Aerobic oxidation of alcohols over carbon nanotube-supported Ru ...

Applied Catalysis A: General 382 (2010) 131–137Contents lists available at ScienceDirectApplied Catalysis A: Generaljournal homepage: www.elsevier.com/locate/apcataAerobic oxidation of alcohols over carbon nanotube-supported Ru catalystsassembled at the interfaces of emulsion dropletsXiaomin Yang a , Xiuna Wang a , Jieshan Qiu a,b,∗a Carbon Research Laboratory, Center for Nano Materials and Science, State Key Lab of Fine Chemicals, School of Chemical Engineering,Dalian University of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, Chinab Key Laboratory for Micro/Nano Technology and System of Liaoning Province, Dalian University of Technology, Dalian 116012, ChinaarticleinfoabstractArticle history:Received 6 January 2010Received in revised form 4 April 2010Accepted 26 April 2010Keywords:Aerobic oxidationAlcoholsCarbon nanotubesRuEmulsion catalysisCarbon nanotube (CNT)-supported ruthenium catalysts, assembled at the interfaces of emulsion droplets,show excellent activity, selectivity, and stability for the selective oxidations of benzyl alcohol to benzaldehydewith oxygen or air as oxidant in the presence of water. The selective oxidation of benzyl alcohol overRu/CNTs catalysts is greatly enhanced and quickened due to the presence of water. A reaction pathwayis proposed, in which the promotion effect of water on the catalytic activity of Ru/CNTs is discussed. Theas-made Ru/CNTs catalysts are also active for the aerobic oxidation of a variety of alcohols with a sulfuror nitrogen atom or a carbon–carbon double bond in the multiphase reaction system. Moreover, after thereactions, the catalysts can be easily separated and recycled by sedimentation.© 2010 Elsevier B.V. All rights reserved.1. IntroductionOxidation of alcohols to carbonyl compounds is one of themost fundamental and important processes for synthesis of organicchemicals. A variety of methods for the oxidation of alcohols havebeen developed; nevertheless, traditional methods such as noncatalyticmethods with stoichiometric amounts of heavy metalreagents [1] or moisture-sensitive expensive oxidants [2] are stillwidely used. These processes are often conducted in environmentallyundesirable media such as chlorinated solvents. Fromenvironmental and atom-economical points of view, it would beideal to oxidize alcohols using air or molecular oxygen under atmosphericpressure conditions. Obviously, this cheaper, safer, andmore environmentally benign oxidation process requires novel yethighly efficient heterogeneous catalysts. The research efforts haveled to a number of efficient catalysts for the aerobic oxidationof alcohols such as hydroxyapatite (HAP) bound Ru/HAP [3] and∗ Corresponding author at: Carbon Research Laboratory, State Key Lab of FineChemicals, School of Chemical Engineering, Dalian University of Technology, 158Zhongshan Road, P.O. Box 49, Dalian 116012, China. Tel.: +86 411 39893994;fax: +86 411 88993991.E-mail address: jqiu@dlut.edu.cn (J. Qiu).Pd/HAP [4,5], Ru/Al 2 O 3 [6], Au/CeO 2 [7], and Au–Pd/TiO 2 [8].Uptonow, oxidation reactions at low temperature in appropriate solventsare still highly demanded for alcohols with high meltingpoints or low stability at high temperatures, or in cases wherea minimal amount of an alcohol is needed for scientific research[9–14].With the discovery of carbon nanotubes (CNTs) and their largescale production, much attention has been paid to their potentialapplications such as in electronic devices, biosensors, and catalysisdue to their unique electron conductivity, thermal stability,and high mechanical strength [15–23]. Compared with traditionalcatalyst supports, CNTs with high external surface area and aspectratio display unusual behaviors, such as being able to significantlyincrease the contact surface between the reactants and active sitesof catalysts, and to greatly minimize the diffusion limitations. Thesefeatures make CNTs very attractive as catalyst supports in liquidphase reactions. Here we report that, with CNT-supported rutheniumas catalysts, several types of alcohols with a sulfur or nitrogenatom, or a carbon–carbon double bond in the molecule, can be effectivelyoxidized to their corresponding carbonyl compounds usingoxygen or air as oxidant under mild conditions, as illustrated inEq. (1). It is interesting to note that the CNT-supported rutheniumcatalysts assemble and form droplets in the emulsion-like multi-0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2010.04.046

