Marc Bée

Dynamics of ethane and propane in zeolite ZSM-5 studied by quasi-elastic neutron scattering

Abstract Quasi-elastic neutron scattering has been used to study the dynamics of ethane and propane in zeolite ZSM-5. The experiments were performed at different loadings for the two alkanes and at different temperatures for propane. The long-range translational motion of the molecules has been observed and has been interpreted with a jump diffusion model, the mean jump lengths being of the order of 10 A, slightly decreasing with increasing loading. The self-diffusion coefficients of ethane are about 2 × 10 −9 m 2 s −1 at 300 K, those of propane at the same temperature being smaller by a factor of two. An activation energy of 5 kJ mol −1 is obtained for the self-diffusion of propane. These results are in good agreement with the pulsed-field gradient n.m.r. measurements. In addition to the translational motion, a rotational motion of the two molecules is also observed; it is described by an uniaxial rotational diffusion model and rotational diffusion constants are derived.

Translational and rotational dynamics of methane in ZSM-5 zeolite: A quasi-elastic neutron scattering study

Abstract The translational and rotational motions of methane adsorbed at different loadings in NaZSM-5 have been studied by quasi-elastic neutron scattering at two temperatures: 200 and 250 K. The translational motion does not simply follow Ficks law, but a jump diffusion model with a Gaussian distribution of jump lengths satisfactorily simulates the experimental results. The diffusion coefficient that is obtained for long-range translational motion does not vary much on the loading in the range that was studied: it is of ⋍ 2.7 × 10 −5 cm 2 s −1 at 200 K and ⋍ 5.5 × 10 −5 cm 2 s −1 at 250 K. Good agreement is found between the neutron and n.m.r. results for this motion, but large discrepancies are observed with the macroscopic measurements. The rotational motion is well described by an isotropic rotational diffusion model, and this motion is found to be much slower in the zeolite than in physisorbed layers or in bulk solid methane.

Molecular self-diffusion of methane in zeolite ZSM-5 by quasi-elastic neutron scattering and nuclear magnetic resonance pulsed field gradient technique

The translational mobility of methane sorbed on an NaZSM-5 sample of proven crystallinity has been studied by quasi-elastic neutron scattering (QENS) and an n.m.r. pulsed field gradient technique (n.m.r. PFGT) at different loadings and temperatures. In both methods, long-range self-diffusion is detected. However, owing to the different timescales of the two experimental methods applied, the mean diffusion paths followed are of different magnitude: in QENS, molecular translation is measured up to 6nm; in n.m.r. PFGT, the mean molecular displacements amount to some µm. Nevertheless, since the r.m.s. molecular displacements followed by both methods considerably exceed the distances between the pore intersections of the ZSM-5 channel network, in both techniques the translational self-diffusion coefficient of guest molecules inside the zeolite pores is detected. For the intracrystalline self-diffusion coefficient, D, as well as for the activation energy of self-diffusion, Ea, the values determined by the two independent methods agree very well. In the temperature region 200–250 K, the intracrystalline self-diffusion coefficients of methane in ZSM-5 are found to be of the order of 10–5–10–4 cm2 s–1, with only slight concentration and temperature dependence. The activation energy determined by both methods amounts to 4–5 kJ mol–1. Further agreement between QENS and n.m.r. PFGT is obtained by comparing the mean molecular jump lengths, which by both techniques are found to be ca. 1 nm for light hydrocarbons in ZSM-5, slightly decreasing with increasing loading.

Influence of the cation composition on the dynamics of xylenes in X-type zeolites

The rotational and translational dynamics of para- and meta-xylene in NaX and BaX zeolites have been measured by quasi-elastic neutron scattering. Two different neutron spectrometers were used to cover time scales ranging from a picosecond to a few nanoseconds. At short times, rotational motions of the methyl groups and oscillations of xylenes adsorbed in front of cations in the supercages are observed. In NaX, larger amplitude motions are found compared with BaX. Even at 460 K, only the methyl groups of para-xylene are rotating in BaX, the aromatic ring is immobile on a time scale of 10 ps. At longer times, diffusion between cages can be measured in NaX, the diffusivity of the para-isomer being slightly larger than that of the meta-isomer. However, for both xylenes in BaX, only jumps between adsorption sites within a supercage are observed on the same time scale, indicating a much lower intercage migration in this zeolite.

