Fumed silica, SBA-15 and MCF silicas for CO2 capture

https://doi.org/10.1002/ente.201402211

“The CO₂ capacity of the chitosan/silica composites was measured with two different methods: volumetric and gravimetric adsorptions. The volumetric CO₂ adsorption capacity of the 19 wt % chitosan/fumed silica sample was found to be 0.29 mmol g⁻¹ and that measured gravimetrically was 0.09 mmol g⁻¹ (Table 3). No uptake of CO₂ was recorded from the gravimetric measurement for a pure fumed silica sample (see Figure S5 in the Supporting Information), which indicates that the CO₂ uptake from these samples is attributed to the coated chitosan. With consideration that pure chitosan has a CO₂ adsorption capacity of approximately 0.02 mmol g⁻¹,21 our results show a remarkable 23-fold increase in capacity per unit mass of chitosan (0.47 mmol g), a unique property of these chitosan/silica composites. The higher capacity measured from the volumetric analysis than from the gravimetric measurement is consistent with other works in the literature.26 The difference between these two measuring systems is that the gravimetric analysis was carried out in a dynamic flow system, whereas a static closed system was used for the volumetric measurement. Moreover, a 50 % CO₂ in N₂ purge stream was used for the gravimetric measurements, instead of 100 % CO₂ for the latter. In the next section, we will discuss the effect of CO₂ concentration in the gas adsorbate on the CO₂ adsorption capacity.”

“The results of the volumetric CO₂ adsorption analysis for the chitosan on mesoporous silica supports (SBA-15 and MCFs) are shown in Figure 5 and Table 3. The chitosan/MCF-3 sample has the highest adsorption capacity at 0.98 mmol g⁻¹, whereas other samples vary from 0.75–0.80 mmol g⁻¹. These recorded capacities are comparable with other high-surface-area nanomaterials designed for CO₂ adsorption, such as metal organic frameworks (for example, MOF-5 with a capacity of approximately 2 mmol g⁻¹, measured with a volumetric method).27 Indeed, we can compare this result in terms of adsorption efficiency (that is, CO₂/NH₂ ratio) with an adsorbent of similar structure, NH₂-MCM-41, which gives a CO₂ adsorption capacity of 1.0 mmol g⁻¹ at P=1 atm and T=20 °C.15 NH₂-MCM-41 has a concentration of 2.48 mmol g⁻¹ of NH₂ groups, which results in a CO₂/NH₂ ratio of 0.4. The chitosan/MCF-3 sample has a concentration of approximately 0.76 mmol g⁻¹ of NH₂ groups (based on the elemental analysis) and a CO₂/NH₂ ratio of 1.3, which can be viewed as a 3-fold increase in efficiency compared with that of NH₂-MCM-41. In theory, the adsorbent with the highest BET surface area should adsorb the highest amount of CO₂ as a result of physisorption. However, this was not observed in this case because chitosan/MCF-3 had the lowest BET surface area among all of the samples except chitosan/fumed silica (Table 1). There is also no clear correlation between the pore volume of the adsorbents and their CO₂ adsorption capacity (Table 1 and 3), which suggests a complex relationship between adsorption and the structural parameters (surface area and pore volume). Notably, these structural parameters were measured for the whole composite, with no distinction between the gas adsorption (N₂ or CO₂) from the silica surface or from the chitosan coating. It would be difficult to pinpoint the critical structural factor influencing the overall CO₂ adsorption capacity.”

“Figure 5 – Volumetric CO₂ adsorption isotherms of chitosan/fumed silica (◊), chitosan/SBA-15 (○), chitosan/MCF-3 (×), chitosan/MCF-6 (□) and chitosan/MCF-10 (▵). The CO₂ adsorption capacity recorded is in the order: chitosan/MCF-3>chitosan/SBA-15>chitosan/MCF-10>chitosan/MCF-6>chitosan/fumed silica.”

“Similar to that for chitosan/fumed silica, the adsorption results from the gravimetric analysis show a lower capacity for all chitosan on mesoporous silica samples relative to the adsorption capacity from the volumetric method (Table 3). Nonetheless, the CO₂ adsorption capacity of all chitosan/silica composite samples follows the same trend; the gravimetric adsorption is approximately one third of the volumetric adsorption capacity. On comparison between these two techniques, the CO₂ adsorption capacity measured with the gravimetric method is more representative of the real adsorption from a flowing flue gas, in contrast to the volumetric measurement from a closed system. Therefore, many recent research works in the literature have adopted the gravimetric method for measuring the CO₂ adsorption capacity for adsorbents.28–30”

https://doi.org/10.1002/ente.201402211