Room Temperature Batch and Continuous Flow Synthesis of Water

Jul 12, 2016 - Constructing Ultraporous Covalent Organic Frameworks in Seconds via an Organic Terracotta Process. Suvendu Karak , Sharath Kandambeth ...
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Room Temperature Batch and Continuous Flow Synthesis of WaterStable Covalent Organic Frameworks (COFs) Yongwu Peng,†,§ Wai Kuan Wong,†,§ Zhigang Hu,† Youdong Cheng,† Daqiang Yuan,‡ Saif A. Khan,*,† and Dan Zhao*,† †

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002 China

‡

S Supporting Information *

ABSTRACT: Covalent organic frameworks (COFs) have recently emerged as a new class of crystalline porous materials with many potential applications. The development of facile and effective synthetic methods of COFs is highly desirable for their large-scale applications. Herein, we demonstrate the room temperature batch synthesis of three classical two-dimensional (2D) COFs with various types of linkage, namely, COF-LZU1 (imine-linked), TpPa-1 (enamine-linked), and N3-COF (azinelinked). These obtained COFs exhibit good crystallinity and high porosity comparable to their counterparts synthesized solvothermally at higher temperatures. The facile formation of these COFs under such mild synthetic conditions can be attributed to (1) high solubility of monomers and (2) the strong π−π stacking interactions between monomers and π-systems of oligomers during the initial and the subsequent error-correction crystallization process. Based on this conclusion, two new iminelinked COFs named NUS-14 and NUS-15 were successfully synthesized with good crystallinity under ambient conditions. Moreover, continuous flow synthesis has been demonstrated in COF-LZU1 with a production rate of 41 mg h−1 at an extremely high space-time yield (STY) of 703 kg m−3 day−1. This study represents the first example of synthesizing COFs by continuous processes, which sheds light on the scaled-up synthesis of these promising materials.

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INTRODUCTION Covalent organic frameworks (COFs) are a novel class of porous crystalline materials constructed from purely organic building blocks linked by reversible covalent bonds featuring highly ordered and predesignable two-dimensional (2D) and three-dimensional (3D) architectures.1−5 Due to the covalent periodic ordering linkage as well as the elaborate control of organic units including geometry, size, and functionality, COFs have emerged as tailor-made porous materials and exhibit great potential in various applications such as gas storage and separation,6 optoelectronics,7−11 catalysis,12−15 etc. The last two decades have witnessed blooming studies of COFs, in which most of the reported COFs are synthesized under harsh solvothermal conditions such as reactions in sealed Pyrex tubes with inert atmosphere, high temperature/pressure, and long reaction time,16−19 which are inconvenient for scaled-up production. Moreover, some of the reported COFs, especially those with borate linkages, suffer from moisture instability which remains as the Achilles’ Heel for their broader application.20 Recently, several new synthetic methods have been developed for COF synthesis, such as microwave heating,21,22 ionothermal,23 sonochemical,24 and mechanochemical synthesis,25 etc. These methods may help to reduce the reaction time, while the reaction efficiency and COF products’ crystallinity and porosity remain to be further © 2016 American Chemical Society

improved. For example, liquid-assisted grinding (LAG) under ambient temperature has been demonstrated as a rapid and effective approach for the synthesis of chemically stable COFs, but the crystallinity and porosity of the obtained COFs are somehow inferior to those synthesized through conventional solvothermal methods.25 Microwave-assisted solvothermal methods have been used for the synthesis of several COFs with high crystallinity and porosity, while the throughput remains to be further improved.22 The facile synthesis of COFs with good crystallinity, high porosity, and stability remains challenging but highly desirable for their scaled-up production and wide applications. Very recently, room temperature steam-assisted conversion (SAC) has been proven as a facile route for the synthesis of 2D COFs as bulk powder or thin films with high porosity and good crystallinity.26,27 On the other hand, continuous flow synthesis has been demonstrated in some crystalline materials such as metal−organic frameworks (MOFs)28−32 and porous organic cages (POCs)33 with high space−time yields. These facts prompt us to explore the possibility of synthesizing COFs using batch solution-suspension approaches (SSA) or continuous Received: May 14, 2016 Revised: June 21, 2016 Published: July 12, 2016 5095

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Figure 1. (a) Schematic representation of the synthesis of COF-LZU1 (SSA), TpPa-1 (SSA), N3-COF (SSA), and NUS-14 (SSA) with 2D hexagonal skeletons via the room temperature solution-suspension approach (SSA); (b) Simulated and experimental PXRD patterns of COF-LZU1 (SSA), TpPa-1 (SSA), N3-COF (SSA), and NUS-14 (SSA) assuming 2D eclipsed stacking (inset images show crystal structures viewed through [001] (left) and [100] (right) directions).

flow systems under ambient conditions for their large-scale production.

