A Highly Ordered Nanoporous, Two-Dimensional Covalent Organic

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A highly-ordered nanoporous, two-dimensional covalent organic framework with modifiable pores, and its application in water purification and ion sieving. Valerie Kuehl, Jiashi Yin, Phuoc Duong, Bruce Mastorovich, Brian S. Newell, Katie Dongmei Li-Oakey, Bruce A Parkinson, and John O Hoberg J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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Journal of the American Chemical Society

A highly-ordered nanoporous, two-dimensional covalent organic framework with modifiable pores, and its application in water purification and ion sieving. Valerie A. Kuehl1, Jiashi Yin2, Phuoc H. H. Duong2, Bruce Mastorovich1, Brian Newell3, Katie Dongmei Li-Oakey2*, Bruce A. Parkinson1*, and John O. Hoberg1*. 1Department

of Chemistry, University of Wyoming, Laramie, Wy 82071 USA. of Chemical Engineering, University of Wyoming, Laramie, Wy 82071 USA. 3Central Instrument Facility, Colorado State University, Fort Collins, CO 80523 USA. KEYWORDS Nanoporous, modifiable materials, membrane filtration, covalent organic framework, COF 2Department

ABSTRACT: The preparation of membranes with high selectivity based on specific chemical properties such as size and charge would impact the efficiency of the world’s energy supply, the production of clean water and many other separation technologies. We report a flexible synthetic protocol for preparing highly ordered two dimensional nanoporous polymeric materials (termed covalent organic frameworks or COFs) that allow for placing virtually any function group within the nanopores. We demonstrate that membranes, fabricated with this new family of materials with carboxylated pore walls, are very water permeable, as well highly charged and size selective.

1. INTRODUCTION The past decade has seen an explosion of interest in twodimensional (2D) materials that started with the demonstration of the extraordinary properties of graphene, and has been extended to other 2D materials, such as transition metal dichalcogenides and other elemental 2D phases (germanene, silicene, etc.).1 The promise of 2D materials is largely based on their unique single-layer electrical, optical, and magnetic properties. However, current 2D materials are not easily modified to suit a given application: that is, there is very little flexibility in adjusting the materials performance beyond their intrinsic properties. This lack of adaptability presents significant barriers to technological implementation and broad use. Attempts have been made to achieve this goal by modifying graphene. For example, a top down approach using ion bombardment,2 etching3 or oxidations,4 produces graphene oxide (GO) with pores containing a high degree of polydispersity in both size and density. When produced at high density, these randomly produced pores start to overlap producing both larger openings and weakening the material. In fact, “variations in the degree of oxidation caused by differences in starting materials (principally the graphite source) or oxidation protocol can cause substantial variation in the structure and properties of the material”.5 As a result, “permeation (flux) through GO membranes remains insufficient to technically compete with current commercial pressure-driven membranes”.6 This challenging goal of creating atomically precise nanopores, without destroying the material itself, has thus remained elusive. However, recently a bottom-up synthesis of a nanoporous “graphene” was reported,7 providing a material with ordered nanopores while maintaining the integrity of the graphene. Although this bottom-up strategy proved to be successful in the monolayer regime, the nine-step synthesis provides only nanogram

quantities and did not produce a material capable of pore functionalization. Liu and co-workers recently reported on modifiable aromatic framework, but the electronic demands of the system appear to only allow for placement of electron deficient substituents in the pores.8 Nanoporous metal organic framework (MOF) materials have also been investigated for membrane production. However, their 3D structures are problematic for membrane fabrication since the small grains of these materials allow species to transport in the spaces between grains rather than through the porous structure. In contrast, a 2D nanoporous material can naturally produce a membrane that minimizes or eliminates interstitial transport path via the natural stacking of the 2D sheets. Additionally, 2D materials with uniformly-sized, ordered and functionalized pores may provide direct transport channel through the pores for small molecules like water, instead of the restricted but tortuous diffusion path between multiple layers. Therefore, uniform sized, ordered and functionalized pores will have great efficacy for multiple applications, including high performance separations. Separations are fundamental to life processes, analytical protocols and industrial processes and consumes greater than 10% of world energy production.9 Many of the conventional separation techniques, such as distillation, extraction and chromatographies, are both time and energy intensive. More recently, MOFs and carbon nanotubes have been reported in membrane composites for water purification methods.10–12 Membrane separations are attractive due to their small energy requirements and their potential for fast and selective separations. In addition, ion or gas permeable membranes are vital to the operation of virtually all electrochemical devices including batteries, fuel cells, electrolyzers and desalinization systems. An ultimate membrane would therefore have both high throughput and highly selective transport or rejection of

