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CO2 separation
Separation and Purification Technology 223 (2019) 10–16
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Covalent organic frameworks combined with graphene oxide to fabricate membranes for H2/CO2 separation ⁎
T ⁎
Yucheng Tanga, Shou Fenga, Lili Fana,b, , Jia Panga, Weidong Fana, Guodong Konga, Zixi Kanga,b, , Daofeng Suna,b a b
College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, PR China School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Covalent organic frameworks Graphene oxide Composite membrane Gas separation
Covalent organic frameworks (COFs) are getting more attention by virtue of their intriguing architectures and properties. However, the obstacle for fabricating sequential COF-based membranes for gas separation has become a challenge. In this research, a series of TpPa COFs are successfully synthesized by a facile approach of mechanical grinding. To alleviate the dilemma of membrane formation, we introduce graphene oxide (GO) that owns two-dimensional flexibility to sufficiently assist the COFs to form lamellar membranes. By directly mixing the solutions of GO and COFs, robust and continue COFs/GO composite membranes are prepared without defects via the vacuum filtration. The thickness and the H2/CO2 selectivity of the membranes can be regulated by increasing the amount of GO. The optimal membrane obtained with a GO/COFs ratio of 33.33% (TpPa-1-30/GO10) exhibits the H2/CO2 selectivity of 25.57 with H2 permeance of 1.067 × 10−6 mol·m−2·s−1·Pa−1. This work provides a simple, universal and applicable approach to prepare COF-based composite membranes for separation.
1. Introduction Hydrogen, as one promising clean energy, can be used in many domains, such as hydrogenation of fuels, power fuel cell vehicles, etc. To utilize hydrogen preferably, it's important to purify it by removing the impurity like CO2, which remains a bottleneck to hydrogen utility [1]. Although efforts have been devoted to development of approaches for H2/CO2 separation, such as absorption and adsorption, these conventional processes are either complicated or not environmentally friendly [2,3]. Among various separation techniques, membrane-based separation technology is more competitive owing to its advantages of operability, scalability, and eco-friendly [4,5], which provides a cost-effective implement for chemical plants, particularly for the small scaled one to meet the CO2 emission standards [6]. The membrane functionality comes from the composed material of the membrane. So far, the mostly used polymeric membrane suffers from a trade-off issue between permeability and selectivity [7,8]. Hence, many porous materials like metal-organic frameworks (MOFs) have been invented attempting to balance the membrane permeability and selectivity by utilizing their uniform framework and well-defined porosities [6,9–15]. However, the
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stability, inter-crystal defect and scale-up issues hinder the practical application of this type of membrane material. Currently, a novel kind of nanoporous material named covalent organic frameworks (COFs) [16,17], which consist of light elements (C, N, O, B) and are purely built from strong covalent bond, have attracted more attention due to the merit of low framework density, long-range ordering, controllable pore apertures, and especially high chemical and thermal stability. To prepare COFs into membranes, there are two challenges: 1) Although ionothermal [18], sonochemical [19], microwave [20], and microfluidic [21,22] syntheses routes have been used to prepare COFs, most of them are either complicated or difficult to scaleup; 2) It is difficult to prepare COFs into a sequential membrane, which limits their application as membrane material. For the first concern, Banerjee et. al. reported a TpPa series of COFs (TpPa-1 and so forth) with decent crystallinity and ultrahigh porosity via a simple mechanochemical grinding method [23,24], which makes the synthesis of COFs more accessible. Previous work has also confirmed that it is possible to delaminate COFs into layered morphology by grinding [25]. To solve the second issue, Caro et. al. applied a temperature-swing solvothermal approach, by which, a COF-COF bilayer continuous membrane was obtained [26]. Qiu and Ben et al. combined the COF material with MOF
Corresponding authors at: School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, PR China. E-mail addresses: [email protected] (L. Fan), [email protected] (Z. Kang).
https://doi.org/10.1016/j.seppur.2019.04.069 Received 26 February 2019; Received in revised form 20 April 2019; Accepted 20 April 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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through keeping the machine running for another 0.5 h with the rotating speed of 270 rpm. Then, the suspension was further separated by centrifugation and washed with N,N-dimethylformamide (DMF), DI water, acetone and DI water, respectively. Finally, the orange bulk powder (TpPa-1) was collected after drying at 40 °C overnight in programmable oven.
to fabricate a COF-MOF composite membrane [27]. In our previous work, COF nanosheets were blended with polymer to form mixed matrix membrane for H2/CO2, while the low gas permeability of polymer limits the further improvement of separation performance [28]. Though several efforts have been made attempting to solve these issues, facile syntheses of a COF-based sequential membrane and their application in gas separation has rarely been reported, which looks down upon its potential trait of porosity and devisable structure. Graphene oxide (GO), as an oxidized derivative of graphene material, is highlighted by its outstanding features encompassing two-dimensional flexibility and abundant oxygen-containing groups [29]. Through the application of GO material in the syntheses of COF membranes, it can be more efficient to keep COFs in two-dimensional structure and combine the unique merits of COF and GO phase [30]. Thus, in this study, we fabricate a series of COF/GO composite membranes through a simple solvent-free mechanochemical grinding and vacuum filtration. TpPa-1, TpPa-2 and TpPa-NO2 (Fig. S1), three different COFs are firstly prepared by ball milling and then mixed respectively with GO in dispersion, which serves as an inductive agent to lead an interaction with COFs [31]{Li, 2018 #87}{Li, 2018 #87}{Li, 2018 #87}{Li, 2018 #87}. After sufficiently dispersing, the mixtures are fabricated into membranes by vacuum filtration and applied to H2/ CO2 gas separation (as schematically elucidated in Fig. 1). This syntheses process shows universal applicability and can be readily extend to other COF/GO membranes.
