Experimental and computational characterization of phase transitions in CsB3H8.

Metal hydroborates are versatile materials with interesting properties related to energy storage and cation conductivity. The hydrides containing B3H8- (triborane, or octahydrotriborate) ions have been at the center of attention for some time as reversible intermediates in the decomposition of BH4- (3BH4-↔ B3H8- + 2H2), and as conducting media in electrolytes based on boron-hydride cage clusters. We report here the first observation of two phase transitions in CsB3H8 prior to its decomposition above 230 °C. The previously reported orthorhombic room temperature phase (here named α-CsB3H8) with the space group Ama2 changes into a new phase with the space group Pnma at 73 °C (here named ß-CsB3H8), and then into a face-centered cubic phase, here named γ-CsB3H8, at 88 °C. These phases are not stable at room temperature thus requiring in situ measurements for their characterization. The phase transitions and decomposition pathway of CsB3H8 were studied with in situ synchrotron powder X-ray diffraction (SR-PXD), in situ and ex situ vibrational spectroscopies (Raman and FTIR), and differential-scanning calorimetry combined with thermo-gravimetric analysis (DSC-TGA). The structure determination was validated by vibrational spectroscopy analysis and modeling of the periodic structures by density functional methods. In γ-CsB3H8, a significant disorder in B3H8- positions and orientations was found which can potentially benefit cation conducting properties through the paddle mechanism.

Effect of additives, ball milling and isotopic exchange in porous magnesium borohydride.

Magnesium borohydride (Mg(BH4)2) is a promising material for solid state hydrogen storage. However, the predicted reversible hydrogen sorption properties at moderate temperatures have not been reached due to sluggish hydrogen sorption kinetics. Hydrogen (H) → deuterium (D) exchange experiments can contribute to the understanding of the stability of the BH4 - anion. Pure γ-Mg(BH4)2, ball milled Mg(BH4)2 and composites with the additives nickel triboride (Ni3B) and diniobium pentaoxide (Nb2O5) have been investigated. In situ Raman analysis demonstrated that in pure γ-Mg(BH4)2 the isotopic exchange reaction during continuous heating started at ∼80 °C, while the ball milled sample did not show any exchange at 3 bar D2. However, during ex situ exchange reactions investigated by infrared (IR) and thermogravimetric (TG) analyses a comparable H → D exchange during long exposures (23 h) to deuterium atmosphere was observed for as received, ball milled and γ-Mg(BH4)2 + Nb2O5, while the Ni3B additive hindered isotopic exchange. The specific surface areas (SSA) were shown to be very different for as received γ-Mg(BH4)2, BET area = 900 m2 g-1, and ball milled Mg(BH4)2, BET area = 30 m2 g-1, respectively, and this explains why no gas-solid H(D) diffusion was observed for the ball milled (amorphous) Mg(BH4)2 during the short time frames of in situ Raman measurements. The heat treated ball milled sample partially regained the porous γ-Mg(BH4)2 structure (BET area = 560 m2 g-1). This in combination with the long reaction times allowing for the reaction to approach equilibrium explains the observed gas-solid H(D) diffusion during long exposure. We have also demonstrated that a small amount of D can be substituted in both high surface area and low surface area samples at room temperature proving that the B-H bonds in Mg(BH4)2 can be challenged at these mild conditions.

Isotopic Exchange in Porous and Dense Magnesium Borohydride.

Magnesium borohydride (Mg(BH4)2) is one of the most promising complex hydrides presently studied for energy-related applications. Many of its properties depend on the stability of the BH4(-) anion. The BH4(-) stability was investigated with respect to H→D exchange. In situ Raman measurements on high-surface-area porous Mg(BH4 )2 in 0.3 MPa D2 have shown that the isotopic exchange at appreciable rates occurs already at 373 K. This is the lowest exchange temperature observed in stable borohydrides. Gas-solid isotopic exchange follows the BH4(-) +D˙ →BH3D(-) +H˙ mechanism at least at the initial reaction steps. Ex situ deuteration of porous Mg(BH4)2 and its dense-phase polymorph indicates that the intrinsic porosity of the hydride is the key behind the high isotopic exchange rates. It implies that the solid-state H(D) diffusion is considerably slower than the gas-solid H→D exchange reaction at the surface and it is a rate-limiting steps for hydrogen desorption and absorption in Mg(BH4)2.

H2 interaction with divalent cations in isostructural MOFs: a key study for variable temperature infrared spectroscopy.

Systematic studies of H2 adsorption by variable temperature infrared (VTIR) spectroscopy have added value in the characterization of hydrogen storage materials. As a key study to describe the potential of the method, here we report VTIR spectroscopy results of H2 adsorption at isostructural MOFs CPO-27-M (M = Mg, Mn, Co, Ni, Zn). The strongest perturbation of H2 vibrational frequency is due to the interaction with an open metal site. Although ionic radius is an empirical value, the direct correlation between ionic radii of the metal cation and H2 interaction energy is found in MOFs of the same topology. The highest enthalpy of hydrogen adsorption 15 ± 1 kJ mol(-1) was found for Ni(2+). VTIR results of H2 adsorption at isostructural MOFs CPO-27-M (M = Mg, Mn, Co, Ni, Zn) were compared with data obtained from analogous studies performed on a large variety of microporous materials (MOFs and zeolites), underlining the relevance of the approach to get reliable energetic and entropic (ΔH(0) and ΔS(0)) values to be compared with computational data and isosteric heats.

