Visible-Light-Driven Cycloaddition of CO2 with Epoxides Catalyzed by Recyclable Mg2⁺/TiO2 Photocatalyst Operating at Room Temperature

The utilization of CO₂ as a C₁ building block remains a key challenge in sustainable organic synthesis. We now report a recyclable metal ion modified titanium oxide (Mⁿ⁺/TiO₂, where Mⁿ⁺ represents Mg²⁺, Rh³⁺, Cr³⁺, Mn²⁺, Fe²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Al³⁺) photocatalytic system used with a tetrabutylammonium halide (TBAX, X = I, Br, Cl) as a co-catalyst, which enables the cycloaddition of CO₂ with various epoxides under visible light irradiation at room temperature. This catalyst system exhibits a broad range of responsiveness to visible light and is compatible with simulated solar light, and operates solvent-free conditions without the need for high pressure. Excellent yields and selectivities were achieved by the Mg²⁺/TiO₂ photocatalyst, and the method was successfully scaled to gram-scale synthesis without any loss of efficiency. Simulated post-combustion exhaust gas (CO₂: 7.5%, N₂: 92.5%, relative humidity: 90%) was concentrated to 97% and used for the reaction, yielding 88% styrene carbonate from styrene oxide by the catalysis of Mg²⁺/TiO₂. Furthermore, the Mg²⁺/TiO₂ catalyst could be readily recovered and reused over multiple cycles. Mechanistic investigations supported by density functional theory (DFT) calculations indicated that the bromine radical (Br•), generated via photooxidation of bromide ion from TBABr, initiates the epoxide ring-opening.