Evolution of a Process: off

edited by: W. Conard Fernelius, Harold Wittcoff, and Robert E. Varnerin

The industrial manufacture of diphenyl oxide (DPO) evolved from the phenol-producing chlorohenzene hydrolysis reaction. An understanding of the chemistry involved in this reaction provides the basis for converting an unprofitable process and plant to a profitahle business without a sub- stantial use of capital or a major change in operating proce- dures. As will be seen, the chemist's role was critical in rec- ognizing the desirability of the product, unraveling the chemistry and working with a production unit to develop the necessary technology.

The story begins with phenol. Phenol has been produced for many years and has several important uses. Phenolic resins used as adhesives for plywood account for 49% of the phenol produced.' Manufacture of hisphenol A and caprolactam, used respectively in epoxy resins and Nylon 6 fibers, are the other main uses of phenol.

Natural phenol can he recovered from coal tar and petro- leum. but can contribute to fillina onlv a tiuv nortion of the - "

totalphenol demand. Several commercial synthetic routes were available for the manufacture of phenol: however, the cumene peroxidation process developed in Germany in 1944, proved most practical. This process involves the alkylation of benzene followed by air oxidation and rearrangement to phenol and acetone as shown.

Since the industrial introduction of the cumene peroxidation process in 1955, this process has steadily become the eco- nomical method of choice for phenol production and in 1975 accounted for 90% of the phenol produced in the United States. However, the phenol process used by the Dow Chem- ical Company since 1924 had been chlorobenzene hydrolysis as shown below.

PH

The Manufacture of Diphenyl Oxide

Harold G. Fravel, Jr. Project Leader

Dow Chemical Company

The hydrolysis of chlorohenzene with caustic was first de- scribed in 1849, and in 1917 Alysworth obtained a patent for the continuous svnthesis of ohenol in a tubular autoclave. Improwd p n w & and engineering technolorn m ~ d e it s \,int~le alternariw 10 the benzene ;ulfw~;rrim imce.;s. In 1924, r~hmol was first produced commercially using the chlorob&zene process. Chlorohenzene and aqueous caustic are mixed and heated a t 3001100°C and 4000-6000 psi in a tubular-type autoclave which permits continuous operation. The reaction mixture is cooled and separated in a decanter into organic and aqueous phases. The organic layer contains unreacted chlo- robenzene which can he recycled, as well as the coproduct DPO. Although DPO was sold in 1924 for heat-transfer media, a very small market existed and the chlorobenzene process was made economical by recycling DPO back to the process where it was hydrolyzed back to phenol. The aqueous phase, con- taining mainly sodium phenate, is neutralized and phenol is collected. The resultant brine stream is recycled to the chlor-alkali electrolytic cells in which sodium chloride is converted to sodium hydroxide and chlorine.

Inspection of the phenol and DPO produced from the chlorohenzene hydrolysis shows that a simple displacement of chlorine by hydroxide or phenate ion has occurred. How- ever, several other by-products are not as easily explained. A benzyne mechanism had been proposed, and in 1956 Roberts and Bottini2 showed that in actuality a benzyne mechanism was predominately operative under the reaction conditions. Abstraction of the ortho hydrogen followed by loss of chloride produces a highly reactive henzyne intermediate which could react with hydroxide to produce phenol or with phenoxide to give DPO.

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Volume 57, Number 12, December 1980 / 873

There is some S N ~ reaction competing with the benzyne pathway. This became apparent when Roberts allowed chlo- robenzene labeled in the 1-position with carbon-14 to react with caustic and he found 58% of the phenol labeled in the 1-position and 42% of the 2-isomer. If the mechanism was totally benzyne, one would expect a 1:l ratio of the two iso- mers. The high reactivity of benzyne can also he invoked to explain the formation of DPO and several other products formed during the chlorobenzene process. Reaction of a henzvne intermediate with ohenate ion and its resonance form; can afford DPO, OPP i o r t h ~ - ~ h e n ~ l ~ h e n o l ) and P P P (para-ohenvl~henol). All of these compounds are indeed Lrmed in the chlorobenzene hydrolysis~along with smaller amounts of di- and triaryl ethers.

r 0s n o 1

Dm PPP

While the chemistrv of the process was being unravelled in the sixtirs, thi, cumene pemxidation was making phend pnl- duction \,ia the chlt,roben~.ene hydrvlvh unrwnomical.

However. the increasine osace of heat transfer fluids con- taining DPO and the devekpm;nt of DPO-based surfactants gave production of DPO greater significance. One heat transfer fluid contains DPO and the di- and triaryl ethers formed in the chlorohenzene hydrolysis reaction. These fluids have several desirable properties such as low freezing point, low volatility and great thermal stability. In addition, OPP is a potent antimicrobial agent which had developed an im- portant market. Abandonment of the chlorobenzene hydrol- ysis would have eliminated these valuable by-products of the

more expensive phenol process. On the other hand, continued operation producing phenol mainly for these by-products was not economically possible since the high cost of the phenol would ereatlv offset the value of the DPO obtained. Direct " . routes to DPO were possible but were even more expensive and failed to produce the useful by-products.

Laboratory work indicated a means of using the chloro- benzene process bvshiftinc the reaction to vield mainlv DPO. In a number of laboratory experiments, it was shown that adjustment of the caustic and chlorobenzene ratios and the addition of phenol to the feed stream would increase the DPO level produced and other by-products.

CsHsOH + NaOH - C6HsONa + HzO (1)

CsHsCl + 2NaOH - CGHSONB + H20 + NaCl (2)

CsHsCI + CeHsONa - CsHs0CsH5 + NaCl (3)

Phenol is formed during the hydrolysis (eqn. (2)); however, phenol is also consumed during the reaction (eqn. (3)). If we consider the net phenol, or (phenol final - phenol initial), we can have three possible situations. If the phenol produced is equal to the amount added initially, the net phenol is equal to zero. Similarlv if more nhenol is obtained than added. the net phenol will he positive:~astly one can consume phenoiand have a negative net phenol production. All three situations are possible by careful attention to reactant levels. In some in- stances bhenol mav actuallv have to be bought and added to the initial mixture o h t a i n b ~ ~ as the prim&y product. The ohenol process has been converted to a phenol consuming pr<rress and former by-prod~~rts are now the main products. I'sinr the uroccw chemistr\, devi~lopi~d, the ol~solrsctmt phmol process via chlorobenzene hydrolysis has been conve&ed to a DPO producing process. The transition was made without maim chances in eaoioment or o~era t ine orocedures therebv saGing capiial and adiitional opkrating-&anges.

Intimate knowledge of the problem with the chlorobenzene hydrolysis process permitted the change from phenol to DPO uroduction. Without the chemists involvement. valuable broducts such as DPO and OPP may have been lost as the old phenol process became ohsolete.

874 1 Journal of Chemical Education