MgO

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Effects of Biomolecules on the Selectivity of Biosynthesized Pd/MgO Catalyst toward Selective Oxidation of Benzyl Alcohol Xinde Jiang,†,‡ Hai Liu,§ Hanfeng Liang,‡ Guixian Jiang,† Jiale Huang,‡ Yingling Hong,‡ Dengpo Huang,‡ Qingbiao Li,‡,∥ and Daohua Sun*,‡ †

College of Science, Nanchang Institute of Technology, Nanchang 330099, China Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Fujian Provincial Key Laboratory of Chemical Biology, Xiamen University, Xiamen 361005, China § College of Chemistry and Chemical Engineering, Beifang University of Nationalities, Yinchuan 750021, China ∥ College of Chemistry & Life Science, Quanzhou Normal University, Quanzhou 362000, China ‡

ABSTRACT: Catalysts prepared through biosynthesis have aroused wide concern recently, but the detailed role of biomolecules and the identification of the main components have not been figured out yet. In this study, biogenic Pd/MgO catalysts were prepared through sol-immobilization (SI) and absorption-reduction (AR) methods, respectively, using Artocarpus heterophyllus Lam leaves extracts as reductant and stabilizer. The catalyst prepared by SI method showed obviously higher benzaldehyde selectivity than that by AR method under the same conditions. The difference can be attributed to the different oxidation behaviors of phenolic hydroxyls between the two synthetic procedures. In the SI procedure, the phenolic hydroxyls were oxidized to semiquinone, while in the AR procedure, the phenolic hydroxyls were oxidized to hydroxyquinone, resulting in less carboxyls binding onto the surface of the Pd nanoparticles (NPs), thus inhibiting the formation of the byproducts. Moreover, gallic acid, chlorgenic acid, and rutin were identified as the most active ingredients in the Artocarpus heterophyllus Lam leaves extracts. Similar catalytic performances were achieved when the model solution containing the three components was used instead of the original plant extracts. The biogenic Pd/MgO-SI catalyst also showed superior stability toward the oxidation of benzyl alcohol.

1. INTRODUCTION Selective oxidation of alcohols to their corresponding aldehydes or acids plays a central role in organic chemistry and is of importance to industrial fine chemistry.1 Conventionally, the oxidation of alcohols in industry is mainly based on the use of transition metal or halogenoxo salts, such as permanganate, chromate, bromate, or other stoichiometric oxidants, which usually generate high amounts of toxic wastes.2−4 Until recently, highly active supported metallic catalysts with high selectivity and reusability have been developed for the aerobic oxidation of alcohols under atmospheric pressure. Ruthenium,5,6 platinum,7 palladium,8,9 and gold10,11 catalysts become the most active ones for these oxidation reactions.12 The selectivity and activity are two important factors to that should be considered for effective oxidation of benzyl alcohol.13 High selectivity to benzaldehyde (more than 99%) can be achieved in supported metallic catalysts depending on the reaction conditions and/or the support employed: (a) pH. The selectivity of the reaction can be strongly affected by the presence of a base,14 which is believed to cleave the O−H bond of the alcohol to form an alkoxide intermediate. (b) Solvent. The effect of solvent was investigated by Mori et al.15 and Villa et al.16 Mori found the trifluorotoluene (with a selectivity of 99%) was the most effective solvent, while aprotic polar solvents such as acetonitrile, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO) turned out to be not effective. (c) Support. The selectivity to benzaldehyde was highly dependent on the support. 15,17 For example, the selectivity for benzaldehyde upon hydroxyapatite (HAP)-supported catalyst © 2014 American Chemical Society

