Energy Fuels 2009, 23, 5049–5054 Published on Web 09/03/2009
: DOI:10.1021/ef900619y
Mechanochemical Synthesis of Sodium Borohydride by Recycling Sodium Metaborate Lingyan Kong, Xinyu Cui, Huazi Jin, Jie Wu, Hao Du, and Tianying Xiong* State Key Laboratory of Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Received November 9, 2008. Revised Manuscript Received August 21, 2009
Hydrogen can be easily produced from the catalytic hydrolysis of sodium borohydride (NaBH4), with sodium metaborate (NaBO2) being the co-product. If NaBO2 can be economically recycled in the process, NaBH4 could be considered as a promising hydrogen carrier in fuel cells because of its high hydrogen content. In this paper, we report our investigation into the synthesis of NaBH4 from NaBO2 via ball milling. The starting materials for the synthesis were NaBO2 and magnesium hydride (MgH2). After ball milling at ambient temperature in inert gas, NaBH4 and magnesium oxide (MgO) were produced. The synthesized NaBH4 was extracted from the admixture by isopropylamine. Our results indicated that the yield of NaBH4 from this process was 71 wt % when the MgH2/NaBO2 mole ratio, ball/powder ratio (BPR), inert gas (argon) pressure, and milling time were 2.07:1, 50:1, 200 kPa, and 2 h, respectively. The mole ratio of the reactants (MgH2 and NaBO2), ball-milling time, BPR, and milling atmosphere were found to have significant influence over NaBH4 synthesis. We present the results and discuss the effects of these key parameters in this paper. Bayer process,9,10 mechanochemical (ball-milling) process,11 electrolytic process,12,13 and radiolysis process.14 The modified Bayer process was developed by several groups in Japan.9,10 Magnesium (Mg) or magnesium hydride (MgH2) is the common reducing agent for this process. In this process, magnesium replaces sodium used in the traditional Bayer process for NaBH4 synthesis. Theoretically, the reaction could occur at room temperature in accordance with the standard Gibbs free energy given in eq 2. However, the investigators found that the process should be carried out at high temperature because of poor mass transfer between the solid particles of the reactants, as well as high hydrogen pressure providing MgH2 decomposition at high temperatures.
Introduction NaBH4, which is widely used as a common reducing agent in organic synthesis, wastewater treatment, and paper pulp bleaching, has attracted considerable attention as a hydrogen storage material because of its high hydrogen content (10.6 wt % H-).1-7 Hydrogen and NaBO2 can be produced via catalyzed hydrolysis of NaBH4 in accordance with eq 1. catalyst
NaBH4 þ2H2 O sss f NaBO2 þ4H2 v
ð1Þ
For NaBH4 to be used as a viable hydrogen fuel, an economical and environmentally sound process for recycling NaBO2 to NaBH4 is needed. Several processes to prepare NaBH4 from NaBO2 have been reported recently.8 The notable ones include the modified
NaBO2 þ2MgH2 f NaBH4 þ2MgO ΔG0 ¼ -269:7 kJ=mol
It was reported that NaBH4 produced from the modified Bayer process is in molten form and does not decompose at 550 °C and 7 MPa (under hydrogen atmosphere) for 2 h.9 Because the modified Bayer process needs to be conducted under severe reaction conditions with hazardous materials, it is a high-risk process, especially if one contemplates largescale industrial production. Furthermore, an effective method to separate the reaction products is required to obtain a reasonable yield of NaBH4.8 The electrolytic process, which was first developed by Cooper,12 was thought of as the most attractive process for NaBO2 recycling. The electrolytic reaction (eq 3) was carried out in the NaBO2 caustic solution. ð3Þ anode reaction: 4H2 O f 2O2 þ8Hþ þ8e
*To whom correspondence should be addressed. Telephone: þ86-2423971746. Fax: þ86-24-23971746. E-mail:
