Letter www.acsami.org
Nanoconfined Zeolitic Imidazolate Framework Membranes with Composite Layers of Nearly Zero Thickness Naixin Wang,* Xiaoting Li, Lin Wang, Lilong Zhang, Guojun Zhang, and Shulan Ji Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, China S Supporting Information *
ABSTRACT: The key to preparing dense composite membranes is reducing the thickness of the composite layer with stable separation performance. Herein, we report a nanoconfined composite membrane prepared by in situ growth of Zeolitic Imidazolate Framework (ZIF) nanocrystals in the nanoporous layer of the substrate via a fine-tuning contra-diffusion method. The thickness of the composite layer on the membrane surface was nearly zero. The formed ZIF nanoconfined composite membranes showed state-of-art flux and high stability in removing dyes from water. This new strategy is expected to offer great opportunities for the potential practical application of polymer-supported metal−organic framework (MOF) composite membranes. KEYWORDS: nanoconfined membrane, ZIF membrane, polymeric substrate, zero thickness, nanofiltration, high flux
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concept of this microstructure is that the fabricated composite membranes almost have no barrier layer on top surface of the substrate. The nanoporous particles are embedded in the nanopores of the subsurface to form a selective mass transfer channel. The comparison of the microstructure for traditional TFC/TFN membranes and NCC membranes is shown in Figure 1. In general, the substrate consists of three layers, which are the support layer (usually nonwoven), the finger-like pores layer and the nanoporous skin layer. For the TFN membrane, the composite layer, which is composed of polymer and nanoparticles, is usually deposited on the surface of the substrate. In contrast, for the NCC membrane, the porous nanoparticles are formed in the nanoporous skin layer of the substrate. The nanopores in the substrate are used as a template to confine the size and distribution of the particles. The thickness of the composite layer on the substrate surface is nearly zero, which greatly reduces the mass transfer resistance. Consequently, the separation performance of the NCC membranes is significantly improved. To the best of our knowledge, such a nanoconfined structure has not been used to prepare composite membranes. This technique will contribute a new methodology for the creation of nanohybrid composite membranes for different separation applications. In this study, NCC membranes were fabricated using a finetuning contra-diffusion method. Metal organic framework (MOF) nanocrystals were formed in the nanoporous layer of
embrane separation is a clean, convenient, and energy efficient technology that has been used in many fields, such as wastewater treatment, gas separation, seawater desalination, fuel cells, and the petrochemical industry.1−3 Currently, composite membranes constructed from separating barrier layer and porous support have emerged as excellent candidates in a wide variety of separation applications.4,5 Within the composite membrane, a defect-free film is coated on top of a microporous substrate. The substrate usually provides the mechanical strength, whereas the film is responsible for the high separation performance.6 To reduce the mass transfer resistance in the separation process, thin film composite (TFC) membranes have undergone extensive development.7−9 However, developments in the TFC membrane have reached a limit in the trade-off between permeability and selectivity because of the polymeric materials.10−12 For TFC membranes, the thinner the selective layer, the lower the transfer resistance. Enhancing the permeation flux will sacrifice the selectivity. To overcome this limitation, thin film nanocomposite (TFN) membranes were developed.13 They are formed by embedding molecular sieve nanoparticles throughout the thin film layer. The ultrathin separation layer of TFN membranes require nanoscale particles. However, the nanoparticles easily agglomerate during the membrane preparation process, forming nonselective voids between the phases.14 Furthermore, the control of the size and dispersion of nanoparticles in the polymer are also challenges in the fabrication of TFN membranes.15,16 Herein, we present a new concept of the composite membrane, namely nanoconfined composite (NCC) membranes, which can control the size and distribution of particles by using the confined space within the substrate. The key © XXXX American Chemical Society
Received: July 13, 2016 Accepted: August 15, 2016
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DOI: 10.1021/acsami.6b08581 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. Comparison of the microstructure of the TFC/TFN membrane and the NCC membrane.
