Enhanced Catalytic Activities and Characterization of Ruthenium-Grafted HalogenousHydroxyapatite Nanorod Crystallites

Yanjie Zhang,† Junhu Wang,*,† Jie Yin,†,‡ Kunfeng Zhao,†,‡ Changzi Jin,† Yuying Huang,§

Zheng Jiang,§ and Tao Zhang*,†

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457Zhongshan Road, Dalian 116023, China, Graduate School of Chinese Academy of Sciences, Beijing 100049, China,and Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences,Shanghai 201204, China

ReceiVed: May 1, 2010; ReVised Manuscript ReceiVed: August 18, 2010

The nanorod crystallites of ruthenium-grafted halogenous hydroxyapatite (RuXAp, X ) F, Cl or Br) werenewly developed through a facile method and identified as highly efficient catalysts for the aerobic oxidationof alcohols. Compared with RuHAp for selective oxidation of benzyl alcohol, the existence of F, Cl, and Brelements in hydroxyapatite dramatically enhanced the catalytic activity with a prominent selectivity of farmore than 99%. In particular, the RuClAp and RuFAp catalysts, respectively, showed the excellent catalyticactivity of TOF ) ∼233 and 210 h-1, which was nearly 3 times higher than that of RuHAp. The RuFApcatalyst was furthermore demonstrated to be recyclable and available to be applied for various alcohols. Onthe basis of the DRIFT and XAFS results, the enhanced activities could be preliminarily ascribed to theelectron-withdrawing effect of halogens and the greater amounts of active species existing in the surface ofRuXAp compared with that of RuHAp.

1. Introduction

The selective oxidation of alcohols to the correspondingaldehydes and ketones is a pivotal intermediate process for thepharmaceutical and fine-chemical industries. Because of thetoxicity and large amounts of wastes, the use of traditionaloxidants (such as chromates, hypochlorites, and permanganates)has been limited. In view of the economy and environment,catalytic oxidation with molecular oxygen or air is particularlyattractive as a “green” technology. In the past years, there hasbeen a growing demand for effective catalysts in the selectiveoxidation of alcohols. Many supported noble metals (such aspalladium, ruthenium, and gold) solid catalysts have beendeveloped and thoroughly investigated.1-4 Supported Au andPd catalysts are more active and achieve remarkable reactionrates (TOF up to 269 000 and 12 500, respectively, for Au-Pd/TiO2 and Au/CeO2; see Table S1 in the Supporting Information)under solvent-free conditions. However, the poor selectivity andrequired high reaction temperature (433 K) limits its application.Just as reported in Kobayashi’s review article,2 a wide substratescope and high selectivity toward the target compound are veryimportant for selective oxidation of alcohols. Supported ruthe-nium catalysts, such as Ru/Al2O3,5 Ru(OH)x/TiO2,6 RuO2/FAUzeolite,7 Ru-Co-Al-hydrotalcite,8 Ru(III)/TiO2 nanotube,9 andRu/Ni(OH)2 composite,10 have drawn tremendous interests fortheir unique characteristics, including widest substrate scope,moderate activity, high selectivity to aldehydes or ketones, andmild operating conditions.

Recently, hydroxyapatite (HAp)-based materials have at-tracted more and more attention as solid and recyclable catalysts.

Rutheniuim hydroxyapatite (RuHAp) has been demonstrated tobe an efficient catalyst on the aerobic oxidation of alcohols,11-13

amines,14 and organosilanes.15 RuHAp is a highly selectivecatalyst (>99%) for the partial oxidation of alcohols. However,only a low alcohol oxidation activity (TOF ) 2 h-1) could beobtained.11 To date, much effort has been focused on theimprovement of RuHAp’s catalytic activity, such as the additionof a metal promoter (RuCoHAp, TOF ) 78 h-1),16 the additionof γ-Fe2O3 in the HAp matrix (TOF ) 196 h-1),17 and theorganic modification (RuHAp-BAcid, TOF ) 242 h-1).13 Theorganically modified RuHAp achieved high activity; however,its application was limited by the complicated preparationprocess. Therefore, the design and development of a moreefficient Ru-based catalyst with a low cost and environmentalfriendliness still remains a challenge for its application in theselective oxidation of alcohols to the corresponding aldehydesand ketones.

The anion exchange property of HAp allows us to designand develop the novel HAp-based materials for certain specificapplications. It has been well-known that fluorine, chlorine, andbromine can be incorporated into an HAp crystal lattice, partlyor totally replacing the hydroxyl group.18,19 Incorporation offluoride ions dramatically improves the surface stabilization ofthe crystal and is an important contributor to the higher thermalstability, chemical durability, and lower solubility.20-22

Up to now, to the best of our knowledge, there is no approachfor promoting the catalytic activity of RuHAp by focusing onthe modification of the hydroxyl group in HAp. It is knownthat the fundamental properties of HAp, such as solubility, acidfastness, acid-base property, and crystallinity, are easily tunedby modifying the anion group. On the basis of the aboveconsiderations, we present a new strategy for the design of ananostructured heterogeneous catalyst with the substitution ofthe hydroxyl group in the HAp crystal. Surface stabilization byhalogen ions is an important contributor to the lower solubility.

