This study gives insights into the advantages and constraints of post-synthetic NH₄F treatments in the effective production of hierarchical zeolite catalysts.

Abstract

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 (~50 % of bioethanol conversion at 34 h TOS).