(461g) A Novel Computational Framework for the Rational Design of Shape Selective Separation and Catalysis

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).