Computation zeolite

Raman spectra for SO -deactivated HM, CuHM and CuNZA were examined by a Spex 20 spectrometer equipped with a Spex Datamate computer. Zeolite samples were excited using the 514.5 nm line of a Spectra Physics 171 Ar ion laser powered with a Spectra Physics 265 exciter. To minimize local heating effects where the laser beam impinges on the catalyst surface, a rotating lens assembly was employed. 

Presented text does not give a final word about what model is the best suited for applications in the zeolite science. We have attempted to show that the choice of the model depends primarily on the property under investigation. It is important to stress that other factors must be also taken into consideration, e. g., software and hardware limitations. It is always a difficult decision to find a suitable compromise among the size of the model and reliability of the method (under the constraint of available computational resources). It is equally difficult to judge the reliability of any computational study in the zeolite science. We hope that the present manuscript helps the reader to acquire some initial orientation in the computational zeolite science. 

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The state-of-the-art methodologies in computational zeolite catalysis are based on DFT calculations with periodic boundary conditions. These allow theoretical analysis of structure and chemical properties of zeolites with moderate-sized unit cells using a real crystal structure as a model (Fig. 2C). Such periodic DFT calculations of zeolites are mostly limited to the GGA density functionals. 

Figure B3.3.14. Template molecule in a zeolite cage. The CFIA stmcture (periodic in the calculation but only a fragment shown here) is drawn by omitting the oxygens which are positioned approximately halfway along the lines shown coimecting the tetrahedral silicon atoms. The molecule shown is 4-piperidinopiperidine, which was generated from the dicyclohexane motif suggested by computer. Thanks are due to D W Lewis and C R A Catlow for this figure. For fiirther details see [225].

A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

For adsorption in zeolites, the biased Monte Carlo method as developed by Smit is an excellent method to determine the free energies of molecules adsorbed on zeolites [9bj. This method can be used to compute the concentration of molecules adsorbed on zeolites, as we discuss below. 

In Table 1.1 a comparison is made of the differences in free energies for two different zeolites. Note the large repulsive energies computed for the intermediates and their sensitivity to zeolite structure. 

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. 

The tortuosity for pore-filling liquids is ideally a purely geometric factor but can, in principle, depend on the fluid-surface interaction and the molecular size if very small pores are present such as in zeolites (see Chapter 3.1). To obtain a measure for a realistic situation, we have used n-heptane as a typical liquid and have computed x... 

Sauer, J., 1998, Zeolites Applications of Computational Methods in Encyclopedia of Computational Chemistry, Schleyer, P. v. R. (Editor-in-Chief), Wiley, Chichester. 

Sefcik J, Demiralp E, Cagin T, Goddard WA (2002) Dynamic charge equilibration-morse stretch force field application to energetics of pure silica zeolites. J Comput Chem 23(16) 1507-1514... 

Selectivity to ethers over H-BEA-25 and H-FAU-30 are always higher than 90 % at 160 - 200°C, similarly as in case of thermally stable resin Amberlyst 70, but zeolites are far less active [5,10], Activation energies for investigated catalysts computed from initial reaction rates (Table 3) are comparable to those for Amberlyst 70 (119 + 4 kJ/mol for DNHE synthesis [10], and 115 + 5 kJ/mol for DNPE synthesis [5], respectively)... 

The location of boron or aluminum sites in zeolites is of utmost importance to an understanding of the catalytic properties. Due to the inherent long-range disorder of the distribution of these sites in most zeolites, it is difficult to locate them by diffraction methods. The aforementioned methods to measure heteronuclear dipolar interactions can be utilized to determine the orientation between the organic SDA and A1 or B in the framework. The SDA location may be obtained by structure refinement or computational modeling. For catalytic reactions, the SDA must be removed from the pores system by calcination. 

The characterization OF catalyst structures has undergone revolutionary developments in recent years. Powerful novel techniques and instrumentation are now used to analyze catalyst structure before, during, and after use. Many of these advances are responsible for placing the field of catalysis on an improved scientific basis. These developments have resulted in a better understanding of catalytic phenomena, and therefore improvements in commercial catalysts and the discovery of new systems. The application of advanced electronics and computer analysis has optimized many of these analytical tools. These developments are especially evident in spectroscopy, zeolite structure elucidation, and microscopy several other techniques have also been developed. Thus, the difficult goal of unraveling the relationships between the structure and reactivity of catalytic materials is finally within reach. 

The strong interaction of methanol with the surface hydroxyl groups leads to a methylated surface which is clearly shown by Figure 3. The CP/MAS C-NMR line of 59.9 ppm is attributed to surface methyl groups (10). On a NaGeX zeolite, the amount of these groups was computed from the comparison of the relative amounts of CH3OH and of (CI O obtained from either mass spectrometry data or C-NMR measurements (45). ... 

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