Published: June 06, 2011 r2011 American Chemical Society 8187 dx.doi.org/10.1021/la2012765 | Langmuir 2011, 27, 8187–8197 ARTICLE pubs.acs.org/Langmuir Enhanced CO 2 Solubility in Hybrid MCM-41: Molecular Simulations and Experiments Linh Ngoc Ho, † Javier Perez Pellitero, † Fabien Porcheron,* ,† and Roland J.-M. Pellenq ‡,§ † IFP Energies nouvelles, Rond-Point de l'  Echangeur de Solaize, BP 3, 69360 Solaize, France ‡ Centre Interdisciplinaire des Nanosciences de Marseille, CNRS, Campus de Luminy, 13288 Marseille, Cedex 09, France § Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ’ INTRODUCTION Controlling and reducing the concentration of CO 2 in the atmosphere has been gaining significant public and scientific attention as an effective mitigation measure against global warming. The most dominant source of CO 2 has been inarguably pinpointed to be due to ever-growing industry activities in the last century, particularly from the use of fossil fuels. For instance, one important source of carbon dioxide emissions comes from coal-fired power stations where the flue gas at atmospheric pressure is predominantly composed of N 2 (∼90%) with a small fraction of CO 2 (∼10%). Until now, a large number of technol- ogies have been adopted to removing CO 2 from flue gas streams, for example, amine scrubbing, adsorption, membrane diffusion, and cryogenic distillation. 1À5 From these conventional techniques, the combined advantages of both absorption (a high selectivity and capacitive solvent) and adsorption (absence of corrosion, reduced solvent loss) have recently been materialized in a new generation of hybrid adsorbents. 6 However, the greatest challenge for this approach is to synthesize suitable adsorbents displaying high CO 2 selectivity and an affordable regeneration cost. A majority of work in this area has focused on grafting amine molecules into a solid support. 7À10 In most of those studies, the authors expect a strong interaction between amines and CO 2 , similar to the mechanism of a conventional chemical absorption process. An additional advantage of this approach is that solids are simple to handle and do not give rise to corrosion problems. As a result, the use of physical solvents in hybrid adsorbents has yet to receive adequate attention from researchers because of their characteristic low CO 2 solubility at low pressures. In recent work, Song et al. 11,12 observed the apparition of enhanced CO 2 solubility in a hybrid adsorbent composed of an aminated polymer confined within MCM-41. In other words, the CO 2 solubility in the hybrid adsorbent was found to be greater than that in the bulk fluid and in the raw solid. The microscopic mechanisms governing this behavior, however, were not fully established. Moreover, Miachon et al. 13,14 noted that a remark- able enhancement of hydrogen and light hydrocarbon solubility can be achieved when solvents are confined in mesoporous solids. For instance, by confining hexane in low-density mesoporous materials (i.e., silica aerosol), the authors observed an “oversolubility” Received: April 7, 2011 Revised: May 19, 2011 ABSTRACT: Grand canonical Monte Carlo simulations are performed in a hybrid adsorbent model in order to interpret the CO 2 solubility behavior. The hybrid adsorbent is prepared by confining a physical solvent (OMCTS) into the pores of a mimetic MCM-41 solid support. As a result, simulated adsorp- tion isotherms of CO 2 nicely match the experimental data for three distinctive systems: bulk solvent, raw MCM-41, and hybrid MCM-41. The microscopic mechanisms underlying the apparition of enhanced solubility are then clearly identified. In fact, the presence of solvent molecules favors the layering of CO 2 molecules within the pores; therefore, the CO 2 solubility in the hybrid adsorbent markedly increases in comparison to that found in the raw adsorbent as well as in the bulk solvent. In addition, a good understanding of confined solvents’ properties and solid surface structures is essential to fully evaluate the efficiency of hybrid adsorbents in capturing CO 2 . The sorbentÀsolid interactions along with the solvent molecular size’s impact on CO 2 solubility are therefore investigated in this study. We found that an ideal hybrid system should possess a weak solventÀsolid interaction but a strong solventÀCO 2 interaction. Besides, an optimal solvent size is obtained for the enhanced CO 2 solubility in the hybrid system. According to the simulation results, the solvent layer builds pseudomicropores inside the mesoporous MCM-41, enabling more CO 2 molecules to be absorbed under the greater influence of spatial confinement and surface interaction. In addition, the molecular sieving effect is clearly observed in the case of larger solvent molecular sizes.