Understanding HZSM-5's shape-selectivity improvement through tiny autonomous dealumination

The decisive factor that determines HZSM-5's shape-selective capability is obscure due to the interference from zeolite acidity, pore size and crystal size. p-xylene selectivity was found to increase from 45% to 82% in toluene alkylation reaction with the same HZSM-5 catalyst but storage period difference. However, no instrument detectable zeolite acid sites strength, type and quantity changes were observed. NMR revealed slight progressive increase of extra-framework aluminum by dealumination, and nitrogen adsorption-desorption isotherm verified the tiny decrease of micropore volume. Pulse chromatography tests confirmed p-xylene diffusion resistance increase in HZSM-5's channels. The finding made it possible to clarify the interactions between shape-selectivity improvement, p-xylene diffusion resistance increase and extra-framework aluminum while excluding all the other interfering factors, indicating the diffusion resistance control of molecules in HZSM-5's channels is the key to improve its shape-selectivity.