Surface Heterogeneity Affects Adsorption Selectivity for CO2 Over CH4 in Bare Mesostructured Silica with 2D Hexagonal Symmetry and Different Pore Size

Mesoporous silica-based materials are used as sorbents and supports in many fields. The ordered pore architecture of MCM-41, and the absence of interconnections, make it suitable as a model system. The surface is natively functionalized by many silanol groups, endowing the material with a polar surface to directly interact with the target species or to mount additional functional groups. Either bare or functionalized, surface silanol arrangement is crucial to material performance. In the case of CO2 capture with amine-functionalized silica, silanol groups are responsible for fundamental H-bonds during chemisorption, but they also modulate the effect and weight of humidity on the material performance. In addition, the silanol groups can also tune the weight of physisorption over chemisorption. The relationship between these aspects and the textural features of mesostructured silica has not been addressed in detail. In this study, computer models are employed to investigate the adsorption capacity and selectivity of bare mesostructured silica with three different pore sizes. Results are rationalized in terms of silanol surface density and pore curvature. The importance of the energetic inequivalence between Q3 and Q2 sites on the silica surface is emphasized, as this causes adsorption behavior to deviate from ideality.