Abstract:
A method for preparing a metal borohydride salt, M(BH 4 ) n , where n is 1 or 2, from a slurry of sodium borohydride and a sodium alkoxide in a liquid hydrocarbon, wherein M is Li, K, Rb, Cs, Mg, Ca, Sr or Ba.

Description:
[0001]    This application claims the benefit of priority under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 60/801,917 filed on May 19, 2006. 
     
     BACKGROUND 
       [0002]    This invention relates generally to a method for preparing borohydride salts from a slurry of sodium borohydride and a sodium alkoxide in a liquid hydrocarbon. 
         [0003]    Processes for production of potassium borohydride from alkoxide-containing mixtures are known, but are inefficient in that they do not result in useful products from both the borohydride and alkoxide components. For example, U.S. Pat. No. 2,720,444 describes production of potassium borohydride from a mixture containing sodium borohydride and sodium methoxide. However, the process combines the sodium borohydride with 10% excess potassium hydroxide, and no data are provided on the disposition of the excess potassium hydroxide, nor is it suggested that high-purity sodium methoxide could be isolated as a co-product. 
         [0004]    The problem addressed by this invention is to provide a process for producing a metal borohydride, other than sodium borohydride, and sodium methoxide. 
       STATEMENT OF INVENTION 
       [0005]    The present invention is directed to a method for producing a metal borohydride, M(BH 4 ) n , where n is 1 or 2, from a slurry of sodium borohydride and a sodium alkoxide in a liquid hydrocarbon. The method comprises steps of: (a) combining said slurry with 0.99 to 1.01 equivalents of a metal salt, M(X) n ; and methanol; wherein M is Li, K, Rb, Cs, Mg, Ca, Sr or Ba; and X is halide, alkoxide or acetate; (b) filtering to collect M(BH 4 ) n ; and (c) separating oil and methanol liquid phases to obtain sodium methoxide in methanol solution. 
     
     DETAILED DESCRIPTION 
       [0006]    Unless otherwise specified, all percentages herein are stated as weight percentages and temperatures are in 0° C. 
         [0007]    In one embodiment of the invention, the metal, M, is Li, K, Ca, Sr or Ba. In a preferred embodiment of the invention, M is K. In one embodiment of the invention, the anion, X, in the metal salt is alkoxide, chloride, bromide or iodide. In a preferred embodiment of the invention, X is chloride or alkoxide. Particularly preferred metal salts include potassium methoxide, potassium chloride, calcium chloride, strontium chloride and barium chloride. 
         [0008]    The liquid hydrocarbon used in the present invention is any hydrocarbon which is liquid at 25° C. Suitable hydrocarbons include alkanes, e.g., mineral oil; and aromatics. Mineral oil is particularly preferred. Preferably, the amount of liquid hydrocarbon is from 0.01 L/g NaBH 4  to 10 L/g NaBH 4 , alternatively from 0.2 L/g NaBH 4  to 1 L/g NaBH 4 . 
         [0009]    In one embodiment of the invention in which X is an alkoxide, the alkoxide is a C 1 -C 12  alkoxide, alternatively a C 1 -C 8  alkoxide, alternatively a C 1 -C 4  alkoxide. In one preferred embodiment, the alkoxide is methoxide, ethoxide, isopropoxide or t-butoxide. Methoxide is particularly preferred. Alkoxides can be generated from hydroxides and alcohols. 
         [0010]    In one embodiment of the invention, the amount of metal salt used is from 0.995 to 1.005 equivalents with respect to the amount of sodium borohydride in the slurry, alternatively about one equivalent. In one embodiment of the invention, the sodium alkoxide and sodium borohydride in the slurry are in a molar ratio of about 3:1, alkoxide:borohydride. 
         [0011]    For the case where M is a monovalent metal (n=1), and sodium alkoxide and sodium borohydride in the slurry are in a molar ratio of about 3:1, alkoxide:borohydride, an equation describing the reaction is as follows: 
         [0000]      3NaOR+NaBH 4 +MX→MBH 4 +NaX+3NaOR 
         [0000]    MBH 4  is insoluble in the methanol/hydrocarbon reaction medium, and thus will form a precipitate. NaX may also be insoluble, depending on the nature of X. In cases where NaX is soluble, the MBH 4  can be isolated by filtration, with the filtrate containing a hydrocarbon phase, and a methanol/NaX phase. When M is divalent (n=2) and X is chloro, the reaction is as follows: 
         [0000]      6NaOR+2NaBH 4 +MCl 2 →M(BH 4 ) 2 +2NaCl+6NaOR 
         [0000]    In this case, both M(BH 4 ) 2  and NaCl are insoluble. NaCl can be removed from the metal borohydride by washing the solids with water. 
         [0012]    Preferably, the reaction temperature is from 0° C. to 50° C., alternatively from 15° C. to 40° C. Preferably, the amount of methanol added is from 10 g/g NaBH 4  to 50 g/g NaBH 4 , alternatively from 12 g/g NaBH 4  to 30 g/g NaBH 4 . 
       EXAMPLES 
       [0013]    Preparation of KBH 4  (KBH)—A two liter three neck flask equipped with an overhead stirrer, thermometer, liquid addition funnel and nitrogen purge was charged with 658.2 grams of a mineral oil slurry containing 20.2% NaOCH 3  and 4.6% NaBH 4 . The addition funnel was charged with a solution of 49.01 g of solid KOH and 406.1 g of methanol. This material was added to the stirred slurry at a rate such that the reaction temperature of the slurry did not rise above 40° C. The consistency of the reaction slurry changed from a free flowing slurry to a clumpy dough material to a partial three-phase system. An addition 400 g of methanol was added, resulting in a three-phase system having two liquid phases and a white powdery solid in the top layer. 
         [0014]    This three-phase slurry was initially filtered through a grade “B” ultra-coarse glass frit (70-100 μm) but no solids were caught by the filter. The slurry was then passed through a grade “E” extra-fine glass frit (2 to 8 μm). This filtration was done under a dynamic oil pump vacuum. The resulting solid was washed multiple times with 742 grams of hexane and the resulting solid was dried under vacuum at room temperature. The resulting two liquids were separated using a separatory funnel. T 
         [0015]    he two liquids and solids were analyzed for % NaOCH 3  (NaOMe), % KBH, % NaOH, % oil and by ICP analysis. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Analysis of product from KOH in methanol 
               
