Towards ‘greener’ catalyst manufacture: Reduction of wastewater from the preparation of Cu/ZnO/Al2O3 methanol synthesis catalysts

The generation of large volumes of nitrate-containing wastewater is a major issue in the industrial production of solid catalysts such as Cu/ZnO/Al2O3 employed in methanol synthesis. Extensive washing with water is needed to remove nitrate (and sodium) residues in the as-precipitated metal hydroxy-carbonate catalyst precursors. Residual nitrate species promote severe metal agglomeration during the subsequent calcination stage, while Na can act as a catalyst poison. The use of (NH4)HCO3 rather than Na2CO3 as precipitating agent results in Na-free metal hydroxy-carbonate precursors. Here we show that the calcination conditions applied for the Na-free, unwashed hydroxy-carbonate precursors can be adjusted to prevent metal agglomeration. Conventional calcination of unwashed precipitates in stagnant air results in poor Cu dispersions and catalytic activities. In contrast, efficient removal of the nitrate decomposition products by decomposing the unwashed precursors in a gas stream prevents metal agglomeration. Decomposition of the precipitate in a gas flow, particularly in O2-free gases, leads to catalysts as active as those obtained by conventional calcination after removal of the nitrate species by extensive washing of the precipitates. Hence, rational adjustment of the calcination conditions can significantly reduce the production of nitrate-containing wastewater during catalyst manufacture.