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the species of interest based on size, charge or other molecular properties. This is in fact, “a difficult task to construct atomic scale porous membranes that will selectively allow the separation of the species from solutions depending on the molecular and ionic size”.13 We report fabricating such a membrane that allows for high water flux, ion size selective separation and functionalization from one of our new COFs. In this paper, we present a new family of two-dimensional covalent organic framework (COF) polymers that have a highly stable semiconducting aromatic backbone with intrinsically ordered nanometer sized pores that, unlike other COFs,14,15 can be functionalized, either pre or post synthesis, with almost any desired functional groups. We highlight an example where a highly ordered COF is synthesized with ionizable carboxylate groups in 2.8 nm diameter pores and demonstrate both high water flux and high selectivity of a membrane fabricated with this material to selectively conduct cations smaller than a precise size threshold. Additionally, related materials can be easily synthesized to either increase or reduce this size threshold or make anion selective membranes. Previously reported COFs that were employed in size selective separations are dependent on modification of the organic linker size within the backbone, necessitating a complete new synthesis of a COF for each separation.16,17 Our new bottom up approach to synthesizing 2D-COF materials achieves the goal of a highly ordered and modifiable material with both a very stable aromatic backbone and functionalized pores. Finally, these can be easily synthesized by condensation polymerization of easily obtainable small precursor molecules in concert with well-known high yield coupling reactions to replace bromines placed on the aromatic rings that line the pores to produce diverse pore functionality. 2. RESULTS AND DISCUSSION The condensation of amines with ketones is a wellestablished method for polymer production including the formation of 2D-COFs.18–25 Of particular relevance to our work was the disclosure by Jiang and co-workers18 who used a C3-symmetric hexamine and C2-symmetric tetraone to produce a highly ordered COF in very good yield. This strategy produces well-ordered materials by preventing the incorporation of errors in the polymerization step that is inherent in many 2D-COF formations26, using materials such as tetra- and hexamino benzene.27 However, the use of C3symmetric triphenylene hexamine is problematic due to cost and the multiple synthetic manipulations resulting in modest overall synthetic yield.28 We therefore developed an inexpensive, facile and scalable synthesis, as illustrated in Figure 1, to prepare a similar material but one with ready functionalization of the pores. Microwave induced condensation of hexaketocyclohexane29 (HKH) and benzenetetramine produces hexamine 1,30 which is

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subsequently brominated to give 2. Formation of 2 allows for the use of metal catalyzed coupling reactions, developed over the past several decades, and provides any number of substituents that can be incorporated into the pores of the 2DCOF. Additionally, the formation of hexaazatrinaphthalene 1 produces an increased pore size compared to the use of the triphenylene hexamine used by Jiang.18 Several palladium catalyzed reactions were performed to produce 3-5, with yields ranging from 62% for the carbonylation to give 3, and up to 91% for 4 and 5. The R groups illustrate the range that can be incorporated from neutral (1), positive (5, R = amine) and negative (3 and 5, R = carboxylate) charges, in addition to varying the size of the pore with the incorporation of the alkyne moiety (3 vs 5). Furthermore, functionalization of pyrene tetraone was also performed via the coupling of known dibromo tetraone with propiolic acid to produce 7. Microwave induced condensation of the hexaamines with pyrene tetraone resulted in crystalline black powders in yields of 80-95%. Interestingly, under solvothermal conditions in solvents such as N-methylpyrolidinone or acetic acid poor results were obtained for the formation of ordered, crystalline materials. The success observed from the microwave reaction may be due to an increase in material-microwave interactions during the course of the reaction.31 We hypothesize that growth is driven by the increasing conductivity of the developing extended π system resulting in increased microwave absorption, thus favoring the activation of larger grains for polymer growth over additional nucleation. A recent publication also emphasized the importance of controlling nucleation and growth to obtain ordered 2D COF growth albeit with less ordered material than our materials.32 The synthesis of COFs 8-10 best serve to illustrate the potential of our strategy. With this overall four-step synthesis, we have synthesized over 25 different COFs in overall yields of 5070% and can produce materials in gram amounts. COFs 8 and 9 best illustrate the formation of positive or negatively charged pores while inserting a total of 12 such groups in the pore. Alternatively, COF 10 incorporates only six groups inside the pore and demonstrates the controlled reduction in pore size. The space-filling model in Figure 1 depicts the impact on pore size with these three COFs in comparison to an unfilled pore (A). We have also performed post modification of a COF, in which propiolic acid was coupled with a COF where R1 = H and R2 = Br to produce COF 10. Comparison of IR (Figure S16 supplementary) and TEM data of 10 in which the propiolic acid was attached pre-COF formation vs post formation displayed subtle differences. The IR of post COF modification also indicated the presence of bromine atoms, which was verified with a flame test. Thus, it is possible to perform post modifications of brominated COFs (R1 or R2 = Br) however, it is preferable to modify the starting hexamine 2 prior to COF formation to obtain complete functionality.


