Oxidation of phenol to p-benzoquinone in acetonitrile/methanol cosolvent
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The process of this invention is related to that claimed in U.S. Pat. No. 3,987,068 and filed in Class 260/396 R. 
     2. Description of the Prior Art 
     P-benzoquinone is an intermediate in the formation of hydroquinones which are useful as antioxidants, reducing agents, polymer intermediates, etc. 
     U.S. Pat. No. 3,987,068 concerns a process for making p-benzoquinone wherein phenol is contacted with a source of oxygen in the presence of a copper salt catalyst in a nitrile solvent such as acetonitrile. The patent contains no suggestion of an acetonitrile/methanol cosolvent system. 
     U.S. Pat. No. 3,210,384 teaches the making of a tetrasubstituted dibenzoquinone by oxidizing disubstituted phenol in the presence of a copper salt catalyst in a solvent comprising acetonitrile. Cosolvents such as methanol, ethanol, acetone, trichloroethylene, etc., can be employed &#34;providing they do not interfere or enter into the oxidation reaction&#34; (column 6, lines 49 and 50). 
     U.S. Pat. No. 3,870,731 teaches the oxidation of phenol to p-benzoquinone in the presence of copper salt catalysts, a catalyst promoter, and methanol. The patent does not disclose acetonitrile solvent. 
     U.S. Pat. No. 3,859,317 teaches the cobalt and manganese-catalyzed oxidation of phenol to p-benzoquinone in the presence of a solvent such as methanol. Acetonitrile is not disclosed. 
     There is nothing in the prior art to suggest the improvement in yield and productivity made possible by employing the specific acetonitrile/methanol cosolvent system described herein. 
     SUMMARY OF THE INVENTION 
     This invention concerns an improvement in the process for making p-benzoquinone (quinone), ##STR1## from phenol, ##STR2## and an oxidant in the presence of a copper salt catalyst. The improvement comprises employing a cosolvent system comprising acetonitrile and methanol, the weight ratio of acetonitrile to methanol being about 1 to 20:1. The preferred ratio of acetonitrile to methanol is 2 to 10:1. 
     The oxidant can be oxygen, or, preferably, an oxygen-containing gas. The reaction is carried out under pressure, the partial pressure of oxygen being about 10 to 100 atmospheres (1.0 to 10.1 MPa). Reaction temperatures are normally about 20° to 80° C. 
     The copper salt catalysts can be one or more salts selected from the group consisting of cuprous chloride, cuprous bromide, and cupric chloride. 
     The term &#34;consumption&#34; employed herein is defined as the number of moles of phenol that went into the reaction minus the number of moles that came out of the reaction, divided by the number of moles in; said fraction multiplied by 100. &#34;Consumption&#34; is a measure of the percent of starting phenol used up. 
     The term &#34;yield&#34; which is a measure of the selectivity and efficiency of the process of this invention is defined as the number of moles of quinone produced, divided by the consumption; said fraction multiplied by 100. 
     The term &#34;productivity&#34; is defined for a continuous reactor at steady state as the number of gram moles of benzoquinone (BQ) produced per hour, divided by the number of liters of liquid reaction mixture in the reactor, g moles/L-hr. 
     DETAILS OF THE INVENTION 
     An oxygen partial pressure of about 1.0 to 10.1 MPa will insure that the reaction is directed toward production of p-benzoquinone and away from production of polymeric products. Typically, an acetonitrile/methanol solution of phenol and copper salt catalyst is contacted with air, oxygen, or oxygen diluted with any other gas which does not interfere with the reaction. When air is used as the oxidizing agent, air pressures of about 5 to 50 MPa, preferably 7 to 35 MPa, are employed. CAUTION SHOULD BE EXERCISED WHEN THE OXIDANT IS UNDILUTED OXYGEN. 
     The amount of copper salt used should be sufficient to provide a ratio of about 5 to 25 moles of the phenol per mole of copper salt. With higher molar ratios, the amount of copper salt is so small that the phenol consumption, as well as the amount thereof converted to the benzoquinone drops. No advantage in terms of yield is achieved in using an amount of copper salt in excess of that which gives about a 5/1 molar ratio of phenol to copper salt. 
     The concentration of the acetonitrile/methanol solution of the phenol can vary widely. There is no minimum concentration to be adhered to except that which is dictated by practical considerations. At concentrations of about 50% or more of phenol based on the weight of the acetonitrile/methanol, the phenol consumption tends to decrease. Therefore, a preferred concentration range is about 10% to 50%. The molar ratio of solvent to copper salt is in excess of about 10/1, and often in excess of about 100/1. 
