Abstract:
A stabilized aqueous mixture containing at least one borohydride compound and at least one metal hydroxide compound. The mixture has improved stability with regard to decomposition of borohydride, especially at elevated temperatures.

Description:
[0001]    This application claims the benefit of priority under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 60/818,609 filed on Jul. 5, 2006. 
         [0002]    This invention relates to an aqueous sodium borohydride mixture having increased stability, especially at high temperatures. 
         [0003]    Aqueous borohydride solutions having relatively good stability are known. For example, solutions containing about 12% sodium borohydride and about 40% sodium hydroxide are sold commercially. In some applications, a stable borohydride-containing solution having less sodium hydroxide is desired, e.g., in hydrogen fuel cells. Such a solution is described in U.S. Pat. No. 6,866,689; it contains about 44% sodium borohydride and about 0.2% sodium hydroxide. However, this solution exhibits relatively high rates of borohydride decomposition at elevated temperatures. 
         [0004]    The problem addressed by this invention is to find an aqueous formulation of borohydride that has improved stability at elevated temperatures. 
       STATEMENT OF INVENTION 
       [0005]    The present invention provides an aqueous mixture comprising: (a) from 15% to 65% of at least one borohydride compound; (b) from 1% to 10% of at least one metal hydroxide; and (c) from 0.1% to 20% of at least one absorbent polymer or excipient. The invention is further directed to a method for stabilizing aqueous borohydride mixtures by adding at least one absorbent polymer or excipient. 
     