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slowly added, and yellow precipitate was formed during this process. The vial was then sealed and left undisturbed for 3 days at room temperature. The yielded yellow precipitate was collected by centrifugation and washed with anhydrous tetrahydrofuran, anhydrous acetone, and anhydrous dichloromethane, separately. The collected powder was then activated by solvent exchange with anhydrous methanol for 3 times and dried at 120 °C under vacuum for 12 h to give a yellow powder with 84% isolated yield and a molecular formula of (C18H12N3)n (% calc/found: C 79.98/77.42, H 4.47/4.70, N 15.55/ 14.32). Synthesis of COF-LZU1 using a continuous flow system: stock solutions of TFB (0.17 M) and PDA (0.25 M) were prepared separately in a mixture of dioxane/3 M acetic acid (5:1, v/v, 0.7 mL) and then injected together into a flow reactor (PTFE tube, 10 cm 0.3 mm ID). The synthesis was conducted at room temperature with an individual flow rate of 20 μL min−1 for both TFB and PDA stock solutions, giving a residence time of 11 s. The obtained yellow product was collected and washed with anhydrous tetrahydrofuran, anhydrous acetone, and anhydrous dichloromethane, separately. The collected powder was then activated by solvent exchange with anhydrous methanol 3 times and dried at 120 °C under vacuum for 12 h to give a yellow powder with 76% isolated yield.

EXPERIMENTAL SECTION

Materials and Equipment. All chemicals and reagents were commercially available and used without further purification. 1,3,5Triformylphloroglucinol (TFP),34 1,3,5-tris(4-formyl-phenyl)benzene (TFPB),35 and 1,3,5-tris(4-formyl-phenyl)triazine (TFPT)13 were synthesized according to published procedures. Fourier transform infrared spectroscopy (FTIR) spectra were obtained with a Bio-Rad FTS-3500 ARX FTIR spectrometer under N2 atmosphere. Elemental analyses were performed on a Vario MICRO series CHNOS elemental analyzer. Solid-state nuclear magnetic resonance (NMR) data were collected on a Bruker Avance 400 MHz NMR spectrometer (DRX400) with cross-polarization magic-angle spinning (CP/MAS). Powder X-ray diffraction (PXRD) patterns were obtained on a Bruker D8 Advance X-ray powder diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å) at a scan rate of 0.02° s−1. Thermogravimetric analyses (TGA) were performed using a Shimadzu DTG-60AH in the temperature range of 50 to 800 °C under flowing N2 (30 mL min−1) with a heating rate of 10 °C min−1. Scanning electron microscopy (SEM) was conducted on a JEOL-JEM5600 Lab-SEM (15 kV) equipped with an energy dispersive spectrometer. Samples were treated via Pt sputtering before observation. The Brunauer−Emmett− Teller (BET) surface areas were calculated from N2 sorption isotherms at 77 K using a Micromeritics ASAP 2020 surface area and pore size analyzer. Pore size distribution data were calculated based on the nonlocal density functional theory (NLDFT) model in the Micromeritics ASAP2020 software package. The continuous flow system was composed of two syringes equipped with T-micro mixer and 0.3 mm i.d. polytetrafluoroethylene (PEFE) coil connected in series. Synthetic Procedure. In a typical procedure, exemplified by COF-LZU1, by using room temperature solution-suspension approach (SSA): A 10 mL glass vial was charged with 1,3,5-triformylbenzene (TFB) (32 mg, 0.2 mmol), p-phenylenediamine (PDA) (32 mg, 0.3 mmol), and dioxane (2 mL). The mixture was sonicated for 10 min to get a clear solution. Subsequently, acetic acid (3 M, 0.4 mL) was

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RESULTS AND DISCUSSION

Synthesis and Characterization. We first attempted the room temperature SSA synthesis of three water-stable 2D COFs with various types of linkage, namely, COF-LZU1 (imine-linked),36 TpPa-1 (enamine-linked),37 and N3-COF (azine-linked)13 using D3h symmetric aldehydes as vertices and D2h symmetric amine or hydrazine as edges, which can be covalently linked affording 2D hexagonal skeletons (Figure 1a). COF-LZU1 and TpPa-1 were synthesized by the cocondensation of 1,3,5-triformylbenzene (TFB) or 1,3,5triformylphloroglucinol (TFP) with p-phenylenediamine (PDA), respectively. N3-COF was synthesized from the reaction of 1,3,5-tris(4-formyl-phenyl)triazine (TFPT) with 5096

