Catalytic interplay of metal ions (Cu2+, Ni2+, and Fe2+) in MFe2O4 inverse spinel catalysts for enhancing the activity and selectivity during selective transfer hydrogenation of furfural into 2-methylfuran

Abstract

2-Methylfuran obtained via the hydrogenation of furfural is an important biomass-derived liquid fuel. However, the large-scale production of 2-methylfuran from furfural requires cost-effective, active, and selective heterogeneous catalysts. Herein, we have performed experimental and DFT investigations on MFe₂O₄ (M = Cu²⁺, Ni²⁺, and Fe²⁺) inverse spinel catalysts to selectively hydrogenate furfural into 2-methylfuran via transfer hydrogenation, and to determine the nature of active sites. CuFe₂O₄ afforded 99.4% furfural conversion and 97.6% 2-methylfuran selectivity at 200 °C in 1.5 h. In contrast, NiFe₂O₄ and Fe₃O₄ catalysts afforded furfuryl alcohol as a major product. The comprehensive characterization of the catalysts revealed that the acidity of these inverse spinel catalysts originates from the Lewis acid sites. Further, the strength of Lewis acid sites and the associated catalytic activity trend directly correlate with the binding energy of Feⁿ⁺ ions in MFe₂O₄ catalysts. DFT calculations revealed the energetically favorable interactions of furfural and furfural alcohol with the Fe³⁺ sites of Fe₃O₄ and Fe sites (most likely Fe³⁺) of CuFe₂O₄ and NiFe₂O₄ catalysts, whereas Cu²⁺ and Ni²⁺ sites present in CuFe₂O₄ and NiFe₂O₄ catalysts have stronger interactions with the isopropanol molecule. Hence, it is proposed that the Fe³⁺ sites present in CuFe₂O₄ are the active Lewis acid centers to selectively convert furfural into 2-methylfuran via transfer hydrogenation. Literature reports on the metal oxide catalyzed transfer hydrogenation of furfural into 2-methylfuran are rare. The present findings on the elucidation of the active sites of cost-effective, recyclable mixed metal oxide catalysts for the selective transfer hydrogenation of furfural into 2-methylfuran using isopropanol are attractive from the green chemistry perspective, and therefore extremely important for the academic catalysis community and to industrialists.