution
    Millot, Yannick; Hervier, Antoine; Ayari, Jihed; Hmili, Naoures; Blanchard, Juliette; Boujday, Souhir
    Journal of the American Chemical Society (2023), 145 (12), 6671-6681CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Silica surface functionalization is often done through the condensation of functional silanes on silanols, silica surfaces' terminal groups. APTES, aminopropyltriethoxysilane, is widely used due to its assumed high reactivity with silanols, kinetically promoted by the catalytic action of the terminal amine function. Here, we revisit, based on a quant. anal. by solid-state 29Si NMR, the assembly of this silane on silica surfaces to investigate whether its presence results from grafting, i.e., hetero-condensation with silanol groups or from homo-condensation of silane mols. in soln. leading to polycondensates physisorbed on silica. We investigate the interaction of APTES with a cryst. layered silicate, ilerite, and with amorphous nonporous silica. We also studied a second silane, cyanopropyltrichlorosilane (CPTCS), terminated with a nitrile group. Our results undoubtedly prove that while CPTCS is grafted on the silica surface, the presence of APTES on silica and silicate materials is only marginally assocd. with silanol consumption. The anal. of the signal related to silicon atoms from silanes (Tn species) and those from silica (Qn species) allowed for the accurate estn. of the extent of homo-condensation vs. grafting based on the ratio of T-O-T/Q-O-T siloxane bridges. These findings deeply question the well-established certainties on APTES assembly on silica that should no longer be seen as grafting of alkoxysilane by hetero-condensation with silanol groups but more accurately as a homo-condensed network of silanes, predominantly physisorbed on the surface but including some sparse anchoring points to the surface involving less than 6% of the overall silanol groups.
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  32. 32
    Cui, J.; Chatterjee, P.; Slowing, I. I.; Kobayashi, T. In Situ 29Si solid-state NMR study of grafting of organoalkoxysilanes to mesoporous silica nanoparticles. Microporous Mesoporous Mater. 2022, 339, 112019,  DOI: 10.1016/j.micromeso.2022.112019
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    32
    In Situ 29Si solid-state NMR study of grafting of organoalkoxysilanes to mesoporous silica nanoparticles
    Cui, Jinlei; Chatterjee, Puranjan; Slowing, Igor I.; Kobayashi, Takeshi
    Microporous and Mesoporous Materials (2022), 339 (), 112019CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
    Since the catalytic activity and the stability of silica-bound organometallic complexes are affected by their interactions with hydroxyl groups on the surface, isolated hydroxyls are often created prior to the introduction of catalytic species. Here, we investigate a method to remove the indigenous hydroxyls and create new isolated hydroxyls by grafting organo-trimethoxysilane (R-TMS) to generate a silicon T2 site, (=SiO-)2SiR(-OH). We used in situ 29Si solid-state NMR expts. to monitor the evolution of Tn sites, (=SiO-)2SiR(-OH)3-n (n = 1, 2, 3). The study indicates that i. the grafting proceeds in a consecutive manner as T1 → T2 → T3, and ii. the kinetics depend on the type of functional groups in the silane. However, the rates of T1 formation and T2 → T3 conversion are also controlled to a significant extent by the entropy loss assocd. to the initial silane binding and the spatial arrangement of surface hydroxyls, resp. The grafting of R-TMS with a basic functional group leads to a lower concn. of T1 sites. The nucleophilicity of the functional group facilitates the grafting process by lowering the enthalpy barrier, while the T1 formation rate is more influenced by the entropy barrier than the T1 → T2 conversion rate. Thus, the basic functional group promotes the T1 → T2 conversion more than the T1 formation, resulting in a lower concn. of T1 sites.
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  33. 33
    van Meerten, S. G. J.; Franssen, W. M. J.; Kentgens, A. P. M. ssNake: A cross-platform open-source NMR data processing and fitting application. J. Magn. Reson. 2019, 301, 56– 66,  DOI: 10.1016/j.jmr.2019.02.006
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    ssNake: A cross-platform open-source NMR data processing and fitting application
    van Meerten, S. G. J.; Franssen, W. M. J.; Kentgens, A. P. M.
    Journal of Magnetic Resonance (2019), 301 (), 56-66CODEN: JMARF3; ISSN:1090-7807. (Elsevier B.V.)
    For solid-state NMR or for unconventional expts. only a very limited no. of modern processing and simulation software packages are available. For this reason, we have developed ssNake, an NMR processing program which provides both interactive and script-based processing tools. ssNake is aimed at solid-state NMR expts., but can also be used for liq.-state expts. It can read various data formats, including those from all major spectrometer vendors. It has extensive fitting capabilities, which can be used for spectrum deconvolution. ssNake also provides the unique feature of being able to fit multiple spectra (or curves) simultaneously, where some or all of its parameters are shared. This method can be used, for example, to fit quadrupole spectra at various magnetic fields simultaneously. This allows the quadrupole and chem. shift parameters to be accurately detd. ssNake also provides a method of fitting using external simulation programs, such as SIMPSON. This makes fitting very versatile, as it brings together exptl. data and simulation software.
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  34. 34
    (a) Liu, C. C.; Maciel, G. E. The Fumed Silica Surface: A Study by NMR. J. Am. Chem. Soc. 1996, 118 (21), 5103– 5119,  DOI: 10.1021/ja954120w
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    34a
    The Fumed Silica Surface: A Study by NMR
    Liu, Changhua C.; Maciel, Gary E.
    Journal of the American Chemical Society (1996), 118 (21), 5103-5119CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    High-resoln. solid-state NMR techniques were used to study the surface structure of Cab-O-Sil fumed silica. 1H NMR results obtained from CRAMPS, MAS-only, and relaxation studies reveal the existence of both H-bonded silanols and isolated silanols on the Cab-O-Sil surface. A systematic dehydration study of fumed silica was carried out, with results on the quantity of each type of silanol on the surface at various dehydration stages. 29Si CP-MAS expts., including CP spin dynamics studies and various other relaxation studies, were used to probe H bonding and the local structural environments of various hydroxyl groups of silica surfaces. 29Si CP-MAS expts. on water-treated and D-exchanged Cab-O-Sil indicate the existence of interparticle silanols and internal silanols in fumed silica. 1H and 29Si NMR show that for fumed silica both isolated and H-bonded silanols are present on the surface of an untreated sample, in contrast to the case of silica gel, where all silanols of an untreated sample are H bonded.
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    (b) Lippmaa, E.; Maegi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A. R. Structural studies of silicates by solid-state high-resolution silicon-29 NMR. J. Am. Chem. Soc. 1980, 102 (15), 4889– 4893,  DOI: 10.1021/ja00535a008
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    Structural studies of silicates by solid-state high-resolution silicon-29 NMR
    Lippmaa, E.; Maegi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A. R.
    Journal of the American Chemical Society (1980), 102 (15), 4889-93CODEN: JACSAT; ISSN:0002-7863.
    The high-resoln. 29Si NMR spectra of solid silicates and aluminosilicates were studied. High-speed magic angle sample spinning in combination with high-power proton decoupling and, wherever possible, polarization transfer was used to achieve high (1 ppm) resoln. Although ionization and cation influence are reflected on 29Si chem. shifts, the isotropic 29Si chem. shifts in solids and solns. are generally the same and depend mainly on the degree of condensation of Si-O tetrahedra. In solid aluminosilicates, addnl. paramagnetic shifts appear, which correlate well with the degree of Si substitution by Al.
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    (c) Lechert, H. G.; Engelhardt und, D. G. Engelhardt und D. Michel:High Resolution Solid State NMR of Silicates and Zeolites. John Wiley & Sons, Chichester, New York, Brisbane, Toronto, Singapore, 1987. 485 Seiten, Preis: $ 55.-. Ber. Bunsenges. Phys. Chem. 1988, 92 (9), 1059,  DOI: 10.1002/bbpc.198800267
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    (a) Cheng, R.; Liu, X.; Fang, Y.; Terano, M.; Liu, B. High-resolution 29Si CP/MAS solid state NMR spectroscopy and DFT investigation on the role of geminal and single silanols in grafting chromium species over Phillips Cr/silica catalyst. Appl. Catal., A 2017, 543, 26– 33,  DOI: 10.1016/j.apcata.2017.05.011
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    35a
    High-resolution 29Si CP/MAS solid state NMR spectroscopy and DFT investigation on the role of geminal and single silanols in grafting chromium species over Phillips Cr/silica catalyst
    Cheng, Ruihua; Liu, Xuee; Fang, Yuwei; Terano, Minoru; Liu, Boping
    Applied Catalysis, A: General (2017), 543 (), 26-33CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
    Phillips Cr/silica catalyst is industrially important in ethylene polymn. The high-resoln. solid state 1H MAS NMR and 29Si CP/MAS NMR allowed the identification of the type and amt. of silanols: geminal vs. single (isolated and vicinal) silanols on Phillips catalysts calcined at different temps., which were compared with those of the bare silica gel counterparts. The residual silanols on the catalyst and silica gel samples were all decreased with increasing calcination temps. from 120 to 800 °C. For the catalysts treated at temps. lower than 300 °C, the amt. of residual silanol groups were much lower than those of the silica gel counterparts. It suggested that the chromium species were mainly grafted on the silica gel through esterification reaction with surface silanols below 300 °C. The geminal silanols almost disappeared on the catalysts at 120 °C, while that for the silica gel occurred at 300 °C. Further increasing the calcination temps. from 300 to 800 °C, the amt. of single silanols were slower decreased for the catalysts than that for the silica gel samples. It indicated that the presence of the grafted chromate species obstructed further removal of the residual single silanols. The role of silanols on the formation of surface chromate species on the well-defined polyoligomericsilsesquioxane (POSS) models contg. various types of silanols was theor. studied by d. functional theory (DFT) method. It was shown that one silanol of the geminal pair and another adjacent single silanol was the most thermodynamically favored for grafting chromium species. The priority of the reaction between chromium species and different types of surface silanol groups during calcination for Phillips catalysts were exptl. and theor. elucidated for the first time.
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    (b) Bruch, M. D.; Fatunmbi, H. O. Nuclear magnetic resonance analysis of silica gel surfaces modified with mixed, amine-containing ligands. J. Chromatogr. A 2003, 1021 (1–2), 61– 70,  DOI: 10.1016/j.chroma.2003.08.093
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    35b
    Nuclear magnetic resonance analysis of silica gel surfaces modified with mixed, amine-containing ligands
    Bruch, Martha D.; Fatunmbi, Hafeez O.
    Journal of Chromatography A (2003), 1021 (1-2), 61-70CODEN: JCRAEY; ISSN:0021-9673. (Elsevier Science B.V.)
    Different approaches for quant. anal. by 29Si and 13C CP/MAS NMR of silica gel chem. modified by a mixt. of long and short chain amines, -(O)3Si(CH2)3N(CH3)2(CH2)13CH3 and -(O)3Si(CH2)3N(CH3)3, are compared to elemental anal. Unlike 29Si NMR, variable contact time data are necessary for accurate quant. anal. by 13C NMR. Surprisingly, spectral overlap does not interfere with this approach. Surfaces prepd. from reaction mixts. that consisted of 67 and 33% (vol./vol.) long chain are found to actually contain 37 and 16% long chain amines, resp. The mixed phase surfaces have more extensive crosslinking and fewer unreacted hydroxyls than single phase surfaces.
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    Srikanth, C. S.; Chuang, S. S. C. Spectroscopic Investigation into Oxidative Degradation of Silica-Supported Amine Sorbents for CO2 Capture. ChemSusChem 2012, 5 (8), 1435– 1442,  DOI: 10.1002/cssc.201100662
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    Spectroscopic Investigation into Oxidative Degradation of Silica-Supported Amine Sorbents for CO2 Capture
    Srikanth, Chakravartula S.; Chuang, Steven S. C.
    ChemSusChem (2012), 5 (8), 1435-1442, S1435/1-S1435/2CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Oxidative degrdn. characteristics of silica-supported amine sorbents with varying amts. of tetraethylenepentamine (TEPA) and polyethylene glycol (PEG; P200 or P600 represents PEG with mol. wts. of 200 or 600) have been studied by IR and NMR spectroscopy. Thermal treatment of the sorbents and liq. TEPA at 100 °C for 12 h changed their color from white to yellow. The CO2 capture capacity of the TEPA/SiO2 sorbents (i.e., SiO2-supported TEPA with a TEPA/SiO2 ratio of 25:75) decreased by more than 60 %. IR and NMR spectroscopy studies showed that the yellow color of the degraded sorbents resulted from the formation of imide species. The imide species, consisting of NH assocd. with two C=O functional groups, were produced from the oxidn. of methylene groups in TEPA. Imide species on the degraded sorbent are not capable of binding CO2 due to its weak basicity. The addn. of P200 and P600 to the supported amine sorbents improved both their CO2 capture capacities and oxidative degrdn. resistance. IR spectroscopy results also showed that TEPA was immobilized on the SiO2 surface through hydrogen bonding between amine groups and the silanol groups of SiO2. The OH groups of PEG interact with NH2/NH of TEPA through hydrogen bonding. Hydrogen bonds disperse TEPA on SiO2 and block O2 from accessing TEPA for oxidn. Oxidative degrdn. resistance and CO2 capture capacity of the supported amine sorbents can be optimized through adjusting the ratio of hydroxyl to amine groups in the TEPA/PEG mixt.
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  37. 37
    Vallet-Regí, M.; Schüth, F.; Lozano, D.; Colilla, M.; Manzano, M. Engineering mesoporous silica nanoparticles for drug delivery: where are we after two decades?. Chem. Soc. Rev. 2022, 51 (13), 5365– 5451,  DOI: 10.1039/D1CS00659B
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    Engineering mesoporous silica nanoparticles for drug delivery: where are we after two decades?
    Vallet-Regi, Maria; Schuth, Ferdi; Lozano, Daniel; Colilla, Montserrat; Manzano, Miguel
    Chemical Society Reviews (2022), 51 (13), 5365-5451CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. The present review details a chronol. description of the events that took place during the development of mesoporous materials, their different synthetic routes and their use as drug delivery systems. The outstanding textural properties of these materials quickly inspired their translation to the nanoscale dimension leading to mesoporous silica nanoparticles (MSNs). The different aspects of introducing pharmaceutical agents into the pores of these nanocarriers, together with their possible biodistribution and clearance routes, would be described here. The development of smart nanocarriers that are able to release a high local concn. of the therapeutic cargo on-demand after the application of certain stimuli would be reviewed here, together with their ability to deliver the therapeutic cargo to precise locations in the body. The huge progress in the design and development of MSNs for biomedical applications, including the potential treatment of different diseases, during the last 20 years will be collated here, together with the required work that still needs to be done to achieve the clin. translation of these materials. This review was conceived to stand out from past reports since it aims to tell the story of the development of mesoporous materials and their use as drug delivery systems by some of the story makers, who could be considered to be among the pioneers in this area.
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  38. 38
    (a) Mafra, L.; Čendak, T.; Schneider, S.; Wiper, P. V.; Pires, J.; Gomes, J. R. B.; Pinto, M. L. Structure of Chemisorbed CO2 Species in Amine-Functionalized Mesoporous Silicas Studied by Solid-State NMR and Computer Modeling. J. Am. Chem. Soc. 2017, 139 (1), 389– 408,  DOI: 10.1021/jacs.6b11081
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    38a
    Structure of Chemisorbed CO2 Species in Amine-Functionalized Mesoporous Silicas Studied by Solid-State NMR and Computer Modeling
    Mafra, Luis; Cendak, Tomaz; Schneider, Sarah; Wiper, Paul V.; Pires, Joao; Gomes, Jose R. B.; Pinto, Moises L.
    Journal of the American Chemical Society (2017), 139 (1), 389-408CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Two-dimensional (2D) solid-state NMR (SSNMR) expts. on samples loaded with 13C-labeled CO2, "under controlled partial pressures", have been performed in this work, revealing unprecedented structural details about the formation of CO2 adducts from its reaction with various amine-functionalized SBA-15 contg. amines having distinct steric hindrances (e.g., primary, secondary) and similar loadings. Three chemisorbed CO2 species were identified by NMR from distinct carbonyl environments resonating at δC ≈ 153, 160, and 164 ppm. The newly reported chemisorbed CO2 species at δC ≈ 153 ppm was found to be extremely moisture dependent. A comprehensive 1H-based SSNMR study [1D 1H and 2D 1H-X heteronuclear correlation (HETCOR, X = 13C, 29Si) expts.] was performed on samples subjected to different treatments. It was found that all chemisorbed CO2 species are involved in hydrogen bonds (HBs) with either surface silanols or neighboring alkylamines. 1H chem. shifts up to 11.8 ppm revealed that certain chemisorbed CO2 species are engaged in very strong HBs. The authors demonstrate that NMR may help in discriminating among free and hydrogen-bonded functional groups. 13C{14N} dipolar-recoupling NMR showed that the formation of carbonate or bicarbonate is excluded. D. functional theory calcns. on models of alkylamines grafted into the silica surface assisted the 1H/13C assignments and validated various HB arrangements that may occur upon formation of carbamic acid. This work extends the understanding of the chemisorbed CO2 structures that are formed upon bonding of CO2 with surface amines and readily released from the surface by pressure swing.
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    (b) Vieira, R.; Marin-Montesinos, I.; Pereira, J.; Fonseca, R.; Ilkaeva, M.; Sardo, M.; Mafra, L. Hidden” CO2 in Amine-Modified Porous Silicas Enables Full Quantitative NMR Identification of Physi- and Chemisorbed CO2 Species. J. Phys. Chem. C 2021, 125 (27), 14797– 14806,  DOI: 10.1021/acs.jpcc.1c02871
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    38b
    Hidden CO2 in Amine-Modified Porous Silicas Enables Full Quantitative NMR Identification of Physi- and Chemisorbed CO2 Species
    Vieira, Ricardo; Marin-Montesinos, Ildefonso; Pereira, Joao; Fonseca, Rita; Ilkaeva, Marina; Sardo, Mariana; Mafra, Luis
    Journal of Physical Chemistry C (2021), 125 (27), 14797-14806CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Although spectroscopic investigation of surface chemisorbed CO2 species has been the focus of most studies, identifying different domains of weakly interacting (physisorbed) CO2 mols. in confined spaces is less trivial as they are often indistinguishable resorting to (isotropic) NMR chem. shift or vibrational band analyses. Herein, we undertake for the first time a thorough solid-state NMR anal. of CO2 species physisorbed prior to and after amine-functionalization of silica surfaces; combining 13C NMR chem. shift anisotropy (CSA) and longitudinal relaxation times (T1). These methods were used to quant. distinguish otherwise overlapping physisorbed CO2 signals, which contributed to an empirical model of CO2 speciation for the physi- and chemisorbed fractions. The quant. measured T1 values confirm the presence of CO2 mol. dynamics on the microsecond, millisecond, and second time scales, strongly supporting the existence of up to three physisorbed CO2 species with proportions of about 15%, 15%, and 70%, resp. Our approach takes advantage from using adsorbed 13C-labeled CO2 as probe mols. and quant. cross-polarization magic-angle spinning to study both physi- and chemisorbed CO2 species, showing that 45% of chemisorbed CO2 vs. 55% of physisorbed CO2 is formed from the overall confined CO2 in amine-modified hybrid silicas. A total of six distinct CO2 environments were identified from which three physisorbed CO2 were discriminated, coined here as "gas, liq., and solid-like" CO2 species. The complex nature of physisorbed CO2 in the presence and absence of chemisorbed CO2 species is revealed, shedding light on what fractions of weakly interacting CO2 are affected upon pore functionalization. This work extends the current knowledge on CO2 sorption mechanisms providing new clues toward CO2 sorbent optimization.
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  39. 39
    dos Santos, T. C.; Bourrelly, S.; Llewellyn, P. L.; de Carneiro, J. W.; Machado Ronconi, C. Adsorption of CO2 on amine-functionalised MCM-41: experimental and theoretical studies. Phys. Chem. Chem. Phys. 2015, 17 (16), 11095– 11102,  DOI: 10.1039/C5CP00581G
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    Adsorption of CO2 on amine-functionalised MCM-41: experimental and theoretical studies
    dos Santos, Thiago Custodio; Bourrelly, Sandrine; Llewellyn, Philip L.; de M. Carneiro, Jose Walkimar; Machado Ronconi, Celia
    Physical Chemistry Chemical Physics (2015), 17 (16), 11095-11102CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Adsorption of CO2 on MCM-41 functionalised with [3-(2-aminoethylamino)propyl]trimethoxysilane (MCM-41-N2), N1-(3-trimethoxysilylpropyl)diethylenetriamine (MCM-41-N3), 4-aminopyridine (MCM-41-aminopyridine), 4-(methylamino)pyridine (MCM-41-methylaminopyridine) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (MCM-41-guanidine) was studied. The amine-functionalised materials were characterized by 29Si and 13C solid-state NMR, N2 adsorption/desorption isotherms, x-ray diffraction and TEM. CO2 adsorption at 1.0 bar and 30° showed that the amt. of CO2 (nads/mmol g-1) adsorbed on MCM-41-N2 and MCM-41-N3 is approx. twice the amt. adsorbed on MCM-41. For MCM-41-aminopyridine, MCM-41-methylaminopyridine and MCM-41-guanidine, the CO2 adsorption capacity was smaller than that of MCM-41 at the same conditions. The p affinity (computed with wB97x-D/6-311++G(d,p)) of the secondary amino groups is higher than that of the primary amino groups; however, the relative stabilities of the primary and secondary carbamates are similar. The differential heat of adsorption decreases as the no. of secondary amino groups increases.
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    Desforges, A.; Backov, R.; Deleuze, H.; Mondain-Monval, O. Generation of Palladium Nanoparticles within Macrocellular Polymeric Supports: Application to Heterogeneous Catalysis of the Suzuki-Miyaura Coupling Reaction. Adv. Funct. Mater. 2005, 15 (10), 1689– 1695,  DOI: 10.1002/adfm.200500146
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    Generation of palladium nanoparticles within macrocellular polymeric supports: Application to heterogeneous catalysis of the Suzuki-Miyaura coupling reaction
    Desforges, Alexandre; Backov, Renal; Deleuze, Herve; Mondain-Monval, Olivier
    Advanced Functional Materials (2005), 15 (10), 1689-1695CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We present new hybrid org./inorg. materials dedicated to heterogeneous catalysis. The systems are obtained by the polymn. of a high internal phase reverse emulsion (the so-called polyHIPE porous materials) and have been further functionalized with various org. groups in order to promote the growth of palladium nanoparticles on its surface. Final supports are then tested for their ability to catalyze the Suzuki-Miyaura coupling reaction, and one material exhibits better activity than the well-known Pd@C powder system. Furthermore, the catalytic activities of these materials are close to those obtained with their homogeneous catalysis counterpart. These new supports remain active towards a wide range of substrates assocd. with Suzuki-Miyaura carbon-carbon coupling reactions.
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    Yamashita, T.; Hayes, P. Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci. 2008, 254 (8), 2441– 2449,  DOI: 10.1016/j.apsusc.2007.09.063
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    Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials
    Yamashita, Toru; Hayes, Peter
    Applied Surface Science (2008), 254 (8), 2441-2449CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Samples of the iron oxides Fe0.94O, Fe3O4, Fe2O3, and Fe2SiO4 were prepd. by high temp. equilibration in controlled gas atmospheres. The samples were fractured in vacuum and high resoln. XPS spectra of the fractured surfaces were measured. The peak positions and peak shape parameters of Fe 3p for Fe2+ and Fe3+ were derived from the Fe 3p XPS spectra of the std. samples of 2FeO.SiO2 and Fe2O3, resp. Using these parameters, the Fe 3p peaks of Fe3O4 and Fe1-yO were analyzed. High resoln. XPS techniques can be used to det. the Fe2+/Fe3+ ratios in metal oxides. The technique has the potential for application to other transition metal oxide systems.
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    The authors use high purity Fe oxide nanoparticles to confirm the Fe 2p x-ray photoemission peak attributions made in previous study of Fe nanoparticles and the initial stage of their oxidn. To accomplish this, the authors found it necessary to consider the spectral contributions of the ligand field of the Fe-O crystal structure, the cryst. disorder at the nanoparticle surface, and the Russell-Saunders broadening of the FeIII components of the Fe 2p spectra.
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    Limiting our plastic waste and finding greener, more sustainable solns. for disposal is currently an environmental priority. Polyethylene terephthalate (PET), one of the more prominent single-use plastics, has recently been under investigation for chem. recycling as a means to ameliorate the environmental impact. This work reports a dual-catalytic approach to the chem. recycling of PET, aiming to combine inexpensive, readily available Lewis acid-base pairs to exhibit cooperative catalytic activity.
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    Low-Temperature Glycolysis of Polyethylene Terephthalate
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    ACS Sustainable Chemistry & Engineering (2022), 10 (24), 7965-7973CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    Due to the antibiodegradable properties, numerous plastics have been accumulated in the ecosystem and aggravate ecol. pollution. Poly (ethylene terephthalate) (PET) is among the most used plastics. Glycolysis of PET is a useful approach to solve the waste PET pollution and obtain bis(2-hydroxyethyl) terephthalate (BHET). In this paper, waste PET was efficiently depolymd. through glycolysis catalyzed by cyanamide. In particular, compared with the previously reported catalyst, cyanamide is more readily available and can be used directly in catalysis without a complex prepn. process. Under optimal conditions, PET was completely depolymd. with up to nearly 100% BHET yield. Even at a temp. as low as 150°C, a good BHET yield can be obtained. The application potential of this glycolysis procedure was demonstrated by its excellent performance in the glycolysis of various real PET wastes like transparent and opaque PET samples and polyester foam and by the high quality of the obtained BHET products. The mechanism was studied by 1H NMR anal., and DFT calcns. showed that the higher activity of cyanamide than its trimer, melamine, is due to the stronger hydrogen bonds formed between cyanamide and PET or ethylene glycol.
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  • Abstract

