A fluorine-containing hydrophobic covalent triazine ...

Journal of

Materials Chemistry A PAPER

Cite this: J. Mater. Chem. A, 2018, 6, 6370

A fluorine-containing hydrophobic covalent triazine framework with excellent selective CO2 capture performance† Guangbo Wang,a Karen Leus, a Himanshu Sekhar Jena,a Chidharth Krishnaraj,a Shuna Zhao,a Hannes Depauw,a Norini Tahir,a Ying-Ya Liub and Pascal Van Der Voort *a In this article, a set of fluorine functionalized covalent triazine-based frameworks have been designed and synthesized with 2,20 ,3,30 ,5,50 ,6,60 -octafluoro-4,40 -biphenyldicarbonitrile as the monomer under typical ionothermal conditions. A prominent defluorination process during synthesis etches the networks to release CFn, resulting in a significant loss of fluorine and carbon. Notably, the synergistic effects of polar C–F bonds and rich CO2-philic N sites bestow upon the framework an excellent H2 uptake (1.77 wt%, 77 K and 1 bar) as well as a significantly high CO2 adsorption capacity (5.98 mmol g1, at 273 K and 1 bar), surpassing all related CTF materials measured under identical conditions that have been reported

Received 10th October 2017 Accepted 6th March 2018

in the literature to date. Additionally, the material also exhibits a high CO2/N2 selectivity of 31 as predicted by the Henry model. The hydrophobicity of the CTF materials has been significantly enhanced

DOI: 10.1039/c7ta08913a rsc.li/materials-a

owing to the incorporation of hydrophobic fluorine groups, which was further confirmed by ambient water vapor sorption.

Introduction The development of effective technologies or strategies for capturing or eliminating CO2 produced from various processes, in order to minimize its inuence on climate change, has remained an urgent and challenging task.1–4 Recently, a new class of designable and robust porous organic polymers (POPs), including metal organic frameworks (MOFs)5–7 and covalent organic frameworks (COFs),8,9 have demonstrated signicant promise for gas sorption and separation. To address and tackle the CO2 challenges, we have focused on a nitrogen-rich subclass of POPs, namely covalent triazine frameworks (CTFs), which exhibits promising potential in a variety of applications including gas adsorption and separation,10–15 heterogeneous catalysis12,16–18 and waste water treatment19,20 owing to their remarkably large surface area, low skeleton density, and good thermal and chemical stabilities combined with enhanced tunability and functionality. Since the pioneering work of Kuhn et al. in 2008, who developed the rst covalent triazine framework, denoted as CTF-1, by trimerization of 1,4-dicyanobenzene

a

Department of Inorganic and Physical Chemistry, Ghent University, COMOC-Center for Ordered Materials, Organometallics and Catalysis, Krijgslaan 281-S3, 9000 Ghent, Belgium. E-mail: [email protected]

b

State Key Laboratory of Fine Chemicals, Dalian University of Technology, 116024 Dalian, P. R. China † Electronic supplementary 10.1039/c7ta08913a

information

(ESI)

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available.

See

DOI:

in the presence of ZnCl2 under typical ionothermal conditions,10 considerable efforts have been devoted to the design and synthesis of various CTF materials and exploring their potential applications.21–23 In principle, the structure and functionalities of this type of material are easily adjusted by careful selection of structure-directing monomers as building blocks. As far as the functionalization of the monomer is concerned, replacing hydrogen by nitrogen or uorine atoms has proven to be benecial for gas sorption/purication and heterogeneous catalysis due to the strong electrostatic interactions of specic gas molecules or catalytically active metal sites with the heteroatoms in the pore walls of porous materials.24–26 Within this context, Lotsch et al. reported a series of nitrogenrich CTFs as high-performance platforms for selective CO2 capture and storage. Notably, the CO2 uptake of a bipyridineCTF synthesized at 600 C reached up to 5.58 mmol g1 at 273 K and 1 bar.15 Very recently, Palkovits et al. reported a class of N-doped CTF materials, that exhibit the highest H2 (2.63 wt% at 1 bar and 77 K) and CO2 (5.97 mmol g1 at 273 K and 1 bar) adsorption capacities to date.12 Besides the introduction of nitrogen groups, the incorporation of uorine functional groups into porous materials (e.g. MOFs and COFs) has been extensively studied because of their hydrophobicity as well as their potential inuence on gas sorption properties and catalytic activities.27–29 However, reports on uorinated CTF materials remain few.14,30 Firstly, Han et al. synthesized a peruorinated covalent triazine-based framework, named FCTF-1-600, with tetrauoroterephthalonitrile as the monomer,

