New Polymer Colloidal and Carbon Nanospheres: Stabilizing

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New Polymer Colloidal and Carbon Nanospheres: Stabilizing Ultrasmall Metal Nanoparticles for Solvent-Free Catalysis Tao Wang,† Pengfei Zhang,‡ Yan Sun,† Bing Liu,† Yunling Liu,† Zhen-An Qiao,*,† Qisheng Huo,† and Sheng Dai‡ †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, Jilin 130012, China Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

‡

S Supporting Information *

ABSTRACT: Herein, we report the synthesis of new colloidal polydiaminopyridine (PDAP) nanospheres with uniform particle size tuned from 76 to 331 nm. The polymer colloidal nanospheres and thin films assembled by the nanospheres exhibit amphiphilic or superhydrophilic properties, originated from the different polymer surfactants. In addition, the polymer nanospheres can be easily carbonized into microporous carbon nanospheres with high N content up to 24 wt %, thus enabling a preferred basicity for CO2 adsorption. Assynthesized PDAP show good compatibility that can not only encapsulate typical nanoparticles to form core−shell composite nanospheres but also stabilize noble metal ions to obtain ultrasmall metal nanoparticles (e.g., Pd and Au) during the thermal reduction process by the nitrogen sites from sphere frameworks. The nanocomplex Pd/PDAP-500 shows exceptional activity and high selectivity in the solvent-free oxidation of alcohols with O2.

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INTRODUCTION The development of novel methodologies and the design of highly functional systems for the production of polymer and carbon colloidal nanospheres stand out as appealing goals enthusiastically pursued by the chemical community, because polymer and carbon nanospheres with uniform size and tunable architecture are vibrant materials for catalysis, sustainable energy, environment protection, and biomedical applications.1,2 In the past several years, scientists have made a lot of efforts in the synthesis, characterization, and application of polymer nanospheres and a variety of polymers have been successfully used to synthesize uniform nanospheres, such as polystyrene, poly(methyl methacrylate), poly(hydroxyethyl methacrylate), polyaniline-co-polypyrrole, polydopamine, and phenol-formaldehyde resin.3−9 The conversion of polymer nanospheres into carbon nanospheres under inert gas by the pyrolysis process is an important way to achieve higher stability, larger surface area, higher catalytic activity, and so on.10,11 In particular, N-doped carbon materials as emerging carbon materials have gained much attention because of their enhanced electrical conductivity, chemical reactivity, specific capacitance, and adsorption affinity toward CO2.12−16 Toward the design of high quality polymer/carbon nanospheres, a few issues still remain such as poor thermal stability, long synthesis time, extreme conditions, and high cost of raw material, which limit their large scale synthesis and industrial applications.17 Because of the lack of cross-linkable organic groups for a stable network, traditional polymer colloidal nanospheres like polystyrene, poly(methyl methacrylate), and poly(hydroxyethyl methacrylate) were typically used as high monodispersed © 2017 American Chemical Society

templates to synthesize porous or hollow materials, but the nanospheres themselves failed to convert to carbon nanospheres.2,18 Phenol-formaldehyde resin has a highly cross-linked structure with a good thermal stability and could be carbonized to carbon nanospheres without deformation of spherical shape;8 thus, several methods have been developed to synthesize uniform microporous or mesoporous carbon nanospheres based on phenol-formaldehyde resin.2,8,19−21 Recently, our group developed a facile “silica-assisted” synthesis for diverse carbon spheres, a category that covers mesoporous carbon nanospheres, hollow mesoporous carbon nanospheres, and yolk−shell mesoporous carbon nanospheres.19 Compared to phenol-formaldehyde resin, carbon nanospheres based on polydopamine22 and polyaniline-co-polypyrrole6 are nitrogenous carbon materials and further developed to achieve highly efficient oxygen-reduction catalysts. However, the synthesis of polydopamine and polyaniline-co-polypyrrole nanomaterials is relatively expensive, time-consuming, or temperature dependent. Hence, the synthesis of new uniform polymer colloidal and porous N-doped carbon nanospheres is indeed difficult and remains a grand challenge.11,23 Herein, we report a facile strategy to rapidly synthesize a new type of polymer colloidal nanospheres with 2,6-diaminopyridine (DAP) as precursor, polymer surfactants (poly vinylpyrrolidone (PVP K-30), Brij-58, Pluronic P123, or Pluronic F127) as the protective agent, and ammonium peroxydisulfate Received: February 21, 2017 Revised: April 17, 2017 Published: April 18, 2017 4044

