Reactive CaCO Formation from CO and Methanolic Ca(OH) 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 (CHOH/HO) systems. Mid-infrared (mid-IR) analysis shows that in the absence of carbon dioxide (CO) Ca(OH) establishes a reaction equilibrium with CHOH, 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 CHOH reaction. Time-resolved XRD indicates that in the presence of HO the Ca(OCOOCH) ester releases CHOH and CO, forming ACC, which subsequently transforms into vaterite and then calcite. TGA reveals that thermal decomposition of Ca(OCOOCH) in the absence of HO mainly leads to the reformation of Ca(OCH), but this is accompanied by a significant parallel reaction that releases dimethylether (CHOCH) and CO. CaCO is the final product in both decomposition pathways. For CHOH/HO mixtures containing more than 50 mol % HO, 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 % CHOH systems. In the presence of ≤20 mol % HO, hydrolysis of the ester led to the formation of an ACC sol-gel. In both the 90 and 100 mol % CHOH 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.