CAREER: Molecular Transport Theory for Nanoporous Solids

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CTS-9734153 Auerbach Most environmentally friendly technologies rely on separations that remove pollutants from benign materials. Zeolite molecular sieves, consisting of regularly spaced, molecular-sized cages shown below, are ideally suited for these separations. Although much is known about zeolites, no simple theory exists for accurately predicting how molecules diffuse through them. This is because of the difficulty in modeling infrequent event dynamics coupled with strong adsorbate-adsorbate interactions. Furthermore, zeolites are ignored by the single most important class in physical science: freshman chemistry. To address these issues, the Principal Investigator will develop novel educational techniques and molecular transport theories aimed at accelerating material and process screening of emerging chemical technologies. The mobility of many hydrocarbons in industrially important zeolites is exceedingly sluggish, precluding the use of straightforward molecular dynamics simulations, and hence requiring modeling techniques specialized for infrequent events. Previous transition state simulations of diffusion in zeolites were performed at low loadings, so that adsorbate-adsorbate interactions could be neglected. These conditions are irrelevant to industrial processes, which are carried out at essentially full hydrocarbon loading. To model transport in zeolites under industrial conditions, the Principal Investigator will develop a hierarchy of three new techniques with varying accuracy and speed. An accelerated molecular dynamics algorithm that adds a novel boost potential for reducing site-to-site activation energies will predict essentially exact diffusivities provided that the boost avoids transition states. A lattice Monte Carlo simulation will be developed that includes a new treatment of activation energies modified by adsorbate-adsorbate interactions. Finally, a completely analytical theory will be derived that synthesizes for the first time fluctua ting activation energies with the mean field theory of fluids. This new hierarchy provides the opportunity of choosing a model based on the need for speed or accuracy in process optimization. These models will be incorporated into a freshman chemistry module entitled: "Zeolites in Industry," involving new Web-based, interactive software that mimics the physical and chemical properties of these systems. This research will impact science and society on many levels. First, incorporating zeolite science and technology into freshman chemistry will excite and inform thousands of students about real-world benefits of industrial chemistry and engineering. Second, this novel combination of chemical reaction dynamics and mean field theory will yield the first truly molecular theory for transport in zeolites, elucidating how the competition between host-guest and guest-guest interactions controls these transport processes. This will guide and complement new diffusion measurements, and will accelerate screening of new chemical processes. Third, when applied to aromatic separations and hydrofluorocarbon separations in large-pore zeolites, these calculations will help guide scientists and engineers in choosing optimal zeolite Si:Al ratios, zeolite channel structures, temperatures and pressures for carrying out efficient and cost-effective separations.