Interconnected Hierarchical ZSM-5 with Tunable Acidity Prepared by a Dealumination–Realumination Process: A Superior MTP Catalyst

Abstract

ZSM-5 that uses TPAOH as a template has an Al-rich exterior and defective Si-rich interior; thus, a simple base leaching selectively removed the Si-rich interior while the Al-rich exterior was protected. This catalyst showed no change in stability comparing with parent ZSM-5 during the MTP reaction that was attributed to the enclosed hollow structure and richly acidic outer shell. A preliminary fluorination, however, both removed defective Si-sites and caused distortion in tetrahedral aluminum that made the outer shell susceptible to alkaline treatment. These distorted tetrahedral Al were mostly leached out by NaOH in 1 min. Furthermore, aluminum in the filtrate was slowly redeposited onto the zeolite, serving as external pore-directing agents to control silicon dissolution from the Si-rich interior. This dealumination–realumination alkaline treatment process led to a higher solid yield and a uniform opened-mesopore structure with mesopores around 13 nm in diameter. This material was characterized by SEM, TEM, N 2 adsorption, and mercury porosimetry. In addition, NH 3 -TPD, OH-IR, 27 Al MAS NMR, and 1 H MAS NMR results demonstrated that the reinserted Al were unlike the framework Al, contributing less to acidity. The dealumination–realumination process, therefore, was also capable of tuning the acidity of the mesoporous ZSM-5. This mesoporous catalyst exhibited a longer lifetime and a higher propylene selectivity than other catalysts with an enclosed mesopore structure.