Theoretical Study of the Hydrogen Release from Ammonia Alane and the Catalytic Effect of Alane

Electronic structure calculations at the CCSD(T) level with the aug-cc-pVnZ and aug-cc-pV(n+d)Z basis sets (n = D, T, and Q) were employed to construct the potential energy surfaces for H 2 release from ammonia alane without and with the presence of alane (AlH 3 ). In the AlH 3 NH 3 monomer, although the energy barrier for H 2 loss of ∼29 kcal/mol is comparable with the energy for Al−N bond cleavage (∼27 kcal/mol), kinetics calculations show that only the latter is important at 298 K. The calculated results demonstrate that alane can play the role of an efficient bifunctional catalyst for H 2 release from ammonia alane. The transition state for H 2 production from AlH 3 NH 3 + AlH 3 is located 4.3 kcal/mol lower in energy than the separated reactants and 18.7 kcal/mol above the complex AlH 3 NH 3 ···AlH 3 . A systematic comparison with the reaction pathways for H 2 loss from ammonia borane (BH 3 NH 3 ) with AlH 3 or BH 3 as the catalyst shows that alane is a better catalyst than borane. The predicted kinetic rate constants for hydrogen elimination from ammonia alane with alane as the catalyst are k (298 K) = 2.6 × 10 -2 s -1 and k (400 K) = 8.4 × 10 1 s -1, including tunneling corrections.