Abstract:
The invention relates to a process for the removal of m-dichlorobenzene from dichlorobenzene mixtures by chlorination in the liquid phase at elevated temperature in the presence of Friedel-Crafts catalysts, the chlorination being carried out with addition of sulphur, sulphur compounds, iodine and/or iodine compounds and the chlorination mixture then being worked up in the usual way.

Description:
BACKGROUND OF THE INVENTION 
     1. FIELD OF THE INVENTION 
     The invention relates to a process for the removal of m-dichlorobenzene from dichlorobenezene mixtures with an m-dichlorobenzene content of up to 35% by weight, relative to the total amount of dichlorobenezene in the mixture, by chlorination in the liquid phase at elevated temperature in the presence of Friedel-Crafts catalysts. 
     BACKGROUND INFORMATION 
     From U.S. Pat. No. 4,089,909, a process is known for the separation of dichlorobenzene isomers in which, in a dichlorobenzene mixture, m-dichlorobenzene is preferentially chlorinated to 1,2,4-trichlorobenzene and higher poly-chlorobenzenes with elemental chlorine in the liquid phase in the presence of Friedel-Crafts catalysts. The residual o-/p-dichlorobenzene can then be separated from higher chlorinated chlorobenzenes by conventional fractional distillation and crystallization. 
     In this separation process, it is disadvantageous that not only the m-dichlorobenzene in the mixture is chlorinated, but also, to a significant degree, the valuable o- and p-dichlorobenzene, as the tables in the U.S. Pat. No. 4,089,909 mentioned. 
     Thus, significant losses of the desired o- and p-dichlorobenzene are unavoidable during the separation and purification of the dichlorobenzene isomers according to the process described in U.S. Pat. No. 4,089,909. The economics of this process are therefore poor. 
     SUMMARY OF THE INVENTION 
     A process has now been found for the removal of m-dichlorobenzene from dichlorobenzene mixtures with an m-dichlorobenzene content of up to 35% by weight, relative to the total amount of dichlorobenzene in the mixture, by chlorination in the liquid phase at elevated temperature in the presence of Friedel-Crafts catalysts which is characterized in that the chlorination is carried out with addition of sulphur, sulphur compounds, iodine and/or iodine compounds and the chlorination mixture is then worked-up in the usual way. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the process according to the invention, dichlorozenzene mixtures as obtained in the nuclear chlorination of benzene or chlorobenzene, for example, in the liquid phase in the presence of Friedel-Crafts catalysts, can be used (cf. Ullmanns Encyclopadie der technischen Chemie, volume 5, page 463 and Houben-Weyl, Methoden der organischen praparativen Chemie, volume 5, page 653). Such dichlorobenzene mixtures contain, depending on the preparative method, about 0.5 to 3% by weight of m-dichlorobenzene and about 60 to 70% by weigth of o- and p-dichlorobenzene, the ortho/para ratio fluctuating in the range of about 3.5:1 to 1.5:1 (p-:o-dichlorobenzene). The remainder consists of unreacted benzene and monochlorobenzene as well as more highly chlorinated chlorobenzenes. 
     Benzene and chlorobenzene are distilled off in the usual way from the chlorination mixture obtained during the nuclear chlorination and, if necessary, the remaining dichlorobenzene mixture is subjected to a further separation by distillation. Depending on the number of separation stages, an m-dichlorobenzene-enriched dichlorobenzene mixture is obtained which contains up to 35% by weight, preferably 0.3 to 5% by weight, particularly preferably 1 to 3% by weight of m-dichlorobenzene, about 0.2 to 40% by weight, preferably 25 to 38% by weight, of o-dichlorobenzene and about 50 to 99% by weight, preferably 90 to 98% by weight, of p-dichlorobenzene. The remainder of the mixture consists of trichlorobenzenes and higher chlorinated chlorobenzenes. 
     The m-dichlorobenzene-enriched dichlorobenzene mixture is used in the process according to the invention as described above. 
     The chlorination of the dichlorobenzene mixture is usually carried out at temperatures of about 30° to 120° C., preferably 35° to 70° C. 
     As Friedel-Crafts catalysts, the following can, for example, be used: iron(III) chloride, aluminium chloride, zinc chloride and/or tin chloride, preferably iron(III) chloride. The amount of Friedel-Crafts catalyst to be used is not critical and usually amounts to less than 5% by weight, relative to the amount of dichlorobenzene, preferably 0.1 to 4% by weight. 
     In the process according to the invention, sulphur, sulphur compounds, iodine and/or iodine compounds are added to the Friedel-Crafts catalyst. As sulphur compounds, the following may be mentioned: sulphur chlorides, such as S 2  Cl 2 , iron(II) sulphide, mercaptans and thioethers; as iodine compounds, the following may be mentioned: alkyl iodides and aromatic iodine compounds. Preferably, sulphur, S 2  Cl 2  and/or iodine are added. 
     The amounts of sulphur, iodine, sulphur compounds and/or iodine compounds which are added to the Friedel-Crafts catalyst are usually about 0.01 to 5% by weight, preferably 0.01 to 1% by weight, relative to the total amount of dichlorobenzene in the mixture. 
     The ratio of the amount of Friedel-Crafts catalyst to the amount of iodine or sulphur is about 1:1 to 5:1, preferably 1.2:1 to 3:1. 
     The chlorination of the dichlorobenzene mixture is conveniently continued until the m-dichlorobenzene content in the mixture lies below about 0.05% by weight, preferably about 0.02% by weight. It is however also possible to continue the chlorination, if this becomes necessary, until the m-dichlorobenzene content in the mixture lies under 0.005% by weight. m-Dichlorobenzene contents of significantly more than 0.05% by weight in the dichlorobenzene mixture are disadvantageous as the separation of the dichlorobenzene isomers is complicated by this. 
     In general, a slight excess of chlorine, relative to the aromatics to be chlorinated, is used in the chlorination according to the invention. About 1.0 to 1.5 moles of chlorine, preferably 1.0 to 1.05 moles of chlorine, are used per mole of the aromatics to be chlorinated. 
     After the chlorination is completed, the dichlorobenzene mixture obtained can be fractionally distilled to separate o- and p-dichlorobenzene. 
     In the process according to the invention, it is surprising that, as a result of the addition of sulphur, iodine, sulphur compound and/or iodine compounds, practically only the m-dichlorobenzene is chlorinated to higher chlorobenzenes during the chlorination of m-dichlorobenzene-enriched dichlorobenzene mixtures and that the o- and p-dichlorobenzene is hardly chlorinated any further under thse chlorination conditions. Because of this, hardly any losses of the desired o- and p-dichlorobenzene occur. In addition, the amount of chlorine to be used in the chlorination of the mixture is reduced by the addition of sulphur, iodine, sulphur compounds and/or iodine compounds. 
     The following examples are intended to illustrate the process according to the invention, but without limiting it to these examples. 
    