132 X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137phase reaction system. After the reaction, the catalysts can be easilyseparated from the liquid products by sedimentation and can berecycled.90 min, followed by evacuation at 450 ◦ C for 4 h. Before the measurement,the catalysts were evacuated at 40 ◦ C for 2 h.(1)2. Experimental2.1. MaterialsCNTs with a diameter of 10–20 nm, a length of 1–2 m, and N 2surface area of 151 m 2 /g were used. Coconut shell based activecarbon (AC) with N 2 surface area of 986 m 2 /g was purchasedfrom Beijing Broad Activated Carbon Co., Ltd. (Beijing, China). TiO 2(Degussa P 25 )withaN 2 surface area of 59 m 2 /g was purchasedfrom Shanghai Haiyi Scientific & Trading Co., Ltd. (Shanghai, China).RuO 2 was purchased from Alfa Aesar Co., Ltd. Oxygen with a purityof 99.995% was purchased from Dalian Guangming Special TypeGas Co., Ltd. (Dalian, China). All reagents used in this work were ofanalytical grade.2.2. Catalyst preparation2.4. Alcohol oxidationAlcohol oxidation reactions were carried out in a 50 mL threeneckround-bottom flask under magnetic stirring. For a typicalrun, 0.2 g of Ru/CNTs catalyst, 2 mmol of benzyl alcohol, 10 mL oftoluene, and 5 mL of water were placed in a flask that had beenflushed with flowing oxygen or air at atmospheric pressure, andheated at 85 ◦ C for 3 h. The products were analyzed by GC (Agilent6890N) and GC–MS (HP6890–HP5973). After the reaction, theRu/CNTs catalyst was separated from the reaction products by sedimentation,and was subsequently washed first with acetone, thenwith an aqueous solution of NaOH (0.5 M), and finally with water.Thereafter it was dried at 100 ◦ C for 8 h under vacuum before recycling.Before use, pristine CNTs were refluxed in a mixture of HNO 3and H 2 SO 4 (v/v 2:1) at 120 ◦ C for 4 h, then filtered and washed withdeionized water until the pH value of the filtrate reached 7, thendried at 130 ◦ C for 12 h under vacuum. Pristine AC was crushed,washed, and sieved to 200–320 mesh, then treated by the sameprocedure for modifying the CNTs as described above.Ruthenium catalysts supported on CNTs were prepared by awetness impregnation method. The modified CNTs were impregnatedin an aqueous solution of RuCl 3 under ultrasonic conditionsfor 30 min, and then the mixture was incubated at room temperature(RT) for 12 h. The CNT-supported RuCl 3 samples were dried at110 ◦ C for 12 h under vacuum, and reduced at 400 ◦ C in flowing H 2for 2 h, and then cooled to RT in argon, yielding Ru/CNTs catalystswith different Ru loadings. For the Ru/AC and Ru/TiO 2 catalysts, thesame procedure as described above was adopted.2.3. Catalyst characterizationFT-IR spectra of pristine and modified CNTs were recorded ona Jasco-430 spectrometer. The BET surface areas of the modifiedCNTs were measured by N 2 physisorption at −196 ◦ C (MicromeriticsASAP 2020). Ruthenium reducibility was determined bytemperature-programmed reduction (TPR) using a MicrometricsChemisorb 2720 instrument equipped with a thermal conductivitydetector. A non-reduced RuCl 3 /CNTs sample was heated to 750 ◦ Cat 10 K min −1 in a gas stream of 10% hydrogen in argon.The crystalline structure of Ru/CNTs was analyzed by powderX-ray diffraction (XRD, D/MAX-2400) using Cu-K radiationwith a scanning rate of 2 ◦ min −1 . Transmission electron microscopy(TEM) examination was conducted using a Philips Tecnai G 2 20microscope equipped with a CCD camera operated at 200 kV. Thecomposition of the catalyst sample was analyzed by Energy DispersiveX-ray (EDX) Spectrometry.The dispersion of exposed Ru atoms on the supported Ru catalystswas analyzed by H 2 chemisorption using a QuantachromeAutosorb-1 instrument at 40 ◦ C, assuming the chemisorption stoichiometryof H/Ru = 1/1 [24,25]. Before chemisorption, catalystswere heated from room temperature to 450 ◦ Cat4 ◦ Cmin −1 in flowingH 2 (99.999%). The catalysts were reduced in H 2 at 450 ◦ C for3. Results and discussion3.1. Catalyst characterizationThe modification of the CNTs aims to remove amorphous carbonand to increase the number of surface oxygen-containing functionalgroups that are useful for the metal deposition and dispersion.The FT-IR spectra of the CNTs before and after the modificationare shown in Fig. 1, showing that new peaks at 1710 cm −1 and1190 cm −1 appear after modification, which indicates that the carboxylicgroups are formed on the CNTs. The BET surface area of themodified CNTs is 162 m 2 /g.The TPR profile of RuCl 3 /CNTs is shown in Fig. 2. The peak with amaximum at about 155 ◦ C is similar to the reduction temperature ofunsupported RuCl 3 [26,27]. Peaks with maxima at about 248 ◦ C and301 ◦ C are attributed to the reduction of oxygen-containing functionalgroups on the surface. The peak with a maximum at about554 ◦ C is due to the methanation of CNTs. The RuCl 3 /CNTs catalystswere reduced at 400 ◦ C in flowing H 2 for 2 h; under these conditionsRuCl 3 could be completely converted into metallic Ru.Fig. 1. FT-IR spectra of pristine CNTs and modified CNTs.