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chemical structure with the chemical shift of keto-form carbonyl carbon at 183.2 ppm (Figure S1b). The permanent porosity of these COFs synthesized through the room temperature batch approach was investigated by measuring N2 sorption isotherms at 77 K (Figure 2). Both

hydrazine (HZ). In a typical procedure, a mixture of monomers and solvent in a 10 mL vial was mixed and sonicated to obtain a homogeneous suspension, which was sealed and left undisturbed at room temperature for 3 days. The yielded precipitate was collected by centrifugation, washed with anhydrous tetrahydrofuran, anhydrous acetone, and anhydrous dichloromethane, separately, and dried under vacuum to afford the target products named COF-LZU1 (SSA), TpPa-1 (SSA), and N3-COF (SSA). These COFs were insoluble in water and common organic solvents and were formulated to be (C18H12N3)n for COF-LZU1 (SSA), (C18H12N3O3)n for TpPa-1 (SSA), and (C24H15N6)n for N3-COF (SSA) based on elemental analyses performed on guest-free samples. The obtained COFs exhibit good crystallinity as revealed by the powder X-ray diffraction (PXRD) patterns (Figure 1b). The PXRD patterns of COF-LZU1 (SSA) and TpPa-1 (SSA) demonstrate main diffraction peaks arising from the (100) facet at 4.74 and 4.68°, respectively, as well as some diffractions from (110) (8.10°), (200) (9.42°), (210) (12.66°), and (001) (25.58°) for COF-LZU1(SSA) and (110) (7.94°), (210) (11.9°), and (001) (26.76°) for TpPa-1(SSA). The presence of slightly broad peaks in TpPa-1 (SSA) observed at higher 2θ angles of the (001) facet may originate from the defects in the π−π stacking between the successive COF layers.25,37 The π−π stacking distances between the COF layers calculated from (001) facet diffraction were estimated to be 3.70 and 3.40 Å for COF-LZU1 (SSA) and TpPa-1 (SSA), respectively (Figure 1b). The PXRD pattern of N3-COF (SSA) exhibits an intense peak at 3.60° along with some minor peaks at 6.11, 7.13, 9.44, and 12.68°, which can be assigned to the (100), (110), (200), (210), and (220) facets, respectively. The presence of the (001) facet at 25.92° indicates that the periodicity of the 2D sheets is extended to the third dimension,38 and the π−π stacking distance between the COF layers was calculated to be 3.47 Å (Figure 1b). The experimental PXRD patterns of these COFs obtained through room temperature batch synthesis match well with the simulated patterns obtained using the eclipsed 2D stacking crystal models and are also comparable with the PXRD patterns of their solvothermal counterparts. Fourier transform infrared (FT-IR) spectra of COF-LZU1 (SSA) and N3-COF (SSA) demonstrate a typical stretching band arising from CN at 1620 cm−1. In contrast, the aldehyde band of the starting material (1697 cm−1) in COFLZU1 (SSA) is strongly attenuated, indicating the formation of imine-linked bonds (Figure S1a). The FT-IR spectra of TpPa-1 (SSA) reveal the disappearance of carbonyl stretching band of TFP (1643 cm−1) along with a series of new characteristic stretching bands at 1597 and 1284 cm−1 originating from the CC and CN stretching bands, respectively, confirming the condensation reaction and the formation of enimine-linked linkages (Figure S1b). No carbonyl stretching bands can be found corresponding to the starting monomers (TFPT around 1705 cm−1) in the FT-IR spectra of N3-COF (SSA), demonstrating that the product is a fully azine-linked network (Figure S1c). The chemical structures of these COFs were further confirmed by solid-state 13C cross-polarization magicangle-spinning nuclear magnetic resonance (13C CP/MAS NMR), with the characteristic chemical shifts of 156.9 ppm for COF-LZU1 (SSA) and 159.2 ppm for N3-COF (SSA) attributing to the imine carbon of the imine (CN ArNC) and diazabutadiene (CN−NC) linkers, respectively (Figure S1a,c). The 13C CP/MAS NMR spectrum of TpPa-1 (SSA) also confirms the proposed

Figure 2. N2 sorption isotherms measured at 77 K (top) and pore size distribution (bottom) of COF-LZU1 (SSA), TpPa-1(SSA), N3-COF (SSA), NUS-14 (SSA), and COF-LZU1 (FS). Adsorption, closed; desorption, open.

