Photo-tailored heterocrystalline covalent organic framework membranes for organics separation.

Organics separation for purifying and recycling environment-detrimental solvents is essential to sustainable chemical industries. Covalent organic framework (COF) membranes hold great promise in affording precise and fast organics separation. Nonetheless, how to well coordinate facile processing-high crystalline structure-high separation performance remains a critical issue and a grand challenge. Herein, we propose a concept of heterocrystalline membrane which comprises high-crystalline regions and low-crystalline regions. The heterocrystalline COF membranes are fabricated by a two-step procedure, i.e., dark reaction for the construction of high-crystalline regions followed by photo reaction for the construction of low-crystalline regions, thus linking the high-crystalline regions tightly and flexibly, blocking the defect in high-crystalline regions. Accordingly, the COF membrane exhibits sharp molecular sieving properties with high organic solvent permeance up to 44-times higher than the state-of-the-art membranes.

Introduction

Global demand for organic solvents is expected to surpass 20 million tons by 2023 for use in chemical, fragrance, and pharmaceutical processes due to the solubility and product selectivity requirements. These solvents must be separated from intermediate chemicals or products for recycling and minimizing emission[1-4]. Pressure-driven membrane separation operating at mild conditions without phase change holds great promise due to its much lower energy consumption and organic vapor emission compared with the commonly used thermal-based distillation technology[5-8]. However, conventional membranes which are fabricated from network polymers are often difficult to simultaneously achieve high solvent resistance and fast molecular sieving performance[9,10], necessitating new-generation membrane materials with highly robust, ordered, and porous structures.

Two-dimensional covalent organic framework (COF) is an emerging crystalline framework material comprising rigid molecular building blocks connected by robust in-plane covalent bonds and out-of-plane π-π interactions, exhibiting excellent solvent-resistant properties[11-14]. Moreover, the COF material features an ordered porous structure that has up to 100 times higher surface area than most network polymers[15]. Theoretically, high-crystalline COF membranes (COMs) formed totally by rigid crystallites can harvest high solute selectivity and possible high solvent permeability, however, the defect-free membranes are quite difficult to fabricate[16-18]. Low-crystalline COMs are much easier to fabricate, but often accompany with much less ordered structure and a dramatic sacrifice in solute selectivity as well as possible solvent permeability[19,20]. It proves a grand challenge to acquire good membrane processibility and sufficiently high crystallinity through adjusting the crystallinity of the whole COM. In this case, the COM can be regarded as the homocrystalline COM.

Herein, we propose a concept of heterocrystalline membrane comprised of both high-crystalline regions and low-crystalline regions. The heterocrystalline COM is prepared through a two-step procedure based on sequential Schiff-base reactions where the bond linkage can tautomerize under photo irradiation. The first step is dark reaction to generate the high-crystalline COM. The second step is photo reaction to generate low-crystalline regions in the intercrystalline defect of the COM, thus linking the high-crystalline regions tightly and flexibly, blocking the defect in high-crystalline regions. Photochemistry, which can interfere in proton transfer, is a widely-used and green way to modulate chemical reactions[21]. It has been demonstrated that photo-induced excited-state intramolecular proton transfer (ESIPT) can enable enol-imine to keto-enamine tautomerization[22,23], while the enol-imine linkage is the key for the commonly used Schiff-base COF to form high-crystalline structure[24]. This photo-induced tautomerization provides the chemical basis to control the reactive-crystallization procedure of COF by photo irradiation. In this study, we present a photo-tailoring strategy to prepare the heterocrystalline COM (Fig. 1). Under dark settings, the reversible enol-imine linkage can break up and reform, thus correcting the initial mismatched amorphous structure, allowing the formation of a high-crystalline COM. Subsequently, photo irradiation is introduced and the phototautomerization of enol-imine linkage inhibits the “error-correcting” process, allowing the formation of low-crystalline regions in the intercrystalline defects of the COM. By tuning the photo reaction time, the low-crystalline regions can tightly and flexibly link the high-crystalline regions to obtain the defect-free COM. Meanwhile, the well-preserved high-crystalline regions with ordered porous structures would enable precise and fast organics separation. The resulting COM displayed sharp molecular sieving properties, as manifested by the high organic solvents permeance up to 44-times higher than the state-of-the-art membranes.

Fig. 1

Schematic preparation of heterocrystalline COM.

