Zhifeng Wu and
Heyan Jiang*
Key Laboratory of Catalysis Science and Technology of Chongqing Education Commission, Chongqing Key Laboratory of Catalysis and Functional Organic Molecules, College of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, China. E-mail: orgjiang@163.com; Fax: +86-23-62769652
First published on 8th April 2015
Pd and Ru nanoparticles were synthesized in ionic liquid by using tri(m-sulfonyl)triphenyl phosphine 1-butyl-2,3-dimethyl-imidazolium salt ([BMMIM]3[tppt]) as a stabilizing agent. The well-dispersed Pd and Ru NPs with mean diameters of 2.4 nm and 1.7 nm were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It was demonstrated that [BMMIM]3[tppt] stabilized Pd and Ru NPs displayed high activity and excellent selectivity in the hydrogenation of functionalized olefins, aromatic nitro compounds and aromatic aldehydes. The Pd and Ru NPs showed better catalytic performance than corresponding commercially available Pd/C and Ru/C catalysts. The present catalytic system could be easily reused at least six times without significant decrease in activity and selectivity.
Ionic liquids which have good thermal stability, low saturated vapour pressure and variable physical and chemical properties are deemed to be ideal media for various transformations.14 In recent years, the number of reports on ionic liquids as solvents and/or stabilizers to prepare NPs catalysts in various reactions, such as hydrogenation,15 C–C coupling16 and oxidation,17 has been growing. However, traditional simple ionic liquids could not provide effective stabilization, especially in catalytic recycle and severe reaction conditions, NPs form visible bulk species, and activity decrease obviously.18–20 Functionalized ionic liquids which can provide stronger stability than simple ionic liquids are designed to act as efficient stabilizers for NPs.18 Recently, transition-metal NPs stabilized by N-containing functionalized ionic liquids showed satisfactory performance. Zhao et al.21 used ionic-liquid-like copolymer stabilized rhodium to catalyse the hydrogenation of benzene and other arenes, high reaction rates and conversions were obtained. Dyson et al.22 reported polyvinyl pyrrolidone stabilized rhodium nanoparticles were highly soluble in hydroxyl-functionalized ionic liquids, providing an effective and highly stable catalytic system for biphasic hydrogenation. While Dupont23 found functionalized ionic liquid containing nitrile groups stabilized Ru NPs displayed unusual selectivity towards the hydrogenation of nitrile-containing aromatic compounds.
In this work, Pd and Ru NPs stabilized by novel phosphine-functionalized ionic liquid (PFIL) with characteristic of phosphine ligand as the anion were originally utilized as efficient catalysts in the chemoselective hydrogenation of functionalized olefins, aromatic nitro compounds and aromatic aldehydes. The well-dispersed Pd and Ru NPs were characterized by TEM, XRD and XPS. The Pd and Ru NPs showed better catalytic performance than corresponding commercially available Pd/C and Ru/C catalysts.
The Pd NPs stabilized by [BMMIM]3[tppt] (Scheme 1) were prepared by hydrogenation reduction of Pd(OAc)2 in [BMMIM]PF6. A black powder could be isolated from the obtained Pd NPs by adding acetone and then centrifuging (5000 rpm, 5 min). Washed three times with acetone and dried under reduced pressure. The isolated black powder (Pd-1) was characterized by TEM, XRD and XPS methods.
TEM was used to confirm the formation of Pd particles and observe dispersal. TEM images and distribution histograms of nearly spherical Pd NPs were displayed in Fig. 1, the observation indicated that these particles were well dispersive with an average diameter of 2.4 nm in fresh Pd-1. Furthermore, the Pd NPs after six recycles of styrene hydrogenation were also well-dispersed with an average diameter of 2.76 nm. Size distribution histograms were obtained on a basis of the measurement of 300 particles.
The XRD pattern of Pd-1 (Fig. 2) confirmed that the presence of crystalline Pd(0). The most representative reflections of Pd(0) were indexed as face-centered cubic (fcc) structure. The Bragg reflections at 40.16°, 46.68° and 68.30°, corresponded to the indexed planes of the crystals of Pd(0) (111), (200) and (220).
