Loadings 2-methylpentane

Component 1 in Singapore buildings was correlated with compounds associated with humans and their activities. Human effluents have been reported to contain isoprene (Ellin et al, 1974) while tetrachloroethylene is a VOC found in dry-cleaned clothes worn by building occupants (Wallace, Pellizzari and Wendel, 1991) or from the use of consumer products (Sack et al., 1992). Tetradecane, benzaldehyde, o-xylene, naphthalene are emissions from dry process photocopiers (Leovic et al., 1996). Component 2 with high loadings ofn-decane, n- undecane, toluene, styrene, n-nonane, 1,2,4-trimethyl benzene probably reflects the emissions of carpets and vinyl floorings (Yu and Crump, 1998). Component 3 was primarily correlated with heptane and methylcyclopentane, which could be due to the emissions of water-based paints. Finally, component 4 was associated with 2-methylpentane, hexane, cyclohexane, methylcyclohexane and limonene, which is reflective of the emissions of air fresheners and cleaning products (Sack et al., 1992). 

Figure 9 shows the binary adsorption data of n-hexane and 2-methylpentane at 433 K as a function of the gas-phase ratio of the hydrocarbons. Obviously, the n-hexane loading monotonically decreases upon an increase of the partial pressure and loading of the 2-methylpentane. The total hydrocarbon loading only sUghtly decreases at high 2-methylpentane fraction in the gas phase. The preference for adsorption of n-hexane over the monobranched isomer is in line with the above-mentioned entropic considerations. 

Fig. 9 Loadings of mixture components in silicalite as a function of 2-methylpentane fraction in the gas phase (total hydrocarbon partial pressure 6.6 kPa, T = 433 K)...

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]. 

The loading dependence of 2-methylpentane is similar to earlier results showing a decrease of 2-methylpentane diffusivity with loading in single-... 

The loading of n-hexane in mixtures is somewhat higher than it is expected to be if it were proportional to its partial pressure (Fig. 12). On the contrary, the 2-methylpentane loading is somewhat lower. This points to preferential adsorption of -hexane over isohexane in their mixtures in H-ZSM-5 than in silicalite-1. In earlier experimental [50] and CBMC simulation studies [44] of n-hexane/isohexane mixtures in silicalite-1, a slight preferential adsorption of the linear alkane over the branched one has been found. The most prominent explanation for this preference is the molecular siting of these two hydrocarbon molecules. Whereas -hexane exhibits no clear preference for a position in the micropore system of MFI zeolite, the branched isomer is preferentially located at the channel intersections due to entropic reasons [44]. Consequently, 2-methylpentane will be pushed out from silicalite-1 by -hexane. These effects are even stronger for H-ZSM-5, most likely due to the stronger... 

Table 2 shows the adsorbed concentrations of the pure components. At a partial pressure of 6.6 kPa the amount of n-hexane is just slightly higher than that of isohexane in silicalite-1, while the linear alkane is obviously adsorbed more strongly than 2-methylpentane in H-ZSM-5 due to the stronger interaction with the acid sites. The maximum loading of each component has been measured by a separate adsorption study. The sorption capacity of n-hexane (7 molecules per unit cell), in agreement with earlier studies [48,59-61] exceeds that of 2-methylpentane (4 molecules per unit cell). The latter value equals the number of channel intersections in the MFI pore system per unit cell. Indeed, the sorption of isohexane molecules at... 

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. 

The current work indicates the strong effect of acid sites on the interaction and diffusivity of hydrocarbons. To further study this effect, we determined the single-component diffusion coefficients and specifically the activation energy for diffusion. Activated diffusion is described by the Arrhenius-type Eq. 8. The pre-exponential factor Djnf is related to the jump frequency between adsorption sites in the zeolite lattice, while the exponential expresses the chance that the molecules are able to overcome the free energy barrier - act between these sites. The loadings of n-hexane and 2-methylpentane in H-ZSM-5 and silicalite-1 have been measured at temperatures between 373 and 533 K at intervals of 20 K. The hydrocarbon pressure was taken identical... 

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... 

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. 

A comparison between sihcalite-1 and H-ZSM-5 teaches that acid sites have a profound influence on the self-diffusivity of alkanes. The self-diffusivities of both components decrease strongly, and we observe a significant preferential adsorption of the linear over the branched hexane. This is caused by the relatively stronger interaction of the linear hexane with the acid sites. On the contrary, 2-methylpentane loadings in mixtures in sihcahte-1 and H-ZSM-5 are very close. In H-ZSM-5, the diffusivity of the hnear alkane in mixtures with the branched alkane is influenced by two factors... 

E. J. M. Hensen, A. M. de Jong, and R. A. van Santen have written Chapter 7, which introduces the tracer exchange positron emission profiling (TEX-PEP) as an attractive technique for in-situ investigations, for example, in a stainless steel reactor, of the adsorption and diffusive properties of hydrocarbons in zeolites under chemical steady-state conditions. Self-diffusion coefficients of hydrocarbons, labeled by proton-emitting C at finite loadings and even in the presence of another imlabeled alkane, may be extracted. The method is illustrated by adsorption and diffusion measurements of linear (n-hexane) and branched (2-methylpentane) alkanes in Fl-ZSM-5 and silicalite-1. 

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