Elsevier

Journal of Catalysis

Volume 250, Issue 1, 15 August 2007, Pages 33-40
Journal of Catalysis

Ionic-liquid-like copolymer stabilized nanocatalysts in ionic liquids: II. Rhodium-catalyzed hydrogenation of arenes

https://doi.org/10.1016/j.jcat.2007.05.014Get rights and content

Abstract

Rhodium nanoparticles stabilized by the ionic-liquid-like copolymer poly[(N-vinyl-2-pyrrolidone)-co-(1-vinyl-3-butylimidazolium chloride)] were used to catalyze the hydrogenation of benzene and other arenes in ILs. The nanoparticle catalysts can endure forcing conditions (75 °C, 40 bar H2), resulting in high reaction rates and high conversions compared with other nanoparticles that operate in ILs. The hydrogenation of benzene attained record total turnovers of 20,000, and the products were easily separated without being contaminated by the catalysts. Other substrates, including alkyl-substituted arenes, phenol, 4-n-propylphenol, 4-methoxylphenol, and phenyl-methanol, were studied and in most cases were found to afford partially hydrogenated products in addition to cyclohexanes. In-depth investigations on reaction optimization, including characterization of copolymers, transmission electron microscopy, and an infrared spectroscopic study of nanocatalysts, were also undertaken.

Introduction

Soluble nanoparticle catalysts with unique properties have aroused increasing interest in recent years due to their potentially high catalytic efficiency [1], [2]. Unlike their counterparts restricted on solid surfaces, soluble nanoparticles with their rotational freedom and spherically symmetrical geometry are, at least in principle, more active [3].

Ionic liquids (ILs) provide the opportunity to combine the advantages of both homogeneous and heterogeneous processes in a single system [3], [4]. Immobilization of nanoparticles by “supporting” them in an IL rather than on a surface preserves the rotationally free catalytic centers, in keeping with soluble nanoparticle systems. Moreover, the low interfacial tension and structural organization of ILs are believed to be effective for preparing small nanoparticles and for controlling extended ordering of nanoscale structures [5]. In addition, evidence has been found relating to the fact that imidazolium-based ILs are good stabilizers for transition-metal nanoparticles [6].

Generally, the most pertinent problem encountered in soluble nanoparticle applications is the need to stabilize the particles against aggregation or agglomeration, which tends to be done by adding stabilizers. Examples of stabilizers used in conjunction with catalytically active soluble nanoparticles include solvents [7], [8], soluble polymers [9], quaternary ammonium salts [10], and polyoxoanions [11]. It must be kept in mind that very stable nanoparticles are not the ultimate aim in catalysis, because they are generally less active, due to “overprotection” of the catalytically active nanoparticle surface. In our previous communication, rhodium nanoparticles protected by “IL-like” copolymers were shown to catalyze benzene hydrogenation with a record of total turnover (TTO) of 20,000 [12], demonstrating that an optimum balance between stability and reactivity to be a critical parameter.

The hydrogenation of benzene and arenes, which is accomplished using heterogeneous catalysts almost exclusively [13], [14], molecular precatalysts immobilized on heterogeneous surfaces [15], [16], [17], supported nanoparticle [18], or soluble nanoparticle [19], [20] systems, sometimes inadvertently derived from molecular precursors [21], [22], represents an important industrial catalytic transformation, particularly for the production of cleaner-burning, low-aromatic diesel fuels [13]. The partial hydrogenation of arenas, which is far more difficult to realize, is equally important because it can help to simplify many multistage synthetic procedures and allow the use of alternative precursors [23]. The catalytic procedure for partial hydrogenation of arenes generally is not simple; for example, the selective hydrogenation of benzene to cyclohexene performed on an industrial scale involves a multiphase process using a ruthenium-based heterogeneous catalyst, which results in 90% conversion and 60% selectivity [24].

In an extension to our preliminary report on Rh nanoparticles that catalyze the hydrogenation of benzene in ILs with unprecedented lifetimes [12], we now describe their application to the hydrogenation of benzene and other arenes in greater detail. IL-like copolymers were evaluated in terms of composition and average molecular weight and compared with poly(N-vinyl-2-pyrrolidone) (PVP), a widely used protecting agent [9], [25]. The hydrogenation of arene substrates with various alkyl and other substituents was then investigated, and activities were correlated to nanoparticle structure. Moreover, we found that these rhodium nanoparticles are highly active catalysts for the partial hydrogenation of arene substrates in ILs, with very high selectivity toward the monoene in some cases.

Section snippets

Synthesis of 1-vinyl-3-butylimidazolium chloride ([VBIM]Cl)

1-Vinylimidazole (5.00 g, 53 mmol) and butyl chloride (18.50 g, 200 mmol) were mixed and stirred vigorously at 70 °C for 24 h. The residual mixture was cooled to 0 °C, and then the upper liquid layer was removed by decantation. The residue was then washed with ethyl acetate (3×30 ml), evaporated under vacuum, and dried to yield a pale-yellow solid [VBIM]Cl (7.74 g, 75%).

1H NMR δ (300 MHz, D2O) 0.98 (t, J=7.6, 3H, CH3), 1.41 (m, 2H, CH2), 1.95 (m, 2H, CH2), 4.41 (t, J=7.6, 2H, CH2), 5.39 (dd, J=

Hydrogenation of benzene and arene derivatives

As reported previously, the rhodium nanoparticles stabilized by an IL-like copolymer [poly(NVP-co-VBIMCl)] showed unprecedented lifetime and activity for benzene hydrogenation under relatively mild conditions with a TTO of 20,000 (from five recyclings of 4000 TTOs per batch) and TOFs exceeding 200 h−1 [12]. It was shown that the Rh0 nanoparticles could be used at least 5 times without deterioration in activity (i.e., quantitative conversion under the conditions used) for benzene hydrogenation,

Conclusion

Rhodium nanoparticles protected by IL-like copolymers are highly active catalysts for the hydrogenation of arenes. The nanoparticle catalysts protected by the IL-like copolymer [poly(NVP-co-VBIMCl)] immobilized in ILs can endure forcing reaction conditions, resulting in high reaction rates and high conversions. The solubility of the substrates in the reaction media and the steric/electronic properties of the substituents on the aromatic ring influence the rate of catalytic hydrogenation and the

Acknowledgements

This work was supported by the National Science Foundation of China (projects nos. 20533010 and 20473002). The authors thank Professor Zi-Chen Li and Dr. Yong-Quan Dong, Department of Polymer Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, for supplying the poly(NVP-co-VBIMCl) sample used in this study.

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