N-Pentane. isomerization

Koradia, P., Kiovsky, J.R., and Asim, M.Y. (1980) Optimization of SiOy/AlyOy, mole ratio of mordenite for n-pentane isomerization. . Catal., 66, 290-293. 

The ability of water molecules to promote a reaction depends on many factors. In most cases, zeolites with monovalent cations have low activity. However, the addition of water molecules to X and Y zeolites with monovalent ions increased the isomerization of cyclopropane (63). De-cationized zeolites can be promoted readily with water, and the process is reversible (2, 60, 64). It was shown (2) that the promoting ability of water molecules in faujasites is less when the Si02/Al203 increases. Dealu-minated faujasites are even more difficult to promote. For erionite and mordenite the maximum effect of water was observed only after treatment with liquid water and subsequent heating (2). The effect of water on zeolites saturated with polyvalent cations is less pronounced (65, 66, 67). However, the presence of multivalent cations stabilizes the catalytic activity. Water and alcohols were reported to promote ion exchanged zeolites for n-pentane isomerization (68) and n-hexadecane hydrocracking (69). 

Catalysts Based on Mordenite. Isomerization of paraffins over H-mordenite based catalysts has been described (6, 7,14, 0, 21). Minachev (7) reports that cyclohexane isomerization activity of Na-H-mordenite catalysts increases linearly with H+ concentration in the zeolite for 25-94% exchange. He further observed that H-mordenite is deactivated by other cations such as Li, K, Mg, Cd, Zn, and Al. This agrees with Bryant s work (6) he reported that, compared with Pd-H-mordenite, samples in which hydrogen was partly replaced by Ca or Zn had an appreciably lower n-pentane isomerization activity. 

The mechanism and kinetics of pentane, hexane, and cyclohexane isomerization over Pd-H-mordenite have been extensively investigated by Bryant (6), Hopper (21), and Beecher (20). They assume a conventional dual function mechanism as described earlier. Bryant (6) pointed out that H-mordenite itself has a high activity for pentane isomerization and that impregnation of a noble metal does not change the rate of the isomerization reaction. This exceptional activity of mordenite has since been reported by Benesi (14) and Minachev (7) as well. In Mina-chev s paper the reaction mechanism of n-pentane isomerization over H-mordenite is discussed in some detail. The rate of reaction is inversely proportional to the hydrogen pressure, and it is concluded that the reaction proceeds according to the following scheme ... 

Figure 2. n-Pentane isomerization activity of mordenite. Platinum stabilizes conversion and increases selectivity. 

Fig. 5. Isomerization rate versus pentene partial pressure (S4). Comparison of n-pentane isomerization rate over platinum-alumina catalyst with the rate of skeletal isomerization of 1-pentene over the platinum-free catalyst 372°C.

Thus, at 372°C. and over the range of pressures and hydrogen to n-pentane ratios covered in the investigation, it appears that the proposed mechanism can account in large part for the observed kinetic data. However, Starnes and Zabor (S8) have proposed an alternative mechanism, based on their studies of n-pentane isomerization over platinum-alumina-halogen catalysts. They postulate that the paraffin is adsorbed on platinum sites with dissociation of a hydrogen atom, followed by polarization of the adsorbed species. 

Figure 5.11. Variation of the composition of the catalytic phase as a function of the SbF5 concentration in the n-pentane isomerization in HF—SbF5.90 T — 15°C, pm = 5 bars, volume of the catalytic phase = 57 ml. , Mass of C5+, o, mass of C5H (Freon-113 extract) a, % weight of Cs+ + C5H (methylcyclopentane extract).

Pt-Re A1203 support Partial coke burning. C6H6 hydrogenation and n-pentane isomerization. 

Figure 4. Relative metallic or acidic activity as a function of relative residual carbon content on the "burned1 catalyst. The unity of catalytic activity is the percentage of cyclohexane produced by hydrogenation of benzene (a), or i-pentane produced by n pentane isomerization (b), over the completely decoked catalyst. The unity of residual carbon is the percentage of carbon in the initial catalyst (Table 1). I, catalyst I burnt with ozone-air II, catalyst II burnt with 02-N2 III, catalyst III burnt with 02-N2.

Keywords alkanes-isomerization iron-promoter n-pentane-isomerization platinum-promoter selectivity tungsten oxide zirconia-tungstated-acidity zirconia-tungstated-redox properties... 