    
     EXAMPLE 1 
     Mixture used: 
     95.0% of para-dichlorobenzene (p-DClB) 
     4.5% of meta-dichlorobenzene (m-DClB) 
     0.2% of ortho-dichlorobenzene (o-DClB) 
     0.3% of trichlorobenzene, benzene and monochlorobenzene 
     
         __________________________________________________________________________                           TriClB +      Cl.sub.2             more highlyTemp.    Catalyst      consumed            p-DClB                 m-DClB                      o-DClB                           chlorinated benzenes°C.    % by weight      %     %    %    %    %__________________________________________________________________________55  0.1 FeCl.sub.3       0    95.0 4.5  0.20 0.30    (comparison)       68   93.3 0.16 0.11 6.43       76   82.3 0.11 0.10 17.49       84   80.2 0.07 0.09 19.64       92   79.1 0.05 0.09 20.76      100   77.5 0.03 0.09 22.3855  0.1 FeCl.sub.3 +       0    95.0 4.5  0.20 0.30    0.1 iodine      100   92.25                 0.02 0.09 7.6455  0.1 FeCl.sub.3 +       0    95.0 4.5  0.20 0.30    0.05 iodine      100   91.7 0.03 0.14 8.1355  0.1 FeCl.sub.3 +       0    95.0 4.5  0.20 0.30    0.02 sulphur      100   94.2 0.03 0.13 5.64__________________________________________________________________________ 
    
     The example shows that p- and o-dichlorobenzene are consumed markedly during the chlorination with FeCl 3  without addition of iodine or sulphur. 
     EXAMPLE 2 
     Mixture used: 
     73.75% of p-DClB 
     25.40% of o-DClB 
     0.35% of m-DClB 
     0.50% of trichlorobenzene (TriClB) 
     