DOI: 10.1021/acs.chemmater.7b00710 Chem. Mater. 2017, 29, 4044−4051

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Chemistry of Materials

2400CHN analyzer. Raman spectra of PMO was performed on a Renishaw inVia Raman microscope. The catalytic products were analyzed with a gas chromatograph system (Shimadzu GCMS-QP 2010 Plus) with a flame ionization detector (FID) using tetralin as the internal standard. Preparation of PDAP Nanospheres. Typically, PDAP-V nanospheres with particle sizes of around 200 nm were synthesized as below. DAP (0.15 g), PVP K30 (0.075 g, Mw ca. 40 000), 0.045 g of NaOH, and 9.2 mL of deionized water were combined and stirred to form a clear solution at ambient temperature. Then, 0.8 mL of APS solution (1.46 M) was added under stirring. The color of solution changed immediately, and the reaction finished in 5 min. PDAP-V nanospheres with different average sizes were obtained by modulating the addition of APS solution or ethanol. PDAP-B, PDAP-P, or PDAPF nanospheres were obtained by replacing PVP with the same weight of Brij-58 (Mw ca. 1123), P123 (Mw ca. 5800), or F127 (Mw ca. 12 600), respectively. The product was collected by centrifugation and washed with water and ethanol several times. Fabrication of PDAP Films. Five mg of PDAP-V nanospheres was homogeneously dispersed in 20 mL of ethanol by ultrasonication. Then, a clean glass slide was immersed into the solution, pulled up, and dried in air several times until a thin film had been coated on the slide. The fabrication of PDAP-B, PDAP-P, or PDAP-F film was the same as the fabrication of PDAP-V film. Preparation of CeO2 Nanospheres. CeO2 nanospheres were synthesized according to the literature.24 Briefly, 3.0 g of Ce(NO3)3· 9H2O, 3 mL of glacial acetic acid, and 2.5 mL of water were combined and stirred to form a clear solution at ambient temperature. Then, the solution was transferred to an autoclave and heated at 140 °C for 12 h. The product was collected by centrifugation and washed with water and ethanol several times. Then, the product was dried at room temperature and calcined at 600 °C for 3 h under air. Preparation of CeO2@PDAP Core−Shell Nanospheres. The as-synthesized CeO2 nanospheres (0.06 g) and PVP (0.1 g) were homogeneously dispersed in 9.7 mL of water by ultrasonication. Then, 0.15 g of DAP and 0.045 g of NaOH were added under stirring. 0.8 mL of APS solution (1.46 M) was added under stirring after the dissolution of DAP. The product was collected by centrifugation and washed with water and ethanol several times. Carbonization of PDAP Nanospheres. The obtained PDAP-V nanospheres were dried in air under room temperature and heated under a nitrogen atmosphere at 2 °C/min from room temperature to 350 °C and kept at 350 °C for 2 h. The temperature was then raised at 5 °C/min to 800 °C and kept at 800 °C for 1 h. The product was denoted as PDAP-800. PDAP-700 and PDAP-600 were obtained by changing the final temperature to 700 or 600 °C while PDAP-350 was obtained without the second heating step. The carbonization of CeO2@PDAP core−shell nanospheres had the same heating steps as PDAP-800. Preparation of Pd/PDAP-500. 1.2 g of PDAP-V was dispersed in 50 mL of water. Then, 1.7 mL of K2PdCl4 solution (10 g/L) was added and then stirred at room temperature for 12 h. The powdery solid was collected by centrifugation, washed with water, and dried at room temperature. Subsequently, the Pd-loaded PDAP-V was thermally treated at 500 °C for 2 h in a reducing atmosphere (H2/ N2 = 1/99, volume ratio). The palladium loading in terms of total weight of the obtained catalyst (Pd/PDAP-500) is ∼0.76 wt %. Catalytic Oxidation of Hydrocarbons and Alcohols. In a typical oxidation, 10 g of benzyl alcohol and 20 mg of Pd/PDAP-500 catalysts were added into a 25 mL three-neck glass reactor. The reaction was performed at 120 °C in an oil bath with magnetic stirring. A stream of O2 was introduced into the reaction mixture at a constant flow rate (20 mL/min). The liquid phase of the reaction mixture was collected by filtration after completion of the reaction. Then, 50 μL of the liquid mixture and 50 μL of internal standard were added to 5 mL of ethanol for GC-MS analysis. Catalyst Recycling Test. After the first oxidation, the catalyst was collected by centrifugation, washed twice with ethanol, and dried in a vacuum oven at 65 °C for 12 h. Finally, the recovered Pd/PDAP-500 was used in the subsequent reaction.

