Article pubs.acs.org/JPCC
Facile Solvent-Free Synthesis of Anhydrous Alkali Metal Dodecaborate M2B12H12 (M = Li, Na, K) Liqing He,† Hai-Wen Li,*,‡,§ Son-Jong Hwang,# and Etsuo Akiba†,‡,§ †
Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan International Research Center for Hydrogen Energy, Kyushu University, Fukuoka 819-0395, Japan § WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan # Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States ‡
S Supporting Information *
ABSTRACT: Metal dodecaborate, widely regarded as an obstacle of the rehydrogenation of high-density hydrogen storage materials metal borohydrides M(BH4)n, is generally synthesized using liquid-phase process followed by a careful dehydration process. In this study, we propose a new and facile solvent-free synthesis process of dodecaborates using B10H14 with a low melting point of 99.6 °C as a boron source. As a case study, our first challenge focused on the syntheses of anhydrous M2B12H12 (M = Li, Na, and K) by heat treatment of starting materials (a) 2MH + 1.2B10H14 or (b) 2MBH4 + B10H14 at 200−450 °C conditions, which have been proved to be successful for the first time by X-ray diffraction (XRD), Raman, and NMR analysis. Starting materials (b) 2MBH4 + B10H14 shows better reactivity than that of (a) 2MH + 1.2B10H14, which demonstrates that synthesis of anhydrous M2B12H12 by heat treatment of 2MBH4 + B10H14 is a feasible solvent-free process.
1. INTRODUCTION Metal borohydride M(BH4)n has been studied for a decade as potential hydrogen storage material because of its high gravimetric and volumetric hydrogen storage densities.1,2 Dehydrogenation of M(BH4)n proceeds via multistep reactions accompanied with the formation of several kinds of intermediate compounds, such as [B 3 H 8 ] − , [B 5 H 9 ] 2− , [B12H12]2−, and so on.3,4 Among them, metal dodecaborateM2/nB12H12 has been widely regarded as an obstacle of the rehydrogenation because of the strong B−B bond in the icosahedral boron cage, resulting in the degraded reversibility of M(BH4)n upon cycling.3−15 Systematic investigation of fundamental thermodynamic and kinetic properties of M2/nB12H12, therefore, becomes important in order to substantially improve the hydrogen storage performance of M(BH4)n. M 2/nB 12 H 12 is generally synthesized via liquid-phase processes using organic reagent triethylamineborane (Et3NBH3) and inorganic reagent M(OH)n solution. The typical multistep processes include16−19 2Et3NBH3 + 5B2H6 → (Et3NH)2 B12H12 + 11H 2
The product obtained from these liquid-phase processes is M2/nB12H12·xH2O, which then needs a careful dehydrating process to remove crystal water.15,20 Because some M2/nB12H12, such as MgB12H12, tend to react with crystal water during the dehydrating process, the liquid-phase processes may be limited to the synthesis of alkali metal dodecaborate M2B12H12.21 Therefore, development of solvent-free processes for synthesis of anhydrous M2/nB12H12 is in great need. In this contribution, we propose a new and facile solvent-free synthesis process of anhydrous M2/nB12H12 by heat treatment of metal hydrides MH or metal borohydrides M(BH4)n using B10H14 with low melting point (99.6 °C) as a boron source on the basis of the following equations: 2MH + 1.2B10H14 → M 2B12H12 + 3.4H 2
(4)
2MBH4 + B10H14 → M 2B12H12 + 5H 2 (M = Li, Na, K)
and
(2)
(3)
Received: January 9, 2014 Revised: February 28, 2014 Published: February 28, 2014
(Et3NH)2 B12H12 + 2/n M(OH)n → M 2/ nB12
H12 + 2Et3N + 2H 2O © 2014 American Chemical Society
(5)
To prove the feasibility of this new synthesis process, our first challenge focused on the syntheses of alkali metal dodecaborate as a case study. Also, we have compared the reactivity between MH and MBH4 routes in order to optimize the solvent-free synthesis process. This study will not only provide a novel
(1)
2Et3NBH3 + B10H14 → (Et3NH)2 B12H12 + 3H 2
or
(M = Li, Na)
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3. RESULTS AND DISCUSSION 3.1. Synthesis from 2MH + 1.2B10H14 (M = Li, Na). Figure 2a and b shows XRD patterns and Raman spectra of samples synthesized from 2MH + 1.2B10H14 with and without heat treatment. After ball milling, only diffraction peaks originating from starting materials of MH and B10H14 are identified from the XRD patterns, and the only observation of B10H14 from Raman spectra consistently indicates that no chemical reaction between MH and B10H14 occurred during
solvent-free synthesis process of M2/nB12H12 but will also give insightful information for understanding the formation mechanism of M2/nB12H12, which is of great importance for further improvement of hydrogen storage performance of M(BH4)n.
