The effect of metal-free synergistic binary deep eutectic solvents based on p-toluenesulfonic acid for efficient CO2 fixation on cyclic carbonates

A metal-free binary deep eutectic solvent (DES) was prepared from 1-butyl-4-(dimethylamino)pyridinium bromide and p -toluenesulfonic acid ( p -TSA). This DES was employed as a liquid catalyst for the cycloaddition of CO₂ to epoxides under mild conditions. The catalyst provides a complementary Brønsted acidic and nucleophilic environment that efficiently promotes the reaction. Under 1 atm CO₂ and 70 ° C, styrene oxide, considered a challenging substrate, was converted to its cyclic carbonate with 99.6 % conversion and 99.7 % selectivity within 3 h, demonstrating the high efficiency of the catalyst. The DES exhibited good recyclability and maintained its catalytic performance over multiple reaction cycles. Thermal analysis confirmed the notable thermal stability of the DES. The methodology is also applicable to other epoxides, demonstrating high conversions and selectivities. This work highlights the potential of rationally designed DESs as sustainable, recyclable catalysts for CO₂ fixation to cyclic carbonates under mild and practical conditions. • A metal-free, recyclable binary DES ([C4DMAP]Br/p-TSA) catalyzes CO₂ cycloaddition to epoxides. • High conversion/selectivity achieved under mild, metal-free conditions (70°C, 1 atm). • Synergy between Br⁻ and p-TSA is essential for activating epoxides and fixing CO₂. • The catalyst is recyclable and sustainable, promoting green CO₂ utilization.