COF-LZU1 (SSA) and TpPa-1 (SSA) exhibit reversible type I isotherms indicating their microporous structures (pore size less than 2 nm). Meanwhile, N3-COF (SSA) adopts a reversible type I/IV isotherm with a step observed at P/P0 = 0.10−0.20 suggesting narrowly distributed mesopores (pore size between 2 and 50 nm) matching well with the crystal structure. The Brunauer−Emmett−Teller (BET) surface areas were calculated to be 412 m2 g−1 for COF-LZU1 (SSA), 834 m2 g−1 for TpPa-1 (SSA), and 1722 m2 g−1 for N3-COF (SSA), which are comparable to that of their solvothermally synthesized counterparts: COF-LZU1 (410 m2 g−1),36 TpPa-1 (535 m2 g−1),37 and N3-COF (1537 m2 g−1).13 It is interesting to note that the BET surface areas of COFs synthesized via room temperature SSA are higher than that of the COFs synthesized solvothermally proving the effectiveness of this synthetic approach and can be attributed to its milder synthetic conditions allowing for better crystallization process.39 Pore size distribution calculated based on N2 sorption isotherms reveals average pore sizes of ca.18 Å for both COF-LZU1 5097

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interlayer packing under ambient conditions. This result clearly indicates the important role of π−π stacking interactions between π-systems of monomers and oligomers as the driving force toward the formation of crystalline COF products under mild conditions. On the other hand, replacing TFPT with TFPB in N3-COF (SSA) affords another COF named TFPBHZ (SSA) (aka N-COF)13 with a much lower BET surface area and poor stability (Figures S4−7). Considering the similar chemical structure and molecular weight of N3-COF and TFPB-HZ, the huge difference in BET surface area may come from the possible steric hindrance between the central phenyl ring and the peripheral phenyl rings of TFPB units in TFPBHZ (SSA) resulting in less favorable interlayer stacking and weakened crystallinity,49 which has been confirmed in other COFs.50,51 It has been reported in MOFs that such steric hindrance is absent in TFPT units because of the central triazine ring,49 which may cause more effective π−π stacking interactions and better crystallinity of resultant COFs.41,52 Room Temperature Batch Synthesis of NUS-14 and NUS-15. So far, we have reached the conclusion that room temperature SSA synthesis of COFs should work as long as the monomers have good solubility, with rigid conformation and strong π interactions serving as driving forces for effective interlayer packing and crystal growth. Following these rules, we have successfully synthesized two new COFs named NUS-14 and NUS-15 by matching TFPT (NUS-14) or TFPB (NUS15) with PDA through room temperature SSA synthesis (see Supporting Information for details). These two new COFs were insoluble in water and common organic solvents and were formulated as (C 33 H 21 N 6 ) n for NUS-14 (SSA) and (C36H24N3)n for NUS-15 (SSA) based on the elemental analyses performed on guest-free samples. The expected chemical compositions of these two COFs were confirmed by FT-IR and 13C CP/MAS NMR (Figure S8). Because of the nearly planar conformation and strong π interactions of monomers, NUS-14 and NUS-15 exhibit excellent crystallinity matching well with the eclipsed 2D structural models (Figure 1, Figure S9). Both NUS-14 and NUS-15 adopt reversible type I/ IV isotherms with steps observed at P/P0 ≈ 0.3 indicating their mesoporous structures (pore size between 2 and 50 nm) agreeing well with the crystal structures. The BET surface areas of NUS-14 and NUS-15 were calculated to be 1164 m2 g−1 and 322 m2 g−1, respectively (Figure 2, Figure S10). The difference in BET surface area between NUS-14 and NUS-15 may also be attributed to the slight difference in conformation between TFPT and TFPB. Room Temperature Continuous Flow Synthesis of COF-LZU1. As an alternative synthetic route to conventional batch reactions, continuous flow systems have been renowned as one of the most promising synthetic methodologies and have become increasingly popular in the past two decades due to their advantages of rapid heat and mass transfer resulting in better reaction kinetics and improved reaction yields.53−55 Recently, continuous flow systems have been deployed to produce crystalline porous materials such as MOFs28−32 and POCs33 with promising results. With the above results of room temperature batch synthesis of COFs in hand, we then further explored the possibility of synthesizing COFs using a simple continuous laminar flow system, which consists of two syringes acting as reservoirs, leading to a T-micromixer and a 0.3 mm i.d. polytetrafluoroethylene (PTFE) coil of 0.1 m length connected in series (Figure 4). COF-LZU1 was chosen as the target material due to its facile synthesis under mild

