Synthesis of Nanostructured Molybdenum Carbide as Catalyst for the Hydrogenation of Levulinic Acid to γ-Valerolactone

Abstract

The effect of the morphology and size of unsupported molybdenum carbide (β-Mo₂C) was investigated in the selective hydrogenation of levulinic acid to γ-valerolactone (GVL) in aqueous phase. Nanostructured β-Mo₂C was synthetized by two different approaches: (i) using multiwalled carbon nanotubes (CNT) as both hard template and source of carbon and; (ii) using 1D nanostructured α-MoO₃ as precursor. Depending on the type of synthesis used, the morphology of the resulting β-Mo₂C was different. Well-oriented β-Mo₂C nanoparticles with a fibril morphology were formed when CNTs were used as hard template and source of carbon at 700 °C for 6 h under inert environment, while well-defined β-Mo₂C 1D nanostructures were formed after carburization of the nano-sized α-MoO₃ precursor at 650 °C/2 h under 20 % (v/v) CH₄/H₂ atmosphere. The catalytic performance of the materials was investigated at 30 bar H₂ and 180 °C in a batch reactor and compared with a Mo₂C synthesized by temperature-programmed carburization of commercial MoO₃. The β-Mo₂C 1D nanostructures presented a relatively higher activity than the others probably as a result of more exposed active sites, confirmed by the higher CO chemisorption uptake. All of the catalysts were highly selective to GVL (>85 %). Deep hydrogenation products such as 1,4 pentanediol and methyltetrahydrofuran were observed in minor amounts, underlining the hydrogenation potential of molybdenum carbide based materials.