2.3. Preparation of TpPa-2 and TpPa-NO2 TpPa-2 and TpPa-NO2 were synthesized by the same method mentioned above for TpPa-1. The only difference in the synthesis is the amount of corresponding diamines, which is 61.3 mg of Pa-2 for TpPa-2 and 68.9 mg of Pa-NO2 for TpPa-NO2, respectively. 2.4. Preparation of TpPa-1/GO composite membrane
2. Experimental and characterization methods
10 mg of GO was dispersed in 1000 mL DI water by ultra-sonication for 0.5 h to obtain the GO suspension (0.01 mg/mL). 10 mg as-synthesized TpPa-1 was sufficiently dispersed in 200 mL DMF by ultra-sonication for 2 h. Afterwards, in a typical membrane preparation (Table S1), 30 mL homogeneous dispersed TpPa-1 solution was directly mixed with different volume (5 mL, 10 mL, 20 mL and 30 mL) of GO suspension (0.01 mg/mL) before ultra-sonication for 0.1 h in order to study the impact of GO amount on the degree of two-dimension and thickness of the membranes. Then the composite membranes were fabricated on the nylon substrate via vacuum filtration, stored in plastic utensils and dried at 40 °C in vacuum oven overnight.
2.1. Materials and equipment
2.5. Preparation of TpPa-2/GO and TpPa-NO2/GO composite membranes
All chemicals and reagents were commercially available and used without purification. p-Toluenesulfonic acid (PTSA) was purchased from Energy Chemical Reagent Co., Ltd (China). 1,3,5Triformylphloroglucinol (Tp) was purchased from Beijing HWRK Chem Co., Ltd. p-Phenylenediamine (Pa-1) was purchased from Energy Chemical. 2,5-Dimethyl-1,4-phenylenediamine (Pa-2) and 2-nitro-1,4phenylenediamine (Pa-NO2) were provided by J&K Scientific. And graphene oxide (GO) was obtained from XFNANO Co., Ltd. The ball mill machine (DECO-PBM-AD-0.4L) was purchased from Changsha Deco Equipment Co., Ltd.
The preparation of TpPa-2/GO and TpPa-NO2/GO membranes is the same as TpPa-1/GO membrane mentioned above. 2.6. Characterization methods The structures of COFs were characterized by X-ray diffraction (XRD, Ultima, XRD-6000, Japan) at a scanning rate of 3.0° min−1 and 3–50° angular range. Solid state NMR spectra were recorded (SSNMR) on 300 MHz NMR spectrometer (JNM-ECZ600R). Carbon chemical shifts are expressed in parts per million (δ scale). The Thermal gravimetric analysis (TGA) test was performed using a TG-DTA, DT-40 system under nitrogen atmosphere. The morphologies of all materials and membranes were observed with a scanning electron microscope (SEM) (HITACHI, S4800). Transmission electron microscope (TEM) images were performed with a JEM-2100 (JEOL Co. Japan) at the accelerating voltage of 200 kV. The particle size of COFs was analyzed by dynamic light scattering (DLS) method using a Brookhaven Instrument
2.2. Preparation of TpPa-1 A mixture of 475 mg PTSA, 48.6 mg Pa-1 and 63 mg Tp were added into a jar with some balls with different sizes. Then, the ball milling machine run for 3 h with a pre-set program. After adding 50 mL deionized water, the obtained precursor was separated from the jar firstly
Tp PTSA
balls in different size
Pa-1 jar DMF
filtration
run for 3 h
TpPa-1 wash & centrifugation
GO
ultrasonic dispersion
directly mix
DI water
Fig. 1. the fabrication of TpPa-1/GO composite membrane and the sketch of gas separation test. 11
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where i, j represent the two components in the mixture and X, Y are the mole fractions in the permeate and feed solution, respectively.
proceeded reaction. This observation is further demonstrated by the solid-state 13C NMR spectrum (Fig. 2b). It shows a characteristic peak at ∼181 ppm, which belongs to the carbonyl carbon (C]O) [23]. The XRD results of the other two COFs (Fig. S2) are compared with the simulated patterns as well. Several peaks of TpPa-2 and TpPa-NO2 were not observed, due to the exfoliation of COFs by mechanical grinding [24]. However, the FTIR and NMR results of TpPa-2 and TpPa-NO2 summarized in Figs. S3–S4 can confirm the successful constructing of COF structures by ball milling method. The thermogravimetric analysis (TGA) in N2 atmosphere shows that the synthesized TpPa COFs are thermally steady up to 300 °C (Fig. S5), indicating their good thermal stability. The morphology of the COFs are characterized with SEM and TEM. The SEM image (Fig. 2d) illustrates a flower-like feature that matches the previous reports [22]. Meanwhile, the magnified TEM image (Fig. 2e) reveals the internal morphology of this material, which can be exfoliated into layered structure according to the intrinsic two-dimensional hexagonal topology and the π-π stacking structure between the sequential COF layers. Therefore, it's potential to delaminate COF bulks into sheet-like morphology after the grinding and ultra-sonication. Thus, the interaction with GO could happen easily. The relative characterization of TpPa-2 and TpPa-NO2 can be found in the Fig. S6. Furthermore, to analyze the particle size distribution and topography of COF, dynamic light scattering (DLS) and AFM are utilized after dispersed in ethanol and sonication adequately. The results are shown in Figs. S7 and S8. DLS results give the particle size of the COFs ranged from 150 nm to 250 nm. From the AFM images, it’s clearly to see that the COFs were exfoliated into plates with the thickness of 4–10 nm, which is suitable to be inserted between the GO layers. The BET surface areas of COFs are calculated based on the N2 adsorption isotherms at 77 K (Fig. S9), which shows significant and comparable results in this route of synthesis (40.96 m2 g−1 for TpPa-1, 38.03 m2 g−1 for TpPa-2 and 29.57 m2 g−1 for TpPa-NO2), matching well with the values of other COFs prepared by grinding method [37]. H2 and CO2 sorption isotherms on TpPa-1 are also collected at 298 K to evaluate the selective gas sorption properties (Fig. S10). It’s obviously that COF TpPa-1 adsorbs much more CO2 than H2 at room temperature. The interaction between TpPa-1 framework and CO2 will hinder the diffusion of gas molecules, benefiting for H2/CO2 separation.