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    Scheme 1

    Scheme 1. (a) Synthesis Pathways to Prepare Mesoporous SiO2 with Five Different Organic Capping Ligands SiO2–NH2 (1); SiO2–NH2–SB (2); SiO2–NH–NH2 (3), SiO2–NH–NH2–SB (4) and SiO2–Pincer (5), and (b) the Catalysts Preparation Scheme
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    Figure 1

    Figure 1. XPS analysis of the (a) N 1s spectra and (b) O 1s core level for bare silica and the functionalized SiO2 with –NH2 (1), –NH2 SB (2), –NHNH2 (3), –NHNH2 SB (4), and pincer (5), respectively.

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    Figure 2

    Figure 2. 1D (a) 29Si MAS NMR spectra together with their deconvolution of bare silica and (b) functionalized SiO2–NH2 (1). (c) Stacked plot of the 29Si MAS NMR spectra of all samples; bare silica, SiO2–NH2 (1), SiO2–NH2–SB (2), SiO2–NH–NH2 (3), SiO2–NH–NH2–SB (4), and SiO2–pincer (5); (d) 1H → 13C CPMAS NMR spectra of the functionalized silica with ligands (1) to (5).

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    Figure 3

    Figure 3. TEM images of SiO2 support (a) before Fe-ion immobilized, (b) after calcinations, and (c) after chemical reduction. (d) XRD of SiO2 catalysts prepared by calcination and chemical reduction, (e) high angle angular dark field image of SiO2 catalyst after chemical reduction, and associated (f) Si, (g) Fe, and (h) O EDX maps. Scale bars in (e,f) are 100 nm.

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    Figure 4

    Figure 4. XPS core level spectra of (a) Fe 2p (b) O 1s and (c) N 1s for the SiO2–NH-SB and the corresponding Fe2O3 NP catalysts prepared via calcination (C) and chemical reduction (R).

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    Figure 5

    Figure 5. Percentage conversion of PET (blue) and isolated yield of BHET (green) obtained from glycolysis reaction catalyzed by (a) organic ligand modified SiO2 (before metal loading), (b) iron–ion-immobilized SiO2, (c) Fe2O3–NP SiO2 via calcination, and (d) Fe2O3–NP SiO2 prepared by chemical reduction.

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    Figure 6

    Figure 6. Optimized geometries for the different SiO2/ligands coordinating the Fe3+ ion. Color scheme: gray (C), white (H), blue (N), orange (Fe), red (O), and beige (Si).

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    Figure 7

    Figure 7. (a) Optimized geometry of the adsorbed PET molecular cluster over the Fe3+ ion. (b) Optimized geometry of the adsorbed PET over the iron oxide NC. Color scheme: gray (C), white (H), blue (N), orange (Fe), red (O, and beige (Si). (c) Relative Gibbs energy profiles at 190 °C (in kcal·mol–1) for PET glycolysis considering EG as a solvent, via uncatalyzed (blue color), catalyzed by a single atom Fe3+ (red color), and catalyzed by the iron oxide nanocluster (purple color).

