Preparation of sodium borohydride by the reaction of MgH2 with dehydrated borax through ball milling at room temperature

Article abstract of DOI:10.1016/S0925-8388(02)00872-1

A convenient method was developed to synthesize NaBH₄ by the reaction of MgH₂ with Na₂B₄O₇ through ball milling at room temperature. In order to improve the sodium borohydride yield, Na compounds were added to compensate the Na insufficiency in reactants when MgH₂ instead of NaH was used as the reducing agent. It was found that Na₂CO₃ addition was better than NaOH or Na₂O₂ addition in increasing the borohydride yield.

Full text of DOI:10.1016/S0925-8388(02)00872-1

Journal of Alloys and Compounds 349 (2003) 232–236
L
www.elsevier.com/locate/jallcom
Preparation of sodium borohydride by the reaction of MgH with
2
dehydrated borax through ball milling at room temperature
*
Z.P. Li , N. Morigazaki, B.H. Liu, S. Suda
Department of Environmental and Chemical Engineering, Kogakuin University, Nakano-machi 2665-1, Hachioji-shi, Tokyo 192-0015, Japan
Received 2 May 2002; received in revised form 10 June 2002; accepted 10 June 2002
Abstract
A convenient method was developed to synthesize NaBH4 by the reaction of MgH2 with Na B O through ball milling at room
2
4
7
temperature. In order to improve the sodium borohydride yield, Na compounds were added to compensate the Na insufficiency in
reactants when MgH instead of NaH was used as the reducing agent. It was found that Na CO addition was better than NaOH or Na O
2
2
3
2
2
addition in increasing the borohydride yield.

2002 Elsevier Science B.V. All rights reserved.
Keywords: Metal hydrides; Energy storage materials; Chemical synthesis; Crystal structure; X-ray diffraction
1
. Introduction
4NaH 1 B(OCH ) 5 NaBH 1 3NaOCH
3
(2)
3
3
4
Borohydrides are well-known reducing agents. They
At higher temperatures (330–350 8C), sodium hydride
and boric oxide react to produce up to 60% yields of
sodium borohydride by the following reaction:
often have quite specific uses in organic and inorganic
2
chemistry, where they may also be the sources of H other
than simple reductants. Recently they have attracted more
attention as a hydrogen storage medium due to their high
4
NaH 1 B O 5 NaBH 1 3NaBO
(3)
2
3
4
2
hydrogen storage capability. For example, NaBH contains
4
Sodium borohydride can also be synthesized by heating
1
0.6 wt.% hydrogen which is much more than what most
a mixture of dehydrated borax, quartz and sodium metal
under hydrogen gas to higher temperatures of 450–500 8C
through the following reaction [2].
hydrogen storage alloys have. Furthermore, the hydrolysis
of borohydride is of interest in hydrogen generation
because half of the generated hydrogen is from the
borohydride and the other half is from water. NaBH can
4
16Na 1 8H 1 Na B O 1 7SiO 5 4NaBH 1 7Na SiO
2
2
4
7
2
4
2
3
generate 10.8 wt.% of hydrogen based on the following
hydrolysis reaction.
(
4)
Besides sodium hydride, calcium hydride has been used
to react with NaBO2 to prepare NaBH4 in the same
temperature range [3]
NaBH 1 2H O 5 NaBO 1 4H
2
(1)
4
2
2
Its hydrogen generation rate, when using some catalysts,
is rather high at room temperature compared with the
hydrogen desorption rate of metal hydrides. However, only
very few investigations have been done on borohydride
synthesis since the 1950s, when Schlesinger et al. [1]
proposed a rapid reaction at 225–275 8C of 1 mol of
methyl borate with 4 mol of sodium hydride
2CaH 1 NaBO 5 NaBH 1 2CaO
(5)
2
2
4
All these reactions mentioned above were conducted at
higher temperatures. In this paper, a new method is
proposed to prepare sodium borohydride through a mech-
ano–chemical reaction at room temperature. The mech-
ano–chemical reaction was conducted by ball milling the
reactants in a planetary ball mill. Mg hydrides were used
as the reducing agent to react with dehydrated borax,
*
Corresponding author.
E-mail address: bq96001@ns.kogakuin.ac.jp (Z.P. Li).
considering that MgH (7.60 wt.%) contains more hydro-
2
0
925-8388/02/$ – see front matter
 2002 Elsevier Science B.V. All rights reserved.
PII: S0925-8388(02)00872-1

