Janus N-Doped Carbon@Silica Hollow Spheres as Multifunctional

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Functional Nanostructured Materials (including low-D carbon)

Janus N-doped Carbon@Silica Hollow Spheres as Multifunctional Amphiphilic Nanoreactors for Base-free Aerobic Oxidation of Alcohols in Water Jinyu Dai, Houbing Zou, Zhiqiang Shi, Hengquan Yang, Runwei Wang, Zongtao Zhang, and Shilun Qiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11888 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 8, 2018

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ACS Applied Materials & Interfaces

Janus N-doped Carbon@Silica Hollow Spheres as Multifunctional Amphiphilic Nanoreactors for Base-free Aerobic Oxidation of Alcohols in Water Jinyu Dai,† Houbing Zou,*†‡ Zhiqiang Shi,† Hengquan Yang,‡ Runwei Wang,*† Zongtao Zhang,† Shilun Qiu† †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, 130012, China

‡

School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China

ABSTRACT: The hydrophobicity/hydrophilicity of nanocatalysts has a significant impact on their performances via modulating the adsorption, transfer and desorption of reactants/products. In this work, we reported a novel multifunctional amphiphilic nanoreactor composed of Janus nitrogen-doped carbon@silica hollow nanostructure and ultrasmall Pt nanoparticles. The core/shell polybenzoxazine@mesosilica spheres were used as the precursor for pyrolysis. It was found that the internal polybenzoxazine was decomposed from interior to exterior and transformed into nitrogen-doped carbon hollow shell that partly embedded into the mesosilica layer, forming the Janus hollow spheres. The obtained nanoreactor showed remarkable activity and selectivity for base-free aerobic oxidation of alcohols in water using air as the oxidant. One-pot oxidation-condensation cascade reaction was also successfully demonstrated to synthesize imines from alcohols and amines with good yields. The sorption analyses revealed that the superior hydrophilicity/hydrophobicity strengthened both adsorption of hydrophobic alcohols from water and desorption of byproduct water molecules from the active sites. The doped nitrogen atoms in carbon matrix were not only severed as anchoring sites for stabilizing ultrasmall Pt nanoparticles but also as basic active sites for accelerating deprotonation process. Moreover, due to the anchoring effect of nitrogen and the extremely stable amphiphilicity, this nanoreactor exhibited excellent catalytic stability. KEYWORDS: Janus nanostructure, hollow nanostructure, amphiphilic nanoreactor, green aerobic oxidation, cascade reaction

INTRODUCTION Recently, hollow nanostructures have attracted particular attention and have been well recognized to be severed as highly efficient nanoreactors for various catalysis applications.1-9 Since the permeable hollow shell can not only effectively isolate catalytic species but also provide unique reaction microenvironment, these nanoreactors often display outstanding catalytic activities and selectivities. Up to date, different hollow nanostructures based on mesoporous silica,10-14 zeolite,15-16 nanoporous polymer/carbon17-19 and metal organic framework20-23 have been continuously reported as novel nanoreactors and presented enhanced performances in various catalytic reactions. Among these nanocatalysts, amphiphilic nanoreactors remain a great surge of research interest because the hydrophobicity/hydrophilicity has a significant impact on their catalytic performances via modulating the adsorption, transfer and desorption of reactants and intermediates as well as products.24-27 For example, Shi et al. tailored the surface wettability of the hollow structured Au@SiO2 for enhanced reduction of nitroaromatic compounds in aqueous phase.28 A positive correlation was found between the reaction rate and the hydrophobicity of the substrate. Zhang et al.29 and Lee’s group30 independently reported inorganic micellar catalysts with controllable hydrophobic/hydrophilic interfaces, which exhibited extraordinary performance in terms of catalysis and oil/water separation. Our group also developed a series of amphiphilic hollow porous nanomaterials via tuning the proportion of the inorganic (-SiO2-) and organic (–O1.5Si–R–SiO1.5–) unit.31-34

The hydrophilic external surfaces and hydrophobic hollow cavities made these materials disperse well in water yet still enrich organics from aqueous solution, then leading to excellent performances in sorption and catalysis. While significant advance have been achieved, the fabrication of these amphiphilic nanoreactors is highly dependent on the selective surface modification or framework functionality, which results in an instable amphiphilicity. This is powerless for the catalytic reactions occurred at high temperature (≥ 100 oC). Importantly, implanting multiple active sites into these amphiphilic nanocatalysts still remains a great challenge because it is very difficult to introduce different accessible active sites in conjunction with tuning complex hydrophobicity/hydrophilicity. Over the past decade, Janus particle consisting of two surfaces or internal materials is increasingly emerging as one new generation of smart functional materials and presents some promising applications in biomedicine and catalysis.35-39 Because of distinct physical and chemical properties of two sides, Janus particles can be viewed as solid surfactants and often shows excellent amphiphilicity.40-42 Obviously, a straightforward and efficient approach to construct amphiphilic nanocatalysts is rational designing Janus silica/carbon nanoparticles with well-defined proportions and positions, and various active sites can be easily introduced into their frameworks via molecular functionality or heteroatom doping.43-44 For instance, Yang’s group recently designed an anisotropic dumbbellshaped mesoporous carbon@organosilica Janus particle and found that the particle possessing both hydrophilic side and

