Enhanced Catalytic Activities and Characterization of Ruthenium

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

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Enhanced Catalytic Activities and Characterization of Ruthenium-Grafted Halogenous Hydroxyapatite 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, 457 Zhongshan 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) were newly developed through a facile method and identified as highly efficient catalysts for the aerobic oxidation of alcohols. Compared with RuHAp for selective oxidation of benzyl alcohol, the existence of F, Cl, and Br elements in hydroxyapatite dramatically enhanced the catalytic activity with a prominent selectivity of far more than 99%. In particular, the RuClAp and RuFAp catalysts, respectively, showed the excellent catalytic activity of TOF ) ∼233 and 210 h-1, which was nearly 3 times higher than that of RuHAp. The RuFAp catalyst was furthermore demonstrated to be recyclable and available to be applied for various alcohols. On the basis of the DRIFT and XAFS results, the enhanced activities could be preliminarily ascribed to the electron-withdrawing effect of halogens and the greater amounts of active species existing in the surface of RuXAp compared with that of RuHAp. 1. Introduction The selective oxidation of alcohols to the corresponding aldehydes and ketones is a pivotal intermediate process for the pharmaceutical and fine-chemical industries. Because of the toxicity and large amounts of wastes, the use of traditional oxidants (such as chromates, hypochlorites, and permanganates) has been limited. In view of the economy and environment, catalytic oxidation with molecular oxygen or air is particularly attractive as a “green” technology. In the past years, there has been a growing demand for effective catalysts in the selective oxidation of alcohols. Many supported noble metals (such as palladium, ruthenium, and gold) solid catalysts have been developed and thoroughly investigated.1-4 Supported Au and Pd catalysts are more active and achieve remarkable reaction rates (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 and required high reaction temperature (433 K) limits its application. Just as reported in Kobayashi’s review article,2 a wide substrate scope and high selectivity toward the target compound are very important for selective oxidation of alcohols. Supported ruthenium catalysts, such as Ru/Al2O3,5 Ru(OH)x/TiO2,6 RuO2/FAU zeolite,7 Ru-Co-Al-hydrotalcite,8 Ru(III)/TiO2 nanotube,9 and Ru/Ni(OH)2 composite,10 have drawn tremendous interests for their unique characteristics, including widest substrate scope, moderate activity, high selectivity to aldehydes or ketones, and mild operating conditions. Recently, hydroxyapatite (HAp)-based materials have attracted more and more attention as solid and recyclable catalysts. * To whom correspondence should be addressed. Tel: +86 411 84379015 (T.Z.). Fax: +86 411 84691570 (T.Z.). E-mail: [email protected] (T.Z.), [email protected] (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.

Rutheniuim hydroxyapatite (RuHAp) has been demonstrated to be an efficient catalyst on the aerobic oxidation of alcohols,11-13 amines,14 and organosilanes.15 RuHAp is a highly selective catalyst (>99%) for the partial oxidation of alcohols. However, only a low alcohol oxidation activity (TOF ) 2 h-1) could be obtained.11 To date, much effort has been focused on the improvement of RuHAp’s catalytic activity, such as the addition of a metal promoter (RuCoHAp, TOF ) 78 h-1),16 the addition of γ-Fe2O3 in the HAp matrix (TOF ) 196 h-1),17 and the organic modification (RuHAp-BAcid, TOF ) 242 h-1).13 The organically modified RuHAp achieved high activity; however, its application was limited by the complicated preparation process. Therefore, the design and development of a more efficient Ru-based catalyst with a low cost and environmental friendliness still remains a challenge for its application in the selective oxidation of alcohols to the corresponding aldehydes and ketones. The anion exchange property of HAp allows us to design and develop the novel HAp-based materials for certain specific applications. It has been well-known that fluorine, chlorine, and bromine can be incorporated into an HAp crystal lattice, partly or totally replacing the hydroxyl group.18,19 Incorporation of fluoride ions dramatically improves the surface stabilization of the crystal and is an important contributor to the higher thermal stability, chemical durability, and lower solubility.20-22 Up to now, to the best of our knowledge, there is no approach for promoting the catalytic activity of RuHAp by focusing on the modification of the hydroxyl group in HAp. It is known that the fundamental properties of HAp, such as solubility, acid fastness, acid-base property, and crystallinity, are easily tuned by modifying the anion group. On the basis of the above considerations, we present a new strategy for the design of a nanostructured heterogeneous catalyst with the substitution of the hydroxyl group in the HAp crystal. Surface stabilization by halogen ions is an important contributor to the lower solubility.

