Due to concerns about sustainability issues, we have been interested in the synthesis of water-soluble MNPs for their use as catalysts in water or under biphasic liquid–liquid conditions. For this purpose, we took inspiration from organometallic catalytic systems in water and considered common ligands employed to stabilize complexes such as 1,3,5-triaza-7-phosphaa- damantane (PTA) and sulfonated diphosphines.
4.3.1.2.1 PTA-stabilized Pt and Ru Nanoparticles. The synthesis of PTA- stabilized Ru and PtNPs was carried out by decomposition of [Ru(COD) (COT)] and [Pt(CH3)2(COD)] precursors in the presence of 0.8 equiv. of PTA (P(H2)¼ 3 bar; THF; 70 1C) (see Figure 4.4).44The resulting NPs displayed a spherical shape and low size dispersity. They were purified by washing Figure 4.3 Synthesis of Ru/NHC NPs and their catalytic performance in hydrogen- ation of ortho-methylanisole (0.5 M) in different solvents (Ru/IPr0.2 (0.3% Ru); 40 bar H2; 298 K : pentane (black), THF (red), methanol (blue), no-solvent (green, 0.1% Ru).43
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Figure 4.4 Synthesis of PTA-stabilized NPs and TEM images in THF and in water for Ru NPs (left) and Pt NPs (right).44 Organometallic Approach for the Synthesis o f Noble Metal Nanoparticles 53 23/06/2014 08:14:50.
with pentane and filtration, and were further dissolved in water without any change in dispersion and in their mean diameter, with a value ofB1.4 nm andB1.1 nm for the RuNPs and the PtNPs, respectively. Aqueous sus- pensions of these NPs were stable for weeks when kept under an argon at- mosphere. 1H, 13C, 31P solution and solid-state NMR studies showed the strong coordination of PTA at the surface of the particles by the phosphorous atom.
Biphasic liquid–liquid hydrogenation was investigated with model olefins and aromatic substrates using aqueous colloidal solutions of Ru/PTA and Pt/PTA NPs as catalysts, with Roucoux et al. (see Table 4.1).45 Octene and dodecene were totally converted into the corresponding alkanes (r.t.; 1 bar H2), with moderate activities. An increase in the hydrogen pressure (P(H2)¼ 10 bar) was not detrimental for the colloidal suspension stability. Complete hydro- genation of toluene into cyclohexane was observed overnight with Ru/PTA NPs whereas 60% of m-methoxymethylcyclohexane was formed from methox- ymethylanisole. In comparison, the reduction with Pt/PTA NPs was achieved after 16 h even if very low conversions were obtained after 2 h with 8% and 15% cyclohexyl derivatives, respectively. In summary, these PTA-stabilized Ru and Pt NPs were active in the hydrogenation of olefinic and aromatic compounds under mild conditions despite the change of environment that they under- went after their dissolution into water.
4.3.1.2.2 Sulfonated Diphosphine-stabilized Ru Nanoparticles. By apply- ing the same procedure as previously described with the PTA ligand (see Figure 4.5), we used sulfonated diphosphines to stabilize RuNPs.46In this collaborative work with Roucoux et al. and Monflier et al., the RuNPs Table 4.1 Hydrogenation of olefins and aromatic derivatives with aqueous Ru/PTA
and Pt/PTA colloidal solutions.a
Catalyst Substrate P H2(bar) Time (h) Conversion (%)f
Pt/PTAb Octene 1 2 100 Pt/PTAb Octene 1 2 68g Pt/PTAc Toluene 10 2 8 Pt/PTAc Toluene 10 16 100 Pt/PTAc m-Methylanisole 10 2 15 Pt/PTAc m-Methylanisole 10 16 100 Ru/PTAd Dodecene 1 5 100h Ru/PTAd Octene 10 1 100 Ru/PTAe Toluene 10 16 100 Ru/PTAe m-Methylanisole 10 16 60
aReaction conditions: [substrate]–[metal]
¼ 100, T ¼ 201C stirred at 1500 mn1.
b
Pt (0.0119 mmoL); substrate (1.19 mmoL).
cPt (0.0122 mmoL); substrate (1.22 mmoL). d
Ru (0.0159 mmoL); substrate (1.59 mmoL).
e
Reaction conditions: Ru (0.020 mmoL); substrate (2 mmoL).
f
Substrate conversion determined by gas chromatography analysis.
g
Recycling of the aqueous suspension of entry 1.
hDetermined by1H and13C NMR.
