Modeling of Adsorption in Templated Porous Materials

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ABSTRACTCTS-9876599VanTassel, P. RWayne State U.Materials with controllable pore structures are extremely useful as adsorbents catalysts, and sensors. A challenge is to engineer a pore architecture tailored to a specific application. A promising new approach is to form the material via a gelation of monomer units in the presence of a removable template. Once the gelation is complete, the template is removed and, in theory, the final pore structure will mimic the structure of the template. By selecting the right template, one can in principle create a material with a pore architecture specific for a given purpose. Templated materials have tremendous potential as adsorbents, sensors, gas separation membranes, and molecular recognition agents.A current limitation to developing template approaches is the lack of a universal, quantitative understanding of the influence of template morphology and concentration on the material's pore structure and, subsequently, on the behavior of molecules adsorbed in the material. What is needed is a theoretical description sufficiently general so that template strategies may be intelligently formulated for novel materials of diverse chemical compositions. Currently available models of porous materials are suboptimal because they do not accountexplicitly for the influence of the template. It is proposed to establish a theoretical framework through which the structure of templated porous materials and the behavior of their adsorbedmolecules may be modeled.Templated porous materials are usually formed by gelation in the presence of template molecules. Following aging and drying, the templates are removed from the solid by thermal or other treatment, leaving behind their molecular scale imprints. This process inspires the model that we propose. Our model begins with a binary system of particles representing matrix and template components. Following equilibration, the particles are quenched, that is, frozen in space. This mimics the experimental gelation step. Next, the template particles are removed. The remaining matrix particles are the model porous material. To these immobile matrix particles mobile (model) adsorbate molecules are added and their properties are investigated. The project begins by establishing molecular computer simulation and integral equation techniques to investigate this model system. Matrix, template, and adsorbate are considered to be simply shaped particles (spheres, spherocylinders, and/or linked chains of spheres) that interact via potential functions such as hard particle, Lennard-Jones, and continuum Lennard- Jones. Grand canonical Monte Carlo and Ornstein-Zernike integral equation techniques will be used to calculate the adsorption isotherm, phase diagram, and heat of adsorption. Grand canonical molecular dynamics will be used to calculate the rate of permeation through a templated material. The principal goal is to understand the effect that template size, shape, and concentration have on the behavior of adsorbed phase.Next, efforts are to be directed toward modeling specific systems for which experimental data are available: small molecules adsorbed in organic templated silica gels [2,3]. The density of matrix particles will serve as a single adjustable parameter; we intend to relate this parameter to the synthesis conditions (drying rate, temperature of template removal, etc.). The principal goal here is to establish a connection between our model and experiment. Finally, a web page is to be established where our computer programs, written specifically to apply the models developed here to experimental data, can be obtained by other researchers. The significance of this work is in establishing and disseminating a universal theoretical framework - capturing the essential features of disorder, pore space connectivity, and template removal - for modeling adsorption phenomena in templated porous materials.