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A new interatomic potential for simulating the properties of zeolites synthesized through the fluoride route has been derived. Hexafluoro compounds have been used as a basis for deriving a force field for the interaction of Si/Ge/Al ions with fluoride ions, as a starting point for describing the chemical behavior of the fluoride anions in the cavities of zeolites. From this, as well as ab initio data concerning the interaction of fluoride with framework, the equilibrium location of F^- in the different small cavity environments found in zeolites can be calculated. It is found that the pentacoordination of Si due to formation of Si−F bonds is predicted by our model. The calculated Si−F distances for each case are found to be in excellent agreement with recent ¹⁹F NMR and single-crystal XRD results. Furthermore, our force field can be used to predict distances in any zeolite, and in particular to establish the effect of Ge−F interactions and the corresponding equilibrium distances, which in some cases are not yet available experimentally. This new force field can be used in zeolites containing any compositional range with Si/Ge/Al as framework atoms, in the presence or absence of fluoride. Aluminogermanates and silicoaluminogermanates can therefore also be studied and the results used as a guide for new synthesis experiments using the fluoride route.

Molecular mechanics techniques and the use of atomic force fields have been used to calculate the energy of the system zeolite + structure directing agent (SDA), as well as the different energetic terms with their respective weights in deciding the final zeolite synthesis product. A new SDA has been found that discriminates energetically between two closely related zeolitic structures, ISV and BEC, that strongly compete during the crystallization process. The subsequent synthesis experiments with this new SDA led to the selective formation of ISV, thus supporting the predictions made by the computational chemistry calculations.

An interatomic potential for Ge/Si-zeolites has been developed by fitting to the experimental properties of quartz, GeO2(quartz), and GeO2(rutile). Testing the force field by comparison of the experimental and computed structural properties of octadecasil, ASU-7, and ASU-9 shows errors within 1−2% in the cell parameters and interatomic distances. Furthermore, Ge−O−Ge angles are well reproduced and predicted to lie within the range of 130−138° as compared to the experimental values in the range 126−140°. The force field can be used for new zeotype structures containing germanium and silicon as tetrahedral atoms in order to predict structural properties, as well as the energetic stability of different Ge and Si orderings. Improved accuracy is achieved, relative to earlier force fields, through the use of different polarizabilities for the oxide ion in distinct environments.

Molecular dynamics techniques have been used to simulate the diffusion of n-heptane in the supercage system of purely siliceous MCM-22 zeolite at 750 K. Diffusion through the 12-member-ring (MR) large cavities is dominated by intracage motion due to the presence of preferential minimum-energy positions at the top and bottom of the supercage forming the 12-MR pockets. Molecules in singly occupied supercages show higher diffusivity because there is no impediment for intracage motions other than the small activation energy. Molecules in doubly occupied supercages show lower diffusivity and intermolecular contacts preclude intracage jumps. Finally, higher loadings allow the intercage region to be more populated and the possibility of intercage diffusion when the n-heptane orientation is favorable. The intercage diffusion is analyzed on the basis of the energy path for the intercage motion and on the orientational probability for intercage jumps.

Force field atomistic simulations have been used to investigate the Al preferential location and proton siting in zeolites, and the role of the template is shown to be crucial in order to elucidate the Al and acid site distributions. Therefore, it is not the energy of the final structure what controls the Al distribution, but rather the energetics during the synthesis process (zeolite+SDA) that has to be considered, and in this sense the structure directing agent (SDA) accommodation within the microporous space acts not only as a structure director but also as a director of the Al distribution. Once the Al distribution is known, the positively charged SDA can be removed from the calculation and substituted by protons and the population of the four protons in each Al center can be calculated from the corresponding energies. Also, this methodology allows to compute the OH stretching frequencies and, therefore, a map of acid site frequencies, and relative population is obtained which gives a simulation of the OH stretching IR bands in terms of frequencies and intensities. The methodology has been validated by applying this general approach to the case of the newly synthesized ITQ-7 zeolite and the corresponding calculated IR spectrum shows a reasonable agreement with the experimental spectrum. Two bands centered at 3577 and 3624 cm^-1 result from the simulation, which compare to the experimental values of 3595 and 3629 cm^-1. Preferential proton siting has been also studied and energy differences in each AlO4 tetrahedra of about 30−40 kJ/mol are found, in qualitative agreement with ¹H NMR results.

The role of trispyrrolidinium cation as the structure-directing agent of the ZSM-18 zeolite structure is explained by a close matching between the template and the zeolite walls. Two template orientations are proposed, and their respective structure-directing abilities are discussed in terms of template−zeolite energetic interactions. The relation of the template orientation and the Al distribution of the ZSM-18 structure is explained, and the preferential substitution of Al in the three-membered rings of ZSM-18 is explained from the results of the calculations, in accordance with the XRD solution of the structure.

