Liquid-Phase Furfural Hydrogenation over Ni/Alumina Catalysts

Furfural is an important platform chemical for producing value-added biobased molecules and materials as alternatives to fossil-derived chemical building blocks. Furfuryl alcohol (FALC) is one such valuable product, whose sustainable synthesis requires the catalytic reduction of furfural over Earth-abundant elements under mild conditions. Here, we report the liquid-phase hydrogenation of furfural over Ni nanoparticles prepared by either wet impregnation of alumina or exsolution from a NiAl layered double hydroxide (LDH). Exsolved and calcined Ni nanoparticles (NPs) spanned 11-18 nm, whereas the wet impregnation of [γ+δ]Al₂O₃ yielded large Ni particles (24-101 nm) indicative of weak metal-support interactions. All catalysts exhibited moderate acid loadings (0.3-0.9 mmol·g^-1) and weak basicity. Furfural conversion at 10 bar H₂ and 165 °C is inversely proportional to Ni particle size and structure-insensitive. Ni metal is the active site for furfural hydrogenation to FALC (specific activity of 84 mmol.g_(Ni)^-1·h^-1 for NiAl-LDH, six times faster than Al₂O₃-supported Ni analogues with similar loading, and superior to many precious metal catalysts). FALC was the primary product at isoconversion with 60% selectivity but prone to secondary reactions at high furfural conversion, notably hydrogenolysis to 2-methylfuran (2-MF) or ring hydrogenation to tetrahydrofuryl alcohol (THFA). THFA was itself susceptible to hydrodeoxygenation over small Ni NPs at 10 bar H₂ in the presence of an acidic support to form 2-methyltetrahydrofuran (2-MTHF) via a previously unreported pathway. Higher hydrogen pressures favored FALC ring hydrogenation to THFA. Furfural hydrogenation to FALC was structure-insensitive for Ni NPs spanning 11-101 nm; however, secondary reactions of FALC were structure-sensitive. LDH-derived catalysts with 11 nm Ni NPs achieved a high yield of 2-MTHF (73%), a green solvent, liquid electrolyte, and high-density fuel additive. Furfural inhibited ring hydrogenation of reactively formed FALC (versus its hydrogenolysis or HDO), suppressing THFA and 2-MTHF production. However, ring hydrogenation of reactively formed FALC is favored at 25 bar H₂, albeit with THFA, the dominant product.