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Article

Thermal Decomposition of Anhydrous Alkali Metal Dodecaborates M2B12H12 (M = Li, Na, K)

by
Liqing He
1,
Hai-Wen Li
2,3,* and
Etsuo Akiba
1,2,3
1
Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan
2
International Research Center for Hydrogen Energy, Kyushu University, Fukuoka 819-0395, Japan
3
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan
*
Author to whom correspondence should be addressed.
Energies 2015, 8(11), 12429-12438; https://doi.org/10.3390/en81112326
Submission received: 16 July 2015 / Revised: 8 October 2015 / Accepted: 22 October 2015 / Published: 4 November 2015
(This article belongs to the Special Issue Hydrides: Fundamentals and Applications)
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Versions Notes

Abstract

:
Metal dodecaborates M2/nB12H12 are regarded as the dehydrogenation intermediates of metal borohydrides M(BH4)n that are expected to be high density hydrogen storage materials. In this work, thermal decomposition processes of anhydrous alkali metal dodecaborates M2B12H12 (M = Li, Na, K) synthesized by sintering of MBH4 (M = Li, Na, K) and B10H14 have been systematically investigated in order to understand its role in the dehydrogenation of M(BH4)n. Thermal decomposition of M2B12H12 indicates multistep pathways accompanying the formation of H-deficient monomers M2B12H12−x containing the icosahedral B12 skeletons and is followed by the formation of (M2B12Hz)n polymers. The decomposition behaviors are different with the in situ formed M2B12H12 during the dehydrogenation of metal borohydrides.

1. Introduction

Metal dodecaborates M2/nB12H12 (n is the valence of M) have been widely regarded as a dehydrogenation intermediate of metal borohydrides M(BH4)n with a high gravimetric hydrogen density of 10 mass% [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. The formation of M2/nB12H12, despite is still controversial, largely depends on the dehydrogenation temperature, hydrogen backpressure, particle size and sample pretreatment [11,12,13,14,15,16,17,18,19,20,21]. Due to the strong B-B bonds in an icosahedral boron cage B12, the intermediate comprising of polyatomic anion [B12H12]2− has been widely regarded at the main obstacle for the rehydrogenation of M(BH4)n [5,22,23,24]. Systematic investigation on the thermal decomposition of M2/nB12H12 is therefore of great importance to understand their role in the dehydrogenation of M(BH4)n.
M2/nB12H12 is generally synthesized using liquid phase reactions, followed by careful dehydration processes [25]. However, in some M2/nB12H12 such as MgB12H12, the coordination water tends to form a hydrogen bond with the polyatomic anion [B12H12]2−, resulting in the failure of dehydration [26]. To solve such problems, we have recently successfully developed a novel solvent-free synthesis process, i.e., sintering of M(BH4)n and B10H14 with stoichiometric molar ratio. Several anhydrous metal dodecaborates M2/nB12H12 (M = Li, Na, K, Mg, Ca, LiNa), so far, have been successfully synthesized via the newly developed method [21,27,28].
In this work, we carefully investigate the thermal decomposition behaviors of anhydrous alkali metal dodecaborates M2B12H12 (M = Li, Na, K) that are synthesized using the reported method [27]. Furthermore, the roles of M2B12H12 (M = Li, Na, K) in the dehydrogenation of corresponding borohydrides MBH4 are discussed, based on the comparison of the decomposition pathways of M2B12H12 and those in situ formed during the decomposition of MBH4.

