Synthesis strategy of atomically dispersed Au clusters induced by NH3 on TS-1: Significantly improve the epoxidation activity of propylene

Abstract

The epoxidation of propylene (C3H6) over TS-1 molecular sieve supported with nano-Au clusters by deposition–precipitation (DP) method has always been the focus of the catalytic field. A quantity of atomically dispersed Au clusters can be synthesized induced by NH3 as an assistant in the DP system. These smaller Au clusters show 14.3% C3H6 conversion, 91.7% propylene oxide (PO) selectivity, 10.6% H2 efficiency and 477.4 gPO·kgcat-1·h-1 PO productivity in the epoxidation reaction of C3H6. The interaction between NH3 and [AuCl4]- in aqueous solution is analyzed by means of experiments and Density Functional Theory (DFT), and it is concluded that at least 2 times of NH3 is needed to make Au precipitate completely. Similarly, the mechanism of C3H6 epoxidation catalyzed by Aun (n = 2, 3, 4) clusters in coordination with Ti sites is calculated with the help of DFT. It is believed that the in-situ formation of Ti-OOH structure at Ti sites through H2 and O2 is the key to initiate C3H6 epoxidation. The general rules of catalytic mechanism under different size of Au clusters are also analyzed.