Tandem Diels-Alder Reaction of Dimethylfuran and Ethylene and Dehydration to para-Xylene Catalyzed by Zeotypic Lewis Acids

The zeotypic Lewis acids Sn-BEA, Zr-BEA, and Ti-BEA have recently been reported to catalyze the synthesis of p-xylene by dehydrative aromatization of the Diels-Alder product between 2,5-dimethylfuran and ethylene. Although it has been shown that these Lewis acids can catalyze the dehydration of the Diels-Alder cycloadduct, the tandem scheme precludes decoupling of the two steps needed to infer whether these same catalysts can catalyze the Diels-Alder step. We have employed electronic structure calculations and microkinetic modelling to investigate the Diels-Alder aromatization of 2,5-dimethylfuran and ethylene to p-xylene over the Lewis-acidic zeotypes Sn-, Zr-, and Ti-BEA. We show that there is only minor catalysis of the Diels-Alder reaction, solely attributable to phenomena varying with the translational freedom allowed to the species inside the zeolite. Microkinetic modelling and sensitivity analysis of the computed rates show that the heterogeneous Diels-Alder pathway does not contribute to the overall rate, and that the homogeneous cycloaddition is rate-limiting at high acid site concentrations. Only the partially hydrolyzed (open) Lewis acid sites are found to be catalytically active, with moderately Bronsted-acidic silanol groups formed, which catalyze 2,5-dimethylfuran hydrolysis. Of the Lewis acids tested in this work, Zr-BEA and Sn-BEA have similar activities, in agreement with experiment, whereas Ti-BEA is found to be inactive, suggesting that the recently reported Ti-BEA activity was likely a result of Bronsted-acidic defect sites.