Paraffin, skeletal isomerization

Paraffin skeletal isomerization is the second of the reaction-producing high-octane hydrocarbons, though TMPs cannot be obtained by using diverse isomerization catalysts, even the Pt/Cl-Al Oj, has been used on the industrial scale in n-butane isomerization... 

Pt supported on an acidic support is a typical catalyst for the skeletal isomerization of light n-paraffins. The acidic supports can be acidic oxides, e.g., halogenated (Cl, F) alumina or sulfated zir-conia (Zr02/S04), or an appropriate zeolite, e.g., Mordenite. Pt-(C1, F)-alumina catalysts have a high performance at low temperatures and efficiently operate at temperatures between 115 and 150°C. Such low temperatures thermodynamically favor isomerization and thus, highly branched products are obtained. Zeolite supports are less active at lower temperatures and have to be operated at about... 

Olefins, unlike paraffins, do not show significant gains in octane number with skeletal isomerization (see Table 14.2). As a result, olefin isomerization is not a useful octane boosting strategy. However, tertiary olefins (olefins with three alkyl substituents on the double bond), do react fairly readily with olefins to form ethers, which do have good octane numbers-for example, methyl tert-butyl ether (MTBE). 

Olefins are formed by dehydrogenation of the n-paraffin feed over the metallic hydrogenation-dehydrogenation function and are adsorbed on the acidic surface of the catalyst as carbonium ions by proton addition. After skeletal isomerization they are desorbed as isoolefins and subsequently hydrogenated to the corresponding isoparaffins. The net result (i.e., the formation of carbonium ions) of the action of metal and acid in dual function catalysis is, on pure Friedel-Crafts type catalysts, described by the scheme ... 

The skeletal isomerization of straight-chain paraffins is important for the enhancement of the octane numbers of light petroleum fractions. The isomerization of H-butane to isobutane has attracted much attention because isobutane is a feedstock for alkylation with olefins and MTBE synthesis. It is widely believed that the low-temperature transformation of n-alkanes can be catalyzed only by superacidic sites, and this reaction has often been used to test for the presence of these sites. 

Pdi.5PW,2O40 supported on Si02 catalyzes the skeletal isomerization of C5 and C6 paraffins (378-380). The presence of H2 is necessary to maintain the high activity in the stationary state. This requirement indicates the bifunctional character of the catalysis, with protons being generated by the reduction of Pd2 +, as shown in Eq. (45). 

Alumina is one of the most widely used catalyst supports in the petroleum industry because it is robust, porous, relatively inexpensive, and—what is especially important—it is capable of contributing acid-catalyzed activity that can be tailored to suit the requirements of a diverse array of catalytic processes. These include reforming (52, 55), hydrotreating (84, 55), and paraffin isomerization (56-55). Since pure alumina is relatively inactive for the skeletal isomerization reactions that are necessary in such processes, its acid activity is promoted through the addition of catalyst components such as fluoride, chloride, phosphate, silica, or boria. After a discussion of pure alumina itself, we will review pertinent studies of surface acidity and catalytic activity of the promoted aluminas. 

Another important and well studied paramagnetic ion in the lattice of oxide semiconductors is Zr3+ in Zr02. Zirconia dioxide is widely used both as a catalyst of different chemical processes, and as a carrier for constructing supported metal-complex catalysts. In the last years, sulfated zirconia attracted significant interest as an active and selective catalyst in skeletal isomerization of normal alkanes at low temperatures, cracking of paraffins, alkylation and acylation of aromatics [42, 53 and Refs therein]. The appropriate experimental data are collected in the following Table 8.2. 

Here we will describe the main aspects of the chemistry involved in selected zeolite-catalyzed processes in the field of oil refining and petrochemistry, such as short paraffin aromatization, skeletal isomerization of n-paraffins and n-olefins, isoparaffin/olefin alkylation, and catalytic cracking. 

