N2 Separation with Fluorinated Carbon Molecular Sieve Membranes

Article

Surpassing Robeson Upper Limit for CO2/N2 Separation with Fluorinated Carbon Molecular Sieve Membranes Zhenzhen Yang, Wei Guo, Shannon Mark Mahurin, ..., Wanqin Jin, Ilja Popovs, Sheng Dai [email protected] (I.P.) [email protected] (S.D.)

HIGHLIGHTS Realizing the fabrication of CO2philic fluorinated trazine-based membranes Functionalization of the membrane skeleton with fluorine, ether, and triazine groups Preparation of nitrogen, oxygen, and fluorine-doped carbon molecular sieve membranes Achieving enhanced CO2 separation performance surpassing Robeson upper limit

Fluorinated membranes based on covalent triazine frameworks are prepared through rational design of aromatic nitrile monomers containing fluorine and ether groups via a sol-gel polymerization process. The CO2 separation performance is markedly enhanced with the increase of fluorine content in the membrane. With functionalized triazine units, fluorine and ether groups, the carbon molecular sieve membranes obtained after pyrolysis exhibit intrinsic ultra-micropores, high surface areas, and outstanding CO2 separation performance.

Yang et al., Chem 6, 1–15 March 12, 2020 Published by Elsevier Inc. https://doi.org/10.1016/j.chempr.2019.12.006

Please cite this article in press as: Yang et al., Surpassing Robeson Upper Limit for CO2/N2 Separation with Fluorinated Carbon Molecular Sieve Membranes, Chem (2019), https://doi.org/10.1016/j.chempr.2019.12.006

Article

Surpassing Robeson Upper Limit for CO2/N2 Separation with Fluorinated Carbon Molecular Sieve Membranes Zhenzhen Yang,1,2 Wei Guo,1 Shannon Mark Mahurin,2 Song Wang,3 Hao Chen,1 Long Cheng,4 Kecheng Jie,1 Harry M. Meyer III,2 De-en Jiang,3 Gongping Liu,4 Wanqin Jin,4 Ilja Popovs,2,* and Sheng Dai1,2,5,*

SUMMARY

The Bigger Picture

The CO2-philic properties of powdered fluorinated triazine frameworks make them promising candidates for fabrication of porous CO2 separation membranes. This is rarely reported because of lack of suitable synthetic approaches. Here, fluorinated membranes based on covalent triazine frameworks are prepared through a rational design of aromatic nitrile monomers containing fluorine and ether groups via a solgel polymerization process. The CO2 separation performance rises significantly with the fluorine content in the membrane. With functionalized triazine units, fluorine, and ether groups, these carbon molecular sieve membranes obtained after pyrolysis exhibit intrinsic ultra-micropores, high surface areas and excellent thermal stability under air. Excellent CO2 permeability and CO2 to N2 selectivity surpassing the Robeson upper bound are achieved. Our general design and synthesis protocol allow an easy access to fluorinated porous membranes, thus significantly expanding the currently limited library of CO2-philic and chemically stable membranes for highly efficient CO2 separation.

Rational design of robust and highly selective separation processes leading to an efficient sequestration of anthropogenic CO2 is one of the most important problems, especially considering the effects of climate change. Unsurprisingly, the fabrication of highly selective CO2 separation membranes, especially those capable of overcoming undesirable trade-off relationship between permeability and selectivity, is a vibrant and evergrowing field. However, there are only a handful examples of membranes that reportedly overcome the Robeson upper limit in CO2 separations. In this work, we present an efficient strategy that addresses a rational design of materials that exhibit remarkable affinity toward CO2 by introducing CO2-philic fluorinecontaining substituents into their structure, via a bottom up polymerization and pyrolysis approach and a precise control over the ultra-microporosity, resulting in some of the most efficient and promising CO2 separation media reported thus far.

INTRODUCTION Emission of CO2 arising from the combustion of fossil fuels has raised widespread environmental concern.1,2 Exploring cost-effective and scalable technologies for carbon capture from emission sources is regarded as one of the most efficient strategies to reduce anthropogenic CO2 emissions.2,3 Various carbon capture technologies, such as physical adsorption,3,4 chemical absorption,5 and membrane separation,6 have been widely investigated. Compared with other modes, membrane separation possesses many inherent advantages, such as high energy efficiency, low capital investments, operational simplicity, and flexibility to scale up and is considered a very promising technology.6–11 Membrane separation is based on the difference of gas permeation rates through membrane barriers. Although polymeric membranes have been successfully commercialized since the 1980s, unfortunately, the performance is limited by an undesirable trade-off relationship between permeability (P) and selectivity (a), and thus constrained by an upper limit, which is illustrated empirically by Robeson12 and theoretically by Freeman.13 Intensive research efforts have been devoted to the rational design and preparation of novel nanoporous polymeric materials to overcome this limit, for example, by using thermally rearranged (TR) polymers,14 polymers of intrinsic microporosity (PIMs),15,16 and mixed-matrix membranes (MMMs).6,17 The most significant feature in most of these approaches is the addition of entropic selectivity to improve molecular discrimination between similarly sized gas molecules.18

