This invention relates to a process for purifying an alkylated anthraquinone.

It is well known that a commercial alkylated anthraquinone will contain impurities which may in¬ clude: anthraquinone/ o-benzoylbenzoic acid, and related compounds. The presence of such impurities is undesirable, particularly when the alkylated an¬ thraquinone is to be used to manufacture hydrogen peroxide by the reduction and oxidation of a working solution comprising an alkylated anthraquinone.

In U. S. Patent No. 2,842,563, Hinegardner et al recognize the problem caused by anthraquinone as an impurity in commercial t-butylanthraquinone when the latter is used as the working compound for the pro¬ duction of hydrogen peroxide. Hinegardner et al teach that an alkylated anthraquinone can be further purified when dissolved in a non polar solvent after a preliminary treatment with activated alumina by adding a polar solvent such as alconol to super¬ saturate the solution with respect to anthraquinone; the supersaturated solution is then seeded with anthraquinone and after standing from four hours to four days the crystallized anthraquinone is separated from the solution. The process of Hinegardner et al has the disadvantage in that it is incapable of com¬ pletely removing the anthraquinone from the solution. In addition, the added polar solvent must be removed from the solution requiring at least one additional step.

The process of the present invention can remove an unalkylated anthraquinone from a solution of an alkylated anthraquinone in an inert solvent or mix¬ ture of inert solvents. The process comprises re- ducing the solution containing the impure alkylated anthraquinone sufficiently to convert at least part of the unalkylated anthraquinone to the reduced form. f OMP

The reduced unalkylated anthraquinone is extracted from the solution by contacting the reduced solution in the absence of oxygen with an alkaline aqueous solution. Any inert solvent for either the alkylated an¬ thraquinone or anthrahydroquinone may be used in the process. For example, if it is more economical to remove part of the anthrahydroquinone by a physical separation process such as filtration prior to caus- tic extraction, it is desirable to use an aromatic solvent for the process to ensure that substantially all of the alkylated anthraquinone is in solution prior to the reduction step. On the other hand it may be more convenient to merely form a slurry of the alkylated anthraquinone in a polar solvent to ensure that substantially all of the anthrahydroquinone formed is in solution prior to the extraction step. Conventionally, in such a purification process it is desirable to select a solvent or solvent mixture so that no solid phase is present during the process.

If the purified alkylated anthraquinone is to be used as a solution in a process to produce hydrogen peroxide it is preferable for the solvent in the present process to be a solvent ^' or a mixture of sol- vents used in said hydrogen peroxide process. If desired the purified alkylated anthraquinone may be subsequently separated from the solvent by a conven¬ tional separation process.

This invention is based on the observation that an unalkylated anthraquinone is more easily reduced than an alkylated anthraquinone. If it is desirable to merely decrease the ratio of unalkylated anthra¬ quinone to alkylated anthraquinone then it is neces¬ sary to reduce only part of the unalkylated anthra- quinone in the solution. Generally it is preferable to remove substantially all of the unalkylated an¬ thraquinone when the alkylated anthraquinone is to

used to produce hydrogen peroxide.

Any suitable means for reducing the unalkylated anthraquinone may be used for the present process. The desirable reducing agents include: hydrogen in the presence of a catalyst such as nickel or palla¬ dium either as a fixed bed or a fluid bed sodium dithionate or any other dithionate salt or the re¬ duced form (anthrahydroquinone) of an alkylated an¬ thraquinone. One skilled in the art will recognize that the present process will remove other impurities which can be either reduced and extracted from the solution or extracted from the solvent by the alka¬ line aqueous solution. For example, o-benzoylbenzoic acid and its alkylated derivatives which are inter- mediaries in the synthesis of the respective anthra- quinones would be extracted by the alkaline aqueous solution.

The present process can be conducted at any con¬ venient temperature between the boiling point and freezing point of the solution. The process is not pressure sensitive and can be conducted at a pressure that is convenient in view of the temperature selec¬ ted.

Any alkali capable of forming a water soluble salt with an anthraquinone is satisfactory for the present process; sodium hydroxide and potassium hydroxide are preferred.

For the purpose of this invention the term "alkylated anthraquinone" includes both the alkylated anthraquinone, its tetrahydro derivative, and the isomers of both if any. For example 2-amylanthra- quinone includes the various primary, secondary, and tertiary amyl isomers of the anthraquinone and the tetrahydroanthraquinone. The specific alkylated anthraquinone selected for a commercial hydrogen peroxide process will depend on plant design factors well known to those skilled in the art. The pre-

ferred alkylated anthraquinone usually is one or more of the compounds selected from the group consisting of 2-ethylanthraquinone, 2-butylanthraquinone, and 2- amylanthraquinone, the isomers thereof, and the cor- responding tetrahydroanthraquinones.

The best mode of practicing the present invention will be evident to one skilled in the art from the following non-limiting example:

Example 1 A solution was prepared by dissolving 1600 g of a commercial 2-amylanthraquinone (contained in 3100 g of a C9-C10 aromatic solvent) . The solution- was hydrogenated in the presence of the palladium cata¬ lyst described in U. S. Patent No. 3,635,841 at 43°C at a pressure of about 250 kPa until the hydrogen taken up by the solution was equivalent to the amount of the anthraquinone orginally present in the 2- amylanthraquinone. The hydrogenated solution was maintained in an inert atmosphere of nitrogen while it was ultimately contacted three successive times with 500 ml portions of a 10% aqueous solution of sodium hydroxide; after the final separation from the sodium hydroxide, the solution was then thoroughly washed with water. After the solution was separated from the washwater the solvent was removed by distil¬ lation to permit analysis of the dissolved alkylated anthraquinone. The residue after distillation assayed over 99% amylanthraquinone; the anthraquinone content was found to be reduced from about 2% to less than 0.1%.

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