Diffusion of n-hexane

Table 3. Activation energy for diffusion of n-hexane in samples A, B and C.

Although the Na ions must enhance the activation energy for diffusion of n-hexane molecules in the channel system of Theta-1 the SHR method of analysis assumes that desorption is not diffusion controlled and thus any increase in Ed with Na content cannot be interpreted as due to an increase in the activation energy of diffusion. Although the activation energy of diffusion of n-hexane has not been measured, as yet, as a function of cation content or type, it is certain that this energy will be... 

P-08 -Transient uptake measurements using an oscillating microbalance effect of acid leaching on the diffusivity of n-hexane in Pt/H-Mordenite... 

Fig. 18 Apparent coefficients of intracrystalline self-diffusion of n-hexane as observed by time- and space-resolved PFG NMR in a bed of zeolite Na-X with restricted ( ) and unrestricted ( ) sorbate supply in dependence on the sorbate concentration. The real diffusivities open symbols) were calculated from these values by using the correspondence presented by Fig. 3. The full line with the indicated error bars represents the range of intracrystalline diffusivities as observed in previous PFG NMR studies with closed sample tubes. From [163] with permission...

Counter-Diffusion of n-Hexane vs Neopentane and Neopentane vs n-Hexane by "Micro"-FTIR... 

Fig. 32 Counter-diffusion of n-hexane (preloaded) versus neopentane (subsequently admitted)...

Figure 11 shows the self-diffusion coefficients obtained from the TEX-PEP experiments for both alkanes as a function of the gas-phase mixture composition. Evidently, we find that the self-diffusivity of n-hexane is an order of magnitude higher than that of the 2-methylpentane. Indeed, the kinetic diameter of n-hexane (4.3 A) is smaller than that of isohexane (5.0 A) [51]. Moreover, we observe a decrease in mobility with increasing fraction of the branched alkane in the gas phase. Analogous behavior was found for CH4/CF4 mixtures, where the self-diffusivity of both components decreased as the loading of the slower diffusing tetrafiuoromethane increased [52]. 

Figure 13 displays the self-diffusivities of n-hexane and 2-methylpentane in silicalite-1 and H-ZSM-5 as a function of the ratio of the hydrocarbons. The self-diffusivities of both hexanes linearly decrease with increasing gas-phase fraction of the branched hexane in the gas phase for the non-acidic and acidic zeolite. In H-ZSM-5, the mobility of alkanes is approximately two times slower than in silicalite-1. Obviously, the presence of acid sites strongly affects the molecular transport due to stronger interactions with the n-hexane molecules. A similar effect of Bronsted sites on the single component diffusion of aromatics was observed in MFI zeolites with different concentration of acid sites [63-65]. The frequency response (FR) technique provided similar results... 

Around a value of the gas-phase fraction of 2-methylpentane of about 0.83, the influence of the acid sites on the n-hexane diffusivity is not dominant anymore in comparison to the pore occupation of slow-diffusing 2-methyl-pentane. Figure 14 shows the dependence of the diffusivities of both components versus the concentration of adsorbed 2-methylpentane in terms of molecules per unit cell. The diffusivities of n-hexane in silicalite-1 and H-ZSM-5 become nearly equal when the concentration of 2-methylpentane reaches approximately 2.75 molecules per unit cell. For 2-methylpentane we And that the self-diffusivity in silicalite-1 becomes very close to the value in H-ZSM-5 at the same loading. 

Figure 15 displays the loadings of n-hexane and 2-methylpentane in both zeolites. Under similar conditions, the adsorbed concentration of n-hexane is higher than that of 2-methylpentane, especially at lower temperatures. The interaction with n-hexane results in higher loadings for H-ZSM-5 than for silicalite-1. From the temperature dependence of the diffusivity of n-hexane in both zeolites, the apparent activation energy has been deduced and the results are collected in Table 3. Corresponding Arrhenius plots are shown... 

Fig. 16 Arrhenius plots for diffusivities of n-hexane left) and of 2-methylpentane right) in silicalite-1 and H-ZSM-5...

Summarizing, we observe that the presence of acid sites causes a decrease in the self-diffusivity of n-hexane and 2-methylpentane. In H-ZSM-5, we find that the diffusivity of n-hexane in mixtures with its branched isomer is determined by two factors (i) the interaction with acid sites, strong for the linear alkane, which decreases the diffusivity and (ii) the presence of 2-methylpentane which has an order of magnitude lower diffusivity. At low 2-methylpentane loadings the influence of the acid sites is dominating. However, at a loading of about 2.7 molecules per unit cell, the effect of pore blocking by the preferential location of the branched alkane in the intersections dominates. The diffusivities are then more or less equal in silicalite-1 and H-ZSM-5. 

We have discussed the adsorption and diffusion of binary mixtures of hnear (n-hexane) and branched (2-methylpentane) alkanes in silicahte-1. It turned out that not only the size but also the siting of the molecules in the particular zeohte plays an important role in the behavior of the mixture components. A shght preference for the adsorption of n-hexane over 2-methyl-pentane was observed because of the higher packing efficiency of the hnear alkane. This is due to the preferential location of the branched alkane in the zeohte intersections. A consequence of this is that the diffusivity of n-hexane... 

The Knudsen diffusion, viscous flow and surface diffiision for strongly adsorbing vtqx>rs are well described at low range of pressures in this paper. The collision-reflection factor for Knudsen diffusion is found to not constant but exhibit a modest increase with an increase in pressure. The dependence of the Knudsen diffusion for n-hexane on pressure is stronger than that of the other vapors. Moreover the activation energy for the surfoce diffusion of n-hexane exhibits a faster decreasing behavior in comparison with the others. Conclusively, the reason for the minimum appearance in the total permeability of n-hexane can be attributed by dte interplay between the Knudsen diffusion and surface diffusion. 

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