Ultrafine Ag Nanoparticles Encapsulated by Covalent Triazine

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Energy, Environmental, and Catalysis Applications

Ultrafine Ag Nanoparticles Encapsulated by Covalent Triazine Framework Nanosheets for CO2 Conversion Xing-Wang Lan, Cheng Du, Lili Cao, Tiantian She, Yiming Li, and Guoyi Bai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14743 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 21, 2018

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ACS Applied Materials & Interfaces

Ultrafine

Ag

Nanoparticles

Encapsulated

by

Covalent Triazine Framework Nanosheets for CO2 Conversion Xingwang Lan,* † ‡ Cheng Du,† Lili Cao,† Tiantian She,† Yiming Li,† and Guoyi Bai*† †Key

Laboratory of Chemical Biology of Hebei Province, College of Chemistry and

Environmental Science, Hebei University, Baoding, Hebei, 071002, P.R.China. ‡Key

Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical

Engineering and Technology Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University Weijin Road 92, Tianjin, 300072, P.R.China. KEYWORDS covalent triazine framework nanosheets; ultrasonic exfoliation; silver nanoparticles; CO2 conversion; catalysis

ABSTRACT. This paper describes the fabrication of covalent triazine framework nanosheets encapsulated Ag nanoparticles (Ag0@CTFN) via a simple combination of ultrasonic exfoliation and solution infiltration method. The as-prepared Ag0@CTFN displays an order layered-sheet structure with abundant micropores and mesopores, while ultrafine Ag nanoparticles are confined and stabilized in their interlayers through the interaction between N sites of triazine units and Ag nanoparticles. Considering that the Ag0@CTFN possesses the merits of high

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nitrogen, low density, and abundant basic sites, it was thus believed to have enough abilities to adsorb and activate CO2 in the CO2 conversion and catalysis. Importantly, the Ag0@CTFN, as a heterogeneous catalyst, showed highly catalytic activity in the carboxylation of various alkynes with CO2 at ambient pressure and low temperature. This catalyst also exhibited good functionalgroup tolerance and excellent stability without any significant loss of its activity after six recycles. This work not only achieves valuable and novel composite material but also provides the first application of covalent triazine framework nanosheets in chemical conversion of CO2, opening a new field in preparing recyclable heterogeneous catalysts to accelerate the utilization of CO2.

INTRODUCTION Carbon dioxide (CO2) is an abundant, safe, low-cost and renewable C1 resource in organic synthesis.1-4 The conversion of CO2 into value-added chemicals has been spurred by emerging environmental and energy issues in recent years. Great efforts have been made to develop chemical approaches for the utilization of CO2.5-7 In particular, the direct carboxylation of terminal alkynes with CO2 as a C1 building block to produce alkynyl carboxylic acids, valuable synthetic intermediates as well as privilege motifs in organic and medical chemistry, has attracted special interests. Various homogeneous catalytic systems have been developed for this attractive reaction.8-10 However, either troublesome recycling or harsh reaction conditions are involved in those homogeneous catalytic systems, which severely impeded their further applications. Alternatively, heterogeneous catalysts are more attractive for the conversion of CO2 owing to its easy-separated and recyclable nature.11 Up to now, there are only a few heterogeneous catalysts described for this transformation.12-15 For example, Ma and Cheng et


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al.13 reported that Ag@MIL-101 catalyst gave a high yield of alkynyl carboxylic acids at ambient pressure. Very recently, Yang and Huang et al.15 developed a Schiff base modified silver catalyst by an in-situ reduction method for the direct carboxylation of terminal alkynes with CO2. Although several elegant catalytic systems have been established, most of these catalysts still suffer from more or less limitations to some extent. Thus, design and synthesis of a robust and efficient heterogeneous catalyst for the direct carboxylation of terminal alkynes with CO2 is still of great importance. Covalent organic frameworks (COFs) have currently attracted enormous interest

