17724 | Chem. Commun., 2015, 51, 17724--17727 This journal is©The Royal Society of Chemistry 2015

Cite this:Chem. Commun., 2015,

51, 17724

Hollow Co@C prepared from aCo-ZIF@microporous organic network: magneticadsorbents for aromatic pollutants in water†

Seokjo Hong,a Jin Yoo,a Nojin Park,a Sang Moon Lee,a Je-Geun Park,b

Ji Hoon Parkc and Seung Uk Son*a

This work shows the new engineering strategy of magnetic adsor-

bents by the combination of zeolitic imidazolate framework (ZIF)

and microporous organic network (MON) chemistry. ZIF-67 nano-

particles containing Co2+ ions were coated with MON. The thermo-

lysis of ZIF-67@MON under argon resulted in hollow carbon materials

bearing cobalt nanoparticles which showed promising performance

as magnetic adsorbents for aromatic pollutants in water.

Zeolitic imidazolate frameworks (ZIFs) are coordination polymersconsisting of transition metal ions and imidazole derivatives.1

Due to their chemical and structural diversity, various transitionmetal ions have been screened as connectors to form ZIFs. Forexample, the coordination networking of cobalt(+2) ions with2-methylimidazoles resulted in the formation of polyhedralZIF-67.2 For application as the precursors of functional materials,ZIF nanoparticles have been prepared via controlled growth.3 Inaddition, the thermolysis of coordination polymers including ZIFmaterials has been studied to prepare metal oxide4 and compositematerials.5

As another recent class of molecular materials, microporousorganic networks (MONs) have been prepared by the chemicalcoupling of organic building blocks.6 The thermolysis of theMON materials can result in the formation of carbon materials.7

Our research group has shown that MON materials can be intro-duced into silica, metal oxides and porous materials.7b,8 It can bespeculated that the thermolysis of ZIF–MON composite materialsmay result in the generation of new materials. However, as far as we

are aware, there were no reports on the synthesis of functionalmaterials via the thermolysis of ZIF–MON composites.

Magnetic separation is an attractive work-up method due toits facile and fast process.9 Thus, magnetic adsorbents for theremoval of target chemicals in water have attracted significantattention of scientists.10 Due to the diffusion pathway effect ofadsorbates in the interaction with adsorbents, porous adsorbentswith a hollow structure can show more effective adsorptionbehaviors, compared with non-hollow adsorbents.11 Moreover,magnetic inorganic materials can be located in the hollowspace to form magnetic composite adsorbents.11c In this work,we report the preparation of magnetic hollow Co@C materialsvia the thermolysis of ZIF-67@MON materials and their adsorp-tion properties towards aromatic pollutants in aqueous solution.

Fig. 1 shows a synthetic scheme for the preparation of hollowCo@C materials.

ZIF-67 nanoparticles were prepared by the procedure reportedin the literature.3 After being dispersed in a mixture of triethyl-amine and methanol, ZIF-67 particles (0.18 g) were coated with MONvia the Sonogashira coupling of tetrakis(4-ethynylphenyl)methane

Fig. 1 Synthetic scheme of ZIF-67@MON, Co@C, and hollow MON. (Referto the Experimental section in the ESI† for detailed procedures.)

a Department of Chemistry and Department of Energy Science,

Sungkyunkwan University, Suwon 440-746, Korea. E-mail: sson@skku.edu;

Fax: +82-031-290-4572b Department of Physics & Astronomy, Seoul National University, Seoul 151-747,

Koreac Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea

† Electronic supplementary information (ESI) available: Experimental details,additional characterization data of ZIF-67, ZIF-67@MON, Co@C-500, Co@C-600-2,Co@C-700, and Co–C materials, characterization data of the recovered adsorbentsafter five adsorption cycles, and desorption behaviors of Co@C-600-2. See DOI:10.1039/c5cc06873h

Received 15th August 2015,Accepted 13th October 2015

DOI: 10.1039/c5cc06873h

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(40 mg) and 1,4-diiodobenzene. The heat-treatment of ZIF-67@MONat 600 1C under argon resulted in the formation of Co@C materials(denoted as Co@C-600-2). The materials were investigated bymicroscopic analysis.

