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660d

A Quantum-Mechanics/Molecular-Mechanics Study of Potential Steps in Direct Propylene Epoxidation Using H2 and O2 on Au/Titanium-Silicalite-1 Catalysts

Ajay M. Joshi1, W. Nicholas Delgass2, and Kendall T. Thomson2. (1) School of Chemical Engineering, Purdue University, FRNY 117 B, 480 Stadium Mall Drive, West Lafayette, IN 47907, (2) Chemical Engineering, Purdue University, Forney Hall of Chemical Engineering, 480 Stadium Mall Drive, West Lafayette, IN 47907

Abstract

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[1], 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 ). We considered both non-defect and Si-vacancy defect Ti-sites with and without Au3 adsorbed on them[9] and investigated OOH/H2O2 formation pathways[10]. Consistent with experiments on Au/SiO2[11] 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[10]. 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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