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hydrophobic side showed well-defined amphiphilicity which is similar to that of molecular surfactant.45 However, although some silica/carbon composites have been reported for enhanced catalytic applications,46-49 developing silica/carbonbased amphiphilic nanocatalysts has been still rarely explored, due to the great challenge in precisely controlling their structures. Herein, we prepared a Janus nitrogen-doped carbon@silica hollow nanostructure via confined pyrolysis to construct a novel multifunctional amphiphilic nanoreactor. The nitrogendoped carbon partly embedded into the mesoporous silica shell, leading to the formation of external silica surface and internal carbon surface as well as buffered carbon/silica interface. Such a unique Janus structure endowed the material superior hydrophobicity/hydrophilicity, and then presenting extreme affinity towards organics in water. Additionally, the doped nitrogen atoms in carbon matrix can not only provide anchoring sites for stabilizing ultrasmall metal nanoparticles but also serve as basic active sites. On the basis of these merits, our amphiphilic nanoreactor showed significantly enhanced activity and selectivity for base-free aerobic oxidation of various alcohols in water using air as the oxidant. Moreover, one-pot synthesis of imines from alcohols and amines could be achieved with good yields via oxidationcondensation tandem catalysis. Furthermore, owing to the high stability of framework for silica and carbon, the nanoreactor possessed extremely stable amphiphilicity, thus leading to an excellent catalytic stability.

EXPERIMENTAL SECTION Synthesis of Janus N-doped Carbon@mesosilica Hollow Spheres (NxC@mSiO2). In a typical synthesis, 0.12 g of CTAB was dissolved in a mixture of ethylenediamine (EDA), ethanol (15 mL) and deionized water (35 mL). Then, 0.16 g of resorcinol and 0.24 mL of formaldehyde (37 wt %) were slowly added into above solution. After stirring for 2 h at room temperature, polybenzoxazine (PB) spheres were obtained. Subsequently, 50 mg of CTAB and 0.6 mL of TEOS were introduced into the resultant PB solution for coating mesosilica (mSiO2). After stirring for 2 h at room temperature, the core/shell structured PB@mSiO2 nanospheres were collected by centrifugation and then washed with water and ethanol several times. Before pyrolysis, the solid product was dried at room temperature for 2 days. Finally, the hybrid hollow nanostructure NxC@mSiO2 was obtained by simply heating the PB@mSiO2 nanospheres under argon atmosphere. The temperature was raised from 30 oC to 900 oC with a heating rate of 5 oC/min and kept at 900 oC for 3 h. Constructing Multifunctional Amphiphilic Nanoreactor via immobilizing Pt nanoparticles (Pt/NxC@mSiO2). 0.2 g of NxC@mSiO2 was dispersed in 40 mL of ethanol solution by ultrasonication. Then, 20 mM H2PtCl6 solution was dropwise added and the resultant mixture was stirred for 2 h at room temperature. The solvent was removed by rotary evaporation, and the obtained solid material was transferred to hydrogen atmosphere and heated for 2 h at 200 oC, presenting multifunctional amphiphilic nanoreactor Pt/NxC@mSiO2. The loading amount of Pt is identified to be 5 wt%. Catalytic Tests. For the catalytic oxidation study, 0.5 mmol of alcohols, Pt catalysts (1.0 mol %) and 4.0 mL of H2O were mixed in a 25 mL round-bottomed flask equipped with a reflux condenser. The reaction was performed at 80 oC in an oil bath under magnetic stirring. After completion of the reaction,