Isomerization of n-paraffins in the C5-C6 range, such as those present in the LSR (light straight run) fraction, is industrially carried out to improve the octane number of the gasoline. Skeletal isomerization of n-paraffins is an acid-catalyzed reaction that is thermodynamically favored at lower temperatures. Therefore, acid catalysts with strong acidity have to be used in order to perform the reaction at temperatures as low as possible. The process is carried out in the presence of hydrogen and a bifunctional catalyst, which typically consists of a noble metal (Pt) supported on an acidic carrier. 

In order to confirm the acidity results measured using the indicators shown in Table 17.1, we have investigated as many acid-catalyzed reactions as possible. The reactions are summarized in Table 17.4 [43, 48, 118, 119]. Among them, the skeletal isomerization of light paraffins, in particular butane and pentane, has been the most widely applied. The isomerization of butane at room temperature was a well known test reaction for superacidity at the beginning of this work [43, 48, 118]. The activity for many of the reactions tested correspond to the acidities as determined by use of the Hammett indicators. 

Isomerization of n-paraffin, especially normal pentane to iso-pentane is essential for making high octane gasoline with low aromatics content. Isomerization of lower paraffins has been conducted in the solid catalyzed gas-phase reaction system by using noble metal-supported solid acid under hydrogen atmosphere. The most predominant reaction mechanism for the isomerization of alkane is as follows (1) the dehydrogenation of alkane to alkene on the supported metal (2) proton addition to the alkene to form carbenium ion on the acidic component (3) skeletal isomerization of the carbenium ion on the acidic component (4) deprotonation of the isoraerized carbenium ion to form alkene on the acidic component (5) hydrogenation of the alkene to alkane on the metal [1]. 

On the other hand, it was proposed that acid catalyzed reactions such as skeletal isomerization of paraffin [2], hydrocracking of hydrocarbons [3] or methanol conversion to hydrocarbon [4] over metal supported acid catalysts were promoted by spillover hydrogen (proton) on the acid catalysts. Hydrogen spillover phenomenon from noble metal to other component at room temperature has been reported in many cases [5]. Recently Masai et al. [6] and Steinberg et al. [7] showed that the physical mixtures of protonated zeolite and R/AI2O3 showed high hydrocracking activities of paraffins and skeletal isomerization to some extent. 

At 435°C the olefin concentration is found to be only about 0.02% at 30 atm. partial pressure of hydrogen. Thus, if we were to carry out paraffin isomerization by successive and separate steps of dehydrogenation of ri-paraffin to n-olefin, followed separately by skeletal isomerization of the n-olefin produced to iso-olefin (and subsequent rehydrogenation), the over-all conversion of such a scheme could be, at best, 0.02%. Thus, the paraffin isomerization, if accomplished in a bifunctional reaction system with a high conversion as might be described by formula (2), is an example of a nontrivial case as defined by (3) above. 

It is found that Mode E behaves similarly to the zeolite free Pt-Re/Al203 Both catalysts have a relatively high proportion of isomer products which could be formed over the metal surface via a bond-shift mechanism [8]. Isomers are formed by doublebond isomerization and skeletal isomerization reactions at both the acid sites of the alumina support and the metal sites. The later provides a dehydrogenation-hydrogenation function and the acid sites an isomeiization function for the olefins to dehydrogenate from paraffins over the metal function, since it is known that olefin isomerization proceeds much quicker than the respective paraffin isomerization [8]. On the other hand, branched paraffins are less easily cracked than linear ones [10]. Therefore, once isomers are formed over conventional reforming catalysts, they are likely to be the final products. Evidently, the isomerization of paraffin requires the metal function in the bimetallic catalyst, and so does the paraffin aromatization. This can also explain the obseiwed decrease in the isomers and aromatics production with time-on-Hne since it is well- known that coke preferentially deposits on a metal surface first [14]. 

