Effects of alkali-metal carbonates and nitrates on the CO2 sorption and regeneration of MgO-based sorbents at intermediate temperatures

Synthesis, characterization, CO2 mineralization in air, and thermal decomposition of nano- C8H10MgO10·4H2O powder

Effective solutions for efficient carbon dioxide (CO₂) capture in air at room temperature are in high demand due to the major impacts CO₂ has on global climatic changes. Solid adsorbents materials for CO₂ capture received great attention over the past years, among them, magnesium-based sorbents have been identified as a promising solution for CO₂ capture at intermediate temperatures. This study reports for the first time (1) the synthesis of monoclinic magnesium malate tetrahydrate by combining electrospinning and aging processes, and (2) its room temperature CO₂ adsorption and mineralization in air. Commercial magnesium hydroxide (Mg(OH)₂) powder was used as raw material in the synthesis of magnesium carbonate hydrates (MCH), by three processes; (1) direct calcination, (2) electrospinning and calcination, and (3) electrospinning, calcination, and aging (at room temperature and in air to incubate CO₂ mineralization). The synthesized powder samples were characterized thoroughly using XRD, SEM, EDS, and TGA analyses. Effects of calcination temperature/aging time on CO₂ adsorption (at room temperature), crystallization, and mineralization of MCH were studied. Interestingly, the results showed that the 6-month aged samples (via the third synthesis process above), recorded a CO₂ adsorption capacity of 15.5 wt% within 90 min at 30 °C. Subsequently, three novel mechanisms of thermal decomposition CO₂ adsorption/mineralization were proposed, and a theoretical upper limit of carbon saving potentials was estimated, i.e., 8 mol CO₂ per 1 mol MgO. This work provides a novel CO₂ mineralization approach that results in (1) effective and practical solutions of carbon dioxide (CO₂) emission management and which holds (2) great potential for novel carbon-based fuels development.

NaNO3 -Promoted Mesoporous MgO for High-Capacity CO2 Capture from Simulated Flue Gas with Isothermal Regeneration

NaNO₃ -promoted MgO composite materials have been prepared and their ability to sorb CO₂ at a concentration relevant to CO₂ capture from flue gas is explored. The uptake kinetics and capacities of sorbents of different NaNO₃ /MgO ratios are measured at intermediate temperatures of 230-300 °C. The sorbent with a NaNO₃ /MgO ratio of 0.10 has the highest 12 h sorption capacity among sorbents with different NaNO₃ loadings, and the highest sorption capacity of 11.2 mmol CO 2  g^-1 is observed at 260 °C. Intriguingly, an induction period is observed in the initial stage of CO₂ sorption. In situ XRD analysis, in situ FTIR spectroscopy, and a comparison of the CO₂ sorption behavior under simulated flue gas conditions in comparison to prior studies employing pure CO₂ indicated that the sorption of CO₂ occurred through nucleation of MgCO₃ crystallites in the material. The data indicate that the concentration of CO₂ within the molten medium of NaNO₃ , which is affected by both the solubility of CO₂ in molten NaNO₃ and the partial pressure of CO₂ in the surrounding atmosphere, has a critical impact on the length of the induction period. A partially desorbed sample after sorption of CO₂ displays much-improved sorption kinetics in the next cycle and was able to sorb and desorb CO₂ over multiple cycles at isothermal conditions by simply switching the feed gas from CO₂ to inert gas.

Stabilization of NaNO3-Promoted Magnesium Oxide for High-Temperature CO2 Capture

NaNO₃-promoted MgO sorbents are known to achieve enhanced CO₂ sorption uptake but fail to maintain their capacity after multiple sorption-regeneration cycles. In this study, commercially available hydrotalcites (Pural Mg30, Pural Mg70, and synthetic hydrotalcite) were used as stabilizers for NaNO₃-impregnated MgO (MgONaNO₃) sorbents to improve their cyclic stability. Results show that the Mg30-stabilized MgONaNO₃ attained higher and stable overall CO₂ sorption performance as compared to bare MgONaNO₃ after multiple sorption cycles. XRD analyses reveal that the hydrotalcites act as templates for MgCO₃ by restricting the formation of large and nonuniform product crystallites. Furthermore, CO₂-TPD results show that the hydrotalcites cause a change in the basic sites of the sorbent, which may be attributed to its high interaction with both MgO and NaNO₃. This interaction becomes stronger as cycles proceed due to the structural rearrangements occurring, thus contributing to the stable behavior of the sorbents. However, these characteristics were not found on MgONaNO₃ and the α-Al₂O₃-stabilized samples, thus proving the unique ability of hydrotalcites. From these results, we then derived the formation scheme of MgCO₃ on the hydrotalcite-stabilized sorbents. This study presents a simple yet effective method of improving the stability of molten salt-promoted sorbents with promising potential for industrial use.

Mechanisms of absorption and desorption of CO2 by molten NaNO3-promoted MgO

We present details of the mechanism of absorption and desorption of carbon dioxide by molten NaNO₃-promoted MgO and their implications for the applications of alkali nitrate-promoted MgO absorbents with many repeated absorption and desorption cycles.