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the reaction mixture was extracted by ethyl acetate for 3 times. The liquid phase was subsequently analyzed by gas chromatography-mass spectrum Shimadzu GCMS-QP2010 Plus with a flame ionization detector (FID), and dodecane was used as the internal standard. For the recycling test, the catalyst was collected using centrifugation. The residual catalyst was washed with water and ethanol for several times and used directly for the next catalytic reaction. For the direct imine formation by oxidative coupling of alcohols and amines, 0.5 mmol of alcohols, 0.6 mmol of amines, Pt catalysts (5.0 mol %) and 4.0 mL of H2O were mixed in a 25 mL round-bottomed flask equipped with a reflux condenser. The reaction was performed at 120 oC in an oil bath under magnetic stirring. After completion of the reaction, the reaction mixture was extracted by ethyl acetate for 3 times. The liquid phase was subsequently analyzed by GC-MS. Materials Characterization. Scanning electron microscope (SEM) images were taken on a JEOL JSM-6700F fieldemission electron microscope. Transmission electron microscope (TEM) images were obtained from an FEI Tecnai G2 F20s-twin D573 field emission transmission electron microscope at an accelerating voltage of 200 kV. Powder XRD patterns were obtained by using a Rigaku 2550 diffractometer with Cu Ka radiation (λ=1.5418 Å). N2 adsorption-desorption isotherms were obtained at -196 oC on a Micromeritics ASAP 2010 sorptometer. Samples were degassed at 120 oC for a minimum period of 12 h prior analysis. Brunauer-Emmett-Teller (BET) surface areas were calculated from the linear part of the BET plot. Pore size distribution was estimated from the adsorption branch of the isotherm by the DFT method. Raman spectroscopy was performed using a Horiba LabRAM HR Evolution. CHN elemental analyses were obtained using an Elementer vario MICRO cube analyzer. The thermogravimetric analysis (TGA) was obtained in a flow of nitrogen from 30 to 800 oC (10 oC min-1) using a Netzsch STA 449F3 thermogravimetric analyzer. UV-visible spectra were recorded on a Shimadzu UV-2450 spectrometer. Inductively coupled plasma mass spectrometry (ICP) analyses were carried out on a NexION 350 ICP-MS instrument. The X-ray photoelectron spectroscopy (XPS) spectra were acquired using an ESCALAB250 spectrometer. Pickering emulsions were observed with an optical microscope (XSP-8CA, Shanghai).

RESULTS AND DISSCUSSION Preparation and Characterization of Multifunctional Amphiphilic Nanoreactor Pt/NxC@mSiO2. The multifunctional amphiphilic nanoreactor was prepared via a confined pyrolysis strategy followed by loading Pt nanoparticles, which is illustrated in Figure 1a. Firstly, we synthesized polybenzoxazine (PB) spheres with an average particle diameter of 240 nm (Figure S1a) via a sol-gel method where resorcinol and formaldehyde were used as carbon precursor and ethylenediamine was used as both base catalyst and nitrogen source. And then the tetraethoxysilane was subsequently introduced into the above synthesis system for uniformly coating a mesosilica (mSiO2) layer on the PB spheres, forming core/shell structured PB@mSiO2. The SEM and TEM images (Figure S1b-d) clearly indicate that every particle has a uniform core/shell structure and the thickness of mSiO2 shell is approximately 30 nm. After carbonizing in inert atmosphere at 900 oC, the internal PB spheres transformed into nitrogen-doped carbon (NxC) hollow shell under the protection of external mSiO2 layer which partly embedded into the mSiO2 shell,


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Figure 1. (a) Schematic illustration for the preparation of multifunctional amphiphilic nanoreactor. SEM image (b), TEM images (c, d), HRTEM image (e), HAADF-STEM image (f), elemental mapping images (g-j), nitrogen adsorption-desorption isotherm (k), Raman spectrum (m) and N 1s XPS spectrum (n) of the Janus NxC@mSiO2 hollow spheres. The inset in k is the corresponding DFT pore size distribution.

leading to formation of Janus NxC@mSiO2 hollow spheres. Finally, catalytically active Pt ultrasmall nanoparticles were implanted into the NxC matrix via an impregnation-reduction approach, presenting the multifunctional amphiphilic nanoreactor Pt/NxC@mSiO2. We firstly characterized the Janus NxC@mSiO2 hollow nanostructure in detail. The SEM image (Figure 1b) shows uniform spherical morphology with an average particle size of 300 nm. The broken sphere directed by the red arrows suggests the particles are hollow. The low-magnification TEM image in Figure 1c further confirms the hollow nanostructure. The shell thickness is about 40 nm which is distinct larger than the mSiO2 shell thickness (30 nm) of the sample PB@mSiO2,

suggesting that the internal surface is NxC. From the highmagnification TEM images (Figure 1d-e), it is surprising to observe that the NxC is partly embedded into the mesochannels of mSiO2. The perpendicular mesochannels directed by red arrows could be also seen in the mSiO2 layer. To verify this unique Janus hollow nanostructure, we removed the NxC and mSiO2 from the NxC@mSiO2 hollow sphere via calcining at air atmosphere and etching using hydrofluoric acid, respectively. Their TEM images (Figure S2) clearly show that the mSiO2 shell thickness is about 30 nm that is in good agreement with that of the sample PB@mSiO2 and the NxC shell thickness is about 20 nm. This result strongly reveals that there is a junction between internal NxC and external mSiO2.



