Catalytic Pathways for Furfural Hydrogenation on Transition Metals in Aqueous Medium

Catalytic Pathways for Furfural Hydrogenation on Transition Metals in Aqueous Medium

Junnan Shangguan and Ya-Huei (Cathy) Chin*

Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada.

  *cathy.chin@utoronto.ca

Furfurals are derivatives from xylose dehydration and are precursors used for synthesizing sustainable chemicals and fuels in catalytic hydrogenation and oxygen removal reactions [1]. Here, we report the catalytic pathways and kinetic requirements for furfural hydrogenation in aqueous medium under mild conditions (0-60 bar, 413-523 K) on Group VIII metal clusters (Ru, Pd, and Pt) dispersed on high surface area carbon supports. Furfurals undergo C=O bond hydrogenation to furfuryl alcohol intermediates, followed by their sequential reactions via the four competitive steps of: (1) direct hydrogenation of the C=C bonds in the furanic ring, producing tetrahydro-2-furanmethanol (Pathway 1), (2) ring-rearrangement reaction with water, producing cyclopentanol (Pathway 2), (3) deoxygenation via C-O bond cleavage, producing 2-methylfuran (Pathway 3), and (4) polymerization, forming water insoluble dimer/trimers, which precipitate into the solid phase (Pathway 4). On Ru clusters, the ring-rearrangement pathway (Pathway 2) is the favorable pathway for furfural reactions at low temperatures (413 K, 79% carbon selectivity, 0.42 M furfural, 50 bar H₂, 0.1g Ru/C) whereas the C=C bond hydrogenation of the furanic ring (Pathway 1) remains kinetically insignificant (carbon selectivity 4%). As the temperature increases from 413 K to 523 K, furfural conversion concomitantly increases from 16% to 91% (0.42 M furfural, 50 bar H₂, 0.05 g Ru/C). The increase in temperature promotes polymerization (Pathway 4, 85% carbon selectivity at 523 K) and leads to carbon losses from liquid phase, as polymerization products precipitate. A change in the metal identity from Ru to Pd or Pt affects the overall turnover rates and selectivities. Pd is more effective for furfural activation than Pt and Ru (, where  denotes the pseudo 1^st order rate constant for furfural conversion at 0.42 M furfural, 413 K, 50 bar H₂, 0.05 g Pd/C and Pt/C or 0.1 g Ru/C) and the major catalytic pathways on Pd are Pathways 1, 2, and 4 (carbon selectivities 23%, 39%, and 38%, respectively). Differed from Ru and Pd, Pt catalyzes both the ring-rearrangement reactions (Pathway 2, carbon selectivity 44%) and the condensation reaction that forms dimeric and trimeric species (Pathway 4, carbon selectivity 54%). The knowledge of these catalytic pathways and the effects of metal identity on rates and selectivities allow us to design catalysts for the selective conversion of furfurals to retain their carbon atoms in the liquid phase while suppressing the undesired polymerization reactions.

References

[1] J.Q. Bond, A.A. Upadhye, H. Olcay, G.A. Tompsett, J. Jae, R. Xing, D.M. Alonso, D. Wang, T. Zhang, R. Kumar, A. Foster, S.M. Sen, C.T. Maravelias, R. Malina, S.R.H. Barrett, R. Lobo, C.E. Wyman, J.A. Dumesic, G.W. Huber, Production of renewable jet fuel range alkanes and commodity chemicals from integrated catalytic processing of biomass, Energy & Environmental Science, 7 (2014) 1500-1523.