Insights into tuning toluene oxidation intermediates by phase structures of Mn-based catalysts

MnOx catalysts with different active sites were prepared through a Mn-MOF-derived method, and their active structures gradually changed from Mn3O4-Mn2O3, Mn2+-coupled-Mn2O3, and finally to Mn2O3 by increasing calcination temperature. The optimum MnOx-500 catalyst calcined at 500ºC, in the form of Mn2+ coupled Mn2O3 dual catalytic sites, exhibited the best catalytic performance and stability for toluene oxidation due to the enhanced low-temperature redox properties, weakest Mn-O bond and the highest percentage of Mn2+ species. The possible route of toluene oxidation was proposed based on in-situ FTIR, and interestingly, all the reaction intermediates, including benzyl alcohol, benzaldehyde, benzoate, phenol, and maleate, were produced on MnOx-400, while for MnOx-500, Mn2+-coupled-Mn2O3, only benzaldehyde, benzoate, and maleate were formed, and the rate-determining step for conversion of benzoate into ring-oping products requires less energy. The work opens a new avenue for the precise construction of active sites based on multivalent metal oxide catalysts for the degradation of volatile organic compounds.