Abstract
AbstractPlatinum nanoparticles stabilized by imidazolium‐based phosphine‐decorated Polymer Immobilized Ionic Liquids (PPh2‐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@PPh2‐PEGPIILS) more active than its N‐alkylated counterpart (PtNP@PPh2‐N‐decylPIILS). The maximum initial TOF of 169 moleH2.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 (Ea) of 23.9 kJ mol−1 for the hydrolysis of NaBH4 catalyzed by PtNP@PPh2‐PEGPIILS is significantly lower than that of 35.6 kJ mol−1 for PtNP@PPh2‐N‐decylPIILS. Primary kinetic isotope effects kH/kD of 1.8 and 2.1 obtained with PtNP@PPh2‐PEGPIILS and PtNP@PPh2‐N‐decylPIILS, respectively, for the hydrolysis with D2O 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@PPh2‐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.
Highlights
There is currently an urgent need to reduce our reliance on fossil fuels as this resource is finite and dwindling[1] and, the resulting emissions have already increased the atmosphere CO2 concentration from the pre-industrial level of 250 ppm to theWhile hydrogen release from NaBH4 has been catalyzed by homogeneous systems,[7] the use of supported metal nanoparticles has recently received increasing attention because their size, surface area, morphology as well as catalyst-support interactions can be tuned to control their efficacy.[8]
As the hydrolysis of sodium borohydride occurs under ambient conditions, hydrogen evolution from a solution of sodium borohydride was measured as a function of time at various temperatures ranging from 21 to 40 °C and all the data obtained during this project was corrected by subtracting the background hydrogen gas generated in the absence of catalyst under the appropriate conditions
Polymer immobilized ionic liquid stabilized PtNPs catalyze the hydrolytic evolution of hydrogen from sodium borohydride with remarkable efficiency and catalyst stabilized by PEG-modified imidazolium-based polymer (PtNP@PPh2-PEGPIILS) is markedly more active than its N-decyl counterpart (PtNP@PPh2-NdecylPIILS)
Summary
There is currently an urgent need to reduce our reliance on fossil fuels as this resource is finite and dwindling[1] and, the resulting emissions have already increased the atmosphere CO2 concentration from the pre-industrial level of 250 ppm to the. While hydrogen release from NaBH4 has been catalyzed by homogeneous systems,[7] the use of supported metal nanoparticles has recently received increasing attention because their size, surface area, morphology as well as catalyst-support. We have recently initiated a program to develop heteroatom donor-decorated PIIL-stabilized nanoparticles on the basis that covalent attachment of the ionic liquid (IL) to the polymer would combine the favorable properties of an IL such as weak electrostatic stabilization of NPs[17] with the advantages associated with attachment to a solid support including facile product separation, catalyst recovery and recycling.[18]. Heteroatom donors (HADs) were initially incorporated into the polymer to supplement the stabilization of nanoparticle by the ionic liquid, there is evidence that these donors may well enable the surface electronic structure to be modified to improve catalyst performance and/or the size and morphology of the nanoparticles to be controlled through metalheteroatom donor interactions.[19]. We compare the efficiency of a PEG-imidazolium and an N-alkyl-imidazolium-based phosphinemodified polymer immobilized ionic liquid stabilized PtNPs as catalysts for the hydrolytic generation of hydrogen from NaBH4 kinetic studies to explore the mechanism and details of a tandem reaction that used the hydrogen generated from NaBH4 in D2O for the hydrogenation of 1,1-diphenylethene; eight isotopologues were identified, assigned and quantified by NMR spectroscopy confirming that H-D scrambling was rapid and that -hydride elimination-reinsertion occurred during the hydrogenation
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