Process for producing aromatic peroxycarboxylic acids
FIELD OF THE INVENTION 
     This invention relates to a process for producing an aromatic peroxycarboxylic acid by aerobic oxidation of an aromatic aldehyde. 
     BACKGROUND OF THE INVENTION 
     Aromatic peroxycarboxylic acids are important oxidizing agents used in various reactions such as epoxidation of olefins, oxidation of ketones to esters or lactones, phenols to p-quinones, and sulfides to sulfoxides. A heretofore known process for producing aromatic peroxycarboxylic acids involves oxidizing an aromatic aldehyde with oxygen in the presence of a catalyst. Among the proposed catalysts for this process are homogeneous catalysts, such as conjugated imines as described in Japanese Patent Publication (Kokai) Shou 55-43058 (1980), and cobalt halides as described in Japanese Patent Publication (Kokai) Hei 1-117859. 
     However, conducting the aforementioned process with homogeneous catalysts results in various disadvantages. The yield of aromatic peroxycarboxylic acids is low when the catalyst is based on a conjugated imine. Serious safety hazards arise when the homogeneous catalyst is a cobalt halide. The aromatic peroxycarboxylic acids are produced in high yield, but are simultaneously decomposed in the presence of the cobalt halide catalyst which increases the risk of an explosion. Consequently, these homogenous catalysts are unsatisfactory for use in producing aromatic peroxycarboxylic acids on an industrial scale. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide a catalyst which provides aromatic peroxycarboxylic acids effectively by aerobic oxidation of the corresponding aldehydes and does not decompose the produced peroxycarboxylic acids. This object was achieved on the basis of the finding that by using heterogeneous transition metal oxides as a catalyst, the desired aromatic peroxycarboxylic acids are safely and easily obtained in a high yield without causing decomposition of the products. 
     Thus, the present invention provides a process for producing an aromatic peroxycarboxylic acid which comprises reacting an aromatic aldehyde with oxygen in the presence of at least one oxide of a transition metal selected from chromium, manganese, iron, cobalt, nickel and copper in a solvent. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the process of the present invention, aromatic aldehydes include, for instance, benzaldehyde, o-tolualdehyde, m-tolualdehyde, p-tolualdehyde, p-ethylbenzaldehyde, p-isopropylbenzaldehyde, p-tert-butylbenzaldehyde, 2,4-dimethylbenzaldehyde, 2,4,6-trimethylbenzaldehyde, p-anisaldehyde and m-chlorobenzaldehyde. 
     The present process is conducted in the presence of a transition metal oxide as a heterogenous catalyst. Preferred transition metal oxides are chromium(III) oxide, chromium(IV) oxide, manganese(IV) oxide, iron(III) oxide, iron(II, III) oxide, cobalt(II) oxide, cobalt(II, III) oxide, nickel(II) oxide, nickel(III) oxide, copper(I) oxide and copper(II) oxide are exemplified. Iron(III) oxide, iron(II, III) oxide, manganese(IV) oxide, copper(I) oxide and copper(II) oxide. The amount of the transition metal oxide is not particularly limited, but it is usually in a range of 0.0 to 50 mol % based on the aromatic aldehyde, and preferably is in a range of 0.1 to 10 mol %. 
     In the present invention, oxygen can be supplied as pure oxygen gas or molecular oxygen-containing gas such as air. The reaction can be carried out under elevated pressure or atmospheric pressure. When pure oxygen gas is used, atmospheric pressure is good enough to carry out the reaction. More particularly, when the oxygen is supplied as oxygen gas, the reaction can be carried out at atmospheric pressure, although the pressure can be from above about atmospheric to 5 atm. When the oxygen is supplied as an oxygen-containing gas, the reaction can, in general, be carried out at a pressure above about atmospheric pressure to about 100 atm, and more specifically up to 60 atm. 
     Exemplary solvents for use in the present process include aromatic hydrocarbons such as benzene, toluene, xylene, monochlorobenzene and dichlorobenzene, halogenated hydrocarbons such as dichloromethane, chloroform and ethylene dichloride, esters such as ethyl acetate, nitriles such as acetonitrile, ketones such as acetone and methyl ethyl ketone. The aromatic hydrocarbons are preferably used since they are easily handled from an industrial point of view. 
     The amount of the solvent used is determined in accordance with the reaction conditions such as the kind of the starting aromatic aldehyde and the reaction temperature and is not particularly limited. But it is usually in a range of 1 to 40 by weight and preferably is in a range of 2 to 20 times by weight based on the aromatic aldehyde used. 
     The reaction temperature is usually in a range of 0° to 70° C., preferably in a range of 20° to 50° C. 
     After the reaction, the desired aromatic peroxycarboxylic acid can easily be obtained, for example, by filtering off insoluble materials such as the transition metal oxide. When the desired compound is used as an oxidizing agent, the resulting solution can be used as it is. 
     According to the present invention, aromatic peroxycarboxylic acids can safely be obtained from the corresponding aromatic aldehydes in high yield without causing decomposition of the product. 
    
