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Letter Cite This: ACS Macro Lett. 2017, 6, 1444−1450

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General Route to High Surface Area Covalent Organic Frameworks and Their Metal Oxide Composites as Magnetically Recoverable Adsorbents and for Energy Storage Yaozu Liao,*,† Jiahuan Li,† and Arne Thomas*,‡ †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials & College of Materials Science and Engineering, Donghua University, Shanghai 201620, China ‡ Department of Chemistry, Functional Materials, Technische Universität Berlin, Berlin 10623, Germany S Supporting Information *

ABSTRACT: Two-dimensional (2D) imine-linked covalent organic frameworks (COFs) have attracted great interest for gas uptake, catalysis, drug delivery, electronic devices, and photocatalytic applications. The synthetic methodologies involved in imine-linked COF formations such as solvothermal synthesis usually require harsh experimental conditions. In this work, we show for the first time how highly crystalline COFs with very high surface areas (3.6 times higher than using conventional approaches) can be prepared by combining a mechanochemical and crystallization approach. More importantly, this facile method is a general route to novel composites of COF and metal oxides including Fe3O4, Co3O4, and NiO. The composites can be used as magnetically recoverable adsorbents and show a strong redox-activity making them interesting for applications in electrochemical energy storage.

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redox-active compounds such as metal oxides and conducting polymers opens the possibility to utilize COFs even for this application.23,28,29

ovalent organic frameworks (COFs) have attracted great interest for various applications such as gas storage and separation, (photo)catalysis, or drug delivery.1−5 Among them, imine-linked COFs synthesized via Schiff base reaction have extensively studied.6 The synthetic methodologies involved in the preparation of imine-linked COFs, such as solvothermal synthesis, usually require relatively harsh experimental conditions.7−12 Recently, the mechanochemical synthesis has gained interest as a fast and environmentally friendly alternative.13−15 Unfortunately, a simple neat mechanical grinding normally leads to COFs with low crystallinity and porosity compared to COFs obtained via the conventional solvothermal synthesis. More recently, modified methods including liquid-,16 sonication-,17 microwave-,18,19 and ptoluenesulfonic acid (p-TSA)-assisted20 mechanical syntheses have been reported to prepare COFs in shorter time, under less harsh conditions, or with improved crystallinity. For example, with the introduction of p-TSA during the synthesis led to highly crystalline porous COFs in 60 s.20 Very recently, a crystallization approach has been proposed to prepare crystalline COFs from amorphous polymer network precursors.21−23 On the other hand, the preparation of COF composites23−25 with metals or metal oxides, can broaden the spectrum of possible applications of pure COFs. As an example, it has been noted that most imine-linked COFs show a low redox-activity,26,27 a property critical for applications such as electrochemical energy storage in batteries. Hybridization with © XXXX American Chemical Society

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RESULTS AND DISCUSSION Herein we present a facile route for the preparation of COF/ metal oxide composite materials, in which mechanochemical grinding and crystallization approaches are combined for the first time to yield highly crystalline COFs (Figure 1a). This route facilitates the synthesis of high-quality COFs and their metal oxide composites. Here we used the simplest imine-linked COF (COF-LZU1) and iron oxide (Fe3O4) to present the concept in detail. However, also other metal oxides such as cobalt oxide (Co3O4) and nickel oxide (NiO) can be applied similarly (Figure S1). COF-LZU1 is conventionally synthesized via a solvothermal technique.24 In contrast, here a mechanical grinding of 1,3,5-triformylbenzene (TFB) and 1,4-diaminobenzene (DAB) with the assistance of a few drops of 1,4-dioxane/ acetic acid was applied, which first led to an amorphous polymer network (COF-0d, Figure 1b). Subsequently, a mild crystallization reaction in the presence of a low amount of 1,4-dioxane/ mesitylene/acetic acid was carried out at 70 °C for 3, 5, and 7 Received: October 29, 2017 Accepted: November 30, 2017

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DOI: 10.1021/acsmacrolett.7b00849 ACS Macro Lett. 2017, 6, 1444−1450

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ACS Macro Letters

Figure 1. (a) Schematic illustration for the synthesis of COFs and Fe3O4/COF composites, (b, c) mechanochemical syntheses of amorphous polymer network as the COF precursor (b) and Fe3O4/amorphous polymer network composite (c), and (d) COFs and (e) Fe3O4/COF composites synthesized via crystallization for different days.