was as high as 99%, while upon other supports (Al2O3, SiO2, and C) the selectivities were less than 50%. Besides conditions above, the synthetic method is also essential. Recently, bioreduction methods have emerged as an ecofriendly process, which uses cheap plants extracts as reducing agents and stabilizers. The reaction is performed under mild conditions without importing any additional base or organic solvents. The reported biosynthesis of catalysts was conducted in two ways: (1) Sol-immobilization (SI) method. Metal ions were reduced to form metal nanoparticles (NPs) first, and then, the metal NPs were immobilized on the support. (2) Absorption-reduction (AR) method. Metal ions were immobilized on the support first, then the ions were reduced to form NPs on the support surface. Du et al.18 reported that Au/ TS-1 catalysts prepared via SI method using C. platycladi extracts, which showed excellent activity and high selectivity for propylene epoxidation at a relatively high reaction temperature. The performance might be attributed to the existence of residual biomolecules on the catalysts. Zhan et al.19 also found that biomolecules were related to the aerobic oxidation of benzyl alcohol, for supported Au−Pd catalysts synthesized both in the SI and AR method. Whichever synthesis route is employed, biomolecules are believed to play an important role in the catalytic reaction. However, to the best of our Received: Revised: Accepted: Published: 19128

August 18, 2014 November 21, 2014 November 22, 2014 December 2, 2014 dx.doi.org/10.1021/ie503290c | Ind. Eng. Chem. Res. 2014, 53, 19128−19135

Industrial & Engineering Chemistry Research

Article

was used to correlate the measured electrophoretic mobility to zeta potentials. The biomolecules bound to the catalysts were analyzed by FTIR on Nicolet N6700 (Nicolet, USA). Mass Spectra of the biomolecules were recorded on an Esquire 3000 Quadrupol Ion Trap (Bruker, U.S.A.) operated in negative mode using full scan from 100 to 800 m/z. 2. 4. Catalytic Tests. The oxidation of benzyl alcohol was carried out in a three-neck glass reactor (50.0 mL) by dispersing the catalysts into 30.0 mL solution consisting of 3.0 mL benzyl alcohol and 27.0 mL deionized water at 90 °C under magnetic stirring. Analytical samples (0.5 mL) were taken periodically from the reactor and extracted in 1.0 mL ethyl acetate and centrifuged to separate the catalysts from the mixture. The products were analyzed by a gas chromatography equipped with FID detector (SE-30 capillary column), and external calibration method was used for quantitative analysis of the amounts of reactants consumed (conversion) and products generated (selectivity). Benzaldehyde selectivity was defined as the amount of benzaldehyde produced divided by the amount of benzyl alcohol consumed, and benzaldehyde yield (%) was defined as the conversion of benzyl alcohol (%) multiplied by the selectivity of benzaldehyde (%).

knowledge, the influence of preparation route on the surface and catalytic properties of the biosynthesized supported catalysts has not been investigated. Additionally, how the biomolecules interact with the catalyst and the substrate or intermediate products is still unclear. Herein, Pd/MgO catalysts were prepared by the SI and AR routes respectively, using Artocarpus heterophyllus (A. heterophyllus) Lam leaves extracts as reductant and stabilizer. The catalytic activities of the two Pd/MgO catalysts toward the oxidation of benzyl alcohol with molecular oxygen in aqueous medium were examined. With the aid of transmission electron microscopy (TEM), thermogravimetric (TG) analysis, Brunauer−Emmett−Teller (BET), zeta potential, and Fourier Transform Infrared Spectroscopy (FTIR), the difference that was caused by the two preparation routes was systematically investigated. The main components in the biomass extracts involved in the synthesis of the Pd/MgO catalysts were identified. Moreover, the interactions between the biomolecules and the support and/or intermediate products in the different synthesis routes were clearly distinguished.

