Nanoscale

PAPER

Cite this: Nanoscale, 2016, 8, 1049

Received 9th November 2015,Accepted 15th November 2015

DOI: 10.1039/c5nr07897k

www.rsc.org/nanoscale

Water-soluble metal nanoparticles stabilized byplant polyphenols for improving the catalyticproperties in oxidation of alcohols†

H. Mao,a Y. Liao,a J. Ma,a S. L. Zhao*a and F. W. Huo*b

Plant polyphenols extracted from plants are one of the most abundant biomasses in nature, which are

typical water soluble natural polymers. Herein, we reported a facile approach for the synthesis of platinum

nanoparticle (PtNP) aqueous colloid by utilizing black wattle tannin (BWT, a typical plant polyphenol) as

amphiphilic stabilizer. The phenolic hydroxyls of BWT provide the PtNPs with enough hydrophilicity, and

their reduction ability could protect the PtNPs from deactivation caused by oxygen atmosphere. Addition-

ally, the hydrophilic nature of BWT could efficiently promote the oxidation of alcohols in water, mean-

while, the hydrophobic and rigid backbones of plant polyphenols are able to suppress the PtNPs from

aggregating, thus ensuring the high dispersion of the PtNPs during reactions. Under mild aerobic con-

ditions, the as-prepared BWT-Pt colloid catalyst exhibited high activity in a series of biphasic oxidation of

aromatic alcohols and aliphatic alcohols. As for the cycling stability, the BWT-Pt catalyst showed no

obvious decrease during the 7 cycles, revealing superior cycling stability as compared with the counter-

parts using PVP or PEG as the stabilizer.

Introduction

Selective oxidation of alcohols to aldehydes plays a crucial rolein organic synthesis due to the fact that aldehydes are valuableintermediates and as high-value components for the perfumeindustry.1–3 Oxidation reactions carried out using various typesof transition metallic nanoparticles (MNPs) have attractedmuch attention, especially in the reactions under mild reac-tion conditions.4–6 In general, homogeneous MNP catalystsexhibit high activity during the reactions, but the separation ofthe catalysts from the reaction system is quite difficult, whichoften leads to poor cycling stability.7,8 Heterogeneous MNPcatalysts are more attractive due to easy recycling compared tothe homogeneous counterparts.9,10 Many types of hetero-geneous MNP catalysts have been designed for the oxidation ofalcohols with high efficiency.11–14 Unfortunately, the oxidationreactions utilizing heterogeneous MNP catalysts usually showthe lower activity under mild reaction conditions, thus oftenperforming in organic solvents and/or under harsh reaction

conditions.15,16 Ideally, an environmentally benign alternativeis needed, which performs the catalytic reactions in waterunder mild reaction conditions with reusable catalysts.

In response to this, water-soluble MNP colloid catalysts aresuitable potential candidates. The reactions catalyzed by watersoluble MNPs could avoid excessive use of harmful organic sol-vents, and the catalysts can be easily separated from the reac-tion system by simple filtration.17–20 Usually, the MNPs arestabilized by water-soluble organic small molecules or amphi-philic polymers to realize their solubility.21–23 The MNPs stabil-ized by hydrophilic organic small molecules generally sufferfrom aggregating and leaching due to the limited stability.24

Conversely, amphiphilic polymers with abundant hydrophilicfunctional groups could stabilize the MNPs efficiently duringthe reactions. Previous research studies have been reportedabout the aerobic oxidation of some aliphatic and alicyclic alco-hols in water with reusable MNPs stabilized by amphiphilicpolymers,25–28 while the strong interactions between amphiphi-lic polymers and the NPs may profoundly alter the catalytic pro-perties of the MNPs. Additionally, the synthesis of theseamphiphilic polymers required complex synthetic procedures,which also involves the use of toxic organic solvents. Hence, thegreen strategy to prepare the MNP colloid catalysts with goodwater-solubility and reusability by using a suitable stabilizer isstill deserved in the selective oxidation reaction of alcohols.

