Phases of Cu/Zn/Al/Zr precursors linked to the property and activity of their final catalysts in CO₂ hydrogenation to methanol

Abstract

Cu/ZnO/Al2O3/ZrO2 catalysts have been studied in methanol synthesis by CO2 hydrogenation. However, their property and activity were rarely linked to the phase of coprecipitated Cu/Zn/Al/Zr precursor although a lot of similar works were done for Cu/ZnO/Al2O3 and Cu/ZnO/ZrO2. Herein, a series of Cu/Zn/Al/Zr precursors were prepared with a series of Cu/Zn/Al and Cu/Zn/Zr precursors for comparison, where the Cu/Zn ratio was fixed at 70:30. The characterization results of Cu/Zn/Al/Zr precursors indicate that the Al/Zr ratio determines the relative phase population of hydrotalcite (HT) to zincian malachite (zM). At a high Al/Zr ratio, HT phase was present in a lower abundance due to the inhibition effect of Zr4+, compared to Cu/Zn/Al precursor with the identical Al composition. This resulted in a high Cu surface area and enhanced methanol synthesis activity. At a low Al/Zr ratio, zM phase was dominant along with negligible or small existence of HT phase. Since unincorporated Al3+ behaves like Zr4+, such precursor was transformed into a final quaternary catalyst showing the activity equivalent or superior to Cu/ZnO/ZrO2 with the identical Zr composition. Consequently, the controlled formation of HT phase relative to zM phase in Cu/Zn/Al/Zr precursor is important in acquiring an active Cu/ZnO/Al2O3/ZrO2 catalyst.