Sorption of n-hexane

The identification of the solid phases and the evaluation of their crystallinity were performed by X-ray powder diffraction while the pore volume of the ZSM-48 materials was evaluated by isothermal (90 °C) sorption of n-hexane, followed in the thermobalance (Stanton Redcroft ST-780 combined TG-DTA-DTG thermoanalyser). 

Table V. Sorption of n-Hexane on Zeolite Omega Influence of Thermal and Ion-Exchange Treatments0...

Sorption of n-hexane was shown to be a remarkably simple and accurate method to measure the total length of the medium sized channels of a zeolitic matrix, as these molecules perfectly fill the pore system by achieving an end to end configuration (40, 41). From the n-hexane sorption values on Nu-10 reported by Harrison et al. (42) and by Hogan et al. (29), we found respectively 13.3 A and 12.03 A for the total length of the channel system per unit cell [the sorption values of Hogan et al. had to be extrapolated for 25 C (40)]. 

In this study, we present the results of experiments performed on the sorption of n-hexane in HZSM-5 single crystals of different sizes. To examine the influence of the Si/Al ratio, ngj/Ai. on the sorption properties, a silicalite sample has additionally been studied. 

MH zeolites (Si/Al=35 and 480) were prepiared as described elsewhere [14]. The products were calcined at 773 K and exchanged with NH4CL IM. MH(35) was modified by steaming at 673 K for 25 h. All catalysts were characterized using XRD, electron microscopy, FTIR, solid state A1 and Si MAS NMR and sorption of n-hexane. The catalysts used and their properties are listed in Table 1. 

The adsorption capacities of M-ETS-10 (M = Li, Na, K, Rb, Cs) samples for n-hexane are presented in Table 1. The sorption of n-hexane is found to decrease with increase in the size of the alkali metal cation. 

Figures 10.13 and 10.14 show the kinetic plots for the sorption of n-hexane vapors on Styrosorb 2 at 20 and 40°C [16]. The anomalous character of kinetics shown in the nonlinear dependence of 7 versus y/r is clearly seen at the low and moderate extents of bead saturation. By replacing the curved kinetic plots with a set of short stra ht lines and calculating the effective diffusion coefficients, Dg, according to Eq. [10.2] within each linear interval, one arrives at the important conclusion that the diffusion rate of the sorbate molecules increases by more than 1 order of magnitude (F. 10.15) during the sorption process.

June et al. investigated the sorption and spatial distribution of butane and three hexane isomers within the pores of silicalite, using a Metropolis MC method (87) and MD simulations (85). Perturbations of conformation as a result of confinement within the pore were also reported. Heats of adsorption and Henry s law coefficients were found to be in good agreement with experimental values for butane (48-51 kJ/mol) (142,148,150,163-165) and n-hexane (70-71 kJ/mol) (163, 166, 167). The heats of sorption of the other two hexane isomers, 2- and 3-methylpentane, were predicted to be 5 kJ/mol lower than that of n-hexane. 

Sorption Properties. Sorption isotherms were determined of n-hexane and 2,3-dimethylbutane on variously pretreated samples of zeolite by a gravimetric method using a Cahn electrobalance. No shape-selective sorption was observed for these sorbates, which bespeaks a pore size greater than about 0.5 nm. The sorption capacity of S2 was appreciably lower than that of zeolite X, Y, or mordenite. Routine sorption capacities were determined by a simple procedure of pore filling with benzene at room temperature after calcination of the samples at various temperatures. 

Despite the fact that both normal and monomethyl-substituted paraffins readily enter the pores of ZSM-5 and ZSM-11, preferential sorption of the normal isomer is observed under thermodynamic equilibrium, non-kinetically controlled conditions. Whereas small-pore zeolites, such as 5A and erionite, totally exclude branched hydrocarbons, and large-pore zeolites exhibit little preference, the intermediate pore-size zeolites ZSM-5 and ZSM-11 show a marked preference for sorption of the linear paraffin, even under equilibrium conditions. Competitive liquid phase sorption studies at room temperature indicated selectivity factors greater than ten in favor of n-hexane relative to... 

Figure 7. Effect of temperature on localization of n-hexane sorption in a zeolite.

Fig. 6. Sorption kinetics of n-hexane in a SO/SO blend of polystyrene and polyphenylene oxide illustrating C se 11 sorption kinetics. Note that Mt/M is proportional to time rather than to the square root of time in the initial stages of sorptimi ...

Sorption kinetics of n-hexane in MFI-type zeolites investigated by micro-Fl lR spectroscopy... 