Molecular dynamics of xylene in Na, Yb-Y and ZSM-5 studied by quasi-elastic neutron scattering

The quasi-elastic neutron scattering of xylene adsorbed in Na, Yb-Y and ZSM-5 has been studied in the temperature range 100–460 K. In both zeolites the translational motion is too slow to be observed on the timescale of the experiment so that only the rotational motions can be characterized. At 100 K only the methyl groups are rotating for xylene in Na, Yb-Y and with increasing temperature a uniaxial rotation of the whole molecule sets in. For high loadings of p-xylene in Na, Yb-Y the amplitude of oscillation remains at lower values at the same temperature as for the low loadings, indicating molecule–molecule interactions. The experimental elastic incoherent structure factors (EISFs) of p-xylene in ZSM-5 do not vary over the whole examined temperature range, the aromatic ring stays fixed and only one of the methyl groups is rotating.

On the translational mobility of benzene adsorbed on NaX-type zeolites

Application of the NMR pulsed field gradient technique and of quasi-elastic neutron scattering to molecular self-diffusion studies of benzene in zeolite NaX leads to coinciding results in both the absolute values and the concentration dependence.

Mobility of cyclohexane in a microporous silica sample: a quasielastic neutron scattering and NMR pulsed-field gradient technique study

Abstract The mobility of cyclohexane in a microporous silica powder has been studied by neutron scattering and NMR techniques at different temperatures and loadings. The silica powder has the same characteristics as the corresponding supported membrane. Self-diffusion coefficients of the order of 10 −10 m 2 s −1 are obtained both techniques at 300 K, the activation energy being ca. 11 kJ mol −1 . The same value was also found for benzene diffusing in the same silica sample, this implies that there are no specific interactions with silica. The dimension of the pores between the voids of the non-porous silica particles is of the order of 1 nm.

Measurement of the diffusivity of benzene in microporous silica by quasi-elastic neutron scattering and NMR pulsed-field gradient technique

The diffusivity of benzene in a microporous silica powder has been measured by neutron scattering and NMR techniques. The measurements have been performed on un-supported silica but the powder has the same characteristics as the active layer of a real membrane. Self-diffusion coefficients of the order of 10−10 m2s−1 are found at 300 K by both techniques so that the model of Knudsen diffusion is not valid for benzene in this microporous material. Due to the presence of small pores, the diffusion of benzene in the membrane-material approaches the diffusion regime usually observed in zeolites. Furthermore, the diffusivity of benzene follows an Arrhenius law with an activation energy of 11 kJ mol−1.

Quasi-elastic neutron scattering study of benzene adsorbed in ZSM-5

The quasi-elastic neutron scattering of benzene adsorbed at two different loadings in H-ZSM-5 has been studied in the temperature range 90–400 K. The translational motion is too slow to be observed on the timescale of the experimental so that only the rotational motions can be characterized. The model of uniaxial rotation in an N-fold cosine potential is used to interpret the experimental data. On decreasing temperature, there is a progressive blocking of the molecules for both loadings but the amplitude of motion is more restricted at higher loading, at the same temperature, indicating benzene–benzene interactions. Isotropic spherical rotation is not observed, in agreement with n.m.r. results, and the C6 uniaxial rotation is as fast as in the liquid, with a correlation time τ= 3.2 × 10–12 s at 300 K.

Microdynamics of Guest Molecules in Zeolites Studied by Quasielastic Neutron Scattering and Nmr Pulsed Field Gradient Technique

Summary Application of the quasi-elastic neutron scattering and NMR pulsed field gradient technique to molecular self diffusion studies of hydrocarbons in zeolites ZSM-5 and NaX leads to coinciding results in both the absolute values, the concentration dependence and activation energy of the intracrystalline self-diffusion coefficients. Further agreement between the neutron and NMR diffusion data is obtained by comparing the mean molecular jump lengths which by both techniques are found to be of the order of 1.2…0.25 nm, slightly decreasing with increasing loading. Due to the different time scales of the two experimental methods applied, the mean diffusion paths followed are of different magnitude: in neutron scattering experiments molecular translation is measured up to 6 nm; in the NMR pulsed field gradient technique the mean molecular displacements amount to some nm. However, in both methods, longrange self-diffusion is detected.