Aromatization n-hexane

Benzene formation from all isohexanes had a similar energy of activation value. With platinum this was nearly twice as high as that of n-hexane aromatization (62) with palladium black, however, nearly the same values were found for -hexane and isohexanes (97a). This indicates a common rate-determining step for aromatization with skeletal rearrangement. This is not the formation and/or transformation of the C5 ring. We attribute benzene formation to bond shift type isomerization preceding aromatization. It requires one step for methylpentanes and two steps for dimethyl-butanes this is why the latter react with a lower rate, but with the same energy of activation. 

Different charge-compensating cations in zeolite L have been tested for their promotional effect in n-hexane aromatization. Apparently, high basicity of the alkaline and alkaline earth promoter favors n-hexane aromatization. Basicity and selectivity both increase from Li and Cs 331) and from Mg to Ba (22,25). Bezouhanova et al. studied the FTIR bands of linearly adsorbed CO in the range of 2060-2075 cm . One band at 2075 cm", which is also found on unsupported Pt, is attributed to extrazeolite Pt particles, a second band shifts from 2060 cm" for Li to lower wavenumbers with K and Rb 331). Another criterion, used by Larsen and Haller, is the measured rate of competitive hydrogenation of benzene and toluene, which has been found to correlate with the zeolite basicity (25). As described in a previous section, this method had previously been used by Tri el al. to probe for the electron deficiency of Pt particles in acidic zeolites 332). The rate data are analyzed in terms of a Langmuir-Hinshelwood model and the ratio of the adsorption coefficients of toluene and benzene, A, /b, is determined. It was found to decrease from 8.6 for Pt/Si02, and 5.4 for Pt/MgL, to 4.4 for Pt/BaL. As direct electron transfer from the cations to neutral Pt particles is unlikely, an interaction of Pt with the zeolite framework or with... 

Conversion of n-hexane over Pt-Re/Al203 catalysts depends on the temperature, the velocity of alkane feed, and the catalyst composition. Pt-Re/Al203 catalysts are widely used in industrial reforming. The effect of Pt/Re ratio on the n-hexane aromatization was investigated. 

Influence of Pt-content on n-hexane aromatization Catalyst, Cs-ETS-10 WHSV(h ) = 2 H2/nC6(mole) ( ) 723K (A) 773K... 

The results of n-hexane aromatization over a commercial Pt-A Oa sample containing 0.6 wt.% Pt are presented in Fig. 3 and Table 4. It is immediately apparent that Pt-ETS-10 produces many times more benzene, the benzene selectivity over Pt-Cs-ETS-10 being 34.5%... 

Fig. 5. Product accumulation curves measured as a function of reaction time at 573 K for n-hexane aromatization catalyzed over platinum single-crystal surfaces. ( ), Pt( 10,8,7) (O), Pt(lll) (A), Pt(100) (A), Pt(l3,l,l). H2/HC = 10. P1M = 220 torr. From Ref. 54.

Roessner et al. [134] found that Zn,H-ZSM-5 or Zn-ZSM-5 obtained via SSIE possessed catalytic activities in n-hexane isomerization similar to those exhibited by Zn,H-ZSM-5 catalysts that had been prepared by conventional methods, i. e., either through ion exchange in aqueous solutions of ZnfNOjlj or by the incipient-wetness technique. The latter method seemed to be in between exchange in aqueous solution and a solid-state reaction (cf. Sect 6.1). Also, Zn-ZSM-5 produced via SSIE proved to be almost as equally active in ethane arom-atization as conventionally modified Zn-ZSM-5 prepared by exchange in aqueous Zn(N03)2 solution [135]. Finally, as was shown by Rojasova et al. [143], incorporation of zinc into NH4-Y by solid-state reaction with, e.g., ZnO yielded catalysts active in n-hexane aromatization. Under equal conditions, ZnO alone did not catalyze this reaction. 

Petroleum ether fractions free from aromatic hydrocarbons are marketed, as are also n-hexane and n-heptane from petroleum. 

Triton X-100 polycyclic aromatic hydrocarbons 10-fold 1% solution in n-hexane optimal emission after 60 min 10% zone enlargement [234]... 

