J. Phys. Chem. C 2010, 114, 10045–10047
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Deuterium-Hydrogen Exchange in Solid Mg(BH4)2 Hans Hagemann,*,† Vincenza D’Anna,† Jean-Philippe Rapin,‡ and Klaus Yvon‡ Department of Physical Chemistry and Laboratory of Crystallography, UniVersity of GeneVa, 1211 GeneVa, Switzerland ReceiVed: February 18, 2010; ReVised Manuscript ReceiVed: April 29, 2010
Deuterium-hydrogen exchange in solid R-Mg(BH4)2 is demonstrated. Compared to the previously reported exchange reactions in the alkali borohydrides, the temperature at which isotope exchange starts to take place is significantly lower (132 °C vs 200 °C for LiBH4). The activation energy for the deuterium-hydrogen exchange reaction is estimated to be 51 ( 15 kJ/mol. Almost complete isotope exchange was observed by treating solid Mg(BH4)2 for 72 h at 172 °C with 42 bar of D2. Preliminary experiments indicate that under these conditions Ca(BH4)2 also undergoes isotope exchange. Introduction Inorganic borohydrides are actively studied in view of their potential as hydrogen storage materials.1 Mg(BH4)2 with 14.9 wt % of hydrogen has been subject to recent theoretical and experimental investigations concerning its crystal structure and hydrogen desorption behavior [see refs 2-9]. Hydrogen-deuterium exchange reactions are interesting for several reasons. One aspect is the preparation of deuteriumlabeled materials for neutron diffraction experiments or for mechanistic studies when the borohydride is used as reducing agent in organic synthesis. On the other side, in view of potential hydrogen storage applications, the hydrogen-deuterium exchange reactions considered in this work involve breaking of a B-H bond and formation of a B-D bond in the solid. These reaction steps are potentially involved also during a dehydrogenation (respectively rehydrogenation) of Mg(BH4)2 and can thus contribute to an improved understanding of these potential hydrogen storage materials. Hydrogen-deuterium (tritium) exchange reactions have been reported previously for LiBH4 (T g 200 °C),10,11 NaBH4 (T g 350 °C),10,11 and KBH4 (T g 500 °C).11,12 The deuterium hydrogen exchange in LiBH4 has been studied in more detail on LiBH4 exposed to 20 bar of D2 at 538 K (260 °C) up to 23 h and revealed statistical isotopic distribution of the partially deuterated samples.13,14 In this study we show that hydrogen-deuterium exchange can take place in solid Mg(BH4)2 at temperatures as low as 132 °C. This temperature is significantly lower than the previously reported exchange temperatures in LiBH4.10,11,13,14 Preliminary results for Ca(BH4)2 are also presented.
and the mixture was stirred for 2 days. The solid was filtered and put to dry under vacuum 1 night at room temperature, yielding a light gray (almost white) powder. The solvated Et3N (as seen in the IR spectra of the product) was completely removed by progressive heating over several days under vacuum up to 170 °C, yielding 6.85 g of >95% pure Mg(BH4)2 with some unreacted MgH2 and Mg as impurities. Ca(BH4)2 was prepared by pumping under vacuum up to 130 °C a commercial sample of Ca(BH4)2 · 2THF. X-ray diffraction shows (see Supporting Information) that the sample is in fact a mixture of R (59%), β (13%), and γ (28%) Ca(BH4)2. Deuterium Exchange Reactions. The sample (ca. 0.25 g) was loaded into the autoclave in the glovebox. After being pumped for 1 h at room temperature, the system was heated to the desired temperature (within ca. 10 K) overnight (14 h) under continuous pumping. Next 35 bar of D2 was introduced, the temperature was stabilized at the final temperature, and the pressure was adjusted to 40 or 42 bar. The system was left for a specified time (20-23 h in the first experiments) under these conditions, then the pressure was reduced to 1 bar and the system was cooled back to room temperature. Another series of experiments were carried out at 84 bar, but otherwise the same experimental conditions were used. IR spectra were obtained at room temperature with a Specac “Golden Gate” diamond ATR setup located in a Biorad “Excalibur” FT-IR instrument. The samples were loaded in a glovebox, and the nominal resolution was set to 1 cm-1. Raman spectra were obtained (using 488 nm laser radiation) at room temperature, using a Kaiser Optical Holospec monochromator in conjunction with a liquid nitrogen cooled CCD camera. Results and Discussion
Experimental Section Synthesis of Mg(BH4)2. Mg(BH4)2 was prepared by a slight variation of the procedure reported by Chlopek et al.5 A 5 g sample of MgH2 was ball milled for 2 h. Then 60 mL of Et3NBH3 was added and the mixture was heated for 1 h to 100 °C, let cool and stirred overnight, and next heated for 6 h to 145 °C and cooled, then 180 mL of was cyclohexane was added * To whom correspondence should be addressed. Phone +41 22 379 6539. Fax +41 22 379 6103. E-mail:
[email protected]. † Department of Physical Chemistry. ‡ Laboratory of Crystallography.