2. EXPERIMENTAL SECTION Commercial LiH (98%, Alfa Aesar), NaH (95%, Aldrich), B10H14 (99%, Wako), LiBH4 (95%, Aldrich), NaBH4 (99.99%, Aldrich), and KBH4 (98%, Aldrich) were all stored in a glovebox and were used without further purification. Mechanical milling was carried out using Fritsch P-7 ball milling machine. Different proportions of starting materials such as 2LiH + 1.2B10H14, 2NaH + 1.2B10H14, 2LiBH4 + B10H14, 2NaBH4 + B10H14, and 2KBH4 + B10H14 were first mechanically milled at room temperature using a planetary ball mill (Fritsch P-7) with 10 steel balls (7 mm in diameter) in a hardened steel vial (30 cm3 in volume) under 0.1 MPa Ar for 5 h (15 min milling, 5 min pausing). Subsequently, the ball-milled products were sealed into stainless steel crucibles for heat treatment at 200−450 °C with a time range from 10 to 20 h. The schematic illustration of the heating apparatus is shown in Figure 1.
Figure 1. Schematic illustration of the heat treatment apparatus.
Powder X-ray diffraction (XRD) patterns were recorded using Rigaku Ultima IV X-ray diffractometer with Cu−Kα radiation using 40 kV/40 mA as accelerating voltage/tube current. The sample powders were placed in a glass plate sealed by Scotch tape to avoid air exposure during the measurement. Raman spectra were examined by RAMAN-11 VIS-SS (Nanophoton) using a green laser with a wavelength of 532 nm. Solidstate magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were recorded using a Bruker DSX-500 spectrometer and a boron background free 4 mm Bruker MAS probe at room temperature. NMR sample preparations were always handled in a glovebox filled with purified Ar gas, and dry N2 gas was used for sample spinning. 11B MAS NMR spectra were obtained after a short pulse (0.5 μs − π/12 pulse) and with strong 1H decoupling pulses. 11B NMR chemical shifts were referenced to BF3OEt2 (δ = 0.00 ppm). All the sample preparations were always handled in a glovebox filled with purified Ar gas.
Figure 2. (a) XRD patterns and (b) Raman spectra of synthesized samples from 2MH + 1.2B10H14 at different reaction conditions. (i) The 5 h ball-milled 2LiH + 1.2B10H14; (ii) 5 h ball-milled 2NaH + 1.2B10H14; (iii, iv, v, and vi) 5 h ball-milled 2LiH + 1.2B10H14 followed by heat treatment at 200 °C for 10 h, 200 °C for 15 h, 250 °C for 15 h, and 300 °C for 15 h, respectively; (vii) 5 h ball-milled 2NaH + 1.2B10H14 followed by heat treatment at 450 °C for 20 h; (viii, ix, and x) starting materials of LiH, NaH, and B10H14, respectively. 6085
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ball milling. This suggests that the ball-milled 2MH + 1.2B10H14 samples may be a physical mixture of starting reactants. To improve reaction between the starting materials of MH and B10H14, heat treatment was performed under argon atmosphere. For 2LiH + 1.2B10H14 heat-treated at 200 °C for 10 h, there were still no new diffraction peaks or Raman bands except for those of the starting materials of LiH and B10H14. When the heating was increased to 15 h, a new diffraction peak at 2θ = 15.80° identified to Li2B12H12 appeared in the XRD patterns, whereas no distinct Raman spectra of Li2B12H12 or starting materials were observed because of the strong fluorescence emission.22 Then, we further raised the heattreatment temperature to 250 and 300 °C. Under both conditions, the diffraction peaks of starting