Production of Synthetic Cresols

Along with the olefins, particularly ethylene and propylene, aromatic hydrocarbons, more specifically benzene, toluene, para- and or /io-xylenes, for long had been the key building blocks for synthesis into a number of critical organic chemicals and intermediates. 

Among aromatic hydrocarbons benzene, toluene, and xylenes (BTX), toluene was earlier considered commercially the least important, and a number of processes were developed for conversion of surplus toluene into more lucrative benzene and Cs aromatics. More recently, demand of toluene has started picking up partly for augmenting octane number of gasoline, and more importantly, as a critical feedstock for a few important chemicals as shown in Fig. 2.1. 

Earlier hydrodealkylation of toluene into benzene, disproportionation of toluene into benzene and xylenes and transalkylation of toluene and C9 aromatics into xylenes were developed and commercialized to strike a right balance in desired aromatic 

Wherever the situation warranted either one or all of the processes were used to maximize benzene and xylenes production in lieu of toluene which used to be produced in surplus quantities in a catalytic reforming unit of a petroleum refinery. 

Commercial processes for conversion of toluenes and in pure fractions to benzene and other aromatic hydrocarbons are shown in Table 2.1 [9]. 

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Mixed cresols, also known as cresylic acids, the lowest among the alkyl phenols, were primarily produced as by-products from coal carbonization plants or recovered from the petroleum refinery caustic washes. These cresols obtained from natural sources were known to the chemical industry for the last 75 years and had limited uses. Production of synthetic cresols from toluene opened up new avenues for these products... 

Cresols are rapidly absorbed thru the skin causing severe burns (Ref 6). They are used as disinfectants, flotation agents ink, paint varnish removers lube oil additives, organic intermediates, and for the production of synthetic resins (Ref 7)... 

Resorcinol, dihydroxy benzene, and naphthols both a and P, are the most important hydroxy-aromatics having wide applications and are produced from benzene and naphthalene, respectively, using similar technologies as in production of synthetic phenol from benzene or cresols from toluene. It may be relevant to discuss briefly about these products. 

General Electric of USA have been producing 10,000 tpa o-cresol and 70,000 tpa 2,6-xylenols, in USA, and in Holland further, 4000 tpa o-cresol and 16,000 tpa 2,6-xylenols. Synthetic Chemicals, UK (later on taken over by Inspec and now Laporte) have been producing 8000 tpa o-cresol and 2,6-xylenols in UK. There has been production of o-cresol and 2,6-xylenols in Japan, Russia, and Czech Republic [1,6]. 

In 1865 just prior to Kekule s synthesis of phenol, Joseph Lister (1827-1912) was experimenting with carbolic acid as an aid to antiseptic surgery which he had pioneered. A mixture of crystallised carbolic acid and shellac (lac plaster) was employed in the finally adopted mode of application. The requirement of phenol for the manufacture of picric acid during the Boer war and other uses resulted in a demand which soon outstripped the resources of phenol/cresols available from coal distillation. Synthetic phenol thus became a potentially important intermediate. The lengthy processing involved in the separation of phenol and the isomeric cresols led to the desirability for specific syntheses. In 1978 of the world production of phenol only 3% came from coal sources by extraction of the mixed phenols (about 1.5% in coal tar) with 10% sodium hydroxide, acidification with carbon dbxide and separation. Phenolic compounds are also formed during catalytic cracking processes in the petroleum industry. There are historically six industriai processes for the production of synthetic phenol, variously from benzene and toluene, some of which are also applicable to the cresols and the dihydric phenols. 

A number of years ago. coal tar was the primary, if not the sole, source lor hundreds of important organic chemicals and derivatives, notably the phenols, cresols, naphthalene, and anthracene, as well as other important coal lar end-prralucls, such as solvent naphtha and pitch. In recent years, synthetic processes Tor the production of phenol, the cresols and later the xylcnols. have been developed and thus, to a large extent, have pushed coal lar into the background as a source of feedstocks for (he chemical industry. 

For example, the solvent products of a liquefaction carried out with a synthetic solvent (80% 2-methyl naphthalene, 18% p-cresol and 2% y-picoline) were shown (by gc/ms) to have formed a variety of dimeric products Figure 7 presents the gas chromatogram of this solvent after reaction in which the major components were identified ... 

The electro-Fenton method (or EFR) was initially used for synthetic purposes considering the hydroxylation of aromatics in the cathodic compartment of a divided cell. Thus, the production of phenol from benzene (Tomat and Vecchi 1971 Tzedakis et al. 1989), (methyl)benzaldehydes and (methyl)benzyl alcohols from toluene or polymethylbenzenes (Tomat and Rigo 1976,1979,1984,1985) by adding Fe3+ to generate Fe2+ via reaction (19.13), as well as benzaldehyde and cresol isomers from toluene or acetophenone and ethylphenol isomers from ethylbenzene (Matsue et al. 1981) with direct addition of Fe2+, have been described. Further studies have reported the polyhydroxylation of salicylic acid (Oturan et al. 1992)... 

From Natural Sources.—In addition to their synthetic preparation many of the phenols are obtained by the distillation of natural substances, e.g.y coal (coal tar), wood, resins, etc. Phenol is obtained from coal tar, while the hydroxy toluenes or cresols are obtained from both wood tar and coal tar as indicated by the term creosote for the crude distillation products of wood. 

Synthetic cresols production from toluene feedstock will be discussed in some details in a subsequent chapter. A brief review of cresols or cresylic acid production from coal/lignite is presented here. 

