Isotherm benzene

The isotherms are presented in Figure 1 and the results are summarised in Table 1 together with nitrogen, water and benzene isotherm data3 to aid comparison. 

A selection of hydrocarbon vapour isotherms obtained with dimethyldioctadecyl-ammonium bentonite is displayed in Figure 11.10. The toluene and ethyl benzene isotherms are almost linear over a wide range of p/p° and are more like the sorption isotherms given by organic polymers than by inorganic porous adsorbents. On the other hand, the Type lib character of the iso-octane and cyclohexane isotherms is apparent (Barter, 1978, p. 455). 

In view of die difference in the benzene adsorption energies on NaY and NaX, we would expect to find a difference in the benzene isotherms - especially at low loadings. The results of Kacirek et al. (1980) confirm that the benzene adsorption affinity of NaX is indeed significandy higher than that of NaY. 

With this sample both methanol and dichloromethane give reversible isotherms and the same micropore volume. A much smaller micropore volume is given by benzene indicating that almost 40% of the pores or pore entrances are of size between that of dichloromethane and benzene. On the other hand, only a relatively small amount of low pressure hysteresis is present on the benzene isotherm indicatii that the remaining pores and pore entrances are significantly larger than the benzene molecule. A more complete characterisation of this sample would require the use of a larger probe molecule, such as neopentane. 

It is agreed with literary data of absence of steric hindrance for adsorption of normal hydrocarbons on zeolites of a pentasile type [5-8]. Adsorption isotherms of 3-methylpentane lay below ones of n-hexane and starting US-69 sample at identical p/ps, the decrease of adsorption volume capacity made about 10%. In the investigated p/ps range adsorption isotherm of benzene on US-69 sample lays below, than for 2,3-dimethylbutane, and in initial area - even is lower than for cyclohexane contrary to a ratio of there kinetic diameters. At the same time, benzene isotherm increases faster, so that level of saturation for benzene can lay above, than for 2,3-dimethylbutane. Apparently, such character of benzene isotherm is connected to a feature of packing of molecules in pentasile channels, and also with stronger interaction adsorbate-adsorbate in comparison with interaction adsorbate-adsorbent. 

Fig. 5. The X-ray diffraction patterns and corresponding benzene isotherms for MCM-41 materials formed from alkyltrimethylammonium salts having alkyl chain lengths of n= 12,14, and 16...

Benzene adsorption has advantages over nitrogen adsorption in that nitrogen is limited to a maximum relative pressure of around 0.99 due to pressure fluctuations, whereas with benzene the adsorption temperature is close to room temperature so that fluctuations in (PIPq) are small at pressures near Pq. Further (t/Vi/RT) for benzene is 2.2 times the value for nitrogen so that a lower relative pressure is required for a given pore radius, so that benzene isotherms give information down to a smaller size (1.58 cf 25) nm. 

Dubinin et al. evaluated the amount surface area from the benzene isotherm measured for the nonporous reference adsorbent /7/, we, however estimated the specific surface area Sme of the mesopores from the adsorption isotherm studied /8/, In calculations of the mesopore size distribution and the specific surface area Sme it has been assumed that the parallel - sided slits are rigid and the size distribution does not extend continuously from the mesopore into both the macropore and micropore range. We have used the desorption branch of the hysteresis loop of the isotherm for the computation. The procedure of B.F. Roberts /9/ has been applied. In this computation, which is a rigorous application of the concept of simultaneous capillary condensation and multilayer adsorption, the adsorbed volume is first expressed as a function of pore size then it is converted to pore volume. A standard t - curve /lO/, which represents the benzene adsorption onto nonporous carbon blacks, has been used for correction for multilayer thickness. 

Table 3 shows results obtained from a five-component, isothermal flash calculation. In this system there are two condensable components (acetone and benzene) and three noncondensable components (hydrogen, carbon monoxide, and methane). Henry s constants for each of the noncondensables were obtained from Equations (18-22) the simplifying assumption for dilute solutions [Equation (17)] was also used for each of the noncondensables. Activity coefficients for both condensable components were calculated with the UNIQUAC equation. For that calculation, all liquid-phase composition variables are on a solute-free basis the only required binary parameters are those for the acetone-benzene system. While no experimental data are available for comparison, the calculated results are probably reliable because all simplifying assumptions are reasonable the... 

