Pd NP-Loaded and Covalently Cross-Linked COF Membrane

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Energy, Environmental, and Catalysis Applications

Pd NPs Loaded and Covalently Cross-Linked COF-Membrane Microreactor for Aqueous CBs Dechlorination at Room Temperature Bing-Jian Yao, Jiang-Tao Li, Ning Huang, Jinglan Kan, Liang Qiao, Luo-Gang Ding, Fei Li, and Yu-Bin Dong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04022 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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ACS Applied Materials & Interfaces

Pd NPs Loaded and Covalently Cross-Linked COFMembrane Microreactor for Aqueous CBs Dechlorination at Room Temperature Bing-Jian Yao,

‡a

Jiang-Tao Li,

‡a

Ning Huang,b Jing-Lan Kan,a Liang Qiao,c Luo-Gang Ding,a Fei Li,a Yu-

Bin Dong*a a

College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation

Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, P. R. China. b

Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United

States c

Department of Chemical and Biological Engineering, University at Buffalo (SUNY), Buffalo,

New York 14260, United States. KEYWORDS. COF, Pd NPs, covalently cross-linked membrane, continuous-flow, CBs dechlorination in water

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ABSTRACT. We report an allyl-decorated and hydrazine-connected covalent organic framework (COF-AO, 1) which could support Pd nanoparticles (Pd NPs) to generate (Pd@COFAO, 2). The incorporation of 2 with thiol-functionalized polysiloxane (termed as PSI-SH) via thiol-ene click reaction provided the stand-alone and elastic membrane (3). The obtained COFinvolved and Pd NPs loaded covalently linked membrane of 3 is robust, permanently porous, uniform, processable and water permeable. Moreover, it can be used to construct highly efficient membrane-based microreactor for continuous-flow operation to catalyze CBs dechlorination in water at room temperature. The provided approach herein allows the processability and practical application of the powdered COF materials to be feasible.

INTRODUCTION Chlorobenzenes (CBs),1 as an important class of hazardous pollutants, objectively and widely exist in various water sources such as wastewater, groundwater, river and estuaries.2-5 Owing to their high toxicity, persistence, and bioaccumulate, CBs have been listed as the priority pollutants by the Environmental Protection Agency (EPA) in many countries such as China, the United States and European Community. Up to now, some CBs degradation methods, such as aerobic biodegradation,6-8 catalytic hydrogenolysis and oxidation,9-10 photo-chemical treatment11 and transition metal-assisted hydrogenation, have been successfully developed.12 On the other hand, the C-Cl bond has been found to be relatively inactive,13 so a general and facile CBs dechlorinating approach that is able to achieve the highly efficient C−Cl bond cleavage under mild conditions is extremely desired. Among various CBs detoxification techniques, metal nano particles (M NPs)-based catalytic hydrogenation is regarded as a powerful and economical way whereby the C-Cl bond cleavage in

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CBs can be realized under mild conditions. For example, palladium nano particles (Pd NPs) catalyzed hydrodechlorination is simple, safe and highly efficient, 14-17 and it can be employed to treat the CBs in water at ambient temperature, furthermore, facilitates the dechlorinated product recycling with lower net impact on the environment.

18-19

aggregate into Pd black due to their high surface energy.

Pd NPs, however, are easily to

20-21

For addressing such an issue,

chemists have ingeniously made use of porous materials to immobilize the metal NPs. For instance, Pd NPs can be well dispersed and stabilized into metal-organic frameworks (MOFs), in which their uneven dispensability is effectively avoided. 22-25 Covalent organic frameworks (COFs)26-33 have currently gained a great attention owing to their unique properties and wide range of potential applications.34-35 Compared to MOFs, their good aqueous chemical stability would make COFs an ideal type of supports to upload active Pd NPs 36-39 for CBs dechlorination, especially in water. So far, various organic functional linkages, such as boronate ester and borosilicate,27 Schiff base,27 triazine,28 hydrazone and squaraine,29 and so on were used to build COFs. Among them, the hydrazone-based 2D COFs, which were reported by Yaghi40 and Zhao groups,41 are interesting due to their excellent water stability. Hopefully, such hydrazone-connected COFs could be used as a platform to support Pd NPs for the composite catalytic systems, furthermore, applied to the aqueous CBs dechlorination. On the other hand, the aqueous continuous-flow microreactor is very attractive

