Preparation of recoverable Ru catalysts for liquid-phase oxidation and hydrogenation reactions

Applied Catalysis A: General 360 (2009) 177–182

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Preparation of recoverable Ru catalysts for liquid-phase oxidation and hydrogenation reactions Marcos J. Jacinto a, Osvaldo H.C.F. Santos a, Renato F. Jardim b, Richard Landers c, Liane M. Rossi a,* a

Instituto de Quı´mica, Universidade de Sa˜o Paulo, 05508-000, Sa˜o Paulo, SP, Brazil Instituto de Fı´sica, Universidade de Sa˜o Paulo, CP 66318, 05315-970, Sa˜o Paulo, SP, Brazil c Instituto de Fı´sica, UNICAMP, 13083-970, Campinas, SP, Brazil b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 23 January 2009 Received in revised form 10 March 2009 Accepted 14 March 2009 Available online 25 March 2009

We here report the synthesis, characterization and catalytic performance of new supported Ru(III) and Ru(0) catalysts. In contrast to most supported catalysts, these new developed catalysts for oxidation and hydrogenation reactions were prepared using nearly the same synthetic strategy, and are easily recovered by magnetic separation from liquid phase reactions. The catalysts were found to be active in both forms, Ru(III) and Ru(0), for selective oxidation of alcohols and hydrogenation of olefins, respectively. The catalysts operate under mild conditions to activate molecular oxygen or molecular hydrogen to perform clean conversion of selected substrates. Aryl and alkyl alcohols were converted to aldehydes under mild conditions, with negligible metal leaching. If the metal is properly reduced, Ru(0) nanoparticles immobilized on the magnetic support surface are obtained, and the catalyst becomes active for hydrogenation reactions. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Magnetite nanoparticles Magnetic recovery Ruthenium Hydrogenation Oxidation

1. Introduction The search for ‘clean’ or ‘green’ technologies merges with the search for catalytic systems that activate clean oxidizing agents (e.g. O2) or reducing agents (e.g. H2) for replacement of classical organic routes employing stoichiometric amounts of inorganic reagents [1,2]. Stoichiometric amounts of metal oxidants such as chromium(VI) and manganese compounds or metal reductants (Na, Mg, Fe, Zn) and metal hydrides (NaBH4 and LiAlH4) are still commonly used, despite the formation of large amounts of toxic and hazardous waste. Within this scenario, the use of catalyzed reactions is very attractive for waste minimization in fine chemicals manufacture. The selective oxidation of alcohols to carbonyl compounds is the foundation of many important industrial and fine-chemical processes [3]. Research has been focused on the development of catalytic system that could be competitive to a stoichiometric approach involving strong and toxic oxidizing agents [4]. Thanks to its improved selectivity and activity, the interest in homogeneous catalysts for oxidation reactions has grown dramatically. Nevertheless, one key challenge for the commercial development and practical use of homogeneous catalysts is the separation of product from the catalytic media. This process is often complicated and

* Corresponding author. Tel.: +55 11 30912181; fax: +55 11 3815 5579. E-mail address: [email protected] (L.M. Rossi). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.03.018

usually accomplished by means of complex work-up procedures. Attempts to improve catalyst recovery and recycling include the use of biphasic systems [5–10] or the immobilization of catalysts on solids [11–15]. Anchoring metal complexes often requires chemical modifications on the support and ligands, which can decidedly affect the catalytic activity. For these systems, the most common used supports are silica, alumina, and carbon, and the product separation is frequently achieved by a physical method such as filtration or even centrifugation. However, as the size of the support decreases, the separation becomes a difficult task. For instance, in systems comprised of very thin solids, simple filtration constitutes an inefficient tool to accomplish product isolation. In such circumstances, an alternative that has drawn attention lately is the use of magnetically recoverable solid supports to attain easy and fast catalyst recovery from the reaction medium [16–31]. These supports, mainly constituted of superparamagnetic nanoparticles, can be attracted to relatively low static magnetic field strengths and are easily recovered since no residual magnetization is observed when the magnetic field is removed. The use of such materials in catalysis is usually accomplished by the direct immobilization of the catalyst on the magnetic nanoparticles [16–24]. An alternative is the use of coated magnetic nanoparticles for the metal immobilization [25–32], which provides the catalyst, especially nanoparticle-type, with enlarged stability. Here we focus on the immobilization of ruthenium on a functionalized silica-coated magnetic support with great catalytic and separability properties. The new supported Ru(III) catalyst