16,17

due to

their unique features such as low density, permanent micro- or mesopores and structural diversity. These merits of COFs endow them with superior potential in gas storage and separation, electrochemical and clean energy, catalysis, and chromatography separation.18-30 In particular, the COFs can serve as an excellent platform for CO2 capture and conversion.31-34 Although metal-organic frameworks (MOFs) reported have also shown dominant performances in CO2 capture and conversion, the COFs possess their all the characteristics as well as higher CO2 capture capacity and better stability especial in organic and aqueous medium. Covalent triazine frameworks (CTFs), as one kind of unique COFs, exhibit high nitrogen content, high thermal and chemical stability, and large structure design variety.35-38 These characteristics facilitated them to be promising alternatives because unique framework structures and triazine units can provide chemospecific tailoring of the adsorptive surfaces.39 It is thus believed that the CTFs can promote the conversion of CO2, but exploring COFs/CTFs‐catalyzed the conversion of CO2 is still in infant. This can be due to the fact that CTFs itself have no enough catalytic activity for inert CO2 in spite of its high capture capacity. To enhance its catalytic performance,


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constructing hybrid material by introducing metal active species into CTFs probably is an ideal strategy for CO2 catalysis.40-42 Silver (Ag) nanoparticles loaded on supports as heterogeneous catalysts have shown prominent advantages in catalysis.43-45 Although supported Ag catalysts have been developed, the unstable supports still limited their applications. On the other hand, Ag nanoparticles easily aggregated during catalytic processes due to their high surface energy, decreasing their active sites and surface area, which might result in reducing catalytic activity and recycling ability.46 Therefore, controllable preparation of Ag nanoparticles with a small size range on a stable support is very challenging. To solve these problems, we herein decide to apply CTF nanosheets (CTFN) to support Ag nanoparticles (Ag0@CTFN), which can serve as a template to direct the fabrication of nanoparticles with restricted size and to stabilize nanoparticles without aggregating on their surface due to their interaction between N sites of triazine units and Ag nanoparticles. The unique functionality of CTFN and Ag nanoparticles can be logically integrated into one hybrid material, thereby endowing the catalyst with high CO2 adsorption and catalysis capacity. As expected, the as-prepared Ag0@CTFN catalyst shows excellent catalytic activity and stability in the conversion of CO2 into alkynyl carboxylic acid under mild conditions. It is worthy of note that CTF nanosheets are first employed as support to confine and stabilize ultrafine Ag nanoparticles, and the metal-functionalized covalent triazine frameworks Ag0@CTFN hybrid catalyst is first applied into the conversion of CO2. EXPERIMENTAL SECTION Materials. Piperazine, cyanuric chloride, alkyne compounds, silver nitrate (AgNO3), cesium carbonate (Cs2CO3) were purchased from J&K Scientific LTD. All the organic solvents were obtained from local chemical suppliers. 1,4-Dioxane was redistilled over calcium hydride, and


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other solvents were directly used without further purification. Deuterated solvents for NMR measurement were purchased from J&K Scientific LTD. Synthesis of CTF. To a mixture of piperazine (0.516 g, 6 mmol) and anhydrous K2CO3 (1.656 g, 12 mmol) in anhydrous 1,4-dioxane (50 mL), anhydrous 1,4-dioxane (20 mL) solution of cyanuric chloride (0.732 g, 4 mmol) was introduced dropwise with cotton-like precipitate appearing continually, and then was stirred for 2 h at room temperature. Following that, the reaction mixture was heated at 90 °C for 36 h, yielding an off-white precipitate, which was isolated by vacuum filtration, washed with a large amount of dichloromethane, ethanol, and deionized water orderly to eliminate the unreacted piperazine and cyanuric chloride. The product was finally dried under oven at 70 °C for 12 h to afford CTF. Preparation of Ag0@CTFN. First, the bulk CTF (0.2 g) was exfoliated by ultrasound irradiation in 10 mL of acetonitrile for 1 h, yielding the desired CTFN. After that, AgNO3 (20 mg) was directly added in the resultant solution of CTFN without any pretreatment, and the mixture was continuously stirred for 4 h at room temperature. The solid was separated by centrifugation, washed by acetonitrile for three times, activated at 100 oC under vacuum. Subsequently, the obtained solid was mixed with a solution of NaBH4 (50 mg) in ethanol (4 mL), and the mixture was constantly vigorous stirred at room temperature for an additional 4 h. The solid was isolated by centrifugation and washed by ethanol for three times. The as-prepared sample was then dried at 70 °C for 12 h under vacuum. Finally, the yellow solid Ag0@CTFN was obtained and preserved in Ar atmosphere for further use. General procedure for carboxylation of terminal alkynes with CO2. Typically, the indicated amount of Ag0@CTFN catalyst was added into a mixture of terminal alkynes (0.051 g, 0.5 mmol), Cs2CO3 (0.326 g, 1.0 mmol) in DMSO (3 mL) in the reaction flask (10 mL). The flask


