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

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.