Adsorption and diffusion of n-hexane/2-methylpentane mixtures in zeolite silicalite:: Experiments and modeling

With the tracer-exchange positron emission profiling (TEX-PEP) technique, the reexchange process of radioactively labeled molecules with a steady-state feed stream can be measured inside a zeolite-packed bed reactor. When the experimental tracer-exchange curves are modeled, values for the micropore diffusion and adsorption constant can be obtained. As one can choose which component to label, this technique is ideally suited for studying multicomponent diffusion. In the present study, this technique has been used to measure the diffusive and adsorptive properties of an n-hexane/2-methylpentane mixture in zeolite silicalite. The measurements were performed at different ratios of n-hexane and 2-methylpentane in the gas phase at a constant total hydrocarbon pressure of 6.6 kPa and a temperature of 433 K. A slight preference for the adsorption of n-hexane was found because it is entropically more favorable to adsorb these molecules as they have no preferential siting in the zeolite pores; The diffusivity of the slowly moving 2-methylpentane is not strongly affected by the presence of the fast moving n-hexane. The mobility of the linear alkane however strongly decreases with increasing 2-methylpentane ratio and suddenly drops at a loading of approximately three 2-methylpentane molecules per unit cell. This is caused by the fact that the branched alkanes are preferentially sited in the intersections between the straight and zigzag channels of silicalite and therefore effectively block the zeolite pore network. These results show that the adsorptive properties of the components and the structure of the zeolite network play an important role in the behavior of multicomponent mixtures in zeolites.