Organic acids and process for preparing same
This application is a continuation-in-part application of our U.S. Patent application Ser. No. 696,752, filed June 16, 1976, entitled Organic Acids and Process for Preparing Same, now U.S. Pat. No. 4,052,448. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a mixture of polycyclic aromatic polycarboxylic acids that is substantially insoluble in acetone and substantially insoluble in water and a process for preparing the mixture by treating a carbonaceous material with nitric acid. 
     2. Description of Prior Art 
     Treating a carbonaceous material, such as coal, with nitric acid to obtain carboxylic acids is shown in U.S. Pat. Nos. 2,555,410 to Howard, 2,726,262 to Grosskinsky et al., 2,949,350 to Heinze et al., 2,991,189 to Rickert and 3,173,947 to Benning et al. In each of these the acids obtained are said to be water soluble. Creigton et al. in U.S. Pat. No. 3,468,943 are interested in passing coal through a screw conveyor and at spaced apart intervals feeding appropriate quantities of concentrated nitric acid so that it is completely reacted with the coal before the coal arrives at the next nitric acid feed point to obtain humic acids which are said to be partially soluble in sodium hydroxide solution but substantially insoluble in water. 
     SUMMARY OF THE INVENTION 
     We have prepared novel mixtures of polycyclic aromatic polycarboxylic acids that are substantially insoluble in acetone and substantially insoluble in water. The individual components of said mixtures are believed to be composed of condensed and/or non-condensed benzene rings, with an average number of benzene rings in the individual molecules ranging from two to about ten, but generally from three to about eight. On the average, the number of carboxyl groups carried by the individual molecules are believed to range from about four to about ten, generally from about six to about eight, and the average number of nitro groups from about 0 to about four, generally from about 0 to about two. The average molecular weight of the mixture is believed to range from about 600 to about 3000, generally from about 1000 to about 3000, and the average neutral equivalent from about 150 to about 300, generally from about 175 to about 250. A typical analysis of the novel mixture is defined below in Table I in approximate amounts. 
     
                       TABLE I                                                     
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            Weight Per Cent                                               
            Broad Range                                                   
                       General Range                                      
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Carbon        50 to 65     52 to 62                                       
Hydrogen      3 to 5       3.2 to 4                                       
Nitrogen      2 to 6       2.2 to 4                                       
Oxygen        20 to 40     23 to 30                                       
Sulfur        0.1 to 0.6   0.2 to 0.5                                     
Ash           5 to 10      6 to 9                                         
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     A preferred and novel procedure for obtaining the above novel mixtures is described in reference to FIG. I. There is introduced into reactor 2 by line 4 an aqueous solution of nitric acid and by line 6 a carbonaceous material. The nitric acid can have a concentration of about five to about 90 percent, but preferably will be in the range of about 10 to about 70 percent. The carbonaceous material is preferably a solid in the form of a slurry, for example, an aqueous slurry containing the carbonaceous material in particulate form and from about 50 to about 90 weight percent of water. 
     The solid carbonaceous material that can be used herein can have the following composition on a moisture-free basis: 
     
                       TABLE II                                                    
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            Weight Per Cent                                               
            Broad Range                                                   
                      Preferred Range                                     
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Carbon        45-95       60-92                                           
Hydrogen      2.5-7        4-6                                            
Oxygen        2.0-45       3-25                                           
Nitrogen      0.75-2.5    0.75-2.5                                        
Sulfur        0.3-10      0.5-6                                           
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     The carbon and hydrogen content of the carbonaceous material will reside primarily in multi-ring aromatic compounds (condensed and/or uncondensed), heterocyclic compounds, etc. Oxygen and nitrogen are believed to be present primarily in chemical combination. Some of the sulfur is believed to be present in chemical combination with the aromatic compounds and some in chemical combination with inorganic elements associated therewith, for example, iron and calcium. 