Plant polyphenols, typical natural amphiphilic polymerswith good water-solubility extracted from plants, are one of themost abundant biomasses in nature. There are large numbers

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr07897k

aCollege of Chemistry and Materials Science, The Engineering Center for the

Development of Farmland Ecosystem Service Function, Sichuan Normal University,

Chengdu 610068, P. R. China. E-mail: Zhaoslin@aliyun.combKey Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials

(IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials

(SICAM), Institute of Advanced Materials, Nanjing Tech University (NanjingTech),

Nanjing 211816, P. R. China. E-mail: iamfwhuo@njtech.edu.cn

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of phenolic hydroxyls in their molecules, which has highaffinity to various metal ions, including Fe3+, Cr3+ andAl3+.29,30 The reductive phenolic hydroxyls of plant polyphe-nols not only provide the MNPs with good water-solubility, butalso could protect the MNPs from deactivation caused byoxygen atmosphere.31,32 Furthermore, the hydrophobic andrigid backbones of plant polyphenols are able to prevent theMNPs from aggregating, thus ensuring the high dispersion ofthe NPs during reactions.33,34 As a consequence, it is possibleto prepare highly active and recyclable MNP catalysts by utiliz-ing plant polyphenols as amphiphilic stabilizer.

Herein, black wattle tannin (BWT), a typical plant poly-phenol, was used as natural amphiphilic stabilizer to prepareaqueous platinum nanoparticle colloid (BWT-Pt). Comparedwith the traditional amphiphilic stabilizer, BWT is one kind ofgreen stabilizer obtained from the natural plants. Meanwhile,the phenolic hydroxyls of BWT supply good water-solubility forPtNPs and protect the PtNPs from deactivation caused byoxygen atmosphere. Notably, the prepared BWT-Pt catalystswere used for aerobic oxidation reaction of alcohols inaqueous media under mild reaction conditions, which exhibitshigh activity and cycling stability.

ExperimentalSynthesis of BWT stabilized Pt nanoparticles

15 mg of BWT was dissolved in 5 ml of deionized water, andthen 3 ml of deionized water containing 20 μmol Pt4+ wasadded into the BWT solution. The resultant mixture wasstirred constantly for 1 h to allow sufficient chelating of BWTmolecules towards Pt4+ via their hydroxyls. Subsequently, theBWT-Pt4+ solution was reduced by 2 ml of NaBH4 (10 mg) solu-tion to obtain the BWT-Pt catalyst. Additionally, BWT-Pt cata-lysts with BWT content of 2 mg, 5 mg, 10 mg, 30 mg and60 mg were prepared by using the above procedure.

Characterization

Fourier transform infrared (FT-IR) spectra were recorded witha JASCO FT-IR 480 Plus spectrophotometer. Inductivelycoupled plasma (ICP) spectroscopy was conducted on a Dual-view Optima 5300 DV ICP-OEM system. Ultraviolet-visible (UV-vis) spectra were recorded with a Shimadzu UV-2501PC. Trans-mission electron microscopy (TEM) images were taken byusing a JEOL JEM 2010F at an accelerating voltage of 200 kV.

Typical catalytic oxidation of alcohols

In a typical reaction, 1 mmol alcohol was added to a mixtureof 5 ml of water and 0.5 mmol K2CO3. Subsequently, 10 ml ofthe BWT-Pt catalyst solution was added into mixture. The airflow was bubbled at 20 ml min−1, controlled by using a massflow meter, into the mixture to initiate the reaction. Themixture was stirred at 50 °C for 24 h. After reaction, 0.2 mmoldodecane was added as an internal standard. The productswere extracted twice using 3 ml of diethylether. When theextraction was completed, the biphasic system was allowed to