(APS) as the oxidizer. The obtained polydiaminopyridine (PDAP) nanospheres have a uniform particle size distribution (76−331 nm), and the size of colloidal PDAP can be precisely adjusted by two tunable factors, the concentration of APS and the amount of ethanol. The surfactant around the PDAP surface influences the hydrophilicity of PDAP nanospheres (amphiphilic or superhydrophilic properties), which can be used to paint PDAP thin films with different wettability. PDAP nanospheres were carbonized under protection of an inert gas to produce microporous N-doped carbon nanospheres, which showed very high CO2 areal capacities (∼9.8 μmol m−2). The as-synthesized PDAP provides a new choice for coating oxides or metal particles to form core−shell nanostructures and stabilizing ultrasmall noble metal nanoparticles by nitrogen sites to fabricate nanocomposites (Scheme 1). Pd nanoparticles Scheme 1. Synthesis and Applications of PDAP Colloidal Nanospheres

stabilized by PDAP show excellent catalysis activity for solventfree aerobic oxidation of diverse alcohols with high turnover frequencies (TOFs), high selectivity, and good recyclability, which show great potential for prospective applications in industrial heterogeneous catalysis for petroleum refining and fine chemical production.

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EXPERIMENTAL SECTION

Materials and Methods. DAP, Brij-58, Pluronic P123, and Pluronic F127 were purchased from Sigma-Aldrich. PVP K-30, NaOH, glacial acetic acid, ethanol, and K2PdCl4 were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Ce(NO3)3· 9H2O, APS, benzyl alcohol, 1-phenylethanol, cinnamic alcohol, 1octanol, 2-octanol, and 3-octanol were purchased from Aladdin. All of the chemicals were used without further purification. The structure and morphology of PDAP nanospheres were analyzed with a FEI Tecnai G2 F20 s-twin D573 field emission transmission electron microscope operated at 200 kV and JEOL JSM-6700F fieldemission scanning electron microscope operated at 5 kV. The sizes of PDAP nanospheres were measured by photon correlation spectroscopy employing a Nano ZS90 laser particle analyzer (Malvern Instruments, UK) at 25 °C. Water contact angles (CAs) on PDAP films were measured using a DSA 100 drop shape analyzer at ambient temperature, and each was obtained by measuring more than five different positions on the same sample. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a ESCALAB250 system with Al Kα radiation (1486.6 eV). Powder X-ray diffraction (XRD) patterns were collected by using a Rigaku 2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å). N2 adsorption− desorption isotherms were obtained at −196 °C on a Micromeritics 2420 instrument. CO2 gas adsorption measurements were performed on a Micromeritics ASAP 2020 instrument. Samples were degassed at 150 °C for a minimum of 12 h prior to analysis. The thermal gravimetric analyses (TG) were performed on a TGA Q500 thermogravimetric analyzer used in N2 with a heating rate of 10 °C min−1. The CHN element analyses were done on a PerkinElmer 4045

