Hexane, adsorption isotherms

The water and n-hexane adsorption isotherms of the zeolitic mesoporous materials obtained are compared to that of a 4S0 nm colloidal silicalite-1 in Figure 5. The water adsorption isotherms are distinctively type HI, whereas the n-hexane isotherms are type 1. The lowest water isotherm was for the colloidal silicalite-1, where the first point measured for the n-hexane isotherm was already at 80 mg g. The amount of n-hexane adsorbed reached 2S0 mg g at high pressure, which roughly corresponds to the filling of silicalite-1 micropores. 

In the case of vermiculite, the short-chain organic cations do not penetrate into the interlayer space, and are adsorbed only on the external surface of the mineral. This is clearly seen from the analytical measurements, showing that only 0.06 meq/g of Na" " ions is substituted when Na-vermiculite is treated with tetramethylammonium salt solution, the total exchange capacity being equal to 1.5 meq/g. Therefore, the values of specific surface area obtained from water and hexane adsorption isotherms for the initial and [(CH3)4N] modified vermiculite are virtually the same (Table 3). Such a pronounced difference in the... 

FIGURE 1.295 (a) Hexane adsorption isotherms and distribution functions of (b) pore size of SiOj... 

Figure 1 shows experimental and predicted (Nitta et al., [11]) n-pentane and n-hexane adsorption isotherms between 373-573K obtained in a flow gravimetric system operated at atmospheric pressure (Silva and Rodrigues, [19], [20]). 

Finally, adsorption kinetics of various species confined in CNTs have been reported [99-104]. Formation of two condensed phases of Ar in the interior of SWCNTs has been observed at 77 K [101]. The similarity between the hexane adsorption isotherms and those of other organic molecules demonstrates that the adsorption mechanisms are not spedlic to hexane, and that this methodology can be potentially applicable to other sorbates [99]. 

For hexane adsorption isotherms have been measured by Stach et al. [124] and Richard and Rees [128]. We have used the average of the two Henry coefficients, namely 3.05 [mmol/g/Pa]. For the longer alkanes we could not find sufficiently reliable isotherms at low pressures to compute a Henry coefficient at room temperature. 

Still another manifestation of mixed-film formation is the absorption of organic vapors by films. Stearic acid monolayers strongly absorb hexane up to a limiting ratio of 1 1 [272], and data reminiscent of adsorption isotherms for gases on solids are obtained, with the surface density of the monolayer constituting an added variable. 

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. 5.10 The adsorption isotherms of n-hexane (A) and of water (B) on graphitized carbon black.Solid symbols denote desorption. (After...

Isotherms for H2O and / -hexane adsorption at room temperature and for O2 adsorption at Hquid oxygen temperature on 13X (NaX) zeoHte and on the crystalline Si02 molecular sieve siHcaHte are are shown in Figure 8 (43). SiHcaHte adsorbs water very weaMy. Further modification of siHcaHte by fluoride incorporation provides an extremely hydrophobic adsorbent, shown in Figure 9 (44). These examples illustrate the broad range of properties of crystalline molecular sieves. 

Fig. 8. Adsorption isotherms of H2O, O2, and / -hexane on 2eolite NaX (open symbols) and silicalite (filled symbols). Oxygen is at — 183°C and water and...

NaY yields a compietely reversible type I isotherm, characteristic of micropore filling common in many zeolites. However, USY-B and DAY yield an isotherm close to type IV. Similar differences in adsorption isotherms were observed for n-hexane, cyclohexane, n-pentane and benzene. Furthermore, many of the isotherms measured on DAY zeolites showed hysteresis loops (Figure 6). 

Beyer and Belenykaia (27) have investigated the sorption properties of DAY zeolites prepared from Y zeolite and SiCl vapors. They reported a very low adsorption capacity for water and ammonia, similar to that of the almost aluminum-free silicalite (49). The low adsorption capacity for water is indicative of a hydrophobic zeolite surface. The adsorption isotherms for n-butane, benzene and n-hexane obtained on the aluminum-deficient zeolite have a shape similar to those obtained on NaY zeolite and are characteristic for micropore structures. They show the absence of secondary pores in this DAY zeolite. 