* To whom correspondence should be addressed. Tel: +86 411 84379015(T.Z.). Fax: +86 411 84691570 (T.Z.). E-mail: taozhang@dicp.ac.cn (T.Z.),wangjh@dicp.ac.cn (J.W.).

† Dalian Institute of Chemical Physics, Chinese Academy of Sciences.‡ Graduate School of Chinese Academy of Sciences.§ Shanghai Institute of Applied Physics, Chinese Academy of Sciences.

J. Phys. Chem. C 2010, 114, 16443–16450 16443

10.1021/jp1039783 2010 American Chemical SocietyPublished on Web 09/01/2010

The incorporation of different anion groups (F, Cl, and Br) intoHAp promotes a network of surface hydrogen bonding withsurface OH groups and, consequently, enhances the stability ofsurface atoms. The improvement of surface stability of the HApcrystal is considered to change the chemical surroundings ofRu active sites and finally influence its catalytic performance.

Here, we report the effect of anion exchange on the catalyticproperties of RuHAp and used the aerobic oxidation of benzylalcohol as a model reaction. In this study, RuFAp, RuClAp,and RuBrAp were identified as more efficient catalysts comparedwith RuHAp for the selective oxidation of benzyl alcohol tobenzaldehyde. A particular order of their catalytic activity wasconfirmed as RuClAp > RuFAp > RuBrAp > RuHAp. TheRuFAp catalyst was furthermore confirmed to have a highactivity and selectivity for the oxidation of a wide range ofalcohols and could be recyclable. The RuXAp catalysts weremainly investigated by XAFS and DRIFT studies of COadsorption to gain insight into the nature of catalytic perfor-mance. The results indicated that the improved catalyticactivities were resulted from the electron-withdrawing effect ofhalogens and the greater amounts of active species existing inthe surface of RuXAp.

2. Experimental Section

2.1. Preparation of Catalysts. The chemical precipitationmethod was used for synthesizing the XAp (X ) F, Cl, andBr) crystal. Separately, using aqueous calcium nitrate (0.03 mol),diammonium hydrogen phosphate (0.018 mol), and 0.006 molof NaF (or NaCl, NaBr) as precursor solutions (pH ) ∼2,adjusted by nitric acid), the precipitate was obtained when thepH value of the mixed solution was adjusted to ∼8 by additionof ammonia under vigorous magnetic stirring. The suspensionwas sealed in a jar and then kept inside a water bath for 24 hat 60 °C. The precipitate was separated by centrifugation,washed with deionized water several times until the pH of thefiltrate was ∼7, and dried at 60 °C. Finally, the precursorpowders (XAp) were obtained. There was no further heattreatment for XAp in this method, which was usually carriedout at 500 °C in other former reports.10 The RuXAp catalysts(Ru ) 1 wt %) were separately prepared by immersion of 1 gof XAp in a 70 mL aqueous RuCl3 solution (1.41 × 10-3 M)with vigorous magnetic stirring under ambient conditions. Fora comparison, the RuHAp catalyst (Ru ) 1 wt %) was alsoprepared by the chemical precipitation and then immersionmethods as described above without using the sodium halideprecursor solution. The actual weight contents of Ru wereseparately measured by ICP as 1.17, 1.21, 0.85, and 1.02% forRuHAp, RuFAp, RuClAp, and RuBrAp catalysts.

2.2. Catalytic Performance Test. Oxidations of alcoholswere typically carried out as follows: A suspension of theprepared 0.1 g of Ru-based catalyst in toluene (10 mL) wasmagnetically stirred, and the substrate (1 mmol) was then added.The resulting mixture was kept at 80 °C under an O2 flow (20mL/min) for proceeding with the aerobic oxidation of alcoholreactions. The selectivity and conversion were determined byGC analysis (Agilent 6890, equipped with an HP-FFAP capillarycolumn and FID detector) using mesitylene as an internalstandard. For the RuFAp catalyst, the reusability was also testedby using the aerobic oxidation of benzyl alcohol as a modelreaction. Furthermore, the RuFAp catalyst was applied toaerobically oxidize a wide range of alcohols, including severalrepresentative benzylic, allylic, and alphatic alcohols.

