Hydroxyl engineering of precipitated silica to construct hierarchically layered ZSM-5 zeolites for efficient methanol-to-propylene

Abstract

Non-classical crystallization pathways have emerged as a pivotal strategy for the synthesis of zeolites with tailored structures, with the initial feed composition being one of the most critical factors influencing this process. As primary raw material for the formation of zeolite networks, the physicochemical properties of Si sources significantly affected structural and acid properties of zeolites via non-classical crystallization pathways. While, the present operating mechanism between each other was still confusing. In this work, the precipitated silica with tunable Si-hydroxyls was home-made using one-step synthesis of fluorosilicic acid and ammonia aqueous in a SK static reactor. Leveraging this tailored silica, a series of hierarchically layered L-ZSM-5 zeolites were successfully prepared by a non-classical crystallization pathway regulated by Si-hydroxyl engineering. The findings revealed a linear positive correlation between the percentage of layered structures and framework single Al distribution of L-ZSM-5 zeolites and the number of silanol nests present in the precipitated silica. Under comparable conditions, the as-prepared hierarchical L-ZSM-5(7/6) zeolite with more layered structures and framework Al located in the straight/sinusoidal channels exhibited excellent propylene selectivity (44.09 %) and reaction lifetime (25 h, WHSV = 6 h⁻¹) in methanol-to-propylene reaction. This work elucidates a decisive relationship between Si-hydroxyls engineering and zeolite crystallization, offering novel insights into the synthesis of zeolites with specific structural features through non-classical crystallization pathways.