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    Figure 8

    Figure 8. (a) Optimization of catalyst loading (w.r.t the SiO2 mass) and reaction temperature, (b) PET/EG ratio optimization, (c) time series of FeCl3, SiO2–Fe2O3 pincer NP and SiO2–Fe2O3–NH2SB NP both prepared via a reducing agent. (d) Application of heterogeneous catalysts to colored postconsumer PET.

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      Sustainability (2021), 13 (19), 10528CODEN: SUSTDE; ISSN:2071-1050. (MDPI AG)
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      Hydrolysis of polyethylene terephthalate (PET) is an efficient strategy for the depolymn. of waste PET to terephthalic acid (TPA), which can be used as a fundamental building block for the repolymn. of PET or for the synthesis of biodegradable plastics and metal-org. frameworks. However, most of the reported hydrolysis catalysts are strong acids or bases, which are sol. in reaction media and difficult to sep. after the reaction, leading to high prodn. costs and a profound influence on the environment. Herein, we propose the use of TPA, the basic unit of PET, as an acid catalyst to promote the hydrolysis of PET. Under optimized conditions, i.e., 2.5 g of PET, a TPA concn. of 0.1 g/mL, mass ratio PET:H2O of 1:8, 220°C of temp., and 180 min of reaction time, a PET conversion of up to 100.0% and a TPA yield of 95.5% were achieved. Furthermore, the produced TPA exhibited a high purity of 99%, similar to that of fresh TPA, and was easily recoverable for PET hydrolysis without tedious workup and purifn. processes. More importantly, the hydrolysis efficiency was maintained over eight consecutive reaction cycles. Overall, this study provides a green, easy, and low-cost technol. to recover and reuse TPA for waste PET hydrolysis.
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      6a
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      Chen, Jinyang; Lv, Jingxiao; Ji, Yimei; Ding, Junying; Yang, Xuanyu; Zou, Mihua; Xing, Luyao
      Polymer Degradation and Stability (2014), 107 (), 178-183CODEN: PDSTDW; ISSN:0141-3910. (Elsevier Ltd.)
      Ionic liq. (IL) as cosolvent is a new way to accelerate polymer solvation degrdn., and thus several common imidazole ILs have been studied as cosolvent to improve the alcoholysis of PET with isooctyl alc. (2-EH) to produce dioctyl terephthalate (DOTP) at the reflux temp. (190∼200 °C). Owing to its best effect and relatively inexpensiveness, [Bmim]Cl has been adopted to assist the alcoholysis. Both tetra-Bu titanate (Ti(OC4H9)4) and zinc acetate (ZA) have good catalysis for the alcoholysis, but because ZA is solid and difficultly dissolved in alc. and IL, it is easily to be sepd. out and more feasible to be used as catalyst. With [Bmim]Cl and ZA as cosolvent and catalyst, the optimal alcoholysis of PET has been detd. to be as follows: reflux temp., reaction time 5 h, wt. ratio of IL:2-EH:PET 2:2:1 and wt. ratio of catalyst/PET 1.2%, and then the degrdn. rate of PET is almost 100% and the yield of DOTP is 93.1%. Comparing with traditional alcoholysis without IL as cosolvent, the reaction time decreases greatly from more than 10 h to several hours. Furthermore, the IL has been repeated four times and the degrdn. rate of PET and the yield of DOTP nearly have not been changed.
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      (b) Scremin, D. M.; Miyazaki, D. Y.; Lunelli, C. E.; Silva, S. A.; Zawadzki, S. F. PET Recycling by Alcoholysis Using a New Heterogeneous Catalyst: Study and its Use in Polyurethane Adhesives Preparation. Macromol. Symp. 2019, 383 (1), 1800027,  DOI: 10.1002/masy.201800027
      There is no corresponding record for this reference.
    7. 7
      Wang, L.; Nelson, G. A.; Toland, J.; Holbrey, J. D. Glycolysis of PET Using 1,3-Dimethylimidazolium-2-Carboxylate as an Organocatalyst. ACS Sustainable Chem. Eng. 2020, 8 (35), 13362– 13368,  DOI: 10.1021/acssuschemeng.0c04108
      7
      Glycolysis of PET Using 1,3-Dimethylimidazolium-2-Carboxylate as an Organocatalyst
      Wang, Lei; Nelson, Gareth A.; Toland, Jeni; Holbrey, John D.
      ACS Sustainable Chemistry & Engineering (2020), 8 (35), 13362-13368CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      The use of 1,3-dimethylimidazolium-2-carboxylate as an organocatalyst for the glycolysis of waste PET, poly(ethylene terephthalate), is reported for the first time. Postconsumer PET was completely depolymd. in less than 1 h at 180°C with up to 60% of bis(2-hydroxyethyl terephthalate) (BHET) recovered by pptn. after cooling the reaction mixt. Under comparable conditions, the basic ionic liq., 1,3-dimethylimidazolium acetate, was a significantly less effective catalyst suggesting that catalysis occurs through formation of a nucleophilic N-heterocyclic carbene. Carbene formation from 1,3-dimethylimidazolium-2-carboxylate generates superior performance in glycolysis of PET compared to corresponding basic 1,3-dimethylimidazolium acetate ionic liq. catalyst.
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    8. 8
      Uekert, T.; Singh, A.; Desveaux, J. S.; Ghosh, T.; Bhatt, A.; Yadav, G.; Afzal, S.; Walzberg, J.; Knauer, K. M.; Nicholson, S. R. Technical, Economic, and Environmental Comparison of Closed-Loop Recycling Technologies for Common Plastics. ACS Sustainable Chem. Eng. 2023, 11 (3), 965– 978,  DOI: 10.1021/acssuschemeng.2c05497
      8
      Technical, Economic, and Environmental Comparison of Closed-Loop Recycling Technologies for Common Plastics
      Uekert, Taylor; Singh, Avantika; DesVeaux, Jason S.; Ghosh, Tapajyoti; Bhatt, Arpit; Yadav, Geetanjali; Afzal, Shaik; Walzberg, Julien; Knauer, Katrina M.; Nicholson, Scott R.; Beckham, Gregg T.; Carpenter, Alberta C.
      ACS Sustainable Chemistry & Engineering (2023), 11 (3), 965-978CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      Over 400 million metric tons of plastic waste are generated globally each year, resulting in pollution and lost resources. Recycling strategies can recapture this wasted material, but there is a lack of quant. and transparent data on the capabilities and impacts of these processes. Here, we develop a data set of material quality, material retention, circularity, contamination tolerance, min. selling price, greenhouse gas emissions, energy use, land use, toxicity, waste generation, and water use metrics for closed-loop polymer recycling technologies, including mech. recycling and solvent-based dissoln. of polyethylene, polyethylene terephthalate (PET), and polypropylene, as well as enzymic hydrolysis, glycolysis, and vapor methanolysis of PET. Mech. recycling and PET glycolysis display the best economic (9%-73% lower than competing technologies) and environmental (7%-88% lower) performances, while dissoln., enzymic hydrolysis, and methanolysis provide the best recyclate material qualities (2%-27% higher). We identify electricity, steam, and org. solvents as top process contributors to these metrics and apply sensitivity and multicriteria decision analyses to highlight key future research areas. The ests. derived in this work provide a quant. baseline for comparing and improving recycling technologies, can help reclaimers identify optimal end-of-life routes for given waste streams, and serve as a framework for assessing future innovations.
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    9. 9
      Chen, F.; Wang, G.; Shi, C.; Zhang, Y.; Zhang, L.; Li, W.; Yang, F. Kinetics of glycolysis of poly(ethylene terephthalate) under microwave irradiation. J. Appl. Polym. Sci. 2013, 127 (4), 2809– 2815,  DOI: 10.1002/app.37608
      9
      Kinetics of glycolysis of poly(ethylene terephthalate) under microwave irradiation
      Chen, Feifei; Wang, Guanghui; Shi, Chuan; Zhang, Yichen; Zhang, Long; Li, Wei; Yang, Feng
      Journal of Applied Polymer Science (2013), 127 (4), 2809-2815CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
      The glycolysis of poly(ethylene terephthalate) (PET) was carried out using excess ethylene glycol (EG) in the presence of zinc acetate as catalysts under microwave irradn. The effects of particle size, microwave power, the wt. ratio of EG to PET, the wt. ratio of catalyst to PET, reaction temp. and stirring speed on the yield of bis(hydroxyethyl terephthalate)(BHET) were studied. The glycolysis rate was significantly influenced by stirring speed and initial particle size. The optimal parameters of glycolysis reactions were the wt. ratio of catalyst to PET of 1%, the wt. ratio of EG to PET of 5,500 W and 196 °C, the yield of BHET reached to 78% at only 35 min. The glycolysis products were analyzed and identified by FTIR, differential scanning calorimetry, and elemental anal. The kinetics of glycolysis of PET under microwave irradn. could be interpreted by the shrinking core model of the film diffusion control. The apparent activation energy was evaluated using the Arrhenius equation and it was 36.5 KJ/mol, which was lower compared to the same process using conventional heating. The reaction time was significantly decreased under microwave irradn. as compared with it by conventional heating. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2012.
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    10. 10
      Vollmer, I.; Jenks, M. J. F.; Roelands, M. C. P.; White, R. J.; van Harmelen, T.; de Wild, P.; van der Laan, G. P.; Meirer, F.; Keurentjes, J. T. F.; Weckhuysen, B. M. Beyond Mechanical Recycling: Giving New Life to Plastic Waste. Angew. Chem., Int. Ed. 2020, 59 (36), 15402– 15423,  DOI: 10.1002/anie.201915651
      10
      Beyond Mechanical Recycling: Giving New Life to Plastic Waste
      Vollmer, Ina; Jenks, Michael J. F.; Roelands, Mark C. P.; White, Robin J.; van Harmelen, Toon; de Wild, Paul; van der Laan, Gerard P.; Meirer, Florian; Keurentjes, Jos T. F.; Weckhuysen, Bert M.
      Angewandte Chemie, International Edition (2020), 59 (36), 15402-15423CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Increasing the stream of recycled plastic necessitates an approach beyond the traditional recycling via melting and re-extrusion. Various chem. recycling processes have great potential to enhance recycling rates. In this Review, a summary of the various chem. recycling routes and assessment via life-cycle anal. is complemented by an extensive list of processes developed by companies active in chem. recycling. We show that each of the currently available processes is applicable for specific plastic waste streams. Thus, only a combination of different technologies can address the plastic waste problem. Research should focus on more realistic, more contaminated and mixed waste streams, while collection and sorting infrastructure will need to be improved, i.e., by stricter regulation. This Review aims to inspire both science and innovation for the prodn. of higher value and quality products from plastic recycling suitable for reuse or valorization to create the necessary economic and environmental push for a circular economy.
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    11. 11
      Duque-Ingunza, I.; López-Fonseca, R.; de Rivas, B.; Gutiérrez-Ortiz, J. I. Process optimization for catalytic glycolysis of post-consumer PET wastes. J. Chem. Technol. Biotechnol. 2014, 89 (1), 97– 103,  DOI: 10.1002/jctb.4101
      11
      Process optimization for catalytic glycolysis of post-consumer PET wastes
      Duque-Ingunza, I.; Lopez-Fonseca, R.; Rivas, B.; Gutierrez-Ortiz, J. I.
      Journal of Chemical Technology and Biotechnology (2014), 89 (1), 97-103CODEN: JCTBED; ISSN:0268-2575. (John Wiley & Sons Ltd.)
      Glycolysis has been the subject of increased interest as a valuable feedstock recycling for poly(ethylene terephthalate) (PET). However, there are no reports in the tech. literature that deal with the design and optimization of the global process. Conversion into bis(2-hydroxyethyl) terephthalate (BHET) of the non-glycolyzed solid was feasible, carried out in the presence or absence of fresh PET. The yield varied between 63 and 80%. The monomer was recovered by extn. with hot water followed by crystn. at 4 °C. The optimized H2O:BHET ratio was 6.7 mL g-1. The EG(ethylene glycol)/H2O mixt. was effectively sepd. by vacuum distn. and EG of 99.6% purity was recovered, which could be used again. Finally, the highly active catalytic role of sodium carbonate salt for glycolysis was evident not only for transparent PET wastes but also for complex wastes. The recirculation of the solid residue from extn., the sepn. of EG/H2O mixt. from crystn. and the subsequent refeeding of the org. reactant into the reactor, the minimization of the amt. of water to efficiently recover the monomer and the suitability of the chem. recycling of complex PET wastes are operational aspects that significantly increased the global efficiency. © 2013 Society of Chem. Industry.
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    12. 12
      (a) Shukla, S. R.; Kulkarni, K. S. Depolymerization of poly(ethylene terephthalate) waste. J. Appl. Polym. Sci. 2002, 85 (8), 1765– 1770,  DOI: 10.1002/app.10714
      12a
      Depolymerization of poly(ethylene terephthalate) waste
      Shukla, S. R.; Kulkarni, K. S.
      Journal of Applied Polymer Science (2002), 85 (8), 1765-1770CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
      Poly(ethylene terephthalate) waste was depolymd. with ethylene glycol in the presence of different catalysts, two conventional metal catalysts (zinc acetate and lead acetate) and two alkalies (sodium carbonate and sodium bicarbonate). The resulting monomer bis(2-hydroxyethyl) terephthalate was characterized by thin layer chromatog., m.p., IR spectroscopy, differential scanning calorimetry, and elemental anal. The results show that the qual. and quant. yields of the monomer obtained with alkalies as catalysts were most comparable with the conventional heavy metal catalysts, thus providing a further advantage for the recycling of polyester waste for the cause of environmental pollution abatement.
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      (b) López-Fonseca, R.; Duque-Ingunza, I.; de Rivas, B.; Flores-Giraldo, L.; Gutiérrez-Ortiz, J. I. Kinetics of catalytic glycolysis of PET wastes with sodium carbonate. Chem. Eng. J. 2011, 168 (1), 312– 320,  DOI: 10.1016/j.cej.2011.01.031
      12b
      Kinetics of catalytic glycolysis of poly(ethylene terephthalate) wastes with sodium carbonate
      Lopez-Fonseca, Ruben; Duque-Ingunza, Itxaso; de Rivas, Beatriz; Flores-Giraldo, Laura; Gutierrez-Ortiz, Jose I.
