Insights into the size and structural effects of zeolitic supports on gaseous toluene oxidation over MnOx/HZSM-5 catalysts

HZSM-5 zeolites with micro/nano crystallites and hollow structure were respectively synthesized for the preparation of supported MnOx catalysts, and the size and structural effects of zeolitic supports on both the physicochemical properties and toluene oxidation performances of MnOx/HZSM-5 catalysts were extensively investigated by numerous characterizations and experimental evaluation. Beneficially from the higher surface area and hierarchical porosity of nanoscale hollow HZSM-5 (namely as H-Z5) zeolite, good surface dispersion of MnOx nanoparticles was acquired over the resultant supported MnO, catalyst (namely as MnOx/H-Z5), simultaneously achieving great enhancement of its low-temperature reducibility, surface oxygen mobility and more plentiful surface Mn4+ cations. The superiority in these physicochemical characteristics was reasonably correlated with the optimum catalytic activity and good catalytic durability for toluene oxidation over MnOx/H-Z5. The results of coke analysis indicated that MnOx/H-Z5 exhibited relatively higher coke-resistant ability since the highly dispersed MnOx nanoparticles on zeolite surface effectively promoted the oxidative destruction of toluene and other organics species. A possible reaction mechanism was proposed based on the in situ DRIFTS results, where the key reaction intermediates including benzaldehyde, benzoic acid and cyclic anhydride and the relevant conversion pathways were determined. The surface active oxygen originating from gas-phase oxygen activation by MnOx nanoparticles played a crucial role in the oxidative elimination of coke precursors into carbon dioxide and water under air atmosphere.