CO2 capture using different hierarchically ordered porous silica materials

Figure 6 shows the CO2 adsorption isotherms of HSMs(D) and HSMs(DC), and Table 1 lists the CO2 adsorption capacities measured at 273, 298 K, and 1.0 bar. As can be seen in Figure 6, the amount of CO2 adsorption for the unmodified series of HSMs(D) and HSMs(DC) ranged from 1.00 to 1.64 mmol/g. All the isotherms exhibited low adsorption contents at low pressure and high adsorption contents at high pressure, suggesting that the isotherm adsorption profiles were attributable to the predominance of physical adsorption processes. Moreover, it was observed that these samples of CO2 adsorption performance correlated with the pore structure. As shown in Figure 6, the order of adsorption capacity for HSMs(D) series at 273 K and 1.0 bar was HSMs(D)-80 > HSMs(D)-30 > HSMs(D)-50; the adsorption performance was not directly related to the specific surface area and pore volume. As shown in Table 1, although HSMs(D)-30 had the lowest BET-specific surface area and pore volume, its CO2 adsorption capacity of 1.1 mmol/g was slightly more than that of HSMs(D)-50 (1.0 mmol/g) because of the increased microporous content of HSMs(D)-30 (Smi = 210 m2/g) compared with that of HSMs(D)-50 (Smi = 70 m2/g). Moreover, HSMs(D)-80 had the highest CO2 adsorption capacity of 1.22 mmol/g due to the relatively high specific surface area and appropriate micropore content. As shown in Figure 6, the CO2 adsorption capacity was in the following order: HSMs(DC)-0.15 > HSMs(DC)-0 > HSMs(DC)-0.5 > HSMs(DC)-2, and the highest CO2 adsorption amount reached 1.64 mmol/g for HSMs(DC)-0.15 at 273 K and 1.0 bar. Comparability, for the HSMs(DC) series, an increase in CO2 adsorption capability was observed. This result was mainly attributed to the higher micropore surface area and larger pore volume compared with those of HSMs(D). According to the previous discussion, high CTAB content in the composite template was beneficial for improving the micropore surface area and mesopore size of the hierarchical porous silica. Therefore, the effect of the pore structure of HSMs(DC) played an important role in the improvement of CO2 adsorption capability. The samples with higher micropore surface areas and pore volumes exhibited an enhanced ability to capture CO2 molecules. Furthermore, the existence of mesopores in the hierarchical porous silica material facilitated the flow of CO2 into the material because of the smaller diameter of the CO2 molecule (CO2: 3.3 Å) compared with that of the mesopore [47]. In addition, as the adsorption temperature rose to 298 K, as shown in Figure S4 (Supporting Information), the CO2 adsorption capability of the samples significantly declined, confirming that the interaction between the CO2 molecule and sorbent surface was attributable to physical absorption.”


Figure 6. CO2 adsorption isotherms of samples at 273 K.”


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