- Email: [email protected]
Nitrogen-rich hierarchical porous carbon supported Ag nanoparticles for efficient nitrophenol reduction
Microporous and Mesoporous Materials 290 (2019) 109672
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Nitrogen-rich hierarchical porous carbon supported Ag nanoparticles for efficient nitrophenol reduction
T
Qiuliang Wang, Yunfei Liu, Qiuyu Meng, Yiling Zhu, Jun Xie, Yali Luo∗, Yinong Lyu∗∗ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous polymer Silver nanoparticles Support/carrier Catalytic reduction
Porous carbons with predefined porosity and customizable pore structures are promising templates for the synthesis of highly dispersed metal nanoparticles. Herein, nitrogen-rich hierarchical porous carbon was synthesized using rationally designed microporous organic polymer as the precursor by high-temperature treatment with potassium hydroxide activation. Due to the meso-microporous structure and high Brunauer-Emmett-Teller (BET) surface area (2585 m2 g−1), the resulting porous carbon has been employed as the catalyst support and silver nanoparticles (Ag NPs) were evenly dispersed on the surface or embedded within carbon matrix. Compared with polymer precursor-supported silver nanoparticles, the porous carbon-supported silver nanoparticles show higher catalytic activity in 4-nitrophenol reduction, which can be attributed to the smaller silver particles size (3.9 nm) and the hierarchical porous structure of the support material. The pseudo-first-order rate constant (k) and catalytic activity parameter (turnover frequency, TOF) is estimated to be 8.7 × 10−1 min−1 and 1.90 × 10−5 mol g−1 s−1, respectively. This is the highest value reported to date for Ag-based catalysts. More importantly, the porous composite catalyst is highly stable, easy to recycle and reused without losing its catalytic activity. The present work opens new opportunities for the design and preparation of metal nanoparticles/ porous carbon composite materials.
1. Introduction Over the past decades, fine and highly dispersed metal nanoparticles (NPs) have caused wide concern in catalysis science due to efficiency, greenness, convenient use, and improved understanding of reaction mechanisms. These metal nanoparticles have been diffusely used in numerous organic catalytic reactions, including hydrogenation [1], methanol oxidation [2], and C–C cross-coupling [3]. It has been shown that the smaller the size of NPs is, the higher catalytic activity the NPs will have [4]. However, small NPs are often unstable and may aggregate irreversibly during catalytic reactions on account of their high surface energies, which can lead to rapid degradation of the catalytic activity [5,6]. Moreover, small NPs are generally difficult to separate from catalytic reaction systems. This also hinder their practical application. To overcome such issues, the immobilization of NPs onto a high surface area supporting substrate, such as nanoporous carbon [7,8], porous silica [9,10], metal organic frameworks (MOFs) [11], polymers [12,13], has been proven to be an effective method. Among all these supports, porous carbon materials are the strongest contenders owing to the large surface area, great chemical stability, fast kinetics, versatility,
∗
and excellent mechanical strength. To date, various kinds of carbon materials have been extensively studied in heterogeneous catalysis, including active carbons, mesoporous carbon, carbon nanotubes, carbon nanofibers, and graphene. Traditionally, these porous carbons can be easily fabricated through template synthesis, pyrolysis and chemical or physical activation of raw materials such as coal, wood or fruit shells. Despite the great progress in this field, porous carbon materials as the catalyst supports still suffer from some limitations for industrial applications. For example, the micropores of the active carbons are easily blocked by metal NPs, which limits the mass transfer of molecules in reactions [14]. Moreover, the weak interaction between metal NPs and the supports, metal NPs attached to carbon nanotubes and graphene always undergo leaching during the catalytic process. Thus, the development of novel strategy to produce new porous carbon materials is highly desirable. In recent years, utilization of MOFs as templates or precursors for the construction of porous carbon materials are being actively pursued [15]. Compared to conventional carbon materials, MOF-derived carbons can have a fine control in the porosity and morphology, originating from the inherent diversity of MOFs. There have been intensive
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Luo), [email protected] (Y. Lyu).
∗∗
https://doi.org/10.1016/j.micromeso.2019.109672 Received 19 March 2019; Received in revised form 13 July 2019; Accepted 20 August 2019 Available online 20 August 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
procedure to obtain notable dilation of the main pore size from micropores (< 2 nm) to mesopores (2–8 nm) and accompanying formation of the hierarchical porous carbon. Furthermore, the BET surface area (from 821 to 2585 m2 g−1) area and pore volume of the hierarchical porous carbon were significantly improved. Highly dispersed Ag nanoparticles on the CCTPB and CCTPB-K were prepared and completely characterized. The synthesized Ag@CCTPB and Ag@CCTPB-K showed great catalytic activity in 4-nitrophenol reduction within 25 and 3.5 min, respectively. The effect of the structure and component of Ag@CCTPB/Ag@CCTPB-K on recyclability and particle size was also studied in this article.
research efforts to construct diverse composite catalysts with MOFs as precursors. For example, Dong et al. [14] reported magnetic porous carbon, derived from MOFs, exhibiting large surface area, magnetic separability, and reusability act as a support for Au and Pd nanoparticles. Wu et al. [8] prepared highly dispersed Pt nanoparticles and amorphous Ni-based 3D mesoporous carbon catalyst by the carbonization of Cu-MOF (HKUST-1), which exhibited outstanding property in the reduction of nitrophenol. However, the ligands used in the synthesis of MOFs are generally expensive, which limits their large-scale industrial production, and the yield of porous carbon prepared by MOFs is low. In contrast, another new type of porous organic polymers has developed rapidly over the past decade, and its strong covalent bond connection gives the material a high specific surface area and high stability. Besides that, the high carbon content of these materials also makes them very suitable as carbon precursors. Gu and co-workers had synthesized some kinds of hierarchical porous carbon with the high surface area (1348–1824 m2 g−1) through chemical activation method [16]. Hierarchical porous carbons exhibit superior performances in catalytic applications because they can retain nanoparticles in the substrate to avoid aggregation and make NPs reused without significantly reducing of catalytic activity [14,17]. In addition, heteroatom doped, particularly N-doped, the porous carbons, which supply many active N sites to enhance electron density, promote the dispersion of metal NPs, making the N-doped porous carbons wonderful substrate to anchor metal NPs, with stability, small size and high dispersion [18,19]. 4-Nitrophenol (4-NP), derived from the industrial manufacturing processes of petrochemicals and pharmaceuticals, is identified as one of the most hazardous and toxic pollutants [20,21]. There are various techniques utilized to remove 4-NP from wastewater and catalytic reduction has been proven to be an efficient and cost-effective method [22]. More importantly, the reduction product 4-aminophenol (4-AP) is an extremely useful organic compound for the manufacture of analgesic and antipyretic drugs. For instance, Li et al. reported that hypercrosslinked β-cyclodextrin porous polymer/gold nanoparticle composites could serve as an effective catalyst to activate the reduction of 4NP [13]. Dong et al. successfully fabricated two novel catalysts based on Au and Pd nanoparticles supported on magnetic porous carbon (MPC) from metal organic framework with high catalytic activity for 4NP reduction. The reaction rate constant was 1.0 × 10−2 s−1 and 1.2 × 10−2 s−1 for the reaction catalyzed using Au/MPC and Pd/MPC nanocatalysts, respectively [14]. Recently, reduced graphene oxide supported Ag nanoparticle (rGO-Ag-U) also showed a high catalytic activity toward the 4-NP reduction with reaction rate constant of 2.0 × 10−2 s−1 [23]. Despite of the effort made in recent years for nanocatalyst preparation, the study of the porous organic polymer derived carbon as a carrier for the growth of metal NPs is little. Here, we reported the easy synthesis of the hierarchical porous carbon derived from CCTPB (Scheme 1) by chemical activation. More specifically, the CCTPB with abundant micropores and N element was synthesized by Friedel-Crafts reaction. After that, the CCTPB was chemically activated via a common
2. Experimental sections 2.1. Materials All regents and solvents were purchased from commercial suppliers and used without further purification, unless otherwise noted. N, N, N′, N′-tetraphenylbenzidine (TPB), cyanuric chloride (CC), o-dichlorobenzene, silver nitrate solution (AgNO3, 0.1 M), sodium borohydride (NaBH4), methane-sulfonic acid and 4-nitrophenol (4-NP) were obtained from Aladdin Reagent Limited Company (Shanghai, China). KOH was supplied by Shanghai Lingfeng Chemical Reagent Co., Ltd.
