Mechanochemical reactions of alkali borohydride with CO2 under ambient temperature

Article abstract of DOI:10.1016/j.jssc.2019.07.037

Many efforts have been made to study the reactions of the borohydride with CO₂ in solution. However, it is scarce to achieve the hydrogenation of CO₂ using borohydrides as hydrogen source by the solid-gas direct reaction in the condition of ball milling under ambient temperature. Herein, we investigated the solid-gas noncatalytic reaction of ABH₄ (A = Li or Na) with CO₂ under mechanochemical conditions for the first time. It is found that hydrogen and trimethylborane are formed in the gas phase, as well as borate, formate, and methoxy species are formed in the solid phase.

Full text of DOI:10.1016/j.jssc.2019.07.037

Accepted Manuscript
Mechanochemical reactions of alkali borohydride with CO under ambient
2
temperature
Wei Zhu, Juan Zhao, Lu Wang, Yun-Lei Teng, Bao-Xia Dong
PII:
S0022-4596(19)30371-8
https://doi.org/10.1016/j.jssc.2019.07.037
YJSSC 20876
DOI:
Reference:
To appear in: Journal of Solid State Chemistry
Received Date: 27 March 2019
Revised Date: 17 July 2019
Accepted Date: 21 July 2019
Please cite this article as: W. Zhu, J. Zhao, L. Wang, Y.-L. Teng, B.-X. Dong, Mechanochemical
reactions of alkali borohydride with CO under ambient temperature, Journal of Solid State Chemistry
2
(2019), doi: https://doi.org/10.1016/j.jssc.2019.07.037.
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ACCEPTED MANUSCRIPT
Mechanochemical reactions of alkali borohydride with CO2 under ambient
temperature
wei zhu; juan zhao; lu wang; Yun-Lei Teng *; baoxia dong *;
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Mechanochemical reactions of alkali borohydride with CO2 under ambient
temperature
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Wei Zhu, Juan Zhao, Lu Wang, Yun-Lei Teng,* Bao-Xia Dong*
School of Chemistry and Chemical Engineering, Yangzhou University,
Yangzhou, 225002, P. R. China.
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Abstract: Many efforts have been made to study the reactions of the borohydride
with CO2 in solution. However, it is scarce to achieve the hydrogenation of CO2 using
borohydrides as hydrogen source by the solid-gas direct reaction in the condition of
ball milling under ambient temperature. Herein, we investigated the solid-gas
noncatalytic reaction of ABH4 (A = Li or Na) with CO2 under mechanochemical
conditions for the first time. It is found that hydrogen and trimethylborane are formed
in the gas phase, as well as borate, formate, and methoxy species are formed in the
solid phase.
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Keywords: Carbon dioxide; Borohydride; Ball milling; Ambient temperature
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*Corresponding author. E-mail: ylteng@yzu.edu.cn; bxdong@yzu.edu.cn.
24 1. Introduction
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As the increasing use of carbon-based fuels by humans, the amount of carbon
dioxide (CO2) emitted to the atmosphere has also risen sharply. The vast amount of
CO2 causes significant and negative effects on the global environment [1]. In 2016,
the volumetric concentration of CO2 in the earth's atmosphere exceeded 400 ppm for
the first time, which was much higher than the 280 ppm before industrialization [2].
Therefore, it is important to take effective measures to reduce and treat CO2 emissions.
As an economical, safe, and sustainable carbon resource compound, CO2 has great
potential for development into liquid fuels and chemicals which conforms to the green
chemistry principles [3-5].
Borohydrides are conventional hydride-donating agents in use today. The chemical
properties of borohydride are affected by bonding between metallic atoms and
borohydride ligands and the electron structure. The H in [BH4]- is mainly bonded to B
by a covalent bond, resulting in that the thermodynamic properties are very stable
[6,7]. Therefore, borohydrides often exhibit different property with that of salt-like
hydrides such as LiH, NaH, and MgH2. Borohydrides have been widely used in
organic synthesis to reduce various functional groups because they are stable
reduction reagents in air and solution [8,9]. In addition, due to their high weight
energy densities. Borohydrides are also widely used ad hydrogen source. A significant
effort has been made to produce hydrogen by hydrolysis or by combining with other
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hydrides [10-14].
