The transformation of CO2 into a precipitated mineral carbonate through an ex situ mineral carbonation route is considered a promising option for carbon capture and storage (CCS) since (i) the captured CO2 can be stored permanently and (ii) industrial wastes (i.e., coal fly ash, steel and stainless-steel slags, and cement and lime kiln dusts) can be recycled and converted into value-added carbonate materials by controlling polymorphs and properties of the mineral carbonates. The final products produced by the ex situ mineral carbonation route can be divided into two categories-low-end high-volume and high-end low-volume mineral carbonates-in terms of their market needs as well as their properties (i.e., purity). Therefore, it is expected that this can partially offset the total cost of the CCS processes. Polymorphs and physicochemical properties of CaCO3 strongly rely on the synthesis variables such as temperature, pH of the solution, reaction time, ion concentration and ratio, stirring, and the concentration of additives. Various efforts to control and fabricate polymorphs of CaCO3 have been made to date. In this review, we present a summary of current knowledge and recent investigations entailing mechanistic studies on the formation of the precipitated CaCO3 and the influences of the synthesis factors on the polymorphs.

Mineral carbonation, which involves spontaneous reactions of CO2 with alkaline earth metals such as calcium or magnesium, is considered one of the most attractive options to sequester CO2 because CO2 can be permanently stored in an inert solid forming stable inorganic carbonate. Moreover, the precipitated CaCO3 has various potential applications in industrial areas, including adhesives, sealants, food, pharmaceuticals, paints, coating, paper, cement, construction materials, etc. In particular, it is expected that the total cost of the carbon capture and storage process could be partly offset by producing value-added CaCO3 materials. In order to add value to the precipitated CaCO3 produced in the ex-situ mineral carbonation process, CaCO3’s polymorphs, as well as other properties such as particle size, shape, density, color, and brightness, must be finely tuned. Among CaCO3’s polymorphs, calcite, aragonite, and vaterite, calcite is considered to be the most thermodynamically favorable structure at ambient temperatures. However, kinematic constraints in the crystallization induced by synthetic factors are known to significantly affect the formation of polymorphs as well. Here, we revisited the effects of the synthetic factors such as pH, temperature, feeding order, and concentration and molar ratio of Ca2+ and CO3 2– on the formation of CaCO3’s polymorphs to provide fundamental insight into how to control the polymorphism of CaCO3 with the ultimate goal of creating value-added mineral carbonation products. ATR FT-IR spectroscopy and a powder X-ray diffraction analysis were performed on the precipitated CaCO3 using model chemicals, K2CO3 and CaCl2, and CaCO3’s thermal stability was also investigated.

Carbon capture is an important and effective approach to control the emission of CO2 from point sources such as fossil fuel power plants, industrial furnaces and cement plants into the...

For the first time, the use of stannate-based sorbents was investigated as high temperature CO2 sorption to evaluate their potential to contribute towards reducing carbon emissions. The sorption capacity and kinetics of commercial tin oxide, sodium, potassium and calcium stannates and lab synthesised potassium stannates were tested using thermogravimetric analysis. Commercial K2SnO3 was found to possess the largest CO2 uptake capacity (2.77 mmol CO2/g or 12.2 wt%) at 700 °C, which is among the highest for potassium sorbents, but the CO2 desorption was not successful. On the contrary, the in-house synthesised K-stannate (K-B) using facile solid-state synthesis outperformed the other sorbents, resulting in a CO2 uptake of 7.3 wt% after 5 min, an adsorption rate (0.016 mg/s) one order of magnitude higher than the other stannates, and stability after 40 cycles. The XRD and XPS analyses showed that K-B contains a mixture of K2SnO3 (76%) and K4SnO4 (21%), while the Scherrer crystal sizes confirmed good resistance to sintering for the potassium stannates. Among the apparent kinetic model tested, the pseudo-second order model was the most suitable to predict the CO2 sorption process of K-B, indicating that chemical adsorption is dominant, while film-diffusion resistance and intra-particle diffusion resistance governed the sorption process in K-B. In summary, this work shows that solid-state synthesised potassium stannate could be an effective sorbent for high temperature separation, and additional work is required to further elucidate its potential.

Hybrid ultramicroprous materials (HUMs) with a number of different variations of the inorganic pillars as well as surface functional groups have been tested as CO2 sorbent for low-concentration CO2 capture. Specifically well-known HUM NbOFFIVE-1-Ni (NbOFFIVE=(NbOF5)2-, 1=pyz, Ni=nickel(II)) like HUMs were synthesized in this study by replaces Nb with V and Ta. Replacing the metal center from Nb to V or Ta showed that the CO2 adsorption isotherm, in particular at low partial pressures, adopted different shapes and gradient. This study shows that the CO2 adsorption properties at low partial pressures on HUMs can be affected by the metal present in the inorganic pillars.