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was excluded and charged with CO2 three times for gas exchanging process, and then the flask was attached to a CO2 balloon (99.99%). Following that, the reaction mixture was stirred at 60 °C for 24 h. Upon completion, the flask was cooled to room temperature, diluted with H2O (10 mL) and the catalyst was separated by centrifugation. Then, the mixture was washed with CH2Cl2 and the aqueous layer was acidified with concentrated HCl to pH = 1. The acidified solution was then extracted with ethyl acetate (3×30 mL). The obtained organic layer was washed with cooled saturated NaCl solution, dried over anhydrous Na2SO4, filtered, and the organic phase was removed in vacuo to afford the desired products. Procedure for the recycle of the catalyst. After the reaction, the Ag0@CTFN catalyst was separated and recovered from the organic phase by centrifugation, washed with ethanol three times, dried under vacuum at 70 °C. And then the recovered solid was directly used as the recycled catalyst in subsequent runs. Catalyst characterization. Fourier transform infrared (FT-IR) spectra were collected on a Nicolet iS10 spectrometer (Thermo Fisher Scientific Co. LTD, US) equipped with a DTGS room temperature detector and operated at above 0.4 cm-1 resolution.

13C

cross-polarization magic angle spinning (CP-MAS)

spectrum was acquired by an infinity plus 300 MHz spectrometer with a 0.7 T wide-bore magnet. The Ag loading of the Ag0@CTFN catalyst was determined by inductively coupled plasma-mass spectrometry (ICP-MS; Varian Vista MPX). X-ray diffraction (XRD) patterns were carried out on a Bruker D8-Advance X-ray diffractometer using a Cu Kα radiation with the scanning range from 0° to 80° at the rate of 6°/min. Nitrogen sorption isotherms were measured by a Autosorb-iQ-MP (Quantachrome Instruments, US) surface area and pore size analyzer. Scanning electron microscopy (SEM) was recorded on a JSM-7500 (JEOL LTD, Japan).


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Transmission electron microscopy (TEM) was performed using a Tecnai G2 F20 S-TWIN electron microscope (FEI, US) with a maximum accelerating voltage of 200 kV. The average particle sizes were calculated according to 60 particles. X-ray photoelectron spectroscopy (XPS) was obtained by a EXCALAB 250Xi (Thermo Fisher Scientific Co. LTD, US) with Mg Kα Xray source. Thermogravimetric analysis (TGA) was tested on the STA449C thermogravimetric analyzer (Netzsch, Germany) under nitrogen atmosphere. CO2-Temperature-programmed desorption (CO2-TPD) was carried out on the Micromeritics Autochem II 2920 instrument. RESULTS AND DISCUSSION The covalent triazine framework CTF was synthesized by the nucleophilic substitution of cyanuric chloride and piperazine. Under the solvothermal conditions (anhydrous 1,4-dioxane, K2CO3, 90 °C, 36 h), CTF was finally obtained as off-white powder (see Supporting Information, Figure S1). The as-synthesized CTF was characterized by FT-IR and 13C CP-MAS spectroscopy (see Supporting Information, Figure S2 and S3). The FT-IR spectrum of the obtained CTF clearly implies the almost disappearance of the C-Cl stretching vibration at 854 cm-1 of cyanuric chloride and the strong N-H stretching vibration of piperazine at 3211 cm-1. In addition, the C-H stretching vibrations for methylene of piperazine (2860 and 2921 cm-1) and CN stretching vibrations of triazine (1492 and 1366 cm-1) can be still observed.