10.1021/jp1039783  2010 American Chemical Society Published on Web 09/01/2010

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The incorporation of different anion groups (F, Cl, and Br) into HAp promotes a network of surface hydrogen bonding with surface OH groups and, consequently, enhances the stability of surface atoms. The improvement of surface stability of the HAp crystal is considered to change the chemical surroundings of Ru active sites and finally influence its catalytic performance. Here, we report the effect of anion exchange on the catalytic properties of RuHAp and used the aerobic oxidation of benzyl alcohol as a model reaction. In this study, RuFAp, RuClAp, and RuBrAp were identified as more efficient catalysts compared with RuHAp for the selective oxidation of benzyl alcohol to benzaldehyde. A particular order of their catalytic activity was confirmed as RuClAp > RuFAp > RuBrAp > RuHAp. The RuFAp catalyst was furthermore confirmed to have a high activity and selectivity for the oxidation of a wide range of alcohols and could be recyclable. The RuXAp catalysts were mainly investigated by XAFS and DRIFT studies of CO adsorption to gain insight into the nature of catalytic performance. The results indicated that the improved catalytic activities were resulted from the electron-withdrawing effect of halogens and the greater amounts of active species existing in the surface of RuXAp. 2. Experimental Section 2.1. Preparation of Catalysts. The chemical precipitation method was used for synthesizing the XAp (X ) F, Cl, and Br) crystal. Separately, using aqueous calcium nitrate (0.03 mol), diammonium hydrogen phosphate (0.018 mol), and 0.006 mol of NaF (or NaCl, NaBr) as precursor solutions (pH ) ∼2, adjusted by nitric acid), the precipitate was obtained when the pH value of the mixed solution was adjusted to ∼8 by addition of ammonia under vigorous magnetic stirring. The suspension was sealed in a jar and then kept inside a water bath for 24 h at 60 °C. The precipitate was separated by centrifugation, washed with deionized water several times until the pH of the filtrate was ∼7, and dried at 60 °C. Finally, the precursor powders (XAp) were obtained. There was no further heat treatment for XAp in this method, which was usually carried out at 500 °C in other former reports.10 The RuXAp catalysts (Ru ) 1 wt %) were separately prepared by immersion of 1 g of XAp in a 70 mL aqueous RuCl3 solution (1.41 × 10-3 M) with vigorous magnetic stirring under ambient conditions. For a comparison, the RuHAp catalyst (Ru ) 1 wt %) was also prepared by the chemical precipitation and then immersion methods as described above without using the sodium halide precursor solution. The actual weight contents of Ru were separately measured by ICP as 1.17, 1.21, 0.85, and 1.02% for RuHAp, RuFAp, RuClAp, and RuBrAp catalysts. 2.2. Catalytic Performance Test. Oxidations of alcohols were typically carried out as follows: A suspension of the prepared 0.1 g of Ru-based catalyst in toluene (10 mL) was magnetically stirred, and the substrate (1 mmol) was then added. The resulting mixture was kept at 80 °C under an O2 flow (20 mL/min) for proceeding with the aerobic oxidation of alcohol reactions. The selectivity and conversion were determined by GC analysis (Agilent 6890, equipped with an HP-FFAP capillary column and FID detector) using mesitylene as an internal standard. For the RuFAp catalyst, the reusability was also tested by using the aerobic oxidation of benzyl alcohol as a model reaction. Furthermore, the RuFAp catalyst was applied to aerobically oxidize a wide range of alcohols, including several representative benzylic, allylic, and alphatic alcohols. 2.3. Characterization. Phase analysis of the prepared catalysts was conducted using X-ray diffraction (XRD) with Cu

