Microkinetic Modeling of Benzyl Alcohol Oxidation on Carbon Supported Pd and AuPd Nanoparticles

Abstract

Experiments were conducted on the liquid-phase oxidation of benzyl alcohol over Pd and AuPd nanoparticles, with the aim of determining the reaction mechanism. It was determined that there are two primary reaction paths: A) an alkoxy pathway leading to toluene, benzaldehyde, and benzyl ether, and B) a carbonyloxyl pathway (\u201cneutral carboxylate\u201d) leading to benzoic acid, benzene, and benzyl benzoate. Therefore, microkinetic modelling using the obtained mechanism with surface intermediates was capable of producing all experimentally observed trends with mostly quantitative agreement. 1. Scope Metal particles can directly catalyze the liquid phase oxidation of alcohols using molecular oxygen as the oxidant.1 In particular, benzyl alcohol oxidation to benzaldehyde is of practical use for pharmaceutical, perfume, dye, and agricultural industries.2 Depending on the reaction conditions (temperature, solvent, oxygen pressure), many side products including benzene, toluene, benzoic acid, benzyl benzoate, and benzyl ether have been reported to be formed in addition to the main product, benzaldehyde. Tentative mechanisms were proposed. However, a detailed and complete mechanism of Pd (AuPd) catalyzed benzyl alcohol oxidation in organic solvent has not yet been proposed. In this study, we have performed experiments in which the temperature, gas-phase oxygen pressure, and initial concentration of the benzyl alcohol were varied to elucidate the mechanism.3 Furthermore microkinetic modeling (simulation and fitting) of the reaction were performed.4,5 The objectives of this microkinetic modeling are threefold: 1) to provide additional evidence for the mechanism used by verifying that kinetic modeling with this mechanism can reproduce the kinetic behavior observed experimentally (absolute quantities produced and selectivity trends), 2) to identify which reactions are the most kinetically significant, and 3) to extract kinetic parameters for use in future modeling/studies. 2. Results and discussion The liquid-phase oxidation of benzyl alcohol over Pd and AuPd nanoparticles supported on a cwtiavsa tpeedr fcoarrmboend. xEyxlpeenreim aesn tsth ew esroel vpeenrtf oarnmde dc oinnt ian uboautsc hg arse acptuorrg iwngit ho fp atrhae- hvaeraidesdp:a cthee. iTnhiteia l foblelnozwyiln ga lcoexhpoel ricmonencetanlt raptiaorna,m tehteer so xwygeerne pFarortmia lt rpernedsss uinre t hine cthoen cheenatdrastpiaocne p, raonfdil etsh ea nrde aicnttoerg rtaetmedp eprraotduurec.t ion of each product, it was determined that Scheme 1. Proposed mechanism for benzyl alcohol oxidation.3 tehtheerer, aarned t wBo) ap rciamrbaroyn yreloaxcytilo np apthawthasy: A(\u201c)n eaunt raallk ocxaryb opxaythlawtea\u201dy) lleeaaddiinngg ttoo tboeluneznoeic, baceindz,a bldeenhzyednee,, aanndd bbeennzzyyll bTehnez omatiec r(oSkcihneemtice 1m).o deling in this work was able to reproduce the selectivities and trends observed for the production of both the main product (benzaldehyde) and the byproducts (benzene, toluene, benzoic acid, benzyl benzoate, and benzyl ether). The present study suggests that the most important activation energies are those of k2, k5, and k6 (Scheme 1), which we estimate as Ea2=57.9 kJmol-1, Ea5=129 kJmol-1, and Ea6= 175 kJmol- 1 corresponding to alcohol dissociation, alkyl hydrogenation, and reaction of alkyl species with alkoxy species. Under the same reaction conditions, AuPd/C has a lower activity compared to Pd/C and shows a different product distribution with less formation of products from the \u201ccarbonyloxyl\u201d pathway (benzene, benzoic acid, benzoate). It was found that the selectivity changes can be explained by this change in k1, which corresponds to oxygen adsorption (Figure 1). 3. Conclusions Quantitative kinetic analysis of benzyl alcohol oxidation over carbon-supported Pd and AuPd nanoparticles has enabled us to elucidate the reaction mechanism. The proposed mechanism suggests that the selectivity is influenced by not only temperature and the coverage of the reactants, but also by the side reaction of oxygen scavenging surface hydrogen. Additional insights on how the rate constants affect the production of each product (and thus selectivities) were gained from the analytical equations that were derived from the microkinetic model. References 1. M. Besson, P. Gallezot, Catal. Today 2000, 57, 127-141. 2. T. Mallat, A. Baiker, Chem Rev 2004, 104, 3037-3058. 3. A. Savara, C. E. Chan-Thaw, I. Rossetti, A. Villa, L. Prati, ChemCatChem 2014, 6, 3464\u20133473. 4. A. Savara, I. Rossetti, C. E. Chan-Thaw, L. Prati, A. Villa, ChemCatChem 2016, 8, 2482 \u20132491. 5. A. Savara, C. E. Chan-Thaw, J. E. Sutton, D. Wang, L. Prati, A. Villa, ChemCatChem DOI: 10.1002/cctc.201601295