Toluene oxidation to benzoic acid

The oxidation of / -xylene to terephthalic acid is by far the most important process based on the oxidation of methyl aromatics. However other similar processes are also operated industrially and oxidize toluene to benzoic acid or m-xylene to isophthalic acid. The latter is used as comonomer with terephthalic acid in bottles for carbonated drinks, and for special polyesters, and its production is roughly 2% of that of the terephthalic derivatives. 

Isolated V oct. sites are probably responsible for the selective behavior in toluene oxidation to benzaldehyde as indicated by the characterization of Vx-HMS catalysts. It should be remarked that unsupported or supported vanadium oxides oxidize toluene to benzoic acid [12] or a mixture of benzaldehyde and benzoic acid [2], whereas all V-containing zeolites tested form only benzaldehyde. The nature of the zeolite influences the nature of isolated species, as well as the ratio between isolated to polynuclear vanadium species. 

Several other companies oxidize toluene to benzoic acid, as above. Except for Dow and Snia Viscosa (nylon-6), outlets are mainly into speciality uses. Benzaldehyde is often recovered as a by-product, while Rhone-Poulenc and others oxidize toluene to give mainly benzaldehyde and benzyl alcohol (phenylmethanol). However, many derivatives, including the major benzyl esters, are produced via the side-chain chlorination of toluene. 

Strong oxidizing agents oxidize toluene to benzoic acid. The oxidation can be carried out by the action of hot alkaline potassium permanganate. This method gives benzoic acid in almost quantitative yield ... 

Molecular oxygen oxidizes toluene to benzoic acid and p-xylene to terephthalic acid via p-toluic acid. These reactions are catalyzed by [Co(OAc)3] or [Mn(OAc)3]. The Mn salts, less oxidizing than those of Co , catalyze the oxidation of these alkylaromatics, but not the oxidation of alkanes whose redox potentials are higher than those of alkylaromatics. It is probable that the M ions are regenerated from and the ArCH200" radicals (mechanism below). 

Oxygen Nucleophiles. A reagent such as permanganate oxidizes toluene to benzoic acid, whereas benzylic oxidation by palladium acetate results in benzyl alcohol derivatives. The oxidation is favored by electron-releasing substituents in the phenyl ring. Catalytic amounts of palladium acetate and tin diacetate, in combination with air, effects an efficient palladium-catalyzed benzylic oxidation of toluene and xylenes. For the latter substrates, the Q, Q -diacetate is the main product.A mixed palladium diacetate-copper diacetate catalyst has also been found to selectively catalyze the benzylic acyloxylation of toluene (eq 64). ... 

Obtained synthetically by one of the following processes fusion of sodium ben-zenesulphonate with NaOH to give sodium phenate hydrolysis of chlorobenzene by dilute NaOH at 400 C and 300atm. to give sodium phenate (Dow process) catalytic vapour-phase reaction of steam and chlorobenzene at 500°C (Raschig process) direct oxidation of cumene (isopropylbenzene) to the hydroperoxide, followed by acid cleavage lo propanone and phenol catalytic liquid-phase oxidation of toluene to benzoic acid and then phenol. Where the phenate is formed, phenol is liberated by acidification. 

The three chemical reactions in the toluene—benzoic acid process are oxidation of toluene to form benzoic acid, oxidation of benzoic acid to form phenyl benzoate, and hydrolysis of phenyl benzoate to form phenol. A typical process consists of two continuous steps (13,14). In the first step, the oxidation of toluene to benzoic acid is achieved with air and cobalt salt catalyst at a temperature between 121 and 177°C. The reactor is operated at 206 kPa gauge (2.1 kg/cm g uge) and the catalyst concentration is between 0.1 and 0.3%. The reactor effluent is distilled and the purified benzoic acid is collected. The overall yield of this process is beheved to be about 68 mol % of toluene. 

Benzaldehyde. Annual production of ben2aldehyde requires ca 6,500—10,000 t (2-3 x 10 gal) of toluene. It is produced mainly as by-product during oxidation of toluene to benzoic acid, but some is produced by hydrolysis of ben2al chloride. The main use of ben2aldehyde is as a chemical intermediate for production of fine chemicals used for food flavoring, pharmaceuticals, herbicides, and dyestuffs. 

Oxidation catalysts are either metals that chemisorb oxygen readily, such as platinum or silver, or transition metal oxides that are able to give and take oxygen by reason of their having several possible oxidation states. Ethylene oxide is formed with silver, ammonia is oxidized with platinum, and silver or copper in the form of metal screens catalyze the oxidation of methanol to formaldehyde. Cobalt catalysis is used in the following oxidations butane to acetic acid and to butyl-hydroperoxide, cyclohexane to cyclohexylperoxide, acetaldehyde to acetic acid and toluene to benzoic acid. PdCh-CuCb is used for many liquid-phase oxidations and V9O5 combinations for many vapor-phase oxidations. 

Benzaldehyde is prepared by hydrolysis of benzal chloride, for example in acidic media in the presence of a catalyst such as ferric chloride, or in alkaline media with aqueous sodium carbonate. Part of the commercially available benzaldehyde originates from a technical process for phenol. In this process, benzaldehyde is a byproduct in the oxidation, with air, of toluene to benzoic acid. 

Recently, interesting results have been obtained using the electrochemically generated ruthenium(IV) complex [(trpy)(bpy)RuO] in the oxidation ofp-xylene and p-toluic acid to terephthalic acid and of toluene to benzoic acid . The current yield is practically quantitative. After 100 turnovers about 75% of the redox catalyst can be recovered giving a turnover number of about 400 (Eq. (16)). 

