Production of cupric and manganous alkanoates
THE INVENTION 
     This invention relates to a process for producing a mixture of cupric and manganous alkanoates containing from 6 to 9 carbon atoms. These alkanoates can then be used as catalysts for the oxidation of C 6  to C 9  saturated aliphatic aldehydes to their respective acids. This process involves the initial thermal decomposition of a mixture of cupric and manganous oxalates in the presence of one or more C 6  -C 9  saturated aliphatic monoocarboxylic acids and in the absence of oxygen. Manganous alkanoate is immediately produced, together with metallic copper. The copper is then oxidized to the cupric form, which reacts with saturated aliphatic monocarboxylic acid to form cupric alkanoate, thus giving a mixture of cupric and manganous alkanoates which can be separated into its individual constituents or used as such. 
     BACKGROUND OF THE INVENTION 
     The overall objective when producing organic saturated aliphatic monocarboxylic acids containing from 6 to 9 carbon atoms by oxidation from their corresponding aldehydes, is to obtain the highest yields and product efficiencies at the highest conversion levels. The use of mixtures of copper and manganese compounds soluble in the acid product as catalysts helps achieve this objective, and is disclosed in copending U.S. patent application Ser. No. 345,890, filed on Feb. 4, 1982 in the names of Hobbs, Jr. and Thigpen and assigned to Celanese Corporation. However, copper has a tendency to plate out in the distillation apparatus used for acid product recovery, resulting in undesirable mechanical problems such as erosion of reboilers and pump impellers and rapid pump seal failures. 
     A method of overcoming this copper plating problem, involving adding oxalic acid to the acid product to precipitate copper and manganese as the oxalates, then filtering the oxalates from the acid product prior to the acid distillation step, is described in U.S. Pat. No. 4,289,708, issued Sept. 15, 1981 to Scott et al and assigned to Celanese Corporation. Copper and manganese can also be separated from the acid product, again as the oxalates, by adding aqueous oxalic acid to precipitate manganese and copper oxalates into the aqueous phase, which is then separated from the acid product by decantation of the acid. The acid can then be further purified by distillation. This latter process is described in U.S. Pat. No. 4,246,185, issued Jan. 12, 1981 to Wood, Jr. and assigned to Celanese Corporation. 
     The foregoing separation processes result in the production of substantial amounts of cupric and manganous oxalates. It is the purpose of this invention to convert these oxalates to alkanoates useful as catalysts for recycle in the oxidation of C 6  -C 9  saturated aliphatic aldehydes to the corresponding monocarboxylic acid. 
     U.S. Pat. No. 3,133,942, issued May 19, 1964 to Hahl and assigned to Badische Anilin- &amp; Soda-Fabrik Aktiengesellschaft, describes a technique for the production of metal salts, including copper salts of organic acids, including saturated aliphatic monocarbolic acids having 1 to 8 carbon atoms, oleic acid and benzoic acid, which involves reacting the metal with the acid in the presence of free oxygen. The reactions of n-butyric, n-valeric and n-caprylic acids with copper powder in the presence of oxygen to produce copper (II) butyrate, copper (II) valerate and copper (II) caprylate, respectively, are specifically disclosed. Hahl makes no mention, however, of conducting such reactions in the presence of manganese compounds such as manganese alkanoate and the products obtained by the thermal decomposition of manganous oxalate and cuprous oxalate in the presence of a C 6  -C 9  saturated aliphatic monocarboxylic acid. In particular, Hahl has no teaching of the conversion of the oxalates suspended in an alkanoic acid to form metallic copper and manganese alkanoate directly. Finally, Hahl&#39;s teaching does not include the treatment of manganese in any form whatever to obtain a manganese alkanoate since the normal potential of manganese is -1.029 volt and thus does not fall within the range of the normal potential of -0.80 and +0.5 volt described by Hahl. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The mixture of manganous and cupric oxalates utilized as the starting material for the process of the present invention is first combined with one or more organic saturated aliphatic monocarboxylic acids containing from 6 to 9 carbon atoms, such as hexanoic acid, heptanoic acid, nonanoic acid, and the like. The resulting mixture of oxalates and free acids is then subjected to thermal decomposition conditions, which essentially involve reaction at temperatures in the range from about 225° C. to about 300° C., preferably from about 225° C. to about 275° C., in an atmosphere substantially free of oxygen, for a sufficient period of time to yield manganous oxide and metallic copper as well as carbon monoxide, carbon dioxide and water. Oxygen is excluded from the decomposition reaction to avoid further oxidation of the organic acids present. Manganous oxide readily reacts with the C 6  -C 9  acid or acids present to form the corresponding manganous alkanoates having C 3  -C 9  carbon atoms. Metallic copper, however, does not react with these acids under these conditions. 
     The solution containing manganous alkanoate, acid and metallic copper is cooled to a temperature at which metallic copper can be oxidized to the cupric form, but at which the acid will not oxidize. Such temperatures range from about 50° C. to about 125° C., and preferably from about 80° C. to about 110° C. The cooled solution is then contacted with oxygen, in the form of air at a pressure maintained from about 200 to about 300 psig, preferably from about 225 to about 275 psig, for a period of time sufficient to convert the metallic copper to the cupric form. Ordinarily, this will take from about 1/2 to aout 10 hours, and preferably from about 1 to about 6 hours. The cupric ions produced by oxidation of metallic copper immediately react with available organic acid to produce the corresponding cupric alkanoate containing 6 to 9 carbon atoms. Thus, for example, if heptanoic acid was used as the acid in which cupric oxalate and manganous oxalate were thermally decomposed, manganous heptanoate and eventually cupric heptanoate would be produced. The resulting mixture of cupric and manganous alkanoates, e.g., cupric and manganous heptanoate, can be used as the catalyst for the oxidation of heptanal to heptanoic acid. 
     The invention will be illustrated by the following example. 
    
    
     EXAMPLE 
     In a stirred 300 cc stainless steel autoclave, 100 milliliters of heptanoic acid and 10 milliliters of a slurry containing 60 weight percent water and 40 weight percent of a mixture of cupric and manganous oxalates (equal weights of each oxalate) were heated to 250° C. for 4 hours at autogenous pressures. This resulted in a mixture containing heptanoic acid, manganous heptanoate and finely divided elemental copper. This mixture was cooled to 100° C., then pressured with air at 250 psig and stirred for 6 hours. During this time, all of the finely divided copper was oxidized to the cupric form which immediately reacted with heptanoic acid to form cupric heptanoate. The resulting solution, containing 1 weight percent each of cupric heptanoate and manganous heptanoate in heptanoic acid, is suitable as the oxidation catalyst to produce heptanoic acid from heptanal using the same process described in the above-mentioned copending U.S. patent application, Ser. No. 345,890. These catalysts can be recycled to the catalytic heptanal oxidation process for reuse.