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J. Phys. Chem. C 2008, 112, 16110–16117
Support Effect of New Au/FeOx Catalysts in the Oxidative Dehydrogenation of r,ω-Diols to Lactones Jie Huang, Wei-Lin Dai,* and Kangnian Fan Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: May 19, 2008; ReVised Manuscript ReceiVed: August 1, 2008
FeOx of different crystal phases were synthesized by altering the calcination temperature of the precursors obtained by a hydrothermal method. Crystal transformation from maghemite to hematite begins at calcination temperature of 573 K. Gold was deposited on the support uniformly with average size of approximately 5 nm. It is found that the as-prepared catalysts show superior activity and selectivity to the commercial Fe2O3 supported gold catalysts in the oxidation of 1,4-butanediol and 1,5-pentanediol to the corresponding lactones. Support shows dramatic influence on the intrinsic properties of gold particles including particle size and electronic nature. Strong interaction between gold and the nanosized ferric oxide support was studied by XPS and TEM. 1. Introduction There are increasing interests in gold catalysis, especially the catalytic oxidation reactions.1-3 Intensive researches have been performed to study the nature of gold catalysts in CO oxidation reaction, for example, the gold particle size effect,4 role of gold oxidation state,5,6 intrinsic ability of active sites,7,8 and the influence of the support.9 CO oxidation reaction is of great significance in the removal of CO pollution and elimination of CO in the fuel cell system,10 as well as an important reaction in revealing the essence of the catalyst. However, at present, more and more attention is being paid on liquid phase reactions, and a series of alcohol oxidation reactions such as benzyl alcohol oxidation and selective oxidation of 1,2-diols, glycerol, or glucose are being studied because of their importance in the synthesis of many fine chemicals. The early researches on the oxidation of alcohols are mainly focused on Au/C11-13 and colloidal gold catalysts.14,15 More recently, it is found that many oxide supported gold catalysts are active in alcohol oxidation, including Au/TiO2,16 Au/CeO2,17 even Au/Cu/Mg/Al-mixed oxides,18,19 and so forth. Catalysts on supports of different component,20 crystalline phase,21 morphology,22,23 porosity,24 reducibility, and surface nature differ greatly in catalytic activity and stability.25 The metal oxide support influences the catalytic activity by functioning as electronic promoter, structure promoter, or catalytic component. The strong metal support interaction26 is regarded to be beneficial to the high activity of gold catalysts. It is reported that both oxidized gold27 and negatively charged gold28 can be generated through strong interaction with the support. The oxidized gold was served as the active sites in many reactions.27,29 Whereas the negatively charged gold formed by electron donating from the metal oxide support was found to exhibit excellent adsorption properties.30,31 Metal oxide supports of different properties bring about the variation in gold particle size, morphology,32 electronic property, and thus the activity of the catalysts. For this purpose, it is of great value and * To whom all correspondence should be addressed. E-mail: wldai@ fudan.edu.cn. Fax: (+86-21) 65642978.
significance to investigate the support effect to reveal the essence of the supported gold catalysts. Iron oxide is a potential support due to its reducibility, stability, and the ability for electron transfer. The coprecipitated Au/Fe2O3 is one of the earliest gold catalysts intensively studied, which is highly active in water-gas shift reaction33 and selective hydrogenation of R,β-unsaturated aldehydes or ketones to the corresponding alcohols.34 Iron oxide has several crystal forms, among which the most common are magnetite (Fe3O4), maghemite (γ-Fe2O3), and hematite (R-Fe2O3). Crystal type of magnetite and maghemite is inverse spinel showing ferromagnetic property. In Fe3O4, Fe2+ ions occupy octahedral sites, and Fe3+ ions are distributed evenly over octahedral and tetrahedral sites. In γ-Fe2O3, the Fe2+ ions in the octahedral sites are replaced by vacancies and Fe3+ ions. Hematite has a corundum structure and presents antiferromagnetic property. Hematite is widely used as catalyst support,35,36 while the other two are applied in many fields including biomedical,37 magnetic storage devices, ferrofluids, magnetic refrigeration systems, and so forth due to their ferromagnetic property. Separation of the substrate and recycling of the catalyst would be more convenient by using ferromagnetic oxide as support, since it can be simply collected by a permanent magnet. Effect of crystal form of the support can be investigated by using iron oxides with different crystal types as support for gold catalyst. Lactones is a group of chemicals that can be applied as solvent,38 extraction agent, and intermediate of many biomedical products, fibers, and pesticides;39 hence, they are widely used in agriculture, petroleum industry, pharmaceutics, resins, and fibers. They can be synthesized by dehydrogenation or oxidation of the corresponding R,ω-diols and cycloketones. Current synthesis procedures involve with fierce reaction conditions and oxidants,40,41 or the sacrifice of a series of cooxidants such as aldehydes,42,43 N-oxide, and so forth. Abundant wastes are generated, and the process is not in accordance with the requirements of green chemistry and sustainable development. Several improved catalytic systems were developed, including the dehydrogenation of 1,4-butanediol to γ-butyrolactone by Cubased44 and Ru complex catalysts.45 Cu catalysts need high reaction temperature and give low conversion, while homoge-