DOI: 10.1021/acs.chemmater.7b00710 Chem. Mater. 2017, 29, 4044−4051

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Figure 1. (a) SEM images of PDAP-V nanospheres with different sizes. (b) Photographs of PDAP-V and PDAP solutions. (c) PDAP-V nanospheres with different sizes adjusted by the concentration of APS and the amount of ethanol. (d) Relationship between the concentration of APS and nanosphere sizes of PDAP-V. (e) Relationship between the amount of ethanol and nanosphere sizes of PDAP-V. Scale bar: 200 nm.

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RESULTS AND DISCUSSION Synthesis of PDAP Nanospheres. Under the protection of PVP, PDAP nanospheres with different average sizes (76− 331 nm) can be obtained by modulating the concentration of APS and the amount of ethanol and characterized by the dynamic light scattering method (DLS, Figure 1c) and scanning electron microscopy (SEM, Figures 1a and S1). The size of PDAP-V nanospheres increased from 76 to 255 nm with the increasing concentration of APS (Figure 1d) in the absence of ethanol. Varying the ethanol/water volume ratio from 0 to 1.5 still has a dramatic effect upon the size of PDAP nanospheres, and the average diameter of PDAP nanospheres can be tailored from 132 to 331 nm at the constant concentration of APS (0.07 M, Figure 1e). On the basis of these observations, a good linear function of size vs the concentration of APS (Figure 1d) and a good quadratic function of size vs the amount of ethanol (Figure 1e) were obtained through calculation. Compared to a traditionally single-variable control system, the method we reported has two tunable variables that provide more composite modes and predictable nanosphere sizes. After centrifugation of the reaction mixture, the obtained PDAP nanospheres attaching centrifuge tubes show green luster (Figure 2a), which indicates the existence of locally ordered packing structure (Figure 2b), owing to the uniform particle size of PDAP nanospheres. It is worth noting that PDAP-V nanospheres can be rapidly formed within 5 min under a wide range of other experimental conditions such as pH (3 to 10) and temperature (0 to 50 °C, Figure S1). About 11 g of PDAP powder can be obtained from 750 mL of the reaction solution, still with high yield of ∼95% (Figure 2e). Thus, the synthesis approach is considered to be robust under complex conditions and very suitable for industrial production. Various polymer surfactants can be used as the protective agent in the synthesis of PDAP nanospheres, such as PVP, Brij58, Pluronic P123, and Pluronic F127. These nanospheres stabilized by different surfactants are correspondingly denoted as PDAP-V, PDAP-B, PDAP-P, and PDAP-F (Figure S3a) and can be used to paint films on the glass surface by an easy

Figure 2. Photograph (a) and SEM image (b) of PDAP-V nanospheres after centrifugation. (c) Photographs of PDAP-F dispersed in various solvents. (d) Photograph of PDAP-V film. (e) Photograph of 11 g of PDAP-V powder. (f) Water CA images of PDAP films.

evaporation of the suspension (Figures 2c and S3b). As shown in Figure 2f, all the PDAP films are hydrophilic and the water apparent contact angles (CAs) range from 3° to 30°. The PDAP-V film shows superhydrophilicity, on which the water apparent CA is less than 5°. PDAP-F nanospheres show their 4046