2.2. Synthesis of CCTPB CCTPB was synthesized using the procedure described by Xiong et al. [24] Briefly, methanesulfonic acid (70.0 mmol, 6.73 g) was added to a solution of N, N, N′, N′-tetraphenylbenzidine (3.75 mmol, 1.83 g) and cyanuric chloride (5 mmol, 0.92 g) in o-dichlorobenzene (100 mL). The mixture was refluxed at 140 °C for 24 h. After the reaction was complete, the solid product was isolated by filtration, washed with methanol, tetrahydrofuran, and acetone. Further purification was done by soxhlet extraction with methanol. The product was dried under vacuum at 60 °C for 24 h to afford CCTPB as a yellowy brown colored solid (Yield: 90%). Elemental analysis calcd. (wt%) for C20H12N3: C, 81.61; H, 4.11; N, 14.28. Found: C, 80.23; H, 3.12; N, 11.76.
2.3. Synthesis of potassium hydroxide activated carbon material CCTPB and KOH were uniformly mixed in an agate mortar at a mass ratio of CCTPB: KOH = 1:2. The mixed powder was placed in a nickel crucible and then taken into the tubular furnace. The chemical activation was executed under N2 protection at a ramp rate of 2 °C/min and held at 800 °C for 2 h. After cooling to room temperature, the product was completely washed by HCl aqueous solution (V/V = 1:1). Then the powder was rinsed with deionized water until the water became neutral. The carbonized black sample was dried under vacuum at 70 °C for 24 h and denoted as CCTPB-K.
Scheme 1. Synthesis route to the precursor of CCTPB. 2
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
methanesulfonic acid (Scheme 1) [24]. Carbonization was performed by heating the CCTPB/KOH powder mixture to the target temperature of 800 °C. We used wet-chemistry method to synthesize Ag@CCTPB and Ag@CCTPB-K with NaBH4 as reducing agent. CCTPB, CCTPB-K, Ag@CCTPB and Ag@CCTPB-K are insoluble in water and general organic solvents hexane, DMSO, Tetrahydrofuran, and DMF. FT-IR was used to determine the functional groups of the precursor CCTPB and the carbonized material. As seen in Fig. S1, compared to CC, the disappearance of C–Cl stretching vibration absorption at 850 cm−1 indicates complete conversion of C–Cl [25]. Further evidence for the formation of the target framework is given by the fierce bands at 1506 (in plane deformation vibration of triazine ring) and 822 cm−1 (out-ofplane deformation vibration) [24]. A new band of bending vibration of C–H in 1,4-disubstitued benzene at 810 cm−1 appearances. The structure of the polymer was also offered by 13C CP-MAS NMR spectra (Fig. S2). The obvious signals at 171 ppm could be corresponded to the sp2 carbons of the triazine ring [26]. The primary signals at around 147, 135 and 127 ppm should be contributed to the substituted aromatic carbon bonded to nitrogen atom and substituted and un-substituted aromatic carbon of the aromatic ring connected with the benzene ring, respectively. Fig. S3a is the FT-IR spectrum for the sample CCTPB-K showing that the intense peak at 1088 cm−1 is corresponded to CO–O–C stretching vibrations, which is different from CCTPB. The peak around 1400 cm−1 may be the -COO- stretching vibrations [16]. Oxygen is produced by KOH etching the polymer skeleton during activation. Similar peaks can also be found in other KOH activated MOPs [16]. X-ray photoelectron spectra (XPS) (Figs. S3b–d) was also carried out to analyze the chemical structure of the hierarchical porous carbon. The carbon atoms in four different environments were found in the C 1s spectrum of CCTPB-K (Fig. S3c): C]C (284.8 eV), C−O (285.9 eV), C] O (288.1 eV), and O−C]O (289.2 eV). In addition, the O 1s spectrum (Fig. S3d) shows the main signals of C]O, C−O, and C−OH. The chemical structure of CCTPB-K is completely different from that of CCTPB, indicating the structural rearrangement for the productive hierarchical porous carbons occurred during the pyrolysis process. No significant diffraction peaks were observed in the powder X-ray diffraction (PXRD) patterns of CCTPB and CCTPB-K (Fig. S4), indicating their amorphous nature. The PXRD pattern (Fig. 1) of Ag@CCTPB and Ag@CCTPB-K demonstrates the formation of the face-centered cubic structure of Ag nanoparticles. The diffraction peaks at 38.17°, 44.25°, 64.45° and 77.49° are attributed to the (111), (200), (220) and (311) crystal faces of the Ag nanoparticles, respectively, which demonstrate successful loading of Ag NPs on CCTPB and CCTPB-K [27]. The valence state of the Ag nanoparticles was analyzed by X-ray photoelectron spectra (XPS) (Fig. 2). For Ag@CCTPB and Ag@CCTPB-K, the Ag 3d spectra show peaks at 368.3 eV and 374.3 eV, which are assigned to core-level Ag 3d5/2 and Ag 3d3/2 signals of zero-valence metallic Ag, respectively (Fig. 2b and d) [20]. Besides, the Ag MNN Auger spectrum
2.4. Synthesis of Ag@CCTPB, Ag@CCTPB-K and Ag NPs The catalyst Ag@CCTPB or Ag@CCTPB-K was fabricated using AgNO3 as the silver source and CCTPB or CCTPB-K as the supporting substrate. In a typical procedure, 3 mL of AgNO3 solution (10 mM) was added dropwise to a well-dispersed suspension of as-prepared CCTPB or CCTPB-K (30 mg) in deionized water (60 mL). The mixture stirred for 1 h to deposit metal precursor in polymer support. Subsequently, freshly prepared NaBH4 solution (0.01 M, 10 mL) was added dropwise and the mixture was stirred for 2 h. Thus, the Ag nanoparticles were formed and anchored on the wall of porous channels. Then, the product was collected through centrifugation, washed with deionized water several times, and dried in a vacuum at 50 °C overnight. For comparison, Ag NPs without the porous support were also prepared by the similar procedure. 2.5. Common procedure of the reduction of 4-nitrophenol First, freshly prepared aqueous NaBH4 solution (0.5 M, 0.15 mL) was added into 3 mL of 4-nitrophenol solution (1 × 10−4 M), which resulted in the formation of deep yellow solution. Then, a suspension of catalyst in water (0.5 mg mL−1, 50 or 150 or 300 μL) was added into the yellow solution and the reaction was monitored by the UV–vis spectroscopy. The effect of the pH value of reaction mixture and the temperature on the catalytic reduction was also taken into account. For the recycle test, after each catalytic reaction, the catalyst was collected by centrifugation, washed thoroughly with deionized water, and dried in a vacuum at 50 °C overnight for the next catalytic cycle. 2.6. Characterization Fourier transform infrared (FTIR) spectra were collected on KBr pellets using a Thermo Nicolet Nexus 670 FTIR spectrometer. Solidstate cross polarization magic angle spinning (CP/MAS) NMR spectrum was recorded on a Bruker Avance III 600 spectrometer. Elemental compositions (C, H, and N) were determined using an elemental analyzer (Vario EL cube, Germany). Morphology and energy dispersive Xray spectroscopy (EDS) were carried out on a scanning electron microscopy (SEM) (JEOL JSM-6510, Japan) and TEM (JEOL JEM2100Plus, Japan). Thermogravimetric analysis (TGA) were performed on a Netzsch STA449 F3 Jupiter thermal analyzer in the temperature range of 40–800 °C at a heating rate of 10 °C min−1 under an atmosphere of nitrogen. The powder X-ray diffraction (PXRD) patterns of the samples were recorded with SmartLab TM 3 kW (Rigaku Corporation, Japan). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Fisher ESCALAB 250Xi with Al Kα radiation. The kinetic energy (KE) in the Ag MNN region of the Auger transitions was measured and the modified Auger parameter (α′) was used to characterize the chemical state of silver. In order to determine pore textural properties including the specific Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size distribution, nitrogen adsorption/desorption isotherm on the samples at 77.3 K were measured in a Micromeritics ASAP 2020 M surface area and porosity analyzer (Micromeritics, USA). Prior to the measurements, samples (0.2 g) in the analysis chamber were subject to a vacuum of 10−5 bar at 150 °C for 12 h. For Ag content analysis, the sample was measured using a Solarix 70-FT-MS high resolution mass spectrometer (ICP-MS). UV–vis spectra were measured on a Puxi TU-1810 spectrophotometer. 3. Results and discussion 3.1. Structural and morphological characterization The precursor, a nitrogen-rich porous organic polymer (CCTPB), was synthesized by simple Friedel-Crafts reaction of N, N, N′, N′-tetraphenylbenzidine with cyanuric chloride under the catalysis of
Fig. 1. PXRD patterns of the Ag@CCTPB and Ag@CCTPB-K nanocomposites. 3
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
Fig. 2. XPS spectra Ag@CCTPB and Ag@CCTPB-K; (a, c) survey scan, (b, d) Ag 3d high-resolution, and (e, f) Ag MNN Auger region.