Zhang et al. KR radiation (λ ) 1.5418 Å). The diffractometer (X’pert Pro Super, PANAnalytical) was operated at a 2θ range of 10-80° with a step size of 0.02°. The prepared catalysts were also characterized by Fourier transform-infrared spectroscopy (FTIR, Equinox 55, Bruker) in the range of 4000-400 cm-1. Their diffuse reflectance infrared (DRIFT) spectra were collected with an 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 for 30 min. The morphology and grain sizes of the powders were observed through transmission electron microscopy (TEM, Philips CM200 and JEM-2000EX). The chemical composition of the prepared catalysts was separately obtained by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Thermo IRIS Intrepid II spectrum apparatus) and X-ray fluorescence spectrometry (XRF, PANAnalytical, Axios 2.4 kw). Ru K-edge X-ray absorption fine structure (EXAFS and XANES) spectra of the prepared catalysts were made at the BL14W1 beamline of SSRF, SINAP (Shanghai, China), with the use of a Si(111) crystal monochromator. The storage ring was operated at 3.5 GeV with injection currents of 100 mA. Ru foil and RuCl3 · 3H2O were used as reference samples, and their X-ray absorption spectra were measured in the transmission mode. All spectra of the prepared catalysts were conducted in the fluorescence mode. The raw data were energy-calibrated (Ru K-edge energy of Ru foil ) 22117 eV, first inflection point), background-corrected, and normalized using the IFEFFIT software. Fourier transformation of the EXAFS data was applied to the k2-weighted functions, respectively. For the curve-fitting analysis, theoretical backscattering phases and amplitudes for Ru-O bonding were calculated from the data of RuO2. 3. Results and Discussion 3.1. Properties of the Catalysts. Information on the morphology, crystallinity, and chemical composition of the prepared catalysts was extracted from the results of TEM, XRD, FT-IR, and XRF characterizations, respectively. Figure 1 shows TEM micrographs of the prepared catalysts along with the results of HAp and FAp for a comparison. Figure 1a,b shows the TEM micrographs of precursor powders for HAp and FAp, respectively. The rodlike particles with a length of less than 50 nm were observed in Figure 1a. Taking into account the HAp lattice constants (a ) 9.422 Å and c ) 6.883 Å) and the hexagonal symmetry with the space group P63/m, its unit cell will be arranged along the c axis. The Ostwald ripening occurred during the aging process and facilitated the growth habit of HAp along the c axis. As seen from Figure 1b, the legible rodlike morphology was obtained for FAp with a length of clearly more than 50 nm, which indicated that introducing a F anion enhanced the crystallinity and facilitated the growth of the HAp crystal along the c axis. It was clear that the prepared RuHAp and RuFAp catalysts had similar rodlike morphologies, as shown in Figure 1c,d. Meanwhile, the typical sizes of the rodlike particles in Figure 1c,d were, respectively, found to be identical to those in Figure 1a,b, demonstrating that the crystal morphology almost did not change when the Ru ions were exchanged during the immersion process. The typical size of the RuFAp crystal was 60-80 nm in length, and it had an aspect ratio of 4-5. Both of the prepared RuClAp and RuBrAp catalysts had very similar morphology and particle size with that of RuHAp, as shown in Figure 1e,f. Figure 2 shows XRD patterns of the prepared catalysts also including the result of HAp for a comparison. It could be observed that all of the prepared catalysts were single phase

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

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

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

be detectable after either the introduction of different halogen ions or the grafting of Ru ions during the immersion process. The typical FT-IR spectra of the prepared RuHAp and RuFAp catalysts are shown in Figure 3. The characteristic bands of 3571 and 632 cm-1 were assigned to the hydroxyl group in the HAp crystal, whereas the broad band at 3427 cm-1 was attributed to the adsorbed water. The bands at 1096, 1032, 962, 603, and 564 cm-1 were attributed to PO43- ions (ν1s962 cm-1, ν3s1032 and 1096 cm-1, ν4s564 and 603 cm-1). No HPO42- group was found due to the absence of the band at 875 cm-1. The FT-IR spectrum (Figure 3, spectrum b) of RuFAp differs significantly from that of RuHAp (Figure 3, spectrum a). The stretching vibration of the hydroxyl group at 3571 cm-1 was not observed for RuFAp, indicating that almost all of the hydroxyl groups in RuHAp were substituted by fluorine ions. The FT-IR spectrum of RuClAp indicated that only part of the hydroxyl groups were substituted by chloride ions because of the existence of the stretching vibration in 3572 cm-1, as shown in Figure S1 in the Supporting Information. As listed in Table 1, the contents of Ru and halogen elements in the prepared catalysts were determined by ICP-AES and XRF, respectively. The XRF results were consistent with the FT-IR spectroscopic studies, and the substitution values (n) of the hydroxyl groups by the halogen ions were separately evaluated to be 1.84, 0.2, and 0.34 for

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).