Z.P. Li et al. / Journal of Alloys and Compounds 349 (2003) 232–236
233
Fig. 1. Scheme of experimental procedures.
gen than NaH (4.17 wt.%) and CaH2 (4.76 wt.%).
2.2. Mechano–chemical reaction of dehydrated borax
with Mg hydride
Sequential experiments were planned to explore the possi-
bility of borohydride synthesis at room temperature as
shown in Fig. 1. On the other hand, in order to improve the
conversion rate from Na B O to NaBH , some Na
The mechano–chemical reaction was conducted by ball-
milling a mixture of magnesium hydride, dehydrated borax
and some Na compounds in a planetary ball mill at room
2
4
7
4
compounds were selected as additives to investigate their
effects in improving borohydride formation.
temperature. The amount of MgH varied from 0.835 g
2
2
2
(
(
3.175310 mol), i.e. 1.392 g of 60% MgH to 4.821 g
2
0.183 mol), i.e. 8.035 g of 60% MgH . The amount of
2
2
2
dehydrated borax was fixed at 2.661 g (1.323310 mol).
2
. Experimental
The amounts of Na compounds were fixed at 1.098 g
2
2
22
(
1.323310
mol) of Na CO , 1.058 g (2.645310
2 3
22
2
.1. Preparation of Mg hydride
mol) of NaOH and 1.032 g (1.323310 mol) of Na O ,
2
2
respectively. The reaction components were put into a
45-ml grinding bowl with seven grinding balls having
diameters of 13 mm. The grinding bowls and balls were
made of stainless steel. Ball milling was then conducted at
room temperature at 2750 rpm for 60 min. Fig. 4 shows
the grinding bowl and the grinding mechanism.
Mg powders with a purity of 99.95% (size: ,75 mm)
were hydrogenated at 350 8C under 1 MPa hydrogen
pressure after degassing for 2 h. Mg hydrides with a
hydrogenation rate of 60% were formed through 24-h
hydrogenation. The absorbed hydrogen was measured by a
volumetric method with a Sieverts apparatus. Figs. 2 and 3
show the XRD pattern and microstructure of the hydro-
genated Mg sample, respectively.
The samples were subjected to a qualitative XRD
Fig. 2. XRD pattern of hydrogenated Mg.
Fig. 3. Microstructure of hydrogenated Mg particle.

2
34
Z.P. Li et al. / Journal of Alloys and Compounds 349 (2003) 232–236
Fig. 4. Grinding bowl and grinding mechanism of the ball mill.
Table 1
Purities of the used chemicals
NaH was used as the reducing agent to react with
Na B O , it can be expected that the low Na content in the
2
4
7
Chemicals
Purity (%)
Na B O
Na CO
NaOH
98
Na O
reactants would restrain the borohydride formation because
the atomic ratio of Na and B in the reactant Na B O is
2
4
7
2
3
2
2
99
99
98
2
4
7
1
:2, but that in the product NaBH is 1:1. Therefore, in
4
order to improve the borohydride yield, some sodium
compounds should be supplied to compensate for the Na
insufficiency. Na CO , NaOH and Na O were selected as
analysis after ball milling. The product (NaBH ) was
extracted by anhydrous ethylenediamine with a purity of
4
2
3
2
2
9
9% under an Ar atmosphere. Hardened filter paper was
test materials. As a comparison, borohydride yields with-
out Na compound additive were also investigated. The
borohydride yield rate is referred to as the amount of
sodium borohydride formed by 1 mol of Na B O to the
used to separate the extraction solution from MgO and
remaining reactants. The solid product (NaBH ) was
4
obtained by evaporating the extraction solution under 0.05
MPa at room temperature. A cold trap was applied to
capture the solvent vapor (ethylenediamine) at 210 8C.
The obtained powders were subjected to XRD analysis for
qualitative identification of the borohydride formation. The
2
4
7
theoretical value based on the corresponding reaction (see
reactions (6)–(9) in Table 2). The Gibbs standard reaction
energy (DG8) is listed in Table 2. The calculation was done
based on data of the Gibbs standard formation energy
(298.15 K) [4]. From the thermodynamic calculation, it
follows that all these reactions are possible in practice as
regards thermodynamics.
amount of formed NaBH was quantitatively determined
4
by iodimetric analysis [5]. The purities of the chemicals
used are listed in Table 1.
Fig. 5 shows the XRD qualitative analysis of the sample
after ball milling a mixture of MgH , Na B O and
2
2
4
7
3
. Results and discussion
Na CO in a mole ratio of 9.24:1:1 based on reaction (6).
2 3
The X-ray diagram verified the existence of NaBH and
MgO. Fig. 6 gives the XRD result of this sample after
extraction, filtration and drying. The XRD peaks clearly
4
From published results [1–3], it is known that hydrides
react with borates to form borohydride and oxide. Accord-
ing to the mass conservation law, when MgH instead of
show the existence of NaBH . These results prove that
2
4
Table 2
Expected reactions of sodium borohydride formation and the Gibbs standard reaction energy and maximum yield of NaBH4
Reaction DG8
kJ/mol NaBH4)
Max. yield
(%)
(
8
9
9
4
MgH 1Na B O 1Na CO 54NaBH 18MgO1CO (6)
2254.4
2360.6
2438.4
2239.7
78
64
67
43
2
2
4
7
2
3
4
2
MgH 1Na B O 12NaOH54NaBH 19MgO12H
2
(7)
(8)
(9)
2
2
4
7
4
MgH 1Na B O 1Na O 54NaBH 19MgO1H
2
2
2
4
7
2
2
4
MgH 1Na B O 52NaBH 14MgO1B O
3
2
2
4
7
4
2

