Interzeolite Conversion and the Role of Aluminum: Toward Generic Principles of Acid Site Genesis and Distributions in ZSM-5 and SSZ-13

The performance of zeolite catalysts depends not only on the strength and number of Bronsted acid (or exchange) sites but also on synergistic effects derived from their proximity, in particular, and their distribution, in general. Little is known on the genesis of acid sites and site distributions in hydrothermal zeolite synthesis. By an extensive study of five crystallization systems yielding ZSM-5 (MFI) and SSZ-13 (CHA), with a focus on interzeolite conversion (IZC) methods, several synthesis factors and mechanisms that are key in determining the output acid site distribution have been identified. Key in this study were temporal synthesis profiles while probing the distribution and evolution of proximal acid sites with divalent cation capacity measurements. Over the course of different crystallizations, changing local charge distributions are detected, notably within crystalline materials upon prolonged exposure (maturation). Aluminum is clearly the key driver in IZC syntheses, from charge, dissolution, concentration, and mobility points of view. Quasigeneric principles for IZC syntheses are proposed, distinguishing between Al-loving and Al-averse systems, enabling a new degree of control over the acidity and ion-exchange properties of zeolites, of use to tailoring catalytic activity.

Automated summary

The performance of zeolite catalysts relies on the strength and distribution of Bronsted acid sites. By studying synthesis factors and mechanisms, as well as using divalent cation capacity measurements to investigate the distribution and evolution of acid sites, control over zeolite acidity and ion-exchange properties can be achieved. Aluminum plays a key role in interzeolite conversion (IZC) syntheses, with proposed principles to differentiate between Al-loving and Al-averse systems for tailoring catalytic activity.