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A Novel Computational Framework for the Rational Design of Shape Selective Separation and Catalysis

Chrysanthos E. Gounaris, Christodoulos A. Floudas, and James Wei. Chemical Engineering, Princeton University, Princeton, NJ 08544

The use of zeolites as molecular sieves and catalysts has today been well established in a wide variety of processes. However, almost the totality of applications involves a very small number of nearly circular structures, like Linde Type A, Faujasite and ZSM-5. These structures are usually modified to meet the specific needs of each process. Modification techniques, such as ion exchange or coke deposition, usually result in a distribution of pore sizes and shapes, something that retards the ability of the zeolite to be highly selective. On the other hand, there is a great variety of natural and synthetic zeolites that have been developed, but no significant effort is being made to find potential catalysis and separation applications for them. There can very well be existing structures that are highly selective in their unmodified state, or requiring a small amount of modification, just because their windows happen to be of the proper size and shape. The principal aim of this work is to develop a systematic computational framework that can identify such zeolite structures and provide researchers with a rigorous way to determine the best candidate portals for the process of their interest.

In this work, we propose new mathematical models for the identification of the optimal molecular orientations that are likely to be explored when a guest molecule is approaching a host portal. These orientations usually result to projections on the portal plane (called footprints) that are as compact as possible. Various criteria are used. All of them aim to minimize the area of enclosing regular shapes or minimize sums of distances involving the molecule's atom nuclei.

In the cases where the molecule or the portal are highly non-regular (as it is very often the case), one has to take simultaneously into account the exact shapes of the guest molecule and the host portal. In order to address this issue, we introduced also a method that has an energetic basis and is based on the concept of Strain Index, which is a measure of the distortion needed for a given molecule to penetrate through a given portal. According to this approach, a molecule and a portal are sets of soft spheres that can be squeezed so as penetration to take place. An optimization framework was developed that robustly calculates the host / guest conformation that exhibits the least distortion. Given that different molecules require different amounts of distortion (expressed as activation energies), one can use the strain index results to calculate selectivities between sets of molecules, and identify the most potential structures to be used in separation or catalysis applications. Computations were performed for a wide collection of 38 molecules and 217 zeolite windows (used as portals) and the results are compiled in a large database. Selectivity results, along a wide range of temperatures were studied for commercially interesting applications [1,2].


[1]     C. E. Gounaris, C. A. Floudas, and J. Wei, Rational Design of Shape Selective Separation and Catalysis: I. Concepts and Analysis. Chemical Engineering Science, submitted for publication (2006).

[2]     C. E. Gounaris, J. Wei, and C. A. Floudas, Rational Design of Shape Selective Separation and Catalysis: II. Mathematical Model and Computational Studies, Chemical Engineering Science, submitted for publication (2006).