Recyclable Amphiphilic Metal Nanoparticle Colloid Enabled

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Recyclable amphiphilic metal nanoparticle colloid enabled atmospheric oxidation of alcohols Baicun Hao, Meng Xiao, Yujia Wang, Hongyan Shang, Jun Ma, Yang Liao, and Hui Mao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12989 • Publication Date (Web): 18 Sep 2018 Downloaded from http://pubs.acs.org on September 20, 2018

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ACS Applied Materials & Interfaces

Recyclable amphiphilic metal nanoparticle colloid enabled atmospheric oxidation of alcohols Baicun Hao a, Meng Xiao a, Yujia Wang a, Hongyan Shang

a,b

, Jun Ma

a,b

, Yang Liao

*a,b

, Hui

Mao*a,b a

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, P.

R. China. b

Key Laboratory of land Resources Evaluation and Monitoring in Southwest, Sichuan Normal

University, Ministry of Education, Chengdu 610066, P. R. China. * To whom correspondence should be addressed. E-mail: [email protected] (H. Mao) KEYWORDS: linear plant polyphenols, amphiphilic PdNPs colloid, biphasic catalysis, atmospheric oxidation, activity, cycling stability.

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Developing amphiphilic colloid catalysts are essentially important for realizing environmentally benign biphasic catalysis under atmospheric conditions. Herein, a linear structured plant polyphenol was employed amphiphilic stabilizer for preparing a series of amphiphilic Pd nanoparticles (PdNPs) colloids. For the as-prepared PdNPs colloids, the phenolic hydroxyls of plant polyphenols were responsible for the stabilization of PdNPs, while the rigid aromatic scaffold of plant polyphenols effectively suppressed the PdNPs from aggregation by providing high steric effect. Thanks to the coexistence of hydrophilic phenolic hydroxyls and hydrophobic aromatic rings, the plant polyphenols provided the PdNPs tunable amphiphilic properties, which allowed an easier wetting of PdNPs to the substrate molecules. By tuning the content of plant polyphenols in the colloid, the particle size (3.17-4.73 nm) and dispersity of PdNPs were facilely controlled. When applied for atmospheric oxidation of insoluble alcohols in water by air, the amphiphilic PdNPs preferentially absorbed the alcohol substrates to create a relatively high substrate concentration microenvironment, which improved the mass transfer in the biphasic catalysis, allowing the proceeding of low-temperature (50 oC) atmospheric oxidation of diverse alcohols with high catalytic conversion, including aliphatic alcohols, cyclic aliphatic alcohols and aromatic alcohols. Furthermore, the amphiphilic PdNPs colloid also exhibited excellent reusability with a conversion yield high up to 97.96% in the 5th cycle. In contrast, the control catalysts of PVP-and PEG-stabilized PdNPs were completely inactived in the 5th cycle. As a consequence, our findings provided a new route for developing environmentally benign aqueous


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colloid catalyst that are both highly active and recyclable for mild biphasic oxidation reaction systems.

INTRODUCTION

Catalytic oxidation of alcohols is one of the most important transformation routes for the synthesis of fine chemicals, such as dyes, medicines and perfumes.1-3 The application of Pd catalysts in selective oxidation of alcohols has attracted considerable attention for their high activity and selectivity. The oxidation of alcohol has been catalyzed by a variety of recyclable heterogeneous Pd catalysts supported onto different matrices, including activated carbon,4 Al2O3,5 SBA-15,6 carbon nanotube,7 grapheme,8 etc. However, these catalytic oxidation reactions are often carried out in environmentally unfriendly organic solvents in order to promote the solubility of substrates.9,10 Normally, pressurized pure oxygen (e.g. 1.0 MPa) and/or high reaction temperature are required for obtaining a high substrate conversion yield.11,12 In considerations of the ever-growing environmental concerns, there is an urgent need to explore green reaction media and mild reaction system for achieving catalytic oxidation of alcohol. The use of water as reaction media for the aerobic alcohol oxidation would be preferable, since water is cheap, nontoxic, nonflammable, which allows an easy recovery of the products due to the insolubility of organic products in water. However, alcohols, comprised of more than 4 carbons in the backbone chain, have very low solubility in water, which would dramatically constrain the catalytic conversion yield of substrate since the reaction rate is mainly determined by the



