Light-Driven CO₂ Fixation into Epoxides Using an Al₂O₃/CoAl₂O₄ Composite Photocatalyst

Utilization of carbon dioxide (CO₂) as a C1 feedstock to synthesize value-added chemicals using a catalyst made from earth-abundant elements and under mild conditions is a sustainable approach toward carbon neutrality but difficult to achieve. Herein, the CoAl₂O₄/Al₂O₃ composite catalyst is developed and used for the light-driven epoxide to value-added cyclic carbonate conversion using CO₂. CoAl₂O₄/Al₂O₃ composite catalysts (x% Co-Al₂O₃) are prepared by calcining cobalt-incorporated Al-oxy-hydroxide at 500 °C under an air atmosphere. The composite 15% Co-Al₂O₃ (57% Al₂O₃ and 43% CoAl₂O₄) shows the highest photothermal conversion efficiency (η = 66%) as well as catalytic activity toward CO₂ fixation into epoxides to generate cyclic carbonates under 1 atm CO₂ pressure and solvent-free conditions (300 W xenon lamp). The catalyst displays good selectivity for the synthesis of a series of cyclic carbonates (>95%) with good yield in the presence of tetra butyl ammonium iodide (TBAI) as a cocatalyst (2 mol % with respect to epoxide). Under the optimized reaction parameters, 15% Co-Al₂O₃ retains its catalytic activity up to 8 cycles of catalysis without losing its chemical integrity. The reaction mechanism is proposed based on a structure-photothermal conversion-catalytic activity relationship study and a few control experiments. The design and development of a photocatalyst from the earth-abundant Al element under user-friendly conditions make this approach sustainable for the CO₂ economy.