42

as it allows

to get over the catalyst separation and regeneration issues usually associated with batch CBs dechlorination, moreover, enhance the operability in large-scale water treatment. In this context, Pd NPs loaded and COF-based catalytic membrane should be the ideal candidate for fabrication of the continuous-flow water treatment microreactor. In principle, the polymerizable groups attached COF particles and their oligomer counterparts could be covalently coupled via

3



copolymerization into composite membrane. In this way, COF particles could be uniformly and stoutly tied up in the generated polymeric phase. Thus, this post-synthetic covalent copolymerization (termed as PSCCP) strategy should be an alternative approach to access COFbased membranes with good flexibility, dispersion, and permeability. In this contribution, we report for the first time a Pd NPs loaded covalently cross-linked COFmembrane which was generated from allyl-functionalized Pd-loaded COF and thiol-decorated polysiloxane (PSI-SH) via thiol-ene click reaction. Notably, the Pd NPs-loaded COF membrane can be used to set up highly efficient membrane-based microreactor for continuous-flow operation to promote CBs dechlorination in water at room temperature. EXPERIMENTAL SECTION Materials and Instrumentation. All chemicals were obtained from commercial sources (Acros) and used without further purification. Triformylphloroglucinol was synthesized according to the reported literature.43 The thiol chain decorated polysiloxane (PSI-SH) was synthesized according to the literature method.44 All the characterization data such as IR and NMR for the known compounds provided in ESI were well consistent with those reported results. 1H NMR spectra were obtained on a Bruker AVANCE-400 spectrometer, and the chemical shifts are reported in δ relative to TMS. Infrared spectra performed on a Bruker ALPHA FTIR spectrometer were obtained in the 400-4000 cm-1 range. Scanning electron micrographs (SEM) were conducted on a Gemini Zeiss SUPRA scanning electron microscope equipped with an energy-dispersive X-ray detector. The X-ray diffraction pattern was measured by a D8 ADVANCE X-ray powder diffractometer with the Cu Kα radiation (λ = 1.5405 Å). Inductively coupled plasma (ICP) data was obtained on an IRIS Intrepid (II) XSP and Nu AttoM.

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The N2 adsorption-desorption isotherms were performed on an ASAP 2020/TriStar 3000 (Micromeritics). Transmission electron microscopy (TEM) was performed at an accelerating voltage of 120 kV on JEM-1400 electron microscope (JEOL). X-ray photoelectron spectroscopy (XPS) were performed with a PHI 5000 Versaprobe II (VP-II) electron spectrometer (Ulvac-Phi) using 300W Al Kα radiation with a base pressure of 3 × 10-9 mbar. The binding energies of the elements present in the air-facing side of the specimen perpendicular to the electron beam was calculated according to that of the C1s line at 284.8 eV from adventitious carbon. Thermogravimetric analyses (TGA) curves were obtained on a TA Instrument Q5 simultaneous TGA at a heating rate of 10°C/min from room temperature to 650oC under flowing nitrogen. GC spectrum was obtained using Angilent7890A device. The elemental analysis was performed on Perkin-Elmer Model 2400.

Scheme 1. Synthesis of dimethyl 2,5-bis(allyloxy)terephthalohydrazide. Synthesis of Dimethyl 2,5-Bishydroxyterephthalate. As shown in Scheme 1, a mixture of 2,5-bishydroxyterephthalic acid (1.98 g, 10 mmol), methanol (100 mL) and concentrated sulfuric acid (4 mL) was refluxed for 12 h with magnetic stirring. After cooling to the room temperature, the pH value was adjusted to ca. 7.0 by Na2CO3 saturated aqueous solution. The crude product was extracted with ethyl acetate and then purified by column chromatography on silica gel to generate dimethyl 2,5-bishydroxyterephthalate as yellow powder (1.88 g, 95 %). 1H NMR (400 MHz, DMSO-d6, δ, ppm) 9.8 (s, 2H), 7.27 (s, 2H), 3.8 (s, 6H). ESI-MS: calcd for (C10H10O6) M+