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Membrane materials with high CO2 permeability and ease of membrane fabrication are crucial to large-scale applications.19,20 As the membrane separation process is controlled by coupled sorption-diffusion processes, generally, introduction of small pores and surface functionalities contributes significantly to the increase of the membrane selectivity, whereas high surface area results in a considerable improvement in gas permeability.14 Nitrogen-rich covalent triazine frameworks (CTFs), a subclass of porous organic polymers (POPs), are promising solid-state candidates for carbon capture and sequestration (CCS) with the benefits of abundant micropores, excellent thermal stabilities, and highly basic functionalities.21–27 Particularly, incorporation of fluorine functional groups into the skeleton of CTF-derived materials has been shown to significantly enhance the CO2 sorption properties and catalytic activities.26–30 The perfluorinated FCTF-1-600 exhibited much higher CO2 uptake capacity (5.53 mmol g-1) than the non-fluorinated analog (3.82 mmol g-1) at 273 K, 1 bar and an exceptional CO2/N2 selectivity of 77 under kinetic flow conditions.26 CO2 adsorption capacity was further increased to 5.98 mmol g-1 by fluorinated F-DCBP-CTF-127 and 6.58 mmol g-1 by F12CTF-3 at 273 K and 1 bar.31 The CO2philic nature of fluorine functionalities has been witnessed also in the field of fluoropolymer synthesis in super critical CO2 (scCO2),32,33 CCS by fluorinated metal organic frameworks (MOFs),34,35 and CO2 conversion into value-added chemicals.28–30 CTF-derived materials are ideal for making porous membranes for separation of CO2 because of the highly cross-linked and chemically stable framework structures. However, the high-temperature synthesis method involving molten ZnCl2 cannot be adapted for synthesis of the corresponding membranes. We hypothesized that judicious tailoring of the pore surface of CTF-based membrane with fluorine functionalities will bring benefits in CO2 capture from post-combustion gases.32,33,36 This kind of membrane is rarely reported owing to the paucity of availability and poor reactivity of nitrile monomers. In this work, fluorinated triazine-based membranes (FTMs) are prepared through a rational design of aromatic nitrile monomers containing fluorine atoms and ether groups by using superacid-catalyzed trimerization procedure. The CO2 adsorption and separation performance of TMs is markedly improved with the increase of fluorine content within the membrane backbone. With functionalized triazine units, fluorine atoms and ether groups, N-, O-, and F-doped nanoporous carbon molecular sieve membranes obtained after pyrolysis exhibit intrinsic ultra-micropores (0.5–1.0 nm) and surface areas up to 733 m2 g
1. Excellent CO2 adsorption capacity of 4.92 (273 K) and 3.88 (298 K) mmol g
1 together with high single gas CO2/N2 selectivity of 48.7 G 1.2 (273 K) and 41.0 G 1.1 (298 K) indicate the membranes to be promising candidates for CO2 separation. Good ideal CO2 permeability and CO2/N2 selectivity of 2,140 barrer, 36 (F8TM-1–500) and 3,743 barrer, 23 (F8TM1–600), exceeding the Robeson upper bound, are achieved by tuning the pyrolysis temperature. In addition, the N, O, F-doped porous carbonaceous membranes exhibited excellent thermal stability under air up to 585 C. Through this general design and protocol, fluorinated membranes can be derived, significantly expanding the currently limited library of CO2-philic membranes for highly efficient CO2 separation.

RESULTS Synthetic Approach and Characterization of the Fluorine-Functionalized Membranes Previously, we have reported the formation of polymeric membranes through the trimerization of aromatic nitriles with a superacid catalyst (trifluoromethanesulfonic acid, CF3SO3H) through a sol-gel polymerization process.37 The polymers derived

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1Department

of Chemistry, The University of Tennessee, Knoxville, TN 37996, USA

2Chemical

Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

3Department

of Chemistry, University of California, Riverside, CA 92521, USA

4State

Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing, 210009, China

5Lead

Contact

*Correspondence: [email protected] (I.P.), [email protected] (S.D.) https://doi.org/10.1016/j.chempr.2019.12.006