X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137 133Fig. 2. TPR curve of RuCl 3/CNTs.Fig. 3. XRD patterns of CNTs and Ru/CNTs.For Ru/CNTs samples with different Ru loadings, no diffractionpeaks related to metallic Ru are detected (Fig. 3), which impliesthat Ru nanoparticles are of nanometer scale. The XRD pattern ofCNTs is remarkably similar to that of graphite, while the XRD patternof activated carbon indicates an amorphous structure (Fig. S1).For CNTs, there exists a symmetrical and sharp 0 0 2 peak at 26 ◦ ,and a broad 1 0 0 peak at 42 ◦ . The average interlayer spacing is calculatedto be 0.34 nm [28,29]. It has been noted that the layeredstructure of CNT is favorable for the transfer of electrons and for aclose interaction between the metal and support by intercalation[30]. Typical TEM images of the 6.0 wt% Ru/CNTs sample, along withits corresponding EDX spectrum, are shown in Fig. 4, which furtherevidences the presence of metallic Ru species. It can be seen fromthe TEM images that the Ru particles with an average size of 2.0 nmare highly dispersed on the surface of CNTs.3.2. Alcohol oxidationThe selective oxidation of benzyl alcohol to benzaldehyde wasused to probe the catalytic activities of the Ru-based catalysts. Theeffect of Ru species, catalyst support, and Ru loading on the catalyticactivity of the supported ruthenium catalyst are shown inTable 1. Among various Ru species tested (entries 1–3), Ru/CNTsshows the highest catalytic activity for the oxidation of benzyl alcohol.For TiO 2 and AC supported Ru catalysts with the same metalloading (entries 4 and 5), the catalytic activity is lower than the CNT-Fig. 4. TEM images and corresponding EDX spectrum of 6.0 wt% Ru/CNTs.

134 X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137Table 1Effects of Ru species, catalyst support, and Ru loading on the activity of the supportedruthenium catalysts for the selective oxidation of benzyl alcohol. a,b .Entry Catalyst Substrate/Ru Conversion (%)1 6.0 wt% RuCl 3/CNTs 11 102 RuO 2 17 153 6.0 wt% Ru/CNTs 17 984 6.0 wt% Ru/TiO 2 17 415 6.0 wt% Ru/AC 17 806 4.0 wt% Ru/CNTs 26 777 2.0 wt% Ru/CNTs 50 41a Reaction conditions: substrate (2 mmol), solvent (toluene: 10 mL, H 2O: 5 mL),85 ◦ C, p(O 2) = 0.1 MPa, 3 h.b Selectivity of benzaldehyde is 100%.supported one, indicating that CNT is a suitable support for alcoholoxidation reactions. As the Ru loading increases from 2 wt% to 6 wt%(entries 7, 6, and 4), the conversion of benzyl alcohol increases correspondingly,while the benzaldehyde selectivity remains at 100%,which is rationalized by the fact that even at higher catalyst loading,small clusters are still dominant.The effect of solvents on the activity of Ru/CNTs catalysts isshown in Table 2. Under identical reaction conditions, the catalyticactivity of Ru/CNTs in water is much higher than in typical polarand non-polar organic solvents such as ethanol and toluene. Whenwater is used as solvent, the main product is benzoic acid, implyingthat water promotes carboxylic acid formation. With tolueneas solvent, a higher conversion is achieved in comparison to thatobtained with the ethanol solvent, and at the same time, a muchhigher selectivity to benzaldehyde is also obtained. With ethanolas solvent, the main product is ethyl benzoate. It is known thatthe solubility of O 2 in the organic solvents is much higher than inwater. Obviously, the concentration of O 2 dissolved in a solvent isnot the governing factor for the remarkable changes in the oxidationof benzyl alcohol in water. In fact, it is water, a weak base,which plays an important role during the reaction [31]. Water canfacilitate H abstraction from alcohol, thus promoting the reactivity.Table 3Effect of bi-solvents on the catalytic activity of Ru/CNTs for the selective oxidationof benzyl alcohol. a .Toluene (mL) Water (mL) Conversion (%) Selectivity