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TABLE 1: Oxidation of Benzyl Alcohol with Molecular Oxygen over the Prepared Ru-Based Catalysts catalyst

time conversiona selectivity Ru halogenc substitution (h) (%) (%) (wt %) (wt %) value (n)

RuHApb RuFAp RuClAp RuBrAp

1 0.5 0.5 0.5

69.6 >99 >99 >99

>99 >99 >99 >99

1.17 1.21 0.85 1.02

0 3.25 0.7 2.57

0 1.84 0.2 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 GC using an internal standard technique. b A Ru content of 1.17 wt %, TOF ) 60 h-1 (average turnover frequency, related to the total amount of Ru3+). c Halogen content was detected by XRF, and the substitution value was calculated based on the general chemical formulation Ca10(PO4)6(OH)2-n(X)n (X ) OH, F, Cl, or Br; 0 e n e 2).

RuFAp, RuClAp, and RuBrAp based on the general chemical formulation Ca10(PO4)6(OH)2-n(X)n (X ) OH, F, Cl, or Br; 0 e n e 2). 3.2. Benzyl Alcohol Oxidation. The influence of halogens on the catalytic activity of RuHAp was investigated in the liquidphase aerobic oxidation of benzyl alcohol, which was the most commonly used model compound in alcohol oxidation. Also, a relative mild reaction temperature (353 K) was employed in the reaction. The average turnover frequency (TOF) was introduced approximately to assess the performance of catalysts. The total amount of Ru3+ was used in the calculation of the TOF due to the difficulty in estimating the population of active sites. As shown in Table 1, the introduction of halogen elements dramatically enhanced the activity of the RuHAp catalyst, even ∼3 times the activity of RuHAp in the case of RuFAp and RuClAp. 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 achieved for the halogenous Ru-based catalysts of RuFAp, RuClAp, and RuBrAp. For all of the four catalysts, the selectivity for benzaldehyde was always far more than 99%. Other mild reaction conditions were employed to distinguish the activity of those catalysts, which were 0.05 g of catalyst for 0.5 h. Thus, with the average TOF as a basis for activity comparison, the particular order of activity was obtained as RuClAp > RuFAp > RuBrAp > RuHAp (Figure 4), confirming that the existence of F, Cl, and Br elements dramatically enhanced the catalytic activity of RuHAp for aerobic oxidation of benzyl alcohol to the corresponding benzaldehyde. In the present study, the catalytic performance for the RuHAp and RuFAp catalysts differed from that reported by To˜nsuaadu et al., in which the catalytic ability was reported to be higher for RuHAp compared with RuFAp in the aerobic oxidation of benzyl alcohol.23 The difference could probably be ascribed to the large size and different morphology. Therefore, the RuXAp (X ) F, Cl, Br) crystallites prepared in this study were highly expected to be the novel series of catalysts for the efficient aerobic oxidation of alcohols under mild reaction conditions with a low Ru content of 1 wt %. Among them, RuFAp catalyst was the preferential candidate because of its stability in chemical composition. 3.3. XAFS Analysis. X-ray absorption near-edge fine structure spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) were employed to investigate the Ru species grafted on the prepared catalysts. Figure 5A shows the Ru K-edge XANES spectra of RuXAp as well as RuCl3 · 3H2O and Ru foil. By comparing to the reference spectra of RuCl3 · 3H2O and Ru foil, it could be concluded that Ru3+

