METHOD FOR PRODUCING BORAZANE

Abstract

A method for producing borazane, includes the synthesis of the borazane by reacting (metathesis reaction) at least one ammonium salt with at least one borohydride, the at least one borohydride being selected from the alkali metal borohydrides and the alkali-earth metal borohydrides. The (metathesis) reaction is carried out with gaseous ammonia flushing; gaseous ammonia being co-liquefied with the at least one borohydride and gradually with borazane as the borazane is synthesized.

Claims

1. A method for producing borazane, comprising the synthesis of said borazane by reaction of at least one ammonium salt with at least one borohydride, said at least one borohydride being chosen from alkali metal borohydrides and alkaline-earth metal borohydrides, wherein said reaction is performed while flushing with gaseous ammonia; gaseous ammonia being co-liquefied with said at least one borohydride and gradually with borazane as said borazane is synthesized.

2. The method as claimed in claim 1, wherein said reaction is performed at a pressure of between 110.sup.5 and 210.sup.5 Pa (1 and 2 atm) and at a temperature of between 5 C. and 25 C.

3. The method as claimed in claim 1, wherein said reaction is performed in the absence of organic solvent.

4. The method as claimed in claim 1, wherein the gaseous ammonia is used in a gaseous ammonia/at least one borohydride mole ratio of greater than 1.

5. The method as claimed in claim 1, wherein the at least one ammonium salt is used in excess relative to the stoichiometry.

6. The method as claimed in claim 1, wherein it comprises: making a mixture of said at least one ammonium salt with said at least one borohydride; flushing with gaseous ammonia to bring said mixture into contact with gaseous ammonia; gaseous ammonia then being co-liquefied with said at least one borohydride; and reacting said at least one co-liquefied borohydride and said at least one ammonium salt while flushing with gaseous ammonia; the synthesized borazane being co-liquefied with gaseous ammonia gradually as it is synthesized.

7. The method as claimed in claim 1, further comprising: flushing with gaseous ammonia to bring said at least one borohydride into contact with gaseous ammonia; gaseous ammonia being co-liquefied with said at least one borohydride; adding, to said at least one borohydride co-liquefied with gaseous ammonia, said at least one ammonium salt; and reacting said at least one co-liquefied borohydride and said at least one ammonium salt, while flushing with gaseous ammonia; the synthesized borazane being co-liquefied with gaseous ammonia gradually as it is synthesized.

8. The method as claimed in claim 1, wherein said at least one ammonium salt is selected from ammonium carbonate, ammonium bicarbonate, ammonium sulfate, ammonium chloride, ammonium fluoride, ammonium nitrate, ammonium acetate and ammonium formate; and/or said at least one borohydride is chosen from sodium borohydride, lithium borohydride and potassium borohydride.

9. The method as claimed in claim 6, further comprising, on conclusion of the reaction: stopping the flushing with gaseous ammonia; removing the ammonia; the ammonia removal causing the precipitation of a solid phase containing the synthesized borazane; and selectively dissolving said synthesized borazane in a solvent and then removing, by evaporation under vacuum, said solvent for recovery of said synthesized borazane.

10. The method as claimed in claim 9, wherein said solvent is selected from tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, acetonitrile, ethyl acetate, isopropyl acetate and dimethyl carbonate, and a mixture thereof.

11. The method as claimed in claim 6, further comprising, on conclusion of the reaction: stopping the flushing with gaseous ammonia; filtering the reaction medium to remove the solids; removing the ammonia; the ammonia removal causing the precipitation of a solid phase constituted virtually exclusively of said synthesized borazane.

12. The method as claimed in claim 1, further comprising: making a pulverulent mixture of said at least one ammonium salt with said at least one borohydride; the at least one ammonium salt being used in slight excess, relative to the stoichiometry; flushing with gaseous ammonia to bring said mixture into contact with gaseous ammonia; and then, on conclusion of the reaction, stopping the flushing with gaseous ammonia; removing the ammonia; said ammonia removal generating a solid phase containing the synthesized borazane; and selectively dissolving said synthesized borazane in a solvent and then removing, by evaporation under vacuum, said solvent for the recovery of said synthesized borazane.

13. The method as claimed in claim 2, wherein said reaction is performed at atmospheric pressure.

14. The method as claimed in claim 4, wherein the gaseous ammonia is used in a gaseous ammonia/at least one borohydride mole ratio greater than 5.

15. The method as claimed in claim 5, wherein the at least one ammonium salt is used in slight excess relative to the stoichiometry.

16. The method as claimed in claim 8, wherein said at least one ammonium salt consists of ammonium carbonate and/or said at least one borohydride consists of sodium borohydride.

17. The method as claimed in claim 8, wherein said at least one ammonium salt consists of ammonium carbonate and said at least one borohydride consists of sodium borohydride.

