Nonane adsorption

Adsorption of n-nonane on both types of "as received" (AR) fibers was studied at 30 0.05°C. In the case of T-300T fibers, the column temperature was maintained constant at 60 0.05°C except for n-nonane adsorption, which was also studied at 50 and 70°C. For P-55T fibers, the column temperature was maintained constant at 75 0.05°C except for n-nonane adsorption, which was also studied at 50, 60 and 70°C. The temperature was controlled with a circulating water bath (Lauda K4R). 

Table II. BET Parameters for n-Nonane Adsorption on (HT) Carbon Fibers at 70°C and the Corresponding Specific Surface Areas ...

Fig. 5 shows the low pressure adsorption isotherms of n-nonane by the micropore entrance modified ACF and the pristine ACF. These adsorption isotherms were determined under the almost equilibrium conditions. A remarkable enhancement of n-nonane adsorption with the micropore entrance modification is observed in the low P/Po region, although the adsorption amounts at high P/Pq region almost coincide with each other. The fractional filling of n-nonane at saturation is almost constant irrespective of the surface modification with TTS the ratios of the saturation n-nonane adsorption Wo(nonane) to the saturation Nj adsorption Wo(N2) for the modified ACF and ACF were 0.72 and 0.70, respectively. Thus, the low pressure uptake depends sensitively on the chemical state of the external surface, while the fractional filling at saturation does not change. Consequently, the slight uptake of the pristine ACF should be caused by the limitation of micropore diffusion. The diffusion limitation can be removed by application of n-nonane pressure of P/Pq >0.1 according to the result shown in Fig. 5. Accordingly, a marked enhancement of low pressure adsorption by the micropore-entrance modification is associated with enrichment of n-nonane molecules at the entrance of the micropore due to favourable interaction of n-nonane with hydrocarbon chains of TTS. The amount of the n-nonane enrichment can be estimated from the comparison of both adsorption isotherms in Fig. 5. With the adsorption amount indicated by the horizontal broken line, the equal amount of adsorption for both samples is obtained at different relative pressures of 0.065 (for ACF) and 0.02 (for TTS-modified ACF). That is, application of P/Pq = 0.065 is necessary for the prescribed adsorption in the case of ACF, whereas the TTS-modified ACF does not need such a high P/Pq. Application of P/Po = 0.02 is sufficient for the adsorption by the TTS-modified ACF. Thus, the TTS-modification increases the concentration...

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... 

Adsorption of nitrogen at 77 K on a microptorous carbon after pre-adsorption of n-nonane (cf. Fig. 4.13)... 

Fig. 4.13 The pre-adsorption method (a) adsorption isotherms of nitrogen at 77 K on a sample of Mogul I carbon black charged with different amounts x of pre-adsorbed nonane. Values ofx (mg g (A) 63 (B)48 (C) 29 (D) 16 (E) 0. (See Table 4.5.) (Some points at low pressures omitted for the sake of clarity.)...

It would clearly be of interest to discover how far the nonane method can be used with adsorbates other than nitrogen. A study along these lines has been carried out by Tayyab, but a discussion of his rather unexpected results is best deferred until the role of fine constrictions has been considered (p. 228). Meanwhile it may be noted that the applicability of the technique seems to be limited to adsorptives such as nitrogen or argon which have negligible solubility in solid or supercooled liquid n-nonane. 

The intercept on the adsorption axis, and also the value of c, diminishes as the amount of retained nonane increases (Table 4.7). The very high value of c (>10 ) for the starting material could in principle be explained by adsorption either in micropores or on active sites such as exposed Ti cations produced by dehydration but, as shown in earlier work, the latter kind of adsorption would result in isotherms of quite different shape, and can be ruled out. The negative intercept obtained with the 25°C-outgassed sample (Fig. 4.14 curve (D)) is a mathematical consequence of the reduced adsorption at low relative pressure which in expressed in the low c-value (c = 13). It is most probably accounted for by the presence of adsorbed nonane on the external surface which was not removed at 25°C but only at I50°C. (The Frenkel-Halsey-Hill exponent (p. 90) for the multilayer region of the 25°C-outgassed sample was only 1 -9 as compared with 2-61 for the standard rutile, and 2-38 for the 150°C-outgassed sample). 

Fig. 4.15 a,-plots for the adsorption of nitrogen on a sample of microporous titania, before and after nonane treatment. Curve (A), before nonane pre-adsorption curves (B), (C), (D) after nonane pre-adsorption, followed by outgassing at (B) 250° (C) 150°C (D) 25°C. The a,-plots were based on standard nitrogen isotherms having the same c-values as the isotherms of... 

Fig. 4.16 Comparison plots for a microporous sample of y-Mn02 after outgassing at various temperatures, also after pre-adsorption of nonane. The adsorption on the sample is plotted against the adsorption on a reference sample of synthetic MnOOH. (Courtesy Lee and Newnham.) Outgassing temperature (K) Curve A, 9, room B, O, 393 C, , 443 D, A, 493 K. Curve E, pre-treated with nonane.

