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.

Atmospheric carbon dioxide (CO₂) concentrations have increased rapidly in recent decades due to the burning of fossil fuels, deforestation, and other industrial practices. The excessive accumulation of CO₂ in the atmosphere leads to global warming, ocean acidification, and other environmental imbalances, which may ultimately have wider societal implications. One potential solution to closing the carbon cycle is utilizing CO₂, rather than fossil fuels, as the carbon source for fuels and chemicals production. This lowers atmospheric CO₂ levels while simultaneously providing an economic incentive for capturing and converting CO₂ into more valuable products. This dissertation includes studies on three hybrid catalytic reactor systems coupling electrochemistry, thermochemistry, and plasma chemistry for the conversion of CO₂ into value-added oxygenates, such as methanol and C3 oxygenates (propanal and 1-propanol). First, a tandem two-stage system is described where CO₂ is electrochemically reduced into syngas followed by the thermochemical methanol synthesis reaction. The work here specifically focuses on the electrochemical CO₂ reduction reaction to produce syngas with tunable H₂/CO ratios. Using a combination of electrochemical experiments, in-situ characterization, and density functional theory calculations, palladium-, gold-, and silver-modified transition metal carbides and nitrides were found to be promising catalysts for enhancing electrochemical activity while reducing the overall precious metal loading. Second, another tandem two-stage system is demonstrated where CO₂ is electrochemically reduced into ethylene and syngas followed by the thermochemical hydroformylation reaction to produce propanal and 1-propanol. The CO₂ electrolyzer was evaluated with Cu catalysts containing different oxidation states and with modifications to the gas diffusion layer hydrophobicity, while the hydroformylation reactor was tested over a Rh₁Co₃/MCM-41 catalyst. The tandem configuration achieved a C₃ oxygenate selectivity of ~18%, representing over a 4-fold improvement compared to direct electrochemical CO₂ conversion to 1-propanol in flow cells. Third, a hybrid plasma-catalytic system is investigated where CO₂ and ethane are directly converted into multi-carbon oxygenates in a one-step process under ambient conditions. Oxygenate selectivity was enhanced at lower plasma powers and higher CO₂ to C₂H₆ ratios, and the addition of a Rh₁Co₃/MCM-41 catalyst increased the oxygenate selectivity at early timescales. Plasma chemical kinetic modeling, isotopically-labeled CO₂ experiments, and in-situ spectroscopy were also used to probe the reaction pathways, revealing that alcohol formation occurred via the oxidation of ethane-derived activated species rather than a CO₂ hydrogenation pathway. It is critical to assess whether the proposed CO₂ conversion strategies consume more CO₂ than they emit. A comparative analysis of the energy costs and net CO₂ emissions is conducted for various reaction schemes, including four hybrid pathways (thermocatalytic-thermocatalytic, plasma-thermocatalytic, electrocatalytic-thermocatalytic, and electrocatalytic-electrocatalytic) for converting CO₂ into C₃ oxygenates. The hybrid processes can achieve a net reduction in CO₂ provided that low-carbon energy sources are used, however further catalyst improvements and engineering optimizations are necessary. Hybrid catalytic systems can provide an alternative approach to traditional processes, and these concepts can be extended to other chemical reactions and products, thereby opening new opportunities for innovative CO₂ utilization technologies.