Figure 1 compares the IR spectra between 1500 and 2700 cm-1 of several Mg(BH4)2 samples subjected to deuterium exchange for 23 h at 132, 152, and 162 °C (405, 425, and 435 K) with unreacted Mg(BH4)2. In this spectral region, one expects to observe B-H stretching bands (around 2300 cm-1) and B-D stretching bands (around 1700 cm-1).2 All samples studied are perfectly water free, as no water vibrational modes at ca. 3500 and 1630 cm-1 were observed. It appears that at the lowest temperature (405 K), the onset of deuterium exchange can be observed. This temperature is, to the best of our knowledge, the lowest deuterium exchange temperature in solid borohydrides
10.1021/jp101484f 2010 American Chemical Society Published on Web 05/10/2010
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J. Phys. Chem. C, Vol. 114, No. 21, 2010
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Figure 1. IR spectra of Mg(BH4)2 and partially deuterium exchanged samples reacted for 23 h at 132, 152, and 162 °C. Figure 3. Relative hydrogen concentration vs time (expressed in hours) for the deuterium exchange reaction at 162 °C (435 K).
Figure 2. IR spectra of and partially deuterium exchanged Mg(BH4)2 samples reacted at 162 °C for 3, 6, 9, 15, and 23 h, respectively.
reported so far. With increasing reaction temperature, an increasing deuteride content is observed (see Figure 1). In previous studies on deuterium-hydrogen exchange reactions in LiBH4,14 it was shown that the exchange reaction results in the statistical distribution of BH4-xDx- ions in the crystal. In our present IR spectra, the contributions of the individual isotopomers are not resolved, which is in part also related to the complex crystal structure of Mg(BH4)2 with several different possible sites and orientations for the borohydride group. Room temperature Raman spectra (shown in Figure S3 of the Supporting Information) allow the identification of B-D stretching bands associated with BD4- (1607 cm-1), BD3H- (1641 cm-1), and BD2H2- (1675 cm-1). It is interesting to note that in the B-H stretching mode region (around 2300 cm-1) no similar sharp bands are observed. In a second set of experiments, a Mg(BH4)2 sample was subjected to 5 successive 3 h reactions with deuterium (at 42 bar) at 162 °C (435 K), cooled, and a small sample (ca. 30-40 mg) taken for IR characterization before the next reaction step. Figure 2 shows the successive progress of the exchange reaction as seen by the relative intensity change of the B-H and B-D stretching modes. For further analysis, we used the approximation that the relative B-H content (respectively B-D content) is proportional to the relative IR intensity of the B-H/B-D stretching modes. The validity of this approximation was tested by analyzing the weight increase of the samples of the first reaction series. The sample heated to 425 K showed a weight increase of 0.0136 g (initial weight 0.2315 g), corresponding to a final composition of Mg(BH2.4D1.6)2, while the observed B-H to B-D intensity ratio is close to 1:1. This estimate from the weight increase is, however, a lower limit, as small weight losses during the transfer into the autoclave and back are neglected. Within this approximation, one can estimate the pseudo-first-order (as deuterium is present in excess) reaction
Figure 4. IR spectrum of Mg(BH4)2 reacted for 72 h at 445 K and of Ca(BH4)2 reacted for 24 h at 445 K.
