Optimal binding site of a methane molecule on the silanol covered (010) surface of silicalite-1: ONIOM calculations.

The binding energies and the corresponding structures of a methane molecule on the silanol covered (010) surface of silicalite-1 have been investigated using ab initio methods. Different levels of calculations, HF/6-31G(d), MP2/6-31G(d) and ONIOM (MP2/6-31G(d):HF/6-31G(d)) including the correction of an error due to an unbalance of the basis set, known as basis set super position error (BSSE), as well as the size of the cluster representing the silicalite-1 surface, were systematically examined to validate the model used. The ONIOM method with the BSSE correction was found to be a compromise between accuracy and computer time required. The optimal binding site on the silicalite-1 surface was observed at the configuration where the methane molecule points one H atom toward the O atom of the silanol group. The corresponding binding energy is -1.71 kJ/mol. This value is significantly higher than that of -5.65 kJ/mol when the methane molecule approaches the center of the straight channel. At this configuration, the C atom of methane was observed to locate exactly at the center of the channel. This leads to the conclusion that the methane molecule will relatively seldom be adsorbed on the silanol covered (010) surface of silicalite-1. Instead, the adsorption process will take place directly at the center of the straight channel.

Concentration dependence of the methane structure in silicalite-1: a molecular dynamics study using the Møller-Plesset-based potential.

Møller-Plesset perturbation-based potentials have been used in molecular dynamics simulations to examine methane diffusion in silicalite-1. The simulation box contains 2 unit cells of silicalite-1 and varying loading numbers (n(ld)) from 1, 2, 3, to 4 methane molecules per intersection, corresponding to 8, 16, 24, and 32 molecules per simulation box, respectively. Consistent with the previous study, a preferential diffusing path for methane is close to the channel axes. The structure of the methane molecules in the silicalite-1 pore was exhibited in terms of the methane-methane radial distribution function (RDF) in which the first peak appears at 6.4 A for n(ld) < or = 2, becomes a broad maximum at n(ld) = 3, and splits into two sharp peaks centered at 6.4 and 8.6 A at n(ld) = 4. This fact can be clearly described by an intensification of the methane density in the straight and zigzag channels but a decrease in the intersection when the loading increases. These features of the observed RDFs are in contrast with the previous report using a molecular dynamics force-field potential in which the RDFs for all concentrations show first maxima at approximately 4 A. The analysis of the relative residence times of methane at different sites inside the silicalite suggests that the zigzag channel is the most favored location. The computed self-diffusion coefficients as well as the heat of adsorptions are in reasonable agreement with the available values.

Structure and energetics of water-silanol binding on the surface of silicalite-1: quantum chemical calculations.

Quantum mechanical calculations have been carried out to investigate the structural properties and the interaction between water molecules and silanol groups on the surface of silicalite-1. The (010) surface, which is perpendicular to the straight channel, has been selected and represented by three fragments taken from different parts of the surface. Calculations have been performed using different levels of accuracy: HF/6-31G(d,p), B3LYP/6-31G(d,p), HF/6-31++G(d,p), and B3LYP/6-31++G(d,p). The basis set superposition error has been taken into account. The geometry of the silanol groups and that of the water molecules have been fully optimized. The results show that the most stable conformation takes place when a water molecule forms two hydrogen bonds with two silanols, with only one silanol lying on the opening of the pore of the straight channel. The corresponding binding energy is -48.82 kJ/mol. These areas are supposed to be the first binding sites which have to be covered when the water molecule approaches the surface. When the water loading increases, the next favorable silanols are those of the opening of the pore in which the four possible complex conformations yield a binding energy between -25.62 and -37.41 kJ/mol. It was also found that the calculated O-H bond length of the silanol in the free form was slightly shorter than that in the complex. In terms of the stretching frequency, the complexation leads to a red shift of the O-H stretching of the silanol group.