Comparing the effects of hollow structure and mesoporous structure of ZSM-5 zeolites on catalytic performances in methanol aromatization

Abstract

Comparing the effects of hollow and mesoporous structures over ZSM-5 zeolites on catalytic behaviors in methanol aromatization was done. Hollow zeolites possessing larger voids, increasing Bronsted acid sites and more framework Al in channel crossings promoted higher benzene, toluene and xylene (BTX) selectivity. Zero-length column (ZLC) desorption of cyclohexane stated that higher diffusion rate was observed over mesopores zeolite and hollow zeolite. However, lower effective diffusion constants (Deff) and higher activation energy of hollow zeolite demonstrated that mass transfer was not only limited by microporosity but by surface barriers resulting from surface defects and pore blockage. The highest diffusion rate of cyclohexane over hollow zeolite was presented indicating that surface barrier was not dominant in diffusion limitation. The stability of hollow ZSM-5 was lower compared to mesoporous ZSM-5 as a result of more strong acid sites and higher external silanol groups, which favored to carbon deposition. Additionally, the collapse of hollow structures with thinner shells also contributed to higher deactivation rate.