Difference of Oxidation Mechanism between Light C3–C4 Alkane and Alkene over Mullite YMn₂O₅ Oxides’ Catalyst

Abstract

Revealing the catalytic oxidation mechanism of volatile organic compounds (VOCs) is insightful for the development of efficient catalysts. However, because of the complicated interactions and a large number of intermediate species during the reactions, the analysis of the entire reaction mechanism (including the activation modes of reactant molecules and the rate-limiting step) remains a great challenge. Herein, the YMn2O5 mullite catalyst was proposed to demonstrate how to distinguish the deep oxidation difference among C3–C4 alkanes and olefins via combining experiments and theoretical calculations. The YMn2O5 catalyst prepared via the hydrothermal method displayed a superior catalytic behavior with a low T90 temperature (C3H8 at 250 °C; C3H6, C4H10, and C4H8 less than 200 °C) (1000 ppm of C3–C4 and 10% O2 balanced with He, WHSV = 30 000 mL/g·h). The catalytic activity remained the same after continuous reaction for 100 h at 275 °C for each reactant. Overall, the YMn2O5 mullite catalyst exhibits excellent durability with no activity declines for 400 h. Combined with TPD, DRIFTS, XPS, and DFT analysis, surface oxygen species were found to be active for the oxidation. Owing to the difference of the HOMO induced partial charge distributions between alkanes and alkenes, the dehydrogenation of the end-site C–H of propane is the first step prior to the crucial conversion of acrylate over surface labile oxygen in an octahedral ligand unit. For propene oxidation, the C═C double bond is preferentially attacked by two surface oxygen atoms belonging to the octahedral and pyramid ligand units with the crucial step of acetate decomposition. These findings provide insights into the oxide catalyst design toward the complicated VOCs oxidation from a fundamental point of view.