Synthesis of styrene carbonate (SC) via the fixation of CO2 with styrene oxide (SO) has been investigated using a combination of zinc bromide (ZnBr2) and tetrabutylammonium halides (TBAX) as acid-base binary homogeneous catalysts. The combination of ZnBr2 and TBAB had a synergistic effect, which led to about 6-fold enhancement in the rate of SC formation as compared to using TBAB alone as a catalyst. Propylene carbonate (PC) was chosen as a green solvent for a comprehensive study of reaction kinetics. The reaction followed a first-order kinetics with respect to SO, CO2, and TBAB, whereas a fractional order was observed for the ZnBr2 when used in combination with the TBAB. Arrhenius and Eyring’s expressions were applied to determine the kinetic and thermodynamic activation parameters, where activation energy (Ea) of 23.3 kJ mol−1 was obtained for the SC formation over the temperature range of 90-120°C. The thermodynamic analysis showed that positive values for enthalpy (ΔH‡ = 18.53 kJ mol−1), Gibbs free energy (ΔG‡ = 79.74 kJ mol−1), whereas a negative entropy (ΔS‡ = –162.88 J mol−1 K−1) was obtained. These thermodynamic parameters suggest that endergonic and kinetically controlled reactions were involved in the formation of SC from SO and CO2.
The development of a limonene epoxidation process using environment-friendly H2O2, with high H2O2 conversion (~95%) and selectivity to the epoxide (100%), is reported in this paper. Parametric studies of temperature, oxidant, solvent, acid concentration and sodium sulphate amounts were performed with the focus on establishing a rapid and highly selective process. Approximately 95% conversion of H2O2 at 100% selectivity to limonene-1,2-epoxide was achieved in 15 minutes with a single-step addition of oxidant.The operating conditions included a 323 K temperature in a solvent-free environment, with a limonene/H2O2/catalyst molar ratio of 4:1:0.005, using a tungsten-based polyoxometalates. To prevent the hydrolysis of the epoxide, the reaction mixture was saturated with sodium sulphate. An acid concentration of lower than 0.04 M was used and found to have significant effect on the selectivity. Kinetic studies were performed to allow modelling of the reaction scheme. The activation energy was determined to be ~36 kJ mol -1 .
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