Adsorption and Diffusion of n-Hexane/2-Methylpentane Mixtures in Zeolite Silicalite:  Experiments and Modeling

Abstract

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.

Integration of MFI Zeolite Membranes in the Light Gasoline Isomerisation Process

“…Several authors have already observed this trend, on both C4 and C6 paraffin cuts [10,11]; -the phenomenon whereby the fast compounds are slowed by the slowest compounds only occurs if the coverage rates (ratio of the local concentration of the adsorbed species in its zeolite over the concentration at saturation) of the branched compounds in the zeolite micro-porosity are high. This has been previously observed with a mixture of hexane/3-methylpentane isomers: the diffusion coefficient of the linear compound (normal hexane) in silicalite (a MFI zeolite containing only silica) was equal to 4.5 × 10 -11 m 2 /s when 3-methylpentane was absent from the micro-pores, whereas it was lowered to 10 -11 m 2 /s when 83% of the micro-pore volume was filled with 3-methylpentane [12]. As the adsorption of branched paraffin isomers in MFI zeolite was decreasing with increasing temperature [13], the diffusion selectivity between the linear (faster) compounds and the branched compounds would therefore be more favoured at high temperatures.…”

Positron Emission Profiling Study of the Diffusion of 3-Methylpentane in Silicalite-1

“…An example fit of the experimental data is shown in figure 2. In our previous study [2], we have shown that the presence of intracrystalline diffusion limitations has a significant influence on the shape of the concentration profiles causing the asymmetry: the lower the diffusivity D c , the stronger the tailing of the concentration profile. Parameter K a is related to the delay of the profiles: the smaller the delay, the lower the value of K a .…”

“…The model used to describe the molecular transport in steady-state experiments (TEX-PEP experiments [2]) was applied to the pulse experiments, but with different initial conditions. A detailed description of the model that is basically a modification of the equations used by Noordhoek et al [21], and more information about its application can be found in the work of Schuring et al [2]. The model used to analyze the pulse experiments will be only briefly discussed below.…”

“…Diffusion through the laminar fluid film surrounding the particles can be neglected, as this process is much faster than diffusion inside the zeolite crystal. Adsorption/desorption process at the crystal boundary was discussed in our earlier study [2] and it was shown that it is fast compared to the diffusion inside the zeolite crystal, and therefore, the term N c is determined by diffusion to the crystal boundary:…”

“…Diffusion of the reactant/product mixture might play a significant role in the selectivity of the catalytic process. In our previous study of the n-hexane/ isohexane mixture diffusion in silicalite [2], it has been shown that the diffusion of both alkanes is controlled by the concentration of the slow component. Therefore, the concentration dependence of the diffusion coefficient, especially for bulky alkanes (as isohexane) is an important issue.…”