Continuous dimethyl carbonate synthesis from CO₂ and methanol over Bi_xCe_1-xO_δ monoliths: Effect of bismuth doping on population of oxygen vacancies, activity, and reaction pathway

We evaluated bismuth doped cerium oxide catalysts for the continuous synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide in the absence of a dehydrating agent. Bi_xCe_1-xO_δ nanocomposites of various compositions (x = 0.06-0.24) were coated on a ceramic honeycomb and their structural and catalytic properties were examined. The incorporation of Bi species into the CeO₂ lattice facilitated controlling of the surface population of oxygen vacancies, which is shown to play a crucial role in the mechanism of this reaction and is an important parameter for the design of ceria-based catalysts. The DMC production rate of the Bi_xCe_1-xO_δ catalysts was found to be strongly enhanced with increasing O_V concentration. The concentration of oxygen vacancies exhibited a maximum for Bi_0.12Ce_0.88O_δ, which afforded the highest DMC production rate. Long-term tests showed stable activity and selectivity of this catalyst over 45 h on-stream at 140 ℃ and a gas-hourly space velocity of 2,880 mL·g_cat^-1·h^-1. In-situ modulation excitation diffuse reflection Fourier transform infrared spectroscopy and first-principle calculations indicate that the DMC synthesis occurs through reaction of a bidentate carbonate intermediate with the activated methoxy (-OCH₃) species. The activation of CO₂ to form the bidentate carbonate intermediate on the oxygen vacancy sites is identified as highest energy barrier in the reaction pathway and thus is likely the rate-determining step.