     The solution of the phenol in the solvent containing the copper complex is contacted with the oxidant for about 10 minutes or more, the specific time required in any given case depending on the reaction temperature and the molar ratio of phenol to copper salt. There is no advantage to reaction times exceeding about two hours. 
    
    
     The following Examples illustrate various embodiments of the invention. 
     EXAMPLES 1 TO 11 AND COMPARISONS A TO C 
     Batch Operation 
     The general procedure utilized for the batch runs was as follows. Phenol and catalyst were dissolved in acetonitrile/methanol cosolvent in a Hastelloy &#34;C&#34; reactor. The reactor was then pressurized with air and heated with agitation. The reactor was then cooled to about room temperature and vented. Specific conditions and results are noted in Table 1. 
     
                       TABLE 1                                                     
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[Reaction time: 1 hour; 10 grams of phenol in 50-60 mL                    
of acetonitrile/methanol; all runs employed air at                        
2000 psig (13.8 MPa)].                                                    
                                     Phenol                               
                         Wt.         Con-                                 
                 Wt.     per-        sump- Yield                          
                 of      cent of     tion  of BQ                          
Ex. or                                                                    
      Cata-      Cata-   Meth- Temp. (Per- (Per-                          
Comp. lyst       lyst    anol  °C.                                 
                                     cent) cent)                          
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1     CuCl.sub.2 .                                                        
                 0.85/   20    50    61    59                             
      2H.sub.2 O/CuCl                                                     
                 0.50                                                     
2     CuCl.sub.2 .                                                        
                 1.15/   20    50    57    65                             
      2H.sub.2 O/CuCl                                                     
                 0.35                                                     
3     CuCl.sub.2 .                                                        
                 0.85/   33    50    45    62                             
      2H.sub.2 O/CuCl                                                     
                 0.50                                                     
4     CuCl.sub.2 .                                                        
                 1.15/   33    50    40    65                             
      2H.sub.2 O/CuCl                                                     
                 0.35                                                     
5     CuCl.sub.2 .                                                        
                 1.7     20    40    36    42                             
      2H.sub.2 O                                                          
6     CuCl.sub.2 .                                                        
                 1.7     20    50    48    75                             
      2H.sub.2 O                                                          
7     CuCl       1.0     20    60    88    61                             
8     CuCl.sub.2 .                                                        
                 1.7     17    50    62    71                             
      2H.sub.2 O                                                          
9     CuCl.sub.2 .             50    57    74                             
      2H.sub.2 O                                                          
10    CuCl.sub.2 .                                                        
                 1.7     50    50    49    59                             
      2H.sub.2 O                                                          
11    CuCl       1.0     50    50    60    62                             
A     CuCl.sub.2 .                                                        
                 1.7     100   50    15    --                             
      2H.sub.2 O                                                          
B     CuCl.sub.2 .                                                        
                 1.7     0     50    76    53                             
      2H.sub.2 O                                                          
C     CuCl       1.0     0     50    84    48                             
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     EXAMPLES 12 TO 63 AND COMPARISONS D TO G 
     Continuous Operation 
     In continuous operation of the present process, a solution of phenol and copper salt catalyst in acetonitrile/methanol and an oxidant were fed continuously into a stirred tank reactor. Product solution was removed continuously from the reactor. 
     Tables 2 to 4 summarize a number of continuous runs in acetonitrile/methanol, i.e., according to the process of this invention. In addition, a number of runs are summarized of comparative processes, i.e., wherein no methanol cosolvent was employed. The Comparative Examples serve to highlight the benefits that flow from the process of this invention. 
     In these Tables, &#34;Short&#34; holdup times are about 0.25 hr, &#34;Medium&#34; holdup times are about 0.40 hr, and &#34;Long&#34; holdup times are about 0.80 hour. &#34;Holdup time&#34; is defined as liters of liquid reaction mixture in the reactor, divided by the liquid feed rate in liters per hour. Pressures given are air pressures. Catalyst concentrations vary from 0.1 to 0.3 g mole per liter of feed as indicated. 
     Comparison of the continuous operation of the process employing cosolvent vs. the process employing only acetonitrile (and no cosolvent) demonstrates an increase in yield of up to about 20% and an increase in productivity of up to about 15% for the cosolvent process of this invention. 