3. Results and discussion
3.2. Characterization of COFs/GO membranes
3.1. Characterization of COFs
Several attempts have been carried out to fabricate the COF/GO composite membranes with good performance, and the optimized approach comes to directly mixing of COF and GO dispersions. Fig. 3a–b and S11-12 show the SEM images of the surface and cross-sectional morphologies of the membranes, which illustrate that the membranes own continuous smooth surface and dense layer structure. Additionally, through the energy dispersive spectroscopy (EDS) mapping (Fig. S13), we can find that N element distributes uniformly in the membrane, suggesting the well dispersion of COFs in the GO matrix. These characterizations reveal that both COFs and GO are sufficiently distributed in the membranes without any defect, although slight agglomerations are formed when the amount of GO increases. The interaction between COF and GO is demonstrated by the FTIR spectra (Fig. 3c and S14). With respect to GO before and after incorporating into the membrane, the O-H stretching vibration at 3416 cm−1 and the C]O stretching vibration at 1731 cm−1 moderately shift to lower frequency at 3301 cm−1 and 1631 cm−1 respectively, while the characteristic peaks related to COF (C]C at 1535 cm−1 and CeN at 1274 cm−1, with insignificant shift comparing to the pure COF sample) remain after involved in the membrane. The observations denote that the interaction of hydrogen bonds between different groups on GO and COF is formed during the dispersion process. Moreover, the chemical structures of COF are not damaged over the process of sonication and stacking [38–40]. Therefore, the role of GO in the membrane can be further confirmed
(BI-APDX, 31210). Atomic force microscope (AFM) instrument (Bruker) was used to investigate the microcosmic structure of the prepared COFs. In order to obtain AFM images, samples were dripped and dried on the Si wafer before dispersing in ethanol and characterized on a commercial MultiMode Scanning Probe Microscope with a NanoScope IVa controller in contact mode. For the membrane samples, the surface morphology was tested with SHIMADZU SMP-9700 instrument. BET surface areas of the samples was calculated from the N2 adsorptiondesorption isothermal curves at 77 K collected employing Micro ASAP2020. The powder samples were tested with Micro ASAP2020 to analyze the gas adsorption-desorption performance of H2 (99.995%) and CO2 (99.995%) at 298 K. 2.7. Gas separation test For the experimental setup of gas-separation measurement, the membrane was set in a stainless steel cell at room temperature and standard atmospheric pressure. One side of the membrane was swept by argon while the other side was exposed to single gases or gas mixtures. A soap-film flow meter was used to measure the gas fluctuation. The volume ratio of the gases in the binary mixture was 1:1 and the gases that penetrated the membrane was analyzed by a gas chromatograph (SHIMADZU GC-2014C). The permeability, termed permeance (Pi (mol·m−2·s−1·Pa−1)), of the membrane was calculated with the following Eq. (1):
Pi =
Ni Δpi × A
(1) −1
where Ni (mol·s ) is the permeate flow rate of component i, Δpi (Pa) is the trans-membrane pressure drop of component i, and A (m2) is the membrane area. The membrane permselectivity was evaluated by the separation factor (αi,j), which was obtained according to Eq. (2):
αi / j =
x i /xj yi / yj
(2)
Most COFs are built by combination of organic building units that always own symmetrical structures through condensation reaction, and these materials’ building units are covalently linked into ideally ordered formation in theory [32–36]. Solvothermal synthesis method is commonly used for building COF materials. Herein, we prepare COFs in a rapid and easier approach through ball milling, which is easier to mass production. As revealed from the XRD patterns (Fig. 2c) of TpPa-1, the first intense peak at ∼4.6° (2θ) corresponding to the (1 0 0) reflection plane shows good crystallinity of COFs, which matches well with the simulated pattern. Fourier transform infrared spectroscopy (FTIR) and the solid-state 13C NMR spectra are further used to confirm the successful fabrication of TpPa COFs. As can be seen from the FTIR patterns of Tp, Pa-1 and TpPa-1 (Fig. 2a), the carbonyl stretching bands of Tp (CeH at 2894 cm−1 and C]O at 1640 cm−1) disappear while some new characteristic peaks emerge at 1587, 1517 and 1267 cm−1 that can be attributed to the stretching vibrations of C]O, C]C and CeN, respectively, matching well with the previous literature [35]. It indicates the successful condensation reaction and the existence of tautomerization. The special double peaks of amidogen in diamines also disappear (3375 and 3309 cm−1 for Pa-1) and alter into one single peak (3429 cm−1) in the as-synthesized COFs, suggesting the change from primary amine to secondary amine, which further confirms the 12
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Fig. 2. (a) FTIR spectra of Tp, Pa-1 and TpPa-1, respectively. (b) the solid-state images of TpPa-1.
13
C NMR spectrum of TpPa-1. (c) the XRD pattern of TpPa-1. SEM (d) and TEM (e)
3.3. Gas separation performance
from the above-mentioned results, namely, inducing the COF into layer structure and combining with GO nanosheets to fabricate more robust and stable membrane after a vacuum filtration and drying process. The AFM images shown in Fig. S15 further illustrate the continuous surface of the membranes after doping of COF fillers, and the roughness parameters were calculated as summarized in Table S2, which confirm the smooth surface of membrane.