[email protected]. (1) Kong, V. C. Y.; Foulkes, F. R.; Kirk, D. W.; Hinatsu, J. T. Int. J. Hydrogen Energy 1999, 24, 665–668. (2) Aiello, R.; Sharp, J. H.; Mathew, M. A. Int. J. Hydrogen Energy 1999, 24, 1123–1130. (3) Amendola, S. C.; Goldman, S.; Janjua, M. S.; Kelly, M. T.; Petillo, P. J.; Binder, M. J. Power Sources 2000, 85, 186–189. (4) Suda, S.; Sun, Y. M.; Liu, B. H.; Zhou, Y.; Morimitsu, S.; Arai, K.; Tsukamoto, N.; Uchida, M.; Candra, Y.; Li, Z. P. J. Appl. Phys. A 2001, 72, 209–212. (5) Wu, C.; Zhang, H.; Yi, B. Catal. Today 2004, 93-95, 477–483. (6) Zaluska, A.; Zaluski, L. J. Alloys Compd. 2005, 404-406, 706–711. (7) Liu, B. H.; Li, Z. P.; Suda, S. J. Alloys Compd. 2006, 415, 288–293. (8) Wu, Y.; Kelly, M. T.; Ortega, J. V. Review of chemical processes for the synthesis of sodium borohydride, 2004 (http://www1.eere.energy. gov/hydrogenandfuelcells/pdfs). (9) Kojima, Y.; Haga, T. Int. J. Hydrogen Energy 2003, 28, 989–993. (10) Li, Z. P.; Liu, B. H.; Zhu, J. K.; Morigasaki, N.; Suda, S. J. Alloys Compd. 2007, 437, 311–316. (11) Li, Z. P.; Liu, B. H.; Arai, K.; Morigazaki, N.; Suda, S. J. Alloys Compd. 2003, 356-357, 469–474. (12) Cooper, H. B. H. U.S. Patent 3,734,842, 1973. (13) Calabretta, D. L.; Davis, B. R. J. Power Sources 2007, 164, 782– 791. (14) Bingham, D.; Wendt, K.; Wilding, B. U.S. Appl. 2005/0077170 A1, 2005. r 2009 American Chemical Society
ð2Þ
cathode reaction: BO2 - þ6H2 Oþ8e- f BH4 - þ8OHtotal reaction: NaBO2 þ2H2 O f NaBH4 þ2O2 Obviously, plenty of OH- in caustic solution could be electrolyzed prior to BO2-, and thus, the electrolysis of 5049
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BH4-
to in aqueous media proved to be difficult, if not impossible. Recently, Calabretta and Davis13 demonstrated an anhydrous molten Na-B-O-H system, which could be used as a potential medium in the electrolysis of BO2- to BH4-. This process is currently under investigation.13 The radiolysis process was developed by Bingham.14 Preliminary results indicated that when an aqueous solution of sodium metaborate was subjected to irradiation by waste nuclear energy, sodium metaborate could be converted to sodium borohydride. The efficacy and economics of this process are not yet clear, and furthermore, the mechanism of this process is uncertain. The term mechanochemical synthesis generally refers to processes in which chemical and physicochemical transformation of substances proceeds in solids because of the application of mechanical energy.15-19 Because ball milling is the most commonly used method in supplying mechanical energy to mechanochemical synthesis processes, in this paper, the term ball milling is thus used interchangeably with mechanochemical synthesis. Mechanochemical synthesis of metal hydrides has a long history. For example, in 1963, Ashby reported the synthesis of complex metal hydrides, such as KAlH4 and NaAlH4, in organic solvent at 140-200 °C and a hydrogen pressure of 7-35 MPa during ball milling.15 Although the yield for the process was high (97 wt %),15 in comparison to the Schlesinger-Brown process, it appears to be more energyintensive and hazardous because of the high operating temperature and pressure. To the best of our knowledge, the only known industrial application of mechanochemical synthesis (via ball milling) was the oxide-dispersion strengthened (ODS) process developed by Benjamin in 1966.16 Recently, Li et al. reported the preparation of potassium borohydride (KBH4) from its anhydrous metaborate (KBO2)18 and NaBH4 from sodium borax (Na2B4O7).19 The authors also indicated that, in their experiments, NaBH4 was prepared via ball milling with NaBO2 as the starting material in accordance with eq 2, but few experimental details were given.11 From eq 2, it can be seen that the only byproduct for NaBH4 synthesis via NaBO2 is MgO, which could be recycled to produce Mg through the molten electrolysis process.20 This suggests that the mechanochemical synthesis of NaBH4 from NaBO2 and MgH2 could be both environmentally benign and fairly economical, because the MgO byproduct can be recycled by electrolysis. In accordance with eq 2, the reaction could be spontaneous at standard conditions, as indicated by the standard molar Gibbs free energy (-269.7 kJ/mol). In view of the above, NaBH4 synthesis from NaBO2 and MgH2 via ball milling could be an attractive process for recycling NaBO2 to produce NaBH4. In this paper, we report our investigation into mechanochemical synthesis of NaBH4 from NaBO2 and discuss several process variables that have significant influence on the formation of NaBH4.
