Continuous flow hydrogenolysis of 5-hydroxymethylfurfural into 2,5-dimethylfuran over alumina-supported nickel–iron alloy catalysts

Abstract

Catalytic transformation of furan derivatives into the next generation of biofuels in a continuous operation is a promising platform for application in sustainable biorefining. In this study, as-synthesized γ-Al₂O₃ supported-NiFe catalysts with different Ni to Fe molar ratios were applied for the continuous hydrogenolysis of 5-hydroxymethylfurfural (5-HMF) to 2,5-dimethylfuran (2,5-DMF). The Ni to Fe molar ratio of 0.74 : 0.97 revealed the highest yield of 2,5-DMF in comparison to monometallic NiAl and FeAl catalysts. The comprehensive characterizations disclosed that Ni added into Fe/Al₂O₃ catalysts alleviated the metal and supported the interactions and reduction behavior; however, the number of acidic sites was increased by the Ni species. The in situ XANES experiments confirmed the coexistences of major amounts of metallic Ni and Fe with minor amounts of NiO and FeO_x species in the bimetallic Ni_0.74Fe_0.97Al catalyst, while the alloy NiFe structure was noticed in the XRD analysis. Additionally, the time-resolved XANES investigations elucidated the evolution of the catalyst structure's transformation during the H₂-reduction process. Under the optimizing conditions of weight hourly space velocity (WHSV), the reaction temperature, and the hydrogen pressure, the Ni_0.74Fe_0.97Al catalyst gave a maximum 90.5% yield of 2,5-DMF with 100% 5-HMF conversion at a reaction temperature of 160 °C, H₂ pressure of 40 bar, and WHSV of 0.3 h⁻¹. The possible reaction mechanism was explained in detail based on the structural characterizations and major product distributions.