Figure 2. (a−f) SEM images of (a, d) COF-0d, (b, e) COF-5d, and (c, f) Fe3O4/COF-5d and (g−i) TEM images of Fe3O4/COF-5d at different magnifications.

days. Washing the products with toluene and methanol then yielded crystalline COF-3d, COF-5d, and COF-7d (Figure 1d), respectively. The same procedure (Figure 1c), however, with adding commercially available Fe3O4 powder, yielded Fe3O4/ COF composites, that is, Fe3O4/COF-0d, -3d, -5d, and -7d (Figure 1e).

The iron content of the composites was found to be 14.2, 16.3, 19.2, and 23.9 wt %, respectively, as determined by inductively coupled plasma atomic emission spectroscopy (ICPAES). Scanning electron microscope (SEM) images showed that the morphology of COFs and their composites consist of aggregated microparticles (Figure 2a−f). Transmission electron 1445


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Figure 3. (a) FT-IR spectra, (b) solid-state 13C CP/MAS NMR spectra, and (c, d) powder XRD patterns of COFs and Fe3O4/COF composites.

crystallinity with five peaks corresponding to (100), (110), (200), (210), and (001) facets, exactly matching the reported structure of COF-LZU1 obtained by a solvothermal method.24 For the composites, additional peaks owing to (311), (422), and (440) facets of Fe3O4 crystals are clearly visible in all materials, beside the peaks at lower angles for the COF. Again, optimal crystallization time is achieved at 5 days (Figure 3d). The porosity and surface areas of the obtained COFs and their Fe3O4 composites were measured by nitrogen sorption analysis at 77 K (Figure 4a,c). The application of the Brunauer− Emmett−Teller (BET) model resulted in low surface areas of 32 and 28 m2/g for COF-0d and Fe3O4/COF-0d, respectively, which are largely improved to 1500 m2/g (COF-5d) and 872 m2/g (Fe3O4/COF-5d) upon crystallization for 5 days (Table S1). Further crystallization for 7 days, however, decreased the surface area again. Surface area determination using nitrogen adsorption isotherms demonstrate that the COFs are microporous materials with a remarkable nitrogen uptake at low relative pressures. A pronounced hysteresis extending down to low relative pressures is observed, which is often found for relatively soft microporous materials such as polymers of intrinsic microporosity (PIMs)32 or amorphous microporous polymer networks,33−36 but also for crystalline COFs.24 The hysteresis could result from partially swelling of the material during gas adsorption or restricted access of the adsorbate to some of the micropores,36 for example, due to small mismatches in the actually eclipsed layer stacking. Notably, the surface area for COF-5d (1500 m2/g) improved by more than 3.6× compared to COF-LZU1 (410 m2/g),24 and still 2.5× compared to TAPB-PDA COF gained by one-step crystallization approach (600 m2/g).22 The total pore volumes were calculated to be as high as 0.77 and 0.45 cm3/g (at p/p0 = 0.95) for COF-5d and Fe3O4/COF-5d, respectively. The improved porosity is also confirmed by CO2 adsorption (Figure S5). COF-0d and Fe3O4/ COF-0d only take 2.3 and 1.5 wt % CO2 at 273 K and 1 bar; while crystallized COF-5d and Fe3O4/COF-5d can take up much more CO2 with values up to 8.1 and 7.3 wt % at the same

