Tuning ZSM-22 zeolite through NH4F etching enables enhanced performance and tailored product distribution in n-alkane isomerization for sustainable aviation fuels

Abstract

Isomerization of n-alkane remains a significant strategy for producing high-quality sustainable aviation fuels (SAF). Improving zeolite catalyst performance and optimizing iso-product distribution to meet the SAF specification are still challenging. As the limitation mainly originated from the strict shape selectivity of the microporous zeolite structure, feasible methods were highly demanded. In this work, the NH4F etching approach was utilized to controllably regulate the framework structure of ZSM-22 zeolite, leading to the rational variation in the crystallinity, porosity, morphology, and acidity, which were closely correlated with the etching severity. Compared to the elliptical 10-membered ring micropore mouths of the parent zeolite, the micropore mouths of the etched ZSM-22 zeolite tended to be circular, as observed by HRTEM and strongly supported by Raman, realizing the regulation of zeolite structure. For n-decane isomerization, the etched catalysts exhibited a considerable increase in catalytic performance, surpassing the parent Pt/ZSM-22 catalyst and the highest isomer yield of 82.1% at the conversion of 91.4% was achieved. The optimized iso-product distribution with approximatively equal amounts of several mono-branched isomers and much higher contributions of the originally disfavored isomers was also observed due to the tuned shape selectivity and the balanced bifunctionality. This work presented an efficient method to design zeolite catalysts for linear paraffin isomerization beyond the limit set by the zeolite structure and shape selectivity that meets the strict standard specification for the freezing point of the SAF.