Controlled feed process for making 3, 5-diaminobenzotrifluoride
This application is a CIP of application Ser. No. 07/685,106, filed Apr. 15, 1991, now U.S. Pat. No. 5,347,052. 
     This application is a continuation-in-part of application Serial No. 07/685,106, filed April 15, 1991, by David E. Albright, Jr., titled &#34;Preparation of 3,5-Diaminobenzotrifluoride.&#34; 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method of making 3,5-diaminobenzotrifluoride (DABTF) from 4-chloro-3,5-dinitrobenzotrifluoride (CDNBTF). In particular, it relates to a process in which a solution of CDNBTF is added at a controlled rate to a slurry of a palladium catalyst, magnesium oxide, and an alcohol solvent. 
     In U.S. patent application Ser. No. 07/685,106, there is described a process for preparing DABTF from CDNBTF by reacting the CDNBTF with hydrogen in the presence of magnesium oxide, a palladium catalyst, and methanol. While that process is successful in the laboratory, it has since been found that upon scaling up to quantities of three gallons or more, the yield falls, the solution becomes dark and gummy, and tars and other undesirable compounds form, making it difficult to isolate the product. 
     Analysis showed that the starting material, CDNBTF, was reacting with the product, DABTF, to produce a dimer according the equation ##STR1## where R is NH 2  if the reaction is performed in the presence of hydrogen and a Pd/C catalyst and is NO 2  if hydrogen and the Pd/C catalyst are not present. Other undesirable but unidentified compounds also seemed to be present. In addition, dechlorination did not occur completely so that 4-chloro-3,5-diaminobenzotrifluoride (CDABTF) was also formed, apparently due to deactivation of the catalyst. 
     SUMMARY OF THE INVENTION 
     I have discovered that these scale-up problems can be overcome so that large quantities of DABTF can be produced from CDNBTF in yields in excess of 90% without the formation of tars and compounds that make the solution dark and gummy. I am able to accomplish this by adding a solution of the CDNBTF to a slurry of MgO and the catalyst at a rate that is no greater than the reaction rate of CDNBTF so that the CDNBTF does not accumulate in the reaction mixture. The absence of accumulated CDNBTF in the reaction mixture is the key to preventing the formation of these tars and other undesirable compounds. 
     DESCRIPTION OF THE INVENTION 
     In the process of this invention, a solution of CDNBTF, a commercially available chemical, is prepared in a suitable alcohol, preferably a 1-alkanol to C 4  (i.e., methanol, ethanol, n-propanol, or n-butanol). The preferred solvent is methanol as it is inexpensive and is more effective than the other alcohols. However, methanol vapor may spontaneously combust in the presence of the catalyst and air so precautions must be taken to exclude air when methanol is contacted with the catalyst; use of a water-wet catalyst is also advisable. The solution of CDNBTF in the alcohol can be saturated but more dilute solutions are preferred as that produces a higher yield and fewer side products. Preferably, the concentration of CDNBTF in the alcohol should be less than 0.3 g/mL and a solution of about 5 to about 30 wt % works well. It is not necessary to pre-heat the solution of CDNBTF, though it can be pre-heated if desired. 
     Separately, a reactor is charged with a palladium catalyst and magnesium oxide. The palladium is preferably on a suitable support such as carbon, barium sulfate, or alumina; carbon is preferred as it is most effective, easiest to recycle, and inexpensive. Typically, the catalyst is sold as 1 to 10 wt % palladium on carbon. It is preferable to use about 5 to about 10 wt % of the catalyst, based on CDNBTF weight, as less catalyst will result in a slower reaction and may lead to the formation of side products and more catalyst is unnecessary. 
     The reactor is also charged with magnesium oxide, MgO. At least one equivalent of magnesium oxide is required for each equivalent of CDNBTF and it is preferable to use excess magnesium oxide to insure a complete reaction. About one to about two equivalents magnesium oxide is preferable per equivalent CDNBTF. 