Figure 1. Schematic diagram of the gas switchboard for Mg hydrogenation and ball milling: 1, H2/Ar; 2, ball mill; 3, high-pressure vessel; 4, surge flask; 5, vacuum pump; 6, reducing valve; 7, gas filter; 8 and 9, pressure gauge; 10, needle valve; 11, stop valve; and 12, vent valve.
Experimental Section Commercial Mg powders with particle sizes of less than 74 μm and purity of 99.5 wt % were used to prepare MgH2 by hydrogenation at 300 °C and in 6 MPa of hydrogen in a highpressure reaction vessel (GCF-1, Weihai Jinghong Chemical Machinery Co. Ltd., China) for 24 h. The yield of the asprepared MgH2 was determined by a RH-404 hydrogen analyzer (LECO). Anhydrous NaBO2 was obtained by dehydrating NaBO2 3 4H2O (99.0%). First, the NaBO2 3 4H2O was heated to 120 °C in a vacuum and held at that temperature for 2 h. Then, the obtained powders were further heated to 270 °C and held for an additional 4 h in a vacuum. The dehydrated NaBO2 were cooled to room temperature in desiccators and stored there. Mechanochemical synthesis of NaBH4 was carried out by mixing and ball milling the prepared NaBO2 and MgH2 in a high-speed vibrating mill (QM-3A, Nanjing University Instrument Plant, China). H2 (99.999%) and Ar (99.999%) were both tested as the protective gases during the ball-milling process. A gas switchboard (schematics as shown in Figure 1) was used to change the gas type in the ball mill as desired. The product mixture was first extracted by isopropylamine under Ar atmosphere and then filtered under vacuum. The filtrate was then dried under vacuum (to evaporate off isopropylamine) to obtain raw NaBH4 product in the form of white solids. The obtained NaBH4 was further dried at 30 °C in vacuum and then weighed. The NaBH4 yield is calculated in accordance with the following equation: yield ðNaBH4 Þ ¼
(15) Ashby, E. C.; Bredel, G. J.; Redman, H. E. Inorg. Chem. 1963, 2, 499–504. (16) Benjamin, J. S. Sci. Am. 1976, 234, 40–48. (17) Ronnebro, E.; Majzoub, E. H. J. Phys. Chem. B 2007, 111, 12045–12047. (18) Li, Z. P.; Liu, B. H.; Morigasaki, N.; Suda, S. J. Alloys Compd. 2003, 354, 243–247. (19) Li, Z. P.; Morigasaki, N.; Liu, B. H.; Suda, S. J. Alloys Compd. 2003, 349, 232–236. (20) Liang, B.; Liang, Y. T. Light Met. (Qing JinShu) 2007, 6, 50–51.
obtained mass NaBH4 100% theoretical mass NaBH4
X-ray diffraction (XRD) analysis of the powders was conducted by D/Max-2500PC (Rigaku) using Cu KR radiation (λ = 0.154 18 nm) at operating parameters of 250 mA, 56 kV, step size of 0.02°, and speed at 2°/min. The morphology of the powders was inspected by a JSM 6301F (JEOL) scanning electron microscope (SEM). 5050
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Figure 2. XRD pattern of MgH2 prepared through Mg hydrogenation at 300 °C and 6 MPa H2.
Figure 4. XRD pattern of NaBO2 obtained after dehydration of NaBO2 3 4H2O.
Figure 3. SEM images of the (a) Mg and (b) MgH2 (synthesized at 300 °C and 6 MPa of H2).
Results and Discussion
Figure 5. XRD patterns of various samples from the mechanochemical synthesis of NaBH4 (via ball milling): (a) reaction mixture, (b) retentate (solids after filtration), and (c) dried filtrate.