         __________________________________________________________________________                           TriClB      Cl.sub.2             more highlyTemp.    Catalyst      consumed            p-DClB                 m-DClB                      o-DClB                           chlorinated benzenes°C.    % by weight      %     %    %    %    %__________________________________________________________________________50  0.1 FeCl.sub.3       0    73.75                 0.35 25.40                           0.50    (comparison)      25    71.00                 0.29 24.40                           4.31      62    69.40                 0.14 21.60                           8.86      100   66.40                 0.04 17.50                           16.0675  0.1 FeCl.sub.3 +       0    73.75                 0.35 25.40                           0.50    (comparison)      20    70.90                 0.33 23.60                           5.17      40    69.60                 0.24 21.70                           8.46      100   64.40                 0.06 15.30                           20.2450  0.1 FeCl.sub.3 +       0    73.75                 0.35 25.40                           0.5    0.02 sulphur      50    73.40                  0.100                      24.40                           2.1      75    73.30                  0.050                      23.70                           2.95      100   73.00                  0.025                      23.00                           3.950  0.1 FeCl.sub.3 +       0    73.75                 0.35 25.40                           0.50    0.1 iodine      33    72.65                 0.18 23.30                           3.9      66    71.50                 0.06 20.70                           7.7      100   70.90                  0.023                      19.10                           10.0__________________________________________________________________________ 
    
     EXAMPLE 3 
     Mixture used: 
     55.0% of p-DClB 
     35.5% of o-DClB 
     2.8% of m-DClB 
     6.7% of trichlorobenzene 
     
         __________________________________________________________________________                           TriClB      Cl.sub.2             more highlyTemp.    Catalyst      consumed            p-DClB                 m-DClB                      o-DClB                           chlorinated benzenes°C.    % by weight      %     %    %    %    %__________________________________________________________________________50  0.1 FeCl.sub.3       0    55.0 2.8  35.5 6.7      100   43.8 0.03 13.3 42.8735  0.2 FeCl.sub.3 +       0    55.0 2.8  35.5 6.7    0.08 sulphur       50   54.5 0.2  32.5 12.80      100   54.4 0.02 30.5 15.0850  0.02 FeCl.sub.3 +       0    55.0 2.8  35.5 6.7    0.008 sulphur      100   52.4 0.02 26.7 20.8850  0.1 FeCl.sub.3 +       0    55.0 2.8  35.5 6.7    0.1 iodine       50   53.2 0.5  29.0 17.30      100   52.4 0.02 21.2 26.38__________________________________________________________________________ 
    
     EXAMPLE 4 
     The chlorination was carried out in a manner corresponding to Table II of U.S. Pat. No. 4,089,909 at 66° C. The mixture used corresponded approximately to the mixture listed in Table II of the U.S. Pat. No. 4,089,909. It comprised: 0.14% by weight of benzene, 2.062% by weight of monochlorobenzene, 35,701% by weight of m-dichlorobenzene, 35.295% by weight of p-dichlorobenzene and 26.787% by weight of o-dichlorobenzene. 1000 g of the mixture were chlorinated. 
     
         __________________________________________________________________________Composition of the mixture used (% by weight)__________________________________________________________________________Benzene  MClB m-DClB            p-DClB                 o-DClB                      1,2,4-TriClB                             1,2,3-TriClB                                    More highly0.14   2.062       35.701            35.295                 26.787                      --     --     chlorinated benzenes__________________________________________________________________________                                    Higher chlorinated                                               Introduced       m-DClB            p-DClB                 o-DClB                      1,2,4-TriClB                             1,2,3-TriClB                                    benzenes   chlorineExample  Catalyst       (%)  (%)  (%)  (%)    (%)    (%)        (g)__________________________________________________________________________1      FeCl.sub.3       0.024            21.99                  3.291                      52.427 8.211  13.994     450U.S. Pat. No.  (0 1%)4 089 9092      FeCl.sub.3       0.024            34.997                 20.09                      39.618 3.578  1.655      245  (0.1%) +  S  (0.04%)3      FeCl.sub.3       0.024            31.569                 10.987                      46.417 6.181  4.804      350  (0.1%) +  I.sub.2  (0.1%)__________________________________________________________________________ 
    
     The example shows that p- and o-dichlorobenzene react markedly during the chlorination with FeCl 3  without addition of sulphur or iodine. In addition, a significantly greater excess of chlorine is necessary. 
     It will be appreciated that the instant specification and claims are set forth by way of illustration and not limitation, and that various modifications and changes may be made without departing from the spirit and scope of the present invention.