Impact of anionic species on the crystallization and aluminum localization in the zeolite framework

Abstract

The distribution of acid sites, which are derived from tetrahedrally coordinated aluminum (Al) atoms within zeolite framework plays a critical role in the acidic catalyzed reactions, is strongly influenced by electrostatic interactions between cationic structure-directing agents (SDAs) and anionic charges. Herein, we synthesized various ZSM-5 zeolites without OSDAs and conducted an in-situ analysis to investigate the impact of anionic species from various Al sources on the crystallization and acid site localization in the framework. The results demonstrated that when Al(NO₃)₃ is used as the Al source (denoted as Z5-N), the electronic environment influenced by NO₃⁻ alters the framework formation pathway, leading to the creation of distorted, tetracoordinated Al intermediates. This process enhances Al atoms incorporation during the early stages of crystallization, resulting in partially bonded framework Al due to lower polarization ability on Na⁺. In contrast, using Al(OH)₃ as the Al source (denoted as Z5-H) accelerates the nucleation process and leads to the formation of smaller particle size. Notably, the ²⁷Al multiple quantum magic angle spinning nuclear magnetic resonance (MQMAS NMR) results revealed that pure OH⁻ ions facilitate the preferential localization of Al atoms at T11 sites within the ZSM-5 framework, where the acid strength is reduced due to the larger Si-O-Al bond angle. The unique Al distributions are attributed to the strong polarization ability induced by the electrostatic attraction of OH⁻ ions to Na⁺. This study enhances our understanding of how different anionic environments influence Al placement, enabling more targeted design and synthesis of zeolite materials with tailored catalytic properties.