stand, and then, the organic phase was taken out by using asyringe, and dried over Na2SO4. The resultant organic phasewas analyzed by GC-MS and the obtained aldehyde was deter-mined by GC. The remaining aqueous solution of BWT-Pt wasrecovered and directly used as catalyst in subsequent cycles ofbiphasic reactions. For the analysis of aldehyde, we usedGC-MS to analyze the organic phase to confirm the existenceof the target product of aldehyde. After that, GC was conductedto quantitatively determine the amount of obtained aldehydesby using dodecane as an internal standard. To establish thestandard curves, different mixtures containing varied molarratios of aldehyde/dodecane were measured by GC, and thecorresponding peak area ratios of aldehyde/dodecane wereused as the function of molar ratios of aldehyde/dodecane.During the GC analysis, the peak location of aldehyde pro-ducts in our samples must perfectly match with that of chro-matographic grade aldehydes (the standard chemicals) underthe same GC conditions, and the content of target aldehydewas calculated according to the standard curves. The conver-sion yield of obtained aldehyde was calculated based on theuse of initial mole of alcohol.

Results and discussion

PtNPs with strong oxidation ability have been demonstrated inthe oxidation reaction of alcohols.35 Unfortunately, theirefficient reuse has always been hindered in the oxidation ofalcohols due to the deactivation by oxygen and aggregation aswell as leaching along cycling.36 However, the BWT, a typicalplant polyphenol, was employed as natural amphiphilic stabil-izer to maintain the activity and stability of the PtNPs. In atypical experiment, BWT-Pt was prepared by chelating Pt4+ withBWT in aqueous solution, followed by the reduction of NaBH4

(Fig. 1). Two adjacent hydroxyls on the B rings of BWT can act asbidentate ligands to chelate with one Pt4+, giving rise to the for-mation of a highly stable five-membered chelating ring.37 Sub-sequently, the chelated Pt4+ ions are further reduced by NaBH4

to form the PtNPs, which are still stabilized by the hydroxyls ofBWT via electron donating–accepting interactions.38 Further-more, the aromatic framework of BWT also prevents the PtNPsfrom aggregating by providing enough steric hindrance, leading

Fig. 1 Schematic illustration showing the preparation of the BWT-Ptcatalyst.

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to the formation of highly dispersed PtNPs with small particlesize. In this way, the BWT molecules could provide excellentwater-solubility as well as stability of the PtNPs.

In order to further investigate the preparation process ofBWT-Pt, the UV-vis spectra, FTIR spectra and XPS analysiswere employed to track the whole reaction process. A purePtCl6

2− solution has a strong adsorption peak at λmax =258 nm. After the reaction with BWT, the peak at 258 nmshifts to lower energy, demonstrating the chelating of BWTmolecules with Pt4+ ions via their phenolic hydroxyls.39 Afterreduction of the composite, the spectrum shows higheradsorption intensity at low energy and a new adsorption peakat 282 nm appears, which results from the formation ofBWT-PtNPs (Fig. S1†). The FTIR spectra of BWT and BWT-Pt(Fig. S1†) show that the stretching vibration peak of the pheno-lic hydroxyls of BWT at 3400 cm−1 is narrowed after chelatingwith the Pt4+ ions, which suggests that the PtNPs are stabilizedby the phenolic hydroxyls of BWT.40 Furthermore, the inter-actions between Pt and BWT were studied by XPS analysis. TheO 1s peak at 532.6 eV (Fig. 2b) belongs to the phenolichydroxyls of BWT. The O 1s spectrum of BWT-Pt (Fig. 2c)shows a major peak at 533.1 eV and a relatively small peak at531.8 eV, suggesting that a part of phenolic hydroxyls of BWTbind with the PtNPs.31