13C

CP-MAS

NMR spectrum of the CTF shows the characteristic resonances at 45 and 164 ppm which can be assigned to piperazinyl and triazinyl.47 These reliable results indicate the formation of the CTF material. Morphological structures of the COFs generally present crystalline and amorphous form, or their mixed form,48 but it is very difficult to control their morphology in their synthesis. Particularly, the morphology of the CTFs mainly exists in the form of bulk with layered


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structures formed by stacking among sheets to generate a layered eclipsed structure. Irregular morphology is unexpected for catalysis, and it is thus desirable to optimize the morphology of the CTFs and prepare order and regular metal-CTFs hybrid nanocomposites. To address this issue, the covalent triazine framework nanosheets (CTFN),49,50 a new member of 2D materials family, may be a good choice. CTFN are generally gained by exfoliating disorder bulk CTFs materials through mechanical delamination,51 chemical exfoliation,52 and ultrasonic exfoliation.53 In the direction, the Ag0@CTFN was innovatively fabricated and the synthetic procedure was depicted in Figure 1. Owing to the resultant CTF frameworks in the form of bulks and sheets, therefore, the CTF was first exfoliated by ultrasonication to form layered CTF nanosheets. Next, silver nanoparticles were in-situ deposited on the CTFN by the solution infiltration method with the off-white sample finally turning to yellow, suggesting that Ag0@CTFN was successfully obtained. The Ag0@CTFN with the Ag loading of 1.37 wt% was determined by ICP-MS. SEM images showed that the non-exfoliated CTF presents the disorder structures containing both bulks and sheets, after supported Ag nanoparticles, whereas typical layered-sheet structures can be clearly observed from the resulting Ag0@CTFN. Importantly, the as-synthesized CTF and Ag0@CTFN showed high chemical stability in acidic (5 M HCl aqueous solution) and alkaline media (5M NaOH aqueous solution) as well as common organic solvents (DMSO, CH2Cl2, and DMF) for 7 days (Figure S4).


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Figure 1. Illustrated design and preparation procedure of Ag0@CTFN nanocomposites. The photographs and SEM images of mixed bulk and sheet-like CTF and layered-sheet Ag0@CTFN samples are inserted. Ag nanoparticles are as yellow balls in the interlayers.

The small-angle X-ray diffraction (SAXRD) patterns of CTF and Ag0@CTFN, as shown in Figure 2a, both exhibit a typical low-angle reflection characteristic peak at about 2θ = 1.8o, suggesting that CTF framework exists the ordered mesostructure, which can be still maintained even after modified with Ag nanoparticles. The wide-angle X-ray diffraction (WAXRD) pattern of Ag0@CTFN in Figure 2b shows the nearly complete disappearance of some crystal diffraction peaks of CTF in range of 2θ =10-30o, but still exists the broad signal at around 2θ = 22.3o, which can be ascribed to the order interlayer for the sheets,47 indicating that microcrystalline structure


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of CTF framework has been successfully exfoliated from the sheets. Meanwhile, the WAXRD pattern of Ag0@CTFN shows two major diffraction peaks at 2θ = 38.1o and 44.3o, corresponding to the Ag (111) and (200) reflection,45,46 respectively, demonstrating that Ag nanoparticles have been successfully immobilized on the CTFN. Particularly, the weak and broad characteristic diffraction peaks suggest that Ag nanoparticles distributed at a very small size.

Figure 2. (a) SAXRD and (b) WAXRD patterns of CTF and Ag0@CTFN.