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O O H 2N H 2N

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Figure 1. Schematic representation of the synthesis of COFs 8-10 in which wavy line indicates extension of the periodic structure, and space-filling model illustrating the individual pore structure (does not imply a mixed COF), black = carbon, red = oxygen, blue = nitrogen, yellow = hydrogen. COF formation: 8 E). The presence of 2D crystalline structures up to 300 nm on a side in the TEM images is exceptional for a 2D-COF Transmission electron microscopy (TEM) of the asmaterial. In addition, the powder x-ray diffraction pattern of synthesized COFs showed thin crystalline hexagonal COF 9 (Figure 3) shows the expected peaks for this type of structures that produced very sharp hexagonal diffraction 2D-COF material.18 A typical example in the literature patterns (Figure 2 and S12-14 supplementary). The hexagonal generally shows only 3 broad peaks with line widths of ~4.5° symmetry of the diffraction pattern was consistent with a in comparison with our material multiple peaks with 0.52° line simulated pattern generated by performing a 2D Fourier widths for the sharpest lines. transform of a space-filling model of COF 9 (Figure 2 D and



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Figure 2. TEM image showing well-ordered, 2D flakes on the order of 500 nm (A - C) of COF 9. Expansion of area in red box of A is illustrated in B. (E) 2D Fourier transform of simulated COF (D) from real to reciprocal space. The reciprocal space images correspond exactly to the electron diffraction pattern seen in (F). The bright spots in the image describe the reciprocal lattice of the synthesized flakes.

Figure 3. Powder x-ray diffraction of COF 9 from Figure 2. X-ray line widths of 0.52° are consistent with the grain size of the crystallite structures in the TEM images. Nitrogen sorption isotherms were also measured, however unlike MOFs, where BET isotherms can be a good indicator of pore size, we do not yet know the 2D-COFs defined 3D structure even though the 2D sheets are highly ordered as indicated by the sharp PXRD. As in most 2D materials, stacking polytypes most likely exist for our 2D-COFs.33 In dried 2D-COF materials, used for BET analysis, the more compact stacking of the 2D sheets may not have resulted in defined pores since much smaller voids result if the layers on either face of a single sheet are offset so as not to produce a passage for nitrogen gas in the structure. The BET data (see supplementary Figure S22) appears to be only an indicator of the 2D surface area of the sheets. COF 8, with its more bulky tertiary amine pore substitution, inhibits close packing of the

2D sheets and displays a higher BET surface area indicating that the nitrogen can reach the volume between the sheets. In contrast, for COF membranes that contain hydrophilic, functional groups, such as COF 9 with its 12 carboxylate functional groups per pore, a very different structure may be present when exposed to electrolyte solutions where the carboxylates are ionized and hydrated, with the presence of electrostatic double layer affecting the structure of the membrane. Hydrogen bonding interactions may also contribute to the complexity of the solid-liquid interface between carboxylate groups of different COF layers and surrounding water molecules. In conjunction with the above characterization, acid-base titration of COF 9 was also performed to quantify the level of carboxylic acid groups in the pores. The titration consists of two steps as illustrated in Figure 4. A sample of COF 9 was sonicated with NaOH to produce the sodium carboxylate and to disperse the individual sheets and then back titrated with HCl. The titration consists of two steps as illustrated in Figure 4. Specifically, HCl first reacts with the excess NaOH (Step 1, blue zone), and subsequently HCl reacts with the 2D-COFCOONa (Step 2, green zone). The quantity of carboxylate groups in 9 could be calculated by the additional consumption of the standardized HCl in Step 2 and found to be only 9% lower than the theoretical structure (supplementary Table S1). Since the edge of the 2D-COF sheets can affect the theoretical values and the pores often absorb water, this small discrepancy between titration and theoretical calculation further confirms successful COF formation with fully functionalized pores. Not surprisingly, protonation of the imine moieties in the COF did not occur, which is in agreement with the very low pKa’s of polyaromatic nitrogen heterocycles such as phenazines (pKa = 1.2).34


