Abstract:
1,1,4,4-Tetrabromobutadiene is produced by oxybrominating butane with free oxygen and bromine, the reaction being conducted in gaseous phase; and the butadiene is recovered by selective condensation of an effluent of the reaction medium.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The invention relates to the production of tetrabromobutadiene from butane. 
     B. The Prior Art 
     Tetrabromobutadiene, which has long been known as one of many organic intermediates (having functionality useful in the production of compounds based on one of the most common of the plentiful n-paraffinic compounds) has achieved more recent notability as a monomeric vehicle for incorporating bromine compounds into polymers for fire retardant purposes. 
     Heretofore, 1,1,4,4-tetrabromobutadiene was prepared in a complex 2-step process involving intermediate purification and resulting in low selectivities and poor yields. 
     Oxychlorination has been employed for introducing functionality into olefinic hydrocarbons, but attempts to similarly activate paraffins have been much less successful due to the lack of selectivity obtained at reasonable conversion rates. Were it possible to oxyhalogenate n-butane with reasonably acceptable yields and rates of selectivity of the butadiene tetrahalide, such a process would be a significant advance in the art and is an object of this invention. 
     SUMMARY OF THE INVENTION 
     Butane is continuously oxybrominated with free oxygen and bromine in the presence of a stabilized cupric bromide catalyst on a high surface area catalyst support comprising activated alumina or silica by feeding the oxygen, bromine and butane in gaseous phase, at controlled rates and temperatures into a reactor maintained under conditions of pressure and temperature, the rates and conditions being preselected according to the particular apparatus, catalyst stabilizer and concentration of catalyst to form 1,1,4,4-tetrabromobutadiene in a reaction medium comprising oxygen, bromine and butane. The catalyst may be stabilized with potassium bromide, ammonium bromide, or a combination thereof. The product is isolated by selective condensation of an effluent of the reaction medium. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The butane which has been found to be suitable as the starting material in this reaction is normal butane. 
     I have discovered that when bromine, as opposed to the other halogens, is used under circumstances where an appropriate catalyst can be made stable enough to survive, oxyhalogenation of the n-butane can be successfully employed in the preparation of a brominated butadiene, offering distinct advantages over the expensive and impractical prior art processes. 
     I have also found that if a stabilized cupric bromide catalyst is presented to the oxybromination reaction on a high surface area support comprising activated alumina and/or silica, the reaction can easily be made to favor the 1,1,4,4-tetrabromobutadiene. Presently known examples of activated alumina or silica which are suitable for supporting the reaction include: gamma alumina (γ-Al 2  O 3 ), and all high surface area silicates including aluminum derivatives of silicates such as Al 2  O 3 .SiO 2 . Surprisingly, no other metal classes of catalyst supports have been found which will favor this particular reaction. High surface area supports are those in which the surface area is in excess of about 100 m 2  /g. Presently manufactured catalyst supports run from about 5 to about 350 m 2  /g. 
     Stabilization of the cupric bromide catalyst is achieved by presenting it to the reaction mixed wtih a relatively small amount of a potassium or ammonium halide. Maintenance of stability of the catalyst is essential, and (for oxybromination of butane to tetrabromobutadiene) no other method of maintaining such stability than by the use of these particular compounds as part of the catalyst mixture has yet been found. Salts of the alkaline metals other than potassium do not satisfactorily maintain stability of the cupric halide. While the amount of stabilizing salt is not critical, satisfactory results have been achieved employing stabilizing salts in the amount of 23-50% of the total weight of the catalyst mixture. 
     In a preferred preparation of the catalyst for presentation, the cupric halide and the potassium or ammonium halide is dissolved in a suitable solvent which is used to saturate the catalyst support, subsequently to be removed by filtration and/or evaporation as appropriate, the support then being dried under a nitrogen flow at about 150° C. to leave a catalyst mixture concentration of about 5-10% by weight based on the catalyst support. Solvents known to be suitable for catalyst preparation as described are water and methanol. 
     Depending upon the particular apparatus, catalyst concentration, choice of stabilizer and feed rate of various components, a variety of conditions of pressure and temperature may be selected which will satisfactorily produce the tetrabromobutadiene at yields and rates of selectivity far in excess of those known to be reported in the prior art. The vapor phase reaction is preferably carried out in a fluid or fixed bed single-stage reactor at temperatures above 250° C. and below about 300° C. at atmospheric pressure. I have found that excessive cracking and charring of the reactants in the products will occur when the reaction is conducted at temperatures in excess of 300° C. 
     In order to avoid unreacted bromine and to prevent over-bromination an excess of the butane over the bromine and the oxygen should be employed, preferably by way of a mole feed ratio of oxygen and bromine to that of the butane of about 0.2-0.6 and 0.1-0.5, respectively. The feeding gases are preferably but not necessarily diluted with nitrogen so as to achieve the above-described desirable mole feed ratios, and to avoid the flammability region for the butane which is about 1.8-49 and 1.9-8.5 mole percent butane in oxygen and air, respecitvely. Of course the feed rates of the various components should be individually adjusted so as to provide maximum selectivity of the desired product with respect to the particular apparatus employed, catalyst concentration, stabilizer choice and concentration, pressure and temperature. 
     Any effective selective condensation method may be employed to recover the brominated butadiene. Preferred is a 3-stage condensation which may be operated at successively decreasing temperatures. 
     EXAMPLES 
     The reactor was a 2× 50 cm Pyrex tube to which a gas inlet manifold preheater was attached. A fluidized sand bath was employed to maintain the temperature of the reactor and preheater. A series of condenser traps were provided to accept the effluent of the reactor. These traps were adjusted to provide a first condensation temperature of 25° C., a second at 0° C., and a third at -78° C. The catalyst support in each example (as described) was impregnated with cupric bromide and potassium bromide in the amounts indicated. Temperature of the reactor and feed rates of the gases contact time and product analysis are as shown in the following table: 
     
                                           TABLE I__________________________________________________________________________                                         Product                   Res.                       Gas Feed Rates at Mole %Example             Temp.                   Time                       25° C. &amp; 1 atm                                         1,1,4,4-tetraNo.  Catalyst Composition               ° C.                   Sec.                       C.sub.4 H.sub.10                            Br.sub.2                                O.sub.2                                     N.sub.2                                         bromobudadiene__________________________________________________________________________1    15% CuBr.sub.2 + 10% KBr on γ-               250 5.1 100  23  50   400 90alumina (Girdler T-126)3/16 inch pelletssurface area 300 m.sup.2 /g2    10% CuBr.sub.2 + 3% KBr on γ-               290 1.7 100  24  50   400 85alumina 80- 200 meshHarshaw Al-1401Psurface area 180 m.sup.2 /g3    20% CuBr.sub.2 + 12% KBr on               250 5.1 100  23  50   400 90silica (α-quartz) GirdlerT-1571 3/16 inch spheressurface area 130 m.sup.2 g4    15% CuBr.sub.2 + 10% KBr on               270 4.9 100  24  50   400 60Silica aluminasurface area 100 m.sup.2 /g3/16 inch pelletsGirdler T-1219__________________________________________________________________________