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

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 À 1 .min À 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 À 1 for the hydrolysis of NaBH 4 catalyzed by PtNP@PPh 2 -PEGPIILS is significantly lower than that of 35.6 kJ mol À 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 OÀ 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.

Highly efficient and selective partial reduction of nitroarenes to N-arylhydroxylamines catalysed by phosphine oxide-decorated polymer immobilized ionic liquid stabilized ruthenium nanoparticles

“…We have recently incorporated heteroatom donors (HAD) into polymer immobilised ionic liquids (PIILs) as supports to stabilise nanoparticles, reasoning that the covalently attached ionic liquid would provide stabilisation through weak electrostatic interactions to the surface of the NP while the heteroatom donor would coordinate to the surface metal atoms and provide additional stabilization to prevent aggregation under the conditions of catalysis [96][97][98][99][100][101][102][103][104]. In addition, the reports described above suggest that modification of immobilized ionic liquids with a heteroatom donor could also enable the surface electronic structure and steric environment to be modified and/or the size and morphology of the NP to be controlled and, as such, HAD-modified polymer immobilised ionic liquids may well be versatile supports for tuning and optimising catalyst performance 105].…”

“…Our initial endeavours in this area have been extremely promising as PdNPs stabilised by a polyethylene glycol-modified phosphinedecorated PIIL is a remarkably active catalyst for aqueous phase Suzuki-Miyaura cross-couplings [98], the selective hydrogenation of a,b-unsaturated ketones in water [99] and the aqueous phase hydrogenation and transfer hydrogenation of nitroarenes [100], while gold nanoparticles stabilized by a phosphine oxidedecorated polymer immobilised ionic liquid is a highly efficient and selective catalyst for the partial reduction of nitrobenzene to either N-phenylhydroxylamine or azoxybenzene and complete reduction to aniline [101]. Our most recent endeavours in this area have revealed that ruthenium nanoparticles stabilized by the same polymer is a remarkable efficient catalyst for the hydrogenation of aryl and heteroaryl ketones to the corresponding alcohol and levulinic acid to c-valerolactone [102] as well as the hydrolytic evolution of hydrogen from sodium borohydride [103][104]. Herein, we report that RuNP stabilised by a phosphine oxide-decorated polymer immobilised ionic liquid catalyses the partial reduction of a wide range of nitroarenes in ethanol under mild condition and with remarkable efficiency to afford high yields of the corresponding N-arylhydroxylamine with exceptional selectivity (>99 %); in stark contrast, the corresponding AuNP-based system catalyses the sodium borohydride-mediated reduction of nitrobenzene in ethanol to give quantitative conversion to azoxybenzene as the sole product (Scheme 1).…”

Membrane Nanofiber-Supported Cobalt–Nickel Nanoparticles as an Effective and Durable Catalyst for H2 Evolution via Sodium Borohydride Hydrolysis

“…In this respect, significant increases in the rate of NaBH 4 hydrolysis have been accomplished for a variety of metallic NPs [ 68 , 69 ]. However, recent research has revealed that NaOH has a deleterious effect on Pt and Pd NPs [ 70 , 71 ]. These nanoparticles have a very electron-rich structure that allows them to readily help in the oxidative addition of an O–H bond to a surface-bound activated H-bonded adduct [BH 3 H − ]–H–OH.…”

Heteroatom Modified Polymer Immobilized Ionic Liquid Stabilized Ruthenium Nanoparticles: Efficient Catalysts for the Hydrolytic Evolution of Hydrogen from Sodium Borohydride*

“…While support-grafted ionic liquids have been used to stabilise catalysts for a wide range of transformations, there appear to be only two reports of their use to support nanoparticle catalysts for the hydrolytic evolution of hydrogen from hydrogen-rich boron derivatives, which is somewhat surprising as polymer immobilised ionic liquids are functional and tuneable supports for molecular and nanoparticle catalysts. An imidazolium-based organic polymer has recently been used to prepare highly dispersed ultrafine AuPd alloy NPs for the hydrolytic release of hydrogen from ammonia borane which outperformed both its monometallic counterparts [124] and we have recently reported that phosphine decorated polymer immobilized ionic liquid stabilized PtNPs are highly efficient catalysts for the hydrolytic generation of hydrogen from NaBH 4 [125]. This study has now been extended to investigate the efficacy of phosphine oxide and amine-decorated polymer immobilised ionic liquid stabilised RuNPs as catalysts for the hydrolysis of NaBH 4 on the basis that the heteroatom donor could disrupt the key hydrogen-bonded surface-coordinated ensemble between the acidic hydrogen of water and the hydridic hydrogen of borohydride and thereby influence catalyst performance.…”