Discovery of structure-activity relationship for the synthesis of DMC from methanol and CO2 over CeO2 based catalysts
Résumé
DMC is used in the chemical industry in transesterification reactions for the production of other carbon-ates such as diphenylcarbonate, as alkylating or carboxylating agent. Due to its low toxicity and excellent biodegradability, DMC is an attractive building block for other chemicals [1]. However, conventional DMC production routes involve many drawbacks because they use explosive and hazardous reagents and generate toxic waste. Direct synthesis of DMC starting from methanol and CO2 (Eq.1) is an interesting approach in terms of carbon dioxide utilization to value-added chemicals. (Eq.1) However, the thermodynamic equilibrium is strongly in favour of DMC hydrolysis, e.g. the DMC yield is below 1% under usual conditions at 120-140°C and 100-200 bar. At these low conversion levels, any trace of water will have a significant effect on the DMC yield. Therefore a strict control of the water content of the catalyst, methanol and CO2 is necessary in order to obtain reproducible results. Moreover, controlling the amount of CO2 and an accurate product analysis at very low conversion are paramount to properly rank catalyst activities. Among various catalysts reported [2], zirconia- and ceria-based heterogeneous catalysts exhibit good ac-tivity and DMC selectivity. Although it is generally acknowledged that the catalytic sites shall be composed of well balanced acid and basic sites, the exact nature of the most efficient sites has not been characterised so far, preventing further rational design and catalyst synthesis[3]. The aim of this work is to identify structural features of doped ceria catalysts for DMC synthesis . We have explored a large library of ceria-based solid solutions, such as CexZr1-xO2. We discovered that the crystallinity and the shapes of the crystallites (platelets and square based pyramids) are important parameters that control the activity. This has allowed us to synthesize the most active heterogeneous catalysts reported so far. This rational catalyst design would not have been possible without the strict experimental testing protocol that we have developed, enabling the measurement of the intrinsic catalyst activity.