Isomerization hydrocarbons with acid catalysts

Concurrently, a series of papers began to appear on the isomerization and hydrogen-exchange reactions of hydrocarbons with acid catalysts. Over slightly hydrated aluminum bromide at room temperature propane containing carbon-13 at one end (C Hq-I-C )was found to isomerize toward a statistical mixture with... 

Positional Isomerization. A different type of isomerization, substituent migration, takes place when di- and polyalkylbenzenes (naphthalenes, etc.) are treated with acidic catalysts. Similar to the isomerization of alkanes, thermodynamic equilibria of neutral arylalkanes and the corresponding carbocations are different. This difference permits the synthesis of isomers in amounts exceeding thermodynamic equilibrium when appropriate reaction conditions (excess acid, fast hydride transfer) are applied. Most of these studies were carried out in connection with the alkylation of aromatic hydrocarbons, and further details are found in Section 5.1.4. 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

Several metal oxides could be used as acid catalysts, although zeolites and zeo-types are mainly preferred as an alternative to liquid acids (Figure 13.1). This is a consequence of the possibility of tuning the acidity of microporous materials as well as the shape selectivity observed with zeolites that have favored their use in new catalytic processes. However, a solid with similar or higher acid strength than 100% sulfuric acid (the so-called superacid materials) could be preferred in some processes. From these solid catalysts, nation, heteropolyoxometalates, or sulfated metal oxides have been extensively studied in the last ten years (Figure 13.2). Their so-called superacid character has favored their use in a large number of acid reactions alkane isomerization, alkylation of isobutene, or aromatic hydrocarbons with olefins, acylation, nitrations, and so forth. 

At higher temperatures, C—H and C—C bonds may be similarly broken. Thus, zeolite catalysts may be used for (i) alkylation of aromatic hydrocarbons (cf. the Friedel-Crafts reactions with AICI3 as the Lewis acid catalyst), (ii) cracking of hydrocarbons (i.e., loss of H2), and (Hi) isomerization of alkenes, alkanes, and alkyl aromatics. 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

Alkane isomerization equilibria are temperature-dependent, with the formation of branched isomers tending to occur at lower temperatures (Table 4.1). The use of superacids exhibiting high activity allows to achieve isomerization at lower temperature (as discussed below). As a result, high branching and consequently higher octane numbers are attained. Also, thermodynamic equilibria of neutral hydrocarbons and those of derived carbocations are substantially different. Under appropriate conditions (usual acid catalysts, longer contact time) the thermodynamic... 

To conclude this section, it is necessary to state that besides their application in catalytic cracking, amorphous silica-alumina acid catalysts have been applied in other hydrocarbon transformations, such as isomerization of olefins, paraffins, and alkyl aromatics, the alkylation of aromatics with alcohols and olefins, and in olefin oligomerization [55],... 

It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel et al. (312)). Tetra-cyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. 

Zeolites are solid acid catalysts which are widely used in hydrocarbon processing, such as naphtha cracking, isomerization, dispropornation and alkylation. During reactions carbonaceous materials called coke deposit on the zeolite and reduces its activity and selectivity. Coke deposited not only covers the acid sites of the catalyst, but also blocks the pores, and restrain reactants from reaching the acid sites, leading to the decrease in the apparent reaction rate (1, 2). Here, we will mainly deal with the intracrystalline diffusivity of zeolites, and will discuss the relationship between it and the change in catalyst selectivity. 

In the process of catalytic cracking, characteristic reactions such as chain scission, hydrogen transfer and condensation take place under certain temperature and pressure conditions and when an appropriate catalyst is utilized, products with certain range of molecular weights and structures are obtained. Catalysts with surface acid sites and with the ability of hydrogen ion donation such as silica-alumina and molecular sieve catalyst have been already widely utilized. These catalysts can also enhance the isomerization of products and increase the yield of isomeric hydrocarbons. However, large amounts of coke will deposit on the surface of catalysts and consequently lead to their deactivation. Therefore, the recycling of catalysts is difficult to achieve. 

Basic zeolites are able to catalyze double bond isomerization of olefins [133]. Although this can also be achieved with acidic zeolites, the lower reactivity of basic zeolites towards hydrocarbons (i.e., the complete absence of skeletal isomerization) leads to higher yields [134]. A good example for this is the double bond isomerization of 1-octene over potassium loaded NaY. It is claimed that high yields can be achieved in that way and that the impregnation of the zeolite with an excess of alkali cations is important to obtain a good catalyst [135]. 

When talking about bifunctional catalysis, one thinks immediately of catalysts possessing metal and acid functions. It is well known that traces of olefins accelerate the acid-catalyzed conversion of hydrocarbons and that such a catalysis usually results in rapid deactivation. More stable catalytic activity for the isomerization of paraffins is achieved by bifunctional catalysis, i.e., the association of a hydrogenation function of a metal with an acidic function of a support. In this case, the amount of olefins is controlled by the hydrogenation-dehydrogenation equilibrium. This topic has received considerable attention and has been earlier reviewed by Weisz [130]. However, bifunctional catalysis cannot be restricted to catalysts composed of metal and support with acid sites, but also with supports possessing acid-base pairs, basic or redox sites [131]. This is illustrated by some upcoming short examples. 

CgHi2 bicycloolefin isomerizations are summarized in Fig. 2. At 260°, over a silicophosphoric acid catalyst, bicyclo[2.2.2]-2-octene (4) is converted to the bicyclo[3.3.0]-2-octene (6), with intermediate formation of bicyclo[3.2.1]-2-octene (5). The yield of hydrocarbon 5 passes through a maximum and then vanishes when the conversion of 4 increases 8-10, 18a). This isomerization also slowly forms a mixture... 

By analogy with the results obtained with the phenyl-substituted analogues, chemisorption of isobutane might be expected to proceed by hydride ion abstraction to form a tertiary-butyl carbonium ion and it would indeed be convenient to assign the 3000 A band to this ion. Although such alkyl carbonium ions have been useful for the mechanistic description of a variety of reactions of hydrocarbons over heterogeneous acid catalysts, in recent years numerous observations have been reported which apparently are not in accord with this concept. Notable in this respect is the remarkable degree of stereoselectivity associated with the acid-catalyzed double bond isomerization of butenes... 

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