2. EXPERIMENTAL SECTION 2. 1. Chemicals and Reagents. All chemicals (PdCl2, benzyl alcohol, and MgO) were purchased from Sinopharm Chemical Reagent Co Ltd., China. The A. heterophyllus Lam leaves used in this study were harvested from the campus of Xiamen University (China) and milled approximately to 200 mesh after drying at room temperature, then 2.0 g of the milled powder was dispersed in 100 mL deionized water in a water bath shaker for 4 h. The obtained mixture was then filtrated to remove the residual insoluble biomass, and the resulting filtrate was centrifuged at 8000 rpm for 5 min; then, the extracts were used for the synthesis of Pd NPs and defined as 20 mg/mL. 2. 2. Catalyst Preparation. Catalysts were prepared via the SI and AR method, respectively. For the former, 0.25 mL of 112.8 mM PdCl2 was added into 20 mL 10 mg/mL A. heterophyllus Lam leaves extracts for 12 h at room temperature under continuous stirring, then 0.3 g MgO was added immediately into the mixture. After 4 h, the mixture was filtered; the obtained precipitate was washed thoroughly with distilled water, dried in vacuum oven at 50 °C overnight, and further calcined at 200 °C for 4 h. This catalyst was named as Pd/MgO-SI. As for AR method, 0.25 mL 112.8 mM PdCl2 was first impregnated in a solution containing 0.3 g MgO and 10 mL H2O under stirring for 4 h, then 10 mL 20 mg/mL A. heterophyllus Lam leaves extracts were added, the reduction was carried out for 12 h at room temperature, and the catalyst named as Pd/MgO-AR was obtained after washing and annealing as mentioned above. 2. 3. Catalyst Characterization. TEM was performed on a Tecnai F30 electron microscope (FEI, Netherlands) operated at an accelerating voltage of 300 kV. The samples were prepared by drop-casting ethanol dispersion of catalysts onto carbon-coated copper TEM grids. Size distribution and average size of the Pd NPs were estimated using the software SigmaScan Pro 4. TG analysis was carried out on a thermo balance (Netzsch TG209F1) under atmospheric air at a heating rate of 10 K/min. N2 physisorption isotherms were recorded on a porosimetry analyzer (Micromeritics TriStar 3000) at 77 K. Zeta potential of the nanocatalyst was measured on a ZetaPALS instrument (Brookhaven, U.S.A.) at 25 °C, each sample was measured for three times with each measurement consisting of about 100 acquisitions, the Helmholtz−Smoluchowski equation

3. RESULTS AND DISCUSSION 3. 1. Performances of the Catalysts Prepared by the SI and AR Methods. Figure 1 shows the oxidation of benzyl

Figure 1. Catalytic performances of Pd/MgO-SI and Pd/MgO-AR catalysts (Conditions: benzyl alcohol 29 mmol; catalyst 0.2 g; Pd loading 1.0 wt %; temperature 90 °C; O2 90 mL/min).

alcohol over the as-prepared Pd/MgO as a function of reaction time. The conversion of benzyl alcohol, benzaldehyde selectivity, and benzaldehyde yield were monitored at different reaction time intervals. For Pd/MgO-SI catalyst, it was observed that benzyl alcohol conversions increased with time. Benzaldehyde is the major product and small quantity of benzyl benzoate can be detected as byproduct. The maximum benzaldehyde yield was obtained for both Pd/MgO-SI and Pd/MgO-AR catalysts at reaction time of 10 h. It is worth noting that, under this condition, the conversions of benzyl alcohol over two catalysts are very close, while benzaldehyde selectivity of Pd/MgO-SI (95.7%) is higher than that of Pd/ MgO-AR (83.4%). 3. 2. Difference between Pd/MgO-SI and Pd/MgO-AR Catalysts. Based on the above results, it is believed that the synthetic procedure of the catalysts has a significant impact on their catalytic performances. To gain insight into the difference between the procedures of the two synthetic routes (SI and 19129

dx.doi.org/10.1021/ie503290c | Ind. Eng. Chem. Res. 2014, 53, 19128−19135


Article

Figure 2. TEM images of Pd NPs (a), Pd/MgO- SI (b) and Pd/MgO-AR catalysts (c). Insets are size distributions of Pd nanoparticles.

