The importance of the aging time to prepare Cu/ZnO/Al₂O ₃ catalyst with high surface area in methanol synthesis

Ternary Cu/ZnO/Al₂O₃ catalysts were prepared by a co-precipitation method. The precursor structures were monitored during the aging. The first precipitate structure was amorphous georgeite, which transformed into the unknown crystalline structure. The transition crystalline structure was assigned to the crystalline georgeite, which was suggested with elemental analysis, IR and XRD. The final structure of precursors was malachite. The Cu surface area of the resulting Cu/ZnO/Al₂O₃ was maximized to be 30.6 m²/g at the aging time of 36 h. The further aging rapidly decreased Cu surface areas of Cu/ZnO/Al₂O₃. ZnO characteristic peaks in oxide samples almost disappeared after 24 h aging, indicating that ZnO was dispersed in around bulk CuO. TOF of the prepared catalysts of the Cu surface area ranges from 13.0 to 30.6 m²/gcat was to be 2.67 ± 0.27 mmol/m².h in methanol synthesis at the condition of 250 °C, 50 atm and 12,000 mL/g_cat. h irrespective of the XRD and TPR patterns of CuO and ZnO structure in CuO/ZnO/Al ₂O₃. The pH of the precipitate solution during the aging time can be maintained at 7 by CO₂ bubbling into the precipitate solution. Then, the decrease of Cu surface area by a long aging time can be prevented and minimize the aging time to get the highest Cu surface area.