Acidity of Supports. Most of the side reactions encountered in the attempted dehydrogenation of normal paraffins to linear monoolefins are affected by the acidity of the catalyst. For example, strongly acidic catalysts would be expected to promote skeletal isomerization, cracking, de-hydrocyclization, and coking. Pines and Haag (10) have shown that catalyst supports themselves may exhibit intrinsic acidity and that model reactions may be used to characterize the acidity of the support. Since all of the catalysts covered in the present work contained activated alumina as a major component, an attempt was made to characterize the acidity of several aluminas. The results are given in Table II. We decided to use the amount of LB formation (which includes cracked... 

Skeletal isomerization of n-paraffin is a crucial reaction to enhance the octane number and produce clean fuels. Liquid acids such as HF and H2SO4 catalyze this reaction, but they lead to problems of pollution, corrosion, and toxicity. A solution is thus to replace these catalysts by acidic solids, which are more respectful towards the environment and limit corrosion problems. Among various acidic solids sulfated zirconia attracts the attention, because it exhibits an exceptional acidity and a high catalytic activity in many reactions such as the isomerization of n-paraffin. In 1962 V, C. F. Holm and G. C. Bailey described at first the catalytic properties of sulfated zirconia [1], then in 1979 Hino and... 

Cracked next to ring Double bond shifts rapidly extensive skeletal isomerization Hydrogen transfer is an important reaction and is selective for tertiary olefins Crack at much higher rate than corresponding paraifins Crack at about same rate as paraffins with equivalent structural groups (9)... 

Bifunctional (metallic and acidic functions) catalysts are applied to a variety of oil-refining and petrochemical processes. For example, paraffin hydroisomerization involves n-paraffins dehydrogenation to a-olefins over metal, skeletal isomerization of n-olefins to isoolefins over acidic sites, followed by hydrogenation of isoolefins to isoparaffins over metal. Zeohte-supposted noble metal catalysts are often used for these types of reactions, which... 

These mechanistic features were elucidated in detail in the 1960s. Based on the pioneering work of Mills et al. and Weisz ", a carbenium ion mechanism was proposed, similar to catalytic cracking plus additional hydrogenation and skeletal isomerization. More recent studies of paraffin hydrocracking over noble metal-loaded, zeolite based catalysts have concluded that the reaction mechanism is similar to that proposed earlier for amorphous, bifunctional hydrocracking catalysts. ... 

Skeletal isomerization of paraffins such as butane, pentane, etc. is not catalyzed even by 100% H2SO4. It was found, however, that Zr02 S04 , Ti02 — S04 , and Fe203 — S04 " catalyzed the skeletal isomerization of butane at 293 — 323 K, the main products being isobutane. The activity of the solid superacids is lowered as the reaction proceeds probably due to coke formation. To prevent the catalyst from its deactivation, a catalyst on which a small amount of Pt, Ni etc. was added was developed. Over a Pt — Z1O2 — S04 catalyst, no deactivation was observed for more than 100 h for the skeletal isomerization of pentane at 413 K under 20 kg/cm of hydrogen pressure. 

Skeletal isomerization of n-Cs, Cs paraffins to corresponding isoparaffins is important for improving the octane number as they are mixed in gasoline. Since low temperature is favored for the equilibrium of this reaction, catalysts active at low temperatures are desirable. Noble metals loaded on zeolites such as Pt —Y zeolite with low Na content are effective and used at about 520 Fig 4.3 shows the effect of Na content of zeolite on the catalydc activity for hexane isomerization. As the acidity increases with decreasing Na content the optimum temperature of operadon b gready suppressed. 

The fast reaction is the reversible skeletal isomerization of normal hexane to isohexane. Isomerization reactions are an important building block in the production of high-octane gasoline, as the octane numbers of branched paraffins are substantially higher than those of normal paraffins with the same number of carbon atoms. The slow reaction is the hydrogenolysis (hydrocracking) of normal hexane to lower paraffins such as methane. This reaction is undesirable, as these light paraffins have a much lower economic value. 

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