In situ high pressure NMR study of the direct synthesis of LiAlH4

Abstract

Li wide-line NMR spectroscopy incorporating a high pressure NMR apparatus has allowed the first in situ study of the solvent mediated, direct synthesis of an alanate, thus overcoming the dearth of analytical techniques available to study phenomena occurring in a pressurised slurry. In contrast to the decomposition reaction, the elucidated hydrogenation pathway does not proceed through the hexahydride intermediate.The practical utilisation of hydrogen as an energy carrier awaits the development of high-capacity, hydrogen storage materials that can be recharged under moderate conditions. A viable onboard hydrogen carrier must have: high gravimetric and volumetric hydrogen capacities; thermodynamic properties that are within rather stringent limits; and dehydrogenation and rehydrogenation kinetics that allow hydrogen cycling at moderate temperatures and pressures.1,2 Although no directly reversible hydrogen material has yet met all of these criteria, a great deal of progress as have been made towards harnessing the high storage capacity, relatively low desorption temperatures, and comparative ease of hydrogenation of sodium alanate (NaAlH 4 ) and lithium alanate (LiAlH 4 ). 3It is well established that the dehydrogenation of both undoped and Ti-doped LiAlH 4 (Al(Ti)) proceeds via Li 3 AlH 6 as an intermediate before decomposition into LiH and Al as seen in eqn (1) and (2). 4-73LiAlH 4 / Li 3 AlH 6 + 2Al + 2H 2 (1)The in situ decomposition of LiAlH 4 has been studied previously by DSC, 7,8 X-ray and neutron diffraction measurements 9 and NMR spectroscopy. 10 The direct re-hydrogenation of LiH and Al to LiAlH 4 is challenging as the reaction in eqn (1) While Ashby et al. 13 found that THF solvated LiAlH 4 could be obtained in 96% yield following 5 h or reaction at 393 K under 340 bar of H 2 .More recently, Wang et al. reported the utilisation of high pressure ball-milling to form crystalline LiAlH 4 with a desolvation step following the reaction.14 Graetz et al. subsequently demonstrated the reversibility of this material using PCT isotherms (eqn (3)).15 Hydrogenation was reported to occur at room temperature and 13 bar H 2 forming the LiAlH 4 $4THF adduct from a THF slurry of LiH and Al(Ti), with the removal of the adduct at 333 K in vacuo to form crystalline LiAlH 4 . In a nal development, the complication of a requisite side process to remove the adduct prior to dehydrogenation was eliminated by Liu et al. who reported a remarkably mild and simple process to generate LiAlH 4 from the dehydrogenation products (eqn (4)).

In situ high pressure NMR study of the direct synthesis of NaAlH4

“…This work, in tandem with our parallel study of the direct synthesis of LiAlH 4 (ref. 13) demonstrates impressively how high-pressure NMR studies can shed important new light on the intimate mechanisms of a wide variety of systems that undergo reversible hydrogenation, and shows the technique to be a valuable new weapon in the armory of synthetic chemists and material scientists engaged in the development of advanced, solid state hydrogen storage systems. This journal is c the Owner Societies 2013…”

Ni–B-doped NaAlH4 hydrogen storage materials prepared by a facile two-step synthesis method

“…In recent years, solid light metal complex hydrides [1,2] drew intensive research interest due to their high hydrogen capacities and moderate operating conditions. Among various light metal complex hydrides [3][4][5][6][7][8][9][10], sodium aluminum hydride (NaAlH 4 ) was widely studied after the pioneering researches of Bogdanović and Schwickardi [11]. Recently, many researches on exploring new kind of catalysts or modifying the microstructure to enhance the synthesis efficiency and dehydrogenation properties of NaAlH 4 were carried out.…”

Probing the unusual anion mobility of LiBH4 confined in highly ordered nanoporous carbon frameworks via solid state NMR and quasielastic neutron scattering

“…[1][2][3][4][5][6][7][8][9][10][11] In spite of many achievements in hydrogen storage research, no material can meet all of the U.S. Department of Energy FreedomCAR requirements for vehicular transportation applications. Complex metal hydrides have received considerable attention owing to their high hydrogen capacity, but the high dehydrogenation temperatures and limited kinetics on hydriding have prevented their use in practical applications.…”