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from our superacid-based method are highly cross-linked so as not to be susceptible to swelling induced by soluble gases or solvents and can serve as an ideal precursor for the synthesis of carbonaceous membranes through controlled pyrolysis.18 The attempts to prepare membranes by using 2,20 ,3,30 ,5,50 ,6,60 -octafluoro-4,40 -biphenyldicarbonitrile (F8DCBP) failed (see Supplemental Information for details). Therefore, rational design of the fluorinated aromatic nitrile monomers is needed to increase their reactivity. We hypothesized that aromatic nitrile monomers containing both fluorine and additional ether groups would be amenable to acid promoted polymerization reaction (Scheme S1). Introduction of ether groups into the para-position with respect to cyano group somewhat counteracts electron-withdrawing effect of fluorine atoms, hence increasing the trimerization reactivity of aromatic nitrile monomers. On the other hand, oxygen-containing functionalities show CO2-philic nature in CO2 capture and separation applications. For example, oxygen-doped HAT-CTFs produced through an in situ doping strategy exhibited an exceptionally high CO2 uptake capacity (up to 6.3 mmol g
1 at 273 K and 1 bar).21 To validate our assumption, we selected 4,40 -((perfluoro-[1,10 -biphenyl]-4,40 -diyl)bis(oxy))dibenzonitrile (F8CN-1) as the precursor for the membrane synthesis. Figure 1A provides a schematic representation of the corresponding polymerization procedure (for detailed synthetic process, see the Experimental section). Subjecting this starting material to acid resulted in the formation of transparent and flexible polymeric membrane (F8TM-1) (Figure 1C). Typical stress-strain curve is measured to evaluate the mechanical property of the membrane F8TM-1, which shows a tensile strength of 41.0% with an ultimate stress of 37.4 MPa (Figure S1). Notably, the size and thickness of the membrane could be easily tuned by appropriately selecting the size of the petri dish during the membrane cast process. For the sake of comparison, the non-fluorinated TM-1 membrane was also synthesized using CN-1 as the monomer. As expected, the synthesized membranes are completely insoluble in common organic solvents such as dichloromethane, tetrahydrofuran, and ethanol. The successful polymerization through the formation of triazine framework is further verified spectroscopically. In the Fourier-transform infrared (FT-IR) spectrum (Figure 1D), the existence of triazine units is characterized by two strong absorption bands at 1,508 and 1,363 cm
1 corresponding to the aromatic C–N stretching and breathing modes in the triazine unit, respectively, together with the characteristic peak for C=N located at 1,592 cm
1.38 Additionally, protonated nitrile groups (C= NH+) are also observed with characteristic peak at 1,683 cm
1 because of the presence of excess strong acid during the fabrication process. However, characteristic signal for nitrile groups at 2,230 cm
1 is evidently not present.37 Compared with TM-1, an additional peak located at 1,070 cm
1 for F8TM-1 is assigned to C–F bond stretching. As evidenced by the cross-polarization magic-angle spinning (CP-MAS) 13 C NMR spectrum (Figure 1E), the presence of sp2 carbons at 172.1 ppm confirmed the formation of triazine building block (d). The existence of the C–O bond is evidenced by the signal at 162.0 ppm (c). The signal of C–F bonds in F8TM-1 are located at 144.3 ppm, which is absent in TM-1 (e). In addition, the chemical shifts for aromatic carbons in F8TM-1 (133.1 ppm) are a little higher than those in TM-1 (130.6 ppm) because of the electron-withdrawing nature of fluorine atoms (label b). Solid state 19 F NMR of F8TM-1 spectrum also confirms the presence of C–F bonds within the membrane skeleton; there were two peaks located at
139.5 and
158.0 ppm (Figure S2), which is in accordance with the peaks in F8CN-1 monomer. The successful formation of the triazine frameworks in F8TM-1 is further confirmed by X-ray photo electron spectroscopy (XPS) analysis (Figure 1F). In the C1s spectrum, three peaks with BEs at 285.2, 287.9, and 293.4 eV can be deconvoluted, which are ascribed to carbons of aromatic C, the C–O and C–N bond in triazine ring, and C–F bond,

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Figure 1. Synthetic Route and Characterization of the Fluorinated CTF Membranes in This Work (A and B) Synthetic route and structures of membranes in this work using monomers with two (A) or three (B) cyano groups. (C) Pictures of F 8 TM-1 membrane. (D) FTIR spectra of F 8 TM-1 and TM-1. (E) CP/MAS 13 C NMR spectra. (F) XPS (C1s, F1s, N1s, and O1s) spectra of F 8 TM-1. (G) TGA analysis results obtained under N2 with ramping rate of 10  C min
1 .

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Figure 2. Electron Microscopy Characterization of the Fluorinated Membranes (A) Top view of F 8 TM-1. (B) Cross-section of F 8 TM-1. (C) Amplification of selected section in the red dotted box of (B). (D) Top view of F 8 TM-1–500. (E) Cross-section of F 8 TM-1–500. (F) Amplification of selected section in the blue dotted box of (E). Scale bars, 100 nm for (A), (C), (D), (F) and 3 um for (B) and (E).

respectively.26 The N1s spectrum also show the presence of triazine functionalities with BE = 398.9 eV, together with protonated terminal nitrile group (BE = 400.8 eV).37 Both fluorine and ether groups are detected in F1s spectrum for C–F bond (BE = 688.8 eV) and O1s spectrum for C–O–C bond (BE = 532.8 eV). Some of the ether groups are hydrolyzed to form O–H bond (BE = 534.2 eV) under strongly acidic conditions, or perhaps nitrile groups are hydrolyzed to form carboxylic acid groups. Comparatively, the signals for cyano groups in C1s (BE= 286.5 eV) and N1s (BE = 399.3 eV) spectra of F8CN-1 disappear (Figure S3A). The structure of TM-1 is also confirmed by XPS analysis in comparison with CN-1, except that no C–F bond is observed (Figures S3B and S3C). The powder X-ray diffraction (PXRD) pattern exhibited amorphous nature of the membranes (Figure S4). The morphology and nanostructure of F8TM-1 is further characterized by scanning electron microscopy (SEM) (Figures 2A–2C). The top view of F8TM-1 membrane exhibits a smooth, nearly defect-free surface and a non-porous structure. The polymeric membrane produced has a thickness of 81 um. The observation with the SEM at higher magnification shows a homogeneous stripy morphology of the cross-section for F8TM-1. In order to obtain a detailed morphology information of the membranes, transmission electron microscopy (TEM) images of F8TM-1 are collected after grinding the membranes into a powder, which showed that the membrane was composed of abundant layered structures, forming the strip-like morphology through compact stacking (Figure S5). Investigation of Other Aromatic Nitrile Monomers Other triazine-based membranes were also made using various fluorinated aromatic nitriles as starting materials to investigate the influence of the position of