     In addition to the above the solid carbonaceous material being treated herein will also contain solid, primarily inorganic, compounds which will not be converted to the desired organic mixture claimed herein, which are termed ash, and are composed chiefly of compounds of silicon, aluminum, iron and calcium, with smaller amounts of compounds of magnesium, titanium, sodium and potassium. The ash content of the carbonaceous material treated herein will amount to less than about 50 weight percent, based on the moisture-free carbonaceous material, but, in general, will amount to about 0.1 to about 30 weight percent, usually about 0.5 to about 20 weight percent. 
     Anthracitic, bituminous and subbituminous coal, lignitic materials, and other type of coal products referred to in ASTM D-388 are exemplary of the solid carbonaceous materials which can be treated in accordance with the process defined herein to produce the claimed organic mixture. Some of these carbonaceous materials in their raw state will contain relatively large amounts of water. These can be dried prior to use herein. The carbonaceous material, prior to use, is preferably ground in a suitable attrition machine, such as a hammermill, to a size such that at least about 50 percent of the carbonaceous material will pass through a 40-mesh (U.S. Series) sieve. As noted, the carbonaceous material is slurried in a suitable carrier, preferably water, prior to reaction with nitric acid. If desired, the carbonaceous material can be treated, prior to reaction herein, using any conventional means, to remove therefrom any materials forming a part thereof that will not be converted in reaction with nitric acid herein. 
     The reactant mixture in reactor 2 is stirred while being maintained at a temperature of about 15° to about 200° C., preferably about 50° to about 100° C., and a pressure of about atmospheric to about 1000 pounds per square inch gauge (about atmospheric to about 70 kilograms per square centimeter), preferably about atmospheric to about 500 pounds per square inch gauge (about atmospheric to about 35 kilograms per square centimeter) for about 0.5 to about 15 hours, preferably about two to about six hours. In order to obtain the desired mixture herein without losing appreciable amounts of carboxyl and/or nitro groups on the acids that are formed during the oxidation, and to obtain the desired acids in high yields in reactor 2, it is absolutely critical that the reaction conditions therein, namely nitric acid concentration, temperature, pressure and reaction time, be so correlated to minimize and, preferably, to avoid decarboxylation and/or denitrofication. Gaseous products, such as nitrogen oxides, can be removed from reactor 2 by line 8. 
     The reaction product is removed from reactor 2 by line 10. We have found that the reaction product is soluble in, or reactable with, sodium hydroxide. At this point it is necessary to separate the oxidized product from the water and nitric acid associated therewith. This separation must be accomplished in a manner so that the carboxyl groups are not removed from the acid product. Distillation for the removal of water will not suffice, because under the conditions required for such separation, a significant loss of carboxyl groups would occur. Accordingly, we have found that a mechanical separation will suffice. The reaction product in line 10 is therefore led to a separator 12, which can be, for example, a filter or a centrifuge. 
     The solids that are recovered in separator 12, also soluble in sodium hydroxide, are led by line 14 to a separator 16 wherein they are subjected to extraction with acetone that is introduced therein by line 18. Such separation can be carried out at a temperature of about 20° to about 60° C., preferably about 25° to about 50° C., and a pressure of about atmospheric to about 500 pounds per square inch gauge (about atmospheric to about 35 kilograms per square centimeter), preferably about atmospheric to about 100 pounds per square inch gauge (about atmospheric to about seven kilograms per square centimeter). The novel solid material obtained herein, insoluble in acetone, is removed from separator 16 by line 20 and the acetone solution of the acid mixture by line 22. The acetone solution can then be led to drier 24 wherein acetone is separated therefrom by line 26 and the acetone-soluble, water-insoluble polyaromatic, polycarboxylic acid mixture is recovered in line 28. As before, the acid mixture in drier 24 can be treated by so correlating the conditions therein to remove acetone therefrom in such manner so as to minimize and, preferably, avoid, decarboxylation. The temperature can be in the range of about 10° to about 60° C., preferably about 20° to about 50° C., the pressure about 10 millimeters of mercury to about atmospheric, preferably about 30 millimeters of mercury to about atmospheric, for about 0.5 to about 24 hours, preferably about one to about five hours. 
