Evidence of solvent-mediated proton transfer during H2O2 activation in titanosilicate-catalyzed oxidation systems.

ABSTRACT

The catalytic performance of titanosilicates involving hydrogen peroxide (H2O2) as the oxidant is strongly influenced by the solvents. Until now, there is still a lack of a universal principle that can guide the choice of a solvent. Herein, the kinetics of H2O2 activation catalyzed by various titanosilicates in different solvents is investigated, and an isokinetic compensation effect is concluded. This indicates that the solvent participates in the H2O2 activation process for the formation of a Ti-OOH species. Additionally, the results of isotopically labeled infrared spectra preliminarily confirm that the solvent acts as the mediator to promote the proton transfer during the H2O2 activation process. The catalytic activities of a series of TS-1 catalysts toward 1-hexene epoxidation are compared, which include Ti(OSi)3OH species with a range of densities but a constant total Ti content. This reveals that the solvent effect is closely related to the Ti active sites of these TS-1 catalysts. Based on these results, a principle for the rational choice of solvent for this catalytic process is proposed. ROH is found to be the mediator for Ti(OSi)4 sites, and methanol, which has a strong proton-donating ability, is the best solvent for these sites. However, for the Ti(OSi)3OH sites, water (H2O) is the mediator, and a weaker hydrogen bonding between H2O molecules promotes proton transfer more effectively. The solvent influences the catalytic performance by perturbing the hydrogen bonds between the H2O molecules, and aprotic acetonitrile, which has a strong ability to break the hydrogen bonding network between H2O molecules, is the best solvent for Ti(OSi)3OH sites. This study provides experimental evidence that the solvent promotes the catalytic performance of titanosilicates by assisting the proton transfer during the catalytic H2O2 activation process, which will pave the way toward the rational choice of solvent for the titanosilicate-catalyzed oxidation systems.