(SSA) and TpPa-1 (SSA) while ca. 24 Å for N3-COF (SSA) matching well with their crystal models. Mechanism Study. The above characterization results have unambiguously proven the successful synthesis of these 2D COFs by room temperature SSA. Compared to the reported COFs synthesized by solvothermal approaches,13,36,37 the obtained COFs in this study display similar or even better properties in terms of crystallinity and porosity. In addition, the scalability of the room temperature SSA demonstrated in this study facilitates massive production capability, which is far superior to the solvothermal approaches. The next question we seek to answer isunder what conditions does the room temperature SSA work? Most COFs are constructed from organic building blocks via reversible covalent bonds under dynamic covalent chemistry.16−18 The reversibility of the reactions provides an error-correction mechanism that enables the conversion of kinetic intermediates (amorphous) to thermodynamically stable forms (crystalline). Such an errorcorrection process may proceed under mild conditions (e.g., ambient temperature) given sufficient driving forces,40−42 which has been well recognized in the synthesis of other crystalline materials such as supramolecular assemblies,43,44 POCs,45,46 etc. In this study, we have noticed that good solubility of monomers might be one of the driving forces for the formation of COFs under ambient conditions. In addition, the rigid conformation of monomers with strong π interactions also plays an important role. We speculate that the synergistic effects from the π−π stacking interactions between monomers and π-systems of oligomers during the initial and the subsequent error-correction process should have a major influence on the crystallinity and porosity of resultant COFs. To verify this hypothesis, we conducted several comparative experiments by matching monomers with different conformation and π systems. As shown in Figure 3, four D3h symmetric

Figure 3. Combination study of COF synthesis via room temperature solution-suspension approach by matching D3h symmetric aldehydes as vertices with D2h symmetric amine or hydrazine as edges.

vertices were chosen in which TFPB and TFPT have nearly planar conformation and stronger π systems than TFB and TFP. Accordingly, two D2h symmetric edges were chosen where PDA has a stronger π system than HZ. Similar room temperature SSA syntheses were applied, and the products were subject to FT-IR, 13C CP/MAS NMR, and PXRD tests to verify their structures. Replacing PDA in COF-LZU1 (SSA) and TpPa-1 (SSA) with HZ afforded two COFs named TFBHZ (SSA) (aka ACOF-1)47 and TFP-HZ (SSA) (aka NUS2),48 whose chemical structures can be confirmed by FT-IR and NMR (Figure S2) but with much worse crystallinity (Figure S3) possibly due to the ineffective error correction and 5098

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an important step toward the massive production of COFs for practical 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/acs.chemmater.6b01954. Experimental procedures and additional figures and tables (PDF)

Figure 4. Representative chart for the continuous flow synthesis of COF-LZU1 (FS).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.Z.). *E-mail: [email protected] (S.A.K.).

conditions. After several rounds of parameter optimization, COF-LZU1 can be successfully produced through the flow system when 0.17 M TFB and 0.25 M PDA dissolved in dioxane/3 M acetic acid (5/1 v/v) solutions were simultaneously injected at a flow rate of 20 μL min−1 (corresponding to a residence time of merely 11 s). The resultant COF [denoted as COF-LZU1 (FS), FS indicates flow synthesis] exhibits identical FT-IR and 13C NMR spectra (Figure S11) and PXRD patterns (Figure S12) to the one synthesized through SSA, confirming the expected chemical structure and excellent crystallinity. To our surprise, COF-LZU1 (FS) has a higher BET surface area than COF-LZU1 (SSA) (453 m2 g−1 vs 412 m2 g−1), possibly due to the improved crystallization process under flow conditions enabled by high local supersaturation at the interface between the two flowing streams. It is worth of noting that given the optimized reaction conditions, we were able to continuously produce 41 mg of COF-LZU1 (FS) in 1 h from this simple single reactor unit, and the resultant space-time yield (STY) of this process, which is defined as kg of product per m3 of reaction tube per day of synthesis, was estimated to be 703 kg m−3 day−1. To the best of our knowledge, this study represents the first example of COF synthesis through continuous flow systems, which greatly facilitates the scaled-up production of these promising materials.

