Improving the selectivity of nickel-based catalysts
for vapour-phase furfural hydrogenation

Abstract

Furfural is a key bio-based platform chemical, and its hydrogenation allows access to a variety of
important chemical intermediates, particularly furfuryl alcohol, which is widely used to make resins.
Here, nickel and tin-nickel catalysts have been thoroughly investigated for vapour-phase furfural
hydrogenation as a replacement for the highly carcinogenic and environmentally damaging current
commercial catalyst, copper chromite. The addition of tin to incipient wetness impregnation (IWI)
nickel-based catalysts (metal particle size: 9 ± 4 nm) resulted in a markedly improved selectivity to
furfuryl alcohol of up to 86 %; however mass activity was decreased by an order of magnitude. A
commercially available copper chromite catalyst was used for comparison and exhibited similar
mass activity and stability to the tin-doped IWI catalysts but with a slightly higher furfuryl alcohol
selectivity of around 90 %. To understand the selectivity change that occurred upon the
introduction of tin, more well-defined and uniform SnNi-based catalysts were prepared by
synthesising colloidal SnNi bimetallic nanoparticles. Smaller nanoparticles (~4 nm) could be
successfully synthesised using only an amine capping agent, however larger nanoparticles
(12 – 15 nm) required a phosphine capping agent. The use of a phosphine capping agent led to
phosphorus incorporation into the surface of the nanoparticles (as shown by near ambient pressure
X-ray photoelectron spectroscopy (NAP-XPS)), even when the bulk structure exhibited no evidence
of phosphorus incorporation. Silica supported small phosphorus-free SnNi nanoparticles (Ni:Sn
molar ratio of ~3) had a superior furfuryl alcohol selectivity (96 %) when compared to the IWI
catalysts, however significant sintering occurred in the in situ reduction step prior to reaction (i.e.
the active catalyst was sintered). The silica supported nanoparticles prepared with a phosphine
capping agent exhibited minimal to no sintering. The presence of phosphorus in nickel
nanoparticles led to an increased furfuryl alcohol selectivity (~67 %) when compared to IWI nickel
and amine-capped nickel nanoparticle catalysts (~50 %). The furfuryl alcohol selectivity of the
phosphine capped nanoparticles could be further improved by the introduction of small amounts
of tin, reaching around 92 % for nanoparticles with a Ni:Sn molar ratio of ~18. NAP-XPS was used
to investigate the structure and composition of the nanoparticles, revealing the top surface of both
the small phosphorus-free and larger phosphorus-containing nanoparticles consisted of a tin-nickel
phase with a Ni:Sn molar ratio of ~3, followed by a tin rich layer and then a nickel-based core. This
provides a potential explanation for the similar furfuryl alcohol selectivities achieved by the
nanoparticle catalysts, despite the very different bulk Ni:Sn molar ratios. Overall, the introduction
of tin to nickel catalysts was found to drastically improve the furfuryl alcohol selectivity during
vapour-phase furfural hydrogenation, affording a potential less toxic alternative to
chromium-based catalysts for a future bio-refinery process.