Furfural is formed from biomass and can be converted to other useful compounds. In particular, it has been suggested, based on the results from first-principles density functional theory calculations (Wang et al, ACS Catalysis, 5 (2015) 104-112) that the hydrogen pressure during the catalytic hydrogenation of furfural can influence the selectivity such that furan and tetrahydro furan (THF) are formed preferentially at low hydrogen pressures, while higher hydrogen pressures increase the selectivity to 2-methyl furan. Further, this is proposed to be caused by co-adsorbed hydrogen changing the orientation of adsorbed furfural, with low hydrogen coverages favoring flat-lying adsorption, while higher coverages inducing the formation of tilted species. The proposal is tested by carrying out catalytic reactions on a model Pd(111) single crystal to most closely mimic the system on which the quantum calculations were performed. Measuring the product distribution under catalytic conditions yielded products that were in accord with the postulate, with 100% selectivity to furan and tetrahydrofuran for hydrogen pressure below ~50 Torr, with the selectivity to 2-methyl furan increasing as the hydrogen pressure increases. The origin of the effect is explored by investigating the effect of co-adsorbed hydrogen on the structure of furfural on the surface of the Pd(111) model catalyst by reflection-absorption infrared spectroscopy. This showed that co-adsorbed hydrogen did influence the structure of the co-adsorbed furfural in a way predicted by theory.

Understanding the effect of hydrogen pressure on the selectivity of furfural conversion

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