How non-aqueous media direct the reaction of Ca(OH)2 with CO2 to different forms of CaCO3: operando mid-infrared and X-ray absorption spectroscopy studies

Time-resolved structural changes taking place during the reaction of Ca(OH)2 and CO2 forming different CaCO3 polymorphs, in aqueous and non-aqueous environments, were recorded operando using mid-infrared (mid-IR) and X-ray absorption near-edge structure (XANES) spectroscopy. Results show that Ca(OH)2 directly transforms into calcite in a pure water dispersion. In methanolic media with low water content, calcium di-methylcarbonate (Ca(OCOOCH3)2) is formed, which is hydrolysed to amorphous calcium carbonate (ACC) and vaterite in the presence of sufficient water. The addition of toluene shifts the equilibrium composition further from Ca(OH)2 to ACC and the crystalline forms of CaCO3, probably by affecting the activity of the methoxide intermediate. It can facilitate the formation of aragonite. No Ca(OH)2 conversion was detected in pure ethanol, isopropanol and toluene dispersions, except for nanoscale Ca(OH)2 in ethanolic dispersion, which formed calcium di-ethylcarbonate (Ca(OCOOCH2CH3)2). Our findings underline that vaterite formation is driven by the solution and solid state chemistry related to the reaction via alkoxides and carbonic acid esters of the alcohols, rather than the nucleation process in solution. The alcohol in these systems does not just act as a solvent but as a reactant.