Switching Hydrogenation Pathways of Furfural via Reduction-Degree Engineering of Ni-Based Catalysts
The selective hydrogenation of biomass-derived furfural (FAL) to high-value chemicals such as furfuryl alcohol (FOL) or tetrahydrofurfuryl alcohol (HFOL) is pivotal yet challenging due to the need for precise control over reaction pathways. In this study, a Ni2Al-LDO (layered double oxide) catalyst with highly dispersed surface NiO was synthesized via structural topological transformation of layered double hydroxides. The catalyst exhibited excellent performance in furfural hydrogenation, achieving a 91.42% yield of FOL at 160 °C and 1.4 MPa H2. Gradual reduction of Ni2Al-LDO produced Ni/NiO mixtures, enabling a tunable shift from FOL to HFOL as NiO content decreased and metallic Ni content increased. After reduction at 700 °C for 2 hours, the HFOL yield reached 93.95% under identical conditions. CO2-TPD, NH3-TPD, and FT-IR analyses revealed that variations in reduction degree influenced furfural adsorption behavior. NiO species selectively adsorb the C=O group of furfural, with isopropanol serving as the hydrogen source via the Meerwein-Ponndorf-Verley (MPV) pathway, yielding FOL. In contrast, metallic Ni0 surfaces facilitate flat adsorption, enabling simultaneous activation of both the furan ring and carbonyl group, and can activate both H2 and isopropanol, with H2 as the primary hydrogen source, leading to complete hydrogenation to HFOL. This work elucidates a clear structure-activity relationship centered on the metal oxidation state and provides a practical reduction-engineering approach for designing adaptable catalysts in biomass upgrading.