The sorption kinetics of n-hexane in MFI-type zeolites of different sizes have been measured by means of micro-FTIR spectroscopy. To check for an influence of the Si/Al ratio, nsj/Ai, on the sorption characteristics, a sample of silicalite was also investigated. The measured transport diffiisivities show ndther a dependence on the crystal size nor on the Si/Al ratio. The temperature dependence is shown to follow an Arrhenius-type law. The results of this study compare well with literature data obtained by different techniques. 

Figure 5. Uptake imder 3.1 mbar of n-hexane in HZSM-5, sample A at various sorption temperatures. Solid lines calculated uptake curves, symbols A - 398 K, 0 - 423 K, o - 448 K and -473K.

The decomposition of the zeolite frameworks was monitored by IR spectroscopy and calculated on the basis of n-hexane, nitrogen, and water sorption uptake. 

Sorption. ZSM-5 calcined in air at 550 sorbed only 3 % cyclohexane and 6.2% water, but sorbed 9.5% by weight of n-hexane. These results suggest access to the framework is via ports of about 0.6 nm which is in accord with the structure. There is marked hydrophobicity, since n-hexane/water = 1.5 as compared to typically hydrophilic zeolites A, X, Y, and Nu-1 which have ratios of about 0.5. 

Zeolites of type Y are prepared by either primary or secxindary synthesis. Structures include zeolite Y in t)oth the cubic and hexagonal forms, SAPO-37 and faujasitic frameworks containing Ga or Zn. These materials are characterised using solid state NMR, X-ray powder diffracticai, infrared jectrosccpy, surface aneilysis and sorption. Catalysts are then evaluated for the conversion of n-hexane, cyclohexane and gas-oil. Results are interpreted in terms of the effectiveness of catalytic sites in alkane activaticxi and in the effect of both density and distribution of active sites. 

Fig. 25 Uptake of n-hexane under 3.1 mbar partial pressure with helium as carrier gas at various sorption temperatures monitored via micro-FTIR spectroscopy. Solid lines calculated uptake curves symbols A = 398 K, <> = 423 K, o = 448 K, = 473 K...

Table 2 shows the adsorbed concentrations of the pure components. At a partial pressure of 6.6 kPa the amount of n-hexane is just slightly higher than that of isohexane in silicalite-1, while the linear alkane is obviously adsorbed more strongly than 2-methylpentane in H-ZSM-5 due to the stronger interaction with the acid sites. The maximum loading of each component has been measured by a separate adsorption study. The sorption capacity of n-hexane (7 molecules per unit cell), in agreement with earlier studies [48,59-61] exceeds that of 2-methylpentane (4 molecules per unit cell). The latter value equals the number of channel intersections in the MFI pore system per unit cell. Indeed, the sorption of isohexane molecules at... 

Figure 10.8 Sorption isotherms for vapors of n-hexane on Styrosorb 1 (MCDE, 100%) at different temperatures. (After [7].)...

Figure 10.9 Sorption isotherms for vapors of n-hexane at 20°C on (1) Styrosorb 1 (MCDE, 100%), (2) zeolite NaX, (3) activated carbon AR-3, (4) semicoke, and (5) macroporous polystyrene sorbent Polysorb 1. (After [7].)...

In Figure 2 the solubility isotherms of n-hexane in PTMSP are reported at temperatures of 300 K and 330 K. The behaviour is qualitatively similar to that observed for n-pentane sorption. Both isotherms exhibit a downward curvature and the solubility coefficient in the low pressure limit. So, is high. For n-hexane. So 25 and 16 at 300 and 330 K, respectively. Also for n-hexane sorption, the effect of temperature on solubility indicates a large negative enthalpy of mixing since the solubility decreases substantially with increasing temperature. 

The column used was 25 cm long, 4.6 mm in diameter, and packed with silica gel particle (diameter 5 pm) giving an maximum efficiency at the optimum velocity of 25,000 theoretical plates. The mobile phase consisted of 76% v/v n-hexane and 24% v/v 2-propyl alcohol at a flow-rate of 1.0 ml/min. The steroid hormones are mostly weakly polar and thus, on silica gel, will be separated primarily on a basis of polarity. The silica, however, was heavily deactivated by a relatively high concentration of the moderator 2-propyl alcohol and thus the interacting surface would be covered with isopropanol molecules. Whether the interaction is by sorption or displacement is difficult to predict. It is likely that the early peaks interacted by sorption and the late peaks by possibly by displacement. 

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. 

If cyclohexane is added as a second component to n-hexane, a similar increase of its radioactivity should be observed if it is really produced from -hexane. The appearance of radioactivity in the assumed intermediate can be observed even if its concentration in the gas phase does not correspond to sorption equilibration. 

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