ofbenzaldehyde (%)10 1 67 10010 3 92 10010 5 98 10010 7 96 10010 10 67 100a Reaction conditions: benzyl alcohol (2 mmol), Ru/CNTs (Ru: 5.9 mol%), 85 ◦ C,p(O 2) = 0.1 MPa, 3 h.In addition, water can react with the aldehyde to form an aldehydehydrate, which further reacts with oxygen to form carboxylic acid.The effect of co-solvents on the catalytic activity of Ru/CNTscatalysts is shown in Table 3. With toluene as solvent, the conversionof benzyl alcohol is 50% and the selectivity to benzaldehyde is100%. When water is added, a substantial increase in the conversionof benzyl alcohol is obtained. When 5 mL of water and 10 mL oftoluene are used as solvents, the highest conversion of benzyl alcoholof 98% is achieved, and the TOF is 178 h −1 after the first 0.5 hof reaction. Nevertheless, more water is not favored, which is evidencedby the fact that the conversion of benzyl alcohol decreaseswhen more water is added to the system.In the reaction system adopted here, four phases are involved:vapor, organic phase, aqueous phase, and solid catalyst. The aqueousphase and the organic phase are mixed homogeneously byvigorous agitation during the reaction. The aqueous phase isdispersed homogeneously in the organic phase in the form of emulsiondroplets under stirring. Optical microscopy showed that theRu/CNTs catalysts assemble at the surface of the emulsion droplets(Fig. 5). It is known that lipophilic powder-like solids, such as carbonblack and graphite powder, can be used as emulsifying agentsfor a W/O emulsion, where the size of the solid particulates ismuch smaller than that of the dispersed phase. Furthermore, theTable 2Effect of solvents on the catalytic activity of Ru/CNTs for the selective oxidation of benzyl alcohol. a .Solvent Conversion (%) Selectivity (%)Benzaldehyde Benzoic acid EsterEthanol 7 31 0 69Toluene 50 100 0 0Water 100 30 70 0a Reaction conditions: substrate (2 mmol), Ru/CNTs (Ru: 5.9 mol%), solvent (15 mL), 85 ◦ C, p(O 2) = 0.1 MPa, 3 h.Fig. 5. Optical micrographs of emulsion system a) during the process of reaction and b) after reaction formed with 0.2 g of Ru/CNTs catalyst, 2 mmol of benzyl alcohol, 10 mLof toluene, and 5 mL of water. In order to clearly identify the interphases of the water phase and oil phase, we marked water and oil with methylene blue and methyl orange,respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137 135Scheme 1. Schematic model of multiphase reaction system for the selective oxidation of benzyl alcohol to benzaldehyde over Ru/CNTs.solid particulates with rough surfaces and asymmetric shape forma firm solid film at the interfaces of the emulsion droplets, whichmaintains the stability of the emulsion droplets. In the presentstudy, Ru/CNTs with the above-mentioned characteristics also actas emulsifying agents. Here the Ru/CNTs catalysts have two roles:firstly, to provide stability for the emulsion droplets by forminga solid film around the dispersed water droplets and by decreasingthe interfacial tension; secondly, to provide higher interfacialsurface area where the oxidation of alcohol molecules takes place.The reactants and products can be transferred between differentphases via dissolution, diffusion, and extraction. The interfaces ofthe emulsion droplets also favor the quick mass transfer; as theTable 4Aerobic oxidation of alcohols over Ru/CNTs. a .Entry Alcohol Product Conversion (%) Selectivity (%) TOF c (h −1 )1 98 100 692 100 100 713 100 100 714 89 100 635 88 100 626 92 100 657 b 64 100 458 b 72 100 519 b 16 100 1110 24 100 1711 b 85 100 60a Reaction conditions: substrate (2 mmol), Ru/CNTs (Ru: 5.9 mol%), solvent (toluene: 10 mL, H 2O: 5 mL), 85 ◦ C, p(O 2) = 0.1 MPa, 3 h.b Reaction conditions: substrate (2 mmol), Ru/CNTs (Ru: 5.9 mol%), solvent (H 2O: 15 mL), 85 ◦ C, p(O 2) = 0.1 MPa, 3 h.c TOF was calculated on the basis of the analysis data after 3 h reaction.

