High yield of ethyl valerate from the esterification of renewable valeric acid catalyzed by amino acid ionic liquids

Abstract

Ethyl valerate (EV) as a promising fuel additive was produced by esterification of valeric acid with ethanol over Brønsted acidic amino acid ionic liquids. Hammett method and density functional theory (DFT) calculations were preformed to evaluate the acidities of the catalysts. The composition of catalyst, reaction temperature, reaction time, molar ratio of reactants, amount of catalyst, and recycling ability of the catalyst were investigated. Proline bisulfate (ProHSO₄) ionic liquid has the highest catalytic activity and the best recyclability under the optimized esterification conditions. A high conversion of valeric acid (>99.9%) was obtained for 7 h at 80 °C, with 100% selectivity of EV. The density, viscosity, melting point, boiling point, elemental analysis and heat of combustion of the EV product were measured. The density of EV is 0.896 g cm⁻³. The viscosity of EV was 1.7 cP at room temperature. The heating values of EV are 4158.1 kJ mol⁻¹ and 31.9 kJ g⁻¹. EV obtained from esterification has higher energy density than methanol, ethanol, γ-valerolactone, and valeric acid, which illustrates that EV is a promising biofuel candidate.