Isomerization of n-hexane

Guisnet, M. and Eouche, V. (1991) Isomerization of n-hexane on platinum dealuminated mordenite catalysts II. Kinetic study. Appl. Catal, 71, 295-306. 

It has been claimed that noble metal dual function catalysts based on H-mordenite are more active for paraffin isomerization than their counterparts based on H-zeolite Y (25). For both zeolites the isomerization activity depends strongly on the degree of sodium removal and comparison of low sodium Pd-H-mordenite and low sodium Pd-H-zeolite Y for isomerization of n-hexane at 250° C shows that both materials have about the same activity (Table IV), the Y sieve based material being slightly more active. 

Table IV. Isomerization of n-Hexane Over Pd-H-Zeolite Catalysts Comparison of Mordenite and Zeolite Y...

Figure 20.8 (a and b) show a comparison of the Pt// -zeolite catalyst which activated molybdenum oxycarbide catalysts, prepared from both Mo2C and Mo03, for the isomerization of n-heptane and n-octane at elevated pressure. For the platinum catalyst the chain length is important, as explained above, with the isomerization selectivity obtained for n-octane dropping more quickly with increasing conversion than that for n-heptane over this plantinum catalyst the isomerization of n-hexane can be... 

Figure 20.4 Deactivation of a Pt/Al203 catalyst for the isomerization of n-hexane. , rate ,...

Isomerization of n-hexane 6 in superacid proceeds by three steps formation of the carbenium ion (step 1, Scheme 5.10), isomerization of the carbenium ion via hydride shift, alkyde shift, and protonated cyclopropane (for the branching step) (step 2,... 

FIG. 13. Common intermediate for dehydrocyclization and isomerization of n-hexane and hydrogenolysis of methylcyclopentane (61). 

Figure 3 A Isomerization of n-hexane over PtHMOR catalysts31 and of dichlorobenzene over HMOR catalyst 2. B n-heptane cracking over HFAU samples41 dealuminated by steaming (i.e. with a large amount of extraframework Al species (a) followed by acid treatment, i.e. with a low amount of these species (b). Initial activity Ao vs.Xai. ...

The isomerization of n-hexane at 453° C and a total pressure of 1.6 bars has been investigated by Christoffel and Schuler [15]. They used a single pellet reactor with a pellet thickness L of 6.22 xlO-3 m. The porous pellet material contained 0.35 wt % Pt on A O,. The flow rate along the pellet was so high that the observed conversion rate was independent of the flow rate. The partial pressure of n-hexane in the feed was varied between 15 and 160 mbar. The ratio of the n-hexane concentration in the space below the pellet CAb and in the gas mixture above the pellet CAj was found to be independent of Caj CaJCa, = 0.191. The apparent conversion rate was found to be proportional to the n-hexane concentration in the feed and to be represented by RA = 0.184x0 krnol m 3 s 1. From these results the following information can be obtained ... 

K. Hashimoto, T. Masuda and M. Arai Influence of solid acidity of catalyst on selectivity of isomerization of n-hexane over Y type zeolite supporting platinum catalyst, Sekiyu Gakkaishi, 33, 166-172 (1990). 

The rates at which the above conversions take place vary considerably as shown in Table II. The interconversion of 2-methylpentane and 3-methylpentane is extremely rapid while the isomerization of 2,3-dimethylbutane to 2,2-dimethylbutane is very slow. Thus, the ratedetermining step in the formation of 2,2-dimethylbutane (neohexane) from n-hexane is the slow isomerization of 2,3-dimethylbutane to 2,2-dimethylbutane. In the isomerization of w-hexane to 2,3-dimethylbutane, the rate-determining step is the isomerization of n-hexane to the methylpentanes. 

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. 

The theoretical analysis presented in the previous section was applied to two model SCF isomerization systems. In one scheme, the catalytic isomerization of n-hexane is carried out using supercritical CO2 as solvent medium. CO2 also acts as activator for the Lewis acid catalyst promoted with hydrogen bromide or hydrogen chloride. Reported advantages over conventional processing include better mass transfer characterisitlcs, better selectivity towards isomerization products, and easier separation of reactants, product Isomers and catalyst (15). 

