In-depth understanding of acid catalysis of solvolysis of propene oxide over titanosilicates and titanosilicate/H2O2 systems

A comprehensive investigation was carried out to understand the acidic nature of titanosilicates and titanosilicate/H2O2 catalytic systems. According to a typical probe reaction, the ring opening of propylene oxide (PO) with CH3OH and H2O based on the side reaction in the hydrogen peroxide-propylene oxide (HPPO) process, as well as on the characterization of the IR spectrum, four types of acid sites were confirmed and their alcoholysis ability decreased in the order Ti-OOH > Si-OH groups generated by defect sites >> tetrahedral Ti4+ centers approximate to Ti-OH. The Bronsted acid sites (BAS) of the Si-OH groups generated by defect sites and the Ti-OOH species should be mainly responsible for the ring opening of epoxides. Meanwhile, the alcoholysis products ratio of 1-methoxy-2-propanol (PPM) to 2-methoxy-1-propanol (SPM) was demonstrated to be correlated with the specific acid sites for the first time: BAS tend to generate SPM, while Lewis acid sites (LAS) of tetrahedral Ti4+ centers tend to form PPM. It is found that the reaction activation energy of alcoholysis is higher than that of hydrolysis, since hydrolysis could proceed directly without acid sites. Fine ion exchange and proper additives were adopted to suppress corresponding specific acid sites, which could obviously enhance the PO selectivity in propylene epoxidation. (C) 2016 Elsevier Inc. All rights reserved.