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 (AlH3). In the AlH3NH3 monomer, although the energy barrier for H-2 loss of similar to 29 kcal/mol is comparable with the energy for Al-N bond cleavage (similar to 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, release from ammonia alane. The transition state for H-2 production from AlH3NH3 + AlH3 is located 4.3 kcal/mol lower in energy than the separated reactants and 18.7 kcal/mol above the complex AlH3NH3.. AlH3. A systematic comparison with the reaction pathways for H? loss from ammonia borane (BH3NH3) with AlH3 or BH3 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 x 10(-2) s(-1) and k(400 K) = 8.4 x 10(1) s(-1), including tunneling corrections.