Ultrafine Silver Nanoparticles Supported on a Conjugated

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Ultrafine Silver Nanoparticles Supported on a Conjugated Microporous Polymer as High-Performance Nanocatalysts for Nitrophenol Reduction Hai-Lei Cao,† Hai-Bo Huang,† Zhi Chen,† Bahar Karadeniz,‡ Jian Lü,*,†,‡ and Rong Cao*,‡ †

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Fujian Provincial Key Laboratory of Soil Environmental Health and Regulation, College of Resources and Environment, Fujian Agriculture and Forestry University, Fuzhou 350002, P.R. China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P.R. China S Supporting Information *

ABSTRACT: A conjugated microporous polymer (CMP) material was designed with pore function of cyano and pyridyl groups that act as potential binding sites for Ag+ ion capture. Ultrafine silver nanoparticles (less than 5 nm) were successfully supported on the predesigned CMP material to afford Ag0@CMP composite materials by means of a simple liquid impregnation and light-induced reduction method. Spherical Ag0 nanoparticles with a statistical mean diameter of ca. 3.9 nm were observed and characterized by scanning electron microscopy and transmission electron microscopy. The Ag0@CMP composite materials were consequently exploited as high-performance nanocatalysts for the reduction of nitrophenols, a family of priority pollutants, at various temperatures and ambient pressure. Moreover, the composite nanocatalysts feature convenient recovery and excellent reusability. This work presents an efficient platform to achieve ultrafine metal nanoparticles immobilized on porous supports with predominant catalytic properties by virtue of the structural design and spatial confinement effect available for conjugated microporous polymers. KEYWORDS: conjugated microporous polymer, ultrafine metal nanoparticle, silver, composite, nanocatalyst, nitrophenol reduction

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INTRODUCTION Ultrafine metal nanoparticles (UMNPs) with narrow size distribution have been shown as atom-efficient materials with high density of active sites available for catalysis, favored by the generally high surface-to-volume ratio of these nanocatalysts.1 Nevertheless, UMNPs can be extremely unstable and tend to aggregate during catalytic processes, which causes problems with the maintenance of catalytic efficiency and recycling ability. Despite the vast research effort that has been endeavored, efficient synthetic methods to achieve UMNPs are still rare and highly desirable.2−4 Recently, immobilization of UMNPs on porous framework materials has been greatly advanced as an atom efficiency-enhancing pathway in catalytic processes by means of delicate control of the nucleation and growth of UMNPs, the effective prevention of aggregation, and the efficient harvest of the synergistic effect between UMNPs and supports.5,6 Such composite nanocatalysts produce exciting opportunities for the investigation of both new and existing reactions, taking advantage of the unique catalytic properties of UMNPs. Porous organic polymers (POPs), classified as covalent organic frameworks (COFs),7−10 polymers of intrinsic microporosity (PIMs),11−14 and conjugated micro- and meso-porous polymers (CMPs),15−18 represent a widely investigated family of porous organic materials that are typically prepared from © 2017 American Chemical Society

organic coupling reactions of selected and/or designed precursors. Worldwide research interests of POPs span environmental fields, i.e. storage, separation, and purification of greenhouse gas emissions and volatile organic substrates15,16 and renewable energy sources such as electronics17 and catalysis.18 Of special note, POPs with permanent porosity, unlike zeolites and porous carbons, have inherent design flexibility because the choice of organic building units can be varied and tuned to incorporate multifunctional components. Moreover, the control and modulation on the structures of those materials enable intelligent and delicate design strategy, which is both crucial and challenging for advanced science and technology. More interestingly, catalytic reactions in specific and confined space might display improved catalytic performance due to the so-called confinement effect, which has been demonstrated by several previous studies.19,20 In this context, CMPs might be rational candidates to host and support UMNPs.21−23 However, much to our surprise, the current research of UMNPs@CMP materials has undergone an unparalleled development regarding the structural analogues of metal−organic frameworks (MOFs) and COFs.3,4,10,24,25 Received: October 18, 2016 Accepted: January 25, 2017 Published: February 6, 2017 5231