18. The method as claimed in claim 9, wherein the ammonia is removed by evaporation under vacuum.

19. The method as claimed in claim 11, wherein the ammonia is removed by evaporation under vacuum.

20. The method as claimed in claim 12, wherein the ammonia is removed by evaporation under vacuum.

Description

EXAMPLE

[0060] The metathesis reaction was performed in a three-necked round-bottomed flask (with a volume of 100 mL), equipped with stirring means.

[0061] The following were placed in said three-necked round-bottomed flask: [0062] 2.3 g of sodium borohydride (NaBH.sub.4), and [0063] 2.9 g of ammonium carbonate ((NH.sub.4).sub.2CO.sub.3),
in powder form (said ammonium carbonate was thus introduced in slight excess relative to the stoichiometry).

[0064] An ice bath was placed under said three-necked round-bottomed flask to ensure a temperature of 0 C. in the volume of said three-necked round-bottomed flask.

[0065] A stream of gaseous ammonia (about 45 mL/min) was then introduced, via one of the necks of the flask, at atmospheric pressure. The gaseous ammonia introduced, which was not liquefied, was removed via another of the necks of the flask.

[0066] From the moment of introduction of the stream of gaseous ammonia, the appearance of a heterogeneous reaction medium with a very large solid content was observed in the flask. Said heterogeneous medium (solid ((NH.sub.4).sub.2CO.sub.3)/liquid (co-liquefied sodium borohydride and ammonia) had a volume of about 5 mL. The reaction (solid ((NH.sub.4).sub.2CO.sub.3)/liquid (liquefied sodium borohydride) was performed in such a heterogeneous medium (of foamy white appearance (foamy due to the formation of hydrogen). It is recalled that the successive steps of co-liquefaction and of metathesis reaction are represented schematically as follows:

##STR00003##

[0067] The reaction was continued for 3 hours while flushing with a stream of gaseous ammonia and with stirring.

[0068] On conclusion of these 3 hours (the inventors checked that the sodium borohydride (the starting 2.3 g) was then consumed), the feeding with gaseous ammonia was stopped. The flask was then degassed under vacuum. Its content once again became solid (powder).

[0069] The borazane thus synthesized was then recovered by selective dissolution, from said solid (from said powder).

[0070] 2-Methyltetrahydrofuran (50 mL) was introduced, into said flask, onto the solid. Said 2-methyltetrahydrofuran ensured selective dissolution of the borazane; this resulted in the production of a solid phase dispersed in a liquid phase containing said borazane dissolved in said 2-methyltetrahydrofuran. This solid phaseconstituted essentially of the product obtained together with the borazane (co-product: Na.sub.2CO.sub.3) and of the excess (NH.sub.4).sub.2CO.sub.3, predominantly of said product obtained together with the borazane (co-product: Na.sub.2CO.sub.3)was removed by filtration.

[0071] The synthesized borazane, dissolved in the 2-methyltetrahydrofuran, was then recovered by evaporation of said 2-methyltetrahydrofuran (evaporation performed at 30 C., under 100 mbar for 1 hour).

[0072] 1.3 g of borazane (NH.sub.3BH.sub.3) were thus recovered. This corresponds to a synthesis yield of 70% and to a synthesis productivity of close to 260 g of NH.sub.3BH.sub.3/L of reaction medium (the volume of the reaction medium (about 5 mL (see above), comprising 2.3 g of NaBH.sub.4, 2.9 g of (NH.sub.4).sub.2CO.sub.3 and the liquefied NH.sub.3 (8.1 L)) has been likened here to the volume of solvent (see the above definition of the synthetic productivity). The indicated value of 260 g/L (1.3/510.sup.3) is thus in fact lowered). Given that for a metathesis reaction performed conventionally by the Applicant (on a pilot scale) in organic solvent (2-methyltetrahydro-furan), the synthetic productivity is only about 26 g of NH.sub.3BH.sub.3/L of solvent (1.13 kg of NH.sub.3BH.sub.3 are obtained with 44 L of solvent (such an amount of solvent being necessary to selectively dissolve the synthesized borazane (the co-product obtained (Na.sub.2CO.sub.3) remaining insoluble)), the value of the synthesis according to the invention (implementation under gaseous ammonia, without solvent, under mild temperature and pressure conditions) is thus appreciated.

[0073] The borazane synthesized according to the invention moreover had an advantageous degree of purity. Specifically, the presence of impurities at detectable levels was not demonstrated either by .sup.1H NMR (a given mass of the synthesized borazane was dissolved in a solvent (THF); benzene being added to the analyzed solution as standard), or by infrared, i.e. any impurity potentially present was present only in a content of less than 1 wt. % (in point of fact, according to the infrared technique, any impurity potentially present was only present in a content very much less than 1 wt. %).