For propane, n-pentane and n-hexane the differential heats of adsorption over FER dropped more rapidly, right after 1 molecule was adsorbed per Bronsted acid site. Similar results were obtained with TON. In contrast, with MOR and FAU the drop in the differential heats of adsorption for n-alkanes occurred at lower coverages, indicating that only a certain fraction of the Bronsted acid sites were accessible to the adsorbing alkane probe molecules. With MFI the drop did not occur until 2 molecules of n-alkane were adsorbed per Bronsted acid site, suggesting perhaps a higher stoichiometry of about two n-alkanes per Bronsted acid site. In the cases of i-butane and i-pentane the drop occurred around one alkane per Bronsted acid site. Finally, n-butane adsorption isotherms measured over TON framework type catalysts having three different A1 contents (Si/Al2 = 90, 104, 128) showed Henry coefficients to increase with increase in the A1 content [5], Based... 

Figure 13.11 CBMC simulations of adsorption isotherm (a) and adsorption selectivity (b) for a 50 50 mixture of n-hexane and 3-methylpentane over MFI at 362°K [6].

Based on their molecular properties as well as the properties of the solvent, each inorganic or organic contaminant exhibits an adsorption isotherm that corresponds to one of the isotherm classifications just described. Figure 5.1 illustrates these isotherms for different organic contaminants, adsorbed either from water or hexane solution on kaolinite, attapulgite, montmorillonite, and a red Mediterranean soil (Yaron et al. 1996). These isotherms may be used to deduce the adsorption mechanism. 

Fig. 5.1 Examples of adsorption isotherms. S-type aldrin on oven dry kaolinite from aqueous solution. L-type parathion on oven-dry attapulgite from hexane solution. H-type methylene blue at pH = 6 on montmorillonite from aqueous solution. C-type parathion on clay soil from hexane solution (Yaron et al. 1996)...

The adsorption isotherms of xenon were measured at 34°C using a classical volumetric apparatus. The 29xe-NMR measurements were performed at the same temperature on a Bruker CXP-200 spectrometer operating at 55.3 MHz. The n-hexane adsorptions were conducted at 90°C on a Stanton Redcroft STA-780 thermoanalyzer. The samples were submitted to a preliminary calcination under dry air up to 650°C with a heating rate of 10°C/min. 

The CB-MC method has been used to simulate the adsorption isotherms of various alkanes in silicalite (170, 171). Using potential parameters that were fitted to obtain good agreement with experimental Henry s law coefficients, Smit and Maesen (170,171) have simulated the adsorption isotherms of straight-chain alkanes in silicalite. Good agreement was obtained for ethane and propane in comparison with the different type-I curves measured experimentally. The overall agreement with experimental isotherms was found to be satisfactory with hexane and heptane, and a kink is seen... 

The early work of Kiselev (1957) revealed that the adsorption isotherms of n-pentane and n-hexane on non-porous quartz were intermediate in character between Types II and m. Values of C(BET) <10 were obtained and the differential enthalpies of adsorption decreased steeply at low surface coverage. More recently, the isotherms of isobutane (at 261 K) and neopentane (at 273 K) on TK800 have been found to be of a similar shape (Carrott et al., 1988 Carrott and Sing, 1989). Unlike those of benzene, these alkane isotherms do not undergo a pronounced change of shape as a result of surface dehydroxylation. This is consistent with the non-specific nature of their molecular interactions (see Chapter 1). 

Finally, it should be noted that calorimetric measurements can also be used to monitor adsorption phenomena at the solid-liquid interface (in a solvent). This method has been used to measure the adsorption heats evolved upon injection of dilute solutions of pyridine in alkanes ( -hexane, cyclohexane) onto an acidic solid itself in a slurry with -hexane. The amount of free base in solution is measured separately with a UV-Vis spectrometer, leading to an adsorption isotherm that is measured over the range of base addition used in the calorimetric titrations. The combined data from the calorimetric titration and adsorption measurements are analyzed simultaneously to determine equihbrium constants, quantities of sites per gram and acid site strengths for different acid sites on the solid. 

Fig. 1 shows the uptake of pure hexane and decane frx)m their mixture with iso-octane on ZSM-S. The amounts adsorbed in the zeolite remain constant after 20 hours, indicating that the experiments were performed under equilibrium conditions. In order to verify that iso-octane can be used as inert solvent, a comparative experiment was performed in which the binary adsorption isotherm of hexane and decane was determined using isooctane and l,3,S-trimethyl benzene as respective solvents. The same adsorption isotherms are obtained with both solvent (Fig. 1 b), demonstrating that iso-octane does not interfere with the adsorption of the linear alkanes, and that the relative adsorption of the short and long n-alkanes is not influenced by the nature of the solvent, given that the solvent is not able to enter the pore system. 

