Isomerization hexane

Penex [Pentane and hexane isomerization] A process for converting n-pentane and n-hexane and their mixtures into branched-chain pentanes and hexanes of higher octane number by catalytic isomerization. The catalyst is similar to the Butamer catalyst. The product is used in high-octane gasoline. First commercialized by UOP in 1958. More than 75 units were operating as of 1996. 

The values of these Arrhenius parameters contrast dramatically with those obtained for the bicyclo[2,2,0]hexane isomerization. In this compound there is no weak bridgehead bond, and hence the reaction path is more closely akin to that for cyclobutane itself. The similarity of the A factors for this reaction and that for other simple cyclobutanes supports this contention. If this is so, then the lowering of the energy of activation in this bicyclic compound by some 7 kcal mole from that observed in the alkylcyclobutanes is to be attributed to extra strain energy in this molecule. 

Noble metals (e.g., Pt) can be introduced within the micropores of zeolites by exchange with a complex cation (e.g., Pt(NH3)4 ) followed by calcination and reduction. This mode of introduction generally leads to very small clusters of Pt (high Pt dispersion) located within the micropores. Pt supported on acid zeolites are used as bifunctional catalysts in many commercial processes. The desired transformations involve a series of catalytic and diffusion (D) steps, as shown in n-hexane isomerization over Pt acidic zeolite (Equation 12.1). 

Under the operating conditions, the reaction intermediates (w-hexenes and i-hexenes in n-hexane isomerization) are thermodynamically very adverse, hence appear only as traces in the products. These intermediates (which are generally olefinic) are highly reactive in acid catalysis, which explains that the rates of bifunctional catalysis transformations are relatively high. The activity, stability, and selectivity of bifunctional zeolite catalysts depend mainly on three parameters the zeolite pore structure, the balance between hydrogenating and acid functions, and their intimacy. In most of the commercial processes, the balance is in favor of the hydrogenation function, that is, the transformations are limited by the acid function. 

Table 12.3 Zeolitic pentane and hexane isomerization catalysts. 

Butane isomerization and pentane-hexane isomerization are the two most important isomerization processes. Isobutane is utilized primarily as alkylate feedstock. Isopentanes and isohexanes have become valuable high-octane blending components in gasoline. 

Catalyst Testing. The hexane isomerization activity was measured for several catalysts containing about 0.2 wt % Pt. Appreciable differences in activity were evident which depended upon the method of preparation (Table VI). None of the catalysts is particularly active (c/. equilibrium values in Table VI). The surface areas of the catalysts (Table VI) are somewhat less than expected, and thus one can speculate that better activation procedures will lead to some improvement in performance. 

Figure 20.5 Activation period for n-hexane isomerization over Mo2C-oxygen-modified at atmospheric pressure (623 K, p(C6) = 5 Torr).

However, Pdi,5PWi204o is active for esterification and MTBE synthesis even in the absence of H2 (378). Therefore, it is concluded that this catalyst is not as simple as Ag3PWi2O40. The catalytic activity of PdrH3 tPW,2O40/SiO2 for hexane isomerization is plotted as a function of x in Fig. 67. The addition of a... 

Acid Catalysis tt-Hexane isomerization to methylpentanes AlBr3, A1C13, BF3... 

The SZNbPt catalyst shows a higher isomerization selectivity and activity for all hydrocarbons. While n-pentane and n-hexane isomerization selectivity is very similar on both catalysts, important differences occur with n-heptane. When comparing both catalysts at the same conversion level (60%), SZNbPt shows a higher selectivity (— 5096) than the conventional sulfated zirconia ( 30%). However, the isomerization selectivity of n-heptane SZNbPt is still too low when the temperature is increased to achieve high C5-C6 isomerization yields. 

Another example of secondary shape selectivity is shown by John and co-workers (77,73). They found that the hydroisomerization/hydroeracking of n-hexane over Pt/H-mordenite is significantly inhibited by the presence of benzene. They also found a correlation between the aromatic size relative to zeolite pore size on the inhibition of the hexane reaction and the changes in isomer selectivities. Figure 4 illustrates the relation between the various aromatics co-fed and the n-hexane isomerization rates on H-mordenite. From Figure 4, it is shown that as the kinetic diameter of the aromatics is increased, the isomer formation rate appears to pass through a minimum. This result can be explained by considering the size of the zeolite pore and the kinetic... 

Figure 6 Polyhedron of material balance (a) and thermodynamic tree (b) of hexane isomerization reaction. T = 600 K, P — 0.1 MPa.

A relation exists between the T-O-T bond angles and the acid strength of zeolites20. Thus, the protonic sites of HMOR (bond angle range of 143-180°) and HMFI (133-177°) zeolites are stronger than those of HFAU (133-147°). This explains why HMOR is active for butane and n-hexane isomerizations at 200-250°C which require very strong acid sites whereas it is not the case for HFAU. 

Fig. 1.4 Bifunctional mechanism of n-hexane isomerization over Pt H zeolites...

Silica Si02 Al203 and Pt on Si02 Hexane isomerization... 

Poly step Hexane Isomerization on Coarse Catalyst Mixtures... 

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