(612g) High Performance Mesoporous Silica-Based Materials for CO2 Adsorption in Humid Conditions

Adsorbents for CO₂ capture are the subject of numerous developments to improve their performance. The presence of water in post-combustion emissions can affect the efficiency of the porous materials studied for CO₂ adsorption. Many materials have been studied to date, but few with the aim of controlling the competition/cooperation of H₂O on CO₂ adsorption. Furthermore, understanding the CO₂ adsorption process on these materials, particularly in the presence of water, is complex and requires further research.

In this work, a specific study is carried out on the CO₂ adsorption properties in the presence of water of adsorbents that have been synthesized or modified to improve their hydrophobicity. To achieve this objective, not only CO₂ and H₂O adsorption isotherms are measured by manometry, but also CO₂/H₂O co-adsorption experiments are performed either in a dynamic process (fixed-bed breakthrough) or static process (thermogravimetry). In addition, calorimetric adsorption measurements provide information on the energy involved and the type of interaction between CO₂ and the adsorbent.

For this challenging task, a mesoporous silica type SBA-15 material has been chosen whose hydrophobicity can be tuned by two processes either within the solid matrix or on the surface. Thus SBA-15 materials have been synthesized by adding a silica source such as BTESE (1,2-Bis(triethoxysilyl)ethane) to ethylorthosilicate, or by grafting APTES (3(triethoxysilyl)propylamine) onto standard SBA-15 materials. The first solution aims to modify the structure by adding ethyl group in the pore walls silica. The second way is to add amino groups to change the surface properties.

CO₂ adsorption capacities are discussed in relation to the textural characteristics measured by the standard N₂ adsorption technique at 77K. For example, the quantities of CO₂ adsorbed are relative to the surface area or pore volume, which significantly decrease in the case of the grafted samples.

On the one hand, an enhanced affinity between this grafted silica and CO₂ results in an adsorption capacity of 1.25 mmol.g^-1at 1bar 303K, higher than the standard SBA-15. Calorimetric measurements confirmed the high-energy interaction between CO₂ and the grafted materials compared with SBA-15, leading to a partly irreversible adsorption. This adsorbent shows a lower affinity for water, but remains not negligible (due to the presence of amine groups), which means that competition between CO₂ and H₂O is possible.

On the other hand, for materials structurally modified by incorporating alkyl groups into the pore walls, contrary to the expected effect, affinity for water is retained due to the presence of residual silanols. The surfactant extraction process used during their synthesis preserves them to a greater extent than conventional calcination. Due to the presence of these SiOH groups, there is a real competition between CO₂ and H₂O.

Competitive breakthrough adsorption experiments have confirmed the competitive adsorption on the same sites for CO₂ and H₂O molecules for both optimized materials but also highlighted that, in certain conditions, H₂O adsorption promotes CO₂ adsorption in the form of bicarbonates or others carbonates.

On the optimized modified SBA-15 material, this synergistic effect between silanols and H₂O molecules to increase CO₂ adsorption capacity, does not prevent reversibility, so the competition between H₂O and CO₂ can be used to recover CO₂ through desorption. Whereas the optimized grafted SBA-15 material leads to high selectivity towards CO₂ compared with H₂O, which could be useful for CO₂ storage.

In conclusion, these optimized silicas are promising materials for significantly capturing CO₂ in the presence of water, a breakthrough in energy-efficient capture gas and storage processes.