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exhibits noticeable activity towards the selective conversion of alcohols to the corresponding aldehydes and ketones. If the metal is properly reduced, Ru(0) nanoparticles immobilized on the magnetic support surface are obtained, and the catalyst becomes active for hydrogenation reactions. 2. Experimental 2.1. Preparation of the Ru(III) catalyst The preparation of the catalyst comprises few steps. The first consists of the synthesis of the magnetic support which is comprised of spherically silica-coated Fe3O4 nanoparticles, as reported elsewhere [32]. In a typical experiment, 44.6 g of Polyoxyethylene(5) isooctylphenyl ether were dispersed in 700 mL of cyclohexane. Then, 200 mg of previously prepared Fe3O4 dispersed in cyclohexane were added. The mixture was stirred until it became transparent. After this step, 9.44 mL of ammonium hydroxide (29%) were added to form a reverse microemulsion. Finally, 7.70 mL of tetraethylorthosilicate (TEOS) were added. The solution was gently stirred for 16 h. The core-shell nanocomposite Fe3O4/SiO2 was precipitated with methanol and collected by centrifugation at 7000 rpm. After being washed with ethanol, the collected material was dried in vacuum, resulting in 1 g of material. Then, the solid was modified with amino groups by reaction with 3-aminopropyltriethoxysilane (APTES) in dry toluene under N2. The amine-functionalized solid (Fe3O4/SiO2– NH2) was washed with toluene, separated by centrifugation, and dried at 100 8C for 20 h. Finally, the catalyst was prepared as follows: 50 mg of Fe3O4–SiO2/NH2 were added to 10 mL of an aqueous Ruthenium chloride solution (500 mg/L). The mixture was kept under stirring at 25 8C for 2 h. The solid was then magnetically collected from the solution, washed twice with distilled water and acetone and dried in vacuum. 2.2. Preparation of the Ru(0) catalyst The Ru(0) catalyst was prepared by reduction of a Ru(III) precursor, which was obtained by the same synthetic strategy used in Section 2.1, by sodium boronhydride in ethanol solution. Typically, 100 mg of Fe3O4/SiO2–NH2–Ru(III) was added to 20 mL of ethanol containing 10 mg of NaBH4. Thereafter, the material was washed twice with distilled water and acetone and dried in vacuum. 2.3. Catalyst characterization High-resolution transmission electron microscopy (HRTEM) and X-ray energy dispersive spectroscopy (EDS) were performed at the Laborato´rio Nacional de Luz Sincroton (LNLS, Campinas, SP) using a Jeol-3010 ARP microscope. Samples for TEM observations were prepared by placing a drop containing the nanoparticles in a carbon-coated copper grid. Magnetization measurements M(T, H) were performed by using a superconducting quantum interference device magnetometer from Quantum Design. Applied magnetic fields H between 70 and 70 kOe and temperatures ranging from 5 to 300 K were used in the experiments. In the zero-field cooled (ZFC) experiments, the sample was first cooled down to 5 K under zero applied magnetic field, the magnetic field was set, and the magnetization data were collected upon warming up to 300 K. In the field cooled (FC) process, the data were collected upon cooling the sample from 300 K down to 5 K under an applied magnetic field. In the hysteresis loops M(H) measurements, the sample was ZFC down to a desired temperature and the magnetic field sweep ( 70 kOe  H  70 kOe) was performed. These steps were repeated for several different measuring temperatures. The X-

