Structure-Directing Behaviors of Tetraethylammonium Cations toward Zeolite Beta Revealed by the Evolution of Aluminosilicate Species Formed during the Crystallization Process

Abstract

Organic structure-directing agents (OSDAs) have been widely used for the synthesis of zeolites. In most cases, OSDAs are occluded in zeolites as an isolated cation or molecule geometrically fitted within the zeolite cavities. This is not the case for zeolite beta synthesized by using tetraethylammonium (TEA+) cation as an OSDA, in which a cluster/aggregate of ca. six TEA+ cations is occluded intact in the cavity (i.e., the channel intersection) of zeolite beta. The structure direction of TEA+ in such a nontypical, clustered mode has remained elusive. Here, zeolite beta was hydrothermally synthesized using TEA+ in the absence of other alkali metal cations in order to focus on the structure-directing behaviors of TEA+ alone. The solid products formed throughout the hydrothermal synthesis were analyzed by an array of characterization techniques including argon adsorption-desorption, high-energy X-ray total scattering, Raman and solid-state NMR spectroscopy, and high-resolution transmission electron microscopy. It was revealed that the formation of amorphous TEA+-aluminosilicate composites and their structural, chemical, and textural evolution toward the amorphous zeolite beta-like structure during the induction period is vital for the formation of zeolite beta. A comprehensive scheme of the formation of zeolite beta is proposed paying attention to the clustered behavior of TEA+ as follows: (i) the formation of the TEA+-aluminosilicate composites after heating, (ii) the reorganization of aluminosilicates together with the conformational rearrangement of TEA+, yielding the formation of the amorphous TEA+-aluminosilicate composites with the zeolite beta-like structure, (iii) the formation of zeolite beta nuclei by solid-state reorganization of such zeolite beta-like, TEA+-aluminosilicate composites, and (iv) the subsequent crystal growth. It is anticipated that these findings can provide a basis for broadening the utilization of OSDAs in the clustered mode of structure direction in more effective ways.