This journal is © The Royal Society of Chemistry 2018

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exhibiting extremely high CO2 uptake (1.76 mmol g1 at 273 K and 0.1 bar and 5.53 mmol g1 at 1 bar) and an exceptional CO2/ N2 selectivity of 77 under kinetic ow conditions.14 Apart from peruorinated CTFs, uorinated microporous organic polymers (F-MOPs) showed a twofold increase in the CO2 adsorption capacity in comparison to the corresponding non-functionalized MOPs.31 In this work, we report on a set of uorine-containing covalent triazine frameworks, identied as F-DCBP-CTFs, by trimerization of 2,20 ,3,30 ,5,50 ,6,60 -octauoro-4,40 -biphenyldicarbonitrile (F-DCBP) under ionothermal synthesis conditions at 400 C using different ZnCl2/monomer ratios (Fig. 1) (see the detailed synthesis and characterization in the ESI, Fig. S1–S3†). The obtained F-DCBP-CTF materials were comprehensively characterized. Remarkably, they exhibit relatively large surface areas and pore volumes, signicantly high CO2 and H2 adsorption capacities and high CO2/N2 selectivity, outperforming most of the POPs reported to date. Furthermore, water vapor adsorption measurements conrmed signicant enhancement of the hydrophobicity of the materials in comparison to the non-functionalized analogues.

Results and discussion Synthesis and characterization of the CTF materials The porous materials were prepared at 400 C under typical ionothermal conditions using molten ZnCl2 as the Lewis acid catalyst. The obtained F-DCBP-CTFs were identied as F-DCBPCTF-1 and F-DCBP-CTF-2 when a ZnCl2/monomer ratio of 5 and 10 was applied, respectively. For comparison, non-functionalized CTFs based on 4,40 -biphenyldicarbonitrile (DCBP) as the monomer were also prepared and will be referred to as DCBP-CTF-1 and DCBP-CTF-2 in the following. It should be noted that at a high temperature, the uorine groups partially decomposed, conrmed by the gas release while opening the glass ampoules. Such a gas release was also observed in the earlier reported FCTF1 and the as-formed gases were identied as CF4, C2F4 and F2.14 The crystallinity of the synthesized materials was examined by powder X-ray diffraction (Fig. S4, ESI†). As expected, all the CTFs are amorphous. The successful trimerization reaction was

Fig. 1 Reaction scheme and schematic structure of F-DCBP-CTFs. (* 10% of theoretical fluorine atoms are retained in the framework).