     The reactor is then purged with an inert gas to remove air. Nitrogen is preferable for this purpose, although argon or other inert gases could also be used. Sufficient alcohol is then added to the reactor to form a slurry with the palladium catalyst and the magnesium oxide. 
     The presence of a hydrogen-donating reducing agent is also required. Examples of suitable hydrogen-donating reducing agents include hydrogen gas and Raney nickel. Sufficient hydrogen-donating reducing agent should be present to provide at least seven moles of hydrogen per mole of CDNBTF. The preferred hydrogen-donating reducing agent is hydrogen gas as it is most effective. If hydrogen gas is used, the slurry of catalyst and magnesium oxide in the alcohol is stirred and purged with the hydrogen gas, preferably at a pressure of about 50 to 100 psi. Other reducing agents can be simply added to the slurry. 
     The slurry is heated to a temperature of about 75° to about 100° C. and is maintained within that temperature range during the addition of the CDNBTF. At lower temperatures the reaction is too slow and side products may be produced and higher temperatures are unnecessary and lead to high vapor pressures. Because the reaction is highly exothermic, it is usually not necessary to add additional heat during the reaction and occasionally cooling may even be necessary. 
     The critical step in the process of this invention is the addition of the solution of CDNBTF. The solution of CDNBTF must be added to the slurry with stirring at a rate that is slow enough to prevent the accumulation of CDNBTF in the reaction vessel. This can be accomplished if the reaction is rapid and the rate of addition is slow. In other words, the CDNBTF should be added at a rate that does not exceed its reaction rate. This normally requires one to two hours. While I do not wish to be bound by any theories, I believe that the process of this invention proceeds according to the following equation ##STR2## Since the nitro groups are reduced before the chlorine is removed, the reaction proceeds through the intermediate, CDABTF. The completion of the reaction can be determined by taking samples of the solution and testing for the presence of CDABTF using a gas chromatograph (GC). The reaction will normally proceed to at least 90% completion and 100% completion can be achieved under optimized conditions. When the reaction is finished, stirring is terminated and the solids are permitted to settle out. The solution containing the DABTF can then be decanted off. A centrifuge could also be used to separate the solution from the solids. It is preferable to leave a small amount of alcohol remaining in the reactor. Additional magnesium oxide and some (about 10%) fresh catalyst can then be added to the reactor to prepare a new batch of product. 
     The DABTF can be recovered as a solid precipitate from the alcohol in the decanted solution by distillation of the alcohol, which is recovered and recycled. The magnesium chloride formed in the hydrodechlorination reaction is very soluble in the alcohol solvent and will precipitate with the DABTF. It can be separated from the DABTF by extraction with water. Two consecutive extractions using 80 to 100 g. of water per 100 g. of molten DABTF near a temperature of 100° C. remove the magnesium chloride to less than 500 ppm. Since more than 95% of the magnesium chloride is removed by the first extraction, the water from the second extraction can be recycled to the first extraction of another batch. After water extraction, the molten crude DABTF (density 1.28 g/cc) is easily settled from the water which can then be cooled and decanted. The small amount of DABTF which is solubilized in the water (typically 2 g. per 100 cc of water) can be recovered by extraction with an organic solvent such as butyl acetate. 
     Pure DABTF can be produced from the crude material by distillation. If any CDABTF is left in the product, the resulting DABTF can be purified by selective precipitation of its hydrochloride salt (see U.S. Pat. No. 5,130,490, herein incorporated by reference). DABTF is useful in the production of high performance polyimides, polyamides, polyurethanes, and epoxies. 
     The following examples further illustrate this invention. Examples 8, 9, 11, and 12 specifically illustrate the claimed process and the other examples illustrate the chemistry of the claimed process. 
    
