A Composite of Complex and Chemical Hydrides Yields the First Al‐Based Amidoborane with Improved Hydrogen Storage Properties

Abstract

The first Al-based amidoborane Na[Al(NH2 BH3 )4 ] was obtained through a mechanochemical treatment of the NaAlH4 -4 AB (AB=NH3 BH3 ) composite releasing 4.5 wt % of pure hydrogen. The same amidoborane was also produced upon heating the composite at 70 °C. The crystal structure of Na[Al(NH2 BH3 )4 ], elucidated from synchrotron X-ray powder diffraction and confirmed by DFT calculations, contains the previously unknown tetrahedral ion [Al(NH2 BH3 )4 ](-) , with every NH2 BH3 (-) ligand coordinated to aluminum through nitrogen atoms. Combination of complex and chemical hydrides in the same compound was possible due to both the lower stability of the AlH bonds compared to the BH ones in borohydride, and due to the strong Lewis acidity of Al(3+) . According to the thermogravimetric analysis-differential scanning calorimetry-mass spectrometry (TGA-DSC-MS) studies, Na[Al(NH2 BH3 )4 ] releases in two steps 9 wt % of pure hydrogen. As a result of this decomposition, which was also supported by volumetric studies, the formation of NaBH4 and amorphous product(s) of the surmised composition AlN4 B3 H(0-3.6) were observed. Furthermore, volumetric experiments have also shown that the final residue can reversibly absorb about 27 % of the released hydrogen at 250 °C and p(H2 )=150 bar. Hydrogen re-absorption does not regenerate neither Na[Al(NH2 BH3 )4 ] nor starting materials, NaAlH4 and AB, but rather occurs within amorphous product(s). Detailed studies of the latter one(s) can open an avenue for a new family of reversible hydrogen storage materials. Finally, the NaAlH4 -4 AB composite might become a starting point towards a new series of aluminum-based tetraamidoboranes with improved hydrogen storage properties such as hydrogen storage density, hydrogen purity, and reversibility.

Mono- and Bimetalic Amidoboranes

“…In fact, the heavy-atom sublattice of Ca(AB) 2 may be symmetrized to centrosymmetric C2/m at crude threshold. LiNa(AB) 2 and NaAl(AB) 4 have the lowest (triclinic) symmetry among all amidoborane salts [39,41]. Na 2 Mg(AB) 4 and K 2 Mg(AB) 4 form isostructural crystals with the highest symmetry among amidoborane salts (I4 1 /a) [44,45].…”

“…Crystal structure of Y(AB) 3 [38] and NaMg(AB) 3 [41] were not solved but their powder X-ray patterns were successfully indexed and fitted using the LeBail method. Crystal structures of other synthesized amidoborane salts have not yet been determined.…”

“…In LiNa(AB) 2 [39], Li + cations are coordinated by three N atoms and one hydridic H atom, each in a different top of a distorted tetrahedron, while in α-LiAB [19] and β-LiAB [21] Li + cations are surrounded by one N atom and hydridic H atoms, five and six, respectively, where one or two H atoms sit in one top of a distorted tetrahedron. Na + is coordinated by four AB´anions in NaAB [21], by five AB´anions in LiNa(AB) 2 [39], and by six ABá nions in NaAl(AB) 4 [41] and Na 2 Mg(AB) 4 [44], but only in NaAB [21] Na + are coordinated by N atoms. Metal cations in KAB [25], RbAB [26], CsAB [26] and Ca(AB) 2 [19] are coordinated by five or six AB´anions, where two of them are facing metal cations with N atmos.…”

“…In bimetallic LiNa(AB) 2 [39], NaAl(AB) 4 [41], Na 2 Mg(AB) 4 [44] and Na 2 Mg(AB) 4 [45], tetrahedral complex anions are formed by Li + , Al 3+ and Mg 2+ cations that are coordinated by four AB anions through their N atoms [39,41,44,45]. Respective counter ions, Na + [39,41,44] and K + [45], are coordinated only by hydridic H atoms form amidoborate groups. , β-LiAB [21], NaAB [21], KAB [25], RbAB [26], CsAB [26], Ca(AB)2 [19], Sr(AB)2 [33], LiNa(AB)2 [39], NaAl(AB)4 [41], Na2Mg(AB)4 [44], and K2Mg(AB)4 [45].…”

“…, β-LiAB [21], NaAB [21], KAB [25], RbAB [26], CsAB [26], Ca(AB)2 [19], Sr(AB)2 [33], LiNa(AB)2 [39], NaAl(AB)4 [41], Na2Mg(AB)4 [44], and K2Mg(AB)4 [45]. , β-LiAB [21], NaAB [21], KAB [25], RbAB [26], CsAB [26], Ca(AB) 2 [19], Sr(AB) 2 [33], LiNa(AB) 2 [39], NaAl(AB) 4 [41], Na 2 Mg(AB) 4 [44], and K 2 Mg(AB) 4 [45]. [19], β-LiAB [21], NaAB [21], KAB [25], RbAB [26], CsAB [26], Ca(AB)2 [19], Sr(AB)2 [33], LiNa(AB)2 [39], NaAl(AB)4 [41], Na2Mg(AB)4 [44], and K2Mg(AB)4 [45]) emphasizing the cation-anion connectivity.…”