2. Results and Discussion

2.1. Decomposition of Anhydrous Li2B12H12

Figure 1 shows the thermogravimetry (TG) and mass spectrometry (MS) measurement results of anhydrous Li2B12H12. Only hydrogen is detected in MS, indicating that the weight loss upon heating results from the dehydrogenation. The weight loss starts at approximately 200 °C and the dehydrogenation amount reaches 5.1 mass% (approximately 66% of the theoretical hydrogen content in Li2B12H12) when heated up to 700 °C. The decomposition proceeds with multistep reactions, as shown in TG and MS results.
In order to investigate the decomposition process of anhydrous Li2B12H12, the sample was heated to respective temperatures and subsequently cooled down to room temperature. The changes examined by X-ray diffraction (XRD), Raman and nuclear magnetic resonance (NMR) are shown in Figure 2 and Figure 3, respectively. When the temperature is increased to 250 °C, no obvious changes of XRD patterns, Raman spectra and the main resonance at −15.3 ppm for Li2B12H12 are observed, whereas the resonance at −41.3 ppm originated from residual LiBH4 decreases significantly and those at 11.2 ppm and −36.0 ppm for the unknown side product disappears (Table 1). This indicates that anhydrous Li2B12H12 is stable up to 250 °C and the weight loss of 0.3 mass% up to 250 °C is originated from decomposition of the residual LiBH4 and side product. When the temperature is increased to 300 °C, intensities for the diffraction peaks (2θ = 15.8° and 18.4°) and Raman spectra (between 500–1000 cm−1 and around 2500 cm−1) attributed to Li2B12H12 decrease. This indicates that anhydrous Li2B12H12 starts to decompose above 250 °C, similar to the reported temperature [16]. The resonance at −29.8 ppm originated from Li2B10H10 and that at −17.5 ppm for the unknown side product become significantly weaker when heated up to 300 °C and completely disappear when heated up to 500 °C. When the temperature is increased to 600 °C, diffraction peaks and Raman spectra attributed to Li2B12H12 nearly disappear, and the main resonances at −15.3 ppm for 11B and at 1.4 ppm for 1H originated from Li2B12H12 decrease significantly without any change in the chemical shift. This indicates that a major part of B–H bond in the icosahedral polyatomic anion [B12H12]2− has been broken to release hydrogen [21]. The dehydrogenation amount reaches 3.7 mass% including the contribution from the residual LiBH4 and side products, suggesting that the decomposition product is probably H-deficient Li2B12H12−x (x < 5.3) that remains the icosahedral B12 skeletons [16,21]. No signals in the solution-state 11B NMR were detected, implying that the formed Li2B12H12−x is DMSO insoluble. When the temperature is further increased to 700 °C, the dehydrogenation amount reaches 5.1 mass%, the major resonance of Li2B12H12 at −15.3 ppm in 11B MAS NMR shifts to −11.9 ppm, and that at 1.4 ppm in 1H MAS NMR changes to several weak resonance peaks from −10 ppm to 10 ppm. This suggests that Li2B12H12−x continuously releases hydrogen accompanied by the polymerization of the icosahedral B12 skeletons and the formation of (Li2B12Hz)n polymers [15,21,29], insoluble in water and DMSO.
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Figure 1. Thermogravimetry (TG) curve and mass spectrometry (MS) signals of anhydrous Li2B12H12 (mass numbers 2 and 27 represent H2 and B2H6).
Figure 1. Thermogravimetry (TG) curve and mass spectrometry (MS) signals of anhydrous Li2B12H12 (mass numbers 2 and 27 represent H2 and B2H6).
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Table
Table 1. Relative amount of the B-H species in synthesized Li2B12H12 when heated up to respective temperatures (≤500 °C), estimated from the peak fitting of 11B MAS NMR spectra shown in Figure 3.
Table 1. Relative amount of the B-H species in synthesized Li2B12H12 when heated up to respective temperatures (≤500 °C), estimated from the peak fitting of 11B MAS NMR spectra shown in Figure 3.
Temperature, °C[B12H12]2−, %[B11H11]2−, %[B10H10]2−, %[BH4]−, %Unknown, %
20060.939.0318.765.146.14
22565.829.5012.163.768.76
25070.1710.0017.212.620
30071.6211.7016.200.480
40086.52013.4800
5001000000
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Figure 2. Ex-situ (a) X-ray diffraction (XRD) patterns and (b) Raman spectra of anhydrous Li2B12H12 and heated up to respective temperatures.
Figure 2. Ex-situ (a) X-ray diffraction (XRD) patterns and (b) Raman spectra of anhydrous Li2B12H12 and heated up to respective temperatures.
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Figure 3. Ex-situ 11B and 1H NMR spectra of anhydrous Li2B12H12 and heated up to respective temperatures: (a) solid-state 11B MAS NMR spectra; (b) solution-state 11B NMR spectra measured in DMSO-d6 and (c) solid-state 1H MAS NMR spectra. Resonance assignments of 11B spectra: −15.6 ppm [B12H12]2−, −35.6 ppm [BH4]−, −0.9 & −28.8 ppm [B10H10]2−, −16.8 ppm [B11H11]2−, −20.3 (−20.8) ppm [B9H9]2− [30]. Resonance assignments of 1H spectra: 1.2 ppm [B12H12]2− [10,16].
Figure 3. Ex-situ 11B and 1H NMR spectra of anhydrous Li2B12H12 and heated up to respective temperatures: (a) solid-state 11B MAS NMR spectra; (b) solution-state 11B NMR spectra measured in DMSO-d6 and (c) solid-state 1H MAS NMR spectra. Resonance assignments of 11B spectra: −15.6 ppm [B12H12]2−, −35.6 ppm [BH4]−, −0.9 & −28.8 ppm [B10H10]2−, −16.8 ppm [B11H11]2−, −20.3 (−20.8) ppm [B9H9]2− [30]. Resonance assignments of 1H spectra: 1.2 ppm [B12H12]2− [10,16].
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The thermal decomposition pathway of anhydrous Li2B12H12 up to 700 °C is, therefore, summarized based on the abovementioned experimental results:
Step 1: Li2B12H12 → Li2B12H12−x + x/2H2
Step 2: nLi2B12H12−x → (Li2B12Hz)n + z’H2
The decomposition pathway is similar to those of anhydrous MgB12H12 and CaB12H12 [21]. It is worth noting that the thermal decomposition behaviors of anhydrous Li2B12H12 are different from that in situ formed during the dehydrogenation of LiBH4, like those of anhydrous MgB12H12 and CaB12H12 [21]. Anhydrous Li2B12H12 shows a lower initial decomposition temperature and a wider decomposition temperature range of 250 ~ >700 °C than those in situ formed during dehydrogenation of LiBH4. The formation of Li2B12H12 during the dehydrogenation of LiBH4 generally experiences complicated condensation process with sluggish kinetics [11], attributing to the higher initial decomposition temperature than that of anhydrous Li2B12H12. On the other hand, the high activity of the in situ formed Li2B12H12 together with the concurrent formation of LiH facilitate the decomposition of Li2B12H12 [8], resulting in the lower temperature of complete decomposition than that of anhydrous Li2B12H12.