To test the gas separation properties of as-prepared TpPa/GO composite membranes, mixed gas (H2:CO2 = 50:50) is used as the model system to be separated and experienced different conditions to obtain the optimal membrane with the best performance. All the assynthesized membranes were performed at 298 K and each membrane
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Fig. 3. SEM images of the surface (a) and cross-section (b) of TpPa-1-30/GO-10 membrane; (c) Comparison of FTIR spectra of TpPa-1/GO membranes with GO, TpPa1 and the nylon substrate.
Fig. 4. (a) H2/CO2 mixed gases separation performances on the TpPa-1/GO composite membranes with different volume of GO at 25 °C. (b) Testing condition of time and temperature for TpPa-1-30/GO-10 composite membrane on H2/CO2 mixed gases separation performances.
that the combination of GO and COF is helpful to enhance the gas separation performance. The COF plays the role of hindering the diffusion of CO2, while GO fixes the gaps between the COF nanosheets to form the matrix of the membrane. When the GO/COF ratio is increased to 66.67% (TpPa-1-30/GO-20 membrane), the separation factor of the membrane begins to reduce. This could be attributed to two reasons: (1) the membranes become too thick to cause the reduction of gas permeation; (2) the interaction between GO and COFs is over mighty so that the agglomerations or defects would be produced during the
was examined and repeated at least three times to assure the reliability of the results. The small window of COF and the random narrow interlayer passages as well as the strong solubility of CO2, could block the transport pathways for CO2, while the small gas molecules of H2 that exhibits weak affinity would simply pass through. As can be seen from Fig. 4a and Table S3, the highest H2/CO2 selectivity is 25.57 obtained from the TpPa-1-30/GO-10 composite membrane (thickness is 0.3 μm) with high H2 permeance of 1.067 × 10−6 mol·m−2·s−1·Pa−1 and CO2 permeance of 0.51 × 10−7 mol·m−2·s−1·Pa−1. These results confirm 14
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filtration and drying process [9], which can be confirmed by Fig. S11. Therefore, it demonstrates that there is a balance affecting the interaction between GO and COFs nanosheets, which can be achieved by adjusting the ratio of GO and COFs. The separation results of TpPa-2/ GO and TpPa-NO2/GO membranes are also shown in Fig. S16 and Table S4, the best performance of these membranes all come from the GO/ COF ratio of 16.67%. That is because more defects would be induced by agglomeration as the content of TpPa-2 or TpPa-NO2 increases. Furthermore, no significant improvement is observed though the pore size has been reduced to some extent by inducing the methyl and nitro groups. That can be attributed to the original large pore size of the COF, which weakens the sieving effect. Nevertheless, the performance of all membranes surpasses the 2008 Robeson upper bound [8] for H2/CO2 and are comparable with other membranes for H2/CO2 separation (Fig. S17 and Table S5). As the best separation performance is obtained by TpPa-1-30/GO-10 membrane, afterwards research is mainly focused on this membrane. In order to examine the thermal stability of the TpPa-1-30/GO-10 membrane, tests were carried out for a given setting (25-50-100-5025 °C, each stage tested for 2 h). It's apparent to see from Fig. 4b, as the temperature increases, the gas permeance of the membrane increases remarkably on account of the higher diffusion rate of gas molecules, but the selectivity degrades accordingly. The reduction of selectivity could be due to the weakened selective adsorption of CO2. It should be emphasized that the membrane stays steady during each testing stage, demonstrating the good thermal stability of membrane. To further examine the hydrothermal stability of the composite membrane, we let the membrane exposed in the steam at 130 °C for 18 h. After this treatment, FTIR and SEM characterizations were carried out. The results in Fig. S18 reveal that the membrane morphology is barely changed, proving the high hydrothermal stability of the composite membrane. Consequently, the combination of GO and mechanical grinding attained COF is an efficient strategy to fabricate the potential and stable membranes in the field of gas separation. According to the above discussion, TpPa-1/GO composite membranes exhibit the merits of GO and COF with uniform pore size and functional group that could be designed to regulate the gas separation performance. Furthermore, during the vacuum filtration, the nanosheets dispersed in the solution are stuck onto the substrate randomly, regulating the interlayer pathways of membranes [41]. Meanwhile, the adsorption of CO2 in COF and GO causes the hurdle for the bigger CO2 molecules to pass through. Thus, this approach would be beneficial to enhance the gas separation performance, making this type of membranes very promising in the territory of gas separation application.