caused by the binding of biomolecules. VP and DP decrease slightly after the incorporation of the biomolecules and the Pd NPs into the mesoporous channels of the support. Compared with MgO support, the Pd/MgO-AR catalyst also shows an increase in SBET and a decrease in VP and DP. It is noted that the range of these parameters are lower than that of Pd/MgO-SI, which can be attributed to the less ligands retained in the Pd/ MgO-AR. Zeta potential can be used to qualitatively predict the extent of electrostatic stabilization and specific surface-ions interactions, which is helpful to understand the chemical environment of the solid. The value of the zeta potential depends on the polarity of the anchored ligands, the ions adsorbed on the surface of the solid, and the ion concentration in liquid. The zeta potential profiles of two catalysts and MgO support are shown in Table 2. The surfaces of the two catalysts are negative at pH 9.0, while the support MgO is positively charged. This behavior of the catalysts is mainly attributed to the presence of negatively charged groups, which are deprotonated at high pH value. Compared to Pd/MgO-SI, the zeta potential of Pd/ MgO-AR shows less negative. Since there is no difference in the type and concentration of ions, the zeta potential differs from the polarity of the anchored ligands, indicating that there are less negative groups anchored on the surface of Pd/MgO-AR.20 TG was conducted to examine the residual plant biomasses on the biosynthesized Pd catalysts. The decomposition temperature of the biomass was in the range 280−370 °C, and the capping action of some complicated biomolecules over the catalysts would be released at that temperature.19 Evidently, Figure 3a shows that the biomasses account for 10.4% of the

Table 1. Structural Properties of Pd/MgO-SI and Pd/MgOAR Catalysts and MgO Support catalyst

SBET (m2 g−1)

VP (cm3 g−1)

DP (nm)

MgO support Pd/MgO-SI Pd/MgO-AR

35.092 39.258 36.996

0.123 0.090 0.120

5.029 4.193 4.676

Table 2. Zeta Potentials of Pd/MgO-SI and Pd/MgO-AR Catalysts and MgO Support catalysts

zeta potential (mV)

half width (mV)

Pd/MgO-SI Pd/MgO-AR MgO support

−21.56 ± 0.58 −13.35 ± 1.25 13.60 ± 0.17

5.00 ± 0.25 5.95 ± 1.89 4.43 ± 0.52

AR), the physical and chemical properties of two catalysts were characterized by TEM, BET, zeta-potential, TG, and FTIR. For the SI process, the representative TEM images of the formed Pd NPs with the size of 4.10 ± 0.76 nm are given in Figure 2a. After being immobilized on the MgO support, no obvious aggregation occurred (Figure 2b). With respect to the AR process, spherical Pd NPs are also well dispersed on the support with diameters around 4.35 nm (Figure 2c). Therefore, particle size is not the major reason for the different catalytic performances between Pd/MgO-SI and Pd/MgO-AR. Pore structure parameters, namely specific surface area (SBET), pore volume (VP), and pore diameter (DP) of the two catalysts and MgO support are listed in Table 1. It can be found that the SBET of catalysts are slightly higher than that of MgO, which is probably due to the roughness of the catalysts

Figure 3. (a) TG curves and (b) FTIR spectra of Pd/MgO-SI and Pd/MgO-AR catalysts. 19130



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Figure 4. Schematic illustrations of the reaction occurred during the SI and AR procedure.

Figure 5. (a) Elution curve of biomolecules from the laden-absorbed AB-8 resin. (b) The catalytic performances of the catalysts prepared by different eluents (Conditions: benzyl alcohol 29 mmol; catalyst 0.2 g; Pd loading 1.0 wt %; temperature 90 °C; O2 90 mL/min).

