Heteroatom modified polymer immobilized ionic liquid stabilized ruthenium nanoparticles: efficient catalysts for the hydrolytic evolution of hydrogen from sodium borohydride
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Type
Journal Article
Abstract
Ruthenium nanoparticles stabilised by polymer immobilized ionic liquids catalyse the hydrolytic release of hydrogen from sodium borohydride. The composition of the polymer influences performance and ruthenium nanoparticles stabilised by an amine-decorated imidazolium-based polymer immobilised ionic liquid (RuNP@NH<inf>2</inf>-PIILS) was the most efficient with a maximum initial turnover frequency (TOF) of 177 mole<inf>H2</inf>.mol<inf>Ru</inf><sup>−1</sup>.min<sup>−1</sup>, obtained at 30°C with a catalyst loading of 0.08 mol%; markedly higher than that of 69 mol<inf>H2</inf>.mol<inf>Ru</inf><sup>−1</sup>.min<sup>−1</sup> obtained with 5 wt% Ru/C and one of the highest to be reported for a RuNP catalyst. The apparent activation energy (Ea) of 38.9 kJ mol<sup>−1</sup> for the hydrolysis of NaBH<inf>4</inf> catalysed by RuNP@NH<inf>2</inf>-PIILS is lower than that for the other polymer immobilized ionic liquid stabilised RuNPs, which is consistent with its efficacy. Comparison of the initial rates of hydrolysis in H<inf>2</inf>O and D<inf>2</inf>O catalysed by RuNP@NH<inf>2</inf>-PIILS gave a primary kinetic isotope effect (k<inf>H</inf>/k<inf>D</inf>) of 2.3 which supports a mechanism involving rate limiting oxidative addition of one of the O-H bonds in a strongly hydrogen-bonded surface-coordinated [BH<inf>3</inf>H<sup>−</sup>]—-H<inf>2</inf>O ensemble. The involvement of a surface-coordinated borohydride is further supported by an inverse kinetic isotope effect of 0.65 obtained from a comparison of the initial rates for the hydrolysis of NaBH<inf>4</inf> and NaBD<inf>4</inf> under the conditions of catalysis i.e., at a high hydride/catalyst mole ratio. Interestingly though, when the comparison of the initial rates of hydrolysis of NaBH<inf>4</inf> and NaBD<inf>4</inf> was conducted in dilute solution with a hydride/catalyst mole ratio of 1 a kinetic isotope effect (k<inf>H</inf>/k<inf>D</inf>) of 2.72 was obtained; this would be more consistent with concerted activation of both an O-H and B-H bond in the rate limiting step, possibly via a concerted oxidative addition-hydride transfer in the surface-coordinated hydrogen-bonded ensemble. Catalyst stability and reuse studies showed that RuNP@NH<inf>2</inf>-PIILS retained 71% of its activity over five runs; the gradual drop in the initial TOF with run number appears to be due to passivation of the catalyst by the sodium borate by-product as well as an increase in viscosity of the reaction mixture rather than leaching of the catalyst.
Date Issued
2022-08-01
Date Acceptance
2022-06-24
Citation
Molecular Catalysis, 2022, 528
ISSN
2468-8231
Publisher
Elsevier
Journal / Book Title
Molecular Catalysis
Volume
528
Copyright Statement
© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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Publication Status
Published
Article Number
112476
Date Publish Online
2022-07-01