The Comparison in Dehydrogenation Properties and Mechanism between MgCl₂(NH₃)/LiBH₄ and MgCl₂(NH₃)/NaBH₄ Systems

Abstract

The dehydrogenation properties and mechanism of MgCl 2 (NH 3 )/MBH 4 (here, M is Li or Na) were investigated by thermogravimetric analysis and mass spectrometry, X-ray diffraction (XRD), solid-state 11 B NMR, Fourier transform infrared, and differential scanning calorimetry (DSC). As for the MgCl 2 (NH 3 )/LiBH 4 system, it was found that a new phase, namely, MgCl 2 (NH 3 ) • LiBH 4 , to which the following dehydrogenation relates, is formed after ball milling. Judging from the reaction products, it is confirmed that MgCl 2 is inclined to work as an ammonia carrier, and the ligand NH 3 , transferring from MgCl 2 , is able to combine with the LiBH 4 to release H 2 with a trace of ammonia at ca. 240 °C. With the increase of LiBH 4 content in the mixture, the emission of ammonia was totally suppressed, and Mg(BH 4 ) 2 was produced by the decomposition reaction of MgCl 2 with the excessive LiBH 4 after the ligand NH 3 was exhausted, resulting in an improved dehydrogenation in the whole system. As for the MgCl 2 (NH 3 )/NaBH 4 system, no new phases are detected by XRD after ball milling. The MgCl 2 works as a BH 4acceptor, and the ligand NH 3 stays with Mg 2+ to combine with the BH 4 -, which transfers from NaBH 4 to Mg 2+ , resulting in a totally different decomposition route and thermal effects as compared with the MgCl 2 (NH 3 )/LiBH 4 system. DSC results revealed that the decomposition of MgCl 2 (NH 3 )/LiBH 4 presented an exothermic reaction with an enthalpy of -3.8 kJ mol -1 H 2 , while the MgCl 2 (NH 3 )/NaBH 4 showed two apparent endothermic peaks associated with its two-step dehydrogenation with enthalpies of 8.6 and 2.2 kJ mol -1 H 2 , respectively. Moreover, the MS profiles of the MgCl 2 (NH 3 )/ 2NaBH 4 , with excessive BH 4 -, still released a trace of NH 3 , indicating that the NaBH 4 is not so effective in suppressing the emission of NH 3 as LiBH 4 did.

Lewis Base Complexes of Magnesium Borohydride: Enhanced Kinetics and Product Selectivity upon Hydrogen Release

“…See Figure S2 for a reference spectrum of unsolvated Mg(BH 4 ) 2. The downfield shift of the THF solvated BH4 − complex is comparable to the downfield shift reported for Mg(BH4)2·4NH3 [37]. Upon heating the two peaks collapse into a single narrow peak at 90 °C.…”

Structure and decomposition of zinc borohydride ammonia adduct: towards a pure hydrogen release

“…À groups have a number of favorable attributes including a high hydrogen capacity, 17.6 wt% for NH 3 and 26.7 wt% for BH 4 À ; and highly mobile H d+ /H dÀ atoms which may lead to a rapid generation of H 2 and subsequent desorption at low temperature. 26,27 However, despite their promising characteristics, it has been observed that across a range of AMBs they display a variety of different thermal decomposition properties. LiBH 4 $NH 3 and Ca(BH 4 ) 2 $2NH 3 mainly release ammonia rather than hydrogen during decomposition in argon, which is attributed to the weak coordination between Li + (Ca 2+ ) with NH 3 in the material.…”

The superior desorption properties of MgCl2-added ammonia borane compared to MgF2-added systems—the unexpected role of MgCl2 interacting with [NH3] units

“…Furthermore, except for the similar tridentate 11 B NMR spectra that are possibly related to the single or two overlapping quadrupolar resonances of BN 3 , BN 2 H or even a mixture of both, an additional peak at approximately À35 ppm was obtained for the MgCl 2 -doped sample aer heating, which can be most likely assigned to the unknown phase, as shown in Fig. 3a, i.e., the complex of MgCl 2 with AB aer heating at 130 C. 25,26 Third, we further employed in situ Raman spectroscopy to examine the entire process of thermal decomposition. Fig.…”