Studies on the Synthesis, Reactivity, and Stability of Au Nanoparticles in TS-1 Zeolites for Propylene Epoxidation within H2/O2 Mixtures

The increasing global demand for energy and materials has highlighted the importance of efficient production of chemical intermediates like propylene oxide (PO), a key compound in manufacturing polyurethanes, propylene glycol, and other industrial products. Traditional methods, such as the chlorohydrin process and organic peroxide methods, pose environmental and economic challenges due to hazardous by-products and complex processes. The need for greener alternatives led to the exploration of gold (Au) catalysts for the direct catalytic epoxidation of propylene using hydrogen (H2) and oxygen (O2). This research focused on understanding the propylene epoxidation process over Au catalysts, specifically Au nanoparticles (AuNPs) supported on titanium-silicalite (TS-1). The tandem catalytic system using Au/TS-1 demonstrated exceptional performance, producing PO with selectivity exceeding 95%. The catalytic mechanism involved the in-situ generation of hydrogen peroxide (H2O2) or hydroperoxyl species (H-M-OOH) on the Au surface, which then interacted with titanium active sites to form PO. This single-step, gas-phase process proved to be more environmentally friendly, generating water as the only by-product. Encapsulation techniques, with the use of 3-mercaptopropyl-trimethoxysilane ligands, were employed to protect AuNPs from sintering and agglomeration. These ligands provided chemical protection against the reduction of Au3+ precursors and promoted the nucleation of silicate oligomers, improving the catalyst’s stability and performance. Key findings revealed that the performance of Au@TS-1 catalysts was highly selective to PO, but conversion rates were low. Also, Au@TS-1 catalysts exhibited higher PO selectivities (>99%) but faced challenges in terms of water formation and hydrogen efficiency. Characteristic analysis showed that encapsulating Au nanoparticles within TS-1 zeolites restricted particle sizes and stabilized specific transition states (Ti), leading to improved catalytic efficiency. The encapsulated Au catalysts achieved PO production rates that were 5 times higher than those of non-encapsulated catalysts, reaching values as high as 3.4x10-2 molPO∙(molAu -1∙s-1) in optimized conditions. This approach demonstrated a substantial increase in both selectivity and production rates compared to traditional methods. Additionally, the apparent activation energy (Eapp) for these reactions was measured at approximately 20 kJ/mol, indicating that these catalysts were less sensitive to temperature variations while maintaining stability under operational conditions. Reaction orders for propylene (C3H6) and oxygen (O2) were found to be fractional, ranging from 0.2 to 0.6, indicating complex surface interactions on the catalyst. However, some encapsulated catalysts showed negative reaction orders for hydrogen (-0.2 ~ -0.6) and water (-0.2 ~ -0.6), suggesting that alkali cations could promote the formation of intermediates, of which the breakdown could form water and reduce the availability of active sites for the main epoxidation pathway. In conclusion, the encapsulation of Au nanoparticles in microporous zeolites proved to be a highly effective method for stabilizing the catalyst and improving the efficiency of propylene epoxidation. This work provided a pathway for developing sustainable, high-performance catalytic processes for industrial PO production, exploring key parameters in catalyst design, stability, and selectivity.