Greener and facile synthesis of Cu/ZnO catalysts for CO2 hydrogenation to methanol by urea hydrolysis of acetates

Abstract

Cu/ZnO-based catalysts for methanol synthesis by CO_x hydrogenation are widely prepared via co-precipitation of sodium carbonates and nitrate salts, which eventually produces a large amount of wastewater from the washing step to remove sodium (Na⁺) and/or nitrate (NO₃⁻) residues. The step is inevitable since the remaining Na⁺ acts as a catalyst poison whereas leftover NO₃⁻ induces metal agglomeration during the calcination. In this study, sodium- and nitrate-free hydroxy-carbonate precursors were prepared via urea hydrolysis co-precipitation of acetate salt and compared with the case using nitrate salts. The Cu/ZnO catalysts derived from calcination of the washed and unwashed precursors show catalytic performance comparable to the commercial Cu/ZnO/Al₂O₃ catalyst in CO₂ hydrogenation at 240–280 °C and 331 bar. By the combination of urea hydrolysis and the nitrate-free precipitants, the catalyst preparation is simpler with fewer steps, even without the need for a washing step and pH control, rendering the synthesis more sustainable.