N< hexane isomerization

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. 

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

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... 

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...

Fig. 15. Observed variation of n-hexane isomerization selectivity with degree of activity of the X-oomponent in a platinum (X) acidic oxide (F) catalyst.

R. Zaera, D. Goodbey, and G.A. Somoiiai, Methylcyclopentane Conversion Over Platinum Single Crystal Surfaces Evidence for the Cyclic Mechanism of n-Hexane Isomerization, J. Catal., 101 (1986) 73. 

The objective of this paper is to demonstrate the importance of phase and reaction equilibria considerations in the rational development of SCF reaction schemes. Theoretical analysis of phase and reaction equilibria are presented for two relatively simple reactions, viz., the isomerizations of n-hexane and 1-hexene. Our simulated conversion and yield plots compare well with experimental results reported in the literature for n-hexane isomerization (4) and obtained by us for 1-hexene isomerization. Based on our analysis, the choice of an appropriate SCF reaction medium for each of these reactions is discussed. Properties such as viscosity, surface tension and polarity can affect transport and kinetic behavior and hence should also be considered for complete evaluation of SCF solvents. These rate effects are not considered in our equilibrium study. 

Product distibution in n-hexane isomerization and cracking, at 250°C (Space velocity = 90... 

Methods of synthesis of superacid catalysts based on WOs/ZrOa system (surface area 40 - 250 m /g) are developed. This catalysts exhibit high activity in n-hexane isomerization reaction yield of branched isomers at 230 - 250°C amounts to 65 - 70% with selectivity for i-Ce of 70 - 94% and 80% conversion of n-hexane. Promoting of the catalysts with Fe and Mn compounds does not effect their activity in n-hexane isomerization. 

In this article the methods of synthesis of W03/Zr02 systems with surface area from 40 to 250 m /g are described. These samples exhibit high activity in n-hexane isomerization and gas-phase benzene nitration reactions. 

For testing in n-hexane isomerization reaction the catalysts were promoted with 0.5wt % of Pt by impregnation with H2PtCb solution. Prior to testing the platinated catalysts were reduced for 4 h at 400°C in dihydrogen. 

Investigations carried out have shown that controlled hydrolysis co-precipitation method and sol-gel method with use of polyvinylalcohol as template allow to prepare superacid WO3/Z1O2 materials which are highly active in n-hexane isomerization and vapour-phase benzene nitration with nitric acid. Doping of WO3/Z1O2 with Fe and Mn ions, as well as with silica and alumina [6] does not improve the catalytic properties in n-hexane isomerization. 

The bifunctional catalytic n-hexane isomerization was performed in a microflow reactor under atmospheric pressure, using a mechanical mixture (Iw/w) of catalyst and a standard Pt/A Os reforming catalyst (0.35 wt% Pt). The latter was previously reduced at 723 K for 4 h. The catalytic test was performed as follows The mixture Pt/A Os + catalyst was pretreated first under He flow at 723K for 2 hours and then under H2 for 30 min at 493K. 

Comparison between aerogel and xerogel in n-hexane isomerization reaction at 493K. 

Comparative study of the sulfur loss in the xerogel and aerogel sulfated zirconia calcined at different temperatures effect on n-hexane isomerization... 

The catalytic properties of the aerogel and the xerogel calcined at different temperatures are tested in the n-hexane isomerization in the range 150-220°C. All catalysts are mechanically mixed with Pt/Al203 in the ratio 1/1 (w/w). The presence of platinum is necessary to form the hydrogen species that... 

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