Dehydra-Decyclization of Tetrahydrofuran on H-ZSM5: Mechanisms, Pathways, and Transition State Entropy

Butadiene is an important monomer for rubbery and hard polymeric materials, and it can be produced efficiently from biomass-derived tetrahydrofuran (THF) using solid-acid zeolite catalysts. In this work, electronic structure calculations, kinetic experiments, and microkinetic modeling were applied to investigate the THF dehydra-decyclization reaction to butadiene as well as its retro-Prins fragmentation to the side product propene on a Brønsted acid site within H-ZSM5. A comprehensive reaction network consisting of 15 elementary surface reactions was investigated, and a microkinetic model was parametrized using computed energetics to compare with experimental kinetic data. Among the proposed reaction pathways, THF dehydra-decyclization primarily proceeds via an alkenol intermediate species, 2-buten-1-ol, while retro-Prins fragmentation to propene occurs through a direct pathway. Two other alkenol species that could be involved in the reaction network, 3-buten-1-ol and 3-buten-2-ol, do not substantially contribute to THF conversion. While multiple elementary steps were found to be kinetically relevant, the Brønsted acid-catalyzed ring opening of THF is the predominantly rate-limiting surface reaction. The apparent activation energies (ca. 30 kcal mol^-1 for both butadiene and propene in the temperature range of 220-270 °C), reaction orders, and selectivity, as well as absolute rates predicted by the model are in agreement with experimental values, provided that the modeled entropy of activation is calculated to account for translational freedom for transition states that exhibit complete proton transfer from the solid acid site.