Engineering a Coordinatively Unsaturated Au–O–Ti3+ Structure Toward Unprecedented H2 Efficiency for Low-Temperature Propene Epoxidation with H2 and O2
Zhaoning Song, Hao Yan, Juncong Yuan, Hongfei Ma, Jianlin Cao, Yongxiang Wang, Qiang Wang, Chong Peng, Feng Deng, Xiang Feng, De Chen, Chaohe Yang, Yongkang Hu
Abstract
Since 1998, the Au–O–Ti4+ sites of Au/Ti-based catalysts have been widely accepted as the active sites for propene epoxidation with H2 and O2 at a relatively high temperature, although they are limited by poor H2 efficiency. Herein, we demonstrate a novel Au–O–Ti3+ active site aiming at low-temperature propene epoxidation. Notably, this active site results in a sharp shift in the optimum temperature, from 200 to 138 °C, and allows the catalyst to maintain an unprecedented H2 efficiency of 43.6%, a high propylene oxide (PO) selectivity of 90.7%, and a stability of over 100 h. The Au–O–coordinatively unsaturated Ti3+ active site is quantitatively constructed by tuning the amount of Si–OH and Bu3NH+ in post-treated silicalite-1 seeds. Through operando ultraviolet–visible (UV–vis) spectroscopy, the dynamic evolution of the Ti–OOH intermediate was investigated. It was found that the Ti–OOH generation rate is higher on Au–O–Ti3+ than on conventional Au–O–Ti4+ sites. Moreover, ammonia temperature-programmed desorption (NH3-TPD) and X-ray photoelectron spectroscopy (XPS) characterizations, together with density-functional theory (DFT) calculations, demonstrated that the coordinatively unsaturated Ti3+ sites promote electron transfer between Au and Ti3+, thereby enhancing the O2 adsorption ability of the catalyst and promoting the in situ formation of H2O2 and the Ti–OOH intermediate, even at a low temperature. The insights and methodology reported here not only shed new light on maximizing H2 efficiency over a coordinatively unsaturated Ti3+ structure of titanium silicate-1 but also open up new opportunities for industrial direct gas-phase propene epoxidation in a low temperature range.