(621do) 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

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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.