Process for producing dimethyl formamide
This invention relates to a novel process for producing dimethyl formamide by making monomethyl formamide react with trimethylamine and carbon monoxide in the presence of a catalyst. 
     Conventionally, dimethyl formamide is produced by making dimethylamine react with carbon monoxide or methyl formate. Dimethylamine as a raw material in said conventional process is produced usually by dehydration reaction of methyl alcohol and ammonia, but a large amount of monomethylamine and trimethylamine is inevitably by-produced at the same time in the production of dimethylamine. However, commercial demands for monomethylamine and trimethylamine are poor, as compared with that for dimethylamine, and most of these two by-produced amines are usually recycled to methylamine synthesis system, and converted to dimethylamine. Therefore, use of monomethylamine and trimethylamine having less commercial demands as the raw material for dimethyl formamide in place of dimethylamine is expected to bring about a remarkable rationalization to the methylamine industry and also a great commercial significance. 
     To meet such expectation, U.S. Pat. No. 2,677,706 proposes a process for producing dimethyl formamide by making monomethylamine and/or dimethylamine or further together with trimethylamine react with carbon monoxide in the presence of cuprous chloride, cupric chloride, ammonium chloride, potassium acetate or boron trifluoride as a catalyst. However, the proposed process has low selectivities of monomethylamine and trimethylamine to dimethyl formamide, and thus is not satisfactory as a commercial process. 
     As a result of extensive studies of the process for producing dimethyl formamide from monomethylamine or trimethylamine as a raw material, the present inventors have found a process for producing dimethyl formamide in high yield by making monomethyl formamide, which can be readily prepared by reaction of monomethylamine with carbon monoxide or methyl formate, react with trimethylamine and carbon monoxide. 
     That is, the present invention provides a novel process for producing dimethyl formamide from monomethyl formamide, trimethylamine and carbon monoxide as raw materials. In the present invention, it is presumed that reaction of the following reaction formula takes place. 
     
         (CH.sub.3).sub.3 N + HCONHCH.sub.3 + CO → 2HCON(CH.sub.3).sub.2 
    
     the reaction is carried out under a pressure of 20 kg/cm 2  gage or higher, preferably 30 to 600 kg/cm 2  gage. The reaction can proceed under a lower pressure than 20 kg/cm 2  gage, but side reactions are readily promoted, and the process becomes unpractical. A pressure higher than 600 kg/cm 2  gage is useful to carry out the reaction of the present invention, but not practical in view of economy of the process. 
     The reaction is carried out at a temperature of 150° to 300° C, preferably 160° to 280° C. A temperature outside said range should be avoided on account of the reaction rate and selectivity. 
     Ratio by mole of trimethylamine to monomethyl formamide is 0.1 to 10, preferably 0.5 to 5. A ratio outside said range will increase the amount of unreacted raw materials, and is not economical. 
     Carbon monoxide is a raw material and also plays a role of maintaining the reaction pressure in the present invention, and thus must be used in much excess. But a gas mixture of carbon monoxide and an inert gas can be used on condition that the partial pressure of carbon monoxide is higher than 10 kg/cm 2  gauge. 
     The reaction proceeds without a catalyst, but a catalyst is economically indispensable for the present process. Catalysts effectively used in the reaction of the present invention include halogen elements such as fluorine, chlorine, bromine, iodine, etc.; halides such as metal halides, ammonium halides, hydrohalogenic acids, alkyl halides, etc.; inorganic acids such as boric acid, sulfuric acid, phosphoric acid, etc., solid acids such as silca-alumina, etc., metal carbonyls such as carbonyls of iron, cobalt, nickel, rhodium and ruthenium. Among these catalysts, halogen elements, halides and metal carbonyls are preferable, and iron carbonyl is more preferable. 
     The catalysts can be used alone or in a combination of two or more of these. In the case of the metal carbonyls as the catalyst, it is not always necessary to add the metal carbonyl to the reaction system, and addition of a metal or metal compound capable of being converted to the metal carbonyl under the reaction conditions of the present process, such as iron, cobalt, nickel, rhodium and ruthenium and their oxides, hydroxides and salts of organic or inorganic acid such as oxalate, naphthenate, sulfate, nitrate and carbonate, is effective. 
     The halogen element and the halide as the catalyst are used in an amount of 0.1 to 350 mg-atoms, preferably 1 to 300 mg-atoms, in terms of halogen per mole of monomethyl formamide. 
     The inorganic acid and metal carbonyl as the catalyst is used in an amount of 0.1 to 350 mg-moles, preferably 1 to 300 mg-moles per mole of monomethyl formamide. 
     The solid acid as the catalyst is used in an amount of 0.1 to 50 g, preferably 1 to 30 g, per mole of monomethyl formamide. 
     When the amount of the catalyst used is smaller than the lower limit of said ranges, the yield of dimethyl formamide is decreased, whereas when the amount of the catalyst is larger than the upper limit of said ranges, hydrogen, carbon dioxide and methane are readily by-produced. 
     The reaction can be carried out in any manner, for example, batchwise or continuously. 
     According to the present invention, dimethyl formamide can be produced in high yield with high selectivity. Furthermore, monomethyl formamide, one of the raw materials used in the present invention, can be readily prepared from monomethylamine and carbon monoxide or methyl formate. Therefore, by adopting a manner that monomethyl formamide is prepared, first of all, from these materials, and the resulting monomethyl formamide is used as the raw material of the present invention, both monomethylamine and trimethylamine having less commercial demands, among three amines formed from methanol and ammonia, can be effectively utilized to produce dimethyl formamide. That is, the present invention has a very important commercial significance in this respect. Of course, monomethyl formamide obtained according to other processes can be used as the raw material in the present invention. 
     Now, the present invention will be described below in detail, referring to Examples. 
     In the examples, dimethyl formamide yield to monomethyl formamide and selectivity of monomethyl formamide to dimethyl formamide are calculated according to aforementioned reaction formula. 
     REFERENCE EXAMPLE 
     Monomethylamine was made to react with carbon monoxide under a reaction pressure of 20 kg/cm 2  gage at a reaction temperature of 75° C, using 40 mg-atoms of sodium methylate in terms of sodium per mole of monomethylamine, whereby monomethyl formamide was obtained in yield of 97%. 
     EXAMPLE 1 
     10 Grams of monomethyl formamide obtained in Reference Example, 10 g of trimethylamine, and 1.6 g of iodine as catalyst were charged into an autoclave having a capacity of 100 ml, and carbon monoxide was charged to the autoclave under pressure up to 95 kg/cm 2  gage. Reaction was conducted at a temperature of 260° C for 4 hours. Maximum pressure during the reaction was 236 kg/cm 2  gage, and pressure at the end of reaction was 77 kg/cm 2  gage. Products were analyzed by gas chromatography. 82.1% of monomethyl formamide charged was converted, and 15.0 g of dimethyl formamide was obtained. 
     Dimethyl formamide yield to monomethyl formamide was 60.6%, and selectivity of monomethyl formamide to dimethyl formamide was 73.8%. 
     EXAMPLES 2 - 18 
     Reaction was carried out in the same manner as in Example 1, changing ratios by mole of the raw materials, kind and amount of the catalyst, temperature, and pressure. The results are shown in the following table together with the results of Example 1. 
     
