Hydrogen Generation From Catalytic Hydrolysis Of Ammonia Borane With N-Pyridylpyrazole Iridium Complexes

With the rapid development of economy of all the countries in the world, energy demand is increasing significantly. Further considering the gradual depletion of three traditional fossil fuels, development of alternative energy is highly desirable. Hydrogen is the the most promising new energy because it has high combustion heat and release no pollutant. Efficient hydrogen storage limited the development of hydrogen energy. Ammonia borane (AB) is one the most important hydrogen storage materials due to its many favorable properties. It has high hydrogen gravimetric capacity of 19.6 wt%, and it is non-toxic, thermally and chemically stable. Hydrogen generation from catalytic hydrolysis of AB become a hot research topic because of it characteristics of moderate hydrogen release temperature, releasing a large quantity hydrogen and environment-friendly etc. Heterogeneous catalysts are suffering from high catalyst loading, low hydrogen generation rate, and releasing toxic by-product etc. In contrast, homogeneous catalyst has great advantages in catalytic activity and selectivity which is due to the well distributed catalytic centers. However, research of homogeneously catalytic hydrolysis of AB is barely reported. The main reason is the lack of stable and efficient water-soluble catalysts. Therefore, development of highly efficient and selective and water-soluble homogeneous catalyst is important in the view point of academics and industry.We design and synthesize four kinds of water-soluble iridium complexes with N-pyridylpyrazole bidentate ligands, which were used in hydrogen generation from catalytic hydrolysis of AB. First, we synthesized N-pyridylpyrazole bidentate ligands bearing substituents of Me, MeO, and HO. Four kinds of water-soluble catalysts were synthesized by the reaction of these ligands and [Cp*Ir(H2O)3]SO4 (Cp*=C5Mes). Subsequently, we use these catalysts for the hydrogen generation in water. [Cp*Ir(6-OH-py-pz)(H2O)]SO4 was used to optimize of reaction conditions, including reaction temperature, solution pH, substrate concentration, and catalyst loading. Under the optimal reaction conditions, we examined the activity of the four catalysts. [Cp*Ir(6-OH-py-pz)(H2O)]SO4 exhibited the best catalytic performance and gave an average turnover frequency (TOF) of 360 h-1 during the reaction. AB was completely hydrolyzed after 1.5 hours. In 0.3 M AB, the turnover number (TON) reached 1622 after 9 hours with releasing of 3 equivalents of hydrogen. However, [Cp Ir(6-Me-py-pz)(H2O)]SO4 showed a lower catalytic activity and slowly release hydrogen at 0.25 h after the catalyst was added. [Cp*Ir(4-Me-py-pz)(H2O)]SO4 released hydrogen immediately after addition of catalyst and showed a higher catalytic activity than [Cp*Ir(6-Me-py-pz)(H2O)]SO4. It suggests that the steric effect of ortho-substituted methyl inhibit the catalytic reaction considerably. [Cp*Ir(6-MeO-py-pz)(H2O)]SO4 showed the lowest activity and gave a conversion of 50%after 31 h.Based on the literatures and our experimental results, we proposed a mechanism for hydrolysis of AB catalyzed by [Cp*Ir(6-OH-py-pz)(H2O)]SO4. The hydroxyl group imparted the complex a better water solubility and thus improved the activity. More importantly, under basic conditions, the generated oxygen anion and metal center form hydrogen bonds respectively with N-H and B-H of AB. This synergistic effect promotes the dehydrogenation of AB, and consequently enhances the activity of the catalyst remarkably.