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.