rXXXX American Chemical Society 201 DOI: 10.1021/jz900043c |J. Phys. Chem. Lett. 2010, 1, 201–204 pubs.acs.org/JPCL Thermal Decomposition Behavior of Hydrated Magnesium Dodecahydrododecaborates Xuenian Chen, †,‡ Hima Kumar Lingam, † Zhenguo Huang, † Teshome Yisgedu, † Ji-Cheng Zhao,* ,† and Sheldon G. Shore ‡ † Department of Materials Science & Engineering, The Ohio State University, Ohio 43210, and ‡ Department of Chemistry, The Ohio State University, Ohio 43210 ABSTRACT MgB 12 H 12 is an intermediate in the hydrogen desorption and sorption processes of magnesium borohydride, which is an important candidate material for hydrogen storage. It is thus highly desirable to synthesize anhydrous MgB 12 H 12 in order to study its properties and its role in the hydrogenation and dehydrogena- tion of magnesium borohydride. Contrary to the literature claim, we find that anhydrous MgB 12 H 12 cannot be obtained from simple thermal decomposition of Mg(H 2 O) 6 B 12 H 12 3 6H 2 O (1) which has different thermal decomposition behavior from that of most hydrated alkali and alkaline earth salts of dodecahydrododeca- borates. Thermal decomposition of 1 involves both dehydration and dehydrogen- ation processes in three steps, resulting in the formation of complexes Mg- (H 2 O) 6 B 12 H 12 (2), Mg(H 2 O) 3 B 12 H 12 (3), and Mg(μ-OH) x B 12 H 12-x (4) that were characterized by XRD, IR, and 11 B NMR. Dehydrogenation was also confirmed by both the generation of hydrogen observed in TPD-MS spectra and the formation of polyhydroxylated complexes. SECTION Energy Conversion and Storage M agnesium borohydride, Mg(BH 4 ) 2 , with a high hy- drogen storage capacity (14.9 mass%) has recently attracted considerable attention as a potential hy- drogen storage material. 1- 14 Recent theoretical and experi- mental results clearly show that an intermediate phase, MgB 12 H 12 , forms during hydrogen desorption from Mg(BH 4 ) 2 before the formation of the final products. 15-18 Through the formation of this key intermediate, 6.1% hydrogen can be reabsorbed into the final thermal decomposition product at appropriate (albeit a high pressure of hydrogen) conditions. 18 It is thus very important to prepare an anhydrous and solvent- free compound of MgB 12 H 12 and study its properties in order to elucidate its role as an intermediate in the hydrogen desorption and sorption processes of magnesium boro- hydride and to develop strategies for improved reversibility at moderate conditions. While most anhydrous alkali and alkaline earth metal salts of dodecahydrododecaborates can be obtained through ther- molysis of the hydrated salts, 19,28 hydrated magnesium do- decahydrododecaborate, Mg(H 2 O) 6 B 12 H 12 3 6H 2 O (1), has a different thermal decomposition behavior that produces poly- hydroxylated boron cage compounds rather than the antici- pated anhydrous MgB 12 H 12 . Here, we present the results of thermal treatment at different conditions and the correspond- ing products formed during the decomposition of the hy- drated magnesium dodecahydrododecaborates. The hydrated magnesium salt of dodecahydrododecabo- rate was prepared from the reaction of MgCO 3 with a free acid aqueous solution of dodecahydrododecaborate that was ob- tained through cation exchange from sodium or cesium salts of dodecahydrododecaborate. 20 Single-crystal X-ray diffrac- tion analyses indicate that 12 water molecules are present in the structure of the hydrated compound 1. Six water mole- cules are directly coordinated to magnesium, and other so- called lattice waters are embedded in the lattice by hydrogen bonds. 20 The thermogravimetric analysis (TGA) of 1 in Figure 1 shows a three-step decomposition process. The weight losses of the first two steps correspond to elimination of six and three water molecules, respectively (curve a). On the basis of the weight loss, two intermediates Mg(H 2 O) 6 B 12 H 12 (2) and Mg- (H 2 O) 3 B 12 H 12 (3) were expected to form after the first and second steps. We were able to isolate compounds 2 and 3 by carefully controlling the experimental conditions. The TGA curve of 1 revealed that the six lattice water molecules were removed rapidly at temperatures lower than 150 °C (Figure 1, curve a, first step, 28.0% weight loss). These six water molecules could also be withdrawn by the presence of P 2 O 5 at room temperature under a static vacuum condition or by pumping under a dynamic vacuum condition. It is surprising that these six waters could even be removed by an argon gas flow at room temperature (Figure 2S, Supporting Information). After the removal of six lattice water molecules, hexahydrated compound 2 was formed. For the remaining six water molecules directly bonded to magnesium, the weight Received Date: September 24, 2009 Accepted Date: November 10, 2009