Adsorption of n-alkanes

Evidence of a different kind is furnished by the fact that the Gurvitsch rule (p. 113) is often obeyed by systems showing Type I isotherms " the amounts of different adsorptives taken up by a given adsorbent, when expressed as a volume of liquid, agree within a few per cent. The order of agreement is illustrated by the typical examples in Table 4.1 for the adsorption of n-alkanes on ammonium phosphomolybdate, and in Table 4.2 which refers to a variety of adsorptives on a silica gel. It must be admitted, however, that there are cases where considerable deviations from the Gurvitsch mle are found, even though the isotherms are of Type 1. Thus, in Table 4.3 the variation in values of the saturation uptake is far outside... 

Perhaps the most direct method of evaluating microporosity is to fill up the micropores with some suitable adsorbate whilst leaving the mesopores, macropores and external surface free. The use of n-nonane as a preadsorbate was proposed by Gregg and Langford on the basis of earlier work on the adsorption of n-alkanes C, to C, on ammonium phos-phomolybdate, a microporous solid. This work had shown that the rate at... 

Figure 40 (a) Plot of free energy of adsorption of n-alkane probes vs carbon number 110°C,... 

Storey et al. (1995) reported Kiasmf values for the adsorption of n-alkanes and PAHs from air to quartz at 25 - 30% RH and 70 - 75% RH, respectively. When plotting lnATiasurf versus In piL (Eq. 11-11) for the various data sets, the following slopes m are obtained ... 

In the case of the adsorption of n-alkane probes which are only able to undergo London interaction with solid surfaces, i.e., (—AG P) = 0 in Eqs. (49) and (50), we have ... 

Figure 1.6. Zero coverage energy of adsorption of n-alkanes versus carbon number Nc, on three microporous solids (and two others, for comparison). Molecular sieve carbon, after Carrott and Sing (1987) Silicalite after Canott and Sing (1986) NaX and macroporous silica, after Kiselev (1967) non-porous carbon, after Carrott and Sing (1987) and Avgul and Kiselev (1965).

Figure JO. Experimental heats of adsorption of n-alkanes after Vlugt at at ...

Heats of adsorption of n-alkanes on ZSM-5. [Reprinted from J. Wei, Adsorption and Cracking of N-Alkanes over ZSM-5 Negative Activation Energy of Reaction, Chem. Eng. Sci., 51 (1996) 2995, with permission from Elsevier Science.]... 

Kinetic studies of ion exchange on partially ion-exchanged type A zeolites of Mg Ca and Mn " revealed that mini-mums and maximums characterize the differential coefficients of internal diffusion for every exchange of 2 Na " ions for one divalent cation per unit cell of the zeolite. On the basis of these observations, assuming definite interactions between the cations and the zeolite lattice, predictions can be made concerning the distribution and arrangement of cations in the unit cells of a type A zeolite. Research on liquid phase adsorption of n-alkanes on partially ion-exchanged type A zeolites indicated that the differential diffusion coefficients for alkane adsorption are influenced likewise by cation distribution in the unit cells of the zeolite. 

Zero Coverage. The peaks at Infinite dilution were slightly skewed (skew ratio 0.8), with virtually no dependence of retention volume on Injection size. Instead of the peak maximum method, retention volumes were measured by the method proposed by Conder and Young (32). To ensure that the adsorption of n-alkanes on carbon fibers was taking place under equilibrium conditions, the flow rate was varied In the range 20 to 32 cm3 min-1. The net retention volumes were essentially Independent of flow rate. 

Table I shows that the differential heat of adsorption of n-alkane on "as received" carbon fibers Is low and closely approximates Its heat of liquefaction. This Indicates a low concentration of high energy sites on the "as received" fibers. The differential heat of adsorption on "cleaned fibers, especially T-300, Is greater than on "as received" fibers, suggesting that some of the high energy sites on the carbon fiber surfaces were occupied by physically adsorbed species. GC analysis of desorption products, collected In a liquid nitrogen trap, showed the presence of water and carbon dioxide.

Figure 3. Relation between AH0 and AG° of adsorption of n-alkanes on precipitated silicas (PI), methylated (PC ) and hexadecylated (PC16) silica samples.

Figure 1. Isosteric heat Qa for the adsorption of , n-alkanes O, iso-alkanes and , cyclanes on various adsorbents plotted against their molecular mass M.

Fig.ldl) Heats of Adsorption of n.alkane on Different Zeolite Snrfaces... 

Fig. 1. Adsorption of n-alkanes and polar probes (trichloromethane, ethyl acetate and tetrahy-drofuran) on a photocured aromatic methacrylate resin (The ordinate is numaically equivalent to AGa in kJ mol )...

The same technique was used by Clint et al. [108] for the determination of the surface of carbon black by the adsorption of n-alkanes. Equation (5.21) was used with the following correction for small particles where the thickness of the adsorbed layer t was not negligible in comparison with the particle radius r ... 

The fact that the samples decomposed at 300°C are almost completely rehydrated indicates that most of the mlcropores. Including those inaccessible to nitrogen, are accessible to the smaller water molecule. The molecular sieving of nitrogen is therefore probably associated with constrictions occurring near points of contact of adjacent microcrystallites, rather tluui with very narrow micropores per se. The presence of constrictions is supported by gas chromatographic measurements of isosterlc heats of adsorption of n-alkanes at infinite dilution Il5]. 

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