Moreover, the high-magnification SEM and TEM images exhibit that the external surface of NxC hollow sphere is very rough, further proving that the NxC is partly embedded into the mesochannels of mSiO2. Furthermore, the HAADF-STEM images (Figure 1f and the inset) also confirm the unique Janus hollow shell, as evidenced by the distinct contrast. The elemental mapping testifies the existence of element Si, O, C and N in the particles (Figure 1g-j), and it can be clearly seen that most of the element C is distributed inside the hollow spheres (Figure S3). It is worth mentioning that there are some NxC fragments in the hollow cavity of the sample NxC@mSiO2, which is very beneficial to stably immobilize ultrasmall metal nanoparticles. The key to form such a Janus hollow nanostructure was the decomposition of the polymer PB spheres from interior to exterior under the protection of mSiO2 shell (Figure 1a). Although confined pyrolysis has been recently reported to prepare different porous carbon nanostructures,50-53 the internal polymer was often shrunk from outside to inside, leading to formation of yolk/shell nanostructure50-51 and even nanoframe structure52. Hollow carbon spheres with foam-like shells could be also obtained via carbonizing sandwich-like SiO2@RF@SiO2 spheres.53 In our synthesis, the PB spheres with a low crosslinking degree were served as the carbon precursor. When carbonizing RF@mSiO2 spheres under the identical conditions, we obtained yolk/shell structured carbon@mSiO2 spheres (Figure S4a-b), similar to previous reports. Moreover, if a thermal treatment was carried out to enhance the crosslinking degree of PB spheres before pyrolysis, decomposition of the PB spheres from exterior to interior was also observed (Figure S4c-d). These results clearly suggested that choosing a polymer that is easy to decompose at high temperature is essential for presenting the inside out decomposition type.54 On the other hand, coating mSiO2 shells could provide a hard PB-mSiO2 interface that guided the shrinking directions of the PB spheres from interior to exterior during pyrolysis.53 Without the mSiO2 shell, directly carbonizing the PB spheres led to formation of NxC spheres with decreased particle sizes (Figure S4e-f). It is also worth noting that the surfactant CTAB in the mesochannels of mSiO2 shell converted into carbon matrix during carbonization, also beneficial to the formation of this Janus NxC@mSiO2 hollow shell. The nanoporous structure and chemical composition of the Janus NxC@mSiO2 hollow sphere were further analyzed using nitrogen sorption and different spectroscopies. As shown in Figure 1k, the nitrogen adsorption-desorption isotherm displays distinct uptakes at both low (0-0.01) and high (0.3-0.9) relative pressure P/P0, which should be assigned to the microporous structure of NxC and the mesoporous structure of mSiO2, respectively. A hysteresis loop of the H3 type in the P/P0 range of 0.5-0.9 is also observed in the isotherm, suggesting existence of a hollow cavity inside the particle. The corresponding density-functional-theory (DFT) pore size distribution exhibits two different nanopores centered at 0.6 and 1.3 nm, respectively. The Brunauer-Emmett-Teller (BET) surface area is 227 m2 g-1, among which the micropore area is 161 m2 g-1. The uniform microporous and mesoporous structure were further confirmed by the nitrogen sorption analysis of NxC and mSiO2 hollow sphere (Figure S5), respectively. The Raman spectrum shows two well-defined peaks located at 1350 and 1580 cm-1 (Figure 1m), assigned to typical D-band and G-band of carbon, respectively. The appearance of G-band indicates the graphitic nature of the NxC matrix. The high ID/IG value

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suggests that there are some defect structures distributing in the carbon framework, which might be arisen from the disturbance of mSiO2 shell during carbonization. The X-ray photoelectron spectroscopy (XPS) analysis shows that the sample is composed of C, H, N, O and Si elements. High resolution XPS spectrum of N 1s can be resolved into three peaks located at 401.4 eV, 400.5 eV, and 398.6 eV (Figure 1n), attributed to graphitic-N, pyrollic-N and pyridinic-N, respectively. The elements analysis revealed that the carbon content was 34 wt% (Table S1), close to the result of thermogravimetric (TG) analysis (Figure S6). The nitrogen content in carbon matrix was as high as 2.0 wt%. These results clearly demonstrate that the Janus NxC@mSiO2 hollow sphere was composed of microporous NxC and mesoporous mSiO2.

Figure 2. (a) Appearance of the mixture of water and toluene in presence of mSiO2, NxC and NxC@mSiO2 hollow spheres. Optical micrograph of different emulsions stabilized by NxC@mSiO2 (b), mSiO2 (c) and NxC (d) hollow spheres. The insets in a show the corresponding water contact angles.

The unique Janus structure of NxC@mSiO2 can endow the material superior hydrophilicity/hydrophobicity.40-42 The water contact angle measurements, as shown in the inset of Figure 2a, reveal that the mSiO2 hollow sphere was full hydrophilic (23.5o) and the NxC hollow sphere was hydrophobic (83o), suggesting that the NxC@mSiO2 hollow sphere had hydrophilic external surface yet hydrophobic interior. Moreover, in spite of the hydrophilic surface (with a water contact angle of 28.8o), the sample NxC@mSiO2 could serve as a solid surfactant for emulsifying toluene-water biphasic system whereas the sample mSiO2 and NxC respectively dispersed in water and toluene (Figure 2a, c, d). The optical micrograph in Figure 2b exhibits well-defined oil-in-water droplets with an average diameter of 150 µm. Even after one month, the morphologies and diameters of these droplets were well retained, indicating the NxC@mSiO2 hollow sphere could stably assembly at the oil-water interface. These results clearly testify that the sample NxC@mSiO2 possesses unique amphiphilicity, just like molecular surfactants. It is worth noting that this complex hydrophilicity/hydrophobicity is extremely stable because the frameworks of carbon and silica are stable enough and the sample NxC@mSiO2 was obtained at high temperature. Combined all above results, we have successfully prepared a novel amphiphilic hollow nanostructure based-on silica/carbon composites via a simple confined pyrolysis strategy.


