Advanced Silica-Based Catalysts from Rice Husk Ash for CO₂ Fixation: Synergistic Effects of Metal Oxides and Ionic Liquids in Glycerol Carbonate Production

This review explores the catalytic conversion of carbon dioxide (CO2) into glycerol carbonate (GC), positioning this pathway as a sustainable strategy that couples environmental mitigation with the valorization of surplus glycerol from biodiesel production. Glycerol carbonate maintains extensive industrial utility as a green solvent, chemical intermediate, and functional component in polymers, cosmetics, and packaging. Distinct from prior literature, this study systematically integrates the evaluation of catalysts derived from agro-industrial waste and hybrid catalytic systems, correlating their structural architectures with catalytic efficiency. The review evaluates diverse catalytic frameworks, with a primary focus on heterogeneous systems. Silica-based materials are highlighted, particularly those synthesized from rice husk ash, an abundant amorphous silica source. The sol–gel method is identified as a robust route for engineering porous matrices with high surface areas and tunable structural properties. Furthermore, the doping of silica with metal oxides, such as niobium oxide (Nb2O5) and nickel oxide (NiO), is discussed as a strategic approach to introduce synergistic acid–base sites and redox properties that facilitate CO2 activation. The integration of ionic liquids into hybrid systems is also examined as a promising frontier to enhance reaction kinetics and selectivity. Finally, this review delineates the nexus between agro-industrial waste management and the reduction of greenhouse gas emissions, proposing a circular economy framework for the biodiesel value chain.