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... 

J. C. Vartuli, J. G. Santiesteban, P. Traverso, N. Cardona-Martinez, C. Chang, and S. A. Stevenson, Characterization of the Acid Properties of Tungsten/Zirconia Catalysts Using Adsorption Microcalorimetry and n - Pentane Isomerization Activity, J. Catal. 187, 131-138 (1999). 

Kuba et al. (2003) monitored a WC>3/Zr02 catalysts with and without platinum during n-pentane isomerization and hydroisomerization at 523 K their equipment consisted of a reactor placed next to an integrating... 

Isopentane is available in large amounts in Cj cuts from catalytic craddnj (see Section 2J.1X and can be produced by n-pentane isomerization. However, since it is widely sought after for the manufacture of gasolines due to its high octane number, this raw material is difficult to secure and costly for petrochemicals. 

Sinfelt, J. H., H. Hurwitz, and J. C. Rohrer, Kinetics of n-pentane isomerization over Pt-Al203 catalyst, J. Phys. Chem., 64, 892-894 (1960). 

The catalytic properties of H-, Li-, Na-, K-, Mg-, Ca-, Zn-, Cd-, and Al-forms of synthetic mordenite in the reactions of cyclohexane and n-pentane isomerization and benzene hydrogenation have been studied. The cation forms of mordenite that do not involve the metals of column VIII of the Mendeleyev Table show high activity in these reactions. To elucidate the mechanism of n-pentane isomerization, the kinetics of the reaction on H-mordenite have been studied. Carbonium ion is supposed to result from splitting off hydride ion from hydrocarbon molecule. Na-mordenite catalytic activity in benzene hydrogenation reaction decreases linearly with the increase of decationization. This indicates that cations are responsible for the catalytic activity of zeolite. The high activity of cations of nontransition metals in oxidation-reduction reactions seems to be quite unexpected and may provide evidence for some uncommon mechanism of benzene hydrogenation. 

The high catalytic activity of H-mordenite seems to be uncommon, since (6, 9) amorphous aluminum silicates and zeolites which do not involve the metals of column VIII are not active in isomerization of saturated hydrocarbons under similar conditions. In addition, as has been shown by this research, the introduction of metal into H-mordenite does not increase the catalyst activity in n-pentane isomerization. For example, the yield of isopentane on H-mordenite and 0.5% Pd/HM at p = 30 atm and V = 1 hour" is about 50 wt % (12). 

The study of n-pentane isomerization kinetics on H-mordenite enabled us to bring out some peculiarities of this catalyst. In the absence of hydrogen, the reaction proceeded at a low rate. This probably results from hydrogen rearrangement. For example, in the presence of nitrogen at 280°C and 30 atm, the amounts of isopentane and cracking products were 5% and 5%. 

Thus, study of the kinetics of n-pentane isomerization on H-mordenite leads to the conclusion that the mechanism of the reaction in question is different from that of isomerization on bifunctional and metal-zeolite catalysts. This difference lies in the manner of carbonium ion formation. With bifunctional catalysts, carbonium ion originates with the attachment of a proton to the olefin molecule, while with H-mordenite it originates as a result of splitting off hydride ion from the saturated molecule of the starting hydrocarbon by mordenite proton, as has been suggested by the above reaction scheme. 

Figure 4 shows that the relative activity for n-pentane Isomerization drops linearly with the amount of carbon deposited on the acid function of the catalyst. This is so because isomerization of n-pentane is a typical bifunctional reaction controlled by the acid function of the catalyst. Hydrocracking to propane shows... 

Fig. 8 (left). Relative catalytic activity as a function of carbon elimination from the commercially coked catalyst. , benzene hydrogenation. , n-pentane isomerization... 

It has been shown that over NiSMM without Pd or Pt the rate of n-pentane isomerization depends on the metal function ( 5). Pt-NiSMM and Pd-NiSMM catalysts are only about 2-5 times more active than pure reduced NiSMM ( 5), so it is questionable whether the mere addition of Pt or Pd is sufficient to optimize the acidic properties of NiSMM. 

Figure 8 shows that the relative activity for n-pentane isomerization drops during the whole run and that the decay is approxi-... 

Figure 37 Relative activity of benzene hydrogenation ( ) and n-pentane isomerization (O) as a function of carbon eliminated by burning from a Pt-Re/Al203...

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