solubility of the alcohols.13 To realize efficient catalytic oxidation of alcohol in water, the aqueous-organic biphasic catalysis provides us a promising solution, where the water soluble colloidal nanoparticles (CNPs) as aqueous catalysis phase while the insoluble substrate of alcohol directly serves as immiscible organic phase. For this biphasic catalysis, mass transfer resistance between aqueous catalysis phase and organic substrate phase is critical to determine the reactivity. Therefore, the synthesis of CNPs is essential for obtaining high-performance catalytic oxidation of alcohol in biphasic system since a rational design of CNPs is capable of decreasing the interfacial resistance, which is also possible to realize a mild atmospheric oxidation of alcohol by air.

Aqueous reduction is the most commonly used synthetic strategy for the controlled synthesis of CNPs. Typical aqueous reduction is usually carried out by reducing metal precursors (e.g. metal salts) in the presence of a stabilizer. Various stabilizers, including solvent molecules,14 ion pairs,15 surfactants,16 ligands,17 dendrimers18 and polymers,19 were used to prevent agglomeration and control over the growth of CNPs. However, these stabilizers have limited interfacial activation capability, and usually fail to reduce the interfacial resistance between CNPs solution and organic phase. Ideally, the stabilizer employed for the synthesis of CNPs should be amphiphilic, and have high affinity towards metal species. The high affinity of stabilizer to metal species can help to ensure the stability and dispersity of CNPs, while the amphiphilic property of stabilizer is able to enhance the mass transfer with significantly decreased interfacial resistance.20 Moreover, the amphiphilic stabilizer might also enrich the


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substrate molecules around the catalytically active CNPs, beneficial for obtaining a high activity and selectivity under alleviated reaction conditions.

Plant polyphenols are complex phenolic secondary metabolites extracted from plants.21 The distinct property of plant polyphenols is their chelating capability towards metal ions due to that the ortho phenolic hydroxyls of plant polyphenols are able to form stable five-membered chelating rings with various metal ions, including Cr3+,22 Fe3+,23 Al3+,24 etc. These phenolic hydroxyls are also capable of stabilizing the CNPs upon the reduction of chelated metal ions.25-27 Furthermore, the rigid aromatic scaffold of plant polyphenols could also effectively prevent the aggregation of metal NPs by providing high steric effect. More importantly, plant polyphenols are typical amphiphilic macromolecules due to the coexistence of hydrophilic phenolic hydroxyls and hydrophobic aromatic skeleton. In our previous research, we have utilized black wattle tannins (BWT), a type of plant polyphenols, as stabilizers for preparing water solution RhNPs.28 The as-prepared RhNPs were found to be highly efficient in biphasic hydrogenation of diverse C=O containing unsaturated compounds due to the amphiphilic nature BWT. Although amphiphilic BWT is able to decrease interfacial resistance for the biphasic hydrogenation of substrate, the obvious steric hindrance of BWT determined by its nature as dimensional polymer is the barrier for substrate to access the active CNPs. Therefore, there is a necessity to properly select plant polyphenols with suitable molecular structure for preparing highly efficient CNPs that are both highly active and recyclable in atmospheric oxidation of alcohol at alleviated temperature condition in water.