DOI: 10.1021/acsami.6b13186 ACS Appl. Mater. Interfaces 2017, 9, 5231−5236

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Illustration of Synthetic Pathway and Pore Structure of CMP for Silver Nanoparticle Immobilization

phthalaldehyde (134 mg, 1.0 mmol) were added to 10 mL of glacial acetic acid. The reaction mixture was purged with N2 for 30 min and reacted at 125 °C for 48 h. Yellow precipitate formed during the reaction was filtered and washed with hot acetic acid, water, and ethanol and dried in air. The as-prepared solids were further treated by solvothermal reactions in N,N-dimethylformamide (DMF) at 90 °C for 3 days to yield targeting CMP material (ca. 57%).31 IR (KBr, υmax, cm−1): 2191 (s), 1691 (m), 1640 (s), 1597 (s), 1508 (m), 1423 (m), 1382 (w), 1291 (w), 1195 (m), 1058 (w), 1018 (w), 967 (w), 856 (m), 816 (m), 775 (m), 689 (m), 603 (w). Elemental analysis for C17H6N3 (found/calcd): C, 80.94/76.89; H, 2.40/2. 58; N, 16.66/ 17.69. Activation of the CMP Material. As-prepared CMP was exchanged with acetone 10 times within 3 days and dried in fume hood overnight to afford the activated samples of CMP for silver loading. Guest exchange and removal were monitored by TGA, which confirmed that the removal of acetic acid molecules being trapped in the pores of as-prepared CMP required a fairly high temperature (ca. 200 °C versus ca. 100 °C) in comparison with the removal of acetone molecules presented in activated samples (Figure S1). Preparation of Ag0@CMP Composite Materials. Ten milligrams of CMP was added to 10 mL of AgNO3 solution (0.1 mmol L−1) and stirred overnight in the dark. The grayish yellow sample was collected by centrifugation and washed three times with distilled water, and the solution was added to 20 mL of distilled water to be stirred for another 4 h under visible light irradiation. Upon light-induced reduction from Ag+ to Ag0, Ag0@CMP composite materials were formed, recovered by centrifugation, and further washed three times with distilled water. The Ag content was determined by ICP with a mean loading of ca. 0.73 wt %. Catalytic Experiments. The catalytic activity of Ag0@CMP was tested by reduction reaction of n-nitrophenol (n-NP, n = 2−4) into naminophenol (n-AP) in the presence of sodium borohydride. In a typical procedure, 7.4 mg of Ag0@CMP nanocatalyst (ca. 0.5 mol % loading of Ag) was added to the reaction mixture of n-NP (10 mL, 10 mmol L−1), water (40 mL), and NaBH4 (1.0 mL, 1.0 mol L−1) solution at various temperatures. The reduction of n-NP to n-AP was monitored by UV−vis spectra, and references were made with reactions without catalysts. Upon completion of each catalytic reaction, the mixture was centrifuged for the recovery of Ag0@CMP nanocatalysts. Consecutive recycling reactions were performed to examine the recovery and reusability of nanocatalysts. Between each two consecutive reactions, Ag0@CMP nanocatalysts were recovered by repeating operation of centrifugation and washing with ethanol three times with special care to avoid catalyst loss, and the recovered nanocatalysts were used for the next runs. Rate constants (k) and the activation energy (Ea) were calculated using eqs 1 and 2, respectively, as follows:

In this contribution, a conjugated microporous polymer was designed with a rigid 2D chemical structure and cyano-/pyridylfunctionalized pores, which are demonstrated as viable coordinating groups to capture silver ions.14,26,27 The CMP material was further used for confined reduction and immobilization of ultrafine silver nanoparticles. The composite Ag0@CMP materials were further exploited as nanocatalysts for the reduction of environmental pollutants, here nitrophenols, with fairly high catalytic activity, convenient recovery, and excellent reusability.28

