Intermediate Phase Selectivity in ZSM-5 Zeolite Synthesis and Framework Aluminum Distribution

Abstract

The current understanding of zeolite crystallization is still far from well understood. Herein, we show that the crystallization pathway of high-silica ZSM-5, one of the most industrially important zeolites, can be altered not only by the type of organic structure-directing agents (OSDAs) at a gel Si/Al ratio of 50 but also by the simultaneous use of Na+ and Cs+ ions as inorganic structure-directing agents (ISDAs). The use of the diquaternary OSDA 1,8-bis(N-methylpyrrolidinium)octane (1,8-MPO) instead of tetrapropylammonium (TPA), the most common OSDA for the synthesis of ZSM-5, in the presence of Na+ led to zeolite β as an intermediate phase; otherwise, no crystalline intermediate was observed. Also, the addition of a small amount of Cs+ ions (gel Cs/Na = 0.14) as a secondary ISDA to the 1,8-MPO-Na+ and TPA-Na+ systems gave small-pore zeolites bikitaite and analcime as an intermediate phase during synthesis, respectively. More importantly, we found that the framework Al distribution in the final product ZSM-5 could differ according to the in situ transformation pathway of intermediate phases, as well as to the type of OSDA used. This study provides a new strategy for directly controlling the intrazeolitic location of Brønsted acid sites, as well as the nucleation pathway in the synthesis of high-silica zeolites.