Preparation of heterocrystalline COM by subsequent dark reaction and photo reaction using interfacial polymerization, where the top light-blue layer is the aqueous phase and the bottom navy-blue layer is the organic phase. 1,3,5-triformylphloroglucinol (Tp) and 2,2′-bipyridine-5,5′-diamine (Bpy) are used as aldehyde monomer and amino monomer, respectively, to prepare Schiff-base Tp-Bpy COM. The inset digital photograph is the photo-tailored heterocrystalline COM deposited on non-woven fabrics with the size of ~15 cm × 8 cm (Supplementary Fig. 1).

Results

Photo-tailored reactive-crystallization of Schiff-base COM

The synthetic routes of COMs by either dark reaction or photo reaction were illustrated in Fig. 2a. Initially, precursor trialdehyde (Tp) and diamine (Bpy) would polymerize into an amorphous network via enol-imine linkage[25]. During dark reaction, the reversible enol-imine linkage breaks and reforms slowly, thus converting the initial amorphous network into the thermodynamically stable crystalline framework as a result of the “error-correcting” process[26]. Then, the enol-imine linkage tautomerizes irreversibly to stable keto-enamine form because the basicity of three imine nitrogens (C = N) dominates over the aromaticity of the central benzene ring[27,28]. The optimal reaction time was set at 96 h, and the Bpy and Tp concentrations were set at 0.30 and 0.20 mmol L−1, respectively (Supplementary Fig. 2, 3). Fourier transform infrared (FTIR), solid-state 13C nuclear magnetic resonance (NMR) and X-ray photoelectron spectrometer (XPS) spectra confirm the formation of the keto-enamine-linked COM by dark reaction (DCOM), as indicated by the C = C stretching band at ca. 1566 cm−1 (Supplementary Fig. 4), enamine carbon resonance at 151.0 ppm (Supplementary Fig. 5), and secondary amine (=C-NH) with binding energy of 399.8 eV (Supplementary Fig. 6). X-ray diffraction (XRD) pattern suggests the high crystallinity of the DCOM, which shows an intense and sharp peak at ~3.5° corresponding to the reflection from the 100 crystal plane (Fig. 2b)[29].

Fig. 2

Photo-tailored reactive-crystallization of Schiff-base COM.

a Synthetic route (blue) of COM by dark reaction (DCOM) and synthetic route (yellow) of COM by photo reaction (PCOM) using reactive monomers of 2,2′-bipyridine-5,5′-diamine (Bpy) and 1,3,5-triformylphloroglucinol (Tp). b XRD patterns of the DCOM (blue) and PCOMs (yellow) formed under varied irradiation intensity ranging from 1.5 to 9.0 mW cm–2. c Schematic illustration of the excited-state intramolecular proton transfer (ESIPT) process of enol-imine linkage. d Steady-state photoluminescence emission spectra of the initial amorphous material dispersed in either aqueous phase (solid lines) or organic phase (dashed lines). These samples were excited at 300 nm.

The light source of photo reaction is supplied by a xenon lamp (λ = 200–400 nm) with an irradiation intensity from 1.5 to 9.0 mW cm−2. During photo reaction, the initially formed reversible enol-imine linkage tautomerizes rapidly into keto-enamine form via photo-induced ESIPT (Fig. 2a, c, kESIPT > 1012 s−1)[22,23], resulting in a decrease of reversible enol-imine linkage and thus inhibiting the “error-correcting” process. This photo-induced tautomerization from enol-imine to keto-enamine is confirmed by the steady-state photoluminescence emission spectrum. Figure 2d shows the fluorescence properties of the initial amorphous material dispersed in either the aqueous phase or the organic phase of interfacial polymerization. A dual fluorescence emission phenomenon is observed, with the wavelength emission at 400–450 nm representing the enol-imine form (normal emission) and the wavelength emission at 580–630 nm reflecting the keto-enamine tautomeric form (ESIPT emission)[30,31], confirming photo-induced enol-imine to keto-enamine tautomerization. The COMs formed by photo reaction (PCOMs) display a substantially weaker and wider (100) diffraction peak than DCOM (Fig. 2b), even under low irradiation intensity (1.5 mW cm−2), indicating the pronounced influence of photo irradiation on the COM crystallization. The crystallinity of the PCOMs decreases with the increase of the irradiation intensity (Fig. 2b), offering a facile approach to control the crystalline structure. NMR spectra reveal that the resonance peaks of PCOM are wider and less resolved than those of DCOM, confirming the poor development of crystalline structure in PCOM (Supplementary Fig. 5)[32]. FTIR spectra demonstrate that the C = C stretching band in the keto-enamine linkage of PCOMs was more intense than that of DCOM (Supplementary Fig. 4), ascribing to the small energy barriers of enol-keto phototautomerization[33,34]. These findings demonstrate a simple and effective strategy for tailoring the crystalline structure of Tp-Byp COM. To evaluate the generality of this strategy, we further prepared two kinds of Schiff-base COM, Tp-Tta and Tp-Azo. It has been found that both the Tp-Tta and Tp-Azo COM formed by photo reaction exhibit notably less crystallinity than those formed by dark reaction (Supplementary Fig. 7). This strategy offers the possibility to tailor heterocrystalline COM by controlling the dark and photo reactions during membrane formation.