Figure 6. (a) FTIR spectra and (b) ESI-MS identification of the F17 eluents.

cm−1 can be attributed to hydroxyl groups formed on the MgO surface. The FTIR spectrum of Pd/MgO-AR also presents vibrational peaks at 3700, 1632, and 1415 cm−1. However, the peak at 1109 cm−1 representing C−O group in semiquinone weakened, indicating that biomolecules in the extract were oxidized to different degrees. Phenolic acids, widely existing in A. heterophyllus Lam leaves extracts21 can be oxidized to corresponding semiquinone or hydroxyquinones. In the SI process, metal NPs were synthesized first, then the biomolecules reacted with the Pd ions in aqueous solution without any blockages, the strong reducing groups (phenolic hydroxyls) would be preferentially

total weight of the Pd/MgO-SI, while only 7.6% biomasses are bounded on the Pd/MgO-AR catalyst, which means that different amounts of biomolecules are involved in the stabilization of Pd NPs. FTIR analysis was further performed for the characterization of the two catalysts (Figure 3b). The FTIR spectrum of Pd/MgO-SI shows bands at 3424, 1630, 1417, and 1109 cm−1 along with a sharp absorbance at 3700 cm−1. The bands at 1630 and 1417 cm−1 should be assigned as asymmetric and symmetric vibration of carboxylates, while the 1109 cm−1 band is attributed to C−O group in semiquinone. The band at 3424 cm−1 is characteristic of the hydroxyl functional groups in alcohols or water and the band at 3700 19131



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Figure 7. TEM images of Pd NPs and Pd catalysts prepared from F17 (a and b) and model solution (c and d). Insets are size distributions of Pd nanoparticles.

Figure 9. Catalytic performance of recycled Pd/MgO-SI catalyst.

mainly established by coordinating of O (CO) to Pd atoms,22 so the same amount of CO can be bounded on the Pd NPs’ surface when equal amount of Pd is loaded on the support. Since the ratio of COO− to CO in semiquinone (1:1) is higher than that of hydroxyquinone (1:2), therefore, the Pd/MgO-SI possesses more COO− and has a higher negative zeta potential and a wider variation of pore structure parameters (SBET, VP, and DP) than Pd/MgO-AR (Figure 4). Similar to Brønsted-base sites, the catalysts with more negative charges would inhibit benzaldehyde being further oxidized,23 which is why the Pd/MgO-SI catalyst exhibits higher selectivity to benzaldehyde than the Pd/MgO-AR during the oxidation of benzyl alcohol. 3.3. Identification of Active Compounds in the Extracts Involved in the Preparation of Pd/MgO-SI. In order to understand the role of the biomass, the main

Figure 8. Catalytic performance of Pd/MgO catalysts prepared by original plant extract, F17, model solution, plant extract without polyphenols (Conditions: benzyl alcohol 29 mmol; catalyst 0.2 g; Pd loading 1.0 wt %; temperature 90 °C; O2 90 mL/min; time 10 h).

oxidized and then the partially oxidized biomolecules (semiquinone, 1109 cm−1) would be anchored on to the Pd NPs. While in the AR process, the Pd ions were first absorbed onto MgO support and then reduced to atoms or NPs. Meanwhile, biomolecules (phenolic hydroxyls) were oxidized to semiquinones. Due to the steric hindrance of the semiquinones, other biomolecules had little chance to interact with the neighboring Pd ions that immobilized on the support, and these unsaturated semiquinones would further undergo oxidation, leading to the formation of hydroxyquinones. The interaction between the biomolecules and Pd/MgO catalyst are 19132



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Table 3. Catalytic Performance of Pd/MgO-SI and Reported Pd or Au−Pd Catalysts Towards Oxidation of Benzyl Alcohol

a

catalyst

T (K)

particle size (nm)

catalyst amount (mg)

Au−Pd/APS Au−Pd/TiO2 Pd/Al2O3 Au−Pd/C Pd/HAP Pd/CNTs Pd/NMC Pd/MgO-SI

413 373 393 393 373 373 433 363

2.8 3.8 6.5 3.2 20 3.9 6.6 4.1

20 500 100 50 100 10 20 200

substrate amount metal loading (wt %) (mg) 1.4 5.0 2.6 1.0 10.8 1.0 1.0 1.0

50 1156 300 385 1.0 12.5 23.5 28.9

solvent

selectivity (%)

yield (%)

ref.

