L. Leherte

Determination of the self-diffusion coefficient of water in ferrierite by molecular dynamics

The self-diffusion coefficient D of water in ferrierite at an average density of 6.6 molecules per unit cell and 298 K has been computed from a molecular dynamics simulation and the Einstein relation. The estimated value of D is 0.5 × 10−5 cm2 s−1 and agrees with experimental results.

APPROXIMATION OF THE MULLIKEN-TYPE CHARGES FOR THE OXYGEN ATOMS OF ALL-SILICEOUS ZEOLITES

Distributed multipole analysis on the basis of periodic Hartree-Fock PHF calculations, using the CRYSTAL code, is applied to 12 all-siliceous zeolite models, plus one H-form type. A simple approximation of the dependence of the Mulliken-type charge of the framework oxygens, calculated with two Gaussian basis sets, is found with respect to the average Si-O distance and Si-O-Si angle. These results allow the estimation of the oxygen charges within zeolites with larger elementary unit cells which are still hardly tractable with the presently available computing facilities. The validity of such an estimation for the oxygens of silicalite is shown by comparison with results of direct PHF calculations. q 1998 Elsevier Science B.V. All rights reserved.

A New Graph Descriptor for Molecules Containing Cycles. Application as Screening Criterion for Searching Molecular Structures within Large Databases of Organic Compounds

The search of molecular structures inside a large database of chemical compounds is a critical step for many computer programs used in several domains of chemistry. During the last years, the size of many chemical databases has dramatically increased, hence in the meantime, search engines needed to be more and more powerful. The speed and the efficiency of screening processes of the chemical compounds are thus essential. Looking forward for algorithms dedicated to structure and substructure search, we have developed a new graph descriptor for structures containing cycles in order to find efficient indexation and classification criteria of molecular structures. This graph descriptor can be used as a screening criteria for structure and substructure search in large databases of organic compounds.

Theoretical estimation of the vibrational perturbation of the molecular properties of hydrogen adsorbed within a zeolite A framework

Abstract An iterative numerical procedure is proposed to evaluate the variation of the dependence versus the internuclear distance of several molecular properties (polarizabilities, multipole moments) of hydrogen adsorbed within zeolite A. Dealing with a method which includes only the vibrational perturbation, it is shown that the dependence on internuclear distance of the properties of H2 does not change upon adsorption in NaA as compared to the gas.

Molecular dynamics studies of sorbates in zeolites : water in ferrierite

Abstract In this paper we present molecular dynamics (MD) simulations of water molecules inside a ferrierite-type framework. We stress the importance of introducing the long-range electrostatic effects to the chosen adsorbate-adsorbent interaction potential, and present thermodynamic, structural, and dynamic results computed from the MD trajectories. Detailed analyses of the translational and rotational properties of the water molecules allow us to characterize their behavior inside the ferrierite channels. Simulated infrared and neutron scattering spectra are interpreted and compared with the experimentally available results.

Effects of Long-Range Interactions in Zeolites-Like Systems: Dynamical Behavior of Water in Ferrierite From Molecular Dynamics Simulation

The Ewald Summation technique is applied to compute the Coulombic expression of an empirical water-ferrierite pair potential utilized for Molecular Dynamics simulations including long-range electrostatic interactions. The trajectories of 33 water molecules inside 4 unit cells (11% of free capacity) created in the phase space allow the calculation at room temperature of energetic and dynamical properties as time-averages and of frequency spectrum characterizing the translational and reorientational molecular motions inside the framework.

Storing, retrieving, and analyzing experimental catalyticdata with the help of artificial intelligence methods

Within the framework of the development of a decision support system devoted to the prediction of the best catalyst and reaction conditions for industrially important aromatic reactions catalyzed by zeolites, we propose some techniques to derive correlations from experimental catalytic data stored in a knowledge base by means of artificial intelligence (AI) methods. Having collected miscellaneous experimental informations available from the literature regarding the alkylation of toluene and benzene with light alcohols and olefins, several tools have been developed to retrieve them easily and rapidly. Plotting graphs after several selections imposed by users constraints provides conclusions on the catalytic behavior of the zeolite(s). This approach constitutes the first step on the development of an expert system for the prediction of a suitable catalytic system for a given type of reaction>[1].

Monte Carlo Simulations of Water Interaction with a Ferrlerite Type Zeolite Structure

Abstract In connection with our on-going work on the characterization of structural and thermodynamic properties of zeolites, we performed Monte Carlo simulations of water distributions in ferrierite type zeolite. The zeolite/water interaction energy is expressed in terms of atom-atom pair potentials composed of a Lennard-Jones and an electrostatic contribution. For the water/water interaction, we used the already well tested Matsuoka-Clementi-Yoshimine (MCY) ab initio potential. Ensemble average positions, probability density, and thermodynamic values obtained by the MC simulations, based on the above-cited models, are used to study the thermodynamic properties and the repartition of water molecules as functions of sorption coverage.

Linear dependence of the interaction energy on intramolecular distance for adsorbed or clustered diatomic molecules

The dependence of several molecular properties (polarizabilities, multipole moments) of adsorbed diatomic molecules on the intramolecular distance are discussed within time-independent perturbation theory. The nearly linear dependence of the intermolecular interaction energy and molecular properties rationalizes the small variation of the property functions compared with those for the gas state. This relative independence of the property functions was demonstrated numerically in a previous paper using an iterative procedure for molecular hydrogen adsorbed in a zeolite-A-type framework.

Molecular Modeling and Molecular Graphics of Sorbates in Molecular Sieves

This contribution demonstrates the usefulness of molecular graphics, molecular mechanics, and molecular dynamics in the study of the structural, thermodynamic, and transport properties of different sorbates within various molecular sieves. Five significant applications to zeolite chemistry are reviewed.