Triethylamine–Water as a Switchable Solvent for the Synthesis of Cu/ZnO Catalysts for Carbon Dioxide Hydrogenation to Methanol

Abstract

Cu/ZnO catalyst precursors for industrial methanol synthesis catalysts are traditionally synthesised by coprecipitation. In this study, a new precipitation route has been investigated based on anti-solvent precipitation using a switchable solvent system of triethylamine and water. This system forms a biphasic system under a nitrogen atmosphere and can be switched to an ionic liquid single phase under a carbon dioxide atmosphere. When metal nitrate solutions were precipitated from water using triethylamine–water as the anti-solvent a hydroxynitrate phase, gerhardite, was formed, rather than the hydroxycarbonate, malachite, formed by coprecipitation. When calcined and reduced, the gerhardite precursors formed Cu/ZnO catalysts which showed better productivity for methanol synthesis from CO2 hydrogenation than a traditional malachite precursor, despite their larger CuO crystallite size determined by X-ray diffraction. The solvents could be recovered by switching to the biphasic system after precipitation, to allow solvent recycling in the process, reducing waste associated with the catalyst synthesis.

A structural underpinning of the lower critical solution temperature (LCST) behavior behind temperature-switchable liquids

“…2,3 Recently, switchable biphasic mixtures have come into focus as an emerging separation method, where the application and removal of a stimulus (e.g., temperature, pH, pressure, light, or CO2) triggers a drastic change in material properties, such as mutual solubility of two liquids. 4 For example, amine-based switchable solvents have shown potential for desalination, [5][6][7][8][9] water-softening, 10 anti-solvent precipitation, 11 and oil extraction. 12,13 2 2A).…”