2.2. Decomposition of Anhydrous Na2B12H12

Figure 4 shows the TG and MS measurement results of anhydrous Na2B12H12. Only hydrogen is detected in MS, indicating that the weight loss is originated from the dehydrogenation. The weight loss starts at approximately 580 °C and the dehydrogenation amount reaches 1.9 mass%, which is approximately 29% of the theoretical hydrogen capacity in Na2B12H12. The value is comparable to the reported one (~1.5 mass% at 697 °C) [31].
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Figure 4. TG curve and MS signals of anhydrous Na2B12H12 (mass numbers 2 and 27 represent H2 and B2H6).
Figure 4. TG curve and MS signals of anhydrous Na2B12H12 (mass numbers 2 and 27 represent H2 and B2H6).
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The changes of anhydrous Na2B12H12 heated to respective temperatures and subsequently cooled down to room temperature examined by XRD, Raman and 11B NMR are shown in Figure 5 and Figure 6, respectively. When the temperature is increased to 500 °C, no obvious changes of diffraction peaks, Raman spectra and the major resonance at −15.7 ppm for Na2B12H12 are observed, whereas the resonances originated from NaBH4 and Na2B10H10 nearly disappear. This suggests that the small amount of side product Na2B10H10 and the residual NaBH4 (<8 mass%) start to decompose below 500 °C without detected weight loss. When the temperature is increased to 600 °C, diffraction peaks and Raman spectra attributed to Na2B12H12 becomes weak, indicating that Na2B12H12 starts to decompose at 600 °C. When the temperature is further increased to 700 °C, diffraction peaks and Raman spectra from Na2B12H12 are hardly observed, the main resonance at −15.7 ppm significantly weakens and a broad resonance between −12.4 ppm and −14.8 ppm appears. This suggests that the major dehydrogenation of Na2B12H12 to H-deficient Na2B12H12−x and the polymerization of Na2B12H12−x to water and DMSO insoluble (Na2B12Hz)n polymers start to take place at 700 °C [15,21,29].
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Figure 5. Ex-situ (a) XRD patterns and (b) Raman spectra of anhydrous Na2B12H12 as synthesized and heated up to respective temperatures.
Figure 5. Ex-situ (a) XRD patterns and (b) Raman spectra of anhydrous Na2B12H12 as synthesized and heated up to respective temperatures.
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Energies 08 12326 g006 1024
Figure 6. Ex-situ 11B NMR spectra of anhydrous Na2B12H12 as synthesized and heated up to respective temperatures: (a) solid-state 11B MAS NMR spectra and (b) solution-state 11B NMR spectra measured in DMSO-d6. Resonance assignments: −15.6 ppm [B12H12]2−, −35.9 ppm [BH4]−, −28.8 ppm [B10H10]2− [32].
Figure 6. Ex-situ 11B NMR spectra of anhydrous Na2B12H12 as synthesized and heated up to respective temperatures: (a) solid-state 11B MAS NMR spectra and (b) solution-state 11B NMR spectra measured in DMSO-d6. Resonance assignments: −15.6 ppm [B12H12]2−, −35.9 ppm [BH4]−, −28.8 ppm [B10H10]2− [32].
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2.3. Decomposition of Anhydrous K2B12H12

