Paraffinic hydrocarbons dehydrocyclization

The product coming out of the reactor consists of excess hydrogen and a reformate rich in aromatics. Typically the dehydrogenation of naphthenes approaches 100%. From 0% to 70% of the paraffins are dehydrocyclized. The liquid product from the separator goes to a stabilizer where light hydrocarbons are removed and sent to a debutanizer. The debutanized platformate is then sent to a splitter where Cg and C9 aromatics are removed. The platformate splitter overhead, consisting of benzene, toluene, and nonaromatics, is then solvent extracted (46). 

At the present time, the industrial production of benzene and its homologs is implemented by coal carbonization, dehydrocyclization of the usual paraffin hydrocarbons and dehydrogenation of cyclohexane hydrocarbons with catalytic reforming of directly distilled gasoline fractions. The petroleum refining industry is the main source for meeting the demand for benzene and its reserves can fully meet the increasing demand for this compound. 

The hydrocarbon-type analysis of the Platformate discussed above was based on the product having an octane number of 92.9 (F-l plus 3 ml. of tetraethyllead per gallon). The aromatic content (based on charge) increases continually with increased severity. At the two highest severities, the aromatic yield (based on the charge) is in excess of the total naphthenes and aromatics present in the charge. This indicates the participation of the dehydrocyclization reaction of paraffins to form aromatics. 

Paraffins with more than eight carbon atoms can dehydrocyclize to form bicyclic products. According to Shuikin and Bekauri, bicyclic products can be formed from paraffins by either successive dehydrocyclization or by simultaneous closure of several carbon-carbon bonds (35). The second possibility follows Balandin s sextet model (56). A large number of hydrocarbons follow the consecutive mechanism (27). Thus far there is no evidence for simultaneous closure. 

Catalytic reforming rearranging hydrocarbon molecules in a gasoline-boiling-range feedstock to produce other hydrocarbons having a higher antiknock quality isomerization of paraffins, cyclization of paraffins to naphthenes (q.v.), and dehydrocyclization of paraffins to aromatics (q.v.). 

In the field of hydrocarbon conversions, N. D. Zelinskii and his numerous co-workers have published much important information since 1911. Zelinskii s method for the selective dehydrogenation of cyclohexanes over platinum and palladium was first applied to analytical work (155,351,438,439), but in recent years attempts have been made to use it industrially for the manufacture of aromatics from the cyclohexanes contained in petroleum. In addition, nickel on alumina was used for this purpose by V. I. Komarewsky in 1924 (444) and subsequently by N. I. Shuikin (454,455,456). Hydrogen disproportionation of cyclohexenes over platinum or palladium discovered by N. D. Zelinskii (331,387) is a related field of research. Studies of hydrogen disproportionation are being continued, and their application is being extended to compounds such as alkenyl cyclohexanes. The dehydrocyclization of paraffins was reported by this institute (Kazanskil and Plate) simultaneously with B. L. Moldavskil and co-workers and with Karzhev (1937). The catalysts employed by this school have also been tested for the desulfurization of petroleum and shale oil fractions by hydrogenation under atmospheric pressure. Substantial sulfur removal was achieved by the use of platinum and nickel on alumina (392). 

Supported chromia catalysts have a wide range of applications such as hydrogenation and dehydrogenation reactions of hydrocarbons, the dehydrocyclization of paraffins, dehydroisomerization of paraffins, olefins, and naphthenes, and the polymerization of olefins [1-3]. In order to improve the activity and selectivity, characterization of some critical parameters for both fresh and spent catalysts is necessary. 

The kinetics of 1-5 ring closure were investigated in parallel for aliphatic and aromatic hydrocarbons on Pt-C (755-757). The apparent activation energy for dehydrocyclization is always higher (by 7-15 kcal/mol) in the case of monosubstituted benzenes (n-propyl-, sec-butyl-, and isobutyl-benzenes) than in the case of paraffins (ethylpentane, isooctane, n-hexane). The same is not true, however, for dehydrocyclization of o-ethyltoluene and isooctane, which occur with similar activation energies (757). This result is quite understandable if one considers that the first elementary step in the dehydrocyclization of monosubstituted benzenes but not of disubstituted benzenes results in a loss of aromaticity. 

Figure 5 shows the relative catalytic activities for n-heptane dehydrocyclization to toluene and for dehydrogenation of cyclohexane. On this catalyst, dehydrocyclization of paraffins can be produced simultaneously by a monofunctional metallic mechanism and a bifunctional one controlled by the acid function (refs. 16-18). The deactivation produced by a small coke deposition should correspond to the deactivation of the contribution of the metallic mechanism to dehydrocyclization. This linear deactivation for the rest of the coke deposition should correspond to the deactivation of the contribution of the bifunctional acid controlled mechanism. The decrease in dehydrocyclization observed during the lineout period is smaller than the decrease in gas formation (by hydrocracking-hydrogenoly5is)i therefore, the selectivity to aromatic hydrocarbons increases during this period. 

A special case is the dehydrocyclization of paraffins—a complicated reaction similar to the previous one. Aromatic hydrocarbons are formed in it by way of dehydrogenation of the intermediate six-membered rings... 

Lately, dehydrocyclization has been extended to the closing of five and six-membered rings in paraffins with the formation of cyclo-pentanc hydrocarbons (151, 152). For the mechanism of these reactions see Section I,D. 

Cyclization—The conversion of aliphatic hydrocarbons containing six or more carbon atoms in a chain to the corresponding aromatic hydrocarbon is known as dehydrocyclization. The reaction sequence is believed to involve dehydrogenation of a paraffin to an olefin followed by ring closure and subsequent dehydrogenation of the ring compound to an aromatic. In many cases, isomerization (see 18) reactions also take place. 

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