The Role of Organic and Inorganic Structure-Directing Agents in Selective Al Substitution of Zeolite

Abstract

Organic and inorganic structure-directing agents (SDAs) impact Al distributions in zeolite, but the insights into how SDAs manipulate Al distribution have not been elucidated yet. Herein, the roles of different SDAs such as cyclohexylamine (CHA), hexamethylenimine (HMI), and Na⁺ in selective Al substitution of MCM-49 zeolite are investigated comprehensively by multinuclear solid-state NMR. The results demonstrate that MCM-49 synthesized with HMI shows relatively more T₆ and T₇ Al, while more T₂ Al is observed using CHA. The formation of T₂ Al in both MCM-49(HMI) and MCM-49(CHA) is derived from Na⁺, while protonated HMIs show bias in incorporation of T₆ and T₇ Al. Most HMIs are occluded in protonated status, and about half of CHAs are occluded in nonprotonated status. The close spatial proximity between nonprotonated CHAs and Na⁺ synergistically promotes the formation of zeolite structure, leading to more Na⁺ ions occluded in the zeolite channel with preferential T₂ Al substitution.