Recently, many efforts have been made to study the reactions of the borohydride
with CO2 in solution. Kyle et al. reported that sodium borohydride (NaBH4) could
react with CO2 and bicarbonate (HCO3 ) in aqueous solution to form formate (HCOO-)
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[15]. Zhao et al. have developed a process for converting CO2 to HCOO- using
sodium borohydride at 318 k in a solution of the PH of 9.0 [16,17]. Waheed et al.
achieved the reduction of CO2 to ethanol by a two-step process using methyl
magnesium bromide CH3MgBr (Grignard reagent) and sodium borohydride in
tetrahydrofuran [18]. Knopf et al. reported that sodium borohydride in butyronitrile
solution could be used to reduce CO2 to formate without the use of expensive borane
or complex catalysts [19]. Hao et al. used NaBH4 as a reducing agent in dimethyl
sulfoxide to achieve reductive acylation of amine and CO2 at 100 °C without a
catalyst, resulting in a series of formylation products [20]. Fletcher et al. provided an
efficient method for producing formic acid in high yield (24%) by reducing CO2 with
potassium borohydride(KBH4) in aqueous solution under ambient conditions [21].
Sabet-Sarvestani et al. explored the reduction of CO2 to methanol utilizing
borohydride compounds as a reducing agent [22].
During investigating reactions between borohydride and CO2, the reduction of CO2
to carbon by solid-solid and solid-gas interaction are also explored. Lou et al.
synthesized Y-junction carbon nanotubes by reducing CO2 with NaBH4 at 700 °C [23].
Vitillo et al. found that the main products in the reaction of γ-Mg(BH4)2 with CO2 are
formate and alkoxide compounds [24]. Zhang et al. found that sodium borohydride
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reacted with CO2 at 500 °C to produce boron-doped porous carbon [25]. Zhang et al.
also demonstrated that under moderate conditions, CO2 could be reduced to graphene
oxide using ammonia borane by two consecutive steps [26]. Picasso et al. studied the
solid-gas noncatalytic reaction between KBH4 and CO2 under thermal induction
conditions and found potassium formyl hydroborate, K [HxB(OCHO)4-x] (x = 1-3), is
the main product [27].
However, it is scarce to achieve the hydrogenation of CO2 using borohydrides as
hydrogen source by the direct solid-gas reaction in the condition of ball milling under
ambient temperature.
We believe that the solid-gas reaction proceeded by
mechanochemistry is convenient, safe, and scalable. Besides, little attention in
previous experiments was paid to the gaseous products released from the reaction
between borohydride with CO2. In this work, we studied the reactions between ABH4
(A = Li or Na) and CO2 in the absence of a catalyst under mechanochemical
conditions. It is found that hydrogen and trimethylborane are formed in the gas phase,
as well as borate, formate, and methoxy species are formed in the solid phase.
81 2. Experimental methods
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2.1 Materials
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Lithium borohydride (LiBH4) with a purity of 97% was obtained from Aladdin.
Sodium borohydride (NaBH4, 98%) was supplied by Shanghai Ruihua Chemical Co.
Ltd. Carbon dioxide (CO2 with a purity of 99.999%) was acquired from Shanghai
Chemical Industry Park Pujiang Special Gas Co. Ltd.
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2.2. The mechanochemical solid-gas reaction of ABH4 (A = Li or Na) with CO2
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The mechanochemical solid-gas reaction of ABH4 (A = Li or Na) with CO2 was
performed in an 80 mL stainless steel grinding bowl using a planetary ball milling
apparatus (QM-3SP4) at 450 rpm. ABH4 and CO2 at the initial ratio of 1:1 mol/mol
with 30 stainless steel balls (6 mm diameter, about 27 g) were used to ball mill. The
initial pressure of CO2 is controlled at 0.25 MPa by a pressure sensor. Moreover, the
mass of LiBH4 and NaBH4 is 0.1787 g and 0.3104 g, respectively. ABH4 and CO2
were ball milled for 23 cycles, with a cycle duration of 1 hour and a pause of 30 min.
