Synthesis of limonene and styrene carbonate via CO2 cycloaddition.

Abstract

Global concerns about high CO2 levels and the dwindling supply of fossil resources are increasing. As a consequence, the utilization of CO2 and waste biomass as renewable resources into valuable products is highly desirable as part of a sustainable future of the chemical process industry. Synthesis of cyclic carbonates by CO2 cycloaddition to epoxides is a highly promising reaction in terms of “green chemistry” due to its 100% atom-economy. This work reports bio-based limonene carbonate (LC) synthesis by CO2 cycloaddition to terpene-derived limonene oxide (LO). The reaction was carried out with high stereoselectivity using commercially available, inexpensive tetrabutylammonium chloride (TBAC) as a homogeneous catalyst. High yield (86%) of limonene carbonate (LC) was obtained at 140 °C, 40 bar CO2 pressure using 6 mol% TBAC after 20 h. Moreover, the detailed study of reaction kinetics revealed first order dependence with respect to LO, CO2 and TBAC concentrations. Bio-derived cyclic carbonates are of significant research interest as building blocks for non-isocyanate polyurethanes (NIPUs). Synthesis of cyclic carbonate from epoxide and CO2 is a typical gas-liquid multiphase catalytic process involving a gas-liquid mass transfer in the reactor and catalytic cycloaddition reaction in the liquid phase. Therefore, it was anticipated that an efficient reactor design and a continuous flow approach could mitigate many of the limitations observed in the batch reactor. In this study, a continuous styrene carbonate (SC) synthesis from styrene oxide (SO) and CO2 was demonstrated in a ‘tube-in-tube’ gas-liquid reactor. The exceptionally high permeation of CO2 through the membrane, combined with the high surface area to volume ratio resulted in the quantitative conversion of SO in a significantly reduced reaction time i.e. 45 min at 120 °C and 6 bar p (CO2) using a 1:4 ZnBr2/TBAB catalyst ratio. Synthesis of SC was also investigated in the presence of pyrrolidinopyridinium iodide (PPI) in combination with zinc halides (ZnX2) as a binary homogeneous catalyst system. The synergistic effect between PPI and ZnI2 resulted in a ~10-fold increase in the reaction rate compared to using PPI alone as a catalyst. The potential for the carbonation reaction was further intensified by heterogenising the PPI onto silica. An increase in catalytic activity was observed due to the synergistic effect between halide anion and acidic Si–OH (silanol) surface. In all cases, the detailed studies of reaction kinetics were carried out to determine the corresponding kinetic values (k and Ea) and thermodynamic activation parameters (ΔH‡, ΔS‡ and ΔG‡).