Efficient Hydrolytic Hydrogen Evolution from Sodium Borohydride Catalyzed by Polymer Immobilized Ionic Liquid‐Stabilized Platinum Nanoparticles
Simon Doherty, Julian G. Knight, Hussam Y. Alharbi, Reece Paterson, Corinne Wills, Casey Dixon, Lidija Šiller, Thomas W. Chamberlain, Anthony Griffiths, Sean M. Collins, Ke‐Jun Wu, M. D. Simmons, Richard A. Bourne, Kevin R. J. Lovelock, Jake M. Seymour
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.