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EXPERIMENTAL SECTION

General Methods. Elemental analyses (C, H, and N) were performed on a CE-440 elemental analyzer. Ag was determined using a Jobin Yvon Ultima2 inductively coupled plasma (ICP) atomic emission spectrometer. Infrared (IR) spectra were recorded on PerkinElmer Spectrum One instrument with KBr pellets in the range 400−4000 cm−1. 1H NMR spectra were recorded on Bruker DPX-400 spectrometer. Thermal gravimetric analyses (TGA) were performed under a flow of nitrogen (10 mL min−1) with a heating rate of 10 °C min−1 using a TA SDT-600 thermogravimetric analyzer. Powder X-ray diffraction (PXRD) measurements were carried out at room temperature on PANalytical X’Pert Pro diffractometer using Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectra (XPS) were collected at a takeoff angle of 45° using a PHI Quantum 2000 scanning ESCA microprobe (Physical Electronics, United States) with an Al Kα X-ray line (1486.6 eV). UV−vis spectra were recorded with a UV-2600 spectrophotometer (SHIMADZU). Scanning electron microscopy (SEM) was performed using a JEOL JSM-6700F microscope. Transmission electron microscopy (TEM) was performed using a JEOL-2010 FEI Tecnai G20 field-emission microscope (JEOL, Tokyo, Japan) operated at 200 kV. Photoreduction of Ag+ ions was carried out using a CHF-XM35-500W xenon lamp as a light source. Chemicals. Commercially available reagents and organic solvents were used as received without further purification unless otherwise specified. Synthesis of Amino-[4-(amino-cyano-methyl)-phenyl]-acetonitrile (1). Terephthalonitrile (128 mg, 1.0 mmol), MeCN (164 mg, 4.0 mmol), and potassium tert-butoxide (672 mg, 6.0 mmol) were added to toluene (40 mL), and the reaction mixture was stirred at ambient temperature for 48 h. Saturated NaHCO3 solution (30 mL) was used to quench the reaction, and the resultant solid crude product of 1 was collected by filtration, washed three times with NaCl solution, and dried in air. Yield: ca. 48%. 1H NMR (DMSO-d6): 7.6 (s, 4H, phenyl-H); 6.7 (s, 4H, NH); 4.3 (s, 2H, C−H−CN) ppm. IR (KBr, υmax, cm−1): 2179 (m), 1662 (s), 1583 (s), 1526 (s), 1425 (s), 1281 (m), 859 (m), 795 (s), 693 (s), 609 (s). Elemental analysis for C12H10N4 (found/calcd): C, 67.66/68.5; H, 4.48/4.79; N, 27.59/ 26.65%. Synthesis of CMP. The synthetic procedure was adapted from literature 29,30 as follows: 1 (421 mg, 2.0 mmol) and 1,45232

ln c /c0 = − kt + C

(1)

ln k = ln A − Ea /RT

(2) DOI: 10.1021/acsami.6b13186 ACS Appl. Mater. Interfaces 2017, 9, 5231−5236

Research Article

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ACS Applied Materials & Interfaces

RESULTS AND DISCUSSION The conjugated microporous polymer CMP was designed and synthesized through condensation of aminoacrylonitrile with phthalaldehyde, by which cyano and pyridyl functional groups are introduced as potential interacting sites for Ag+ capture. The CMP material is insoluble in common organic solvents and water and exhibits excellent chemical and thermal stability. Moreover, the CMP material can be recovered and reused, which is crucial for practical applications in catalysis. The synthetic pathway and pore functionalization of the CMP material are illustrated in Scheme 1. Characterization of CMP Materials. The as-prepared CMP has been characterized by Fourier transform infrared spectroscopy (FT-IR). The nearly complete disappearance of CO resonance in IR spectra indicates dialdehydes were mostly consumed during the reaction. In addition, strong stretching vibration bands around 2190 cm−1 were observed, which are assigned to the characteristics of cyano groups (Figure S2). 13C CP-MAS NMR spectrum of the CMP material showed intense peaks between 110 and 150 ppm (centered at ca. 130 ppm) which are assigned to the aromatic carbon centers, and the minor peak at ca. 100 ppm belongs to the cyano groups (Figure S2). The peaks beyond 200 ppm are characteristics of acetic acid molecules trapped in the CMP material. Other characteristic peaks are possibly identified to the oligomeric cross-linking fragments (Figure S3). Powder Xray diffraction (PXRD) for CMP within the 2θ range of 5−50° shows a dispersion peak, which is a characteristic of the amorphous structure of conjugated CMP materials. These results confirmed the successful preparation of CMP as proposed from organic syntheses (Scheme 1). The field emission SEM (FE-SEM) image showed that the CMP material has dispersive sphere morphology with particle size of 1−2 μm (Figure 1a). Characterization of Ag0@CMP Nanocatalysts. Upon visible light irradiation on the activated and Ag+-loaded CMP

