Zeolites computational aspects

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. 

The SIESTA approach is especially designed to handle large systems so that it is not surprising that it has been quite extensively used in order to study minerals, clathrates, and zeolites. Quite often a key aspect in studying processes in these systems is the determination of the distribution of cationic species in the large cavities they exhibit. To handle this question extensive computations with methods giving reliable energy differences are needed. The SIESTA approach seems to fulfill both the precision and efficiency requirements in order to be a useful tool in this area. Here we discuss some recent applications of the SIESTA approach to this kind of materials. 

The experimental methods of dilfraetion and spectroscopy are uniquely applicable to the study of crystalhne microporous solids and their chemistry. Nevertheless, there are important aspects of zeolite science that are not readily accessible to these techniques the species involved in nucleation and crystal growth, the structure of sites (often present at low concentration) that are active for adsorption and catalysis or the reaction intermediates present in catalysis. In these cases computational atomistic simulation offers great possibilities for improved understanding. Furthermore, many experimental measurements, such as calorimetric studies of heats of adsorption, and NMR or neutron scattering studies of dynamics, may be very expensive and time-consuming. Computer simulation methods, which promise to predict the performance of materials as adsorbents and catalysts rapidly and at reasonable expense, are therefore highly attractive. Excellent recent texts and useful reviews are available that deal with the simulation of microporous materials. Here I summarise the most widely used methods and the information they give. 

A class of porous alumino-sihcate materials called zeolites has been known for almost 300 years. They are best known for their role as catalysts. In chapter four, Marcel AUavena and David White present a review of applications of computational chemistry to the proton transfer, primary process for acid-base chemistry on zeolites. Recent ab initio results are compared to experimental studies and critically reviewed. Future directions of the field are given, and the importance of the Car-Parrinello method in exploring the dynamic aspects of reactivity in zeolites is discussed. 

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