Synergistic effect of nanoionic destabilization and partial dehydrogenation for enhanced ionic conductivity in MBH4-C60 (M = Li+, Na+) nanocomposites

Journal of Solid State Electrochemistry - In this work, we evaluate the electrochemical properties of MBH4-C60 (M = Li+, Na+) nanocomposites for potential use as a solid-state electrolyte in...     

Probing the structure, stability and hydrogen storage properties of calcium dodecahydro-closo-dodecaborate

Calcium borohydride can reversibly store up to 9.6 wt% hydrogen; however, the material displays poor cyclability, generally associated with the formation of stable intermediate species. In an effort to understand the role of such intermediates on the hydrogen storage properties of Ca(BH₄)₂, calcium dodecahydro-closo-dodecaborate was isolated and characterized by diffraction and spectroscopic techniques. The crystal structure of CaB₁₂H₁₂ was determined from powder XRD data and confirmed by DFT and neutron vibrational spectroscopy studies. Attempts to dehydrogenate/hydrogenate mixtures of CaB₁₂H₁₂ and CaH₂ were made under conditions known to favor partial reversibility in calcium borohydride. However, up to 670 K no notable formation of Ca(BH₄)₂ (during hydrogenation) or CaB₆ (during dehydrogenation) occurred. It was demonstrated that the stability of CaB₁₂H₁₂ can be significantly altered using CaH₂ as a destabilizing agent to favor the hydrogen release.     

Delamination of layered zeolite precursors under mild conditions: synthesis of UCB-1 via fluoride/chloride anion-promoted exfoliation

New material UCB-1 is synthesized via the delamination of zeolite precursor MCM-22 (P) at pH 9 using an aqueous solution of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride at 353 K. Characterization by powder X-ray diffraction, transmission electron microscopy, and nitrogen physisorption at 77 K indicates the same degree of delamination in UCB-1 as previously reported for delaminated zeolite precursors, which require a pH of greater than 13.5 and sonication in order to achieve exfoliation. UCB-1 consists of a high degree of structural integrity via (29)Si MAS NMR and Fourier transform infrared spectroscopies, and no detectable formation of amorphous silica phase via transmission electron microscopy. Porosimetry measurements demonstrate a lack of hysteresis in the N(2) adsorption/desorption isotherms and macroporosity in UCB-1. The new method is generalizable to a variety of Si:Al ratios and leads to delaminated zeolite precursor materials lacking amorphization.     

Molecular motion of tethered molecules in bulk and surface-functionalized materials: a comparative study of confinement

Achieving high degrees of molecular confinement in materials is a difficult synthetic challenge that is critical for understanding supramolecular chemistry on solid surfaces and control of host-guest complexation for selective adsorption and heterogeneous catalysis. In this Article, using 2H MAS NMR spectroscopy of tethered carbamates as a molecular probe, we systematically investigate the degree of steric confinement within three types of materials: two-dimensional silica surface, bulk amorphous microporous silica, and bulk amorphous mesoporous silica. The resulting NMR spectra are described with a simple two-site hopping model for motion and prove that the bulk silica network severely limits the molecular mobility of the immobilized carbamate at room temperature to the same degree as surface-functionalized materials at low-temperatures (approximately 210 K). Raising the temperature of the bulk materials to 413 K still demonstrates the effect of confinement, as manifested in significantly longer characteristic times for the immobilized carbamate relative to surface-functionalized materials at room temperature. The environment surrounding the carbonyl functionality of the immobilized carbamate is investigated using FT-IR spectroscopy, which shows the carbonyl stretching band to be equally shifted for all materials to lower wavenumbers relative to its noninteracting value in carbon tetrachloride solvent. These results suggest that electrostatic interactions between the carbonyl of the immobilized carbamate and silica surface may play an important role in confining the immobilized carbamate and nucleating the formation of a pore wall close to the immobilized carbamate during bulk materials synthesis.     

Studies on the role of fluoride ion vs reaction concentration in zeolite synthesis

This work is an experimental response to an intriguing paper recently published by Catlow and co-workers, which looked at the computational feasibility of fluorine location in three different all-silica zeotypes (Attfield, M. P.; Catlow, C. R. A.; Sokol, A. A. Chem. Mater. 2001, 13, 4708). The materials were chosen as representative of three unique host locations. Our present work examined the synthesis of zeotypes AST, IFR, and MTT using organo-cations with a strong preference for crystallizing these structures. We studied the effect of reaction time and the H(2)O/SiO(2) reactant ratio. The latter is probably the most important function in these zeolite crystallizations that use HF. As reaction conditions became more dilute, AST gave way to SGT and IFR to MTW as host structures, while the MTT synthesis was invariant. Our reactions were studied in terms of product yield vs time, product organo-cation content, fluorine content, and the representative (29)Si and (19)F NMR spectra for certain samples. A single crystal study was carried out for a sample of MTT. Our results showed that, consistent with other recent studies, low H(2)O/SiO(2) reactant ratios lead to more open framework host structures (i.e., IFR vs MTW), and there is typically a higher uptake of organo-cation and fluorine. The structure may well contain a higher population of 4-rings within the silicate substructure. While MTT that contains no 4-rings was chosen as the best possible candidate to achieve an ion-pair for the organo-cation and fluoride anion within the silicate host, both NMR and single crystal work confirm that fluoride is bonded to a 5-coordinate silica center within the lattice.     

