A novel crystalline azine-linked three-dimensional covalent organic framework for CO2 capture and conversion

Pengxin Guan a, Jikuan Qiu a, Yuling Zhao a, Huiyong Wang *a, Zhiyong Li a, Yunlei Shi b and Jianji Wang *a
aCollaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R. China. E-mail: Jwang@htu.cn; hywang@htu.cn
bCollege of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, P. R. China

Received 23rd July 2019 , Accepted 13th September 2019

First published on 14th September 2019


The targeted synthesis of three-dimensional covalent organic frameworks (3D COFs) is a great challenge, especially those synthesized by using a new kind of organic linkage. Herein, for the first time, a novel 3D azine-linked COF (3D-HNU5) has been synthesized and characterized. It is shown that the obtained 3D COF has a 2-fold interpenetrated diamond topology, and shows good chemical/thermal stability and a narrow pore size distribution, which exhibits excellent performance in the selective uptake of CO2 over N2. Moreover, the 3D-HNU5 is found to be an efficient catalyst for the cycloaddition of propargylic alcohols with CO2 into carbonates with excellent catalytic activity under mild conditions.


Covalent organic frameworks (COFs) represent an emerging class of crystalline materials with accurate periodicity in the skeleton and predesignable porosity.1,2 Since their first discovery by Yaghi in 2005, a large number of COF materials have been synthesized and employed in many fields. Among these COF materials, various two dimensional (2D) COFs with eclipsed stacking structures have been reported and exhibit excellent nature in various applications such as gas adsorption and storage,3,4 optoelectronics,5,6 catalysis,7–9 and others.10–14

In recent years, three-dimensional COFs (3D COFs) have found interesting applications and received considerable attention owing to their high specific surface area and numerous open sites.15–24 However, different from 2D COFs, the design and synthesis of new 3D COFs are still a great challenge.25–27 Among the reported materials, 3D COFs are mainly synthesized using boroxine,28–31 imine,32–34 amide35 and silicate linkages36,37 under hydrothermal conditions. The limited structures of the linkages and the harsh synthesis conditions greatly limit their applications in some cases. Thus, exploration of new crystalline 3D COF materials with novel organic linkages is highly demanded in the field of basic and applied research.

Herein, we report for the first time the construction of a novel 3D COF with an azine linkage (Scheme 1, 3D-HNU5). The material has been easily prepared through the condensation reaction of hydrazine with aldehydes (tetrakis(4-formylphenyl)-methane) in an ionic liquid [Bmim][Tf2N] (1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide) at room temperature. Based on the X-ray diffraction measurements and detailed simulations, it is found that the as-prepared azine-linked 3D COF features two-fold interpenetrated diamondoid (dia) nets, is highly crystalline and exhibits good chemical and thermal stability. Owing to the high porosity and narrow pore size distribution, this 3D-HNU5 exhibits a high selectivity of CO2 absorption over N2. Also, it can serve as a support for nano-Ag catalysts and shows excellent catalytic performance for CO2 chemical transformation. To the best of our knowledge, this is the first report on the cycloaddition of propargylic alcohols with CO2 by using a 3D COF as an efficient catalyst.


image file: c9cc05710b-s1.tif
Scheme 1 Schematic representation for the synthesis of the azine-linked 3D-COF (a), and its 3D porous structure and 2-fold interpenetrated diamond topology (b). C, blue; H, white; and N, red.

Typically, the synthesis of 3D-HNU5 through the condensation reaction of tetrakis(4-formylphenyl)-methane (TFPM) with hydrazine was performed in a [Bmim][Tf2N]/acetic acid mixture at room temperature for 24 hours (see the ESI for details). The detailed structure of a 3D azine-based COF was subsequently confirmed by several analytical techniques. The morphology of the 3D-HNU5 was studied by scanning electron microscopy (SEM), and a rod-like morphology was revealed with dimensions ranging from 0.5 to 1 μm (Fig. S1, ESI). The Fourier transform infrared (FT-IR) spectrum of 3D-HNU5 showed a characteristic peak of the imine linkage at 1625 cm−1, while the peaks of the aldehydic unit at 2734 cm−1 and 1694 cm−1 in TFPM disappeared, suggesting the successful condensation of the aldehyde group (Fig. S2, ESI). As shown in Fig. S3 (ESI), the typical signal at 161 ppm in the 13C cross-polarization magic-angle spinning (CP/MAS) NMR spectrum of 3D-HNU5 confirmed the formation of a C[double bond, length as m-dash]N bond.

