Giuseppe Baldovino Suffritti

Application of the Wolf method for the evaluation of Coulombic interactions to complex condensed matter systems: Aluminosilicates and water

The application of the method recently proposed by Wolf et al. [J. Chem. Phys. 110, 8254 (1999)] for the evaluation of Coulombic energy in condensed state systems by spherically truncated, pairwise r−1 summation is verified for liquid water and anhydrous and hydrated aluminosilicates. Criteria for the estimation of the optimum values for the truncation radius and the damping parameter are discussed. By several examples it is verified that the new method is computationally more efficient than the traditional Ewald summations. For the considered systems the performances of the new method are good, provided that the truncation radius and the damping parameter are carefully chosen.

About the influence of lattice vibrations on the diffusion of methane in a cation-free LTA zeolite

Abstract The influence of lattice vibrations on the diffusion of methane in a cation-free zeolite of structure Type LTA is examined. It is shown that contrary to earlier published results the self-diffusion coefficients obtained with flexible and with rigid lattices are practically the same. This finding is true over a wide range of temperatures and for different interaction parameters. The reason why earlier papers did not state this independence of D on the lattice vibrations is explained.

Molecular dynamics studies on zeolites. II: A simple model for silicates applied to anhydrous natrolite

Abstract A simple model potential in two forms (harmonic and anharmonic) is proposed to be used in molecular dynamics simulations of silicate frameworks. This model is applied to the calculation of structural and vibrational properties of anhydrous natrolite, and the results are compared to experimental data. Despite their crudeness, the proposed models succeed in representing satisfactorily the main features of the silicate framework structure and dynamics.

NMR studies of carbon dioxide and methane self-diffusion in ZIF-8 at elevated gas pressures

Self-diffusion measurements with methane and carbon dioxide adsorbed in the Zeolitic Imidazolate Framework-8 (ZIF-8) were performed by 1H and 13C pulsed field gradient nuclear magnetic resonance (PFG NMR). The experiments were conducted at 298 K and variable pressures of 7 to 15 bar in the gas phase above the ZIF-8 bed. Via known adsorption isotherms these pressures were converted to loadings of the adsorbed molecules. The self-diffusion coefficients of carbon dioxide measured by PFG NMR are found to be independent of loading. They are in good agreement with results from molecular dynamic (MD) simulations and resume the trend previously found by IR microscopy at lower loadings. Methane diffuses in ZIF-8 only slightly slower than carbon dioxide. Its experimentally obtained self-diffusion coefficients are about a factor of two smaller than the corresponding values determined by MD simulations using flexible frameworks.

Molecular dynamics investigation of the diffusion of methane in a cubic symmetry zeolite of type ZK4

Extensive molecular dynamics simulations of methane diffusion in a cubic-type zeolite ZK4 have been performed by using a model including the vibrations of the silicate framework. In order to understand the role of the lattice dynamics in assisting the diffusive motion of the sorbed molecules we studied the same system also in the rigid framework approximation. Even more evidently than in the case of zeolite structures where the micropores are essentially cylindrical, the framework oscillations are confirmed to be an essential factor in determining the magnitude of the intracrystalline diffusion coefficient.

Dynamical Properties of Confined Water Nanoclusters: Simulation Study of Hydrated Zeolite NaA: Structural and Vibrational Properties

Water nanoclusters confined to zeolitic cavities have been extensively investigated by various experimental techniques. We report a series of molecular dynamics simulations at different temperatures and for water nanoclusters of different sizes in order to attempt an atomistic interpretation of the properties of these systems. The cavities of zeolite NaA are spherical in shape and about 1 nm in diameter and can host nanoclusters of water containing nearly up to 24 water molecules. A modified interaction potential, yielding a better reproduction of experimental hydration energy and water diffusivity across a number of different zeolites, is proposed. Molecular dynamics simulations reproduce the known experimental structural features obtained by X-ray diffraction. Variations of simulated vibrational IR and IINS spectra with temperature and size of nanoclusters are in good agreement with experiment. The simulated water nanoclusters in zeolite NaA are found to be too small to crystallize and, at low temperature, behave as amorphous ice, in agreement with recent experimental results for similar water nanoclusters in reverse micelles.

The behaviour of water confined in zeolites: molecular dynamics simulations versus experiment

In order to study the behaviour of water adsorbed in zeolites, which are microporous crystalline aluminosilicates, whose channels and cavities of nanometric dimensions can host many different molecules, we developed a sophisticated empirical potential for water, including the full flexibility of the molecule and the correct response to the electric field generated by the cations and by the charged atoms of the aluminosilicate framework. The reproduction of experimental data by our potential model is similar or even better than that obtained from the first principles methods. The results of molecular dynamics simulations of water confined in a variety of zeolites (worm-like clusters in silicalite, spherical nanoclusters in zeolite A and ice-like nanotubes in AlPO(4)-5 and SSZ-24) at different temperatures and coverage (loading) are discussed in connection with the experimental data, whose overall good reproduction encourages the attempt of an atomic-scale description of structural and dynamical phenomena occurring in confined water, in particular in the supercooled regime. The results are also compared with simulations and experimental data on bulk water.

Molecular dynamics studies on zeolites. Part 5.—Discussion of the structural changes of silicalite

Molecular dynamics simulations of silicalite, with and without sorbed methane molecules, at different temperatures, have been performed using a simple model potential. The results for sorption energetics and diffusion of methane were satisfactory, but the structural properties of the silicalite framework and their dependence on temperature and sorbate loading were reproduced only qualitatively. Suggestions for improvement of model potentials for zeolites are presented.

A molecular dynamics study of diffusion of methane in silicalite molecular sieve at high dilution

The diffusion of methane in silicalite at high dilution was studied by molecular dynamics both using a vibrating framework model, and keeping the framework fixed. Methane molecules were represented by Lennard-Jones particles. The diffusion coefficients resulted in agreement with experiment. The effect of the vibrating framework on the diffusive process is discussed and a detailed analysis of the behaviour of methane molecules in silicalite is reported.

Two-step model of molecular diffusion in silicalite

The influence of the particle “memory” on long-range diffusion in the channel network of silicalite is taken into account by considering pairs of subsequent steps between the channel intersections. It is shown that in this case the correlation rule between the principal elements of the diffusion tensor has to be modified by including an additional term, which takes account of the deviation of molecular propagation from complete randomness. The obtained relations are discussed in terms of molecular dynamics simulations of ethane in silicalite.