      Chemical Engineering Journal (Amsterdam, Netherlands) (2011), 168 (1), 312-320CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      The kinetics of glycolysis of poly(ethylene terephthalate) wastes with ethylene glycol to give highly pure bis(2-hydroxyethyl terephthalate) was examd. in a batch reactor. An excess of ethylene glycol (EG:PET molar ratio of 7.6:1) was used and the reaction was carried out in the presence of sodium carbonate as active catalyst. The influence of several operating conditions covering temp. (165-196°), mean particle size (0.14-3 mm), stirring rate (50-800 rpm), reaction time (0-10 h), and catalyst type and concn. was analyzed. The selected PET particle size and stirring rate for kinetic calcns. were 0.25 mm and 600 rpm, resp. Using a PET:catalyst molar ratio of 100:1 about 80% BHET yield was attained at 196° after 1 h. A simple theor. power-law model was developed to predict the time evolution of conversion. This kinetic model was found to be consistent with exptl. data. The rate consts. for both direct and reverse reactions were estd. Also the values of the activation energy (185 kJ mol-1) and enthalpy of reaction (12 kJ mol-1) were derived.
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      (c) Fang, P.; Liu, B.; Xu, J.; Zhou, Q.; Zhang, S.; Ma, J.; lu, X. High-efficiency glycolysis of poly(ethylene terephthalate) by sandwich-structure polyoxometalate catalyst with two active sites. Polym. Degrad. Stab. 2018, 156, 22– 31,  DOI: 10.1016/j.polymdegradstab.2018.07.004
      12c
      High-efficiency glycolysis of poly(ethylene terephthalate) by sandwich-structure polyoxometalate catalyst with two active sites
      Fang, Pengtao; Liu, Bo; Xu, Junli; Zhou, Qing; Zhang, Suojiang; Ma, Junying; lu, Xingmei
      Polymer Degradation and Stability (2018), 156 (), 22-31CODEN: PDSTDW; ISSN:0141-3910. (Elsevier Ltd.)
      Catalyst in the process on glycolysis of poly (ethylene terephthalate) (PET) wastes is a significant crit. factor, which det. the efficiency and the cost of PET degrdn. In this study, a kind of transition-metal-substituted polyoxometalates (POMs) Na12 [WZnM2(H2O)2(ZnW9O34)2] (M = Zn2+, Mn2+, Co2+, Cu2+, Ni2+) which have a sandwich-structure and more than two transition metal active sites show excellent catalytic performance in the glycolysis of PET under mild conditions. We investigated the effects of temp., reaction time and catalyst amt. on PET degrdn. and obtained the glycolysis optimal conditions. The PET could be fast and completely degraded at 190 °C for 40 min with low catalyst/PET molar ratio (0.018%) and high PET/Ethylene Glycol (EG) wt. ratio (1:4), and the yield of bis(hydroxyethyl) terephthalate (BHET) is higher than 84.5%. After four-times recycling, the conversion of PET and the yield of BHET can still reach 100% and 84.5%. The exact structure of POMs Na12 [WZnM2(H2O)2(ZnW9O34)2] is confirmed by Single Crystal X-ray Diffraction (SC-XRD). Compared with traditional heteropolyacid catalysts, this catalyst possessed of at least two transition metal active sites, which lead to its an excellent catalytic capacity. The possible coordination activates mechanism for PET glycolysis is also proposed.
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    13. 13
      (a) Chen, F.; Wang, G.; Li, W.; Yang, F. Glycolysis of Poly(ethylene terephthalate) over Mg-Al Mixed Oxides Catalysts Derived from Hydrotalcites. Ind. Eng. Chem. Res. 2013, 52 (2), 565– 571,  DOI: 10.1021/ie302091j
      13a
      Glycolysis of Poly(ethylene terephthalate) over Mg-Al Mixed Oxides Catalysts Derived from Hydrotalcites
      Chen, Feifei; Wang, Guanghui; Li, Wei; Yang, Feng
      Industrial & Engineering Chemistry Research (2013), 52 (2), 565-571CODEN: IECRED; ISSN:0888-5885. (American Chemical Society)
      Poly(ethylene terephthalate) (PET) was depolymd. by ethylene glycol (EG) in the presence of Mg-Al hydrotalcites and their corresponding mixed oxides as solid base catalysts. Mg-Al hydrotalcites with different Mg/Al molar ratios were prepd.; it was confirmed by powder X-ray diffraction (XRD) that the materials had hydrotalcite structure. Compared with their corresponding precursors, Mg-Al mixed oxides obtained by the calcination of hydrotalcites exhibited higher catalytic activity for the glycolysis of PET. Furthermore, Mg-Al mixed oxides calcinated at 500 °C with Mg/Al molar ratio of 3 offered the highest catalytic activity for the glycolysis of PET. The exptl. results showed that the basicity of catalyst played an important role on the glycolysis activity. The solid catalyst could be easily sepd. and reused after calcination again. The influences of exptl. parameters on the yield of bis(hydroxyethyl terephthalate)(BHET) were investigated. The evolution of the glycolysis of PET was described by IR spectroscopy (IR), viscosity-av. mol. wt., and scanning electron microscope (SEM).
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      (b) Fuentes, C. A.; Gallegos, M. V.; García, J. R.; Sambeth, J.; Peluso, M. A. Catalytic Glycolysis of Poly(ethylene terephthalate) Using Zinc and Cobalt Oxides Recycled from Spent Batteries. Waste Biomass Valorization 2020, 11 (9), 4991– 5001,  DOI: 10.1007/s12649-019-00807-6
      13b
      Catalytic Glycolysis of Poly(ethylene terephthalate) Using Zinc and Cobalt Oxides Recycled from Spent Batteries
      Fuentes, Cynthia A.; Gallegos, Maria V.; Garcia, Juan R.; Sambeth, Jorge; Peluso, Miguel A.
      Waste and Biomass Valorization (2020), 11 (9), 4991-5001CODEN: WBVAAG; ISSN:1877-2641. (Springer)
      The chem. recycling of polyethylene terephthalate (PET) to bis(2-hydroxyethyl) terephthalate (BHET) was studied using recycled metal oxides. Recovered zinc (RZnO) and cobalt (RCoO) oxides were obtained after a biohydrometallurgical process to recycle spent alk. and lithium-ion batteries (LIBs), resp. Besides, a mixed oxide (Co/RZnO) was prepd. by mech. milling of 2.5 wt% of RCoO on RZnO. The structural, textural, and acidity properties of the catalysts were analyzed by XRD, XANES, SEM, TEM, FT-IR, SBET and pyridine-TPD. The depolymn. of PET (from soft-drink bottles) was carried out with ethylene glycol (EG) at 196°C for 2 h, using PET/catalyst and PET/EG ratios of 100:1 and 1:8, resp. The yields of the BHET monomer in the presence of RZnO, RCoO and Co/RZnO as catalysts were 50%, 10% and 80%, resp. The highest catalytic activity of Co/RZnO could be attributed to the presence of weak and strong acid sites, its overall higher concn. of acid sites and a synergetic effect between Co3O4 and ZnO. The obtained BHET was characterized by DSC, FT-IR, 1H NMR and 13C NMR analyses, which confirmed the purity and structure of the monomer. Metal oxides obtained using spent alk. and lithium-ion batteries as raw materials could be used as catalysts for waste PET treatment and pure BHET monomer synthesis. Graphic Abstr.: [Figure not available: see fulltext.].
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    14. 14
      (a) Wang, Q.; Geng, Y.; Lu, X.; Zhang, S. First-Row Transition Metal-Containing Ionic Liquids as Highly Active Catalysts for the Glycolysis of Poly(ethylene terephthalate) (PET). ACS Sustainable Chem. Eng. 2015, 3 (2), 340– 348,  DOI: 10.1021/sc5007522
      14a
      First-Row Transition Metal-Containing Ionic Liquids as Highly Active Catalysts for the Glycolysis of Poly(ethylene terephthalate) (PET)
      Wang, Qian; Geng, Yanrong; Lu, Xingmei; Zhang, Suojiang
      ACS Sustainable Chemistry & Engineering (2015), 3 (2), 340-348CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      First-row transition metal-contg. ionic liqs. (ILs) were synthesized and used to catalyze the degrdn. of poly(ethylene terephthalate) (PET) in ethylene glycol (EG). One important feature of these IL catalysts is that they have good thermal stability, and most of them, esp. [bmim]2[CoCl4] (bmim = 1-butyl-3-methyl-imidazolium) and [bmim]2[ZnCl4], exhibit higher catalytic activity, compared with traditional catalysts, conventional IL catalysts, and some functional ILs. For example, utilizing [bmim]2[CoCl4] as catalyst, the conversion of PET, selectivity of bis(hydroxyethyl) terephthalate (BHET), and mass fraction of BHET in products reach up to 100%, 81.1%, and 95.7%, resp., under atm. pressure at 175 °C for only 1.5 h. Another important feature is that BHET can be easily sepd. from these IL catalysts and has high purity. Moreover, recycling results show that [bmim]2[CoCl4] worked efficiently after being used six times. These all show that [bmim]2[CoCl4] is an excellent IL catalyst for the glycolysis of PET. Finally, based on in situ IR spectra and exptl. results, the possible mechanism of degrdn. with synthesized IL is proposed.
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      (b) Yue, Q. F.; Xiao, L. F.; Zhang, M. L.; Bai, X. F. The Glycolysis of Poly(ethylene terephthalate) Waste: Lewis Acidic Ionic Liquids as High Efficient Catalysts. Polymers 2013, 5 (4), 1258– 1271,  DOI: 10.3390/polym5041258
      14b
      The glycolysis of poly(ethylene terephthalate) waste: Lewis acidic ionic liquids as high efficient catalysts
      Yue, Qun Feng; Xiao, Lin Fei; Zhang, Mi Lin; Bai, Xue Feng
      Polymers (Basel, Switzerland) (2013), 5 (4), 1258-1271, 14 pp.CODEN: POLYCK; ISSN:2073-4360. (MDPI AG)
      Poly(ethlyene terephthalate) waste from a local market was depolymd. by ethylene glycol (EG) in the presence of Lewis acidic ionic liqs. [Bmim]ZnCl3 and the qual. anal. showed that bis(hydroxyethyl) terephthalate was the main product. Compared with ionic liq. [Bmim]Cl, the Lewis acidic ionic liqs. showed highly catalytic activity in the glycolysis of poly(ethylene terephthalate) PET. Significantly, the conversion of PET and the yield of bis(hydroxyethyl) terephthalate were achieved at 100% and 83.8% with low catalyst ([Bmim]ZnCl3) loading (0.16 wt %). Investigation also showed that the catalytic activity of [Bmim]ZnCl3 was higher than that of [Bmim]MnCl3. Catalyst [Bmim]ZnCl3 can be reused up to five times and 1H-NMR results show that the recovered catalyst is similar to the fresh one. A mechanism of the glycolysis of PET catalyzed by [Bmim]ZnCl3 was proposed.
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    15. 15
      Lalhmangaihzuala, S.; Laldinpuii, Z.; Lalmuanpuia, C.; Vanlaldinpuia, K. Glycolysis of Poly(Ethylene Terephthalate) Using Biomass-Waste Derived Recyclable Heterogeneous Catalyst. Polymers 2020, 13 (1), 37,  DOI: 10.3390/polym13010037
      There is no corresponding record for this reference.
    16. 16
      (a) Yunita, I.; Putisompon, S.; Chumkaeo, P.; Poonsawat, T.; Somsook, E. Effective catalysts derived from waste ostrich eggshells for glycolysis of post-consumer PET bottles. Chem. Pap. 2019, 73 (6), 1547– 1560,  DOI: 10.1007/s11696-019-00710-3
      16a
      Effective catalysts derived from waste ostrich eggshells for glycolysis of post-consumer PET bottles
      Yunita, Isti; Putisompon, Siraphat; Chumkaeo, Peerapong; Poonsawat, Thinnaphat; Somsook, Ekasith
      Chemical Papers (2019), 73 (6), 1547-1560CODEN: CHPAEG; ISSN:1336-9075. (Springer International Publishing AG)
      Herein, we report an effective chem. recycling of poly(ethylene terephthalate) (PET) using sustainable sources of catalysts, calcium oxide (CaO) derived from ostrich eggshells. The active catalysts were demonstrated in the chem. depolymn. of post-consumer PET bottles. Beverage bottles were proceeded with 1 wt% catalyst derived from ostrich eggshells in the presence of ethylene glycol at 192 °C under atm. pressure to give the major product as bis(2-hydroxyethyl terephthalate) (BHET) which was confirmed by m.p., IR spectroscopy, 1H-, 13C-NMR spectroscopy and mass spectrum. The catalyst could fully depolymerize PET within 2 h, producing a good yield of highly pure BHET monomer. The catalysts were successfully characterized by X-ray powder diffraction, XPS, field-emission SEM with energy dispersive X-ray spectroscopy anal., and thermo-gravimetric anal. Furthermore, catalysts derived from chicken eggshells, geloina, mussel, and oyster shells were run to compare the catalytic activities. For better understanding of catalytic parameters, effects of calcination temps. of catalyst, wt. ratio of catalyst, ratio of wt. of solvent, and time of depolymn. for the ostrich eggshells catalyst were also investigated.
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      (b) Laldinpuii, Z.; Lalhmangaihzuala, S.; Pachuau, Z.; Vanlaldinpuia, K. Depolymerization of poly(ethylene terephthalate) waste with biomass-waste derived recyclable heterogeneous catalyst. Waste Manage. 2021, 126, 1– 10,  DOI: 10.1016/j.wasman.2021.02.056
      16b
      Depolymerization of poly(ethylene terephthalate) waste with biomass-waste derived recyclable heterogeneous catalyst
      Laldinpuii, Zathang; Lalhmangaihzuala, Samson; Pachuau, Zodinpuia; Vanlaldinpuia, Khiangte
      Waste Management (Oxford, United Kingdom) (2021), 126 (), 1-10CODEN: WAMAE2; ISSN:0956-053X. (Elsevier Ltd.)
      Poly(ethylene terephthalate) (PET) is one of the most widely used polymeric materials in chem. industry representing about 13% of the world's prodn. With the exponentially increasing consumption of plastics combined with its non-biodegradability, the accumulation of plastic waste in the environment rises steeply and its recycling has attracted enormous attention among researchers in recent years. In this present work, we describe bamboo leaf ash (BLA) as a bio-waste derived recyclable heterogeneous catalyst for the depolymn. of waste PET. The prepd. catalyst was characterized by FT-IR, XRD, SEM, TEM, EDX, TGA and BET analyses to assess its morphol. and compn. Postconsumer PET bottles were shredded and processed with 20 wt% BLA and 16 equiv of ethylene glycol (EG) at 190°C for 3.5 h under atm. pressure to give recrystd. bis(2-hydroxyethyl) terephthalate (BHET) monomer in 83% yield. The catalyst can be reused for four catalytic cycles and the residual EG was recovered for subsequent catalytic reactions. Excellent activity, cost-free, environmental-friendliness and ease of prepn., handling and reusability of the catalyst with simple work-up procedure are the notable advantages of this protocol.
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    17. 17
      Veregue, F. R.; Pereira da Silva, C. T.; Moisés, M. P.; Meneguin, J. G.; Guilherme, M. R.; Arroyo, P. A.; Favaro, S. L.; Radovanovic, E.; Girotto, E. M.; Rinaldi, A. W. Ultrasmall Cobalt Nanoparticles as a Catalyst for PET Glycolysis: A Green Protocol for Pure Hydroxyethyl Terephthalate Precipitation without Water. ACS Sustainable Chem. Eng. 2018, 6 (9), 12017– 12024,  DOI: 10.1021/acssuschemeng.8b02294
      17
      Ultrasmall Cobalt Nanoparticles as a Catalyst for PET Glycolysis: A Green Protocol for Pure Hydroxyethyl Terephthalate Precipitation without Water
