Diffusion of Isobutane in ZSM-5 Zeolite: A Comparison of Quasi-Elastic Neutron Scattering and Supported Membrane Results

Abstract

Isobutane diffusivities in ZSM-5 zeolite have been measured by quasi-elastic neutron scattering (QENS) and supported membrane techniques. This is the first diffusivity measurement of a branched alkane inside a zeolite of MFI type using a microscopic method. The self-diffusion coefficient derived from QENS is 3 × 10 -12 m 2 /s at 500 K. The diffusivity obtained with the supported membrane is 1 order of magnitude larger. In view of the large differences usually reported in the literature between microscopic and macroscopic techniques, the comparison between QENS and supported membrane data is quite satisfactory. The activation energy for diffusion determined from QENS is 17 kJ/mol. Because of the variation of loading due to temperature changes, an apparent activation energy is obtained with the supported membrane; it is 34 kJ/mol. In this zeolite, the diffusion of branched hydrocarbons is much slower than that of linear alkanes; the diffusion coefficient of isobutane is found to be 3 orders of magnitude lower than that of n-butane by QENS.

Experimental study and numerical simulation of hydrogen/isobutane permeation and separation using MFI-zeolite membrane reactor

“…Comparable results have been obtained for isobutane adsorption in silicalite-1 [42][43][44]. Table 1 gives the membrane geometric parameters and Table 2 the adsorption data in MFI material [14,41] for hydrogen and isobutane, respectively.…”

“…The effective layer thickness of the composite membrane, L, has been determined with the same approach used in the same paper. Adsorption data employed in this study were obtained from Bakker et al [14] for hydrogen, and Millot et al [41] for isobutane. Comparable results have been obtained for isobutane adsorption in silicalite-1 [42][43][44].…”

“…Among the molecules available for this test, sulfur hexafluoride (kinetic diameter: d k = 0.55 nm) [33], 2,2-dimethylbutane (d k = 0.60 nm) [12,36], o-xylene (d k = 0.68 nm) [36] and 1,3,5-triisopropylbenzene (d k = 0.74 nm) [41] were tested. In these references, the authors noted a size exclusion for these molecules and thus concluded that their membranes were defect-free.…”

Diffusion à longue distance

“…Les spectres DQEN qui ont été obtenus dans cette structure sont plus proche de la Figure 15 que de la Figure 16 ainsi que celle de l'isobutane, l'alcane branché le plus simple, a été récemment mesurée sur IN16 [58,62].…”

“…Ii est intéressant de noter, pour les applications éventuelles des membranes silicalite, que la diffusion des alcanes linéaires est beaucoup plus rapide que celle des branchés, le coefficient de diffusion du n-butane étant de 3 ordres de grandeur supérieur à celui de l'isobutane [62].…”