(520e) Understanding the p-Xylene Formation Mechanism from Dimethylfuran and Ethanol

Understanding the
p-Xylene Formation Mechanism from Dimethylfuran and
Ethanol

Pavlo Kostetskyy¹, Ivo Teixeira²,
Michail Stamatakis³, Shik Chi Edman Tsang²
and Giannis Mpourmpakis¹

¹Department of Chemical Engineering, University
of Pittsburgh, Pittsburgh, PA 15621, USA

²Wolfson Catalysis Centre, Department of
Chemistry, University of Oxford, Oxford, OX1 3QR, UK

³Department of Chemical Engineering, University
College London, Torrington Place, London WC1E 7JE, UK

Abstract:

Renewable,
biomass-based chemical feedstocks as alternatives to petrol-based commodities
are of high industrial relevance^1,2 and have attracted recent
attention. A number of platform chemicals have been identified as
industrially-important, including cyclic oxygenates such as 2,5-Dimethylfuran (DMF)^2,3,
which can be catalytically converted to p-xylene, an important precursor in the
production of polymers. Acidic heterogeneous catalysts can catalyze the
conversion of DMF to p-xylene by a Diels-Alder (DA) reaction with ethylene. The
reaction can be divided into two stages: DA cycloaddition and cycloadduct water
elimination (dehydration), with the DA being the rate-controlling step in the Bronsted
acid-catalyzed (BA) reactions³. In this work we propose an alternative
route for DMF conversion to p-xylene using BA zeolite catalyst and ethanol as
the dienophile source. Using electronic structure calculations, we elucidate
the detailed reaction pathways and associated energetics and demonstrate that
the ethanol-based pathway is preferred to ethylene in terms of overall reaction
rates. We demonstrate that the sequence of reactions in the ethanol route
follows a proton affinity⁴ thermodynamic preference of the reacting
species, initiated by ethanol dehydration to ethylene. Improved performance is
rationalized by the generation of a water molecule in this first step of the
mechanism. While absent in the ethylene route, the presence of additional water
in the system reduces the entropy loss in key elementary steps and facilitates
proton transfer reactions. As a result, activation free energy barriers for the
catalytic cycle decrease relative to the ethylene pathway. The calculated
activation free energy difference (using energetic span model) between the two
routes is in excellent agreement with kinetic experiments.

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Phys. Chem. C 2015, 119, 16139-16147