Benzoic Acid. The industrial-scale oxidation of toluene to benzoic acid is carried out with cobalt catalysts.973,978,982 983 The process, a free-radical autoxidation, is significantly promoted by bromide ions.984 Under these conditions bromine atoms rather than alkylperoxy radicals serve as a regenerable source of chain-initiating free radicals. Substantial rate increase can be achieved by the addition of manga-nese(II) ions.984... 

Liquid-phase oxidation of toluene to benzoic acid (Dow process,980,985,986 Mid-Century process987) may be carried out in acetic acid or without solvent. Cobalt(II) naphthenate or cobalt(II) 2-ethylhexanoate promoted by bromine is used as the catalyst at 140-190°C, at about 8-10 atm pressure. The highly exothermic oxidation is... 

A major problem associated with such autoxidations is that they are largely indiscriminate, i.e. they exhibit poor chemo- and regio- selectivities. They are synthetically useful only with relatively simple substrates containing one reactive position, e.g. the oxidation of toluene to benzoic acid or p-xylene to terephthalic acid. Any catalytic oxidation has to complete with this non-catalytic pathway. Moreover, the situation is further complicated by the fact that transition metal ions also catalyze autoxidations by mediating the decomposition of trace amounts of hydroperoxides into chain-initiating radicals, via the so-called Haber-Weiss mechanism ... 

Assumption of a similar metabolic change might clear up some aspects of the biological oxidation of toluene to benzoic acid. Bray, Thorpe and White131 have studied the kinetics of the oxidation of both benzyl alcohol and benzaldehyde to benzoic acid. The velocity constant for the oxidation of the alcohol is 1.0, and that for the aldehyde is only 0.3, indicating that both cannot be intermediates in the oxidation of toluene. Since the alcohol has already been shown to be an intermediate, it follows that the aldehyde is not. They pointed out that hydrate formation and D-glu-curonic conjugation may precede oxidation. 

As examples of oxidation processes, two processes are available for the manufacture of phenol, and both involve oxidation. The major process involves oxidation of cumene to cumene hydroperoxide, followed by decomposition to phenol and acetone. A small amount of phenol is also made by the oxidation of toluene to benzoic acid, followed by decomposition of the benzoic acid to phenol. 

The selectivity of palladium and gold for alkene oxidation to aldehydes 28,29,170) was attributed initially to adsorption strength. However, electrooxidation in the presence of palladium ions indicates possible homogeneous alkene insertion, similar to the Wacker process 304). Homogeneous reaction is also involved in redox oxidations of hydrocarbons. In this case, the nature of the metal ions is expected to control selectivity. Indeed, toluene yields 20% benzaldehyde in electrolytes containing Ce salts, while oxidation proceeds to benzoic acid with Cr redox catalysts 311). In addition, the concentration of redox catalysts appears to affect yields in nonelectrochemical oxidation of ethylene large amounts of palladium chloride promote butene formation at the expense of acetaldehyde 312). Finally, the role of the electrolyte and solvent should not be ignored. For instance, electrooxidation of ethylene on carbon, in aqueous solution of acetic acid yields acetaldehyde 313) in the... 

According to reactions (12)-(16), CO2 results from the oxidation of toluene to benzoic acid whose traces were found in our experiments and its subsequent photodecarboxylation. It must be reported that some runs carried out by using benzoic acid at the same experimental conditions used for toluene photooxidation, afforded benzene and CO2 in large amounts. It is well known [7, 8] that ethanoic acid is easily photodecarboxylated in a gas-sohd regime in the presence of irradiated polycrystaUine semiconductor oxides. The small amounts of phenol are probably due to an attack of benzene by OH radicals (see eqn.(16)). 

The low value of the raw materials compared to the premium price paid for chloride-free benzaldehyde and benzoic acid makes the commercial utilization of the process attractive despite the difficulties involved. The production of benzoic acid by the direct oxidation of toluene lias not reached the proportions that benzaldehyde has. lthough the production of beuzoic acid directly is feasible with the correct catalysts, small quantities only from this source are being marketed and are obtained as a by-product of the catalytic oxidation of toluene to benzaldehyde. Because of the difficulty in the selection of correct catalysts, the oxidation oi toluene to benzoic acid is complicated by the oxidation going too far with resultant loss of raw material or by the formation of gummy condensation products intermixed with the benzaldehyde, benzoic acid, maleic acid and anthra-quinone. These complications have deterred commercial application of the oxidation process in the face of the newer process for forming benzoic acid by the decarboxylation of phthalic anhydride. 

Benzylic oxidation of aromatic side-chains is also a well established technology in the bulk chemicals arena, e. g. toluene to benzoic acid and p-xylene to ter-ephthalic acid [1,2]. These processes involve homogeneous catalysis by, e. g., cobalt compounds, however, and also fall outside the scope of this book. Ammoxi-dation of methyl-substituted aromatic and heteroaromatic compounds is performed over heterogeneous catalysts in the gas phase but this reaction is treated elsewhere (Section 9.5). Transition metal-substituted molecular sieves have been widely studied as heterogeneous catalysts for oxidation of aromatic side-chains in the liquid phase, but there are serious doubts about their heterogeneity [5,6]. Here again, a cursory examination of the literature reveals that supported palladium seems to be the only heterogeneous catalyst with synthetic utility [4]. 

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