In considerations of above issues, bayberry tannin (BT), a linear structured plant polyphenol, was herein used as the amphiphilic stabilizer to prepare recyclable and highly efficient PdNPs colloid catalyst that is capable of realizing environmentally benign atmospheric oxidation of alcohols (Figure S1). By tuning the ratio of BT over Pd, PdNPs colloid catalyst with controllable particle size and dispersity were obtained. When applied for atmospheric oxidation of alcohol by air in water, the amphiphilic BT molecules stabilized with PdNPs preferentially absorbed the alcohol substrate, which allowed for obtaining a relatively high substrate concentration in the microenvironment surrounding the PdNPs. This distinct advantage significantly improved the mass transfer in the biphasic catalysis, eventually resulting in high catalytic conversion of diverse alcohols, including n-pentanol, n-hexanol, n-octanol, cyclohexanol, benzyl alcohol, cinnyl alcohol and phenethyl alcohol. For example, when BT stabilized PdNPs (BT-PdNPs) were brought into contact with benzyl alcohol under atmospheric air, the conversion yield reached 98.38% after reacted 12 h at a mild temperature of 50 oC. Besides the high catalytic activity under mild conditions, the amphiphilic BT-PdNPs aqueous colloid also exhibited excellent reusability as compared with PVP and PEG stabilized PdNPs colloid. In the 5th cycle, the conversion yield of benzyl alcohol over BT-PdNPs was still high up to 97.96%. In contrast, the control samples of PVP-PdNPs and PEG-PdNPs were completely inactived. These results manifested the distinctive roles of BT as efficient amphiphilic stabilizer for developing environmentally benign colloid catalyst that is highly active in mild biphasic oxidation of alcohols.


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Scheme 1 The schematic illustration showing the proposed role of BT-PdNPs in mild biphasic oxidation of alcohols.

EXPERIMENTAL

Materials. Bayberry tannin (BT) was purchased from Plant of Forest Products in Guangxi province (China). PdCl2, NaBH4, benzyl alcohol and other chemicals were all analytical reagents and purchased from Aladdin cooperation.

Synthesis of amphiphilic BT-PdNPs aqueous colloid. 20 µmol Pd2+ was dissolved into 5.0 mL of deionized water, and then different amounts of BT (2 mg, 5 mg, 10 mg, 15 mg, 30 mg and 60 mg) were added into the Pd2+ solution. The resultant BT-Pd2+ solution was stirred constantly for 30 min. Subsequently, the BT-Pd2+ solution was reduced by 4.0 mL of 2.5 g.L-1 NaBH4 aqueous solution, and the resultant materials were amphiphilic BTx-PdNPs aqueous colloid, where x is the content of BT in the colloid. PVP-PdNPs and PEG-PdNPs were prepared by the



similar procedures by using PVP (molecular weight of 4000) and PEG (molecular weight of 4000) as the stabilizer, respectively.

Characterization and catalytic experiments. Fourier Transform Infrared spectroscopy (FT-IR) analyses of BT and BT-PdNPs samples were tested by Lambda 950 NIR spectrophotometer. The UV-vis spectra were measured by Lambda 950 UV-vis instrument. The size and distribution of PdNPs was determined using Transmission Electron Microscopy (TEM, Tecnai G2 F20 S-TWIN). The water contact angle of BT solutions on glass substrate was measured by a contact angle goniometer (Krüss, DSA30, Germany). 1.0 mmol of benzyl alcohol was added with 5.0 mL, 0.1 mol.L-1 K2CO3 solution, and 10.0 mL of BT-PdNPs aqueous colloid was added into the K2CO3 solution. The reaction of the resultant biphasic system was carried on different temperatures (30 oC, 40 oC, 50 oC, 60 oC, 70 oC, 80 oC) and reaction times (3 h, 5 h, 10 h, 12 h, 24 h, 36 h, 48 h,) with continuous air bubbling and stirring (500 rpm) at atmospheric pressure. When the reaction was completed, 0.2 mmol of dodecane was added as internal standard, and the product was extracted with ethyl acetate. After the extraction, the organic phase was taken out by using a syringe, and dried over Na2SO4. The resultant organic phase was analyzed by a gas chromatograph equipped with a FID detector and a SGE AC20 capillary column using N2 as the carrier gas and the remaining BT-PdNPs aqueous colloid was directly used for subsequent catalytic cycles.

