Porous Organic Polymer Hybrid Nanosheets

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Porous Graphene Oxide/Porous Organic Polymer Hybrid Nanosheets Functionalized Mixed Matrix Membrane for Efficient CO2 Capture Rongrong He, Shenzhen Cong, Jing Wang, Jindun Liu, and Yatao Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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Porous Graphene Oxide/Porous Organic Polymer Hybrid Nanosheets Functionalized Mixed Matrix Membrane for Efficient CO2 Capture Rongrong He, Shenzhen Cong, Jing Wang*, Jindun Liu,Yatao Zhang** School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China


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Abstract:

The computational simulation of porous graphene oxide (PGO) indicated that it has great potential for the preparation of gas separation membranes. However, scaling up the manufacture of multi-layer, defect-free porous graphene oxide membrane with consistently sized nanopores is extremely challenging. Here, we prepared layer-by-layer CO2-philic Pebax@1657 membranes that functionalized by o-hydroxyazo-hierarchical porous organic polymers (o-POPs) and PGO. The d-spacing of pristine PGO could be finely regulated through CO2-philic o-POPs to facilitate the permeability of CO2. In addition, the o-POPs exhibit “N2-phobic, CO2-philic” properties with the phenolic hydroxyl and the azo group. The best of the POP-PGO membrane exhibit the CO2 permeability and ideal selectivity of CO2/N2 are 232.7 Barrer and 80.7, respectively, and it has surpassed the Robeson’s upper bound (2008).

Keywords: Porous Organic Polymer, Porous Graphene Oxide, Gas Separation Membranes, Nanosheet, N2phobic, CO2-philic, Dual-sheet and Dual-channel

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1. Introduction Membrane-based separation processes can be used to separate gases or collect green energy in an energy-efficient way.1-2 Recently, the conventional two-dimensional (2D) nanosheet materials, such as Graphene Oxide (GO),3-4 Porous Graphene Oxide (PGO),5 have attracted great attention for gas separation. The PGO is one of the 2D laminar material with all kinds of oxygencontaining groups via oxidizing the nature graphene, providing an extensively of opportunities for research nanomaterials science and technology, the resulting membrane have promise for thinness, flexibility, mechanical strength, chemical stability and an atom-thick 2D carbon nanomaterial. Plenty of theoretical studies have proved the unique structure of PGO guided by these theories,6 some experimental studies have begun to explore the PGO in membranes for molecular separation.7-9 PGO with nano-sized pore structure can be used to make the new kind of efficient selective membranes for waste water purity,6,10 CO2/N2,11 H2/CH412-13 and H2/N2.14 However, most researches on PGO based membrane only focus on theoretical calculations and simulations, it makes the research and application of PGO in gas separation very difficult.6,1011,15-17

The o-POP as one new kind of polymeric materials only constructed by the organic

covalent bonds.18-22 o-POPs as an amorphous porous organic polymer material has been widely used for many important application, such as gas adsorption, optoelectronics,23-27 catalysts,28-30 proton conductivity,31 chemical sensors,32-34 drug delivery,35 and energy storage.10 Normally, it can be synthesized based on diazo-coupling reaction of di/tri-phenols with aryl tri/diamines .36 O-POPs exhibit greatly "N2-phobic, CO2-philic" properties with nitrogen-rich functional groups and phenol groups, such strong interaction with CO2 backbones can make the o-POPs exhibits high CO2 adsorption capacity.25 In addition, the porous structure of the o-POPs is also beneficial to the construction of molecular sieve channels.

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Here, in order to provide molecular-sieving channels for efficient CO2 separation, we combined o-POPs with PGO through bonding effect to establishment an ordered stack of dual-sheet and dual-channel PGO sieving membranes as schematically shown in Figure 1. This two species material could generate the quinone bond via the reaction of phenol -OH with the carboxyl group. The presence of a large amount of phenol -OH on o-POPs makes it possible to form quinone bonds with PGO. The quinone bond can make the o-POPs homogeneous dispersed in PGO to form layer-by-layer PGO membrane. The EO segments of Pebax@1657 also has a promoting effect on the CO2 permeability; nitrogen-rich porous polymer, the azo-linked polymer and found that their azo groups (–N=N–) reject N2 selectively. As a two-dimensional porous material, o-POPs have plenty of -OH groups and azo groups (–N=N–). The -OH groups of oPOPs interact with carboxyl groups in PGO to form the quinone bond and the Pebax@1657 contains the flexible EO segments to increase CO2 permeability. In addition, the azo groups (N=N-) of o-POPs has a repulsive effect on N2. Thus, the optimal CO2 permeability of 232.7 Barrer and the best selectivity of CO2/N2 are up to 80.7.