material, the colorless bulky sample gradually turned gray, suggesting a successful reduction from Ag+ to Ag0. Morphology and particle size of the silver nanoparticles in Ag0@CMP were studied by TEM (Figure 1b and Figure S4). It is clear that monodispersed spherical silver nanoparticles with a mean diameter of ca. 3.9 ± 0.7 nm were observed (Figure 1d and Figure S4). The CMP support played an important role in stabilizing and immobilizing the ultrafine crystalline silver nanoparticles from aggregation.32 High-resolution TEM image (HR-TEM, Figure 1b insert and S4) and selected area electron diffraction (SAED) confirmed the Ag nanoparticles were in crystalline phase with a crystal plane spacing of 0.235 nm, which corresponds to the [111] lattice plane of Ag0 NPs. However, PXRD of the resultant gray solid sample showed an absence of the typical characteristics of silver nanoparticles, which indicated low Ag loading within the Ag0@CMP composite material. The ICP result confirmed a low silver loading of ca. 0.73 wt %. The presence of Ag0 nanoparticles in the composite material was demonstrated by XPS, in which the characteristics of Ag0 species were observed at binding energies of 368 eV (3d5/2) and 374 eV (3d3/2), respectively (Figure 2c). Catalytic Activity for Nitrophenol Reduction. Nitrophenols, listed as “priority pollutants” by the United States Environmental Protection Agency (EPA), are probably the most-studied nitroaromatics because they are environmentally toxic and difficult to remove by natural degradation due to their chemical and biological stability. Meanwhile, the reduction products of nitrophenols, the aminophenols, are synthetic intermediates in the manufacture of pharmaceuticals, dyes, polymer stabilizers, imaging agents, etc. However, nitrophenol reduction reactions normally require the use of expensive Pt/ Pd catalysts in acid medium, so there is an obvious demand for developing affordable substitute catalysts. In this context, the discovery of viable catalysts for the reduction of nitrophenols into aminophenols becomes much more fascinating for environmental management as well as sustainable chemistry.28 Therefore, the reduction of nitrophenols was selected as a model reaction to evaluate the catalytic activity of the Ag0@ CMP composite material (Figure 2a). Catalytic reductions of 4-nitrophenol (4-NP) were first performed in the presence of NaBH4 and Ag0@CMP nanocatalysts, which were constantly monitored by UV−vis spectra. Notably, the characteristic peaks at ca. 400 nm, originated from the transformation of 4-NP (ca. 316 nm) to 4nitrophenolate (4-NP′) after the addition of NaBH4, decreased gradually with the time of reaction (Figure 2b). Furthermore, the new characteristics appearing alongside at ca. 295 nm suggested the formation of 4-aminophenol (4-AP; Figure 2b), as also indicated by color change from lime-green to colorless upon completion of the reaction within 1 h. By contrast, control experiments were performed in the absence of Ag0@ CMP nanocatalysts, which showed a nearly unchanged maximum absorbance at ca. 400 nm, indicating an extremely slow progress of nitrophenol reduction.33−35 Furthermore, the catalytic reduction of 4-NP at different temperatures was studied, and rate constants were calculated as listed in Table 1 (entries 1−4). The reactions were considered as pseudo-firstorder in the presence of excess NaBH4, and apparent rate constants were calculated by plotting ln c/c0 versus time to be 0.082 min−1 with a normalized rate constant (knor) of 1.84 mmol−1 s−1, which is comparable to some of the recently reported knor for catalytic nitrophenol reduction with noble metal nanoparticles.28,36−38 Rate constants for the catalytic

Figure 1. (a) SEM micrograph of as-prepared CMP material. (b and c) TEM images of Ag0@CMP before and after five runs of catalytic reactions. (d) Size distribution of silver nanoparticles (statistical data for over 150 particles). 5233

DOI: 10.1021/acsami.6b13186 ACS Appl. Mater. Interfaces 2017, 9, 5231−5236

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Figure 2. (a) Schematic presentation of the proposed process of nitrophenol reduction on the Ag0@CMP nanocatalyst. (b) Pseudo-first-order plot of ln C/C0 versus time at ambient temperature and pressure. (c) XPS spectra of Ag0@CMP. (d) Rate constants k for the reduction reactions of 4-NP in five consecutive runs.