Z.P. Li et al. / Journal of Alloys and Compounds 349 (2003) 232–236
235
Fig. 5. XRD pattern of the sample after ball milling based on the reaction 8MgH 1Na B O 1Na CO 54NaBH 18MgO1CO .
2
2
4
7
2
3
4
2
NaBH can be synthesized by mechano–chemical reduc-
Na B O 1 2NaOH 5 4NaBO 1 H O
(10)
4
2
4
7
2
2
tion through ball milling at room temperature.
Fig. 7 shows the effect of the sodium compound
The produced H O would consume some NaBH to
2 4
addition on improving the NaBH formation. The experi-
generate H2 according to reaction (1) so that the boro-
hydride yield was decreased. Therefore, in order to im-
prove the borohydride formation through Na compensa-
tion, the selected Na compound should avoid reacting with
the reactant (Na B O ) and the product (NaBH ). Na CO
4
ments were performed through changing the MgH supply
2
but the amounts of dehydrated borax and added sodium
compound were fixed to the stoichiometric amounts based
on reactions (6)–(9). It was found that the Na compound
2
4
7
4
2
3
addition effectively promoted the NaBH formation com-
proved to be a successful example in our research.
4
pared with reaction (9).
It was noted that an excessive amount of MgH was
2
Of all these additives, Na CO was the most effective in
needed to form the borohydride and improve the boro-
hydride yield. This can be attributed to the fact that some
2
3
increasing the borohydride yield (Table 2). It is believed
that Na CO is a stable salt with a very low reactivity to
MgH was desorbed due to high speed collisions with the
2
3
2
consume the product NaBH . However, when Na O was
balls during ball milling.
4
2
2
used as Na compensation agent, it was impossible to avoid
oxidation of the formed NaBH4 due to its strong ox-
idizability. As to the NaOH addition, Na B O could react
It was also noted that the amount of formed borohydride
increased with increasing the amount of MgH supplied,
2
but decreased when further increasing the amount of
magnesium hydride after the borohydride yield reached an
extreme value. It was beyond our expectation that the
2
4
7
with NaOH to form H O and NaBO through the follow-
2
2
ing reaction
Fig. 6. XRD pattern of the product after vaporizing the extracted solution based on the reaction 8MgH 1Na B O 1Na CO 54NaBH 18MgO1CO .
2
2
4
7
2
3
4
2

2
36
Z.P. Li et al. / Journal of Alloys and Compounds 349 (2003) 232–236
Fig. 7. NaBH4 formation when increasing the amount of MgH2.
borohydride yield would remain constant after reaching its
Acknowledgements
extreme value. When increasing the amount of MgH , it
2
was found that the reactants easily stuck onto the walls of
the bowl and the grinding balls, which resulted in decreas-
ing the efficiency of the ball-milling for borohydride
formation. In fact, in addition to their role as additives
Na B O etc. also worked as a dispersant to avoid the
This research was financed by the New Energy and
Industrial Technology Development Organization (NEDO)
of Japan.
2
4
7
sticking of the materials onto the walls of the bowl. It is
suggested that the relative decrease in the amount of
Na B O and Na compounds is the reason for the decrease
in borohydride yield when excessive amounts of Mg
hydrides are added.
References
[1] H.I. Schlesinger, H.C. Brown, A.E. Finholt, J. Am. Chem. Soc. 75
2
4
7
(
1953) 205–209.
2] F. Schubert, K. Lang, W. Schabacher, A. Burger, Patent US
077376, 1963.
[
3
[
[
3] D. Goerrig, W. Schabacher, F. Schubert, DBP. 1036222, 1956.
4] D.D. Wagman, W.H. Evans, V.B. Parker, G.H. Schumm, I. Halow,
S.M. Bailey, K.L. Churney, R.L. Nuttall, The NBS tables of
chemical thermodynamic properties, in: J. Phys. Chem. Ref. Data,
Vol. 11, American Chemical Society and American Institute of
Physics for National Bureau of Standards, 1982.
4
. Conclusions
It has been proved that sodium borohydride can be
formed from hydrated borax and magnesium hydride
through ball milling at room temperature. The sodium
borohydride yield can be improved by Na compensation
through adding Na compounds as reactants.
[5] D.A. Lyttle, E.H. Jensen, W.A. Struck, Anal. Chem. 24 (1952) 1843.