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

species existed in the prepared catalysts. All the spectra of RuXAp exhibited no significant difference between their K-edge energies. Their near-edge features were identical, which indicated the similar short-range structures among all of RuXAp. Figure 5B shows the Fourier transform (FT) of the k2-weighted Ru K-edge EXAFS of the different samples. Obviously, the shape of the spectra in Figure 5B was similar for all the samples, and no Ru-Ru bonding contribution was observed. Representative experimental and theoretical fittings of R space for the EXAFS signals are shown in Table 2. RuXAp showed the similar nearest neighbors with two and four oxygen atoms attributed 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 were consistent with the results of RuHAp obtained by the other research groups.24 Figure S2 (Supporting Information) shows the relationship between the TOF and coordination number (CN) of nearest-neighbor Ru atoms. The TOF increased with the increase in the CN (the increase in the amounts of the hydrated Ru3+ oxide species), reached a maximum with RuClAp, and then decreased a little with RuFAp. A particular tendency could be found that RuClAp exhibited the shortest Ru-O1 bond (1.836 Å) in all the prepared catalysts. The decrease of bond distance (or increase of bond energy) implied the presence of Ru species with a higher oxidation state in the RuClAp crystal. 3.4. DRIFT Results. The use of CO as a probe molecule allowed us to determine the coordination environment and the structure of the Ru active site grafted in the prepared catalysts. Baiker’s group reinvestigated the RuHAp catalyst by a DRIFT study of CO adsorption and attributed the real active sites to small hydrated Ru3+ oxide nanoparticles grafted on the surface of HAp.25,26 The time-dependent adsorption of CO on the prepared RuHAp and RuFAp catalysts at 353 K is shown in Figure 6A,B, respectively. For the spectra of RuFAp (Figure 6B), the increased intensity with adsorption time of the three bands at 2067, 1985, and 1954 cm-l was due to the CO adsorption. The bands at 2177 and 2117 cm-l originated from the signals of gaseous CO adsorption. In the high-frequency region (Figure S3 in the Supporting Information), the negative band at 3570 cm-l indicated the removal of the OH group in the HAp crystal during the CO adsorption process. In Figure S4 (Supporting Information), the negative feature at 3570 cm-l was absent because of the substitution of hydroxyl groups by

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J. Phys. Chem. C, Vol. 114, No. 39, 2010 16447 TABLE 2: Structural Parameters of RuXAp Determined by the First Shell Fitting of EXAFS Spectra at the Ru K-Edge

samples

shell

RuHAp Ru-O1 Ru-O2 Ru-Ru RuFAp Ru-O1 Ru-O2 Ru-Ru RuClAp Ru-O1 Ru-O2 Ru-Ru RuBrAp Ru-O1 Ru-O2 Ru-Ru

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

fluorine ions in the RuFAp crystal. The slow adsorption of CO was observed in time-resolved DRIFT spectra (Figure 6C,D) of RuClAp and RuBrAp. The spectra obtained after 10 min of adsorption time for all the catalysts are shown in Figure S5 (Supporting Information. The weak adsorption signals were obtained for RuClAp and RuBrAp even after 10 min of adsorption of CO. Adsorption of CO caused the reduction of Ru3+ ions in those samples. Following assignments discussed in the literature,25,27-31 the band at 2067 cm-l in the catalysts could be assigned to tricarbonyl species [Run+(CO)3]. In the Ru-RuOx/TiO2 catalyst,29 the asymmetric broadening below 2000 cm-l was assigned to the monocarbonyl form of CO linearly bonded with ruthenium of different oxidation states [RuOx(CO) ) 2000-1985 cm-l

interatomic distance coordination Debye-Waller R/Å number factor δ2/Å2 R-factor 1.919 1.997 3.129 1.903 1.998 3.130 1.836 1.998 3.127 1.937 2.050 3.172

2.4 4.4 1.5 2.0 4.8 3.1 2.6 4.4 2.9 1.7 4.2 2.4

0.0133 0.0095 0.0062 0.0166 0.0099 0.0132 0.0047 0.0030 0.0110 0.0006 0.0054 0.0091