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m/z 225.04, found m/z 225.04; FTIR (KBr, cm−1): 3489 (m), 3237(m), 2959 (s), 1683 (s), 1496 (m), 1442(s), 1332 (s), 1198 (s), 1099 (s), 1080 (m), 957 (s), 791 (s), 724(m). Synthesis of Dimethyl 2,5-Bis(allyloxy)terephthalate. K2CO3 (2.76 g) was added to a DMF solution (35 mL) of dimethyl 2,5-dihydroxyterephthalate (0.99 g, 4.4 mmol). After addition of allyl bromide (836 µL, 9.68 mmol), the mixture was heated at 85°C for 3 h. After removal of the residual allyl bromide in vacuum, the crude product was washed with water and dried in vacuum to afford dimethyl 2,5-bis(allyloxy)terephthalate as white floccus solids (0.5 g, 50 % yield). 1H NMR (400 MHz, DMSO-d6, δ, ppm), 7.36 (s, 2H), 6.1-5.8 (m, 2H), 5.5-5.2 (m, 4H), 4.6 (d, 4H), 3.8 (s, 6H). ESI-MS: calcd for (C16H18O6) M+ m/z 307.11, found m/z 307.11 M+Na+ m/z 329.10, found m/z 329.10; FTIR (KBr, cm−1): 3067 (w), 3014(m), 2955 (s), 1709 (s), 1508 (m), 1435 (s), 1410 (s),1301 (s), 1210 (s), 1124 (m), 1008 (m), 933 (s), 894 (m), 799 (m), 784 (m). Synthesis

of

2,5-Bis(allyloxy)terephthalohydrazide.

A

mixture

of

dimethyl

2,5-

bis(allyloxy)terephthalate (0.67 g, 2.2 mmol) and hydrazine hydrate (6 mL) in methanol (45 mL) was refluxed for 12 h. After cooling to room temperature, the obtained crystals were thoroughly washed with water and methanol to afford 2,5-bis(allyloxy)terephthalohydrazide (0.57 g, 85 % yield). 1H NMR (400 MHz, DMSO-d6, δ, ppm), 9.29 (s, 2H), 7.3 (s, 2H), 6.15-5.9 (m, 2H), 5.455.35 (m, 2H), 5.31-5.25 (m, 2H), 4.69-4.58 (m, 4H), 4.60-4.55 (m, 4H). ESI-MS: calcd for (C14H18N4O4) M+ m/z 307.13, found m/z 307.14; FTIR (KBr, cm−1): 3292(s), 3198(m), 2921(m), 2850(m), 1626(m), 1496(s), 1408(s), 1276(s), 1216(s), 1023 (m), 987(m), 921(s), 817(s).

13

C

NMR (400 MHz, DMSO-d6, δ, ppm): 165, 150, 134, 125, 117, 115, 70. Synthesis of COF-AO (1). A 1,4-dioxane/mesitylene (1.5/4.5 mL) solution of 2,5bis(allyloxy)terephthalohydrazide (46 mg, 0.15 mmol), triformylphloroglucinol (21 mg, 0.1 mmol), acetic acid (0.6 mL, 6.0 M) in a pyrex tube was flash frozen in a liquid nitrogen bath and

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degassed. Upon warming to room temperature, the tightly capped tube was heated at 120°C for 3 days. The obtained precipitate was collected by centrifugation and completely washed with acetone and ethanol, separately. The collected solids were dried in vacuum to generate the assynthesized COF-AO (1) as yellow crystalline solids (51 mg, 75 % yield). Elemental analysis: cald for (C145H144N30O42)n (%): C 58.54, H 4.42, N 13.65; found (%): C 58.65, H 4.41, N 13.27. FTIR (KBr, cm−1): 3270 (w), 1633 (s), 1456 (m), 1414 (m), 1216(w), 1192(m), 995(s), 804(m), 785(m); Solid-state 13C CP-MAS NMR (500 MHz, δ, ppm): 189, 168, 162, 157, 149, 132, 122, 116, 104, 99, 70. Synthesis of Pd@COF-AO (2). A mixture of COF-AO (1, 200 mg) and palladium acetate (65 mg, 0.29 mmol) was stirred in toluene (30 mL) for 1 h at room temperature. The product was isolated by centrifugation and completely washed with toluene and diethyl ether, separately. The collected green-yellow crystalline solid was mixed with NaBH4 (30 mg, 0.79 mmol) in water (80 mL), and stirred at room temperature for an additional 5 h to afford Pd@COF-AO (2) as dark brown crystalline solids. The obtained crystalline solids were washed with ethanol thoroughly and dried in vacuum. ICP measurement indicated that the loading amount of Pd NPs in the Pd@COF-AO (2) is 4.6 wt %. Fabrication of Pd@COF-AO (2)-based Membrane (3). The as-synthesized Pd@COF-AO (2, 32.3 mg) was mixed with PSI-SH oligomer (67.7 mg) in anhydrous THF (5 mL) in the presence of 2, 2-dimethoxy-2-phenylacetophenone (5 mg). The mixture was sonicated for 30 min with stirring. Then, the mixture was deposited into a clean PTFE mold and irradiated with UVlight (365 nm, 100 W) for ca. 30 min to afford the stand-alone and elastic Pd NPs-loaded and 2based membrane (3) with ca. 30 wt % COF-AO (1) loading. FTIR (KBr, cm−1): 2961 (s), 1690 (s), 1634 (m), 1450 (m), 1260 (s), 1100 (m), 1064 (m), 802 (s), 703 (m).

