Pentanes, isomerization equilibria

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]. 

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83-85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of l,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170-180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as l,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86. 

Equilibria. The equilibrium distributions of butane, pentane, and hexane isomers have been experimentally determined (5, 16) and are diagrammed in Figure 2. In each case, lower temperatures favor the more highly branched structures. At the approximately 200° F. temperature usually employed for isomerization, the butane equilibrium mixture contains about 75% isobutane. That for pentane contains about 85% isopentane.. In the case of hexane, the equilibrium product contains about 50% neohexane and has a Motor octane rating of about 82. In all cases, of course, the yield of the desired isomers can be increased by fractionation and recycle. 

The temperature of isomerization controls equilibrium isomer composition, and thereby product octane. Figure 4.8 is a plot of isopentane in the C5 product as a function of temperature. The data are from pilot plant runs with three types of commercial UOP isomerization catalysts. The feedstock was a 50/50 mixture of normal pentane and normal hexane, containing about 6% cyclics. The 1-8 and I-80 catalysts are very active at a low temperature, where equilibrium isopentane content is highest. The acid functions in 1-8 and 1-80 are chlorided aluminas. The zeolitic catalyst, HS-10 , requires relatively high temperatures of operation. The LPI-100 catalyst contains sulfated zirconia as the acid function and falls in the middle of the temperature range (12). Due to the equilibrium constraints, a lower temperature operation yields a higher octane product. The 1-8 and 1-80 catalysts yielded Research Octane Numbers of 82-84, as compared to 80-82 for LPI-100 catalyst and 78-80 for HS-10. 

Estimate the equilibrium composition of the reaction n-pentane —> neopentane, at 500 K (440° F) and 10.13 MPa (100 atm), if the system initially contains 1 g mol n-pentane. Ignore other isomerization reactions. 

The solid-state structure of octacarbonyidicobalt (13) is represented in (I). It has been established (14, 15) that Co2(CO)b exists in solution in two isomeric forms. One form (II) corresponds to the structure of the crystalline substance while the second isomer (III) has no bridging carbonyl groups. From a study of the temperature dependence of the infrared spectrum of Co2(CO)a in pentane, Noack (14) has determined that the unbridged isomer (III) is the major component in the equilibrium... 

Since alkylate compositions from the four butene isomers are basically similar, the butenes are thought to isomerize considerably, approaching equilibrium composition prior to isobutane alkylation. Such a postulation is at variance v/ith previously published alkylation mechanisms. The Isomerization step yields predominantly isobutene which then polymerizes and forms a 2,2,4-trimethylpentyl carbonium ion, a precursor of 2,2,4-trimethylpentane, the principal end product. The 2,2,4-trimethylpentyl ion is also capable of isomerization to other trimethylpentyl ions and thus yields other trimethylpentanes, principally 2,3,4-trimethyl-pentane and 2, 3, 3-trimethylpentane. 

The data given for a reaction temperature of 300°C clearly showed the mordenite catalyst to be the more active for isomerization of both the C5 and Ce fractions. Conversions quoted were precious-metal-H-mordenite C5 —65 wt %, Cq >— 15 wt % precious metal-H-Y C5 — 40 wt %, Ce 4 wt %. These data suggest that the pentane fraction may be slightly easier to isomerize over mordenite than the hexane. The equilibrium conversions to isopentane and 2,2-DMB at 300°C are in the vicinity of 65 and 18 wt %, respectively. A possible explanation is that impurities present—e.g., cyclohexane and/or benzene—aifect the rate of 2,2-DMB formation more than that of the isopentane. 

Besides the limiting of isomerization by equilibria, the rate at which equilibrium is approached is also important. Butanes and pentanes... 

A simple naphtha isomerization process has a feed of 10,000 barrels per day (bpd) of a 50 wt% mixture of n-hexane and methyl pentane. The feed is heated and sent to a reactor, where it is brought to equilibrium at 1300 kPa and 250°C. The reactor products are cooled to the dew point and fed to a distillation column operated at 300 kPa. The bottoms product of the distillation is rich in n-hexane and is recycled to the reactor feed. An overall conversion of n-hexane of 95% is achieved. 

Bis(phospholyl)zirconocene 282 was obtained as a 63 37 mixture of rac- and meso-isomers in the crude product from salt metathesis of a phospholyl anion and Z1CI4 in THF (Scheme 59).226 Washing the crude product with pentane enhanced the raclmeso ratio to 80 20, but a THF solution of this mixture reached back to the equilibrium ratio of 63 37 in <15 min, indicating facile isomerization processes among these diastereomers. Slow addition of a THF solution of ((R)-Kinap)RhlCOD)]OTf to a THF solution of 282 produced a single diastereomer of the bimetallic r -phosphazirconocene 283, which accomplishes the dynamic resolution of phosphazirconocene 282 (COD = 1,5-cyclooctadiene). 

Little has been published about the commercial results obtainable by the use of dual-function catalysts. However, it can be stated that, when either n-butane or n-pentane is charged, isomerization proceeds to thermodynamic equilibrium, with selectivities in the range of 90-95%. 

The early study of the equilibrium constants of pentanes is open to criticism because the isomerization was accompanied by excessive decomposition (Moldavskii, Nizovkina, and Shtemer, 33 Shuit, Hoog, and Verheus, 4). In order to establish the equilibrium constant, the isomerization from both the normal and the isopentane side was studied under experimental conditions designed to reduce to a minimum the amount of secondary reactions (Pines et al., 13). On the basis of experimental results obtained the concentration of isopentane and n-pentane in liquid and vapor phase was calculated as a function of temperature the formation of neopentane... 

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