ray photoelectron spectra were obtained with a VSW HA-100 spherical analyzer using an aluminum anode (AlKa line, hn = 1486.6 eV) X-ray source. The high-resolution spectra were measured with constant analyzer pass energies of 44 eV, which produce a full width at half-maximum (fwhm) line width of 1.7 eV for the Au 4f7/2 line. The powdered samples were pressed into pellets and fixed to a stainless steel sample holder with doublefaced tape and analyzed without further preparation. To correct for charging effects the spectra were shifted so that the Si(2p) binding energy of SiO2 was 103.5 eV. Curve fitting was performed using Gaussian line shapes, and a Shirley type background was subtracted from the data. 2.4. Catalytic experiments The hydrogenation and oxidation reactions were carried out in a modified Fischer–Porter glass reactor. H2 and O2 were used as the source of atoms for hydrogenation and oxidation reactions, respectively. In a typical oxidation reaction, the glass reactor was loaded with the supported Ru(III) catalyst (4.0 mol.% Ru), the substrate (0.18 mmol), and 1 mL of toluene under inert atmosphere. The reactor, immersed in an oil bath at 100 8C, was loaded with O2 under a pressure of 3 atm. The temperature was maintained by a hot-stirring plate connected to a digital controller (ETS-D4 IKA). The reactions were conducted under magnetic stirring (700 rpm) for the desired time. The catalyst was magnetically recovered by placing a permanent magnet in the reactor wall and the products were collected and analyzed by gas chromatography (GC) and GC–MS. For hydrogenation experiments, the Ru(0) catalyst (50 mg, 6.8 mmol Ru) and the substrate cyclohexene (15 mmol) were added to the reactor under inert atmosphere. The reactor was evacuated and attached to the hydrogen supply. The reactor was loaded with hydrogen at 6 atm (constant), and placed in an oil bath at 75 8C under stirring. The reaction was followed by the fall in the pressure of the hydrogen tank connected to the reactor. When hydrogen consumption stopped, the reactor was cooled down, the remaining hydrogen gas relieved and the catalyst recovered magnetically by placing a magnet in the reactor wall. The products were collected with a syringe and analyzed by gas chromatography (GC) and GC–MS. Turnover frequency (TOF) corresponds to the molar amount of substrate converted per molar amount of metal per hour of reaction (considering the time hydrogen consumption stopped). The isolated catalyst could be reused when a new amount of substrate was added in both oxidation and hydrogenation reactions. 3. Results and discussion Recently, we have reported the first example in the literature of very nicely distributed Rh nanoparticles on magnetic particles spherically coated with silica [32]. The Rh catalyst showed outstanding results for hydrogenation reactions followed by negligible leaching of metal, after being used in 20 subsequent reactions. The same catalytic support was used in this study for trapping a Ru molecular catalyst. The material is comprised of core-shell silica-coated Fe3O4 nanoparticles with average size 60 nm and exhibits superparamagnetism in a large range of temperature T, as displayed in Fig. 1. Such a superparamagnetic behavior is identified by the occurrence of a maximum close to the blocking temperature TB 150 K in the ZFC curve taken under H = 50 Oe. In addition, the shift of TB for lower temperatures 80 K when the applied magnetic field is further increased to H = 0.5 kOe confirms a commonly feature seen in superparamagnetic systems [33].

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Fig. 1. Temperature dependence of the magnetization (FC and ZFC curves) under magnetic field H = 50 Oe and 500 Oe for the catalyst support.

The catalyst supports were found to display excellent magnetic properties for the recovery of catalyst components, as can be inferred from the magnetic field dependence of the magnetization displayed in Fig. 2. The first relevant feature here is related to the very low coercivity field 2 Oe of the support at room temperature, as displayed in the inset of Fig. 2. Such a very low coercivity field is negligible when compared to the one of 240 Oe at T = 5 K. The other excellent feature of the support relies on its very high saturation magnetization MS at room temperature. Considering that the support contains 9 wt.% of Fe3O4, as determined by ICPOES, we found a MS 69 emu g 1 of Fe3O4 at 70 kOe, a value smaller than the one expected (92 emu g 1) for bulk Fe3O4 [34]. The reduction of the saturation magnetization observed here may be attributed to the surface disorder or spin canting at the particles surface. The magnetic properties described above are suitable for the rapid separation of the catalyst from the reaction media, a process usually followed by an easy catalyst recycling. Besides, it

Fig. 2. Hysteresis loops ( 70 kOe  H  70 kOe) for the catalyst support at 300 K and 5 K (M given in emu g 1 of Fe3O4 present in the catalyst support). Inset: enlarged view ( 4 kOe  H  4 kOe) at 300 K and 5 K.