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Batch Experiment for the 2-Catalyzed Dechlorinating Reaction. A mixture of ArCl (0.25 mmol), HCO2NH4 (2.5 mmol) and Pd@COF-AO (2) (5.8 mg, 1 mol % Pd) in water (2.5 mL) was stirred at room temperature (monitored by GC). Catalyst Recovery. After each catalytic run, 2 was collected by centrifugation, washed with MeOH and dried in air for the next catalytic run. Solid catalyst of 2 could be reused for at least five catalytic runs for the model reaction. Continuous Flow-Through Operation. The aqueous solution (5 mL) of ArCl (0.5 mmol) and HCO2NH4 (5 mmol) was pumped through a membrane microreactor (39 mg, Pd 1.44 wt%, diameter 25 mm, effective area 314 mm2) using a peristaltic pump with a flow rate of 1.0 mL min-1. The reaction process was monitored by GC. For the amplified experiment, the aqueous (25 mL) solution of ArCl (2.5 mmol) and HCO2NH4 (25 mmol) was pumped through the seriesconnected five pieces of membranes (Pd 1.44 wt% per piece) by peristaltic pump with a flow rate of 1.0 mL min-1. The reaction was monitored by GC and the products were determined by 1

H NMR (DMSO-d6) spectrum. Simulated Structure of COF-AO (1). Structural modeling of COF-AO (1) was performed by

the Materials Studio (ver. 5.0) suite of programs. Molecular geometry optimization was conducted with MS DMol3 module. The initial lattice was created by starting with the space group P-3. The a and b lattice parameters (initially 32.960 Å) were estimated according to the center to center distance between the vertices of the COF. The constructed model was geometry optimized using the Forcite module (Universal force fields, Ewald summations). Then the calculated PXRD pattern was generated with the Reflex Plus module. Finally, Pawley refinement was applied for profile fitting, producing the refined PXRD profile with the lattice parameters of a = b = 32.960 (± 0.004) Å and c = 3.6 (± 0.001) Å. Rwp and Rp values converged to 6.64 and

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4.76%, respectively. AA staggered arrangement for COF-AO (1) was also constructed with the space group P-3. Comparison of the measured and simulated PXRD patterns suggested that the preferable structure of COF-AO (1) is the eclipsed arrangement (see Figure S11). RESULTS AND DISSCUSSION

Scheme 2. Synthesis of COF-AO (1), Pd NPs-loaded Pd@COF-AO (2) and schematic illustration of fabrication of covalently cross-linked COF membrane (3). The sample photographs of 1 and 2 were inserted. As shown in Scheme 2, vinyl-functionalized and hydrazone-bridged COF-AO (1) was obtained as yellow crystalline solids by the reaction of 1,3,5-triformylphloroglucinol (TFP) and 2,5-allyloxy-terephthalohydrazide with the aid of HOAc under solvothermal conditions (120 oC, 72 h, see Figure S1-S10). The as-synthesized COF-AO (1) was firstly characterized by FT-IR and 13C cross-polarization magic angle spinning (CP-MAS) spectroscopies. The FT-IR spectrum

