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

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 T-6 and T-7 Al, while more T-2 Al is observed using CHA. The formation of T-2 Al in both MCM-49(HMI) and MCM-49(CHA) is derived from Na+, while protonated HMIs show bias in incorporation of T-6 and T-7 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 T-2 Al substitution.

Automated summary

This study comprehensively investigated the roles of different SDAs in selective Al substitution in MCM-49 zeolite using multinuclear solid-state NMR. The results showed that different SDAs have distinct impacts on Al distributions in the zeolite structure, with varying levels of T-2, T-6, and T-7 Al observed depending on the type of SDA used.