The surface characteristics of Pd-1 were investigated by XPS (Fig. 3). Pd 3d5/2 and Pd 3d3/2 signals, with binding energies of 335.8 eV and 341.2 eV, were observed respectively. In addition, the main peak of Pd 3d5/2 shifted 0.8 eV higher in binding energy compared to the specimens of the giant clusters of Pd(0) (335.0 eV). The XPS observation was consistent with previous literature.26 We deduced there had a tendency that the electron clouds transferred from Pd(0) particles surface to [BMMIM]3[tppt].27 On the other hand, the Pd 3d spectrum indicated the presence of two chemical states of Pd at the nanoparticle surface with distinct binding energies; the main contribution was related to Pd(0) (Pd–Pd bonds, Pd5/2 at 335.8 eV) and the other corresponding to Pd–F bonds (Pd5/2 at 337.3 eV).28
In short, the results of TEM, XRD and XPS indicated that Pd(II) species were completely reduced to Pd(0) NPs by molecular hydrogenation.
The effect of [BMMIM]3[tppt] in catalyst is noteworthy. Due to the strong coordination capacity of the anion of [BMMIM]3[tppt], Pd NPs were well dispersed and highly stable in catalytic hydrogenation. On the basis of these results, it was deduced that the [P(C6H4-m-SO3)3−] ions formed a layer around the surface of the Pd NPs, leading to a sphere of negative charge, and then the cation of [BMMIM]3[tppt] became arranged as an outer layer for charge conservation (Fig. 4). Recently, several groups have demonstrated ionic liquids possess self-organized structures, which can create an external layer around the surface of the metal NPs to protect them from aggregation.29–31
Initially, we chose styrene as a model substrate to explore the catalytic performance of Pd-1. Fig. 5 showed the effect of the conversion versus time. The hydrogenation of styrene produced only ethylbenzene in the test. It was found that the conversion increased linearly with time, showing no induction period, proving that this catalyst did not convert into other catalytically active species. Furthermore, in order to ascertain the involvement of metallic Pd in the hydrogenation reactions, mercury-poisoning experiments were run as they could selectively poison metal nanoparticles, by forming an amalgam with mercury, to help distinguish between homogeneous and heterogeneous catalysts.32 An excess of Hg0 (300 equiv.) was added to the reaction mixture after 2 h (about 60% conversion) and then the catalytic system was performed under standard hydrogenation conditions. The catalytic activity was completely suppressed and no catalytic activity was observed even though the catalytic system had been vigorously stirred with Hg0 for another 3 h. All the results above strongly support proposal that the reaction progresses under heterogeneous catalysis rather than homogeneous catalysis.
In order to investigate the recyclability of catalysts and the stabilization effect of BMMIM]3[tppt] to Pd NPs, both Pd-1 and Pd NPs with no addition of [BMMIM]3[tppt] in the preparation (Pd-2) were tested in the hydrogenation of styrene (Fig. 6). After the hydrogenation, diethyl ether was added to extract products, and then the catalysts were washed with diethyl ether three times and further treated under vacuum for the next run. It was found that the catalyst Pd-1 could be reused at least six times without significant decrease in activity. However, the activity of Pd-2 diminished dramatically after the third cycle. The recyclability results demonstrated the remarkable performance of Pd-1 catalyst and the strong stabilization effect of [BMMIM]3[tppt] to Pd NPs.
Before the chemoselective hydrogenation of olefins, we chose hexane and cyclohexene as substrates to test the catalytic performance of Pd-1. The results showed that complete conversion could be achieved in 8 h and 5 h respectively. We further investigated chemoselective hydrogenation of various olefins (Table 1). Pd-1 dispersed in [BMMIM]PF6 showed excellent chemoselectivity in the hydrogenation of various functionalized olefins. The fact that styrene and ethyl acrylate (Table 1, entries 1–2) exhibited higher activity than hexane and cyclohexene indicated the catalytic hydrogenation was apparently affected by the steric effect and the electronic effect. Furthermore, 4-phenyl-3-buten-2-one and 1, 5-cyclooctadiene (Table 1, entries 3–4), could be completely converted into the corresponding saturated C–C single-bond compounds with the aromatic ring and carbonyl group remained.
