Convergent Modulation of CHA Zeolites by Substituted Phenols and Ammonium Salts

Abstract

High-quality chabazite (CHA-type) zeolites are highly desirable for their excellent catalytic performance without significant cost. However, their optimal synthesis remains challenging due to the difficulty in simultaneously controlling crystallinity and hierarchical porosity, as well as the persistent issue of polymorph competition. To overcome these hurdles, we present a one-step approach for synthesizing high-quality hierarchical CHA zeolites through controlled etching-assisted crystallization. Moving beyond conventional ammonium fluoride (NH4F) and phenol, we systematically investigated a series of para-substituted phenols and structurally diverse ammonium salts as a single additive to manipulate CHA crystallization. Our research reveals that the etching capacity of the additives dictates properties of the resulting zeolites. By balancing dissolution and growth kinetics, we achieve high crystallinity and tailored porosity while circumventing polymorphism and amorphization. Strikingly, the judicious selection of etchant acid strength and amount consistently yields optimized mesoporous CHA zeolites with similar structural features and catalytic performance in selective catalytic reduction with ammonia (NH3-SCR). We identify an inverse correlation between the optimal in-situ nucleophile amount (defined as the etchant quantity multiplied by its number of dissociable protons) and its acid dissociation constant (Ka) for modulating zeolite synthesis, which closely aligns with the ex-situ dissolution rate, underscoring the importance of employing mild etchants. This correlation unifies the mechanism of etching-assisted zeolite crystallization. The demonstrated wide applicability of mild in-situ etchants encompassing organic or inorganic compounds for crystallization control offers a versatile approach for rational design of targeted zeolites with tailored functional properties.