DOI: 10.1021/acs.chemmater.7b00710 Chem. Mater. 2017, 29, 4044−4051

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according to the summary of the reported synthesis methods of doped carbons.26 X-ray photoelectron spectroscopy (XPS) was used to confirm the presence of nitrogen and the binding between carbon and nitrogen in PDAP-800. Typically, the C 1s peak is observed at ca. 284.8 eV (consistent with sp2 graphitic carbon) and displays a slightly asymmetric nature (Figure S4b). The N 1s spectra (Figure 3e) demonstrates four types of nitrogen atoms and the peaks located at 398.3, 400.5, 401.2, and 402.9 eV in a percentage of 48.7%, 30.5%, 14.9%, and 5.8%, related to the pyridinic nitrogen, pyrrolic nitrogen, graphitic nitrogen, and oxidized nitrogen, respectively.27 Powder X-ray diffraction (PXRD) pattern exhibits two broad low intensity peaks at 2θ = 24.3° and 43.8° that correspond to the (002) and (100) planes of graphite, indicating the amorphous carbon structures in PDAP-800 (Figure S4c). The Raman spectra of PDAP-800 (Figure 3f) shows two bands at ca. 1345 cm−1 (D band) and ca. 1575 cm−1 (G band), which further verified the amorphous carbon nature of the pyrolysis product.28 CO2 Adsorption Capacities of N-Doped Carbons. Extensive research efforts have been undertaken worldwide to develop feasible materials for CO2 capture, because the CO2 emission associated with industrial production has a serious impact on the global warming and climate change.29 We have investigated CO2 adsorption capacity of the obtained N-doped carbon nanospheres, because nitrogen-containing porous carbons with extensive basicity are potential materials for CO2 capture.13,30,31 First, the CO2 adsorption capacities of PDAP-800, PDAP-700, and PDAP-600 were investigated at 273 and 298 K (Figures 4c and S4d). Though SBET of PDAP-

unique ability to dissolve in most of the common solvents, including such extremes as hexane and pure water (Figure 2c), and can be easily moved to the water/hexane interface to form a black thin film at the interface by shaking or sonication (Figure S 3d), which demonstrates the amphiphilic nature of nanospheres. PDAP-P nanospheres show the similar amphiphilic behavior as PDAP-F nanospheres, mainly because of the highly similar chemical composition of the protective agents of F127 and P123. PDAP-V and PDAP-B nanospheres are not amphiphilic, which can be dissolved in most of the solvents except hexane (Figure S3c). Fourier transform infrared (FTIR) spectroscopy was used to identify the polymer surfactants coated on PDAP nanospheres. The principal absorption peaks of PDAP-V, PDAP-B, PDAP-P, and PDAP-F (Figure S3e) between 2850 and 3000 cm−1 are assigned to C−H stretching vibrations of PVP, Brij-58, P123, and F127, respectively,25 which indicates the amphiphilic or hydrophilic behavior of the PDAP nanospheres originated from the different polymer surfactants on the nanospheres. Carbonization of PDAP Nanospheres. PDAP nanospheres show good thermal stability that can be directly converted to N-doped carbon nanospheres with a yield of 37 wt % at 800 °C by a carbonization process, according to the thermogravimetry (TG) curve of PDAP-V in the atmosphere of N2 (Figure S4a). The pyrolysis product of PDAP-V is denoted as PDAP-x, where x is the calcination temperature (in °C). As an example, PDAP-800 nanoparticles performed a linear shrinkage of 25% on SEM images (Figure 3a,b) and disordered

Figure 3. SEM images of PDAP-V before (a) and after (b) carbonization at 800 °C. (c) N2 adsorption−desorption isotherms of PDAP-x at 77 K. TEM images (d), XPS spectra (e), and (f) Raman spectra of PDAP-800.