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Figure 3. Textural Properties, CO2 Uptakes, and CO2 Separation Performances of the Membranes in This Work (A) Double logarithmic plots of selectivity versus permeability for CO 2 /N 2 showing Robeson’s 2008 upper bound, with data from this work. (B) CO 2 adsorption isotherms of F 8 TM-1 and TM-1 measured at 273 K and 298 K. (C) CO2 uptake capacities at 273 K and 298 K of membranes including F 8 TM-1 and TM-1, as well as these after pyrolysis at 400–600  C. (D) N 2 adsorption and desorption isotherm curves at 77 K. (E) Pore-size-distribution curves obtained from the adsorption branches by using NLDFT method. (F) Plots of selectivity versus permeability for CO 2 /N 2 , with data from this work and literatures. The following data were shown: membrane [(CO 2 permeability, CO 2 /N 2 selectivity), BET surface area]. N-PCM [(1,149 barrer, 43), 382 m 2 g
1 ], 39 TFM-1 [(518 barrer, 29), 738 m 2 g
1 ], 37 TZPIM-2 [(3,076 barrer, 30.5), 30 m 2 g
1 ], 15 PIM-EA-TB [(7,140 barrer, 13.6), 1,028 m 2 g
1 ], 16 PBI (450) [(,1624 barrer, 26.2), 447 m 2 g
1 ]. 40

the fluorine substituents (Scheme S1; Figure 1B) (see Supplemental Information for details). From the results we concluded that for the successful preparation of fluoro-containing CTF membranes, no fluorine atoms should occupy the ortho-positions relative to the cyano groups. In addition, the fluorine content in the membrane framework can be modulated depending on the structure of the monomers. Gas Separation Properties of the Fluorinated Membranes The CO2 separation performance of the membranes are examined by using a nonsteady-state permeation cell at 298 K and a pressure difference of 45 KPa. A plot of an ideal CO2/N2 selectivity (a(CO2/N2)) versus CO2 permeability (PCO2) is shown in Figure 3A. As expected, triazine-based membranes containing fluorine atoms exhibit better CO2 separation performance, that is, with the values much closer to the upper bound of the Robeson plot compared with non-fluorinated analogs.12 Among these, F8TM-1 shows the best performance with PCO2 of 384 barrer and a(CO2/N2) of 35, demonstrating separation enhancement effect exerted by introducing fluorine atoms within the membrane framework. F8TM-1 also exhibits enhanced CO2 uptake [0.65 (273 K) and 0.40 mmol g
1 (298 K)] in comparison with TM-1 [0.54 (273 K) and 0.33 mmol g
1 (298 K)] at CO2 pressure of 1 bar (Figure 3B) (see Supplemental Information for detailed comparison of various CTF membranes in CO2 adsorption and separation).

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Fabrication, Characterization, and CO2 Uptake Properties of N, O, and FDoped Nanoporous Carbon Molecular Sieve Membranes As stated above, the advantages of introducing C–F bonds into the membrane backbone are obvious and lead to the significant increase in the CO2 separation performance and uptake capacity. However, the CO2 permeability is still unsatisfying for practical applications because of low Brunauer-Emmett-Teller (BET) surface area (<1 m2 g
1) of the obtained membranes derived from the nitrile monomers containing relatively flexible ether groups. In order to obtain fluorinated triazine-based nanoporous carbon molecular sieve membranes, TGA of F8TM-1, F12TM-1, and TM-1 is firstly conducted under N2 up to 800 C to measure the thermal stability of the membranes (Figure 1G). The results indicate that F12TM-1 has poor thermal stability, and the membrane starts to decompose rapidly at 300 C, which makes it an inferior candidate for the fabrication of carbonaceous membrane. Comparably, F8TM-1 and TM-1 have similar stabilities, and there are two decomposition stages beginning at 300 C (loss of trapped solvent and monomers) and 520 C (decomposition and rearrangement of the polymer skeleton), respectively. Residual weight of 45 wt % is still retained at 800 C, demonstrating the possibility to fabricate F, O, and N-doped porous carbonaceous membranes by carefully adjusting the pyrolysis temperature. First, F8TM-1 is taken as an example to investigate the influence of pyrolysis temperature on the resultant membranes, denoted as F8TM-1-T (T = pyrolysis temperature). After treating F8TM-1 at 400 C under N2 to remove the locked solvents and unreacted monomers, the obtained F8TM-1–400 still has low surface area (<1 m2 g
1), however, its CO2 uptake capacity slightly increases to 0.94 (273 K) and 0.6 mmol g
1 (298 K) at CO2 pressure of 1 bar (Figure 3C). Further increasing the pyrolysis temperature to 500 C leads to the formation of F8TM-1–500 (Figure S6A). The top view SEM image of F8TM-1–500 clearly reveals that the outer continuous network of the surface is composed of small polymer nanoparticles (Figure 2D). The thickness of F8TM-1–500 has slightly decreased to 75 mm (Figure 2E). Closer examination on the cross-section of the membrane reveals that randomly distributed blind holes or defects with diameters of 10–50 nm are observed throughout the interior of the membrane (Figure 2F). TEM images of F8-TM-1–500 after grinding also reveals that the layered structure is well preserved without the obvious presence of the mesopores (Figures S6B and S6C). Textural information on F8TM-1–500 is obtained via N2 sorption analysis at 77 K, which shows a first step at P/P0 < 0.05 and exhibits a typical type I isotherm, implying a solely microporous structure (Figure 3D). Its BET surface area is calculated to be 462 m2 g
1 (Figure S7) and had a total pore volume of 0.21 cm3 g
1. As previously reported, the presence of ultra-micropores with the dimensions close to the kinetic diameters of CO2 (0.330 nm) within the materials could significantly enhance the CO2 uptake and separation performance, as well as selectivity toward other gases.7,41 Notably, abundant ultra-micropores with the pore size distribution of 0.5–1.0 nm (Figure 3E), obtained from the adsorption branches by using nonlocal density functional theory (NLDFT) method, renders the obtained carbonaceous N-, O-, and F-functionalized membrane a good candidate for CO2 separation. Accordingly, the CO2 uptake capacity of F8TM-1–500 shows an obvious increase to 2.40 (273 K) and 1.56 mmol g
1 (298 K) at CO2 pressure of 1 bar (Figure 3C). In order to study the potential use of the membranes in gas separation, single component adsorption isotherms for N2 are also collected. CO2/N2 selectivities are obtained using initial slope ratios estimated from Henry’s law constants. The CO2/N2 selectivities of F8TM-1–500 are calculated to be 48.7 G 1.2 (273 K) and 41.0 G 1.1 (298 K) (Figure S8), which are higher than the most values reported previously for the carbonaceous membranes (Table S1).