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Figure 1. Schematic illustration for design of POP-PGO MMMs. 2. EXPERIMENTAL 2.1 Materials. 1, 3, 5-Tris (4-aminophenyl) benzene, 1, 4-diphenol was purchased from Aladdin, all of which were used as they were. Natural graphite powder (about 45 mm) was obtained from Si-nopharm Chemical Regent and used as it is. The concentration is phosphoric acid (H2SO4, 98% by weight), sodium nitrite (NaNO2), phosphoric acid (H3PO4, 85 wt%) by Sigma-Aldrich, hydrochloric acid (HCl) and hydrogen peroxide (H2O2, 30wt%). China Tianjin Feng chuan Chemical Reagent Technology Co., Ltd. provided. Potassium permanganate (KMnO4) and sodium hydroxide (Na2CO3) were purchased from J&K. Pebaxs@1657 purchased from Shanghai Rongtian Chemical Co., Ltd. All these chemical reagents were used as received.

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2.2 Synthesis of o-POPs. The synthetic route for the POP-1 is presented in Scheme 1. At beginning, 1, 3, 5-tris (4-aminophenyl) benzene (1.5 mM) is added to a 250 ml round bottom flasks containing 100 ml of demonized water, and then 1 ml of concentrated hydrochloric (HCl) acid is added. With stirring for 15 min at 0-5 oC, 30 ml of sodium nitrite (4.05 mM) was pour into the solution, stirring for 25 min, and next 30 ml of the 1, 4-p-diphenol (1 mM) solution with sodium carbonate (2 mM) neutralized was added. After keep the temperature at 0-5 oC stirred for 12 h, then though filtration to separate the solid sample from the reaction solution and washed in the order of deionized water, absolute methanol, THF, absolute methanol, deionized water.37, 38 Freeze in a refrigerator and freeze-dry to give a sample in a yield of 89.6%.

Scheme 1. The chemical structures of the o-POP and its synthetic procedure through diazocoupling reaction 2.3 Preparation of porous graphene oxide (PGO). Porous graphene oxide (PGO) was obtained by a battery of chemical reaction on graphene oxide (GO). The detailed synthesis process can be described as follows: Taken natural graphite powders as the raw material to synthesize the GO based on an improved method.37-38 In the first place, 360 ml of H2SO4 and 40 ml of H3PO4 were poured into a 1000 ml three-necked flask, and then 18g of potassium permanganate (KMnO4) 5



was added to the flask. Subsequently, 3g of nature graphite powder was added to the threenecked flask, and heated the mixture at 50 oC oil baths under mechanical stirring for 22 h with reflux. The above completed liquid and hydrogen peroxide (H2O2) alternately slowly added 1000 ml beaker, and cooled with ice, and then ultrasound 1h. The solution was centrifuged 5 ~ 7 min at 1000 r/min obtained the crude GO, and the supernatant obtained was centrifuged at 7000 ~ 8000 r/min for 5 ~ 7 min get precipitate. Then washing the precipitate with 30 wt% HCl for 3 times, then washing with absolute ethanol for 3 times and finally washing with deionized water for 3 times, taking the precipitate into a freezer to freeze and then freeze-drying to obtain GO. The preparation of the PGO can be descript as follow: Firstly, 0.3 g of the GO prepared above was dispersed in 200 ml of DI with ultrasonic dispersion for about 20 min, after that pour into 2 g of sodium hydroxide (NaOH) to the above solution, and magnetically stirred for 1h. Next, the treated GO solution was centrifugal, the centrifugal solution is predisposed into 200 ml of DI water, 5 ml of hydrochloric acid is added to the solution, and then magnetic stirring is performed for 1 h. Then, it is changed with acetone and deionized water for 2-3 times to conduct centrifugation. Finally, the desired porous graphene oxide can be obtained by vacuum drying at 60 oC. 2.4 Gas permeation measurements. The generally accepted theory for gas transmission properties of dense membranes can be explained through a solution–diffusion mechanism (Supporting Information).39-41 3. RESULTS AND DISCUSSION 3.1 Characterization of Porous Organic Polymer and Porous Graphene Oxide. Triazepine coupling reaction based on aromatic amine and phenol in water, the o-POPs is synthesized 6