Table 1. Rate Constants of Reactions with Ag0@CMP Nanocatalysts under Various Conditions 1 2 3 4 5 6 7 8 a

substrate

T (°C)a

k (min−1)

knor (mmol−1 s−1)

4-NP 4-NP 4-NP 4-NP 2-NP 3-NP 2-CH3-4-NP 2-Cl-4-NP

RT 40 50 60 RT RT RT RT

0.0815 0.163 0.309 0.597 0.0635 0.0125 0.0283 0.0412

1.84 3.62 6.87 13.27 1.41 0.28 0.63 0.92

reduction reactions of nitrophenol homologues were studied to further evaluate the substituent effect in these systems (Table 1; entries 7 and 8), for which the rate constants decreased greatly with the insertion of either electron withdrawing (−Cl) or electron-donating (−CH3) substituent groups on the ortho position of 4-NP. These results confirm that the Ag0@CMP nanocatalysts are viable choices in reduction of nitrophenols into aminophenols. Recovery and Reusability of the Ag0@CMP Nanocatalysts. Stability and reusability of Ag0@CMP nanocatalysts are crucial for their practical applications; thus, the nanocatalysts were tested in five consecutive reactions of 4-NP reduction and monitored by UV−vis spectra. The nanocatalysts are able to preserve more than 80% of the initial catalytic ability after 5 runs, as indicated by calculated rate constants (Figure 2d). The slight decrease in catalytic ability might be originated from local particle aggregation (Figures 1c and S4) as well as the insignificant loss of nanocatalysts during the recovery processes, which is negligible in solution as confirmed by ICP measurements.

RT = 20 ± 2 °C.

reduction of 4-NP at different temperatures were further studied, and the activation energy was calculated to be in the range of 42.5−66.2 kJ mol−1 by applying the Arrhenius equation (Table 1; entries 1−4). Moreover, it was confirmed that rate constants for reactions under certain conditions increased with elevated dosage of Ag0@CMP nanocatalysts. The role of the Ag0@CMP nanocatalyst can be decisive for the reduction reactions, in which electron transfer from BH4− to 4NP depends enormously on their adsorption to the surface of silver nanoparticles,39,40 and the increasing amount of nanocatalysts favors substrates and reduction species adsorption by providing a larger substrate-accessible surface area of ultrafine silver nanoparticles. On the other hand, the Ag0@CMP nanocatalysts are applicable widely for reduction of isomers and homologues of 4-NP (Table 1; entries 5−8). It is not surprising that the reduction of 3-NP is less efficient than those of 2-NP and 4-NP, in which the negative charges on nitroxides are delocalized throughout the benzene rings due to both inductive effect and conjugative effect.41 Thus, the rate constants for reduction reactions of nitrophenol isomers are expected to follow the order of K4‑NP > K2‑NP > K3‑NP, which is in accordance with the experimental results (Table 1; entries 1, 5, and 6). Moreover,

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CONCLUSION

In summary, a CMP material was designed with the pore function of cyano and pyridyl groups that act as potential interacting sites for Ag+ ions. Thus, the CMP material can be used as an efficient confinement material and support for the immobilization of ultrafine silver nanoparticles (under 5 nm). Interestingly, the composite material Ag0@CMP exhibits excellent catalytic activity for the reduction of nitrophenols, a family of priority pollutants, at ambient temperature and pressure. Furthermore, the Ag0@CMP features convenient recovery and excellent reusability that is potentially promising for the practical use in catalysis. This work defines an efficient platform to achieve ultrafine metal nanoparticles immobilized on porous supports as high-performance nanocatalysts by virtue 5234

DOI: 10.1021/acsami.6b13186 ACS Appl. Mater. Interfaces 2017, 9, 5231−5236

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of the structural design and spatial confinement effect available for conjugated microporous polymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13186. TGA, IR, and 13C solid-state NMR spectra of CMP and TEM images of Ag0@CMP materials (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jian Lü: 0000-0002-0015-8380 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.L. gratefully acknowledges the National Natural Science Foundation of China (NSFC, Grant 91622114), the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (Grant 20170032), and the Fujian Agriculture and Forestry University (Grant 61201401307) for funding. R.C. thanks the financial support from the 973 program (Grant 2014CB845605), the NSFC (Grants 21520102001, 21521061, and 21331006), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB20000000). H.L.C. thanks the Foundation of Fujian Educational Committee (Project JA15150) for financial support. Z.C. acknowledges the Fujian-Taiwan Joint Innovative Center for Germplasm Resources and Cultivation of Crops for funding (FJ 2011 Program 2015-75, China).

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DOI: 10.1021/acsami.6b13186 ACS Appl. Mater. Interfaces 2017, 9, 5231−5236

Research Article

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DOI: 10.1021/acsami.6b13186 ACS Appl. Mater. Interfaces 2017, 9, 5231−5236