Zeolite force field calculations

The unit cells of zeolites are large, so that full ab-initio calculations are expensive. Force field calculations (70) have been employed to help understand the spectra. Molecular dynamics simulations provide good results (71) at modest cost for a range of zeolites. For both types of calculation, the agreement between observed and calculated results is good, although there are limitations the LO-TO splitting... 

Both the template molecules and the zeolite frameworks were treated as flexible, using the zeolite force field developed in Delft together with the MM3 force field developed by Allinger et Calculations were carried... 

Owing to the computational simplicity of force-field calculations, very large systems can be examined in short periods of time. These are the methods of choice for studying the structure of synthetic polymers, proteins, nucleic acids, and inorganic networks such as zeolites. Force-fields are also used to provide the energies in molecular dynamics calculations, in which the time evolution of structures can be examined. 

Force-field calculations could be simple energy minimization or advanced monte-carlo and molecular dynamics calculations. The major assumption here is the transferability of force-field parameters among the related materials. These calculations can provide wealth of information such as the relative ordering of adsorption sites on surface, diffusion mechanism of molecules particularly inside zeolites, energy barrier for difihision, diffusion coefficients, heats of adsorption and more importantly, the effect of temperature on all these properties. 

The selectivity of required product in a catalysed reaction is crucial from the economical and environmental points of view. The diffusion behavior of reactant, intermediate and product molecules needs to be understood to predict the selectivity. The alkylation reaction is a typical reaction, where several isomers and side-products are possible. A simple method based on force-field calculations has been devised[19], which can be applied to calculate the difiusion energy barriers. This method is demonstrated as an efficient MMM to screen several zeolite catalysts before carrying out the actual experiments. 

In the present study, we report our molecular graphics investigations and force field calculations for the diffusion of isomers of IBEB in mordenite, mazzite and faujasite. The dimensions of the pores in these zeolites as well as the molecules are listed in Table 1. Using... 

Trichlorobenzene (TCB), a well known termite exterminator, is prepared selectively from 1,2-dichlorobenzene (DCB) using zeolite K-L as a catalyst and monochloroacetic acid as a promoter. An attempt has been made to apply the combination of molecular graphics, force field calculations and quantum chemical calculations to understand the mechanism of selective chlorination of 1,2-DCB to 1,2,4-TCB over K-L promoted by monochloroacetic acid. It was found that the zeolite lattice plays an important role in polarising the molecules. The peculiar "barrel shaped pore architecture allows zeolite L to act as a reactor vessel where monochloroacetic acid, chlorine and 1,2-DCB can be accommodated on a molecular level. 

The role of CI2 and monochloroacetic acid in the selective chlorination is a difficult problem to understand from the experimental studies. There are several possible orientations for the reactant, product and promoter molecules inside the complex structure of zeolite-L. In this context, it is pertinent to note that molecular modelling techniques are contributing in considerable amount to understand the reaction mechanisms. Molecular modelling includes force field based calculations [3] such as energy minimisation, Monte Carlo, and molecular dynamics calculations and quantum chemical calculations [4 ] such as EHMO, CNDO/INDO, MOPAC, Hartree-Fock and density functional theory calculations. In this study, we have attempted to apply the combination of molecular graphics, force field calculations and quantum chemical calculations to understand the mechanism of selective chlorination of DCB to TCB over zeolite K-L promoted by monochloroacetic acid. 

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. ... 

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. 

Contrary to the previous steps of the catalytic process, we cannot use force-field-based techniques because the available force fields are unable to describe the breaking and formation of chemical bonds. Thus, the chemical reaction step must be investigated by quantum mechanical techniques. Right away this imposes some limitations on the size of the cluster to be used in the calculations. In principle, since the catalytic sites are well localized within the zeolite framework, one should expect the chemical reactions to occur at very locahzed points of the zeoUtic structure. Thus, one could think of representing the acid sites by much smaller clusters than the ones used in the diffusion and adsorption studies. 

The results for the diffusion coefficients obtained from MD calculations follow the expected trend, based on the differences of mass and shape of the molecules. When compared to the available experimental data, the MD results indicate that the simulations can indeed provide a realistic representation of the microscopic process of diffusion of light alkanes in the pores of zeolites as long as reliable force fields to represented the zeolite and sufficiently long trajectories are used. 

Our calculation is based on the Burchartl.01-Dreiding2.21 force field that combines the Burchart force field [8], which is used to treat the frameworks of zeolites, and Dreiding II... 

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... 

---