According to scanning (SEM) and transmission electronmicroscopy (TEM), ZIF-67 particles have a polyhedron shapeand a B560 nm diameter (Fig. 2a and b). TEM analysis of ZIF-67@MON showed a B660 nm diameter and the homogeneouscoating of MON with a B50 nm thickness on the surface of ZIF-67 particles12 (Fig. 2c and Fig. S1 in the ESI†). To confirm theMON coating on ZIF-67 particles, inner ZIF-67 in ZIF-67@MONwas etched by the treatment of acetic acid to form hollow MONmaterials. The SEM and TEM analysis clearly showed thehollow characteristic and MON shells with a B50 nm thickness(Fig. 2d–f). Co@C-600-2 prepared by the heat-treatment of ZIF-67@MON at 600 1C showed a hollow structure with a B570 nmdiameter and a B45 nm shell thickness. Cobalt particles with aB50 nm size were observed inside of Co@C-600-212 (Fig. 2g–i).

Actually, Co@C-600-2 materials were obtained through screen-ing synthetic conditions. When ZIF-67@MON was treated at 500 1Cor 700 1C, irregular cobalt particles were formed inside of Co@Cmaterials due to the incomplete formation and thermal aggrega-tion of cobalt particles, respectively. (The materials were denoted asCo@C-500 and Co@C-700, respectively. See Fig. S2 in the ESI.†)When ZIF-67@MON was heated at 600 1C, cobalt particles wererelatively well distributed. Combustion elemental analysis showedno significant difference of carbon composition for Co@C-500(45.8 wt% carbon), Co@C-600-2 (47.3 wt% carbon), and Co@C-700 (49.6 wt% carbon).

Three Co@C-600 materials (Co@C-600-1, Co@C-600-2, andCo@C-600-3) were prepared by changing the amount of MONcoating (Fig. 3a). The dramatic effect of MON quantity in the

reduction of Co2+ ions of ZIF-67 was observed. When 20 mg ofthe alkyne building block was used, cobalt particles with a

Fig. 2 SEM images of (a) ZIF-67 particles, (d and f) hollow MON materials,and (i) Co@C-600-2. TEM images of (b) ZIF-67 particles, (c) ZIF-67@MON,(e) hollow MON materials, and (g and h) Co@C-600-2.

Fig. 3 (a) Illustration of the Co@C materials obtained by the thermolysis of ZIF-67@MON at 600 1C which were prepared by using 20 mg, 40 mg, and 60 mg ofalkyne building blocks. SEM and TEM images of (b and c) Co@C-600-1 and(d and e) Co@C-600-3. (f) N2 adsorption–desorption isotherm curves at 77 K, poresize distribution diagrams (DFT method, inset), and (g) PXRD patterns of hollowMON, Co@C-600-1, Co@C-600-2, and Co@C-600-3. (h) Solid phase 13C NMRspectrum of hollow MON. (i) Magnetic hysteresis loops of Co@C-600-1, Co@-600-2, and Co@C-600-3 measured at 300 K. (j) Photographs of the magneticseparation process of Co@C-600-2 from aqueous solution using a magnetic bar.

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B20 nm size were formed in the resultant Co@C-600-1 (Fig. 3band c). The diameter and shell thickness of Co@C-600-1 werereduced to B530 nm and B25 nm, compared with those(B570 nm and B45 nm) of Co@C-600-2. When 60 mg of thealkyne building block was used, the resultant Co@C-600-3 showeda B630 nm diameter and a B60 nm shell thickness (Fig. 3d and e).Relatively big cobalt particles with a B90 nm size were observedinside of Co@C-600-3. These observations imply that the carbonmaterials formed via the thermolysis of MON coating serve asreductants of cobalt ions in ZIF-67. As more alkyne building blockswere used, carbon contents in Co@C gradually increased from38.1 wt% (Co@C-600-1) to 47.3 wt% (Co@C-600-2) and 54.6 wt%(Co@C-600-3). As a control test, when ZIF-67 particles withoutMON coating were treated at 600 1C, non-hollow Co–C materialswere obtained3,13 (Fig. S3 in the ESI†). The diameter of Co–C wasreduced to B400 nm, compared with a B560 nm diameter ofZIF-67 particles. Very small cobalt particles with a B5 nm sizeembedded in the carbon matrix of Co–C were observed.