                                           Table                                   
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Example No.   1      2   3     4    5     6    7     8    10              
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Catalyst                                  conc.           36%             
  Kind        I.sub.2                                                     
                    I.sub.2                                               
                         I.sub.2                                          
                               I.sub.2                                    
                                    TiI.sub.4                             
                                          H.sub.2 SO.sub.4                
                                               Br.sub.2                   
                                                     NH.sub.4 I           
                                                          HCl             
  Amount(g)   1.6   0.8  0.8   2.1  0.85  0.85 0.5   0.9  0.6             
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Monomethyl formamide (g)                                                  
              10.0  10.0 5.0   5.0  10.0  10.0 9.9   10.2 9.9             
Trimethylamine (g)                                                        
              10.0  10.0 15.5  15.0 10.0  10.0 10.3  10.1 10.5            
Pressure of CO charged                                                    
(kg/cm.sup.2 gage)                                                        
              95    145  95    140  95    90   93    92   90              
Maximum CO pressure                                                       
(kg/cm.sup.2 gage)                                                        
              236   340  242   312  245   234  233   232  245             
CO pressure at end of                                                     
reaction (kg/cm.sup.2 gage)                                               
              77    125  81    120  93    87   93    84   88              
Reaction temperature (° C)                                         
              260   255  260   220  265   265  265   259  265             
Reaction time (hr)                                                        
              4     4    4     4    4     4    4     4    4               
Monomethyl formamide                                                      
conversion (%)                                                            
              82.1  74.5 90.7  95.2 79.0  41.2 71.6  75.5 56.0            
Dimethyl formamide                                                        
yield (g)     15.0  14.1 8.7   9.7  12.2  5.4  10.0  13.4 8.8             
Dimethyl formamide                                                        
yield to monomethyl                                                       
              60.6  57.0 70.3  78.4 49.3  21.8 40.8  53.1 35.9            
formamide (%)                                                             
Selectivity of monomethyl                                                 
formamide to dimethyl                                                     
              73.8  76.5 77.5  82.3 62.4  53.0 57.0  70.3 64.1            
formamide (%)                                                             
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 Note:                                                                    
 *Al.sub.2 O.sub.3 13 %                                                   
 (a) FeC.sub.2 O.sub.4 . 2H.sub.2 O                                       
 (b) (NH.sub.4).sub.3 Fe(C.sub.2 H.sub.4).sub.3 . 3H.sub.2 O              
 
    
     
          -                                                                        
10     11      12     13     14     15     16      17    18               
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Silica-                                                                   
alumina*                                                                  
       Co.sub.2 (CO).sub.8                                                
               CH.sub.3 I                                                 
                      Fe(CO).sub.5                                        
                             (a)    (b)    Fe.sub.2 O.sub.3               
                                                  FeSO.sub.4              
2      1       1.1    1.3    1.2    2.8    0.5    0.4    1.1              
10.3   10.0    9.8    10.7   10.2   10.2   10.6   10.6   10.6             
10.4   10.3    9.8    10.3   10.3   9.8    10.6   10.6   10.6             
100    92      92     90     90     99     92     90     90               
248    234     229    250    250    270    250    250    250              
107    85      83     81     82     93     84     85     85               
265    260     260    248    239    238    250    240    240              
4      4       4      4      4      4      4      4      4                
44.0   58.2    76.4   91.1   65.1   48.3   86.1   51.7   50.4             
4.4    9.7     12.2   14.6   12.7   6.6    13.1   7.8    8.0              
17.3   39.2    50.3   55.1   50.3   26.1   49.9   29.7   30.5             
39.2   67.3    65.8   60.5   77.3   54.1   58.0   57.5   60.5             
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