Characterization of precursors of methanol synthesis catalysts, copper zinc aluminum oxides, precipitated at different pHs and temperatures

Catalyst precursors for copper/zinc aluminum mixed oxide catalysts with an atomic ratio of 6/3/1 were prepared by co-current precipitation at different pH levels and different temperatures. The precursors, catalysts and reduced catalysts were characterized by means of infrared spectroscopy, X-ray diffraction, thermal analysis, temperature-programmed oxidation and BET surface measurements. The catalysts were also tested for methanol synthesis activity in a synthesis gas containing a small amount of carbon dioxide. When the precipitation was conducted at pH = 7.0, the precipitates mainly consisted of the malachite-like phase and the catalysts therefrom were more active. Precipitation at pH less than or equal to 6 favored the formation of hydroxynitrate, which led to less active catalysts. Although the composition of the precursors obtained at pH = 7.0 and temperatures lower than 50 degrees C was the same as those at pH = 7.0 and T = 70 degrees C, the activity of the catalysts was much lower. It was also found that the aging temperature was an important factor in determining the interdispersion and activity of the final catalysts. Although the effects of both the pH value and the temperature on activity of the catalysts can be ascribed to a similar reason, i.e., the different interdispersion of copper and zinc, it seems that the pH value exerts its effect through altering the phase composition of the precursors, while the temperature functions through the precipitation kinetics of the precursors with a same composition.