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

High Resolution Image Download MS PowerPoint Slide A combination of ex situ and in situ characterization techniques was used to determine the mechanism of calcium carbonate (CaCO 3 ) formation from calcium hydroxide (Ca(OH) 2 ) dispersions in methanol/water (CH 3 OH/H 2 O) systems. Mid-infrared (mid-IR) analysis shows that in the absence of carbon dioxide (CO 2 ) Ca(OH) 2 establishes a reaction equilibrium with CH 3 OH, forming calcium hydroxide methoxide (Ca(OH)(OCH 3 )) and calcium methoxide (Ca(OCH 3 ) 2 ). Combined ex situ 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 2 reveals the formation of calcium dimethylcarbonate (Ca(OCOOCH 3 ) 2 ). This strongly suggests that carbonation takes place by reaction with the Ca(OCH 3 ) 2 formed from a Ca(OH) 2 and CH 3 OH reaction. Time-resolved XRD indicates that in the presence of H 2 O the Ca(OCOOCH 3 ) 2 ester releases CH 3 OH and CO 2, forming ACC, which subsequently transforms into vaterite and then calcite. TGA reveals that thermal decomposition of Ca(OCOOCH 3 ) 2 in the absence of H 2 O mainly leads to the reformation of Ca(OCH 3 ) 2, but this is accompanied by a significant parallel reaction that releases dimethylether (CH 3 OCH 3 ) and CO 2 . CaCO 3 is the final product in both decomposition pathways. For CH 3 OH/H 2 O mixtures containing more than 50 mol % H 2 O, direct formation of calcite from Ca(OH) 2 becomes the dominant pathway, although the formation of some Ca(OCOOCH 3 ) 2 was still evident in the in situ mid-IR spectra of 20 and 40 mol % CH 3 OH systems. In the presence of ≤20 mol % H 2 O, hydrolysis of the ester led to the formation of an ACC sol–gel. In both the 90 and 100 mol % CH 3 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) 2 in a methanolic phase have been systematically and experimentally characterized.