     
                       TABLE 2                                                     
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[Air Oxidation; 60° C.; 20.7 MPa; 0.10 g mol of CuCl               
and 1.06 g mol of phenol per liter of phenol/solvent/                     
catalyst feed]                                                            
                              Phenol                                      
                              Con-                                        
      Co-     Wt.             sump- BQ    Produc-                         
      sol-    Percent         tion  Yield,                                
                                          tivity                          
Ex. or                                                                    
      vent    Co-      Holdup (Per- (Per- g moles/                        
Comp. Used    solvent  Time   cent) cent) L-hr                            
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D     None    --       Long   74.8  53.9  0.57                            
12    MeOH    21.7     Long   68.5  66.1  0.63                            
13    MeOH    21.7     Long   46.9  78.7  0.53                            
E     None    --       Medium 61.0  56.6  0.90                            
14    MeOH    21.7     Medium 57.0  66.7  1.05                            
15    MeOH    21.7     Short  45.9  66.8  1.24                            
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                       TABLE 2A                                                    
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[Same conditions as in Table 2 except 2.13 g mole of                      
phenol per liter of feed]                                                 
             Wt.            Phenol                                        
     Co-     Per-           Consump-                                      
                                    BQ    Produc-                         
     sol-    cent           tion    Yield,                                
                                          tivity                          
     vent    Co-     Holdup (Per-   (Per- g moles/                        
Ex.  Used    solvent Time   cent)   cent) L-hr                            
______________________________________                                    
16   MeOH    21.7    Long   45.3    49.0  0.60                            
17   MeOH    21.7    Long   34.0    77.8  0.76                            
18   MeOH    21.7    Medium 40.0    42.5  0.96                            
19   MeOH    21.7    Short  22.9    39.7  0.78                            
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                       TABLE 3                                                     
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[Air oxidation, 60° C.; 20.7 MPa; 0.20 g mole of                   
CuCl and 1.06 g mole of phenol per liter of phenol/                       
solvent/catalyst feed]                                                    
                              Phenol                                      
                              Con-                                        
      Co-     Wt.             sump- BQ    Produc-                         
      sol-    Percent         tion  Yield,                                
                                          tivity                          
Ex. or                                                                    
      vent    Co-      Holdup (Per- (Per- g moles/                        
Comp. Used    solvent  Time   cent) cent) L-hr                            
______________________________________                                    
20    MeOH    21.7     Long   59.6  70.2  0.60                            
21    MeOH    21.7     Long   87.2  61.9  0.73                            
F     None    --       Medium 81.5  46.8  1.01                            
22    MeOH    21.7     Medium 70.4  62.2  1.14                            
23    MeOH    21.7     Medium 68.2  61.0  1.16                            
G     None    --       Short  81.6  43.2  1.50                            
24    MeOH    21.7     Short  60.8  57.6  1.60                            
25    MeOH    21.7     Short  60.1  54.7  1.50                            
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                       TABLE 3A                                                    
______________________________________                                    
[Same conditions as in Table 3 except 2.13 g mole of                      
phenol per liter of feed]                                                 
             Wt.            Phenol                                        
     Co-     Per-           Consump-                                      
                                    BQ    Produc-                         
     sol-    cent           tion    Yield,                                
                                          tivity                          
     vent    Co-     Holdup (Per-   (Per- g moles/                        
Ex.  Used    solvent Time   cent)   cent) L-hr                            
______________________________________                                    
26   MeOH    21.7    Long   52.3    82.8  1.23                            
27   MeOH    21.7    Long   60.6    74.5  1.31                            
28   MeOH    21.7    Long   63.1    63.0  1.13                            
29   MeOH    21.7    Medium 49.4    59.7  1.55                            
30   MeOH    21.7    Short  48.1    44.5  1.80                            
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                       TABLE 4                                                     
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[Air oxidation; 60° C.; 20.7 MPa; 0.30 g mole of                   
CuCl and 1.06 g mole of phenol per liter of phenol/                       
solvent/catalyst feed]                                                    
             Wt.            Phenol                                        
     Co-     Per-           Consump-                                      
                                    BQ    Produc-                         
     sol-    cent)          tion    Yield,                                
                                          tivity                          
     vent    Co-     Holdup (Per-   (Per- g moles/                        
Ex.  Used    solvent Time   cent)   cent) L-hr                            
______________________________________                                    
31   MeOH    21.7    Short  66.4    52.7  1.31                            
32   MeOH    21.7    Medium 71.9    60.9  1.21                            
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     TABLES 5 TO 8 
     Table 5 summarizes additional runs employing acetonitrile/methanol cosolvent at pressures of 34.5 MPa. Table 6 shows that the most preferred process conditions have methanol concentrations in the range of about 10 to 22 weight percent at short and medium holdup times. Table 7 explores the effect of varying temperatures on the process of this invention. Table 8 explores the use of cupric chloride, cuprous bromide and cuprous chloride. 
