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 ₃ @CTF-bipyridine). This material and the counterpart AlH ₃ @CTF-biphenyl rapidly desorb H ₂ between 95 and 154 °C, with desorption complete at 250 °C. Sieverts measurements, ²⁷ Al MAS NMR and ²⁷ Al{ ¹ H} REDOR experiments, and computational spectroscopy reveal that AlH ₃ @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 ₃ binding to bipyridine results in single-electron transfer to form AlH ₂ (AlH ₃ ) _n clusters. The resulting size-dependent charge redistribution alters the dehydrogenation/rehydrogenation thermochemistry, suggesting a novel strategy to enable reversibility in high-capacity metal hydrides.
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