with each other during the activation and show a fluffy and porous morphology. The typical transmission electron microscope (TEM) images of Ag@CCTPB and Ag@CCTPB-K (Fig. 3a and b) indicate that the Ag NPs were uniformly dispersed in the CCTPB and CCTPB-K. The statistic histogram of randomly selected particles (Fig. 3c and d) shows that the average size of Ag NPs in CCTPB is 5.35 nm and the average size in CCTPB-K is 3.91 nm, respectively. Thermogravimetric analysis (TGA) was performed in continuous flow of nitrogen to estimate the stability of our materials (Fig. S9 and Fig. S10). CCTPB shows about 10% mass loss at 277 °C, in contrast to CCTPB-K at 391 °C, suggesting carbonized structures are more robust than the polymer. Nitrogen sorption experiments were performed at 77.3 K to examine surface area and pore size distribution of CCTPB, CCTPB-K, Ag@CCTPB and Ag@CCTPB-K. As shown in Fig. 4a, CCTPB, CCTPB-K, Ag@CCTPB and Ag@CCTPB-K show reversible type I and type IV sorption
of both composites was also measured and the modified Auger parameter (α′) was calculated. The Auger spectra (Fig. 2e–f) presents a distinct peak at 358 eV in the MNN region, with α′ value of 726.3 eV, corresponding to metallic silver [28,29]. All these results confirm that Ag nanoparticles have been successfully grown on CCTPB and CCTPBK. The inductively coupled plasma (ICP) analysis shows that the Ag content in the Ag@CCTPB and Ag@CCTPB-K is 9.15 wt% and 9.23 wt %, respectively, which is in good agreement with our theoretical amount of Ag (9.73 wt%). The composition of Ag@CCTPB and Ag@CCTPB-K were further analyzed by energy dispersive spectrum (EDS). As shown in Fig. S5 and Fig. S6, the peaks corresponding to Ag, C, O and N revealing the compositions of Ag@CCTPB and Ag@CCTPBK. SEM images (Figs. S7 and S8) display that CCTPB consisted of irregular small particles, whereas for CCTPB-K these small particles fused 4
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
Fig. 3. TEM images of Ag nanoparticles on the CCTPB/CCTPB-K (a-b) and the statistic histogram of the Ag nanoparticles size distribution (c-d).
the presence of the abundant micropores (Fig. 4b). The main pore sizes of CCTPB are at 0.68 and 1.21 nm, respectively (Table 1). After carbonization, CCTPB-K shows a wider pore size distribution. Besides the smaller pore sizes at 0.63 and 1.12 nm, some new mesopores centered at 1.39 and 3.81 nm are also observed, which could be produced from the hard template effect of the adsorbed KOH (Fig. 4b). Similar
isotherms. The adsorption isotherm for CCTPB shows a steep gas uptake at low relative pressure and a flat course in the intermediate section, suggesting the microporous nature of the polymer network. The Brunauer-Emmett-Teller (BET) surface area is obtained to be 821 m2 g−1 and the total pore volume is 0.49 cm3 g−1 (Table 1). The pore size distribution (PSD) determined from the NLDFT method also confirms
Fig. 4. (a) N2 adsorption (solid symbols)/desorption (open symbols) isotherms of CCTPB, CCTPB-K, Ag@CCTPB and Ag@CCTPB-K at 77 K and (b) NLDFT pore size distribution of the polymer and carbon material. 5
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
Table 1 Pore parameters of the polymer, the carbon and the catalysts. Samples
SBETa (m2 g−1)
SLangb (m2 g−1)
VTotalc (cm3 g−1)
VMicrod (cm3 g−1)
Dominant pore sizee (nm)
CCTPB CCTPB-K Ag@CCTPB Ag@CCTPB-K
821 2585 702 2050
916 3125 794 2412
0.49 1.39 0.37 1.11
0.35 0.98 0.29 0.79
0.68, 0.63, 0.51, 0.73,
a b c d e
1.21 1.12, 1.39, 3.81 1.29 1.12, 1.49, 3.54
Surface area was calculated from the nitrogen adsorption branch according to the BET model. Surface area was calculated from the nitrogen adsorption branch based on the Langmuir model. The total pore volume was calculated from the single point nitrogen uptake at P/P0 = 0.99. The micropore volume was estimated by the t-plot method. Pore size derived from N2 isotherm with the NLDFT approach.
Ag@CCTPB-K, the TOFs are estimated to be 2.67 × 10−6 and 1.90 × 10−5 mol g−1 s−1 respectively. The catalytic activities of Ag@CCTPB and Ag@CCTPB-K were also compared with some reported work on the selective reduction of 4-NP to 4-AP by metal catalysts [20,27,30,31]. As shown in Table S1, the Ag@CCTPB-K exhibit relatively faster catalytic efficiency, due to the combined effects of large surfaces and hierarchical porous structures of the particular support. In addition, we investigated the influence of the amount of catalyst and the temperature on the catalytic reduction of 4-NP. As shown in Figs. S12 and S13, the reduction rate increases with the increase of the catalyst dosage and temperature. This is due to the fact that the higher temperature will promote the diffusion of the reactant through the porous carbon and its arrival at Ag NPs surface [32]. The reduction reaction was also performed under different pH values by keeping the other variables constant. The linear relationship of ln (Ct/C0) on reaction time indicates pseudo first-order kinetics (Fig. S14). With the increases of pH of medium from 4 to 10, the values of k increase according. This can be explained by the high electron density over the surface of Ag NPs at alkaline medium. High electron density of Ag NPs will induce more 4-NP molecules to be attached by electrostatic force, which will enhance the reduction rate. Similar phenomenon was also be reported by Begum et al. [33]. Cycle stability and reusability are important parameters for evaluating the performance of heterogeneous catalysts. In order to perform the stability tests, both Ag@CCTPB and Ag@CCTPB-K were applied to five cycle under the same reaction conditions. The catalyst was simply isolated by centrifugation, washed with deionized water and dried overnight after each use. As shown in Fig. S15, the conversion of 4nitrophenol in each reaction cycle exceeded 90%, indicating that the catalyst is stable under mild reaction conditions without no significant loss of activity. After reaction, the catalyst can be easily recycled through centrifugation or natural settling. In addition, the structural stability of the catalysts after the catalytic reactions were also checked by PXRD. As shown in Fig. S16, the PXRD patterns of Ag@CCTPB and Ag@CCTPB-K after five cycles also exhibit characteristic Ag NP diffraction peaks (2θ = 38.17°, 44.25°, 64.45° and 77.49°) corresponding to (111), (200), (220) and (311) reflections, respectively, which further confirms the excellent stability of Ag@CCTPB and Ag@CCTPB-K catalyst.