Pressure and temperature dependence of the decomposition pathway of LiBH4

The decomposition pathway is crucial for the applicability of LiBH(4) as a hydrogen storage material. We discuss and compare the different decomposition pathways of LiBH(4) according to the thermodynamic parameters and show the experimental ways to realize them. Two pathways, i.e. the direct decomposition into boron and the decomposition via Li(2)B(12)H(12), were realized under appropriate conditions, respectively. By applying a H(2) pressure of 50 bar at 873 K or 10 bar at 700 K, LiBH(4) is forced to decompose into Li(2)B(12)H(12). In a lower pressure range of 0.1 to 10 bar at 873 K and 800 K, the concurrence of both decomposition pathways is observed. Raman spectroscopy and (11)B MAS NMR measurements confirm the formation of an intermediate Li(2)B(12)H(12) phase (mostly Li(2)B(12)H(12) adducts, such as dimers or trimers) and amorphous boron.     

Investigation of the core-shell interface in gold@silica nanoparticles: a silica imprinting approach

The nature of the self-assembled core-shell interface in gold@silica nanoparticles synthesized via a 3-aminopropyltrimethoxysilane (APTMS) route is investigated using materials synthesis as a sensitive tool for elucidating interfacial composition and organization. Our approach involves condensation of the gold@silica nanoparticles within a silica framework for synthesis of a composite gold-silica material containing approximately 30 wt % gold. This material contains one of the highest gold loadings reported, but maintains gold core isolation as ascertained via a single surface plasmon resonance absorption band frequency corresponding to that of gold nanoparticles in dilute aqueous solution. The immobilized gold cores are subsequently etched using cyanide anion for the synthesis of templated porosity, which corresponds to the space that was occupied by the gold. Characterization of immobilized amines is performed using probe molecule binding experiments, which demonstrate a lack of accessible amines after gold removal. Solid-state 13C CPMAS NMR spectroscopy on these materials demonstrates that the amount of amine immobilization must be less than 10% of the expected yield, assuming that all of the APTMS becomes bound to the gold nanoparticle template. These results require a core-shell interface in the gold@silica nanoparticles that is predominantly occupied by inorganic silicate species, such as Si-O-Si and Si-OH, rather than primary amines. Such a result is likely a consequence of the weak interaction between primary amines and gold in aqueous solution. Our method for investigating the core-shell interface of gold@silica nanoparticles is generalizable for other interfacial structures and enables the synthesis of bulk imprinted silica using colloidal templates.     

Organocations in zeolite synthesis: fused bicyclo [l.m.0] cations and the discovery of zeolite SSZ-48

A set of zeolite synthesis experiments is described where lattice substitution is varied in the context of the structure of particular structure-directing organocations (at times referred to as templates). In this particular series, the organocations are constructed as members of a fused bicyclo organonitrogen class of compounds, described as having ring construction [l.m.n], where n = 0. We show that these compounds can best be achieved from starting cyclic ketones that are converted to imines via a Beckman rearrangement reaction. A particular approach to the Beckmann reaction works best in our hands. In some instances isomeric organocations are made and separated. Often their use in zeolite synthesis led to different products. There is a high correlation for the space-filling details of the guest organocations and the type of crystalline host lattice developed in the synthesis. In one instance involving isomers of a decahydroquinoline derivative, a new zeolite, SSZ-48, is discovered and contains only one of the isomers. Characterization of the isomers and their use in the zeolites is followed by (13)C MAS NMR analyses. Some details of the new zeolite are given and it is shown that a reasonable symmetry operation predicting a 14-ring zeolite could be generated under similar conditions to SSZ-48 (a 12-ring zeolite).     

Crystal structure of Li2B12H12: a possible intermediate species in the decomposition of LiBH4

The crystal structure of solvent-free Li2B12H12 has been determined by powder X-ray diffraction and confirmed by a combination of neutron vibrational spectroscopy and first-principles calculations. This compound is a possible intermediate in the dehydrogenation of LiBH4, and its structural characterization is crucial for understanding the decomposition and regeneration of LiBH4. Our results reveal that the structure of Li2B12H12 differs from other known alkali-metal (K, Rb, and Cs) derivatives.