Cryo-TEM and electron tomography reveal leaching-induced pore formation in ZSM-5 zeolite

Zeolites are the catalytic workhorses of the refinery and chemicals production industry. Their inherent micropores lead to remarkable shape-selectivity, but also present diffusion limitations on reactions. One scalable approach to further their functionality is to introduce mesopores into the individual zeolite crystal by base leaching. Using cryogenic transmission electron microscopy (cryo-TEM), we are able to capture the pore formation process in ZSM-5 zeolites in the solution state, first forming mesopores, to eventually yielding hollow crystals. Electron tomography indicates that the larger pores in the initial leaching stages tend to exist at the boundary between the aluminum-rich shell and the aluminum-poor core, while multiple small pores are also present within the aluminum-poor core. This indicates that pore formation is based on crystalline and chemical inhomogeneities at the single crystal level.