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 mu m, 0.1-1 mu m 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, N-2 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 degrees C more easily sublimated to form Mo-species at a atomic/molecular level, diffused into the channels of ZSM-5 and reacted with the Bronsted acid to form Mo-O-Al species, which resulted in the decrease of the Bronsted acid sites in the catalysts. The catalytic results confirmed that nano MoO3 modified ZSM-5 zeolites calcined at 500 degrees 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 degrees C) or calcined at 500 degrees 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.