Typical Run Conditions for Mechanochemical Synthesis of NaBH4. The MgH2 used in our experiments was lab-synthesized by Mg hydrogenation in a pressure vessel at 300 °C and 6 MPa of H2 for 24 h. Figure 2 shows the XRD pattern of the MgH2 prepared from this typical procedure. As can be seen, the only observable impurity was unreacted Mg. The purity for the synthesized MgH2 was determined to be 79.3 wt %. Figure 3 shows the morphology of MgH2 and that of the reactant Mg. It can be seen that the particles of the synthesized MgH2 (Figure 3b) were larger than the feed reactant Mg (Figure 3a) with crumbling on the surfaces of the particles. Figure 4 shows the XRD pattern of the obtained NaBO2 after dehydration. The impurity found in the mixture was NaBO2 3 2H2O. In our experiments, NaBO2 and MgH2 powders prepared by the procedures outlined earlier were mixed at a predetermined mole ratio and put into the ball mill without preblending. The mixture was ball-milled for 2 h under various conditions for the mechanochemical synthesis of NaBH4. After ball milling, isopropylamine was used to extract NaBH4 from the product mixture. A filter was then used to separate solid particles (retentate) from the isopropylamine solvent (filtrate that contains NaBH4). The NaBH4 product was obtained after the filtrate was dried at 30 °C under vacuum to evaporate off the isopropylamine solvent. The XRD patterns of (a) the product mixture obtained after ball milling, (b) the retentate powder mixture, and (c) the filtrate obtained after drying under vacuum are shown in curves a, b,
and c in Figure 5, respectively. It can be seen from Figure 5a that the reaction mixture contains NaBH4, MgO, and unreacted MgH2. After isopropylamine extraction, only MgO and MgH2 remained in the retentate (Figure 5b). The filtrate, which was subjected to drying in vacuum, yielded the NaBH4 product in the form of white powder (Figure 5c). Figure 6 shows the microstructure of the NaBH4 product, which appears to be cubic with a typical particle dimension of approximately 30 30 30 μm. The NaBH4 product yield was determined to be 71 wt %. Li et al. reported a similar NaBH4 synthesis process, in which Na2B4O7 was used as the starting reagent with a NaBH4 yield of 43 wt %.18 4MgH2 þNa2 B4 O7 f 2NaBH4 þ 4MgOþB2 O3 2 In comparison to that of Li et al., our NaBH4 synthesis process started with MgH2 and NaBO2 as the reactants and the NaBH4 yield was relatively higher. The reaction products were NaBH4 and MgO. As noted earlier, the separation of NaBH4 from the reaction mixture is relatively easy (via isopropylamine extraction) and commercial processes are available to cycle the MgO byproduct to Mg. The later can be hydrogenated to produce MgH2. In view of this, we believe that our process might have fewer obstacles in scaling up. Reaction Time (Ball-Milling Time). Figure 7 illustrates the influence of the ball-milling time on product yields. It shows that the reaction occurred after ball milling for 0.5 h. However, 5051
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Figure 8. Effects of the reactant molar ratio on NaBH4 yields and product distribution: (a) MgH2/NaBO2=1.54:1, (b) MgH2/NaBO2= 2:1, and (c) MgH2/NaBO2=2.07:1 (time=2 h; BPR=50:1; and P= 200 kPa Ar).
Figure 6. SEM photograph of the dried filtrate from isopropylamine extraction.
Figure 9. XRD patterns of the retentate: (a) MgH2/NaBO2=1.54:1, (b) MgH2/NaBO2=2:1, and (c) MgH2/NaBO2=2.07:1 (time=2 h; BPR=50:1; and P=200 kPa Ar).
