Effects of the metal ionic radius at lattice site on surface active oxygen and reducibility over doped mesoporous ZSM-5 for catalytic oxidation VOC

Abstract

Heteroatomic mesoporous ZSM-5 (Zeolite Socony Mobil–5) as an important catalyst is widely applied in catalytic field, however, its catalytic performance in VOCs oxidation is not clear. Particularly, the effects of the metal ionic radius at lattice site on surface active oxygen and reducibility of ZSM-5 are confused. Herein, Cu-, Mn-, Cr- and Co-doped mesoporous ZSM-5 were synthesized by a hydrothermal method to investigate the effects and altered mechanisms for hexane oxidation via the catalysts containing transition metals with different ionic radii at lattice sites in mesoporous ZSM-5, and the physical/chemical characteristics of all the catalysts were characterized. The results show that surface active oxygen species and low-temperature reducibility as critical factors in hexane oxidation are relevant to the transition metal ion radius. And, moderate-length ionic radius can significantly facilitate the increase in the amount and transfer capacity of the surface reactive oxygen species and the reducibility of catalysts, which exhibits a typical volcano-type curve relationship. Among the prepared catalysts, Cu-HZ (tetra-coordination Cu2+ radius: 0.57 Å), which possesses the largest concentration of superficial lattice oxygen, the highest reducibility and an adequate degree and strength for surface acidity, exhibits the highest catalytic activity, the highest CO2 yield and selectivity. Additionally, compared with the supported mesoporous ZSM-5, the reaction mechanism of hexane oxidation over doped mesoporous ZSM-5 can be intensified by the synergistic effects between the lattice transition metal atom as the acidic adsorption site and the surface reactive oxygen generated by lattice doping of transition metal. We believe that this work can provide new insights into the design of catalysts for VOC degradation.