Density functional theory studies of furfural hydrodeoxygenations on various catalysts

Abstract

Furfural conversions via hydrodeoxygenation pathways were thoroughly investigated by using density functional theory (DFT) calculations on the Ru/Co3O4 surface, Re/Pt bimetallic system, and Ni2P (0001) surface, respectively. On the Ru/Co3O4 surface, it was found that an oxygen vacancy was necessary to be generated in the form of water for the subsequent hydrodeoxygenation of furfural. In order to generate 2-methylfuran, the reaction initiated from hydrogenation of furfural into furyl–CH2O alkoxide intermediate, followed by C–O bond cleavage, and finally the hydrogenation of the unsaturated furyl–CH2 species. This reaction pathway was both kinetically and thermodynamically facile. Comparing with the group X metals and ruthenium, the decarbonylation pathway to produce furan and carbon monoxide was inhibited on Ru/Co3O4 surface by the adsorption geometry. In the Re/Pt bimetallic system, it was found that the kinetically favoured product was furfuryl alcohol, while the 2-methylfuran and water were the thermodynamically favoured products. Based on the calculations, the hydrodeoxygenation product 2-methylfuran was achievable via the hydrogenation of furfural into hydroxyalkyl species, followed by C–OH bond cleavage, and successive hydrogenations of the furyl–CH intermediate. However, the production of 2-methylfuran was prohibited as the oxidised Re surface cannot accept further oxygen deposition, because the removal of oxygen in the form of water via hydrogenations was difficult at the experimental condition. Comparing the results from the Re/Pt system with those on a monometallic flat Pt surface, we were able to demonstrate that incorporation of the oxophilic metals to active hydrogenation metals could promote the hydrodeoxygenation route by reducing the barrier of C–O bond cleavage. As the Ni3P2-termination was more stable than the Ni3P1-termination in Ni2P (0001) surface, it was mainly focused in the calculations. The generation of 2-methylfuran was favoured via hydrogenation to hydroxyalkyl species, followed by the cleavage of C-OH bond and successive hydrogenations, which indicated that the furfuryl alcohol was not a necessary intermediate for the 2-methylfuran formation. During the further conversion of 2-methylfuran, the ring-hydrogenation pathway to generate 2-methyltetrahydrofuran was kinetically favoured than the ring-opening pathway to produce 2-pentanone. The formation of difurfuryl ether could not be achieved without the participation of phosphorus. The adsorption geometry of furfural was the main factor which inhibited the decarbonylation reaction to generate furan on Ni3P2-termination. Based on the comparisons of these three systems for the hydrodeoxygenation of furfural, Ru/Co3O4 system was the best choice which led to a high selectivity of the desire product 2-methylfuran while inhibited the by-products formation.