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of 1 showed that the characteristic stretching vibrations at 1626 and 1643 cm-1 for C=O groups in the starting materials disappeared after the reaction. Meanwhile, the 1633 cm-1 for the C=O stretching mode of the amide linkage was observed.40-41 Additional support for the formation of the extended COF network was evidenced by the existence of stretching modes at 3270 and 1192 cm-1 that are characteristic of N-H and C-N species in 1 (see Figure S12). Solid-state

13

C CP-

MAS NMR was also used to establish the connectivity of the COF species. The peaks at 189 and 168 ppm correspond to the carbon atoms of the carbonyl groups.40-43 The peaks at 149, 133 and 70 ppm can be assigned to the allyl groups attached to the phenyl ring (see Figure S13), which showed a typical keto structure caused by the keto-enol tautomerism that has been proposed by Banerjee and co-works.45-47

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Figure 1. a) Crystal packing pattern viewed down the crystallographic c axis, single hexagonal unit and crystal packing pattern viewed down the crystallographic [110] direction. b) PXRD patterns of simulated 1, measured 1, 2 and 3. c) SEM image of 1. The obtained COF-AO (1) shows good crystallinity which was revealed by the experimental powder X-ray diffraction (PXRD) measurement. The structural modelling was thus conducted using the software of Materials Studio (ver. 5.0).

48

The most possible structure of 1 was

simulated, analogous to that of 1 as a 2D eclipsed stacking structure using the space group of P-3 with the optimized parameters of a = b = 32.960 Å and c = 3.6 Å (residuals wRp = 6.64 % and Rp = 4.76 %, ESI). As indicated in Figure 1a, the eclipsed 2D structure contains a hexagonal structural unit, in which the opposite Callyl···Callyl distance is ca. 2.1 nm. The close-packed 2D layers with an interlayer distance of 3.6 Å indicated that adjacent layers in 1 are in a weak π-π contact. PXRD pattern of this unit cell is well consistent with the experimental profile (Figure 1b), but it is clearly different from its starting building blocks (see Table S1). A strong peak at 3.4° along with some relatively weaker peaks at 5.9°, 9.0°, 12.2°, 14.9°, and 26.8° were observed, which were assigned to (100), (110), (200), (210), (220) and (001) reflections, respectively. The broad PXRD peaks observed at 26.8° for measured data might be generated from the defect in π-π stacking between adjacent COF layers. 45 The morphology of the COF was also investigated by SEM. A large quantity of uniform nanofibers with diameter of about 50 nm and lengths up to tens of micro-meters implied its phase purity (Figure 1c). Furthermore, thermogravimetric analysis (TGA) indicated that almost no weight loss was observed up to 280 °C for 1, indicating its good thermal stability (see Figure S14). The relatively large pore and the involved heteroatoms would allow 1 to be an ideal support to upload and stabilize Pd NPs.

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Pd NPs loaded Pd@COF-AO (2) was simply prepared by reduction (NaBH4, water, 5h, r.t.) of Pd(II)@COF that was obtained by impregnating 1 in a solution of Pd(OAc)2 in toluene at room temperature for 1h. The formation of 2 was accompanied by a dramatic color change from yellow to dark brown (Scheme 2). Except Pd, no other moieties such as C=N and C=C were reduced under the reaction conditions which was supported by the 13C NMR analysis (see Figure S15).31 The loading amount of Pd, as determined by inductively coupled plasma (ICP) measurement, is up to 4.60 wt %. The oxidation state of the encapsulated Pd species before and after reduction was examined by XPS (see Figure S16). The observation of Pd d5/2 and d3/2 peaks at 335.8 and 340.9 eV in the XPS spectrum of 2 demonstrated the successful reduction from Pd(II) to Pd(0),

49-50

and the atomic lattice fringes had an interplanar spacing of 0.24 nm

belonging to the 1/3 (422) fringes of face-centered cubic (fcc) Pd (see Figure S17).51 The highresolution transmission electron microscopy (HRTEM) showed that the Pd NPs were highly dispersed in porous COF matrix with a particle size of ca. C=C< groups disappeared after the polymerization,44 indicating the UV-triggered thiol-ene click reaction herein is clean and highly efficient.

