Influence of precipitation conditions on precursor particle size distribution and activity of Cu/ZnO methanol synthesis catalyst

Abstract

Binary Cu/ZnO methanol synthesis catalysts were prepared by the co-precipitation of copper and zinc hydroxycarbonates using different initial solution concentrations, stirring rates and aging times and temperatures during precipitation, and different calcination temperatures of the precipitated hydroxycarbonates. The precipitates (catalyst precursors), fresh catalyst and were characterized at appropriate stages by nitrogen adsorption–desorption isotherm, X-ray diffraction (XRD), temperature programmed reduction (TPR), N2O chemisorption and particle size distributions measurement techniques. All catalysts were tested for methanol synthesis activity in fixed-bed reactor under conditions similar to that of commercial operation (503K, 50bar and GHSV of 17,250h−1), using of H2, CO, and CO2 (80/12/8mol ratio) mixture as the feed. During aging of the precursor, a sharp pH drop along with color change (from blue to green) was observed after certain aging time. Further, the particle size of precursors showed a decrease after color change. The time of this change was strongly dependent on the aging temperature and decreased by increasing aging temperature. At 40°C aging temperature, the catalyst activity increased with aging time but the resulting catalyst exhibited poor performance because the color change did not occur even after 65h of aging of the mother liquor. At 80°C aging temperature, a color change took place rapidly after about 0.5–0.75h of aging, and the catalyst activity increased with aging time followed by a decrease upon further aging. At a constant aging time of 5h, the catalyst activity increased with temperature in the range of 40–60°C and then decreased when temperature rose further from 60 to 80°C. The highest methanol synthesis activity (555gMeOH/kgcath) was observed for the catalyst prepared from 1M initial solution, 500rpm stirring rate and aged at 60°C. This was attributed to the small CuO crystallite size and large Cu surface area of the resulting catalyst.