Ionic-liquid-like copolymer stabilized nanocatalysts in ionic liquids: II. Rhodium-catalyzed hydrogenation of arenes
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 (), 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, , 3H, CH3), 1.41 (m, 2H, CH2), 1.95 (m, 2H, CH2), 4.41 (t, , 2H, CH2), 5.39 (dd,
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.
References (40)
- et al.
Catal. Today
(2002) - et al.
Appl. Catal. A Gen.
(2005) - et al.
Tetrahedron: Asymmetry
(1996) - et al.
Inorg. Chim. Acta
(1997) - et al.
J. Mol. Catal. A Chem.
(2004) - et al.
J. Mol. Catal. A Chem.
(1996) - et al.
J. Mol. Catal. A Chem.
(2003) - et al.
Angew. Chem. Int. Ed.
(2000) - et al.
Chem. Rev.
(2002) - et al.
Phys. Chem. Chem. Phys.
(2006)
Chem. Eur. J.
J. Am. Chem. Soc.
Inorg. Chem.
J. Phys. Chem. B
J. Am. Chem. Soc.
J. Am. Chem. Soc.
J. Am. Chem. Soc.
Catal. Rev. Sci. Eng.
J. Am. Chem. Soc.
J. Am. Chem. Soc.
Cited by (93)
Ionic liquid-assisted synthesis of 2-amino-3-cyano-4H-chromenes: A sustainable overview
2022, Journal of Heterocyclic ChemistryControllable production of guaiacols and phenols from lignin depolymerization using Pd/C catalyst cooperated with metal chloride
2018, Chemical Engineering JournalBiological valorization of low molecular weight lignin
2016, Biotechnology AdvancesMaltose hydrogenation over ruthenium nanoparticles impregnated in hypercrosslinked polystyrene
2015, Chemical Engineering JournalCitation Excerpt :However, the main problem of this type of catalysts is leaching of nickel and rapid catalyst deactivation. Hydrogenation over ruthenium supported on MgO, SiO2, Al2O3 and TiO2 gives increased maltose hydrolysis rates and excessive sorbitol formation, therefore selectivity decreased to 20–80% [6,9]. Ru supported on different kinds of carbon shows high activity in maltose hydrogenation with the selectivity up to 96–97% [3].
Hydrogenation of arenes, nitroarenes, and alkenes catalyzed by rhodium nanoparticles supported on natural nanozeolite clinoptilolite
2015, Journal of Molecular Catalysis A: ChemicalCitation Excerpt :Many attempts have been made by researchers to solve these problems. Most of the published works have focused on increasing the efficiency of the utilization of Rh that showed high activity and selectivity [7–51]. Nanozeolite clinoptilolite (NZ-CP) is attractive in material supports due their versatile structural features such as ease of availability of the natural, inexpensive, high specific surface area, large pore volumes, good thermal and chemical stability and non-toxicity.