Role of intrinsic zeolite properties on mesopore formation by desilication of MFI structures

In this contribution the alkaline treatment of both commerical and synthesized MFI zeolites has been studied for creation of hierarchical porosity. The influence of Si/Al ratio, crystal size, and counter-cation on the mesoporosity development was investigated. The Si/Al ratio is of crucial importance in the mesoporosity development, as framework aluminium proves to be the pore directing agent. An optimal Si/Al range of 25–50 is found for Si extraction leading to well-controlled mesopore formation. At lower Si/Al ratios the extraction of framework Si is inhibited due to the protective role of Al, while at higher ratios excessive Si extraction leads to formation of large pores. Variations in the zeolite crystal size reveal a decreased mesoporosity development in crystals>3 μm, where both crystal size and an Al gradient can be held responsible. The type of counter-cation (Na, NH4, and H) that, balances the negative charge of the Al tetrahedra does not significantly affect the mesoporosity development.