Adsorption of n-hexane

Fig. XVII-21. Continued) (c) Isosteric heats of adsorption of n-hexane on ice powder Vm = 0.073 cm STP. (From Ref. 125). (d) Isosteric heats of adsorption of Ar on graphitized carbon black having the indicated number of preadsorbed layers of ethylene. (From Ref. 126.)...

Fig. XVII-23. (a) Entropy enthalpy, and free energy of adsorption relative to the liquid state of N2 on Graphon at 78.3 K (From Ref. 89.) b) Differential entropies of adsorption of n-hexane on (1) 1700°C heat-treated Spheron 6, (2) 2800°C heat-treated, (3) 3000°C heat-treated, and (4) Sterling MT-1, 3100°C heat-treated. (From Ref 18.)...

Data were obtained for the equilibrium adsorption of n-hexane on silica... 

Figure 1 shows the crystallization kinetics of ZSM-48. A good agreement is shown between the crystallinity evaluated by X-ray and adsorption of n-hexane. These kinetic curves confirm the metastability of ZSM-48 zeolite. Indeed the conversion of ZSM-48 into cristobalite, a dense and stable phase, occurs for long reaction times. The difference between the two curves at start reaction times is due to the presence of hydrated silica (Aerosil) that also adsorbs n-hexane. 

Adsorption of n-Hexane from a Natural Gas with Silica Gel... 

The low-coverage energy data for the adsorption of n-hexane and benzene on various non-porous solids in Table 1.4 illustrate the importance of the surface structure of the adsorbent and the nature of the adsorptive. Since n-hexane is a non-polar molecule, Em > Esp, and therefore the value of E0 is dependent on the overall dispersion forces and hence on the density of the force centres in the outer part of the adsorbent (i.e. its surface structure). Dehydroxylation of a silica surface involves very little change in surface structure and therefore no significant difference in the value of E0 for n-hexane. However, replacement of the surface hydroxyls by alkylsilyl groups... 

Figure 11.17. Differential energy of adsorption of n-hexane or benzene versus amount adsorbed by NaY zeolite (after Schirmer et al., 1980).

The process of adsorption of n-hexane in a bed of zeolite Na X was monitored by NMR imaging in combination with PFG NMR (99).The intracrystalline diffusivities were found to depend exclusively on the given sorbate concentration, independent of the time interval elapsed since the onset of the adsorption process. The authors may conclude that adscnhent accommodation during the process of adsorption is not of significant influence on the molecular mobility (99). 

Fig. 13.11. Heat of adsorption of n-hexane correlated to the pore size of materials.

This is also experimentally well documented as one sees a continuous decrease of the heat of adsorption of n-hexane as the pore size increases (see Fig. 13.11 from Ref. [27]). Note that the curve also suggests that the pore size is more important for the strength of interaction than the chemical composition of the porous material. 

However, when hydrocarbons with different shape are adsorbed on activated carbons the values of Vg are not related to a property depending on the molecular weight [14]. This occurs with the adsorption of n-hexane (linear), benzene and cyclohexane (cyclic), and 2,2 dimethyl butane (2,2 DMB, branched) on activated carbons obtained from olive stones (Table 1). In order to explain these results we must consider the relationship between the molecular dimension of the adsorbates and the shape and size of the pores. 

On adsorption of benzene, n hexane and carbon tetrachloride from their mixture on hydroxylated silica the excess adsorption of benzene is positive in all concentration region with the accordance of the higher heats of benzene adsorption in compared with the heats of adsorption of other components. The heats of adsorption of n-hexane and carbon tetrachloride on silica are similar and small positive excess adsorption of carbon tetrachloride is determined by difference in molecular area of molecules on the silica surface which is large for n-hexane. 

These results are further evidence that Ed is closely related to the heat of adsorption of n-hexane rather than the activation energy for diffusion in these frameworks. The higher Ed values for ZSM-5 and ZSM-11 over those for Theta-1 are the result of the stronger electrostatic interactions which exist in the frameworks with higher A1 concentrations. 

Theoretically calculated values of the heat of adsorption for n-hexane and 2-methylpentane are 70 kj mol and 65 kj mol, respectively [46,47], which is in agreement with the average values determined by Zhu et al. [48]. As the heats of adsorption of these alkanes are very close, the difference in adsorption is caused by an entropic effect. Indeed, the conformations of the bulkier branched alkanes are much more restricted in the narrow pores of the medium-pore MEI zeoUte. Eor the branched isomer in siUcaUte-1 there is a large difference in the adsorption entropy between the molecular locations in the intersections and in the channels as shown by Zhu et al. [48]. Therefore, the adsorption of 2-methylpentane from the gas phase leads to a higher reduction in entropy compared to adsorption of n-hexane. This makes it en-tropically less favorable to adsorb the branched isomer [44]. 

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. 

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... 

Similarly, insight has been gained into the pore structure of zeolites of type MWW, ITQ-2 and IM-5 through microcalorimetric studies of adsorption of n-hexane, toluene, m- and o-xylenes, 1,2,4- and 1,3,5-trimethylbenzene [61,67]. Another study [68] has shown that the y-cages of H-ZK-5 and K-ZK-5 are the... 

Therefore, in the system under study nonaqueous solvents exert an essential effect on proceeding of colloid-chemical processes which result in structurization of a porous material. The outcome of such an effect is, as a rule, a decrease (sometimes rather substantial) in the specific surface value as well as a decrease in the sorptive volume of pores and increase in their size. Analogous inferences were also made by the authors of reference [19] who considered the isotherms of adsorption of n-hexane by samples 13-19. AU the curves (with the exception of the isotherm for sample 19) were S-shaped and were distinguished for a steep rise of hysteresis loop at P/Pq > 0.5. This is known to be characteristic of mesoporous adsorbents [27]. The isotherm for sample 19 prepared in the presence of DMF was more flat and differed markedly from the isotherms of samples 13-18. The observation is in complete agreement with the porous structure parameters of this sample (see Table 33.1). 

Further, it was of interest to compare adsorptive properties of samples with mono- and bifunctional surface layer with respect to sorbates of various nature [33]. Figure 33.6 shows isotherms of adsorption of n-hexane, acetonitrile, and acetic acid by samples 14 and 36 in the relative pressure interval 0.0-0.5 (it should be noted here that the isotherms for samples 8, 13, 31, and 34 are analogous to those for sample 14). It is known that... 

Figure 7.9 Experimentally determined heats of adsorption of n-hexane plotted against eage size of channel diameter for a series of pure silica zeolite polymorphs and aluminophosphates.

A classical curve, published by Isirikyan and Kiselev (1961) showing the variation of enthalpy of adsorption of n-hexane on a graphitized carbon black at 20 °C, is as shown in Figure 4.43. Values of A// (calmoU ) initially decrease as the sites of highest adsorption are occupied first. As the monolayer is established, so the dispersion forces between the adsorbed molecules of n-hexane increase in magnitude reaching a maximum on completion of the monolayer. 

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