phenomenon was also observed in other KOH-activated carbonized materials [16]. The BET model was applied to the data for CCTPB-K using the N2 adsorption branch to provide surface area of 2585 m2 g−1 (Table 1). After loading Ag nanoparticles, the BET surface area is 702 and 2050 m2 g−1 for Ag@CCTPB and Ag@CCTPB-K, respectively. The appreciable decrease in both the N2 adsorption and BET surface area of Ag@CCTPB and Ag@CCTPB-K in comparison to CCTPB and CCTPB-K indicates that the pore surface in the carrier is occupied by finely dispersed Ag NPs. A NLDFT pore size distribution analysis based on the N2 isotherms at 77.3 K indicated that the pore size of Ag@CCTPB is distributed around 0.5–3.2 nm, and that of Ag@CCTPB-K is distributed around 0.6–5.4 nm (Fig. 4b). The apparent hysteresis loop over the entire relative pressure range of Ag@CCTPB-K indicates the relatively strong gas bond on the pore surface, which is consistent with the coexistence of micropores and mesopores in the framework. 3.2. Catalytic properties In order to determine the catalytic activities of the Ag@CCTPB and Ag@CCTPB-K nanocomposites, the selective reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) is chosen as a model reaction. It is well known that 4-aminophenol (4-AP) is an important chemical and pharmaceutical intermediate body in many applications, including analgesic and antipyretic drugs, photographic developers, corrosion inhibitors and anticorrosion lubricants [30]. As a comparative experiment, we tested the catalytic activity of CCTPB and CCTPB-K (0.045 mg mL−1, 150 μL) for the reduction of 4-nitrophenol. The peak of the mixture of 4nitrophenol and NaBH4 at 400 nm remained almost unchanged after 30 min (Fig. S11), indicating that CCTPB or CCTPB-K alone had no catalytic activity. In contrast, when the Ag@CCTPB and Ag@CCTPB-K catalysts (0.5 mg mL−1, 150 μL) were added to the solution, the reduction was started immediately, showing that the 4-nitrophenol peak rapidly decreased at 400 nm. Meanwhile, a new peak of 4-aminophenol appeared at 300 nm (Fig. 5a, c) [31]. The solution was completely discolored from bright yellow to colorless at a rate visible to the naked eye. (Fig. 5f). When the same amount of Ag NPs without carrier were used as the catalysts for the reduction of 4-nitrophenol (Fig. 5e), a longer reaction time (about 60 min) was required to reach the same conversion, which may be due to their ease of aggregation. As shown in Fig. 5b, d, the reaction conversion was calculated from Ct/C0, where Ct and C0 were the 4-NP concentrations at time t and 0, respectively [18]. Due to the excessive concentration of NaBH4, this reduction reaction can be considered as a pseudo first order reaction with respect to the 4-NP solution. Fig. 5b and d shows linear relationship between ln (Ct/C0) and t, which confirms the reduction as firstorder. The rate constants (k) are calculated from the slope to be 1.28 × 10−1 min−1 and 8.7 × 10−1 min−1 for the Ag@CCTPB and Ag@CCTPB-K, respectively. The turnover number (TON) and the turnover frequency (TOF) of the catalyst are two important parameters commonly used to compare catalyst efficiency. In heterogeneous catalysis, TON is the number of reactant molecules that can be converted by 1 g of catalyst, and the TOF value is TON/time. For the Ag@CCTPB and
4. Conclusions In conclusion, we synthesized facile chemical activation of the single microporous organic polymer (CCTPB) produced a porous carbon with hierarchical meso-microporous structure and high BET surface area (2585 m2 g−1). The carbon material was used for the growth of Ag nanoparticles (NPs). The results showed that the Ag nanoparticles are uniformly distributed in CCTPB-K without aggregation. Due to the coordinated interaction between the porous structure and the nitrogen atom, the Ag nanoparticles can be immobilized, and the catalyst had excellent catalytic activity, and there was no significant loss of activity after repeated use for 5 times. Therefore, we believe that this nitrogen6
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
Fig. 5. Time-dependent UV–vis spectra of the reduction of 4-nitrophenol (0.3 μmol) catalyzed by Ag@CCTPB (a), Ag@CCTPB-K (c), and Ag NPs (e); the pseudo-firstorder kinetic plots of Ag@CCTPB (b) and Ag@CCTPB-K (d); and Ultraviolet–visible spectra of 4-nitrophenol before and after catalytic reduction by the catalyst (f).
rich hierarchical porous carbon will provide worthy ideas for immobilizing a variety of noble metal NPs for diverse practical applications.
Appendix A. Supplementary data
Acknowledgements
References
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.micromeso.2019.109672.
This work was supported by the National Natural Science Foundation of China (no. 51403100), the Natural Science Foundation of Jiangsu Province (no. BK20140949), and the Jiangsu Overseas Visiting Scholar Program for University Prominent Young & Middle-aged Teachers, China and Presidents and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, China.
[1] D. Laprune, A. Tuel, D. Farrusseng, F.C. Meunier, Appl. Catal. Gen. 535 (2017) 69–76. [2] S. Siwal, S. Matseke, S. Mpelane, N. Hooda, D. Nandi, K. Mallick, Int. J. Hydrogen Energy 42 (2017) 23599–23605. [3] Q. Deng, Y. Shen, H. Zhu, T. Tu, Chem. Commun. 53 (2017) 13063–13066. [4] M.Q. Yang, X.Y. Pan, N. Zhang, Y.J. Xu, CrystEngComm 15 (2013) 6819–6828. [5] L. Shang, Y. Liang, M. Li, G.I.N. Waterhouse, P. Tang, D. Ma, L.-Z. Wu, C.-H. Tung, T. Zhang, Adv. Funct. Mater. 27 (2017) 1606215. [6] W. Li, X. Ge, H. Zhang, Q. Ding, H. Ding, Y. Zhang, G. Wang, H. Zhang, H. Zhao,
7
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
[20] G. Xiao, Y. Zhao, L. Li, J.O. Pratt, H. Su, T. Tan, Nanotechnology 29 (2018) 155601. [21] W.L. Shen, Y.Y. Qu, X.F. Pei, S.Z. Li, S.N. You, J.W. Wang, Z.J. Zhang, J.T. Zhou, J. Hazard Mater. 321 (2017) 299–306. [22] A.T.E. Vilian, S.R. Choe, K. Giribabu, S.C. Jang, C. Roh, Y.S. Huh, Y.K. Han, J. Hazard Mater. 333 (2017) 54–62. [23] Z. Mohammadi, M.H. Entezari, Ultrason. Sonochem. 44 (2018) 1–13. [24] S. Xiong, X. Fu, L. Xiang, G. Yu, J. Guan, Z. Wang, Y. Du, X. Xiong, C. Pan, Polym. Chem. 5 (2014) 3424–3431. [25] T. Geng, Z. Zhu, W. Zhang, Y. Wang, J. Mater. Chem. 5 (2017) 7612–7617. [26] Y. Luo, J. Liu, Y. Liu, Y. Lyu, J. Polym. Sci. A Polym. Chem. 55 (2017) 2594–2600. [27] J. Wang, X. Yang, A. Li, X. Cai, Mater. Lett. 220 (2018) 24–27. [28] S.G. Aspromonte, R.M. Serra, E.E. Miró, A.V. Boix, Appl. Catal. Gen. 407 (2011) 134–144. [29] S.G. Aspromonte, M.D. Mizrahi, F.A. Schneeberger, J.M.R. López, A.V. Boix, J. Phys. Chem. C 117 (2013) 25433–25442. [30] N. Arora, A. Mehta, A. Mishra, S. Basu, Appl. Clay Sci. 151 (2018) 1–9. [31] N. Li, Z. Ji, L. Chen, X. Shen, Y. Zhang, S. Cheng, H. Zhou, Appl. Surf. Sci. 462 (2018) 1–7. [32] X. Chen, Z. Wang, S. Bi, K. Li, R. Du, C. Wu, L. Chen, Chem. Eng. J. 295 (2016) 518–529. [33] R. Begum, K. Naseem, E. Ahmed, A. Sharif, Z.H. Farooqi, Colloid. Surf. Physicochem. Eng. Asp. 511 (2016) 17–26.
Inorg. Chem. Front. 3 (2016) 663–670. [7] D. Xu, Y. Pan, L. Zhu, Y. Yusran, D. Zhang, Q. Fang, M. Xue, S. Qiu, CrystEngComm 19 (2017) 6612–6619. [8] X.-Q. Wu, J. Zhao, Y.-P. Wu, W.-w. Dong, D.-S. Li, J.-R. Li, Q. Zhang, ACS Appl. Mater. Interfaces 10 (2018) 12740–12749. [9] S.S. Beigbaghlou, R.J. Kalbasi, K. Marjani, A. Habibi, Catal. Lett. 148 (2018) 2446–2458. [10] W. Jiang, L. Dong, H. Li, H. Jia, L. Zhu, W. Zhu, H. Li, J. Mol. Liq. 274 (2019) 293–299. [11] N. Tsumori, L. Chen, Q. Wang, Q.-L. Zhu, M. Kitta, Q. Xu, Chem 4 (2018) 845–856. [12] S. Lu, Y. Hu, S. Wan, R. McCaffrey, Y. Jin, H. Gu, W. Zhang, J. Am. Chem. Soc. 139 (2017) 17082–17088. [13] H. Li, B. Meng, S.-H. Chai, H. Liu, S. Dai, Chem. Sci. 7 (2016) 905–909. [14] Z. Dong, X. Le, Y. Liu, C. Dong, J. Ma, J. Mater. Chem. 2 (2014) 18775–18785. [15] L. Oar-Arteta, T. Wezendonk, X. Sun, F. Kapteijn, J. Gascon, Mater. Chem. Front. 1 (2017) 1709–1745. [16] S. Gu, J. He, Y. Zhu, Z. Wang, D. Chen, G. Yu, C. Pan, J. Guan, K. Tao, ACS Appl. Mater. Interfaces 8 (2016) 18383–18392. [17] S. Wang, S. Gao, Y. Tang, L. Wang, D. Jia, L. Liu, J. Solid State Chem. 260 (2018) 117–123. [18] X.X. Wu, H. Zhou, New J. Chem. 41 (2017) 10245–10250. [19] Z.-D. Ding, Y.-X. Wang, S.-F. Xi, Y. Li, Z. Li, X. Ren, Z.-G. Gu, Chem. Eur J. 22 (2016) 17027–17034.