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that water transports predominantly through GO membranes via an interlayer flow between the nano-sheets (Figure 5B).37– 39 Although interlayer flow is also feasible for the COF 9 membrane, the high density of hydrophilic nanopores allows for facile water transport through the pores resulting in a far superior water permeance of ~2260 Lm-2h-1bar-1, as compared to ~7.6 Lm-2h-1bar-1 for the thinner GO membrane, as illustrated in Figure 5C (the GO layer thickness was measured at 170 nm vs 800 nm for the COF 9 membrane, see supplementary Figure S20). Membrane compaction, under pressure-driven membrane separation processes, is a well-known issue that reduces membrane performance.40 Therefore, we compared the performance of GO and COF 9 membranes under various applied transmembrane pressures from 1 to 6 bar using DI water as a feed. Given that the AAO support membrane has a high rigidity with negligible compaction under high pressure,41 and no membrane fouling or concentration polarization is expected to occur with DI water, the reduction of water permeance should only be due to the compaction of GO or COF 9 layers. Figure 5D illustrates a significant reduction of the water flux of the GO membrane to only 15% of its value at 1 bar upon increasing the transmembrane pressure to 6 bar. This significant flux reduction is caused by the elastic deformation of GO constricting the inter-sheet nano-channels.42,43 In contrast, COF 9 membranes retain a high water flux under high transmembrane pressure (~82% at 6 bar, Figure 5E), further indicating that the nanopores of COF 9 are the main water transport pathway versus tortuous path of flow through interlayer spacing. In fact, the small loss of water flux of the COF 9 membrane could be inhibition of the interlayer flow pathway.

Figure 4. Titration curve of COF 9 dispersed in NaOH solution. Having verified the structure and crystalline order of the materials, membranes were constructed with both COF and GO supported on a porous anodic aluminum oxide (AAO) filter to compare both their stability and flux. Photographs of hydrated GO and COF 9 membranes using the same fabrication method and conditions were obtained under dry conditions before and after their exposure to water and are presented in Figure 5A. It is apparent that COF 9 membranes remained intact while the GO membrane prepared under the same vacuum filtration membrane fabrication conditions disintegrated after 30 min in deionized (DI) water, indicating the stronger interaction between the nanosheets of 9 prevents the re-dispersion of the nanolayer-stacked film. We note that although GO membranes prepared from doctor blade casting are mechanically strong in aqueous solutions,35 GO membranes prepared under vacuum filtration often lose their mechanical integrity as recently reported.36 The water permeance of the membranes was then measured under a transmembrane pressure of 1 bar. It is well established (A)

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Figure 5. Comparisons of GO and COF 9 membranes: (A) digital photographs of as-prepared GO and COF 9 membranes (see Figure S19-21 for SEM images of GO and COF 9 membranes in both top surfaces and cross-sections. Images also include before