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It is not surprising that F8TM-1–600 shows higher BET surface area (733 m2 g
1) (Figure S7) and total pore volume (0.31 cm3 g
1) compared with F8TM-1–500, because of decomposition of polymer fragment with low polymerization degree. However, homogeneous distribution of ultra-micropores is still maintained with pore size distribution of F8TM-1–600 in the range of 0.5–1 nm, and without the presence of the mesopores, as shown by the type I N2 adsorption isotherm (Figure 3D) and poresize-distribution curve (Figure 3E). Accordingly, CO2 uptake capacity (1 bar) up to 4.92 (273 K) and 3.88 mmol g
1 (298 K) is achieved (Figure 3C), which are higher than the most values previously reported for membranes based on microporous organic materials (Table S1).7 Relatively high CO2/N2 selectivity of F8TM-1–600 is maintained as 29.9 G 1.1 (273 K) and 26.4 G 0.5 (298 K) (Figure S8). Pyrolysis of TM-1 is conducted for comparison. Notably, TM-1–500 shows a BET surface area of only 0.7 m2 g
1, indicating that the pore formation effect by cleavage of C–F bond in F8TM-1 is much more efficient than that of C–H bond in TM-1, as the Van der Waals radius of fluorine (147 pm) is much bigger than that of hydrogen (53 pm).42 Accordingly, although CO2 uptake capacity (1 bar) of TM-1–500 is slightly increased to 0.75 (273 K) and 0.45 mmol g
1 (298 K), this value is still much lower than that of F8TM-1–500 (Figure 3C). Increasing the pyrolysis temperature to 600 C leads to the formation of TM-1–600 having surface area of 598 m2 g
1 (Figure S7) together with the total pore volume of 0.24 cm3 g
1 and pore size distribution of 0.5–1.4 nm (Figure 3E). The abundant micropores within TM1–600 also supply the favorable factor for CO2 adsorption, with the capacity of 3.78 (273 K) and 2.40 mmol g
1 (298 K) at CO2 pressure of 1 bar being achieved, together with CO2/N2 selectivities of 28.4 G 0.4 (273 K) and 23.7 G 0.4 (298 K) (Figures 3C and S8). For both F8TM-1 and TM-1, the membranes are decomposed and cannot be retained at pyrolysis temperature of 700 C. The detailed chemical structures of membranes obtained after pyrolysis, including F8TM1–500, F8TM-1–600, and TM-1–600, are characterized by FT-IR, solid state 13C and 19F NMR, and XPS techniques (Figures S9–S11). FTIR spectra (Figure S9A) reveal that (1) the peak for C=NH+ (1,683 cm
1) disappeared after thermal treatment and simultaneously signal for nitrile groups (2,230 cm
1), although very weak, is observed, indicating the existence of unreacted nitrile groups after removing the residual acid by heating; (2) the triazine units with characteristic peaks at 1,363, 1,508, and 1,592 cm
1 are well preserved in F8TM-1–500, which become pretty weak in F8TM-1–600, indicating the rearrangement the polymer skeleton and partial cleavage of the triazine units at high temperature; and (3) C–F bond is clearly observed in F8TM-1–500 but almost disappears in F8TM1–600. Reduced intensity of the signal for triazine units has also been observed in the FTIR spectrum of TM-1–600 (Figure S9B). All these results verified the partial cleavage of the C–N, C=N, and C–F bonds has occurred during the thermal treatment, especially at 600 C. Solid-state 13C NMR of F8TM-1–500 spectrum shows the existence of carbons in benzene ring (label a and b), C–O bond (label c), triazine unit (label d), and C–F bond (label e), but with decreased intensity compared with those in F8TM-1 (Figures S10A and 1E). However, in the solid state 13C NMR spectra of F8TM-1–600 and TM-1– 600, only a broad peak is present indicating the aromatic carbon region. Lack of the signals for C–O, C–F, and triazine is caused by decreased content of these functionalities after heating at 600 C, considering the low sensitivity of solid state 13C characterization. In the solid state 19F NMR spectrum of F8TM-1–500 (Figure S10B), signals corresponding to the C–F bonds can be clearly distinguished (label a and b). However, the intensity is significantly decreased for F8TM-1–600, and only one broad peak (label b) is reserved, indicating that the C–F bond of type a is more easily cleaved. XPS analysis of the carbon molecular sieve membranes is further conducted to get an additional insight of the chemical structure features (Figure S11). From the XPS analysis, the surface fluorine content in

