Top-down engineering of zeolite porosity

Synthetic zeolites present a near-ideal environment for catalytic conversion and separation of molecules. They have framework structures with discrete uniform micropores allowing selective processing and sorption based on the size and shape of molecules. Zeolites have been deployed in numerous industrial applications motivating continuous innovation efforts to synthesise new structures and pore systems. The conventional zeolite syntheses are carried out directly as hydrothermal bottom-up assemblies from molecular or amorphous precursors, producing robust hard-to-modify structures/crystals. To diversify zeolite structures, especially to alleviate diffusional limitations and active site accessibility, top-down modification approaches have been initiated. This article presents an overview of the various top-down methods for modifying already synthesised zeolite crystals or precursors to engineer additional porosity and functionality. They include: demetallation focused on Si and Al but also Ge and Ti, formation of micro/mesoporous hybrids by recrystallisation, mechanochemical methods, pore engineering with low-dimensional zeolite forms, especially 2D and nanozeolites, and treatments by microwaves, ultrasounds, plasma and lasers. The discussion presents illustrative examples of relevant properties, such as textural, acidic and catalytic, of materials obtained by the applied treatments. The methods and descriptors used to characterise changes in porosity are described in detail. An extended compilation of reported materials with textural properties is provided.