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through the azo reaction of 1, 3, 5-Tris (4-aminophenyl) benzene and 1, 4-diphenol under mild conditions. PGO can be obtained via a series of chemical treatments from GO. Since o-POPs and PGO are partially combined through bonding effect, o-POPs loaded on the PGO surface results in a decreased specific surface area and makes PGO more porous. The effective surface area of synthesized o-POPs reached 668 m2/g, the ideal selectivity of CO2/N2 up to 83 in 298 K. However, when o-POPs combined with PGO, the surface area of o-POPs have 616 m2/g, and CO2/N2 ideal selectivity of 82 in 298 K. The o-POPs combined with PGO via the quinone bond have no effect on the o-POP' azo groups (–N=N–) for gas adsorption performance. Therefore, PGO only have minimal effect on the ideal selectivity of CO2/N2. Here, CO2-philic dual-sheet and dual-channel POP-PGO membranes are prepared by pouring the obtained solution into a PTFE tray and then solvent evaporation method. O-POPs not only accurately adjust the dspacing of PGO through their interaction with PGO, but construct molecular sieve channels for CO2 separation. The morphology of the polymers membranes was observed via the scanning electron microscope (SEM), transmission electron microscope (TEM) and atomic force microscope (AFM). SEM images show that o-POP is composed of anomalous tiny particles (less than 100 nm), this morphology is same with the classic structure of the POP polymer (Fig. S5). In addition, SEM images further confirm that the o-POP combined with the PGO and make the surface of PGO defects (Fig. 2a). The TEM analysis of the polymer indicates that the microporous structure of the o-POPs, which crossing one another to form different mesopore. The TEM of the o-POPs combined with PGO indicates that the o-POPs particles were attached on the surface of the PGO nanosheet (Fig. 2b). The AFM of the POP-PGO composite material indicated that the size of the o-POPs nanosheet and the PGO nanosheet, as well as the o-POPs combined PGO are hundreds

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of nanometers. It indicated that the o-POPs don’t affect the PGO size. O-POPs are porous, amorphous covalent organic polymer. PGO is a two-dimensional sheet material with a defectforming pore that is epitaxially expanded with a monoatomic structure. Besides, the quinone bonds form between the large number of phenolic hydroxyl groups on o-POP sand the carboxyl groups on PGO. By adjusting the amount of o-POPs, the interlayer spacing of PGO can be precisely controlled, and thus conducive to the tunable CO2 permeability. The CO2-philic mixed matrix membranes were prepared refer to our previous work.41 The prepared membranes was named as POP-PGO X-Y (the X, Y represents the mass concentration of o-POP and PGO). Fig. 2c also indicated the optics photographs of the as-prepared freestanding POP-PGO 10-2 membrane. The prepared membrane appears yellow and transparent, indicating homogeneous o-POPs and PGO dispersion.

Fig. 2. (a). Scanning electron microscope images of POP-PGO; (b) Transmission electron microscope images of the o-POPs combined with PGO; (c) Scanning electron microscope micrograph of the cross section of the POP-PGO 10-2 membrane (insert: POP-PGO physical 8


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map); (d) Scanning electron microscope of cross-sectional morphology of POP-PGO 10-2 composite membrane; Enlarged image (the red circled area shows precise control of the o-POPs in the PGO layer spacing). 3.2 Characterization of membranes. The cross-sectional morphologies of the POP-PGO MMMs were observed via SEM analysis as presented in Fig. 2c The cross-sectional SEM of POP-PGO 10-2 composite membrane indicated that the thickness of POP-PGO 10-2 membrane is ca.55 µm. The high magnification SEM image of the POP-PGO 10-2 membrane images the layer-by-layer stacking of o-POPs regulated the PGO (Fig. 2d). The thermal stability of different POP-PGO membranes were investigated via Thermo gravimetric analysis (TGA) (Fig. 3d), and the results indicated that no significantly decompose of the membranes can be observed before 350 oC under N2 atmosphere (Fig. S4). Plenty of the carboxyl groups (-COOH) in PGO is a necessary condition for its efficient functionalization. The initial Pebax@1657 C=O functional groups based on X-ray photoelectron spectroscopy (XPS) analysis (Fig. S9). The pristine pebax@1657 can be observe at 285.840 eV and represent the C=O. The POP-PGO 0-2 and POP-PGO 10-2 membranes were 285.840, 285.568, and 288.561 eV, respectively. The slight difference of the bonding energy may stem from the increase of -COOH in Pebax@1657, PGO, and the hydrogen bonds between PGO and Pebax@1657. The FT-IR of the POP-PGO membranes was further investigated. The FT-IR spectrum peaks are: 1095, 3298, and 1637 cm-1, respectively represent C-O-C, -OH, and C=O from Fig. 3c. When the PGO dispersed to the o-POPs solution, the phenolic-OH of the o-POPs rapidly combined with the PGO surface via hydrogen bonds, forming chemical bonds with phenolic hydroxyl groups and carboxyl groups over time. The absorption the intensity shifted to higher value of the POP-PGO membrane at 1637 cm-1 corresponds to the C=O covalent bond. 9



