Diffusion benzene

In order to apply this technique for quantitative characterization of counter-transport phenomena in zeolites it had to be checked first for the more simple case of single-component diffusion. As an example, benzene diffusion in H-ZSM-5 was chosen, because results for this case had been already reported by several authors. Their results, obtained from sorption kinetics as well as from NMR experiments, were in very good agreement and, thus, provided a reliable basis for comparison [13,14]. 

The adsorption and diffusion properties of benzene are of immense interest in zeolite research aromatics play important roles in a number of zeolite-catalyzed processes. Theoretical simulations of benzene diffusion first began to be published in the late 1980s. The first studies evaluated and minimized the potential energy of a molecule such as benzene within the channels, a method less computationally demanding than the MD simulations that followed. Most recent studies have used the TST formalism. 

Benzene Benzene diffuses much less rapidly than cyclohexane and the activation energy is higher ( 5 kcal/mole). Detailed studies by Lechert and co-workers(49-51) reveal that in zeolite Y, which has no delocalized cations, the benzene molecules tend to occupy tetrahedrally disposed sites with the molecules oriented parallel to the six ring at a distance 3.2l from the S2 cation. 

S. M. Auerbach,/. Chem. Phys., 106, 7810 (1997). Analytical Theory of Benzene Diffusion in Na-Y Zeolite. 

Ballach, J., Greuter, G., Shultz, E., and Jaeshke, W., Variations of uptake rates in benzene diffusive sampling as a function of ambient conditions, Sci. Total Environ., 243/244, 203-217, 1999. 

T > 2500 s in which is characteristic of benzene diffusion in the alumina oxide pores. 

Figure 9(a) Phase lag versus frequency curves for benzene diffusing in... 

Figure 9(b) Phase lag versus frequency curves for benzene diffusing in NaX at 468 K. (0, x) denote equilibrium pressures of 1, 2 and 3 Torr, respectively. Solid lines are the best theoretical fits. 

TTie diffusion coefficients obtained for benzene diffusing a Silicalite-1 and HZSM-5 in the present study agree well with those reported in the literature.[7] Figure 10(a) shows that our... 

In conclusion, therefore, these studies have shown that benzene diffuses 2-3 orders of magnitude faster in the large channels of NaX at 400K than in the smaller channels of ZSM-5/Silicalite-l. Secondly, the single-step method can follow sorption/desorption processes which only take -200 ms to attain equilibrium. 

Figure 6.2 Decreases of the benzene diffusion coefficient due to its inclusion in micelles...

Figure 6.2 illustrates the decrease of the benzene diffusion coefficient in various micellar solutions. Table 6.1 lists the diffusion coefficients of various solutes in different micellar media. Depending on the solute affinity for the micellar phase, the D , diffusion coefficient can be decreased by a factor varying from 2 to 10 (Figure 6.2). Such a decrease may highly increase the B and C terms of the Knox equation as...

Young et al. suggested the following kinetic situation under the toluene disproportionation conditions. The transalkylation reaction to form benzene and xylenes within the pores is relatively slow. Benzene diffuses out of the pores rapidly. The xylenes isomerize rapidly within the pores. (Xylene isomerization is about 1000 times faster than toluene disproportionation.) para-Xylene diffuses out moderately fast while the ortho and meta isomers move within the pores relatively slowly and further convert to para isomer before escaping from the channel system. 

Even this is not the entire story, however. The properties of the bilayer, and hence the diffusion rates, also depend on the composition and physical state of the bilayer. For example, in the studies of benzene diffusion in a dimyristolylphosphatidyl-choline (DMPC) bilayer mentioned above, temperature was varied in a controlled manner. As the temperature varied, so did the distribution of free volume. At low temperatures (still above the gel to liquid-crystal phase transition) the free volume was largely localized in large voids in the center of the... 

Fig. 3.3-5. Examples of benzene diffusion and convection. In the dilute solution at the left, the exact results are close to the approximate ones in Eq. 2.2-10. In the concentrated case at the right, they are not.

Diffusion coefficients in liquids are about ten thousand times slower than those in dilute gases. To see what this means, we again calculate the penetration distance which was the distance we found central to unsteady diffusion. As an example, consider benzene diffusing into cyclohexane with a diffusion coefficient of about 2 10" cm /sec. At time zero, we bring the benzene and cyclohexane into contact. After 1 second, the diffusion has penetrated 0.004 cm, compared with 0.3 cm for gases after 1 minute, the penetration is 0.03 cm, compared with 4 cm after 1 hour, it is 0.3 cm, compared with 30 cm. 

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