Lee, J.K. and Rhee, H.K. Sulfur tolerance of zeolite beta-supported Pd-Pt catalysts for the isomerization of n-hexane. Journal of Catalysis, 1998, 111, 208. 

The catalyst thiotolerance trend found for the isomerization of n-hexane to i-C6 (Table 3, column 4), an acid-controlled reaction, was different than that stated above for the metal-controlled reactions. It has to be noted here that the acidity of all the catalysts was similar since they contained similar chlorine level (wt% Cl = 0.7-0.8) over the same support. Sulfur practically did not affect the isomerization activity of the Pt-Ge catalyst and this is attributable... 

Paraffin isomerization over dual function catalysts based on zeolite Y and mordenite has been reviewedand a reaction mechanism was proposed in which olefin-paraffin equilibrium is established and carbonium ions are formed from both paraffins and olefins. The isomerization of n-hexane and hydrocrack-... 

The isomerization of n-hexane at 250 °C, 26 bar. a H2/hydrocar-bon (HC) molar ratio of 22.4, and a WHSV of 3.3 g.g". h over fresh Pd-NiSMM led to a conversion of 56 %, almost without cracking. This isomerization activity was totally and irreversibly destroyed after injection of about 10 molecules pyridine per g Pd-NiSMM. Benzene hydrogenation over the poisoned catalyst (260 °C, 26 bar, H2/HC = 25) showed that the hydrogenation function of the catalyst was still active enough to hydrogenate benzene totally to cyclohexane, indicating that the metallic sites had only been partially poisoned, it at all. 

Superacid W0x/Zr02 catalysts for isomerization of n-hexane and for nitration of benzene... 

While isomerization of n-hexane appears to hinder cyclization of the resultant molecule, higher derivatives should isomerize to branched structures which can still form five-membered rings and ultimately yield aromatics. The conversion of higher paraffin homologs to aromatics should therefore be facilitated. This can be illustrated by comparing the earlier hexane results with data in Table IX on the reforming of heptane. Both the conversion and selectivity are higher for... 

On the other hand, R-exchanged zeolites have also been used in combination with a metal function, for carrying out the isomerization and hydrocracking of paraffins and cycloalkanes. This is the case for isomerization of n-hexane to isohexane and 2,2-dimethyl-butane (Rabo et al. 1961) the isomerization of n-undecane to mixed Ci 1 isomers at 275°C on Pt/Ce-Y zeolite (Weitkamp et al. 1985) the isomerization of c (7o-exo-tricyclo[5.2.1.02,6]-decane or exo-tricyclo[6.2.1.02,7]-undecane into adamantane or 1-methyladamantane, respectively, on R-Y at 150-270°C (Lau and Maier 1987) the isomerization of tetrahydrodicyclopentadiene into adamantane on Re-Y in a H2/HC1 atmosphere at 250°C (Honna et al. 1986) or the double bond relocation of 2-alkyl acrolein into fran.j-2-methyl-2-alkenals over Ce,B-ZSM-5 (Fisher et al. 1986). Recently, it has been reported that Ce-promoted Pd/ZSM-5 is an active and selective catalyst in the dehydroisomerization of a-limonene to / -cymene (Weyrich et al. 1997). 

Alloying with active or inactive metals can both accelerate desired reactions and suppress undesired reactions. For example, the addition of Sn to Pd gives selective catalysts for the removal of acetylene from ethylene streams. Similar effects are also observed for Zn, Pb, Ag, and Au [T41]. On alloying Pd with inactive An, the rate of the reaction between H2 and O2 is increased by a factor of 50. As an additive to Pt, Au increases the rate of isomerization of n-hexane tenfold. These effects are explained in terms of a widening of the metal-metal bond by another metal. 

Duchet. J.C. Guillaume, D. Monnier. A. van Gestel. J. Szabo, G. Nascimento. P. Decker. S. In Isomerization of n-Hexane over Sulfated Zirconiu Influence of Hydrogen... 

Isomerization of n-hexane is superacids can be depicted by the three steps shown in Scheme 3. 

---