8
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Nitrogen-rich hierarchical porous carbon supported Ag nanoparticles for efficient nitrophenol reduction
T
Qiuliang Wang, Yunfei Liu, Qiuyu Meng, Yiling Zhu, Jun Xie, Yali Luo∗, Yinong Lyu∗∗ State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous polymer Silver nanoparticles Support/carrier Catalytic reduction
Porous carbons with predefined porosity and customizable pore structures are promising templates for the synthesis of highly dispersed metal nanoparticles. Herein, nitrogen-rich hierarchical porous carbon was synthesized using rationally designed microporous organic polymer as the precursor by high-temperature treatment with potassium hydroxide activation. Due to the meso-microporous structure and high Brunauer-Emmett-Teller (BET) surface area (2585 m2 g−1), the resulting porous carbon has been employed as the catalyst support and silver nanoparticles (Ag NPs) were evenly dispersed on the surface or embedded within carbon matrix. Compared with polymer precursor-supported silver nanoparticles, the porous carbon-supported silver nanoparticles show higher catalytic activity in 4-nitrophenol reduction, which can be attributed to the smaller silver particles size (3.9 nm) and the hierarchical porous structure of the support material. The pseudo-first-order rate constant (k) and catalytic activity parameter (turnover frequency, TOF) is estimated to be 8.7 × 10−1 min−1 and 1.90 × 10−5 mol g−1 s−1, respectively. This is the highest value reported to date for Ag-based catalysts. More importantly, the porous composite catalyst is highly stable, easy to recycle and reused without losing its catalytic activity. The present work opens new opportunities for the design and preparation of metal nanoparticles/ porous carbon composite materials.
1. Introduction Over the past decades, fine and highly dispersed metal nanoparticles (NPs) have caused wide concern in catalysis science due to efficiency, greenness, convenient use, and improved understanding of reaction mechanisms. These metal nanoparticles have been diffusely used in numerous organic catalytic reactions, including hydrogenation [1], methanol oxidation [2], and C–C cross-coupling [3]. It has been shown that the smaller the size of NPs is, the higher catalytic activity the NPs will have [4]. However, small NPs are often unstable and may aggregate irreversibly during catalytic reactions on account of their high surface energies, which can lead to rapid degradation of the catalytic activity [5,6]. Moreover, small NPs are generally difficult to separate from catalytic reaction systems. This also hinder their practical application. To overcome such issues, the immobilization of NPs onto a high surface area supporting substrate, such as nanoporous carbon [7,8], porous silica [9,10], metal organic frameworks (MOFs) [11], polymers [12,13], has been proven to be an effective method. Among all these supports, porous carbon materials are the strongest contenders owing to the large surface area, great chemical stability, fast kinetics, versatility,
∗
and excellent mechanical strength. To date, various kinds of carbon materials have been extensively studied in heterogeneous catalysis, including active carbons, mesoporous carbon, carbon nanotubes, carbon nanofibers, and graphene. Traditionally, these porous carbons can be easily fabricated through template synthesis, pyrolysis and chemical or physical activation of raw materials such as coal, wood or fruit shells. Despite the great progress in this field, porous carbon materials as the catalyst supports still suffer from some limitations for industrial applications. For example, the micropores of the active carbons are easily blocked by metal NPs, which limits the mass transfer of molecules in reactions [14]. Moreover, the weak interaction between metal NPs and the supports, metal NPs attached to carbon nanotubes and graphene always undergo leaching during the catalytic process. Thus, the development of novel strategy to produce new porous carbon materials is highly desirable. In recent years, utilization of MOFs as templates or precursors for the construction of porous carbon materials are being actively pursued [15]. Compared to conventional carbon materials, MOF-derived carbons can have a fine control in the porosity and morphology, originating from the inherent diversity of MOFs. There have been intensive
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Luo), [email protected] (Y. Lyu).
∗∗
https://doi.org/10.1016/j.micromeso.2019.109672 Received 19 March 2019; Received in revised form 13 July 2019; Accepted 20 August 2019 Available online 20 August 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
procedure to obtain notable dilation of the main pore size from micropores (< 2 nm) to mesopores (2–8 nm) and accompanying formation of the hierarchical porous carbon. Furthermore, the BET surface area (from 821 to 2585 m2 g−1) area and pore volume of the hierarchical porous carbon were significantly improved. Highly dispersed Ag nanoparticles on the CCTPB and CCTPB-K were prepared and completely characterized. The synthesized Ag@CCTPB and Ag@CCTPB-K showed great catalytic activity in 4-nitrophenol reduction within 25 and 3.5 min, respectively. The effect of the structure and component of Ag@CCTPB/Ag@CCTPB-K on recyclability and particle size was also studied in this article.
research efforts to construct diverse composite catalysts with MOFs as precursors. For example, Dong et al. [14] reported magnetic porous carbon, derived from MOFs, exhibiting large surface area, magnetic separability, and reusability act as a support for Au and Pd nanoparticles. Wu et al. [8] prepared highly dispersed Pt nanoparticles and amorphous Ni-based 3D mesoporous carbon catalyst by the carbonization of Cu-MOF (HKUST-1), which exhibited outstanding property in the reduction of nitrophenol. However, the ligands used in the synthesis of MOFs are generally expensive, which limits their large-scale industrial production, and the yield of porous carbon prepared by MOFs is low. In contrast, another new type of porous organic polymers has developed rapidly over the past decade, and its strong covalent bond connection gives the material a high specific surface area and high stability. Besides that, the high carbon content of these materials also makes them very suitable as carbon precursors. Gu and co-workers had synthesized some kinds of hierarchical porous carbon with the high surface area (1348–1824 m2 g−1) through chemical activation method [16]. Hierarchical porous carbons exhibit superior performances in catalytic applications because they can retain nanoparticles in the substrate to avoid aggregation and make NPs reused without significantly reducing of catalytic activity [14,17]. In addition, heteroatom doped, particularly N-doped, the porous carbons, which supply many active N sites to enhance electron density, promote the dispersion of metal NPs, making the N-doped porous carbons wonderful substrate to anchor metal NPs, with stability, small size and high dispersion [18,19]. 4-Nitrophenol (4-NP), derived from the industrial manufacturing processes of petrochemicals and pharmaceuticals, is identified as one of the most hazardous and toxic pollutants [20,21]. There are various techniques utilized to remove 4-NP from wastewater and catalytic reduction has been proven to be an efficient and cost-effective method [22]. More importantly, the reduction product 4-aminophenol (4-AP) is an extremely useful organic compound for the manufacture of analgesic and antipyretic drugs. For instance, Li et al. reported that hypercrosslinked β-cyclodextrin porous polymer/gold nanoparticle composites could serve as an effective catalyst to activate the reduction of 4NP [13]. Dong et al. successfully fabricated two novel catalysts based on Au and Pd nanoparticles supported on magnetic porous carbon (MPC) from metal organic framework with high catalytic activity for 4NP reduction. The reaction rate constant was 1.0 × 10−2 s−1 and 1.2 × 10−2 s−1 for the reaction catalyzed using Au/MPC and Pd/MPC nanocatalysts, respectively [14]. Recently, reduced graphene oxide supported Ag nanoparticle (rGO-Ag-U) also showed a high catalytic activity toward the 4-NP reduction with reaction rate constant of 2.0 × 10−2 s−1 [23]. Despite of the effort made in recent years for nanocatalyst preparation, the study of the porous organic polymer derived carbon as a carrier for the growth of metal NPs is little. Here, we reported the easy synthesis of the hierarchical porous carbon derived from CCTPB (Scheme 1) by chemical activation. More specifically, the CCTPB with abundant micropores and N element was synthesized by Friedel-Crafts reaction. After that, the CCTPB was chemically activated via a common
2. Experimental sections 2.1. Materials All regents and solvents were purchased from commercial suppliers and used without further purification, unless otherwise noted. N, N, N′, N′-tetraphenylbenzidine (TPB), cyanuric chloride (CC), o-dichlorobenzene, silver nitrate solution (AgNO3, 0.1 M), sodium borohydride (NaBH4), methane-sulfonic acid and 4-nitrophenol (4-NP) were obtained from Aladdin Reagent Limited Company (Shanghai, China). KOH was supplied by Shanghai Lingfeng Chemical Reagent Co., Ltd.
