(511g) Towards Catalyst Design for the Conversion of 2,5-Dimethylfuran to p-Xylene

In this paper we use electronic structure calculations to investigate the mechanism and energetics of the synthesis of p-xylene by Diels-Alder [4+2] cycloaddition between 2,5-dimethylfuran (DMF) and ethylene and subsequent dehydration of the resulting oxa-norbornene derivative. Using the zeolite Y as a catalyst, this two-step process was recently demonstrated to be highly selective to p-xylene ^[1] and holds great promise for the sustainable synthesis of aromatics from renewables such as biomass, since DMF can be obtained from carbohydrates.

We have carried out mechanistic studies of the two main reactions in the presence of Lewis and Brønsted acid zeolite Y catalysts, which we have modeled with active site clusters. ^[2] We shall show that only alkali-exchanged zeolites Y can accelerate the rate of the cycloaddition and that Brønsted catalysis is beneficial only to the dehydration of the resulting oxa-norbornene derivative, a reaction that cannot proceed uncatalyzed. Using our data, we shall argue that the proposed synthesis of p-xylene from DMF and ethylene over the zeolite HY is rate-limited by the uncatalyzed Diels-Alder cycloaddition, and thus explain why kinetic studies show that the rate of p-xylene production over HY is independent of the density of active sites. In addition, we have mapped out the mechanism of the most important side reaction – the Brønsted-catalyzed hydrolysis of DMF and shown, for the first time, that the hydrolysis of furans follows general acid catalysis. Finally, by combining the calculated activation and reaction energies with microkinetic modelling, we shall suggest strategies toward catalyst design.

[1]           C. L. Williams, C.-C. Chang, P. Do, N. Nikbin, S. Caratzoulas, D. G. Vlachos, R. F. Lobo, W. Fan, P. J.
                 Dauenhauer, ACS Catal., 2012, 2, 935-939

[2]           N. Nikbin, P. T. Do, S. Caratzoulas, R. F. Lobo, P. J. Dauenhauer, D. G. Vlachos, J. Catal., 2013, 297,
                35-43