Gas-phase total oxidation of benzene, toluene, ethylbenzene, xylenes using manganese oxides and copper manganese oxide catalysts.

Abstract

Volatile organic compounds (VOCs) continue to be the major source of direct and indirect air pollution. Here, cryptomelane-type octahedral molecular sieve (OMS-2) manganese oxide, amorphous manganese oxide (AMO), and mixed copper manganese oxide (CuO/Mn2O3) nanomaterials were synthesized and, together with commercial MnO2, characterized by various techniques. These catalysts were investigated for gas-phase total oxidation of six VOCs under air atmosphere. Using OMS-2 at 250 °C, the average conversions for toluene, benzene, ethylbenzene, p-xylene, m-xylene, and o-xylene were 75%, 61%, 45%, 23%, 13%, and 8%, respectively, whereas using CuO/Mn2O3, 72%, 44%, 37%, 29%, 27%, and 26%, respectively, were obtained. Generally, the conversion of VOCs to CO2 using the synthesized catalysts increased in the order: o-xylene ≈ m-xylene < p-xylene < ethylbenzene < benzene < toluene. However, using commercial MnO2, benzene (44% conversion) was more reactive than toluene (37%), and the xylenes showed similar reactivities (13–20%). Differences in reactivity among VOCs were rationalized in terms of degree of substrate adsorption and structural effects. For example, the reactivity of xylenes was dictated by the shape-selectivity of stable OMS-2. The higher oxidative activities exhibited by OMS-2, AMO, and CuO/Mn2O3 as compared to commercial MnO2 were attributed to a combination of factors including structure, morphology, hydrophobicity, and redox properties. The mobility and reactivity of active oxygen species were strongly correlated with catalytic activities. Lattice oxygen was involved in the VOC oxidation, suggesting that the reaction could proceed via the Mars–van Krevelen mechanism.