Zeolites computer simulations

ABSTRACT Zeolite Y modified with chiral sulfoxides has been foimd catal rtically to dehydrate racemic butan-2-ol enantioselectively depending on the chiral modifier used. Zeolite Y modified with R-l,3-dithiane-1-oxide shows a higher selectivity towards conversion of S-butan-2-ol and the zeolite modified with S-2-phenyl-3-dithiane-1-oxide reacts preferentially with R-butan-2-ol. Zeolite Y modified with dithiane oxide demonstrates a significantly higher catalsdic activity when compared to the unmodified zeolite. Computational simulations are described and a model for the catalytic site is discussed. 

Figure 4.37 MAS NMR of a calcined Y zeolite. The top spectrum is the experimental Si MAS NMR spectrum and the middle spectrum is based on computer simulation...

The five years since last considering specifically recent developments in X-ray and neutron diffraction methods for zeolites [1] have witnessed substantial progress. Some techniques, such as high resolution powder X-ray diffraction using synchrotron X-rays, have blossomed from earliest demonstrations of feasibility to widespread and productive application. Others, such as neutron powder diffraction, have shown steady progress. For still others, notably microcrystal diffraction, a variety of circumstances have contributed to extended gestation periods. Additionally, opportunities scarcely considered earlier (such as single crystal Laue diffraction, and certain developments in computer simulations that complement diffraction work) now command broad attention and warrant the commitment of substantial further investment. 

In the last few years, computer graphics with colour display are being more commonly used not only to visualize complex structures better, but also to examine unusual structural features, defects and transformations as well as reactions. In Fig. 1.45, we show the presence of a Nal" cluster within the sodalite cage of zeolite Y as depicted by computer graphics the cluster fits well within the cavity bounded by the van der Waals surface (net) of the framework atoms. The immense power of computer graphics has been exploited widely in recent years. Structural transitions in solids and sorbate dynamics in zeolites are typical areas where computer simulation and graphics have been used (Ramdas et al., 1984 Rao et al., 1992). 

Fig. 11. High-resolution 29Si MAS NMR spectra of synthetic zeolites Na-X and Na-Y at 79.80 MHz (58). Experimental spectra are given in the left-hand columns Si(nAl) signals are identified by the n above the peaks. Computer-simulated spectra based on Gaussian peak shapes and corresponding with each experimental spectrum are given in the right-hand columns. Individual deconvoluted peaks are drawn in dotted lines.

Figure 4.11 shows an example of how ZSM-5 is applied as a catalyst for xylene production. The zeolite has two channel types - vertical and horizontal - which form a zigzag 3D connected structure [62,63]. Methanol and toluene react in the presence of the Bronsted acid sites, giving a mixture of xylenes inside the zeolite cages. However, while benzene, toluene, and p-xylene can easily diffuse in and out of the channels, the bulkier m- and o-xylene remain trapped inside the cages, and eventually isomerize (the disproportionation of o-xylene to trimethylbenzene and toluene involves a bulky biaryl transition structure, which does not fit in the zeolite cage). For more information on zeolite studies using computer simulations, see Chapter 6. 

Figure 4.11 Computer simulation of the MFI-type zeolite H-ZSM5 (left), and scheme showing the shape-selective formation ofp-xylene inthecage (right).Thanks to Dr. Edith Beerdsen forthe simulation snapshot.

Chatteijee, A., Bhattacharya, D., Iwasaki, T. and Ebina, T. A computer simulation study on acylation reaction of aromatic hydrocarbons over acidic zeolites, J. Catal., 1999,185,23-32. 

From these expressions, we derive M1 = 4x. Thus the center of gravity, M1, deviates substantially from Loewenstein s M1 = 4x/(1-x), as has been pointed out by Melchior, et al.(A) A computer simulation of the completely random A1 occupation on the square planar lattice and the zeolite X lattice resulted in the NMR intensities predicted by Equation (6). 

CONTENTS Introduction, Thom H. Dunning, Jr. Electronic Structure Theory and Atomistic Computer Simulations of Materials, Richard P. Messmer, General Electric Corporate Research and Development and the University of Pennsylvania. Calculation of the Electronic Structure of Transition Metals in Ionic Crystals, Nicholas W. Winter, Livermore National Laboratory, David K. Temple, University of California, Victor Luana, Universidad de Oviedo and Russell M. Pitzer, The Ohio State University. Ab Initio Studies of Molecular Models of Zeolitic Catalysts, Joachim Sauer, Central Institute of Physical Chemistry, Germany. Ab Inito Methods in Geochemistry and Mineralogy, Anthony C. Hess, Battelle, Pacific Northwest Laboratories and Paul F. McMillan, Arizona State University. 

