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A detailed investigation was conducted on the factors influencing the properties of silica-supported tungsten oxide catalysts for propene metathesis. A principal goal of this work was to identify the processes involved in the formation of catalytically active sites. To probe the influence of dispersion, samples were prepared across a range of W loadings using two methods of catalyst preparation: incipient wetness impregnation of amorphous silica and ion exchange of mesoporous SBA-15. The samples were characterized by nitrogen adsorption, UV-vis, Raman, and X-ray absorption spectroscopy (XAS). Catalytic activity was observed to increase with W surface concentration up to the point where WO3 nanoparticles formed. The catalytic performance of all samples was enhanced 2-fold by pretreatment in He, in comparison to pretreatment in air. In situ characterization of samples pretreated in He by Raman and XAS shows an increase in the relative concentration of isolated dioxo W(6+) species relative to mono-oxo W(6+) species, and in situ XAS data collected during propene metathesis indicated that a similar conversion occurs for air-pretreated samples in the presence of propene. For both air- and He-pretreated catalysts an activation period was observed, during which the activity increased and attained steady-state activity. This period was significantly longer for air-pretreated catalysts and was accompanied by the transient formation of acetone. While acetone was not observed during the much shorter transient of He-pretreated samples, in situ XAS provided evidence of reduction occurring in these samples upon contact with propene. It is also notable that, independent of the manner of catalyst preparation or pretreatment, the rate of propene metathesis is first order in propene and exhibits an activation energy of 200 kJ/mol. A model is proposed to explain why only a fraction of the isolated tungstate species is active for propene metathesis (∼5%) and why this fraction increases with increasing concentration of W dispersed on silica. (Chemical Equation Presented).

A sustainable route has been reported for the production of terephthalic acid (PTA) from 5-(hydroxymethyl)furoic acid (HMFA) and ethylene, both of which can be derived from biomass. This process starts with the production of 4-(hydroxymethyl)benzoic acid (HMBA) from HMFA and ethylene catalyzed by Sn-BEA. The subsequent oxidation of HMBA leads to PTA. The present study reports the results of a detailed computational investigation of the mechanism of HMBA synthesis from ethylene and HMFA mediated by Sn-BEA. Density functional theory calculations show that the formation of HMBA proceeds via Diels-Alder cycloaddition of HMFA and ethylene, which is rate-limiting, followed by Lewis acid-catalyzed dehydration. The solution-phase reaction and six different pathways in Sn-BEA, including one pathway on the Si site and five different pathways on the Sn site, are investigated for the Diels-Alder cycloaddition of HMFA and ethylene. Energy decomposition analysis (EDA) shows that the Sn site stabilizes the transition state of the Diels-Alder reaction electrostatically instead of facilitating charge transfer between HMFA and ethylene. Therefore, the preferred pathway for the Diels-Alder reaction starts with binding HMFA to the Sn site by the carbonyl oxygen, which is the configuration that maximizes electrostatic interactions between substrates and the catalyst in the transition state. The effect of substituting Sn in the active site by Zr and Ti was examined and the highest reaction barriers were for the Ti sites. Using EDA, we found that though the barriers of the Sn and Zr site are comparable, the individual contributing effects are different: lower energy penalty associated with distortion of the geometry of the Zr site overcomes less favorable electrostatic and charge transfer effects compared to the Sn site.

The partition functions, heat capacities, entropies, and enthalpies of selected molecules were calculated using uncoupled mode (UM) approximations, where the full-dimensional potential energy surface for internal motions was modeled as a sum of independent one-dimensional potentials for each mode. The computational cost of such approaches scales the same with molecular size as standard harmonic oscillator vibrational analysis using harmonic frequencies (HO(hf)). To compute thermodynamic properties, a computational protocol for obtaining the energy levels of each mode was established. The accuracy of the UM approximation depends strongly on how the one-dimensional potentials of each modes are defined. If the potentials are determined by the energy as a function of displacement along each normal mode (UM-N), the accuracies of the calculated thermodynamic properties are not significantly improved versus the HO(hf) model. Significant improvements can be achieved by constructing potentials for internal rotations and vibrations using the energy surfaces along the torsional coordinates and the remaining vibrational normal modes, respectively (UM-VT). For hydrogen peroxide and its isotopologs at 300 K, UM-VT captures more than 70% of the partition functions on average. By contrast, the HO(hf) model and UM-N can capture no more than 50%. For a selected test set of C2 to C8 linear and branched alkanes and species with different moieties, the enthalpies calculated using the HO(hf) model, UM-N, and UM-VT are all quite accurate comparing with reference values though the RMS errors of the HO model and UM-N are slightly higher than UM-VT. However, the accuracies in entropy calculations differ significantly between these three models. For the same test set, the RMS error of the standard entropies calculated by UM-VT is 2.18 cal mol(-1) K(-1) at 1000 K. By contrast, the RMS error obtained using the HO model and UM-N are 6.42 and 5.73 cal mol(-1) K(-1), respectively. For a test set composed of nine alkanes ranging from C5 to C8, the heat capacities calculated with the UM-VT model agree with the experimental values to within a RMS error of 0.78 cal mol(-1) K(-1), which is less than one-third of the RMS error of the HO(hf) (2.69 cal mol(-1) K(-1)) and UM-N (2.41 cal mol(-1) K(-1)) models.

