A- -o-xylene

Figure 5.76. The fixed-bed reactor and the reaction scheme. Symbols A = o-xylene, B phthalic anhydride, C = waste gaseous products (CO2 and CO).

Symbols A=o-xylene, B = phthalic anhydride, C=waste gaseous products (C02 and CO). 

Figure 1. FT-IR spectra of the surface species arising from adsorption of toluene (A), o-xylene (B), m-xylene (C) and p-xylene (D) on vanadia-titania. Full lines after outgassing at 350 K.

Figure 6.13.1 Axial profiles of temperature (tube axis) (a), o-xylene conversion (b), and selectivity to PA (c) in the multi-tubular reactor for catalytic o-xylene oxidation with different inlet temperatures [fin = Tool, two-dimensional model, Eqs. (6.13.3), (6.13.15)-(6.13.22) parameters seeTable 6.13.1].

Figure 13.5 shows a flowsheet for the manufacture of phthalic anhydride by the oxidation of o-xylene. Air and o-xylene are heated and mixed in a Venturi, where the o-xylene vaporizes. The reaction mixture enters a tubular catalytic reactor. The heat of reaction is removed from the reactor by recirculation of molten salt. The temperature control in the reactor would be diflficult to maintain by methods other than molten salt. 

Oxidation of a side chain by alkaline permanganate. Aromatic hydrocarbons containing side chains may be oxidised to the corresponding acids the results are generally satisfactory for compounds with one side chain e.g., toluene or ethylbenzene -> benzoic acid nitrotoluene -> nitrobenzoic acid) or with two side chains e.g., o-xylene -> phthalic acid). 

In the nitration and acetoxylation of o-xylene the addition of acetic acid increased the rate in proportion to its concentration, the presence of 3-0 mol 1" accelerating the rate by a factor of 30. In the presence of a substantial concentration (2-2 mol 1 ) of acetic acid the rate of reaction obeyed the following kinetic expression... 

More information has appeared concerning the nature of the side reactions, such as acetoxylation, which occur when certain methylated aromatic hydrocarbons are treated with mixtures prepared from nitric acid and acetic anhydride. Blackstock, Fischer, Richards, Vaughan and Wright have provided excellent evidence in support of a suggested ( 5.3.5) addition-elimination route towards 3,4-dimethylphenyl acetate in the reaction of o-xylene. Two intermediates were isolated, both of which gave rise to 3,4-dimethylphenyl acetate in aqueous acidic media and when subjected to vapour phase chromatography. One was positively identified, by ultraviolet, infra-red, n.m.r., and mass spectrometric studies, as the compound (l). The other was less stable and less well identified, but could be (ll). 

Benzene, toluene, and a mixed xylene stream are subsequently recovered by extractive distillation using a solvent. Recovery ofA-xylene from a mixed xylene stream requires a further process step of either crystallization and filtration or adsorption on molecular sieves. o-Xylene can be recovered from the raffinate by fractionation. In A" xylene production it is common to isomerize the / -xylene in order to maximize the production of A xylene and o-xylene. Additional benzene is commonly produced by the hydrodealkylation of toluene to benzene to balance supply and demand. Less common is the hydrodealkylation of xylenes to produce benzene and the disproportionation of toluene to produce xylenes and benzene. 

The first of the benzene polycarboxyUc acids to become a commercial product was phthabc acid, mosdy in the form of the anhydride. The anhydride is obtained by the catalytic vapor-phase air oxidation of o-xylene or naphthalene. The lUPAC name of phthabc anhydride is 1,3-isobenzofurandione... 

Naphthalene (qv) from coal tar continued to be the feedstock of choice ia both the United States and Germany until the late 1950s, when a shortage of naphthalene coupled with the availabihty of xylenes from a burgeoning petrochemical industry forced many companies to use o-xylene [95-47-6] (8). Air oxidation of 90% pure o-xylene to phthaUc anhydride was commercialized ia 1946 (9,10). An advantage of o-xylene is the theoretical yield to phthaUc anhydride of 1.395 kg/kg. With naphthalene, two of the ten carbon atoms are lost to carbon oxide formation and at most a 1.157-kg/kg yield is possible. Although both are suitable feedstocks, o-xylene is overwhelmingly favored. Coal-tar naphthalene is used ia some cases, eg, where it is readily available from coke operations ia steel mills (see Steel). Naphthalene can be produced by hydrodealkylation of substituted naphthalenes from refinery operations (8), but no refinery-produced napthalene is used as feedstock. Alkyl naphthalenes can be converted directiy to phthaUc anhydride, but at low yields (11,12). 

U.S. capacity is 455 x 10 t among five producers, as shown in Table 9 (33). All U.S. producers use o-xylene as a feedstock, although Koppers can switch to coal-tar naphthalene. Europe has 20 producers operating 25 plants, 18 of which use o-xylene. Demand growth in North America has averaged less than 3% annually since 1981. Prices in 1992 were 0.77—0.95/kg for molten, and 0.82—1.01/kg for flake (33). 

Vapor-phase catalytic oxidation of dutene is a mote direct route to the dianhydtide. Hbls in Europe apparently uses this route, which eliminates the need for a separate dehydration step and for handling of any oxidants or solvents. Continuous operation is faciHtated, corrosion is minimized, and product recovery is simplified. The vapor-phase oxidation of dutene is similar to that of o-xylene to phthaHc anhydtide, and phthaHc anhydtide units can be... 

Another sulfur dioxide appHcation in oil refining is as a selective extraction solvent in the Edeleanu process (323), wherein aromatic components are extracted from a kerosene stream by sulfur dioxide, leaving a purified stream of saturated aHphatic hydrocarbons which are relatively insoluble in sulfur dioxide. Sulfur dioxide acts as a cocatalyst or catalyst modifier in certain processes for oxidation of o-xylene or naphthalene to phthaHc anhydride (324,325). 

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