      Veregue, Fernanda Reis; Pereira da Silva, Cleiser Thiago; Moises, Murilo Pereira; Meneguin, Joziane Gimenes; Guilherme, Marcos Rogerio; Arroyo, Pedro Augusto; Favaro, Silvia Luciana; Radovanovic, Eduardo; Girotto, Emerson Marcelo; Rinaldi, Andrelson Wellington
      ACS Sustainable Chemistry & Engineering (2018), 6 (9), 12017-12024CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      Polyethylene terephthalate (PET) is a very stable polymer widely used in the modern world. Due to its stability, this polymer can remain in the environment for several years before its complete degrdn. The glycolysis reaction of PET has emerged as a green approach to obtain the PET monomer, thus avoiding such environmental problems and adding value to this waste. PET waste was depolymd. by glycolysis using ultrasmall cobalt nanoparticles (1.5%) as the catalyst for the prodn. of bis-2-hydroxyethyl terephthalate (BHET). A capping agent (tannic acid, TA) and a borohydride redn. approach were used to obtain such ultrasmall cobalt nanoparticles (∼3 nm). A PET depolymn. yield of 96% was achieved within 3 h at 180°. The pptn. of 77% of pure BHET was achieved without the need for water. The remaining ethylene glycol soln. contg. the ultrasmall cobalt nanoparticle catalyst was reused five times for this glycolysis process, demonstrating the feasibility of solvent reuse without the need for any treatment. A reaction mechanism is proposed to explain the high BHET yield obtained by this ultrasmall cobalt nanoparticle catalyst stabilized with TA.
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    18. 18
      Du, J.-T.; Sun, Q.; Zeng, X.-F.; Wang, D.; Wang, J.-X.; Chen, J.-F. ZnO nanodispersion as pseudohomogeneous catalyst for alcoholysis of polyethylene terephthalate. Chem. Eng. Sci. 2020, 220, 115642,  DOI: 10.1016/j.ces.2020.115642
      18
      ZnO nanodispersion as pseudohomogeneous catalyst for alcoholysis of polyethylene terephthalate
      Du, Jin-Tao; Sun, Qian; Zeng, Xiao-Fei; Wang, Dan; Wang, Jie-Xin; Chen, Jian-Feng
      Chemical Engineering Science (2020), 220 (), 115642CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
      Chem. depolymn. and recycling of polyethylene terephthalate (PET) is a sustainable way to preserve the resources and protect the environment. In this work, methanol and ethylene glycol dispersions of ultrasmall ZnO nanoparticles are firstly adopted as pseudohomogeneous catalysts for alcoholysis of PET. The as-prepd. ZnO nanoparticles have a uniform size of 4 nm and can be stable in dispersions for 6 mo. In the methanolysis process of PET, the effects of various parameters on the conversion of PET and the yield of di-Me terephthalate (DMT) were investigated. The results show that higher temp. (170°C) was beneficial to the conversion of PET and the yield of DMT, which can reach about 97% and 95% after 15 min, resp. The excellent activity of 553 g PET h-1 (g ZnO)-1 was achieved. Furthermore, the methanolysis of PET have shorter reaction time (1/4) and higher activity (4.7 times) than the glycolysis of PET.
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    19. 19
      Sun, Q.; Zheng, Y.-Y.; Yun, L.-X.; Wu, H.; Liu, R.-K.; Du, J.-T.; Gu, Y.-H.; Shen, Z.-G.; Wang, J.-X. Fe3O4 Nanodispersions as Efficient and Recoverable Magnetic Nanocatalysts for Sustainable PET Glycolysis. ACS Sustainable Chem. Eng. 2023, 11 (19), 7586– 7595,  DOI: 10.1021/acssuschemeng.3c01206
      19
      Fe3O4 Nanodispersions as Efficient and Recoverable Magnetic Nanocatalysts for Sustainable PET Glycolysis
      Sun, Qian; Zheng, Yuan-Yuan; Yun, Ling-Xia; Wu, Hao; Liu, Rong-Kun; Du, Jin-Tao; Gu, Yu-Hang; Shen, Zhi-Gang; Wang, Jie-Xin
      ACS Sustainable Chemistry & Engineering (2023), 11 (19), 7586-7595CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      Polyethylene terephthalate (PET), as one of the most indispensable synthetic org. compds. with high strength and transparency properties, can be widely used for textile and food packaging. With the increasing demand for PET prodn., the recycling of discarded PET has attracted great interest. In this work, we first proposed ethylene glycol (EG) dispersions of highly dispersed Fe3O4 nanoparticles, which were prepd. through a co-pptn. route, as efficient and recoverable nanocatalysts for a PET chem. depolymn. achieved by a glycolysis reaction. The as-prepd. Fe3O4 nanoparticles have an av. size of 11 nm and can be stably dispersed in EG for up to 6 mo. This glycolysis process was optimized in terms of catalyst concns., EG dosages, degrdn. temp., and reaction time. Furthermore, the possible reaction mechanism of PET glycolysis using Fe3O4 as a catalyst was presented. More importantly, 100% PET conversion was achieved, and the bis(2-hydroxyethyl) terephthalate (BHET) yield reached more than 93% under optimal conditions (Fe3O4/PET = 2%, EG/PET = 13, 210°C 30 min) even after three cycles. The Fe3O4 nanocatalysts are relatively stable during recycling and have great application prospects in chemocatalysis for future research.
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    20. 20
      Son, S. G.; Jin, S. B.; Kim, S. J.; Park, H. J.; Shin, J.; Ryu, T.; Jeong, J.-M.; Choi, B. G. Exfoliated manganese oxide nanosheets as highly active catalysts for glycolysis of polyethylene terephthalate. FlatChem 2022, 36, 100430,  DOI: 10.1016/j.flatc.2022.100430
      20
      Exfoliated manganese oxide nanosheets as highly active catalysts for glycolysis of polyethylene terephthalate
      Son, Seon Gyu; Jin, Se Bin; Kim, Seo Jin; Park, Hong Jun; Shin, Junho; Ryu, Taegong; Jeong, Jae-Min; Choi, Bong Gill
      FlatChem (2022), 36 (), 100430CODEN: FLATAL; ISSN:2452-2627. (Elsevier B.V.)
      Glycolysis of poly(ethylene terephthalate) (PET) is one of the main methods by which PET is recycled, which is in the need of an inexpensive and efficient catalyst. We report a glycolysis reaction of PET into bis(hydroxyethyl) terephthalate (BHET) catalyzed by ultrathin exfoliated MnO2 nanosheets (e-MON). The exfoliation of MnO2 was achieved using a shear flow system with a mixing behavior that facilitates the intercalation of potassium ions into the interlayer voids of MnO2, delaminating MnO2 into the mono- and few-layer e-MON. Glycolysis was optimized by varying the reaction temp., time, and loading of e-MON. The e-MON catalyst provided a BHET yield of 100% for a reaction lasting 30 min at 200°C, which was much higher than the 77.6% yield achieved with bare bulk MnO2. The high yield persisted over five cycles, demonstrating the reusability of the e-MON catalyst.
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    21. 21
      Cao, J.; Lin, Y.; Jiang, W.; Wang, W.; Li, X.; Zhou, T.; Sun, P.; Pan, B.; Li, A.; Zhang, Q. Mechanism of the Significant Acceleration of Polyethylene Terephthalate Glycolysis by Defective Ultrathin ZnO Nanosheets with Heteroatom Doping. ACS Sustainable Chem. Eng. 2022, 10 (17), 5476– 5488,  DOI: 10.1021/acssuschemeng.1c08656
      21
      Mechanism of the Significant Acceleration of Polyethylene Terephthalate Glycolysis by Defective Ultrathin ZnO Nanosheets with Heteroatom Doping
      Cao, Jingjing; Lin, Yuheng; Jiang, Wei; Wang, Wei; Li, Xiaodong; Zhou, Tianpeng; Sun, Ping; Pan, Bingcai; Li, Aimin; Zhang, Quanxing
      ACS Sustainable Chemistry & Engineering (2022), 10 (17), 5476-5488CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      The current scale of polyethylene terephthalate (PET) prodn. and unreasonable handling have caused widespread environmental, economic, and health-related consequences. The catalytic glycolysis of PET waste is one effective method to solve this issue. Recently, improving yield and selectivity has become a major focus in the glycolysis of PET. Herein, we report a study on the synthesis of Co/ZnO and Mo/ZnO nanosheets that have been employed as efficient catalysts in the glycolysis of PET. Under optimized conditions, the bis(hydroxyethyl) terephthalate prodn. rate and the space time yield are about two times and six times higher for Mo/ZnO nanosheets compared to those of a conventional catalyst prepd. with Zn(OAc)2, resp. We demonstrated that Mo and Co were atomically dispersed over ZnO nanosheets via oxygen bridge bonds, thus forming Mo-O or Co-O bonds on the catalyst surface. The dynamically constructed Mo or Co species drive holes into oxygen ligands to facilitate intramol. oxygen coupling, thereby triggering lattice oxygen activation to form Mo-Zn or Co-Zn dual sites as ultimate catalytic centers with highly intrinsic activity. Besides, the cooperation between the Zn-O bond and Mo-Zn or Co-Zn dual sites resulted in an active interfacial structure for PET activation and conversion. This work paved the way for engineering efficient catalysts for PET conversion through tunable compns. and electronic structures.
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    22. 22
      Suo, Q.; Zi, J.; Bai, Z.; Qi, S. The Glycolysis of Poly(ethylene terephthalate) Promoted by Metal Organic Framework (MOF) Catalysts. Catal. Lett. 2017, 147 (1), 240– 252,  DOI: 10.1007/s10562-016-1897-0
      22
      The Glycolysis of Poly(ethylene terephthalate) Promoted by Metal Organic Framework (MOF) Catalysts
      Suo, Qianqian; Zi, Jiangzhi; Bai, Zilong; Qi, Shoushan
      Catalysis Letters (2017), 147 (1), 240-252CODEN: CALEER; ISSN:1011-372X. (Springer)
      Three metal org. framework catalysts (ZIF-8, ZIF-67, MOF-5) were prepd. and used in the glycolysis of poly(ethylene terephthalate) (PET). All catalysts showed excellent catalytic activities, esp. ZIF-8. The raw material ratio in ZIF-8 catalysts was optimized. The effects of reaction conditions on the conversion of PET and yield of bis(hydroxyethyl) terephthalate (BHET) were examd.
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    23. 23
      Wu, Y.; Wang, X.; Kirlikovali, K. O.; Gong, X.; Atilgan, A.; Ma, K.; Schweitzer, N. M.; Gianneschi, N. C.; Li, Z.; Zhang, X. Catalytic Degradation of Polyethylene Terephthalate Using a Phase-Transitional Zirconium-Based Metal-Organic Framework. Angew. Chem., Int. Ed. 2022, 61 (24), e202117528  DOI: 10.1002/anie.202117528
      23
      Catalytic Degradation of Polyethylene Terephthalate Using a Phase-Transitional Zirconium-Based Metal-Organic Framework
      Wu, Yufang; Wang, Xingjie; Kirlikovali, Kent O.; Gong, Xinyi; Atilgan, Ahmet; Ma, Kaikai; Schweitzer, Neil M.; Gianneschi, Nathan C.; Li, Zhong; Zhang, Xuan; Farha, Omar K.
      Angewandte Chemie, International Edition (2022), 61 (24), e202117528CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Polyethylene terephthalate (PET) is utilized as one of the most popular consumer plastics worldwide, but difficulties assocd. with recycling PET have generated a severe environmental crisis with most PET ending its lifecycle in landfills. We report that zirconium-based metal-org. framework (Zr-MOF) UiO-66 deconstructs waste PET into the building blocks terephthalic acid (TA) and mono-Me terephthalate (MMT) within 24 h at 260°C (total yield of 98% under 1 atm H2 and 81% under 1 atm Ar). Extensive structural characterization studies reveal that during the degrdn. process, UiO-66 undergoes an intriguing transformation into MIL-140A, which is another Zr-MOF that shows good catalytic activity toward PET degrdn. under similar reaction conditions. These results illustrate the diversity of applications for Zr-MOFs and establish MOFs as a new class of polymer degrdn. catalysts with the potential to address long-standing challenges assocd. with plastic waste.
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    24. 24
      Shukla, S. R.; Palekar, V.; Pingale, N. Zeolite catalyzed glycolysis of poly(ethylene terephthalate) bottle waste. J. Appl. Polym. Sci. 2008, 110 (1), 501– 506,  DOI: 10.1002/app.28656
      24
      Zeolite catalyzed glycolysis of poly(ethylene terephthalate) bottle waste
      Shukla, S. R.; Palekar, Vikrant; Pingale, Navnath
      Journal of Applied Polymer Science (2008), 110 (1), 501-506CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
      Polyethylene terephthalate (PET) bottle waste was depolymd. using excess of ethylene glycol (EG) in the presence of zeolites, β-zeolite and Y-zeolite as transesterification catalyst. The glycolysis reaction was carried out under reflux in excess of ethylene glycol up to 8 h. The product of glycolysis was mainly the virtual monomer, bis(2-hydroxyethyl) terephthalate (BHET) admixed with dimer as residue. The BHET was obtained in pure cryst. form. Influence of the reaction time, PET: EG ratio, type, and concn. of catalyst on the yield of the glycolysis products was investigated. The characterization of the purified monomer was carried out by elemental anal., m.p., IR spectroscopy, DSC, and NMR. The yield of BHET monomer was more than 60%, which is comparable with the conventionally used heavy metal catalysts such as zinc acetate and lead acetate. This process of glycolysis of PET is economically viable and the catalysts are environment friendly.
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    25. 25
      Yang, R.-X.; Bieh, Y.-T.; Chen, C. H.; Hsu, C.-Y.; Kato, Y.; Yamamoto, H.; Tsung, C.-K.; Wu, K. C. W. Heterogeneous Metal Azolate Framework-6 (MAF-6) Catalysts with High Zinc Density for Enhanced Polyethylene Terephthalate (PET) Conversion. ACS Sustainable Chem. Eng. 2021, 9 (19), 6541– 6550,  DOI: 10.1021/acssuschemeng.0c08012
      25
      Heterogeneous Metal Azolate Framework-6 (MAF-6) Catalysts with High Zinc Density for Enhanced Polyethylene Terephthalate (PET) Conversion
      Yang, Ren-Xuan; Bieh, Yen-Tsz; Chen, Celine H.; Hsu, Chang-Yen; Kato, Yuki; Yamamoto, Hideki; Tsung, Chia-Kuang; Wu, Kevin C.-W.
      ACS Sustainable Chemistry & Engineering (2021), 9 (19), 6541-6550CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      Polyethylene terephthalate (PET) has been extensively used for the fabrication of various packaging materials, creating million tons of waste per yr. Degrading and recycling PET waste has been identified as a prominent issue. Herein, we demonstrate an effective process to chem. convert PET to bis(2-hydroxyethyl) terephthalate (BHET) through the use of metal azolate framework-6 (MAF-6) as a catalyst in the presence of ethylene glycol. MAFs are a subclass of metal-org. frameworks (MOFs), with MAF-6 comprised of the metal ion Zn2+ and the org. ligand 2-ethylimidazole. We have optimized the reaction temp., reaction time, and catalyst amt. to achieve up to a 92.4% conversion of PET and an 81.7% yield of BHET at 180°C for 4 h. MAF-6 was easily recovered and reused for at least five times. We have also hypothesized a mechanism for the high conversion and yield of the PET glycolysis reaction catalyzed by MAF-6. The use of MAF-6 as a catalyst opens a new route for the postconsumer recycling of PET with remarkable practicality.
