Effect of the particle size of MoO3 on the catalytic activity of Mo/ZSM-5 in methane non-oxidative aromatization

Abstract

Three types of molybdenum oxide with different sizes (i.e. 10–20 μm, 0.1–1 μm and 30–150 nm) were used to modify ZSM-5, which were investigated in the methane non-oxidative aromatization reaction. The structure and surface properties of the prepared catalysts were studied using XRD, SEM, N₂ adsorption/desorption, ICP-AES, NH₃-TPD and the IR spectra of pyridine adsorption. The characterization results showed that MoO₃ with smaller size calcined at 500 °C more easily sublimated to form Mo-species at a atomic/molecular level, diffused into the channels of ZSM-5 and reacted with the Brønsted acid to form Mo–O–Al species, which resulted in the decrease of the Brønsted acid sites in the catalysts. The catalytic results confirmed that nano MoO₃ modified ZSM-5 zeolites calcined at 500 °C showed higher methane conversion (14.1%) and aromatics yield (9.5%) than micron MoO₃ modified ZSM-5. In addition, micron MoO₃ modified ZSM-5 calcined under higher temperature (i.e. 550 °C) or calcined at 500 °C for longer time did not achieve the higher catalytic behaviour. Moreover, nano-MoO₃ modified ZSM-5 showed better catalytic stability when compared with micron-MoO₃ modified ZSM-5.