Figure 7 shows the TG and MS results of anhydrous K2B12H12. Only hydrogen is detected in MS, indicating that the weight loss upon heating results from the dehydrogenation. The weight loss starts at approximately 480 °C and the dehydrogenation amount reaches 4.4 mass% (approximately 80% of theoretical hydrogen capacity in K2B12H12) when heated up to 700 °C. The dehydrogenation proceeds with multistep reactions, as shown in TG and MS results.
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Figure 7. TG curve and MS signals of anhydrous K2B12H12 (mass numbers 2 and 27 represent H2 and B2H6).
Figure 7. TG curve and MS signals of anhydrous K2B12H12 (mass numbers 2 and 27 represent H2 and B2H6).
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The changes of anhydrous K2B12H12 heated up to respective temperatures and subsequently cooled down to room temperature examined by XRD, Raman and 11B NMR are shown in Figure 8 and Figure 9, respectively. When the temperature is increased to 475 °C, no obvious changes of diffraction peaks, Raman spectra and the major resonance at −15.4 ppm attributed to K2B12H12 are seen. The resonance originated from residual KBH4 (−38.2 ppm in solid-state and −35.6 ppm in solution-state 11B NMR) disappears when the temperature is increased to 550 °C, whereas no obvious changes of diffraction peaks, Raman spectra and the major resonance attributed to K2B12H12 are observed. This suggests that the weight loss bellow 550 °C is responsible for the dehydrogenation of residual KBH4. When the temperature is increased to 625 °C, diffraction peaks, Raman spectra and the main resonance attributed to K2B12H12 become weak slightly, indicating the partial decomposition of K2B12H12 at 625 °C. It is worth noting that the initial thermal decomposition temperature increases with the order of Li2B12H12 < Na2B12H12 < K2B12H12, which shows the same trend to the dehydrogenation temperature of corresponding metal borohydrides [33].
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Figure 8. Ex-situ (a) XRD patterns and (b) Raman spectra of anhydrous K2B12H12 as synthesized and heated up to respective temperatures.
Figure 8. Ex-situ (a) XRD patterns and (b) Raman spectra of anhydrous K2B12H12 as synthesized and heated up to respective temperatures.
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Energies 08 12326 g009 1024
Figure 9. Ex-situ 11B NMR spectra of anhydrous K2B12H12 as synthesized and heated up to respective temperatures: (a) solid-state 11B MAS NMR spectra and (b) solution-state 11B NMR spectra measured in DMSO-d6. Resonance assignments: –15.6 ppm [B12H12]2−, –35.6 ppm [BH4]− [32].
Figure 9. Ex-situ 11B NMR spectra of anhydrous K2B12H12 as synthesized and heated up to respective temperatures: (a) solid-state 11B MAS NMR spectra and (b) solution-state 11B NMR spectra measured in DMSO-d6. Resonance assignments: –15.6 ppm [B12H12]2−, –35.6 ppm [BH4]− [32].
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When the temperature is increased to 700 °C, the diffraction peaks and Raman spectra from K2B12H12 are hardly observed, the main resonance at −15.4 ppm in 11B MAS NMR decreases significantly without any change in the chemical shift and a broad resonance between −12.2 ppm and −14.2 ppm appears. It suggests that the major dehydrogenation of K2B12H12 to K2B12H12−x and the polymerization of K2B12H12−x to (K2B12Hz)n polymers start to happen at 700 °C [21], similar to those of Na2B12H12. Like (Li2B12Hz)n and (Na2B12Hz)n polymers, the produced (K2B12Hz)n polymers are also insoluble in water and DMSO. The decomposition behavior of anhydrous K2B12H12 is different from that formed as a dehydrogenation intermediate of KBH4 predicted by theoretical calculation [34], suggesting that the coexisting KH may facilitate the decomposition of K2B12H12.
In summary, the thermal decomposition of anhydrous alkali metal dodecaborates M2B12H12 (M = Li, Na, K) proceeds in two steps: (1) dehydrogenate to produce H-deficient M2B12H12−x containing the icosahedral B12 skeletons and (2) polymerization of M2B12H12−x to form (M2B12Hz)n. Such behaviors are similar to those of anhydrous MgB12H12 and CaB12H12 [21], but fairly differ from those in situ formed during the dehydrogenation of M(BH4)n. These findings suggest that further investigations on the correlation between thermal decomposition behaviors of possible dehydrogenation intermediates and of the corresponding metal borohydrides are of great importance for the clarification of the dehydrogenation mechanism.

