(195c) Acid-Catalyzed Conversion of Dimethylfuran to p-Xylene in Catalytic Zeolite-Y. A Combined Computational and Experimental Study

p-Xylene, conventionally produced by catalytic reforming of petroleum naptha, is used on a large scale for the manufacture of terephthalic acid for polyethylene terephthalate, which is used to make plastic bottles and clothing; several million tonnes are produced annually. We have recently proposed a renewable pathway to p-xylene from biomass-derived carbohydrates by reacting dimethylfuran (DMF) with ethylene in zeolite-Y catalyst.^[1] The reaction proceeds through Diels-Alder cyclo-addition and subsequent dehydration of the resulting oxa-norbonene derivative cyclo-adduct.

In this paper, we use electronic structure calculations to investigate the mechanism and energetics of the reaction under different types of acid catalysis – Lewis vs Brønsted –  map out the network of the most important side reactions and, based on the calculated activation and reaction enthalpies, explore how the selectivity to p-xylene may be optimized via rational design of bifunctional catalysts. Some of the key findings that we shall present include:

The Diels-Alder (DA) reaction proceeds through a concerted mechanism and its rate is accelerated in the case of Lewis acid catalysis. In contrast, we find that Brønsted catalysis of the DA is not feasible and, in fact, adsorption of the dienophile (DMF) on a Brønsted site activates side reactions. The dehydration of the oxa-norbonene derivative cyclo-adduct to p-xylene requires Brønsted catalysis.

Two major side reactions that affect the selectivity to p-xylene are: (1) DMF hydrolysis to 2,5-hexanedione, a reaction that is catalyzed by Brønsted acids; and (2) Further DA cyclo-addition between DMF and the oxa-norbonene intermediate and its isomers.

Finally, by comparing the calculated activation barriers of the pathway from DMF to p-xylene with experimentally observed turnover frequencies, we conclude that the H-Y zeolite is more active than the zeolite-Y with fully exchanged protons with alkali cations (i.e., zeolite-Y with Lewis-acid activity).

[1]          C. Luke Williams, Chun-Chih Chang, Phuong Do, Nima Nikbin, Stavros Caratzoulas, Dionisios G. Vlachos, Raul F. Lobo, Wei Fan, Paul J. Dauenhauer. ACS Catalysis, 2012 , 2, 935-939