Entry | Substrate | Product | t. h−1 | Conv. (%)b | Sel. (%) |
---|---|---|---|---|---|
a Reaction conditions: Pd-1 (2.5 × 10−3 mmol), substrate/Pd = 250, [BMMIM]PF6 (1 mL), 1 MPa initial hydrogen pressure, 45 °C.b GC yield. | |||||
1 | Styrene | Ethylbenzene | 4 | 100 | 100 |
2 | Ethyl acrylate | Ethyl propionate | 4 | 99 | 100 |
3 | 4-Phenyl-3-buten-2-one | 4-Phenyl-2-butanone | 15 | 100 | 100 |
4 | 1,5-Cyclooctadiene | Cyclooctane | 15 | 100 | 100 |
Selective hydrogenation of substituted nitrobenzene to corresponding aniline is still a significant reaction and challenge for chemists, since aniline and its derivatives are important intermediates for dyes, polyurethanes, pharmaceuticals, explosives and agrochemicals.33,34 As in Table 2, Pd-1 catalytic system was very active and exclusively selective towards hydrogenation of the nitro group (Table 2, entry 1). While commercial Pd/C catalyst acted as the heterogeneous catalyst, chemoselectivity to aniline was 98% (Table 2, entry 2). Additionally, Pd-2 showed an unsatisfactory conversion of 80% and 95% chemoselectivity to aniline (Table 2, entry 3). Pd-2 catalytic species formed black particles visible to naked eyes after the hydrogenation. The results testified that traditional ionic liquid ([BMMIM]PF6) could not provide effective stabilization for Pd NPs. The scope of the substrates was also tested. The aromatic nitro compounds bearing different substituent groups (–CH3, –Cl, –F, –CO) (Table 2, entries 4–10) did not show significant influence on the activity and selectivity during the hydrogenation. Furthermore, dehalogenation was not detected in the process of hydrogenation. However, considering substrates with a methoxy substituent, the activity was sharply decreased (Table 2, entries 11–12). The results revealed that Pd-1 showed promising activity for chemoselective hydrogenation of nitrobenzene and its derivatives, which was not apparently affected by the steric effect, but possibly by the electric effect.
Entry | Substrate | Product | T (h) | Conv. (%)b | Sel. (%) |
---|---|---|---|---|---|
a Reaction conditions: catalyst Pd (2.5 × 10−3 mmol), substrate/Pd = 200, [BMMIM]PF6 (1 mL), 5 MPa initial hydrogen pressure, 50 °C.b GC yield.c 10% Pd/C (2.66 mg).d Pd NPs with no addition of [BMMIM]3[tppt] in the preparation (Pd-2). | |||||
1 | Nitrobenzene | Aniline | 12 | 100 | 100 |
2c | Nitrobenzene | Aniline | 12 | 100 | 98 |
3d | Nitrobenzene | Aniline | 12 | 80 | 95 |
4 | p-Nitrotoluene | p-Touidine | 12 | 100 | 100 |
5 | m-Nitrotoluene | m-Toluidine | 12 | 92 | 100 |
6 | p-Chloronitrobenzene | p-Chloroaniline | 8 | 100 | 100 |
7 | o-Chloronitrobenzene | o-Chloroaniline | 10 | 100 | 100 |
8 | p-Fluoronitrobenzene | p-Fluoroaniline | 12 | 97 | 100 |
9 | o-Nitroacetophenone | o-Aminoacetophenone | 12 | 100 | 100 |
10 | m-Nitroacetophenone | m-Aminoacetophenone | 12 | 100 | 100 |
11 | p-Nitroanisole | p-Anisidine | 12 | 38.4 | 100 |
12 | o-Nitroanisole | o-Anisidine | 12 | 22.4 | 100 |
Aromatic alcohols are important intermediates in pharmaceuticals, flavors, fragrances, chemical intermediates and photographic chemicals.35 We further applied Pd-1 in the challenging chemoselective hydrogenation of aromatic aldehydes. Benzaldehyde was selected as the model substrate and relatively low conversion of 27.5% was obtained (Table 3, entry 1). The conversion of commercial catalyst Pd/C was 24% (Table 3, entry 2). Water is an attractive alternative to traditional organic solvents, because it is cheap, readily available, nontoxic, non-flammable and safe to environment. When water was introduced to catalytic system, the activity of Pd-1 sharply increased to 100% (Table 3, entry 3). The conversion of commercial Pd/C catalyst also increased to 93% (Table 3, entry 4). However, Pd-2 showed unsatisfied conversion of just 40% (Table 3, entry 5). Some representative examples are listed in Table 3 for the chemoselective hydrogenation of aromatic aldehydes catalyzed by Pd-1 in a mixture solvent of H2O and [BMMIM]PF6. The catalytic system showed good activity and selectivity to corresponding aromatic alcohols (Table 3, entries 6–10).
