Liquid phase sorption and diffusion of branched and cyclic hydrocarbons in silicalite

Sorption and diffusion of several singly branched, doubly branched and cyclic C-6 and C-7 hydrocarbons (2-methylpentane, 3-methylpentane, 3,3 dimethylbutane, 2,2-dimethylbutane, 2,4-dimethylpentane, cyclohexane, cyclohexene, n-hexane and 2 methyl-l-hexene) in large silicalite crystals, with n-hexane as the carrier, have been studied by liquid chromatography and by liquid ZLC measurements. The equilibrium data show mildly endothermic exchange energies and rather low equilibrium constants. Both the absolute values of the equilibrium constants and the exchange energies are consistent with vapor phase data. The kinetic data are more surprising, yielding exchange diffusivities which are larger by several orders of magnitude relative to the values derived from (single component) vapor phase measurements at low sorbate loading. The diffusivity values derived from the chromatographic and ZLC measurements are self-consistent. A tentative explanation involving expansion of the silicalite lattice under saturation conditions, is suggested, to explain these observations. (C) 1998 Elsevier Science B.V. All rights reserved.