Detailed mechanisms for the synthesis of p-xylene as well as the primary byproducts observed experimentally, 2,5-hexadione and 2,5-dimethyl-3-[(4-methyl-1,3-cyclohexadien-1-yl)methyl]furan, from ethylene and 2,5-dimethylfuran (DMF) mediated by H-BEA are obtained using an extended QM/MM model containing 208 tetrahedral atoms. The formation of p-xylene proceeds via Diels-Alder cycloaddition of ethylene and DMF, which is rate-limiting, followed by Brønsted acid-catalyzed dehydration. Secondary addition of DMF to the substrate following the Diels-Alder reaction leads to 2,5-dimethyl-3-[(4-methyl-1,3-cyclohexadien-1-yl)methyl]furan. The analysis of the free energies associated with the mechanisms suggests that the secondary addition can be eliminated by introducing n-heptane as an inert solvent to decrease the loading of DMF in the zeolite or by using a weak Brønsted acid site to facilitate the dehydration of the Diels-Alder product, for which the rate is determined by the deprotonation via the conjugate base of the active site. Water formed in the dehydration process can react directly with DMF to form 2,5-hexadione, thereby decreasing the yield of p-xylene. However, the free-energy barriers for the formation of 2,5-heaxdione compared to the Diels-Alder reaction indicate that DMF and 2,5-hexadione will be equilibrated. Therefore, the 2,5-hexadione yield can be minimized by operating at a high conversion of DMF.

The effects of the coordination environment and connectivity of Ti on the rate of n-butanal self-condensation over Ti-silica catalysts were investigated. Ti was introduced in two ways, either during the synthesis of mesoporous SBA-15 or via grafting onto amorphous silica with a disordered pore structure. The connectivity of Ti was then characterized by XANES, UV-vis, and Raman spectroscopy. For the lowest Ti loadings, the Ti is found to be predominantly in isolated monomeric species, irrespective of the manner of sample preparation, and as the Ti loading is increased, a progressively larger fraction of Ti is present in oligomeric species and anatase nanoparticles. The turnover frequency for butanal condensation decreased monotonically with increasing Ti loading, and the apparent activation energy increased from 60 kJ mol-1 for monomeric species to 120 kJ mol-1 for oligomeric species. A kinetic H/D isotope effect was observed over isolated titanol and Ti dimer catalysts suggesting that α-H abstraction is the rate-determining step. This conclusion is supported by theoretical analysis of the reaction mechanism. In agreement with experimental results, the calculated activation barrier for alkanal condensation over a Ti dimer is roughly two times greater than that over Ti-OH sites. The cause for this difference was explained by energy decomposition analysis of the enolate formation step which showed that there is a large energetic penalty for the substrate to distort over the Ti-O-Ti dimer than the Ti-OH monomer.

Glucose-fructose isomerization mediated by Sn-BEA is investigated using an extended QM/MM model containing 208 tetrahedral atoms. The isomerization mechanism consists of a sequence of ring-opening, isomerization, and ring-closing processes, consistent with the previously reported experimental observations. In agreement with the experimentally observed kinetic isotope effect, the rate-determining step is found to involve a hydride shift from the C2 carbon to the C1 carbon. The apparent activation energy for the rate-limiting step is 22.3 kcal/mol at 343 K. The difference in the reaction barriers for the partially hydrolyzed and the fully coordinated Sn sites was investigated using energy decomposition analysis. It is found that the higher activity of the partially hydrolyzed site comes from the extra flexibility provided by the defect in the lattice. The effect of substituting Sn in the active site by Ti, Zr, V, Nb, Si, and Ge was examined, and it was found that Sn and Zr are metals that result in the lowest reaction barrier for glucose isomerization. By using energy decomposition analysis, two physical properties are shown to contribute to the magnitude of the reaction barrier: the polarizability of the metal atom in the active site and the Brønsted basicity of the oxygen atom bound to the metal atom. © 2014 American Chemical Society.