RESULTS AND DISCUSSION


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Apparently, the content of BT has significantly influence on the particle size and dispersity of PdNPs contained in the colloid. Fig. 1a-f show the TEM images of a variety of amphiphilic BT-PdNPs aqueous colloid and the corresponding particle size distribution. The PdNPs suffer from obvious aggregation when the content of BT is 5 mg. The aggregation is ascribed to insufficient stabilization of BT with low content. The aggregation is alleviated along with the increasing content of BT. There is almost no aggregation of PdNPs observed in BT-PdNPs aqueous colloid when the content of BT is as high as 15 mg. The average particle size of PdNPs is 4.73 nm in the BT5-PdNPs. The particle size of PdNPs in the colloid is further decreased to 3.23 and 3.17 nm when the content of BT is increased to 15 mg and 60 mg, respectively. Normally, PdNPs with smaller size usually exhibit better catalytic activity.29,30

Fig. 1 TEM and High resolution TEM (HRTEM) images of BT5-PdNPs (a, d), BT15-PdNPs (b, e), BT60-PdNPs (c, f). The insets in a, b, c are the corresponding particles size diameter of



BT5-PdNPs, BT15-PdNPs and BT60-PdNPs.

However, amphiphilic BT molecules tend to self-assemble into supermolecular structure via multiple hydrogen bond and/or hydrophobic interactions among BT molecules.31,32 These BT shells should restrict the accessibility of substrates to the stabilized PdNPs, leading to a low activity. In Fig. 1c, it is quite evident that the PdNPs are dispersed in a continuous film that are formed from the self-assembly of free BT molecules existing in the BT60-PdNPs. As a consequence, the content of BT in the BT-PdNPs aqueous colloid should be rationally controlled in a suitable range to obtain both high dispersity and high activity. In the following experiments, BT15-PdNPs was demonstrated to be highly efficient in terms of activity and cycling stability. The PdNPs in the BT-PdNPs aqueous colloid are highly crystallized due to that their XRD patterns shows the characteristic peak of metallic Pd at 40o. (Figure S2).33,34

The stabilization of BT towards PdNPs was also proved by the FTIR spectra of BT and BT15-PdNPs as shown in Figure S3. In the FTIR spectrum of BT, the peak at 3400 cm-1 is attributed to the stretching vibration of phenolic hydroxyls, and its broad range is due to the formation of hydrogen bonds among BT molecules.35 Peaks in the vicinity of 1614-1458 cm-1 indicate the presence of aromatic rings of BT. In FTIR spectrum of BT-PdNPs, the stretching vibration peak of phenolic hydroxyls is narrowed and the peak intensity of aromatic rings of BT also becomes weak. These changes confirm the stabilization of phenolic hydroxyls of BT towards the PdNPs.36 Fig. S3 shows the UV-vis spectra of BT and BT-PdNPs aqueous colloid. The UV-vis spectrum of BT exhibited a characteristic peak of at 283 nm while this characteristic


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peak disappeared in the UV-vis spectrum of BT-PdNPs. These changes of UV-vis spectra suggest that the phenolic structure of BT is involved in the stabilization PdNPs.37,38 In consideration of the strong chelating ability of adjacent phenolic hydroxyls of polyphenols, the PdNPs should be stabilized by electron donation/accepting interactions between BT molecules and PdNPs.

Fig. 2. Influences of BT content on the catalytic activity of BT-PdNPs (a); Influences of reaction time on the catalytic activity of BT15-PdNPs (b); Influences of temperature on the catalytic activity of BT15-PdNPs (c); Cycling stability of BT5-PdNPs and BT15-PdNPs. Reaction



conditions: 1 mmol alcohol (d), catalyst contains 20 µmol of Pd and 15 mg of BT, 15 ml of water, 0.5 mmol of K2CO3, 50 ℃.