parameters from the monoexponential decrease of hydrogen content versus reaction time. These data are shown in Figure 3, where also the relative intensity observed after 23 h in a single reaction run (without intermediate cooling) is shown. The exponential fit (shown by the solid line) yields a reaction rate of k ) 0.041 h-1 at 435 K. Another series of experiments were performed at this same temperature, but with double the deuterium pressure (84 bar). The corresponding data points are also shown in Figure 3. Finally, an experiment was performed with Mg(BH4)2 at 445 K for 72 h with an intermediate purge of the deuterium after 30 h of reaction. The resulting product showed, as expected, about 95% of deuterium substitution as seen from the IR spectrum (Figure 4). A reaction at this same temperature and pressure for 24 h with Ca(BH4)2 showed also almost complete exchange (Figure 4). Comparing the relative intensities for the reactions after 23 h at 425 and 435 K, in addition to the above-mentioned value of k, one can make a rough estimate of the activation energy of ca. 51 ( 15 kJ/mol for the deuterium-hydrogen exchange reaction. The error estimate is based on the difference of the value of k obtained from the fit (Figure 3) vs the value of k estimated from the single point after 23 h of reaction (0.045 vs 0.041 h-1). Note that this value corresponds to the global contribution of all individual steps in the exchange reaction, in particular the diffusion of hydrogen (molecular or atomic?) and the reaction of hydrogen with the borohydride ion in the solid. This value may be compared to the value of 24 ( 6 kJ/mol reported for the activation energy of the deuterium-hydrogen exchange reaction in Ti-catalyzed NaAlH4.15 An initial idea to explain qualitatively the low temperatures of the isotope exchange reaction was related to the crystal
Deuterium-Hydrogen Exchange in Solid Mg(BH4)2 structure of R-Mg(BH4)22-4 which shows the presence of unoccupied voids. These voids could favor the diffusion of hydrogen into the structure. The observation of hydrogen exchange in Ca(BH4)2, which has no voids in the crystal structure, does not support this idea. In the case of LiBH4, the experimental studies13,14 as well as a molecular dynamics calculation16 favor the diffusion of hydrogen atoms. A recent theoretical study on the decomposition of Mg(BH4)28 shows the possible formation of B2H62- ions in the solid. The addition of H2 (respectively D2) to B2H62- would yield 2BH4- (respectively 2BH3D-) groups. This could suggest the diffusion of molecular hydrogen (deuterium) in Mg(BH4)2. Nevertheless, our experiments give the following additional information with respect to the reaction mechanism. During the reaction, there does not appear to form a passivating layer similar to the one that was shown to slow the solid gas synthesis of LiBD4,17 as the exchange reaction appears to proceed regularly over many hours (see Figure 3) and almost complete exchange could be observed. MgO was reported to improve hydrogen desorption in MgH2.18 In our experiments, we had no evidence to demonstrate the presence or absence of MgO, although our samples contained Mg, which could easily form MgO at its surface. The IR spectra showed that the exchanged samples are water free. On the other side, opening and closing the autoclave after each 3 h exchange step to extract a small sample for IR experiments (see Figures 2 and 3) did not modify significantly the