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2.3. Characterization of gaseous product
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The gaseous products were analyzed by gas chromatography (GC). H2
concentration was detected using an SP-6890 GC (Rainbow Instrument Co., China)
equipped with a TDX-01 column and a thermal conductivity detector (TCD). The gas
Infrared spectroscopy was performed to characterize the gas product. The gas was
introduced into the glass tube with potassium bromide at both ends. The gas Fourier
transform IR spectroscopy (FTIR) were recorded on Bruker Vertex 70 v spectrometer
between 500–4000 cm-1 and with a spectral resolution of 4 cm-1 in the transmission
mode.
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2.4. Characterization of solid product
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The morphologies of the solid products were examined via scanning electron
microscopy (SEM, S-4800II, Japan). Samples S-4800II analyses were placed on the
sample holders supported by carbon paint followed by 1-min sputter coating of gold.
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EDX analysis was accomplished using a link attachment on an S-4800II transmission
electron microscope. Powder X-ray diffraction (XRD) measurement was carried out
using an X-ray diffractometer (AXS D8 ADVANCE, Bruker, Germany) for a 2θ range
of 10–80º, a step size of 0.1º and using graphite monochromatic Cu Kα radiation with
a nickel filter. The tube current was 300 mA with a tube voltage of 40 kV. FTIR
(Fourier-transform infrared) spectra were recorded on a Bruker TENSOR27 FTIR at a
resolution of 4 cm-1 from 500 to 4000 cm-1. 11B and 13C solid-state MAS NMR (magic
angle spinning-nuclear magnetic resonance) spectra were recorded by Bruker
AVANCE III400MHzWB, operating in a 9.4 T magnet. Samples were spun at 10 kHz
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for B and 9 kHz for C with the sample temperature maintained at 298.15 K. The
13C solid-state MAS NMR spectra were referenced to the methylene carbon of
adamantane at 38.48 ppm. 11B NMR spectra were referenced against an external
BF3•OEt2 standard (d = 0 ppm). All data were processed with 100 Hz line broadening.
121 3. Results and discussion
122 3.1. Gaseous product
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ABH4 (A = Li or Na) and CO2 (0.25 MPa) were ball milled at 450 rpm during
different reactions times. To elucidate the reaction process, we analyzed the gaseous
products using GC. Distinct chromatographic peaks of hydrogen (at the retention time
of around 0.28 min) are observed, as shown in Fig. 1. The peak intensity of H2 in the
ABH4 (A = Li or Na)-CO2 system increases with ball milling time increasing. The
detector used in gas chromatography is a thermal conductivity detector (TCD), which
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has low sensitivity and cannot detect a small amount of trimethylborane and other
boron products.
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Fig. 1. GC profiles (tested with a TDX-01 column) of the gaseous products from the
reaction between LiBH4 (NaBH4) and CO2 for 2, 12, and 24 h.
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In order to further determine the gas composition, FTIR spectra of the gaseous
products from the reactions of ABH4 (A = Li or Na) with CO2 by ball milling were
obtained (Fig. 2). The FTIR absorptions due to trimethylborane (B(CH3)3) were
clearly observed at 2971, 2885, 1494, 1363, and 1041 cm−1, which indicates that
B(CH3)3 is formed in the reactions of ABH4 (A = Li or Na) with CO2 [28]. Moreover,
FTIR spectra of the gaseous products from the LiBH4-CO2 system shows the
difference from those of the NaBH4-CO2 system. Some adsorptions clearly reveal the
formation of boron species containing B−H (peaks in the 2626 cm-1 region is due to
ν(B−H), at 1619 cm-1 is due to γ(H−B−H) and around 1187 cm-1 is as result of ν(B−B))
[29].