    
     EXAMPLE 1 
     This example illustrates the making of DABTF from CDNBTF in a variety of alcohols. Using a 500 mL Parr hydrogenator, 10 g. CDNBTF, 1.6 g. MgO, 1.2 g. of water-wet 5% Pd/C catalyst, and 100-120 mL of solvent were added and heated to 80° C. under 50 psig of hydrogen. The following table gives the alcohols used and the results of these experiments. 
     
         ______________________________________                                    
                                DABTF Mass                                
Solvent  DABTF (g)   CDABTF (g) Yield (%)                                 
______________________________________                                    
Methanol 6.29        0.00       97                                        
Ethanol  5.91        0.44       91                                        
1-propanol                                                                
         5.41        0.88       83                                        
Ethyl Acetate                                                             
         3.90        2.62       60                                        
Isopropanol                                                               
         4.02        2.53       62                                        
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     EXAMPLE 2 
     Example 1 was repeated with methanol using various amounts of magnesium oxide. The following table gives the results. 
     
         ______________________________________                                    
CDNBTF             DABTF    CDABTF  DABTF                                 
(g)     MgO (g)    (g)      (g)     Yield (%)                             
______________________________________                                    
10      3.0 (4 eq.)                                                       
                   6.10     0.28    94                                    
10      1.6 (2 eq.)                                                       
                   6.29     0.00    97                                    
10      0.8 (1 eq.)                                                       
                   5.81     0.04    89                                    
10      0.0 (0 eq.)                                                       
                   2.85     0.76    44                                    
20      3.2 (2 eq.)                                                       
                   12.9     0.06    99                                    
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     EXAMPLE 3 
     Example 1 was repeated using different amounts of catalyst and methanol with 10 g. of CDNBTF. The following table gives the results. 
     
         ______________________________________                                    
Methanol         DABTF    CDABTF  DABTF Mass                              
(mL)    Pd/C (g) (g)      (g)     Yield (%)                               
______________________________________                                    
100     1.2      6.29     0.00    97                                      
 60     1.2      5.65     0.08    87                                      
 60     0.8      5.84     0.48    90                                      
100     0.8      6.23     0.03    96                                      
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     The above table shows that better results were achieved with the more dilute solution and that 0.8 g of catalyst was sufficient. 
     EXAMPLE 4 
     Example 1 was repeated with methanol using various bases. The following table gives the results. 
     
         ______________________________________                                    
CDNBTF            DABTF    CDABTF  DABTF Mass                             
(g)     Base      (g)      (g)     Yield (%)                              
______________________________________                                    
10      MgO       6.29     0.00    97                                     
10      CaO       0.90     0.86    14                                     
10      None      2.85     0.76    44                                     
 5      Na.sub.2 CO.sub.3                                                 
                  2.04     0.07    63                                     
10      NaOH      1.99     0.11    31                                     
10      NAOH*     3.62     2.45    56                                     
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 *Added after the initial reduction of the nitro groups without a base    
 present to eliminate the displacement of the labile C1.                  
 
    
     These experiments show that magnesium oxide is the only base that is effective in this reaction. 
     EXAMPLE 5 
     The reactor was charged with 6.32 g. CDABTF and 100 mL methanol. Runs were carried out for 20 minutes using 10 wt % catalyst and different temperatures and pressures. The following table gives the results. 
     
