Computational Study Of Catalyzed Dehydrogenation Of Ammonia Borane For Chemical Hydrogen Storage

Ammonia borane (NH3BH3, AB) has emerged as one of the most compelling candidates for hydrogen storage, owing to its large hydrogen content (19.6wt%) and high thermal stability. Searching for efficient transition metal catalysts for H2release is still critical in dehydrogenation of AB. In this thesis, the mechanism of catalytic dehydrogenation of ammonia borane (NH3BH3, AB) with the metal catalysts has been investigated by the density functional theory (DFT). The main contents of this thesis contain four parts as follows:1. The mechanism of catalytic dehydrogenation of ammonia borane (NH3BH3, AB) catalysed by Ni N-heterocyclic carbene (NHC) complexes has been investigated with the B3LYP/MIXED. Two possible mechanisms of the dehydrogenation of NH3BH3have been theoretically studied:intramolecular reaction at Ni dicarbene and intermolecular reaction at Ni monocarbene and dicarbene. The facile occurrence of the dehydrogenation of AB was demonstrated by the low activation barriers of the rate-determining steps. It was found that the intramolecular pathway is mostly kinetically favorable. In that pathway the lowest activation barrier of the first step is15.51kcal/mol and the activation barrier of rate-determining steps is22.57kcal/mol lower than the intermolecular pathway. Moreover, for intermolecular dehydrogenation of AB, the activation prefers to take place at monocarbene Ni(NHC) catalyst.2. The catalytic dehydrogenation of ammonia borane (NH3BH3) with the Ir pincer complex,[Ir(ItBu’)2]+, has been investigated with the B3LYP/LACVP+**. A mechanism has been proposed for dehydrogenation process involving three possible pathways: stepwise B-H/N-H activation, concerted B-H/N-H activation and proton transfer mechanism. It was found that the reaction barriers of the rate-determining steps for the dehydrogenation of AB catalyzed by [Ir(ItBu’)2]+, are predicted to be38.28kcal/mol (stepwise B-H/N-H activation),33.58kcal/mol (concerted B-H/N-H activation), and28.38kcal/mol (proton transfer pathway), respectively. Thus, the proton transfer pathway is kinetically most favorable. This process differs from the dehydrogenation of AB by (POCOPMe)Ir(H)2, which involves a concerted B-H/N-H bond activation mechanism.3. With the B3LYP method the mechanism of catalytic dehydrogenation of ammonia borane catalysed by [Pd(NHC)(PCy3)] has been studied. It was found that the dehydrogenation processes go through two possible pathways:stepwise N-H/B-H activation and stepwise B-H/N-H activation. The large energy (AG≠) of the the dehydrogenation of AB with [Pd(NHC)(PCy3)] are predicted to be53.78kcal/mol and55.37kcal/mol, respectively. Therefore, both pathways are competitive reactions. The first step of the two pathways is with the high reaction barriers, and then the second step occurs easily with the low barriers.