Dehydrocyclization, paraffins catalysts

Dehydrocyclization refers to the conversion of feed paraffins into alkylcyclohexane and alkylcyclopentane naphthenes. These, in turn, are subsequently converted by isomerization and dehydrogenation into aromatics. Dehydrocyclization is controlled by both acid and platinum functions and is the most sensitive indicator of catalyst selectivity. 

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. 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

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). 

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. 

The results suggest that it may be fhiitful to search for reactions for which supported metal clusters have catalytic properties superior to those of conventional supported metals. The important opportunity in catalysis may be to find reactions for which the activity or selectivity of supported metal clusters is superior to those of conventional supported metals. The high selectivity of Pt/LTL zeolite catalysts for paraffin dehydrocyclization, which is now exploited... 

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. 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

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. 

Coke Deposition on the Catalytic Functions and Their Deactivations. The acid and metal functions of the reforming catalysts are balanced to have the highest possible yield in the bifunctional reaction of paraffin dehydrocyclization. Coke is deposited on both acid and metal sites, decreasing their catalytic activities. It is important to know (under commercial conditions) the deactivation on both sites and which controls the reactions and fixes the catalyst s practical cycle. If the rate of reaction is several orders of magnitude higher on one site than on the other, the deactivation of the first site does not modify the rate of the bifunctional reaction and the deactivation of the second will control the whole reaction. If a reaction is so rapid on a catalytic site that it is under thermodynamic equilibrium, the deactivation of the site will not be noticed if the reaction is kinetically controlled, it is possible to follow the site deactivation by means of this reaction change. 

Zeolites were used in various processes that convert paraffins or olefins into alkyl monoaromatics containing chiefly from six to nine carbon atoms.There are various catalysts, and they involve a base or an acid zeolite, according to the type of process. The first catalyst, identified at the end of the 1970s. is composed of Pt deposited on the L zeolite (Table 2) exchanged with large alkaline ions such as potassium or alkaline earths, such as Ba. which gives it an alkaline nature. This cataiyst is monofunctional and is conceptually different from the conventional bifunctional acid catalysts based on Pt on chlorinated alumina. It selectively dehydrocyclizes the paraffins into aromatics, particularly hexane, which is the least reactive of them. The reaction takes place on the metal, which develops a special selectivity in contact with the alkaline zeolite. This aromatization process has not been successful so far. partly because of the extreme sensitivity of the catalyst to the slightest trace of sulfur compounds. 

The paraffin dehydrocyclization reaction is slower than the dehydrogenation of cyclohexanes, dehydroisomerization of cyclopentane, and isomerization of paraffins. Because the rate is not so high, it is not possible to reach the equilibrium conversion. The larger fraction of the transformation of the paraffins into aromatics occurs in the last reactor of the reforming unit, in which the largest fraction of catalyst is loaded, and the higher operation temperature is used. 

The isoparaffins dehydrocyclize at a lower rate than the n-paraffin wdth the same number of carbon atoms, due to the lower possibility of ring closure wdthout previous rearrangement. For example, the 2,2,4-trimethylpentane cannot dehydrocyclize. It has to be first rearranged to an iso-hexane or iso-heptane compoimd, which does not readily occur with commercial reforming catalysts. 

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