Figure 3. The photographs of the 2-based membrane (3) (a and b). Surface (c and d) and crosssection (e and f) SEM images of 2-based membrane (3). As shown in Figures 3a-b, the obtained stand-alone membrane of 3 is free of macroscopic defects, mechanically robust and flexible, which is a very important feature for their application in building membrane-based microreactor. SEM images of the as-synthesized membrane is provided in Figures 3c-f, the surface of the membrane showed that the COF nanofibers are well “dissolved” in the polymer phase without any detectable interface boundary, indicating their inherent intermiscibility driven by covalent binding between COF and organic polymers based on photo-induced polymerization. The cross-section images showed that the membrane is ca. 113

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µm in thickness, and that the COF are densely packed in the membrane. The uniform texture of the membrane was further demonstrated by the evenly distributed Pd, N, S and Si elements in 3 based on its SEM-EDX measurement (see Figure S23). Compared to the PSI-SH polymer, the N2 adsorbing amount (59.64 cm3/g) of the 2-based membrane (3) at 77 K was significantly enhanced (Figure 2), and the BET surface area correspondingly increased to 110.12 m2/g. Upon incorporation of Pd-loaded COF nanofibers, the membrane thus exhibited a significantly improved capacity, consequently, the pore accessibility. This would be beneficial to its application in continuous-flow microreactor. Notably, the PXRD pattern of the 2-based membrane (3) is identical to that of 1, demonstrating that the COF crystallinity and structural feature were well maintained after the photoinduced polymerization (Figure 1b). As known, the most of reported Pd-catalyzed dechlorination used hydrogen as the reducing agent,

14-17

which is not convenient and safe to use. Therefore, the aid of stable nongaseous

hydrogen source should be an alternative to the catalytic CBs dechlorination by hydrogen gas. On the other hand, the known dechlorinating reactions were usually conducted in organic solvents such as THF and xylene,13 which would not facilitate the removal of chlorinated waste in practical water treatment. For above reasons, we initially used p-chlorophenol dechlorination as the model reaction to examine the catalytic activity and stability of Pd@COF-AO (2) in water. After optimization, we were pleased to find that p-chlorophenol was completely transferred to phenol (> 99 %) in water at room temperature within 3 h in the presence of 2 (1.0 mol % of Pd) (Figure 4a). The high activity of 2 was certainly ascribed to the highly dispersed and small sized Pd NPs in COF matrix, the greater surface area and accessible mesoporous pores of the COF.

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Figure 4. Dehalogenation of p-chlorophenol in water catalyzed by 2. a) Reaction time examination (black line) and leaching test (red line) for the dechlorinating reaction. The solid catalyst of 2 was filtrated from the reaction solution after 1 h, whereas the filtrate was transferred to a new vial and reaction was carried out under the same conditions for an additional 2 h. b) Catalytic cycles. After each run, the catalyst was collected by centrifugation, washed with ethanol, and air dried for next catalytic run under the same conditions. Reaction conditions: pchlorophenol (0.25 mmol), HCO2NH4 (2.5 mmol), 2 (1 mol% Pd), H2O (2.5 mL), room temperature, in air. Yield and product were determined by GC analysis and 1H NMR (DMSO-d6) spectrum, respectively (see Figure S24). The turnover number (TON) and turnover frequency (TOF) for the model reaction are 99 and 33 h-1, respectively. In order to gain insight into the heterogeneous nature of 2, the hot leaching test was performed. As shown in Figure 4a, no further reaction occurred without 2 after ignition of the reaction at 1h, suggesting that 2 exhibits a typical heterogeneous catalyst nature herein. In addition, the recyclability of 2 was also examined (Figure 4b). After each catalytic run, the solid catalyst was readily recovered by centrifugation, washed with ethanol and then dried in air

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for the next run under the same reaction conditions. 2 still showed excellent activity and the dehalogenation yield was even up to 97 % after five catalytic cycles. The PXRD analysis indicated that the crystallinity and structural feature of 2 was well maintained (see Figure S25). The HRTEM image of the reused catalyst revealed that no Pd NPs aggregation occurred (see Figure S26), while XPS analysis indicated that no valence change for Pd species was detected (see Figure S25). ICPAES analysis showed that basically no Pd leaching occurred even after five catalytic runs (see Table S2). Thus, the porous COF-AO (1) herein is an ideal platform to load and support Pd NPs for CBs dechlorination in aqueous solution. In addition, the catalytic activity of Pd@COF-AO (2) with higher Pd NPs loading was examined. For example, the saturated Pd NPs-loaded 2 (22.13 wt %, determined by ICP and XPS) material was prepared and it showed a really bad catalytic performance for this aqueous CBs dechlorination (see Table S3, and Figure S27). The corresponding catalytic yield of pchlorophenol was sharply down to 13 % under the same reaction conditions (see Figure S28), which should be due to the Pd NPs aggregation and leaching (see Table S3 and Figure S27).