The morphology and structural identity of BWT-Pt werestudied by transmission electron microscopy (TEM) and X-raydiffraction (XRD). BWT-Pt was prepared by using 2 mg, 5 mg,10 mg and 15 mg of BWT as the stabilizer. The resultant cata-lysts are denoted as BWT0.5-Pt, BWT1.25-Pt, BWT2.5-Pt andBWT3.75-Pt respectively. As shown in Fig. 3, the BWT-Pt nano-particles prepared with 2 mg, 5 mg and 10 mg BWT suffer

serious aggregating, which is alleviated with the increasing ofBWT amount to 15 mg. It is a fact that the low content (lessthan 15 mg) of BWT is unable to effectively prevent the aggre-gation of PtNPs. In addition, the XRD pattern of BWT3.75-Pt(Fig. S2†) was also measured, which shows the characteristicpeaks of Pt at 40°, 46° and 67°, corresponding to the (111),(200) and (220) planes of Pt, respectively. Other catalysts pre-pared with different contents of BWT also show the existenceof metallic Pt according to XRD measurements. The XPS spec-trum (Fig. 2a) has also confirmed the existence of Pt. More-over, Fig. 4 displays the TEM images of fresh BWT3.75-Ptcatalyst and BWT3.75-Pt catalyst after 4 cycles of biphasicbenzyl alcohol oxidation. The freshly-formed PtNPs have amean diameter of 1.8 nm, which is highly dispersed withoutany obvious aggregation. After 4 cycles of biphasic benzylalcohol oxidation, the average diameter of PtNPs increasesslightly from 1.8 nm to 2.5 nm, which is still highly dispersedwithout any obvious aggregation. These results suggest thatthe content of BWT around 15 mg is enough for achievinggood cycling stability, which is attributed to the steric hin-drance provided by the aromatic molecular backbones of BWT.Hence, the small PtNPs with high dispersion in aqueous col-loids are expected to exhibit high catalytic activity and cyclingstability in the oxidation of alcohols.

To evaluate the catalytic properties of the BWT-Pt catalysts,we conducted the oxidation of a variety of alcohols, includingaromatic and aliphatic alcohols. All the reactions were carriedout under aerobic conditions, and representative results aresummarized in Table 1. It should be noted that there is apossibility for the alcohol to be oxidized to acid as the by-product. To eliminate the influence of acids on the test of alde-hydes, GC-MS and GC together were employed to analyze thecatalysis products. For quantitative determination of theobtained aldehyde, dodecane was used as the internal stan-dard in biphasic catalysis. The detailed analysis procedures areshown in the Experimental section. The BWT-Pt catalyst wasfound to be active in the oxidation of these alcohols in wateras solvent. A strong base potassium carbonate is needed forthe oxidation to proceed.41 With the existence of potassium

Fig. 2 XPS spectra of BWT and BWT-Pt. (a) XPS spectrum of BWT-Pt,(b) XPS O 1s spectrum of BWT, (c) XPS O 1s spectrum of BWT-Pt.

Fig. 4 TEM images, HRTEM images and size distribution of BWT3.75-Pt(a–c) and BWT3.75-Pt after 4 cycles of biphasic benzyl alcohol oxidation(d–f ).

Fig. 3 TEM images of BWT-Pt catalysts with different BWT amounts.BWT0.5-Pt (a, b), BWT1.25-Pt (c, d), BWT2.5-Pt (e, f ) and BWT3.75-Pt (g, h).

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carbonate, the conversion yield of benzyl alcohol over theBWT-Pt catalyst could reach 89% while less than 10% conver-sion was achieved without the base. Hence, all of the oxidationreactions were performed by the use of potassium carbonate.