136 X. Yang et al. / Applied Catalysis A: General 382 (2010) 131–137Scheme 2. Possible reaction mechanism of benzyl alcohol oxidation over the supportedruthenium catalysts.amount of water increases, more interfaces would be available,which results in an increase in the conversion of substrate. Thesolubility of benzaldehyde in toluene is higher than that of benzylalcohol, but its solubility is much lower in water than thatof benzyl alcohol. Toluene extracts benzaldehyde to the organicphase, avoiding further oxidation. As such, a maximum yield canbe obtained at a proper water/toluene ratio. A model is proposedand shown in Scheme 1. Following the classical dehydrogenationmechanism of alcohol oxidation, the possible reaction steps of theoxidation of benzyl alcohol are shown in Scheme 2. The O–H bondof benzyl alcohol breaks upon adsorption at the surface sites, yieldingan adsorbed alkoxide and hydrogen [32]. For the adsorbedalkoxide, the -C–H bond is weaker than other C–H bonds dueto the electron-withdrawing effect of the oxygen atom, resultingin the preferential breaking of the -C–H bond that is the ratedeterminingstep [33], as shown in Scheme 2. Meanwhile, adsorbedactivated oxygen is necessary to oxidize the hydrogen co-product,which helps to shift the equilibrium toward the carbonyl compoundand to accelerate the whole reaction by liberating surfacemetallic sites [34], as schematically shown in Scheme 2. A similarmultiphase reaction system was proposed and discussed before[35,36].The oxidation of other activated and non-activated alcoholswas also tested in the presence of water (Table 4). Four of thesealcohols contain a sulfur or nitrogen atom, or a carbon–carbondouble bond (entries 4–6). The reaction selectivity is 100% in allcases, and all primary and secondary benzylic alcohols are convertedinto the corresponding benzaldehydes and ketones. Primaryallylic alcohol (typical cinnamyl alcohol) affords the correspondingenal (cinnamaldehyde) without intramolecular hydrogen transferor geometrical isomerization of the double bond. The Ru/CNTs systemalso catalyzes the oxidation of alcohols containing nitrogen orsulfur atoms to the corresponding aldehydes in high yields (entries5 and 6), while monomeric Ru complexes cannot effect catalyticoxidation of these alcohols because of strong coordination to themetal center [37]. Alicyclic alcohols such as cyclopentanol andcyclohexanol are selectively oxidized to the corresponding cyclicketones (entries 8 and 9). Less reactive aliphatic alcohols such as 1-octanol and 2-octanol are also oxidized (entries 10 and 11). Whenair is used as oxidant instead of pure O 2 , the conversion of benzylalcohol remains at the same level, demonstrating that the efficientselective oxidation of benzyl alcohol over the as-made Ru/CNTs catalystscan be realized in air, which is of great potential for industrialapplications. When the oxidation of benzyl alcohol is repeated 4times with the same catalyst of Ru/CNTs, the conversion of benzylalcohol remains 92% without any obvious loss in activity, indicatingthat the Ru/CNTs catalyst has excellent stability.4. ConclusionsEffective catalytic oxidation of alcohols under mild conditionshas been realized in an emulsion system, in which CNT-supportedruthenium composites function both as catalysts and as emulsifyingagents. Ru/CNTs catalysts are prepared by the traditionalwetness impregnation method, and show excellent activity, selectivity,and stability for the selective oxidation of benzyl alcohol withoxygen or air as oxidant. The catalytic activity of Ru/CNTs for theselective oxidation of benzyl alcohol is greatly enhanced by water,which is due to the formation of emulsion droplets where the CNTsupportedcatalysts assemble at the interfaces. Ru/CNTs catalystsare of potential for the selective oxidation of a large variety ofalcohols in the presence of water. Furthermore, the catalyst canbe easily separated and recycled by sedimentation.The results reported here also highlight the unique propertyand functions of solid catalysts assembled at the interfaces of twoliquid phases to form special emulsion catalysis systems, whichleads one to anticipate that such emulsion-stabilizing solid catalystsafter being further tailored will be of wide use in a broad rangeof reactions.AcknowledgementsThis work is partly supported by NSFC (Nos. 20725619,20836002). 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