     The filtrate obtained in separator 12 is removed therefrom by line 30. In all cases the filtrate will contain water, nitric acid and most of the inorganic material (ash) that was present in the carbonaceous charge. In addition there can also be present other oxidized material, which are primarily acetone-soluble, water-soluble organic acids. 
     Separation of the filtrate into its component parts can be effected as follows. It can be passed to distillation tower 32 maintained at a temperature of about 50° to about 100° C., preferably about 70° to about 90° C. and a pressure of about 10 millimeters of mercury to about atmospheric, preferably about 30 millimeters of mercury to about atmospheric. Under these conditions nitric acid and water are removed from distillation tower 32 by line 34 and solids by line 36. The solids are led to separator 38 where they are subjected to extraction with acetone introduced therein by line 40. The conditions in separator 38 are similar to those used in separator 16. A mixture of acetone-soluble, water-soluble organic acids is removed from separator 38 by line 42 and substantially all of the inorganic material that was present in the carbonaceous charge by line 44. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Several runs were carried out in which a North Dakota Lignite analyzing as follows, on a substantially moisture-free basis, was subjected to oxidation using nitric acid as the oxidant: 65.03 weight percent carbon, 4.0 weight percent hydrogen, 27.0 weight percent oxygen, 0.92 weight percent sulfur, 0.42 weight percent nitrogen and 0.04 weight percent moisture. The ash was further analyzed and found to contain 43 weight percent oxygen, 7.8 weight percent sulfur and the remainder metals. In each run, the data of which are summarized below in Table III, 70 percent aqueous nitric acid was used. In each run, 100 milliliters of the defined nitric acid was gradually added to the stirred slurry containing 100 grams of powdered lignite defined above (corresponding to 67.5 grams of moisture-free feed) and 370 grams of water while maintaining the contents at selected temperature levels and atmospheric pressure. Nitrogen oxides were permitted to escape from the reaction zone as they evolved. 
     At the end of the reaction period the product slurry was withdrawn from the reaction zone and filtered to obtain a solids fraction and a filtrate. The solids were extracted with acetone at atmospheric pressure. The acetone solution was then subjected to evaporation at reduced pressure to obtain acetone-soluble solids. The novel acetone-insoluble portion was dried at reduced pressure and was found to be readily soluble in sodium hydroxide. 
     The filtrate in Runs Nos. 6 and 7 was found to consist essentially of unreacted nitric acid, water and inorganic materials (ash). However, in each of the remaining runs acetone soluble, water-soluble organic acids were also found. The work-up of the filtrate was carried out as follows. Initially the filtrate was subjected to distillation to separate unreacted nitric acid and water therefrom. The remaining solids were subjected to extraction with acetone. The acetone solution was dried to remove acetone therefrom, resulting in the recovery of the acetone-soluble, water-soluble organic acids substantially completely soluble in sodium hydroxide. The residue was mainly ash. The data obtained are summarized below in Table III. 