2.3. Characterization. Phase analysis of the prepared cata-lysts was conducted using X-ray diffraction (XRD) with Cu

KR radiation (λ ) 1.5418 Å). The diffractometer (X’pert ProSuper, PANAnalytical) was operated at a 2θ range of 10-80°with a step size of 0.02°. The prepared catalysts were alsocharacterized by Fourier transform-infrared spectroscopy (FT-IR, Equinox 55, Bruker) in the range of 4000-400 cm-1. Theirdiffuse reflectance infrared (DRIFT) spectra were collected withan Equinox 55 spectrometer (Bruker) at a resolution of 4 cm-1.The CO adsorption spectra were taken at 80 °C in a 3.9% CO/He flow on the catalyst powders pretreated after a He flow for30 min. The morphology and grain sizes of the powders wereobserved through transmission electron microscopy (TEM,Philips CM200 and JEM-2000EX). The chemical compositionof the prepared catalysts was separately obtained by inductivelycoupled plasma-atomic emission spectrometry (ICP-AES, Ther-mo IRIS Intrepid II spectrum apparatus) and X-ray fluorescencespectrometry (XRF, PANAnalytical, Axios 2.4 kw).

Ru K-edge X-ray absorption fine structure (EXAFS andXANES) spectra of the prepared catalysts were made at theBL14W1 beamline of SSRF, SINAP (Shanghai, China), withthe use of a Si(111) crystal monochromator. The storage ringwas operated at 3.5 GeV with injection currents of 100 mA.Ru foil and RuCl3 ·3H2O were used as reference samples, andtheir X-ray absorption spectra were measured in the transmissionmode. All spectra of the prepared catalysts were conducted inthe fluorescence mode. The raw data were energy-calibrated (RuK-edge energy of Ru foil ) 22117 eV, first inflection point),background-corrected, and normalized using the IFEFFITsoftware. Fourier transformation of the EXAFS data was appliedto the k2-weighted functions, respectively. For the curve-fittinganalysis, theoretical backscattering phases and amplitudes forRu-O bonding were calculated from the data of RuO2.

3. Results and Discussion

3.1. Properties of the Catalysts. Information on the mor-phology, crystallinity, and chemical composition of the preparedcatalysts was extracted from the results of TEM, XRD, FT-IR,and XRF characterizations, respectively. Figure 1 shows TEMmicrographs of the prepared catalysts along with the results ofHAp and FAp for a comparison. Figure 1a,b shows the TEMmicrographs of precursor powders for HAp and FAp, respec-tively. The rodlike particles with a length of less than 50 nmwere observed in Figure 1a. Taking into account the HAp latticeconstants (a ) 9.422 Å and c ) 6.883 Å) and the hexagonalsymmetry with the space group P63/m, its unit cell will bearranged along the c axis. The Ostwald ripening occurred duringthe aging process and facilitated the growth habit of HAp alongthe c axis. As seen from Figure 1b, the legible rodlikemorphology was obtained for FAp with a length of clearly morethan 50 nm, which indicated that introducing a F anion enhancedthe crystallinity and facilitated the growth of the HAp crystalalong the c axis. It was clear that the prepared RuHAp andRuFAp catalysts had similar rodlike morphologies, as shownin Figure 1c,d. Meanwhile, the typical sizes of the rodlikeparticles in Figure 1c,d were, respectively, found to be identicalto those in Figure 1a,b, demonstrating that the crystal morphol-ogy almost did not change when the Ru ions were exchangedduring the immersion process. The typical size of the RuFApcrystal was 60-80 nm in length, and it had an aspect ratio of4-5. Both of the prepared RuClAp and RuBrAp catalysts hadvery similar morphology and particle size with that of RuHAp,as shown in Figure 1e,f.

Figure 2 shows XRD patterns of the prepared catalysts alsoincluding the result of HAp for a comparison. It could beobserved that all of the prepared catalysts were single phase

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and had a high crystallinity. All peaks correspond to HAp basedon the standard XRD pattern card of HAp (JCPDS card no.09-432), indicating that the RuHAp, RuFAp, RuClAp, andRuBrAp were isostructural with the HAp crystal. A detailcomparison confirmed that almost no diffraction peak shift could

be detectable after either the introduction of different halogenions or the grafting of Ru ions during the immersion process.

The typical FT-IR spectra of the prepared RuHAp and RuFApcatalysts are shown in Figure 3. The characteristic bands of 3571and 632 cm-1 were assigned to the hydroxyl group in the HApcrystal, whereas the broad band at 3427 cm-1 was attributed tothe adsorbed water. The bands at 1096, 1032, 962, 603, and564 cm-1 were attributed to PO4

3- ions (ν1s962 cm-1, ν3s1032and 1096 cm-1, ν4s564 and 603 cm-1). No HPO4

2- group wasfound due to the absence of the band at 875 cm-1. The FT-IRspectrum (Figure 3, spectrum b) of RuFAp differs significantlyfrom that of RuHAp (Figure 3, spectrum a). The stretchingvibration of the hydroxyl group at 3571 cm-1 was not observedfor RuFAp, indicating that almost all of the hydroxyl groups inRuHAp were substituted by fluorine ions. The FT-IR spectrumof RuClAp indicated that only part of the hydroxyl groups weresubstituted by chloride ions because of the existence of thestretching vibration in 3572 cm-1, as shown in Figure S1 inthe Supporting Information. As listed in Table 1, the contentsof Ru and halogen elements in the prepared catalysts weredetermined by ICP-AES and XRF, respectively. The XRF resultswere consistent with the FT-IR spectroscopic studies, and thesubstitution values (n) of the hydroxyl groups by the halogenions were separately evaluated to be 1.84, 0.2, and 0.34 for

Figure 1. TEM micrographs of the prepared catalysts also showingthe results of HAp and FAp for a comparison: (a) HAp, (b) FAp, (c)RuHAp, (d) RuFAp, (e) RuClAp, and (f) RuBrAp.