     DETAILED DESCRIPTION 
       [0006]    Percentages are weight percentages and temperatures are in ° C., unless specified otherwise. A “borohydride compound” is a compound containing the borohydride anion, BH 4   − . An “absorbent polymer” is a polymer capable of absorbing water. Preferably, absorbent polymers are chosen from among water-soluble polymers (polymers having water solubility of at least 1%, alternatively at least 5%), cellulose and its derivatives, and cross-linked polymers. Cross-linked polymers preferably have carboxyl, amide, hydroxy, amino, or ether groups, or a combination thereof, to facilitate dispersion in water. “Excipients” are pharmaceutical excipients as defined, e.g., in  Handbook of Pharmaceutical Excipients,  A. Wade &amp; P. J. Weller (1964). Especially preferred excipients are those used as tablet disintegrants, e.g., polyacrylic acids, angelic acid, calcium alginate, carboxymethylcellulose (e.g., calcium or sodium salt), chitosan, croscarmellose sodium, crospovidone, docusate sodium, guar gum, polacrilin potassium, povidone, sodium alginate, sodium starch glycolate, and starch. In one embodiment of the invention, excipients have a water solubility of at least 0.5%, alternatively at least 1%. In one embodiment of the invention, preferred absorbent polymers include polyacrylic acids, polyacrylamide co polyacrylic acid, copolymers of acrylic acid (e.g., polyacrylic acid co polyethylene oxide), polyvinylpyrrolidone, polyvinylpyridine, polyvinylpyridine N-oxide, polyvinylimidazole, ethoxylated polyethylenimine, cellulose esters (e.g., acetate, butyrate), hydroxyalkyl celluloses, and carboxymethyl cellulose. The aqueous mixture of the present invention may be a solution, slurry, gel, or any other fluid mixture at temperatures of at least 40° C.: Mixtures having higher concentrations of borohydride, e.g., at least 50%, typically are solids or thick slurries at ambient temperature of about 25° C., but become fluid at higher temperatures. 
         [0007]    In one embodiment, the amount of borohydride compound(s) is at least 20%, alternatively at least 25%. In one embodiment, the amount of metal hydroxide(s) is no more than 8%, alternatively no more than 6%, alternatively no more than 4%. In one embodiment of the invention, the amount of metal hydroxide is at least 2%, alternatively at least 3%. In one embodiment, the amount of borohydride compound is no more than 55%, alternatively no more than 50%, alternatively no more than 45%, alternatively no more than 40%. Preferably, the borohydride compound is a metal salt which has a metal cation from groups 1, 2, 4, 5, 7, 11, 12 or 13 of the periodic table, or a mixture thereof. In one embodiment, the borohydride compound is an alkali metal borohydride or combination thereof, alternatively it comprises sodium borohydride (SBH) or potassium borohydride or a mixture thereof, alternatively sodium borohydride. Preferably, the metal hydroxide is an alkali metal hydroxide or combination thereof, alternatively sodium, lithium or potassium hydroxide, or a mixture thereof, alternatively sodium hydroxide or potassium hydroxide; alternatively sodium hydroxide. More than one alkali metal borohydride and more than one metal hydroxide may be present. 
         [0008]    In one embodiment of the invention, the amount of absorbent polymer(s) and/or excipient(s) is at least 0.2%, alternatively at least 0.4%, alternatively at least 1%, alternatively at least 2%. In one embodiment of the invention, the amount of absorbent polymer(s) and/or excipient(s) is no more than 15%, alternatively no more than 12%, alternatively no more than 10%, alternatively no more than 8%. In one embodiment of the invention, extremely small amounts of absorbent polymer(s) and/or excipient(s) are required; at least 0.1%, but no more than 1%, alternatively no more than 0.5%. For example, partially neutralized polyacrylic acids, or copolymers containing at least 20% acrylic acid monomer units, alternatively at least 30%, alternatively at least 50%, alternatively at least 75%, may require only very small amounts of absorbent polymer(s) and/or excipient(s). The degree of neutralization of the polyacrylic acids added to the borohydride compound(s) is not critical because the aqueous mixture contains additional base, so that the acids typically will be completely neutralized in the aqueous mixture. For polymers used in this invention, a weight average molecular weight of at least 1500 is preferred, alternatively at least 3000, alternatively at least 5000. 
         [0009]    In one embodiment of the invention, the aqueous mixture is substantially free of substances that catalyze hydrolysis of borohydride, e.g., salts of transition metals such as Co, Ru, Ni, Fe, Rh, Pd, Os, Ir, Pt, or mixtures thereof, and borides of Co and/or Ni. 
         [0010]    The aqueous mixture of this invention also may be used in the fields of synthesis and metal recovery. 
       EXAMPLES 
       [0011]      
         [0000]                                          TABLE 1                   Decomposition rate of solutions containing 25% SBH, 3% NaOH with       additives at room temperature and 60° C.                    Rate of   Rate of                   decomposition   decomposition   % decrease       25% SBH, 3% NaOH   Wt % of   At RT,   at 60° C.   in decomposition       with Additive   additive   % SBH/day   % SBH/day   rate at 60° C.               None, 25% NaBH 4 ,   N/A   −0.12   −0.958   N/A       3% NaOH       Poly acrylic acid   0.5   −0.05   −0.564   41       partial sodium salt,       lightly cross linked       (solid) CAS # 76774-       25-9       Poly acrylamide co   0.5   −0.074   −0.662   31       acrylic acid partial       sodium salt (solid)       M w  = 15,000,000       Poly acrylic acid co   0.5   −0.012   −0.614   36       poly ethylene oxide       (solid) CAS # 27599-       59-56-0       Poly vinyl   0.5   −0.004   −0.734   23       pyrrolidone (solid)       M w  = 10,000; CAS #       9003-39-0       Carboxy methyl   0.5   −0.032   −0.728   24       cellulose sodium salt       (solid) M w  = 70,000;       CAS # 9004-39-4                    
The addition of poly acrylic acid partial cross linked partial sodium salt to higher-concentration sodium borohydride formulations also results in decreasing the hydrolysis rates of the 40 and 50 wt % formulations. An unexpected phenomenon has been observed when these formulations are heated for long periods of time. After an initial induction period, the rate of hydrolysis of the borohydride decreases to nearly zero.
 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Decomposition rate (“DR”) of solutions containing 40% SBH, 
               
               
                 5% NaOH with additives at room temperature and 50° C. 
               