without without without without H2O H2O H2O H2O

91.3 91.6 94.3 67.4 a 73 91.2 95.7

20.4 68.2 75.5 61.2 95 65.7 59.8 79.3

17 30 31 32 29 33 34 b

Not shown. bThis study.

achieved among the former three catalysts. While the one prepared from plant extract absence of polyphenols displayed undesired catalytic performance. Therefore, it can be concluded that polyphenols including gallic acid, chlorgenic acid, and rutin are the most important ingredients in the preparation of the Pd/MgO catalysts. 3.4. Recyclability of Pd/MgO-SI. Figure 9 presents the recyclability of Pd/MgO-SI for eight cycles. After each cycle, the catalyst was recovered, washed with deionized water, dried at room temperature, and reused in a subsequent new catalytic run. It can be observed that the conversion of benzyl alcohol and the selectivity for benzaldehyde keep constant after eight cycles. In addition, Pd cannot be detected by atomic absorption spectrophotomerry (AAS) analysis in the filtrate after the reaction, indicating that no observable leaching of Pd from the catalyst occurred. Table 3 lists the catalytic performance of the as-synthesized Pd/MgO-SI catalyst and other reported Pd or Au−Pd catalysts toward the aerobic oxidation of benzyl alcohol. Although the experimental conditions used in various studies deviated somewhat, a rough comparison is still feasible. Comparing with other catalysts expect for Pd/HAP,29 the Pd/MgO-SI catalyst exhibited better catalytic performance with a benzaldehyde selectivity of 95.7% and benzaldehyde yield of 79.3%. It should be mentioned that the 10 wt % of Pd was loaded on the Pd/HAP catalyst, while only 1.0 wt % was on Pd/MgO-SI catalyst. Moreover, no additives or cocatalysts were introduced to facilitate the Pd/MgO-SI catalyst efficient catalytic conversion of benzyl alcohol to benzaldehyde under atmospheric O2 pressure. Hence, the as-obtained biogenic Pd/ MgO-SI catalyst shows superior catalytic activity and high stability toward the oxidation of benzyl alcohol.

components of biomolecules participating in the synthesis of Pd/MgO-SI were identified. The A. heterophyllus Lam leaves extracts were separated by AB-8 resin, and then divided into 24 eluents (F1−F24).24 After being frozen dried, the dry weights of the eluents were measured and the result is shown in Figure 2a. Most of the biomolecules from the extracts are eluted from F14 to F19. Thus, these six fractions with same dry weights concentration were redissolved in deionized water and used for preparing Pd/MgO-SI catalysts, respectively. Their catalytic performances are given in Figure 5b. The catalysts from F14 and F15 showed benzaldehyde selectivities below 90% in the aerobic oxidation of benzyl alcohol, while other four catalysts had selectivities higher than 95%. Among these, the catalyst prepared by F17 was the most active and were employed for the further experiments. FTIR spectrum of F17 (Figure 6a) shows bands at 3417, 2923, 1714, 1612, 1513, 1441, and 1076 cm−1. The bands at 3417, 1612, 1513, and 1441 cm−1 are assigned to aromatic hydroxyl and benzene ring, representing the presence of phenols in the extracts. The band at 1714 cm−1 is ascribed to carboxyls, indicating the presence of phenolic acids.25 In order to determine the detail components of the F17, the mass spectral analysis of F17 was further carried out (Figure 6b). Three major peaks at 169, 353, and 609 can be observed, which are confirmed to be polyphenols after being tested by Folin− Ciocalteu’s phenol reagent.26 The specific types of polyphenols observed are gallic acid, chlorgenic acid, and rutin, respectively.27 Hence, we speculate that these three components play key roles in the synthesis of Pd/MgO catalysts. To validate this, a model solution of these active components was prepared. The amounts of gallic acid, chlorgenic acid, and rutin in the model solution were similar to that in the F17. This obtained solution was employed for the reduction of the Pd ions. After reaction with the PdCl2 under the same conditions, the obtained Pd NPs were immobilized onto the MgO support. It can be seen from Figure 7 that Pd NPs with a mean size of 4.5 nm that synthesized using F17 and are evenly dispersed on the MgO support. Meanwhile, the Pd NPs prepared using the model solution are with similar sizes (4.8 nm) and well distributed on the support. Hence, no obvious difference in the morphology can be observed between the catalysts prepared using F17 and model solution. Moreover, the catalytic performances of the catalysts toward the oxidation of benzyl alcohol prepared from original plant extract, F17, model solution, and plant extract without polyphenols (removed through polyvinylpolypyrrolidone absorption28) were tested under the same condition and procedures (SI method). From Figure 8, we can see that similar results (selectivity of 95.6%, conversion of 81.9%) were

