On the preparation and NMR spectroscopic characterization of potassium aluminium tetrahydride KAlH4

Potassium aluminium tetrahydride KAlH4 of high phase purity (space group Pnma (62)) was synthesized via a mechanochemical route. The thus obtained material was studied by 27Al and 39K MAS NMR spectroscopy. For both nuclei precise data for the isotropic chemical shift and the quadrupole coupling at T=295 K were derived (27Al: delta_iso=(107.6+-0.2) ppm, C_Q = (1.29+-0.02) MHz and eta = 0.64+-0.02; 39K: delta_iso=(6.1+-0.2) ppm, C_Q = (0.562+-0.005) MHz and eta = 0.74+-0.02). The straightforward NMR spectroscopic approach applied here should also work for other complex aluminium hydrides and for many other materials containing half-integer nuclei experiencing small to medium-sized quadrupole couplings.Comment: 27 pages, 5 figures, final version as published in PCCP, with reviewer reports attached to the ES

On the use of Solomon echoes in 27Al NMR studies of complex aluminium hydrides

The quadrupole coupling constant $C_Q$ and the asymmetry parameter $\eta$ have been determined for two complex aluminium hydrides from $^{27}$Al NMR spectra recorded for stationary samples by using the Solomon echo sequence. The thus obtained data for KAlH4 ($C_Q=(1.30\pm0.02)$ MHz, $\eta=(0.64\pm0.02)$) and NaAlH4 ($C_Q=(3.11\pm0.02)$ MHz, $\eta<0.01$) agree very well with data previously determined from MAS NMR spectra. The accuracy with which these parameters can be determined from static spectra turned out to be at least as good as via the MAS approach. The experimentally determined parameters ($\delta_{iso}$, $C_Q$ and $\eta$) are compared with those obtained from DFT-GIPAW (density functional theory - gauge-including projected augmented wave) calculations. Except for the quadrupole coupling constant for KAlH4, which is overestimated in the GIPAW calculations by about 30%, the agreement is excellent. Advantages of the application of the Solomon echo sequence for the measurement of less stable materials or for in-situ studies are discussed.Comment: 26 pages, 7 figure

Highly microporous monodisperse silica spheres synthesized by the Stöber process

Silica spheres that are prepared by the Stöber process are usually considered non-porous. Here we report on a slightly modified synthesis protocol that allows preparation of microporous Stöber particles. Successive treatment with water and alcohol at room temperature results in substantial reorganization of the silica within the Stöber particles. Hydrolysis of alkoxy groups as well as condensation and re-esterification of silanol groups upon re-immersion in alcohol are crucial for that process. As the result of the silica reorganization, micropore channels are formed within the particles. After a final washing with water to remove all alkoxy groups from the particles, pure microporous silica spheres are obtained. The total pore volumes of these materials are comparable to those of zeolitic materials.Fil: Bazula, Piotr A.. Max Planck Institut für Kohlenforschung; AlemaniaFil: Arnal, Pablo Maximiliano. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Max Planck Institut für Kohlenforschung; AlemaniaFil: Galeano, Carolina. Max Planck Institut für Kohlenforschung; AlemaniaFil: Zibrowius, Bodo. Max Planck Institut für Kohlenforschung; AlemaniaFil: Schmidt, Wolfgang. Max Planck Institut für Kohlenforschung; AlemaniaFil: Schueth, Ferdi. Max Planck Institut für Kohlenforschung; Alemani