Reactive CaCO 3 Formation from CO 2 and Methanolic Ca(OH) 2 Dispersions: Transient Methoxide Salts, Carbonate Esters and Sol-Gels

A combination of and characterization techniques was used to determine the mechanism of calcium carbonate (CaCO ) formation from calcium hydroxide (Ca(OH) ) dispersions in methanol/water (CH OH/H O) systems. Mid-infrared (mid-IR) analysis shows that in the absence of carbon dioxide (CO ) Ca(OH) establishes a reaction equilibrium with CH OH, forming calcium hydroxide methoxide (Ca(OH)(OCH )) and calcium methoxide (Ca(OCH ) ). Combined mid-IR, thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray absorption spectroscopy and scanning electron microscopy examination of the reaction product formed in the presence of CO reveals the formation of calcium dimethylcarbonate (Ca(OCOOCH ) ). This strongly suggests that carbonation takes place by reaction with the Ca(OCH ) formed from a Ca(OH) and CH OH reaction. Time-resolved XRD indicates that in the presence of H O the Ca(OCOOCH ) ester releases CH OH and CO , forming ACC, which subsequently transforms into vaterite and then calcite. TGA reveals that thermal decomposition of Ca(OCOOCH ) in the absence of H O mainly leads to the reformation of Ca(OCH ) , but this is accompanied by a significant parallel reaction that releases dimethylether (CH OCH ) and CO . CaCO is the final product in both decomposition pathways. For CH OH/H O mixtures containing more than 50 mol % H O, direct formation of calcite from Ca(OH) becomes the dominant pathway, although the formation of some Ca(OCOOCH ) was still evident in the mid-IR spectra of 20 and 40 mol % CH OH systems. In the presence of ≤20 mol % H O, hydrolysis of the ester led to the formation of an ACC sol-gel. In both the 90 and 100 mol % CH OH systems, diffusion-limited ACC → vaterite → calcite transformations were observed. Traces of aragonite were also detected. We believe that this is the first time that these reaction pathways during the carbonation of Ca(OH) in a methanolic phase have been systematically and experimentally characterized.