0.0126 0.0156 0.0071 0.020

(LF1, low frequency) and 1950-1935 cm-l (LF2, low frequency)]. In this work, the band below 2000 cm-l also corresponded to the presence of the RuOx (x < 2) species. The intensity of the low-energy shoulder (1954 cm-l for RuFAp and 1963 cm-l for RuBrAp) below 2000 cm-l correlated well with the fraction of the hydrated Ru3+ oxide small nanoparticles.25 Therefore, the fraction of these hydrated Ru3+ oxide small nanoparticles could be roughly evaluated in the sequence RuFAp > RuBrAp > RuHAp, which could be one of the main reasons for resulting in the differences of catalytic performance in these catalysts. In the case of RuClAp, the situation was a little special; the band at 2023 cm-l had already been observed, which was associated with a lower valence state Ru containing carbonyl species reduced by CO.25,27 The more active Ru species should be mainly responsible for the excellent catalytic activity of RuClAp. 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 surface was largely determined by the stronger back-donation of d electrons from the metal into the 2π* orbitals.28 When the more electronegative elements were present, the electron density on Ru species was influenced by the different electronegativity. Therefore, the blue shifts in the DRIFT spectra were apparently ascribed to be originated from the different surroundings of Ru3+ oxide small nanoparticles grafted on FAp, HAp, ClAp, or BrAp containing different elements with different abilities of withdrawing electrons. Briefly, the enhanced activity of the prepared RuXAp catalysts could be preliminarily ascribed to two reasons: First, the greater amounts of active species of small hydrated Ru3+ oxide nanoparticles existing in the surface of RuXAp compared with RuHAp. This was mainly considered to be originated from surface stabilization and the high acid resistance of halogenous hydroxyapatite compared with HAp. Second, the different surroundings of Ru3+ oxide small nanoparticles grafted on FAp, HAp, ClAp, or BrAp with different electron-withdrawing effects. 3.5. Proposed Reaction Mechanism. A reaction mechanism was proposed for the aerobic oxidation of alcohols over RuXAp catalysts. Generally, aerobic alcohol oxidation on a supported ruthenium catalyst involved three steps:24 formation of Rualcoholate species (step 1), β-hydride elimination (step 2), and reoxidation of hydrido-ruthenium species by molecular oxygen (step 3). Dehydrogenation (step 2) played a key role in the reaction mechanism, and it would be the rate-determining step. The reaction rate of the rate-determining β-hydride elimination for different catalysts was investigated through the oxidation

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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) DRIFT spectrum obtained after absorption of CO, followed by desorption in He for 10 min. (C) RuClAp and (D) RuBrAp: (a) DRIFT spectra recorded after 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 of CO for 10 min, RuClAp and RuBrAp for 30 min.

of benzyl alcohol under anaerobic conditions. The conversion of benzyl alcohol under an Ar atmosphere was confirmed to be 16.1, 22.5, 29.1, and 17.8%, respectively, for RuHAp, RuFAp, RuClAp, and RuBrAp, which was consistent with the particular order of activity as RuClAp > RuFAp > RuBrAp > RuHAp.

Therefore, the introduction of halogens into HAp (especially for Cl and F) would effectively facilitate β-hydride elimination. 3.6. Aerobic Oxidation of Other Alcohols and Reusability of the RuFAp Catalyst. To further evaluate the catalytic performance of RuFAp, its recycling use and aerobic oxidation of a wide range of alcohols were also studied. The results demonstrated that the RuFAp catalyst was highly active in the oxidation of various alcohols with molecular oxygen under mild conditions, and in all the reactions shown in Table 3, the corresponding aldehyde was the only detectable product. Several representative benzylic, allylic, and alphatic alcohols listed in Table 3 were selected to investigate the correlation between conversion and time. In particular, benzylic and allylic alcohols showed higher reactivity than alphatic alcohols (entries 1-6). The conversions increased rapidly with time in the initial reaction stage, and then the rate became slow. However, an interesting trend could be observed in the present work. Despite the great difference in activity, there was not much difference for the conversion of benzylic, allylic, and alphatic alcohols except for benzyl alcohol at the initial 1 h (entries 2, 3, and 4). The oxidation of benzyl alcohol proceeded much faster than that of 1-phenylethanol for the RuFAp catalyst. The faster oxidation of benzyl alcohol suggested that it highly tended to form the alcoholate species via the ligand exchange with ruthenium active species in RuFAp.