Entry | Substrate | Co-solventb | Product | Time (h) | Con. (%)c | Sel. (%) |
---|---|---|---|---|---|---|
a Reaction condition: catalyst Pd (2.5 × 10−3 mmol), substrate/Pd = 1![]() ![]() |
||||||
1 | Benzaldehyde | — | Phenylmethanol | 19 | 27.5 | 100 |
2d | Benzaldehyde | — | Phenylmethanol | 19 | 24 | 100 |
3 | Benzaldehyde | H2O | Phenylmethanol | 19 | 100 | 100 |
4d | Benzaldehyde | H2O | Phenylmethanol | 19 | 93 | 100 |
5e | Benzaldehyde | H2O | Phenylmethanol | 19 | 40 | 100 |
6 | p-Methylbenzaldehyde | H2O | p-Tolylmethanol | 40 | 100 | 100 |
7 | p-Chlorobenzaldehyde | H2O | p-Chlorobenzylalcohol | 45 | 99 | 100 |
8 | m-Chlorobenzaldehyde | H2O | m-Chlorobenzylalcohol | 48 | 85.1 | 100 |
9 | o-Chlorobenzaldehyde | H2O | o-Chlorobenzylalcohol | 48 | 97 | 100 |
10 | o-Methoxybenzaldehyde | H2O | o-Methoxybenzylalcohol | 48 | 78.5 | 100 |
Based on the obvious difference in catalytic activity during the chemoselective hydrogenation of olefins and aromatic aldehydes (Table 1 and 3), the hydrogenation intermediates are proposed and shown in Scheme 2. In comparison with the chemoselective hydrogenation of olefins (Scheme 2A), the chemoselective hydrogenation of CO in aromatic aldehydes (Scheme 2B) is less thermodynamically preferable. So, the olefins exhibited much higher hydrogenation activity than aromatic aldehydes. Additionally, the results in Table 3 have clearly shown that water plays a promotional role in the hydrogenation of aromatic aldehydes to aromatic alcohols. We speculate that C
O not only interacts with palladium, but it also forms the hydrogen bond with water (Scheme 2C). As a result, the hydrogen bond between C
O and water improves the activity of Pd-1 for the hydrogenation of aromatic aldehydes to aromatic alcohols.
On the basis of the above results, we examined the use of Ru NPs as active species and extended the application scope of PFIL stabilized transition-metal NPs catalytic system. The Ru nanocatalyst was prepared by hydrogenation reduction of RuO2 hydrate in [BMMIM]PF6 at 75 °C for 4 h. A black powder could be isolated from the obtained Ru NPs by adding acetone and then centrifuging (5000 rpm, 5 min). Washed three times with acetone and dried under reduced pressure. The obtained black species (Ru-1) were characterized by TEM, XRD and XPS methods.
