R. Kahn

Effect of an electric field on a methane molecule. I. Infrared analysis of methane (CH4–CD4) adsorbed in NaA zeolite in the temperature range 150–20 K

The spectra of methane adsorbed at low temperature in NaA zeolite show two strong effects of the field existing in the cavities: (i) the appearance of the ν1 forbidden band which increases when decreasing the temperature and (ii) the splitting of the ν3 degenerate band. The variation with T of its components lead to the conclusion that the molecule progressively is oriented in a C3V configuration with respect to the field.

Molecular dynamics by numerical simulation in zeolites: Methane in NaA

A molecular dynamical study of one methane molecule in a cavity of NaA zeolite is performed in order to compare calculated to experimental data obtained by infrared spectroscopy and neutron scattering experiments in the temperature range 300–30 K. The calculation shows the trajectory of the molecule in the cavity and then the occupied volume as a function of energy. It allows the calculation of average quantities and correlation functions: (i) the mean field felt by the molecule comparable to the field responsible for the induced infrared band ν1, (ii) the average of the potential energy (to be compared to the heat of adsorption) and of the velocity squared, (iii) the external frequency distribution, and (iv) the position autocorrelation function which is related to the dynamical structure factor seen by neutron scattering.

Effect of an electric field on a methane molecule. II. Calculation of the degeneracy splitting of the ν3 band. Expression of the second derivatives of the CH4 dipole moment and evaluation of the second derivative of the C–H bond polarizability

In order to interpret infrared spectra of methane adsorbed in NaA at low temperature, we have calculated the frequencies of CH4 and CD4 perturbed by an electric field. This calculation needs the expressions of the second derivative of the dipole moment and the polarizability. These later were given by Montero and we have expressed the terms ∂2m/∂Si∂Sj using the same approximations: additivity of the CH bond dipole derivatives. We find for the CH bond derivatives: μ’=−0.7×10−10 esu, μ‘=+1.5×10−2 esu. The magnitude of the field (3×105 esu) has been determined by the intensity of the induced ν1 band. The orientation of the field deduced from the experiments, is along a C3 axis of the molecule. Adjusting theoretical and experimental results, we have calculated the parallel and perpendicular second derivatives of the CH bond polarizability 2β‘+δ‘=−6.4×10−8 cm, δ‘− β‘=+5.3×10−8 cm.

Diffusivity of the hydrogen molecule sorbed in NaA zeolite by a neutron scattering experiment

The diffusion of hydrogen in NaA zeolite was studied by incoherent neutron scattering. An experiment was carried out on samples loaded with 1.2 to 3.4 molecules per cavity and at several temperatures from 70 to 150 K. The angular (θ) dependence of the elastic and quasielastic intensities shows that the H2 molecule has a translational motion in a nonrestricted volume. A diffusion model where the molecule undergoes isotropic jumps of mean length l=3.9 A independent of temperature and is at rest for a time τ0 between two jumps accounts for the width of the quasielastic scattering in the entire (θ,T) range (τ0=10.8 ps at T=100 K). This leads to a diffusion coefficient D(cm2/s)=6×10−4 exp(E/RT) with E=2 kJ/mol for the less loaded samples. The diffusion coefficient increases slightly with the loading.

Adsorption sites in Na-A zeolites. Neutron diffraction study of NaA-methane and NaA-acetylene systems*

Neutron diffraction has been used to study the adsorption of methane and acetylene in Na-A zeolites. The experiment has been carried out with a dehydrated powder sample (400°C) and with samples containing one molecule per cavity. The effect of the sorbed gas has been increased by using deuterated molecules. In spite of some dubiousness on the structure of the dehydrated Na-A we have been able to find the region in the zeolitic cavity where the molecule is adsorbed: at room temperature for acetylene and at low temperature (40 K) for methane, molecules are trapped in front of the NaII and NaIII cations. In its equilibrium position, the axis of the acetylene molecules is not parallel to the axis of the window. As the temperature is raised (150 and 300 K) our analysis of the diffraction pattern of the methane Na-A sample shows that the molecule is more and more delocalized. This last result is in agreement with previous quasi-elastic measurements.

Modelization of experimental isotherms of n-alkanes in NaX zeolite

Abstract Isotherms of five n -alkanes: propane, butane, pentane, hexane, and heptane in 13 X zeolite have been measured by microgravimetry. In temperature ranges in which the isotherms are reversible, the sorption is described by the statistical model developed by Dupont-Pavlovsky, showing that the sorbed phase is localized. The heat of sorption and the sorbate-sorbate interaction, obtained from the model, increase with the number of carbons.

Determination of the intracrystalline diffusion coefficient of methane in A zeolites by means of neutron spin-echo experiments

The intracrystalline motion of methane molecules adsorbed on CaNaA (5A) and NaA (4A) zeolites have been studied by means of spin-echo neutron scattering spectroscopy over distances from 15 to 30 A within the time-scale from 10–10 to 10–8 s. In CaNaA zeolite we were able to measure a mean lifetime of 4 × 10–10 s for the molecule in one cavity. In NaA this lifetime is > 10–8 s. The increase in residence time on going from CaNaA to NaA shows the influence of the cations located in the eight-membered oxygen windows. The interpretation of the results in terms of a diffusion-jump model involving discrete sites on a cubic lattice leads to a value of the intracrystalline diffusion coefficient in agreement with that obtained from n.m.r. pulse field-gradient measurement. This shows the influence of crystallinity on the diffusivity.

Effect of an electric field on a methane molecule

In zeolite cavities the strong electric field due to the ionic charges induces forbidden IR bands and splitting of degenerate vibrational modes. This has been observed on the CH4 molecule orientated in tripod configuration in adsorption sites of NaA and CaA zeolites. The two components of the v 3 splitting have similar intensities for a given temperature but evolve differently with the temperature, corresponding possibly to a variable distance of the molecule with respect to the site. We have calculated the infrared intensity, i.e., |∂M/∂Q|2, taking into account the permanent and the induced moment . The calculated intensities for the electric field directed along one of the CH bonds are plotted versus the field amplitude value and compared with the experimental data. For CH4 in NaA this leads to the assignment of each component (A and E symmetry) and to the conclusion that the derivatives of the permanent moment and of the polarizability tensor of CH4 with respect to the normal coordinate Q 3 have opposi...

Diffusion of H2 and HD in Zeolites: Quantum Effects at 100 K

The isotopes of hydrogen adsorbed in zeolites reveal both thermodynamical and dynamical anomalies for temperatures less than 100 K. We propose that quantum effects may explain the observed results, in particular the departure of the diffusion constant from an Arrhenius behaviour. With this hypothesis we find the same activation energy for H2 and HD and a very good order of magnitude for the potential curvature at the bottom of the wells.

Semiempirical calculation of the interaction energy of a CH4 molecule and a Na+ cation

We present a semiempirical calculation of the interaction energy of a CH4 molecule and one Na+ cation of the zeolite Na A. This energy depends on the distance, R, between the Na+ nucleus and the carbon nucleus of CH4, and on the orientation of the molecule. Taking in account the anisotropy of the repulsion, the most favourable orientation, at any R, is such that the carbon-Na+ axis is a C 2V axis with three hydrogens pointing to the cation, and the least favourable is such that the axis is still a C 3V axis but with one hydrogen pointing to the cation. The results are in agreement with the ones of Sauer et al. obtained by an ab initio calculation.