Figure 2. XRD patterns of the prepared catalysts also including the result of HAp for a comparison: (a) HAp, (b) RuHAp, (c) RuFAp, (d) RuClAp,and (e) RuBrAp. All the peaks corresponded to HAp based on the standard XRD pattern card of HAp (JCPDS card no. 9-432).

Figure 3. FT-IR spectra of (a) RuHAp and (b) RuFAp.

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RuFAp, RuClAp, and RuBrAp based on the general chemicalformulation Ca10(PO4)6(OH)2-n(X)n (X ) OH, F, Cl, or Br; 0e n e 2).

3.2. Benzyl Alcohol Oxidation. The influence of halogenson the catalytic activity of RuHAp was investigated in the liquid-phase aerobic oxidation of benzyl alcohol, which was the mostcommonly used model compound in alcohol oxidation. Also, arelative mild reaction temperature (353 K) was employed inthe reaction. The average turnover frequency (TOF) wasintroduced approximately to assess the performance of catalysts.The total amount of Ru3+ was used in the calculation of theTOF due to the difficulty in estimating the population of activesites. As shown in Table 1, the introduction of halogen elementsdramatically enhanced the activity of the RuHAp catalyst, even∼3 times the activity of RuHAp in the case of RuFAp andRuClAp. In Table 1, under mild reaction conditions (catalyst,0.1 g; temperature, 353 K; reaction time, 0.5 h; benzyl alcohol,1 mmol; toluene, 10 mL), a more than 99% yield was achievedfor the halogenous Ru-based catalysts of RuFAp, RuClAp, andRuBrAp. For all of the four catalysts, the selectivity forbenzaldehyde was always far more than 99%.

Other mild reaction conditions were employed to distinguishthe activity of those catalysts, which were 0.05 g of catalystfor 0.5 h. Thus, with the average TOF as a basis for activitycomparison, the particular order of activity was obtained asRuClAp > RuFAp > RuBrAp > RuHAp (Figure 4), confirmingthat the existence of F, Cl, and Br elements dramaticallyenhanced the catalytic activity of RuHAp for aerobic oxidationof benzyl alcohol to the corresponding benzaldehyde. In thepresent study, the catalytic performance for the RuHAp andRuFAp catalysts differed from that reported by Tonsuaadu etal., in which the catalytic ability was reported to be higher forRuHAp compared with RuFAp in the aerobic oxidation ofbenzyl alcohol.23 The difference could probably be ascribed tothe large size and different morphology. Therefore, the RuXAp(X ) F, Cl, Br) crystallites prepared in this study were highlyexpected to be the novel series of catalysts for the efficientaerobic oxidation of alcohols under mild reaction conditionswith a low Ru content of 1 wt %. Among them, RuFAp catalystwas the preferential candidate because of its stability in chemicalcomposition.

3.3. XAFS Analysis. X-ray absorption near-edge fine struc-ture spectroscopy (XANES) and extended X-ray absorption finestructure spectroscopy (EXAFS) were employed to investigatethe Ru species grafted on the prepared catalysts. Figure 5Ashows the Ru K-edge XANES spectra of RuXAp as well asRuCl3 ·3H2O and Ru foil. By comparing to the reference spectraof RuCl3 ·3H2O and Ru foil, it could be concluded that Ru3+

species existed in the prepared catalysts. All the spectra ofRuXAp exhibited no significant difference between their K-edgeenergies. Their near-edge features were identical, which indi-cated the similar short-range structures among all of RuXAp.Figure 5B shows the Fourier transform (FT) of the k2-weightedRu K-edge EXAFS of the different samples. Obviously, theshape of the spectra in Figure 5B was similar for all the samples,and no Ru-Ru bonding contribution was observed. Representa-tive experimental and theoretical fittings of R space for theEXAFS signals are shown in Table 2. RuXAp showed thesimilar nearest neighbors with two and four oxygen atomsattributed to Ru-OH species and Ru-O-P connectivity,respectively. In RuFAp, RuClAp, and RuBrAp crystals, Ru3+

ions were surrounded by six oxygen atoms, which wereconsistent with the results of RuHAp obtained by the otherresearch groups.24 Figure S2 (Supporting Information) showsthe relationship between the TOF and coordination number (CN)of nearest-neighbor Ru atoms. The TOF increased with theincrease in the CN (the increase in the amounts of the hydratedRu3+ oxide species), reached a maximum with RuClAp, andthen decreased a little with RuFAp. A particular tendencycould be found that RuClAp exhibited the shortest Ru-O1 bond(1.836 Å) in all the prepared catalysts. The decrease of bonddistance (or increase of bond energy) implied the presence ofRu species with a higher oxidation state in the RuClAp crystal.

