Methanol synthesis catalysts

Abstract

The commercial synthesis of methanol from CO₂, CO and H₂ is achieved using copper-zinc oxide catalysts based on the low pressure process developed by I. C. I. Copper-zinc oxide based catalysts are generally prepared by the coprecipitation of a hydroxide, hydroxycarbonate or hydroxynitrate precursor followed by calcination and reduction. We have investigated how the structure and composition of precursor phases affects the dispersion of CuO and ZnO in the calcined catalysts and Cu in the reduced catalysts. Parameters which control precursor formation, including the method of mixing the reagents, pH, Cu/Zn ratio and the presence or absence of aluminium and precursor ageing were studied.

Precursor phases which contain all the catalyst components give rise to highly dispersed copper and zinc oxide after reduction. Zincian-malachite (Cu_2-xZn_x(OH)₂CO₃) gives rise to higher copper dispersion than aurichalcite (Cu_5-xZn_x(OH)₆(CO₃)₂) although both contain copper and zinc. This is because the malachite precipitate has a much smaller particle size than aurichalcite which, together with a homogeneous composition hinders growth of CuO and ZnO during calcination giving rise to a high dispersion.

The activity of the reduced catalysts towards methanol synthesis and the reverse water-gas shift reaction were determined and the results support the proposal that the active site for these reactions is located on the metallic copper surface.