Bifunctional carbon Ni/NiO nanofiber catalyst based on 5-sulfosalicylic acid for conversion of C5/C6 carbohydrates into ethyl levulinate

Abstract

A method was developed for preparing bifunctional carbon Ni/NiO nanofiber catalysts that promote efficient one-pot conversion of C5/C6 carbohydrates into levulinate esters in alcohol solvents. The bifunctional catalysts were prepared via solvothermal carbonization of 5-sulfosalicylic acid/NiSO₄ without the use of sulfuric acid or hydrogen gas and had fine particle sizes (d = 5 nm to 50 nm) and contained –NH₂, –SO₃H, –COOH and phenolic –OH functional groups. Under optimal conditions, the catalysts afforded 93% selectivity of ethyl levulinate in ethanol with the major intermediate being 2-(ethoxymethyl)furan, 4,5,5-triethoxypentan-2-one and major byproduct being 2,5,5-triethoxpentan-2-one. Cooperative activity of Lewis acidity, Brønsted acidity and functional group sites of the catalyst is demonstrated for multi-step reaction sequences of C5/C6 carbohydrates with one-pot conversions and alcohols (methanol, ethanol, 1-propanol, 1-butanol) that act as both solvent and hydrogen donor source in which the bifunctional catalyst was shown to be recyclable five times with no apparent change in conversion and ca. 5% change in selectivity.