Efficient Hydrolytic Hydrogen Evolution from Sodium Borohydride Catalyzed by Polymer Immobilized Ionic Liquid-Stabilized Platinum Nanoparticles

Platinum nanoparticles stabilized by imidazolium-based phosphine-decorated Polymer Immobilized Ionic Liquids (PPh 2 -PIIL) catalyze the hydrolytic evolution of hydrogen from sodium borohydride with remarkable efficiency, under mild conditions. The composition of the polymer influences efficiency with the catalyst based on a polyethylene glycol modified imidazolium monomer (PtNP@PPh 2 -PEGPIILS) more active than its N -alkylated counterpart (PtNP@PPh 2 - N -decylPIILS). The maximum initial TOF of 169 moleH 2 .molcat -1 .min -1 obtained at 30 °C with a catalyst loading of 0.08 mol% is among the highest to be reported for the aqueous phase hydrolysis of sodium borohydride catalyzed by a PtNP-based system. Kinetic studies revealed that the activation energy (Ea) of 23.9 kJ mol -1 for the hydrolysis of NaBH 4 catalyzed by PtNP@PPh 2 -PEGPIILS is significantly lower than that of 35.6 kJ mol -1 for PtNP@PPh 2 - N -decylPIILS. Primary kinetic isotope effects k H / k D of 1.8 and 2.1 obtained with PtNP@PPh 2 -PEGPIILS and PtNP@PPh 2 -N-decylPIILS, respectively, for the hydrolysis with D 2 O support a mechanism involving rate determining oxidative addition or sigma-bond metathesis of the O-H bond. Catalyst stability and reuse studies showed that PtNP@PPh 2 -PEGPIILS retained 70% of its activity across five runs; the gradual drop in conversion appears to be due to poisoning of the catalyst by the accumulated metaborate product.