     
                                           TABLE III                               
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          Reaction                                                        
               Acetone-Soluble, Water-                                    
                           Acetone-Insoluble                              
Run                                                                       
   Temperature,                                                           
          Time,                                                           
               Insoluble Organic Acids,                                   
                           Organic Acids,                                 
                                    Analysis of Organic Acids, Weight Per 
                                    Cent                                  
No.                                                                       
   ° C.                                                            
          Hours                                                           
               Grams       Grams    Carbon                                
                                        Hydrogen                          
                                              Nitrogen                    
                                                   Oxygen                 
                                                        Sulfur            
                                                            Ash           
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1  70     2    51.1        18        55.52.sup.a                          
                                        3.72  4.70 35.13                  
                                                        0.30              
                                                            0.63          
                                     60.69.sup.b                          
                                        3.81  3.34 23.70                  
                                                        0.36              
                                                            8.10          
2  80     5    47.1        23.4     52.31                                 
                                        4.13  4.69 38.14                  
                                                        0.33              
                                                            0.40          
                                    58.24                                 
                                        3.47  2.32 29.35                  
                                                        0.31              
                                                            6.31          
3  90     2    52.5        24.5     53.94                                 
                                        4.38  4.61 36.39                  
                                                        0.25              
                                                            0.43          
                                    57.08                                 
                                        3.45  3.77 28.08                  
                                                        0.27              
                                                            7.35          
4  100    2    49          18       52.08                                 
                                        4.16  4.57 38.14                  
                                                        0.28              
                                                            0.77          
                                    57.09                                 
                                        3.37  3.57 27.48                  
                                                        0.42              
                                                            8.07          
5  110    2    35.5        28       54.53                                 
                                        4.36  4.51 35.35                  
                                                        0.27              
                                                            0.98          
                                    55.34                                 
                                        3.72  3.59 28.93                  
                                                        0.33              
                                                            8.09          
6  130    2    7.1         39.4     56.8                                  
                                        4.20  4.31 33.59                  
                                                        0.24              
                                                            0.86          
                                    NT  NT    NT   NT   NT  NT            
7  150    2    4.7         32.8     61.65                                 
                                        5.06  4.03 27.95                  
                                                        0.26              
                                                            1.05          
                                    NT  NT    NT   NT   NT  NT            
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 Row .sup.a in each case is the analysis of the acetone-soluble acids     
 Row .sup.b in each case is the analysis of the acetone-insoluble acids   
 NT means no analysis was made.                                           
 
    
     Although we have stated above that the novel composition is acetone-insoluble and we have shown the use of acetone as suitable in the process defined herein, this has been done merely as a characterization of the composition and to exemplify one embodiment of our process. Many polar solvents can be used in place of acetone herein. Among the polar solvents that have been used are methanol, ethanol, isopropanol, methyl ethyl ketone, tetrahydrofuran, dioxane, cyclohexanone, etc. The use of such solvents, therefore, falls within the scope of the invention claimed herein. 
     Since the novel mixture claimed herein has abundant functionality in both carboxyl and nitro groups, it is apparent that the mixture lends itself to many known chemical reactions, for example, esterification of the carboxyl groups, hydrogenation of nitro groups that may be present to amines, etc. We have found that the novel mixtures defined herein can be converted to their corresponding anhydrides using conventional dehydrating conditions and that such anhydrides can be used as curing agents for epoxy resins to produce a cured epoxy resin suitable for many uses, for example, in encapsulation of electrical parts, such as resistors. Curing of an epoxy resin with the anhydride is illustrated below. 
     RUN NO. 8 
     There was charged to a quart ball mill jar containing two dozen Borundum Balls 45 grams of EPON 1004 (an epoxy resin having an ep equivalent of 0.05 and an epoxy equivalent weight of 900, manufactured by Shell Chemical Co.), 20 grams of the anhydride obtained from the acid mixture resulting from Run No. 1 above, having an anhydride equivalent of 0.05 and an anhydride equivalent weight of 400, and 0.45 gram of tine octanoate catalyst. The mixture was milled for two hours and there was recovered molding powder passing through a 100 mesh screen. Ten grams of the above powder was molded in accordance with the procedure defined in ASTM-D-647 to form a disc 1/8-inch thick having a diameter of two inches. The mold was cured in a steam-heated hydraulic press at 175° C. and 2000 pounds per square inch gauge (140.5 kilograms per square centimeter) for 30 minutes and then cooled to room temperature while maintaining said pressure. The resulting disc was a hard black solid having a smooth, continuous surface. 
     Obviously, many modifications and variations of the invention, as hereinabove set forth, can be made without departing from the spirit and scope thereof and, therefore, only such limitations should be imposed as are indicated in the appended claims.