2.2. Synthesis of CCTPB CCTPB was synthesized using the procedure described by Xiong et al. [24] Briefly, methanesulfonic acid (70.0 mmol, 6.73 g) was added to a solution of N, N, N′, N′-tetraphenylbenzidine (3.75 mmol, 1.83 g) and cyanuric chloride (5 mmol, 0.92 g) in o-dichlorobenzene (100 mL). The mixture was refluxed at 140 °C for 24 h. After the reaction was complete, the solid product was isolated by filtration, washed with methanol, tetrahydrofuran, and acetone. Further purification was done by soxhlet extraction with methanol. The product was dried under vacuum at 60 °C for 24 h to afford CCTPB as a yellowy brown colored solid (Yield: 90%). Elemental analysis calcd. (wt%) for C20H12N3: C, 81.61; H, 4.11; N, 14.28. Found: C, 80.23; H, 3.12; N, 11.76.
2.3. Synthesis of potassium hydroxide activated carbon material CCTPB and KOH were uniformly mixed in an agate mortar at a mass ratio of CCTPB: KOH = 1:2. The mixed powder was placed in a nickel crucible and then taken into the tubular furnace. The chemical activation was executed under N2 protection at a ramp rate of 2 °C/min and held at 800 °C for 2 h. After cooling to room temperature, the product was completely washed by HCl aqueous solution (V/V = 1:1). Then the powder was rinsed with deionized water until the water became neutral. The carbonized black sample was dried under vacuum at 70 °C for 24 h and denoted as CCTPB-K.
Scheme 1. Synthesis route to the precursor of CCTPB. 2
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
methanesulfonic acid (Scheme 1) [24]. Carbonization was performed by heating the CCTPB/KOH powder mixture to the target temperature of 800 °C. We used wet-chemistry method to synthesize Ag@CCTPB and Ag@CCTPB-K with NaBH4 as reducing agent. CCTPB, CCTPB-K, Ag@CCTPB and Ag@CCTPB-K are insoluble in water and general organic solvents hexane, DMSO, Tetrahydrofuran, and DMF. FT-IR was used to determine the functional groups of the precursor CCTPB and the carbonized material. As seen in Fig. S1, compared to CC, the disappearance of C–Cl stretching vibration absorption at 850 cm−1 indicates complete conversion of C–Cl [25]. Further evidence for the formation of the target framework is given by the fierce bands at 1506 (in plane deformation vibration of triazine ring) and 822 cm−1 (out-ofplane deformation vibration) [24]. A new band of bending vibration of C–H in 1,4-disubstitued benzene at 810 cm−1 appearances. The structure of the polymer was also offered by 13C CP-MAS NMR spectra (Fig. S2). The obvious signals at 171 ppm could be corresponded to the sp2 carbons of the triazine ring [26]. The primary signals at around 147, 135 and 127 ppm should be contributed to the substituted aromatic carbon bonded to nitrogen atom and substituted and un-substituted aromatic carbon of the aromatic ring connected with the benzene ring, respectively. Fig. S3a is the FT-IR spectrum for the sample CCTPB-K showing that the intense peak at 1088 cm−1 is corresponded to CO–O–C stretching vibrations, which is different from CCTPB. The peak around 1400 cm−1 may be the -COO- stretching vibrations [16]. Oxygen is produced by KOH etching the polymer skeleton during activation. Similar peaks can also be found in other KOH activated MOPs [16]. X-ray photoelectron spectra (XPS) (Figs. S3b–d) was also carried out to analyze the chemical structure of the hierarchical porous carbon. The carbon atoms in four different environments were found in the C 1s spectrum of CCTPB-K (Fig. S3c): C]C (284.8 eV), C−O (285.9 eV), C] O (288.1 eV), and O−C]O (289.2 eV). In addition, the O 1s spectrum (Fig. S3d) shows the main signals of C]O, C−O, and C−OH. The chemical structure of CCTPB-K is completely different from that of CCTPB, indicating the structural rearrangement for the productive hierarchical porous carbons occurred during the pyrolysis process. No significant diffraction peaks were observed in the powder X-ray diffraction (PXRD) patterns of CCTPB and CCTPB-K (Fig. S4), indicating their amorphous nature. The PXRD pattern (Fig. 1) of Ag@CCTPB and Ag@CCTPB-K demonstrates the formation of the face-centered cubic structure of Ag nanoparticles. The diffraction peaks at 38.17°, 44.25°, 64.45° and 77.49° are attributed to the (111), (200), (220) and (311) crystal faces of the Ag nanoparticles, respectively, which demonstrate successful loading of Ag NPs on CCTPB and CCTPB-K [27]. The valence state of the Ag nanoparticles was analyzed by X-ray photoelectron spectra (XPS) (Fig. 2). For Ag@CCTPB and Ag@CCTPB-K, the Ag 3d spectra show peaks at 368.3 eV and 374.3 eV, which are assigned to core-level Ag 3d5/2 and Ag 3d3/2 signals of zero-valence metallic Ag, respectively (Fig. 2b and d) [20]. Besides, the Ag MNN Auger spectrum
2.4. Synthesis of Ag@CCTPB, Ag@CCTPB-K and Ag NPs The catalyst Ag@CCTPB or Ag@CCTPB-K was fabricated using AgNO3 as the silver source and CCTPB or CCTPB-K as the supporting substrate. In a typical procedure, 3 mL of AgNO3 solution (10 mM) was added dropwise to a well-dispersed suspension of as-prepared CCTPB or CCTPB-K (30 mg) in deionized water (60 mL). The mixture stirred for 1 h to deposit metal precursor in polymer support. Subsequently, freshly prepared NaBH4 solution (0.01 M, 10 mL) was added dropwise and the mixture was stirred for 2 h. Thus, the Ag nanoparticles were formed and anchored on the wall of porous channels. Then, the product was collected through centrifugation, washed with deionized water several times, and dried in a vacuum at 50 °C overnight. For comparison, Ag NPs without the porous support were also prepared by the similar procedure. 2.5. Common procedure of the reduction of 4-nitrophenol First, freshly prepared aqueous NaBH4 solution (0.5 M, 0.15 mL) was added into 3 mL of 4-nitrophenol solution (1 × 10−4 M), which resulted in the formation of deep yellow solution. Then, a suspension of catalyst in water (0.5 mg mL−1, 50 or 150 or 300 μL) was added into the yellow solution and the reaction was monitored by the UV–vis spectroscopy. The effect of the pH value of reaction mixture and the temperature on the catalytic reduction was also taken into account. For the recycle test, after each catalytic reaction, the catalyst was collected by centrifugation, washed thoroughly with deionized water, and dried in a vacuum at 50 °C overnight for the next catalytic cycle. 2.6. Characterization Fourier transform infrared (FTIR) spectra were collected on KBr pellets using a Thermo Nicolet Nexus 670 FTIR spectrometer. Solidstate cross polarization magic angle spinning (CP/MAS) NMR spectrum was recorded on a Bruker Avance III 600 spectrometer. Elemental compositions (C, H, and N) were determined using an elemental analyzer (Vario EL cube, Germany). Morphology and energy dispersive Xray spectroscopy (EDS) were carried out on a scanning electron microscopy (SEM) (JEOL JSM-6510, Japan) and TEM (JEOL JEM2100Plus, Japan). Thermogravimetric analysis (TGA) were performed on a Netzsch STA449 F3 Jupiter thermal analyzer in the temperature range of 40–800 °C at a heating rate of 10 °C min−1 under an atmosphere of nitrogen. The powder X-ray diffraction (PXRD) patterns of the samples were recorded with SmartLab TM 3 kW (Rigaku Corporation, Japan). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Fisher ESCALAB 250Xi with Al Kα radiation. The kinetic energy (KE) in the Ag MNN region of the Auger transitions was measured and the modified Auger parameter (α′) was used to characterize the chemical state of silver. In order to determine pore textural properties including the specific Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size distribution, nitrogen adsorption/desorption isotherm on the samples at 77.3 K were measured in a Micromeritics ASAP 2020 M surface area and porosity analyzer (Micromeritics, USA). Prior to the measurements, samples (0.2 g) in the analysis chamber were subject to a vacuum of 10−5 bar at 150 °C for 12 h. For Ag content analysis, the sample was measured using a Solarix 70-FT-MS high resolution mass spectrometer (ICP-MS). UV–vis spectra were measured on a Puxi TU-1810 spectrophotometer. 3. Results and discussion 3.1. Structural and morphological characterization The precursor, a nitrogen-rich porous organic polymer (CCTPB), was synthesized by simple Friedel-Crafts reaction of N, N, N′, N′-tetraphenylbenzidine with cyanuric chloride under the catalysis of
Fig. 1. PXRD patterns of the Ag@CCTPB and Ag@CCTPB-K nanocomposites. 3
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
Fig. 2. XPS spectra Ag@CCTPB and Ag@CCTPB-K; (a, c) survey scan, (b, d) Ag 3d high-resolution, and (e, f) Ag MNN Auger region.