Impact of metal and anion substitutions on the hydrogen storage properties of M-BTT metal-organic frameworks.

Microporous metal-organic frameworks are a class of materials being vigorously investigated for mobile hydrogen storage applications. For high-pressure storage at ambient temperatures, the M(3)[(M(4)Cl)(3)(BTT)(8)](2) (M-BTT; BTT(3-) = 1,3,5-benzenetristetrazolate) series of frameworks are of particular interest due to the high density of exposed metal cation sites on the pore surface. These sites give enhanced zero-coverage isosteric heats of adsorption (Q(st)) approaching the optimal value for ambient storage applications. However, the Q(st) parameter provides only a limited insight into the thermodynamics of the individual adsorption sites, the tuning of which is paramount for optimizing the storage performance. Here, we begin by performing variable-temperature infrared spectroscopy studies of Mn-, Fe-, and Cu-BTT, allowing the thermodynamics of H(2) adsorption to be probed experimentally. This is complemented by a detailed DFT study, in which molecular fragments representing the metal clusters within the extended solid are simulated to obtain a more thorough description of the structural and thermodynamic aspects of H(2) adsorption at the strongest binding sites. Then, the effect of substitutions at the metal cluster (metal ion and anion within the tetranuclear cluster) is discussed, showing that the configuration of this unit indeed plays an important role in determining the affinity of the framework toward H(2). Interestingly, the theoretical study has identified that the Zn-based analogs would be expected to facilitate enhanced adsorption profiles over the compounds synthesized experimentally, highlighting the importance of a combined experimental and theoretical approach to the design and synthesis of new frameworks for H(2) storage applications.

H2 storage in isostructural UiO-67 and UiO-66 MOFs.

The recently discovered UiO-66/67/68 class of isostructural metallorganic frameworks (MOFs) [J. H. Cavka et al. J. Am. Chem. Soc., 2008, 130, 13850] has attracted great interest because of its remarkable stability at high temperatures, high pressures and in the presence of different solvents, acids and bases [L. Valenzano et al. Chem. Mater., 2011, 23, 1700]. UiO-66 is obtained by connecting Zr(6)O(4)(OH)(4) inorganic cornerstones with 1,4-benzene-dicarboxylate (BDC) as linker resulting in a cubic MOF, which has already been successfully reproduced in several laboratories. Here we report the first complete structural, vibrational and electronic characterization of the isostructural UiO-67 material, obtained using the longer 4,4'-biphenyl-dicarboxylate (BPDC) linker, by combining laboratory XRPD, Zr K-edge EXAFS, TGA, FTIR, and UV-Vis studies. Comparison between experimental and periodic calculations performed at the B3LYP level of theory allows a full understanding of the structural, vibrational and electronic properties of the material. Both materials have been tested for molecular hydrogen storage at high pressures and at liquid nitrogen temperature. In this regard, the use of a longer ligand has a double benefit: (i) it reduces the density of the material and (ii) it increases the Langmuir surface area from 1281 to 2483 m(2) g(-1) and the micropore volume from 0.43 to 0.85 cm(3) g(-1). As a consequence, the H(2) uptake at 38 bar and 77 K increases from 2.4 mass% for UiO-66 up to 4.6 mass% for the new UiO-67 material. This value is among the highest values reported so far but is lower than those reported for MIL-101, IRMOF-20 and MOF-177 under similar pressure and temperature conditions (6.1, 6.2 and 7.0 mass%, respectively) [A. G. Wong-Foy et al. J. Am. Chem. Soc., 2006, 128, 3494; M. Dinca and J. R. Long. Angew. Chem., Int. Ed., 2008, 47, 6766]. Nevertheless the remarkable chemical and thermal stability of UiO-67 and the absence of Cr in its structure would make this material competitive.

FTIR spectroscopy and thermodynamics of CO and H(2) adsorbed on gamma-, delta- and alpha-Al(2)O(3).

The adsorption of CO and H(2) at the surface of transitional (gamma and delta) and corundum (alpha) phases of Al(2)O(3) is studied by means of FTIR spectroscopy at temperature variable in the 293-60 K (CO) and 293-20 K (H(2)) intervals with the aims of better clarifying the nature of the surface Lewis centres and evaluate the thermodynamics of the adsorption process.

Storage of hydrogen as a guest of a nanoporous polymeric crystalline phase.

A mechanism of H(2) uptake, based on adsorption in the ordered cavities of nanoporous polymeric crystalline phases rather than on disordered amorphous polymeric surfaces, has been clearly established, for aerogels of syndiotactic polystyrene (s-PS) exhibiting the nanoporous δ phase. An ordered arrangement of the H(2) molecules is proven by FTIR spectra while the inclusion of H(2) is assessed by gravimetric measurements and molecular simulations.