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    26. 26
      Wang, T.; Shen, C.; Yu, G.; Chen, X. Metal ions immobilized on polymer ionic liquid as novel efficient and facile recycled catalyst for glycolysis of PET. Polym. Degrad. Stab. 2021, 194, 109751,  DOI: 10.1016/j.polymdegradstab.2021.109751
      26
      Metal ions immobilized on polymer ionic liquid as novel efficient and facile recycled catalyst for glycolysis of PET
      Wang, Tianlin; Shen, Chuanchao; Yu, Guangren; Chen, Xiaochun
      Polymer Degradation and Stability (2021), 194 (), 109751CODEN: PDSTDW; ISSN:0141-3910. (Elsevier Ltd.)
      Environmental pollution aroused by accumulation of waste PET has attracted worldwide attention. Metal chloride as Lewis acid catalyst has excellent catalytic performance in enormous org. reaction. However, the high soly. of the metal chloride in ethylene glycol and the residue of metal ions in the product will have an adverse impact on the subsequent process. In this work, polymer ionic liq. contg. [NTf2]- which is insol. in ethylene glycol at room temp. was synthesized and used to immobilize metal ions to catalyze glycolysis of PET. After reaction, the polymer ionic liq.-metal ion catalyst could be recovered at room temp. by simple filtration. PET conversion reached 95.4% and BHET yield reached 77.8% using PIL-Zn2+ as catalyst at 195°C for 120 min. Ionic liq. was characterized by FT-IR, NMR and ESI-MS. Interaction between ionic liq. and metal ions was characterized by FT-IR. The interaction energy of polymer ionic liq.-metal ion complex is higher than that of ethylene glycol-metal ion complex by DFT calcn. (D. functional calcn.). The both content of metal ion in polymer ionic liq.-metal ion complex and catalytic performance of as-synthesized catalysts did not obviously decrease after the catalyst was reused for five times. This work mainly provided a way of recovering catalyst and promote development of glycolysis of PET.
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    27. 27
      Kim, J.; Song, B.; Chung, I.; Park, J.; Yun, Y. High-performance Pt catalysts supported on amine-functionalized silica for enantioselective hydrogenation of α-keto ester. J. Catal. 2021, 396, 81– 91,  DOI: 10.1016/j.jcat.2021.02.001
      27
      High-performance Pt catalysts supported on amine-functionalized silica for enantioselective hydrogenation of α-keto ester
      Kim, Jeongmyeong; Song, Byeongju; Chung, Iljun; Park, Jisu; Yun, Yongju
      Journal of Catalysis (2021), 396 (), 81-91CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Herein, mesocellular silica foams (MCFs) functionalized by primary, secondary, and tertiary amine groups were used as supports for Pt nanoparticles. The catalytic performances of the Pt/amine-functionalized MCFs were evaluated for the enantioselective hydrogenation of Me pyruvate in the presence of cinchonidine. Compared to Pt/MCF, the Pt/amine-functionalized MCFs exhibited enhanced activity and enantioselectivity. Particularly, 0.5 wt% Pt/NH2-MCF showed a superior performance than 5 wt% Pt/Al2O3, a highly efficient catalyst in the enantioselective hydrogenation of α-keto esters, despite a 10 times lower Pt loading. Furthermore, the Pt/NH2-MCF yielded 100% conversion and 96% ee at 0.1 MPa H2 pressure during nine successive cycles, thus showing high reusability. The excellent performance of the Pt/amine-functionalized MCFs is attributed to the formation of electron-deficient Pt species through strong interactions between the Pt nanoparticles and amine groups.
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    28. 28
      Li, H.; Chen, X.; Shen, D.; Wu, F.; Pleixats, R.; Pan, J. Functionalized silica nanoparticles: classification, synthetic approaches and recent advances in adsorption applications. Nanoscale 2021, 13 (38), 15998– 16016,  DOI: 10.1039/D1NR04048K
      28
      Functionalized silica nanoparticles: classification, synthetic approaches and recent advances in adsorption applications
      Li, Hao; Chen, Xueping; Shen, Danqing; Wu, Fan; Pleixats, Roser; Pan, Jianming
      Nanoscale (2021), 13 (38), 15998-16016CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      A review. Nanotechnol. is rapidly sweeping through all the vital fields of science and technol. such as electronics, aerospace, defense, medicine, and catalysis. It involves the design, synthesis, characterization, and applications of materials and devices on the nanometer scale. At the nanoscale, phys. and chem. properties differ from the properties of the individual atoms and mols. of bulk matter. In particular, the design and development of silica nanomaterials have captivated the attention of several researchers worldwide. The applications of hybrid silicas are still limited by the lack of control on the morphol. and particle size. The ability to control both the size and morphol. of the materials and to obtain nano-sized silica particles has broadened the spectrum of applications of mesoporous organosilicas and/or has improved their performances. On the other hand, adsorption is a widely used technique for the sepn. and removal of pollutants (metal ions, dyes, orgs.,...) from wastewater. Silica nanoparticles have specific advantages over other materials for adsorption applications due to their unique structural characteristics: a stable structure, a high sp. surface area, an adjustable pore structure, the presence of silanol groups on the surface which allow easy modification, less environmental harm, simple synthesis, low cost, etc. Silica nanoparticles are potential adsorbents for pollutants. We present herein an overview of the different types of silica nanoparticles going from the definitions to properties, synthetic approaches and the mention of potential applications. We focus mainly on the recent advances in the adsorption of different target substances (metal ions, dyes and other orgs.).
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    29. 29
      Graf, N.; Yegen, E.; Gross, T.; Lippitz, A.; Weigel, W.; Krakert, S.; Terfort, A.; Unger, W. E. S. XPS and NEXAFS studies of aliphatic and aromatic amine species on functionalized surfaces. Surf. Sci. 2009, 603 (18), 2849– 2860,  DOI: 10.1016/j.susc.2009.07.029
      29
      XPS and NEXAFS studies of aliphatic and aromatic amine species on functionalized surfaces
      Graf, Nora; Yegen, Eda; Gross, Thomas; Lippitz, Andreas; Weigel, Wilfried; Krakert, Simone; Terfort, Andreas; Unger, Wolfgang E. S.
      Surface Science (2009), 603 (18), 2849-2860CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      The chem. constitution of functionalized supports is an important parameter that dets. their performance in a broad range of applications, e.g. for immobilization of biomols. Supports with amino functionalized surfaces are also often used for DNA microarray expts. However, spectral data which were reported for surfaces with amino functionalities suffer from some inconsistencies. A detailed XPS and NEXAFS (Near edge x-ray absorption fine structure) database for amino functionalized surfaces is presented. Amino-terminated surfaces prepd. from aliph. and arom. aminosilanes or aminothiols and a field sample are considered. Effects of aging in air and damage by radiation are addressed as well.
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    30. 30
      Paengjun, N.; Vibulyaseak, K.; Ogawa, M. Heterostructural transformation of mesoporous silica-titania hybrids. Sci. Rep. 2021, 11 (1), 3210,  DOI: 10.1038/s41598-020-80584-8
      30
      Heterostructural transformation of mesoporous silica-titania hybrids
      Paengjun, Navarut; Vibulyaseak, Kasimanat; Ogawa, Makoto
      Scientific Reports (2021), 11 (1), 3210CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
      Mesoporous silica (SBA-15 with the BJH pore size of 8 nm) contg. anatase nanoparticles in the pore with two different titania contents (28 and 65 mass%), which were prepd. by the infiltration of the amorphous precursor derived from tetraisopropyl orthotitanate into the pore, were heat treated in air to investigate the structural changes (both mesostructure of the SBA-15 and the phase and size of the anatase in the pore). The mesostructure of the mesoporous silica and the particle size of anatase unchanged by the heat treatment up to 800°C. The heat treatment at the temp. higher than 1000°C resulted in the collapse of the mesostructure and the growth of anatase nanoparticles as well as the transformation to rutile, while the transformation of anatase to rutile was suppressed esp. for the sample with the lower titania content (28 mass%). The resulting mesoporous silica-anatase hybrids exhibited higher benzene adsorption capacity (adsorption from water) over those heated at lower temp., probably due to the dehydroxylation of the silanol group on the pore surface. The photocatalytic decompn. of benzene in water by the present hybrid heated at 1100°C was efficient as that by P25, a benchmark photocatalyst.
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    31. 31
      Millot, Y.; Hervier, A.; Ayari, J.; Hmili, N.; Blanchard, J.; Boujday, S. Revisiting Alkoxysilane Assembly on Silica Surfaces: Grafting versus Homo-Condensation in Solution. J. Am. Chem. Soc. 2023, 145 (12), 6671– 6681,  DOI: 10.1021/jacs.2c11390
      31
      Revisiting Alkoxysilane Assembly on Silica Surfaces: Grafting versus Homo-Condensation in Solution
      Millot, Yannick; Hervier, Antoine; Ayari, Jihed; Hmili, Naoures; Blanchard, Juliette; Boujday, Souhir
      Journal of the American Chemical Society (2023), 145 (12), 6671-6681CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Silica surface functionalization is often done through the condensation of functional silanes on silanols, silica surfaces' terminal groups. APTES, aminopropyltriethoxysilane, is widely used due to its assumed high reactivity with silanols, kinetically promoted by the catalytic action of the terminal amine function. Here, we revisit, based on a quant. anal. by solid-state 29Si NMR, the assembly of this silane on silica surfaces to investigate whether its presence results from grafting, i.e., hetero-condensation with silanol groups or from homo-condensation of silane mols. in soln. leading to polycondensates physisorbed on silica. We investigate the interaction of APTES with a cryst. layered silicate, ilerite, and with amorphous nonporous silica. We also studied a second silane, cyanopropyltrichlorosilane (CPTCS), terminated with a nitrile group. Our results undoubtedly prove that while CPTCS is grafted on the silica surface, the presence of APTES on silica and silicate materials is only marginally assocd. with silanol consumption. The anal. of the signal related to silicon atoms from silanes (Tn species) and those from silica (Qn species) allowed for the accurate estn. of the extent of homo-condensation vs. grafting based on the ratio of T-O-T/Q-O-T siloxane bridges. These findings deeply question the well-established certainties on APTES assembly on silica that should no longer be seen as grafting of alkoxysilane by hetero-condensation with silanol groups but more accurately as a homo-condensed network of silanes, predominantly physisorbed on the surface but including some sparse anchoring points to the surface involving less than 6% of the overall silanol groups.
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    32. 32
      Cui, J.; Chatterjee, P.; Slowing, I. I.; Kobayashi, T. In Situ 29Si solid-state NMR study of grafting of organoalkoxysilanes to mesoporous silica nanoparticles. Microporous Mesoporous Mater. 2022, 339, 112019,  DOI: 10.1016/j.micromeso.2022.112019
      32
      In Situ 29Si solid-state NMR study of grafting of organoalkoxysilanes to mesoporous silica nanoparticles
      Cui, Jinlei; Chatterjee, Puranjan; Slowing, Igor I.; Kobayashi, Takeshi
      Microporous and Mesoporous Materials (2022), 339 (), 112019CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
      Since the catalytic activity and the stability of silica-bound organometallic complexes are affected by their interactions with hydroxyl groups on the surface, isolated hydroxyls are often created prior to the introduction of catalytic species. Here, we investigate a method to remove the indigenous hydroxyls and create new isolated hydroxyls by grafting organo-trimethoxysilane (R-TMS) to generate a silicon T2 site, (=SiO-)2SiR(-OH). We used in situ 29Si solid-state NMR expts. to monitor the evolution of Tn sites, (=SiO-)2SiR(-OH)3-n (n = 1, 2, 3). The study indicates that i. the grafting proceeds in a consecutive manner as T1 → T2 → T3, and ii. the kinetics depend on the type of functional groups in the silane. However, the rates of T1 formation and T2 → T3 conversion are also controlled to a significant extent by the entropy loss assocd. to the initial silane binding and the spatial arrangement of surface hydroxyls, resp. The grafting of R-TMS with a basic functional group leads to a lower concn. of T1 sites. The nucleophilicity of the functional group facilitates the grafting process by lowering the enthalpy barrier, while the T1 formation rate is more influenced by the entropy barrier than the T1 → T2 conversion rate. Thus, the basic functional group promotes the T1 → T2 conversion more than the T1 formation, resulting in a lower concn. of T1 sites.
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    33. 33
      van Meerten, S. G. J.; Franssen, W. M. J.; Kentgens, A. P. M. ssNake: A cross-platform open-source NMR data processing and fitting application. J. Magn. Reson. 2019, 301, 56– 66,  DOI: 10.1016/j.jmr.2019.02.006
      33
      ssNake: A cross-platform open-source NMR data processing and fitting application
      van Meerten, S. G. J.; Franssen, W. M. J.; Kentgens, A. P. M.
      Journal of Magnetic Resonance (2019), 301 (), 56-66CODEN: JMARF3; ISSN:1090-7807. (Elsevier B.V.)
      For solid-state NMR or for unconventional expts. only a very limited no. of modern processing and simulation software packages are available. For this reason, we have developed ssNake, an NMR processing program which provides both interactive and script-based processing tools. ssNake is aimed at solid-state NMR expts., but can also be used for liq.-state expts. It can read various data formats, including those from all major spectrometer vendors. It has extensive fitting capabilities, which can be used for spectrum deconvolution. ssNake also provides the unique feature of being able to fit multiple spectra (or curves) simultaneously, where some or all of its parameters are shared. This method can be used, for example, to fit quadrupole spectra at various magnetic fields simultaneously. This allows the quadrupole and chem. shift parameters to be accurately detd. ssNake also provides a method of fitting using external simulation programs, such as SIMPSON. This makes fitting very versatile, as it brings together exptl. data and simulation software.
