Modification of Zeolite Morphology via NH₄F Etching for Catalytic Bioalcohol Conversion

Various commercial zeolites, including FER, MOR, ZSM-5, BEA, and FAU frameworks, were treated with NH4F aqueous solutions to study the effects of fluoride etching on different zeolite frameworks. NH4F-treated small-medium pore FER, MOR, and ZSM-5 samples showed much higher mesoporosities than the untreated ones without alteration of the structural compositions and acidic properties. On the other hand, the 12-membered ring zeolites BEA and FAU showed severe dissolution of the framework aluminosilicate structure after NH4F etching due to the high accessibility of fluoride species into the framework structures. The effect of NH4F concentration on the fluoride treatment of H-ZSM-5 zeolite was specifically studied. From the results, we observed that structural etching with 20 wt % NH4F was optimal for fabricating open-pore H-ZSM-5 zeolite and resulted in a high mesoporosity with comparable relative crystallinity and acidity with respect to the untreated H-ZSM-5. The catalytic activities of the open-pore H-ZSM-5 were evaluated with acid-catalyzed methanol and bioethanol conversions. Remarkably, the hierarchical open-pore H-ZSM-5 zeolite fabricated via fluoride etching exhibited an enhanced catalytic performance in bioethanol conversion with >85 % conversion over 34 h TOS and a higher catalytic stability in methanol conversion than the parent H-ZSM-5 (similar to 50 % of bioethanol conversion at 34 h TOS).