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The nitrogen adsorption-desorption isotherms of CTF and Ag0@CTFN are shown in Figure 3a, and the pore structure parameters are listed in Table S1. Both the curves demonstrate a steep rise at a low pressure region (P/P0 < 0.02), reflecting the existence of microporous in the materials. The isotherms describe a continuous increase at a high relative pressures (0.4 < P/P0 < 1.0) with a hysteresis loop, implying its permanent mesopores nature. Their pore size distributions calculated by a nonlocal density functional theory (DFT) method in Figure 3b further demonstrate that the porosity is dominated by abundant micropores centered at ~1.1 nm with mesopores centered at around 2.9~8.1 nm. The Brunauer-Emmett-Teller (BET) surface area of CTF and Ag0@CTFN is determined to be 315 m2/g and 246 m2/g, with the total pore volume of 0.80 cm3/g and 0.66 cm3/g, respectively. These comparable results indicate that the involved Ag nanoparticles did not significantly block the pore structures. Also, TGA curves of both CTF and Ag0@CTFN show that the maximum weight loss rate appears between 393 and 520 °C (Figure S5), indicating that the Ag0@CTFN still keeps the high thermal stability of the CTF framework.


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Figure 3. (a) N2 adsorption-desorption isotherms, and (b) pore size distributions of CTF and Ag0@CTFN.

The morphology and porous structure of CTF and Ag0@CTFN were investigated by SEM and TEM analysis. As shown in Figure 4a and Figure S6a-7a, the CTF is relative random morphology with abundant micropores and mesopores due to the stacking between bulks and sheets. The bulks of Ag0@CTFN are disrupted by ultrasonication, while the layered-nanosheets with networks are largely preserved as seen from Figure 4b and Figure S6b-c, S7b, which can


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still clearly observe the existence of micropores and mesopores. The TEM and HRTEM images of the CTF (Figure 4c and Figure S7c) further reveal that the porous structures were composed of micropores and mesopores with microcrystalline structure. In the TEM image of the Ag0@CTFN, as shown in Figure 4d, the Ag nanoparticles can be directly observed. It is postulated that the nanoparticles are in the interlayers consisting of N sites in the triazine units due to their weak interaction. The high-magnification TEM image (Figure 4e and Figure S7d) clearly shows that Ag nanoparticles are highly dispersed among the CTFN without aggregation and abundant microporous structure still exists (see Figure 4e inset). The size distribution of Ag nanoparticles in CTFN in Figure 4f shows a narrow range with an average diameter of about 4.97 nm, demonstrating that the CTFN can well confine and stabilize the ultrafine Ag nanoparticles. As compared to the SEM and TEM images of CTF, the structure of Ag0@CTFN is well maintained after loading Ag nanoparticles, implying the good mechanical and chemical stability of Ag0@CTFN.


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Figure 4. SEM images of (a) CTF and (b) Ag0@CTFN. TEM images of (c) CTF, (d) Ag0@CTFN, and high-magnification TEM image of (e) Ag0@CTFN. The inset figure is the HRTEM for corresponding sample. The Ag nanoparticles size distribution of (f) Ag0@CTFN.

The XPS analysis elucidates the corresponding chemical bonding status of Ag0@CTFN, as shown in Figure 5. The high-resolution C 1s spectrum is deconvoluted into three different peaks at 283.6 eV for C-N=C in triazine units, 284.8 eV for C-C and C-H and 286.7 eV for C-N in piperazine units54 (Figure 5a and 5d). The high-resolution N 1s spectrum can be separated into


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two distinctive peaks at binding energies 397.3 and 398.6 eV,55 which are attributed to C-N bond in piperazine units and the characteristic pyridinic nitrogen in triazine rings (N=C), respectively (Figure 5b and 5d). In addition, the binding energies of Ag 3d3/2 and Ag 3d5/2 appear at 373.3 and 367.3 eV,56-58 respectively, and the splitting of the 3d doublet is 6.0 eV, confirming that Ag exists as a metallic state on the CTFN (Figure 5c and 5d). As compared to the binding energies of the bulk Ag (367.9 eV for 3d5/2 and 373.9 eV for 3d3/2),59 the two peaks of Ag 3d slightly shift toward the lower BE value, which may be due to the ultrafine nanoparticles and the interaction between the Ag nanoparticles and the CTFN.40-42 This strongly suggests that, from these XPS results, the structure of CTFN can be well retained after the load of Ag nanoparticles, and the CTFN plays an important role in immobilizing and confining Ag nanoparticles through the weak interaction between with N sites of triazine rings.

