Hydrogen Generation From Hydrolysis Of Ammonia Borane Catalyzed By Water-soluble Metal Complexes

As a hydrogen storage material,ammonia borane(NH3BH3,AB)has a number of advantages: high hydrogen content(19.6 wt%),easy to store and transport,moderate thermal stability,releasing pure hydrogen.The hydrogen production from ammonia borane hydrolysis has many merits,such as high content of hydrogen release,fast reaction rate,low reaction temperature,convenient for storage and transportation.Therefore it is considerably studied by many researchers in recent years.A thorough literatures review suggests that most of the hydrogen releasing systems need to add tetrahydrofuran,isopropanol,and other organic solvents to increase the solubility of the catalyst.These systems can not release 3 equivalents hydrogen completely,and the reaction mechanism is not clear.In order to solve the problems existing in these systems,a series of metal complexes with good water solubility were designed and synthesized.The effect of reaction temperature,p H value,substrate concentration and catalyst loading on the catalytic performance was investigated.The reaction mechanism of the catalytic reaction was investigated by kinetic isotope effect(KIE).The main research contents include:Synthesis of two kinds of catalysts: substituted pyrimidinyl pyrazole catalysts and bipyridine and bipyrimidine catalyst.(1)The effects of temperature on the catalytic activity were investigated using catalyst 1 and catalyst 11,respectively.The catalytic activity of catalyst 1 at 50-90 °C was studied.When the reaction temperature was 90 °C,3 equivalents of hydrogen could be completely released within 1 h and the TOF could reach 1240 h-1.The catalytic activity of catalyst 11 at 65-85 °C was studied,and the TOF was 14090 h-1 at 80 °C.Considering the different catalytic activity of the two kinds of catalysts at different temperature,we chose 80 °C as the optimized temperature.(2)The effect of p H on the catalytic activity was investigated using catalyst 1,and the catalytic activity of the complex increased first and then decreased with the increase of p H value.At p H 9,the highest catalytic activity was observed.Therefore,p H = 9 is chosen as the optimum p H value.(3)Under the optimal temperature(T = 80 °C)and p H(p H = 9),influence of the concentration of ammonia borane(0.05-0.30 mol/L)on content of hydrogen released and rate of hydrogen release were studied with catalyst 8 and 11.It was found that when the concentration of ammonia borane was 0.1 mol/L,the hydrogen releasing rate was moderate and the TOF values were 3550 h-1 and 14090 h-1,respectively.Therefore,the optimal concentration of substrate was determined as 0.10 mol/L.(4)Under the optimal temperature(T = 80 °C)and p H(p H = 9),the hydrogen releasing rate was investigated with different catalyst loading(0.5-2.0?mol)by using catalyst 10 and 11.It was found that when the catalyst loading was 1 ?mol,3 equivalents of hydrogen released from 1 mmol ammonia borane with high TOF values(3702 h-1 and 14154 h-1).Taking the cost factor into account,we chose 1.0 ?mol as the optimal catalyst loading.(5)The activation energy of each system was obtained by optimizing the reaction temperature with catalyst 1 and catalyst 11.The reaction mechanism of ammonia borane hydrolysis was discussed and verified by kinetic isotope effect(KIE).