N-Pentane hydrogenolysis

TABLE 7.8. Kinetic Parameters for n-Pentane Hydrogenolysis Over Metal Catalysts... 

T. L6pez, A. Lopez-Gaona, and R. Gomez, Deactivation of Ruthenium Catalysts Prepared by the Sol-Gel Method in Reactions of Benzene Hydrogenation and n-Pentane Hydrogenolysis, Langmuir, 6, pp. 1343-46, 1990. 

Other types of non-micro-channel, non-micro-flow micro reactors were used for catalyst development and testing [51, 52]. A computer-based micro-reactor system was described for investigating heterogeneously catalyzed gas-phase reactions [52]. The micro reactor is a Pyrex glass tube of 8 mm inner diameter and can be operated up to 500 °C and 1 bar. The reactor inner volume is 5-10 ml, the loop cycle is 0.9 ml, and the pump volume adds a further 9 ml. The reactor was used for isomerization of neopentane and n-pentane and the hydrogenolysis of isobutane, n-butane, propane, ethane, and methane at Pt with a catalyst. 

The temperature dependence of the selectivity for isomerization versus hydrogenolysis depends on the type of catalyst. Thus, over thick platinum film catalysts this selectivity was temperature independent for the reaction of the butanes and neopentane (24). However, in Boudart and Ptak s (122) reaction of neopentane over platinum/carbon the selectivity to isomerization decreased slightly with increasing temperature while Kikuchi et al. (128) found an increased trend for isomerization in the reaction of n-pentane over platinum/silica and platinum/carbon catalysts. 

Higher hydrocarbon molecules allow study of the unique cracking pattern of metals. These studies are usually carried out at low conversion to observe only primary hydrogenolysis. Nickel exhibits high selectivity to cleave terminal C—C bonds leading to demethylation that is, it cleaves only bonds that involve at least one primary carbon atom. For example, in the transformation of n-hexane, only methane and n-pentane are formed (180°C, Ni-on-silica catalyst, 0.3% conversion), whereas 2-methylpentane and 3-methylpentane yield methane, n-pentane, and isopentane.260 In the transformation of 2-methylpentane, the n-pentane isopentane ratio is close to 2, which corresponds to the statistical value. Under more forcing conditions, successive demethylations lead eventually to methane as the only product. 

When iron is added as a second promoter, the performance of PtFeWZ catalysts is dramatically improved in the presence of dihydrogen in the feed.19,21 Under identical reaction conditions, PtFeWZ(S) is characterized by an n-pentane isomerization rate of 9 x 10 x mol s 1 m 2. Whereas the PtWZ catalyst is characterized by a nearly stable selectivity of about 95% (see Table 2), the PtFeWZ(S) catalyst develops a selectivity (increasing with TOS) of up to 98%, and PtFeWZ(N) shows a stable selectivity greater than 99%. The suppression of the hydrogenolysis products, which are formed on the platinum in PtWZ by the addition of iron as a second promoter, might be a consequence of the suppression of the formation of metallic platinum. Furthermore, the redox properties of the Fe3+/Fe2+ pair in the surface solid solution (see above) might... 

With linear alkanes having five or more carbon atoms, cyclization becomes possible as well as isomerization and hydrogenolysis. With n-pentane, cyclization is minimal and with n-hexane it does not exceed 25% in the range 470-570 K [6] with the latter molecule, isomerization predominates above 520 K. Product selectivities are particle-size sensitive, and Pt/SiC>2 catalysts having lower dispersion give more hydrogenolysis and cyclization. 

Fig. 4 (left). Relative catalytic activity for the n-pentane reactions at 500 C on commercially coked catalysts as a function of carbon content. A, isomerization. , hydrocracking. , dehydrocyclization. o hydrogenolysis... 

On platinum, the a, -dicarbene mechanism which accounts for the hydrogenolysis of cycloalkanes (Scheme 34) is no longer predominant in the hydrocracking of acyclic alkanes. It has already been emphasized that the internal fission of isopentane and n-pentane is related to the metallocyclobutane bond shift mechanism of isomerization (see Section III, Scheme 29), and that in more complex molecules, the favored rupture of the C-C bonds in a p position to a tertiary carbon atom is best explained by the rupture of an a,a,y-triadsorbed species (see Section III, Scheme 30). The latter scheme can account for the mechanism of hydrocracking of methylpentanes on platinum. Finally, the easy rupture of quaternary-quaternary C-C bonds in... 

Reactions 4 and 5 occur by a monofunctional metallic mechanism and Figure 8 shows that they have a rapid initial decay and afterwards reach a steady-state value in agreement with what can be expected according to coke deposition on the metal function shown in Figure 7. Hydrogenolysis of n-pentane is more affected by coke... 

Scheme IU.6. Singte hydrogenolysis ofZ-mcthylpcntanc leading via isomode to n-pentane.

When the integral method of kinetic analysis is applied, numerical integration of the continuity equations containing the rate equations is generally necessary for the comparison of the predicted and experimental responses for each experiment in each iteration cycle of the parameter estimation. Examples can be found in the work of De Pauw and Froment [1975] on n-pentane reforming in the presence of coke formation, in the work of Emig, Hofmann, and Friedrich [1972] on methanol oxidation, and in Example 2.6.4.A on benzothiophene hydrogenolysis. 

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