China (21501198, 21601205, 21771193, and 21571187), Taishan Scholar Foundation (ts201511019) and the Fundamental Research Funds for the Central Universities (18CX02047A, 18CX07001A). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.04.069. References [1] C.A. Scholes, K.H. Smith, S.E. Kentish, G.W. Stevens, CO2 capture from pre-combustion processes—Strategies for membrane gas separation, Int. J. Greenhouse Gas Control 4 (2010) 739–755. [2] J.D. Figueroa, T. Fout, S. Plasynski, H. McIlvried, R.D. Srivastava, Advances in CO2 capture technology—The U.S. Department of Energy's Carbon Sequestration Program, Int. J. Greenhouse Gas Control 2 (2008) 9–20. [3] R.S. Haszeldine, Carbon capture and storage: how green can black be? Science 325 (2009) 1647. [4] P. Luis, T. Van Gerven, B. Van der Bruggen, Recent developments in membranebased technologies for CO2 capture, Prog. Energy Combust. Sci. 38 (2012) 419–448. 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4. Conclusions The COFs prepared via the mechanical ball-milling method were incorporated into GO layers to fabricate the composite membranes by vacuum filtration. GO plays a crucial role in the formation of the composite membranes, while strong interaction between CO2 and COF is benefit for the enhancement of H2/CO2 selectivity. The FTIR results proved that the rich oxygen function groups on the surface of GO, which helps it integrate well with the COFs. The explored optimal membrane of TpPa-1-30/GO-10 exhibits a high gas separation performance of H2/CO2 (H2 permeance of 1.067 × 10−6 mol·m−2·s−1·Pa−1 and H2/CO2 separation factor of 25.57). By the method mentioned above, other COFs/GO membranes (TpPa-2/GO and TpPa-NO2/GO) are successfully prepared as well. Hence, this work provides a simple and universally applicable approach to manufacture COF-based composite membranes for energy-saving molecules separation. Acknowledgements This work was supported by National Natural Science Foundation of 15
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Covalent organic frameworks combined with graphene oxide to fabricate membranes for H2/CO2 separation ⁎
T ⁎
Yucheng Tanga, Shou Fenga, Lili Fana,b, , Jia Panga, Weidong Fana, Guodong Konga, Zixi Kanga,b, , Daofeng Suna,b a b
College of Science, China University of Petroleum (East China), Qingdao, Shandong 266580, PR China School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Covalent organic frameworks Graphene oxide Composite membrane Gas separation
Covalent organic frameworks (COFs) are getting more attention by virtue of their intriguing architectures and properties. However, the obstacle for fabricating sequential COF-based membranes for gas separation has become a challenge. In this research, a series of TpPa COFs are successfully synthesized by a facile approach of mechanical grinding. To alleviate the dilemma of membrane formation, we introduce graphene oxide (GO) that owns two-dimensional flexibility to sufficiently assist the COFs to form lamellar membranes. By directly mixing the solutions of GO and COFs, robust and continue COFs/GO composite membranes are prepared without defects via the vacuum filtration. The thickness and the H2/CO2 selectivity of the membranes can be regulated by increasing the amount of GO. The optimal membrane obtained with a GO/COFs ratio of 33.33% (TpPa-1-30/GO10) exhibits the H2/CO2 selectivity of 25.57 with H2 permeance of 1.067 × 10−6 mol·m−2·s−1·Pa−1. This work provides a simple, universal and applicable approach to prepare COF-based composite membranes for separation.
1. Introduction Hydrogen, as one promising clean energy, can be used in many domains, such as hydrogenation of fuels, power fuel cell vehicles, etc. To utilize hydrogen preferably, it's important to purify it by removing the impurity like CO2, which remains a bottleneck to hydrogen utility [1]. Although efforts have been devoted to development of approaches for H2/CO2 separation, such as absorption and adsorption, these conventional processes are either complicated or not environmentally friendly [2,3]. Among various separation techniques, membrane-based separation technology is more competitive owing to its advantages of operability, scalability, and eco-friendly [4,5], which provides a cost-effective implement for chemical plants, particularly for the small scaled one to meet the CO2 emission standards [6]. The membrane functionality comes from the composed material of the membrane. So far, the mostly used polymeric membrane suffers from a trade-off issue between permeability and selectivity [7,8]. Hence, many porous materials like metal-organic frameworks (MOFs) have been invented attempting to balance the membrane permeability and selectivity by utilizing their uniform framework and well-defined porosities [6,9–15]. However, the
⁎
stability, inter-crystal defect and scale-up issues hinder the practical application of this type of membrane material. Currently, a novel kind of nanoporous material named covalent organic frameworks (COFs) [16,17], which consist of light elements (C, N, O, B) and are purely built from strong covalent bond, have attracted more attention due to the merit of low framework density, long-range ordering, controllable pore apertures, and especially high chemical and thermal stability. To prepare COFs into membranes, there are two challenges: 1) Although ionothermal [18], sonochemical [19], microwave [20], and microfluidic [21,22] syntheses routes have been used to prepare COFs, most of them are either complicated or difficult to scaleup; 2) It is difficult to prepare COFs into a sequential membrane, which limits their application as membrane material. For the first concern, Banerjee et. al. reported a TpPa series of COFs (TpPa-1 and so forth) with decent crystallinity and ultrahigh porosity via a simple mechanochemical grinding method [23,24], which makes the synthesis of COFs more accessible. Previous work has also confirmed that it is possible to delaminate COFs into layered morphology by grinding [25]. To solve the second issue, Caro et. al. applied a temperature-swing solvothermal approach, by which, a COF-COF bilayer continuous membrane was obtained [26]. Qiu and Ben et al. combined the COF material with MOF
Corresponding authors at: School of Materials Science and Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, PR China. E-mail addresses: [email protected] (L. Fan), [email protected] (Z. Kang).
https://doi.org/10.1016/j.seppur.2019.04.069 Received 26 February 2019; Received in revised form 20 April 2019; Accepted 20 April 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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through keeping the machine running for another 0.5 h with the rotating speed of 270 rpm. Then, the suspension was further separated by centrifugation and washed with N,N-dimethylformamide (DMF), DI water, acetone and DI water, respectively. Finally, the orange bulk powder (TpPa-1) was collected after drying at 40 °C overnight in programmable oven.
to fabricate a COF-MOF composite membrane [27]. In our previous work, COF nanosheets were blended with polymer to form mixed matrix membrane for H2/CO2, while the low gas permeability of polymer limits the further improvement of separation performance [28]. Though several efforts have been made attempting to solve these issues, facile syntheses of a COF-based sequential membrane and their application in gas separation has rarely been reported, which looks down upon its potential trait of porosity and devisable structure. Graphene oxide (GO), as an oxidized derivative of graphene material, is highlighted by its outstanding features encompassing two-dimensional flexibility and abundant oxygen-containing groups [29]. Through the application of GO material in the syntheses of COF membranes, it can be more efficient to keep COFs in two-dimensional structure and combine the unique merits of COF and GO phase [30]. Thus, in this study, we fabricate a series of COF/GO composite membranes through a simple solvent-free mechanochemical grinding and vacuum filtration. TpPa-1, TpPa-2 and TpPa-NO2 (Fig. S1), three different COFs are firstly prepared by ball milling and then mixed respectively with GO in dispersion, which serves as an inductive agent to lead an interaction with COFs [31]{Li, 2018 #87}{Li, 2018 #87}{Li, 2018 #87}{Li, 2018 #87}. After sufficiently dispersing, the mixtures are fabricated into membranes by vacuum filtration and applied to H2/ CO2 gas separation (as schematically elucidated in Fig. 1). This syntheses process shows universal applicability and can be readily extend to other COF/GO membranes.