Journal of Materials Chemistry A

indicated by FT-IR measurements (Fig. S5, ESI†). The weak band at 2230 cm1 for F-DCBP-CTFs points to the incomplete polymerization of the nitrile groups, while the characteristic weak bands at 1560 and 1380 cm1 can be assigned to the triazine rings.15,22 Elemental analysis (EA) revealed that the nitrogen and carbon contents of the synthesized DCBP-CTF materials are in accordance with the literature.10 As outlined in Table 1, a ZnCl2/monomer ratio of 5 resulted in CTF materials with higher nitrogen content, surface area and porosity in comparison to the materials obtained using a ZnCl2/monomer ratio of 10. More interestingly, the obtained F-DCBP-CTF materials have a much higher nitrogen content in comparison to the non-functionalized DCBP-CTF compounds, which are supposed to be one of the benecial binding sites for enhanced CO2 adsorption uptake in this framework. In all cases, the total amount of elements determined by EA is always lower than 100%, the residual mass can be assigned to trapped metal salts and water in the pores, which could not be removed completely.10,15 To obtain more detailed information about the structure of the materials, solid state cross-polarization magic angle spinning (CP/MAS) 13C and 19F NMR experiments were performed on all the CTF materials. As shown in Fig. S6, ESI†, the broad peak at 126 ppm corresponds to the aromatic carbons. It is however difficult to distinguish the positions of the various aromatic carbons owing to the partial graphitization, previously reported.22,32 The presence of uorine groups in the structure was conrmed by 19F NMR measurements, since the signal for carbon-uorine at approximately 137 ppm was clearly detected (Fig. S7, ESI†).33 Additionally, X-ray photoelectron spectroscopy (XPS) measurements were performed to get more insights into the chemical states of the elements in the CTF materials (Fig. 2 and S8–S10†). For F-DCBP-CTF-1, the C 1s spectrum can be deconvoluted into three peaks. The dominant peak at 284.8 eV is related to the aromatic sp2 carbon and the peak at 286.1 eV can be assigned to the triazine or nitrile carbon. The less pronounced peak at 288.1 eV is associated with carboxyl groups.14,30 The N 1s spectrum is divided into three peaks as well. The rst peak at 399 eV is related to the nitrile species,34 the peak at 400.5 eV is attributed to the pyrrolic/pyridonic-N species30 and the peak at 403.2 eV is assigned to the oxidized N–O species.34 The single peak at 687.6 eV in the F 1s spectra of the F-DCBP CTFs corresponds to the aromatic C–F bond.20 The uorine content was determined by XPS measurements. We note that the synthesized F-DCBPCTFs only contain about 10% of the theoretical uorine atoms. Specically, the uorine content amounts to 4.20% for F-DCBPCTF-1 and 3.13% for F-DCBP-CTF-2, which is much lower than the theoretical value (43%). The much lower F content is mainly because of the elimination of uorine and the slight difference between F-DCBP-CTF-1 and F-DCBP-CTF-2 is due to the fact that ZnCl2 is able to induce and accelerate the deuorination process, thereby generating different materials for different ZnCl2/monomer ratios.30 High angle annular dark eld scanning-transmission electron microscopy (HAADF-STEM) and corresponding energy dispersive X-ray spectroscopy (EDX) mapping images of F-DCBP-CTF-1 (Fig. 3) and F-DCBP-CTF-2 (Fig. S11, ESI†) show

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Elemental analysis and pore characteristics of all the synthesized CTF materials

Table 1

Elemental analysis (calculated/experimental) Sample

C

H

N

F

SBET (m2 g1)a

V0.1 (cm3 g1)b

Vtot (cm3 g1)c

DCBP-CTF-1 DCBP-CTF-2 F-DCBP-CTF-1 F-DCBP-CTF-2

82.3/84.4 82.3/79.4 48.3/59.7 48.3/59.4

3.92/2.75 3.92/2.83 —/1.24 —/1.97

13.7/6.55 13.7/4.96 8.05/11.3 8.05/10.2

— — 4.20d/43 3.13d/43

2437 2036 1574 1126

1.41 1.51 0.51 0.34

1.48 2.26 1.50 1.56

a BET surface area calculated over the pressure range 0.01–0.05 P/P0 at 77 K. calculated at P/P0 ¼ 0.98. d Determined by XPS.

b

V0.1, pore volume at P/P0 ¼ 0.1 at 77 K. c Vtot, total pore volume

stable up to 400 C, thus slightly lower than the DCBP-CTF materials (Fig. S14, ESI†). This can be attributed to the strong electron withdrawing effect of the uorine atoms in the framework.35 Gas sorption properties

Fig. 2

C 1s, N 1s and F 1s spectra of F-DCBP-CTF-1.

The porosity of all the obtained CTF materials was determined by nitrogen adsorption measurements collected at 77 K (Fig. 4). At a relative low pressure, all the isotherms exhibit a high N2 uptake, indicating a typical microporous character. Notably, for DCBP-CTF-2, a type IV isotherm with a H2 hysteresis loop in the desorption branch is obtained, typical for mesoporous materials according to the IUPAC classication. In contrast to DCBP-CTF-2, the F-DCBP-CTF materials give rise to type I isotherms with a steep rise of the isotherms at high relative pressure (P/P0 > 0.9), indicating the presence of meso-/macropores attributed to interparticulate voids that exist between the highly aggregated particles.36 As summarized in Table 1, the Brunauer–Emmett–Teller (BET) surface area and pore volume of DCBP-CTF-1 is calculated to be 2437 m2 g1 and 1.48 cm3 g1, respectively. In contrast, F-DCBP-CTF-1 exhibits a slightly lower surface area and pore volume of 1574 m2 g1 and 1.5 cm3 g1, respectively. The porous nature and high nitrogen content along with the introduction of uorine groups into the F-DCBP-CTF materials make them promising candidates for gas sorption and

Fig. 3 High angle annular dark field scanning-transmission electron microscopy (HAADF-STEM) and corresponding energy dispersive X-ray spectroscopy (EDX) mapping images of carbon (red), nitrogen (green) and fluorine (blue) in the sample of F-DCBP-CTF-1.

that C, N and F elements are well dispersed and homogeneously distributed throughout the materials. Scanning electron microscopy (SEM) images show that all the CTF materials are composed of tiny particles with irregular shapes and sizes (Fig. S12 and S13, ESI†). The thermal stability of all the studied CTF materials was determined by thermogravimetric analysis (TGA) whose results indicate that the F-DCBP-CTF materials are

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Fig. 4 N2 adsorption (solid symbols) and desorption (open symbols) isotherms of all the obtained CTF materials measured at 77 K.