Notably, the reactivity of the aliphatic alcohols is muchlower than that of aromatic alcohols. Under the same reactionconditions, the conversion yields of benzyl alcohol andhexanol were 89% and 36%, respectively. Meanwhile, the con-version yields of aromatic alcohols with carbon chains ofdifferent lengths were all higher than those of aliphatic alco-hols. According to the literature, one reason is probably thestrong adsorption of the aromatic moieties on the metalsurface.42 Another reason is that the benzene rings in the back-bones of BWT molecules could absorb the aromatic alcoholsby forming hydrophobic bonds, which will promote contactsbetween the reactant and the PtNPs.43 In addition, the sterichindrance of BWT molecules has shown significant influenceon the reactivity of BWT-Pt catalysts in the oxidation of alco-hols. For instance, the aromatic alcohols with larger molecularstructures have suffered lower conversion yield in the oxidationreactions catalyzed by BWT-Pt. This is because the steric hin-drance of BWT molecules makes it more difficult for the reac-tants with large molecular structure to come in contact withthe PtNPs. Moreover, the conversion of benzyl alcoholdecreased from 89% to 31% when the content of BWT withinthe BWT-Pt catalyst increased from 15 mg to 60 mg (Fig. S3†).A higher content of BWT may enhance the steric hindrance

and restrict access of substrates to PtNPs surface due to theincrease of intermolecular interactions within BWT molecules.Thus, the dosage of BWT should be controlled in a properrange to ensure the reactivity and stability of BWT-Pt catalysts.In other words, the accessible area of noble nanoparticles isresponsible for the activity of the catalyst. Usually, N2 adsorp-tion–desorption can be used to characterize the accessibility.However, BWT-Pt is a colloidal solution, and it is impossibleto measure its specific surface area because this measurementrequires that the catalyst should be a solid. We also found thatthe cyclohexanol, a kind of alicyclic alcohol, could be catalyzedby BWT-Pt to produce the corresponding cyclohexanone with47% yield. The corresponding activity is appreciable comparedwith other reported catalysts.44 The catalytic reactivity ofBWT-Pt in the oxidation of non-activated alicyclic alcohols canbe attributed to the amphiphilic nature of the BWT, whichdecreases the interfacial resistance between the aqueous cata-lyst and organic substrates.38 As control, the oxidation ofbenzyl alcohol was carried out by just utilizing BWT as a cata-lyst, which exhibited no catalytic reactivity under the sameexperimental conditions, thus demonstrating that the catalyticactivity of BWT-Pt was completely derived from the PtNPs.

The BWT-Pt catalysts with good cycling stability weredemonstrated in the catalytic oxidation of alcohols. Threetypical alcohols were utilized as substrates to study the cyclingstability of BWT-Pt, including benzyl alcohol, cyclohexanol andhexanol. As shown in Fig. 5, the conversion of the three kindsof alcohols catalyzed by BWT-Pt catalysts shows no significantdecrease during the 7 reaction cycles, revealing high cyclingstability of the catalysts. However, the decrease in reactivity ofMNP catalysts during the cyclic reaction was a commonphenomenon due to the leaching and aggregating of metalNPs.9,17 The leached Pt is negligible as confirmed by ICP-AESanalysis and PtNPs still exhibit good dispersion withoutobvious aggregation as shown in TEM analysis (Fig. 4d–f ) after4 reaction cycles, suggesting that the leaching and aggregationof PtNPs have been effectively suppressed due to the goodstabilization of BWT toward PtNPs.

Table 1 The catalytic activity of BWT-Pt in oxidation of alcohols inaqueous solutiona

Substrates Products Yield (%) Time (hour)

89 24

82 24

45 24

59 24

38 24

47 24

12 24

36 24

a Reaction conditions: 1 mmol of alcohol, catalyst contains 20 µmol ofPt and 15 mg of BWT, 15 ml of water, 0.5 mmol of K2CO3, 50 °C.

Fig. 5 Cycling stability of BWT in oxidation of alcohols. Reaction con-ditions: 1 mmol of alcohol, catalyst contains 20 µmol of Pt and 15 mg ofBWT, 15 ml of water, 0.5 mmol of K2CO3, 50 °C, 24 hours.

