Continuous precipitation of Cu/ZnO/Al₂O₃ catalysts for methanol synthesis in microstructured reactors with alternative precipitating agents

Ternary Cu/ZnO/Al₂O₃ catalyst systems were systematically prepared by innovative synthesis routes in microstructured synthesis setups, allowing to study different types of micromixers. The coprecipitation in the slit plate and valve-assisted mixers was operated continuously under exact control of pH, temperature, concentration and ageing time. Due to the enhanced surface to volume ratio in microstructured reactors, a precise temperature control and efficient mixing of the reactants are enabled. The precipitation was performed with sodium, ammonium and potassium carbonate as well as sodium hydroxide. To evaluate the potential of the novel synthesis routes, reference samples in a conventional batch process were prepared. The catalysts were synthesized according to the constant pH method with a molar ratio of 60:30:10 for copper, zinc and aluminum. The synthesis routes applied have a significant influence on the structures of hydroxycarbonate precursors and on the catalytic activity in methanol synthesis. XRD patterns of hydroxycarbonate precursors from the synthesis in micromixers, especially using ammonium carbonate as precipitating agent, display high crystallinity and sharp reflections of malachite and rosasite. Cu/ZnO/Al₂O₃ catalysts prepared in continuously operated micromixers in general show higher specific copper surface areas than catalysts prepared in conventional batch processes. The highest methanol productivity of all prepared catalyst systems was observed with the catalyst precipitated in the slit plate mixer with ammonium carbonate.