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separation. Accordingly, the CO2 adsorption isotherms for both DCBP-CTFs and F-DCBP-CTFs up to 1 bar at 273 K and 298 K were recorded (Fig. 5 and S15 and S16, ESI†). In general, the materials synthesized with the ZnCl2/monomer ratio of 5 show a better performance in CO2 adsorption compared to those prepared with a ZnCl2/monomer ratio of 10, due to the higher microporosity, nitrogen content and number of uorine groups of the materials (Table 1). It is noteworthy that F-DCBP-CTF-1 exhibits a signicantly high CO2 adsorption capacity of 5.98 and 3.82 mmol g1 at 273 K and 298 K and 1 bar, respectively (Table 2, entry 1). To the best of our knowledge, this value represents the highest CO2 adsorption capacity of all the CTF materials synthesized at 400 C reported thus far, as summarized in Tables 2 and S1, ESI.† It should be noted that direct comparison of the CO2 adsorption capacities of the materials remains difficult as they are synthesized under different conditions and will partially or completely carbonize at high temperatures, and hence they are not CTFs anymore and normally called triazine-based porous carbon materials. More importantly, the realistic ue gas generally contains

Fig. 5 CO2 adsorption (solid symbols) and desorption (open symbols) isotherms of DCBP-CTF-1 and F-DCBP-CTF-1 at both 273 K and 298 K, respectively.

Table 2

approximately 15% CO2 at a total pressure of 1 bar, thus, the adsorption of CO2 at 0.15 bar is more relevant to realistic CO2 capture. Remarkably, at 0.15 bar and 273 K, F-DCBP-1 can adsorb 2.15 mmol g1 CO2, while at 298 K, this capacity only decreases moderately to 1.19 mmol g1, remaining signicantly higher than most of the reported porous organic polymers.37 The remarkably enhanced CO2 adsorption capacity for F-DCBPCTF-1 can be attributed to the strong electrostatic interactions between CO2 molecules and the polar C–F bonds in the skeleton. Additionally, incorporation of nitrogen into POP materials has already been proven to enhance the adsorption capacity of CO2,15,38 an observation in line with our ndings that F-DCBPCTFs have a much higher nitrogen content and for this reason a higher CO2 adsorption capacity compared to the DCBP-CTF materials. The outstanding CO2 uptake at both low and moderate pressure together with the remarkable water tolerance and high thermal and chemical stabilities of the peruorinated CTFs make them promising adsorbents for CO2 capture from ue gas. To provide a better understanding of the CO2 adsorption in the studied CTFs, we have also calculated the isosteric heat of CO2 adsorption (Qst) for both DCBP-CTF-1 and F-DCBP-CTF-1 materials using the Clausius–Clapeyron equation to t CO2 adsorption isotherms at 273 K and 298 K and plotted it as a function of the capacity of adsorbed CO2 (Fig. S17, ESI†). The Qst value at low CO2 adsorption uptake was calculated to be 33.1 kJ mol1 for F-DCBP-CTF-1, indicating the strong dipole–quadrupole interactions between CO2 molecules and the framework of F-DCBP-CTF. Additionally, the lower Qst value also means a much lower regeneration cost of this material compared to conventional amine solutions.39 The incorporation of uorine groups into CTF materials not only resulted in enhanced CO2 adsorption capacities but also afforded more preferential adsorption of CO2 over N2, i.e., higher CO2/N2 selectivity, another crucial factor for realistic CO2 capture. We calculated the selectivity of all the CTF materials using the ratio of the initial slopes in the Henry region of the CO2 and N2 adsorption isotherms at 298 K. (Fig. S18 and S19, ESI†). The calculated values are listed in Table S1† and a remarkable CO2/N2 selectivity of 31 was achieved by F-DCBP-

Summary and comparison of the gas sorption properties of the studied materials and the reported CTF materials CO2 uptake (mmol g1)

H2 uptake (wt%)

N2 uptake (mmol g1)

Sample

273 K

298 K

77 K

298 K

Qmax (kJ mol1) st

CO2/N2 selectivity

Ref.