microscope (TEM) images showed that the COF network deposited on the surface of Fe3O4 particles (Figure 2g−i), forming a 15−35 nm thick organic shell with continuous and smooth appearance (Figure 2h). The lattice fringes of the core are seen in the high-resolution image (Figure 2i) with the adjacent fringe spacing of 0.253 nm,30 corresponding to (311) lattice planes for Fe3O4 crystals. Fourier transform infrared (FT-IR) spectra were obtained for all of the COFs and the respective monomers (Figures 3a and S2). Note that the N−H stretching band between 3300 and 3500 cm−1 seen for the diaminobenzene monomer merged with the stretching frequency of −OH bands which appears most likely due to the moisture included in the pores.31 Noteably, the intense carbonyl (CO) peak seen for 1,3,5-triformylbenzene at 1699 cm−1 nearly disappears already for COF-0d, and merges with a newly formed peak assigned to the formed CN bond at 1602 cm−1 showing that already after the first mechanochemical step high polymerization degrees can be achieved, even though no crystalline framework is observed at this point. Indeed, no significant changes are observed within the IR spectra even after the crystallization step for 5 d. This can be confirmed by solid state 13C cross-polarization magic-angle-spinning nuclear magnetic resonance (CP/MAS NMR) measurements (Figure 3b), which show comparable spectra for both the amorphous COF-0d and the crystallized COF-5d, with five peaks assignable to the respective carbon atoms in the repeating unit. These measurements, together with the very comparable thermogravimetric analysis curves, that is, similar decomposition temperatures (Figures S3 and S4), show that the chemical structure of the COF is already fully formed during mechanochemical synthesis and that the second step is just changing the arrangement of the polymer chains from an amorphous network to a 2D crystalline framework structure. This can be nicely followed by powder X-ray diffraction (XRD) measurements (Figure 3c) showing just a broad peak at 2θ = 20° for COF-0d, revealing an amorphous structure. Upon crystallization for 5 days, COF-5d showed the highest 1446


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Figure 4. (a, c) N2 adsorption/desorption isotherms and (b, d) pore size distribution of COFs and Fe3O4/COF composites; the results of the precursors are also shown for comparisons. Insets in (b) and (d) showing pore size distribution in a zoomed area (0.8−2.2 nm).

Figure 5. (a) Photographs indicate that the magnet used for the recovery of the Fe3O4/COF-5d particles, (b) cyclic voltammetry scans and (c) charge/discharge curves of Fe3O4/COF-5d, and (d) capacitance of COF-0d, COF-5d, and Fe3O4/COF-5d.

(NLDFT) are shown in Figure 4b,d and Table S1. The pore size distribution plots of both COFs and Fe3O4/COFs revealed a narrow pore width distribution at 1.1−1.3 nm, which is smaller than the theoretical value (1.8 nm24), which has been attributed

condition, respectively. The heat of adsorption for CO2 was calculated to 20−35 kJ/mol (Figure S6). The pore size distributions calculated on the basis of N2 adsorption isotherms using nonlocal density functional theory 1447