Fig. III-9. Representative plots of surface tension versus composition, (a) Isooctane-n-dodecane at 30°C 1 linear, 2 ideal, with a = 48.6. Isooctane-benzene at 30°C 3 ideal, with a = 35.4, 4 ideal-like with empirical a of 112, 5 unsymmetrical, with ai = 136 and U2 = 45. Isooctane-

Fig. XI-10. Isotherm of composition change or surface excess isotherm for the adsorption of (1) benzene and (2) n-heptane on Graphon. (From Ref. 141.)...

Fig. XI-11. Relation of adsorption from binary liquid mixtures to the separate vapor pressure adsorption isotherms, system ethanol-benzene-charcoal (n) separate mixed-vapor isotherms (b) calculated and observed adsorption from liquid mixtures. (From Ref. 143.)...

As a quite different and more fundamental approach, the isotherms of Fig. XI-10 allowed a calculation of X as a function of temperature. The plot of In K versus 1 /T gave an enthalpy quantity that should be just the difference between the heats of immersion of the Graphon in benzene and in n-heptane, or 2.6 x 10 cal/m [141]. The experimental heat of immersion difference is 2.4 x 10 cal/m, or probably indistinguishable. The... 

For adsorption on Spheron 6 from benzene-cyclohexane solutions, the plot of N N2/noAN2 versus N2 (cyclohexane being component 2) has a slope of 2.3 and an intercept of 0.4. (a) Calculate K. (b) Taking the area per molecule to be 40 A, calculate the specific surface area of the spheron 6. (c) Plot the isotherm of composition change. Note Assume that is in millimoles per gram. 

One of the factors responsible for the rather wide variation in a values for benzene is the presence of ji-clectrons in the molecule, which can cause its adsorption to acquire a specific character if the adsorbent is polar (Chapter 1, p. 11). On hydroxylated silica, for example, the heat of adsorption is much higher than on the dehydroxylated material - on the latter solid indeed the interaction is so weak that a Type HI isotherm results (Fig. 2.19). Unfortunately c-values are rarely quoted in the literature, but... 

Fig. 2.19 Adsorption isotherm of benzene on (I) hydrated, and (II) dehydrated siliea gel. (After Kiselev .)...

A factor militating against the use of other adsorptives for pore size determination at the present time is the lack of reliable r-curves. The number of published isotherms of vapours such as benzene, carbon tetrachloride or the lower alkanes, or even such simple inorganic substances as carbon dioxide, on a reasonable number of well-defined non-porous adsorbents, is very small. 

Fig. 4.2 Adsorption isotherms of benzene at 25°C on (1) a charcoal from anthracite coal, activated to 56% yield (2) an activated coconut charcoal. (After Cadenhead and Everett.)...

An example from Dubinin s paper illustrates the application of the new equation. For benzene at 293 K, the plot of Equation (4.18) with m = 2 gave a line concave to the log (p°/p) axis. A revised value of m was therefore required, and was obtained as follows. A provisional value of IVq (= 408 mm g ) was estimated from the plateau of the isotherm the relative pressure (p/p°), corresponding to IV/tV = 0-368 was then read off, and a first value of S calculated from... 

For a second active carbon, AG, the DR plot was convex to the logio(p7p) This carbon was believed from X-ray results to have a wider distribution of pores. It was found that the isotherms of both benzene and cyclohexane could be interpreted by postulating that the micropore system consisted of two sub-systems each with its own Wq and and with m = 2 ... 

Fig. 5.8 Adsorption isotherms at 25°C of benzene and cyclohexane on a mesoporous silica gel. Curve (A), benzene curve (B), cyclohexane. Solid symbols denote desorption.

Fig. 5.14 Adsorption isotherms of water on carbon in (a) to f) with corresponding isotherms of nitrogen in (a), (c) and (J), and of benzene in (f>). (a) Charcoal (b) active carbon AY8 (c) charcoal A (J) charcoal (e) a coal tar pitch kilned at 1200°C (/) a charcoal (S600H). (Redrawn from the diagrams in the original papers.)...

Figure 5. Langmuir Adsorption Isotherms for Benzene, Butyl Chloride and Chloroform...

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