Adsorption of hexane

The apparent acidities of zeolite catalysts are characterized by Av0h values induced by adsorption of hexane (Av0h.C6) under the same conditions than those applied during separate catalytic experiments. The Avoh,c6 values for the different zeolite samples shown in fig. 2 were determined as Figure 1. DRIFT spectra measured in the above for the nitrogen probe molecule. vOH region before (solid lines) and after (dashed lines) contacting the samples with N2 at 298 K and 9 bar equilibrium pressure. 

Our third hypothesis, i.e., that the activity enhancement involves the proximity of the zeolite s acid sites, appears to be consistent with the hydrocarbon adsorption experiments, but may also be due to differences in the nickel dispersion arising from surface area differences between the two types of particles. Clearly, the adsorption of hexane at lower temperature on the nickel contaminated zeolitic particles suggests a significantly altered environment from both the uncontaminated and the non-zeolitic materials. 

Adsorption passes the air-solvent vapor mixture through a bed of activated charcoal. Most of the solvent vapors are adsorbed by the charcoal as the air, with some traces of solvent, passes through. When the charcoal is almost saturated with solvent, the air-solvent vapor mixture is sent to a second charcoal adsorber while the first one is cleansed with hot steam to strip out the solvent. The steam-solvent mixture is then condensed and sent to the solvent-water separator. Care should be exercised not to overload the adsorber with hexane vapors. Adsorption of hexane by carbon releases heat. If unmonitored and uncontrolled, the adsorber could catch on fire. This is especially likely to happen if a process upset sends a sudden surge of hexane vapors to the adsorber. 

Figure 4. Distribution of the energy of adsorption of hexane for different silicas. (Reproduced with permission from reference 27. Copyright 1990.)...

For the adsorption of hexane on graphite we may use the experimental results of Isirikyan and Kiselev [18], who found that on graphi-tized carbon black (Graphon) n-hexane occupies 54 sq. A. per molecule and has a heat of adsorption (with liquid hexane as standard state) of 5.0 kcal. per mole. The value of y for graphite which gives a heat of adsorption of 5.0 kcal. per mole is found by Equation 8 to be 108 dynes per cm. 

The loading of n-hexane in mixtures is somewhat higher than it is expected to be if it were proportional to its partial pressure (Fig. 12). On the contrary, the 2-methylpentane loading is somewhat lower. This points to preferential adsorption of -hexane over isohexane in their mixtures in H-ZSM-5 than in silicalite-1. In earlier experimental [50] and CBMC simulation studies [44] of n-hexane/isohexane mixtures in silicalite-1, a slight preferential adsorption of the linear alkane over the branched one has been found. The most prominent explanation for this preference is the molecular siting of these two hydrocarbon molecules. Whereas -hexane exhibits no clear preference for a position in the micropore system of MFI zeolite, the branched isomer is preferentially located at the channel intersections due to entropic reasons [44]. Consequently, 2-methylpentane will be pushed out from silicalite-1 by -hexane. These effects are even stronger for H-ZSM-5, most likely due to the stronger... 

A final possibility is to use the surface of the crystal as the sample itself. Either the surface of the crystal is used as received, as ZnSe on which sodium dodecyl sulfate (Gao and Chorover, 2010) or Ge on which heptyl xanthate (Larsson et al., 2004) formed a monolayer, or the surface was chemically modified and is different from the bulk. Thus, Asay and Kim (2005) studied the adsorption of water molecules on the native layer of silica present on a silicium ATR crystal, or Wang et al. (2006) studied the adsorption of hexane and ethylbenzene from the vapor phase on a layer of zeolite grown directly on the surface of a silicium ATR crystal. Frederiksson and Holmgren (2008) have formed a PbS film on a ZnS ATR crystal by a chemical bath deposition process in order to study the adsorption of heptyl xanthate. In these latter studies, the system is very close to a film obtained by drying of a suspension, but the optical properties are expected to be better. Couzis and Gulari (1993) have deposited 600 A of alumina by sputtering on a ZnSe crystal. 

Singh, K. et al. (2002). Vapor-Phase Adsorption of Hexane and Benzene on Activated Carbon Fabric Cloth Equilibria and Rate Studies. Industrial Eneineerins Chemis-... 

Huddersman, K., Adsorption of hexane isomers on ion exchanged niordenite, AlChE J., 42(10), 2990-2992... 

Co-adsorption of Hexane and C TAB Molecules at the Water/Hexane Vapor Interface... 

The adsorption of hexane from the vapor phase on C12TAB solution drops was studied in [17, 18] using the drop profile analysis tensiometer PAT-1. In these experiments a solution drop was formed in a closed cuvette and after a time of about 300 s 1 ml of hexane was injected into the cuvette. When the cell is... 

From Fig. 2 the significant decrease of interfacial tension due to the co-adsorption of hexane molecules from the vapor phase at the drop surface is easily visible. At the experimental temperature of 25 °C the partial pressure of saturated hexane vapor is about 2.000 Pa. The solid lines in Fig. 2 were calculated using the parameter values required for Eqs. (7) and (8) were given in Table 2. Note, for very low surfactant concentrations, the obtained parameter values describe the adsorption effect of only the hexane molecules. The values di = 6 x 10 1/Pa and (oio = 3.5 X 10 m /mol provide the best fit for the experimental data [17]. From... 

E. Dfaz, S. Ordonez, A. Auroux, Comparative study on the gas-phase adsorption of hexane over ZBoUtes by calorimetry and inverse gas chromatography. J. Chromatogr. A 1095,131-137 (2005). doi 10.1016/j.chroma.2005.07.117... 

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