Figure 2. (a) PXRD patterns of hydrolyzed PAN substrate, ZIF-11/PAN membrane, and simulated ZIF-11 crystals. (b) FTIR spectra of hydrolyzed PAN substrate, ZIF-11/PAN membrane, and as-prepared ZIF-11 crystals. (c) SEM image of the hydrolyzed PAN substrate surface. (d) SEM image of the hydrolyzed PAN substrate cross-section. (e) SEM image of ZIF-11/PAN composite membrane surface. (f) SEM image of ZIF-11/PAN composite membrane cross-section.
our knowledge, there are only a few studies concerning the preparation of pure MOF films on polymer substrates.20−29 Almost all of them were used in gas separation. Only one study
the sheet polyacrylonitrile (PAN) ultrafiltration support membrane. Recently, many researchers have prepared MOF films on the surface of inorganic substrates.17−19 To the best of B
DOI: 10.1021/acsami.6b08581 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 3. (a) Comparison of the dye removal performances of ZIF/PAN membrane with other membranes reported in literatures; (b) stability check of the optimized ZIF-11/PAN membrane in methyl blue removal.
−CN group of the PAN substrate. The presence of the CN group indicated that the surface of the PAN substrate was not completely covered by the ZIF-11 particles. This hypothesis was also confirmed using SEM images. Compared with the cross-section morphologies of the PAN substrate and the ZIF11/PAN composite membrane (Figure 2d, f), the composite layer could barely be observed on the surface of the composite membrane. We also found that the nanoparticles were embedded in the nanoporous layer of the substrate. The EDX characterization illustrated the change of C, Zn, and N elements on the membrane cross-section (Figure S4). The results showed that the highest Zn concentration was at a depth of 100 nm beneath the membrane surface. Because all Zn signals come from the ZIF-11 particles, it indicated that most of the ZIF-11 particles were formed in the subsurface of the PAN substrate at a depth of 100 nm. All of these results confirmed that the ZIF-11 particles were implanted in the nanoporous layer of the PAN substrate to form an NCC membrane with the composite layer on the membrane surface of nearly zero thickness. The nanofiltration separation performance of the ZIF-11/ PAN NCC membranes were evaluated by removing dye from water. The schematic of the nanofiltration system is shown in Figure S2. Under the optimized preparation and operation conditions (the front side of the PAN substrate contact with Zn2+ solutions; liquid level interval 4 cm; Zn2+ concentration 0.022 g/L; diffusion time 2 h; drying temperature 50 °C; operating pressure 0.2 MPa), the rejection and flux of the ZIF11/PAN NCC membrane for the removal methyl blue from water was 98.4% and 464 L/m2hMPa, respectively. The membrane exhibited enhanced nanofiltration performance when compared with other previously reported membranes (Figure 3a; data from Table S1). The flux of the membrane was 3−4 times higher than most of other membranes and simultaneously had excellent rejection. This result can be explained by the unique microstructure of the NCC membrane. The nanoporous layer of the PAN substrate was filled by ZIF11 nanoparticles, which have large cages (14.64 Å) and small apertures (3.0 Å).30 The small aperture allows water molecules to pass through the ZIF-11/PAN nanoconfined layer whereas the methyl blue molecules were rejected. More importantly, the ultrathin thickness of the ZIF-11 layer increases the flux because of its lower diffusional resistance. For practical application, the membrane stability under long-term operation is crucial. Therefore, the stability of the ZIF-11/PAN NCC membrane in the removal of methyl blue from water was tested.