Figure 5. High resolution XPS spectra of (a) C 1s, (b) N 1s, and (c) Ag 3d of the Ag0@CTFN, and (d) its schematic structure.


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The CO2-TPD tests were performed to reveal the surface basicity of CTF and Ag0@CTFN samples. As shown in Figure 6, CTF sample appears two obvious desorption peaks at around 60 oC

and 200 oC, demonstrating the existence of weak and medium-strength basic sites. The higher

area of the peak at 200 oC implies that CTF possesses a large amount of medium-strength basic sites. In contrast, as for Ag0@CTFN, weak basic sites have nearly disappeared, and the number of medium basic sites has also decreased. Notably, the desorption temperature of Ag0@CTFN has an obvious shift towards lower temperature, which can be due to the fact that Ag nanoparticles supported may neutralize some basic sites. These results further suggest the interaction between Ag nanoparticles and CTFN, in accordance with the XPS results. In addition, CO2 desorption at above 300 oC for these two samples is ascribed to the thermal instability of the carbon-based materials.60

Figure 6. CO2-TPD profiles of CTF and Ag0@CTFN.


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Catalytic activity of Ag0@CTFN was tested in the carboxylation of terminal alkynes with CO2. In the initial experiments, we selected the carboxylation of 1-ethynylbenzene with CO2 into phenylpropiolic acid as the model reaction to achieve the optimal conditions (Table 1). Several Ag0@CTFN catalysts with the Ag loading of 1.18 wt%, 1.37 wt%, and 1.49 wt% (determined by ICP, and denoted as Ag0@CTFN-1, Ag0@CTFN-2 and Ag0@CTFN-3) were investigated, respectively. The reaction can also proceed without any catalyst or only with CTF, but only low yields were obtained (entries 1 and 2). All the catalysts had good catalytic effect on the reaction, in particular, Ag0@CTFN-2 and Ag0@CTFN-3 showed the comparative yield in 97% (entries 4 and 5). Considering the superior value of Ag0 : 1-ethynylbenzene, the catalyst Ag0@CTFN-2 was finally chosen as the optimal catalyst for further investigation. Subsequently, the amount of catalyst was also investigated. However, decreasing the amount of catalyst resulted in a dropped yield and increasing the amount cannot achieve a higher yield (entries 6-8). Several kinds of solvents were then scanned. It can be seen that DMF and CH3CN gave a lower yield and 1,4dioxane were not suitable for this reaction (entries 9-11), while DMSO provided the best result. Furthermore, the yield of product was sharply decreased when reaction time shortened to only 12 h (entry 12). The reaction temperatures were also monitored, while reducing temperature (25 oC) caused a remarkably decreased yield and elevating temperatures (80 oC) did not facilitate the reaction (entries 13 and 14). In addition, to compare with the Ag0@CTFN catalyst, we have prepared Ag/C and Ag/C3N4 catalyst and examined their catalytic activity for the model reaction under the same conditions. It was found that 73% and 87% isolated yields were obtained, respectively (entries 15 and 16), that is, Ag0@CTFN catalyst showed more remarkable advantage. These results further indicate that the unique structures and properties of Ag0@CTFN catalyst can efficiently promote the CO2 transformation.


