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Influence of cross-linked materials development and adsorption temperature

 

Fig. 7(a–c), shows that the maximum CO2 adsorption capacity is consistently higher for the lesser cross-linked materials and is consistently highest for 25 : 1 (V). This may be expected given that 25 : 1 (V) has the highest amount of primary and secondary amines relative to triazine cross-linker. 25 : 1 (V) and 10 : 1 (V) show maximum final CO2 uptake at 60 °C, reaching 2.45 mmol g−1 and 1.95 mmol g−1, respectively. Their maximum uptakes at 30 °C are 1.90 mmol g−1 and 1.73 mmol g−1, respectively, but saturation is not reached at this temperature within 2 hours, indicative of slow adsorption kinetics. Their adsorption rates increase with temperature, as after 10 minutes 10 : 1 (V) adsorbs 39.49% of its final capacity at 30 °C, 81.88% at 60 °C and 93.54% at 90 °C, and 25 : 1 (V) adsorbs 38.26%, 78.66% and 92.35%, of its final capacity at 30 °C, 60 °C and 90 °C, respectively. Their slower adsorption and lower uptakes at 30 °C suggest that 10 : 1 (V) and 25 : 1 (V) are limited in their adsorption potential by a diffusional barrier; however, increasing temperature results in decreased hydrogen bonding between polyamine chains and higher chain mobility, enabling CO2 to overcome this diffusion resistance, and uptake increases, as has been frequently observed for PEI-based CO2 adsorbents.23,24,27,56 From 30 °C to 60 °C, the maximum adsorption of 10 : 1 (V) increases by 12.47% (relative to adsorption at 30 °C), while 25 : 1 (V) improves by more than twice that at 29.00%. Although 25 : 1 (V) has better absolute adsorption capacity, 10 : 1 (V) may be experiencing relatively less diffusion resistance at 30 °C, which may suggest a structure that better enables gas transport. Both materials then show a reduction in their adsorption on moving from 60 °C to 90 °C, as previously observed on increasing temperature,23,24 due to the shifting of the thermodynamic adsorption–desorption equilibrium.57

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Fig. 7 TGA–CO2 sorption (mmol g−1) of PEI-4BMFPT 1 : 1 (V), 3 : 1 (V), 5 : 1 (V), 10 : 1 (V), 25 : 1 (V) at (a) 30 °C; (b) 60 °C; (c) 90 °C, under 90% CO2.”

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