Catalytic Conversion of Glycerol to Glycerol Carbonate Utilizing CO₂ and Dimethyl Carbonate

The decline in fossil fuels drives research into alternatives like biofuels. Biodiesel production creates a surplus of crude glycerol, a major bottleneck. Scientists are exploring new, cost-effective uses for this abundant by-product. This research focuses on the catalytic conversion of glycerol to value-added glycerol carbonate using CO2, an abundant, non-toxic greenhouse gas, and dimethyl carbonate (DMC), which yields more glycerol carbonate. The catalysts were prepared by co-precipitation method, and characterized by using XPS, FT-IR, XRD, TGA, SEM, and CO2-TPD techniques. Firstly, metal oxide catalysts of Al2O3, CaO, and MgO were used in the carbon dioxide and glycerol reaction. A glycerol conversion of 24.5±2.2 mol%, and glycerol carbonate yield of 12.8±1.2 mol% under optimized conditions of 5.5% catalyst loading, 150°C temperature, and 8 MPa pressure was achieved by using MgO as the catalyst. Subsequently, novel mixed metal oxides (Ca-Al, Mg-Al, and Mg-Ca) are introduced to the same reaction, and Mg0.75Ca0.25O exhibited the highest glycerol carbonate yield of 16 mol%, under optimized reaction parameters of 8.5% catalyst loading, 180°C temperature, and 7 MPa pressure. BET surface area and total basicity of the catalysts had the most effective role in their performance in the reaction. To boost glycerol carbonate yield, various catalysts were designed for the glycerol and DMC reaction. Lithium oxide combined with synthesized activated carbon, using incipient wetness impregnation method, performed best, achieving an 87.9±2.4% yield due to its high basicity and surface area. The activated carbon was microwave-assisted synthesized from flax shive which is an abundant agricultural residue. Using Central Composite Design optimization, a 75% phosphoric acid concentration and 4 minutes of heating time were found to produce the optimal activated carbon pore width of 3.9 nm. The optimal reaction parameters were found as a temperature of 89.1°C, 3.4 DMC/glycerol molar ratio, and 5.6% catalyst dosage, resulting in a glycerol carbonate yield of 97.7%. The reaction kinetics adhered to a pseudo-first-order rate law with an activation energy of 45.5 kJ/mol. The 20%Li2O/AC catalyst exhibited satisfactory reusability across five cycles of the reaction. This comprehensive exploration underscores the potential for a more sustainable biodiesel industry through innovative waste usage processes.

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