F-DCBP-CTF-1 F-DCBP-CTF-2 DCBP-CTF-1 DCBP-CTF-2 FCTF-1 lut-CTF400 F-CTF-1-600 Bipy-CTF-600 CTF-pyHT HAT-CTFs

5.98 5.23 3.65 3.31 4.67 4.55 5.53 5.58 5.97 6.3

3.82 3.16 2.07 1.84 3.21 2.72 3.41 2.95 4.22 4.8

1.77 — — — — 1.36 — 2.1 2.63 —

0.28 0.29 0.24 0.13 — 0.20 — 0.28 0.25 0.44

33.1 — 24.1 — — 37.5 — 34.4 27.1 27.1

31 22 13 21 31a 63 19a 37 29 126

This work

a

14 15 14 15 12 11

CO2/N2 selectivity was calculated using ideal adsorbed solution theory (IAST).


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CTF-1, much higher than that of DCBP-CTF-1.13 The higher CO2/N2 selectivity further emphasizes the synergistic effects of microporosity and rich CO2-philic sites (N and F) within these polymeric networks. As such, given the exceptionally high CO2 adsorption capacity and the intrinsic high thermal and chemical stabilities, these materials seem to be plausible adsorbents for practical application in a postcombustion CO2 capture. As a potential renewable and clean alternative to current fossil fuels, hydrogen storage remains a major obstacle for their widespread implementation. Recently, adsorptive hydrogen storage in porous materials (e.g. MOFs and COFs) has been explored as a possible approach for hydrogen storage.40–42 For this reason, we further performed H2 adsorption measurements at 77 K up to 1 bar with those materials that showed higher CO2 uptake capacities (Fig. 6 and Table S1, ESI†). As shown in Fig. 6, the H2 adsorption capacity of DCBP-CTF-1 reaches 1.6 wt%, comparable to a previously reported result.10 Notably, F-DCBPCTF-1 exhibits a higher H2 uptake of 1.77 wt%, surpassing most of the reported CTF materials12 and ranking only slightly below -CTF-400 (1.95 wt%)43 and PCTF-1 (1.86 wt%)44 measured under identical conditions. Water vapor sorption To date, there have been only a few studies on the water sorption characteristics of CTFs.13,15 The hydrophobicity of the uorine groups prompted us to explore the potential of the CTF materials for water vapor sorption (Fig. 7 and S20, ESI†). As shown in Fig. 7, both CTF materials exhibit a type V isotherm, in which water molecules are only slightly adsorbed at low relative pressure, revealing the hydrophobic character of the pores, followed by a steep increase of water uptake at high relative pressures. A total water adsorption capacity of 630 cm3 g1 (0.51 g g1) for DCBP-CTF-1 is obtained whereas F-DCBP-CTF-1 only exhibits an adsorption capacity of 253 cm3 g1 (0.21 g g1) at P/P0 ¼ 0.9 and 293 K. This clearly demonstrates that the hydrophobicity of the pore surface of F-DCBP-CTF has been signicantly enhanced aer uorine functionalization, mainly due to the hydrophobicity of the uorine groups present in the framework that hinders the entrance of water molecules into the cavities easily.

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Fig. 7 Water vapor adsorption (solid symbols) and desorption (open

symbols) isotherms of F-DCBP-CTF-1 (red) and DCBP-CTF-1 (black) measured at 293 K.

Conclusions In summary, a class of robust and hydrophobic uorine-containing covalent triazine-based frameworks with large surface areas was synthesized under ionothermal conditions. Trimerization of the peruorinated monomer and the subsequent deuorination carbonization process of the network results in a signicant loss of uorine and carbon, yielding nitrogen-rich materials. The obtained porous materials display exceptionally high CO2 and H2 uptake capacities. The H2 uptake for F-DCBPCTF-1 can reach 1.77 wt% at 77 K and 1 bar, while the maximum CO2 adsorption capacity for F-DCBP-CTF-1 is up to 5.98 mmol g1 at 273 K and 1 bar (2.15 mmol g1 at 273 K and 0.15 bar), exceeding most of the reported POP materials in the literature to date. The uorine-containing CTFs also exhibit relatively high CO2/N2 selectivity compared to the non-functionalized CTFs. Moreover, in comparison to the DCBP-CTF materials, the hydrophobicity of the F-DCBP-CTF materials has been signicantly enhanced due to the hydrophobic nature of the C–F bonds, conrmed by ambient water vapor sorption measurements. These highly uorinated hydrophobic porous materials are not only of great interest for gas sorption and separation, but they also can be used for a wide range of applications including water/oil separation and capture of biorenewable resources from water.