3. Experimental Section

Anhydrous Li2B12H12, Na2B12H12 and K2B12H12 were synthesized by sintering of B10H14 with LiBH4, NaBH4 and KBH4 (Sigma-Aldrich, Ichikawa, Japan), at 200–450 °C for 15–20 h [27]. All the synthesized anhydrous Li2B12H12, Na2B12H12 and K2B12H12 were stored in glove box filled with purified Ar gas.
Powder XRD patterns were recorded by a Rigaku Ultima IV X-ray diffractometer with Cu-Kα radiation (Rigaku, Tokyo, Japan), and the accelerating voltage/tube current were set as 40 kV/40 mA. The sample powders were placed on a zero diffraction plate sealed by Scotch tape to prevent air exposure during the measurement. Raman spectra were obtained from a RAMAN-11 VIS-SS (Nanophoton, Osaka, Japan) using a green laser with 532 nm wavelength. Thermal decomposition was analyzed by TG (Rigaku), with a heating rate of 5 °C/min under a 200 mL/min flow of helium gas. The gas released during the TG measurement was analyzed by a quadrupole mass spectrometer coupled with TG. Solid-state MAS NMR spectra were recorded by a Bruker Ascend-600 spectrometer (Bruker, Yokohama, Japan), at room temperature. NMR sample preparations were always operated in a glove box filled with purified Ar gas and sample spinning was conducted using dry N2 gas. 11B MAS NMR spectra were obtained at excitation pulses of 6.5 μs (π/2 pulse) and with strong 1H decoupling pulses. 11B NMR chemical shifts were referenced to BF3OEt2 (δ = 0.00 ppm). 1H MAS NMR spectra were obtained at excitation pulses of 6.5 μs (π/2 pulse) and the chemical shifts were referred to deuterated water (δ = 4.75 ppm). Solution-state 11B NMR experiments were carried out using the same apparatus of Bruker Ascend-600 (Bruker), dimethyl sulfoxide (DMSO-d6) was used as solvent and saturated B(OH)3 aqueous solution at 19.4 ppm was used as external standard sample.

4. Conclusions

Systematic investigations of thermal decomposition indicate that anhydrous alkali metal dodecaborates M2B12H12 (M = Li, Na, K) firstly dehydrogenate to produce the H-deficient M2B12H12−x containing the icosahedral B12 skeletons, followed by the polymerization of M2B12H12−x to form (M2B12Hz)n polymers, similar to those of anhydrous MgB12H12 and CaB12H12 [21]. No amorphous B was detected in all M2B12H12 samples upon heating up to 700 °C, suggesting that higher temperature is needed for the complete decomposition of (M2B12Hz)n. The initial thermal decomposition temperature increases with the order of Li2B12H12 < Na2B12H12 < K2B12H12, which shows the same trend to the dehydrogenation temperature of corresponding borohydrides. The thermal decomposition behaviors of anhydrous M2B12H12 are fairly different with those in situ formed during the dehydrogenation of corresponding metal borohydrides. Further investigations on the correlation between thermal decomposition of possible dehydrogenation intermediates and of the corresponding metal borohydrides are expected in order to clarify the exact dehydrogenation mechanism of metal borohydrides.

Acknowledgments

We would like to sincerely thank Miho Yamauchi and Motonori Watanabe in I2CNER for their great help on 11B MAS NMR measurement. This study was partially supported by JSPS KAKENHI Grant No. 25709067 and the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology.

Author Contributions

All of the authors contributed to this work. Liqing He and Hai-Wen Li designed and carried out the experiments. Liqing He, Hai-Wen Li and Etsuo Akiba analyzed the experimental results and wrote the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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MDPI and ACS Style

He, L.; Li, H.-W.; Akiba, E. Thermal Decomposition of Anhydrous Alkali Metal Dodecaborates M2B12H12 (M = Li, Na, K). Energies 2015, 8, 12429-12438. https://doi.org/10.3390/en81112326

AMA Style

He L, Li H-W, Akiba E. Thermal Decomposition of Anhydrous Alkali Metal Dodecaborates M2B12H12 (M = Li, Na, K). Energies. 2015; 8(11):12429-12438. https://doi.org/10.3390/en81112326

Chicago/Turabian Style

He, Liqing, Hai-Wen Li, and Etsuo Akiba. 2015. "Thermal Decomposition of Anhydrous Alkali Metal Dodecaborates M2B12H12 (M = Li, Na, K)" Energies 8, no. 11: 12429-12438. https://doi.org/10.3390/en81112326

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