The as-prepared BT-PdNPs aqueous colloids were employed as aqueous catalysis phase in the biphasic atmospheric oxidation of benzyl alcohol. The reaction temperature was kept at 50 oC and air bubble was supplied from the bottom of the catalysis phase as the oxygen source. K2CO3 solution was added to promote the activity and selectivity of BT-PdNPs aqueous colloids.39 As presented in Fig. 2a, the conversion of benzyl alcohol was ~99% when BT2-PdNPs and BT5-PdNPs were used as the catalysis phase. Under the same experimental conditions, the conversion of benzyl alcohol was found to slightly decrease to 98.74% and 98.31% when BTx-PdNPs with higher content of BT was employed in the biphasic oxidation, including BT10-PdNPs and BT15-PdNPs. When BT30-PdNPs was applied in atmospheric oxidation of benzyl alcohol, the conversion of benzyl alcohol began to decrease quickly, which is as low as 96.72%. More obvious decrease of conversion was obtained when BT60-PdNPs was used as the catalysis phase. The corresponding conversion of benzyl alcohol was as low as 86.41%. Apparently, the content of BT has significant influence on the activity of BT-PdNPs aqueous colloid. When the content of BT is in the range of 2 to 15 mg, the activity of BTx-PdNPs aqueous colloid is kept in a high level, while excessive high content of BT causes loss of activity. As shown in the above TEM observations, BT60-PdNPs consists of smaller PdNPs and have better dispersity than BTx-PdNPs (x = 5, 15). However, these catalysis colloids actually exhibit much lower activity, which can be interpreted by the surfactant-like properties of BT. The amphiphilic


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BT molecules are likely to assemble to supermolecules via multiple hydrogen bond and/or hydrophobic interactions. These supermolecules not only give rise to high steric hindrance to restrict the growth of PdNPs but also cause partial inactivation of PdNPs by preventing the access of substrate to the PdNPs.40 In consideration of both activity and cycling stability, the BT15-PdNPs is selected as the most appropriate catalysis phase in subsequent experiments.

Fig. 2b. shows the relationship of reaction time versus the conversion of benzyl alcohol at 50 o

C using BT15-PdNPs as the catalyst. Along with the increase of reaction time from 3 h to 12 h,

the conversion of benzyl alcohol increases from 58.39% to 98.38%. Further prolong the reaction time, the conversion of benzyl alcohol slightly changes, indicating biphasic catalytic oxidation of benzyl alcohol reaches equilibrium at 12 h. Compared with previous reported heterogeneous PdNPs catalysts,41-43 the BT15-PdNPs aqueous colloid shows appreciable catalytic activity and advantages in terms of mild reaction conditions and free from organic solvent. The catalytic performance of BT15-PdNPs is appreciable compared with the other PdNPs in biphasic oxidation of alcohols.44,45

A mild reaction temperature is highly preferred in view point of energy saving in practical application. We thus carried out biphasic atmospheric oxidation of benzyl alcohol in the range of 30 oC to 80 oC. As showed in Fig. 2c, the conversion of benzyl alcohol is abruptly increased from 82.49% to 98.31% when the temperature is increased from 30 oC to 50 oC. Further increasing temperature to 60 oC, the conversion of benzyl alcohol reaches the value of 99.45% and then increased slightly upon the reaction temperature increases to 70 oC and 80 oC. The optimized



reaction temperature for atmospheric oxidation of benzyl alcohol is substantially lower than other reported catalysts. For example, microgel-stabilized Pd nanoclusters synthesized by Andrea Biffis et al. exhibited high activity at 100 oC with the pure O2 as oxidant.46 This advantage of alleviated reaction temperature is likely due to the amphiphilic nature of BT that decreases the interfacial resistance, and is capable of absorbing the organic substrate, creating a microenvironment with relatively high substrate concentration surrounding the PdNPs. We measured the static water contact angle (WCA) of various BT-PdNPs aqueous colloids on glass substrate. It was found that the WCA of BT-PdNPs droplet on the glass substrate is decreased from 66.5o to 38.1o along with the content of BT is increased from 2 to 60 mg, as shown in Fig. 3. The decrease of WCA is due to the increased BT content in water, which minimizes the surface energy of BT-PdNPs droplet, allowing an easier wetting of BT-PdNPs colloid to the substrate surface. These results suggest that amphiphilic properties of BT-PdNPs colloid should be helpful for enhancing the interactions between PdNPs and the substrate molecules. In the following cycling experiments, the biphasic atmospheric oxidation of benzyl alcohol was carried out at 50 o

C.


