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Kinetics of modified LDHs at different temperatures

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

“DAC adsorbents suffer from poor kinetics under ultradilute conditions, commonly requiring several hours for polyamine-impregnated adsorbents to reach half-adsorption capacities.37 In contrast, Figure 4A shows that amine-grafted LDHs exhibited fast CO2 adsorption rates when exposed to N2 flow containing 400 ppm CO2, and the CO2 uptake within 120 min correlates with the capacities determined from isotherm tests. It is worth noting that TRI-Mg0.55Al-a displayed an even higher CO2 uptake within 60 min than the representative class 1 adsorbent PEI-SBA-15. Normalizing CO2 uptake by dividing with the total CO2 capacity (Figure 4B) gives more reliable comparisons, showing that both APS- and TRI-Mg0.55Al-a reached 70% capacity within 30 min, whereas the time required was doubled for PEI-SBA-15 to reach the same capacity. The long diffusion process detected for TRI-SBA-15 further indicates the benefit of using LDH supports with a broader range of pore sizes to avoid amine blockages.”

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Figure 4. CO2 adsorption and desorption kinetics of modified LDHs

(A) CO2 uptakes of amine-grafted samples at 25°C under 400 ppm CO2 in N2 after regeneration at 120°C under pure N2 for 60 min.

(B) Normalized CO2 uptakes after division by the total CO2 uptakes after 120 min.

(C) Normalized CO2 desorbed from 25°C to 120°C at a heating rate of 5°C min−1 after saturation under 400 ppm CO2.

(D) Temperature-programmed desorption (TPD) results calculated from the differential of normalized weight loss in C.

(E) Normalized CO2 uptakes of TRI-Mg0.55Al-a when regenerated at varying temperatures under pure N2 for 60 min.

(F) CO2 uptakes of TRI-Mg0.55Al-a within 120 min at 25°C under 400 ppm CO2 in N2 after regeneration at varying temperatures under pure N2 for 60 min.”

“Amine-grafted LDHs also showed a decrease in desorption temperature peaks (e.g., 75°C) compared to PEI-SBA-15 (100°C), as evidenced by the temperature-programmed desorption (TPD) results at a heating rate of 5°C min−1 (Figures 4C and 4D). The milder regeneration requirements of amine-grafted LDHs provide the opportunity to use industrial waste heat at temperatures <100°C. For instance, TRI-Mg0.55Al-a retained 80% of its total adsorption capacity, with a desorption temperature as low as 80°C (Figure 4C). It should be noted that amine monolayers formed by dry grafting dispersion on exfoliated LDH-derived nanosheets exhibited the lowest CO2 desorption to gas stream pathway. In contrast, polyamine molecules thickened and aggregated on the support surface, and thus required additional energy to transfer desorbed CO2 through amine films.38 In addition, ultrafast CO2 desorption kinetics were observed for TRI-Mg0.55Al-a, which only required 70°C to regenerate 90% of the normalized CO2 uptake in 10 min (Figures 4E and S5). After desorption under a range of temperatures, fast adsorption kinetics were retained in the next cycle (Figure S6), and CO2 uptake within 120 min agreed well with the TPD results (Figure 4F).”

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