Conflicts of interest There are no conicts to declare.

Acknowledgements

Fig. 6 Low pressure H2 adsorption isotherms of DCBP-CTF-1 (black) and F-DCBP-CTF-1 (red) measured at 77 K.

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G. W. thanks the China Scholarship Council (CSC) and UGENT BOF grant for PhD Funding. K. L. acknowledges the nancial support from Ghent University. H. S. J. acknowledges FWO [PEGASUS]2 Marie Sklodowska-Curie grant agreement no 665501 for the incoming post-doctoral fellowship. S.-N. Z. acknowledges the postdoctoral scholarship of the Ghent


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University Special Research Fund (BOF.PDO.2016.0030.01). H. D. is grateful for the Research Foundation Flanders (FWO-Vlaanderen) grant (no. G.0048.13N.). Y.-Y. L. acknowledges the nancial support of the China Ministry of Science and Technology (2016YFE01069800) and National Natural Science Foundation of China (21403025). The authors would like to thank Katrien Haustraete for STEM and EDX mapping measurements. Dr Johannes Schmidt is acknowledged for XPS measurements.

Notes and references 1 Z. J. Zhang, Z. Z. Yao, S. C. Xiang and B. L. Chen, Journal, 2014, 7, 2868–2899. 2 C. A. Trickett, A. Helal, B. A. Al-Maythalony, Z. H. Yamani, K. E. Cordova and O. M. Yaghi, Nat. Rev. Mater., 2017, 2, 17045. 3 Y. Zeng, R. Zou and Y. Zhao, Adv. Mater., 2016, 28, 2855– 2873. 4 S. Chu, Science, 2009, 325, 1599. 5 S. L. James, Chem. Soc. Rev., 2003, 32, 276–288. 6 S. Kitagawa, R. Kitaura and S.-i. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334–2375. 7 H. Furukawa, K. E. Cordova, M. O'Keeffe and O. M. Yaghi, Science, 2013, 341, 1230444. 8 A. P. Cˆ ot´ e, A. I. Benin, N. W. Ockwig, M. Keeffe, A. J. Matzger and O. M. Yaghi, Science, 2005, 310, 1166. 9 H. Furukawa and O. M. Yaghi, J. Am. Chem. Soc., 2009, 131, 8875–8883. 10 P. Kuhn, M. Antonietti and A. Thomas, Angew. Chem., Int. Ed., 2008, 47, 3450–3453. 11 X. Zhu, C. Tian, G. M. Veith, C. W. Abney, J. Dehaudt and S. Dai, J. Am. Chem. Soc., 2016, 138, 11497–11500. 12 G. Tuci, M. Pilaski, H. Ba, A. Rossin, L. Luconi, S. Caporali, C. Pham-Huu, R. Palkovits and G. Giambastiani, Adv. Funct. Mater., 2017, 27, 1605672. 13 S. Dey, A. Bhunia, H. Breitzke, P. B. Groszewicz, G. Buntkowsky and C. Janiak, J. Mater. Chem. A, 2017, 5, 3609–3620. 14 Y. Zhao, K. X. Yao, B. Teng, T. Zhang and Y. Han, Energy Environ. Sci., 2013, 6, 3684–3692. 15 S. Hug, L. Stegbauer, H. Oh, M. Hirscher and B. V. Lotsch, Chem. Mater., 2015, 27, 8001–8010. 16 C. E. Chan-Thaw, A. Villa, P. Katekomol, D. Su, A. Thomas and L. Prati, Nano Lett., 2010, 10, 537–541. 17 J. Roeser, K. Kailasam and A. Thomas, ChemSusChem, 2012, 5, 1793–1799. 18 P. Katekomol, J. Roeser, M. Bojdys, J. Weber and A. Thomas, Chem. Mater., 2013, 25, 1542–1548. 19 W. Zhang, F. Liang, C. Li, L.-G. Qiu, Y.-P. Yuan, F.-M. Peng, X. Jiang, A.-J. Xie, Y.-H. Shen and J.-F. Zhu, J. Hazard. Mater., 2011, 186, 984–990.



