Selective epoxidation of propylene to propylene oxide with H ₂ and O ₂ over Au/Ti-MWW catalysts*

Direct epoxidation of propylene to propylene oxide (PO) with H ₂ and O ₂ has been performed on bifunctional catalysts, Au nanoparticles supported on novel Ti-MWW titanosilicate (Au/Ti-MWW). In comparison to conventional Au/TS-1 catalysts, Au/Ti- MWW exhibited a similar phenomenon with respect to PO formation, that is, the PO selectivity increased with increasing Si/Ti ratio of titanosilicate. However, at optimized Ti contents corresponding to Si/Ti ratio >140, the PO selectivity of Au/Ti-MWW catalysts was lower than 60 % in comparison to ca. 90 % achieved on Au/TS-1. A large number of boron species and defect-site-related hydroxyl groups contained in Ti-MWW were assumed to retard the desorption of PO from the channels or crystallite surface of zeolite, which favors side reactions such as over-oxidation and decomposition of PO. Poststructural rearrangement was then carried out on Ti-MWW with piperidine (PI) solution to improve effectively its hydrophobicity, leading to defect-less Re-Ti-MWW. This enhanced significantly the PO selectivity of Au/Re-Ti-MWW thus prepared, which reached as high as 92 % at Si/Ti ratio of 135. Au/Re-Ti-MWW(135) then gave the highest PO formation rate of 22.0 gPO kg ^-1 h ^-1.