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 Å). 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 (DE_act = 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 (DE_act = 33.2 kcal/mol) and thermodynamically unfavorable (DE = +2.8 kcal/mol). However, it is much easier (DE_act = 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 DE_act = 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 (DE_act = 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 (DE_act = 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.

References (1) In Chemical Engineering Progress, October 2003, 14. (2) Hayashi, T.; Tanaka, K.; Haruta, M. J. Catal. 1998, 178, 566. (3) Nijhuis, T. A.; Huizinga, B. J.; Makkee, M.; Moulijn, J. A. Ind. Eng. Chem. Res. 1999, 38, 884. (4) Stangland, E. E.; Stavens, K. B.; Andres, R. P.; Delgass, W. N. J. Catal. 2000, 191, 332. (5) Landon, P.; Collier, P. J.; Papworth, A. J.; Kiely, C. J.; Hutchings, G. J. Chem. Commun. 2002, 2058. (6) Sivadinarayana, C.; Choudhary, T. V.; Daemen, L. L.; Eckert, J.; Goodman, D. W. J. Am. Chem. Soc. 2004, 126, 38. (7) Yap, N.; Andres, R. P.; Delgass, W. N. J. Catal. 2004, 226, 156. (8) Taylor, B.; Lauterbach, J.; Delgass, W. N. Appl. Catal. A 2005, 291, 188. (9) Joshi, A. M.; Delgass, W. N.; Thomson, K. T. J. Phys. Chem. B 2006, Submitted. (10) Joshi, A. M.; Delgass, W. N.; Thomson, K. T., Manuscript in Preparation. (11) Naito, S.; Tanimoto, M. Chem. Commun. 1988, 832. (12) Wells, D. H.; Delgass, W. N.; Thomson, K. T. J. Catal. 2004, 225, 69. (13) Joshi, A. M.; Delgass, W. N.; Thomson, K. T. J. Phys. Chem. B 2005, 109, 22392.