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CO2 adsorption using PEI modified SCF

https://doi.org/10.1039/C8TA06224B

Fig. 5 shows the adsorption profiles of representative PEI-modified SCF adsorbents for each series of SCF materials synthesised. Despite the considerable variation of optimal adsorption temperatures for different SCFs, the CO2 adsorption capacities were all measured at the same temperature of 75 °C for the sake of consistent comparison. As can be seen from Fig. 5, all SCF samples showed significantly faster adsorption kinetics and higher CO2 capacities than the commercial benchmark silica (PQ Silicas), which is a highly meso-structured siliceous material with its pore volume, pore diameter and window size being similar to or even greater than those of the SCFs (Table 2). The times taken for the SCF adsorbents to achieve 80% and 90% of equilibrium CO2 capacity were found to be just ca. 1.5 min and 3 min, which is nearly 8 times faster than the ca. 12 and 23 min obtained for the PQ silica, respectively. This highlights the novel accessibility of the immobilised amines to CO2 due to the high levels of pore network connectivity in the SCF materials.”

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Fig. 5 CO2 adsorption profiles of representative PEI-impregnated SCF adsorbents in 15% CO2/N2 at 75 °C.”

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Fig. 6 Variation of the CO2 uptake of SCF materials with precursor chemistries and other preparation conditions.”

Table 3 compares the CO2 adsorption performance for all PEI-600-modified SCF materials at different PEI loading levels, while Fig. 6 shows the variation of CO2 uptake with the precursor chemistry and preparation conditions used. All PEI-modified SCF adsorbents showed increased CO2 uptake capacities with increasing levels of PEI but with greater variations in CO2 capacity, determined by the preparation conditions. Among all the materials, SCF-3-120-24 gave rise to the highest CO2 adsorption capacity of 180.5 mg per g-adsorbent at a PEI content of 70 wt%, followed by SCF-2-100-42 (178.4 mg CO2 per g) and SCF-3-100-42 (176.5 mg CO2 per g), which are all significantly higher than those obtained previously for amine-modified mesoporous cellular foam materials38–40 and virtually any other 2D mesoporous silica materials.28,35,50–52 At 50 wt% and 60 wt% PEI, however, the highest CO2 uptake was instead obtained for SCF-1-100-60 at 150.4 and 168 mg g−1, respectively. It is noteworthy that with a total pore volume of just 1.1 cm3 g−1, the SCF material prepared without the pore-expanding agent (SCF-0-100-24) also achieved a higher CO2 capacity of 138 mg-CO2 per g-ads than that of the baseline PQ silica material (127.4 mg CO2 per g-ads) with an over 50% higher meso-pore volume (1.7 cm3 g−1), highlighting the importance of the 3D pore connectivity of the SCF materials. It appears that for the SCF materials prepared with lower TMB concentrations, there generally exists a maximum PEI loading level beyond which the CO2 adsorption capacity decreases with further increase in PEI loading, being consistent with previous findings on amine-modified MCFs.38,53 However, the same trend was not observed for those prepared with higher TMB concentrations or aging temperatures, which showed much lower CO2 uptake capacities but with a sharp continuous increase with increasing PEI loading up to 70 wt%. Nevertheless, no further appreciable increases in CO2 uptake were obtained when levels of PEI impregnation were increased to higher than 70 wt% and the prepared sorbents started to become pasty.”

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