Process for Oxidizing a Phenol to a P-Benzoquinone

It is known in the art to oxidize phenol to p-benzoquinone with oxygen in the presence of a copper ion catalyst and such a process is disclosed in U.S. Pat. No. 3,987,068. In that patent the oxidation is carried out in a nitrile solvent using a complex formed from the copper catalyst and the solvent and the operating conditions are said to be at temperatures of from about 0.degree. to 100.degree. C. and a partial pressure of oxygen of from about 7 to 200 (preferably 14 to 100) atmospheres. As pointed out in U.S. Pat. No. 3,987,068, yield of quinone product increases with increased partial pressure of oxygen and it appears from the data therein that partial pressures of oxygen above about 100 atmospheres are required in order to achieve conversions of phenol to p-benzoquinone on the order of about 75%. Such pressures are too high to be useful in an economical commercial process because they require special equipment of high capital cost.

U.S. Pat. No. 4,208,339 discloses a process for the preparation of p-benzoquinone by oxidation of phenol in the presence of cuprous or cupric ions and in the presence of a metal in the metallic form in a nitrile, amide, alcohol or sulfoxide solvent. Reaction rate is said to be increased by including an alkali metal or alkaline earth metal halide.

It has now been found that the copper catalyzed process for oxidation of a phenol to p-benzoquinone or a substituted p-benzoquinone can be significantly improved so as to enable operation at lower, commercially useful pressures and still achieve an improved conversion and/or improved selectivity to product. In accord with the invention, such objectives are achieved by conducting the oxidation of a phenol in the presence of a copper ion catalyst which is promoted with an oxide or hydroxide of a Group II A metal; e.g., Be, Mg, Ca, Sr, Ba, etc.

According to the invention, phenol and substituted phenols are converted to corresponding p-benzoquinone and substituted p-benzoquinones as illustrated in the following reaction: ##STR1## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 may be the same or different and may comprise hydrogen, halo or cyano, alkyl or alkoxy containing 1 to about 12 carbon atoms; phenyl, naphthyl, phenylalkyl, alkylphenyl, phenoxy or phenalkoxy, containing 7 to about 16 carbon atoms; and R.sub.1 and R.sub.2 or R.sub.3 and R.sub.4 may be joined to form an aliphatic, aryl or hetero ring. Specific R groups include chloro, cyano, methyl, ethyl, propyl, butyl, isobutyl, tert-butyl, pentyl, p-tolyl, p-anisyl, methoxy, t-butoxy, phenoxy, p-methylphenoxy, and the like. Preferred phenols useful in the process are phenol, o-chlorophenol, o-cresol, m-cresol, 2,5- and 2,6-di-t-butylphenol, 2-t-butylphenol, 2,6-dimethylphenol, and 1-naphthol. When R.sub.1 and R.sub.2 or R.sub.3 and R.sub.4 are joined, the resulting p-benzoquinones will be those derived from naphthalene, quinoline, isoquinoline, chroman (dihydrobenzopyran), indole and the like.

In carrying out the process of the invention, conventional temperature conditions, solvent systems and a monovalent or divalent (i.e., cuprous or cupric) copper catalyst may be used. Thus, a temperature of from about 20.degree. to about 100.degree. C. (preferably about 50.degree. to 75.degree. C.) and a nitrile solvent, preferably acetonitrile, is usually employed. The copper catalyst will preferably be a copper halide, preferably chloride, and preferably a monovalent copper catalyst. Copper nitrate is operable and mixtures of such salts also may be used. However, other copper II salts such as acetates, sulfates, benzoates, carbonates, phosphates, and bisulfates have been found not to be effective catalysts for the reaction. The mole ratio of phenol reactant to catalyst may vary widely, on the order of from about 50:1 to 1:1, preferably about 15:1 to 5:1. The Group II A oxide or hydroxide promoters will preferably be those of Be, Mg, and Ca. The amount of promoter to be used will generally be from about 0.1 to about 1.9 mole promoter per mole of catalyst; a mole ratio of about 1:1 has been found to be quite effective. As indicated above, the reaction can be carried out at moderate pressures and such pressures will generally be between about 100 and about 500 psig partial pressure of oxygen, preferably between about 200 and 400 psig. Mixtures of oxygen and nitrogen, air alone, or oxygen alone may be used, but preferably mixtures of oxygen and nitrogen such as air will be employed as the oxygenating medium.

In order to further illustrate the invention, the following examples are given:

A solution of phenol in 5 ml. of acetonitrile which was agitated in a magnetically stirred mini-autoclave under an initial total pressure of 750 psig was oxidized over a three hour period with a mixture of 40% (vol.) oxygen and 60% nitrogen in the presence of a CuCl or CuCl.sub.2 catalyst. The reaction parameters and results obtained are shown in the following Table I.

As can be seen from the above data, significant improvements are obtained by use of the promoters. Thus, with divalent copper catalyst, in Run 2, the BeO promoter results in increased selectivity to p-benzoquinone (PBQ) although conversion and yield are low. With CaO and Ca(OH).sub.2, however, (Runs 3 and 4) significant improvements in conversion and yield over the control (Run 1) are achieved.

With the monovalent copper catalyst, both selectivity and yield of PBQ are significantly improved by use of the promoter (compare Runs 6, 7, and 8 with Run 5).

In experiments which may be conducted substantially as described in example 1, except for substitution of the substituted phenols listed below, good improvements in selectivity to the corresponding substituted benzoquinones, conversion and/or yield are achieved: