Abstract:
A process for the preparation of dibromomethane is described, in which gaseous methyl bromide and bromide are reacted as temperatures of 300° C. or higher. The reaction is highly selective to DBM and almost quantitative Br 2  conversion is obtained in the absence of catalysts.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for the manufacture of dibromomethane. More particularly, the invention relates to a process by means of which dibromomethane is produced by the thermal bromination of methyl bromide. 
     BACKGROUND OF THE INVENTION 
     Dibromomethane (DBM) is an important solvent and chemical intermediate which is used in a large number of industrial applications. DBM is normally produced by the catalyzed halogen exchange of bromine for the chlorine contained in dichloromethane (DCM). One such process is described in German Patent No. 727,690. This process suffers from a number of drawbacks. The source of bromine is dry, gaseous HBr, which often contains considerable amounts of hydrogen, deriving from its manufacturing process. Hydrogen, being an inert gas in the DBM process, flows through the reactor, sweeping out part of the volatile DCM before it has time to react, thus decreasing the efficiency of the conversion. In addition, DCM is an expensive raw material, considering that 83% of its weight is wasted as HCl along with the excess HBr which is employed, making for an ecological problem. Disposal of spent catalyst is an additional ecological problem. 
     Other processes known in the art employ bromine salts instead of HBr. These processes, however, are inefficient and are not used in actual industrial applications. Thus, for example, NH 4  Br in dimethylformamide (Neth. Appli. No. 6,607,498), and NaBr (U.S. Pat. No. 3,923,914) have been used for such purposes. 
     Direct bromination of methane at high temperatures has also been attempted, but resulted in low yields and reaction selectivity. Thus, for instance, German Patent No. 330,642 obtained a maximum of 44,4% DBM at 370° C., in the presence of an iron catalyst. 
     SUMMARY OF THE INVENTION 
     It has now been found, and this is an object of the present invention, that DBM can be prepared by a non-catalytic, non-photochemical bromination of methyl bromide, with a high selectivity to DBM. Under appropriate conditions, an almost quantitative Br 2  conversion is obtained. 
     The selectivity of the process to DBM can be increased even more by recycling the by-product bromoform (and CBr 4 , if any) to the reaction zone where it is debrominated to DBM in a reverse reaction. Therefore, essentially the only process by-product is HBr, which is an industrially useful material and which can be utilized, as will be detailed further below. Thus, no ecological problem exists. On the other hand, bromoform can be obtained as a valuable by-product, if desired. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The process of the invention comprises continuously reacting gaseous methyl bromide and bromine at a reaction temperature of at least 300° C., and with a small methyl bromide conversion per pass. The reaction is strongly dependent on the temperature and, therefore, higher temperatures are preferred for obtaining high conversions. On the other hand, decomposition of DBM is known to set in above 400° C., so that the maximal reaction temperature is limited. The preferred temperature range for the process should be within 350° C.-410° C., 400° C. being the most preferred temperature. 
     A number of acidic and active surface catalysts have been found, which allow for the reduction of the reaction temperature. Thus, for example, with infusorial earth, FeCl 3 , ZnCl 2 , active carbon and pumice, reaction takes place at about 100° C. lower than without catalyst. However, operation in an unencumbered free-space reactor is generally found to be more convenient. As will be apparent to the skilled person, different catalysts can be employed in the process of the invention. The use of a catalyst, however, does not substantially alter the process of the invention and can exploit the advantages provided by the invention. 
     The initial molar ratio of methyl bromide:bromine is also an important parameter, and the selectivity of the reaction depends on the dilution of bromine in the reaction mixture. Thus, at a 1:1 ratio, a typical product distribution is: 42% mole CH 2  Br 2  ; 49% CHBr 3  and 9% CBr 4 . At a molar ratio of 50:1, on the other hand, 96% of the product is CH 2  Br 2 , the remaining 4 mole % being CHBr 3 . 
     It is therefore clear that, in order to obtain high selectivity, it is necessary to work at high initial molar ratios, with consequent low conversions of methyl bromide per pass, and high methyl bromide recycle ratios. When a CH 3  Br:Br 2  molar excess of about 10:1 is employed, a 80% molar selectivity to CH 2  Br 2  is obtained. The selectivity increases to 95-96% molar, when 50:1 molar excesses are used. Unreacted CH 3  Br is recycled to the reactor after condensing out the higher bromomethanes. DBM (b.p. 98° C.) is easily obtained in a pure state, free of bromoform (b.p. 151° C.), by distillation. 
    
    
     The above and other characteristics and advantages of the process of the invention will be better understood through the following illustrative and non-limitative description of experiments and experimental results. 
     EXPERIMENTAL SYSTEM 
     The reaction system comprises a vertically mounted glass column, either empty or filled with Wilson glass helices (d=0.03 cm) and electrically heated by a coil made from heating tape (Electrothermal HC 103). Bromine is fed into the column through a Sage syringe pump, and methyl bromide (kept in a 500 ml Fisher pressure bottle) is fed through a calibrated flow meter. The loss in weight of the methyl bromide container is measured periodically. The products leaving the heating zone are cooled and the liquid products collected in a water cooled receiver, the condensable gaseous products in an ice-cooled trap, and the exit HBr in water. A preheater, kept at 230°-290° C., was used in a few experiments. 
     EXAMPLE 1 
     The glass reactor (L=23 cm, D=2.6 cm) filled with Wilson glass helices (d=0.03 cm) was heated to 390° C. The temperature was measured by means of a thermocouple inserted into the middle of the column. Bromine, 63.8 g (0.4 mole), was placed in a 25 ml syringe in the Sage pump, and the addition adjusted to 0.25 gr/min. Gaseous methyl bromide was fed into the heated column, through the flow meter, at a rate of 0.80 g/min. A total of 200 g (2.1 moles) of CH 3  Br was fed during 251 minutes. The addition of both reagents was constant throughout the reaction. The gases leaving the column were condensed in a water-cooled receiver, then an ice-cooled trap, and finally absorbed in water. 
     At the end of the reaction, after cooling the reactor, a slow N 2  stream was passed through, in order to flush out any residual products from the column. It was then washed with methylene chloride. The two traps and the methylene chloride washing were united, washed with water to remove the HBr, cooled in ice and reacted with a 40% solution of NaHSO 3  to remove traces of residual Br 2 , if present. After separation, the organic phase was dried over anhydrous Na 2  SO 4 . 113.2 g of product were obtained, with 91.9% Br 2  conversion. Product analysis. CH 3  Br: 18.8%; CH 2  Cl 2  (solvent): 38.3%; CH 2  Br 2  : 29.5%; CHBr 3  : 13.3%; CBr 4  : 0.29%. 
     EXAMPLE 2 
     Several experiments were carried out in a manner similar to Example 1, varying the operating parameters. The results of these experimental runs are set forth in Table I below. 
     EXAMPLE 3 
     Example 1 was repeated, but employing a glass column without packing (Wilson helices). The results are detailed in Table II below. 
     EXAMPLE 4 
     Liquid CH 3  Br exits a holding tank (10 liters) and vaporizes in a heat exchanger. The flow is controlled by a micrometric valve and is measured by a thermal mass flowmeter (Porter) and/or a rotameter (Gilmont). The CH 3  Br is mixed with recycle bromoform and CBr 4 , if any, and fed to a glass reactor. Bromine is pumped from a 1 liter bottle with a peristaltic pump (Watson-Marlow). The flowrate is controlled by the pump and measured by means of a balance. 
     A scheme of the reactor is given in FIG. 1. It is made of three sections: 
     Section 1: Preheater (P) - CH 3  Br is heated therein to a temperature 20° to 30° C. below the reaction temperature. Bromine vaporizes in the internal tube. 
     Section 2: Reactor (R) - CH 3  Br (or the mixture CH 3  Br/bromoform) and bromine are mixed by means of an injector at the entrance to the reaction zone. The volume of the reaction zone is 500 ml. The temperature is controlled at the center of the reactor. 
     Section 3: Cooler (C) - The gases are cooled with air. The temperature at the bottom of the cooler is between 100° and 150° C. 
     Before operation, the system is evacuated to eliminate any traces of air. A purge system, constituted of a peristaltic pump (Watson-Marlow), withdraws 5-10% of the circulating stream. The CH 3  Br condenses in an acetone/Dry Ice trap and any air present exits to the atmosphere. The liquid CH 3  Br returns to the recirculating stream through a siphon. 
     CH 3  Br is preheated in the preheater P and then mixed with Br 2  in the injector I. The reaction takes place in the reactor R. After cooling, the liquid fraction which contains organic products, residual Br 2  and dissolved CH 3  Br, are separated from the gas fraction in a mist separator. Most of the CH 3  Br dissolved in the product is separated in a separation column. In the reservoir is obtained a product which contains residual Br 2  and 5-10% CH 3  Br. The gaseous fraction which contains CH 3  Br, HBr and traces of Br 2  and DBM passes through an absorption column and is washed free of HBr and Br 2  with water. It is then pumped by a diaphragm pump and dried in a CaCl 2  column. After adding CH 3  Br make-up (and bromoform, if desired) it is recirculated to the reactor. DBM is isolated from the liquid fraction by distillation. 
     A series of experiments was carried out, according to the above-detailed procedure and under varying conditions. The results obtained are summarized in Table III. 
     EXAMPLE 5 
     Example 4 was repeated at 410° C., and with a 6.5 seconds residence time, using a feed which was made up to contain, in addition to CH 3  Br and Br 2 , also bromoform recovered by distillation from a previous run. The molar ration of CH 3  Br:Br 2  :bromoform in the feed was 10:1:0.1. At a Br 2  conversion of ˜99%, the product distribution (on a CH 3  Br free basis) was (in mole percent): 90% DBM, 9% bromoform and 1% CBr 4 . Thus in this experiment, by recycling the bromoform, essentially only DBM was produced. 
     The crude product was treated with aqueous sodium bisulphite to remove the residual Br 2 , washed with water to neutral pH and distilled under an inert atmosphere (N 2 ) and eventually under reduced pressure, through an Oldershaw plate column (L=50 com, D=2 cm). DBM, &gt;99.5% pure and bromoform 98% pure were obtained. 
     EXAMPLE 6 
     A number of bromination reactions were carried out using different catalysts. When about 0.1 moles of CH 3  Br were reacted with about 0.1 moles of Br 2 , employing pumice as the support, and 10% FeCl 3  ; 10% ZnCl 2  as the catalyst, 30% of bromination was obtained at a reaction temperature of 250°-270° C., 88% bromination at 300°-320° C. and 97% bromination at 350°-360° C. As a comparison, the reaction was carried out with glass tubing (5 mm OD glass tubing, cut to length of about 8 mm), and only 67% of bromination was obtained at 300°-320° C. reaction temperature. Similar temperature reduction can be observed using active carbon. 
     The above description and examples have been given for the purpose of illustration and are not intended to be limitative. Many variations can be effected in the various operating conditions, without exceeding the scope of the invention. 
     
                                           TABLE I__________________________________________________________________________Bromination of Methyl Bromide                        Product Distribution.sup.(b)Temp    Run Time     CH.sub.3 Br/Br.sub.2           Total Feed                  Residence                        CH.sub.2 Br2                             CHBr3                                  CBr4°C.    (Hours)     Ratio Rate (g/hour)                  Time (sec)                        mole %                             mole %                                  mole %__________________________________________________________________________300 1.2   1/1   22     26    53   45   2350 2     1/1   13     40    42   49   9350 2     2/1   18     27    60   35   5350 2     4/1   27     16    75   23   2370.sup.(a)    2     5/1   29     45    73   27   0.3390 4.2   5.3/1 63     9     76   24   0.4390 1.5   7.2/1 87     2     79   21   0.2390 2     14.8/1           31     6     83   16   0.9390 1.6   24.7/1           50     4     92   7    0.2390 1.7   50/1  75     2     96   4    0__________________________________________________________________________ .sup.(a) Preheated. Preheater consisting of column (L = 29 cm, D = 2.6 cm kept at 230° C.. Residence time calculation includes the preheater zone .sup.(b) On methyl bromide free basis. 
    
     
                                           TABLE II__________________________________________________________________________Bromination in Glass Column Without PackingTemp    Run Time     CH.sub.3 Br/Br           Total Feed                  Residence                        CH.sub.2 Br2                             CHBr3                                  CBr4°C.    (Hours)     Ratio Rate (g/hour)                  Time (sec)                        mole %                             mole %                                  mole %__________________________________________________________________________390 2.7   5.2/1 70     12    65   33   2390 1.8   4.3/1 47     18    71   28   1390.sup.(c)    0.93  4.6/1 64     9     76   24   0.4__________________________________________________________________________ .sup.(b) On methyl bromide free basis. .sup.(c) Packed column. 
    
     
                                           TABLE III__________________________________________________________________________                   Product Distribution.sup.(a)Temp    CH.sub.3 Br/Br     Bromine Residence                   CH.sub.2 Br2                        CHBr3                             CBr4°C.    Ratio Conversion %             Time (sec)                   mole %                        mole %                             mole %__________________________________________________________________________390 20:1  97.5    4     91.1  8.8 0.1390 20:1  93.2    2.5   88.7 11.0 0.3390 10:1  94.3    4     85.5 13.9 0.6390 10:1  98.7    6.5   83.6 16.3 0.1390  5:1  86.0    4     72.5 25.3 2.1390 20:1  99.3    6.5   88.5 11.5 0.1390 20:1  95.7    2.5   83.5 15.9 0.6390 10:1  88.0    2.5   83.2 15.9 0.9390 10:1  91.8    4     84.5 14.8 0.7400 10:1  97.4    4     76.3 23.2 0.5400 10:1  95.3    2.5   82.0 17.4 0.5410 10:1  94.0    2.5   80.4 19.0 0.6410 10:1  98.0    2.5   77.6 21.9 0.5410 10:1  99.0    6.5   80.4 19.4 0.2400 10:1  97.2    6.5   74.6 24.9 0.5380 10:1  91.6    6.5   80.6 18.1 1.3370 10:1  85.8    6.5   78.9 19.6 1.5360 10:1  82.2    6.5   82.1 16.3 1.6__________________________________________________________________________ .sup.(a) On methyl bromide free basis.