Methanol Synthesis from CO2 Using Skeletal Copper Catalysts Containing Co-precipitated Cr2O3 and ZnO

Abstract

Skeletal Cu-Cr2O3-ZnO catalysts have been prepared by leaching CuAl2 alloy particles at 273 K using 6.1 M aqueous NaOH solutions containing sodium chromate (Na2CrO4) and sodium zincate (Na2Zn(OH)4). The presence of sodium chromate and sodium zincate in the caustic solution was found to affect the pore structure and surface areas of the resulting catalysts. Both BET and Cu surface areas were increased by increasing the concentration of Na2CrO4 and of Na2Zn(OH)4. Increasing the Na2CrO4 level from 0 to 0.06 M in a 6.1M NaOH solution containing 0.2M Na2Zn(OH)4 caused the content of ZnO in the catalyst to decrease from 8.8 to 3.0 wt% whilst increasing the Cr2 O3 content from 0 to 1.7 wt%, indicating that the presence of Na2CrO4 in the leach liquor not only resulted in deposition of a Cr compound but also inhibited precipitation of zinc hydroxide onto skeletal Cu catalysts. On the other hand, increasing the concentration of Na2Zn(OH)4 from 0 to 0.6 M in a 6.1 M NaOH solution containing 0.008 M Na2 CrO4 resulted in increasing the ZnO loading from 0 to 8.9wt% with an almost constant content of Cr2 O3 (1.3 ± 0.2%) in the catalysts, revealing that sodium zincate only led to precipitation of zinc hydroxide and did not suppress Cr2O3 formation. Hydrogenation of CO2 was studied using a gas mixture of 24% CO2 in H2 at a total pressure of 4MPa, space velocities up to 210000L kg-1h-1 and temperatures in the range 493-533K. The catalysts were found to be both highly active and selective for methanol synthesis. This study confirms the role of ZnO in promoting the activity of copper for methanol synthesis from CO2 and improving the selectivity by inhibiting the reverse water-gas shift reaction. The role of Cr2O3 is to improve the structural development of high surface area skeletal copper.