20 J. Byun, H. A. Patel, D. Thirion and C. T. Yavuz, Nat. Commun., 2016, 7, 13377. 21 L. Tao, F. Niu, C. Wang, J. Liu, T. Wang and Q. Wang, J. Mater. Chem. A, 2016, 4, 11812–11820. 22 K. Wang, H. Huang, D. Liu, C. Wang, J. Li and C. Zhong, Environ. Sci. Technol., 2016, 50, 4869–4876. 23 M. J. Bojdys, J. Jeromenok, A. Thomas and M. Antonietti, Adv. Mater., 2010, 22, 2202–2205. 24 R. Palkovits, M. Antonietti, P. Kuhn, A. Thomas and F. Sch¨ uth, Angew. Chem., Int. Ed., 2009, 48, 6909–6912. 25 H.-R. Yu, S. Cho, B. C. Bai, K. B. Yi and Y.-S. Lee, Int. J. Greenhouse Gas Control, 2012, 10, 278–284. 26 S. Hug, M. E. Tauchert, S. Li, U. E. Pachmayr and B. V. Lotsch, J. Mater. Chem., 2012, 22, 13956–13964. 27 T.-H. Chen, I. Popov, O. Zenasni, O. Daugulis and O. S. Miljanic, Chem. Commun., 2013, 49, 6846–6848. 28 D.-S. Zhang, Z. Chang, Y.-F. Li, Z.-Y. Jiang, Z.-H. Xuan, Y.-H. Zhang, J.-R. Li, Q. Chen, T.-L. Hu and X.-H. Bu, Sci. Rep., 2013, 3, 3312. 29 P. M. Bhatt, Y. Belmabkhout, A. Cadiau, K. Adil, O. Shekhah, A. Shkurenko, L. J. Barbour and M. Eddaoudi, J. Am. Chem. Soc., 2016, 138, 9301–9307. 30 X.-M. Hu, Q. Chen, Y.-C. Zhao, B. W. Laursen and B.-H. Han, J. Mater. Chem. A, 2014, 2, 14201–14208. 31 Z.-Z. Yang, Y. Zhao, H. Zhang, B. Yu, Z. Ma, G. Ji and Z. Liu, Chem. Commun., 2014, 50, 13910–13913. 32 C. Gu, D. Liu, W. Huang, J. Liu and R. Yang, Polym. Chem., 2015, 6, 7410–7417. 33 Q. Chen and K. Schmidt-Rohr, Macromolecules, 2004, 37, 5995–6003. 34 D. Y. Osadchii, A. I. Olivos-Suarez, A. V. Bavykina and J. Gascon, Langmuir, 2017, 33, 14278–14285. 35 P. Pachfule and R. Banerjee, in Encyclopedia of Inorganic and Bioinorganic Chemistry, John Wiley & Sons, Ltd, 2011, DOI: 10.1002/9781119951438.eibc2199. 36 B. Li, X. Huang, L. Liang and B. Tan, J. Mater. Chem., 2010, 20, 7444–7450. 37 L. Zou, Y. Sun, S. Che, X. Yang, X. Wang, M. Bosch, Q. Wang, H. Li, M. Smith, S. Yuan, Z. Perry and H. C. Zhou, Adv. Mater., 2017, 29, 1700229. 38 S.-Y. Ding and W. Wang, Chem. Soc. Rev., 2013, 42, 548–568. 39 W. Lu, D. Yuan, J. Sculley, D. Zhao, R. Krishna and H.-C. Zhou, J. Am. Chem. Soc., 2011, 133, 18126–18129. 40 L. Schlapbach and A. Zuttel, Nature, 2001, 414, 353–358. 41 L. J. Murray, M. Dinca and J. R. Long, Chem. Soc. Rev., 2009, 38, 1294–1314. 42 J. Sculley, D. Yuan and H.-C. Zhou, Energy Environ. Sci., 2011, 4, 2721–2735. 43 S. Hug, M. B. Mesch, H. Oh, N. Popp, M. Hirscher, J. Senker and B. V. Lotsch, J. Mater. Chem. A, 2014, 2, 5928–5936. 44 A. Bhunia, V. Vasylyeva and C. Janiak, Chem. Commun., 2013, 49, 3961–3963.

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