with each other during the activation and show a fluffy and porous morphology. The typical transmission electron microscope (TEM) images of Ag@CCTPB and Ag@CCTPB-K (Fig. 3a and b) indicate that the Ag NPs were uniformly dispersed in the CCTPB and CCTPB-K. The statistic histogram of randomly selected particles (Fig. 3c and d) shows that the average size of Ag NPs in CCTPB is 5.35 nm and the average size in CCTPB-K is 3.91 nm, respectively. Thermogravimetric analysis (TGA) was performed in continuous flow of nitrogen to estimate the stability of our materials (Fig. S9 and Fig. S10). CCTPB shows about 10% mass loss at 277 °C, in contrast to CCTPB-K at 391 °C, suggesting carbonized structures are more robust than the polymer. Nitrogen sorption experiments were performed at 77.3 K to examine surface area and pore size distribution of CCTPB, CCTPB-K, Ag@CCTPB and Ag@CCTPB-K. As shown in Fig. 4a, CCTPB, CCTPB-K, Ag@CCTPB and Ag@CCTPB-K show reversible type I and type IV sorption
of both composites was also measured and the modified Auger parameter (α′) was calculated. The Auger spectra (Fig. 2e–f) presents a distinct peak at 358 eV in the MNN region, with α′ value of 726.3 eV, corresponding to metallic silver [28,29]. All these results confirm that Ag nanoparticles have been successfully grown on CCTPB and CCTPBK. The inductively coupled plasma (ICP) analysis shows that the Ag content in the Ag@CCTPB and Ag@CCTPB-K is 9.15 wt% and 9.23 wt %, respectively, which is in good agreement with our theoretical amount of Ag (9.73 wt%). The composition of Ag@CCTPB and Ag@CCTPB-K were further analyzed by energy dispersive spectrum (EDS). As shown in Fig. S5 and Fig. S6, the peaks corresponding to Ag, C, O and N revealing the compositions of Ag@CCTPB and Ag@CCTPBK. SEM images (Figs. S7 and S8) display that CCTPB consisted of irregular small particles, whereas for CCTPB-K these small particles fused 4
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
Fig. 3. TEM images of Ag nanoparticles on the CCTPB/CCTPB-K (a-b) and the statistic histogram of the Ag nanoparticles size distribution (c-d).
the presence of the abundant micropores (Fig. 4b). The main pore sizes of CCTPB are at 0.68 and 1.21 nm, respectively (Table 1). After carbonization, CCTPB-K shows a wider pore size distribution. Besides the smaller pore sizes at 0.63 and 1.12 nm, some new mesopores centered at 1.39 and 3.81 nm are also observed, which could be produced from the hard template effect of the adsorbed KOH (Fig. 4b). Similar
isotherms. The adsorption isotherm for CCTPB shows a steep gas uptake at low relative pressure and a flat course in the intermediate section, suggesting the microporous nature of the polymer network. The Brunauer-Emmett-Teller (BET) surface area is obtained to be 821 m2 g−1 and the total pore volume is 0.49 cm3 g−1 (Table 1). The pore size distribution (PSD) determined from the NLDFT method also confirms
Fig. 4. (a) N2 adsorption (solid symbols)/desorption (open symbols) isotherms of CCTPB, CCTPB-K, Ag@CCTPB and Ag@CCTPB-K at 77 K and (b) NLDFT pore size distribution of the polymer and carbon material. 5
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
Table 1 Pore parameters of the polymer, the carbon and the catalysts. Samples
SBETa (m2 g−1)
SLangb (m2 g−1)
VTotalc (cm3 g−1)
VMicrod (cm3 g−1)
Dominant pore sizee (nm)
CCTPB CCTPB-K Ag@CCTPB Ag@CCTPB-K
821 2585 702 2050
916 3125 794 2412
0.49 1.39 0.37 1.11
0.35 0.98 0.29 0.79
0.68, 0.63, 0.51, 0.73,
a b c d e
1.21 1.12, 1.39, 3.81 1.29 1.12, 1.49, 3.54
Surface area was calculated from the nitrogen adsorption branch according to the BET model. Surface area was calculated from the nitrogen adsorption branch based on the Langmuir model. The total pore volume was calculated from the single point nitrogen uptake at P/P0 = 0.99. The micropore volume was estimated by the t-plot method. Pore size derived from N2 isotherm with the NLDFT approach.
Ag@CCTPB-K, the TOFs are estimated to be 2.67 × 10−6 and 1.90 × 10−5 mol g−1 s−1 respectively. The catalytic activities of Ag@CCTPB and Ag@CCTPB-K were also compared with some reported work on the selective reduction of 4-NP to 4-AP by metal catalysts [20,27,30,31]. As shown in Table S1, the Ag@CCTPB-K exhibit relatively faster catalytic efficiency, due to the combined effects of large surfaces and hierarchical porous structures of the particular support. In addition, we investigated the influence of the amount of catalyst and the temperature on the catalytic reduction of 4-NP. As shown in Figs. S12 and S13, the reduction rate increases with the increase of the catalyst dosage and temperature. This is due to the fact that the higher temperature will promote the diffusion of the reactant through the porous carbon and its arrival at Ag NPs surface [32]. The reduction reaction was also performed under different pH values by keeping the other variables constant. The linear relationship of ln (Ct/C0) on reaction time indicates pseudo first-order kinetics (Fig. S14). With the increases of pH of medium from 4 to 10, the values of k increase according. This can be explained by the high electron density over the surface of Ag NPs at alkaline medium. High electron density of Ag NPs will induce more 4-NP molecules to be attached by electrostatic force, which will enhance the reduction rate. Similar phenomenon was also be reported by Begum et al. [33]. Cycle stability and reusability are important parameters for evaluating the performance of heterogeneous catalysts. In order to perform the stability tests, both Ag@CCTPB and Ag@CCTPB-K were applied to five cycle under the same reaction conditions. The catalyst was simply isolated by centrifugation, washed with deionized water and dried overnight after each use. As shown in Fig. S15, the conversion of 4nitrophenol in each reaction cycle exceeded 90%, indicating that the catalyst is stable under mild reaction conditions without no significant loss of activity. After reaction, the catalyst can be easily recycled through centrifugation or natural settling. In addition, the structural stability of the catalysts after the catalytic reactions were also checked by PXRD. As shown in Fig. S16, the PXRD patterns of Ag@CCTPB and Ag@CCTPB-K after five cycles also exhibit characteristic Ag NP diffraction peaks (2θ = 38.17°, 44.25°, 64.45° and 77.49°) corresponding to (111), (200), (220) and (311) reflections, respectively, which further confirms the excellent stability of Ag@CCTPB and Ag@CCTPB-K catalyst.
