Catalysts pentane isomerization

Pentane Isomerization. Pentane isomerization, although carried out on a much smaller scale, increased the critical supply of aviation gasolines toward the end of the war. Two pentane processes—one developed by Shell and one by Standard (Indiana) —were commercialized before the end of the war. The principal differences between the butane and pentane processes are the use in pentane isomerization of somewhat milder conditions and the use of an inhibitor to suppress side reactions, principally disproportionation. In general, the problems of the butane processes are inherent also in pentane isomerization, but the quality of the feed stocks is less important. Catalyst life is much... 

The other commercialized pentane isomerization process is that of the Standard Oil Co. (Indiana) (20). This process differs from the Indiana-Texas butane process in that the aluminum chloride is introduced as a slurry directly to the reactor and that about 0.5% by volume of benzene is added continuously in the feed to suppress side reactions. Temperature, catalyst composition, space velocity, and hydrogen chloride concentration are generally similar to those in the corresponding butane process, but the reactor pressure is about 100 pounds lower. The Pan American Refining Co. operated the Indiana pentane isomerization process commercially during the last nine months of the war and produced about 400 barrels of isopentane per calendar day. 

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. 

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. 

Pentafining a pentane isomerization process using a regenerable platinum catalyst on a silica-alumina support and requiring outside hydrogen. 

In order to follow the catalytic recovering produced by the burning of coke, partially regenerated catalyst samples were submitted to standard reaction tests for benzene hydrogenation (metallic function) and normal pentane isomerization (acid function). Benzene hydrogenation was done at 423 K, 0.1 MPa, WHSV = 2 h 1, and molar ratio H2/Bz = 20. 200 mg of catalyst were loaded, which was reduced at 533 K with H2 for 2 h before the test. The isomerization of n-pentane was performed at 773 K, 0.1 MPa, WHSV = 2 h 1 and molar ratio H n = 6. 200 mg of catalyst were loaded, and were reduced with hydrogen at 773 K for 2 h before the test. 

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.

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... 

Commercial zeolite based hydroisomerization catalysts comprise alumina bound and platinum impregnated dealuminated mordenite. The activity and selectivity of the hydroisomerization of n-paraffins is strongly influenced by acid leaching. The influence of silica to alumina ratio has been studied for pentane isomerization over platinum mordenite many times since one of the first papers published (6). 

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 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. 

Fig. 21. Process variables for pentane isomerization. Shell liquid-phase process. Conditions (unless otherwise noted) temperature, 203°F. residence time, 9 minutes Hj pressure, 45-65 p.s.i. HCl, 3.7-4.8 wt.% catalyst-to-hydrocarbon ratio, 1/1.

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... 

Samples of a catalyst commerciaHy coked up to 9.9% C were partially decoked by burning and submitted to test reactions of ben7ene hydrogenation andn-pentane isomerization. Figure 8 shows relative activities for these reactions as a... 

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

The cyclodimerization of cyclopropenes, a novel reaction, was foundto be catalysed by KA and NaX zeolites. Carbanion intermediates were proposed and the selectivity of the reaction was attributed to spatial constraints. Paraffin disproportionation, with isomerization, at about 500 K has been shownto occur over H-mordenite and HZSM-4 catalysts. Synthetic H-ferrierite is an active and very selective catalyst for n-paraffin cracking and hydrocracking. Palladium on zeolite L comparesfavourably with Pd/HY as a catalyst for 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. 

The structure and catalytic behavior of WOx/Zr02 are dependent on WOx concentration, preparation conditions, and thermal treatment. In pentane isomerization, the maximum rates were observed for catalysts with 19-24 wt% WO3 after calcination at temperatures in the range 923 - 1123 K [2, 9, 10, 11]. These conditions correspond to the formation of WO domains of intermediate size on zirconia surface, which seem to be the active species in such reactions [12-14]. These islands appear to provide a compromise... 

Sinfelt, J.H. Hurwitz, H. and Rohrer, J.C. "Kinetics of/7-pentane isomerization over Pt-Al203 catalyst."/. Phys. Chem. 64 892-894 1960. 

Most feeds contain some olefin as an impurity moreover many sulfated zirconia catalysts contain traces of iron or other transition metal ions that are able to dehydrogenate hutane. In the presence of such sites, the olefin concentration is limited by thermodynamics, i.e a high pressure of H2 leads to a low olefin concentration. That aspect of the reaction mechanism has been proven in independent experiments. The isomerization rate over sulfated zirconia was dramatically lowered by H2. This effect is most pronounced when a small amount of platinum is deposited on the catalyst, so that thermodynamic equilibrium between butane, hydrogen and butene was established. In this way it was found that the isomerization reaction has a reaction order of +1.3 in -butane, hut -1.2 in hydrogen [40, 41]. The byproducts, propane and pentane, are additional evidence that a Cg intermediate is formed in this process. As expected, this kinetics is typical for butane isomerization only in contrast pentane isomerization is mainly a monomolecular process, because for this molecule the protonated cyclopropane ring can be opened without forming a primary carbenium ion [42]. 

Curve 2 of Fig. 8 concerns the competitive isomerization and hydrogen-olysis of normal pentane as a function of particle size over Pt/Si02 catalysts (140). Isomerization is favored over large particles and hydrogenoly-sis over small particles. It is clear again that the best catalyst probably corresponds to an intermediate particle size. 

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