The POP-PGO 10-2 membrane were observed by TEM analysis as presented in Fig. 3a, the dspacing of PGO is approximately 0.36 nm was observed. We further proved the interlayer distance of the other POP-PGO membranes by XRD on the basis of Bragg's law. As showed in Figure 3b, the original porous graphene oxide membrane shows weak peaks because of the PGO and POP π-π covalent interactions, resulting in a wide distribution of nano-channel sizes and thus reduced size membrane capacity. From the Bragg equation to subtract the thickness of monolithic graphene by XRD data, and the initial interlayer spacing of PGO was 4.8 Å (Fig. S11). Due to the bonding effect of o-POPs and PGO, the d-spacing of POP-PGO 10-2 membrane was reduced to 3.60 Å, which was between the molecular size of CO2 (3.3 Å) and N2 (3.63 Å), constructed the molecular sieving channel.

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Fig. 3.(a) TEM image of the cross section of OP-PGO 10-2 membrane; (b) X-ray diffraction patterns of the POP-PGO membranes; (c) FTIR-spectra of POP-PGO membranes; and (d) TG of the POP-PGO membranes 3.3 Gas separation performance of the membranes. The pure gas permeation behavior experiments were performed to investigate the effect of o-POPs or PGO concentration on the membrane performances, the pure feed gas (CO2 and N2) and wet mixed feed gas (CO2:N2, 10:90, v:v)were tested via a membrane gas permeation testing apparatus. In all cases, the pure gas permeability and the ideal selectivity of the dual-channel and dual-sheet POP-PGO membranes were observed in dry state. Fig. 4a indicates that the permeability of the pristine pebax@1657 membrane for CO2 and N2 were 94 Barrer and 2.9 Barrer at 30°C, respectively. In our work, the CO2 permeability and the ideal selectivity of CO2/N2 for the POP-PGO membranes have a great improved compared with the original Pebax@1657 membrane. We have compared a series of the POP-PGO membranes performance to explore the reasons. The POP-PGO 10-2 membrane exhibits the best CO2 permeability reach up to 232.7 Barrer, which is 1.51 times than the original Pebax@1657 membrane; the ideal selectivity is approximately 1.53 times higher than the pristine Pebax@1657 membrane and up to 80.7. It was found that the POP-PGO 10-1 membrane shows the best of performance, with the CO2 permeability of 88.9 Barrer and the ideal selectivity of 47.7 for CO2/N2 (Fig. S12).When compare the POP-PGO 10-2 membrane with the other POP-PGO 10-Y membranes, it is found that the POP-PGO 10-2 membrane has the best CO2 permeability and the ideal selectivity of CO2/N2, which the CO2 permeability is 232.7 Barrer and the ideal selectivity of CO2/N2 is 80.7 (Fig. S13). However, when the POP-PGO 20-Y, 30-Y membrane compared with POP-PGO 20-0, 30-0 membrane, the POP-PGO membranes of the CO2 permeability and the CO2/N2 ideal selectivity is lower. Fig. 4b shows that the CO2