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Figure 4.5 Synthesis and TEM images of sulfonated diphosphine-stabilized RuNPs in water with (a) dppb-TS (b) dppp-TS, and (c) dppe- TS with [L]–[Ru]¼ 0.1.46 Organometallic Approach for the Synthesis o f Noble Metal Nanoparticles 55 23/06/2014 08:14:50.
were prepared from [Ru(COD)(COT)] and the diphosphines in THF (3 bar H2; r.t.). Different diphosphines, (1,4-bis[(di-m-sulfonatophenyl)phosphine]- butane¼ dppb-TS, 1,4-bis[(di-m-sulfonatophenyl)phosphine]propane¼ dppp-TS, 1,4-bis[(di-m-sulfonatophenyl)phosphine]ethane¼ dppe-TS), and ligand–Ru ratios were employed in order to analyze the effect of the backbone and the diphosphine concentration on the stability and the size of the NPs and thus on their catalytic properties. Depending on the ligand amount, well-crystallized RuNPs in a mean size range of 1.2–1.5 nm were formed. The coordination of the sulfonated diphosphines at the surface of the RuNPs allowed their further dispersion in water giving rise to very homogeneous and stable aqueous colloidal solutions (up to several months) without any change in their mean sizes.
The catalytic behaviour of these aqueous colloidal solutions showed promising results in terms of reactivity when tested for the hydrogenation of unsaturated substrates (tetradecene, styrene and acetophenone) in biphasic liquid–liquid conditions. Interestingly, minor structural differences in the diphosphine ligands, such as the alkyl chain length, influenced the catalytic activity in styrene hydrogenation significantly, in addition to the positive effect of an increase in temperature (from 20 1C to 50 1C) or pressure (from 1 to 10 bar H2) (see Table 4.2). As the [ligand]–[Ru] ratio increased, conversion and selectivity (expressed as ethylbenzene (EB)–ethylcyclohexane (EC) ratio) Table 4.2 Hydrogenation of styrene Ru/sulfonated diphosphine aqueous colloidal
solutions.a Ru NPs H2 EC + EB ST Nanocatalysts T (1C) Time (h) Product selectivity (%) ST EB EC Ru/dppb-TS 20 1 25 75 0 20 20 0 45 55 20 40 0 3 97 Ru/dppp-TS 20 1 59 40 1 20 20 0 47 53 20 40 0 2 98 Ru/dppe-TS 20 1 75 24 1 20 20 0 41 59 20 40 0 1 99 Ru/dppb-TS 50 1 10 90 0 50 2 0 82 18 50 3 0 0 100
aReaction conditions: [ligand]–[Ru]
¼ 0.1; ruthenium (3.9 105mol), styrene (3.9 103mol), 1 bar H2, water (10 mL).
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also increased at short reaction times. The best results were obtained with the dppb-TS ligand with the longest alkyl chain, giving rise to 75% EB after 1 h compared to 40% EB and 20% EB with dppp-TS and dppe-TS, respect- ively. As all the RuNPs display similar mean sizes, the observed differences in their catalytic properties were correlated to the difference in the flexibility of the alkyl chain. Due to having the highest number of carbon atoms, the dppb-TS ligand had the highest flexibity and therefore favoured a better diffusion of the substrate towards the metal surface. Although the mean size (1.25 nm) of the RuNPs did not alter with the increase of [dppe-TS]–[Ru] from 0.2 to 0.5, the variation in selectivity was explained by a limited access of the aromatic substrate to the NP surface due to increased steric hindrance when more ligands were coordinated. A molar ratio of 0.1 appeared to be a good compromise between the stabilization and the catalytic activity of the NPs. Finally, as also observed for Ru/PTA NP systems, preliminary results of re- cycling were encouraging for the recovery of these water-soluble Ru/sulfo- nated diphosphine nanocatalysts.