3.4. DRIFT Results. The use of CO as a probe moleculeallowed us to determine the coordination environment and thestructure of the Ru active site grafted in the prepared catalysts.Baiker’s group reinvestigated the RuHAp catalyst by a DRIFTstudy of CO adsorption and attributed the real active sites tosmall hydrated Ru3+ oxide nanoparticles grafted on the surfaceof HAp.25,26 The time-dependent adsorption of CO on theprepared RuHAp and RuFAp catalysts at 353 K is shown inFigure 6A,B, respectively. For the spectra of RuFAp (Figure6B), the increased intensity with adsorption time of the threebands at 2067, 1985, and 1954 cm-l was due to the COadsorption. The bands at 2177 and 2117 cm-l originated fromthe signals of gaseous CO adsorption. In the high-frequencyregion (Figure S3 in the Supporting Information), the negativeband at 3570 cm-l indicated the removal of the OH group inthe HAp crystal during the CO adsorption process. In FigureS4 (Supporting Information), the negative feature at 3570 cm-l

was absent because of the substitution of hydroxyl groups by

TABLE 1: Oxidation of Benzyl Alcohol with MolecularOxygen over the Prepared Ru-Based Catalysts

catalysttime(h)

conversiona

(%)selectivity

(%)Ru

(wt %)halogenc

(wt %)substitution

value (n)

RuHApb 1 69.6 >99 1.17 0 0RuFAp 0.5 >99 >99 1.21 3.25 1.84RuClAp 0.5 >99 >99 0.85 0.7 0.2RuBrAp 0.5 >99 >99 1.02 2.57 0.34

a 0.1 g of catalyst, benzyl alcohol (1 mmol), toluene (10 mL),353 K, O2 flow. Conversion and selectivity were determined by GCusing an internal standard technique. b A Ru content of 1.17 wt %,TOF ) 60 h-1 (average turnover frequency, related to the totalamount of Ru3+). c Halogen content was detected by XRF, and thesubstitution value was calculated based on the general chemicalformulation Ca10(PO4)6(OH)2-n(X)n (X ) OH, F, Cl, or Br; 0 e ne 2).

Figure 4. Discriminative catalytic performance of the preparedcatalysts in the aerobic oxidation of benzyl alcohol. 0.05 g ofcatalyst, benzyl alcohol (1 mmol), toluene (10 mL), 353 K, O2 flow.Selectivity for benzaldehyde always > 99%. Average turnoverfrequency calculated by using the total amount of Ru3+ determinedby ICP-AES analysis.

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fluorine ions in the RuFAp crystal. The slow adsorption of COwas observed in time-resolved DRIFT spectra (Figure 6C,D)of RuClAp and RuBrAp. The spectra obtained after 10 min ofadsorption time for all the catalysts are shown in Figure S5(Supporting Information. The weak adsorption signals wereobtained for RuClAp and RuBrAp even after 10 min ofadsorption of CO.

Adsorption of CO caused the reduction of Ru3+ ions in thosesamples. Following assignments discussed in the literature,25,27-31

the band at 2067 cm-l in the catalysts could be assigned totricarbonyl species [Run+(CO)3]. In the Ru-RuOx/TiO2 cata-lyst,29 the asymmetric broadening below 2000 cm-l was assignedto the monocarbonyl form of CO linearly bonded with rutheniumof different oxidation states [RuOx(CO) ) 2000-1985 cm-l

(LF1, low frequency) and 1950-1935 cm-l (LF2, low fre-quency)]. In this work, the band below 2000 cm-l alsocorresponded to the presence of the RuOx (x < 2) species. Theintensity of the low-energy shoulder (1954 cm-l for RuFAp and1963 cm-l for RuBrAp) below 2000 cm-l correlated well withthe fraction of the hydrated Ru3+ oxide small nanoparticles.25

Therefore, the fraction of these hydrated Ru3+ oxide smallnanoparticles could be roughly evaluated in the sequence RuFAp> RuBrAp > RuHAp, which could be one of the main reasonsfor resulting in the differences of catalytic performance in thesecatalysts. In the case of RuClAp, the situation was a little special;the band at 2023 cm-l had already been observed, which wasassociated with a lower valence state Ru containing carbonylspecies reduced by CO.25,27 The more active Ru species shouldbe mainly responsible for the excellent catalytic activity ofRuClAp.

In addition, in Figure 7, a blue shift from 1985 to 1999 cm-l

was observed for the prepared catalysts (RuFAp, 1985 cm-l;RuHAp, 1991 cm-l; RuClAp, 1992 cm-l; RuBrAp, 1999 cm-l).The bonding of CO to Run+ ions grafted on the catalyst surfacewas largely determined by the stronger back-donation of delectrons from the metal into the 2π* orbitals.28 When the moreelectronegative elements were present, the electron density onRu species was influenced by the different electronegativity.Therefore, the blue shifts in the DRIFT spectra were apparentlyascribed to be originated from the different surroundings of Ru3+

oxide small nanoparticles grafted on FAp, HAp, ClAp, or BrApcontaining different elements with different abilities of with-drawing electrons.