     
                       TABLE 5                                                     
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[Air oxidation, 21.7 weight percent of methanol,                          
60° C., 34.5 MPa]                                                  
                            Phenol                                        
     CuCl    Phenol         Consump-                                      
                                    BQ    Produc-                         
     (g      (g             tion    Yield tivity                          
     Mol/    Mol/    Holdup (Per-   (Per- g moles/                        
Ex.  Liter)  Liter)  Time   cent)   cent) L-hr                            
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33   0.10    1.06    Long   61.2    71.6  0.60                            
34   0.20    1.06    Medium 73.2    56.8  1.17                            
35   0.20    1.06    Medium 73.0    62.1  1.11                            
36   0.20    1.06    Short  60.1    71.7  1.72                            
37   0.20    1.06    Short  65.6    59.0  1.64                            
38   0.20    2.13    Long   65.8    68.0  1.27                            
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                       TABLE 6                                                     
______________________________________                                    
[Varying methanol concentrations; 60° C.; Pressures                
of 34.5 MPa for Examples 39 to 44 and 20.7 MPa for                        
Examples 45 to 47]                                                        
             Phe-                Phenol                                   
             nol                 Con-        Pro-                         
     CuCl    (g/           Wt    sump- BQ    duc-                         
     (g      Mol/   Hold-  Per-  tion  Yield tivity g                     
     Mol/    Li-    up     cent  (Per- (Per- moles/                       
Ex.  Liter)  ter)   Time   MeOH  cent) cent) L-hr                         
______________________________________                                    
39   0.20    1.06   Short  10.9  73.0  55.7  1.65                         
40   0.20    1.06   Short  21.7  65.6  59.0  1.64                         
41   0.20    1.06   Short  43.4  61.5  50.9  1.28                         
42   0.20    1.06   Medium 10.9  78.0  66.1  1.37                         
43   0.20    1.06   Medium 21.7  73.2  56.8  1.17                         
44   0.20    1.06   Medium 43.4  68.8  48.7  0.92                         
45   0.10    2.13   Long    5.5  48.3  49.5  0.62                         
46   0.10    2.13   Long   10.9  43.3  48.2  0.59                         
47   0.10    2.13   Long   21.7  45.3  49.0  0.60                         
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                       TABLE 7                                                     
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[Varying temperature; 21.7 weight percent of methanol;                    
pressures of 34.5 MPa for Examples 48 to 53 and                           
20.7 MPa for Examples 54 to 56]                                           
             Phe-                Phenol                                   
             nol                 Con-        Pro-                         
     CuCl    (g                  sump- BQ    duc-                         
     (g      Mol/   Hold-        tion  Yield tivity g                     
     Mol/    Li-    up     Temp. (Per- (Per- moles/                       
Ex.  Liter)  ter)   Time   °C.                                     
                                 cent) cent) L-hr                         
______________________________________                                    
48   0.20    1.06   Medium 40    47.5  66.1  0.92                         
49   0.20    1.06   Medium 60    73.2  56.8  1.17                         
50   0.20    1.06   Medium 70    81.4  64.6  1.42                         
51   0.20    1.06   Short  40    44.6  68.8  1.25                         
52   0.20    1.06   Short  60    65.6  59.0  1.64                         
53   0.20    1.06   Short  70    76.4  58.2  1.88                         
54   0.10    2.13   Long   25    13.8  85.9  0.33                         
55   0.10    2.13   Long   60    45.3  49.0  0.60                         
56   0.10    2.13   Long   80    59.0  49.0  0.76                         
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                       TABLE 8                                                     
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                                      Produc-                             
                  Phenol      BQ      tivity                              
                  Consumption Yield   g moles/                            
Ex.  Variable     (Percent)   (Percent)                                   
                                      L-hr                                
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[CuBr vs. CuCl as catalyst at 60° C.; 0.20 g mole of               
Cu per liter; 1.06 g mol of phenol per liter; 21.7                        
weight percent of MeOH; long holdup time]                                 
57   CuBr (34.5 MPa)                                                      
                  69.2        50.3    0.51                                
58   CuCl (20.7 MPa)                                                      
                  59.6        70.2    0.60                                
59   CuCl (20.7 MPa)                                                      
                  87.2        61.9    0.73                                
[CuBr vs. CuCl at 60° C.; 0.10 g mol of Cu per liter;              
1.06 g mol of phenol per liter; 21.7 weight percent                       
of MeOH, long holdup time; 34.5 MPa]                                      
60   CuBr         37.7        56.1    0.29                                
61   CuCl         61.2        71.6    0.60                                
[CuCl.sub.2 . 2H.sub.2 O vs. CuCl at 60° C.; 0.10 g mol of Cu per  
liter; 1.06 g mol of phenol per liter; 21.7 weight                        
percent of MeOH; medium holdup time; 20.7 MPa]                            
62   CuCl.sub.2 . 2H.sub.2 O                                              
                  34.6        69.7    0.61                                
63   CuCl         57.0        66.7    1.05                                
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