N-hexane diffusivity

Hernandez and Catlow (86) recently reported an investigation of n-butane and of n-hexane diffusion in silicalite the work is similar to that of June et al. (85). Many calculation details were the same as in the earlier work, including the assumption of identical Lennard-Jones coefficients of intermolecular dispersion and repulsion. Simulations were performed at different loadings for butane, namely, 2, 4, 5.3, and 8 molecules per unit cell. In addition, simulations were performed at a constant loading and variable temperature (200, 300, and 400 K) for both butane and hexane. These calculations were performed for 1000 ps, twice the length of those of June et al. The zeolite framework was held rigid. 

Fig. 12. The effect of zeolite loading in the TEOM reactor on the diffusion process for n-hexane diffusing into HZSM-5 at T = 298 K, total flow rate of 250 ml/inin and n-hexane partial pressure of 22.7 mbar. Symbols , 20 mg of zeolite O, 10 mg +, 5 mg.

Similarly, also the n-hexane diffusivity in Na(75),Ca-X is found to be much smaller than in Na-X at small concentrations [152]. In this case, however. 

Comparison of the diffusivities in Na,Ca-X and Na,Ca-A at comparable calcium exchange show [152] that the methane diffusivities are of the same order, while the n-hexane diffusivity in Na,Ca-A is much smaller than in Na,Ca-X. One has to conclude, therefore, that for the diffusion of the longer n-alkanes in Na,Ca-A it is justified to consider the passage through the windows as the rate-limiting step. For methane and ethane, however, the transport properties are dominated by the interaction with the cations. MD simulations of methane in Na,Ca-A are in satisfactory agreement with the NMR data and confirm this conclusion [154]. 

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. 

Summarizing, we conclude that for binary mixtures of a linear and branched hexane in H-ZSM-5 and silicalite-1 two factors influence the respective diffusivities (i) the strong interaction with acid sites preferentially decreases n-hexane diffusivity and (ii) the blocking of intersection adsorption sites by 2-methylpentane decreases n-hexane diffusivity. At high loadings of the branched isomer the latter effect is dominating, and Anally the diffusivity of the linear hexane is totally determined by its branched isomer. 

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