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To further demonstrate the advantages of BWT as a highlyefficient stabilizer of PtNPs, PVP and PEG were also used asstabilizers to prepare PtNP colloid catalysts. As shown in Fig. 6,fresh PVP-PtNPs and PEG-PtNPs have average particle sizes of4.0 and 3.5 nm, respectively. The PVP-PtNPs and PEG-PtNPsexhibit quite poor cycling stability. As shown in Table 2, theyield of benzaldehyde over PVP-PtNPs and PEG-PtNPs in theoxidation of benzyl alcohol decreased from 46.1% (1st cycle) to19.9% (4th cycle) and 38.6% (1st cycle) to 15.3% (4th cycle),respectively. Probably, PVP and PEG just cover up the Pt sur-faces, and thus, the stabilization of PtNPs is weak, which leadsto the aggregation and leaching of PtNPs during cycling. Theseassumptions are confirmed by the ICP measurements andTEM observations. Based on ICP measurements, 6.3% and8.1% of Pt are leached into the organic phase after the 1stcycle of benzyl alcohol oxidation using PVP-PtNPs andPEG-PtNPs as the catalysts, respectively. Although freshPVP-PtNPs and PEG-PtNPs have good dispersion, these par-ticles tend to aggregate together after repeated 4 cycles of cata-lytic reactions. As shown in Fig. 6b and c, the size of most ofthe PVP-PtNPs increased to 15 nm after 4 cycles, and the wideparticle size distribution of PVP-PtNPs is also observed owingto the aggregation of PVP-PtNPs. For PEG-PtNPs (Fig. 6e andf), more serious aggregation of PtNPs is observed, the particlesize of which can even reach 500 nm. On the contrary, the

BWT-Pt catalyst exhibits much higher stability, which exhibits30.5% yield of benzaldehyde in 4th cycle. These resultsstrongly verify that BWT as stabilizer is essential for achievinghigh activity and stability for biphasic oxidation of alcohol. ForBWT-Pt, the recovery of catalyst inevitably leads to some loss ofactivity, which could be responsible for the observed activityloss shown in Table 2.

We can simply mix Au3+ solution with BWT solutiontogether to obtain the BWT-AuNP colloidal solution. As shownin Fig. S4,† the synthesized Au nanoparticle colloidal solutionis dark red, and the surface plasmon resonance (SPR) peak ofAu NP is clearly observed at 535 nm in the UV-vis spectra ofBWT-AuNPs.45,46

Conclusions

In conclusion, a facile strategy was developed to synthesize aPtNP aqueous colloid by utilizing natural plant polyphenols asamphiphilic stabilizers. The as-prepared BWT-Pt catalystsfeature small particle size and good dispersity due to thestabilization effect of BWT derived from its unique molecularstructure. Under mild aerobic conditions, the BWT-Pt catalystsexhibited high activity and cycling stability in the oxidation ofaromatic alcohols and aliphatic alcohols. Additionally, theamphiphilic nature of BWT could efficiently promote the oxi-dation of alcohols in water. The experimental results suggestthat the hydroxyls in the BWT play critical role in stabilizingthe PtNPs, which also prevent the PtNPs from aggregating andleaching during the reactions.

Acknowledgements

This work was financially supported by National NaturalScience Foundation of China (21506134, 51173122, 21406247)and the Engineering Research Center for the Development ofFarmland Ecosystem Service Functions, Sichuan Province Insti-tutions of Higher Education.

Notes and references

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Table 2 Cycling stability of BWT-Pt, PVP-Pt and PEG-Pt as catalysts inbiphasic benzyl alcohol oxidation

Yield (%) Cycles 1 2 3 4

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Reaction conditions: 1 mmol of alcohol, catalyst contains 20 µmol ofPt and 15 mg of stabilizer, 15 ml of water, 0.5 mmol of K2CO3, 50 °C,10 hours.

Fig. 6 TEM images of fresh PVP-Pt (a) and PEG-Pt (d) catalysts andTEM images of PVP-Pt (b, c) and PEG-Pt (e, f ) catalysts after 4 cycles ofbiphasic benzyl alcohol oxidation.

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