Paraffins cyclization

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 (g.v.), dehy-drocyclization of paraffins to aromatics (g.v.). 

For 1-hexene isomerization and for acid catalyzed Cg aromatic reactions all molecular sieves were evaluated in their calcined, powdered state. For the study of Cg aromatics, selected SAPO molecular sieves were aluminum exchanged or steam treated as noted in Table IV. For bifunctional catalysts used in paraffin cyclization/isomerization and ethylbenzene-xylene interconversions, the calcined molecular sieve powder was mixed with platinum-loaded chlorided gamma alumina powder. These mixtures were then bound using silica sol and extruded to form 1/16" extrudates which were dried and calcined at 500°C. The bifunctional catalysts were prepared to contain about 0.54 platinum and about 40 to 504 SAPO molecular sieve in the finished catalysts. 

Paraffin cyclization and isomerization studies were also conducted in the microreactor system. The feed used in this study was technical grade n-hexane containing 874 n-hexane, 94 methyl cyclopentane and 44 isopentanes. Typically about 0.35 g of ground extrudates (20/80 mesh) were mixed with quartz chips and loaded to the reactor. The reactor was heated to the reaction temperature in flowing hydrogen, prior to hexane feed introduction. Runs lasted for 24 hours and samples were collected only after 20 hours on feed to permit catalyst line-out. Liquid and gaseous products were collected and analyzed as described above. 

The mechanistic steps are as follows paraffins dehydrogenate to olefins the olefins oligomerize and cyclize and the cycHcs aromatize. Because the first step is rate controlling, very Httie olefin is actually present. The BTX product is relatively free of nonaromatics and therefore is very desirable as a chemical feed. As in reforming, some C —C2 fuel gas is produced along with a valuable hydrogen stream. Prom a C —feed the BTX product is roughly 35 45 20, respectively. 

The cyclization step has been reported to work well in triglyme, mineral oil, paraffin, Dowtherm A , Ph20 or polyphosphoic acid (PPA). PPA has been used to promote the entire reaction in a single process (vide infra). 

A -acetyl groups attached to the aniline have been shown to withstand the Conrad-Limpach reaction. Phenols and alcohols also survived unless in proximity to a reactive center. Jaroszewski reported the formation of 64 by reaction of aniline 63 with ethyl acetoacetate (5). Cyclization under thermal conditions in paraffin gave a mixture of quinolone 65 and quinoline 66. 

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. 

More often than what has been mentioned above regarding the cyclization of paraffins over the platinum catalyst, the formed olefin species reacts with the acid catalyst forming a carbocation. Carbocation formation may occur by abstraction of a hydride ion from any position along the hydrocarbon chain. However, if the carbocation intermediate has the right configuration, cyclization occurs. For example, cyclization of 1-heptene over the alumina catalyst can occur by the following successive steps ... 

Aromatization of paraffins can occur through a dehydrocyclization reaction. Olefinic compounds formed by the beta scission can form a carbocation intermediate with the configuration conducive to cyclization. For example, if a carbocation such as that shown below is formed (by any of the methods mentioned earlier), cyclization is likely to occur. 

Figure 6. Proposed reaction scheme during dehydro-cyclization of n-paraffins over Te-NaX.

Whereas the cyclization of bis(aminomethylenemalonate) (168, R = H) by heating in paraffin oil at 255°C was not successful, the cyclization did occur in boiling diphenyl ether to give the angular 4,7-phenanthroline-2,9-dicarboxylate (802) in 88% yield (49JCS1017). When the condensation and ring closure were carried out in a one-pot procedure, starting from p-... 

Paracrystallinity, Cu/ZnO/AIjOj, 31 295 2"-Paracyclophanes, 32 453 Paracyclophanes, macrocycles, 29 206-208 Paraffin, see also Alkanes alkylation, 10 165, 27 98 carbon selectivity, bed residence time effects, 39 249-250 cracking, 39 283 cyclization, 28 295 rates, 28 301, 302 double cyclization, 28 312-314 in exhaust gases, 24 66, 67 hydrogenolysis, 30 43-44 hydroisomerization, 39 183 oxidation, 32 118-121 solubility enhanced hydrogeolysis, 39 285 Parahydrogen conversion rate correlations, 27 48-50... 

Light hydrocarbons (Ci to C4) and aromatics (mainly Ce to Ce) were produced by ZSM-5 due to the the conversion of olefins and paraffins. Thus,these results provide evidence for cracking of olefins, paraffins and cyclization of olefins by ZSM-5 at 500 C. The steam deactivated ZSM-5 catalyst exhibited reduced olefin conversion and negligible paraffin conversion activity. 

A 6-oxo bicycle (43) was also produced from base-induced cyclization of the N,N-disubstituted pyrazole 42. The isomer 45 was formed from the cro-tonate 44 in liquid paraffin at 250°C.46 Analogous reactions have been reported in the aminopyrazole series (Section II,7,a). 

Cyclization of ethyl 2-[(2-quinolyl)aminomethylene]cyanoacetates (225) at 100°C under reduced pressure (58JA3066), and by heating in paraffin oil at 320-340°C (74MIP1) or in dibenzylbenzene at 320°C (79MIP2), gave 1-oxo-lH-py rimido-[l, 2-a]quinoline-2-carbonitriles (226). 

The work of Herington and Rideal (H4) showed that a good prediction of the relative rates of cyclization of various paraffins could be made based on considerations of the number of ways in which ring closure could be effected, assuming that cyclization involves two-point adsorption in the manner illustrated in Fig. 8. For example, in the dehydro-cyclization of re-hexane, five two-point contacts with the surface are possible, but only two can lead to formation of a six-membered ring. By assuming the cyclization rate constant to be given by... 

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). 

Platinum-catalyzed cyclization of alkylaromatics is faster than the cyclization of paraffins because the presence of the aromatic ring enhances the rate. The rate of dehydrocyclization further increases with the number of aromatic rings in the feed molecule. A comparative study of the de-... 

C5- and C6- cyclizations are parallel reactions. Csicsery has shown that isomerization of tetralin to methylindan over platinum-alumina at 371°C is extremely slow (22). Davis and Venuto provided further evidence by showing that methylindan is also not converted to tetralin or naphthalene over platinum on silica-alumina (23). This behavior is similar to that observed in the cyclization of aliphatic hydrocarbons. Davis and Venuto also reported that the major aromatic products obtained from ten C8-C9 paraffins and olefins at 482°C are only formed by direct six-membered ring... 

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