The analysis of N2 sorption isotherm curves of Co@C-600-1,Co@C-600-2, and Co@C-600-3 showed microporosity and surfaceareas of 329 m2 g�1, 388 m2 g�1, and 393 m2 g�1, respectively(Fig. 3f). Powder X-ray diffraction (PXRD) studies on Co@C-600-2showed a mixture of hexagonal and cubic zerovalent cobalt withmain PXRD peaks at 41.71, 44.81, 47.61, and 75.91 (Fig. 3g).Interestingly, as the carbon content in Co@C-600 increased, thehexagonal phase of zerovalent cobalt became dominant, indi-cating that MON coating plays an active role in the reduction ofcobalt ions. Hollow MON materials obtained via the etching ofinner ZIF-67 of ZIF-67@MON showed amorphous characteristic,surface area of 739 m2 g�1, and microporosity, matching wellwith the general characteristics of the MON materials preparedvia the Sonogashira coupling6 (Fig. 3f and g). While solid-phase13C nuclear magnetic resonance (NMR) spectroscopy could notbe applied for cobalt-containing materials due to their magneticnature, the 13C NMR spectrum of hollow MON showed 13C peaksfor benzyl carbon, internal alkynes, and aromatic groups at59 ppm, 80–90 ppm, and 120–141 ppm, respectively, supportingthe expected network structure of building blocks (Fig. 3h).

The magnetic properties of Co@C-600 materials were studiedusing a superconducting quantum interference device (SQUID).As shown in Fig. 3i, the magnetic hysteresis loops of Co@C-600materials showed a superparamagnetic behavior at 300 K. It canbe expected that saturation magnetization (Ms) of Co@C materialsmay increase with the increasing cobalt sizes and the decreasingcarbon contents. The Ms values of Co@C-600-1, Co@C-600-2, andCo@C-600-3 were measured to be 42 emu g�1, 41 emu g�1, and31 emu g�1, respectively. In the case of Co@C-600-1, while thecarbon shell thickness (B25 nm) was thinner than that(B45 nm) of Co@C-600-2, the size (B20 nm) of cobalt particleswas smaller than that (B50 nm) of Co@C-600-2. Thus, there wasno big difference in the Ms values of Co@C-600-1 and Co@C-600-2.In the case of Co@C-600-3, while the size (B90 nm) of cobaltparticles was much bigger than that of Co@C-600-2, the carbonshell thickness (B60 nm) was thicker than that of Co@C-600-2.Finally, the Ms value of Co@C-600-3 was smaller than that ofCo@C-600-2. This indicates that the thickness of carbon shells

is a more significant factor than the size of cobalt particles forMs values.

The Ms values of magnetic phenol adsorbents were reportedin the range of 4–20 emu g�1 in the literature.14 Thus, Co@Cmaterials with Ms values of 31–42 emu g�1 are quite promisingas magnetic adsorbents. As shown in Fig. 3j, Co@C materialsare highly dispersible in water and easily separable using amagnetic bar. Considering these facts, we studied the adsorp-tion properties of Co@C materials toward aromatic pollutantsin water. Fig. 4 and Table S1 in the ESI† summarize the results.Fig. 4a shows phenol adsorption behaviors of various Co@Cadsorbents. Among the materials tested, Co@C-600-2 and Co@C-600-3 showed the best performance in the phenol adsorption.However, Co@C-600-3 showed significantly slow and incompletemagnetic separation due to weak magnetic properties (Fig. 3i).While surface areas increased gradually from Co@C-500

Fig. 4 (a) Adsorption behavior of Co@C and Co–C materials toward phenol(Co: 100 mg L�1) in water. (b) Adsorption behavior of Co@C-600-2 toward4-nitrobenzene, phenol, and aniline (Co: 100 mg L�1). (c) Recycled adsorp-tion of Co@C-600-2 toward 4-nitrobenzene, phenol, and aniline. (d) Langmuirand (e) Freundlich isotherms at 298 K of Co@C-600-2 toward 4-nitrobenzene,phenol, and aniline (refer to Table S1 in the ESI†).

