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n MðMeÞm þ 1=2 ðn:m þ nÞB2H6 ! n MðBH4Þm þ m BðMeÞn
where Me ¼ CH3
using LiBH4 (Aldrich, >90%) instead of NaBH4. The mechanochem-
ical reaction was expected to proceed according to reaction (5b),
with (M = Li, Na). The ball-to-powder weight ratio was 44:1. To
avoid temperature increase during the experiment, milling times
of 15 min were alternated with 10 min of rest. Loading and powder
extraction procedures were performed in an argon-filled glove box
(O2 and H2O < 1 ppm).
ð4aÞ
ð4bÞ
3 ZrðORÞ4 þ 8 B2H6 ! 3 ZrðBH4Þ4 þ 4 BðORÞ3
where OR ¼ OC4H9
However, reaction (4b) is not satisfactory for the preparation of
Zr(BH4)4 since both products are present in the resulting solution
and are difficult to separate.
Another general approach is the preparation of covalent
M(BH4)n as solvates in different solvents (such as THF, ethers, pyr-
idine, amines):
The crystalline structure of as-milled samples was character-
ized by X-ray diffraction (XRD) analysis on a Philips PW 1710/01
Instruments with Cu Ka radiation (graphite monochromator). A
tight sealed sample holder was used to completely prevent the
reaction between samples and air during XRD measurement.
FTIR spectra were taken using FTIR Spectrum GX spectrometer.
After milling, Zr(BH4)4 or Zr(BD4)4 were separated from the gas-
phase and solid co-products using a high-vacuum system. First,
the milling vial was cooled to 253 K to eliminate the gas phase.
Then, the vial was heated up to 343 K and simultaneously the col-
lecting bulb was cooled with liquid nitrogen. The Zr(BH4)4 or
Zr(BD4)4 held in a tube at low temperature were freshly sublimed
into a degassed quartz optical cell with NaCl windows and room
temperature gas phase spectra were taken from the sample vapor
(533 Pa). To determine the decomposition temperature of both
compounds, different FTIR spectra were obtained from mixture of
Zr(BH4)4-B2H6 and Zr(DH4)4-B2D6 as a function of the temperature
by continued heating of quartz cell (total pressure 533 Pa). The
spectra were taken each 10 min to analyze the evolution of the
gas mixture from room temperature up to 500 K.
MXn þ n M0BH4 ! MðBH4Þ þ n M0X
ð5aÞ
ð5bÞ
ZrCl4 þ 4 MBH4 ! ZrðBH4Þn þ 4 MCl
4
Metal tetrahydroborates obtained are isolated from solution as
solvates with one o more molecules of solvent [6,12]. Therefore,
this process requires an additional desolvation step, which could
lead to the decomposition of M(BH4)n with hydrogen evolution
preceding the desolvation point.
An alternative and very convenient synthesis method was first
carried out by Reid et al. [11]. It was based in the development
of exchange reaction (5b) in the solid-phase, under dry conditions,
using Ni balls to ‘‘activate mechanically” the reactants. A yield of
90% was obtained by using an excess of LiBH4. Attempts to prepare
Zr(BH4)4 using solid-phase reaction from NaBH4 or KBH4 were
unsuccessful [11]. Moreover, an ulterior elaboration of this early
work was developed by Volkov et al. [5,13]. These authors demon-
strated the possibility to produce Zr(BH4)4 from LiBH4 or NaBH4 by
the exchange reaction (5b) using rotational mills. Very fast forma-
tion kinetics and higher yield of Zr(BH4)4 (about 90% with
ZrCl4:8LiBH4 ratio) were achieved when LiBH4 was used instead
of NaBH4.
Thermal stability of the purified solid compounds was studied
by calorimetric measurements using a heating rate of 5 °C minꢀ1
and an argon flow rate of 122 ml minꢀ1 (DSC 2910, TA Instru-
ments). Sample was prepared inside of glove box using aluminium
sealed-holder to avoid exposing to air.
In the last 20 years, mechanochemical technique has emerged
as an experimental procedure routinely used for both the prepara-
tion and nano-structuring of hydrogen-rich solids [14–16]. Follow-
ing this path, recently high energy ball milling of a complex
hydride mixed with an appropriate metal chloride (as example,
see Eq. (5a)) led to the convenient preparation of hexahydroalumi-
nates and tetrahydroborates, which are hardly accessible thought
the wet-chemistry synthetic routes [17–20]. Therefore, in search-
ing for a simple and economical procedure for the production of
Zr(BH4)4, the last synthesis procedure is the most suitable: it pro-
ceeds under solvent-free conditions and is potentially scaleable.
However, the factors that influence the mechanochemical reaction
and their contribution on the yield of the Zr(BH4)4 are still unclear.
In this work, we describe a remarkable extension of the mechano-
chemical approach for the preparation of pure zirconium tetrahy-
droborate and its deuterated counterpart at room temperature.
To the authors’ knowledge this is the first time that a complex deu-
teride compound is synthesized from NaBD4 using mechanochem-
ical processing. The effect of different synthesis parameters on the
yield of the desired product is presented. Thermal stabilities of the
complex hydride (deuterade) compounds obtained after milling
were investigated by FTIR and thermal analysis. The synthesis
was performed using NaBH4 (NaBD4) since this is a cheaper reac-
tive than LiBH4 (LiBD4).
3. Results
3.1. Synthesis of Zr(BH4)4 by ball milling of ZrCl4:6NaBH4 mixtures
under argon
Screening experiments (Eq. (5b)) were performed using
ZrCl4:6NaBH4 to determine optimum conditions of milling regard-
ing rpm and time, at constant ball-to-powder weight ratio under
argon atmosphere. These results revealed that increasing rpm from
200 to 400 favors the decomposition of the tetrahydroborate an-
ions to diborane, as determined by gas FTIR. This observation is rel-
evant for 400 rpm, where total pressure inside the milling vial
increases noticeably after 1 h of milling. Considering that the for-
mation of diborane involves the loss of BH4 groups from NaBH4,
we discard this last milling condition. The formation of diborane
from mechanical milling of NaBH4 (or LiBH4) in the presence of
anhydrous metal halides (with metal = Sn, Cu, Cr, Zn) was already
reported [5,13]; but it was not previously observed for Zr. We per-
formed additional measurements to clarify the formation of dibo-
rane and they will be analyzed in the next section.
The influence of milling time was further investigated for runs
performed at 200 and 300 rpm. The mechanochemical reaction
(5b) is not complete after 15 h of milling at 200 rpm, whereas both
starting materials, ZrCl4 and NaBH4, are detected by XRD (not
shown). Instead, the situation using 300 rpm is different. Fig. 1
shows the diffraction profiles of the samples obtained after differ-
ent milling times at 300 rpm. Fig. 1A corresponds to the hand-
milled mixture of ZrCl4 and NaBH4 with a composition of 1:6. At
3 h of milling, the starting materials are identified simultaneously
with NaCl. After 6 h of milling, the ‘‘metathesis reaction” (5b) is
practically completed and NaCl is the main phase identified as
crystalline product. Additional milling up to 12 h does not intro-
2. Experimental
Zr(BH4)4 (Zr(BD4)4) was synthesized from a mixture of anhy-
drous ZrCl4 (Merck, 99%) and NaBH4 (Aldrich, >90%) (NaBD4,
Aldrich, >90%). Mixtures of ZrCl4:NaBH4 with different composi-
tions were mechanically milled using a planetary ball mill (Fristch
P-6) in a hardened steel vial (80 cm3 in volume) under 100 kPa of
argon or vacuum for different times. Three runs were performed