micropores on TEM images (Figure 3d) after thermal treatment. The pore characteristics of PDAP-x were examined by a nitrogen adsorption experiment (Figure 3c). The Brunauer−Emmett−Teller surface areas (SBET) of PDAP-600, PDAP-700, and PDAP-800 are 311, 360, and 412 m2 g−1 based on the N2 adsorption isotherm, respectively. The nitrogen adsorption−desorption isotherms of PDAP-600, PDAP-700, and PDAP-800 exhibit characteristics of type I according to the classification of the International Union of Pure and Applied Chemistry (IUPAC), indicating the microporous frameworks of the carbon nanospheres. The N content of the pyrolysis product decreased with the increase of pyrolysis temperature based on the results of CHN elemental analysis, 23.72 wt % for PDAP-600, 18.50 wt % for PDAP-700, and 13.08 wt % for PDAP-800, which are very high values among N-doped carbons

Figure 4. (a) N2 adsorption−desorption isotherms of PDAP-KOH at 77 K. (b) Pore size distribution analysis for PDAP-KOH according to the DFT model. (c) CO2 adsorption isotherms at 273 K of PDAP-x and PDAP-KOH. (d) The isosteric heat of CO2 adsorption on PDAPx and PDAP-KOH calculated from the experimental adsorption isotherms at 273 and 298 K.

600 is the lowest, CO2 adsorption of PDAP-600 is 3.05 mmol g−1, the largest among these three materials at 273 K and 1 bar. Through calculation, PDAP-600 exhibits very high CO2 areal capacities (∼9.8 μmol m−2) at 273 K, higher than that of PDAP-700 (∼8.4 μmol m−2) and more than double that of PDAP-800 (∼4.5 μmol m−2), which indicates the beneficial effect of nitrogen content on CO2 uptake. To determine the 4047

DOI: 10.1021/acs.chemmater.7b00710 Chem. Mater. 2017, 29, 4044−4051

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Figure 5. TEM images of Au@PDAP (a), CeO2@PDAP (b), and CeO2@PDAP-800 (c); the inset shows the magnified TEM image of CeO2@ PDAP-800. (d) TEM image of Pd/PDAP-500; the inset shows Pd particle size distribution. (e) XPS spectra and (g) PXRD patterns for Pd/PDAP500. (f) The reusing tests of Pd/PDAP-500 in the benzyl alcohol oxidation. Reaction conditions: 10 g of benzyl alcohol, 20 mg of catalyst (recovered), O2 1 atm, 120 °C, and 11 h.

creation of additional micropores during the KOH activation process. Thus, a larger SBET is beneficial for improving CO2 adsorption, and N content determines Qst of CO2 adsorption. It is worth mentioning that CO2 areal capacities of PDAP-600 (∼9.8 μmol m−2) are higher than those of previously reported porous carbons, and CO2 adsorption of PDAP-KOH (6.23 mmol g−1) is a very high value among those excellent carbon materials (Table S1).32 PDAP Based Nanocomposites. The tunable structural characteristics and controllable chemical functionalities of the polymer nanospheres provide an exceptional platform for the fabrication of new types of polymer and carbon nanocomposites. The as-synthesized PDAP has been used to encapsulate other nanoparticles, such as ceria, silica, or Au nanoparticles, to form multifunctional core−shell nanocomposites. The core−shell structure can be clearly identified in TEM images (Figures 5a,b and S9), and the shell thickness can be precisely tailored from 4 to 86 nm by the concentration of APS, like the control of the size of PDAP nanospheres (Figure S7). Similarly, porous carbon shelled CeO2 was generated by

strength of the interaction between CO2 molecules and the pyrolysis PDAP, the isosteric heat (Qst) of CO2 adsorption was calculated using CO2 adsorption isotherms at 273 and 298 K based on the Clausius−Clapeyron equation (Figure 4d). The calculated Qst for PDAP-600 is in the range of 46.3−32.7 kJ mol−1, substantially higher than the Qst for PDAP-700, though their CO2 adsorptions are quite close. The Qst for PDAP-800 is the lowest among these three materials, mainly due to its low nitrogen content. PDAP-600 was activated by KOH to enhance CO 2 adsorption capacity. SBET of KOH-activated PDAP-600 (denoted as PDAP-KOH) is 2200 m2 g−1 (Figure 4b), almost seven times SBET of PDAP-600. CO2 adsorption of PDAP-KOH is improved significantly to 6.23 mmol g−1 (273 K, 1 bar), more than double that of PDAP-600 (Figure 4c). The calculated Qst for PDAP-KOH is in the range of 37.5−24.4 kJ mol−1, and the N content is 8.47 wt %. The CO2 adsorption capacity of PDAP350 is 1.15 mmol g−1 because of its very low SBET (∼22 m2 g−1). Obviously, the enhanced CO2 adsorption of PDAP-KOH benefits from the enlargement in surface area, due to the 4048

