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.
The adsorption properties of several branched paraffins from C4 to C6 on MFI type zeolite were investigated.
A temperature-programmed-equilibration method was used for this study, and the analysis of the desorption
rates as a function of temperature showed two desorption peaks. The interpretation of this phenomenon
is discussed, and it is revealed that the high-temperature desorption occurs in conditions close to equilibrium
while low-temperature desorption is subject to diffusional limitations. Furthermore, an XRD study shows
that the phase transition in the crystal structure can be lowered by 70 K when 3-methylpentane is adsorbed.
This property shows that the low-temperature desorption peak is not due to a change of symmetry in the
structure of the zeolite. Modeling of the high-temperature peak provided adsorption isosteric enthalpy and
entropy variations. These values are consistent with literature data. For the first time, adsorption isosteric
entropy variations of sorption are calculated for branched alkanes in silicalite-1.
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