Stability of modified LDHs

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

“Taking advantage of their thermal, hydrothermal, and chemical stability, commercial opportunities for amine-modified LDHs with their promising adsorption capacities and kinetics could be an attractive option for rapid temperature swing adsorption (R-TSA) processes to significantly increase CO₂ productivity and reduce energy consumption in DAC.³⁰ To demonstrate this further, a 50-cycle test was carried out for TRI-Mg_0.55Al-a with adsorption at 25°C in 400 ppm CO₂/N₂ for 60 min followed by desorption at 120°C in pure N₂ for 15 min. Under these working conditions, TRI-Mg_0.55Al-a achieved an average CO₂ uptake of 0.912 mmol g⁻¹ with high stability throughout all of the cycles (Figure 5A). The stability results were compared to the existing data in the literature marked as A-X/Y, where A, X, and Y represent the adsorbent, adsorption, and desorption temperatures, respectively (Figure 5B; Table S5). In general, amine-grafted adsorbents showed better cycling stability under dry conditions than polyamine-impregnated adsorbents because of lower amine leakage during heating. For instance, PEI-SBA-15-50/130 loses >50% of the uptake after only 30 cycles.³⁹ Additional treatments, such as adding nanostructured compounds^40,41 or surface modification,42, 43, 44 are required to strengthen the bond between the polyamine and the support. In comparison, Sayari et al.¹⁷ reported a substantially lower uptake loss of 14% for TRI-PE-MCM-41-50/120 after 40 cycles, with the formation of urea species under CO₂ atmosphere and elevated temperatures as the main deterioration pathway. A later study showed that secondary amines are more stable than primary amines against CO₂-induced deactivation.⁴⁵ Urea formation from the amine groups can be greatly suppressed in the presence of steam¹⁷ but requires a hydrothermally resistant support. It has been shown that commercialized Mg_xAl-CO₃ LDH-derived materials are highly stable during thousands of cycles for CO₂ adsorption in the presence of steam without structural collapse, even at elevated temperatures.⁴⁶ Our previous work also demonstrated that polyamine-functionalized calcined Mg_0.55Al-a lost 14% of the CO₂ capacity after 20 cycles under dry conditions, but were highly stable when regenerated by steam purging.²¹ In addition, humid stream increases the CO₂ uptake by 36.7%, which is mainly ascribed to the shift of the adsorption mechanism to the formation of ammonium bicarbonate.”

“Figure 5. Stability of modified LDHs

(A) A 50-cycle test of TRI-Mg_0.55Al-a when adsorbed at 25°C in 400 ppm CO₂/N₂ for 60 min and desorbed at 120°C in pure N₂ for 15 min.

(B) Comparison of long-term stability of class 2 adsorbents under dry conditions.”