In recent years, the concentration of CO2 in the atmosphere has been constantly increasing, having an impact on global climate change. Therefore, a growing number of countries and researchers are interested in developing technologies to reduce CO2 emissions and developing efficient carbon dioxide utilizations systems. In the BioDME project (founded by the german BMBF) we focus on developing a pilot plant for the hydrogenation of CO2 to gain dimethyl ether (DME) in high amounts and selectivities. The CO2 is developed in a microbiological electrolysis cell out of waste water. The goal is to realize a demonstration plant to further investigate the stability, economy and sustainability of this process in daily operation.
Our CO2 hydrogenation catalyst is a Cu/ZnO/ZrO2 system, which has been approved as a reliable catalyst for MeOH production.[3,4] Because of ZrO2 as promoter, the hydrophilic character hinders the strong adsorption of water and therefore a possible deactivation reaction. This catalyst is combined with an additional solid acid catalyst for the MeOH dehydration to DME. Most commonly used are zeolites like ZSM5, FER and MOR and other easy available systems like BEA.[5,6] The synthesis of the MeOH catalyst is carried out by a precipitation method, in which an aqueous solution of the metal nitrate salts is precipitated with sodium carbonate at a constant temperature and pH value. The resulting raw product is then spray dried and calcined. To be useful as a possible catalyst at an industrial scale, the Cu/ZnO/ZrO2 system has to be synthesizable in a great amount. Therefore, we developed a synthesis route which produces about 7 times more CZZ catalyst than before, based on our earlier synthesis. We scaled the volume of our synthesis reactor up from 1 to 7 L, but maintained the same values for the reaction time, temperature and pH value, respectively. Catalyst tests showed, that multiple up-scaled syntheses and the standard catalyst synthesis batches have similar CO2 conversions and selectivities for MeOH. Alternating temperature experiments also showed, that increasing the temperature over 250°C leads to a decline in CO2 conversion and selectivity for methanol, following the thermodynamic expectations.
Long-term stability tests also showed high stability and with only 9 % a very low decrease of catalytic activity over 7 d.