             
          
           
               
                   
                   
                   
                   
                   
                 DR at 50° C. 
               
               
                   
                   
                   
                 DR at 
                 % 
                 % SBH/Day 
               
               
                 40% SBH, 5% 
                 Wt % 
                 DR at 
                 50° C. 
                 decrease 
                 After 
               
               
                 NaOH with 
                 of 
                 RT % 
                 % SBH/ 
                 in DR 
                 induction 
               
               
                 Additive 
                 additive 
                 SBH/day 
                 Day 
                 at 50° C. 
                 period 
               
               
                   
               
               
                 None, 40% 
                 N/A 
                 0.0549 
                 0.3604 
                 N/A 
                 N/A 
               
               
                 NaBH 4 , 5% 
               
               
                 NaOH 
               
               
                 Poly acrylic 
                 0.5 
                 0.0333 
                 0.1139 
                 68 
                 0.0547 
               
               
                 acid partial 
               
               
                 sodium salt, 
               
               
                 lightly cross 
               
               
                 linked (solid) 
               
               
                 CAS # 76774- 
               
               
                 25-9 
               
               
                 Poly acrylic 
                 4% 
                 0.0339 
                 0.1716 
                 52 
                 0.0084 
               
               
                 acid partial 
               
               
                 sodium salt 
               
               
                 (Liquid) M w  = 
               
               
                 30,000; CAS # 
               
               
                 9003-04-07 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Decomposition rate of solutions containing 50% SBH, 
               
               
                 5% NaOH with additives at room temperature and 58° C. 
               
             
          
           
               
                   
                   
                   
                   
                   
                 DR at 58° C. 
               
               
                   
                   
                   
                 DR at 
                   
                 % SBH/day 
               
               
                 50% SBH, 5% 
                 Wt % 
                 DR at 
                 58° C. 
                 % 
                 after 
               
               
                 NaOH with 
                 of 
                 RT % 
                 % SBH/ 
                 decrease 
                 induction 
               
               
                 Additive 
                 additive 
                 SBH/day 
                 day 
                 in DR 
                 period 
               
               
                   
               
             
          
           
               
                 None, 50% 
                 N/A 
                 0.0031 
                 0.35 
                 N/A 
                 N/A 
               
               
                 NaBH 4 , 5% 
               
               
                 NaOH 
               
               
                 Poly acrylic 
                 0.5% 
                 — 
                 0.03121 
                 91 
                 0.01165 
               
               
                 acid partial 
               
               
                 sodium salt, 
               
               
                 lightly cross 
               
               
                 linked (solid) 
               
               
                 CAS # 
               
               
                 76774-25-9 
               
               
                 Poly acrylic 
                   4% 
                 — 
                 0.01715 
                 95 
                 0.01388 
               
               
                 acid partial 
               
               
                 sodium salt 
               
               
                 (Liquid) 
               
               
                 M w  = 30,000; 
               
               
                 CAS # 
               
               
                 9003-04-07 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Decomposition rate of solutions containing 25% SBH, 5% NaOH with 
               
               
                 different wt % of poly acrylic acid), partial sodium salt solid at 22° C. 
               
             
          
           
               
                   
                 Wt % 
                 Rate of decomposition % 
                   
               
               
                 Temp ° C. 
                 additive 
                 SBH/Day 
                 % Difference 
               
               
                   
               
             
          
           
               
                 22 
                 0 
                 0.026 
                 N/A 
               
               
                 22 
                 0.5 
                 0.011 
                 57.7 
               
               
                 22 
                 1 
                 0.0485 
               
               
                 22 
                 1.5 
                 0.0473 
               
               
                 22 
                 2 
                 0.0473 
               
               
                 22 
                 4 
                 0.025 
                 3.8 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Decomposition rate of solutions containing 25% SBH, 5% NaOH with 
               
               
                 different wt % of poly acrylic acid), partial sodium salt solid at 60° C. 
               
