P-Xylene hydrogenation

This is manufactured in three steps ammoxidation of p-xylene, hydrogenation of the resulting dinitrile to diamine, oxygenation of diamine. 

The model reaction of p-xylene hydrogenation was chosen in order to provide the mild conditions of the experiments in both gas and capillary condensed phases, and to avoid the influence of side reaction and catalyst deactivation. The recycle type of gradientless reactor was used that provides uniform temperature and concentration profiles within all the catalyst packing. The catalyst particles (0.25-0.50 mm) provide a negligible intraparticle limitation of mass- and heat-transfer. 

Bukhavtsova and Ostrovskii [9] have studied the effect of capillary condensation on the reaction rate of p-xylene hydrogenation in transitions by variations of the feedstock flow and temperature. In order to avoid the intraparticle diffusion limitation, the crushed catalyst particles were used (0.25-0.5 mm). 

That the reaction with a lower rate constant is taking place preferentially and that the rate increases during the reaction are phenomena that can also occur with parallel reactions. As an example, Wauquier and Jungers (48), when studying competitive hydrogenation of a series of couples of aromatic hydrocarbons on Raney-nickel, have observed these phenomena for the couple tetraline-p-xylene (Table I). The experimental result was... 

First, the kinetics of the reactions of 0-, m-, and p-xylene as well as of toluene were studied separately (96) at various combinations of initial partial pressures of the hydrocarbon and hydrogen. From a broader set of 23 rate equations, using statistical methods, we selected the best equations for the initial rate and determined the values of their constants. With xylenes and toluenes, these were Eqs. (17a) and (17b). 

Iodo-p-xylene has been prepared by the action of potassium iodide on diazotized p-xylidine (2,5-dimethylbenzenamine) (21% yield),5 from p-xylene with molecular iodine in concentrated nitric acid (50% yield)6 or in ethanol-sulfuric acid in the presence of hydrogen peroxide (64% yield),7 and with molecular iodine in glacial acetic acid-sulfuric acid in the presence of iodie acid as a catalyst (85% yield).8... 

Iridium and rhodium nanoparticles have also been studied in the hydrogenation of various aromatic compoimds. In all cases, total conversions were not observed in BMI PF6. TOFs based on mol of cyclohexane formed were 44 h for toluene hydrogenation with Ir (0) and 24 h and 5 h for p-xylene reduction with lr(0) or Rh(0) nanoparticles, respectively. The cis-1,4-dimethylcyclohexane is the major product and the cisitrans ratio depends on the nature of the metal 5 1 for lr(0) and 2 1 for Rh(0). TEM experiments show a mean diameter of 2.3 nm and 2.1 nm for rhodium and iridium particles, respectively. The same nanoparticle size distribution is observed after catalysis (Fig. 4). 

Finally, Jessop and coworkers describe an organometalhc approach to prepare in situ rhodium nanoparticles [78]. The stabilizing agent is the surfactant tetrabutylammonium hydrogen sulfate. The hydrogenation of anisole, phenol, p-xylene and ethylbenzoate is performed under biphasic aqueous/supercritical ethane medium at 36 °C and 10 bar H2. The catalytic system is poorly characterized. The authors report the influence of the solubility of the substrates on the catalytic activity, p-xylene was selectively converted to czs-l,4-dimethylcyclohexane (53% versus 26% trans) and 100 TTO are obtained in 62 h for the complete hydrogenation of phenol, which is very soluble in water. 

Fig. 5. The 1 2 host-guest complex (l-naphthyl)3SiOH-2p-xylene with hydrogen atoms omitted. The p-xylene molecules are shown drawn with solid lines. Drawn using coordinates taken from the Cambridge Crystallographic Database.

Octafining A process for isomerizing m-xylene to o- and p-xylene, developed by the Atlantic Richfield Company in 1960. The catalyst was originally platinum on an aluminum silicate base now a zeolite base is used. The reaction takes place in a hydrogen atmosphere. Hydrocarbon Research installed units in Argentina and the USSR. 

To simplify the synthetic effort required to deposit such films, attempts were made to deposit films by pyrolyzing tetrafluoro-p-xylene (F4C8H6). Under similar reaction conditions, a polymer film was deposited that was different from poly(tetrafluoro-p-xylylene) as the FTIR spectrum indicates that it contains more hydrogen and less fluorine. Presumably HF is preferentially eliminated rather than H2. 