materials of LiH and B10H14 disappeared and the diffraction peaks of Li2B12H12 became more apparent. The second characteristic peak of Li2B12H12 at 2θ = 18.35° was clearly observed when 2LiH + 1.2B10H14 was heated at 300 °C for 15 h.22 The formation of Li2B12H12 was also confirmed by Raman analysis. The Raman bands at 745 cm−1 and 2500 cm−1 that have been assigned to B−H bending and stretching vibration of Li2B12H12 were clearly observed for the sample heat-treated at 300 °C for 15 h.5,23 In addition, no traces of O−H bond of water were observed from Raman spectra of LiH and B10H14 after heat treatment, which suggests the successful synthesis of anhydrous Li2B12H12. These results demonstrate that appropriate heat treatment can effectively improve the chemical transformation between LiH and B10H14 to synthesize anhydrous Li2B12H12. Similar to that of Li2B12H12, synthesis of Na2B12H12 was also carried out by heat treatment of ball-milled 2NaH + 1.2B10H14. As shown in Figure 2, diffraction peaks of starting materials of NaH and B10H14 become invisible, but those of Na2B12H12 appear at 2θ = 15.15° for 2NaH + 1.2B10H14 heated at 450 °C for 20 h.20 The presence of a diffraction peak at 2θ = 15.80° is identified as Na2B12H12·xH2O (see Figure S1 of the Supporting Information), which suggests that Na2B12H12 may absorb water from the glass plate or the Scotch tape during the XRD measurement. The B−H bending and stretching vibration modes of Na2B12H12 are confirmed at 500−1000 cm−1 and around 2500 cm−1, respectively, in Raman spectra shown as Figure 2b. Both XRD patterns and Raman spectra consistently prove the successful synthesis of Na2B12H12 by heat treatment of NaH and B10H14. 3.2. Synthesis from 2MBH4 + B10H14 (M = Li, Na, K). Figure 3 shows XRD patterns and Raman spectra of synthesized samples from 2MBH4 + B10H14 (M = Li, Na, K) reaction pathway after ball milling with and without heat treatment. After ball milling, only diffraction peaks and Raman spectra originating from starting materials of MBH4 and B10H14 can be confirmed, which suggests that no chemical reaction between MBH4 and B10H14 occurred during ball milling. The ball-milled 2MBH4 + B10H14 samples are a physical mixture of starting materials, similar to that of ball-milled 2MH + 1.2B10H14 samples. The reactivity between MBH4 and B10H14 for the synthesis of M2B12H12 has been largely improved by heat treatment at the following conditions: 2LiBH4 + B10H14 at 200 °C for 15 h, 2NaBH4 + B10H14 at 450 °C for 20 h, and 2KBH4 + B10H14 at 450 °C for 20 h. After heat treatment, diffraction peaks of starting materials of MBH4 and B10H14 disappear and new diffraction peaks are identified as Li2B12H12, Na2B12H12, and K2B12H12 with cubic (Pa3̅), monoclinic (P21/n), and cubic (Fm3̅) structures, respectively.20,24,25 The successful syntheses
Figure 3. (a) XRD patterns and (b) Raman spectra of synthesized samples from 2MBH4 + B10H14 at different reaction conditions: (i) 5 h ball-milled 2LiBH4 + B10H14; (ii) 5 h ball-milled 2NaBH4 + B10H14; (iii) 5 h ball-milled 2KBH4 + B10H14; (iv) 5 h ball-milled 2LiBH4 + B10H14 followed by heat treatment at 200 °C for 15 h; (v) 5 h ballmilled 2NaBH4 + B10H14 followed by heat treatment at 450 °C for 20 h; (vi) 5 h ball-milled 2KBH4 + B10H14 followed by heat treatment at 450 °C for 20 h; (vii, viii, ix, x) are LiBH4, NaBH4, KBH4, and B10H14, respectively.