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Table 1 Synthesis of 3-phenylpropiolic acid from CO2 and 1-ethynylbenzene.a catalyst

CO2

+

(balloon)

a

HCl

Cs2CO3

Ag0 : 1-ethynylbenzene

COOH

solvent

yieldb (%)

0

DMSO

49

CTF (30)

0

DMSO

54

3

Ag0@CTFN-1 (30)

1 : 152

DMSO

88

4

Ag0@CTFN-2 (30)

1 : 132

DMSO

97

5

Ag0@CTFN-3 (30)

1 : 122

DMSO

97

6

Ag0@CTFN-2 (10)

1 : 417

DMSO

87

7

Ag0@CTFN-2 (20)

1 : 200

DMSO

92

8

Ag0@CTFN-2 (40)

1 : 98

DMSO

96

9

Ag0@CTFN-2 (30)

1 : 132

DMF

55

10

Ag0@CTFN-2 (30)

1 : 132

CH3CN

49

11

Ag0@CTFN-2 (30)

1 : 132

1,4-Dioxane

8

12c

Ag0@CTFN-2 (30)

1 : 132

DMSO

79

13d

Ag0@CTFN-2 (30)

1 : 132

DMSO

73

14e

Ag0@CTFN-2 (30)

1 : 132

DMSO

95

15f

Ag/C (50)

1 : 130

DMSO

73

16g

Ag/C3N4 (30)

1 : 137

DMSO

87

entry

catalyst (mg)

1

none

2

(mmol : mmol)

Reaction condition: 1-ethynylbenzene (0.5 mmol), catalyst (Ag0@CTFN-1 = 1.18 wt%

Ag0@CTFN, Ag0@CTFN-2 = 1.37 wt% Ag0@CTFN, Ag0@CTFN-3 = 1.49 wt% Ag0@CTFN), Cs2CO3 (1.0 mmol), CO2 (balloon), 60 °C, solvent (3 mL), 24 h, unless otherwise noted. b Yield of isolated product. c 12 h. d 25 oC. e 80 oC. f 0.83 wt% Ag/C (determined by ICP). g 1.31 wt% Ag/C3N4 (determined by ICP).


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To demonstrate the versatility of this Ag0@CTFN catalyst, the carboxylation of different terminal alkynes with CO2 was investigated (Table 2). As shown in Table 2, all the electronneutral, electron-deficient and electron-rich aromatic alkynes underwent the transformation smoothly with excellent yields of isolated products (entries 1-8), indicating that the electronic effect has no significant influence on this transformation. Moreover, the alkyne with a hetero aromatic ring group (entry 9) could give a 84% yield of the promising product. Even for the long chain alkyl alkynes, the reaction could also proceed smoothly with excellent yields (entries 10 and 11). Collectively, these results demonstrated the excellent catalytic performance and functional-group tolerance of Ag0@CTFN for the carboxylation reaction under mild conditions. Table 2 Synthesis of various alkynyl acids from terminal alkynes and CO2 over [email protected]

entry

alkyne

product

yieldb(%)

1

97

2

92

3

97

4

94

5

96

6

97

7

94

8

93

9

84


19


a

Page 20 of 33

10

96

11

97

Reaction condition: alkynes (0.5 mmol), 1.37 wt% Ag0@CTFN catalyst (30 mg), Cs2CO3 (1.0

mmol), CO2 (balloon), 60 °C, DMSO (3 mL), 24 h. b Yield of isolated product. Next, the recyclability of the Ag0@CTFN catalyst was also investigated in the carboxylation of 1-ethynylbenzene with CO2 (Figure 7a). The catalyst was regenerated by centrifugation, washed with ethanol three times, dried in vacuum after each cycle. Under this process, the Ag0@CTFN catalyst could be reused at least six times without significant loss of catalytic activity, which is much better than other reported catalysts (Table S2). The TEM image of the used catalyst revealed that the sheet-like structure of CTFN was maintained and Ag nanoparticles appeared certain aggregation but most of nanoparticles still retained their sizes. It is thus demonstrated that the robust CTFN could prevent Ag nanoparticles from leaching and aggregation during the reaction, clearly implying the good stability of the Ag0@CTFN catalyst (Figure 7b).

Figure 7. (a) Reuse of Ag0@CTFN catalyst. (b) The TEM image and particles size distribution of the used Ag0@CTFN catalyst after six runs.


