Selective Production of Para-Xylene from Biomass-derived 2,5-Dimethylfuran through Tandem Diels–Alder/Dehydration Reactions with a Bifunctional Ga,Al-Zeolite Catalyst

Abstract

Here we demonstrate that Ga,Al-*BEA zeolites are effective bifunctional catalysts for para-xylene (p-xylene) production from bio-derived 2,5-dimethylfuran (DMF) through tandem Diels-Alder/dehydration reactions. A series of catalysts was synthesized via direct (one-pot) and post-synthesis techniques to introduce Brønsted and Lewis acid sites. The synthesis approach employed in this study avoids cost- and time-intensive processes typically associated with the preparation of metal-substituted *BEA zeolite catalysts for p-xylene production. Our findings reveal that Ga,Al-*BEA catalysts enhance DMF con-version and p-xylene selectivity in comparison to Al-*BEA zeolite. The pairing of Ga and Al in a single catalyst yields fewer byproducts, such as 2,5-hexanedione, 1-methyl-4-propyl-benzene, alkylated products, and oligomers. Comparisons of zeo-lites prepared with different Ga content reveal a higher turnover frequency for DMF conversion to p-xylene over Ga,Al-*BEA catalysts prepared by a one-pot synthesis compared to Al-*BEA catalysts. We observed a correlation between Ga content and p-xylene selectivity and yield, which is attributed to the Brønsted acidity of Ga framework sites (with reduced acid strength compared to Al sites) and to the Lewis acidity of extra-framework Ga species. The latter contribution was confirmed by analysis of Ga-impregnated Al-*BEA zeolites, which are less active than framework species but have a positive effect on p-xylene selectivity. Our collective findings indicate that tuning zeolite acidity by optimizing the amount of heteroatom incorporation in the crystal framework to tailor the speciation and strength of acid sites is beneficial to maximize p-xylene production from renewable resources.