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Fig. 3. The static water contact angle (WCA) of BT2-PdNPs (a), BT5-PdNPs (b), BT10-PdNPs (c), BT15-PdNPs (d), BT30-PdNPs (e) and BT60-PdNPs (f) on glass substrate.

The proper higher content of BT was found to have significant enhancement on the recycling stability of BTx-PdNPs. As shown in Fig. 2d, the conversion of benzyl alcohol catalyzed by BT5-PdNPs was 99.25% in the first cycle, and the substrate conversion in the follow 4 cycles keeps above 90%. However, the conversion of benzyl alcohol suffers from a sharp decline and drops to 29.27% in the 6th cycle. In contrast, the conversion of benzyl alcohol exhibits no significant decrease during the six cycles when BT15-PdNPs was employed as the catalysis phase. According to TEM observations, obvious aggregation of PdNPs was evident for the recycled BT5-PdNPs. Some aggregation of PdNPs even has the particle size of 200 nm (Fig.



4a). These morphology changes of BT5-PdNPs suggest that a low content of BT is insufficient to provide enough stability to the PdNPs, which eventually leads to activity loss after several repeative cycles. In contrast, the recycled BT15-PdNPs colloid still exhibits pretty good dispersity of PdNPs due to the sufficient steric hindrance provided by the proper content of BT (Fig. 4b).

Fig. 4 TEM images of BT5-PdNPs (a) and BT15-PdNPs (b) after cycling.

Subsequently, the catalytic performance of BT15-PdNPs in oxidation of other alcohols was also investigated, including linear aliphatic alcohols, cyclic aliphatic alcohols and aromatic alcohols. All the reactions were carried out under atmospheric conditions at 50 ℃ by air in water. The experimental results are summarized in Table 1. BT15-PdNPs was still active for oxidation of other alcohols under these mild conditions. The conversion of linear and cyclic aliphatic alcohols were all above 90%. It should be noted that the conversion of aromatic alcohols is decreased with the increase of the carbon chain. The relatively low conversion of aromatic alcohols with larger molecular weight could be attributed to the steric hindrance of BT molecules, which


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makes it more difficult for the larger substrate to contact with the PdNPs. Notably, this strong steric hindrance effect of BT-PdNPs might be applied for obtaining high selectivity, and further investigations in this aspect will be carried out.

Table 1 The catalytic activity of BT15-PdNPs in biphasic, low-temperature and atmospheric oxidation of alcohols Substrates

Conversion（%）

Time (hour)

n-pentanol

98.96

10

n-hexanol

98.36

10

n-octanol

98.99

10

Cyclohexanol

98.67

10

Benzyl alcohol

99.89

24

Cinnyl alcohol

92.93

24

Phenethyl alcohol

62.83

24

Reaction conditions: 1 mmol alcohol, catalyst contains 20 µmol of Pd and 15 mg of BT, 15 ml of water, 0.5 mmol of K2CO3, 50 ℃.

The reusability of BT15-PdNPs was also carried out in biphasic atmospheric oxidation of phenylethyl alcohol, cyclohexanol and n-pentanol. As shown in Fig. 5b, the activity of BT15-PdNPs catalyst shows no loss of activity during the five cycles, manifesting the high stability of BT15-PdNPs.



Fig. 5 Cycling stability of BT15-PdNPs, PEG-PdNPs and PVP-PdNPs in biphasic atmospheric oxidation of Benzyl alcohol (a). Cycling stability of BT15-PdNPs in oxidation of different alcohols (b). Reaction conditions: 1 mmol of alcohol, catalyst contains 20 µmol of Pd and 15 mg of BT, 15 ml of water, 0.5 mmol of K2CO3, 50 °C, 24 hours.

