ARTICLE pubs.acs.org/JPCC
Synthesis, Structural Characterization, and Thermal Decomposition Study of Mg(H2O)6B10H10 3 4H2O Teshome B. Yisgedu,† Xuenian Chen,†,‡ Hima K. Lingam,† Zhenguo Huang,† Aaron Highley,§ Sean Maharrey,§ Richard Behrens,*,§ Sheldon G. Shore,*,‡ and Ji-Cheng Zhao*,† †
Department of Materials Science and Engineering and ‡Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, United States § Sandia National Laboratories, Livermore, California 94550, United States
bS Supporting Information ABSTRACT: Compound 1 (Mg(H2O)6B10H10 3 4H2O) was synthesized and characterized using NMR, IR, XRD, and elemental analysis. Its thermal decomposition behavior was studied using Simultaneous Thermogravimetric Modulated Beam Mass Spectrometry (STMBMS), TGA, DSC, IR, and 11 B NMR. The crystal structure of 1 reveals multiple dihydrogen and hydrogen bonding interactions that form a 3D extended structure. A reaction network characterizing the thermal decomposition of 1 and its secondary products over a temperature range from 20 to 1000 C has been developed. Thermal decomposition of 1 is primarily controlled by two competing branches in the reaction network, where coordinated water evolves as either H2O (dehydration) or H2 (dehydrogenation). The extent of reaction to form H2 depends on the fraction of the coordinated water remaining in the sample when its temperature is between 160 and 225 C. The evolution of coordinated water is reversible and controlled by dissociative sublimation. For the release of coordinated water between 160 and 215 C, the vapor pressure of water is given by Loge P (Torr) = 30.4561 12425.2/T (K) and ΔHs = 103.3 ( 0.3 kJ/mol. The nature of the condensed phase secondary product remaining after all coordinated water is removed by either dehydration and/or dehydrogenation depends strongly on the extent of reaction to form Mg(OH)xB10H10x. Results of STMBMS experiments where x varies from 0.2 to ∼4 are used to develop the reaction network that characterizes the thermal decomposition process. Heating of 1 at 205 C resulted in the formation of water-soluble Mg(OH)x(H2O)2xB10H10x, while prolonged heating of 1 at 270 C and heating up to 1000 C led to decomposition.
’ INTRODUCTION Mg(BH4)2 is a promising hydrogen storage material due to its very high volumetric and gravimetric density (15% hydrogen by mass). The synthesis and hydrogen desorption properties of Mg(BH4)2 have been extensively studied.122 MgB12H12 which has been referred to as an unfavorable “sink” was detected by 11B NMR1,2 as an intermediate during the desorption of Mg(BH4)2. Li and co-workers have reported that the completely desorbed Mg(BH4)2 products can be rehydrided to form amorphous MgB12H12 which was confirmed by Raman spectroscopy.3 Recently, Severa and co-workers reported that MgB2 can be recharged to form Mg(BH4)2 at 950 bar hydrogen pressure.6 The MgB12H12 produced as a result of Mg(BH4)2 desorption or MgB2 hydrogenation is always in a mixture with other products. Therefore, it is highly desirable to synthesize pure MgB12H12 to study its role in hydrogen desorption and rehydrogenation of Mg(BH4)2. Our recent detailed study showed that solvent-free MgB12H12 cannot be obtained from Mg(H2O)6B12H12 3 6H2O by pyrolysis.23 Instead, a polyhydroxylated Mg(H2O)(3x)(μOH)xB12H(12x) was produced. The incomplete removal of all the coordinated water molecules is attributed to the dihydrogen r 2011 American Chemical Society
bonding interactions between the water protons and BH hydrides (OH 3 3 3 HB) that result in the release of hydrogen upon heating. Mixtures of B12H122 and B10H102 salts are formed during heating of either decaborane/triethylamine24 or tetraalkylammonium borohydride salts.25 In addition, we have observed partial conversion of B10H102 to B12H122 during the desorption of (NH4)2B10H10.26 Even though MgB10H10 is not observed during the desorption of H2 from Mg(BH4)2, but since MgB10H10 contains a stable closo-boron cage structure next to MgB12H12, it might be helpful to synthesize solvent-free MgB10H10 from Mg(H2O)6B10H10 and compare its thermal properties with Mg(H2O)6B12H12. Here we report the synthesis and characterization of Mg(H2O)6B10H10 3 4H2O by single-crystal XRD, NMR, IR, TGA, elemental analysis, DSC, and the reaction network that controls its thermal decomposition by simultaneous thermogravimetric modulated-beam mass spectrometry (STMBMS). We have also characterized the thermal decomposition residues using NMR and IR. Received: November 19, 2010 Revised: May 5, 2011 Published: May 20, 2011 11793
dx.doi.org/10.1021/jp200541k | J. Phys. Chem. C 2011, 115, 11793–11802
The Journal of Physical Chemistry C
Figure 1. Cross section of reaction cells used in STMBMS experiments.
’ EXPERIMENTAL SECTION (NEt3H)2B10H10 was synthesized from decaborane according to a literature report.24 (MgCO3)4 3 Mg(OH)2 3 5H2O was purchased from Strem Chemicals. Triethylamine and deuterium oxide were purchased from Aldrich. IR spectra were recorded on a Bruker Tensor 27 spectrometer. 1H and 11B NMR spectra were collected using Bruker DPX-250 or DPX-400 spectrometers at 250.1/400.1 MHz and 80.25/128.4 MHz, respectively. Boron spectra were externally referenced to BF3 3 OEt2 in C6D6 (δ = 0 ppm). Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer TGA 7 analyzer with a TAC 7/DS thermal analysis instrument controller. TGA samples were loaded on a quartz crucible under an argon atmosphere with a flow rate of ∼60 cc/min. Thermal stability was measured using a Mettler Toledo high-pressure differential scanning calorimeter (DSC27HP) in an argon glovebox with a ramp rate of 5 C/min and a argon flow rate of 5060 cc/min. Elemental analyses were performed by Galbraith Laboratories Inc. Single-crystal XRD data were collected on a Nonius Kappa CCD diffraction system, which employs graphite-monochromated Mo KR radiation (λ = 0.71073 Å). A single crystal of 1 was mounted on the tip of a glass fiber coated with Fomblin oil (a pentafluoropolyether). Unit cell parameters were obtained by indexing the peaks in the first ten frames. All frames were integrated and corrected for Lorentz and polarization effects using the DENZO-SMN package (Nonius BV, 1999).27 Absorption correction for the structures was accounted for by using SCALEPACK. All non-hydrogen atoms were located and refined anisotropically. All hydrogens were located on difference maps, and their positional and isotropic thermal parameters were refined isotropically. STMBMS instrument28,29 allows the concentration and release rate of each gas-phase species in a reaction cell (Figure 1) to be measured as a function of time by correlating the ion signals at different m/z values measured with a mass spectrometer with the rate of force change measured by a microbalance at any instant. A small sample of 1 (∼10 mg) was placed in an alumina reaction cell that was then mounted on a thermocouple probe, which was seated on a microbalance. The reaction cell was enclosed in a high vacuum environment (
