Dehydrogenative aromatizations examples

On an industrial scale, the elimination generally involves dehydrogenation. For example, styrene, the most important of these compounds—and perhaps the most important synthetic aromatic compound—can be prepared by simply heating ethylbenzene to about 600 in the presence of a catalyst. The ethylbenzene, in... 

Petroleum fractions contain many different hydrocarbon molecules and ever more stringent environmental constraints now determine con iosition and purity requirements of the products. Furthermore, when upgrading different hydrocarbon streams the formation of side-products leads to even more complex mixtures. For example when producing linear olefinic hydrocarbons by paraffin dehydrogenation aromatic side-products are formed [28]. Often, alkane/alkene/aromatic hydrocarbon mixtures have to be separated. For the liquid phase separation of normal alkenes from n-alkene/n-alkane mixtures, the OLEX process was developed [2]. Also, the separation of alkane/alkene mixtures by adsorption via Ji-complexation has been extensively studied [29-31]. However, no industrial adsorptive separation processes are available for the separation of either alkanes or alkenes of different chain length. Rather, a downstream distillation section is used as to separate for exan5)le the linear aZp/jfl-olefins (C4-C10) produced by the AlphaSelect Process (IFP) [32]. 

An example involving a larger hydrocarbon molecule is provided by the dehydrogenation (aromatization) of methyl cyclohexane (C6H11CH3) to toluene (C6H5CH3) catalyzed by platinum. Methyl cyclohexane will be abbreviated as MCH. According to Sinfelt (1981), the mechanism can be written as a sequence of reactions in the forward direction only... 

Dehydrogenation (the conversion of alicycllc or hydroaroraatic compounds into their aromatic counterparts by removal of hydrogen and also, in some cases, of other atoms or groups) finds wide appUcation in the determination of structure of natural products of complex hydroaroraatic structure. Dehydrogenation is employed also for the synthesis of polycyclic hydrocarbons and their derivatives from the readily accessible synthetic hydroaroraatic compounds. A very simple example is the formation of p-raethylnaphthalene from a-tetra-lone (which is itself prepared from benzene—see Section IV,143) ... 

This is an example of the Doebner synthesis of quinoline-4-carboxylic acids (cinchoninic acids) the reaction consists in the condensation of an aromatic amine with pyruvic acid and an aldehj de. The mechanism is probably similar to that given for the Doebner-Miller sj nthesis of quinaldiiie (Section V,2), involving the intermediate formation of a dihydroquinoline derivative, which is subsequently dehydrogenated by the Schiff s base derived from the aromatic amine and aldehyde. 

Like mthenium, amines coordinated to osmium in higher oxidation states such as Os(IV) ate readily deprotonated, as in [Os(en) (NHCH2CH2NH2)] [111614-75-6], This complex is subject to oxidative dehydrogenation to form an imine complex (105). An unusual Os(IV) hydride, [OsH2(en)2] [57345-94-5] has been isolated and characterized. The complexes of aromatic heterocycHc amines such as pyridine, bipytidine, phenanthroline, and terpyridine ate similar to those of mthenium. Examples include [Os(bipy )3 [23648-06-8], [Os(bipy)2acac] [47691-08-7],... 

The generation of caibocations from these sources is well documented (see Section 5.4). The reaction of aromatics with alkenes in the presence of Lewis acid catalysts is the basis for the industrial production of many alkylated aromatic compounds. Styrene, for example, is prepared by dehydrogenation of ethylbenzene made from benzene and ethylene. 

Selecting the naphtha type can be an important processing procedure. For example, a paraffinic-base naphtha is a better feedstock for steam cracking units because paraffins are cracked at relatively lower temperatures than cycloparaffins. Alternately, a naphtha rich in cycloparaffins would be a better feedstock to catalytic reforming units because cyclo-paraffins are easily dehydrogenated to aromatic compounds. Table 2-5 is a typical analysis of naphtha from two crude oil types. 

The second aromatization reaction is the dehydrocyclization of paraffins to aromatics. For example, if n-hexane represents this reaction, the first step would be to dehydrogenate the hexane molecule over the platinum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers, but cyclization to a cyclohexane ring may occur through a different mechanism). Cyclohexane then dehydrogenates to benzene. 

A curious mechanistic phenomenon found in hydrogenation is also found in dehydrogenation. Once again, we see an example of a reaction occurring on both sides of a six-membered ring. A clear example of trams dehydrogenation occurs during the Pd-catalyzed aromatization of the compound in Scheme 5.6.48 See Section 2.1.3.2. for a discussion of trans addition. 

Catalytic reformers. Catalytic reforming is an important step to improve the quality of gasoline. During the reforming process, naphthens are dehydrogenated to aromatics. As a representative example, hydrogen is produced by cyclohexane dehydrogenation to benzene as follows ... 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

Fullerenes can also be obtained by pyrolysis of hydrocarbons, preferably aromatics. The first example was the pyrolysis of naphthalene at 1000 °C in an argon stream [58, 59], The naphthalene skeleton is a monomer of the Cjq structure. FuUerenes are formed by dehydrogenative coupUng reactions. Primary reaction products are polynaphthyls with up to seven naphthalene moieties joined together. FuU dehydrogenation leads to both Cjq as well as C7Q in yields less than 0.5%. As side products, hydrofuUerenes, for example CjqHjj, have also been observed by mass spectrometry. Next to naphthalene, the bowl-shaped corannulene and benzo[k]fluoranthene were... 

See FIGURE 2-8 for an example of dehydrogenation of cycloparaffins to yield aromatics + hydrogen... 

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