Figure 7. Influence of the reaction time on mechanochemical synthesis of NaBH4: (a) NaBO2 prepared, (b) ball milling for 0.5 h, (c) ball milling for 1.0 h, and (d) ball milling for 2.0 h (MgH2/NaBO2 = 2.07:1; BPR=50:1; and P=200 kPa Ar).
present study).21 In view of the above, we believe that the ball-milling time could be one of the key issues that needs to be resolved for the commercial application of mechanochemical synthesis of NaBH4 from NaBO2. Molar Ratio between Reactants. To obtain a reasonable yield of NaBH4, the optimum molar ratio between the two reactants (MgH2 and NaBO2) was studied. Referring to eq 2, the stoichiometric molar ratio between MgH2 and NaBO2 is 2:1. First, we studied the effects of excessive NaBO2 in the reactants (MgH2/NaBO2 molar ratio=1.54:1, which we referred to as our case a study). In this case, XRD analysis indicated that NaBH4 did not appear in the ball-milled mixture even though the byproduct MgO was already formed after ball milling for 2 h (Figure 8a). Furthermore, unreacted MgH2 was not detected in the mixture (Figure 8a). When the MgH2/NaBO2 molar ratio was adjusted to a stoichiometric molar ratio of 2:1 (case b), XRD analysis of the reaction mixture indicated that both MgO and NaBH4 became detectable (Figure 8b). When the molar ratio between MgH2 and NaBO2 was further increased to 2.07:1 (case c), the NaBH4 product and MgO byproduct in crystal forms were clearly visible in the XRD patterns of the reaction mixture, with excessive MgH2 remaining in the mixture (case c in Figure 8c).
with 0.5 h ball-milling time, the products were rather amorphous (as indicated by the XRD pattern in Figure 7b) and the NaBH4 yield was estimated to be 20 wt %. By extending the ball-milling time to 1 h, the peaks for NaBH4 in the XRD pattern were still very broad and weak (Figure 7c). However, after ball milling for 2 h, the peaks for NaBH4 in the XRD pattern became dominant and it was easy to identify the crystal of the product (Figure 7d). The NaBH4 yield was estimated to be 71 wt %, which was the highest yield among the experiments reported here. Our investigation indicated that the ball-milling time was the most important factor affecting the yields of NaBH4. In a batch production process, the minimum ball-milling time required to obtain reasonable yields for NaBH4 will have direct impacts on energy consumption and throughput of the process. Although we used a high-energy vibrating mill in the present study, it was still necessary to operate for 2 h to obtain a reasonable yield of NaBH4. We believe that, in commercial applications, a longer ball-milling time may be required because of the fact that commercial-scale ball mills are normally operated at much lower rotation rates than our high-energy vibrating mill. As a result, power transfer to the reactants in a commercial-scale ball mill could be much lower than that in a high-energy vibrating mill (as used in the
(21) Suryanarayana, C. Prog. Mater. Sci. 2001, 46, 1–184.
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Figure 10. Effects of the ball-milling atmosphere on NaBH4 synthesis: (a) Ar/200 kPa and (b) H2/200 kPa (time=2 h; BPR=50:1; and MgH2/NaBO2 ratio=2.07:1).
Figure 11. Effects of the BPR in mechanochemical synthesis of NaBH4: (a) BPR=10:1, (b) BPR=30:1, and (c) BPR=50:1 (time= 2 h; MgH2/NaBO2 ratio=2.07:1; and P=200 kPa Ar).
The NaBH4 yields for cases a, b, and c (curves a-c in Figure 8) were determined to be 34, 61, and 71 wt %, respectively. The retentate from isopropylamine extraction/filtration was also characterized by XRD (Figure 9). It was found that both MgO and NaBO2 3 2H2O were present in the reaction mixture after running with the MgH2/NaBO2 molar ratio of 1.54:1 and 2:1. Some MgH2 was detected in the case where the mole ratio was 2.07:1, as would be expected. MgH2 in excess of the stoichiometric molar ratio (between MgH2 and NaBO2) promoted a higher yield of NaBH4, which was similar to that reported by Li et al.18,19 Although the analytical method was not the same, the product yield for NaBH4 did not reach 100%, unlike that of the case preparing KBH4.18 In the Schlesinger-Brown NaBH4 synthesis process, a large amount of sodium is consumed as a reducing reagent. In the Schlesinger-Brown process, 4 mol of sodium are required to produce only 1 mol of NaBH4, while 3 mol of sodium were consumed to form useless NaOCH3, as shown in eq 4.
