Gas-phase photooxidation of alkenes by V-doped TiO₂-MCM-41: mechanistic insights of ethylene photooxidation and understanding the structure-activity correlation

Abstract

Nanoparticles of Ti_0.95V_0.05O₂ were found to be impregnated in the hexagonal channels of the MCM-41 host, with a distribution of some particles on the surface, thus leading to an effective variation in the particle size as a function of loading host MCM-41 matrix. These catalysts were subjected to the photocatalytic degradation of alkenes under the ambient conditions in which the photocatalytic activity varied as a function of the loading percentage of Ti_0.95V_0.05O₂ in the host MCM-41. This is explained in light of the structure–activity correlation, and the better catalytic activity can be attributed to an electronic interaction between the host and guest molecules, as established from X-ray photoelectron spectroscopy. To understand the mechanistic aspect of the photooxidation of ethylene on the vanadium-doped titania dispersed in the MCM-41 matrix, extensive in situ FTIR experiments were undertaken. The intermediate species produced on bare Ti_0.95V_0.05O₂ are different from that produced on the Ti_0.95V_0.05O₂/MCM-41 surface. Moreover, different intermediates were produced during ethylene oxidation under UV and visible irradiation, thus leading to different rates. The ethylene decomposition over bare Ti_0.95V_0.05O₂ occurs by means of formation of ethoxy groups, transformed to acetaldehyde or enolates, subsequently to acetates, and then to CO₂ under both UV and visible irradiation. However, in the case of Ti_0.95V_0.05O₂/MCM-41 catalyst with UV irradiation, the adsorbed acetaldehyde thus formed undergoes aldol condensation over the Lewis acid sites to lead to the formation of crotonaldehyde, which is subsequently oxidized to acetate and consequently to CO₂. It was observed that during visible irradiation labile ethyl acetate is produced either by the Tischenko reaction or by the reaction between the labile acetic acid and the unreacted ethoxy groups. The ethyl acetate produces acetic acid monomer, which is oxidized to CO₂. Furthermore, in this work the effects of particle size on the intermediate species were also studied.