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F8TM-1 is 17.8 wt %, which is slightly decreased to 12.6 wt % in F8TM-1–500 and then sharply decreased to 4.7 wt % in F8TM-1–600, in accordance with the changes observed in both the FTIR and solid-state NMR spectra. Compared with F8TM-1 (Figure 1F), after thermal treatment at 500 C (F8TM-1–500) (Figure S11A), the carbon ratio in C–O and triazine functionalities decreases from 44% to 29% because of cleavage of these bonds and loss of the corresponding segments. In addition, rearrangement of the membrane skeletons leads to the formation of N–O bonds and disappearance of the C=NH+ functionalities, as shown in the N1s and O1s spectra. For F8TM-1–600 obtained at higher temperature (Figure S11B), the nitrogen ratio in C=N decreases from 55% to 19% whereas that in N–O bonds increases from 45% to 81% on the surface. The same phenomenon is observed in the XPS spectra of TM-1–600, compared with that of TM-1 (Figures S3C versus S11C). The difference in textural structures as well as the content of fluorine atoms between F8TM-1–500, F8-TM-1–600, and TM-1–600 leads to significant changes in their CO2 separation performance, as shown below. Gas Separation Properties of the N, O, and F-Doped Nanoporous Carbon Molecular Sieve Membranes The detailed separation performances of CO2 over N2 are measured, as shown in Figure 3F. Compared with using F8TM-1, using F8TM-1–500 obtained much higher PCO2 of the 2,140 barrer, together with a(CO2/N2) of 36, which exceeds the upper bound limit suggested by Robeson in 2008.12 As for F8TM-1–600, the PCO2 and a(CO2/N2) are measured to be 3,743 Barrer and 23, respectively, which is superposed with the most recent upper bound and are comparable or better than the most of the carbonaceous membranes described in the literature (Figure 1F and Table S1). Notably, the CO2 separation effect of F8TM-1–500 is comparable with the performance obtained by MOF scaffold coated with poly(ethylene glycol) (a(CO2/N2) =35, PCO2 = 2,700 barrer).43 The CO2/N2 separation performance of TM-1–600 is also tested, which shows a little lower PCO2 of 2,997 together with the same a(CO2/N2) of 23. Generally, the highest a(CO2/N2) is achieved with F8TM-1–500, indicating the enhancement by introducing fluorine species. Higher PCO2 is obtained with TM-1–600 mainly due to its higher surface area (598 m2 g
1) compared with that of F8TM-1–500 (462 m2 g
1). A similar phenomenon has been observed for F8TM-1–600 with even higher surface area of 733 m2 g
1 while maintaining the same a(CO2/N2). Using the same procedure but changing the amount of the monomer F8CN-1 and CF3SO3H, F8TM-1 membranes with various thicknesses were prepared and after pyrolysis, F8TM-1–500 with thicknesses of 30, 75, and 146 mm were fabricated. The CO2 separation performance results show that compared with the membrane with thickness of 75 mm, which had a(CO2/N2) of 36 versus PCO2 of 2,140 barrer, decreasing the membrane thickness to 30 mm led to a comparable a(CO2/N2) of 34 and PCO2 of 2,143 barrer. F8TM-1–500 with a thickness of 146 mm exhibited decreased a(CO2/N2) of 27 versus increased PCO2 of 4,015 barrer. For F8TM-1–500 with thickness of 146 mm, as shown in Figure S12, the time lag (q) for CO2 and N2 is 0.77 and 1.28 min, respectively, which is short compared with the carbon and carbon mixedmatrix membranes derived from the pyrolysis of the polyimide precursor,44 because of the high surface area (462 m2 g
1) and abundant micropores within the structure. Accordingly, the diffusion selectivity and sorption selectivity were calculated to be 1.7 and 15.9, respectively (see Experimental Procedures for a detailed calculation). Therefore, solubility contribution to permeability was much greater than the diffusivity contribution. That is, the higher CO2 uptake, hence the solubility of this gas overwhelms the dynamics of adsorption in the diffusion process, which is the key to achieve such high CO2/N2 selectively in the membrane. These results indicate that