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    34. 34
      (a) Liu, C. C.; Maciel, G. E. The Fumed Silica Surface: A Study by NMR. J. Am. Chem. Soc. 1996, 118 (21), 5103– 5119,  DOI: 10.1021/ja954120w
      34a
      The Fumed Silica Surface: A Study by NMR
      Liu, Changhua C.; Maciel, Gary E.
      Journal of the American Chemical Society (1996), 118 (21), 5103-5119CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      High-resoln. solid-state NMR techniques were used to study the surface structure of Cab-O-Sil fumed silica. 1H NMR results obtained from CRAMPS, MAS-only, and relaxation studies reveal the existence of both H-bonded silanols and isolated silanols on the Cab-O-Sil surface. A systematic dehydration study of fumed silica was carried out, with results on the quantity of each type of silanol on the surface at various dehydration stages. 29Si CP-MAS expts., including CP spin dynamics studies and various other relaxation studies, were used to probe H bonding and the local structural environments of various hydroxyl groups of silica surfaces. 29Si CP-MAS expts. on water-treated and D-exchanged Cab-O-Sil indicate the existence of interparticle silanols and internal silanols in fumed silica. 1H and 29Si NMR show that for fumed silica both isolated and H-bonded silanols are present on the surface of an untreated sample, in contrast to the case of silica gel, where all silanols of an untreated sample are H bonded.
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      (b) Lippmaa, E.; Maegi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A. R. Structural studies of silicates by solid-state high-resolution silicon-29 NMR. J. Am. Chem. Soc. 1980, 102 (15), 4889– 4893,  DOI: 10.1021/ja00535a008
      34b
      Structural studies of silicates by solid-state high-resolution silicon-29 NMR
      Lippmaa, E.; Maegi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A. R.
      Journal of the American Chemical Society (1980), 102 (15), 4889-93CODEN: JACSAT; ISSN:0002-7863.
      The high-resoln. 29Si NMR spectra of solid silicates and aluminosilicates were studied. High-speed magic angle sample spinning in combination with high-power proton decoupling and, wherever possible, polarization transfer was used to achieve high (1 ppm) resoln. Although ionization and cation influence are reflected on 29Si chem. shifts, the isotropic 29Si chem. shifts in solids and solns. are generally the same and depend mainly on the degree of condensation of Si-O tetrahedra. In solid aluminosilicates, addnl. paramagnetic shifts appear, which correlate well with the degree of Si substitution by Al.
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      (c) Lechert, H. G.; Engelhardt und, D. G. Engelhardt und D. Michel:High Resolution Solid State NMR of Silicates and Zeolites. John Wiley & Sons, Chichester, New York, Brisbane, Toronto, Singapore, 1987. 485 Seiten, Preis: $ 55.-. Ber. Bunsenges. Phys. Chem. 1988, 92 (9), 1059,  DOI: 10.1002/bbpc.198800267
      There is no corresponding record for this reference.
    35. 35
      (a) Cheng, R.; Liu, X.; Fang, Y.; Terano, M.; Liu, B. High-resolution 29Si CP/MAS solid state NMR spectroscopy and DFT investigation on the role of geminal and single silanols in grafting chromium species over Phillips Cr/silica catalyst. Appl. Catal., A 2017, 543, 26– 33,  DOI: 10.1016/j.apcata.2017.05.011
      35a
      High-resolution 29Si CP/MAS solid state NMR spectroscopy and DFT investigation on the role of geminal and single silanols in grafting chromium species over Phillips Cr/silica catalyst
      Cheng, Ruihua; Liu, Xuee; Fang, Yuwei; Terano, Minoru; Liu, Boping
      Applied Catalysis, A: General (2017), 543 (), 26-33CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
      Phillips Cr/silica catalyst is industrially important in ethylene polymn. The high-resoln. solid state 1H MAS NMR and 29Si CP/MAS NMR allowed the identification of the type and amt. of silanols: geminal vs. single (isolated and vicinal) silanols on Phillips catalysts calcined at different temps., which were compared with those of the bare silica gel counterparts. The residual silanols on the catalyst and silica gel samples were all decreased with increasing calcination temps. from 120 to 800 °C. For the catalysts treated at temps. lower than 300 °C, the amt. of residual silanol groups were much lower than those of the silica gel counterparts. It suggested that the chromium species were mainly grafted on the silica gel through esterification reaction with surface silanols below 300 °C. The geminal silanols almost disappeared on the catalysts at 120 °C, while that for the silica gel occurred at 300 °C. Further increasing the calcination temps. from 300 to 800 °C, the amt. of single silanols were slower decreased for the catalysts than that for the silica gel samples. It indicated that the presence of the grafted chromate species obstructed further removal of the residual single silanols. The role of silanols on the formation of surface chromate species on the well-defined polyoligomericsilsesquioxane (POSS) models contg. various types of silanols was theor. studied by d. functional theory (DFT) method. It was shown that one silanol of the geminal pair and another adjacent single silanol was the most thermodynamically favored for grafting chromium species. The priority of the reaction between chromium species and different types of surface silanol groups during calcination for Phillips catalysts were exptl. and theor. elucidated for the first time.
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      (b) Bruch, M. D.; Fatunmbi, H. O. Nuclear magnetic resonance analysis of silica gel surfaces modified with mixed, amine-containing ligands. J. Chromatogr. A 2003, 1021 (1–2), 61– 70,  DOI: 10.1016/j.chroma.2003.08.093
      35b
      Nuclear magnetic resonance analysis of silica gel surfaces modified with mixed, amine-containing ligands
      Bruch, Martha D.; Fatunmbi, Hafeez O.
      Journal of Chromatography A (2003), 1021 (1-2), 61-70CODEN: JCRAEY; ISSN:0021-9673. (Elsevier Science B.V.)
      Different approaches for quant. anal. by 29Si and 13C CP/MAS NMR of silica gel chem. modified by a mixt. of long and short chain amines, -(O)3Si(CH2)3N(CH3)2(CH2)13CH3 and -(O)3Si(CH2)3N(CH3)3, are compared to elemental anal. Unlike 29Si NMR, variable contact time data are necessary for accurate quant. anal. by 13C NMR. Surprisingly, spectral overlap does not interfere with this approach. Surfaces prepd. from reaction mixts. that consisted of 67 and 33% (vol./vol.) long chain are found to actually contain 37 and 16% long chain amines, resp. The mixed phase surfaces have more extensive crosslinking and fewer unreacted hydroxyls than single phase surfaces.
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    36. 36
      Srikanth, C. S.; Chuang, S. S. C. Spectroscopic Investigation into Oxidative Degradation of Silica-Supported Amine Sorbents for CO2 Capture. ChemSusChem 2012, 5 (8), 1435– 1442,  DOI: 10.1002/cssc.201100662
      36
      Spectroscopic Investigation into Oxidative Degradation of Silica-Supported Amine Sorbents for CO2 Capture
      Srikanth, Chakravartula S.; Chuang, Steven S. C.
      ChemSusChem (2012), 5 (8), 1435-1442, S1435/1-S1435/2CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Oxidative degrdn. characteristics of silica-supported amine sorbents with varying amts. of tetraethylenepentamine (TEPA) and polyethylene glycol (PEG; P200 or P600 represents PEG with mol. wts. of 200 or 600) have been studied by IR and NMR spectroscopy. Thermal treatment of the sorbents and liq. TEPA at 100 °C for 12 h changed their color from white to yellow. The CO2 capture capacity of the TEPA/SiO2 sorbents (i.e., SiO2-supported TEPA with a TEPA/SiO2 ratio of 25:75) decreased by more than 60 %. IR and NMR spectroscopy studies showed that the yellow color of the degraded sorbents resulted from the formation of imide species. The imide species, consisting of NH assocd. with two C=O functional groups, were produced from the oxidn. of methylene groups in TEPA. Imide species on the degraded sorbent are not capable of binding CO2 due to its weak basicity. The addn. of P200 and P600 to the supported amine sorbents improved both their CO2 capture capacities and oxidative degrdn. resistance. IR spectroscopy results also showed that TEPA was immobilized on the SiO2 surface through hydrogen bonding between amine groups and the silanol groups of SiO2. The OH groups of PEG interact with NH2/NH of TEPA through hydrogen bonding. Hydrogen bonds disperse TEPA on SiO2 and block O2 from accessing TEPA for oxidn. Oxidative degrdn. resistance and CO2 capture capacity of the supported amine sorbents can be optimized through adjusting the ratio of hydroxyl to amine groups in the TEPA/PEG mixt.
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    37. 37
      Vallet-Regí, M.; Schüth, F.; Lozano, D.; Colilla, M.; Manzano, M. Engineering mesoporous silica nanoparticles for drug delivery: where are we after two decades?. Chem. Soc. Rev. 2022, 51 (13), 5365– 5451,  DOI: 10.1039/D1CS00659B
      37
      Engineering mesoporous silica nanoparticles for drug delivery: where are we after two decades?
      Vallet-Regi, Maria; Schuth, Ferdi; Lozano, Daniel; Colilla, Montserrat; Manzano, Miguel
      Chemical Society Reviews (2022), 51 (13), 5365-5451CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. The present review details a chronol. description of the events that took place during the development of mesoporous materials, their different synthetic routes and their use as drug delivery systems. The outstanding textural properties of these materials quickly inspired their translation to the nanoscale dimension leading to mesoporous silica nanoparticles (MSNs). The different aspects of introducing pharmaceutical agents into the pores of these nanocarriers, together with their possible biodistribution and clearance routes, would be described here. The development of smart nanocarriers that are able to release a high local concn. of the therapeutic cargo on-demand after the application of certain stimuli would be reviewed here, together with their ability to deliver the therapeutic cargo to precise locations in the body. The huge progress in the design and development of MSNs for biomedical applications, including the potential treatment of different diseases, during the last 20 years will be collated here, together with the required work that still needs to be done to achieve the clin. translation of these materials. This review was conceived to stand out from past reports since it aims to tell the story of the development of mesoporous materials and their use as drug delivery systems by some of the story makers, who could be considered to be among the pioneers in this area.
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    38. 38
      (a) Mafra, L.; Čendak, T.; Schneider, S.; Wiper, P. V.; Pires, J.; Gomes, J. R. B.; Pinto, M. L. Structure of Chemisorbed CO2 Species in Amine-Functionalized Mesoporous Silicas Studied by Solid-State NMR and Computer Modeling. J. Am. Chem. Soc. 2017, 139 (1), 389– 408,  DOI: 10.1021/jacs.6b11081
      38a
      Structure of Chemisorbed CO2 Species in Amine-Functionalized Mesoporous Silicas Studied by Solid-State NMR and Computer Modeling
      Mafra, Luis; Cendak, Tomaz; Schneider, Sarah; Wiper, Paul V.; Pires, Joao; Gomes, Jose R. B.; Pinto, Moises L.
      Journal of the American Chemical Society (2017), 139 (1), 389-408CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Two-dimensional (2D) solid-state NMR (SSNMR) expts. on samples loaded with 13C-labeled CO2, "under controlled partial pressures", have been performed in this work, revealing unprecedented structural details about the formation of CO2 adducts from its reaction with various amine-functionalized SBA-15 contg. amines having distinct steric hindrances (e.g., primary, secondary) and similar loadings. Three chemisorbed CO2 species were identified by NMR from distinct carbonyl environments resonating at δC ≈ 153, 160, and 164 ppm. The newly reported chemisorbed CO2 species at δC ≈ 153 ppm was found to be extremely moisture dependent. A comprehensive 1H-based SSNMR study [1D 1H and 2D 1H-X heteronuclear correlation (HETCOR, X = 13C, 29Si) expts.] was performed on samples subjected to different treatments. It was found that all chemisorbed CO2 species are involved in hydrogen bonds (HBs) with either surface silanols or neighboring alkylamines. 1H chem. shifts up to 11.8 ppm revealed that certain chemisorbed CO2 species are engaged in very strong HBs. The authors demonstrate that NMR may help in discriminating among free and hydrogen-bonded functional groups. 13C{14N} dipolar-recoupling NMR showed that the formation of carbonate or bicarbonate is excluded. D. functional theory calcns. on models of alkylamines grafted into the silica surface assisted the 1H/13C assignments and validated various HB arrangements that may occur upon formation of carbamic acid. This work extends the understanding of the chemisorbed CO2 structures that are formed upon bonding of CO2 with surface amines and readily released from the surface by pressure swing.
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      (b) Vieira, R.; Marin-Montesinos, I.; Pereira, J.; Fonseca, R.; Ilkaeva, M.; Sardo, M.; Mafra, L. Hidden” CO2 in Amine-Modified Porous Silicas Enables Full Quantitative NMR Identification of Physi- and Chemisorbed CO2 Species. J. Phys. Chem. C 2021, 125 (27), 14797– 14806,  DOI: 10.1021/acs.jpcc.1c02871
      38b
      Hidden CO2 in Amine-Modified Porous Silicas Enables Full Quantitative NMR Identification of Physi- and Chemisorbed CO2 Species
      Vieira, Ricardo; Marin-Montesinos, Ildefonso; Pereira, Joao; Fonseca, Rita; Ilkaeva, Marina; Sardo, Mariana; Mafra, Luis