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permeability first increased and then reduce. It may be that the high concentration of o-POPs caused the pores of the o-POPs sheet blocked and impeded the diffusion of CO2, which make the CO2 permeable slowly. The increased CO2 permeability and the CO2/N2 ideal selectivity of the POP–PGO 10-2 membranes compared with the other POP-PGO membranes may be stated through the principle of dissolution–diffusion in and the molecular permeation the POP–PGO nano-sheets as follows. The PGO nanosheets were combined with the o-POPs via chemical bond can be effectively adjust the interlayer spacing of PGO between the CO2 (3.3Å) and N2 (3.63Å). The d-spacing of the POP-PGO 10-2 membrane is 3.6Å, which is beneficial to CO2 permeate. The phenolic hydroxyl group on the o-POPs forms a chemical bond by acting with the oxidation group on the porous graphene oxide nanosheet, thereby causing defects in the surface of the graphene oxide, resulting in a decrease in selectivity and an increase in CO2 permeability. Meanwhile, the oPOPs connected with PGO nanosheets can adjust the ordered lamination structure of PGO to achieve the CO2 efficient separation. In addition, the ideal adsorption selectivity of o-POPs for CO2/N2 up to 83 at a low pressure (Fig. 4c). However, when the o-POPs combined with the PGO, the ideal adsorption selectivity of o-POPs for CO2/N2is still 82 (Fig. 4d). It can be explained that the o-POPs only regulate the d-spacing of PGO nanosheet and don’t make the PGO defects. The o-POPs have a strong adsorption contribute to the CO2 dissolution and diffusion in the membranes. The highly stack of ordered PGO can effectively decrease the CO2 permeation resistance; the super CO2-philic of o-POPs can improve the CO2 dissolution and diffusion in the membrane. All of these improve the CO2 permeability while ensuring the performance of CO2/N2 ideal selectivity.

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Fig. 4. (a) The performance of the mixed gas of the POP-PGO X-2, and the performance of the pure gas permeability of the POP-PGO X-2;41 (b) The performance of the pure gas of the POPPGO 10-Y; (c) The CO2/N2 ideal selectivity of the o-POPs power at 298K; (d) The ideal Selectivity of CO2/N2 of the POP-PGO power at 298K. The mixed-gas (CO2:N2 10:90) performance of the POP–PGO X-2 and the original Pebax@1657 membranes in the wet state is shown in Figure 4a.41 The experimental result indicated that the membrane's permeability performance is not affected via the feed gas, and the optimal permeability performance and selectivity of CO2 is still POP-PGO 10-2 membrane. However, the membrane with competitive adsorption and water swelling at the wet mixed-gas condition, the performance of the gas permeability is greatly increased and the CO2/N2 13



selectivity is also somewhat improved compared to the pure feed.42 The performance of the membrane have so large change indicated the wet gas have a great influence on gas transport behavior. Moreover, the water molecules can give rise to additional curvature when the gas permeate the membrane.43

Fig. 5 (a) The Robinson’s upper bound (2008) 1 represent performance of POP–PGO 10–2 membrane (pure gas); 2 indicate the performance of POP–PGO 10–2 membrane (mixed-gas); 3 indicate the performance of POP–PGO 10–2 membrane (wet gas); 4 indicate the performance of POP–PGO10–2 membrane (wet mixture-gas); 5 represent the performance of POP–PGO10–0 membrane (pure gas);41 (b) Long-term operation test of the POP-PGO 10-2 membrane for CO2/N2 separation 14


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The CO2/N2 selectivity for POP-PGO membranes and other polymers some are surpassed the Robeson upper bound (2008), with expected “trade-off” behavior between permeability and selectivity, as shown in Fig. 5a. In addition, the POP-PGO membranes performance have a great improved with the wet mixed feed gas. Fig. 5b indicated that the POP-PGO 10-2 membrane have a long stability for CO2/N2 separation. 4. Conclusion In conclusion, the dual-sheet and dual-channel molecular sieving CO2-philic membrane is constructed through the regulation of porous o-POPs on PGO. These two porous nanosheet materials are combined via bonding effect and achieve effective sieving for CO2 and N2for the first time. Due to the sieving effect of the pores and the repulsion of N2 through the azo bond of o-POPs, N2 is blocked and CO2 can pass through. The CO2-philic membranes offer an outstanding performance in pure gas separation, and they have better permeability and selectivity than commercially available and previously reported membranes. This study provides a convenient and practical method to prepare dual-sheet and dual-channel molecular sieving membranes. This approach may improve the rapid development of the synthesis of twodimensional (2D) porous material family, such as g-C3N4, zeolites, and covalent organic framework (COF) to obtain porous nanosheets-based membranes with huge potential for applications in gas separations. Furthermore, the great long-term stability performance has a great advantage to achieve the industry application. Supporting Information Experimental Instrumentation and Gas Permeation Measurement. Schematic diagram of dense membrane gas permeation testing apparatus. FT-IR spectra, 1H-CP/MAS NMR spectra, SEM

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image, TGA trace, XPS analysis, and gas permeability and selectivity vary with different PGO or POPs concentration (PDF). Corresponding Author *E-mail: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. U1704139 and 21376225), Key Science and Technology Program of Henan Province (182102310013), and Training Plan for Young Backbone Teachers in Universities of Henan Province (2017GGJS002).

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