Model zeolites

Figure 3. Schematic representation of a model zeolite cylindrical micropore (as for instance the AlP04-5 zeolite one). The curved inner zeolite surface is expected to influence greatly the confined molecule properties. Indeed, such a highly curved surface can be seen as composed of four surface types top, bottom, left and right surfaces.

The results of experimental studies of the sorption and diffusion of light hydrocarbons and some other simple nonpolar molecules in type-A zeolites are summarized and compared with reported data for similar molecules in H-chabazite. Henry s law constants and equilibrium isotherms for both zeolites are interpreted in terms of a simple theoretical model. Zeolitic diffusivitiesy measured over small differential concentration steps, show a pronounced increase with sorbate concentration. This effect can be accounted for by the nonlinearity of the isotherms and the intrinsic mobilities are essentially independent of concentration. Activation energies for diffusion, calculated from the temperature dependence of the intrinsic mobilitieSy show a clear correlation with critical diameter. For the simpler moleculeSy transition state theory gives a quantitative prediction of the experimental diffusivity. 

Modeling Zeolite Catalyst Deactivation by Coking and Nitrogen Compound Poisoning... 

Ionic phosphonates, which contain loosely bound alkali metal ions, are of interest from the point of view of modeling zeolites containing dissolved metal ions displaying conductive hyperlattices. Reactions of fBuP(0)(OH)2 with alkali metal salts of tetraalkylgallates provide a convenient route to ionic phosphates (Chart ll).31 A... 

Figure 7.3. Schematic illustration of the formation of a model zeolite. Adapted from...

Figure I. Location of a sodium cation at a model zeolite six-ring. Zeolite oxygen centers directed toward inside and outside the cluster are also shown.

There are two extreme views in modeling zeolitic catalysts. One is based on the observation that the catalytic activity is intimately related to the local properties of the zeolite s active sites and therefore requires a relatively small molecular model, including just a few atoms of the zeolite framework, in direct contact with the substrate molecule, i.e. a molecular cluster is sufficient to describe the essential features of reactivity. The other, opposing view emphasizes that zeolites are (micro)crystalline solids, corresponding to periodic lattices. While molecular clusters are best described by quantum chemical methods, based on the LCAO approximation, which develops the electronic wave function on a set of localized (usually Gaussian) basis functions, the methods developed out of solid state physics using plane wave basis sets, are much better adapted for the periodic lattice models. 

We have shown that the changes in the shape selectivity can be explained by changes in diffusivity by using ZSM-5 (MFI type) and Y type zeolites as model zeolites. However, it is very difficult to derive the model equations for representing the deactivation mechanisms for every types of zeolites, since each type of zeolite has different pore structure Hence, the mechanism of deactivation should be clarified for each type of zeolites. Reports on the activity of zeolites which were determined experimentally are omitted here. However, it is still impossible to evaluate physicochemical properties of a catalyst from the spectrum of ammonia TPD, which is usually employed to evaluate the acidic properties of a catalyst, since the spectrum is affected by various factors. Therefore, it is difficult to obtain the exact relationship between acidic properties and the change in activity due to deactivation. However, if an accurate method to evaluate the acidic properties is developed, it is expected that we can clarify whether the coverage of acid sites or pore blockage is the dominant factor of decrease in the activity due to coke deposition. 

De Vos Burchart et al. have recently developed a force field for modeling zeolites.21 The model originally was intended for all-silica zeolites but was quickly extended to aluminum-containing zeolites. The parameters were derived from several sources. Standard bond dissociation energies were used, and the force constants were refined to fit the structure of ZSM-5, the structure and frequencies of a-quartz, in addition to unit cell dimensions of other zeolites. With the all-silica model, the authors were able to calculate heats of formation... 

Finally I shall present some interesting applications of solid state modelling. Zeolites were one of the early success stories and there is still considerable interest in modelling these and related compounds such as ALPOs. Calculations in this area can predict new structures, determine the distribution of different cations over the available sites and throw light on the use of zeolites as catalysts by investigating the interaction of molecules with the framework. 