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Based on these results and previous reports,12,15,61,62 a plausible model of Ag0@CTFN catalyst confining Ag nanoparticles and activating CO2 was constructed, as shown in Figure 8a. The abundant triazine units embedded in the CTFN possess a large amount of basic sites, it is thus speculated that Ag nanoparticles are immobilized in the interlayers of layered-sheet CTFN through the interaction between N sites of triazine units and Ag nanoparticles. Notably, Ag atoms tend to aggregate into nanoparticles even in the assistance with CTFN due to their high surface energy, which is the reason that cannot form single atom state. Meanwhile, the abundant basic N sites can also adsorb CO2 on the surface, pore structures, and interlayers, reducing activation energy of CO2. Following the guidance of the activated model, a reaction mechanism for the carboxylation of terminal alkynes with CO2 is proposed (Figure 8b). The Ag nanoparticles on Ag0@CTFN catalyst can coordinate terminal alkynes with the Cs2CO3 to form a silveracetylide intermediate. Meanwhile, CO2 is adsorbed on the basic sites and activated into electrophilic species, subsequently attacking the silveracetylide to form the silver carboxylate. With the assistance of Cs2CO3 and alkynes, the silver carboxylate are quickly transferred to cesium carboxylate through the transmetallation. Finally, the cesium carboxylate can be acidified to generate carboxylic acid products. The in-depth investigation into the synergistic effect of Ag0@CTFN on adsorbing and activating CO2 is under way in our laboratory.


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Figure 8. (a) A plausible model of Ag0@CTFN catalyst on adsorbing and activating CO2. (b) A proposed mechanism for the carboxylation of terminal alkynes with CO2.

CONCLUSION In summary, we have developed a novel and facile strategy for the fabrication of covalent triazine framework nanosheets encapsulated Ag nanoparticles catalyst via a combination of ultrasonic exfoliation and solution infiltration method. The CTFN derived from the disorder bulks and sheet-like CTFs presented rich nitrogen content and order layered-sheet structure with abundant micropores and mesopores, and served as a platform for confining and stabilizing Ag nanoparticles through the interaction between N sites of triazine units and Ag nanoparticles. With the aid of the CTFN, ultrafine Ag nanoparticles might be effectively confined and grow in the interlayers with an average size of 4.97 nm, which can prevent the aggregation of nanoparticles, contributing to a good stability. The synthesized Ag0@CTFN catalyst showed excellent catalytic activities for the carboxylation of terminal alkynes using CO2 as building block under mild conditions with broad substrate scope and good recyclability. This work not only achieves valuable and novel composite material but also provides the first application of


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covalent triazine frameworks materials in chemical conversion of CO2, opening a new field in preparing recyclable heterogeneous catalysts to accelerate the utilization of CO2.

ASSOCIATED CONTENT Supporting Information. The FT-IR and 13C CP-MAS NMR spectroscopy of CTF material; characterization of catalyst with chemical stability, BET data, SEM, TEM, and TGA. 1H NMR and 13C NMR data and copies of all the products. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Guoyi Bai: 0000-0002-0986-6455 Xingwang Lan: 0000-0003-3178-1113 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China (21676068), Hebei Natural Science Foundation (B2018201118), Hebei Higher Colleges Science and Technology Research Program (QN2018049), and the 63rd China Postdoctoral Science Foundation (2018M631745).

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[62] Dang, Q.-Q.; Liu, C.-Y.; Wang, X.-M.; Zhang, X.-M. Novel Covalent Triazine Framework for High Performance CO2 Capture and Alkyne Carboxylation Reaction. ACS Appl. Mater. Interfaces. 2018, 10, 27972-27978.


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Table of Contents Covalent triazine framework nanosheets encapsulated Ag nanoparticles (Ag0@CTFN) fabricated by ultrasonic exfoliation display an order layered-sheet structure with abundant micropores and mesopores, while ultrafine Ag nanoparticles are confined and stabilized in their interlayers through the interaction between N sites of triazine units and Ag nanoparticles. These characteristics contribute it to adsorb and activate CO2 in the catalytic CO2 conversion.


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Ultrafine Ag Nanoparticles Encapsulated by Covalent Triazine

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