Meanwhile attempts to find an air oxidation route directly from p-xylene to terephthalic acid (TA) continued to founder on the relatively high resistance to oxidation of the /Moluic acid which was first formed. This hurdle was overcome by the discovery of bromide-controlled air oxidation in 1955 by the Mid-Century Corporation [42, 43] and ICI, with the same patent application date. The Mid-Century process was bought and developed by Standard Oil of Indiana (Amoco), with some input from ICI. The process adopted used acetic acid as solvent, oxygen as oxidant, a temperature of about 200 °C, and a combination of cobalt, manganese and bromide ions as catalyst. Amoco also incorporated a purification of the TA by recrystallisation, with simultaneous catalytic hydrogenation of impurities, from water at about 250 °C [44], This process allowed development of a route to polyester from purified terephthalic acid (PTA) by direct esterification, which has since become more widely used than the process using DMT. 

More recently Hartog and Zwietering (103) used a bromometric technique to measure the small concentrations of olefins formed in the hydrogenation of aromatic hydrocarbons on several catalysts in the liquid phase. The maximum concentration of olefin is a function of both the catalyst and the substrate for example, at 25° o-xylene yields 0.04, 1.4, and 3.4 mole % of 1,2-dimethylcyclohexene on Raney nickel, 5% rhodium on carbon, and 5% ruthenium on carbon, respectively, and benzene yields 0.2 mole % of cyclohexene on ruthenium black. Although the cyclohexene derivatives could not be detected by this method in reactions catalyzed by platinum or palladium, a sensitive gas chromatographic technique permitted Siegel et al. (104) to observe 1,4-dimethyl-cyclohexene (0.002 mole %) from p-xylene and the same concentrations of 1,3- and 2,4-dimethylcyclohexene from wi-xylene in reductions catalyzed by reduced platinum oxide. 

The aromatics complex converts approximately 75% of the feed naphtha to petrochemical aromatics with the vast majority of the remainder being exported as raffinate and some hydrogen. With a modern aromatics complex flowscheme, a little over half of the mixed xylenes are produced in the Tatoray unit while the rest are produced in the CCR Platforming unit directly from the naphtha reforming. Having reviewed the framework of an aromatics complex we are now in a better position to understand the context of the continuous countercurrent liquid adsorptive Parex process which produces the primary aromatics complex product, p-xylene. 

Chlorine atoms react with aromatic hydrocarbons, but only at a significant rate with those having saturated side chains from which the chlorine atom can abstract a hydrogen or unsaturated side chains to which it can add. For example, the rate constant for the Cl atom reaction with benzene is 1.3 X 10"15 enr3 molecule-1 s-1 (Shi and Bernhard, 1997). On the other hand, the rate constants for the reactions with toluene and p-xylene are 0.59 X 10-10 and 1.5 X 10-l() enr3 molecule"1 s"1, respectively (Shi and Bernhard, 1997), and that for reaction with p-cymene is 2.1 X 10"10 cm3 molecule"1 s-1 (Finlayson-Pitts et al., 1999). Hence... 

Silica-alumina, either amorphous catalysts or zeolites, are used in several processes.261,282-285 Mobil developed several technologies employing a medium-pore ZSM-5 zeolite. They use a xylene mixture from which ethylbenzene is removed by distillation, operate without hydrogen, and yield p-xylene in amounts... 

Toluene disproportionation and transalkylation are important industrial processes in the manufacture of p-xylene. Toluene disproportionation [Eq. (5.73)] transforms toluene into benzene and an equilibrium mixture of isomeric xylenes. The theoretical conversion of toluene is 55%. Commercial operations are usually run to attain 42 18% conversions. In conventional processes308 309 324 325 alumina-supported noble metal or rare-earth catalysts are used in the presence of hydrogen (350-... 

The product mixture, consisting mainly of p-toluic acid, monomethyl terephtha-late, and some terephthalic acid, is esterified and methyl p-toluate is recycled after separation from the end-product dimethyl terephthalate. The special advantage of cooxidation is that the hydroperoxy radical (111) formed in the autoxidation of p-xylene participates in initiation through hydrogen abstraction from methyl p-toluate, thus promoting oxidation of the latter ... 

Accordingly, hydrogenation of p-xylene yields a 1 3 mixture of cis- and trans-1,4-dimethylcyclohexane. Both observations suggest that dissociation and readsorption of the partially reduced arene take place. 

The hydrogenation of aromatics (benzene, toluene, a-methylstyrene) can be carried out under very low (1 12,000) catalyst substrate ratio, and mild conditions on Rh and Ni organometallic complexes anchored to USY zeolites.474 A Rh complex anchored to functionalized MCM-41 exhibits excellent performance in the hydrogenation of arenes (benzene, toluene, p-xylene, mesitylene) under mild conditions (45°C and 1 atm) 475 A uniquely selective hydrogenation of acenaphthene and acenaphthylene was performed by using a triruthenium carbonyl cluster 476... 

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