Hydrogen gas-free processes for single-step preparation of transition-metal bifunctional catalysts and one-pot γ-valerolactone synthesis in supercritical CO2-ionic liquid systems

Abstract

Hydrothermal carbonization of glucose (180 °C, 4 h) with 5-sulfosalicylic acid and nickel or copper sulfate afforded transition-metal (Ni/NiO, Cu/CuO) functional carbon (FC) catalysts in a single-step without hydrogen gas. Hydrogenation of levulinic acid to γ-valerolactone (GVL) in supercritical carbon dioxide (scCO2)-ionic liquid ([BMIM]Cl) systems with formic acid as H-donor source and Ni/NiO-FC catalysts gave 97% GVL yields (170 °C, 3 h). The Ni/NiO-FC catalysts (d = 50 to 200 nm) had well-dispersed Ni/NiO particles (<5 nm) with –SO3H, COOH and phenolic −OH functional groups; Ni/NiO-FC catalysts were more effective than Cu/CuO-FC catalysts. Ni/NiO-FC catalysts were active for conversion of substrates (ethyl levulinate, fructose, cellobiose or cellulose) to respective products (GVL, 5-HMF, sugars). The role of scCO2 in the reaction system is one of improving mass transport and suppressing side-reactions via GVL product removal. Proposed methods for catalyst synthesis and substrate hydrogenation do not require hydrogen gas and are widely applicable to processing biomass.