(751e) Supported Gold Clusters with Modulated Environment As Highly Stable Catalysts for Propylene Epoxidation

Propylene oxide (PO) is considered a high value-added commodity chemical which is used as an intermediate to produce polyurethane foams and polyesters. Currently, PO is commercially produced using the chlorohydrin process or the hydroperoxide process, which has many limitations and drawbacks. Since Haruta and co-workers discovered the catalytic activity of gold nanoparticles for the direct gas phase epoxidation of propylene to PO using H₂ and O₂, the process has gained considerable attention as a simple, environmentally benign route, using gold, supported on titanium (Ti)-containing catalysts. However, low catalyst stability and poor hydrogen efficiency are key issues that still need improvement. It is known that highly dispersed gold nanoparticles, along with tetrahedrally coordinated Ti⁴⁺ sites in the support are required to generate in situ peroxy species to produce PO [1]. The metal-ligand interaction plays an important role in tuning the electronic structures of these clusters, which influences their catalytic properties [2].

In this study, we report a facile, one pot synthesis of sub-nanometer gold clusters using triphenylphosphine as a stabilizing ligand. Furthermore, the pre-synthesized gold nanoparticles are successfully immobilized onto a titanosilicalite-1 (TS-1) support, and a non-thermal plasma is used for the removal of bound ligands with minimum alteration in the size of the gold particles. The catalytic performance of these supported gold clusters for gas phase propylene epoxidation using H₂ and O₂ is presented. These materials exhibit better hydrogen efficiency, high selectivity towards PO and remarkably improved stability, as compared to previously reported Au/TS-1 catalysts. The structure-function relationships for the enhanced stability and selectivity are elucidated using a range of techniques, including HRTEM, XPS, TGA, DR-UV/VIS and ³¹P MAS NMR.

1. Hayashi, T., Tanaka, K. and Haruta, M. Catal. 178, 566 (1998).
2. Trogadas, P., Nigra, M. M. and Coppens, M. -O. New J Chem. 40, 4016 (2016).