(117d) 2,5-Dimethylfuran As a Model Compound to Investigate Hydrodeoxygenation of Complex Furanyl Compounds
Ying Lin Louie, Konstantinos Goulas, and Alexis T. Bell
Department of Chemical and Biomolecular Engineering,
University of California-Berkeley, Berkeley, California 94720
Several innovative pathways have been explored to produce diesel-range fuel additives via C-C bond coupling reactions of furanyl platform molecules, 5-hydroxymethylfurfural (HMF) and furfural (FUR), obtained from the dehydration of sugars [1, 2]. In each case, however, a final hydrodeoxygenation (HDO) step is required in order to produce a product that is oxygen-free, has a high cetane number, is compatible with conventional diesel fuel, and has minimal impact on the performance current diesel engines. While a number of studies of the hydrodeoxygenation of biomass-derived fuels have been reported , little is understood about the reaction pathways and how catalyst composition affects these pathways. The aim of this work is to develop such knowledge based on studies conducted with model compounds containing structural motifs representative of those found in second-generation biomass-derived diesel.
To understand the HDO chemistry of furanyl compounds and develop suitable HDO catalysts for this task, 2,5-dimethylfuran (DMF) was chosen as a model compound. The sequence of reactions by which DMF undergoes HDO was then investigated in both gas and liquid phase over various bi-functional metal/acid catalysts. Screening studies with carbon-supported Pt, Pd, and Rh revealed that Pt/C exhibits the highest C-O hydrogenolysis activity. Hydrogenation of DMF over Pt/C was found to proceed via parallel pathways of direct ring opening and ring saturation existed. It was observed that under mild reaction conditions, C-O bond hydrogenolysis of DMF to 2-hexanone is preferred over saturation of the furan ring to form 2,5-dimethyltetrahydrofuran (DMTHF). The resulting tetrahydrofuran ring is resistant to ring-opening to form ketones and alcohols. The distribution of products was found to be sensitive to the reaction conditions used. High temperatures and low H2 pressures were found to favor the furan ring-opening pathway, and under optimal conditions, we were able to obtain greater than 95% selectivity toward C-O hydrogenolysis products. Removal of oxygen from 2-hexanone and DMTHF occurred very effectively when Pt was supported on an oxide, such as TiO2, Nb2O5, and NbOPO4, which contain Lewis acid centers. Of the acid catalysts screened, niobium based oxides had stronger acidity and the highest ratio of Lewis acid to Brønsted acid sites , measured by infrared spectroscopy of adsorbed pyridine absorption. Pt/NbOPO4, in particular, was found to be exceptionally active for promoting the HDO of DMF to hexane. The results obtained from our studies of DMF have enabled us to optimize the HDO of more complex substrates, such as bis(5-methylfuran-2-yl)methane (BMFM, C11) and tetra(5-methylfuran-2-yl)methane (TMFM, C16). Greater than 81% yield to C11 was achieved during the HDO of BMFM at 250oC and 500psi H2, conditions that are much milder than those previously reported in the literature.
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