Density Functional Theory Investigation of the Conversion of 5-(Hydroxymethyl)furfural into 2,5-Dimethylfuran over the Pd(111), Cu(111), and Cu₃Pd(111) Surfaces

The conversion of 5-(hydroxymethyl)furfural (5-HMF) into 2,5-dimethylfuran (2,5-DMF) via cascade hydrogenation and hydrogenolysis over metallic catalysts has been considered a promising method to produce high-energy-content biofuel. Understanding the adsorption of reactants, cascade reactions, and desorption of byproducts is essential for developing efficient and selective catalysts. Herein, the most plausible reaction mechanisms for the conversion of 5-HMF to 2,5-DMF over the Pd(111), Cu(111), and Cu3Pd(111) surfaces are investigated using density functional theory calculations. The reaction pathways for the formation of reaction intermediates (2,5-bis(hydroxymethyl)furan, 5-methylfurfural, and 5-methylfurfuryl alcohol (5-MFA)), 2,5-DMF, and byproducts (water, 2,5-dimethyltetrahydrofuran (2,5-DMTHF)) are established. The overall reaction barrier on the Pd(111) surface, which is governed by the hydrogenolysis of the C-OH bonds in 5-MFA, is larger (1.96 eV) than that on the Cu3Pd(111) surface (1.68 eV). In addition, the significantly higher adsorption energy of 2,5-DMF on the Pd(111) surface (-2.47 eV) than on the Cu3Pd(111) surface (-0.18 eV), which is caused by the flat adsorption geometry with a η2-(C-O)-aldehyde configuration, leads to the formation of 2,5-DMTHF via furan ring saturation. Even though the overall energy barrier on the Cu(111) surface (0.84 eV) is much lower than those on the Pd(111) and Cu3Pd(111) surfaces, the weak perpendicular adsorption of 5-HMF in a η1-(O)-aldehyde configuration, highly unfavorable dissociative H2 adsorption, and high energy required for the formation of H2O hinder the conversion of 5-HMF and its intermediates. The favorable adsorption of 5-HMF (-0.54 eV), low overall reaction barrier, facile desorption of 2.5-DMF (-0.18 eV), and low energy barrier for dissociative H2 adsorption render the Pd-Cu alloy catalyst a promising candidate for the selective conversion of 5-HMF to 2,5-DMF.