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Figure 3. TEM image (a), HAADF-STEM image (b), particle size distribution of Pt nanoparticles (c) and Pt XPS spectrum (d) of multifunctional amphiphilic nanoreactor Pt/NxC@mSiO2.

Catalytically active Pt nanoparticles were subsequently introduced into the sample NxC@mSiO2 via a simple impregnation-reduction route for constructing multifunctional amphiphilic nanoreactor Pt/NxC@mSiO2. The TEM and HAADF-STEM images clearly exhibit that the ultrasmall Pt nanoparticles uniformly disperse throughout the hollow sphere and most of Pt nanoparticles are loaded inside the mSiO2 shell (Figure 3a-b). Distinct contrast can be also observed in the NxC@mSiO2 shell (Figure 3b), indicating that the process of loading metal nanopartilces did not have any influence on the unique Janus hollow nanostructure at all. The particle size distribution shows that the diameter of Pt nanoparticle is in the range of 1.4-3.0 nm with an average size of 2.1 nm (Figure 3c). Such an ultrasmall but uniform particle size should be attributed to the strong anchoring effect of the NxC framework. The PXRD pattern also proves the presence of crystalline Pt nanoparticles in the sample (Figure S7). The diffraction peaks at 2θ value of 39.9o, 46.4o and 67.7o could be ascribed to the (111), (200) and (220) plane of face-centered-cube (fcc) Pt, respectively. The Pt 4f XPS spectrum further illustrates that the surface Pt is mainly in the metallic state and the Pt 4f7/2 binding energy is located at 71.5 eV (Figure 3d). Compared to the Pt 4f7/2 peak position of commercial Pt/C (71.3 eV), the Pt 4f7/2 binding energy of the Pt/NxC@mSiO2 is obviously positiveshifted, which might be resulted from the electron transfer from Pt to NxC matrix. Such positive charges on the surface of Pt nanoparticles would be very favorable for activating alcohol molecules.55 For comparison, we also immobilized Pt nanoparticles in the hydrophobic NxC hollow spheres and the hydrophilic mSiO2 hollow spheres via the same method (Figure S8). Catalytic Activity of Multifunctional Amphiphilic Nanoreactor in Base-Free Aerobic Oxidation of Alcohols in Water. Selective oxidation of alcohols is widely recognized as one of the most important and fundamental transformations in both laboratory and industrial synthetic chemistry. However, sustainable aerobic alcohol oxidation under green and mild conditions that is quitely close to the future industrial applications still suffered from the limited activity and selectivity.56-58 Here, in view of the fascinating amphiphilicity and the untraf-

ine metal nanoparticle, we examined the catalytic property of the designed nanoreactor Pt/NxC@mSiO2 in selective oxidation of 4-methoxybenzyl alcohol under extremely green conditions (water as solvent, free-base, air as an oxidizing agent, atmospheric pressure). According to the reaction kinetics as shown in the Figure 4a, our catalyst Pt/NxC@mSiO2 converted 97.7% of alcohol in 8 hours whereas the hydrophobic and hydrophilic counterpart (Pt/NxC and Pt/mSiO2) only presented a conversion of 74.4% and 31.1% in 10 hours, respectively, suggesting that the unique hydrophilicity/hydrophobicity played a significant role. Owing to the mild reaction conditions, excellent selectivity (≥99.9%) could be achieved in all catalysts. When the reaction time was prolonged to 10 hours, a yield of 100% for aldehyde was obtained. Under such green conditions, the conventional catalyst Pt/C and Pt/MCM-41 also gave a very low yield of 68.5% and 57.2%, respectively. Additionally, in comparison to the catalyst Pt/C, Pt/NxC showed a higher activity, revealing that the doped nitrogen in carbon matrix was beneficial to enhancing the oxidation rate. Furthermore, our catalyst Pt/NxC@mSiO2 afforded a turnover frequence (TOF) of 26.7 h-1, which was 1.4-3.9 times higher than that for other hydrophobic or hydrophilic catalysts (Figure 4b). To examine the origin of the significantly enhanced activity, we carefully analyzed both the reactant adsorption and the byproduct desorption on different supports. As shown in the Figure S9, our designed amphiphilic NxC@mSiO2 displays a high sorption capacity of 714.3 mg/g at an equilibration concentration under ambient conditions, which is much higher than that of hydrophilic mSiO2 hollow sphere (434.8 mg/g). In spite of the lower BET surface area, NxC@mSiO2 still shows much higher adsorption than mSiO2 hollow sphere at other temperatures (40 oC and 50 oC). Their adsorption enthalpies (∆rHΘm) were further evaluated by fitting the equilibration data according to the Clausius-Clapeyron equation:59-60 InCe ∆H T RT