Preparation and characterizations of DPCOMs

The morphologies of DCOM formed under dark condition and PCOMs formed under varied irradiation intensity are systematically investigated. For DCOM, fiber-crystallite assembled morphology with tens-of-nanometer-sized intercrystalline defects can be observed (Fig. 3a, Supplementary Fig. 8), revealing poor processibility of high-crystalline COM. In contrast, the morphology of PCOMs changes into a flexible polymer-like structure with inappreciable intercrystalline defects by increasing the irradiation intensity from 1.5 to 9.0 mW cm−2 (Fig. 3a, Supplementary Fig. 9), manifesting the superior processability of low-crystalline COM formed under 9.0-mW cm−2 irradiation intensity.

Fig. 3

Structure and morphology characterizations.

a Top-view SEM images of COMs deposited on track-etched substrate membranes. The intercrystalline defects are marked by the white circle. Scale bar = 500 nm. b TEM images of the COMs. Insets are high-magnification images where the high-crystalline regions show lattice diffraction patterns. Scale bar = 10 nm. c Young’s moduli of COMs tested using the peak force quantitative nano-mechanical property mapping method. Scale bar = 200 nm. d, e XRD patterns (d) and N2 adsorption isotherms (e) of DCOM (blue), DPCOMs (red), and PCOM (yellow). The PCOM here is fabricated under 9.0-mW cm−2 irradiation.

The heterocrystalline COM, denoted as DPCOM, was fabricated by dark reaction first in the same way as DCOM, followed by photo reaction under 9.0-mW cm−2 irradiation (Fig. 1). After photo reaction, the successful incorporation of low-crystalline regions into DPCOM is revealed by FTIR analysis, where the C=C (1566 cm−1) of keto-enamine linkage gradually increases with the increase of photo reaction time (Supplementary Fig. 10). Transmission electron microscope (TEM) analysis indicates that the low-crystalline regions grow in the edge of the high-crystalline regions bearing lattice diffraction pattern and the growth area is proportional to photo reaction time (Fig. 3b). After forming low-crystalline regions, the size and quantity of the intercrystalline defects within DP2hCOM observably decrease (Fig. 3a, Supplementary Fig. 8). Moreover, the thickness of the DP2hCOM does not increase (~55 nm), suggesting that the low-crystalline regions grow in the intercrystalline defects instead of along the thickness of the membrane (Supplementary Fig. 11). This is ascribed to the inherent self-inhibition effect in the interfacial polymerization process, where the monomers dissolved separately in organic and aqueous phase prefer to infuse and polymerize in the defects of interfacial membrane[35]. We also use atomic force microscope (AFM) tip to determine the local Young’s modulus of the individual regions of COMs (Fig. 3c). The DP2hCOM (3.2 GPa) exhibits much higher average modulus than that of PCOM (0.6 GPa) and DCOM (2.0 GPa) due to the rigid high-crystalline regions with large modulus and the decreased defect regions bearing tiny modulus. These results demonstrate that the intercrystalline defects of DP2hCOM are effectively sealed by the low-crystalline regions. Furthermore, our strategy can even seal the several-hundred-nanometer defects of COM (Supplementary Fig. 12).

The photo irradiation would not affect the crystalline structure of high-crystalline regions, which can be proved by the almost unchanged full width at half maximum (FWHM) of the (100) diffraction peak of DPCOM (Fig. 3d)[36]. This high crystallinity endows DP2hCOM with a very porous structure, which shows a Brunauer−Emmett−Teller surface area (SBET) of up to 1443 m2 g−1 (Fig. 3e). This value is slightly lower than that of DCOM due to the incorporation of low-crystalline regions but an order of magnitude higher than previously reported organics separation membranes (Supplementary Table 1). The above results prove that the as-prepared heterocrystalline COMs can simultaneously satisfy the requirements for both membrane processibility and sufficient high crystallinity.