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the F, O, and N-containing CO2-philic nanodomains in the membrane skeleton play an important role in propagating CO2 molecules through the membrane. Subsequently, a binary CO2/N2 test was conducted using F8TM-1–500 at 25 C. Generally, the a(CO2/N2) in mixed gas test is lower than that in pure gas test mainly ascribed to the competitive sorption in membranes.45 With the CO2 and N2, 15 and 85 mol % in feed, a(CO2/N2) of 27 is obtained together with slightly increased PCO2 of 3,712 barrer. In comparison, with the CO2/N2 50/50 mol % in feed, higher PCO2 permeability of 4,660 barrer together with lower a(CO2/N2) of 20 is achieved. Notably, in the field practice, the presence of water in the atmosphere is inevitable, and usually H2O molecule is competing with CO2 for the active sites that provide selectivity. This is well illustrated in the case of MOFs-derived materials, which exhibit good performance in CO2 separation under dry conditions, but diminishes considerably in the presence of water vapor.46 In this study, the CO2 separation performance of F8TM-1–500 was also conducted under high relative humidity (90%) conditions with CO2/N2 50/50 mol % in feed. The results indicate that a relatively high PCO2 of 4,135 barrer can be retained, and the a(CO2/N2) is only slightly decreased to 19. Therefore, the highly hydrophobic nature of F8TM-1–500 provided by both carbon and fluorine functionalities renders this material a promising candidate for practical applications. Overall, F8TM-1–500 displays good performance for selective separation of CO2 from N2, and the plot of a(CO2/N2) versus PCO2 even surpassed the Robeson upper bound. Although F8TM-1–600 exhibits much higher PCO2, and decreased a(CO2/N2), it is still superposed with the most recent upper bound. Considering the pore size distribution of the carbon molecular sieve membranes in this work is in the range of 0.5–1 nm, which is larger than the kinetic diameters of both CO2 (3.30 A˚) and N2 (3.64 A˚), the adsorption and diffusion mechanism is mainly responsible for the high membrane separation performance. The adsorption/diffusion is critically dependent on the properties of the pore surface structure from hetero atoms including F, O, and N. Heats of adsorption (Qst) for CO2 and N2 using F8TM-1–500 were calculated and the results show that the Qst for CO2 achieved 32.0 kJ mol
1 and in comparison, the highest Qst for N2 was only 12.06 kJ mol
1 (Figure S13), indicating the strong interaction of CO2 with the membrane frameworks. Besides good CO2 permeability and high CO2/N2 selectivity, high thermal stability of the membrane, particularly under air is also very crucial for its utilization in flue gas separation, considering the practical high-temperature atmosphere and the presence of O2 (3%–4%) as the most ubiquitous impurity in flue gas.47 TGA under air was conducted to evaluate the thermal stability of the N, O, and F-doped porous carbonaceous membranes in this work (Figure S14). The result showed that all the three kinds of membranes with efficient CO2 separation performance exhibited high thermal stability under air up to 585 C for F8TM-1–500, indicating its potential to be used for practical separation of CO2 in flue gas. Therefore, the fluorinated carbon molecular sieve membranes developed in this work are fabricated from easily synthesized nitrile monomers through a simple sol-gel process, followed by a pyrolysis. The obtained membranes exhibit good CO2 separation performance both under dry and humid conditions and high thermal stability under air atmosphere, which make them promising candidates for future scale-up industrial applications.

DISCUSSION In summary, fluorinated membranes based on CTFs have been obtained through a rational design of the aromatic nitrile monomers. The reactivity of the monomers in a trimerization reaction is enhanced by simultaneously introducing ether groups to the

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para-position and fluorine atoms to the meta position of the benzene rings relative to the cyano groups. A series of transparent and flexible polymeric membranes are obtained under mild reaction conditions. The CO2 separation performance is significantly enhanced with the increase of fluorine content in the membrane. Furthermore, the presence of functionalized triazine units, fluorine atoms, and ether groups, as well as high thermal stability of the membranes make it possible to fabricate N-, O-, and F-doped nanoporous carbon molecular sieve membranes for CO2 separation. Specifically, membranes obtained after pyrolysis show high BET surface areas, high thermal stability under air and ultra-microporosity, as well as higher CO2 adsorption capacity than most of the POPs reported in the literature. All these properties make them excellent candidates for membrane-based CO2 separation. High CO2 permeability together with good CO2/N2 selectivity is observed, exceeding the upper Robeson bound limit. We anticipate that this study will expand the viability of such task-specific functionalities (e.g., triazine, ether, and fluorine) involved membranes to a wide variety of fields.

EXPERIMENTAL PROCEDURES Characterization Liquid 1H and 13C NMR spectra was recorded on Bruker 400 MHz NMR spectrometer. FTIR spectra of the samples were collected on a TENSOR 27 FTIR at a resolution of 2 cm
1 in the spectral range from 4000 to 500 cm
1. Solid-state 13C and 19F NMR was performed by using a Solid State Varian INOVA 400 MHz. X-ray photoelectron spectroscopy (XPS) measurements: XPS experiments were performed with a PHI 3056 spectrometer equipped with an Al anode source operated at 15 KV and an applied power of 350 W and a pass energy of 93.5 eV. Samples were mounted in foil given that the C1s binding energy was used to calibrate the binding energy shifts of the sample (C1s = 284.8 eV). The nitrogen adsorption and desorption isotherms were measured at 77 K under a Gemini 2360 surface area analyzer. The samples were outgassed at 150 C for 16 h before the measurements. Surface areas were calculated from the adsorption data by using Brunauer-Emmett-Teller (BET) methods. The pore-size-distribution curves were obtained from the adsorption branches by using non-local density functional theory (NLDFT) method. SEM imaging was performed by using a Zeiss Auriga microscope with an electron beam operation of 5 keV, which is a dual beam FIB (Focused Ion Beam) with a field-emission electron column for high-resolution electron imaging and a Canion Ga+ column for precision ion beam milling. CO2/N2 Uptake Measurement Gas uptake measurements were obtained using a gravimetric microbalance (Hiden Isochema, IGA) that combines accurate computer control of pressure and temperature with high precision measurements of sample weight changes to acquire gas uptake isotherms. The sample was loaded into a quartz sample container and sealed in the stainless steel chamber. The sample was dried and degassed at a temperature of 160 C and a vacuum pressure of 1 mbar for a minimum of 12 h before the dry mass was recorded. Mass measurements were then acquired at increasing CO2 pressures up to 1 bar, taking into account the buoyancy effect on the mass. The temperature of the sample was maintained using either a constant temperature recirculating water bath for the 298 K measurement or an ice bath for the 273 K measurement. CO2/N2 Membrane Separation Gas permeability measurements were performed using a custom test chamber that has previously been described in detail.48 The samples were not large enough to occupy the entire area of the test chamber (47 mm2), so they were masked by first placing a section of the material on a 47 mm2 piece of adhesive-backed aluminum