The second aromatization reaction is the dehydrocyclization of paraffins to aromatics. For example, if n-hexane represents this reaction, the first step would be to dehydrogenate the hexane molecule over the platinum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers, but cyclization to a cyclohexane ring may occur through a different mechanism). Cyclohexane then dehydrogenates to benzene. 

This is also an endothermic reaction, and the equilibrium production of aromatics is favored at higher temperatures and lower pressures. However, the relative rate of this reaction is much lower than the dehydrogenation of cyclohexanes. Table 3-6 shows the effect of temperature on the selectivity to benzene when reforming n-hexane using a platinum catalyst. 

Even though cyclohexane is rapidly converted into benzene under these conditions, the results in Figure 3 clearly prove that it cannot be a gas phase intermediate in the n-hexane reaction. If it were, there would have been radioactivity in the unreacted cyclohexane when it was mixed with labeled n-hexane none was observed. This proves that the cyclization step must be further along the reaction stream and must not involve an olefin forming cyclohexane which then dehydrogenates to the aromatics. 

As to Irgafos 168 the reader is advised to notice the results of a round-robin involving PP/(Irganox 1076, Irgafos 168) [209a], Ultrasonication at room temperature with anhydrous n-hexane or acetone is a suitable soft extraction mode for the determination of aromatic phosphites and phosphonites, such as Ultranox 626 and Sandostab P-EPQ, which easily degrade in heating extraction procedures [210]. 

The pressurised dissolution/cooling procedure of Macko el al. [490], which uses a UV-transparent low-boiling point solvent, is fast and simple as no additional evaporation of the solvent, preconcentration or redissolution of the additive is necessary. Macko el al. [491] have given an extensive listing of HPLC analyses of aromatic antioxidants and UVAs which can be separated with n-heptane and n-hexane as the main component of the mobile phase. The method was also used for HPLC quantification of thioether antioxidants (Santonox R, Chimox 14 and Irganox PS 802) in MDPE [612],... 

The polymerization is normally carried out in non-aromatic solvents, such as cyclohexane and n-hexane, at temperatures of 50 to 90 °C.Temperatures within this range influence the stereospecificity of the polymerization to only a small extent. These catalysts, unlike ones based on uranium, do not have to be preformed. ... 

Similarly, organic liquids have a variety of applications. For example, hexane, which frequently contains impurities such as aromatic compounds, is used in a variety of applications for extracting non-polar chemicals from samples. The presence of impurities in the hexane may or may not be important for such applications. If, however, the hexane is to be used as a solvent for ultraviolet spectroscopy or for HPLC analysis with UV absorbance or fluorescence detection, the presence of aromatic impurities will render the hexane less transparent in the UV region. It is important to select the appropriate grade for the task you have. As an example, three different specifications for n-hexane ( Distol F , Certified HPLC and Certified AR ), available from Fisher Scientific UK, are shown in Figure 5.5 [10]. You will see that the suppliers provide extra, valuable information in their catalogue. 

The decarbonylations, which do not appear to be affected by light, are reasonably selective with aromatic aldehydes, yielding the expected product however, significant amounts of other products are obtained with non-aromatic substrates (e.g. cyclohexane-aldehyde gives methylcyclopentane and small amounts of n-hexane, as well as the expected cyclohexane and cyclohexen-4-al gives both cyclohexene and cyclohexane). Indeed, the unexpected products perhaps provided a major clue to an understanding of the reaction mechanism(s) involved. 

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

The interaction parameters for binary systems containing water with methane, ethane, propane, n-butane, n-pentane, n-hexane, n-octane, and benzene have been determined using data from the literature. The phase behavior of the paraffin - water systems can be represented very well using the modified procedure. However, the aromatic - water system can not be correlated satisfactorily. Possibly a differetn type of mixing rule will be required for the aromatic - water systems, although this has not as yet been explored. 

ForAs [Am] 1 and /ci+ks [Am] k i, a linear response to the nucleophile concentration, such as that depicted in equation 8, is obtained. This behaviour is characteristic of most base-catalysed reactions. On the other hand, whereas all the studied reactions were base-catalysed in n-hexane, only mild acceleration was observed in benzene9. Also, the reactions seem to be inhibited in benzene and other electron-donor solvents, and Sil-ber and coworkers attributed this effect to a preferential solvation exerted through EDA complex formation with the aromatic substrate, as shown in Scheme 49. 

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