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Fig. 2. IRGA (infrared gas analysis) of the gaseous products from the reaction
between LiBH4 (NaBH4) and CO2 for 2, 12, and 24 h.
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This result indicates that the main gaseous products from the LiBH4-CO2 and
NaBH4-CO2 systems are hydrogen and trimethylborane. As shown in Scheme S1, the
trimethylborane species is possible to be formed due to the interaction of the B atom
with the C atom forming an intermediate with B-C bond, which is further
hydrogenated to form the trimethylborane species. Previous studies reported that
methane could be largely prepared by the reactions of metal hydrides with CO2
[30,31]. However, there is no methane after the reactions of borohydrides with CO2.
These results imply that the mechanism of the reaction of borohydrides with CO2 is
different from that of the reaction between metal hydrides and CO2.
157 3.2. Solid product
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Compared with the gaseous product after the reactions for 2 and12 h, there is no
CO2 in the gaseous product after the reaction for 24 h (Fig. 2). Therefore, the product
after the 24-h reaction is selected for analysis and characterization. To further
elucidate the reaction process, we characterized the solid products by FTIR, XRD,
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and NMR.
The FTIR spectra of the solid products from the reaction between LiBH4 (NaBH4)
and CO2 are displayed in Fig. 3. For the LiBH4-CO2 system, the IR spectra are
relatively simple. The peak at 2289 cm-1 is characteristic of ν(BH4). The FTIR
absorptions are clearly observed at 1627, 1375, 1006, and 698 cm-1, which implies the
formation of ν(C=O), ν(B−O), ν(B−O), and γ(B−O) [32]. According to the existing
literature [32], the infrared telescopic vibration of B-O mainly has the following four
types: the infrared peak at 1450-1300 cm-1 is Asymmetric stretching of B(3)-O (three
coordinate boron); the infrared peak at 1150-1000 cm-1 is Asymmetric stretching of
B(4)-O (four coordinate boron); the infrared peak at 960-890 cm-1 is Symmetric
stretching of B(3)-O; the infrared peak at 890-740 cm-1 is Symmetric stretching of
B(4)-O. However, the IR spectra from the NaBH4-CO2 system is complex (Fig. 3b).
The peaks at 2293 cm-1 and 1278 cm-1 are characteristic of ν(BH4) and σ(BH4). As
shown in Fig. 3b, some of peaks clearly show the production of formate (–OOCH,
peak at 1608 cm-1 is as a result of ν(C=O) mode) and methoxy species (–OCH3, peaks
in the 3000–2800 cm-1 region is as a result of ν(CH3) and approximately 1130 cm-1 is
as a result of ν(C–O)) [24]. Eventually, the absorption bands centering approximately
1350, 1000, 945, and 823 cm-1 were assigned to the ν(B−O) mode (in agreement with
the results of the NMR spectrum as following) [32].
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Fig. 3. FTIR spectra of the solid products from the reacton between LiBH4 (NaBH4)
and CO2 for 24 h.
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The X-ray powder diffraction pattern of the solid-phase product is shown in Fig. 4.
The [BH4]- cannot be observed in XRD patterns (Fig. 4a), but the peak of ν(BH4) is
observed in Fig. 4a. However, Lang et al. studied the X-ray diffraction of unmilled
and milled LiBH4 [33]. They proved the crystal structure of LiBH4 could not be
destroyed. Therefore, we speculate the solid products of LiBH4-CO2 system is
amorphous and attached to the surface of LiBH4. For the NaBH4-CO2 system, Fig. 4b
shows that NaBH4 does not react completely (combined with Fig. 3b) after ball
milling for 24 hours. There are no diffraction peaks due to new species, indicating that
the solid products are amorphous.
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Fig. 4. XRD of the solid products from the reacton between LiBH4 (NaBH4) and CO2
for 24 h.