    
     The present invention is illustrated by the following examples, but the scope of the present invention is not limited to them. Analysis of the product was carried out by iodometry in each example. 
     EXAMPLE 1 
     A mixture of 0.318 g of benzaldehyde, 5 mg of iron(III) oxide and 5 ml of benzene was vigorously stirred at 25° C. under oxygen atmosphere (1 atm ) for 30 minutes. 
     Analysis of the reaction mixture gave the results shown below. 
     
         ______________________________________                                    
conversion of benzaldehyde                                                
                      94%                                                 
selectivity of peroxybenzoic acid                                         
                      80%                                                 
yield of peroxybenzoic acid                                               
                      75%                                                 
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     EXAMPLE 2 
     A mixture of 0.403 g of 2,4-dimethylbenzaldehyde, 2 mg of copper(II) oxide and 5 ml of toluene was vigorously stirred at 25° C. under oxygen atmosphere (1 atm) for 1 hour. 
     Analysis of the reaction mixture gave the results shown below. 
     
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conversion of 2,4-dimethylbenzaldehyde                                    
                          99%                                             
selectivity of 2,4-dimethylperoxybenzoic acid                             
                          80%                                             
yield of 2,4-dimethylperoxybenzoic acid                                   
                          75%                                             
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     EXAMPLE 3 
     A mixture of 0.408 g of p-anisaldehyde, 7 mg of iron(II, III) oxide, and 5 ml of ethyl acetate was vigorously stirred at 25 ° C. under oxygen atmosphere (1 atm) for 1 hour. 
     Analysis of the reaction mixture gave the results shown below. 
     
         ______________________________________                                    
conversion of p-anisaldehyde                                              
                          99%                                             
selectivity of p-methoxyperoxybenzoic acid                                
                          80%                                             
yield of p-methoxyperoxybenzoic acid                                      
                          75%                                             
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     EXAMPLES 4˜9 
     A mixture of 0.318 g of benzaldehyde, 3 mol % based on the aldehyde of a catalyst , and 3 ml of benzene was vigorously stirred at 25° C. under oxygen atmosphere (1 atm) for 1 hour. 
     Analysis of the reaction mixture gave the results shown in Table 1. 
     
                                           TABLE 1                                 
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Example    Conversion of                                                  
                    Selectivity of per-                                   
                              yield of peroxy-                            
No.   Catalyst                                                            
           benzaldehyde (%)                                               
                    oxybenzoic acid (%)                                   
                              benzoic acid (%)                            
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4     Cr.sub.2 O.sub.3                                                    
           92       82        75                                          
5     MnO.sub.2                                                           
           98       88        86                                          
6     Fe.sub.3 O.sub.4                                                    
           97       89        86                                          
7     CO.sub.3 O.sub.4                                                    
           91       84        76                                          
8     Ni.sub.2 O.sub.3                                                    
           90       84        76                                          
9     Cu.sub.2 O                                                          
           96       89        85                                          
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     EXAMPLE 10 
     The reaction mixture obtained in example 1 was allowed to stand for 24 hours at 25° C. After that, it was analyzed and the result is shown below. Decomposition of peroxybenzoic acid was not observed. 
     
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conversion of benzaldehyde                                                
                      100%                                                
selectivity of peroxybenzoic acid                                         
                      79%                                                 
yield of peroxybenzoic acid                                               
                      79%                                                 
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     COMPARATIVE EXAMPLE 1 to 5 
     To the mixture of 100 g of a 6.2% by weight benzene solution of peroxybenzoic acid was added each catalyst shown in Table 2 and the resulting mixture was stirred for 6 hours at 25° C. 
     Analysis of the reaction mixture gave the results shown in Table 2. 
     
                                           TABLE 2                                 
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Comp.       Amount                                                        
                  Concent-                                                
                       Remaining ratio                                    
Ex.         of    ration                                                  
                       of peroxybenzo-                                    
No.  Catalyst                                                             
            catalyst                                                      
                  of metal                                                
                       ic acid Remark                                     
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1    COCl.sub.2.6H.sub.2 O                                                
            2.0 mg                                                        
                  5 ppm                                                   
                       .sup. 41%                                          
                               homogeneous                                
2    FeCl.sub.3                                                           
            1.5   5    29      homogeneous                                
3    CuCl.sub.2                                                           
            1.1   5    54      homogeneous                                
4    MnCl.sub.2.4H.sub.2 O                                                
            1.8   5    22      homogeneous                                
5    Fe.sub.2 O.sub.3                                                     
            0.7   5    99      heterogeneous                              
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     EXAMPLE 11 
     A solution obtained by removing insoluble materials from the reaction mixture obtained in the same manner as in example 1 was added dropwise to 98 mg of cyclohexanone at 25° C. during an hour, and at the same temperature, the mixture was stirred for 12 hours to carry out an oxidation reaction of cyclohexanone. 
     After the reaction, analysis of the reaction mixture gave the results shown below. 
     
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conversion of cyclohexanone                                               
                   99%                                                    
selectivity of caprolactone                                               
                   100%                                                   
yield of caprolactone                                                     
                   99%                                                    
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