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ACS Macro Letters to an offset stacking of the 2D sheets.25 The pore width is very comparable to the value obtained on COF-LZU1 (∼1.2 nm24). In comparison with the pure COFs, no alterations in pore size were found for the Fe3O4/COF composites, indicating that the presence of metal oxide do not block the cavities of COFs. To give a further confirmation, we used the Fe3O4/COF-5d as a magnetically recoverable adsorbent. Upon addition of 3 mg adsorbent into 5 mL of 0.2 g/L iodine aqueous solution (Figure 5a), the liquid phase became colorless upon sonication for 15 s, indicating the efficient adsorption of iodine. An adsorption capacity of up to 797 mg/g was found for iodine on Fe3O4/ COF-5d although it was lower than that of COF-5d (910 mg/ g), as determined by ultraviolet−visible (UV−vis) absorbance spectroscopy (Figure S7). So far, a variety of materials including metal organic frameworks, conjugated microporous polymers, porous carbons, and graphene aerogels have been developed for effective capture and storage of volatile radioiodine existing in vapors and organic solvents.37−40 However, the capture and separation iodine from water at room temperature remains challenging.41 COF-5d and Fe3O4/COF-5d have a much higher iodine adsorption capacity compared to many metal-based adsorbents used in aqueous solutions.42−44 More importantly, Fe3O4/COF-5d can be completely collected with aid of a magnet. Upon methanol washing, the composite adsorbent can be reused without any loss of adsorption ability. Furthermore, no traces of solids can be recognized in the solution, even after five runs of adsorption and washing, showing the durability of the Fe 3O4/COF-5d composite. It can be assumed that the combination of liquidassisted grinding and crystallization offers a general route to COFs and metal oxide/COF composites. Since a redox-active metal oxide can be such easily combined with the COF, this process should provide a facile avenue to improve the redox properties of the latter, as demonstrated by electrochemically cyclic voltammetry (CV) scans. COF-0d and COF-5d exhibited a comparable pair of redox peaks located at around 0.46 and 0.52 V at a rate of 5 mV s−1 (Figure S8a,b). With the 10-fold increase in the sweep rate from 5 to 50 mV s−1, the position of the oxidative peak and reductive peak shifted from 0.52 to 0.57 V and 0.46 to 0.33 V, respectively. Compared to COF-0d and COF-5d, Fe3O4/COF-5d showed similar oxidation/reduction pairs but with higher current densities and less alteration in the position of the pairs (Figure 5b), indicating a better redox property. The high surface area and reversible redox processes of the obtained materials are of interest for pseudocapacitive energy storage devices, in which charge is stored both in the electrochemical double layer and through surface-bound Faradaic (pseudocapacitive) processes. As determined by galvanostatic charge−discharge (GCD) experiments (Figures 5c and S9), COF-0d and COF-5d have a comparable capacitance (e.g., 64 vs 52 F/g), while for Fe3O4/ COF-5d, the capacitance has been significantly enhanced (e.g., 112 F/g) at a current density of 0.5 A/g (Figure 5d). The Nyquist plots indicate that the equivalent series resistances follow the order: COF-0d < Fe3O4/COF-5d < COF-5d (Figure S10a). Upon 2000 GCD cycles, COF-0d retained 52% initial capacitance; COF-5d and Fe3O4/COF-5d retained 77 and 76% initial capacitance (Figure S10b), respectively, indicating an improved cycling stability after the crystallization. Additionally, the Coulombic efficiencies of COFs and the composites are nearly 100% according to the discharge capacitance dividing by the charge capacitance, suggesting stable electrochemical energy storage achieved. We assume that the redox property as well as

energy storage ability may be much enhanced by the introduction of more redox-sensitive components such as cobalt and nickel oxides.45,46 On the basis of BET (Figure S11) and XRD measurements (Figure S12), our initial results show that the composites of Co3O4/COF-5d and NiO/COF-5d can be also prepared using the same procedure. However, a reduced BET surface area is observed for NiO and Co3O4 with respect to the Fe3O4-based composites (Figure S11), which might be due to changing affinity of the oxide surfaces to TFB and DAB due to the different surface area and possibly surface chemistry, which could influence the kinetics of polymer growth and crystallization rate and thus the surface area of the COFs. Moreover, the crystallization rate might be further increased with the aid of the microwave,18,19 which should provide a feasibility to rapid synthesis of COFs and their composites. Optimization the crystallization and relative electrochemical studies on NiO and Co3O4 composites are now in progress in our laboratories.

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CONCLUSIONS In summary, we have presented a simple route to imine-linked covalent organic frameworks (COFs) with a large surface area up to 1500 m2/g through a two-step synthesis involving mechanochemical grinding and crystallization. This method was also applied to synthesize Fe3O4, Co3O4, and NiO-based COF composites. We believe that this method should be generally applicable to many other interesting functional metal oxides and imine-based COFs. With the combination of well-defined pore channel as well as the magnetic and redox-active properties, Fe3O4/COF composites show promising properties for magnetically recoverable adsorbents and electrochemical energy storage applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00849. Experiment section, addtional SEM images, FT-IR sepectra, TGA curves, summarized porosity table, CO2 adsorption/desorption isotherms, the heat of adsorption, CV curves, GCD curves, EIS, GCD cycling tests, and additional BET and XRD results (PDF).

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AUTHOR INFORMATION

Corresponding Authors

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

Yaozu Liao: 0000-0001-9263-6281 Arne Thomas: 0000-0002-2130-4930 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51673039), the Shanghai Pujiang Talent Program (16PJ1400300), and the Fundamental Research Funds for the Central Universities (16D110618). We furthermore acknowledge the support from the Sino-German Center for Research Promotion (GZ879). 1448


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