Figure 5. Continuous-flow setup (graphic and photographic representations) containing single piece membrane of 3 for the continuous catalytic dehalogenation operation. The setup containing series-connected five pieces membranes was shown in Figure S29.

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By taking advantage of the porous covalently cross-linked COF membrane, together with the embedded highly active Pd NPs, we designed and built a continuous-flow setup for continuous catalytic operation for the of p-chlorophenol dechlorination at room temperature (Figure 5). In the typical continuous-flow through experiment, the mixture of p-chlorophenol (0.5 mmol) and HCO2NH4 (5 mmol) in 5 mL water was pumped through a piece of membrane (39 mg, Pd 1.44 wt%, diameter 25 mm, effective area 314 mm2) using a peristaltic pump. Flow rate of 1.0 mL/min was used to realize the reaction system circulation. The reaction system was monitored by GC and the reaction was finished within ca. 3 h (Table 1, entry 1), that is, the reaction solution needed to be passed through the membrane for 36 times (5 min per cycle) to realize the full p-chlorophenol dichlorination under the given conditions. ICP analysis showed that no Pd leaching from the membrane occurred during the continuous-flow catalytic process (see Table S4). Table 1. Pd-Loaded COF Membrane-Catalyzed CBs Dechlorination in Water on the Continuous-Flow Microreactor a entry 1

CBs Cl

product OH

t (h) yield (%)b

OH

3

97

OH

3

93

OH

3

97

Cl

2

OH

Cl

3

18

OH

4

Cl

NH2

NH2

3

96

5c

Cl

NO2

NH2

6

98


Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cl

6c

CN

8

98

CHO

6

97

8

96

8

94

OH

8

97

OH

8

94

8

99

CN

7

Cl

8

Cl

CHO

O

O

Cl O

9

O

Cl

10

Cl

OH

Cl

11

Cl

OH

Cl

12

Cl

13d

Cl

OH

OH

9

94

14e

Cl

OH

OH

3

98

a

Reaction condition: aryl chloride (0.5 mmol), HCO2NH4 (5 mmol), 3 (1.44 wt % Pd per piece, 1 mol% Pd equiv), H2O (5 mL), room temperature, in air. b Yield and product were respectively determined by GC and 1H NMR (DMSO-d6) spectrum (see Figures S30-S43). c Water/ethanol (5 mL, 1:1 v/v) was used. d The amplified p-chlorophenol dechlorination with one piece of membrane 3: p-chlorophenol (1.5 mmol), HCO2NH4 (25 mmol), 3 (1.44 wt % Pd per piece, 0.33 mol % Pd equiv), H2O (5 mL), room temperature, in air. e The amplified p-chlorophenol dechlorination via five pieces of series-connected flow-through membranes: p-chlorophenol (2.5 mmol), HCO2NH4 (25 mmol), 3 (1.44 wt % Pd per piece, 1 mol % Pd equiv), H2O (25 mL), room temperature, in air. With above results in hand, the scope of the reaction was explored (Table 1). It is noteworthy that 2-based catalytic membrane is applicable to wide range of functional groups such as -OH, -

NH2, -CHO, -CN, -COCH3 at different substituted positions on mono- and polychlorinated CBs. As shown in Table 1, the electronic effect of substituents appeared to be crucial for the

