Influence of CO2 concentration on cross linked polymer based carbon capture

“Gravimetric experiments were carried out using dilute CO2 mixtures of 10% CO2/N2 and 400 ppm CO2/N2, simulating the concentration of CO2 in flue gas and air, respectively. Due to the high selectivity for CO2, the weight increase from the gravimetric experiment is interpreted as pure CO2 uptake. As generally expected for an amine-based adsorbent, performance is lower under a more dilute CO2 source.27 Under 10% CO2/N2, 10 : 1 (R) adsorbed 1.50 mmol g−1 CO2 at 30 °C, 1.42 mmol g−1 at 60 °C, and 0.55 mmol g−1 at 90 °C, after 180 minutes dynamic adsorption, as shown in Fig. 13(a) and Table 3 (with normalised adsorption data shown in Fig. S16, ESI). Here, 10 : 1 (R) follows a similar trend of adsorption seen under 90% CO2: maximum adsorption is highest at 30 °C. After 10 minutes at 60 °C, 10 : 1 (R) takes up 1.01 mmol g−1 CO2, while at 30 °C, it adsorbs less than half of this at 0.47 mmol g−1. It is only after 115 minutes that adsorption at 30 °C starts to surpass adsorption at 60 °C; this is much later than under 90% CO2, for which adsorption at 30 °C overtakes that at 60 °C after just 43 minutes. The slow kinetic uptake at 30 °C suggests that under the more dilute CO2 environment of 10% CO2, a higher temperature to overcome diffusion limitation is more beneficial for uptake than it is detrimental, over a longer duration. Under dilute CO2, the probability of each amine coming into contact with CO2 is reduced (therefore maximising the number of accessible amines is imperative). Once the surface amines are saturated, the CO2 reacts with the internalised amines, and due to temperature assisted diffusion enhancement, this occurs more readily at 60 °C than at 30 °C. Given sufficient time at 30 °C, in terms of total uptake, the thermodynamic benefit outweighs the benefit of enhanced diffusion.”


Fig. 13 TGA–CO2 sorption (mmol g−1) of 10 : 1 (R), in (a) 1 atm 10% CO2/N2 (b) 1 atm 400 ppm CO2/N2.”


“Under 400 ppm CO2 10 : 1 (R) behaves entirely differently and uptake is significantly lower, Fig. 13(b). Adsorption is both fastest, and highest at 30 °C, with a final capacity of 0.20 mmol g−1 CO2, decreasing to 0.09 mmol g−1 and 0.01 mmol g−1 at 60 °C and 90 °C, respectively. Uptake is fastest until about 30 minutes, then the rate of adsorption decreases sharply at 30 °C and more so at 60 °C, whereas for 90 °C, desorption occurs after about 20 minutes. Under lower CO2 partial pressures, the heats of adsorption for amine-based adsorbents are higher due to the dominance of the most strongly interacting amine sites.67 It is likely that within the first half-hour, the most reactive surface amines become saturated. The evolved heat may not be fully dissipated, resulting in some desorption after around 40 and 20 minutes at 60 and 90 °C, respectively. After saturation of the surface amines, adsorption is via the less reactive and the more internalised amines, therefore it may occur at a lower rate partly due to slow diffusion of CO2. Rather than promoting diffusion and increasing adsorption, higher temperatures have a severely detrimental effect. This trend has also been reported by Goeppert et al. for silica-supported PEI adsorbents operating under air capture conditions.68 The lower driving force under the lower partial pressure of 400 ppm may overcome the benefit of enhanced diffusion at higher temperatures, as a greater decrease in entropy is required for adsorption.

Comparing the final adsorption capacities at 30 °C under 400 ppm and 10% CO2, adsorption is reduced by a factor of 7.5 at the lower partial pressure. This is in line with what was observed in lower amine-loaded hyperbranched aminosilica adsorbents reported by Choi et al.69 Therefore, it can be inferred that with greater amine functionality, the CO2 capture performance of 10 : 1 (R) under 400 ppm CO2 may be improved.”

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