             
          
           
               
                   
                 Wt % 
                 Rate of decomposition % 
                   
               
               
                 Temp ° C. 
                 additive 
                 SBH/Day 
                 % Difference 
               
               
                   
               
             
          
           
               
                 60 
                 0 
                 0.4334 
                 N/A 
               
               
                 60 
                 0.5 
                 0.3764 
                 13.2 
               
               
                 60 
                 1 
                 0.409 
                 5.6 
               
               
                 60 
                 1.5 
                 0.424 
                 2.1 
               
               
                 60 
                 2 
                 0.4114 
                 5.1 
               
               
                 60 
                 4 
                 0.4201 
                 3.1 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Decomposition rate of solutions containing 25% SBH, 5% NaOH with 
               
               
                 different wt % of poly (acrylic acid sodium salt 30% in water 
               
               
                 M w  = 15,000 at 22° C. (wt. % additive in this table, 
               
               
                 and following tables based on total polymer/water formulation) 
               
             
          
           
               
                   
                   
                 Rate of 
                 Rate of decomposition 
                   
               
               
                   
                 Wt % 
                 decomposition, 
                 of standard, % 
                 % 
               
               
                 Temp ° C. 
                 additive 
                 % SBH/Day 
                 SBH/Day 
                 Difference 
               
               
                   
               
             
          
           
               
                 22 
                 0 
                   
                   
                   
               
               
                 22 
                 10 
                 0.1161 
                 0.0114 
               
               
                 22 
                 5 
                 0.0019 
                 0.1029 
                 98.1 
               
               
                 22 
                 1 
                 0.046 
                 0.0421 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Decomposition rate of solutions containing 25% SBH, 5% NaOH with 
               
               
                 different wt % of polyacrylic acid sodium salt 30% in 
               
               
                 water M w  = 15,000 at 60° C. 
               
             
          
           
               
                   
                   
                   
                 Rate of 
                   
               
               
                   
                   
                 Rate of 
                 decomposition of 
               
               
                   
                 Wt % 
                 decomposition, 
                 standard, % 
               
               
                 Temp ° C. 
                 additive 
                 % SBH/Day 
                 % SBH/Day 
                 % Difference 
               
               
                   
               
             
          
           
               
                 60 
                 0 
                   
                   
                   
               
               
                 60 
                 10 
                 0.3982 
                 0.4993 
                 20.2 
               
               
                 60 
                 5 
                 0.4301 
                 0.4964 
                 13.3 
               
               
                 60 
                 1 
                 0.4236 
                 0.4399 
                 3.7 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 Decomposition rate of solutions containing 25% SBH, 5% NaOH with 
               
               
                 15 wt % of polyethyleneimine 80% ethoxylated at 22° C. 
               
             
          
           
               
                   
                   
                   
                 Rate of 
                   
               
               
                   
                   
                 Wt % 
                 decomposition % 
                 % 
               
               
                 Temp ° C. 
                   
                 additive 
                 SBH/Day 
                 Difference 
               
               
                   
               
             
          
           
               
                   
                 Standard 
                 0 
                 0.0585 
                 N/A 
               
               
                 22 
                 Polyethylenimine 
                 15 
                 0.0116 
                 80.1 
               
               
                   
                 80%, ethoxylated 
               
               
                   
                 37% in water 
               
               
                   
                 M w  = 50,000; CAS 
               
               
                   
                 # 26558-46-8 
               
               
                 22 
                 Polyethylenimine 
                 15 
                 0.0321 
                 45.1 
               
               
                   
                 80% ethoxylated 
                 15 
               
               
                   
                 35 to 40% in 
               
               
                   
                 water M w  = 
               
               
                   
                 70,000; CAS # 
               
               
                   
                 26558-46-8 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 9 
               
             
             
               
                   
               
               
                 Decomposition rate of solutions containing 25% SBH, 5% NaOH with 
               
               
                 15 wt % of polyethyleneimine 80% ethoxylated at 60° C. 
               
