Preparation of core–shell-like zeolites by diffusion controlled chemical etching

The present work reports on the preparation of core–shell-like zeolites with a Si-rich mesoporous shell. This is achieved by diffusion controlled chemical etching of zeolite crystals with a mixture containing oxalic acid and ammonium fluoride (OA–NH4F). The OA–NH4F mixture served as a bifunctional etchant extracting both surface Si and Al and further dealuminating the periphery of the zeolite crystals. The fast dissolution kinetics and the relatively slow intra-crystalline diffusion of the OA–NH4F etching solutions resulted in a distinct core–shell-like structure with a porous and Si-rich shell, preserving the unique advantages of single crystals with interconnected micro- and mesopores.