(307g) Mercaptosilane-Directed Synthesis of Encapsulated Au Clusters in Ti-MFI Zeolites for Direct Epoxidation of Propylene

Encapsulated Au nanoclusters in Ti-MFI zeolites (denoted Au@TI-MFI) were prepared via a ‘one-pot’ hydrothermal synthesis route assisted by 3-mercaptopropyl-trimethoxysilane. The mercaptosilane coupling agent stabilized Au³⁺ precursors (HAuCl₄â‹…3H₂O) to preserve metal dispersion by minimizing premature precipitation and subsequent metal cluster growth during zeolite crystallization.¹ Syntheses were carried out in a static convection oven at 140 °C for 63 h. Following centrifugation and subsequent drying at 80 °C, the as-made samples were thermally treated in either air or nitrogen at elevated temperatures (150-450 °C) to remove the organic precursors. The crystallinity of Au@Ti-MFI zeolites was determined by comparing powder X-ray diffraction patterns of the as-made samples with reference patterns for siliceous MFI zeolites. Bulk elemental compositions were determined by inductively coupled plasma-mass spectrometry and revealed Si/Ti molar ratios of ~100 and Au loadings between 0.2-0.5 wt.%. Calculated absorption energies from diffuse reflectance ultraviolet-visible spectra confirmed isomorphous substitution of tetrahedral Ti⁴⁺ species into the MFI framework. Studies of gas-phase deactivation behavior and steady-state kinetics measurements were performed during propylene epoxidation in the presence of H₂ and O₂ at 1 atm over a temperature range of 140-200 °C in a custom-built, gas‑phase recycle reactor. The encapsulated Au nanoclusters within Ti-MFI exhibited excellent stability with < 3% observable deactivation after 48 h on stream. Kinetic results showed that steady-state PO rates at 200 °C (H₂:O₂:C₃H₆:N₂ = 10:10:10:70 (v%), 36 cm³ min^-1) ranged from 20 to 35 g_PO h^-1 (kg_cat)^‑1 for Au@Ti-MFI, accounting for inhibition by PO,² compared to ~5 g_PO h^-1 (kg_cat)^‑1 for Au/Ti-MFI catalysts prepared by deposition-precipitation (DP). PO selectivities for Au@Ti-MFI and Au/Ti-MFI (DP) were 99.7% and 98.5%, respectively, consistent with reported selectivities > 95% for Au/Ti-MFI (DP).² The increase in PO rates and improved stability of Au@Ti-MFI is attributed to the presence of well-dispersed Au clusters with reduced mobility inside of Ti-MFI.

References:

1. Otto, T.; Zones, S. I.; Iglesia, E., Journal of Catalysis 2016, 339, 195-208.
2. Harris, J. W.; Arvay, J.; Mitchell, G.; Delgass, W. N.; Ribeiro, F. H., Journal of Catalysis 2018, 365, 105-114.