Hexane sorption

The intracrystalline pore volume of the catalysts was evaluated by n-hexane sorption as shown in Fig. 6. Sorption capacities for samples SI to S3 are comparable to that of the zeolite before Ga impregnation and correspond to the value expected for an unaltered ZSM-5 type material (S10). Sorption capacity decreases for samples S3, S4, S5, and S6, because of intracrystalline volume blockage by coke deposits and possibly also (silica)-alumina debris [6] in the aged catalyt S6. In addition, the sorption rate for S6 is about twice the rate observed for the other samples, suggesting that adsorption occurs mostly at the external surface of the S6 catalyst crystallites. Thus, it appears that coke deposited on S6, probably as polyaromatic species, has almost blocked the channel pore mouths and/or practically occupied the whole intracrystalline pore volume. It explains the poor catalytic performance of S6. 

Enscore, D., Hopfenberg, H., and Stannett, V., Effect of particle size on the mechanism controlling n-hexane sorption in glassy polystyrene microspheres, Polymer, Vol. 18, No. 8, 1977, pp. 793-800. 

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

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)]. 

N-hexane sorption experiments with numerous different samples showed major variations in sorption properties (capacities and rates, as shown in Table 1), some of which... 

Chemical analysis and sorption data. Chemical analyses (ICP-AES) of highly crystalline and pure ZSM-20 materials give Si/Al ratios in the range of 3.7 (Table 2) to 4.7 (Table 1). Na/Al ratios of 0.7 are typical, as are (Na+TEA)/A1 ratios of about 1.1. Single point n-hexane sorption capacities at 40 torr and 23°C, after burn off of the TEA at 550°C in air for 3 hours, are invariably in the range of 18% to 20% wt.- values typical for high silica directly synthesized faujasite type product (24.) ... 

Fig. 2 n-Hexane sorption uptake of DAY-S treated 72 h in steam at saturation pressure... 

The sorption values of n-DBA-VPI-5 sample for water, n-hexane and 1,3,5-triisopropylbenzene (kinetic diameters = 2.65, 4.30 and 8.50 A, respectively) at 298K (p/po = 0.5 for water and n-hexane and 0.8 for triisopropylbenzene) were obtained as 33.2%, 18.5% and 11.4%, respectively. The different adsorption capacities for the above molecules is probably related to the differences in packing of these molecules. In fact, in terms of volume, water and n-hexane sorption correspond to 33 ml and 28 ml per 100 g, respectively which are more than the micropore volume of 0.124 ml/g obtained from nitrogen adsorption suggesting the presence of mesoporosity. The sorption of the bigger molecule like triisopropylbenzene... 

Fig. 38. Comparison of theoretical curves and experimental data for w-hexane sorption in the large- and small-diameter spheres at P/Pq = 0.75 and 30°C. A, Fickian model, diameter = 184 /.tm B, Case II model, diameter = 0.534 //m.

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. 

The dominant transport process from water is volatilization. Based on mathematical models developed by the EPA, the half-life for M-hexane in bodies of water with any degree of turbulent mixing (e.g., rivers) would be less than 3 hours. For standing bodies of water (e.g., small ponds), a half-life no longer than one week (6.8 days) is estimated (ASTER 1995 EPA 1987a). Based on the log octanol/water partition coefficient (i.e., log[Kow]) and the estimated log sorption coefficient (i.e., log[Koc]) (see Table 3-2), ii-hexane is not expected to become concentrated in biota (Swann et al. 1983). A calculated bioconcentration factor (BCF) of 453 for a fathead minnow (ASTER 1995) further suggests a low potential for -hcxanc to bioconcentrate or bioaccumulate in trophic food chains. 

The extraction of aromatic chlorophenols (e.g., chloroguaiacols, chloro-catechols) is complicated by the different sorption processes that control their binding within the soil-sediment structure [411-413]. The free, physically adsorbed chlorophenolics can be extracted with solvent, but this may only account for 1-5% of the total concentration of these pollutants in the sediment. Martinsen et al. [414] found that -hexane or cyclohexane and iso-propanol... 

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. 

Liquid Samples. Three procedures were evaluated for extracting pesticides from liquid samples collected at the Horticulture and Agronomy pits. They were the resin sorption method of Junk et al. (2), solvent extraction with hexane-diethylether, and solvent... 

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). 

The sorption of two weak acids (warfarin and thiopentone) and two weak bases (chlorpromazine and diltiazem) into PVC infusion bags was described by a constant partition model. PVC-water partition coefficients were obtained using three different methods equilibrium values for sorption into PVC bags, the sorption versus pH relationship, and partition into PVC strips. The data were compared with similar values derived from a liquid-liquid partition system and different organic solvents (octanol, dichloromethane, carbon tetrachloride, and hexane). Octanol is the preferred solvent, and it is suggested that octanol-water partition data can be used to predict sorption behavior [182]. 

June et al. (85) presented united-atom calculations for butane and for hexane in silicalite, whereby the bond and dihedral angles of the alkanes were allowed to vary. In addition, the calculation of hexane took account of an additional intramolecular Lennard-Jones potential for nonbonded atoms more than three bonds apart (which prevents the alkane crossing over itself). The interaction parameters for the alkane molecules were taken from Ryckaert and Bellmans (3), and those governing the interaction of the alkanes with the zeolite from a previous study of the low-occupancy sorption of alkanes in silicalite (87). Variable loadings of alkanes were considered from 1 to 8 molecules per unit cell were considered, and calculations were allowed to run for 500 ps for diffusion at 300 K. 

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. 

A study is presented of the synthesis and properties of the novel synthetic zeolite omega. The synthesis variables and kinetics of formation are discussed, as well as the ion exchange, sorption, and thermal properties. By decomposition of imbibed tetra-methylammonium ions and exhaustive treatments of the zeolite with ammonium ions, a pure hydrogen form can be obtained which is a suitable substrate for the preparation of hydrocarbon conversion catalysts. Several catalysts were prepared and utilized to isomerize n-hexane, and to hydrocrack a heavy gas oil. 

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. 

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

Highly selective sorption of aromatic compounds from paraffin-containing solutions has been reported for the faujasites NaX, NaY, and HY under equilibrium conditions (2). Thus, benzene is preferentially adsorbed relative to n-hexane or n-decane, p-xylene relative to n-octane, and naphthalene relative to n-decane. The measured separation factors in these systems are so large (K>700) that essentially only one species exclusively occupies the internal volume of the zeolites. 

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