PROCESS FOR PRODUCTION OF
SODIUM BOROHYDRIDE AND DIPHENYL OXIDE
Background
This invention relates generally to a process for production of sodium borohydride
and diphenyl oxide.
Production of sodium borohydride from the reaction of sodium aluminum hydride
with a boric acid ester with conversion of byproduct aluminum alkoxides to aluminum sulfate
and recycle of alcohol is disclosed in U.S. Pat. No. 7,247,286.
The problem addressed by this invention is to find a more efficient and economical
process for production of sodium borohydride from sodium aluminum hydride that extracts
additional value from the byproducts.
Statement of Invention
The present invention is directed to a process for production of an alkali metal
borohydride. The process comprises steps of: (a) combining a phenyl ester of a boric acid
ester precursor with a compound of formula MAlH4_(OPh) , where x is from zero to three, M
is an alkali metal and Ph is phenyl; to produce an alkali metal borohydride and Al(OPh)3; (b)
separating sodium borohydride from Al(OPh)3; and (c) heating Al(OPh) 3 to produce diphenyl
oxide.
Detailed Description
All percentages are weight percentages (wt%), and all temperatures are in °C, unless
specified otherwise. A "boric acid ester precursor" is a compound containing boron and
oxygen, e.g., B(OH)3, which can be converted into a boric acid phenyl ester, e.g., B(OPh)3.
Preferably, a boric acid ester precursor is an acid or salt containing a B O3 , B4(V 2 or B0 2
_1
group. Boric acid esters include boroxine compounds, e.g., (PhOBO)3, typically formed at
higher temperatures and 1:1 stoichiometry between the boric acid ester precursor and phenol
Preferably, the reaction temperature is from 100°C to 300°C, preferably from 110°C to
250°C, preferably from 110°C to 200°C,. Examples of the conversion of a boric acid ester
precursor to a boric acid ester include but are not limited to the following examples:
H3BO3 + 3PhOH ® B(OPh)3 + 3H20
Na2B40 7 + 12PhOH ® 4B(OPh)3 + 2NaOH + 5H20
120°C-180°C
B(OH), + PhOH ► (PhOBO)3 -H20
Preferably, M is lithium, sodium or potassium; preferably lithium or sodium;
preferably sodium. MAlH4_(OPh) may be a mixture of compounds each of which has an
integer value of x from zero to four, in which case x refers to the molar average value of x for
the mixture. Preferably, x is from zero to two.
An alkali aluminum hydride may be produced from its constituent elements at high
temperatures, e.g., according to the following equation, in which M is Na.
Na + Al + 2H2® NaAlH4
For example, U.S. Pat. No. 4,081,524 discloses preparation of sodium aluminum hydride in
hydrocarbon solvents at 160°C and a pressure of 5000 psi (34,000 kPa). Compounds of
formula MAlH4_(OPh) , where x is from one to three, or mixtures of compounds having an
average value of x from one to three, may be produced by combining a compound of formula
(PhO)M with aluminum and hydrogen, as described, e.g., in U.S. Pat. No. 3,728,272.
Preferred solvents for the reaction of a phenyl ester of a boric acid ester precursor
with a compound of formula MAlH4_(OPh) are those in which the sodium borohydride has
limited solubility, e.g., ethers, including 2-methyl-tetrahydrofuran, tetrahydrofuran,
dimethoxyethane, diglyme, triglyme, tetraglyme, diethyl ether, dibutyl ether and dibutyl
diglyme; aromatic solvents; and alkanes. Especially preferred solvents include 2-methyltetrahydrofuran,
tetrahydrofuran and dimethoxyethane. Preferably, this reaction proceeds at a
temperature in the range from 0°C to 50°C, preferably from 10°C to 35°C. Preferably, the
sodium borohydride precipitates from the reaction solvent and is separated, while the
aryloxide salts remain in solution.
