Fixation of carbon dioxide into epoxides to form cyclic carbonates using silicate based catalysts
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This study aimed at developing silicate based catalytic systems for the fixation of carbon dioxide (CO2) into epoxides to form cyclic carbonates. Catalytic activity of selected naturally occurring silicates including: biotite, chlorite, muscovite, phlogopite, talc, vermiculite and synthetic zeolites was evaluated for the cycloaddition of CO2 and epoxide. Effect of reaction parameters such as CO2 pressure, temperature, time, solvent, and co-catalyst were investigated using the talc catalysts coupled with TBAB. Synthetic zeolites doped with metals of known catalytic behavior were also investigated for their catalytic performance of the cycloaddition of CO2 with epoxide. Talc catalyst was characterized using a number of techniques including: Powder X-ray Diffraction, Inductively Coupled Plasma Optical Emission Spectroscopy, BET-N2 physisorption and Fourier Transform InfraRed Spectroscopy to determine the physical, chemical properties. Synthetic zeolites were characterized using Fourier Transform InfraRed Spectroscopy and Powder X-ray Diffraction. Catalyst activity in the cycloaddition of CO2 with propylene oxide and cyclohexene oxide was tested. Natural catalysts exhibited promising activity for cycloaddition of carbon dioxide into propylene oxide to form propylene carbonates in the presence of Lewis- base (co-catalyst). With talc and vermiculite exhibiting an 88.4% (conversion = 98.3%, selectivity = 89.9%) and 68.6% PC yield (conversion = 78.8%, selectivity = 87.1%) respectively, at the optimized reaction conditions. The synthetic zeolites exhibited very low activity (≤ 3.4% PC yield) for the same reaction with a slight improvement in in the presence of a co-catalyst to give a 44.0% PC yield. Furthermore, when the talc catalytic system was tested for cycloaddition reaction of CO2 and cyclohexene oxide to produce cyclohexene carbonate, a 42.3% CHC yield was obtained in 13 h under the optimized conditions for the CO2/PO reaction conditions. Results from this study provide relevance for the investigation of several silicate based catalysts for the cycloaddition of CO2 into epoxides to form cyclic carbonates and the possibility of making their synthetic analogues with tunable properties as a way of improving performance.