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TABLE 3: Aerobic Oxidation of Primary Alcohols and Secondary Alcohols to Carbonyl Compounds Using RuFAp as a Catalyst entry

substrate

1 2

benzyl alcohol 1-phenylethanol

3

cinnamyl alcohol

4 5

1-octanol 1-octanolb

6

2-octanolb

time (h) conversiona (%) selectivity (%) 0.5 1 2.5 1 3 4 1 2 6 2

>99 76 98 64 89 97 54 70 87 75

>99 >99 >99 >99 >99 >99 >99 >99 >99 >99

a 0.1 g of RuFAp (1 wt % Ru), substrate (1 mmol), toluene (10 mL), 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 oxidation of benzyl alcohol. Reaction conditions: catalyst, 0.1 g; benzyl alcohol, 1 mmol; solvent (PhCH3), 10 mL; O2 flow rate, 20 mL/min; temperature, 353K; time, 0.5 h.

The reusability of RuFAp for the aerobic oxidation of benzyl alcohol was also investigated. The recovered catalyst was reused after being separately washed with ethanol and deionized water several times and dried at 333 K. The conversion of benzyl alcohol and the selectivity to benzaldehyde could be kept over 99% in the third recycling experiment, as shown in Figure 8. Therefore, for this substrate, the newly developed RuFAp catalyst can be easily reused at least three times with almost the same high catalytic activity and selectivity as the first run. The reusability of RuFAp for the aerobic oxidation of 1-octanol is shown in Figure S6 (Supporting Information). It showed an ∼20% loss in the conversion of 1-octanol during the second cycle, but the selectivity could be kept far more than 99% even in the third recycling experiment. The reusability of RuFAp was limited for 1-octanol, and the decrease of its catalytic performance was probably attributed to the partial covering of Ru active species. 4. Conclusion A new series of RuXAp (X ) F, Cl, or Br) catalysts with a special rodlike morphology were developed through a facile method as efficient heterogeneous catalysts for the aerobic oxidation of alcohols. The prepared catalysts showed very high activity at a low Ru content (1 wt %) under mild reaction conditions. A particular order of activity was obtained as RuClAp > RuFAp > RuBrAp > RuHAp for the aerobic oxidation

of benzyl alcohol at the same conditions. Further studies demonstrated that the RuFAp catalyst was also highly active in the oxidation of various alcohols and could be easily reused with the high catalytic activity and superior selectivity. DRIFT combined with XAFS spectroscopies and several other techniques provided insight into the nature of the catalytic performance of the catalysts. The enhanced catalytic activities of RuXAp should be related to the electron-withdrawing effect of halogens as well as the structural and physicochemical properties of the HAp’s surface and bulk modified by the addition of halogens. The present study demonstrated that modification of hydroxyl groups in the crystal lattice of HAp by halogens was an effective method for promoting the performance of HApbased heterogeneous solid catalysts. Acknowledgment. Financial support obtained from the Chinese Academy of Sciences for “100 Talents” Project, the Natural Science Foundation of Liaoning Province (No. 20092173), and the National Science Foundation of China for Distinguished Young Scientists (No. 20325620) is greatly acknowledged. This work was also supported by the Science and Technology Commission of Shanghai Municipality of China (Project No. 09JC1417100). The authors also thank the XAFS beamline staff at Shanghai Synchrotron Radiation Facility (SSRF) for assisting with the XAFS data collection and data analysis. Supporting Information Available: Experimental details of the oxidation of benzyl alcohol under anaerobic conditions; comparison of highly selective catalysts in the oxidation of benzyl alcohol to benzaldehyde with oxygen (Table S1); FTIR spectrum of RuClAp (Figure S1); relationship between TOF and coordination number (CN) of nearest-neighbor Ru atoms for RuHAp, RuFAp, RuClAp, and RuBrAp (Figure S2); time-resolved DRIFT spectra of RuHA after 0, 1, 3, 5, and 10 min (Figure S3); time-resolved DRIFT spectra of RuFAp after 1, 3, 5, and 10 min (Figure S4); DRIFT spectra of RuFAp, RuHAp, RuClAp, and RuBrAp after adsorption of CO for 10 min (Figure S5); and reusability of the RuFAp catalyst for the aerobic oxidation of 1-octanol (Figure S6). This material is available free of charge via the Internet at http://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.

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