             
          
           
               
                   
                   
                   
                 Rate of 
                   
               
               
                 Temp 
                   
                 Wt % 
                 decomposition 
                 % 
               
               
                 ° C. 
                   
                 additive 
                 % SBH/Day 
                 Difference 
               
               
                   
               
             
          
           
               
                   
                 Standard 
                 0 
                 0.5782 
                 N/A 
               
               
                 60 
                 Polyethyleneimine 80% 
                 15 
                 0.5096 
                 11.8 
               
               
                   
                 ethoxylated, 37% in water 
               
               
                   
                 M w  = 50,000; CAS # 
               
               
                   
                 26558-46-8 
               
               
                 60 
                 Polyethylenimine 80% 
                 15 
                 0.5052 
                 12.6 
               
               
                   
                 ethoxylated, 35 to 40% in 
               
               
                   
                 water M w  = 70,000; CAS # 
               
               
                   
                 26558-46-8 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 10 
               
             
             
               
                   
               
               
                 Decomposition rate of solutions containing 25% SBH, 5% NaOH with 
               
               
                 15 wt % different MW polyacrylic acid sodium salts at 22° C. 
               
             
          
           
               
                   
                   
                   
                 DR 
                   
               
               
                 Temp 
                   
                 Wt % 
                 % SBH/ 
                 % 
               
               
                 ° C. 
                   
                 additive 
                 Day 
                 Difference 
               
               
                   
               
             
          
           
               
                   
                 Standard 
                 0 
                 0.0585 
                 N/A 
               
               
                 22 
                 Poly (acrylic acid sodium salt 45% 
                 15 
                 0.06493 
                 Higher 
               
               
                   
                 in water M w  = 1,200; CAS # 9003- 
                   
                   
                 by 10 
               
               
                   
                 04-7 
               
               
                 22 
                 Poly (acrylic acid sodium salt 45% 
                 15 
                 0.0257 
                 56.0 
               
               
                   
                 in water M w  = 8,000; CAS # 9003- 
               
               
                   
                 04-7 
               
               
                 22 
                 Poly (acrylic acid sodium salt 40% 
                 15 
                 0.0091 
                 84.4 
               
               
                   
                 in water M w  = 30,000; CAS # 
               
               
                   
                 9003-04-7 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 11 
               
             
             
               
                   
               
               
                 Decomposition rate of solutions containing 25% SBH, 5% NaOH with 
               
               
                 15 wt % of different MW poly acrylic acid sodium salts at 60° C. 
               
             
          
           
               
                   
                   
                   
                 DR 
                   
               
               
                 Temp 
                   
                 Wt % 
                 %SBH/ 
                 % 
               
               
                 ° C. 
                   
                 additive 
                 Day 
                 Difference 
               
               
                   
               
             
          
           
               
                   
                 Standard 
                 0 
                 0.5782 
                 N/A 
               
               
                 60 
                 Poly (acrylic acid sodium salt 45% 
                 15 
                 0.3812 
                 34.0 
               
               
                   
                 in water M w  = 1,200; CAS # 
               
               
                   
                 9003-04-7 
               
               
                 60 
                 Poly (acrylic acid sodium salt 45% 
                 15 
                 0.3607 
                 37.6 
               
               
                   
                 in water M w  = 8,000; CAS # 
               
               
                   
                 9003-04-7 
               
               
                 60 
                 Poly (acrylic acid sodium salt 40% 
                 15 
                 0.3736 
                 35.38 
               
               
                   
                 in water M w  = 30,000; CAS # 
               
               
                   
                 9003-04-7