         ______________________________________                                    
T (°C.)                                                            
        P (psia)    MgO (eq.) Conversion (%)                              
______________________________________                                    
23      29          1.5       15.6                                        
23      62          1.5       13.8                                        
60      29          1.5       58.7                                        
80      29          1.5       86.4                                        
90      29          1.5       98.5                                        
90      29           1.16     96.5                                        
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     The above table shows that temperatures over 60° C. are required for a satisfactory conversion of CDABTF to DABTF. 
     EXAMPLE 6 
     In this example, the reaction was scaled up to a larger reactor. A three gallon Pfaudler reactor was charged with 700 g. of CDNBTF, 7000 mL methanol, 104 g. magnesium oxide, and 84 g. water-wet Pd/C catalyst. The reaction was carried out under 40 to 50 psi of hydrogen and near 80° C. After one hour, a gas chromatogram (GC) sample showed the presence of 9.4 area % CDABTF. After an additional four hours, GC analysis still showed the presence of 9.1 area % CDABTF. This indicated that the catalyst had been deactivated and that no significant further reaction was occurring. GC internal standard analysis of the filtered reaction product showed the formation of 382.3 g. DABTF and 29.6 g. CDABTF. However, the product was highly colored and solvent evaporation showed the presence of a tar-like side product which had not been seen in the small scale reactions. This tar-like substance greatly hindered all further processing, including catalyst recovery. 
     EXAMPLE 7 
     Example 6 was repeated using 700 g. CDNBTF, 7000 mL methanol, 100 g. MgO and 100 g. of water-wet palladium on carbon catalyst in 70 to 85 psi hydrogen and 85°-100° C. GC analysis of the reaction product showed removal of the CDBTF to less than 3%. However, tar formation also increased and the mass yield of DABTF was only about 75%. 
     EXAMPLE 8 
     The reaction of Example 7 was repeated except that the reactor was modified so that a concentrated methanol solution of CDNBTF could be metered into the reactor. The reactor was charged with 2900 mL of methanol, 104 g. MgO and 100 g. of water-wet Pd/C catalyst. The charge was heated and agitated under hydrogen pressure and the addition of 700 g. CDNBTF and 3000 mL of methanol was carried out over 90 minutes at a rate below the reaction rate of the CDNBTF, so that no CDNBTF accumulated. The reaction was under 50 psi hydrogen at about 70°-80° C. Analysis of the reaction product showed that about 12% of the CDABTF intermediate remained unreacted. However, the product was clear with no trace of any tar side product. 
     EXAMPLE 9 
     Example 8 was repeated using an initial charge of 3000 mL of methanol, 100 g. MgO, and 130 g. of water-wet Pd/C catalyst. The mixture was heated and agitated under hydrogen pressure and the addition of 1100 g. CDNBTF and 3500 mL of methanol was carried out over several hours. The rate of addition of the CDNBTF was less than its reaction rate and no CDNBTF accumulated. At the end of the CDABTF intermediate addition, more than 5 wt % CDNBTF remained unconverted. Continuation of agitation and heating between 80°-90° C. over several hours resulted in the complete removal of the CDABTF intermediate. GC analysis showed the formation of 620 g. of pure DABTF (87 wt % mass yield). The reaction products were initially water white but turned amber after exposure to air. No trace of tar side products was present. 
     EXAMPLE 10 
     The reactor was charged with 2700 mL of methanol, 67 g. of magnesium oxide, and only 15 g. of water-wet Pd/C catalyst. 750 g. of CDNBTF in 3000 mL of methanol was then slowly fed over 157 minutes. The temperature was maintained below 60° C. and the hydrogen pressure was kept between 40 and 60 psig. These conditions resulted in incomplete conversion and produced many side products which hindered product recovery. 
     EXAMPLE 11 
     A slurry containing 75 g. of water-wet Pd/C and 75 g. of magnesium oxide in 2000 mL of methanol was maintained near 80° C. under a hydrogen pressure of 90 psig. A dilute solution of 400 g. of CDNBTF in 3000 mL of methanol was added to the reactor over the course of 1 hour so that no starting material accumulated. Following the feed addition, the CDABTF intermediate concentration was approximately 20 percent. Continued agitation for another 6 hours achieved complete conversion of the remaining CDABTF intermediate to the DABTF product. The reaction products were colorless, very pure and easy to process. Distillation of the product proceeded readily with less than 3 percent of the material lost in the still bottoms. The ease of processing the dilute solution makes this an attractive option for dimer minimization and yield improvement. Coupling the dilute solution feed with a slower feed rate should provide the best hydrogenation results. 
     EXAMPLE 12 
     To achieve catalyst recycle in the 3 gallon Pfaudler reactor, solids from each completed hydrogenation were allowed to settle to the reactor bottom and the supernatant liquid containing the DABTF product was siphoned off for processing. Enough liquid was allowed to remain in the reactor to provide a starting slurry for the next batch. Make-up catalyst and MgO were added, the slurry was heated and agitated under hydrogen pressure, and then the next batch of CDNBTF solution was reacted with controlled feed. Five consecutive 3 gallon Pfaudler batches were carried to complete conversion using the Pd/C catalyst from the previous batch plus about 15 percent fresh catalyst make-up. The following table lists the charges and results for each experiment. 
     
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      Controlled  Charged to Pfaudler Prior to Batch                      
Batch Feed Solution                                                       
                  MgO     Pd/C  MeOH   Conversion                         
______________________________________                                    
1     750 g CDNBTF                                                        
                  134 g   100 g 4000 mL                                   
                                       100%                               
      in                                                                  
      3100 mL MeOH                                                        
2     750 g CDNBTF                                                        
                  50 g    20 g  0      100%                               
      in                                                                  
      3000 mL MeOH                                                        
3     750 g CDNBTF                                                        
                  60 g    15 g  0      100%                               
      in                                                                  
      3000 mL MeOH                                                        
4     750 g CDNBTF                                                        
                  56 g    10 g  0      100%                               
      in                                                                  
      3000 mL MeOH                                                        
5     750 g CDNBTF                                                        
                  56 g    15 g  0      100%                               
      in                                                                  
      3000 mL MeOH                                                        
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 Note: Pd/C charged is approximately 50% water by mass.                   
 
    
     The controlled feed solution was added slowly so that no accumulation of the CDNBTF occurred. 
     No significant decrease in catalyst activity was observed over the course of the 5 consecutive runs. GC analysis of the final products showed that the CDABTF intermediate was reduced to negligible levels in each batch. This level of CDABTF removal would normally require about 75 to 100 g. of water-wet Pd/C for each batch for a total of 375 to 500 g. of 5% Pd/C (50% water). However, by recycling the catalyst from the previous batch, these runs required only 160 g. of Pd/C catalyst. Since no catalyst deactivation was observed, a greater reduction in catalyst usage is likely possible.