Fig. 4.18 Plot of log (.x/(mg g" )) against log (p°/p) for the adsorption of nitrogen at 77 K on the samples of manganese dioxide referred to in Fig. 4.16. Outgassing temperature (A) room (B) 393 K (C)443 K (D) 493 K. For the points denoted by V in Curve A, a sample was outgassed at 493 K and charged with nonane before the final outgassing at room temperature. (Courtesy Lee and Newnham.)...

A major difficulty in testing the validity of predictions from the DR equation is that independent estimates of the relevant parameters—the total micropore volume and the pore size distribution—are so often lacking. However, Marsh and Rand compared the extrapolated value for from DR plots of CO2 on a series of activated carbons, with the micropore volume estimated by the pre-adsorption of nonane. They found that except in one case, the value from the DR plot was below, often much below, the nonane figure (Table 4.9). 

These procedures proposed by Dubinin and by Stoeckli arc, as yet, in the pioneer stage. Before they can be regarded as established as a means of evaluating pore size distribution, a wide-ranging study is needed, involving model micropore systems contained in a variety of chemical substances. The relationship between the structural constant B and the actual dimensions of the micropores, together with their distribution, would have to be demonstrated. The micropore volume would need to be evaluated independently from the known structure of the solid, or by the nonane pre-adsorption method, or with the aid of a range of molecular probes. 

As remarked on p. 214, the validity of the nonane pre-adsorption method when adsorptives other than nitrogen are employed for determination of the isotherms, has been examined by Tayyab. Two organic adsorptives, /i-hexane and carbon tetrachloride, which could be used at or near room temperature, were selected and the adsorbents were the ammonium salts of... 

Fig. 4J0 Adsorption isotherms on ammonium silicomolybdate powder. (I), (4). nitrogen at 77 K (2), (3), /t-hexane at 298 K. Isotherms I and 2 were measured before, and 3 and 4 after, pre-adsorption of n-nonane. Open symbols, adsorption solid symbols, desorption. (Adsorption is expressed in mm (liquid.)...

Fig. 4J2 Adsorption isotherms of carbon tetrachloride (at 298 K) on ammonium phosphotungstate compact, (1) before, (2) after preadsorption of n-nonane. (3) is the isotherm of nitrogen, after preadsorption, for reference. Open symbols, adsorption solid symbols,...

It follows that the applicability of the nonane pre-adsorption method for the evaluation of microporosity is restricted to adsorptives such as nitrogen which are used at temperatures far below ambient and which have negligible solubility in soUd or liquid nonane. 

If a Type I isotherm exhibits a nearly constant adsorption at high relative pressure, the micropore volume is given by the amount adsorbed (converted to a liquid volume) in the plateau region, since the mesopore volume and the external surface are both relatively small. In the more usual case where the Type I isotherm has a finite slope at high relative pressures, both the external area and the micropore volume can be evaluated by the a,-method provided that a standard isotherm on a suitable non-porous reference solid is available. Alternatively, the nonane pre-adsorption method may be used in appropriate cases to separate the processes of micropore filling and surface coverage. At present, however, there is no reliable procedure for the computation of micropore size distribution from a single isotherm but if the size extends down to micropores of molecular dimensions, adsorptive molecules of selected size can be employed as molecular probes. 

Figure 9.16 The practical examples of comparative plots (a) N2 adsorption on a graphitized carbon black, modified by physical adsorption of methanol (the numbers correspond to the amount of CH3 OH in the fractions of monolayer capacity) [83] (b) the usual types of comparative plots by [3] and (c) N2 isotherms on microporous titanium oxide after various amount of preadsorbed nonane by [53].

Figure 6 shows the isotherms of the samples using different alkanes as expander. Except the sample obtained with nonane, the adsorption-desorption isotherms of all other compounds are type IV, characteristic of mesoporous materials according to the BDDT classification [21], Isotherms can be decomposed in three parts the formation of the monolayer, a sharp increase characteristic of the capillary condensation of nitrogen within the mesopores and finally a plateau indicating the saturation of the samples. From pentane to decane the relative pressure at which the capillary condensation occurs, increases from 0 30 to 0.60, indicating that the value of the pore diameter increases when the alkane chain length is raised since the p/po position of the inflection point is related to the pore diameter. From undecane, this value decreases to reach 0.40 for dodecane We can conclude that the value of the pore diameter drops from decane to dodecane... 

In a typical experiment, two solutions were prepared A - contained 0.3g p-xylene, 0.3g l,3-di(tri-fluoromethylbenzene, and 11.4g 1,3,5-tri-isopropyl-benzene B - contained l.Og n-nonane, l.Og mesitylene and 6.0g 1,3,5-tri-isopropylbenzene. 3.0g of solution A was added to l.Og H-ZSM-5 at room temperature the sorption of p-xylerie was monitored over a period of several hours by gas chromatography. When the sorption had reached a constant value, 0.6g of solution B was added. The resulting desorption of p-xylene and the adsorption of n-nonane was monitored by gas chromatography. In these experiments, the 1,3-di(trifluoro-methyl)benzene and mesitylene behaved exclusively as non-sorbing internal standards. 