10.1021/jp8043913 CCC: $40.75 2008 American Chemical Society Published on Web 09/18/2008
Support Effect of New Au/FeOx Catalysts neous Ru complex is not easy to be separated from the reaction mixtures. In our previous work, it was found that the Au/TiO2 catalyst was highly active in the oxidative dehydrogenation of 1,4-butanediol to γ-butyrolactone, and above 99% conversion and selectivity was achieved, but catalytic behaviors of other supported gold catalysts are not known.46 There are also other reports on the PVP stabilized Au-Pd bimetallic catalyst that is active in the formation of γ-butyrolactone by oxidation of 1,4butanediol,47 but the intrinsic mechanism is not established. Therefore, it is essential to carry out further research on the catalytic performance of gold catalysts in R,ω-diol oxidation and to extend the reaction to more substrates including 1,5pentanediol. In the present work, we focus on the support effect of Au/FeOx catalysts by preparing FeOx with different crystalline types, which can be altered by calcining precursors obtained from a hydrothermal method at different temperatures in air. Catalysts with different gold particle size and oxidation state are also obtained. The oxidation of 1,4-butanediol and 1,5pentanediol to γ-butyrolactone and δ-valerolactone are utilized as probe reactions to study the support effect of the new nanosized FeOx supported gold catalysts. 2. Experimental Section 2.1. Catalyst Preparation. The nanosized FeOx is synthesized as described elsewhere.48 A 1.67 g sample of FeSO4 · 7H2O and 50 mL ethanol amine is mixed and put into an ultrasonicater for 5 min. Then, 50 mL water is added, and the solution is transferred into a Teflon-lined autoclave. The autoclave is sealed, placed in an oven at 373 K for 5 h. After that, the solution turns to dark suspension, which is centrifuged to give dark precipitates. The resulting precipitates are washed with absolute ethanol four times, collected by centrifugation, and dried at room temperature. The resulted dark brown powders are calcined in air at desired temperatures. Iron oxides calcined at different temperatures are labeled as FeOx-temperature. The commercial γ-Fe2O3 is bought from Fangyuan Nano Material Institute of Anhui University of Technology for comparison. Deposition of gold onto the FeOx substrate is carried out using the homogeneous deposition-precipitation method. A 10 mL sample of 2.43 × 10-2 mol · L-1 HAuCl4 solution is dissolved in 50 mL deionized water. A 2.92 g sample of urea is used as the precipitation agent. A 0.6 g sample of FeOx is then added into the solution. The mixture is kept stirring for 2 h at 353 K. The as-received precipitates are collected by filtration, washed three times with deionized water, and dried in air overnight at 373 K, followed by calcination in air for 4 h at 573 K. Gold catalysts supported on different temperature calcined FeOx are labeled as Au/FeOx-T. The commercial γ-Fe2O3 supported gold catalyst is denoted as Au/γ-Fe2O3-Com. 2.2. Characterization. Specific surface areas of the samples are measured by nitrogen adsorption at 77 K (Micromeritics Tristar ASAP 3000) using Brunauer-Emmett-Teller (BET) method. Simultaneous thermal gravimetric (TG) and differential thermal analysis (DTA) measurements are performed from room temperature to 1173 K on a Perkin-Elmer 7 series thermal analyzer, using Al2O3 as a reference. Samples are heated at a rate of 10 K · min-1 under air. The gold loadings are determined by the inductively coupled plasma method (ICP, thermo E.IRIS). XRD patterns are recorded on a Bruker D8 advance diffractometer with Cu KR radiation (λ ) 0.154 nm), operated at 40 mA and 40 kV. The SEM micrographs are obtained using a Philips XL 30 microscope. The sample is deposited on a sample holder with a piece of adhesive carbon tape and is then sputtered with a thin film of gold. TEM micrographs are obtained on a Joel JEM 2010 transmission electron microscope. The average
J. Phys. Chem. C, Vol. 112, No. 41, 2008 16111
Figure 1. XRD patterns of as-prepared FeOx calcined at (a) 298, (b) 573, (c) 673, (d) 773, (e) 823, (f) 873, and (g) 973 K.
TABLE 1: Specific Surface Areas and Crystal Phase of FeOx sample
SBET (m2/g)
FeOx phase
FeOx-RT FeOx-573 FeOx-673 FeOx-773 FeOx-823 FeOx-873 FeOx-973 γ-Fe2O3-Com
55 76 70 35 35 30 14 79
Fe3O4 γ R+γ R+γ R+γ R R γ
size of Au particles and their distributions are estimated by counting more than 300 Au particles. XPS spectra are recorded under ultra high vacuum (