Many new adsorbents have been developed over the past 20 years including carbon molecular sieves, new zeolites and aluminophosphates, pillared clays and model mesoporous solids. In addition, various spectroscopic, microscopic and scattering techniques can now be employed for studying the state of the adsorbate and microstructure of the adsorbent. Major advances have been made in the experimental measurement of isotherms and heats of adsorption and in the computer simulation of physisorption. 

Framework Dynamics Including Computer Simulations of the Water Adsorption Isotherm of Zeolite Na-MAP. See also J.-R. Hill, C. M. Freeman, and L. Subramanian, in Reviews in Computational Chemistry, K. B. Lipkowitz and D. B. Boyd, Eds., Wiley-VCH, New York, 2000, Vol. 16, pp. 141-216. Use of Force Fields in Materials Modeling. The shell model is also discussed by B. van de Graaf, S. L. Njo, and K. S. Smirnov, in Reviews in... 

Another important aspect of the problem, which can also be addressed using computer simulations, has to do with the distribution of the alkane molecules over the zeolite channels. If one takes into consideration the fact that a zeolite such as ZSM-5, for instance, has 48 different acidic sites, with distinct acidic strengths, the catalytic activity of the zeolite towards the different alkanes will be certainly related to the way the substrate molecules are distributed within the zeolite network. As mentioned in the last section, the previous simulations [24,26-29,31] predicted quite distinct distributions, but considering the variety of different simulation conditions employed, no clear conclusion could be reached. On the contrary, we have used exactly the same conditions (force fields, cluster size, loading, initial distribution of molecules, etc.) with the three methodologies, except in the case of the MM calculations with a single alkane molecule. 

K. P. Schroder, J. Sauer, M. Leslie, C. R. A. Catlow, and J. M. Thomas, Chem. Phys. Lett., 188, 320 (1992). Bridging Hydroxyl Groups in Zeolitic Catalysts A Computer Simulation of Their Structure, Vibrational Properties and Acidity in Protonated Faujasites (H-Y Zeolites). 

The isotropic and anisotropic hyperfine coupling terms in a arise from interactions between electron and nuclear spins, and provide information about the nature of the orbital containing the unpaired electron and the extent to which it overlaps with orbitals on adjacent atoms. The anisotropic term can cause similar difficulties to the g tensor anisotropy in analysing spectra of polycrystalline powders extracting coupling constants from spectra of transition metal ions or radicals in zeolites can be difficult or impossible without computer simulation. 

Computer simulation impacts almost all branches of zeolite research. This orientational overview touches applications to zeolite structural characterization and to the prediction of various properties, particularly those associated with sorption, focusing on the issues associated with applying modeling and simulation effectively in these areas. A small number of specific examples are used to illustrate particular points and opportunity areas for progress over the next five years are identified. 

Computer simulation impacts almost all branches of zeolite research [1-5]. It is worth repeating this simple statement as it underscores the challenge of attempting to present an instructional overview of a very broad and diverse topic. This particular overview is heavily weighted towards the author s recent work and interests. It is necessarily subjective in its coverage. This subjectivity dictates that many areas and many individual seminal papers are not addressed, referenced or dted. 

A. K. Cheetham and J. D. Gale, in Computer Modeling of Structure and Reactivity of Zeolites, C. R. A. Catlow, Ed., Academic Press, London, 1992, pp. 63-78. Computer Simulation of the Structure, Thermochemistry and Dynamics of Adsorbed Molecules in Zeolites and Related Catalysts. 

COMPUTER SIMULATED DEACTIVATION OF A ZEOLITE WITH RANDOMLY DISTRIBUTED CATALYTIC SITES... 

I.Y. Chan, R. Csencsits, M.A. O Keefe, and R. Gronsky, Computer-Simulated Images of Platinum Clusters in the Channels of Y Zeolites Zone-Axis Results. J. Catal., 1987, 103, 466-473. 

Some zeolites provide such an opportunity. Computer simulations of simple molecules in zeolites have been carried out already for more than two decades. The goal of those works was in considerable degree motivated by an attempt to find and test proper relations for various terms of Eq. (4). We do not discuss these works in the present paper and refer an interested reader to a recent article concerning the computer simulation of the adsorption of rare gases in silicalite [41]. A silicalite is a form of porous crystalline Si02... 

It was noted [21, 28] that the value of t for the Ar - oxide ion interaction obtained earlier from analysis of the Henry s Law constants for various zeolites is 226 K. This value was confirmed by independent calculations of different authors (but on essentially the same zeolites). Thus it was concluded that the most important parameter for computer simulation of physical adsorption on oxides - the energy minimum of the atom/oxide ion interaction - is untransferrable from one adsorption system to another. The comparison of the results presented above suggest that this might not be the case the parameter determined from amorphous oxides of different chemical natures (Si02 and Ti02) and... 

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