and after filtrations), using the same fabrication method with a post-treatment at 80 C for 5 min and their stability in water, (B) sketch of water transport pathways through layer-stacked GO and (C) COF 9 membranes, and the effect of the transmembrane pressure on water permeance through (D) GO (post-treatment by drying in air overnight) and (E) COF 9 (post-treatment at 80 C for 5 min). the supplementary section on cation selectivity). However, it is impressive that covering all the 200 nm pores with a layer of Recent work on selective separations using COFs has been COF 9 only decreased the conductivity by about a factor of 16,17,44 reported, however in these cases disadvantages can be three in addition, especially considering that the polystyrene noted when compared to the ease for which our COFs can be sulfonate anions cannot contribute to the ion current through employed. Given these impressive results for water flux, COF the membrane as they could in the control. Figure 6 also 9 with its 12 carboxylate groups per pore, producing highly demonstrates that the COF/TEPC membranes possess negatively charged pores, was further evaluated for their excellent ion selectivity towards the ammonium cations of cation selectivity in membranes. Membranes were fabricated various sizes. The higher cation conductivity of the smaller from a solution of COF 9 exfoliated in basic water that was cations, including NH4, Me4N, Et4N, Bu4N and Hex4N, transferred onto a track-etched polycarbonate membrane indicates that these cations can pass through the COF/TEPC support (TEPC) with a pore size of 200 nm. Vacuum filtration membrane, due to their smaller size (Hex4N radius = 8.2 Å) and curing in a vacuum oven produced a supported COF than the COF pores (radius = 13.5 Å). We note that the 9/TEPC membrane. The simple apparatus for ion selectivity decreasing conductivity trend from NH4+ to Hex4N+ for measurements consists of a two compartment U-shaped tube COF/TEPC is similar to that of bare TEPC membrane due to divided by a membrane (Figure S1). Both compartments were their smaller transference numbers. The significant drop in filled with 0.01M solutions of ammonium and various conductivity of COF/TEPC membranes for the larger cations tetraalkyl ammonium polystyrene sulfonate salts45 dissolved in (Oct4N radius = 10.9 Å and dodecyl = 15.1 Å) reveals the methanol. Polystyrene sulfonate (70 kDa) was used as the nearly complete rejection of larger cations by the COF layer, anion since it is too large to pass through the COF pores demonstrating its high cation size selectivity. The small ion requiring that the ion current across the membrane must be current for the larger cations could be attributed to cation carried by the cations with a larger transference number and transport through a tortuous path between the polycrystalline smaller size. Platinum electrodes were placed in both tubes 2D COF sheets or small pinholes in the membrane. This type and connected to a potentiostat with ion transport being of selectivity for very small ions is implicated for graphene measured by recording current-voltage curves between 0 V oxide membranes that have significantly lower ion and solvent and 2 V. The slope of the linear current-voltage (I-V) curves fluxes than our engineered nanoporous COF materials (see was used to define the resistance of the membrane indicative Figure S18 for comparison of GO vs COF 9). Figure 7 of the ability to support an ion current across it since it is the illustrates the size selectivity of our membrane by depicting a highest resistance element of the cell. single pore from COF 9 and three different sized tetraalkyl Figure 6 reveals that TEPC membranes with large 200 nm ammonium cations. Note also that the 2D nature of the diameter pores facilitate ion transport pathways for all the nanoporous COF provides natural ordering of the sheets unlike cations used in this study, as well as the sulfonated polymer membranes constructed by 3D MOFs, made up of small 3D anions, and was used as a control. Increasing the size of the grains of the material, resulting in larger spaces between the tetralkyl ammonium cations decreased their rates of transport grains, and, consequently, non-selective transport. We further through the TEPC membrane due to their decreasing hypothesize that the 2D stacking of the microcrystalline sheets transference number of the cations of different sizes, with in our COFs is driven by hydrogen bonding, perhaps through + 46 dodecyl4N showing the lowest conductivity. The ion water molecules, between carboxylate groups on different 2D conductivity of the COF/TEPC membranes is lower than layers, which, in turn, may actually align the pores producing control TEPC, indicating that the top COF layer increased the ions channels in at least portions of the membrane. ion transport resistance (for a discussion on the ion flux see 10.0

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Figure 6. Cation transport of a track-etched polycarbonate (TEPC) membrane support and a COF 9/TEPC supported membrane showing rejection of tetraalkyl cations larger than tetrahexyl ammonium.

Figure 7. Space-filling models of a single pore of COF 9 with Bu4N+ (A), Hex4N+ (B) and Oct4N+ (C) cations inside the pore. Note that the alkyl chains are depicted fully extended and that ion pairing with anions will also be probable in ethanol. 3. CONCLUSION In summary, we have designed and synthesized new, highly ordered 2D nanoporous COFs with functionalized pores. The synthesis of multiple pore functionalities has been demonstrated with a series of charged and uncharged systems. We then prepared membranes with a carboxylate functionalized COF that demonstrated both a high-water flux and cation size selectivity. The synthetic flexibility of this system allows for rational design and synthesis of membrane materials for many different types of separations based on size, charge, hydrophobicity and hydrophilicity among other chemical properties with potential applications in desalinization, anti-protein-fouling membranes, fuel cell membranes, redox flow battery membranes, dialysis membranes, gas separation membranes and other technologies requiring membrane separations some of which are already being pursued in our laboratories.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. All experimental and characterization data for the synthesis of 1-10, IR and TEM data of COFs 8-10. Experimental protocol for titrations, membrane formation and testing. AUTHOR INFORMATION Corresponding Authors [email protected] and [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors acknowledge generous support from the University of Wyoming’s School of Energy Resources Carbon Initiative, the UW Center of Produced Water Management and NASA EPSCoR Grant #NNX15AK56A (fellowship for VK). We thank Dr. Thomas Martin for helpful discussions on TEM analysis and Dr. Takashi Yanase for technical discussions.

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TOC O

O

R2

R2 O

O + H 2N

NH 2

R1 R1 H 2N H 2N

R1 N

N

N

N

N

N R1

R1

R1 NH 2 NH 2

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A Highly Ordered Nanoporous, Two-Dimensional Covalent Organic

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