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with a hole cut in the center. The membrane and adhesive aluminum assembly was then attached to a 47 mm2 aluminum disk (1/16 in. thick) whose center was cut with a mating hole corresponding to the hole in the adhesive-backed aluminum, thus creating a sandwich. A thin layer of epoxy was placed on the interface between the membrane and the aluminum to seal the membrane completely and to ensure that the only available diffusion path would be through the membrane. The diameter of the membrane as defined by the holes cut in the center of the aluminum was measured to be 15 mm. The thickness of the membrane was measured using high-resolution calipers. The membrane was then placed in the test chamber and evacuated to 20 mTorr, where it remained overnight. Single-gas permeability measurements were performed by isolating the two parts of the chamber (the permeate side and the retentate side) and introducing each gas to the retentate side until a pressure of 45 kPa was reached. The pressure rise on the permeate side was monitored using a 10 Torr Baratron gauge (MKS Instruments) and a data logging program. After an initial delay time corresponding to the time for diffusion through the membrane, the pressure in the permeate side rose linearly with time. From the slope of this pressure rise and the properties of the membrane, the permeability (P) was calculated using the following equation: P=

Vt dp RTADP dt

where V is the permeate volume, t is the membrane thickness, R is the ideal gas constant, T is the absolute temperature, A is the membrane area, Dp is the pressure difference across the membrane, and dp/dt is the rate of gas pressure increase on the permeate side. Permeability is traditionally reported in the unit of barrer [1 barrer = 10
10$cm3 (STP)$cm/(cm2$s$cmHg)]. Ideal selectivity is typically used to characterize the efficiency of a membrane to separate a faster-permeating species from a slower-permeating species, which can be defined as the ratio of single-gas permeabilities: aA=B =

PA PB

Permeability can be deconvoluted into the product of a kinetic factor (diffusivity) and a thermodynamic factor (sorption coefficient): PA = DA  SA 2

in which D is diffusivity (cm /s) and S is sorption coefficient (cm3 (STP)/cm3$cmHg). The ideal selectivity can be written as the product of diffusion selectivity and sorption selectivity:     DA SA aAB = aD  aS =  DB SB in which aD is diffusion selectivity and aS is sorption selectivity. Diffusivity can be estimated using the time lag method: D=

l2 6q

in which l is thickness of the separation layer and q is the permeation time lag. Gas transport behaviors in the mixed gas permeation tests were measured using constant pressure and variable volume technique. Mixture of CO2 and N2

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(50 vol %:50 vol %, or 15 vol %:85 vol %) was used as a feed gas and Ar was chosen as sweep gas (Figure S14). The pressure at the feed side was 0.3 MPa, and the temperature was set as 25 C. When the membranes were tested in humid state, the feed and sweep gases were both humidified by water bottles. After humidifying, the relative humidity was about 90%. When the system reached steady-state, all the gas permeation measurements were performed more than five times. The selectivity can be obtained by this equation: aA=B =

yA =yB xA =xB

Where x and y are the volumetric fractions of the one component in the feed and permeate respectively, which were detected by the gas chromatography. Preparation of Membranes The membranes were synthesized through a similar procedure in the reported literature with small modifications.37 Briefly, 1 mL of CF3SO3H was added dropwise to 0.1 g of the aromatic nitrile monomers, except that 0.2 g of F8CN-1 and 0.3 g of F12CN-1 were needed per 1 mL of CF3SO3H, at room temperature under stirring. The viscous red solution was continuously stirred for 0.5 h (4 h for F12TM-1) and then poured into a flat bottom glass dish with diameter of 5 cm and allowed to spread into a thin layer. Then, the dish was covered, and heated at 100 C for 20 min (1 h for F12TM-1), and a soft coating film was successfully formed. For the preparation of F12TM-1, reaction time of 4 h at room temperature and 1 h at 100 C was needed. The material was then quenched in cold water and immersed in diluted NaOH solution for 12 h to remove the excess CF3SO3H. After the material was washed with water and ethanol several times, the obtained transparent and flexible film was dried in a vacuum oven at 60 C. The yield was 80%, with majority of the loss caused by the liquid transfer process. For the synthesis of N, O, F-doped carbonaceous membrane, in general, a piece of polymeric membrane precursor with diameter of 12 mm was placed in a tube furnace at certain temperature and maintained at this temperature for 12 h under a nitrogen gas flow (heating rate: 2 C min
1). For the fabrication of F8TM-1–500 with thicknesses of 30, 75 and 146 mm, the precursors F8TM-1 was different thicknesses were prepared in advance by changing the amount of F8CN-1 and CF3SO3H. Briefly, 0.1 g, 0.2, and 0.4 g F8CN-1 together with 0.5 mL, 1 mL, and 2 mL CF3SO3H was added, respectively. Thickness (cross-section) of the membranes is measured by SEM, as shown in Figures 2B and 2D.

SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.chempr. 2019.12.006.

ACKNOWLEDGMENTS The research was supported financially by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy.

AUTHOR CONTRIBUTIONS Z.Y., I.P., and S.D. conceived and designed the experiments; Z.Y., I.J., and W.G. carried out most of the synthesis and material characterization experiments; Z.Y., W.G., S.M., H.C., S.W., D.J., K.J., and I.P. analyzed data; H.M. conducted the XPS

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experiments and analyzed the data. L.C., G.L., and W.J. conducted the mixed gas separation and humidity test; Z.Y. and S.D. wrote the paper; and S.D. directed the project.

DECLARATION OF INTERESTS The authors declare no competing interests. Received: June 20, 2019 Revised: August 5, 2019 Accepted: December 6, 2019 Published: January 16, 2020

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