Paraxylene

The early sihca/alumina catalysts for the isomerization of xylene suffered from deactivation due to the deposition of carbon and the needed frequent regeneration. The process was improved by both the use of catalysts impregnated with platinum and the addition of hydrogen to the reactants, and thrrs led to a reduction in the need for frequent regeneration. These catalysts also converted ethyl benzene to xylenes. High-silica zeolites are now used to produce most of the / -xylene obtained by isomerization, because high selectivity can be achieved of equilibrium conversion. Mobil ZSM-5 is particularly useful because the pore size promotes paraselectivity and controls the unwanted disproportionation reac- 

Silica/alumina 400°-500°C, atmospherie pressure Carbon deposition No ethylbenzene conversion 

Platinum/silica alumina 430°-450°C, 10-25 atm Hydrisomerization (+ H2) No carbon—long life Ethylbenzene conversion 

1% Platinum/AI2O3 + mordenite 425°C, 12 atm Ammonia injection to control zeolite acidity/activity 

Platinum exchanged ZSM-5 420°- 60 C, 14-18 atm Hydrisomerization (+H2) Equilibrium shifted to increase conversion by 4% 

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Ethylbenzene is separated from mixed xylenes by fractionation using 360 trays and a high reflux ratio. Ethylbenzene is separated from the closest isomer paraxylene whose normal boiling point is only 3.90°F higher. The average relative volatility between ethylbenzene and paraxylene in the fractionation is about 1.06. The fractionator feed is entirely Cg aromatics which are prepared by the extraction of powerformate by the sulfolane process and by fractionation of the aromatic extract. 

Paraxylene is recovered from Cg aromatics by crystallization. Fortunately, the solidification point of the para isomer is unusually high, -1-55.9°F, considerably above the meta and orthoxylenes which freeze at -54.2 and -13.3°F, respectively. The separation of para from meta by distillation is impractical because the spread in their normal boiling points is only 1.4°F. 

Paraxylene is used to make terephthalic acid, the raw material for manufacturing polyester fibers, such as Dacron, Kodel, Fortrel and Terylene. 

In order to produce more paraxylene than is available in catalytic reformate, a xylenes-isomerization plant is sometimes included in the processing scheme. The isomerization step uses the effluent (filtrate) from the paraxylene crystallization step as feed. The filtrate contains about 7-9 percent of paraxylene. The isomerization unit brings the concentration back to its equilibrium value of about 20 percent. 

Para-verblndung, /. para compound, -wein-saure, /. paratartaric acid (racemic acid), -xylol, n. paraxylene, p-xylene. parazentrisch, a. paracentric. 

Terephthalic acid is an important monomer for producing polyesters. The main route for obtaining the acid is the catalyzed oxidation of paraxylene. It can also be produced from benzoic acid by a disproportionation reaction of potassium benzoate in the presence of carbon dioxide. Benzene is the coproduct ... 

Figure 25. Ion mass peaks at different two-photon energies. Broadenings of trimethylamine (TMA ) ion peaks as a function of the ionization energy. A hv2 > 3.875 eV B hv2 = 3.688 eV C hv2 = 3.607 eV. Excitation energy of paraxylene (PX) in the Si state = 3.90 eV. The broadenings in B and C correspond to time constants of 160 20 and 200 20 ns, respectively. Peaks corresponding to TMA H+ are also observable. Taken with permission from Int. J. Mass Spectrom. Ion Proc. 1994, 131, 233-264.

Paraxylene and Derivatives - World Supply and Demand Report, PCI-Xylenes and Polyesters Ltd, Guildford, Surrey, UK. 

Present developments. One might think that an established reaction such as aerobic oxidation (or autoxidation) is not the subject of further research and improvement, but this is definitely not the case and both new homogeneous and heterogeneous catalysts are in development. In the introduction we already mentioned the drawbacks of oxidation of cyclohexene to adipic acid and several researchers address this challenge. Also a highly developed reaction such as the oxidation of paraxylene is subject to further improvements. 

S.13i. Terephthalic acid (TPA) is produced by air oxidation of paraxylene at 200 psig. The reaction is exothermic. Nitrogen plus excess oxygen leave the top of the reactor. 

Plee, D. and Methivier, A. (2002) Agglomerated zeolite adsorbents, process for their preparation, and their use for adsorbing paraxylene from aromatic C8 fractions. U.S. Patent 5,410,815. 

Cain, J.J. (2001) Adsorption process for paraxylene purifacation using Cs SSZ-25 adsorbent with benzene desorbent. U.S. Patent 5,281,405. 

Ordering of vacancies also plays a key role in selective oxidation catalysis over perovskite-based catalysts such as CaMnOs oxides. CaMnOs has a CaTiOs (AMO3) perovskite structure which is made up of cations coordinated to 12 0 anions. They, in turn, are connected to corner-sharing MoOe octahedra. CaMnOs was used as a model catalyst on a laboratory scale by Thomas et al (1982) in propene oxidation to benzene and 2-methyl propene to paraxylene. In such reactions the compounds are found to undergo reduction to form anion-deficient metastable phases of the type CaMnOs-x where 0 < x < 0.5, forming several distinct phases. 

Synonym Parathion-Methyl Paraxylene Paris Green Parrot Green Patent Alum PCB PE... 

Displacement of the adsorbate with another substance that is in turn displaced in process is practiced, for instance, in liquid phase recovery of paraxylene from other Cg aromatics. In the Soibex process, suitable desorbents are toluene and paradiethylbenzene. This process is described later. 

Figure 15.19. The Toray simulated continuous adsorption process, (a) Showing the main valving for a seven-chamber adsorption system [Otani et al., U.S. Pat. 3,761,533, (25 Sep. 1973)]. (b) Flowsketch for recovery of paraxylene by continuous adsorption [Otard et al., Chem. Economy Eng. Rev. 3(6), 56-59 (1971)].

Parathyroid hormone Parathyroid hormone Paraxylene Parazylene Parchment... 

Paraxylene and air comprise the chargestock. These materials, along with a proprietary catalyst and solvent, are fed to a liquid-phase oxidation reactor, operated at moderate pressure and temperature. The reaction is ... 

The synthesis of paratolunitrile (PTN) and terephtalonitrile (TPN) by reaction of paraxylene with nitrogen monoxide was studied over a series of aerogel chromium oxide alumina catalysts. The stabilization of the active phase was interpreted on the basis of Cr O support interactions. Kinetic studies show that the reaction follows a "redox" mechanism for the formation of PTN and a Langmuir Hinshelwood mechanism for the production of TPN. 

Earlier work in this laboratory showed that chromium oxide supported on alumina is a good catalyst for the conversion of olefins (ref. 1) as well as paraffins (ref. 2) to nitriles with high selectivities, by reaction of NO with the hydrocarbons (nitroxidation). Recent work (ref. 3) reported preliminary results of the nitroxidation of paraxylene as an extension of the use of C Oj-Al Oj to the catalytic synthesis of aromatic nitriles. It should be mentioned that only few data are available in the literature related to the nitroxidation of aromatic hydrocarbons. Teichner et al (ref. 4 ) reported interesting results of selective synthesis of benzonitrile by nitroxidation of toluene on NiO-AlgO catalysts. Improvements of the catalytic activity and selectivity in this reaction were reached by use of C Og-Al. which also exhibits striking properties in the synthesis of paratolunitrile by contact of NO with paraxylene (ref. 3). 

The present paper deals with the extension of the previous and preliminary study of the nitroxidation of paraxylene on OgOj-A Og (ref. 3), in order to... 

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