The crystalline structure of 3D-HNU5 was investigated by powder X-ray diffraction (PXRD) analysis. The peaks at 7.75, 9.67, 10.97, 13.29, 14.89, 17.42, and 19.16 2θ could be assigned to the (101), (111), (102), (003), (202), (221) and (311) Bragg peaks of P42/N (No. 86) (Fig. 1), respectively. The Pawley refinement patterns matched well with the experimental results with negligible difference. After a geometrical energy minimization (Materials Studio, version 7.0), the unit cell parameters were determined based on the 2-fold interpenetrated dia topology. It was shown that the 3D-HNU5 has tetragonal unit cells with a = b = 15.1435 Å, c = 19.9678 Å, and α = β = γ = 90° (Fig. S4 and Table S1, ESI). We also tried to simulate the other alternative structures for the 3D-COF. None, three, four and five-fold interpenetrated diamond nets for 3D-HNU5 were constructed from the space groups I41/A (No. 88) and P4/N (No. 85), respectively. However, the calculated results for these alternative structures did not agree with the experimental patterns (Fig. S5–S8 and Tables S2–S5, ESI).


image file: c9cc05710b-f1.tif
Fig. 1 PXRD patterns and simulated structures of 3D-HNU5. The experimental pattern is shown in black, the simulated pattern in green, the Pawley refined pattern in red, and the differences between the experimental and refined profiles in blue.

The porosity parameter was evaluated by measuring nitrogen adsorption–desorption isotherms at 77 K. As shown in Fig. 2, the 3D-HNU5 exhibited a typical type of I isotherm, which indicates a character of microporous materials (Fig. 2a). Inclination of the isotherms in the 0.5–1.0 P/P0 range revealed the existence of mesopores, which is the result of COF crystal agglomeration. The BET surface area was estimated to be 864 m2 g−1 for 3D-HNU5, and the total pore volume was calculated to be 0.89 cm3 g−1 (P/P0 = 0.99). Meanwhile, from the determination of the Connolly surface of the crystal models,38 the theoretical surface area was calculated to be 5161 m2 g−1 for the two-fold interpenetrated crystal structure of 3D-HNU5. The theoretical value was much higher than the experimental one, the case of which was consistent with that observed previously for other frameworks.39 The pore size distribution of the 3D-HNU5 was estimated by using nonlocal density functional theory (NLDFT). As shown in Fig. 2b, 3D-HNU5 displayed a narrow pore distribution mainly at 1.01 nm, which is in good agreement with the pore distribution of the proposed model (Fig. 2b).


image file: c9cc05710b-f2.tif
Fig. 2 (a) N2 adsorption and desorption isotherms of the 3D-HNU5 at 77 K and (b) the DFT pore size distribution for the 3D-HNU5.

The chemical stability and thermal stability of the 3D COF were investigated by soaking the 3D material in organic solvents (hexane, ethanol, DMF, DMSO and CHCl3), and acidic and alkaline solutions (1.0 M HCl and 0.1 M NaOH) at room temperature for 3 days. We found that the framework of 3D COF samples was very stable under these conditions. Then, these samples were subjected to XRD measurements after collection, washing with EtOH, and drying under vacuum at 100 °C for 18 h. Surprisingly, the high crystallinity of all the samples was retained, and the PXRD peak intensities remained intact (Fig. 3a). In addition, only slight modification to the nitrogen adsorption isotherms was observed after exposure for a long time under each of the conditions, indicating that the azine-linked framework was chemically stable (Fig. 3b). However, it was noted that the framework was not very stable in alkaline solution in comparison with that in acidic medium due to the reversibility of the reactions in the formation of C[double bond, length as m-dash]N linked COFs. We also tested the chemical stability of 3D-HNU-5 under more harsh conditions such as boiling in water and refluxing in DMF, 12 M HCl and 12 M NaOH for 48 h. It was found that 3D-HNU-5 retained its crystallinity in the case of boiling in water and refluxing in DMF (Fig. 3a). Moreover, the BET surface areas were also retained under these conditions (Fig. 3b). Nevertheless, the framework of 3D-HNU5 collapsed when it was immersed in 12 M HCl and 12 M NaOH, indicating that the sample was not stable under the harsh acid and base conditions (Fig. S9, ESI). Moreover, the 3D-HNU5 showed excellent thermal stability up to 450 °C according to thermogravimetric analysis (TGA) (Fig. S10, ESI). The high stability of the 3D COF may be attributed to the linear and conjugated azine linkage.40–42


image file: c9cc05710b-f3.tif
Fig. 3 Chemical stability of 3D-HNU5 under various conditions: (a) PXRD patterns of the samples treated with different solvents and (b) nitrogen adsorption isotherms for the samples shown in (a).

Among various porous crystalline materials, 3D COFs have attracted considerable attention in gas storage applications owing to their remarkable properties such as high specific surface area, low density, and numerous open sites.22,29,31 Here, we investigate the ability of the azine-linked 3D-COF for the adsorption and selectivity of CO2 over N2. The gas adsorption isotherms of 3D-HNU5 were measured at 273 K from 0 to 1.0 bar. As seen from Fig. 4a, 3D-HNU5 showed rapid CO2 uptake at low pressure and the value of uptake capacity was 123.1 mg g−1. However, the value of N2 uptake by 3D-HNU5 was considerably lower at 273 K, and the value of uptake capacity was only 4.2 mg g−1. It was also found that under the condition of similar BET surface areas, the CO2 uptake capacity of 3D-HNU5 reported in this work was close to that of the reported 3D non-interpenetrated and interpenetrated COF materials (Table S6, ESI). The high CO2 uptake of 3D-HNU5 could be attributed to the high nitrogen heteroatoms contained in the pores, which led to additional Lewis acid/base interactions at low pressure.43–45 In addition, considering the fact that the recyclability of an adsorbent is one of the critical factors for the practical applications, recycling experiments were performed, and the results demonstrated that 3D-HNU5 could be reused without loss of adsorption capacity. For example, 100% adsorption capacity was still maintained after five cycles (Fig. 4b). This result indicates that 3D-HNU5 is highly suitable for CO2 adsorption.


