Zeolite modeling

Sierraalta, A., Anez, R. and Brussin, M.-R. (2002) Theoretical study of N02 adsorption on a transition-metal zeolite model, J. Catal., 205, 107. 

Bastiaan van de Graaf, Swie Lan Njo, and Konstantin S. Smirnov, Introduction to Zeolite Modeling. 

Bandyopadhyay and Yashonath (31), in an extension of their work on MD studies of noble gas diffusion, presented MD results for methane diffusion in NaY and NaCaA zeolites. The zeolite models were the same as those used in the noble gas simulations (13, 15, 17, 18, 20, 28, 29) and the zeolite lattice was held rigid. The methane molecule was approximated as a single interaction center and the guest-host potential parameters were calculated from data of Bezus et al. (49) (for the dispersive term) and by setting the force on a pair of atoms equal to zero at the sum of their van der Waals radii (for the repulsive term). Simulations were run for 600 ps with a time step of 10 fs. 

Fig. 32. (a) Selected internal coordinates and partial charges for p-fluoronitrobenzene adsorbed on the zeolite model. p-Fluoronitrobenzene is weakly hydrogen bonded to the zeolite, but not protonated. Values in parentheses are those of an isolated, neutral p-fluoronitrobenzene molecule, (b) Optimized geometry for p-fluoroaniline adsorbed onto the zeolite. In this case, we started with the proton on the zeolite the optimization resulted in protonation of the adsorbate. (Reprinted with permission from Nicholas et al. (82). Copyright 1995 American Chemical Society.)... 

The utility of zeolite models in this context will be considered later, but Barrer et al. (4) have already pointed out an important resemblance— i.e., like zeolites, many biological systems contain sites of localized polarity or charge arising from the distribution of the opposing polarity or charge over a macromolecular structure., To this, a second generalization may be added in both zeolites (5) and protein structures (6), water seems to exist in some structured state which lies between those of ice and liquid water. 

With these two broad concepts as ideals, we completed preliminary investigations into the validity of zeolite models in two distinct natural systems. 

The Vienna Ab Initio Simulation Package (VASP) has been used to perform the periodic structure calculationsAll atoms have been allowed to relax completely within the periodic unit cell. The large 12-membered ring Mordenite has been used for this study as this zeolite has a relatively small unit cell (i.e. 146 atoms). It has previously been studied by Demuth et al. . For our zeolite model, the Si/Al ratio is 23, and the geometry of the unit cell is described by a = 13.648 k, b = 13.672 A, c = 15.105 A, a = 96.792 >5 =... 

We will use now the same method and Mordenite zeolite model as in the previous part, and investigate the isomerization of xylene isomers. As described in the previous part, this reaction can proceed via two alternative routes, viz. a methyl shift isomerization, and disproportionation reactions. Moreover, we observed than in the case of toluene isomerization, the location of toluene with respect to the Br0nsted acidic site for the shift isomerization was of no consequence for the activation energy barrier. We will check these mechanisms for the three xylenes. 

The optimized sodium cation positions in a six-ring of FAU zeolite structure containing two A1 atoms in para-position (denoted as Na-Al-2p) ° is shown in Figure 1. This position of the cation is representative for Sn cation position in Y and X zeolites," as well as the position of Na in Na-EMT zeolite. As expected, for this and the other zeolite model structures, Na" prefers positions near to oxygen centers bonded to A1 atoms rather than those of Si-O-Si bridges. Also, tbe cation is far from oxygen centers which are connected to compensating cations, an additional proton in this case. 

The zeolite framework was described by a specific force field developed by van Santen et al. [11] while the hydrocarbon molecules and their interaction among themselves and with the zeolite lattice were described by the generic force field Drdding n [12]. All the internal coordinates of the alkane molecules were allowed to fully relax. The nonbonded interactions (electrostatic and van der Waals) were computed for aU atoms within a cutoff-radius of 12A. Periodic boundary conditions were imposed along the three axes of the zeolite model to simulate an infinite crystal. 

Figure 11. Distribution of Magdelung potentials Vj, at the atomic of zeolite models [29]. ( ) NaX, (O) NaY, ( ) Na-mordenite, (v) Na-ZSM-5.

There is an already impressive literature on the application of various first principles and semi-empirical approaches to aspects of zeolite chemistry (see, e.g., [104-107]). Even a cursory overview of this aspect of zeolite modeling and simulation would be beyond the scope of the present paper. However, two recent development areas are noted. 

First principles approaches are important as they avoid many of the pitfalls associated with using parameterized descriptions of the interatomic interactions. Additionally, simulation of chemical reactivity, reactions and reaction kinetics really requires electronic structure calculations [108]. However, such calculations were traditionally limited in applicability to rather simplistic models. Developments in density functional theory are now broadening the scope of what is viable. Car-Parrinello first principles molecular dynamics are now being applied to real zeolite models [109,110], and the combined use of classical and quantum mechanical methods allows quantum chemical methods to be applied to cluster models embedded in a simpler description of the zeoUte cluster environment [105,111]. 

The special properties of zeolites (Figure 2 illustrates the molecular sieve property for zeolite CaA), and the possibility to tailor zeolite materials to suit one s need, have created much interest in zeolite science. Accordingly, thousands of studies appear in the literature each year. Paralleling this is interest in zeolite modeling. About 1000 studies have been reported in the literature during the last decade, and the number of modeling studies is still growing fast. One of the reasons for the interest in theoretical methods is that, in spite of the advanced techniques and instrumentation available at the moment, many aspects of zeolite science are difficult or even impossible to study experimentally. 

The next section reviews the state of zeolite modeling on the basis of a literature survey. However, this survey is not presented in all details because parts of the field of zeolite modeling have been reviewed recently, and it is our opinion that our readers are better served with a selection of reviews rather than with an endless list of references. The sections following the literature survey address models and methods, some selected applications, and further developments. An effort is made to keep mathematical formulas as simple as possible, with an emphasis on the physical meaning of the written equations. 

Many computational approaches have been applied in zeolite modeling. Each uses a different combination of the description of the system (model) and the technique (method) that may be suited for a particular problem (application) (see Figure 3). This section mentions briefly the frequently encountered models and methods in zeolite modeling, to provide a basic understanding. In the following sections, they are discussed in more detail. 

Figure 3 Approaches to zeolite modeling depicted in a three-dimensional graph.

Table 1 gives a summary of the approaches applied in zeolite modeling. 

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