p-Xylene Formation by Dehydrative Aromatization of a Diels-Alder Product in Lewis and Bronsted Acidic Zeolites

Diels-Alder cycloaddition with furans as dienes and subsequent dehydrative aromatization are potentially valuable processes for sustainable conversion of biomass-derived furans to aromatics. We have performed electronic structure calculations to investigate the catalytic activity of HY and of alkali-exchanged Y zeolites in connection with the conversion of 2,5-dimethylfuran and ethylene to p-xylene. We have used two active site settings: an active site cluster model on which we have carried out density functional theory calculations and a mechanically embedded active site cluster model on which we have performed hybrid quantum mechanics/molecular mechanics calculations with the ONIOM scheme. Even though Lewis catalyzed Diels-Alder cycloaddition has received considerable attention over the years, we show that confinement and charge transfer in zeolite catalysts play a significant role in catalysis. Both HY and alkali-Y can catalyze the aromatization of the cycloadduct through dehydration but HY is found to be far more effective. Our analysis shows that the electron withdrawing ability of the cations and the catalytic activity of alkali-Y as Lewis acids are diminished by substrate binding-induced electron density shift from the framework oxygen atoms to the cations. On account of these inductive phenomena, we show that the DMF-ethylene cycloaddition follows a bidirectional instead of normal electron flow mechanism.