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