Defying Thermodynamics: Stabilization of Alane Within Covalent Triazine Frameworks for Reversible Hydrogen Storage.

The highly unfavorable thermodynamics of direct aluminum hydrogenation can be overcome by stabilizing alane within a nanoporous bipyridine-functionalized covalent triazine framework (AlH @CTF-bipyridine). This material and the counterpart AlH @CTF-biphenyl rapidly desorb H between 95 and 154 °C, with desorption complete at 250 °C. Sieverts measurements, Al MAS NMR and Al{ H} REDOR experiments, and computational spectroscopy reveal that AlH @CTF-bipyridine dehydrogenation is reversible at 60 °C under 700 bar hydrogen, >10 times lower pressure than that required to hydrogenate bulk aluminum. DFT calculations and EPR measurements support an unconventional mechanism whereby strong AlH binding to bipyridine results in single-electron transfer to form AlH (AlH ) clusters. The resulting size-dependent charge redistribution alters the dehydrogenation/rehydrogenation thermochemistry, suggesting a novel strategy to enable reversibility in high-capacity metal hydrides.