Author Contributions §

Y.P. and W.K.W. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National University of Singapore (CENGas R-261-508-001-646) and Singapore Ministry of Education (MOE AcRF Tier 1 R-279-000-410-112, AcRF Tier 2 R-279-000-429-112).

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REFERENCES

(1) Côté, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.; Matzger, A. J.; Yaghi, O. M. Porous, crystalline, covalent organic frameworks. Science 2005, 310, 1166−1170. (2) El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortés, J. L.; Côté, A. P.; Taylor, R. E.; O’Keeffe, M.; Yaghi, O. M. Designed synthesis of 3D covalent organic frameworks. Science 2007, 316, 268−272. (3) Mastalerz, M. The next generation of shape-persistant zeolite analogues: covalent organic frameworks. Angew. Chem., Int. Ed. 2008, 47, 445−447. (4) Colson, J. W.; Dichtel, W. R. Rationally synthesized twodimensional polymers. Nat. Chem. 2013, 5, 453−465. (5) Ascherl, L.; Sick, T.; Margraf, J. T.; Lapidus, S. H.; Calik, M.; Hettstedt, C.; Karaghiosoff, K.; Doeblinger, M.; Clark, T.; Chapman, K. W.; Auras, F.; Bein, T. Molecular docking sites designed for the generation of highly crystalline covalent organic frameworks. Nat. Chem. 2016, 8, 310−316. (6) Furukawa, H.; Yaghi, O. M. Storage of hydrogen, methane, and carbon dioxide in highly porous covalent organic frameworks for clean energy applications. J. Am. Chem. Soc. 2009, 131, 8875−8883. (7) Dogru, M.; Handloser, M.; Auras, F.; Kunz, T.; Medina, D.; Hartschuh, A.; Knochel, P.; Bein, T. A photoconductive thienothiophene-based covalent organic framework showing charge transfer towards included fullerene. Angew. Chem., Int. Ed. 2013, 52, 2920− 2924. (8) Dogru, M.; Bein, T. On the road towards electroactive covalent organic frameworks. Chem. Commun. 2014, 50, 5531−5546. (9) Chen, L.; Furukawa, K.; Gao, J.; Nagai, A.; Nakamura, T.; Dong, Y. P.; Jiang, D. L. Photoelectric covalent organic frameworks: Converting open lattices into ordered donor-acceptor heterojunctions. J. Am. Chem. Soc. 2014, 136, 9806−9809. (10) Cai, S. L.; Zhang, Y. B.; Pun, A. B.; He, B.; Yang, J. H.; Toma, F. M.; Sharp, I. D.; Yaghi, O. M.; Fan, J.; Zheng, S. R.; Zhang, W. G.; Liu, Y. Tunable electrical conductivity in oriented thin films of tetrathiafulvalene-based covalent organic framework. Chem. Sci. 2014, 5, 4693−4700. (11) Duhović, S.; Dincă, M. Synthesis and electrical properties of covalent organic frameworks with heavy chalcogens. Chem. Mater. 2015, 27, 5487−5490.

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CONCLUSIONS In summary, we aim to develop facile methods for COF production in this study. Three water stable COFs with similar 2D hexagonal skeletons but different linkages, namely, COFLZU1 (imine-linked), TpPa-1 (enamine-linked), and N3-COF (azine-linked), were successfully synthesized by room temperature solution-suspension batch approach with excellent crystallinity and porosity. The prerequisites for this synthetic approach include good solubility of monomers and strong π interactions serving as driving forces for the crystallization under mild conditions. Following these rules, we have successfully synthesized two new COFs named NUS-14 and NUS-15 using monomers featuring nearly planar conformation and strong π interactions. In addition, we have pioneered the room temperature continuous flow synthesis of COFs exemplified by COF-LZU1, with a production rate of 41 mg h−1 at an extremely high space-time yield of 703 kg m−3 day−1. To the best of our knowledge, this is the first example to synthesize COFs using continuous flow systems. Although some of the prerequisites for this synthetic approach (e.g., good solubility of the monomers) may restrict the feasibility of this methodology to some extent, it is worth of noting that our observations related to the synthetic methodologies will pave 5099

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Chemistry of Materials

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DOI: 10.1021/acs.chemmater.6b01954 Chem. Mater. 2016, 28, 5095−5101

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DOI: 10.1021/acs.chemmater.6b01954 Chem. Mater. 2016, 28, 5095−5101