Design of Hybrid Organic/Inorganic Adsorbents Based on Periodic Mesoporous Silica Christian Schumacher, †,§ Jorge Gonzalez, ‡ Manuel Pe ´ rez-Mendoza, †,| Paul A. Wright, ‡ and Nigel A. Seaton* ,† Institute for Materials and Processes, School of Engineering and Electronics, UniVersity of Edinburgh, King’s Buildings, Mayfield Road, Edinburgh EH9 3JL, U.K., and School of Chemistry, UniVersity of St. Andrews, St. Andrews, Fife KY16 9ST, U.K. We are interested in the design of hybrid organic-inorganic adsorbents (HOIAs) for gas separation and storage. In this paper, we propose a methodology for the design of HOIAs based on periodic mesoporous silicas. We use a method based on kinetic Monte Carlo simulation to generate realistic model adsorbents and then carry out Grand Canonical Monte Carlo simulations of adsorption in these model materials. Good agreement is obtained between the adsorption behavior of the model adsorbents and that of the corresponding real materials, giving confidence that we can identify optimal structures for particular gas-separation applications. We demonstrate this capability using, as an example, the pressure-swing adsorption of carbon dioxide from flue gas, using a HOIA based on MCM-41. Introduction Periodic mesoporous silicas (PMSs) are porous materials prepared from a silica source in a surfactant solution, in which the surfactant molecules form a liquid-crystal phase which serves as a template for the porous material. PMSs are, at an atomic level, either amorphous or, at most, only partially crystalline, but they nevertheless have a well-defined pore structure with long-range order. 1-3 Various pore sizes and shapes can readily be prepared. 4-8 The chemical properties of the pore surface can be modified by lining it with organic groups, either in situ by cocondensation of a triethoxysiloxane containing the desired functional group or postsynthesis by grafting the triethoxy- siloxane to the surface of the previously prepared pure-silica material. 9-13 Because both the pore structure and the surface composition of PMSs can be tailored, they are of interest in applications including catalysis, 14,15 controlled drug release, 16 and adsorption. 17-19 We are interested in the design and synthesis of surface- modified, PMS-based adsorbents for gas separation and storage applications. (In this context, we refer to the materials as hybrid organic-inorganic adsorbents, or HOIAs.) The effective design of these materials requires a method that can relate the structure of the adsorbent to its performance in the application of interest. The material design calculation is then the optimization of the pore structure and surface composition (within the scope of materials that it is likely to be feasible to synthesize) with respect to an appropriate objective function. The optimized structure then becomes a target for the materials synthesis activity, minimizing the experimental effort that would otherwise be involvedsboth in materials synthesis and in measuring the performance of adsorbents under operational conditions. The accuracy of a design method of this type depends on the realism with which it represents the structure of the real material. For crystalline solids, such as zeolites, the structure is known more-or-less exactly (except for defects and other deviations from the crystalline state, such as the charge-balancing cations in zeolites). For noncrystalline materials, such as PMSs, the link between the real and simulated materials is statistical. Several approaches with different levels of detail have been taken to generate models for the amorphous walls of PMSs. For instance, MCM-41, a PMS consisting of straight pore channels in a regular hexagonal arrangement, has been modeled as isolated one-dimensional channels with regular 20 or disordered walls. 21 Models that include the geometric arrangement of neighboring channels were generated by random distribution of atoms around the pore volume, 22 by cutting pores out of solid amorphous silica obtained from the molecular dynamics (MD) simulation of the annealing of molten glass, 23 or by relaxation of randomly generated amorphous silica walls by MD simulation at high temperature. 24 However, these models are not based on the physics of the actual synthesis reaction, so their ability to predict adsorption (without the tuning of the structure for particular adsorption systems) is likely to be limited. Our approach, in contrast, is to generate the model material by carrying out a kinetic Monte Carlo (kMC) simulation of the hydrothermal synthesis and calcination of PMSs, in the expectation that a model based on the physics of the synthesis is likely to be more realistic than models generated in a more ad hoc way. In an earlier paper, we demonstrated that this approach does indeed give realistic structures for a MCM-41, a PMS consisting of cylindrical pores in a hexagonal arrangement. We simulated adsorption in the model MCM-41 materials, using the Grand Canonical Monte Carlo (GCMC) method, and found very good agreement with adsorption in the corresponding real materials. 25 In this paper, we extend this approach to the generation of HOIAs by replacing a number of surface silanol groups by organic functional groups. To validate this method, models of phenyl-MCM-41 and aminopropyl-MCM-41 were generated for the simulation of the adsorption of ethane and carbon dioxide, covering nonpolar and polar functional groups and adsorptive gases. The simulation accurately predicts the experimental isotherms in the case of in situ-functionalized phenyl-MCM- * Address correspondence to this author. E-mail: n.seaton@ed.ac.uk. Tel.: (+44) 131 650 4867. Fax: (+44) 131 650 6551. † University of Edinburgh. ‡ University of St. Andrews. § Current address: Environmental Control Systems, Airbus Deut- schland GmbH, Kreetslag 10, 21129 Hamburg, Germany. | Current address: Departamento de Quı ´mica Inorga ´nica, Facultad de Ciencias, Universidad de Granada, Avda. Fuentenueva s/n, 18071 Granada, Spain. 5586 Ind. Eng. Chem. Res. 2006, 45, 5586-5597 10.1021/ie051191n CCC: $33.50 © 2006 American Chemical Society Published on Web 04/04/2006