Organics separation performance of DPCOMs

The organics separation performance of COMs was evaluated in terms of dye rejection and organic solvent permeance. The DCOM bearing an intrinsic pore of 1.6 nm displays poor rejection (45%) to Evans blue with a dimension of 1.2 × 3.1 nm due to the tens-of-nanometer intercrystalline defects (Fig. 4a). The rejection of DP2hCOM rises to 99%, as high as that of defect-free and dense PCOM prepared under 9.0-mW cm−2 irradiation intensity (Fig. 4a, Supplementary Fig. 13), confirming that the generated low-crystalline regions seal the non-selective intercrystalline defects effectively. We further investigated the rejections of DP2hCOM to a series of dye solutes with various molecular weights (Supplementary Table 2). For all dyes, the adsorption on membranes is as low as 0.41 μg m−2, which suggests that the rejection value is based on size exclusion rather than adsorption (Supplementary Fig. 14). As shown in Fig. 4b, the molecules with molecular weights >800 Da (reactive red 24 and Evans blue) can be effectively rejected by DP2hCOM with rejections more than 90%, while molecules with molecular weights <600 Da (methyl orange and primuline) can easily permeate through the membrane with rejections less than 10%. The D-value of molecular weight cutoff (MWCO, 800 Da) and molecular weight retention onset (MWRO, 600 Da) is only 200 Da. The rejection curve of DP2hCOM is much steeper than that of high-crystalline DCOM, low-crystalline PCOM, and other previously reported membranes attributing to its defect-free and crystalline ordered pore structure (Fig. 4c, Supplementary Table 3). Furthermore, a mixed dye separation experiment of methyl orange (327 Da) and Evans blue (961 Da) was conducted (Supplementary Fig. 15). The Evans blue can be completely rejected by DP2hCOM, while the methyl orange could pass through freely. This precise molecular sieving ability endows DP2hCOM with prospective potential for separating organic mixtures.

Fig. 4

Organics separation performance.

a Pure ethanol permeance and Evans blue rejection of DCOM (blue), PCOM (yellow), and DPCOMs (red). b The rejection of dyes with different molecular weights through DCOM (blue), DP2hCOM (red), and PCOM (yellow). The fitted sigmoidal model of the rejection curve is Doseresp. c Comparison of the rejection curve of DP2hCOM with the reported state-of-the-art membranes. d Permeance of pure organic solvents through the DP2hCOM as a function of their inverse viscosity. e Variation of the ethanol flux of DP2hCOM under varying pressure. f Ethanol permeance of DP2hCOM after solvent immersion. g Comparison of the dye (700–900 Da) rejection and solvent permeance (ethanol or methanol) of DP2hCOM with the reported state-of-the-art membranes. The specific separation data of the reported membrane are listed in Supplementary Table 1, 3. The PCOM here is fabricated under 9.0-mW cm−2 irradiation. All the error bars in this figure represent the average deviation (n = 3 independent experiments), data are presented as mean values ± SD. Dye concentration: 20 ppm for Evans blue, 50 ppm for other dyes.

The ethanol permeance of DP2hCOM is as high as 81 L m−2 h−1 bar−1, 26-times higher than that of PCOM with 3.6-times lower porosity, demonstrating the importance of porous structure in the organic molecules transport (Fig. 4a). The DP2hCOM exhibits superior organophilic behavior (Supplementary Fig. 16), and then we further evaluated its permeation properties for different types of organic solvents including apolar (toluene, n-hexane), polar protic (methanol, isopropanol, butanol), and polar aprotic (acetonitrile, acetone, ethyl acetate, dimethylformamide, tetrahydrofuran) solvents (Fig. 4d). The solvent permeance of DP2hCOM is found to be linearly proportional to the inverse of solvent viscosity (η−1). n-Hexane, with a viscosity of 2.97 × 10−4 Pa·s, gives the highest permeance of 343 L m−2 h−1 bar−1 (Supplementary Table 4). Methanol, the most used model solvent, with a small viscosity of 5.4 × 10−4 Pa·s, also gives high permeances of 264 L m−2 h−1 bar−1. The viscous flow behavior of the solvent through DP2hCOM is ascribed to the solvent resistance and rigid pore structure[37,38]. Moreover, due to this structure, the solvent flux of DP2hCOM increases linearly with an increase in the transmembrane pressure, revealing superior compaction resistance of DP2hCOM (Fig. 4e). And the permeance keeps constant even after 7-day solvent immersion (Fig. 4f, Supplementary Fig. 17).

