Low-Dimensional Zeotypes Templated by Stacked Cyclic Benzimidazolium Revealed by Electron Crystallography

The structural diversity of zeolites depends strongly on the use of organic structure-directing agents (OSDAs) that guide their formation. Low-dimensional zeolitic materials, such as layered or chain-like phases, can serve as key intermediates in topotactic condensation pathways, yet the mechanisms governing their formation and transformation remain poorly understood. Here, we report three low-dimensional zeolitic materials, EMM-75P, EM-L01, and EM-L02, synthesized using benzimidazolium cations as OSDAs. Their structures were determined by three-dimensional electron diffraction (3D ED), including the atomic structure of the OSDAs, revealing their confinement within the framework to shed light on their structure-directing role. The bulky benzimidazolium OSDAs prevent the formation of materials with three-dimensional framework structures and instead direct the formation of low-dimensional zeotypes. Upon calcination, the two-dimensional layered aluminosilicate zeotype EMM-75P undergoes topotactic condensation to form a three-dimensional zeolite, EMM-75, with a previously unreported zeolite framework topology. Aluminosilicate EM-L01 is a 2D analogue of STF/SFF zeolite frameworks and partially condensed to an STF-topology upon calcination, whereas EM-L02, a 1D silicate composed of double 6-ring chains, packed analogous to the CHA zeolite framework, collapses during the thermal treatment. The detailed structural characterization of these three materials provides insights into the mechanism of topotactic condensation and demonstrates how such pathways can lead to new zeolite materials.