Oxidation 9-xylene

As noted, for example by van der Wiele and van den Berg,4 the interaction between the molecule to be oxidized and the catalyst is based on the electron-donating and -accepting properties and can be described in terms of the acid-base properties. Activity and selectivity are thus expected to depend on the relative acidity and basicity. Thus Boreskov et al.80 observed a clear correlation between the acidity of a V205/Ti02 catalyst modified with different alkali metals and the selectivity for 0-xylene oxidation. The most acidic site was assumed to be responsible for the destructive oxidation. Thus the selectivity increased with the basicity, i.e., with the atomic number of the alkali metal. 

Figure llJ.c-4 0-xylene oxidation. Effect of hot spot on phthalic anhydride yield (from Froment [76]). 

Figure llJ.c-6 0-xylene oxidation. Effect of diluting the catalyst bed with inert material. 

Figure lL7,c-7 0-xylene oxidation. Predictions of one-dimen-sional model for influence of inlet temperature (from Fronient (76D. 

Commercial production of acetic acid has been revolutionized in the decade 1978—1988. Butane—naphtha Hquid-phase catalytic oxidation has declined precipitously as methanol [67-56-1] or methyl acetate [79-20-9] carbonylation has become the technology of choice in the world market. By-product acetic acid recovery in other hydrocarbon oxidations, eg, in xylene oxidation to terephthaUc acid and propylene conversion to acryflc acid, has also grown. Production from synthesis gas is increasing and the development of alternative raw materials is under serious consideration following widespread dislocations in the cost of raw material (see Chemurgy). 

This catalyst system is used in about 70% of the -xylene oxidations, and the percentage is increasing as new plants almost invariably employ it. Process conditions are highly corrosive owing to the acetic acid and bromine, and titanium must be used in contact with some parts of the process. 

Manufacture and Processing. Isophthahc acid is synthesized commercially by the Hquid-phase oxidation of / -xylene [108-38-3]. The chemistry of the oxidation is almost identical to that of -xylene oxidation to terephthahc acid, and production facihties can be used interchangeably for these two dicarboxyhc acids. However, because isophthahc acid is more soluble than terephthahc acid in reaction solvents as can be seen by comparing data in Tables 16 and 25, crystallization equipment is more important in isophthahc acid facihties. 

Another approach is to use an easily oxidized substance such as acetaldehyde or methylethyl ketone, which, under the reaction conditions, forms a hydroperoxide. These will accelerate the oxidation of the second methyl group. The DMT process encompasses four major processing steps oxidation, esterification, distillation, and crystallization. Figure 10-16 shows a typical p-xylene oxidation process to produce terephthalic acid or dimethyl terephthalate. The main use of TPA and DMT is to produce polyesters for synthetic fiber and film. 

In Figure 3 the merits of the two processes for p-xylene oxidation are compared. The main disadvantages of the Eastman Kodak/Toray cooxidation method are the need for a cosubstrate (acetaldehyde of methylethylketone) with concomitant formation of a coproduct (0.21 ton of acetic acid per ton product) and high catalyst concentration. The Amoco MC process, on the other hand, has no coproduct and much lower catalyst concentrations but has the disadvantage that the bromide-containing reaction mixture is highly corrosive, necessitating the use of a titanium-lined reactor. 

Terephthalic acid (p-TA or TA), a raw material for polyethylene terephthalate (PET) production, is one of the most important chemicals in petrochemical industry. Crude terephthalic acid (CTA), commonly produced by homogeneous liquid phase p-xylene oxidation, contains impurities such as 4-carboxybenzaldehyde (4-CBA, 2000-5000 ppm) and several colored polyaromatics that should be removed to obtain purified terephthalic acid (PTA). PTA is manufactured by hydropurification of CTA over carbon supported palladium catalyst (Pd/C) in current industry [1]. 

Davey JF, DT Gibson (1974) Bacterial metabolism of para- and mefa-xylene oxidation of a methyl substitnent. J Bacterial 119 923-929. 

The V20s/Si02 catalyst for o-xylene oxidation prepared by wet impregnation under microwave irradiation had several advantages [6] compared with that prepared by the conventional thermal method ... 

The diester/diacid component of PBT is made by oxidizing para-xylene. Oxidation followed by esterification leads to dimethyl terephthalate. 

Para-Xylene Oxidation (wt.%) Phenol Oxidation (wt.%) TBHP Conv. (TOF,h ) H2O2 Conv. (TOF,h )... 

Upon m-xylene oxidation, the above band of m-tolualdehyde disappears near 523 K, while a couple of strong and broad bands is grown near 1530 and 1430 cm. These bands are typical of carboxylates and are again observed, with very weak band shifts, upon oxidation of all methyl-benzenes, as well as of the corresponding aromatic aldehydes. They can be assigned predominantly (if not entirely) to benzoate and toluate anions (28) (in Figure 2, meta-toluate anions). These bands raise their maximum near 523 K in all cases and suddenly disappear above 673 K, when gas-phase CO2 begins to be detectable. 

However, features belonging to other species not involved in this path are also observed upon m-xylene oxidation in Near 433 K two other bands are clearly observed at 1710 and 1670 cm. A feature near 1700 cm persists also near 673 K when a very strong and complex absorption pattern becomes detectable in the 1900-1700 cm region. In this region the couples of bands due to symmetric and asymmetric C=0 stretchings of the 0=C-0-C=0 system of cyclic anhydrides typically fall. The... 

