Dynamic evolution of Al species in the hydrothermal dealumination process of CHA zeolites

Abstract

The hydrothermal stability of zeolites is an important factor being considered as it could restrict their scope of industrial application. Understanding the water-induced dealumination mechanism is crucial for improving the hydrothermal stability of zeolites. Here a series of water-treated H-SSZ-13 samples under hydrothermal conditions was investigated by solid-state NMR methods and we found that dealumination was caused by the hydrolysis of the Al–O bond by water molecules under high temperature and pressure. The hydrolysis of the first Al–O bond could generate framework-associated Al species with a single framework aluminum hydroxyl. The sequential hydrolysis of four Al–O bonds could form the extra-framework aluminum (EFAL) species, which was prone to adsorbing near the Brønsted acid sites (BASs) by electrostatic interaction contributing to stabilizing the molecular sieve framework. Some of the BASs were perturbed by the framework or extra-framework aluminum hydroxyls which was represented by different signals in ¹H MAS NMR due to hydrogen-bonded interactions. The signals between 5 and 8 ppm in ¹H MAS NMR were attributed to the BASs perturbed by framework Al–OH (2.8 ppm), and correspondingly, BASs disturbed by extra-framework Al–OH (2.4 ppm) resulted in the signals appearing between 12 and 15 ppm. These clear attributions were facilitated by ¹H–¹H DQ-SQ and ¹H–²⁷Al S-RESPDOR MAS NMR. More than that, the partial or complete hydrolysis of the framework Al atoms was achieved uniformly by the water-treatment process under conditions of high temperature and pressure.