It can be clearly found in the Figure 4c, d that the ∆rHΘm of NxC@mSiO2 could be predicted to be -9.33 kJ/mol, which is 1.36 times lower than that of mSiO2 hollow sphere (-6.87 kJ/mol), strongly proving that the hydrophobic alcohols are captured from water easier by amphiphilic materials. Furthermore, we examined the adsorption-desorption kinetics of NxC@mSiO2 towards water molecules that are the sole byproduct in the aerobic oxidation of alcohols (Figure 4e, f). As expected, the amphiphilic NxC@mSiO2 showed a much lower adsorption capacity than hydrophilic mSiO2 hollow sphere (110 mg/g Vs 350 mg/g). Importantly, the adsorbed water molecules can fast desorb from the NxC@mSiO2 while the sample mSiO2 displays a distinct hysteresis resulted from the strong hydrogen bond interaction between the water molecules and the polar surface. These sorption experiments clearly demonstrate that the unique amphiphilicity strengthens both adsorption of reactant from water and desorption of byproduct water molucules from the active sites, thus leading to the significantly enhanced catalytic activity.



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Figure 4 (a) Kinetic plots of aerobic oxidation of 4-methoxybenzyl alcohol over different catalysts. (b) Turnover frequencies (TOF) for different catalysts identified from the conversions at 2 hours. (c) Adsorption enthalpy analysis of the NxC@mSiO2 hollow sphere towards 4-methoxybenzyl alcohol in water. (d) Adsorption enthalpy analysis of the mSiO2 hollow spheres towards 4-methoxybenzyl alcohol in water. (e) Water vapor sorption of the NxC@mSiO2 hollow spheres. (f) Water vapor sorption of the mSiO2 hollow sphere. Reaction conditions: 4-methoxybenzyl alcohol (0.5 mmol), H2O (4 mL), catalyst (Pt 1.0 mol%), 80 oC, open air.

Figure 5 Proposed mechanism for the oxidation of alcohols in water over the catalyst Pt/NxC@mSiO2. (a) Diffusion of substrate and product over the catalyst Pt/NxC@mSiO2. (b) Simplified mechanism model for oxidation of alcohols at the surface of Pt/NxC.

According to the catalysis result and the sorption analysis as well as literature reports,57 we proposed a possible mechanism for catalytic oxidation of alcohols in water over the nanoreactor Pt/NxC@mSiO2 (Figure 5). Since the complex wettability throughout particles endowed the material extreme affinity towards organics in water, the alcohol molecules fast diffused into and were enriched in the hydrophobic interior (Figure 5a), leading to a high reactant concentration around the active sites which could promote reaction rate. Subsequently, the alcohol molecules were converted to the corresponding aldehyde at the surface of Pt/NxC via the following steps (Figure 5b): 1) adsorption; 2) forming metal-alcoholate; 3) β-hydride elimination; 4) oxidation of metal-hydride and regeneration of metal surface for next cycle. On the one hand, the NxC matrix could

disperse the charge density of Pt nanoparticles via electron transfer, resulting in partly positive charges on the surface of Pt nanoparticles. This is very benifical to enhancing the adsorption of alcohol molecules and then activating the O-H bond. On the other hand, although any additives (inorganic base) were not involved in current catalysis system, the doped N atoms in carbon matrix (especially pyridinic-N) could be served as a Lewis base to accelerate the deprotonation process for forming metal-alcoholate.55 CO2-TPD analysis clearly verified the existence of a basic site (Figure S10). Finally, when the product aldehyde molecules maybe stay in the hydrophobic interior, the sole byproduct water molecules fast desorbed from the active sites and diffused out of the hydrophobic interior (Figure 5a), which was favorable for the positive reaction


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and then further enhanced the reaction rate. It was worth noting that over-oxidation did not occur to produce corresponding acids due to the extremely mild reaction conditions (base-free) although the aldehyde molecules did not immediately desorbed from the active sites. Encouraged by the superior activity of the multifunctional amphiphilic nanoreactor Pt/NxC@mSiO2, we further explored the aerobic oxidation of other alcohols under this very green condition. The results summarized in Table 1 show that the catalyst was highly active and extremely selective for the oxidation of all substrates. Primary aromatic alcohols with various substituent groups (Entry 1-5), such as benzyl alcohol, 4methylbenzyl alcohol, 4-methoxybenzyl alcohol, 4chlorobenzyl alcohol and 4-fluorobenzyl alcohol were easily oxidized to the corresponding aldehydes with conversions of >95% and selectivities of >99%. Considerable conversions could be also obtained for less active 1-naphthylmethanol and cinnamyl alcohol (Entry 6-7). In addition, secondary aromatic alcohols, viz. (+/-)-1-phenylethanol and 1-(4-methoxyphenyl) ethanol were effectively converted to their corresponding ketones in a conversion of >90% with a selectivity of >99% (Entry 8-9). All these catalytic results verify a high versatility of our Pt/NxC@mSiO2.

