(725e) Investigation of Surface Heterogeneity Behavior of Porous Silica Towards CO2 Adsorption: A Theoretical Approach

In recent years, mesoporous silica materials are profusely used as CO₂ capture adsorbents, because, these materials exhibit a unique combination of high surface area (>1000 m²/g), large pore sizes (3-30 nm) and high pore volume (2.6 cm³/g). In the present work, we address the physical and chemical features characterizing the interaction of CO₂ with the silica surface. For this study, we have considered the silica surface of our recently developed pore-expanded material (MCM-41) (A) (surface area: 1045.21 m²/g, pore volume: 2.58 cc/g and average pore size: 30 nm) through sol-gel method for CO₂ capture. Gravimetric adsorption study of CO₂ on A was conducted at four different temperatures (30, 45, 60, 75 ⁰C) and pressures ranging from 0-25 bar using Rubotherm magnetic suspension balance. Langmuir multi-site model was used to quantitatively determine the individual contribution of different surface groups of silica to the overall CO₂ adsorption capacity. Heat of adsorption values for different active sites was estimated using the temperature dependent form of Langmuir multisite parameter (bj).  The two different values of heat of adsorption (7.75 kJ/mol and 22.19 kJ/mol) clearly indicated that CO₂ interacts with site 1 (siloxane bridges) through dispersive forces and site 2 (silanol groups) through hydrogen bonding.  Interestingly, it was observed that the surface apparently becomes homogeneous in nature as the temperature increases. This is because, at low temperature all the sites don’t have the same energy, and rather there is a distribution of energies. At higher temperature (~75 ⁰C), the adsorbing sites of A became energetically uniform resulting in an almost homogenous surface. To find out the fraction of surface covered by the two functionalities at different temperatures, we developed a methodology relating the maximum adsorption capacity obtained by Langmuir and Langmuir multi-site model. The shift of α value from 0.82 to 0.97 with increase in temperature indicated the less effectiveness of the H-bond and existence of the dispersive force only for CO₂ adsorption.