One might hope that some progress would also be made in areas barely touched upon at all in zeolite modeling at this time. Specifically we think here of synthesis of zeolites and by zeolites, and of zeolite design. Ideally, one would like to model zeolite synthesis by MD simulation however, the time scale of zeolite synthesis, hours to many days, and the complexity of the synthesis mixture represent huge difficulties. Perhaps MC methods, for example, the configurational bias MC method, might be a way out let us wait and see. 

Whereas simulation methods are available to model zeolite structures as a function of their composition as well as their topology [1-9], we do not discuss... 

A combined Monte Carlo and energy minimisation method has been developed to model zeolitic materials with low and medium Si/Al and with a variety of extra-framework species. We present results for Na- and H-Mordenites with Si/Al of 5 and 11. The A1 and cation distributions obtained are in reasonably good agreement with experimental studies. Furthermore, our calculated vibrational spectra are in excellent agreement with experiment, which has allowed us to re-interpret the de-convolution and assignment of the various acid sites. 

Several attempts have been undertaken to derive such MM force constants for modeling zeolite frameworks [39]. Typical examples are the rigid ion and the shell model which assume that the character of the bonds in the lattice is largely ionic. Within the rigid ion model developed by Jackson and Catlow [40], the potential energy is given by... 

Some Selected Examples of Modeling Zeolite Vibrational Spectra... 

Although the molecular mechanics method would seem an obvious method for modelling zeolites, it has not been applied until very recently. To date, with the exception of our own work, there has only... 

C. A. Freeman, C. R. A. Catlow, J. M. Thomas, and S. Erode, Chem. Phys. Lett., 186,137 (1991). Computing the Location and Energetics of Organic Molecules in Microporous Adsorbates and Catalysts A Hybrid Approach Applied to Isomeric Butenes in a Model Zeolite. 

The development of new or improved processes in catalysis and adsorption were in many cases induced by the development of new catalytic materials and adsorbents. In this context, the synthesis of new aluminosilicates is a continuing challenge in zeolite science. The present review, discussing the synthesis principles of selected more recent zeoUtes, has shown that there is still much room for innovation in this field. It can be expected that by the use of new classes of templates (one recent example is that of the metallocenes) new structures wiU be synthesized in the future. Moreover, with the availability of more and more sophisticated tools for modelling zeolite and template structures and their interactions, it will probably be possible to tailor templates for a given (or a theoretical) zeolite structure. Finally, beside the exploration of new templates and new reaction compositions, the influence of the synthesis conditions on the products should not be overlooked, e.g. changing the reaction parameters from subcriti-cal to supercritical conditions could well have an influence on the materials which are formed. 

The differential heats of adsorption of acetone adsorbed on H-ZSM-5 (at 360 K) and silicahte (at 350 K) over a wide range of surface coverage were reported by Sepa et al. [76]. The results were compared with ab-initio calculations of the reaction of acetone with model zeolite structures to form a stoichiometric hydrogen-bonded cluster-molecule complex [76]. The differential heats of acetone adsorption on H-ZSM-5 were approximatively constant around 130 kJ mol up to a coverage of one molecule per Al, after which the heats dropped to ca. 105kjmol [71], while on silicalite the heats of adsorption were constant over the entire range examined and equal to ca. 67 kJ mor. ... 

Before we can discuss in detail the simulation of adsorption and diffusion in zeolites using atomistic simulation we must ensure that the methods and potentials are appropriate for modelling zeolites. The work of Jackson and Catlow reviewed in the previous section shows the success of this approach. Perhaps the most critical test is to apply lattice dynamics and model the effect of temperature as any instability will cause the calculation to fail. Thus we performed free energy minimization calculations on a range of zeolites to test the methodology and applicability to zeolites. As noted in Section 2.2, the extension of the static lattice simulation technique to include the effects of pressure and temperature leading to the calculations of thermodynamic properties of crystals and the theoretical background to this technique have been outlined by Parker and Price [21], and this forms the basis of the computer code PARAPOCS [92] used for the calculations. 

Freeman, C.M., C.R.A. Catlow, J.M. Thomas, and S. Brode. 1991. Computing the location and energetics of organic molecules in microporous adsorbents and catalysts A hybrid approach applied to isomeric butenes in a model zeolite. Chem. Phys. Lett. 186 137-142. 

Figure 2 shows different clusters used to model zeolitic Brpnsted sites. 

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