Regarding the adsorptives used for the characterization of solids by physical adsorption, in principle, it is possible to use any gas or vapor (N2, C02, Ar, He, H20, CH4, benzene, nonane, etc.), but in practice only a limited number of adsorptives are employed. 

Another procedure which may be used for the assessment of microporosity is the preadsorption method. In this approach the micropores are filled with large molecules (e.g. nonane), which are not removed by pumping the adsorbent at ambient temperature. In the most straightforward case, this procedure can provide an effective way of isolating the micropores and leaving the external surface available for the adsorption of nitrogen, or another suitable adsorptive. 

Some more recent investigations of die pre-adsorption method have shown, however, that the results are not always so easy to interpret (Martin-Martinez et al., 1986 Carrott et al., 1989). As would be expected, the nonane molecules are more strongly trapped in ultramicropores than in supermicropores. However, since many microporous adsorbents have complex networks of pores of different size, the retention of the nonane molecules in narrow pores also leads to blocking of some wider pores. 

As discussed in Chapter 8, the pre-adsorption of n-nonane can be used as a means of blocking narrow micropore entrances (see Section 8.2.3). Thus, in the case of an ultramicroporous adsorbent such as Carbosieve, the pre-adsorption of nonane leads to complete blockage of the pore structure. The effect of progressively removing the pre-adsorbed nonane from a supermicroporous carbon is shown in Figure 9.13. The adsorbent used in this work was a well-characterized carbon cloth with the following properties a(BET), 1330 m2g-1 a(ext), 25m2g 1 fp(mic), 0.44 cm3 g-1 wp, 0.6-2.0 nm (Carrott et al., 1989). 

Figure 9.13. Nitrogen adsorption isotherms ar 77 K (a) and corresponding as plots (b) for charcoal cloth JF012 after prc-adsoiption of nonane followed by outgassing at indicated temperature (after Carrott etal., 1989).

A novel method for determining the location of the primary water adsorbing sites has been developed by Bailey et al., (1995). This approach involves the pre-adsorption of naphthalene, which was chosen because of its planar molecular shape and immiscibility with water. With some activated carbons it was found that the growth of the H-bonded water clusters was inhibited by the presence of naphthalene, while in other cases there was very little effect. It was thought that sites in larger micro-pores were prone to obstruction by the pre-adsorbed naphthalene. It is too early to judge the success of this interesting approach, which may turn out to be a useful alternative to pre-adsorption by n-nonane. 

Further confirmation of the development of microporosity has been obtained by the application of the nonane pre-adsorption method of Gregg and Langford (1969). 

Figure 10.5. Nitrogen isotherms (left, with open circles for adsorption and closed circles for desorption) and as plots (right) for precipitated silica VN3 out gassed at (a) 25°C, (b) 110°C, (c) 200°C and (d) 300°C, with ordinate shifted upwards of 1 mmol g 1, for clarity) Run (e) after outgassing at 200°C and n nonane pre-adsotpbon (Carrott and Sing, 1984)...

The molecular sieving behaviour of Silicalite-I, as illustrated in Table 11.5 by the low saturation uptakes of neopentane and o-xylene, is primarily dependent on size exclusion. It is of interest that n-nonane has been found to give an isotherm of essentially Type I character at 296 K (Grillet et al., 1993). The initial part of this isotherm was completely reversible, but a small sub-step at p/p0 0.2 was followed by a long plateau and associated narrow, Type H4, hysteresis loop. The plateau was located at N° = 4 molec uc"1. This level of pre-adsorption was sufficient to block the whole of the intracrystalline pore structure. The accessibility to nitrogen was gradually restored by the progressive removal of the nonane. These results confirm the complexity of the nonane pre-adsorption and entrapment in relation to the pore network and indicate that there is no simple relation between the thermal desorption of n-nonane and the adsorbent pore structure. 

Another procedure proposed for micropore evaluation is by nonane pre-adsorption, which is dependent on the strong retention of n-nonane molecules in the ultramicropores. However, it is now apparent that the extent of the pore blocking is determined by the network connectivity as well as by the micropore size distribution. 

The critical concentration Cc for formation of foam and emulsion bilayers of Do(EO)22 are 4-10 6 mol dm 3 and 1.6 10 5 mol dm 3, respectively, and are in good correlation with the lowest concentrations, 2-31 O 6 mol dm 3 and 10 5 mol dm 3 [421] at which maximum filling of the surfactant adsorption monolayer is attained. It should also be noted that in the case of the emulsion bilayers, CMC < Ce which implies that it is not possible to obtain infinitely stable (i.e. with r = °°) bilayers of Do(EO)22 between two droplets of nonane under the described conditions. For this reason, it may be thought that thermodynamically stable nonane-in-water emulsions stabilised with Do(EO)22 do not exist. 

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