used the polymer-supported MOF membranes for liquid separation.22 They prepared a ZIF-8 composite layer on a poly(ether sulfone) (PES) substrate via an interfacial synthesis method. The thus-formed ZIF-8/PES composite membrane was used for solvent resistant nanofiltration. The NCC membranes were prepared using a homemade contra-diffusion device (Figure S1). The contra-diffusion methods were proposed in 2011 and are considered to be an effective and facile way to directly grow MOF film on flexible porous substrates.21 Metal ion solution (zinc acetate, Zn2+) and organic ligand solution (Benzimidazole, Bim) were separated on each side of the hydrolyzed PAN porous substrate. Both of the solutions diffused to the opposite side through the pores of the substrate. Zn2+ and Bim interacted in the pores to form MOF crystals (ZIF-11) through coordination. The ZIF-11 nuclei were immobilized in the pores because of the coordination interaction between Zn2+ and the carboxylate groups of the hydrolyzed PAN substrate. The nanoparticles consequently grew in this nanoconfined space. Subsequently, the ZIF-11/PAN NCC membranes were finally prepared. The surface morphologies of the hydrolyzed PAN substrate and ZIF-11/PAN composite membranes are shown in Figure 2c, e. Many nanoscale pores can be found on the substrate surface (Figure 2c), whereas these pores were filled by nanoparticles in composite membrane (Figure 2e). The particles size from Figure 2e seems to be larger than the pore size of the substrate in Figure 2c. The reason could because that the high porosity of the PAN substrate lead to the pores very close to each other. The nanoparticles which grew out from the pores were thus intergrown with the neighbor particle to some extent. Therefore, the particles we observed from Figure 2e were composed of some small nanoparticles to make the particle size increase. Energy-dispersive X-ray spectroscopy (EDX) was used to characterize the elemental composition of the composite membrane surface. C, Zn, and N elements were detected and equally distributed on the surface of the composite membrane after assembly (Figure S3). The formation of crystalline ZIF-11 particles inside the PAN substrate was confirmed by powder X-ray diffraction (PXRD). The diffraction peaks related to ZIF-11 were present in the patterns of the ZIF-11/PAN membrane (Figure 2a). Fourier transform infrared (FTIR) spectra (Figure 2b) showed characteristic peaks at 750 cm−1, which was because of the C− H out-of-plane bending vibration from the benzene ring in the Bim and was found on the surface of the ZIF-11/PAN composite membrane. The peak at 2242 cm−1 was due to the C
DOI: 10.1021/acsami.6b08581 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
(Figures S11−S17). As the diffusion time increased, the ZIF-11 particles grew from the nanoporous layer to membrane surface, leading to increased surface roughness, water contact angle, mechanical strength and surface charge of the membrane. We found that the optimum diffusion time was 2 h because the molar ratio of Zn2+/Bim increased with the diffusion time and achieved equilibrium at 2 h. This means that the primary generated ZIF-11 could be nanocrystalline and formed in the nanoporous layer. As the diffusion time was extended, the particles gradually grew large and formed on the surface of the membrane to make ZIF layer thicker. Similarly, increasing the precursor concentration could provide more nuclei in the contact regions and promote the heterogeneous nucleation on the substrates. A large number of crystals grew on the substrate surface, leading to an increase in the mass transfer resistance during the separation process. During the drying process, with the evaporation of solvent, the residual precursor will continue growing into MOF particles. The effects of the drying temperature on membrane morphology, property, and separation performance were subsequently investigated (Figures S18−22). When the drying temperature increased, the amount of ZIF particles increased, making the membrane more compact. Therefore, the rejection increased, and the flux decreased. The nanofiltration performance of the membranes was also affected by the operating pressure (Figure S23). The results indicated that the ZIF-11/ PAN NCC membrane is more suitable for operation at low pressure conditions. To demonstrate the potentially general applicability of the method, we also fabricated ZIF-12/PAN and ZIF-8/PAN nanoconfined composite membranes. The photographs and SEM images of the ZIF/PAN membranes were shown in Figures S24 and S25. The flux for the ZIF-12/PAN and ZIF-8/PAN membranes were 272 and 660 L/m2 h MPa with excellent rejection (see Table S1). The exceptionally high flux shows the great potential of the nanoconfined ZIF/PAN membranes in dye removal by nanofiltration. In conclusion, a new concept of the composite membrane, namely nanoconfined composite membranes, was prepared via the fine-tuning contra-diffusion method. The barrier layer of the formed NCC membrane was of nearly zero thickness, which decreased the mass transfer resistance of water. The formed ZIF-11 nanoparticles were in situ embedded in the nanopores of the PAN substrate. The particle size and distribution can be controlled because of the confinement effect of the substrate. As a result, the formed ZIF/PAN membranes showed dramatically increased flux when compared to other nanofiltration membranes reported in the literature, without sacrificing rejection. The separation performance for the ZIF-11/PAN membrane had a flux of 464 L/m2hMPa and a rejection of 98.4%. This membrane can stably run for 60 h without performance degradation. The excellent separation performance as well as the good stability of the membrane make it has a great potential application in nanofiltration fields. In view of these advantages, we anticipate that this strategy may contribute to the preparation of various MOF membranes on polymer porous substrates in many separation fields.
