Adsorption and diffusion properties of hydrocarbons in zeolites

The adsorption and diffusion behaviour of C1–C6n-alkanes and C5–C8 cyclic hydrocarbons, including benzene, toluene, p-xylene, ethylbenzene, cyclopentane, cyclohexane, p-dichlorobenzene (p-DCB), cis- and trans-1,4-dimethylcyclohexane (c- and t-DMCH), in silicalite-1, theta-1 and AlPO4-5 have been systematically investigated by using gravimetric balances, computer simulation calculations and the frequency-response (FR) technique, respectively. All the C1–C6n-alkane molecules present simple, type-I isotherms in both silicalite-1 and theta-1 zeolites. The deviation of the isotherms from normal Langmuir model can be, however, found for n-pentane and n-hexane adsorbed in silicalite-1 due to the significant interactions between the sorbed molecules. The adsorption of the cyclic hydrocarbons in silicalite-1 show anomalous adsorption properties which can be ascribed to the subtle interplay of increased sorbate–sorbate interactions and decreases in the entropy of sorption at loadings in excess of 4 molecules per unit cell (m/u.c.). For these molecules sorbed in the one-dimensional, 12-ring channels of AlPO4-5, simple adsorption behaviour was observed. Analogous to the adsorption behaviour, the diffusivities of hydrocarbon molecules in zeolites also depend strongly on the characteristics of the sorbate and on the structures of zeolite channel systems. Molecular simulation results have been used to explain the experimental phenomena and to understand, at a molecular level, the underlying adsorption and diffusion features of the systems studied.