India does not produce any quantity of synthetic coumarin. Some quantities of coumarin are produced from natural resources. Atul produces both o-cresol (at Ankleshwar) and phosgene (at Atul). Unfortunately, they have not considered this product seriously, apparently because, they just do not have the right technology. Some companies are, however, working on the synthesis of the product from o-cresol, phenol, and also salicylaldehyde (Perkin reaction) [30]. 

The Company, Gujarat Aromatics Limited, was making synthetic cresols from toluene but since the finished product, mixed cresols (cresylic acid) did not have sufficient market, more often than not the company s manufacturing facilities at Ankleshwar, Gujarat had to be shut down. The company was soon amalgamated with Atul Limited, a big multiproduct Chemical giant of Gujarat, and, was reconstituted as the Aromatics Division of Atul. 

This high reactivity is especially essential for the production of phenol/formal-dehyde condensation products (Bakelite resins) phenol/formaldehyde resins were the first synthetic plastics, with production patented by Leo H. Baekeland in 1907. In addition to phenol, cresols, xylenols and long-chain alkylphenols are used to obtain special quality characteristics. Phenolic resins are used widely for numerous applications. Phenolic resin production in Western Europe in 1985 was around 500,0001, in the USA 1,150,0001 and in Japan 325,0001. 

Another important derivative of m-cresol used in the manufacture of plant protection agents is m-phenoxytoluene, which can be produced from m-cresol and chloro- or bromobenzene at temperatures of 200 °C, with copper catalysts. m-Phenoxytoluene is converted into m-phenoxybenzoic acid methyl ester by oxidation with a cobalt acetate/KBr catalyst and subsequent esterification m-phenoxybenzoic add methyl ester serves as an intermediate in the production of m-phenoxybenzaldehyde, which is used as the raw material in the production of the synthetic pyrethroid insecticide, fenvalerate (see Chapter 6.3.2). The cyanohydrin is formed in-situ, then made to react with 2-isopropyl-(4-chlorophenyl) acetic acid chloride to yield fenvalerate, which was developed by Sumitomo Chemical in 1972. Pyrethroid insecticides are distinguished by their low toxicity and high activity. 

The use of alkyl phenols such as cresol in phenolic resin production reduces reactivity, hardness, cross-linking density and color formation but increases solubility in non polar solvents, flexibility, and compatibility with natural oils. At room temperature, o- and p-cresol are crystalline solids, while m-cresol is a viscous oil. Cresols are less soluble in water than phenol is (Weber and Weber, 2010). Approximately 60% of cresol is obtained from coal tar and crude oil using classical techniques such as distillation and liquid-liquid extraction. The remaining 40% is obtained synthetically by the alkylation of phenol with methanol (Fink, 2005). In addition to phenolic resins, cresols are also used in the production of herbicides, fungicides, disinfectants, plasticizers, epoxy resins and pharmaceuticals (Fink, 2005 Weber and Weber, 2010). 

Crepe Rubber n A type of crude or sometimes synthetic rubber pressed into crinkled sheets Cresol kre- s61 n [ISV, irreg. fr. cresote] (ca. 1869) (hydroxytoluene, methylphenol) H3CC6H4OH. An important family of coal-tar derivatives, occurring in ortho, meta, and para isomers, and used in the production of phenol-formaldehyde resins and tricresyl phosphate, an important plasticizer for PVC. Three cresols are possible, namely (a) o-cresol, mp 30°C bp, 191°C (b) m-cresol, mp 4°C bp, 205°C (c) p-cresol, mp 36°C bp, 201°C, and these are found together in the crude cresylic acid from coal tar. The main use of the cresols is in the manufacture of cresol-formaldehyde resins, and cresylic acid, rich in the meta isomer, is usually chosen for this purpose. Known also as Cresylic Acid. 

Except for the last procedure, these have been tested many times. Most of the thermal treatment plants - which are mainly designed as central facilities and used in The Netherlands and the USA - are based either on the direct or indirect procedure. In general, thermal plants are used to clean soil masses contaminated with the following substances contaminants based on mineral oil and pit-coal secondary products cresols residues of synthetic dyes polyaromatics complexed cyanides and contaminants based on halogentated (chlorinated) hydrocarbons, including PCB s, dibenzo-dioxines/dibenzofuranes, DDT and other insecticides. 

At one time the requirement for phenol (melting point 41°C), eould be met by distillation of eoal tar and subsequent treatment of the middle oil with eaustic soda to extraet the phenols. Such tar acid distillation products, sometimes containing up to 20% o-cresol, are still used in resin manufacture but the bulk of phenol available today is obtained synthetically from benzene or other chemicals by such processes as the sulphonation process, the Raschig process and the cumene process. Synthetic phenol is a purer product and thus has the advantage of giving rise to less variability in the condensation reactions. 

With 3-methylphenol (meto-cresol), around 8 % of dibromo products result when one equivalent of brominating agent is used, and this rises to 23 % of dibromo products on attempted monobromination of 3,5-dimethylphenol with 1 molar equivalent of resin. Nevertheless, the good yields of products obtained from the mono-substituted phenols tried demonstrate that this is a powerful new synthetic method for the organic chemist. 

In the latter part of the nineteenth century new raw materials for the chemical industries became available from city gas and by-product cote oven operations. Benzene, toluene, xylene, naphthalene, phenol, cresols, and xylenols served as crudes for conversion to various intermediates used in the growing new synthetic dyestuff industry. Many of these intermediates and finished products were nitro compounds and found their way into the explosive industry. 

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