Fig. 1. Adsorption isotherms or i-butyric acid at tlie interfaces (l) water-air (2) water-hexane (3) water-benzene (41 water-olive oil. 7 318 K.

Water affects the reaction rate through its effect on reaction kinetics and protein hydration, which is required for optimal enzyme conformation and activity. Enzymes need a small amount of water to maintain their activity however, increasing the water content can decrease the reaction rate as a result of hydrophilic hin-drance/barrier to the hydrophobic substrate, or because of denaturation of the enzyme (189). These opposite effects result in an optimum water content for each enzyme. In SCFs, both the water content of the enzyme support and water solubilized in the supercritical phase determine the enzyme activity. Water content of the enzyme support is, in turn, determined by the distribution/partition of water between the enzyme and solvent, which can be estimated from water adsorption isotherms (141, 152). The solubility of water in the supercritical phase, operating conditions, and composition of the system (i.e., ethanol content) can affect the water distribution and, hence, determine the total amount of water that needs to be introduced into the system to attain the optimum water content of the support. The optimum water content of the enzyme is not affected by the reaction media, as demonstrated by Marty et al. (152), for esterification reaction using immobilized lipase in n-hexane and SCC02- Enzyme activity in different solvents should, thus, be compared at similar water content of the enzyme support. 

Figure 1. Adsorption isotherm of n-hexane vapor on zeolite NaX (x = 2.96) at different temperatures (8)...

With the aid of a computer, about 40 adsorption systems have been analyzed for equilibrium. Typical examples are presented in the graphs of Figures 1 to 4, where the solid curves represent calculated adsorption isotherms and the circles denote experimental points. The temperatures are expressed in degrees centigrade and pressures in mm of Hg (torr). From above-critical temperatures, for instance, n-hexane (tc = 235°C) and acetylene tc = 36°C), effective values have been obtained by extrapolating the linear dependence of p " on t according to Equation 5 for the temperatures indicated. Effective values of Ps for t tc were calculated by the van der Waals equation, which may be written in the form (12)... 

The surface areas determined from the N2 adsorption Isotherms in the low partial pressure region (uptp p/po = 0.05) are in the range of 500 m g-1 for Sn-MFI and Sn-MEL samples and 300 m g" for Sn-MTW samples (Table 1). It is estimated that meso pore areas (determined form the t-plots at higher p/po values) contribute roughly to 10% of the total area. The amount of H2O, cyclohexane and n-hexane adsorbed by the samples at 298 K and at p/po of 0.5 are included in Table 1. From the amount of H2O adsorbed, it may be concluded that the Sn-silicalites are more hydrophilic than the parent Sn-free silicalites. The sorption capacities for n-hexane and cyclohexane in all the samples show that the micropore volumes are maintained and that occluded Sn02 type of species may not be present in them. 

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. 

Figure 3. Adsorption isotherms of C5 hydrocarbons at 180 C on zeolite a - initial US-69 b -modified water solution boron and phosphoric acids BP-US-69(2) c - modified ethers boron and phosphoric acids BP-US-69(1) 1 - n-hexane (0,43 nm), 2 - 3-methylpentane (0,55 nm), 3 - benzene (0,58 nm), 4 - 2,3- dimethylbutan (0,61 nm), 5 - cyclohexane (0,63 nm).

The adsorption isotherms of the pure enantiomers of 3-chloro-l-phenyl-l-propa-nol were measured by FA on a cellulose tribenzoate coated on silica, eluted with a 95/5 mixtture of n-hexane and ethyl acetate. These data were well accounted for by a simple Langmuir isotherm model. The adsorption data measured fitted well to the Langmuir isotherm model. The elution band profiles of large amoimts of... 

Further study showed that adsorption isotherms of BAC-n by using H20, hexane, and cyclohexane as plug gauge molecules were all of the I type. To illustrate these results, H2O adsorption isotherms of BAC-5, -3, and -10 compared with those of NaX, AIPO4-I7, and silicalite-1 are shown in Figure 4.2. 

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