(72b) Kinetic Analysis of Hydrogenation and Hydrogenolysis of Complex Furans over Supported Noble Metals

Upgrading furan-based compounds through mild hydrotreatment is an important step to produce biomass-derived commodity chemicals and transportation fuels. Biomass derived functionalized furans, such as furfural or 5-hydroxyfurfural, are used in a variety of carbon â?? carbon bond coupling chemistries to produce products in the jet, diesel and lubricant range has also been established [2-4]. These products typically retain the furan functional group, which require further hydrotreatment in order to produce hydrocarbons or valued-added oxygenates.

The kinetics and mechanism of hydrogenating the furanyl moiety over carbon-supported noble metals were previously studied using 2,5-dimethylfuran (DMF) as a model substrate, in liquid phase at conditions removed of both internal and external mass transfer effects (333K - 363K, p_H2 = 0.4-0.7MPa). Of the catalysts studied, Pt/C had the greatest activity for C-O hydrogenolysis, producing 2-hexanone as the major product, whereas Pd/C exclusively saturated the aromatic ring into its tetrahydrofuran analog. We proposed a reaction network, supported by a Langmuir Hinshelwood kinetic model, in which not only are ring opening and ring saturation of DMF competitive pathways, but the three main products observed: 2,5-dimethyltetrahydrofuran, 2-hexanone, and 2-hexanol result directly from hydrogenation of DMF over Pt/C and not from hydrogenation in series.

We apply the findings made from our model system to understand the hydrogenation behavior of more complex, substituted aromatic furans with carbon numbers in the diesel range, such as bismethylfuranylmethane (BMFM), a C₁₁ compound containing two adjacent furan moieties. Similar to DMF, ring opening and ring saturation of the aromatic ring of BMFM occurred in parallel, with the former pathway preferred at elevated temperatures and low p_H2 conditions. This behavior was determined to be due to the high activation energy and a low rate order dependence on p_H2 associated with the ring-opening step of DMF. Contrary to DMF, BMFM hydrogenation activity over Pt/C was sluggish when compared to activity over Pd/C, in which rate of conversion occurred 8-fold faster to produce not only ring saturated cyclic ethers, but also significant quantities of ring-opened alcohol products (~30% yield). C-O bond hydrogenolysis over Pd/C occurred selectively on the interior, aromatic C-O bond as opposed to the more accessible exterior C-O bond, accounting for 96% of the ring opened products. These differences in hydrogenation activity of BMFM suggest that geometry of the furanic substrate plays a dominant role in the reaction network.

References

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