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

Abstract

Abstract 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 apparent activation energy (E a ) 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 σ‐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 as well as the increased viscosity of the reaction mixture.