This scheme and our data agree with the product distribution in o-xylene oxidation reported by many authors (1-7) as well as with experiments of oxidation of intermediates (37) o-tolualdehyde and phthalide are observed as the main intermediates in the 523-573 K temperature range, while phthalic anhydride selectivity grows in the 473-573 K range and later only slightly decreases above 600 K when also maleic anhydride appears and conversion is very high. 

The kinetics of the p-xylene oxidation over tin vanadate was studied by Mathur and Viswanath [206], A differential reactor was used at 320— 380°C. p-Tolualdehyde, maleic anhydride and p-toluic acid are the main... 

The mechanism of the toluene and xylene oxidation bears a close resemblance to the oxidation of propene. Abstraction of a H-atom from the reactive methyl group and formation of a complex between the resulting radical and the catalyst is the first and probably the rate-determining step for both. However, the effect of the mesomeric stabilization of this radical complex is different. While a symmetrical allyl structure is formed from propene, an asymmetrical situation occurs for toluene and xylene, which is illustrated below for the case of toluene, viz. 

Not much is known concerning the mechanism of the oxidation of the nucleus. Complete oxidation is the main reaction while minor amounts of maleic anhydride are formed over some catalysts, in particular those based on V2Os. Blanchard and Vanhove [52] demonstrated with 14C labelling that, for o-xylene oxidation over V2Os, anhydride is exclusively formed from nuclear carbon atoms. This may be generalized to other methyl benzenes. 

The kinetics of the o-xylene oxidation appear to be rather complex due to the fact that several reaction steps seem more or less inhibited by reaction products. 

An ESR study by Yabrov et al. [355] revealed that, at least at low V205 content (0.05—5 wt. %), vanadium forms a solid solution of V4+ and V3+ in Ti02. The samples investigated were sealed in the reactor after steady state operation of the o-xylene oxidation at 350°C. The V4+ solid solution, which is considered the active phase, is not formed by the catalyst pretreatment at high temperature, but requires the interaction of the reaction mixture as was shown by the analysis of fresh catalysts. Solid state reactions between V2Os and Ti02 were also studied by Cole et al. [89]. 

Fig. 9. Selectivity and activity as a function of composition of V2Os— Ti02 catalysts for o-xylene oxidation.

Kinetic and mechanistic investigations on the o-xylene oxidation over V205—Ti02 catalysts were carried out by Vanhove and Blanchard [335, 336] using a flow reactor at 450°C. Possible intermediates like o-methyl-benzyl alcohol, o-xylene-a,a -diol, toluic acid and phthalaldehyde were studied by comparing their oxidation product distribution with that of toluene. Moreover, a competitive oxidation of o-methylbenzyl alcohol and l4C-labelled o-xylene was carried out. The compounds investigated are all very rapidly oxidized, compared with o-xylene, and essentially yield the same products. It is concluded, therefore, that these compounds, or their adsorbed forms may very well be intermediates in the oxidation of o-xylene to phthalic anhydride. The ratio in which the partial oxidation products are formed appears to depend on the nature of the oxidized compounds, i.e. o-methylbenzyl alcohol yields relatively more phthalide, whereas o-xylene-diol produces detectable amounts of phthalan. This... 

Regarding the kinetics, the oxidation of o-xylene and o-tolualdehyde were compared for catalysts with different V/Ti ratios (Table 36). The ratio between partial and complete oxidation (X for o-xylene and Y for o-tolualdehyde) are influenced similarly, indicating that a change in the catalyst structure influences all the reaction steps. The oxidation of o-tolualdehyde in mixtures with o-xylene revealed that o-tolualdehyde reduces the o-xylene oxidation rate by a factor of about 2. The authors conclude that a redox model is inadequate and that hydrocarbon adsorption cannot be rate-determining. Adsorption of various products should be included, and equations of the Langmuir—Hinshelwood type are proposed. It should be noted that the observed inhibition is not necessarily caused by adsorption competition, but may also stem from different... 

The gas phase oxidation of naphthalene to phthalic anhydride over V2Os-based catalysts is one of the oldest successful partial oxidation processes and is still of industrial importance today. Common commercial catalysts are modified silica-supported V—K—S—O catalysts and catalysts similar to those used for benzene or o-xylene oxidation. Maximum phthalic anhydride yields of 80—85 mol. % (92—98 wt. %) at 350—400°C are reported. By-products are naphthoquinone (2—5%), maleic anhydride (2— 5%) and carbon oxides. 

During the workup of the o-xylene oxidation run, a strong lachrymator made its presence felt. This was probably a-bromo-o-xylene, although it was not detected in the low voltage mass spectrum. We suspected that a strong peak at mass 104, undoubtedly caused chiefly by a fragment ion derived from o-methylbenzyl alcohol by loss of H20 (I), might also contain a contribution from benzocyclobutene from the interaction of a-bromo-o-xylene with the indium tube used to introduce samples into the spectrometer. To test this possibility, benzyl bromide and a-bromo-o-xylene were run separately under the same conditions. 

We have not been able to unscramble the complex kinetics of p-xylene oxidation. Ravens studied the second stage of oxidation, that of p-toluic acid in acetic acid with cobalt and manganese acetates and sodium bromide (25), and established the rate equation... 

Phosphorus Isocyanate. Reacts violently with acetic acid.13 Potassium feri-Butoxide. Can react violently with acetic acid.14 y>-Xylene. Oxidation with acetic acid can result in an explosive mixture if experimental conditions are not carefully controlled.15... 

---