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The NMR spectra reveal the analogous information. The 13C CP-MAS spectrum of
the LiBH4-CO2 system (Fig. 5a) is characterized by peaks at 172.91, 85.61, and 50.83
ppm, which are characteristic of C=O, O−C, and OCH3, further proving that there are
formate and methoxy species in the solid products. This is in agreement with previous
results [34]. Furthermore, the 13C NMR spectrum of the NaBH4-CO2 system (Fig. 5b)
is similar to the spectrum of the LiBH4-CO2 system (Fig. 5a).
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Fig. 5. The C NMR spectra of the solid products from the reacton between LiBH4
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Fig. 6 shows B NMR spectra of the solid products formed in the reactions of
ABH4 (A = Li or Na) with CO2. In the 11B CP-MAS NMR spectrum of the
LiBH4-CO2 system (Fig. 6a), sharp peaks at 0.78 ppm and -42.7 ppm are observed,
corresponding to tetrahedral BO4 groups (sp3 B) and BH4 anion [21,26]. A weak peak
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at 13.37 ppm represents trigonal BO3 groups (sp2 B) in the NMR spectrum of the
LiBH4-CO2 system [27]. Moreover, the 11B NMR spectrum of the NaBH4-CO2 system
(Fig. 6b) resembles the spectrum of the LiBH4-CO2 system (Fig. 6a). The unmarked
peaks in Fig. 6 are rotating side-bands. The 11B NMR spectra further prove that there
are borate species in the solid products. Therefore, we speculate that the main
solid-phase products are borate, formate, and methoxy species in the
mechanochemical reaction of ABH4(A = Li or Na) with CO2, according to the
characterizations of FTIR, XRD, 13C NMR, and 11B NMR. On the base of the analysis
of the products and the reports in the literature [27], we tentatively proposed the
formation mechanism of the borate, formate, and methoxy species by the reaction of
borohydride with CO2. As shown in Scheme S2, the formation of borate, formate, and
methoxy species is most likely due to the transfer of hydride and the interaction of the
1
boron atom and the oxygen atom. The peaks at 7.7 and 3.8 ppm in the H CP-MAS
spectrum of the NaBH4-CO2 system are probably due to characteristic of borate and
methoxy species (Fig. S3), further demonstrating the presence of borate and methoxy
species in solid products.
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Fig. 6. The B NMR spectra of the solid products from the reacton between LiBH4
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(NaBH4) and CO2 for 24 h.
229 4. Conclusion
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In conclusion, the reactions of borohydrides with CO2 under ambient-temperature
ball milling condition were systematically studied firstly. The gas and solid products
were characterized by GC, XRD, NMR, and FTIR. It is found that the reactions of
borohydrides with CO2 are complex, compared with the reactions of metal hydrides
with CO2. On the basis of experimental results, we infer that hydrogen and
trimethylborane are formed in the gas phase, as well as borate, formate, and methoxy
species are formed in the solid phase.
Acknowledgments
This work was supported by the National Natural Science Foundation of
China (Nos. 21573192 and 21671169); the Priority Academic Program
Development of Jiangsu Higher Education Institutions; and the High-Level
Personnel Support Program of Yang-Zhou University.
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2:1-21.
331 [34]Xiong R, Zhang J, Lee JW (2013) Carbon dioxide-facilitated low-temperature
332
hydrogen desorption from polyaminoborane. J Phys Chem C 117:3799-3803.
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ACCEPTED MANUSCRIPT
Mechanochemical reactions of alkali borohydride with CO2 under ambient
temperature
wei zhu; juan zhao; lu wang; Yun-Lei Teng *; baoxia dong *
For the reaction of alkali borohydride with CO2, hydrogen and trimethylborane are
formed in the gas phase, as well as borate, formate, and methoxy species are formed
in the solid phase.

ACCEPTED MANUSCRIPT
Highlights
▶ The solid-gas reaction of alkali borohydride with CO2 was investigated.
▶ The reaction is carried out at ambient temperature under mechanochemical
condition.
▶ The hydrogen and trimethylborane are formed in the gas phase.
▶ The borate, formate, and methoxy species are formed in the solid phase.
▶ The work provides a new method for converting CO2 using borohydrides.