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(326 m2 g�1) to Co@C-600-2 (388 m2 g�1) and Co@C-700(411 m2 g�1), nitrogen contents decreased from 5.8 wt% to5.2 wt% and 3.6 wt%.15 Thus, we speculate that Co@C-600-2has the optimized chemical and physical situation for phenoladsorption. In comparison, non-hollow Co–C materials showeda relatively poor adsorption performance (Fig. 4a and Fig. S3 inthe ESI†).

Co@C-600-2 showed increasing adsorption capacities (mg g�1)in the order of 4-nitrobenzene, phenol, and aniline16 (Fig. 4b). Thisorder is attributable to the high molecular weight of 4-nitrobenzeneand the acid preference of Co@C-600-2 which originates from thenitrogen content (5.2 wt%) in adsorbents.

As shown in Fig. 4c, while Co@C-600-2 showed good recycl-ability toward aniline and 4-nitrobenzene, it showed relativelypoor recyclability toward phenol, indicating the relatively easydesorption of aniline and 4-nitrobenzene from adsorbents,compared with phenol. The poor reversibility of phenol adsorp-tion on various carbon adsorbents has been well recognized.17

Its origins including the polymerization of phenols on carbon orchemisorption through chemical reactions between carbon andphenol were reviewed in the literature.17 Co@C-600-2 recoveredafter five adsorption processes were investigated by SEM, PXRD,and surface area analysis (Fig. S4 in the ESI†), showing thesignificant retention of original properties in the adsorptioncases of 4-nitrobenzene and aniline and the relatively reducedsurface area of adsorbents in the adsorption case of phenol dueto the possible covalent deposition of phenol.17

The adsorption behavior of Co@C-600-2 matches better withthe Langmuir model than the Freundlich model, consideringR2 values (Fig. 4d and e and Table S1 in the ESI†). According tothe Langmuir model, Co@C-600-2 showed the maximumadsorption capacities (qmax) of 87.2 mg g�1, 75.6 mg g�1, and56.7 mg g�1 toward 4-nitrobenzene, phenol, and aniline, respec-tively. These adsorption capacities are superior to the results(35–60 mg g�1 phenol adsorption) in the literature,14 whichis attributable to the hollow structure of Co@C adsorbents(Fig. S3 in the ESI†).

In conclusion, this work shows that microporous organicnetworks can be applied to form hybrid materials with ZIFs.The thermolysis of MON–ZIF hybrid materials resulted in theformation of carbon–metal composite materials. In the case ofZIF-67 bearing Co2+ ions, Co@C with a superparamagnetic naturewas formed and applied as a magnetic adsorbent for organicpollutants in water. We believe that more various carbon–metalcomposite materials can be engineered by employing various ZIFand MOF materials.

This work was supported by grants NRF-2012-R1A2A2A01045064(Midcareer Researcher Program) through the National ResearchFoundation of Korea. Magnetization measurements were carriedout at the Center for Correlated Electron Systems, Institute for BasicScience (IBS).

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12 Elemental mapping and additional SEM images were obtained toconfirm the core–shell and hollow structure of ZIF-67@MON andCo@C-600-2 (Fig. S5 in the ESI†). For comparison, nonhollow MONpowders and carbon materials were obtained in the absence of ZIFtemplates and characterized (Fig. S6 in the ESI†).

13 N. L. Torad, M. Hu, S. Ishihara, H. Sukegawa, A. A. Belik, M. Imura,K. Ariga, Y. Sakka and Y. Yamauchi, Small, 2014, 10, 2096–2107.

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15 According to XPS analysis, Co@C-600-2 contains a mixture ofpyridinic and graphitic Ns (Fig. S7 in the ESI†).

16 Co@C-600-2 showed adsorbate-dependent desorption behaviors inethanol (Fig. S8 in the ESI†).

17 A. Dabrowski, P. Podkoscielny, Z. Hubicki and M. Barczak, Chemosphere,2005, 58, 1049–1070.

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