As shown in Figure 3b, the ZIF-11/PAN NCC membrane had stable performance over a 60 h time period. The PXRD patterns reveal that the crystal structure of the ZIF-11 was not destroyed after immersion in water for 240 h (Figure S5). As a result, the ZIF-11/PAN NCC membranes may have a good long-term running stability. It is critical to control the crystal growth in the fabrication of the NCC membrane. In this study, the preparation conditions were fine-tuned to form the ZIF-11/PAN NCC membrane. The fine-tuning of the contact and reaction region between Zn2+ and the Bim solution is crucial to form NCC membranes. As shown in Figure S1, the substrate was fixed in the homemade U type diffusion cell with its front side up. The upside cavity of the cell was filled with metal ion solution, and the underside was filled with Bim solution. The membrane obtained under these conditions was defined as membrane A. The as-prepared membrane with the opposite situation was defined as membrane B. The surface morphologies and separation performance of these two membranes are shown in Figures S6 and S7. When the Zn2+ solution had contacted with the top surface of the PAN substrate, the ZIF-11 particles were embedded in the substrate, and their distribution was uniform (Figure S6). The reason could be the different local molar ratios of Zn2+/Bim in the nanoporous layer for these two membranes. In membrane A, the concentration of Zn2+ was much higher than Bim in the nanoporous layer, resulting in high Zn2+/Bim molar ratios in this region. More nucleation sites were provided to promote the growth of crystals in the nanoporous layer. In contrast, the Zn2+/Bim molar ratios for membrane B was close to zero. ZIF-11 crystals grew slowly and formed on membrane surface.21 Consequently, the rejection and flux of the membrane A was higher than that of membrane B (Figure S7). The Zn2+/Bim molar ratio is also affected by the diffusion rate of Zn2+ and Bim. It was controlled through adjusting the liquid level interval of Zn2+ and the Bim solution. The liquid level of the Bim solution in the underside was higher than that of the Zn2+ solution in the topside, causing Bim to diffuse much more readily into Zn2+ solution (Figure S1). With the extension of diffusion time, both Zn2+ and Bim diffused into the opposite solution (Figures S8a and S8b). When the interval increased, the diffusion rate of Zn2+ decreased, and the diffusion rate of Bim increased. As a result, the molar ratios of Zn2+/Bim were also changed with different intervals (Figure S8c). The surface morphology and separation performance of these membranes are shown in Figures S9 and S10, respectively. When the interval was 2 cm, the molar ratio of Zn2+/Bim at the subsurface of PAN substrate was very high at the beginning (Zn2+/Bim = 10) and then decreased (Zn2+/Bim = 4). Therefore, the ZIF-11 crystals were intergrown to make a thick ZIF layer on the surface of the substrate (Figure S9b). The flux of the composite membrane was thus lower (Figure S10). When the interval increased to 4 cm, the molar ratio of Zn2+/Bim greatly reduced (Zn2+/Bim = 0.5), leading to form nanoparticles in the subsurface of the substrate (Figure S9c). The high rejection and flux was obtained. However, if the interval further increased to 6 cm, the molar ratio of Zn2+/Bim reduced to 0.1. Too many nanocrystals were formed in the nanoporous layer to make the membrane much denser. Therefore, the optimum liquid level interval was 4 cm. In addition to the diffusion rate, the diffusion time and concentration of precursor also have an important influence on the morphology and property of the composite membrane
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08581. D
DOI: 10.1021/acsami.6b08581 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
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Preparation process of the membrane; nanofiltration system and experimental section; stability of ZIF-11 particles in water; effect of assembly conditions on membrane morphology, property, and separation performance; preparation of ZIF-8 and ZIF-12 membrane; comparison of the separation performance in different membranes (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21406006, 21576003), National High Technology Research and Development Program of China (Grant 2015AA03A602), Science and Technology Program of Beijing Municipal Education Commission (Grant KM201510005010), and China Postdoctoral Science Foundation funded project (Grant 2015M580954).
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DOI: 10.1021/acsami.6b08581 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
