Harvesting CaCO₃ Polymorphs from In Situ CO₂ Capture Process

Abstract

The in situ sequestration of CO₂ using alkanolamine and organometallic calcium (OMC) offers an ecofriendly method for synthesizing a diverse range of calcite, vaterite, and aragonite polymorphs of CaCO₃. Aqueous <i>N</i>-methyldiethanolamine (MDEA) has high CO₂ loading capacity with low regeneration energy, but rate of CO₂ absorption was found to be slow. The driving force for the binding between CO₂ and MDEA could be enhanced by the presence of bovine carbonic anhydrase (bCA). The absorbed CO₂ was converted to stable carbonates through the addition of an OMC. The bCA enzyme both accelerated the CO₂ absorption and mineralization in the amine–CO₂–OMC system and improved the catalytic efficiency to 1.07 × 10⁴ M^–1 s^–1. The enthalpy of in situ mineralization, the mechanism underlying the CO₂ absorption process, and the formation of an aggregated composition of CaCO₃ were examined using calorimetric, NMR, and X-ray diffraction techniques, respectively. The crystal formation depended crucially on the mineralization process involving the anions of the OMC precursors. The CaO-based sorbents derived from the CaCO₃ polymorphs shows good CO₂ capture capacity on combustion process, and the consecutive re-formation–regeneration cycles of the CaO sorbents followed the trend aragonite > vaterite > calcite. Hence, the MDEA–OMC–bCA system offers a promising method for transitioning between CaCO₃ polymorphs