TEM analysis was displayed in Fig. 7, the metal particle size distribution was estimated from the measurement of about 300 particles. These particles displayed a monomodal size distribution and the average diameter was 1.7 nm. In comparison with Pd NPs, Ru NPs were significantly smaller. This phenomenon could be explained that Pd NPs can easily form aggregation, especially in severe reaction conditions.18
XRD analysis indicated that the solid consists of metal particles of hexagonal close packed (hcp) ruthenium (Fig. 8). The Bragg reflections at 38.41°, 42.03°, 43.79°, 58.13°, 69.06°, 78.44° corresponded to the indexed planes of (hcp) crystals of Ru (0): (100), (002), (101), (102), (110), (103) respectively. XPS analysis revealed the surface composition of Ru NPs. Ru 3d5/2 and 3d3/2 signals with binding energies of 280.0 eV and 284.8 eV were observed, which was consistent with Ru(0).36
We selected nitrobenzene as standard substrate to explore the performance of Ru-1 catalyst. The results revealed that Ru-1 nanocatalyst showed high activity and chemoselectivity in the hydrogenation of nitrobenzene (Table 4, entry 1). However, the catalytic activity of commercial Ru/C catalyst was only 49% (Table 4, entry 2). When there was no addition of [BMMIM]3[tppt] in the preparation of Ru NPs (Ru-2), the Ru-2 catalyst displayed a very poor conversion of only 4.6% (Table 4, entry 3). Ru-2 species formed black particles visible to naked eyes after the hydrogenation. The above results demonstrated that Ru-1 nanocatalyst was more efficient than commercial Ru/C catalyst in chemoselective hydrogenation of nitrobenzene in [BMMIM]PF6 (Table 4, entries 1 vs. 2). The catalytic performance of Ru-2 revealed the strong stabilization effect of BMMIM]3[tppt] to Ru NPs (Table 4, entries 1 vs. 3). Since the percentage of Ru in Ru-1 catalyst is higher than the percentage of Pd in Pd-1 catalyst (Table 4, entry 1 vs. Table 2, entry 1), it is necessary to clarify whether the high catalytic activity of Ru-1 is owing to the high concentration of active metal. Under the same reaction conditions with Pd-1 in Table 2, Ru catalyst with the same mole loading to Pd-1 was also tested in the hydrogenation of nitrobenzene (Table 4, entry 4). Nitrobenzene could be completely hydrogenated to aniline in 2.5 h. However, 12 hours was needed for Pd-1 to achieve the complete hydrogenation (Table 2, entry 1). Substituted nitrobenzenes were also investigated over Ru-1 in [BMMIM]PF6. The substrates were completely converted to corresponding anilines within 4 h (Table 4, entries 5–12). The results revealed that Ru-1 showed promising activity for chemoselective hydrogenation of nitrobenzene and its derivatives, which was not apparently affected by the steric effect and the electric effect.
Entry | Substrate | Product | T (h) | Conv. (%)b | Sel. (%) |
---|---|---|---|---|---|
a Reaction conditions: catalyst Ru (17.75 × 10−3 mmol), [BMMIM]PF6 (1 ml), substrate/Ru = 200, inner hydrogen pressure 5 MPa, 60 °C.b GC yield.c Ru/C (5% wt, 36 mg).d No addition of [BMMIM]3[tppt] in preparation of Ru NPs.e Catalyst Ru (2.5 × 10−3 mmol), 50 °C. | |||||
1 | Nitrobenzene | Aniline | 1.5 | 100 | 100 |
2c | Nitrobenzene | Aniline | 1.5 | 49 | 100 |
3d | Nitrobenzene | Aniline | 1.5 | 4.6 | 100 |
4e | Nitrobenzene | Aniline | 2.5 | 100 | 100 |
5 | p-Nitrotoluene | p-Toluidine | 2 | 100 | 100 |
6 | m-Nitrotoluene | m-Toluidine | 4 | 100 | 100 |
7 | p-Chloronitrobenzene | p-Chloroaniline | 4 | 100 | 100 |
8 | o-Chloronitrobenzene | o-Chloroaniline | 4 | 100 | 100 |
9 | p-Fluoronitrobenzene | p-Fluoroaniline | 4 | 100 | 100 |
10 | p-Nitroanisole | p-Anisidine | 4 | 100 | 100 |
11 | o-Nitroanisole | o-Anisidine | 4 | 100 | 100 |
12 | o-Nitroacetophenone | o-Aminoacetophenone | 4 | 100 | 100 |
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