The stretching frequency of the bridging hydroxyls in MCM-22 zeolite has been calculated by atomistic simulation techniques. In this way our model was able to reproduce the two infrared bands corresponding to bridging hydroxyls detected experimentally in MCM-22. It is shown that the OH stretching frequency correlates with the modulus of the electric field at the proton site, thus showing the influence of both short- and long-range factors on acidity. The relative substitution energy to form the different acid centers was calculated, and a span of 40 kJ/mol was found between the sites, which indicates that preferential siting is expected. The acidity distribution of the MCM-22 structure is presented showing the strength and location of the individual acid sites. This provides an understanding of the overall acidity of this zeolite as well as the acid features of each void system:  the large pore supercage system and the medium pore sinusoidal system. A stronger acidity is predicted for the centers located in the supercages with respect to those in the sinusoidal chanels.

The diffusion of C7 alkanes in the microporous material ITQ-2 has been investigated by molecular dynamics techniques and the results were compared with those obtained for MCM-22. In MCM-22, differences in diffusivity are observed between the isomers n-heptane and 2-methylhexane, especially in the sinusoidal 10 member ring (MR) system. Similar features in the diffusivity are observed in the supercages, and no intercage diffusion has been observed at any of the temperatures of the simulation (ranging from 450 to 650 K), which would suggest that a much longer time scale is needed for intercage diffusion to be observed. Intracage diffusion is a temperature-activated process. In ITQ-2 the diffusion in the supercage system has been studied using 2-methylhexane as the probe molecule. Important differences appear with respect to MCM-22 because of the opening of the supercages giving chalice-type pores. The molecules have a tendency to penetrate into the chalice, but the subsequent out-diffusion is not restrained by the 10 MR windows interconnecting the supercages, as these windows, present in MCM-22, do not exist in ITQ-2. This allows the 2-methylhexane to escape from the 12 MR system more easily than in MCM-22. Possible catalytic applications of ITQ-2 are analyzed in view of the results.

Molecular dynamics simulations have been performed to study the diffusion of a mixture of benzene and propylene, for the cumene synthesis process, in purely siliceous MWW (MCM-22), a zeolite containing two separate channel systems:  the 10-member ring (MR) sinusoidal and the 12-MR supercages interconnected by 10-MR windows system. The diffusion processes in each channel system of MWW at 650 K have been studied independently. We have found that in order to obtain quantitative or semiquantitative diffusion coefficients, the framework should be optimized. A large diffusivity for propylene in both channel systems, and especially in the supercage system, is observed, whereas benzene is not seen to diffuse in either of the two channel systems, and only intracage mobility is seen in the supercage voids. The positions of minimum energy, where the molecules are expected to react, have been located in both channels. The diffusion of benzene in the supercage system seems to be temperature-activated, and when the temperature is increased, intercage diffusion will probably occur. Radial distribution functions show that condensation reactions between benzene−propylene and propylene−propylene are possible, which indicate the necessity of working in an excess of benzene. The results of the simulations of diffusion suggest that the formation of cumene probably occurs at the external surface or close to the external surface of the MCM-22 zeolite crystals.

Molecular dynamics simulations have been performed to study the diffusion of octane in silicalite. When hydrocarbons that are larger than C6 diffuse through this zeolite, the length of the molecule is such that diffusion through the sinusoidal channels becomes difficult. We have investigated the relative diffusivity through each channel and give an interpretation of the effect of temperature on this process. Simulations at 300 K show greater diffusivity in the sinusoidal channels, whereas at 450 K the straight channels show higher diffusion rates. When the temperature increases from 300 to 450 K, the diffusion coefficient in the straight channels increases by a factor of 5.2, whereas the coefficient in the sinusoidal channels increases by only 1.3. The trajectory plots also show larger diffusion paths through the straight channels at higher temperatures.

Molecular dynamics simulations employing a flexible framework are used to simulate the diffusion of o- and p-xylene in purely siliceous zeolite CIT-1. The simulations are performed at 500 K, investigating two loadings (corresponding to 1 molecule/unit cell and 0.25 molecule/unit cell) for the ortho isomer and one loading (0.25 molecule/unit cell) for the para isomer. For the former system the diffusion coefficient decreases from 7.79 × 10^-6 cm²/s for the lower to 3.56 × 10^-6 cm²/s for the higher loading. The diffusion coefficient for the para isomer (25.18 × 10^-6 cm²/s) is substantially greater. Graphical analyses reveal a jump diffusion mechanism, which in the case of the ortho isomer takes place in the 12 MR channels of the structure, while for the para isomer, incursions into the 10 MR channel are observed. The results of the MD simulations are complemented and reinforced by calculations of the activation energies for the diffusion of the two isomers in the two channels. Diffusivity measurements of both isomers in B-CIT-1 (Si/Al = 35) by FTIR have also been carried out in order to compare the values obtained theoretically and experimentally.