ABSTRACT
The metal sites of MIL-100(Fe), MIL-100(Fe,Al), and MIL-100(Al) metal-organic frameworks (MOFs) were decorated with ethylenediamine (EN). Interestingly, the Al-containing MOFs presented hierarchized porosity, and their structural integrity was maintained upon functionalization. Solution and solid-state NMR confirmed the grafting efficiency in the case of MIL-100(Al) and the presence of a free amine group. It was shown that MIL-100(Al) can be functionalized by only one EN molecule in each trimeric Al3O cluster unit, whereas the other two aluminum sites are occupied by a hydroxyl and a water molecule. The -NH2 sites of the grafted ethylenediamine can be used for further postfunctionalization through amine chemistry and are responsible for the basicity of the functionalized material as well as increased affinity for CO2. Furthermore, the presence of coordinated water molecules on the Al-MOF is responsible for simultaneous Brønsted acidity. Finally, the Al-containing MOFs show an unusual carbon dioxide sorption mechanism at high pressures that distinguishes those materials from their iron and chromium counterparts and is suspected to be due to the presence of polarized Al-OH bonds.
ABSTRACT
Ammonia borane, NH3BH3 (AB), is very attractive for hydrogen storage; however, it dehydrogenates exothermally, producing a mixture of polymeric products with limited potential for direct rehydrogenation. Recently, it was shown that AB complexed with Al3+ in Al(BH4)3·AB endothermically dehydrogenates to a single product identified as Al(BH4)3·NHBH, with the potential for direct rehydrogenation of AB. Here we explore the reactivity of AB-derived RNH2BH3 (R = -CH3, -CH2-) with AlX3 salts (X = BH4-, Cl-), aiming to extend the series to different anions and to enlarge the stability window for Al(BH4)3·NRBH. Three novel complexes were identified: Al(BH4)3·CH3NH2BH3 having a molecular structure similar to that of Al(BH4)3·AB but different dehydrogenation properties, as well as [Al(CH3NH2BH3)2Cl2][AlCl4] and [Al(NH2CH2CH2NH2)(BH4)2][Al(BH4)4], rare examples of Al3+ making part of the cations and anions simultaneously. The latter compounds are of interest in the design of novel electrolytes for Al-based batteries. The coordination of two ABs to a single Al atom opens a route to materials with higher hydrogen content.
