Processes for synthesizing alkali metal borohydride compounds

Details Processes for synthesizing alkali metal borohydride compounds Processes for synthesizing alkali metal borohydride compounds

2001-12-18

2003-07-01

Boykin, Terressa M. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

From boron-containing reactant

C528S271000, C528S396000

Reexamination Certificate

active

06586563

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to processes for synthesizing borohydride compounds, and more particularly to more efficient processes for synthesizing borohydride compounds that have decreased sodium or alkali metal requirements.
BACKGROUND OF INVENTION
Sodium borohydride is an important reducing agent for many organic chemical functional groups (including aldehydes and ketones) and metal salts with various applications in pharmaceutical and fine chemical manufacturing. It can also be used as a purification agent to remove metal ions from industrial waste streams or carbonyl and peroxide impurities from process chemicals. Aqueous solutions of sodium borohydride are used in the pulp and paper industries to produce sodium hydrosulfite bleach.
In addition, sodium borohydride is being evaluated as a hydrogen source for fuel cells and hydrogen-burning internal combustion engines. Sodium borohydride can be used directly as an anodic fuel in a fuel cell or as a hydrogen storage medium (e.g., hydrogen can be liberated by the reaction of sodium borohydride with water, which produces sodium borate as a byproduct). As with all fuels, acceptance of sodium borohydride in the commercial market is partially dependent on the availability of industrial scale quantities.
Sodium borohydride is commercially prepared from the conversion of boric acid and methanol into trimethyl borate (B(OCH
3
)
3
) which is then reduced by sodium hydride to produce sodium borohydride (Equation 1). This process is essentially unchanged from that described in Schlesinger, H. I., Brown, H. C., Abraham, B., Bond, A. C., Davidson, N., Finholt, A. E., Gilbreath, J. R., Hoekstra, H. R., Horvitz, L., Hyde, E. K., Katz, J. J., Knight, J., Lad, R. A., Mayfield, D. L., Rapp, L., Ritter, R. M., Schwartz, A. M., Sheft, I., Tuck, L. D., and Walker, A. O., “
New Developments in the Chemistry of Diborane and the Borohydrides. I. General Summary,” Journal of the American Chemical Society
, vol. 75 (1953), pp. 187-190 (“the Schlesinger process”).
B(OH)
3
+CH
3
OH→B(OCH
3
)
3
+4NaH→NaBH
4
+3NaOCH
3
  (1)
The use of large amounts of sodium metal (4 moles needed to produce one mole of sodium borohydride) is a major factor in the manufacturing cost. According to U. S. Geologic Survey reports, the largest single use for metallic sodium in the United States is in sodium borohydride production. Sodium metal is produced by the electrolysis of molten salt mixtures of sodium chloride and calcium chloride in an energy-intensive process. Sodium hydride is prepared on-site by reaction of sodium metal with hydrogen in a mineral oil slurry.
The process described in Equation 1 provides poor molar economy by requiring 4 moles of sodium (as sodium hydride) to produce 1 mole of sodium borohydride. Based on the above stoichiometry, 75% of the sodium required is converted to a by-product, sodium methoxide. This inefficiency limits the scalability of this process. A process that utilizes sodium atoms more efficiently (i.e., more sodium atoms incorporated into desired product) would therefore be desirable. Additionally, both sodium metal and sodium hydride will react violently with water to generate hydrogen gas, and must be protected from all sources of water, usually under an inert gas atmosphere. Special engineering and safety considerations must be made to prevent possible explosive reactions between sodium and water.
The product of Equation 1 is a mineral oil dispersion of sodium borohydride and sodium methoxide. This mixture is hydrolyzed to produce a two-phase aqueous sodium hydroxide-sodium borohydride-methanol mixture; methanol is then removed from this mixture by distillation. The aqueous solution is a major commercial product; however, powder sodium borohydride is the desired reagent for use in pharmaceutical synthesis and for hydrogen generation applications. Additional extraction, evaporation, crystallization, and drying steps are necessary to obtain solid sodium borohydride.
Alternative routes to produce sodium borohydride have been evaluated in an effort to reduce manufacturing costs, but have not replaced the route shown in Equation 1, as discussed in “
Na Borohydride: can cost be lowered?”, Canadian Chemical Processing
, (1963) pp. 57-62. For example. Bayer AG evaluated a solid-state reaction of borax, quartz, and sodium metal (Equation 2) under a hydrogen atmosphere; however, this approach still requires the unfavorable sodium metal stoichiometry of 4 moles per mole of sodium borohydride produced.
Na
2
B
4
O
7
+16Na+8H
2
+7SiO
2
→4NaBH
4
+7Na
2
SiO
3
  (2)
Furthermore, in view of the large quantities of sodium borohydride needed for use as a hydrogen carrier, e.g., as an alternative fuel in the transportation industry, these processes would produce large quantities of waste products such as sodium methoxide or sodium silicate. Further energy and expense is required to separate these byproducts.
Most improvements found in the prior art are simple modifications of the Schlesinger and Bayer processes represented by Equations 1 and 2. Accordingly, such improvements also suffer from the disadvantages stated above and do not provide any improved efficiency. The idea process would be one in which the majority of the sodium atoms are converted to sodium borohydride product.
An alternative process which conceptually could be used to prepare sodium borohydride compounds on a large scale is the disproportionation of diborane with small, hard Lewis bases. The use of methoxide and hydroxide bases is discussed, for example, in U.S. Pat. No. 2,461,662, in Schlesinger, H. I., Brown, H. C., Hoekstra, H. R., and Rapp, L. R., “
Reactions of Diborane with Alkali Metal Hydrides and Their Addition Compounds. New Syntheses of Borohydrides”, Journal of the American Chemical Society
, vol. 75 (1953), pp. 199-204; and in Davis, R. E., and Gottbrath, J. A., “
On the Nature of Stock's Hypoborate,” Chemistry and Industry
, (1961) pp. 1961-1962. Not only does this result in a favorable utilization of sodium atoms as compared to the processes shown in Equations 1 and 2, it would also allow the use of air and moisture stable sodium salts (as compared to sodium hydride), which simplify handling during production.
However, this approach has not been seriously considered for commercial uses. The reaction of diborane with aqueous sodium hydroxide described in the Davis and Gottbrath article referenced above generates a mixture of products, including sodium borohydride. The methodology described in U.S. Pat. No. 2,461,662 and in the Schlesinger, Brown, Hoekstra, and Rapp article referenced above requires that gaseous diborane be condensed onto solid metal alkoxides at low temperature (ca. −100° C.).
In view of the above, there is a need for improved and energy efficient industrial scale manufacturing processes for producing borohydride compounds that eliminate the need for excess sodium or alkali metals. In addition, there is a need for industrial scale processes that reduce or avoid the production of large quantities of waste products.
SUMMARY OF THE INVENTION
The present invention provides processes for producing large quantities of borohydride compounds, which overcome the above-described deficiencies. In addition, the efficiency of the processes of the present invention can be greatly enhanced over the typical processes for producing borohydride compounds. Further, sodium carbonate (soda ash) is a readily available mined chemical mineral and requires no special handling as compared to metallic sodium or sodium hydride.
In one embodiment of the present invention, a process is provided for producing borohydride compounds which includes the reaction of a carbonate of the formula Y
2
CO
3
in aqueous solution at a temperature of about −5 to about 20° C. with diborane to produce the borohydride YBH
4
, where Y is a monovalent cationic moiety.
In another embodiment of the present invention, a process is provided for producing borohydride compounds which includ

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