N-Propylbenzene, isomerization

C5-cyclic intermediates are involved in the l-methyl-2-ethylbenzene n-propylbenzene isomerization ... 

An in situ 13C-NMR study of the mechanism of cumene - n-propylbenzene isomerization over H-ZSM-11... 

To foiiow the fate of carbon atoms in the alkyi chain or aromatic ring during isomerization, cumenes labeiled with at either a-, or p-position of the aikyl chain, or on the aromatic ring were prepared. As over ZSM-5 or ZSM-11 cataiysts, cumene is produced under excess of benzene in order to avoid propyiene oiigomerization and polymerization, the cumene - n-propylbenzene isomerization was studied under similar conditions. 

As the transition state in the cumene - n-propylbenzene isomerization is rather bulky,the selectivity could be altered by modifying the channel or cavity size of the microporous catalysts. 

This mechanism is analogous to that proposed by Shephard and Rooney (95) for the isomerization of o-ethyltoluene to n-propylbenzene on platinum, a reaction which most likely involves a cyclopentane intermediate and was assumed to consist of an alkene-alkyl insertion reaction of a l,2n,5ff-triadsorbed precursor (Scheme 41). 

The reaction pathway of benzene alkylation with propylene catalyzed by acids is very similar to that already reported for EB. The main difference is represented by the tendency of cumene to isomerize to n-propylbenzene, which is thermodynamically more stable at increased temperature. Also, cumene can undergo further alkylation to diisopropylbenzene (DIPB), which could be recovered by transalkylation with benzene to give cumene. The transalkylation reaction requires a higher temperature than the related alkylation. In addition, not all of the alkylation catalysts are suitable for transalkylation. Beta or dealuminated mordenite are suitable catalysts for transalkylation. The first industrial demonstrations of cumene technologies based on zeolite catalysts were started-up in 1996 by Mobil-Raytheon, EniChem and UOP, independently. In 2001, worldwide, 14 cumene units were already operating with zeolite catalysts. Around 98% ofcumene is used to produce phenol and expected world production of cumene in 2008 is around 9 million tons. For cumene, among the 40 units in the world (2004), 14 cumene plants were in operation with zeolite catalysts [222]. Today over 70% of cumene plants use a zeolite as the catalyst. 

Cumene conversion under excess of benzene was studied over H-ZSM-11 in the adsorbed phase at 473 K by in situ C MASNMR. To follow the fate of different carbon atoms during the reaction, cumenes labelled with C-isotopes either on a-or on p-positions of the alkyl chain or in the aromatic ring have been synthesized. The primary product of cumene conversion over H-ZSM-11 was found to be n-propylbenzene. It is formed via intermolecular reaction of cumene and benzene. At long reaction times, the formation of n-propylbenzene is accompanied by complete scrambling of both cumene and n-propylbenzene alkyl chain carbon atoms and formation of toluene, ethylbenzene and butylbenzene. The rate of isomerization is higher than the rate of scrambling and fragmentation. 

Cumene is an important intermediate in the industrial production of phenol, acetone and a-methylstyrene. The large-scale production of cumene is based on the alkylation of benzene with propene over Friedel-Crafts [1] or phosphoric acid on silica catalysts [2]. Zeolites, namely ZSM-5 and ZSM-11, have also been shown to be potential catalysts for this process [3, 4]. However, the formation of cumene (isopropylbenzene. IPB) on this catalysts is accompanied by its isomerization to n-propylbenzene (NPB). The latter is considered as an undesired by-product with respect to further processing of cumene to phenol and acetone. Therefore, preventing the formation of NPB would enable the substitution of the current catalysts used in the industrial process by ZSM-5 or ZSM-11 type solid acids which have major advantages in terms of environmental protection, safety, and avoidance of corrosion. 

At longer reaction times, the formation of n-propylbenzene is accompanied by complete scrambling of the carbon atoms in the alkyl chains of cumene and n-propylbenzene, and by fragmentation towards T, EB and BB. The rate of isomerization is higher than the rate of scrambling and fragmentation. 

This reaction was first reported by Baddeley in 1930s. It is the migration of alkyl groups in polyalkylbenzenes or polynuclear aromatic compounds in the presence of anhydrous aluminum chloride or the mixture of protonic acid and Lewis acid. In one of Badde-ley s experiments, when 1,3,4-tri-n-propylbenzene was warmed with AICI3 at 100°C, the 1,3,5-tri-n-propyl was formed, along with the lower and higher alkylated benzenes. After extensive studies, it was found that the amount of a-isomer can be reduced by the addition to the reaction mixture of a variety of substances (e.g., nitrobenzene and excess acid chloride) that complex strongly with aluminum chloride. Likewise, less a-isomer has been observed when this reaction is carried out in nitrobenzene. In addition, isomerization of hindered aromatic ketones occurs if the ketones are melted with an excess amount of aluminum chloride and sodium chloride. ... 

The reaction is simply illustrated by the isomerization of 1,2,4-tri-n-propylbenzene to 1,3,5-tri-n-propylbenzene in Baddeley s initial work. 

A 100% methanol mobile phase was used in the study of the retention characteristics for over 50 alkyl-substituted benzenes on a porous graphite column [109]. Retention times varied from 1.8 min k = 0.2) for benzene (just slightly more than the void volume) to 58 min (k = 3 7) for pentamethylbenzene. A distinct advantage of this system over a normal Cjg system was the enhanced selectivity of the graphite column towards both isomeric forms as well as methyl and methylene homo logs. The k values and retention times for all analytes are tabulated. The retention differences between polymethylbenzenes and alkyl-substituted benzenes of the same carbon number (e.g., trimethylbenzenes vs. n-propylbenzene) were studied in detail on Cjg and phenyl columns using an 80/20 methanol/water mobile phase. 

Benzene (1 g, 12.5 mmol) is allowed to react with 1-chloropropane (1 g, 12.5 mmol) and AICI3. The product (1.2 g) is subjected to analysis on a GLC equipped with a thermal conductivity detector. The chromatogram shows two product peaks identified as n-propylbenzene (area = 65 mm = 1.06) and isopropylbenzene (area = 113 mm = 1.09). Calculate the percent yield of each of the two isomeric products obtained in this reaction. Note that since each of the products has the same molar mass of 120, the use of weight factors gives both weight and mole percent composition. 

Studies were made of the behavior of both isomeric propylbenzenes in contact with samples of nickel-alumina catalyst containing 10, 20, and 30 % Ni. The hydrocarbons were passed over the catalysts at the rate of 0.8 hr."i with a hydrogen-hydrocarbon ratio of 5 1. The runs with n-propyl-benzene were made at 465° and 25 and 50 atmospheres hydrogen pressure. 

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