We tried to conduct the synthesis in a hydrogen atmosphere (200 kPa). However, we were not able to observe the characteristic peaks of NaBH4 in XRD analysis (Figure 10b). In addition, MgO peaks in the XRD analysis were also relatively weak in comparison to those obtained in an argon atmosphere (Figure 10a), but the yield of NaBH4 did not change much. We have noted that there was a tendency for the powders to stick to the ball and the walls of the mill, especially at low gas pressures. The situation would improve at higher gas pressures, and this would lead to improvement in powder collection yields. However, for practical and operational consideration, the pressure of the inert gas(es) should not be too high. In view of this, we set an operating pressure of 200 kPa in our ball-milling experiments. Under this condition, the powder collection yields were approximately 90 wt % of the total feeds. Ball/Powder Ratio (BPR). The BPR is referred to the mass ratio between the balls to the powder being milled. BPR is an important parameter that determines the energy delivered to the powder in the milling process. We have tested different BPRs in our experiments while keeping the other parameters constant. At the lower end of BPR in our experiment (BPR= 10:1), the reaction still occurred, as indicated by the disappearance of the XRD peak of anhydrous NaBO2 and the appearance of the MgO peak byproduct in the reaction mixture (Figure 11a). However, the XRD peaks of NaBH4 were not observable in the mixture. At the BPR of 30:1 (Figure 11b), the peaks of NaBH4 became identifiable and the peak intensity of the MgO byproduct became much stronger than that at BPR=10:1. At BPR of 50:1, the peaks of NaBH4 and MgO could be seen clearly in the XRD pattern (Figure 11c). It is likely that, as the BPR increases (i.e., higher vial occupancy), the temperature of the system could increase because of increased kinetic energy of the grinding medium and the consequent transfer of more energy into the system. Ball milling at higher BPR could therefore result in more heat and energy transfer to the powders, which could promote more substantial crystallization of the powders.21
4NaHþ BðOCH3 Þ3 f NaBH4 þ3NaOCH3
ð4Þ
In contrast, in the NaBH4 synthesis process (via ball milling) reported here, stoichiometrically, only 2 mol of MgH2 would be consumed to produce 1 mol of NaBH4 (eq 2). Because MgH2 can be obtained from Mg hydrogenation, Mg was in effect used as the reducing agent to replace the expensive sodium used in the Schlesinger-Brown NaBH4 synthesis process. As noted earlier, the MgO byproduct from the mechanochemical synthesis of NaBH4 could be recycled to produce Mg by a traditional molten electrolyze process; thus, energy input into the mechanochemical synthesis process could potentially be the only input for this process. On the basis of this analysis, mechanochemical synthesis of NaBH4 (via ball milling) could be economically superior to the traditional processes (e.g., the Schlesinger-Brown process). Reaction Atmosphere. Our investigation showed that the gas(es) used to pressurize the system played an important role in the NaBH4 synthesis process. Mechanochemical synthesis of NaBH4 cannot be conducted in the presence of air because MgH2 would react primarily with the constituents in air during the high-energy milling process and an explosion could potentially occur because of the highly exothermic nature of MgH2 combustion or hydrolysis.
Conclusions NaBH4 synthesis from NaBO2 and MgH2 via ball milling was investigated and reported in this study. Ball-milling time and BPR are two important parameters that determine the energy delivered to the powders/reactants in the milling 5053
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process. Both parameters were found to have significant influences on the yields of NaBH4. Ball collision energy was found to promote synthesis and crystallization of NaBH4. With 2 h of ball milling (under an Ar atmosphere) and a BPR of 50:1, we were able to achieve a NaBH4 yield of 71 wt %. To avoid undesirable reactions between MgH2 and the constituents in air, the synthesis/ball-milling process needs to be conducted in an inert atmosphere (Ar was used in our experiments for this purpose). Our investigation indicated that the pressure of Ar had little effect on NaBH4 product yield. Nevertheless, higher Ar pressure was found to be beneficial to the amount of powders that could be collected
from balls and vial. For example, when ball milling was conducted under an Ar pressure of around 200 kPa, the powder collection rate was the highest (at ca. 90 wt %). The molar ratio of the reactants also influenced NaBH4 product yields. It was found that the MgH2/NaBO2 molar ratio had to be higher than the stoichiometric ratio (per eq 2) to have reasonable NaBH4 product yields. It is possible that, during the ball-milling/NaBH4 synthesis process, there might be some other minor side reactions that could consume MgH2. Further investigation into the synthesis reaction mechanism should be conducted to provide insight into this uncertainty.
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