Sustainable Synthesis of Dimethyl- and Diethyl Carbonate from CO2in Batch and Continuous Flow─Lessons from Thermodynamics and the Importance of Catalyst Stability

Equilibrium conversions for the direct condensation of MeOH and EtOH with CO2 to give dimethyl- and diethyl carbonate, respectively, have been calculated over a range of experimentally relevant conditions. The validity of these calculations has been verified in both batch and continuous flow experiments over a heterogeneous CeO2 catalyst. Operating under optimized conditions of 140 °C and 200 bar CO2, record productivities of 235 mmol/L·h DMC and 241 mmol/L·h DEC have been achieved using neat alcohol dissolved in a continuous flow of supercritical CO2. Using our thermodynamic model, we show that to achieve maximum product yield, both dialkyl carbonates and water should be continuously removed from the reactor instead of the conventionally used strategy of removing water alone, which is much less efficient. Catalyst stability rather than activity emerges as the prime limiting factor and should thus become the focus of future catalyst development.