2.3. Preparation of TpPa-2 and TpPa-NO2 TpPa-2 and TpPa-NO2 were synthesized by the same method mentioned above for TpPa-1. The only difference in the synthesis is the amount of corresponding diamines, which is 61.3 mg of Pa-2 for TpPa-2 and 68.9 mg of Pa-NO2 for TpPa-NO2, respectively. 2.4. Preparation of TpPa-1/GO composite membrane
2. Experimental and characterization methods
10 mg of GO was dispersed in 1000 mL DI water by ultra-sonication for 0.5 h to obtain the GO suspension (0.01 mg/mL). 10 mg as-synthesized TpPa-1 was sufficiently dispersed in 200 mL DMF by ultra-sonication for 2 h. Afterwards, in a typical membrane preparation (Table S1), 30 mL homogeneous dispersed TpPa-1 solution was directly mixed with different volume (5 mL, 10 mL, 20 mL and 30 mL) of GO suspension (0.01 mg/mL) before ultra-sonication for 0.1 h in order to study the impact of GO amount on the degree of two-dimension and thickness of the membranes. Then the composite membranes were fabricated on the nylon substrate via vacuum filtration, stored in plastic utensils and dried at 40 °C in vacuum oven overnight.
2.1. Materials and equipment
2.5. Preparation of TpPa-2/GO and TpPa-NO2/GO composite membranes
All chemicals and reagents were commercially available and used without purification. p-Toluenesulfonic acid (PTSA) was purchased from Energy Chemical Reagent Co., Ltd (China). 1,3,5Triformylphloroglucinol (Tp) was purchased from Beijing HWRK Chem Co., Ltd. p-Phenylenediamine (Pa-1) was purchased from Energy Chemical. 2,5-Dimethyl-1,4-phenylenediamine (Pa-2) and 2-nitro-1,4phenylenediamine (Pa-NO2) were provided by J&K Scientific. And graphene oxide (GO) was obtained from XFNANO Co., Ltd. The ball mill machine (DECO-PBM-AD-0.4L) was purchased from Changsha Deco Equipment Co., Ltd.
The preparation of TpPa-2/GO and TpPa-NO2/GO membranes is the same as TpPa-1/GO membrane mentioned above. 2.6. Characterization methods The structures of COFs were characterized by X-ray diffraction (XRD, Ultima, XRD-6000, Japan) at a scanning rate of 3.0° min−1 and 3–50° angular range. Solid state NMR spectra were recorded (SSNMR) on 300 MHz NMR spectrometer (JNM-ECZ600R). Carbon chemical shifts are expressed in parts per million (δ scale). The Thermal gravimetric analysis (TGA) test was performed using a TG-DTA, DT-40 system under nitrogen atmosphere. The morphologies of all materials and membranes were observed with a scanning electron microscope (SEM) (HITACHI, S4800). Transmission electron microscope (TEM) images were performed with a JEM-2100 (JEOL Co. Japan) at the accelerating voltage of 200 kV. The particle size of COFs was analyzed by dynamic light scattering (DLS) method using a Brookhaven Instrument
2.2. Preparation of TpPa-1 A mixture of 475 mg PTSA, 48.6 mg Pa-1 and 63 mg Tp were added into a jar with some balls with different sizes. Then, the ball milling machine run for 3 h with a pre-set program. After adding 50 mL deionized water, the obtained precursor was separated from the jar firstly
Tp PTSA
balls in different size
Pa-1 jar DMF
filtration
run for 3 h
TpPa-1 wash & centrifugation
GO
ultrasonic dispersion
directly mix
DI water
Fig. 1. the fabrication of TpPa-1/GO composite membrane and the sketch of gas separation test. 11
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where i, j represent the two components in the mixture and X, Y are the mole fractions in the permeate and feed solution, respectively.
proceeded reaction. This observation is further demonstrated by the solid-state 13C NMR spectrum (Fig. 2b). It shows a characteristic peak at ∼181 ppm, which belongs to the carbonyl carbon (C]O) [23]. The XRD results of the other two COFs (Fig. S2) are compared with the simulated patterns as well. Several peaks of TpPa-2 and TpPa-NO2 were not observed, due to the exfoliation of COFs by mechanical grinding [24]. However, the FTIR and NMR results of TpPa-2 and TpPa-NO2 summarized in Figs. S3–S4 can confirm the successful constructing of COF structures by ball milling method. The thermogravimetric analysis (TGA) in N2 atmosphere shows that the synthesized TpPa COFs are thermally steady up to 300 °C (Fig. S5), indicating their good thermal stability. The morphology of the COFs are characterized with SEM and TEM. The SEM image (Fig. 2d) illustrates a flower-like feature that matches the previous reports [22]. Meanwhile, the magnified TEM image (Fig. 2e) reveals the internal morphology of this material, which can be exfoliated into layered structure according to the intrinsic two-dimensional hexagonal topology and the π-π stacking structure between the sequential COF layers. Therefore, it's potential to delaminate COF bulks into sheet-like morphology after the grinding and ultra-sonication. Thus, the interaction with GO could happen easily. The relative characterization of TpPa-2 and TpPa-NO2 can be found in the Fig. S6. Furthermore, to analyze the particle size distribution and topography of COF, dynamic light scattering (DLS) and AFM are utilized after dispersed in ethanol and sonication adequately. The results are shown in Figs. S7 and S8. DLS results give the particle size of the COFs ranged from 150 nm to 250 nm. From the AFM images, it’s clearly to see that the COFs were exfoliated into plates with the thickness of 4–10 nm, which is suitable to be inserted between the GO layers. The BET surface areas of COFs are calculated based on the N2 adsorption isotherms at 77 K (Fig. S9), which shows significant and comparable results in this route of synthesis (40.96 m2 g−1 for TpPa-1, 38.03 m2 g−1 for TpPa-2 and 29.57 m2 g−1 for TpPa-NO2), matching well with the values of other COFs prepared by grinding method [37]. H2 and CO2 sorption isotherms on TpPa-1 are also collected at 298 K to evaluate the selective gas sorption properties (Fig. S10). It’s obviously that COF TpPa-1 adsorbs much more CO2 than H2 at room temperature. The interaction between TpPa-1 framework and CO2 will hinder the diffusion of gas molecules, benefiting for H2/CO2 separation.
