(660d) A Quantum-Mechanics/Molecular-Mechanics Study of Potential Steps in Direct Propylene Epoxidation Using H2 and O2 on Au/Titanium-Silicalite-1 Catalysts
Production of propylene oxide (PO) in a single step with no side products has been a long-sought industrial target. While a liquid-phase H2O2/TS-1 based route to PO appears imminent, due to handling problems and cost associated with H2O2, researchers have also focused on direct gas-phase propylene oxidation using H2 and O2 over Au/Ti catalysts[2-4]. The assumption that such catalysts operate by (1) H2O2 formation on Au and (2) propylene epoxidation on Ti using that H2O2 is supported by recent literature[5,6]. Since studies of Au/TS-1[7,8] suggest that part of the epoxidation activity is associated with Au/Ti sites inside the zeolite channels, we have employed the hybrid quantum-mechanics/molecular-mechanics (QM/MM) approach, augmented with full thermochemistry (298.15 K, 1 atm), to develop epoxidation mechanisms inside the TS-1 pores (5.5 Å). We considered both non-defect and Si-vacancy defect Ti-sites with and without Au3 adsorbed on them and investigated OOH/H2O2 formation pathways. Consistent with experiments on Au/SiO2 we found that O2 pre-adsorbed on Au3 enhanced the dissociative adsorption of H2 to form stable OOH species (DEact = 7.7 kcal/mol, Au3/Ti-non-defect). We speculate that an H2O2 formation pathway similar to that found on gas-phase Au clusters[12,13] is likely to operate on Au3/Ti-non-defect sites. H2O2 formed on these sites can then migrate to PO-producing sites via diffusion along the pore walls. Assuming such availability of H2O2, we modeled three different sites for propylene epoxidation: (1) Si-defect, (2) Ti-defect, and (3) Au3-Ti-defect. We found that formation of Si-OOH species due to reaction of H2O2 with a metal-vacancy Si-defect sites is both kinetically (DEact = 33.2 kcal/mol) and thermodynamically unfavorable (DE = +2.8 kcal/mol). However, it is much easier (DEact = 16.8 kcal/mol) to form Ti-OOH species (and water) by attacking the Ti-defect site with H2O2 (DE = -8.0 kcal/mol). Propylene reacts with these Ti-OOH species to form propylene oxide with DEact = 15.8 kcal/mol and DE = -51.3 kcal/mol. Interestingly, we predict that the activation barrier to form Ti-OOH species on Au3/Ti-defect sites is significantly higher (DEact = 28.1 kcal/mol) than that for the Ti-defect site without Au3 and that OOH species formed on Au3 in an Au3/Ti-defect site are likely to decompose rapidly to form water (DEact = 1.3 kcal/mol) due to strong interaction with the silanol (Si-OH) groups around the defect. Thus, we conclude that the sequential propylene epoxidation pathway is kinetically unfavorable on the Au3/Ti-defect site but is favorable with a combination of Au3/Ti-non-defect and Ti-defect sites.
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