Influence of water for cross linked polymer based CO2 capture

https://doi.org/10.1039/D1MA01072G

“Amine-based chemisorbents generally increase their CO₂ adsorption capacity in the presence of water due to better diffusion,^26,27 and improved amine efficiency from the formation of ammonium bicarbonate, theoretically enabling each amine to adsorb one molecule of CO₂.⁵⁸ The influence of water on the CO₂ adsorption capacity of 10 : 1 (R) was measured using different techniques. Initially, an investigation using a TGA experiment was conducted in which the adsorbent was pre-humidified for almost four hours using a flow of wet argon at 30 °C, prior to contact with CO₂, Fig. 14. The adsorbent took up a maximum of 0.207 g g⁻¹ water after 94 minutes, then lost some of this over the next hour, but regained weight and began to stabilise until reaching hydration of 0.198 g H₂O g⁻¹ (11.00 mmol g⁻¹). This was taken to be the maximum hydration or saturation adsorption of the material. On subsequent exposure to wet CO₂, sorption was extremely rapid, reaching 0.342 g g⁻¹ within 28 minutes, therefore CO₂ accounting for 0.144 g g⁻¹ (3.27 mmol g⁻¹), exceeding the maximum adsorption under dry conditions (2.31 mmol g⁻¹) by 0.96 mmol CO₂ g⁻¹. During four hours under wet CO₂, the adsorption equilibrium appears to shift resulting in slight desorption followed by re-adsorption until the final total adsorption was 0.347 g g⁻¹. Exposure to dry argon at 30 °C for eight hours reduced the adsorbed loading to 0.088 g g⁻¹via mass displacement. On heating, complete desorption was achieved at 125 °C. Contrary to what we have previously observed in wet CO₂ uptake experiments on crosslinked PEI materials,^26,36 more adsorbate is removed (0.259 g g⁻¹) under the low temperature desorption step than water is adsorbed during initial hydration. Therefore, it can be inferred that at least some CO₂ is desorbed at low temperature, which may have been weakly chemisorbed or possibly physisorbed, while the remaining CO₂ is strongly chemisorbed, most probably in the form of carbonate and bicarbonate species.”

“Fig. 14 TGA–CO₂ and H₂O sorption (g g⁻¹) of 10 : 1 (R) in humid environment at 30 °C, under 80 ml min⁻¹ CO₂, 10 ml min⁻¹ Ar and 21.4% RH.”

“An enhancement in CO₂ uptake and adsorption kinetics was observed under humid conditions. Therefore, it was decided to further investigate the sorbent behaviour when exposed to water vapour using a DVS setup. Fig. 15 shows H₂O uptake by 10 : 1 (R) at 30 and 40 °C at RH values between 0 and 95%. At both temperatures, an H₂O isotherm type III was measured which is characteristic of monolayer–multilayer adsorption onto favourable sites,⁷⁰ common in water adsorption by non-porous or macroporous amine adsorbents. The water capacity increases exponentially with RH, and largely independently of temperature. At 21% RH, the capture capacity measured by DVS was 1.84 mmol H₂O vapour per g, higher than the typical water uptake reported for amine functionalised sorbents (1.10 mmol g⁻¹).⁷¹ It can be seen that the amount of adsorbed H₂O vapour obtained by DVS significantly differs from the water capacity measured by TGA at the same RH. However, comparisons between these experiments are not entirely direct. Firstly, the activation conditions are different which may significantly affect capture capacities. Secondly, the TGA analyser was operated at 1 atm, with the material exposed to both argon and water. For DVS the material was exposed only to water and the pressure inside the DVS chamber was kept at 0.015 bar to reach the desired 21% RH at 40 °C. Such high variation in the gas pressure may result in the reduction of the surface energy, leading to adsorption-induced strains, a deformation of solid surface in non-porous or macroporous materials, which can impact the amount of H₂O adsorbed.⁷²”

“Fig. 15 H₂O isotherms of 10 : 1 (R) at 30 and 40 °C.”