The reaction may also be run without a solvent, e.g., as a slurry process or by grinding
the solid reactants. Grinding of the reactants will accelerate the reaction, and may be
achieved using any method which applies energy to solid particles to induce a
mechanochemical reaction, especially any method which reduces solids to the micron size
range, preferably the sub-micron size range, and continually exposes fresh surfaces for
reaction, e.g., impact, jet or attrition milling. Preferred methods include ball milling,
vibratory (including ultrasonic) milling, air classifying milling, universal/pin milling, jet
(including spiral and fluidized jet) milling, rotor milling, pearl milling. Especially preferred
methods are planetary ball milling, centrifugal ball milling, and similar types of high kinetic
energy rotary ball milling. Preferably, milling is performed in either a hydrogen atmosphere,
or an inert atmosphere, e.g., nitrogen. In an embodiment in which a solvent is used, grinding
of the reactants may be achieved using any method suitable for grinding a slurry. A solvent
facilitates heat transfer, thereby minimizing hot spots and allowing better temperature
control. Recycle of the solvent is possible to improve process economics. Examples of
solvents suitable for use during the process include amines, especially tertiary amines;
alkanes and cycloalkanes, especially C8-Ci2 alkanes and cycloalkanes; ionic liquids; liquid
crown ethers; and for lower-temperature reaction conditions, toluene, glymes and ethers.
Suitable reaction solvents are those in which the borohydride compound is soluble and which
are relatively unreactive with borohydride.
Another method to accelerate the reaction is to use radiation techniques alone or in
combination with reactive milling. For example, microwave irradiation can direct energy at
specific reaction surfaces to provide rapid heating and deep energy penetration of the
reactants. Microwave absorbers such as metal powders, which could be used as milling
media, and dipolar organic liquids may also be added to the reaction system to promote the
reaction. The advantage of these techniques is that high reaction rates may occur at
considerably lower processing temperature than could be obtained with resistive heating
thermal techniques.
Preferably, the sodium borohydride and the Al(OPh) 3 product are separated by
dissolving the aluminum product in a suitable solvent in which the sodium borohydride is
substantially insoluble. Preferably the solvent is a hydrocarbon solvent. Preferably, a solvent
may be used to separate the borohydride product from the aluminum phenoxide. Suitable
solvents are those in which the borohydride compound is soluble and which are relatively
unreactive with borohydride. A solvent in which the borohydride compound is soluble is one
in which the borohydride compound is soluble at 25°C at least at the level of 2%, preferably,
at least 5%. Preferred solvents include liquid ammonia, alkyl amines (primary and
secondary), heterocyclic amines, alkanolamines, alkylene diamines, glycol ethers, amide
solvents (e.g., heterocyclic amides and aliphatic amides), dimethyl sulfoxide and
combinations thereof. Preferably, the solvent is substantially free of water, e.g., it has a water
content less than 0.5%, more preferably less than 0.2%, more preferably less than 0.1%.
Especially preferred solvents include ammonia, Ci-C4 mono-alkyl amines, pyridine, 1-
methyl-2-pyrrolidone, 2-aminoethanol, ethylene diamine, ethylene glycol dimethyl ether,
diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol
dimethyl ether, dimethylformamide, dimethylacetamide, dimethylsulfoxide and combinations
thereof.
The Al(OPh)3 is heated to produce diphenyl oxide and alumina, as shown in the
following equation.
2Al(OPh)3 ® A 120 3 + 3PhOPh
Diphenyl oxide, PhOPh, is a useful product having commercial value; preferably it is sold to
increase the overall economic efficiency of the process. Preferably, aluminum phenoxide is
heated to a temperature from 200-500°C, preferably 300-400°C, as described in U.S. Pat. No.
4,360,699.

Claims
1. A process for production of an alkali metal borohydride; said process
comprising steps of: (a) combining a phenyl ester of a boric acid ester precursor with a
compound of formula MAlH4 _(OPh) , where x is from zero to three, M is an alkali metal and
Ph is phenyl; to produce an alkali metal borohydride and Al(OPh)3; (b) separating sodium
borohydride from Al(OPh)3; and (c) heating Al(OPh)3 to produce diphenyl oxide.
2. The process of claim 1 in which M is lithium, sodium or potassium.
3. The process of claim 2 in which the phenyl ester of a boric acid ester precursor
and the compound of formula MAlH4 _(OPh) are combined in a hydrocarbon solvent.
4. The process of claim 3 in which x is zero.
5. The process of claim 4 in which M is sodium
6. The process of claim 2 in which x is from zero to two.
7. The process of claim 6 in which M is sodium.
8. The process of claim 7 in which the phenyl ester of a boric acid ester precursor
and the compound of formula MAlH4 _(OPh) are combined in a hydrocarbon solvent.
9. The process of claim 8 in which x is from one to two.