of Li2B12H12, Na2B12H12, and K2B12H12 from 2MBH4 + B10H14 are also confirmed by Raman analyses, that is, the bending and stretching vibration modes originating from [B12H12]2− anion are observed at 500−1000 cm−1 and around 2500 cm−1.5 Taking thus synthesized Li2B12H12 as an example, the Raman spectra at 584, 756, 772, and 947 cm−1 are attributed to Hg, Ag, Hg, and Hg modes, respectively, and the peaks at 2476 and 2543 6086
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cm−1 are assigned to Hg and Ag modes.26,27 No traces of O−H bond of water are observed in Raman spectra, which proves the successful synthesis of anhydrous M2B12H12 (M = Li, Na, K) through heat treatment of 2MBH4 + B10H14. The successful synthesis of M2B12H12 from 2MBH4 + B10H14 is further confirmed by 11B MAS NMR measurements; the results are shown in Figure 4. Peaks at approximately −16 ppm
considered to result from the reaction between MBH4 and boranes which may be produced from the individual decomposition of B10H14.22 Further optimization of the heattreatment condition is required to improve the purity of M2B12H12 synthesized from 2MBH4 + B10H14. 3.3. Comparison of the Two Reaction Routes. The above-mentioned experimental results demonstrate that M2B12H12 (M = Li, Na, K) can be readily synthesized from chemical reactions of 2MH + 1.2B10H14 or 2MBH4 + B10H14. Careful comparison on the diffraction peak intensities (Figures 2 and 3) of M2B12H12 between 2MH + 1.2B10H14 and 2MBH4 + B10H14 indicates that MBH4 may show better reactivity with B10H14 than that of MH. Taking the synthesis of Li2B12H12 as an example, a single phase of Li2B12H12 can be clearly identified for 2LiBH4 + B10H14 heat-treated at 200 °C for 10 h, whereas only very weak diffraction peaks of Li2B12H12 start to appear together with a large amount of unreacted starting materials even when the heat-treatment time was prolonged to 15 h for 2LiH + 1.2B10 H14 (see Figure S3 of the Supporting Information). To understand such different reactivity, we compare the reaction enthalpy and Gibbs free energy of eq 4 of 2MH + 1.2B10H14 and eq 5 of 2MBH4 + B10H14 from the viewpoint of thermodynamics. As shown in Table 1, the standard formation
Figure 4. 11B MAS NMR spectra of synthesized samples from 2MBH4 + B10H14 at different reaction conditions (2LiBH4 + B10H14: 5 h ball milling, heat treatment at 200 °C for 15 h; 2NaBH4 + B10H14: 5 h ball milling, heat treatment at 450 °C for 20 h; 2KBH4 + B10H14: 5 h ball milling, heat treatment at 450 °C for 20 h).
Table 1. Standard Formation Enthalpy of Starting Materials and Products at 298 Ka
2− 9,14,28
Compared to the peak line are assigned to [B12H12] . shape at −16 ppm of Li2B12H12, both Na2B12H12 and K2B12H12 appear to reveal sharper resonance superposed onto the broad resonance, which suggests that both dodecaboranes are present in physically two different phases with different mobilities of the [B12H12]2− species. Note that 11B spectral line can be a good probe for molecular motion of metal compounds of icosahedral [B12H12]2−, and 11B MAS NMR spectra of various alkali-metal dodecaboranes show a typical line width of ∼2000 Hz (under similar MAS spinning rate) for the center band when the crystallites are water free.29,30 Our current 11B MAS spectra of both Na2B12H12 and K2B12H12 closely resemble that of MgB12H12(H2O)6 according to the report by Stavila et al.30 Interestingly, 1H MAS spectra of our current Na2B12H12 and K2B12H12 (see Figure S2 of the Supporting Information) did not show a crystalline water peak, excluding a possibility that the mobile portion of [B12H12]2− molecules is associated with possible water contamination in MgB12H12. There appear to be unrecognized factors that caused crystallizations to occur differently to show such mobility change for Na or K dodecaboranes in our solid-state synthesis procedure. The details will be further investigated. The small peaks at ∼0 ppm and −30 ppm are assigned to M2B10H10, while a small peak at about −38 ppm is attributed to the residual MBH4. From the fact that the peak position is 3−4 ppm upfield compared to those of LiBH4 (−41 ppm)9,11 or NaBH4 (−42 ppm)22 of bulk crystalline phase, the residual MBH4 might be present in nanoscale experiencing a different environment from that of the pristine crystalline. Another small resonance (