image file: c9cc05710b-f4.tif
Fig. 4 (a) CO2 and N2 isotherms for 3D-HNU5 at 273 K. (b) Recycle of CO2 adsorption by 3D-HNU5 at 273 K. (c) Catalytic performance of different catalysts for the synthesis of cyclic carbonates. (d) Yields of various cyclic carbonates generated from the reaction of related propargyl alcohols with CO2 by using Ag@3D-HNU5 as catalyst. Reaction conditions: propargylic alcohols (1.0 mmol), catalyst (0.1 mol%, based on loading of Ag), DBU (1.0 mmol), CH3CN (3 mL), CO2 (99.999%, balloon), RT, 12 h. The yields were determined by 1H NMR spectroscopy.

The incorporation of catalytically active moieties into porous materials can effectively protect them from aggregation and facilitate their dispersion. Owing to the permanent porosity, excellent stability and inherent CO2-adsorbing properties of the framework, this 3D azine based COF material would be a highly promising heterogeneous catalyst for CO2 conversion. Herein, we utilized the as-synthesized 3D-HNU5 as a support for Ag catalyst (denoted as Ag@3D-HNU5, Fig. S11, ESI), and the carboxylative cyclization reaction of CO2 with propargyl alcohols was chosen to evaluate its catalytic performance under mild conditions. As confirmed by inductively coupled plasma (ICP) analysis and energy-dispersive spectroscopy (EDS, Fig. S12, ESI), Ag@3D-HNU5 had a Ag loading of 2.32 wt%, and the Ag NPs were homogeneously distributed throughout the three-dimensional material. The BET surface area and CO2 sorption capacity of Ag@3D-HNU5 were 300 m2 g−1 and 83 mg g−1 (Fig. S13 and S14, ESI), respectively. The decrease in the capture capacity would be attributed to the incorporation of the Ag NPs into the pores of 3D-HNU5. The product yields were found to depend on the amount of Ag immobilized on the COF. The reaction did not occur in the absence of a silver catalyst, and the yield of the product was increased up to 99% when the Ag amount was increased to 0.1 mol% (Table S7, ESI). For comparison, several homogeneous silver salts (AgNO3 and AgI) and heterogeneous Ag catalyst (Ag@AC46) were also chosen to manifest the significantly enhanced catalytic efficiency of Ag@3D-HNU5. As expected, by using Ag@3D-HNU5 catalyst, a yield of >99% and an excellent turnover number (TON) of 990 were obtained for the chemical conversion of CO2 into the corresponding product. However, the use of homogeneous catalysts (AgNO3 and AgI) and heterogeneous Ag@AC only generated moderate yields under the same reaction conditions (Fig. 4c).

On the basis of the above results, we deduced that the high catalytic performance may be attributed to the high uptake capacity of CO2 and highly dispersed nano-Ag active sites within the pores of 3D azine-based COF materials. To further investigate the catalytic activity of Ag@3D-HNU5, four other substrates with different substituted groups were selected (Fig. 4d). It was found that all of the reactants could react with CO2 to afford the target products with excellent yields (99%). In virtue of the high stability, the Ag@3D-HNU5 could be regenerated without an obvious decrease in catalytic activity after recycling at least six times (Fig. S15, ESI), as confirmed by PXRD patterns (Fig. S16, ESI) for the structural integrity after catalysis.

In conclusion, we reported for the first time the design and synthesis of a new azine-linked 3D COF (3D-HNU5) for CO2 capture and conversion. It was found that this type of 3D-COF exhibited high crystallinity, high porosity, and good chemical/thermal stability. Based on PXRD and detailed simulations, together with pore size distribution analysis, the 3D-HNU5 was found to be a 2-fold interpenetrated micro-porous framework, and showed an efficient performance in the selective capture of CO2 over N2. Furthermore, the 3D-COF could be used as a support for nano-Ag catalysts to directly catalyze the cycloaddition of propargylic alcohols with CO2 into carbonates with significantly enhanced catalytic efficiency (yield > 99%, and TON = 990) under mild conditions. We believe that this work creates a new number in the family of 3D COFs, and paves the way to explore various applications of such 3D COF materials.

This work is supported by the National Natural Science Foundation of China (No. U1704251, 21733011, and 21773058), the National Key Research and Development Program of China (2017YFA0403 101), and the 111 Project (No. D17007).

Conflicts of interest

There are no conflicts to declare.

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Footnote

Electronic supplementary information (ESI) available: Experimental methods and additional characterization. See DOI: 10.1039/c9cc05710b

This journal is © The Royal Society of Chemistry 2019