Hydrogen Generation from Hydrolysis and Methanolysis of Guanidinium Borohydride

Metal-catalyzed hydrolysis and methanolysis of guanidinium borohydride (C(NH2)3BH4 or GBH) for hydrogen generation are reported. GBH is comparatively stable in water with only 0.3 equiv of H2 liberated in 24 h at 25 °C while it reacts vigorously with methanol, releasing more than 3.2 equiv of H2 within only 17 min. Even at 0 °C, there was still nearly 2.0 equiv of H2 released after 2 h, but no H2 liberation was observed for hydrolysis under the same conditions. Various metal chlorides were adopted to enhance the reaction kinetics of the hydrolysis and methanolysis, of which CoCl2 exhibits the highest activity in both cases. With the addition of 2.0 mol % CoCl2 at 25 °C, the methanolysis of GBH could generate 4 equiv of H2 within 10 min with a maximum hydrogen generation rate of 9961.5 mL·min–1·g–1 while only 1.8 equiv of H2 was obtained under the same conditions at a maximum hydrogen generation rate of 692.3 mL·min–1·g–1 for hydrolysis. Compared with hydrolysis, methanolysis of GBH possesses much faster reaction kinetics, rendering it an advantage for hydrogen generation, especially at subzero areas. It was proposed that the faster reaction kinetics of methanolysis of BH4– containing compounds is ascribed to the more electron donating methoxy group than that of hydroxyl group. Moreover, a comparison between hydrolysis and methanolysis of GBH indicates that the loss of the first H from BH4– controls the hydrolysis kinetics instead of the cleavage of the O–H bond.