Kinetic features of the platinum catalyzed hydrolysis of sodium borohydride from B-11 NMR measurements

A full kinetic analysis of the platinum catalyzed hydrolysis of sodium borohydride (NaBH4) in alkaline media has been performed using 11B NMR (nuclear magnetic resonance) spectroscopy with a Pt/C 5 wt % commercial powder as catalyst. By fitting the NMR data by least-square regression techniques, the rate constants of platinum catalyzed borohydride hydrolysis have been evaluated. Within the investigated [borohydride]/[catalyst] molar ratio of 200-1500, the rate law has been found to be of first-order in catalyst and zero order in borohydride. Whereas no reagent-isotope kinetic effect is observed in the NaBD4/H2O reaction system, the hydrolysis of NaBH4 in deuterated water shows a significant solvent-kinetic isotope effect. In both cases, however, 11B NMR analysis indicates that the main reaction product is the tetrahydroxyborate species (D)nH4-nBO4 - (n ) 1, 2, 3, 4) followed by a minor amount of the partially scrambled BH3D- species while the hydrolysis-intermediates HnB(OD)4-n - (n ) 1, 2, 3) species are not detectable during all the reaction time. These results suggest that, differently from the Pd catalyzed/borohydride(borodeuteride) hydrolytic process, the hydrogen/deuterium exchange is slower than hydrolysis and that the rate-determining step of the overall process is the formation of the monohydroxy-borohydride intermediate BH3OH-. The activation energy of the overall process has been also evaluated by 11B NMR rate measurements taken at different temperatures.