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eliminates the use of any further physical or chemical productisolation procedures. The catalyst was prepared by immobilization of RuCl3 in the NH2 modified silica shell, previously reacted with 3-aminopropyltriethoxysilane. The presence of 1.4 wt.% of Ru in the solid was determined by ICP-OES analyses. Under the same synthesis conditions, the metal loading for the non-functionalized support was found to be 0.6 wt.%. The surface composition of the catalyst has been analyzed by Xray photoelectron spectroscopy (XPS). The main results are shown in Fig. 3, where peaks corresponding to carbon, oxygen, silicon, and ruthenium were observed in the survey scan depicted in Fig. 3(a). Peaks belonging to Ru 3d and C 1s are overlapped. However, they can be clearly identified by taking into account the Ru 3d5/2 and Ru 3d3/2 peak distance of 4.2 eV and the expected ratio of intensities I3/2/I5/2  0.66 [35]. The deconvoluted spectrum, displayed in Fig. 3(b), exhibits a doublet corresponding to an oxidized species of Ru with peak binding energies of 280.4 (Ru 3d5/2) and 284.6 eV (Ru 3d3/2), and two peaks corresponding to C 1s species at 284.63 and 286.9 eV. The oxygen O 1s peak, with binding energy of 532.8 eV, may be attributed to SiO2 rather than to RuO2, as in this case the O 1s would appear at a lower energy [36,37]. Also, the binding energy difference between the Ru 3d5/2 peak and the O 1s peak was 252.4 eV, which is slightly higher than the 249.0 eV found for ruthenium dioxide [36,37]. It is important to mention that peaks belonging to chlorine have not been detected by XPS, a feature that suggests its complete removal during the immobilization of the

Fig. 3. (a) XPS surface survey and (b) deconvolution of the Ru 3d photoemission line of the Ru(III) magnetically recoverable catalyst.

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Table 1 Oxidation of alcoholsa by Ru(III) magnetically recoverable catalyst. Conversion (%)b

Selectivity (%)b

1

>99

>99

2

>99

>99

3

>99

>99

4c

82

>99

5

>99

>99

Substrate

a b c

Product

Reaction conditions: alcohol (0.18 mmol), Fe3O4/SiO2/Ru3+ (Ru: 4 mol%), toluene (1 mL), 3 h, 100 8C, 3 atm O2. Measured by GC–MS. 7 h.

metal ions on the support. Such a result strongly indicates that the metal is actually coordinated as Ru(III) to the amino-modified silica layer of the core–shell support. The catalytic activity of the magnetically separable Ru(III) catalyst (Fe3O4/SiO2/Ru3+) was investigated in the oxidation of a series of aryl and alkyl alcohols. Selected examples are displayed in Table 1. The reactor loaded with the catalyst and the substrate was subjected to the desired oxygen pressure and temperature. After the desired time interval, the reactor was cooled to room temperature and depressurized. The catalyst was easily recovered magnetically by placing a magnet in the reactor wall, as shown in Fig. 4, and the metal-free liquid phase was analyzed by GC–MS. The conversion was usually complete after less than 3 h under mild conditions. All primary and secondary alcohols were converted into the corresponding aldehydes and ketones, respectively, under 3 atm of molecular oxygen and 100 8C. Noticeably, no subproducts such as carboxylic acids have been observed during the oxidation of primary alcohols, which ensures the catalyst selectivity. It is important to notice that even non-activated alcohols such as 1octanol was oxidized (82% conversion and 100% selectivity) under the reaction conditions studied. In addition, the catalyst oxidizes diols to the corresponding diketones, such as 2,4-pentanedione, by molecular oxygen. We also mention that no reaction takes place

