Computational studies, zeolite framework

The quality of these methods depends on the force fields parameters, the way the QM and MM parts are linked, and how QM and MM parts affect each other. The main advantages of QM/MM are to present a limited increase of required computer power as a function of the size of the system. The MM part can be constituted by up to thousands of atoms. A drawback is that it is not easy to define a priori what should be the size of the QM and MM parts. Ramanchandran et al." observed in their periodic study that during transition state electron delocalization from the Br0nsted site to others zeolite framework oxygen atoms was an important phenomenon. Then, large QM part is required which makes more costly calculations. Furthermore, another drawback of QM/MM is the complexity of the tuning which can lead to misleading results. ... 

There are a few ab initio computational studies which deal with the order of stability of acid sites in other zeolite frameworks, where precise experimental data are lacking and only indirect information is available about preferential... 

The choice of a theoretical model usually depends on the goals of the study, but sometimes other considerations, such as computer resources available, can also play a role in this selection process. A study of zeolite framework acidity or catalytic activity requires an explicit consideration of the electronic structure of the system, and quantum mechanical (QM) models are best suited for such investigations. Since the high CPU demands greatly limit the size of the systems that can be simulated with quantum mechanical models, the zeolite framework is often represented in these simulations by a cluster that presumably resembles the active site. The cluster approximation has the obvious drawback that influences of the crystal lattice are neglected. 

There exist several ways to treat the zeolite framework structure in modeling. One can take into account either the periodicity of the full lattice or only a small part of the lattice, the latter sometimes being called the cluster approach. The cluster approach is often used in quantum chemistry studies because it requires less computer time. As long as specific properties connected with the framework topology (e.g., the dimensions of the channels) are not dominating the outcome of the calculations, this approach can provide valuable informa-... 

The large unit cells required to represent zeolites in periodic DFT calculations have motivated many quantum chemistry cluster studies of zeolites. A complication in these studies is that long-range electrostatic contributions from the zeolite framework and steric constraints from the zeolite pore shape are often significant.259 This complication can be overcome by using embedding methods in which quantum chemistry calculations are coupled with classical force fields,260 but the fact that periodic DFT calculations have now become computationally accessible for these systems suggests that these calculations will widely supplant cluster approaches in the future. 

The strength of the interaction of a guest molecules (probe, reactant, product, adsorbate) with a Bronsted acid site is mainly determined by the acidity of the OH group and its accessibility. The acidity of zeolite OH groups can be affected by various factors, most of them studied both experimentally and computationally [126,127] (i) the aluminum content of the zeolite framework ... 

Periodic LCAO-HF calculations have been used to examine the electronic structure and related properties of a number of silaceous structures such as zeolite cages (White and Hess, 1993 Anchell et al., 1994 Nicholas and Hess, 1994) and clay-like minerals (Hess and Saunders, 1992). The computational cost of such studies restricts structural optimization to calculations using minimal basis sets. Nevertheless, important properties of these structures such as the diffusion of cations through the zeolite framework (Anchell et al., 1994) and the position of the hydrogen atoms in the mineral kaolinite (Hess and Saunders, 1992) have been studied. 

In summary, the direct quantum mechanical simulation of zeolite vibrational spectra is evidently a formidable task and is often severely hampered by limited computational resources. Pure ab initio methods are well-suited if local effects or groups with characteristic vibrational frequencies like Bronsted acidic OH groups are under study. In theoretical studies of vibrational spectra of zeolite frameworks and cations on extra-framework sites, QM calculations are of crucial importance in developing force field parameters which can be used in a subsequent step in MM, MD or NMA calculations. Due to the lack of sufficient exper-... 

Jackson and Catlow [12-13] studied the stabilities of various zeolites using the static lattice energy simulation technique. In addition, the energetic distribution of non-framework cations and the relative stabilities as a function of Si/Al ratio in faujasite, zeolite A and silicalite were also studied. Reliable interatomic potentials necessary for such calculations were derived empirically in collaboration with Sanders [14]. These potentials and related force fields commonly used in zeolite computational studies are discussed later in this section. The adsorption of various molecules in zeolites leading to... 

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