Briefly, the enhanced activity of the prepared RuXAp catalystscould be preliminarily ascribed to two reasons: First, the greateramounts of active species of small hydrated Ru3+ oxidenanoparticles existing in the surface of RuXAp compared withRuHAp. This was mainly considered to be originated fromsurface stabilization and the high acid resistance of halogenoushydroxyapatite compared with HAp. Second, the differentsurroundings of Ru3+ oxide small nanoparticles grafted on FAp,HAp, ClAp, or BrAp with different electron-withdrawing effects.

3.5. Proposed Reaction Mechanism. A reaction mechanismwas proposed for the aerobic oxidation of alcohols over RuXApcatalysts. Generally, aerobic alcohol oxidation on a supportedruthenium catalyst involved three steps:24 formation of Ru-alcoholate species (step 1), �-hydride elimination (step 2), andreoxidation of hydrido-ruthenium species by molecular oxygen(step 3). Dehydrogenation (step 2) played a key role in thereaction mechanism, and it would be the rate-determining step.The reaction rate of the rate-determining �-hydride eliminationfor different catalysts was investigated through the oxidation

Figure 5. (A) X-ray absorption near-edge fine structure spectra ofdifferent samples. (B) Fourier-transformed EXAFS data (k2-weighted�(k)-function, 2.2-11.2 Å-1) of (a) RuHAp, (b) RuFAp, (c) RuClAp,and (d) RuBrAp.

TABLE 2: Structural Parameters of RuXAp Determined bythe First Shell Fitting of EXAFS Spectra at the Ru K-Edge

samples shell

interatomicdistance

R/Åcoordination

numberDebye-Waller

factor δ2/Å2 R-factor

RuHAp Ru-O1 1.919 2.4 0.0133 0.0126Ru-O2 1.997 4.4 0.0095Ru-Ru 3.129 1.5 0.0062

RuFAp Ru-O1 1.903 2.0 0.0166 0.0156Ru-O2 1.998 4.8 0.0099Ru-Ru 3.130 3.1 0.0132

RuClAp Ru-O1 1.836 2.6 0.0047 0.0071Ru-O2 1.998 4.4 0.0030Ru-Ru 3.127 2.9 0.0110

RuBrAp Ru-O1 1.937 1.7 0.0006 0.020Ru-O2 2.050 4.2 0.0054Ru-Ru 3.172 2.4 0.0091

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of benzyl alcohol under anaerobic conditions. The conversionof benzyl alcohol under an Ar atmosphere was confirmed to be16.1, 22.5, 29.1, and 17.8%, respectively, for RuHAp, RuFAp,RuClAp, and RuBrAp, which was consistent with the particularorder of activity as RuClAp > RuFAp > RuBrAp > RuHAp.

Therefore, the introduction of halogens into HAp (especiallyfor Cl and F) would effectively facilitate �-hydride elimination.

3.6. Aerobic Oxidation of Other Alcohols and Reusabilityof the RuFAp Catalyst. To further evaluate the catalyticperformance of RuFAp, its recycling use and aerobic oxidationof a wide range of alcohols were also studied. The resultsdemonstrated that the RuFAp catalyst was highly active in theoxidation of various alcohols with molecular oxygen under mildconditions, and in all the reactions shown in Table 3, thecorresponding aldehyde was the only detectable product. Severalrepresentative benzylic, allylic, and alphatic alcohols listed inTable 3 were selected to investigate the correlation betweenconversion and time. In particular, benzylic and allylic alcoholsshowed higher reactivity than alphatic alcohols (entries 1-6).The conversions increased rapidly with time in the initialreaction stage, and then the rate became slow. However, aninteresting trend could be observed in the present work. Despitethe great difference in activity, there was not much differencefor the conversion of benzylic, allylic, and alphatic alcoholsexcept for benzyl alcohol at the initial 1 h (entries 2, 3, and 4).The oxidation of benzyl alcohol proceeded much faster thanthat of 1-phenylethanol for the RuFAp catalyst. The fasteroxidation of benzyl alcohol suggested that it highly tended toform the alcoholate species via the ligand exchange withruthenium active species in RuFAp.

Figure 6. Time-dependent DRIFT spectra of (A) RuHAp and (B) RuFAp: (a) DRIFT spectra recorded after 0, 1, 3, 5, and 10 min, (b) DRIFTspectrum obtained after absorption of CO, followed by desorption in He for 10 min. (C) RuClAp and (D) RuBrAp: (a) DRIFT spectra recordedafter 0, 1, 3, 5, 10, 20, and 30 min, (b) DRIFT spectrum obtained after absorption of CO, followed by desorption in He for 10 min.