DOI: 10.1021/acs.chemmater.7b00710 Chem. Mater. 2017, 29, 4044−4051

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Chemistry of Materials Table 1. Oxidation of Alcohols Catalyzed by Pd/PDAP-500 Catalysts with O2a

Reaction conditions: 10 g of substrate, 20 mg of Pd/PDAP-500, O2 1 atm, 120 °C. Conv., conversion; Sel., selectivity. bThe catalyst is Pd loaded on PDAP bulk and treated the same as Pd/PDAP-500. cThe catalyst is Pd/PDAP-600. dTOF = [reacted mol substrate]/[(total mol metal) × (reaction time)]. The TOFs were measured after the first 0.5 h of reaction. eTON = [reacted mol substrate]/[total mol metal]. fThe oxidation was performed in 5 g of alcohols with 20 mg of Pd/PDAP-500 as catalyst. a

heating the core−shell nanocomposites at 800 °C under N2 and denoted as CeO2@PDAP-800 (Figure 5c). The pore size distribution of CeO2@PDAP-800 was calculated according to the DFT model based on the N2 adsorption isotherm (Figure S8). The micropores below 2 nm are attributed to the carbon shell, and the mesopores above 3 nm are attributed to the inner cores. The porous carbon shell may serve as molecular sieve, not only the mass transfer channel but also the protector of the inner core. Benefiting from abundant amino groups and nitrogen sites,33 the as-synthesized PDAP polymer spheres can be directly used as adsorbent and complexing agent for metal ions, stabilizing metal nanoparticles (e.g., Pd and Au) during the thermal reduction process. The PXRD pattern of obtained Pd/PDAP500 (Figure 5g) shows a series of broad reflection peaks at 2θ = 39.3°, 45.8°, and 66.9°, assigned to (111), (200), and (220) reflection of the cubic (fcc) palladium lattice, respectively. The mean size of Pd nanoparticles is 3 nm, calculated by the Scherrer equation using full-width-at-half-maximum (fwhm). Pd nanoparticles on Pd/PDAP-500 can be identified with the average size of about 4 nm by TEM (Figure 5d), which is in agreement with the XRD pattern calculation. The Pd 3d XPS spectra in Figure 5e consists of two asymmetric peaks related to Pd 3d5/2 and Pd 3d3/2 core levels, which can be fitted using two doublets. The peaks around 335.7 and 341.0 eV are attributed to metallic Pd0, and those around 337.1 and 342.4 eV correspond to Pd2+ species.34 Furthermore, other metal nanoparticles with ultrasmall sizes, such as Au and Pt, can also be stabilized on PDAP by a similar loading process (Figure S12). These nanocomposites verified the excellent compatibility of collidal PDAP. Solvent-Free Oxidation Using Pd/PDAP-500 Catalyst. To investigate the performance of the Pd/PDAP-500 catalyst, solvent-free oxidation of benzyl alcohol under atmospheric O2 pressure, an important step in the synthesis of fine chemicals,