when the support particles, Fe3O4/SiO2–NH2, are used as catalyst. The support is not catalytic active, however provides the metal stability and surface effect which improve its catalytic activity. Further experiments with the substrate 1-phenylethanol were performed to verify the catalyst recyclability. After each oxidation reaction the catalyst was easily separated by the application of a magnetic field, the product removed with a syringe, and new portions of the substrate were added to the reactor. The new Ru(III) magnetically separable catalyst could be reused for up to three successive oxidation reactions without change in substrate conversion after 3 h (>99% conversion). Thanks to the use of the magnetic property, the time interval for both the catalyst separation and product removal has been insignificant when compared to the time consumed during the reaction. Such a procedure also avoids unnecessary use of other separation techniques such as filtration, decantation, and centrifugation, simplifying greatly the workup procedure. After the recycling studies, the organic products were collected and analyzed by ICPOES showing negligible amounts of Ru (<1 ppm). When the Ru(III) magnetically recoverable catalyst was treated with an ethanol solution of NaBH4, the metal ions were reduced to form Ru(0) nanoparticles, as characterized by transmission electron microscopy. The enlarged micrograph shown in Fig. 5a

Fig. 4. Recovery of catalyst by applying an external magnetic field.

Fig. 5. (a) TEM image and (b) EDS analysis of the Ru(0) magnetically recoverable catalyst in different areas shown in image (a): shell NPs (i), core NPs (ii), and total core-shell NPs (iii).

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displays Ru(0) nanoparticles of 2 nm, even dispersed in the spherically coated magnetite nanoparticles. Energy dispersion spectrometry (EDS) analysis confirms the presence of Ru, Fe and Si (Fig. 5b(iii)). A detailed EDS analysis using a 5 nm probe showed the distribution of the different components on the catalyst comprised of Ru NPs decorating the silica layer (Fig. 5b(i)) and Fe3O4 NPs in the core (Fig. 5b(ii)). The catalyst (Fe3O4/SiO2/Ru(0)) is active for hydrogenation of cyclohexene under mild conditions. In a typical solventless experiment, the solid catalyst and the olefin (Ru/olefin molar ratio 1/2100) were added to the reactor under inert atmosphere. The reactor was loaded with hydrogen at 6 atm and heated at 75 8C. The reaction was monitored by the pressure drop in the H2 gas supply connected to the reactor. The hydrogenation of cyclohexene was complete (>99% of conversion) after 5 h of reaction, corresponding to a turnover frequency of 420 h 1. This result is about 18 times lower than the TOF of our previously reported magnetically recoverable Rh catalyst [32], but still relevant for Ru metal. The magnetically recovered Ru catalyst was reused in three successive runs, by addition of new portions of cyclohexene, to give a total turnover (TON) of 6300. The organic products were collected and negligible amounts of Ru (<0.01 ppm) were detected in the leachant, as inferred from ICP-OES analyses. The organic phase is also free of Fe and Si, which corroborates the high stability of our catalyst. The use of silica-coated magnetic nanoparticles, instead of bare Fe3O4 nanoparticles, provides an extra-protection to the magnetite cores that are bound to be further oxidized to less magnetic iron oxides. These results, along with the findings for the oxidation experiments, indicate that not only do the catalysts afford efficient and practical separation, but they also retain the metal on the support satisfactorily, preventing metal leaching which is an influential drawback in heterogeneous systems [38–40]. 4. Conclusions In conclusion, we presented an interesting way to obtain two different Ru recyclable catalysts, for hydrogenation and oxidation reactions, using nearly the same strategy of synthesis. The preparation of both catalysts is based on the uptake of Ru(III) by amino-functionalized silica-coated magnetic nanoparticles. The oxidized Ru(III) form converted aryl and alkyl alcohols to aldehydes, while the reduced Ru(0) form was active for hydrogenation reactions under mild conditions. These catalysts exhibit several interesting attributes for the clean synthesis of fine chemicals. Foremost, the magnetic core makes it possible to easily separate the catalyst from the liquid media. Secondly, the presence of –NH2 groups is doubly important as they improve both uptake and retention of the metal in the support. Thirdly, the intended idea puts forward the use of metal as catalyst under mild operational conditions. All these features combined are in accordance with important prospective green aspects. Our work now is focused on broadening the idea to other catalytic systems,

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