Investigating Reaction Mechanisms for Furfural Hydrodeoxygenation on Ni and the Effect of Boron Doping on the Activity and Selectivity of the Catalyst

Fast
pyrolysis is a promising route for the production of fuels
and high value chemicals from non-fossil fuel resources like biomass.
However, the design of active and inexpensive catalysts that can selectively
convert furfural, an important component in pyrolysis derived bio-oil,
to target chemicals remains a challenge. In this context, Ni-based
catalysts are potential candidates for the vapor phase activation
of furfural in the presence of H2. In this paper, mechanisms
and energetics (kinetics and thermodynamics) of the catalytic conversion
of furfural to furans, furfuryl alcohol, and C4 products
in the presence of H2 on Ni(111) are established, and the
experimentally observed change in the selectivity with temperature
is explained, using first-principles density functional theory. Hydrogen
adsorbs stronger than furfural on the Ni surface. At low operating
temperatures, hydrogen adsorption is spontaneous, leading to high
hydrogen surface coverages that favor furfural hydrogenation and decarbonylation
over ring-opening, to form furfuryl alcohol and furans. At higher
temperatures, hydrogen adsorption is not thermodynamically favorable,
leading to a relatively clean Ni surface on which furfural ring-opening
and decarbonylation are favored, leading to C4 products
and furans. We reveal that the incorporation of subsurface boron in
Ni leads to a corrugated catalyst surface (NiB) on which furfural
adsorbs stronger than hydrogen. The free energy barriers for the formation
of furans and C4 products are also considerably lower in
the presence of boron, suggesting an enhanced catalytic activity of
NiB. Thus, we propose that the boron-doped Ni catalyst is a potential
candidate for selectively converting furfural to furan and C4 products at lower operating temperatures, relative to Ni.