Pt/NxC@mSiO2 presented a considerable yield of 72.1% under the similar conditions for alcohols oxidation (Table S2), much higher that that of commercial Pt/C (55.6%) and conventional Pt/MCM-41 (27.9%). A little of aldehyde intermediates did not convert to the product imine, which might be arisen from the low nitrogen content of Pt/NxC@mSiO2. Notably, using Pt/C as the catalyst and K2CO3 as the additive also only gave a very low yield towards imine (48.2%), revealing that the synergistic effect between the doped nitrogen atoms and Pt nanoparticles was very important for this tandem transformation. Furthermore, we explored the substrate scope of this oxidation-condensation cascade reaction. As shown in the Table 2, various aromatic primary alcohols with different substituent groups such as Me-, MeO-, Cl- and F- can couple with aniline to yield the corresponding imines derivatives in a yields of 6080% (3a-3e). Different aromatic amines including 4aminophenol, 4-methoxyaniline and 4-chloroaniline can be also efficiently converted into the corresponding imines with 4-methoxybenzyl alcohol (3f-3h). These results clearly demonstrate that green yet efficient synthesis of imines from various alcohols and amines can be realized by our nanoreactor Pt/NxC@mSiO2, which is arisen from the combined amphiphilicity with multiple active sites.

Table 1. Aerobic Oxidation of Various Alcohols over the Catalyst Pt/NxC@mSiO2a

Table 2. One-Pot Synthesis of Imines from Different Alcohols and Amines via a Tandem Transformation over the Catalyst Pt/NxC@mSiO2a

Entry

Substrate

Product

Time (h)

Conv. (%)

Sel. (%)

OH R1

1

6

>99

>99

2

6

>99

>99

3

8

98

>99

4

12

98

>99

5

12

96

>99

R2

[Pt] Air Pt/NxC@mSiO2

R1

24

90

>99

7

24

88

>99

8

30

90

>99

9

30

92

>99

a

Reaction conditions: alcohol (0.5 mmol), H2O (4 mL), catalyst (Pt 1.0 mol%), 80 oC, open air.

One-Pot Oxidation-Condensation Cascade Reaction for Synthesis of Imines from Alcohols and Amines. To further demonstrate the advantages of multiple active sites of our nanoreactor, we examined its power for one-pot synthesis of imines from alcohols and amines via an oxidationcondensation tandem catalysis.61-62 Using 4-methoxybenzyl alcohol and aniline as the model substrate, our catalyst

R1 Pt/NxC@mSiO2

N

R2

3

1

3a, 72.1% (94.1%)b

3b, 68.3% (100%)b

3d, 77% (87%)b

3e, 73% (88.5%)b

3g, 78.5% (80%)b 6

NH2

O

3c, 73% (100%)b

3f, 67% (82.5%)b

3h, 62% (88%)b

a Reaction conditions: alcohol (0.5 mmol), amine (0.6 mmol), H2O (4 mL), catalyst (Pt 5.0 mol%), 120 oC, open air, 48 h. b Conversions of alcohols.

Catalytic and Structural Stability of Multifunctional Amphiphilic Nanoreactor. An important advantage of heterogenerous catalysts is the superior reusability. Here the catalytic stability of the nanoreactor Pt/NxC@mSiO2 was tested by using 4-methoxybenzyl alcohol as a model substrate. As shown in Figure 6a, our catalyst Pt/NxC@mSiO2 still gives a conversion of >90% with a selectivity of 99% under the identical conditions after being reused for seven times. The liquid phase of the reaction mixture was collected after the seventh run and analyzed by inductively coupled plasma mass spectrometry (ICP). Only ppb Pt species could be detected, indicating that the Pt nanoparticles were scarcely leached from the sample. The hot

filtration test further reveals that Pt leaching is negligible (Figure S11). The reused catalyst was also recovered and analyzed. The TEM image in Figure 6b reveals that the ultrasmall Pt nanoparticles without distinct change still uniformly disperse

throughout the particle. Any nanoparticles outside the hollow



sphere and nanoparticles aggregation can not be observed in the image. The particle size distribution further confirms that the average diameter of Pt nanoparticle is about 2.7 nm (the inset in Figure 6b), just slight larger than that of fresh Pt/NxC@mSiO2 (2.1 nm). The elements analysis and ICP analysis also indicate that the silica/carbon ratio and the Pt loading retain similar to that of fresh catalyst (Table S1), respectively. These results clearly prove that our nanoreactor Pt/NxC@mSiO2 possesses excellent catalytic stability.