We also carried out the biphasic atmospheric oxidation of benzyl alcohol by using PVP-PdNPs and PEG-PdNPs as the aqueous catalysis phase. As shown in Fig. 5a, the PVP-PdNPs and PEG-PdNPs exhibit substantially lower conversion of benzyl alcohol, which are 99.96% and 99.92%, respectively. Based on the TEM observations in Fig. 6a, b, the particle size of PdNPs in PVP-PdNPs and PEG-PdNPs are 3.43 nm and 3.27 nm, respectively, very close to that of BT15-PdNPs (3.23 nm). These PdNPs in the two colloids are also highly crystallized as confirmed by their XRD patterns (Fig S2). However, the conversion of benzyl alcohol quickly decreases to 87.26% in the 2nd cycle, and 40.86% in the 4th cycle. In the 5th cycle, the PVP-PdNPs completely lost its catalytic activity. The PEG-PdNPs exhibits similar cycling behaviors to those of PVP-PdNPs. The substrate conversion is decreased from 99.92% in the first


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cycle to 84.95% in the 2nd cycle, 40.84% in the 4th cycle and zero in the 5th cycle. Apparently, the stabilization of PVP and PEG to PdNPs is completely incomparable to restrain the aggregation and leaching of PdNPs during the cycles. These assumptions are further confirmed by the TEM observation and ICP-AES measurements. As shown in Fig. 6c, d, serious aggregation of PdNPs both occurs in the recycled PVP-PdNPs and PEG-PdNPs. For example, micro-size aggregation of PdNPs are observed in the recycled PVP-PdNPs. Based on ICP-AES measurements, 9.5% and 10.6% of Pd have been leached into the organic phase after the 1st cycle of benzyl alcohol oxidation using PVP-PdNPs and PEG-PdNPs as the catalyst, respectively.



Fig. 6 TEM images of PVP-PdNPs (a), PEG-PdNPs (b) and PVP-PdNPs (c), PEG-PdNPs (d) after cycling. The insets in a, b are the corresponding particles size diameter of PVP-PdNPs and PEG-PdNPs, respectively.

CONCLUSIONS

In summary, bayberry tannin, a natural plant polyphenol, was utilized as amphiphilic stabilizer for the facile synthesis of BT-PdNPs aqueous colloid. The abundant orthophenolic hydroxyls of BT could be chelated with Pd ions and further stabilize the PdNPs. By changing the content of BT in the colloid catalyst, the average particle size and dispersity of PdNPs were both rationally adjusted. Due to the amphiphilic nature of BT molecules, the as-prepared BT-PdNPs colloid exhibited surfactant-like properties, which was beneficial for improving the mass transfer of biphasic catalysis and creating a microenvironment surrounding the PdNPs to preferentially absorb substrate. As a consequence, the BT-PdNPs colloid was highly active and recyclable in biphasic atmospheric oxidation of a variety of alcohols under mild aerobic conditions, which also showed obvious advantage in terms of cycling stability as compared with the counterparts prepared by using PVP and PEG as stabilizer.

Supporting Information. Structure of bayberry tannin and schematic illustration of biphasic oxidation, XRD patterns, FTIR spectra and XPS spectra.

AUTHOR INFORMATION Corresponding Author Prof. Hui Mao, Email: [email protected]


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Prof. Yang Liao, Email: [email protected] Notes The authors declare no competing financial interest.

Funding Sources This work was financially supported by National Natural Science Foundation of China (21506134, 21776188) and Youth Foundation of Sichuan Scientific Committee (2017JQ0054).

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Table of Contents

We successfully synthesized a series of amphiphilic Pd nanoparticles (PdNPs) colloids catalysts by using a linear structured plant polyphenol as the amphiphilic stabilizer of metal nanoparticles, which promoted the affinity of metal nanoparticle towards alcohols substrates. The resultant catalysts exhibited high activity and good reusability in low-temperature (50 oC) atmospheric oxidation of alcohols.


Recyclable Amphiphilic Metal Nanoparticle Colloid Enabled

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