3. Results and discussion
3.2. Characterization of COFs/GO membranes
3.1. Characterization of COFs
Several attempts have been carried out to fabricate the COF/GO composite membranes with good performance, and the optimized approach comes to directly mixing of COF and GO dispersions. Fig. 3a–b and S11-12 show the SEM images of the surface and cross-sectional morphologies of the membranes, which illustrate that the membranes own continuous smooth surface and dense layer structure. Additionally, through the energy dispersive spectroscopy (EDS) mapping (Fig. S13), we can find that N element distributes uniformly in the membrane, suggesting the well dispersion of COFs in the GO matrix. These characterizations reveal that both COFs and GO are sufficiently distributed in the membranes without any defect, although slight agglomerations are formed when the amount of GO increases. The interaction between COF and GO is demonstrated by the FTIR spectra (Fig. 3c and S14). With respect to GO before and after incorporating into the membrane, the O-H stretching vibration at 3416 cm−1 and the C]O stretching vibration at 1731 cm−1 moderately shift to lower frequency at 3301 cm−1 and 1631 cm−1 respectively, while the characteristic peaks related to COF (C]C at 1535 cm−1 and CeN at 1274 cm−1, with insignificant shift comparing to the pure COF sample) remain after involved in the membrane. The observations denote that the interaction of hydrogen bonds between different groups on GO and COF is formed during the dispersion process. Moreover, the chemical structures of COF are not damaged over the process of sonication and stacking [38–40]. Therefore, the role of GO in the membrane can be further confirmed
(BI-APDX, 31210). Atomic force microscope (AFM) instrument (Bruker) was used to investigate the microcosmic structure of the prepared COFs. In order to obtain AFM images, samples were dripped and dried on the Si wafer before dispersing in ethanol and characterized on a commercial MultiMode Scanning Probe Microscope with a NanoScope IVa controller in contact mode. For the membrane samples, the surface morphology was tested with SHIMADZU SMP-9700 instrument. BET surface areas of the samples was calculated from the N2 adsorptiondesorption isothermal curves at 77 K collected employing Micro ASAP2020. The powder samples were tested with Micro ASAP2020 to analyze the gas adsorption-desorption performance of H2 (99.995%) and CO2 (99.995%) at 298 K. 2.7. Gas separation test For the experimental setup of gas-separation measurement, the membrane was set in a stainless steel cell at room temperature and standard atmospheric pressure. One side of the membrane was swept by argon while the other side was exposed to single gases or gas mixtures. A soap-film flow meter was used to measure the gas fluctuation. The volume ratio of the gases in the binary mixture was 1:1 and the gases that penetrated the membrane was analyzed by a gas chromatograph (SHIMADZU GC-2014C). The permeability, termed permeance (Pi (mol·m−2·s−1·Pa−1)), of the membrane was calculated with the following Eq. (1):
Pi =
Ni Δpi × A
(1) −1
where Ni (mol·s ) is the permeate flow rate of component i, Δpi (Pa) is the trans-membrane pressure drop of component i, and A (m2) is the membrane area. The membrane permselectivity was evaluated by the separation factor (αi,j), which was obtained according to Eq. (2):
αi / j =
x i /xj yi / yj
(2)
Most COFs are built by combination of organic building units that always own symmetrical structures through condensation reaction, and these materials’ building units are covalently linked into ideally ordered formation in theory [32–36]. Solvothermal synthesis method is commonly used for building COF materials. Herein, we prepare COFs in a rapid and easier approach through ball milling, which is easier to mass production. As revealed from the XRD patterns (Fig. 2c) of TpPa-1, the first intense peak at ∼4.6° (2θ) corresponding to the (1 0 0) reflection plane shows good crystallinity of COFs, which matches well with the simulated pattern. Fourier transform infrared spectroscopy (FTIR) and the solid-state 13C NMR spectra are further used to confirm the successful fabrication of TpPa COFs. As can be seen from the FTIR patterns of Tp, Pa-1 and TpPa-1 (Fig. 2a), the carbonyl stretching bands of Tp (CeH at 2894 cm−1 and C]O at 1640 cm−1) disappear while some new characteristic peaks emerge at 1587, 1517 and 1267 cm−1 that can be attributed to the stretching vibrations of C]O, C]C and CeN, respectively, matching well with the previous literature [35]. It indicates the successful condensation reaction and the existence of tautomerization. The special double peaks of amidogen in diamines also disappear (3375 and 3309 cm−1 for Pa-1) and alter into one single peak (3429 cm−1) in the as-synthesized COFs, suggesting the change from primary amine to secondary amine, which further confirms the 12
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Fig. 2. (a) FTIR spectra of Tp, Pa-1 and TpPa-1, respectively. (b) the solid-state images of TpPa-1.