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embedded into the mSiO2 layer, indicating that the unique Janus hollow nanostructure is stable enough. Moreover, the superior amphiphilicity is confirmed by the well-defined Pickering emulsions (Figure S13). Therefore, based-on all above experiment and catalysis data, the excellent catalytic stability of Pt/NxC@mSiO2 can be understood from two points: 1) stability of ultrasmall Pt nanoparticles; 2) stability of hydrophilicity/hydrophobicity. Because of the strong anchoring effect of nitrogen atoms and the encapsulating effect of mSiO2 hollow shell, the ultrasmall Pt nanoparticles could be stably immobilized in the NxC matrix. Zhang and co-workers63 also achieved such a remarkable stability in their nanocatalyst PtrGO@mSiO2 by the anchoring effect and the encapsulating effect. On the other hand, since the complex hydrophilicity/hydrophobicity throughout particle was originated from the unique hybrid hollow nanostructure formed at high temperature (900 oC), the superior amphiphilicity would be extremely stable. This stability of complex wettability is very important for the catalytic stability and is an advantage of our nanoreactor Pt/NxC@mSiO2, which can not be realized in the polymerbased and hybrid silica-based catalysts with tunable wettability (high temperature would destroy their hydrophobic groups).

CONCLUSION

Figure 6 (a) Recyclability of the catalyst Pt/NxC@mSiO2 in the aerobic oxidation of 4-methoxybenzyl alcohol. (b) TEM image and particle size distribution of Pt nanoparticles (the inset) of the recovered catalyst Pt/NxC@mSiO2 after reused for seven times. (c) Kinetic plots of aerobic oxidation of 4-methoxybenzyl alcohol over different catalysts treated in inert atmosphere at 300 oC for 3 h. (d) TEM image and particle size distribution of Pt nanoparticles (the inset) of the treated catalyst Pt/NxC@mSiO2.

To visualize the catalytic stability better, we treated the multifunctional amphiphilic nanoreactor Pt/NxC@mSiO2 and other relevant catalysts in inert atmosphere at 300 oC for 3 h and then tested their catalytic activities in the aerobic oxidation of 4-methoxybenzyl alcohol. Surprised to us, the treated catalyst Pt/NxC@mSiO2 still obtained a conversion of 92.4% with a selectivity of 99% under identical conditions, which is comparative to that of fresh catalyst. Whereas the treated Pt/C catalyst only gave a conversion of 43.2% and the treated Pt/mSiO2 catalyst has been seriously deactivated, only presenting a conversion of 2.9% in 8 h (Figure 6c). According to the reaction kinetics, in spite of slight lower than that of fresh counterpart, the oxidation rate of the treated Pt/NxC@mSiO2 was still much higher than other hydrophobic and hydrophilic catalysts including Pt/NxC, Pt/C, Pt/mSiO2 and Pt/MCM-41. To explore the origin of this excellent catalytic stability, we characterized the treated catalyst Pt/NxC@mSiO2. From the TEM image in Figure 6d, any distinct change is not observed for the ultrasmall Pt nanoparticles. The particle size distribution further confirms that the average diameter of Pt nanoparticle is about 2.4 nm (the inset in Figure 6d), similar to that of fresh Pt/NxC@mSiO2 (2.1 nm). However, severe aggregation and even migration of Pt nanoparticles are distinctly appeared in the Pt/C catalyst and the Pt/mSiO2 hollow spheres (Figure S12). Additionally, the TEM image also shows that the NxC is

In summary, we have reported a new multifunctional amphiphilic nanoreactor composed of ultrasmall Pt nanoparticles and Janus NxC@mSiO2 hollow nanostructure with hydrophilic external surface and hydrophobic internal surface as well as buffered interface. Such a complex wettability throughout particle enhanced both adsorption of hydrophobic alcohols from water and desorption of water molecules from the active sites. Moreover, the doped nitrogen atoms not only severed as anchoring sites for stabilizing ultrasmall Pt nanoparticles but also as basic active sites for accelerating deprotonation process. On the basis of such features, the designed catalyst exhibited excellent activity and selectivity for base-free aerobic oxidation of alcohols in water using air as the oxidant, significantly outperforming other hydrophilic and hydrophobic catalysts. Green yet efficient one-pot synthesis of imines from various alcohols and amines was also realized via tandem catalysis. Furthermore, owing to the extreme stability of such an amphiphilicity, excellent catalytic stability was presented. This work not only provides a new strategy for designing advanced nanocatalysts with tunable hydrophilicity/hydrophobicity but also brings more opportunities for sustainable aerobic oxidation process.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Additional experimental information including the adsorption enthalpy analysis; additional figures including SEM images, TEM images, nitrogen sorption analyses, TG curves, PXRD patterns, adsorption isotherms of 4-methoxybenzyl alcohol and CO2-TPD.

AUTHOR INFORMATION Corresponding Author E-Mail: [email protected]; [email protected].

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the paper.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21390394, 21771082, 21771081 and 21703128), the National Basic Research Program of China (2012CB821700 and 2011CB808703), NSFC (21261130584 and 91022030), the “111” project (B07016), an Award from the KAUST Project (CRG-1-2012-LAI-009) and the Ministry of Education, Science and Technology Development Center Project (20120061130012).

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Janus N-Doped Carbon@Silica Hollow Spheres as Multifunctional

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