was selected as a model reaction. The Pd/PDAP-500 catalyst with the Pd loading amount of 0.76 wt % (Figure S10) exhibited turnover frequencies (TOFs) of 4582 h−1 and a 42.5% benzyl alcohol conversion with very high selectivity (sel.: 99%) to benzaldehyde in 33 h over the reaction period (Entry 3, Table 1). The conversion of benzyl alcohol and the selectivity to benzaldehyde are comparable to the reported excellent catalysts under the similar conditions (Table S2),34−38 such as 74.5% and 91.6% using Au−Pd/TiO2 catalyst reported by Hutchings and co-workers. The Pd-catalyzed oxidation of 1phenylethanol also proceeded very selectively to give acetophenone as product (Entry 6, Table 1). The oxidation of cinnamyl alcohol exhibited a high conversion (52.3%) with selectivity (sel.: 35%) to cinnamaldehyde in 6 h (Entry 7, Table 1). Pd/PDAP-500 catalyst is also suitable for the selective oxidation of 1-octanol, 2-octanol, and 3-octanol to octyl aldehyde, 2-octanone, and 3-octanone (Entries 8, 9, and 10, Table 1), which is inactive with the Au−Pd/TiO2 catalyst.35 As a comparison, Pd loaded on PDAP bulk did not perform as well as Pd/PDAP-500 in solvent-free oxidation of benzyl alcohol (Entry 4, Table 1), which verified the enhancement of uniform nanospheres. Pd/PDAP was carbonized under higher temperature (600 °C) to obtain Pd/PDAP-600. The performance of Pd/PDAP-600 contrasted unfavorably with that of Pd/PDAP500 in solvent-free oxidation of benzyl alcohol (Entry 5, Table 1), because the mean Pd particle size grew up to about 8 nm according to the PXRD pattern of Pd/PDAP-600 (Figure S11). Thus, Pd/PDAP-500 is the optimal catalyst for solvent-free oxidation of benzyl alcohol. The recycling test (Figure 3f) reveals that the Pd/PDAP-500 catalyst worked in a heterogeneous manner with negligible Pd leaching. The TEM image (Figure S13) and XRD pattern (Figure 5g) of recovered Pd/PDAP-500 verified the good morphology and structure stability. Therefore, Pd nanoparticles stabilized by PDAP bring 4049

DOI: 10.1021/acs.chemmater.7b00710 Chem. Mater. 2017, 29, 4044−4051

Article

Chemistry of Materials

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about a new heterogeneous catalyst for solvent-free oxidation of alcohols with high activity and high selectivity.

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CONCLUSION In summary, we have developed a programmable and robust method to synthesis uniform PDAP nanospheres with different average sizes. Amphiphilic or superhydrophilic PDAP nanospheres are directly synthesized by using the corresponding polymer surfactant. Calcined PDAP nanospheres are porous carbon with high N content. As-synthesized PDAP can not only encapsulate other nanoparticles to form core−shell composite nanospheres but also stabilize noble metal ions to obtain uniform noble metal nanoparticles, which show good activity and high selectivity in the solvent-free oxidation of alcohols. These excellent properties enable PDAP nanospheres great potential in many areas, such as surface modification, stabilizing emulsions, CO2 adsorption, composite materials, and catalysis.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00710. SEM images of PDAP-V nanospheres under different synthesis conditions; TG curve of PDAP-V; XPS spectra of PDAP-800; CO2 adsorption isotherms at 298 K of PDAP-x and PDAP-KOH; TEM images of CeO2@ PDAP with different thickness of shells; SEM-EDX spectra of Pd/PDAP-500; characterization of Au/PDAP200 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tao Wang: 0000-0001-5004-160X Pengfei Zhang: 0000-0001-7559-7348 Yunling Liu: 0000-0001-5040-6816 Zhen-An Qiao: 0000-0001-6064-9360 Sheng Dai: 0000-0002-8046-3931 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Young Thousand Talented Program and the National Natural Science Foundation of China (Grant Nos. 21671073, 21671074, 21371067, 21621001, and 21373095).

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REFERENCES

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DOI: 10.1021/acs.chemmater.7b00710 Chem. Mater. 2017, 29, 4044−4051