Dehydrocyclization of paraffins

Increasing the octane number of a low-octane naphtha fraction is achieved by changing the molecular structure of the low octane number components. Many reactions are responsible for this change, such as the dehydrogenation of naphthenes and the dehydrocyclization of paraffins to aromatics. Catalytic reforming is considered the key process for obtaining benzene, toluene, and xylenes (BTX). These aromatics are important intermediates for the production of many chemicals. 

Aromatization. The two reactions directly responsible for enriching naphtha with aromatics are the dehydrogenation of naphthenes and the dehydrocyclization of paraffins. The first reaction can he represented hy the dehydrogenation of cyclohexane to benzene. 

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. 

It should be noted that both reactions leading to aromatics (dehydrogenation of naphthenes and dehydrocyclization of paraffins) produce hydrogen and are favored at lower hydrogen partial pressure. 

The second and third reactors contain more catalyst than the first one to enhance the slow reactions and allow more time in favor of a higher yield of aromatics and branched paraffins. Because the dehydrogenation of naphthenes and the dehydrocyclization of paraffins are highly endothermic, the reactor outlet temperature is lower than the inlet temperature. The effluent from the first and second reactors are reheated to compensate for the heat loss. 

The predominant reaction during reforming is dehydrogenation of naphthenes. Important secondary reactions are isomerization and dehydrocyclization of paraffins. All three reactions result in high-octane products. 

Other reactions may also occur. These include carbon formation, hydrocracking or thermal cracking, dehydrocyclization of paraffins to naphthenes, and dehydrogenation of naphthenes to aromatics. These have been discussed in the deactivation of reforming catalysts, in Section 2. 

McHenry and co-workers (Ml) have suggested that platinum on alumina catalysts, which are active for the dehydrocyclization of paraffins,... 

The formation of aromatics by the catalytic dehydrocyclization of paraffins with chains of six or more carbon atoms has been known for some time. Certain oxides of the 5th and 6th subgroups of the periodic table, such as chromia and molybdena, were shown early to be particularly effective catalysts for the reaction. Consequently, most of the reported studies of the kinetics and mechanism of the reaction have been carried out using these catalysts (P6, H4, H5). Since the available data on the kinetics of dehydrocyclization over oxide catalysts have been reviewed by Steiner (S9) in 1956, only a brief summary of the work will be made here, primarily for the purpose of orientation. The relatively few kinetic data which have been reported for dehydrocyclization over the bifunctional platinum on acidic oxide catalysts will be discussed subsequent to this. 

Bragin and co-workers found that over platinum-on-carbon catalysts, both paraffins and alkylaromatics follow zero-order kinetics. Activation energy for C5-dehydrocyclization in which the new bond is formed between two sp3 hybridized atoms is substantially less than the activation energy of cyclization in which the new bond is formed between one sp3 hybridized atom and the sp2 hybridized carbon atom of the aromatic ring. Over one batch of platinum-on-carbon catalyst, Bragin and co-workers obtained 20 kcal/mol and 27.5 kcal/mol activation energies for the dehydrocyclization of paraffins and monoalkylbenzenes, respectively (6). Another batch of platinum on carbon (which differed only in some minor details of preparation from the first batch), gave 14 kcal/mol for the cyclization of l-methyl-2-ethylbenzene and isooctane, and 29 kcal/mol for the cyclization of secondary butylbenzene ( ) (Fig. 1). 

There are at least two C6-dehydrocyclization mechanisms one of these proceeds through arylalkene intermediates and corresponds to the hexatriene-type C6-dehydrocyclization of paraffins. The other pathway is direct ring closure. It is probably related to C5-dehydrocyclization. 2-Butylnaphthalene may differentiate between the two mechanisms phenanthrene is probably formed by the first reaction, anthracene by the second. 

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). 

In his 1940 review Plate subjected the experimental material on dehydrocyclization of paraffins published to that time to a critical analysis (304) and concluded that aromatization of paraffins at the temperatures employed will depend upon the selection of proper catalysts in order to suppress the competing reactions, that the multiplet theory satisfactorily explains the mechanism of cyclization, and that intermediate formation of olefins is conceivable on oxide catalysts but can hardly occur on platinum. 

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. 

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. 

Dehydrogenation of cyclohexanes Isomerization/dehydrogenation of cyclopentanes Dehydrocyclization of paraffins... 

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... 

Catalytic reforming processes gasolines and naphthas from the distillation unit into aromatics. Four major reactions occur dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins to aromatics, isomerization, and hydrocracking. 

The aromatics are more stable than the paraffins at high temperatures (above 400°C). Therefore, the dehydrocyclization of paraffins is thermodynamically very feasible under these conditions. At low temperatures the opening of the aromatic ring and hydrogenation to the paraffin (eg, from benzene to n-hexane) is favored. 

The chemistry of catalytic reforming includes the reactions listed in Table 18. All are desirable except hydrocracking, which converts valuable Cs-plus molecules into light gases. The conversion of naphthenes to aromatics and the isomerization of normal paraffins provide a huge boost in octane. H2 is produced by dehydrocyclization of paraffins and naphthene dehydrogenation, which are shown in Figure 15. 

The commercial catalysts described and nsed in the Chevron Aromax process were partly exchanged withbarinm ions and contained platinum (0.6-0.8%) as small particles, corresponding to an approximate formula Pt/Ba2K5Al9Si270 72. Direct dehydrocyclization of paraffins is an alternative to isomerization and reforming to increase octane nnmber of light straight-run... 

---