4. CONCLUSIONS In summary, we have reported the preparation of two biogenic Pd/MgO catalysts through the SI and AR routes by using A. heterophyllus Lam leaves extracts as reductant and stabilizer. Their comparative catalytic performances toward the oxidation of benzyl alcohol were also studied. The result shows that the as-synthesized Pd/MgO-SI exhibited higher benzaldehyde selectivity than Pd/MgO-AR. The reason for the difference between two catalysts may be due to the fact that the biomasses were oxidized to semiquinone during the SI process, while most of biomasses were oxidized to hydroxyquinone during the AR process, resulting in less carboxyls binding onto the surface of the Pd NPs. The negatively charged carboxyls significantly inhibit the formation of the byproducts. Furthermore, the main components from the biomasses involved in the synthesis of the Pd/MgO catalysts have been identified. Gallic acid, 19133



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(13) Prati, L.; Villa, A.; Chan-Thaw, C. E.; Arrigo, R.; Wang, D.; Su, D. S. Gold catalyzed liquid phase oxidation of alcohol: the issue of selectivity. Faraday Discuss. 2011, 152, 353. (14) Guo, H.; Al-Hunaiti, A.; Kemell, M.; Rautiainen, S.; Leskelä, M.; Repo, T. Gold catalysis outside nanoscale: Bulk gold catalyzes the aerobic oxidation of π-activated alcohols. ChemCatChem. 2011, 3 (12), 1872−1875. (15) Mori, K.; Takayoshi, H.; Mizugaki, T.; Kohki, E.; Kiyotomi, K. Hydroxyapatite-supported palladium nanoclusters: A highly active heterogeneous catalyst for selective oxidation of alcohols by use of molecular oxygen. J. Am. Chem. Soc. 2004, 126, 10657−10666. (16) Villa, A.; Plebani, M.; Schiavoni, M.; Milone, C.; Piperopoulos, E.; Galvagno, S.; Prati, L. Pd on carbon nanotubes for liquid phase alcohol oxidation. Catal. Today 2010, 150, 8−15. (17) Chen, Y.; Lim, H.; Tang, Q.; Gao, Y.; Sun, T.; Yan, Q.; Yang, Y. Solvent-free aerobic oxidation of benzyl alcohol over Pd monometallic and Au−Pd bimetallic catalysts supported on SBA-16 mesoporous molecular sieves. Appl. Catal., A 2010, 380 (1−2), 55−65. (18) Du, M.; Zhan, G.; Yang, X.; Wang, H.; Lin, W.; Zhou, Y.; Zhu, J.; Lin, L.; Huang, J.; Sun, D.; Jia, L.; Li, Q. Ionic liquid-enhanced immobilization of biosynthesized Au nanoparticles on TS-1 toward efficient catalysts for propylene epoxidation. J. Catal. 2011, 283 (2), 192−201. (19) Zhan, G.; Huang, J.; Du, M.; Sun, D.; Abdul-Rauf, I.; Lin, W.; Hong, Y.; Li, Q. Liquid phase oxidation of benzyl alcohol to benzaldehyde with novel uncalcined bioreduction Au catalysts: High activity and durability. Chem. Eng. J. 2012, 187, 232−238. (20) Tennysonl, D.; Chi-hung, C.; Reghanj, H.; Clemens, B. Nanoparticle ζ-potentials. Acc. Chem. Res. 2012, 45 (3), 317−326. (21) Prakash, O.; Rajesh, K.; Mishra, A.; Gupta, R. Artocarpus heterophyllus (Jackfruit): An overview. Pharmacog. Rev. 2009, 3 (6), 353−358. (22) Evangelisti, C.; Panziera, N.; D’Alessio, A.; Bertinetti, L.; Botavina, M.; Vitulli, G. New monodispersed palladium nanoparticles stabilized by poly-(N-vinyl-2-pyrrolidone): Preparation, structural study, and catalytic properties. J. Catal. 