Engineering (Jun 2023)
Engineering a Coordinatively Unsaturated Au–O–Ti3+ Structure Toward Unprecedented H2 Efficiency for Low-Temperature Propene Epoxidation with H2 and O2
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.