The solute rejection and solvent permeance are compared with reported state-of-the-art organics separation membranes. As demonstrated in Fig. 4g, the membranes prepared by emerging COFs (marked as blue pentagon) present preponderant separation performance but face a trade-off between permeance and rejection. Our photo-tailoring strategy can create heterocrystalline COMs with both high-crystalline regions and low-crystalline regions, allowing for fast and precise organics separation by eliminating non-selective intercrystalline defects. The as-prepared heterocrystalline COM exhibits stable and up to 44-times higher solvent permeance than previously reported COMs with similar rejection. We further evaluate the separation performance of DP2hCOM in cross-flow mode for higher concentration dye solution to investigate its potential in practical application circumstances[39]. Our DP2hCOM can withstand continuous cross-flow shear forces, display high rejection to 500 ppm dye feed solution, and exhibit stable separation performance over 48-hour operation (Supplementary Fig. 18), indicating great potential for large-scale organic molecular separation process.

Discussion

In summary, we propose a concept of heterocrystalline membrane which comprises high-crystalline regions and low-crystalline regions, thus delicately solving the dilemma between high crystallinity and easy fabrication of defect-free membrane. The preparation of heterocrystalline COM is via a two-step procedure, where a high-crystalline COF membrane forms in the first dark reaction step and the low-crystalline regions form in the second photo reaction step. By tuning the photo reaction time, the low-crystalline regions can tightly and flexibly link the high-crystalline regions to acquire the defect-free COMs with ultrahigh porosity. Accordingly, the resulting COM displays sharp molecular sieving properties with remarkable organic solvents permeance up to 44-times higher than the state-of-the-art membranes. We envisage that our strategy of using photo reaction to tailor the crystallinity of COF membranes may enlighten the manufacture of other crystalline polymer materials, and particularly the concept of heterocrystalline membrane will greatly enrich the design of heterostructure membranes for organics separations and other precise separations.

Methods

Preparation of DCOM by dark reaction and PCOM by photo reaction

The DCOMs were prepared by interfacial polymerization. In a typical process, stock solution of Tp was prepared in dichloromethane at a concentration of 0.20 mmol L−1. Stock solutions of Bpy and p-Toluenesulfonic acid (PTSA) were prepared in water/acetonitrile 70:30 (vol/vol) solution at a concentration of 0.30 mmol L−1 and 0.60 mmol L−1. The interfacial polymerization of membrane was conducted in a beaker. First, 100 mL of Tp stock solution was poured into the beaker followed by pouring 60 mL of water/acetonitrile 70:30 (vol/vol) solution as a spacer layer. After that, 100 mL of Bpy and PTSA solution were slowly and evenly added on the top of the spacer solution over 30 min controlled by an autosampler. The reaction mixture was left in a dark box at ambient temperature (25 ± 5 °C) for 96 h. The DCOMs were then formed at the immiscible interface, collected by removing the top aqueous solution, washed with dimethylacetamide (DMAc), acetonitrile, methanol, and acetone in sequence for 48 h to remove the residual monomers and small aggregates/particles. Afterward, they were transferred to substrates and heat-treated in an air oven (90 °C, 48 h).

PCOMs were prepared using the same method of DCOMs but formed under a 300 W Xe lamp (MC-301, Merry Change Technology Co. Ltd., China) with controlled irradiation intensity and equipped with a 200–400 nm UVREF filter.

Preparation of DPCOM by sequential dark reaction and photo reaction

The DCOMs were prepared by interfacial polymerization. In a typical process, stock solution of Tp was prepared in dichloromethane at a concentration of 0.20 mmol L−1. Stock solutions of Bpy and p-Toluenesulfonic acid (PTSA) were prepared in water/acetonitrile 70:30 (vol/vol) solution at a concentration of 0.30 and 0.60 mmol L−1. The interfacial polymerization of membrane was conducted in a beaker. First, 100 mL of Tp stock solution was poured into the beaker followed by pouring 60 mL of water/acetonitrile 70:30 (vol/vol) solution as a spacer layer. After that, 100 mL of Bpy and PTSA solution were slowly and evenly added on the top of the spacer solution over 30 min controlled by an autosampler. The reaction mixture was left in a dark box at ambient temperature (25 ± 5 °C) for 96 h. Then the photo irradiation is in-situ employed on the reaction mixture with intensity of 9.0 mW cm−2. After irradiating of 0–2 h, the DPCOMs were collected, washed with solvents, transferred to substrates, and heat-treated in an air oven using the same method of DCOMs.