             
          
           
               
                   
                   
                   
                 CH 3 OH 
                   
               
               
                   
                 Solids g, 
                 Oil Layer 
                 Layer g, 
               
               
                   
                 (yield) 
                 g (yield) 
                 (yield) 
                 Recovery 
               
               
                   
                   
               
             
          
           
               
                 Wt 
                 46.31 
                 525.24 
                 658.2  
                   
               
               
                 sample(g) 
               
               
                 % KBH 
                 86.21, 86.25 
                 0.0 
                 0.35, 0.34 
               
               
                 ICP, Na 
                 4.34% 
                  29 ppm 
                 10.14% 
               
               
                 ICP, K 
                 61.46 
                 2.6 ppm 
                 967 ppm 
               
               
                 ICP, B 
                 19.42 
                 4.4 ppm 
                 937 ppm 
               
               
                 % NaOMe/ 
                 N/A 
                 0.0 
                 26.19.26.13 
               
               
                 total 
               
               
                 alkalinity 
               
               
                 % NaOH 
                 7.52, 7.52 
               
               
                 % Oil 
                 1964 ppm 
                 N/A 
                 1184 ppm  
               
               
                 Grams KBH 
                 39.82 
                   
                 2.30 g 
                 42.12/48.50 = 
               
               
                   
                 (82.1%) 
                   
                   
                 86.85 
               
               
                 Grams Oil 
                   
                 525.24 
                   
                 525.24/553 = 
               
               
                   
                   
                   
                   
                 94.89 
               
               
                 Grams 
                  4.63 
                   
                 172.11 
                 176.74/198.94 = 
               
               
                 NaOMe/total 
                   
                   
                 ( 86.5%) 
                 89.26 
               
               
                 alkalinity 
               
               
                   
               
             
          
         
       
     
       These results demonstrate that KBH and NaOMe both can be isolated with good purity and yield, especially with regard to K contamination in NaOMe. 
       [0016]      
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Analysis of product from KOCH 3 /methanol 
               
             
          
           
               
                   
                   
                   
                 CH 3 OH 
                   
               
               
                   
                 Solids g, 
                 Oil Layer g 
                 Layer g, 
               
               
                   
                 (yield) 
                 (yield) 
                 (yield) 
                 Recovery 
               
               
                   
                   
               
             
          
           
               
                 Wt sample(g) 
                 55.44 
                 408.82 
                 700.51 
                   
               
               
                 % KBH 
                 75.422 
                 0.014 
                 0.341 
               
               
                 ICP, Na 
                 — 
                 — 
                 — 
               
               
                 ICP, K 
                 — 
                 — 
                 — 
               
               
                 ICP, B 
                 — 
                 — 
                 — 
               
               
                 % NaOMe/ 
                   
                 0 
                 23.060 
               
               
                 total 
               
               
                 alkalinity 
               
               
                 % NaOH 
                 16.403 
                 0 
               
               
                 % Oil 
                 — 
                 N/A 
                 — 
               
               
                 Grams KBH 
                 41.814 
                 0 
                 2.38 
                 44.19/46.75 = 
               
               
                   
                 (89.43%) 
                   
                   
                 94.52 
               
               
                 Grams Oil 
                   
                 530.82 
                   
                 530.82/533.76 = 
               
               
                   
                   
                   
                   
                 99.45 
               
               
                 Grams 
                 9.31 
                   
                 165.32 
                 174.63/191.55 = 
               
               
                 NaOMe/total 
                   
                   
                 (86.30%) 
                 91.17 
               
               
                 alkalinity