Highly selective, sustainable synthesis of limonene cyclic carbonate from bio-based limonene oxide and CO2: A kinetic study

Abstract

Bio-derived cyclic carbonates are of significant research interest as building blocks for non-isocyanate polyurethanes (NIPUs). Cyclic carbonates from limonene are bio-renewable monomers for the production of fully bio-based polymers from citrus waste; however, there are currently very few reports on their synthesis. This work reports the synthesis of five-membered cyclic carbonates from bio-based limonene oxide (LO) and CO2 catalysed by commercially available inexpensive, tetrabutylammonium halides (TBAX). The cycloaddition of CO2 with commercial LO mixture of cis/trans-isomers (40:60) is highly stereoselective and the trans-isomer exhibits considerably higher conversion than the cis-isomer. Therefore, a stereoselective method of (R)-(+)-limonene epoxidation was performed to achieve a significantly higher yield of the trans-isomer (87 ± 2%) than cis-isomer, which leads to high conversion and yield to the corresponding cyclic carbonates. The catalytic effect of halide anions (X¯) and the influence of operational reaction parameters such as temperature, pressure, and catalyst amount were studied. High conversion (87%) was obtained after 20 h at 120 °C, 40 bar CO2 using 6 mol% tetrabutylammonium chloride (TBAC) catalyst. A detailed study of the reaction kinetics revealed the reaction to be first-order in epoxide (LO), CO2 and catalyst (TBAC) concentrations. Moreover, the temperature dependence of the reaction was studied using Arrhenius and Eyring equations. The activation energy (Ea) of the reaction was calculated to be 64 kJ mol–1. The high positive value of Gibbs free energy (ΔG‡ = 102.6 kJ mol–1) and negative value of activation entropy (ΔS‡ = –103.6 J mol–1) obtained as result of the thermodynamic study, indicate that the reaction was endergonic and kinetically controlled in nature.