Fluoride etching opens the structure and strengthens the active sites of the layered ZSM-5 zeolite

Abstract : The NH4F etching is a chemical approach for the preparation of hierarchical zeolites without substantial modification of their framework composition and acidic properties. We employed this approach to modify the 2-dimensional form of ZSM-5 and obtain a catalyst for bulky molecules conversion. The etching conditions were varied in order to obtain hierarchical ZSM-5 with different level of dissolution. Thus, obtained derivatives of 2-dimensional ZSM-5 were thoroughly studied in order to get deep insights into the effect of fluoride treatment. The 2-dimensional ZSM-5 was found unexpectedly stable under etching conditions, which is a consequence of the high quality of zeolite crystals containing a limited number of defect zones and misoriented crystalline domains, in contrast to conventional ZSM-5. Fluoride etching was also used to de-pillar 2-dimensional ZSM-5. The morphology and textural properties of the de-pillared ZSM-5 were very similar to those of the parent layered ZSM-5. Accordingly, amorphous and extra-framework species were dissolved first, resulting in the complete removal of extra-framework Al, thereby strengthening the active sites. In summary, the fluoride etching opens the structure of layered ZSM-5 and strengthens its acid sites when combined with dealumination during calcination.