Mordenite etching in pyridine: Textural and chemical properties rationalized by toluene disproportionation and n-hexane cracking

Abstract

Chemical etching with hydrofluoric acid in pyridine medium has been developed as efficient strategy for the crystal engineering of mordenite zeolites. Contrary to the HF etching in aqueous medium, the etching in pyridine allows to preserves the global composition and acidity of parent mordenites. Zeolites etched in pyridine have a similar distribution of Brønsted acid sites (BAS) as the parent, i.e. 16% in 8-MR channels, 19% at the intersection between side pockets and 12-MR channels and 65% in the 12-MR channels. Etching in aqueous HF failed to preserve BAS distribution due to strong dealumination, which further leads to enlargement of side pockets. Mordenites etched with HF in pyridine allow for superior catalytic performance in both n-hexane cracking and toluene disproportionation. It has been evidenced that only the BAS located in the 8-MR side pockets prove active for the toluene disproportionation at 450 °C, while for n-hexane cracking at 541 °C BAS in both 12-MR channels and 8-MRs pockets catalyze the reaction, yet BAS in side pockets are more active (1.5 times). Moreover, Lewis acid sites generated during the post-synthetic treatments exalt the strength of the active sites, which multiplies TOFs of the active sites by two.