phenomenon was also observed in other KOH-activated carbonized materials [16]. The BET model was applied to the data for CCTPB-K using the N2 adsorption branch to provide surface area of 2585 m2 g−1 (Table 1). After loading Ag nanoparticles, the BET surface area is 702 and 2050 m2 g−1 for Ag@CCTPB and Ag@CCTPB-K, respectively. The appreciable decrease in both the N2 adsorption and BET surface area of Ag@CCTPB and Ag@CCTPB-K in comparison to CCTPB and CCTPB-K indicates that the pore surface in the carrier is occupied by finely dispersed Ag NPs. A NLDFT pore size distribution analysis based on the N2 isotherms at 77.3 K indicated that the pore size of Ag@CCTPB is distributed around 0.5–3.2 nm, and that of Ag@CCTPB-K is distributed around 0.6–5.4 nm (Fig. 4b). The apparent hysteresis loop over the entire relative pressure range of Ag@CCTPB-K indicates the relatively strong gas bond on the pore surface, which is consistent with the coexistence of micropores and mesopores in the framework. 3.2. Catalytic properties In order to determine the catalytic activities of the Ag@CCTPB and Ag@CCTPB-K nanocomposites, the selective reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) is chosen as a model reaction. It is well known that 4-aminophenol (4-AP) is an important chemical and pharmaceutical intermediate body in many applications, including analgesic and antipyretic drugs, photographic developers, corrosion inhibitors and anticorrosion lubricants [30]. As a comparative experiment, we tested the catalytic activity of CCTPB and CCTPB-K (0.045 mg mL−1, 150 μL) for the reduction of 4-nitrophenol. The peak of the mixture of 4nitrophenol and NaBH4 at 400 nm remained almost unchanged after 30 min (Fig. S11), indicating that CCTPB or CCTPB-K alone had no catalytic activity. In contrast, when the Ag@CCTPB and Ag@CCTPB-K catalysts (0.5 mg mL−1, 150 μL) were added to the solution, the reduction was started immediately, showing that the 4-nitrophenol peak rapidly decreased at 400 nm. Meanwhile, a new peak of 4-aminophenol appeared at 300 nm (Fig. 5a, c) [31]. The solution was completely discolored from bright yellow to colorless at a rate visible to the naked eye. (Fig. 5f). When the same amount of Ag NPs without carrier were used as the catalysts for the reduction of 4-nitrophenol (Fig. 5e), a longer reaction time (about 60 min) was required to reach the same conversion, which may be due to their ease of aggregation. As shown in Fig. 5b, d, the reaction conversion was calculated from Ct/C0, where Ct and C0 were the 4-NP concentrations at time t and 0, respectively [18]. Due to the excessive concentration of NaBH4, this reduction reaction can be considered as a pseudo first order reaction with respect to the 4-NP solution. Fig. 5b and d shows linear relationship between ln (Ct/C0) and t, which confirms the reduction as firstorder. The rate constants (k) are calculated from the slope to be 1.28 × 10−1 min−1 and 8.7 × 10−1 min−1 for the Ag@CCTPB and Ag@CCTPB-K, respectively. The turnover number (TON) and the turnover frequency (TOF) of the catalyst are two important parameters commonly used to compare catalyst efficiency. In heterogeneous catalysis, TON is the number of reactant molecules that can be converted by 1 g of catalyst, and the TOF value is TON/time. For the Ag@CCTPB and
4. Conclusions In conclusion, we synthesized facile chemical activation of the single microporous organic polymer (CCTPB) produced a porous carbon with hierarchical meso-microporous structure and high BET surface area (2585 m2 g−1). The carbon material was used for the growth of Ag nanoparticles (NPs). The results showed that the Ag nanoparticles are uniformly distributed in CCTPB-K without aggregation. Due to the coordinated interaction between the porous structure and the nitrogen atom, the Ag nanoparticles can be immobilized, and the catalyst had excellent catalytic activity, and there was no significant loss of activity after repeated use for 5 times. Therefore, we believe that this nitrogen6
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
Fig. 5. Time-dependent UV–vis spectra of the reduction of 4-nitrophenol (0.3 μmol) catalyzed by Ag@CCTPB (a), Ag@CCTPB-K (c), and Ag NPs (e); the pseudo-firstorder kinetic plots of Ag@CCTPB (b) and Ag@CCTPB-K (d); and Ultraviolet–visible spectra of 4-nitrophenol before and after catalytic reduction by the catalyst (f).
rich hierarchical porous carbon will provide worthy ideas for immobilizing a variety of noble metal NPs for diverse practical applications.
Appendix A. Supplementary data
Acknowledgements
References
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.micromeso.2019.109672.
This work was supported by the National Natural Science Foundation of China (no. 51403100), the Natural Science Foundation of Jiangsu Province (no. BK20140949), and the Jiangsu Overseas Visiting Scholar Program for University Prominent Young & Middle-aged Teachers, China and Presidents and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, China.
[1] D. Laprune, A. Tuel, D. Farrusseng, F.C. Meunier, Appl. Catal. Gen. 535 (2017) 69–76. [2] S. Siwal, S. Matseke, S. Mpelane, N. Hooda, D. Nandi, K. Mallick, Int. J. Hydrogen Energy 42 (2017) 23599–23605. [3] Q. Deng, Y. Shen, H. Zhu, T. Tu, Chem. Commun. 53 (2017) 13063–13066. [4] M.Q. Yang, X.Y. Pan, N. Zhang, Y.J. Xu, CrystEngComm 15 (2013) 6819–6828. [5] L. Shang, Y. Liang, M. Li, G.I.N. Waterhouse, P. Tang, D. Ma, L.-Z. Wu, C.-H. Tung, T. Zhang, Adv. Funct. Mater. 27 (2017) 1606215. [6] W. Li, X. Ge, H. Zhang, Q. Ding, H. Ding, Y. Zhang, G. Wang, H. Zhang, H. Zhao,
7
Microporous and Mesoporous Materials 290 (2019) 109672
Q. Wang, et al.
[20] G. Xiao, Y. Zhao, L. Li, J.O. Pratt, H. Su, T. Tan, Nanotechnology 29 (2018) 155601. [21] W.L. Shen, Y.Y. Qu, X.F. Pei, S.Z. Li, S.N. You, J.W. Wang, Z.J. Zhang, J.T. Zhou, J. Hazard Mater. 321 (2017) 299–306. [22] A.T.E. Vilian, S.R. Choe, K. Giribabu, S.C. Jang, C. Roh, Y.S. Huh, Y.K. Han, J. Hazard Mater. 333 (2017) 54–62. [23] Z. Mohammadi, M.H. Entezari, Ultrason. Sonochem. 44 (2018) 1–13. [24] S. Xiong, X. Fu, L. Xiang, G. Yu, J. Guan, Z. Wang, Y. Du, X. Xiong, C. Pan, Polym. Chem. 5 (2014) 3424–3431. [25] T. Geng, Z. Zhu, W. Zhang, Y. Wang, J. Mater. Chem. 5 (2017) 7612–7617. [26] Y. Luo, J. Liu, Y. Liu, Y. Lyu, J. Polym. Sci. A Polym. Chem. 55 (2017) 2594–2600. [27] J. Wang, X. Yang, A. Li, X. Cai, Mater. Lett. 220 (2018) 24–27. [28] S.G. Aspromonte, R.M. Serra, E.E. Miró, A.V. Boix, Appl. Catal. Gen. 407 (2011) 134–144. [29] S.G. Aspromonte, M.D. Mizrahi, F.A. Schneeberger, J.M.R. López, A.V. Boix, J. Phys. Chem. C 117 (2013) 25433–25442. [30] N. Arora, A. Mehta, A. Mishra, S. Basu, Appl. Clay Sci. 151 (2018) 1–9. [31] N. Li, Z. Ji, L. Chen, X. Shen, Y. Zhang, S. Cheng, H. Zhou, Appl. Surf. Sci. 462 (2018) 1–7. [32] X. Chen, Z. Wang, S. Bi, K. Li, R. Du, C. Wu, L. Chen, Chem. Eng. J. 295 (2016) 518–529. [33] R. Begum, K. Naseem, E. Ahmed, A. Sharif, Z.H. Farooqi, Colloid. Surf. Physicochem. Eng. Asp. 511 (2016) 17–26.
Inorg. Chem. Front. 3 (2016) 663–670. [7] D. Xu, Y. Pan, L. Zhu, Y. Yusran, D. Zhang, Q. Fang, M. Xue, S. Qiu, CrystEngComm 19 (2017) 6612–6619. [8] X.-Q. Wu, J. Zhao, Y.-P. Wu, W.-w. Dong, D.-S. Li, J.-R. Li, Q. Zhang, ACS Appl. Mater. Interfaces 10 (2018) 12740–12749. [9] S.S. Beigbaghlou, R.J. Kalbasi, K. Marjani, A. Habibi, Catal. Lett. 148 (2018) 2446–2458. [10] W. Jiang, L. Dong, H. Li, H. Jia, L. Zhu, W. Zhu, H. Li, J. Mol. Liq. 274 (2019) 293–299. [11] N. Tsumori, L. Chen, Q. Wang, Q.-L. Zhu, M. Kitta, Q. Xu, Chem 4 (2018) 845–856. [12] S. Lu, Y. Hu, S. Wan, R. McCaffrey, Y. Jin, H. Gu, W. Zhang, J. Am. Chem. Soc. 139 (2017) 17082–17088. [13] H. Li, B. Meng, S.-H. Chai, H. Liu, S. Dai, Chem. Sci. 7 (2016) 905–909. [14] Z. Dong, X. Le, Y. Liu, C. Dong, J. Ma, J. Mater. Chem. 2 (2014) 18775–18785. [15] L. Oar-Arteta, T. Wezendonk, X. Sun, F. Kapteijn, J. Gascon, Mater. Chem. Front. 1 (2017) 1709–1745. [16] S. Gu, J. He, Y. Zhu, Z. Wang, D. Chen, G. Yu, C. Pan, J. Guan, K. Tao, ACS Appl. Mater. Interfaces 8 (2016) 18383–18392. [17] S. Wang, S. Gao, Y. Tang, L. Wang, D. Jia, L. Liu, J. Solid State Chem. 260 (2018) 117–123. [18] X.X. Wu, H. Zhou, New J. Chem. 41 (2017) 10245–10250. [19] Z.-D. Ding, Y.-X. Wang, S.-F. Xi, Y. Li, Z. Li, X. Ren, Z.-G. Gu, Chem. Eur J. 22 (2016) 17027–17034.
8