In this paper we present the major results of a thermodynamic and structural study of the adsorption of 2-methylpentane (2MP) and 2,2-dimethylbutane (22DMB) on a ZSM-5 zeolite under isothermal and isobaric conditions. A thermodynamically inconsistent behavior is observed with 22DMB at constant pressure: the adsorbed amount increases as the temperature of the zeolite increases from 343 to 443 K. We suspect that this particular phenomenon is the result to the monoclinic/orthorhombic phase transition undergone by the zeolite. This flexibility of the zeolite framework seems to have a significant effect on the separation of both isomers by selective adsorption. Coadsorption experiments show that the ZSM-5 zeolite has a poor selectivity for 2MP with respect to 22DMB when the structure is orthorhombic whereas it is very selective in the monoclinic structure. However the selectivity decreases sharply as the filling increases. The presence of 22DMB molecules located at the intersections of the channels probably obstructs the adsorption of 2MP.
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