Figure 7. DRIFT spectra of RuFAp and RuHAp after adsorption ofCO for 10 min, RuClAp and RuBrAp for 30 min.

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The reusability of RuFAp for the aerobic oxidation of benzylalcohol was also investigated. The recovered catalyst was reusedafter being separately washed with ethanol and deionized waterseveral times and dried at 333 K. The conversion of benzylalcohol and the selectivity to benzaldehyde could be kept over99% in the third recycling experiment, as shown in Figure 8.Therefore, for this substrate, the newly developed RuFApcatalyst can be easily reused at least three times with almostthe same high catalytic activity and selectivity as the first run.The reusability of RuFAp for the aerobic oxidation of 1-octanolis shown in Figure S6 (Supporting Information). It showed an∼20% loss in the conversion of 1-octanol during the secondcycle, but the selectivity could be kept far more than 99% evenin the third recycling experiment. The reusability of RuFAp waslimited for 1-octanol, and the decrease of its catalytic perfor-mance was probably attributed to the partial covering of Ruactive species.

4. Conclusion

A new series of RuXAp (X ) F, Cl, or Br) catalysts with aspecial rodlike morphology were developed through a facilemethod as efficient heterogeneous catalysts for the aerobicoxidation of alcohols. The prepared catalysts showed very highactivity at a low Ru content (1 wt %) under mild reactionconditions. A particular order of activity was obtained asRuClAp > RuFAp > RuBrAp > RuHAp for the aerobic oxidation

of benzyl alcohol at the same conditions. Further studiesdemonstrated that the RuFAp catalyst was also highly active inthe oxidation of various alcohols and could be easily reusedwith the high catalytic activity and superior selectivity. DRIFTcombined with XAFS spectroscopies and several other tech-niques provided insight into the nature of the catalytic perfor-mance of the catalysts. The enhanced catalytic activities ofRuXAp should be related to the electron-withdrawing effect ofhalogens as well as the structural and physicochemical propertiesof the HAp’s surface and bulk modified by the addition ofhalogens. The present study demonstrated that modification ofhydroxyl groups in the crystal lattice of HAp by halogens wasan effective method for promoting the performance of HAp-based heterogeneous solid catalysts.

Acknowledgment. Financial support obtained from theChinese Academy of Sciences for “100 Talents” Project, theNatural Science Foundation of Liaoning Province (No.20092173), and the National Science Foundation of Chinafor Distinguished Young Scientists (No. 20325620) is greatlyacknowledged. This work was also supported by the Scienceand Technology Commission of Shanghai Municipality ofChina (Project No. 09JC1417100). The authors also thankthe XAFS beamline staff at Shanghai Synchrotron RadiationFacility (SSRF) for assisting with the XAFS data collectionand data analysis.

Supporting Information Available: Experimental detailsof the oxidation of benzyl alcohol under anaerobic conditions;comparison of highly selective catalysts in the oxidation ofbenzyl alcohol to benzaldehyde with oxygen (Table S1); FT-IR spectrum of RuClAp (Figure S1); relationship betweenTOF and coordination number (CN) of nearest-neighbor Ruatoms for RuHAp, RuFAp, RuClAp, and RuBrAp (FigureS2); time-resolved DRIFT spectra of RuHA after 0, 1, 3, 5,and 10 min (Figure S3); time-resolved DRIFT spectra ofRuFAp after 1, 3, 5, and 10 min (Figure S4); DRIFT spectraof RuFAp, RuHAp, RuClAp, and RuBrAp after adsorptionof CO for 10 min (Figure S5); and reusability of the RuFApcatalyst for the aerobic oxidation of 1-octanol (Figure S6).This material is available free of charge via the Internet athttp://pubs.acs.org.

References and Notes

(1) Mallat, T.; Baiker, A. Chem. ReV. 2004, 104, 3037–3058.(2) Matsumoto, T.; Ueno, M.; Wang, N.; Kobayashi, S. Chem.sAsian

J. 2008, 3, 196–214.(3) Pinxt, H. H. C. M.; Kuster, B. F. M.; Martin, G. B. Appl. Catal.,

A 2000, 191, 45–54.(4) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.;

Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.;Hutchings, G. J. Science 2006, 311, 362–365.

(5) Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2002, 41, 4538–4542.

(6) Yamaguchi, K.; Kim, J. W.; He, J.; Mizuno, N. J. Catal. 2009,268, 343–349.

(7) Zhan, B. Z.; White, M. A.; Sham, T. K.; Pincock, J. A.; Doucet,R. J.; Rao, K. V. R.; Robertson, K. N.; Cameron, T. S. J. Am. Chem. Soc.2003, 125, 2195–2199.

(8) Matsushita, T.; Ebitani, K.; Kaneda, K. Chem. Commun. 1999, 265–266.

(9) Bavykin, D. V.; Lapkina, A. A.; Plucinski, P. K.; Friedrich, J. M.;Walsh, F. C. J. Catal. 2005, 235, 10–17.

