Effect of Ti incorporated MWW supports on Au loading and catalytic performance for direct propylene epoxidation

Abstract

Two types of novel catalysts consisting of gold nanoparticles (NPs) supported on the Ti-incorporated MWW structures were compared. One is Ti-YNU-1 with 0.67nm micropores between expanded 2D-MWW structures, and the other is the titanosilicate pillared MCM-36 (Si/Ti-MCM-36) with 2nm mesopores. The influences of pore structure and Ti coordination environment on the particle size of anchored Au NPs and their catalytic performances in propylene epoxidation with mixtures of H2 and O2 were investigated. The catalyst materials were characterized with Electron Probe Micro Analysis, X-ray diffraction, nitrogen sorption, UV–visible diffuse reflectance spectroscopy, transmission electron microscopy, and X-ray absorption spectroscopy. The coordination environment of Ti(IV) in Ti-YNU-1 is mainly isolated tetrahedral (Td), while that in Si/Ti-MCM-36 was found to be a mixture of Td and octahedral (Oh). The Au loading was proportional to the amount of Ti incorporated in the porous silica. The particle size of Au NPs in the Si/Ti-MCM-36 support was a little larger than that in the Ti-YNU-1 support, and it is attributed to that the highly isolated Td Ti(IV) in Ti-YNU-1 is favorable for attaining the smaller gold NPs, while the higher loading of Ti in Si/Ti-MCM-36 anchored more Au and resulted in the formation of larger Au particles. On the other hand, the mesopores in Si/Ti-MCM-36 facilitated the diffusion of the produced PO in pore structure, inhibited the further reaction for side product formation, and moderated the deactivation of catalysts.