An investigation of reaction parameters for carbon dioxide utilisation
2017-05-08T11:35:41Z (GMT) by
Carbon dioxide emissions per year have risen exponentially. It is widely known the contribution of CO2 to global warming phenomena, so storage/utilisation of carbon dioxide has become a topical issue and an emerging research area. Despite the fact that utilization of CO2 waste would not solve the problem of the huge quantities going to the atmosphere every year as only less than 1% of it could be reused for the industry, recycled carbon dioxide presents itself as a possible cheap and accessible chemical feedstock. The challenge on recycling CO2 is to minimize energy and cost efficiency of any suitable reaction. On previous investigations the electrochemical synthesis of 5-membered cyclic carbonate from epoxides was accomplished under mild conditions and optimized (1 atm CO2 pressure, 60 mA constant current and 50 °C heating). In order to understand the mechanism of this electrochemical process a deep investigation on the variables of the synthesis of cyclic carbonates was carried out and is presented in this thesis. The variables studied include electrochemical system conditions (application of current through Cu/Mg electrodes, electrodes connected on a closed circuit system with no current, an open circuit system where electrodes were there was no connection between them, and reactions without electrodes), temperature of reaction, solvent screening, catalysts, epoxide substituents, concentration of species and ratio of reactants. As a result of the variables optimization, a new, cheap, simple and relatively fast method (5 to 24 hours of reaction time) for cyclic carboxylation of epoxides with CO2 at atmospheric pressure in acetonitrile in the presence of ammonium salt (TBAI) at mild temperatures (50 - 75 °C) has been developed and improved. The concentration of the reactants, especially of the epoxide, was found to be the most important factor on the success of the reaction. The new reaction conditions also allow converting epoxides to carbonates without the help of any cocatalyst or electrochemical system obtaining excellent yields (50-100%) with the important saving on cost and energy of co-catalyst synthesis and recovery. Chlorostyrene oxide (1 M) reacted almost completely (94%) after 24 hours with TBAI (1 M), in 1 mL of acetonitrile at 75 °C and 1 atm pressure of CO2. Epoxide carboxylation under neat conditions was feasible, producing 44% of chlorostyrene carbonate from chlorostyrene oxide in the presence of TBAI at 75 °C and 1 atm pressure of CO2.