CO2 adsorption capacity of modified LDHs at different temperatures

https://doi.org/10.1016/j.xcrp.2021.100484

“We have measured the CO₂ adsorption isotherms at 25°C for all of the adsorbents prepared (Figure 3A). While the TRI-grafted bulk and stacked LDHs exhibited poor CO₂ capacities under ultradilute conditions, obvious improvements were observed for TRI-grafted AMO-LDHs. Remarkably, TRI-Mg_0.55Al-a achieved a CO₂ uptake capacity of 1.049 mmol g⁻¹ upon exposure to 0.4 mbar CO₂, which is 16.7 times higher than that of TRI-Mg_0.55Al-w (0.063 mmol g⁻¹) and 30% higher than TRI-SBA-15 (0.808 mmol g⁻¹). The fast coprecipitation process did not significantly improve the capacity of TRI-LDHs, which indicates that both reduction in LDH size and thickness and morphology exfoliation play an important role in establishing the desired amine-grafting sites. The monoamine-grafted AMO-LDHs were also tested. They exhibit a very promising capacity of 0.416 mmol g⁻¹ for APS-Mg₂Al-a and 0.778 mmol g⁻¹ for APS-Mg_0.55Al-a in 0.4 mbar.”

“Figure 3. CO₂ adsorption capacity of modified LDHs

(A) Screening of CO₂ adsorption isotherms of amine-grafted samples at 25°C.

(B) CO₂ adsorption isotherms of TRI-Mg_0.55Al-a at 25°C, 45°C, and 65°C.

(C) Isosteric heats of adsorption onto TRI-Mg_0.55Al-a.

(D) CO₂ adsorption capacities under 0.4 mbar and 25°C as a function of amine loadings.”

“Considering the low physisorption capacities of the LDHs under 0.4 mbar at 25°C (Figure S4) and the decreased surface areas after amine grafting, the enhanced CO₂ capacity seen for TRI-LDHs is mainly attributed to chemisorption by the amine functional groups. The isosteric heat of adsorption of TRI-Mg_0.55Al-a samples was calculated according to the Clausius-Clapeyron relation from isotherms measured at 25°C, 45°C, and 65°C (Figures 3B and 3C). The heat of adsorption was underestimated at low CO₂ coverages, but quickly approached 79 kJ mol⁻¹ and remained above 65 kJ mol⁻¹ for loadings of up to 1.2 mmol g⁻¹, which is typical for CO₂ chemisorption on amines.³⁶ At higher surface coverages, physisorption dominates the adsorption process, with the heat of adsorption ranging from 15 to 40 kJ mol⁻¹.³⁵”