19



dehalogenation. For example, the CBs with electron-donating groups were more active for the aqueous dehalogenation under the given reaction conditions (Table 1, entries 1-4), the target dechlorinated products were isolated in excellent yields (93 - 97%) within 3 h. Meanwhile, the CBs with electron-withdrawing groups showed less reactivity under the reaction conditions (Table 1, entries 5-9), and the excellent yields (94 - >99%) were also observed but after a prolonged reaction time (6-8 h). Notably, the dechlorination of 4-chloronitrobenzene was accompanied by a nitro reduction (entry 5), further indicating that 2 is a highly active catalytic material. In addition, the dechlorination of polychlorinated benzenes were also examined. As shown in Table 1 (entries 10-11), dichlorobenzene and trichlorobenzene derivatives (PCBs) could be readily converted into their corresponding dechlorinated products in 94 - 97 % yields within 8 h. Besides, larger sized CBs such as 4-chlorobiphenyl could also be completely dechlorinated with excellent yield (99 %, entry 12) under the reaction conditions. For practical application, this continuous flow-through operation was also conducted with larger quantity of CBs incorporated with membrane 3. For example, 1.5 mmol of p-chlorophenol in 5 mL H2O (Table 1, entry 13, and Figure S42) was passed through one piece of membrane 3 (1.44 wt Pd% per piece), and the reaction finished at a prolonged reaction time (9 h) with a 94 % yield. In other words, around 108 times that the aqueous reaction solution was required to pass through the membrane to fully dechlorinate p-chlorophenol into phenol under the given conditions. Finally, we tried to amplify this continuous-flow dechlorinating scale on a multiplemembrane incorporated setup. In this manner, 2.5 mmol of p-chlorophenol in 25 mL water was pumped through the five pieces series-connected membranes (1.44 wt Pd% per piece) using a peristaltic pump with flow rate of 1mL/min. The dechlorinating yield was up to 98 % within 3 h based on GC analysis (Table 1, entry 14, and Figure S43). Thus, this COF membrane-based

20


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continuous flow-through microreactor for aqueous dechlorination herein was highly efficient, versatile, maneuverable and it could effectively overcome the problems such as catalyst regeneration and product separation which are usually associated with batch CB dechlorination. Hopefully, this COF-membrane-based microreactor model has a great potential application in practical water treatment. CONCLUSION In summary, we have developed a facile post-synthetic covalent copolymerization (PSCCP) strategy for the fabrication of self-standing and covalently cross-linked COF-based and M NPs loaded membrane. Compared to the reported COF-membranes which are fabricated by growing on a support,53-54 interfacial polymerization,55-57 and in situ assembly of organic linkers and coreagents,58 this result has significantly expanded not only the COF-based membrane type, but also its synthetic methodologic scope. The obtained Pd NPs loaded catalytic membrane herein is robust, flexible, water permeable and processable, furthermore, it can be used to build membrane-based

continuous-flow

microreactor

to

effectively

promote

aqueous

CBs

dechlorination under ambient conditions. Compared to a batch dechlorinating approach, the continuous-flow operation is much closer to the practical water treatment. We expect this approach to be viable for the construction of many more new and interesting COF-based catalytic membranes and shaped devices for various applications. ASSOCIATED CONTENT Supporting Information. Additional characterization of COF-AO (1), Pd@COF-AO (2), and the 2-based membrane (3), general procedure for the 2-catalyzed dechlorinating reaction, continuous flow-through operation, simulated structure of COF-AO (1) and GC analysis for the

21



CBs dechlorination catalyzed by 2-based membrane (3) in water at room temperature. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions ‡These authors contributed equally. Notes There are no conflicts to declare. ACKNOWLEDGMENT We are grateful for financial support from NSFC (Grant Nos. 21671122, 21604049 and 21475078), Shandong Provincial Natural Science Foundation (BS2015CL015), China, A Project of Shandong Province Higher Educational Science and Technology Program (J15LC05) and the Taishan Scholar’s Construction Project. REFERENCES (1) Wu, Q.; Milliken, C. E.; Meier, G. P.; Watts, J. E. M.; Sowers, K. R.; May, H. D. Dechlorination of Chlorobenzenes by A Culture Containing Bacterium DF-1, A PCB Dechlorinating Microorganism. Environ. Sci. Technol. 2002, 36, 3290-3294. (2) Oliver, B. G.; Nicole K. D. Chlorobenzenes in Sediments, Water, and Selected Fish from Lakes Superior, Huron, Erie, and Ontario. Environ. Sci. Technol. 1982, 16, 532-536.

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Table of Contents.

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Pd NP-Loaded and Covalently Cross-Linked COF Membrane

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