Fabricating highly dispersed and stable LaMnO3 perovskite oxide catalysts for selective aerobic oxidation of toluene to benzoic acid under solvent-free conditions

Abstract

Chlorination-hydrolysis of toluene or homogeneous Co/Mn/Br catalyzed liquid phase oxidation of toluene to produce benzoic acid suffers from product halogen-contamination, severe equipment corrosion, and costly wastewater treatment industrially. Direct selective oxidation of toluene to benzoic acid under non-homogeneous catalyst, molecular oxygen, and solvent-free conditions will make this transformation more efficient and environmentally friendly. In this work, LaMnO₃ perovskite oxide catalysts were fabricated by sol-gel method followed by etching and heat treatment, and tested via selective aerobic oxidation of toluene to benzoic acid under solvent-free conditions. The etched LaMnO₃ perovskite oxide catalyst possesses a good mesoporous structure, significantly increased specific surface area and pore volume, halved grain size, reduced reduction temperature, and increased density of Mn^4 + ions and oxygen vacancies on the catalyst surface. Further heat treatment significantly improved the catalyst stability while sacrificing some textural properties. The LaMnO₃ perovskite oxide catalyst fabricated under the best conditions achieved a toluene conversion of 24.8 % and a benzoic acid selectivity of 82.7 %, with stable structural properties and catalytic performance after multiple cycles of use. This study can provide beneficial exploration and thinking for the design of perovskite oxide catalysts and their application in selective toluene oxidation.