2010, 272 (2), 246−252. (23) Chen, T.; Zhang, F. Z.; Zhu, Y. Pd nanoparticles on layered double hydroxide as efficient catalysts for solvent-free oxidation of benzyl alcohol using molecular oxygen: Effect of support basic properties. Catal. Lett. 2013, 143 (2), 206−218. (24) Jiang, X.; Sun, D.; Zhang, G.; He, N.; Liu, H.; Huang, J.; Odoom-Wubah, T.; Li, Q. Investigation of active biomolecules involved in the nucleation and growth of gold nanoparticles by Artocarpus heterophyllus Lam leaf extract. J. Nanopart. Res. 2013, 15 (6), 1741. (25) Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric Identification of Organic Compounds, 7th ed.; Wiley: New York, 2005. (26) Everette, J. D.; Bryant, Q. M.; Green, A. M.; Abbey, Y. A.; Wangila, G. W.; Walker, R. B. Thorough study of reactivity of various compound classes toward the Folin−Ciocalteu reagent. J. Agr. Food Chem. 2010, 58 (14), 8139−8144. (27) Vermerris, W.; Nicholson, R. Families of phenolic compounds and means of classification. Phenolic Compound Biochemistry; Springer: Bergisch Gladbach, 2006; pp 1−34. (28) Mitchell, A. E.; Hong, Y.; May, J. C.; Wright, C. A.; Bamforth, C. W. A comparison of polyvinylpolypyrrolidone (PVPP), silica xerogel, and a polyvinylpyrrolidone (PVP)-silica co-product for their ability to remove polyphenols from beer. J. Inst. Brew. 2005, 111 (1), 20−25. (29) Jamwal, N.; Gupta, M.; Paul, S. Hydroxyapatite-supported palladium (0) as a highly efficient catalyst for the Suzuki coupling and aerobic oxidation of benzyl alcohols in water. Green Chem. 2008, 10 (9), 999−1003. (30) Enache, D. I. Solvent-free oxidation of primary alcohols to aldehydes using Au-Pd/TiO2 catalysts. Science 2006, 311 (5759), 362− 365. (31) Pritchard, J.; Kesavan, L.; Piccinini, M.; He, Q.; Tiruvalam, R.; Dimitratos, N.; Lopez-Sanchez, J. A.; Carley, A. F.; Edwards, J. K.; Kiely, C. J.; Hutchings, G. J. Direct synthesis of hydrogen peroxide and

chlorgenic acid, and rutin are confirmed to be the most important ingredients in the A. heterophyllus Lam leaves extracts. Similar catalytic results can be achieved when the original plant extracts were replaced by model solution containing the above three components. The investigation on the role of biomolecules may shed light on the development of catalyst by this promising and eco-friendly method.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (+86) 592-2189595. Fax: (+86)592-2184822. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation projects (21206140 and 21036004), the Science and Technology Key Program of Fujian Province (No. 2013H0044), and the Science and Technology Program of Nanchang Institute of Technology (No. 2014KJ023).

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REFERENCES

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