13
C NMR spectrum of TpPa-1. (c) the XRD pattern of TpPa-1. SEM (d) and TEM (e)
3.3. Gas separation performance
from the above-mentioned results, namely, inducing the COF into layer structure and combining with GO nanosheets to fabricate more robust and stable membrane after a vacuum filtration and drying process. The AFM images shown in Fig. S15 further illustrate the continuous surface of the membranes after doping of COF fillers, and the roughness parameters were calculated as summarized in Table S2, which confirm the smooth surface of membrane.
To test the gas separation properties of as-prepared TpPa/GO composite membranes, mixed gas (H2:CO2 = 50:50) is used as the model system to be separated and experienced different conditions to obtain the optimal membrane with the best performance. All the assynthesized membranes were performed at 298 K and each membrane
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Fig. 3. SEM images of the surface (a) and cross-section (b) of TpPa-1-30/GO-10 membrane; (c) Comparison of FTIR spectra of TpPa-1/GO membranes with GO, TpPa1 and the nylon substrate.
Fig. 4. (a) H2/CO2 mixed gases separation performances on the TpPa-1/GO composite membranes with different volume of GO at 25 °C. (b) Testing condition of time and temperature for TpPa-1-30/GO-10 composite membrane on H2/CO2 mixed gases separation performances.
that the combination of GO and COF is helpful to enhance the gas separation performance. The COF plays the role of hindering the diffusion of CO2, while GO fixes the gaps between the COF nanosheets to form the matrix of the membrane. When the GO/COF ratio is increased to 66.67% (TpPa-1-30/GO-20 membrane), the separation factor of the membrane begins to reduce. This could be attributed to two reasons: (1) the membranes become too thick to cause the reduction of gas permeation; (2) the interaction between GO and COFs is over mighty so that the agglomerations or defects would be produced during the
was examined and repeated at least three times to assure the reliability of the results. The small window of COF and the random narrow interlayer passages as well as the strong solubility of CO2, could block the transport pathways for CO2, while the small gas molecules of H2 that exhibits weak affinity would simply pass through. As can be seen from Fig. 4a and Table S3, the highest H2/CO2 selectivity is 25.57 obtained from the TpPa-1-30/GO-10 composite membrane (thickness is 0.3 μm) with high H2 permeance of 1.067 × 10−6 mol·m−2·s−1·Pa−1 and CO2 permeance of 0.51 × 10−7 mol·m−2·s−1·Pa−1. These results confirm 14
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filtration and drying process [9], which can be confirmed by Fig. S11. Therefore, it demonstrates that there is a balance affecting the interaction between GO and COFs nanosheets, which can be achieved by adjusting the ratio of GO and COFs. The separation results of TpPa-2/ GO and TpPa-NO2/GO membranes are also shown in Fig. S16 and Table S4, the best performance of these membranes all come from the GO/ COF ratio of 16.67%. That is because more defects would be induced by agglomeration as the content of TpPa-2 or TpPa-NO2 increases. Furthermore, no significant improvement is observed though the pore size has been reduced to some extent by inducing the methyl and nitro groups. That can be attributed to the original large pore size of the COF, which weakens the sieving effect. Nevertheless, the performance of all membranes surpasses the 2008 Robeson upper bound [8] for H2/CO2 and are comparable with other membranes for H2/CO2 separation (Fig. S17 and Table S5). As the best separation performance is obtained by TpPa-1-30/GO-10 membrane, afterwards research is mainly focused on this membrane. In order to examine the thermal stability of the TpPa-1-30/GO-10 membrane, tests were carried out for a given setting (25-50-100-5025 °C, each stage tested for 2 h). It's apparent to see from Fig. 4b, as the temperature increases, the gas permeance of the membrane increases remarkably on account of the higher diffusion rate of gas molecules, but the selectivity degrades accordingly. The reduction of selectivity could be due to the weakened selective adsorption of CO2. It should be emphasized that the membrane stays steady during each testing stage, demonstrating the good thermal stability of membrane. To further examine the hydrothermal stability of the composite membrane, we let the membrane exposed in the steam at 130 °C for 18 h. After this treatment, FTIR and SEM characterizations were carried out. The results in Fig. S18 reveal that the membrane morphology is barely changed, proving the high hydrothermal stability of the composite membrane. Consequently, the combination of GO and mechanical grinding attained COF is an efficient strategy to fabricate the potential and stable membranes in the field of gas separation. According to the above discussion, TpPa-1/GO composite membranes exhibit the merits of GO and COF with uniform pore size and functional group that could be designed to regulate the gas separation performance. Furthermore, during the vacuum filtration, the nanosheets dispersed in the solution are stuck onto the substrate randomly, regulating the interlayer pathways of membranes [41]. Meanwhile, the adsorption of CO2 in COF and GO causes the hurdle for the bigger CO2 molecules to pass through. Thus, this approach would be beneficial to enhance the gas separation performance, making this type of membranes very promising in the territory of gas separation application.
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4. Conclusions The COFs prepared via the mechanical ball-milling method were incorporated into GO layers to fabricate the composite membranes by vacuum filtration. GO plays a crucial role in the formation of the composite membranes, while strong interaction between CO2 and COF is benefit for the enhancement of H2/CO2 selectivity. The FTIR results proved that the rich oxygen function groups on the surface of GO, which helps it integrate well with the COFs. The explored optimal membrane of TpPa-1-30/GO-10 exhibits a high gas separation performance of H2/CO2 (H2 permeance of 1.067 × 10−6 mol·m−2·s−1·Pa−1 and H2/CO2 separation factor of 25.57). By the method mentioned above, other COFs/GO membranes (TpPa-2/GO and TpPa-NO2/GO) are successfully prepared as well. Hence, this work provides a simple and universally applicable approach to manufacture COF-based composite membranes for energy-saving molecules separation. Acknowledgements This work was supported by National Natural Science Foundation of 15
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