Reaction kinetics and pathways of p-xylene formation from 2,5-dimethylfuran (DMF) and ethylene via cascade reactions of Diels-Alder cycloaddition and subsequent dehydration over H-BEA zeolite (Si/Al = 12.5) were characterized. Two distinct kinetic regimes were discovered corresponding to the rate limiting reaction, namely Diels-Alder cycloaddition and cycloadduct dehydration, as the concentration of Brønsted acid sites decreases. At catalyst loadings with effective acid site concentrations exceeding a critical value (~2.0 mM), the rate of formation of Diels-Alder products becomes constant. Under these conditions, the measured activation energy of 17.7 ± 1.4 kcal mol -1 and reaction orders correspond to the [4 + 2] Diels-Alder cycloaddition reaction of DMF and ethylene. Conversely, at catalyst loadings below the critical value, the formation rate of p-xylene becomes first order in catalyst loading, and the measured activation energy of 11.3 ± 3.5 kcal mol -1 is consistent with dehydration of the Diels-Alder cycloadduct to p-xylene. Experimental comparison between H-BEA and H-Y zeolite catalysts at identical conditions indicates that the micropore structure controls side reactions such as furan dimerization and hydrolysis; the latter is supported via molecular simulation revealing a substantially higher loading of DMF within H-Y than within H-BEA zeolites at reaction conditions.
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© The Royal Society of Chemistry 2016.