reaction kinetics, i.e., the amount of eventual MgO catalyst did not change. Figure 3 also shows that the rate constant is not significantly changed by doubling the deuterium pressure, which shows that the slow step of this reaction does not take place on the surface involving molecular deuterium. The present experiments show that without intentional addition of a catalyst, the breaking of a B-H bond in Mg (and also in Ca) borohydrides appears to be significantly easier than that for the alkali borohydrides and thus makes these materials more promising for hydrogen storage applications. Further experimental studies on hydrogen exchange studies on Mg(BH4)2 as well as on Ca(BH4)2 are planned to provide more experimental data to confront with new theoretical calculations. It will also be interesting to explore the effect of catalysts on the kinetics of this exchange reaction. The isotope exchange studied in the present work may also be applied to
J. Phys. Chem. C, Vol. 114, No. 21, 2010 10047 prepare larger amounts of deuterated samples for spectroscopic and mechanistic studies. Acknowledgment. This work was supported by the Swiss National Science Foundation. Supporting Information Available: X-ray powder diffraction pattern of Mg(BH4)2 as used for this study, and of Ca(BH4)2 before and after the exchange reaction, and room temperature Raman spectra of three partially deuterated Mg(BH4)2 samples. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Zuttel, A.; Jensen, C. M. Chem. ReV. 2007, 107, 4111. (2) Cˇerny´, R.; Filinchuk, Y.; Hagemann, H.; Yvon, K. Angew. Chem., Int. Ed. 2007, 46, 5765. (3) Her, J.-H.; Stephens, P. W.; Gao, Y.; Soloveichik, G. L.; Rijssenbeek, J.; Andrus, M.; Zhao, J.-C. Acta Crystallogr., Sect. B 2007, 63, 561. (4) Filinchuk, Y.; Cˇerny´, R.; Hagemann, H. Chem. Mater. 2009, 21, 925. (5) Chłopek, K.; Frommen, Ch.; Le´on, A.; Zabara, O.; Fichtner, M. J. Mater. Chem. 2007, 17, 3496–3503. (6) Soloveichik, G. L.; Gao, Y.; Rijssenbeek, J.; Andrus, M.; Kniajanski, S.; Bowman, R. C., Jr.; Hwang, S.; Zhao, J. Int. J. Hydrogen Energy 2009, 34, 916. (7) Li, H.-W.; Kikuchi, K.; Nakamori, Y.; Ohba, N.; Miwa, K.; Towata, S.; Orimo, S. Acta Mater. 2008, 56, 1342. (8) van Setten, M. J.; Lohstroh, W.; Fichtner, M. J. Mater. Chem. 2009, 19, 7081. (9) Fichtner, M.; Zhao-Karger, Z.; Hu, J.; Roth, A.; Weidler, P. Nanotechnology 2009, 20, 204029. (10) Brown, W. G.; Kaplan, L.; Wilzbach, K. E. J. Am. Chem. Soc. 1952, 74, 1343. (11) Than, C.; Moritomo, H.; Andres, H.; Williams, P. G. J. Labelled Compd. Radiopharm. 1996, 38, 693. (12) Mesmer, R. E.; Jolly, W. L. J. Am. Chem. Soc. 1962, 84, 2039. (13) Borgschulte, A.; Zu¨ttel, A.; Hug, P.; Racu, A.-M.; Schoenes, J. J. Phys. Chem. A 2008, 112, 4749. (14) Gremaud, R.; Łodziana, Z.; Hug, P.; Racu, A.-M.; Schoenes, J.; Ramirez-Cuesta, A. J.; Clark, S. J.; Refson, K.; Zu¨ttel, A.; Borgschulte, A. Phys. ReV. B. 2009, 80, 100301. (15) Borgschulte, A.; Zu¨ttel, A.; Hug, P.; Barkhordarian, G.; Eigen, N.; Dornheim, M.; Bormann, R.; Ramirez-Cuesta, A. J. Phys. Chem. Chem. Phys. 2008, 10, 4045. (16) Ramzan, M.; Ahuja, R. Appl. Phys. Lett. 2009, 94, 141903. (17) Friedrichs, O.; Kim, J. W.; Remhof, A.; Wallacher, D.; Hoser, A.; Cho, Y. W.; Oh, K. H.; Zu¨ttel, A., Phys. Chem. Chem. Phys. 2010, 12, 4600. (18) Borgschulte, A.; Bielmann, M.; Zu¨ttel, A.; Barkhordarian, G.; Dornheim, M.; Bormann, R. Appl. Surf. Sci. 2008, 254, 2377.
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