(10) Venkatesan, S.; Senthil Kumar, A.; Lee, J. F.; Chan, T. S.; Zen,J. M. Chem. Commun. 2009, 1912–1914.

(11) Yamaguchi, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K.J. Am. Chem. Soc. 2000, 122, 7144–7145.

(12) Bavykin, D. V.; Lapkin, A. A.; Kolaczkowski, S. T.; Plucinski,P. K. Appl. Catal., A 2005, 288, 175–184.

TABLE 3: Aerobic Oxidation of Primary Alcohols andSecondary Alcohols to Carbonyl Compounds Using RuFApas a Catalyst

entry substrate time (h) conversiona (%) selectivity (%)

1 benzyl alcohol 0.5 >99 >992 1-phenylethanol 1 76 >99

2.5 98 >993 cinnamyl alcohol 1 64 >99

3 89 >994 97 >99

4 1-octanol 1 54 >995 1-octanolb 2 70 >99

6 87 >996 2-octanolb 2 75 >99

a 0.1 g of RuFAp (1 wt % Ru), substrate (1 mmol), toluene (10mL), 353 K, O2 flow. b 0.2 g of RuFAp (1 wt % Ru) was used.

Figure 8. Reusability of the RuFAp catalyst for the aerobic oxidationof benzyl alcohol. Reaction conditions: catalyst, 0.1 g; benzyl alcohol,1 mmol; solvent (PhCH3), 10 mL; O2 flow rate, 20 mL/min; temper-ature, 353K; time, 0.5 h.

Ru-Grafted Halogenous Hydroxyapatite Crystallites J. Phys. Chem. C, Vol. 114, No. 39, 2010 16449

(13) Opre, Z.; Ferri, D.; Krumeich, F.; Mallat, T.; Baiker, A. J. Catal.2006, 241, 287–295.

(14) Mori, K.; Yamaguchi, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K.Chem. Commun. 2001, 461–462.

(15) Mori, K.; Tano, M.; Mizuaki, T.; Ebitani, K.; Kaneda, K. NewJ. Chem. 2002, 26, 1536–1538.

(16) Opre, Z.; Grunwaldt, J.-D.; Maciejewski, M.; Ferri, D.; Mallat, T.;Baiker, A. J. Catal. 2005, 230, 406–419.

(17) Mori, K.; Kanai, S.; Hara, T.; Mizugaki, T.; Ebitani, K.; Jitsukawa,K.; Kaneda, K. Chem. Mater. 2007, 19, 1249–1256.

(18) Kannan, S.; Rebelo, A.; Lemos, A. F.; Barba, A.; Ferreira, J. M. F.J. Eur. Ceram. Soc. 2007, 27, 2287–2294.

(19) Schlesinger, P. H.; Blair, H. C.; Teitalbaum, S. L.; Edwards, J. C.J. Biol. Chem. 1997, 272, 18636–18643.

(20) Bengtsson, Å.; Shchukarev, A.; Persson, P.; Sjoberg, S. Langmuir2009, 25, 2355–2362.

(21) Chaırat, C.; Oelkers, E. H.; Schott, J.; Lartigue, J.-E. Geochim.Cosmochim. Acta 2007, 71, 5888–5900.

(22) Bengtsson, Å.; Shchukarev, A.; Persson, P.; Sjoberg, S. Geochim.Cosmochim. Acta 2009, 73, 257–267.

(23) Tonsuaadu, K.; Gruselle, M.; Villain, F.; Thouvenot, R.; Peld, M.;Mikli, V.; Traksmaa, R.; Gredlin, P.; Carrier, X.; Salles, L. J. ColloidInterface Sci. 2006, 304, 283–291.

(24) Opre, Z.; Grunwaldt, J.-D.; Mallat, T.; Baiker, A. J. Mol. Catal. A2005, 242, 224–232.

(25) Opre, Z.; Ferri, D.; Krumeich, F.; Mallat, T.; Baiker, A. J. Catal.2007, 251, 48–58.

(26) Mondelli, C.; Ferri, D.; Baiker, A. J. Catal. 2008, 258, 170–176.(27) Chin, S. Y.; Williams, C. T.; Amiridis, M. D. J. Phys. Chem. B

2006, 110, 871–882.(28) Chen, H. W.; Zhong, Z.; White, J. M. J. Catal. 1984, 90, 119–

126.(29) Gupta, N. M.; Kamble, V. S.; Iyer, R. M.; Thampi, K. R.; Gratzel,

M. J. Catal. 1992, 137, 473–486.(30) Yokomizo, G. H.; Louis, C.; Bell, A. T. J. Catal. 1989, 120, 1–14.(31) Hadjiivanov, K.; Lavalley, J. C.; Lamotte, J.; Mauge, F.; Saint-

Just, J.; Che, M. J. Catal. 1998, 176, 415–425.

JP1039783

16450 J. Phys. Chem. C, Vol. 114, No. 39, 2010 Zhang et al.

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