      Journal of Physical Chemistry C (2021), 125 (27), 14797-14806CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Although spectroscopic investigation of surface chemisorbed CO2 species has been the focus of most studies, identifying different domains of weakly interacting (physisorbed) CO2 mols. in confined spaces is less trivial as they are often indistinguishable resorting to (isotropic) NMR chem. shift or vibrational band analyses. Herein, we undertake for the first time a thorough solid-state NMR anal. of CO2 species physisorbed prior to and after amine-functionalization of silica surfaces; combining 13C NMR chem. shift anisotropy (CSA) and longitudinal relaxation times (T1). These methods were used to quant. distinguish otherwise overlapping physisorbed CO2 signals, which contributed to an empirical model of CO2 speciation for the physi- and chemisorbed fractions. The quant. measured T1 values confirm the presence of CO2 mol. dynamics on the microsecond, millisecond, and second time scales, strongly supporting the existence of up to three physisorbed CO2 species with proportions of about 15%, 15%, and 70%, resp. Our approach takes advantage from using adsorbed 13C-labeled CO2 as probe mols. and quant. cross-polarization magic-angle spinning to study both physi- and chemisorbed CO2 species, showing that 45% of chemisorbed CO2 vs. 55% of physisorbed CO2 is formed from the overall confined CO2 in amine-modified hybrid silicas. A total of six distinct CO2 environments were identified from which three physisorbed CO2 were discriminated, coined here as "gas, liq., and solid-like" CO2 species. The complex nature of physisorbed CO2 in the presence and absence of chemisorbed CO2 species is revealed, shedding light on what fractions of weakly interacting CO2 are affected upon pore functionalization. This work extends the current knowledge on CO2 sorption mechanisms providing new clues toward CO2 sorbent optimization.
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    39. 39
      dos Santos, T. C.; Bourrelly, S.; Llewellyn, P. L.; de Carneiro, J. W.; Machado Ronconi, C. Adsorption of CO2 on amine-functionalised MCM-41: experimental and theoretical studies. Phys. Chem. Chem. Phys. 2015, 17 (16), 11095– 11102,  DOI: 10.1039/C5CP00581G
      39
      Adsorption of CO2 on amine-functionalised MCM-41: experimental and theoretical studies
      dos Santos, Thiago Custodio; Bourrelly, Sandrine; Llewellyn, Philip L.; de M. Carneiro, Jose Walkimar; Machado Ronconi, Celia
      Physical Chemistry Chemical Physics (2015), 17 (16), 11095-11102CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Adsorption of CO2 on MCM-41 functionalised with [3-(2-aminoethylamino)propyl]trimethoxysilane (MCM-41-N2), N1-(3-trimethoxysilylpropyl)diethylenetriamine (MCM-41-N3), 4-aminopyridine (MCM-41-aminopyridine), 4-(methylamino)pyridine (MCM-41-methylaminopyridine) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (MCM-41-guanidine) was studied. The amine-functionalised materials were characterized by 29Si and 13C solid-state NMR, N2 adsorption/desorption isotherms, x-ray diffraction and TEM. CO2 adsorption at 1.0 bar and 30° showed that the amt. of CO2 (nads/mmol g-1) adsorbed on MCM-41-N2 and MCM-41-N3 is approx. twice the amt. adsorbed on MCM-41. For MCM-41-aminopyridine, MCM-41-methylaminopyridine and MCM-41-guanidine, the CO2 adsorption capacity was smaller than that of MCM-41 at the same conditions. The p affinity (computed with wB97x-D/6-311++G(d,p)) of the secondary amino groups is higher than that of the primary amino groups; however, the relative stabilities of the primary and secondary carbamates are similar. The differential heat of adsorption decreases as the no. of secondary amino groups increases.
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    40. 40
      Desforges, A.; Backov, R.; Deleuze, H.; Mondain-Monval, O. Generation of Palladium Nanoparticles within Macrocellular Polymeric Supports: Application to Heterogeneous Catalysis of the Suzuki-Miyaura Coupling Reaction. Adv. Funct. Mater. 2005, 15 (10), 1689– 1695,  DOI: 10.1002/adfm.200500146
      40
      Generation of palladium nanoparticles within macrocellular polymeric supports: Application to heterogeneous catalysis of the Suzuki-Miyaura coupling reaction
      Desforges, Alexandre; Backov, Renal; Deleuze, Herve; Mondain-Monval, Olivier
      Advanced Functional Materials (2005), 15 (10), 1689-1695CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We present new hybrid org./inorg. materials dedicated to heterogeneous catalysis. The systems are obtained by the polymn. of a high internal phase reverse emulsion (the so-called polyHIPE porous materials) and have been further functionalized with various org. groups in order to promote the growth of palladium nanoparticles on its surface. Final supports are then tested for their ability to catalyze the Suzuki-Miyaura coupling reaction, and one material exhibits better activity than the well-known Pd@C powder system. Furthermore, the catalytic activities of these materials are close to those obtained with their homogeneous catalysis counterpart. These new supports remain active towards a wide range of substrates assocd. with Suzuki-Miyaura carbon-carbon coupling reactions.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXhtFOrtb7J&md5=420df35380f5768b599f3804f48dff2d
    41. 41
      Yamashita, T.; Hayes, P. Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci. 2008, 254 (8), 2441– 2449,  DOI: 10.1016/j.apsusc.2007.09.063
      41
      Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials
      Yamashita, Toru; Hayes, Peter
      Applied Surface Science (2008), 254 (8), 2441-2449CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Samples of the iron oxides Fe0.94O, Fe3O4, Fe2O3, and Fe2SiO4 were prepd. by high temp. equilibration in controlled gas atmospheres. The samples were fractured in vacuum and high resoln. XPS spectra of the fractured surfaces were measured. The peak positions and peak shape parameters of Fe 3p for Fe2+ and Fe3+ were derived from the Fe 3p XPS spectra of the std. samples of 2FeO.SiO2 and Fe2O3, resp. Using these parameters, the Fe 3p peaks of Fe3O4 and Fe1-yO were analyzed. High resoln. XPS techniques can be used to det. the Fe2+/Fe3+ ratios in metal oxides. The technique has the potential for application to other transition metal oxide systems.
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    42. 42
      Poulin, S.; França, R.; Moreau-Bélanger, L.; Sacher, E. Confirmation of X-ray Photoelectron Spectroscopy Peak Attributions of Nanoparticulate Iron Oxides, Using Symmetric Peak Component Line Shapes. J. Phys. Chem. C 2010, 114 (24), 10711– 10718,  DOI: 10.1021/jp100964x
      42
      Confirmation of X-ray Photoelectron Spectroscopy Peak Attributions of Nanoparticulate Iron Oxides, Using Symmetric Peak Component Line Shapes
      Poulin, S.; Franca, R.; Moreau-Belanger, L.; Sacher, E.
      Journal of Physical Chemistry C (2010), 114 (24), 10711-10718CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The authors use high purity Fe oxide nanoparticles to confirm the Fe 2p x-ray photoemission peak attributions made in previous study of Fe nanoparticles and the initial stage of their oxidn. To accomplish this, the authors found it necessary to consider the spectral contributions of the ligand field of the Fe-O crystal structure, the cryst. disorder at the nanoparticle surface, and the Russell-Saunders broadening of the FeIII components of the Fe 2p spectra.
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    43. 43
      Delle Chiaie, K. R.; McMahon, F. R.; Williams, E. J.; Price, M. J.; Dove, A. P. Dual-catalytic depolymerization of polyethylene terephthalate (PET). Polym. Chem. 2020, 11 (8), 1450– 1453,  DOI: 10.1039/C9PY01920K
      43
      Dual-catalytic depolymerization of polyethylene terephthalate (PET)
      Delle Chiaie, Kayla R.; McMahon, Fergus R.; Williams, Esme J.; Price, Matthew J.; Dove, Andrew P.
      Polymer Chemistry (2020), 11 (8), 1450-1453CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
      Limiting our plastic waste and finding greener, more sustainable solns. for disposal is currently an environmental priority. Polyethylene terephthalate (PET), one of the more prominent single-use plastics, has recently been under investigation for chem. recycling as a means to ameliorate the environmental impact. This work reports a dual-catalytic approach to the chem. recycling of PET, aiming to combine inexpensive, readily available Lewis acid-base pairs to exhibit cooperative catalytic activity.
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    44. 44
      Le, N. H.; Ngoc Van, T. T.; Shong, B.; Cho, J. Low-Temperature Glycolysis of Polyethylene Terephthalate. ACS Sustainable Chem. Eng. 2022, 10 (51), 17261– 17273,  DOI: 10.1021/acssuschemeng.2c05570
      44
      Low-Temperature Glycolysis of Polyethylene Terephthalate
      Le, Ngan Hong; Ngoc Van, Tran Thi; Shong, Bonggeun; Cho, Joungmo
      ACS Sustainable Chemistry & Engineering (2022), 10 (51), 17261-17273CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      In this work, we developed a new catalytic method of glycolysis to efficiently convert post-consumer polyethylene terephthalate (PET) into bis(2-hydroxyethyl) terephthalate (BHET). The addn. of an arom. compd. possessing the alkoxy group (e.g., anisole) to the glycolysis reaction system facilitated the conversion of PET to BHET at a reaction temp. near 153°, which is much lower than that of catalytic glycolysis without a co-solvent (>197°), while overall catalytic performance remains almost unchanged. We found that an inexpensive metal salt or org. guanidine base could be used as an effective catalyst for the low-temp. glycolysis. Under the optimal reaction conditions catalyzed by alkali metal (Na or K) acetate, PET completely decompd. in 2 h with a BHET yield of 86%. We also investigated detailed reaction behaviors and possible intermol. interactions between anisole and other chem. species that facilitate catalytic glycolysis. Based on the exptl. results, the most probable reaction steps were proposed and a kinetic model mechanistically describing the overall reaction behavior was developed. The estd. apparent activation energy for PET decompn. in the co-solvent-assisted glycolysis system was found to be a low value of 80.9 kJ mol-1, which is responsible for the high reactivity even at a much lower reaction temp. than that of glycolysis without the co-solvent.
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    45. 45
      Wang, Z.; Jin, Y.; Wang, Y.; Tang, Z.; Wang, S.; Xiao, G.; Su, H. Cyanamide as a Highly Efficient Organocatalyst for the Glycolysis Recycling of PET. ACS Sustainable Chem. Eng. 2022, 10 (24), 7965– 7973,  DOI: 10.1021/acssuschemeng.2c01235
      45
      Cyanamide as a Highly Efficient Organocatalyst for the Glycolysis Recycling of PET
      Wang, Zishuai; Jin, Yu; Wang, Yaoqiang; Tang, Zequn; Wang, Shaojie; Xiao, Gang; Su, Haijia
      ACS Sustainable Chemistry & Engineering (2022), 10 (24), 7965-7973CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      Due to the antibiodegradable properties, numerous plastics have been accumulated in the ecosystem and aggravate ecol. pollution. Poly (ethylene terephthalate) (PET) is among the most used plastics. Glycolysis of PET is a useful approach to solve the waste PET pollution and obtain bis(2-hydroxyethyl) terephthalate (BHET). In this paper, waste PET was efficiently depolymd. through glycolysis catalyzed by cyanamide. In particular, compared with the previously reported catalyst, cyanamide is more readily available and can be used directly in catalysis without a complex prepn. process. Under optimal conditions, PET was completely depolymd. with up to nearly 100% BHET yield. Even at a temp. as low as 150°C, a good BHET yield can be obtained. The application potential of this glycolysis procedure was demonstrated by its excellent performance in the glycolysis of various real PET wastes like transparent and opaque PET samples and polyester foam and by the high quality of the obtained BHET products. The mechanism was studied by 1H NMR anal., and DFT calcns. showed that the higher activity of cyanamide than its trimer, melamine, is due to the stronger hydrogen bonds formed between cyanamide and PET or ethylene glycol.
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    46. 46
      González, D.; Camino, B.; Heras-Domingo, J.; Rimola, A.; Rodríguez-Santiago, L.; Solans-Monfort, X.; Sodupe, M. BCN-M: A Free Computational Tool for Generating Wulff-like Nanoparticle Models with Controlled Stoichiometry. J. Phys. Chem. C 2020, 124 (1), 1227– 1237,  DOI: 10.1021/acs.jpcc.9b10506
      46
      BCN-M: A Free Computational Tool for Generating Wulff-like Nanoparticle Models with Controlled Stoichiometry
      Gonzalez, Danilo; Camino, Bruno; Heras-Domingo, Javier; Rimola, Albert; Rodriguez-Santiago, Luis; Solans-Monfort, Xavier; Sodupe, Mariona
      Journal of Physical Chemistry C (2020), 124 (1), 1227-1237CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      A thorough knowledge of the at. structure of nanomaterials is of high importance to understand their properties. This requires developing nanoparticle models, which is not always straightforward, particularly in the case of nonpure metallic systems. The bulk cut nanoparticle model (BCN-M) computational tool generates Wulff-like models for binary materials with controlled stoichiometry automatically with none or little need for further manipulation from the user. The models are obtained exclusively by introducing the structure of the bulk material, its symmetry, the surface energies of the most representative surfaces, and information about surface termination as input data. The algorithm produces different structural model sets, and the quality of these models is evaluated using different criteria: (i) the deviation from an ideal Wulff shape, (ii) the global coordination of surface metal atoms, and (iii) the polarity of the model. BCN-M has been applied to 15 different materials, leading to a variety of models that cover the most relevant binary ionic structures and symmetries (cubic, tetragonal, hexagonal, and monoclinic). The resulting models can be used for structure anal. of ideal systems as well as their simulation. BCN-M is available as a free web platform (https://bcnm.qf.uab.cat) or as a downloadable utility, and it is expected to be an important tool for the design of future nanomaterials.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acssuschemeng.3c03585.

    • Comparison of heterogeneous catalysts in the literature, catalyst synthesis and DFT computation details, SEM images of unfunctionalized, ion-immobilized SiO2 and NP-immobilized SiO2, Si 2p, Fe 3p, O 1s, and N 1s XPS of catalysts, NMR of BHET product, 13C solid-state NMR spectra of functionalized SiO2, ICP-MS analysis for Fe quantification, and calculated relative Gibbs energies for the modeled reaction systems, catalyst recyclability evaluation (PDF)


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