Ultrathin MetalâOrganic Framework Nanosheets as a Gutter Layer for

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Ultrathin Metal-Organic Framework Nanosheets as A Gutter Layer for Flexible Composite Gas Separation Membranes Min Liu, Ke Xie, Mitchell D. Nothling, Paul Andrew Gurr, Shereen Siew Ling Tan, Qiang Fu, Paul A. Webley, and Greg G. Qiao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06811 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Ultrathin Metal-Organic Framework Nanosheets as A Gutter Layer for Flexible Composite Gas Separation Membranes Min Liu, Ke Xie, Mitchell D. Nothling, Paul Andrew Gurr, Shereen Siew Ling Tan, Qiang Fu*, Paul A. Webley*, Greg G. Qiao* Department of Chemical Engineering, The University of Melbourne, Parkville, VIC 3010, Australia. *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected].

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ABSTRACT

Ultrathin metal-organic framework (MOF) nanosheets show great potential in various separation applications. In this study, MOF nanosheets are incorporated as a gutter layer in high performance, flexible thin film composite membranes (TFCMs) for CO2 separation. Ultrathin MOF nanosheets (~3-4 nm) were prepared via a surfactant-assisted method, and subsequently coated onto a flexible porous support by vacuum filtration. This produced an ultrathin (~25 nm), extremely flat MOF layer, which serves as a highly permeable gutter with reduced gas resistance when compared with conventional polydimethylsiloxane (PDMS) gutter layers. Subsequent spincoating of the ultrathin MOF gutter layer with a polymeric selective layer (Polyactive®) afforded a TFCM exhibiting the best CO2 separation performance yet reported for a flexible composite membrane (CO2 permeance of ~2,100 GPU with a CO2/N2 ideal selectivity of ~30). Several unique MOF nanosheets were examined as gutter layers, each differing with regard to structure and thickness (~10 and ~80 nm), with results indicating that flexibility in the ultrathin MOF layer is critical for optimised membrane performance. The inclusion of ultrathin MOF nanosheets into next-generation TFCMs has the potential for major improvements in gas separation performance over current composite membrane designs.

KEYWORDS: flexible composite membrane; CO2 capture; metal-organic framework; nanosheet; gutter layer.


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Carbon dioxide (CO2) emissions from coal fired power stations and natural gas industries have contributed significantly to global warming and climate change.1 Currently, the most commonly employed technologies to capture CO2 in industry include liquid-phase absorption and cryogenic expansion. However, the intensive energy consumption and high costs associated with these technologies limits their widespread feasibility in future installations. Membrane-based CO2 sequestration technology has been considered a viable economic alternative due to its small environmental footprint and excellent energy efficiency.2 To date, polymer-based membrane designs hold the greatest potential due to their ease of processability and lower cost. The key issue with this technology however, lies in the limited gas permeance (normally expressed as the gas permeation unit (GPU), where 1 GPU = 10-6 cm3(STP) cm-2 s-1 cmHg-1) polymeric membranes. As an example, a single 1,000 MW power station would require a polymeric CO2 separation membrane with a surface area in excess of hundreds of millions of square meters due to the low CO2 permeance.3 Using such membranes at a cost of ~US$50 m-2, would result in the final CO2 capture cost being similar to absorption technology.4 In order obtain cross-membrane gas flow and ensure high gas permeance, engineers will inevitably be required to either compress gas upstream or decrease the downstream pressure to vacuum. However, calculations show that compressing the feed to just 5 bar will use 20% of the energy production of the power plant.3 Even though vacuum operation is favoured because of its lower energy consumption, some other issues associated with this approach, such as the lack of large-scale industrial vacuum equipment need to be addressed. Consequently, the development of polymeric membrane materials with enhanced gas permeance, while maintaining high CO2 selectivity is a significant target for research. To date, there has been limited progress in the development of membranes exhibiting both high CO2 permeance (> 1,000 GPU), as well as good gas selectivity (> 20), which may be capable of meeting the industrial


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demands. While many commercially available membrane materials can deliver acceptable selectivities for the separation of CO2 over light gases (such as N2, CH4 and H2), maintaining this separation efficiency while increasing CO2 flux remains a grand challenge. In comparison with traditional dense polymeric membranes (including mixed matrix membranes,4-13 MMMs), ultrathin film composite membranes (TFCMs) have received increasing attention for industrial-scale gas separation due chiefly to their high gas permeance. Most TFCMs are prepared as a three-layer system, consisting of (i) a highly-porous bottom support layer for overall structural integrity, (ii) an intermediate gutter layer to reduce the geometric restriction of the porous support and prevent the selective polymer solution from penetrating into the porous substrate; and (iii) an ultrathin (< 200 nm) top selective layer for discriminate gas separation.14-20 In order to increase the gas permeance of TFCMs, the most straightforward and practical route is frequently achieved by decreasing the membrane thickness. State-of-the-art selective polymer layers can be prepared with thicknesses of around ~100 nm by leveraging techniques such as dipcoating,14,

21

interfacial polymerization,22 or continuous assembly of a polymer (CAP)

technologies.16 However, further decreasing the thickness of the selective layer while maintaining membrane stability and integrity, is limited for current techniques.19 An alternative target for increasing TFCM gas permeance is to reduce the gas permeation resistance of the intermediate gutter layer. Based on the well-established resistance model of TFCMs,20 the intermediate gutter layer contributes significantly to the overall gas diffusion resistance observed for these assemblies.16,

20, 22-24

A previous study in our laboratory examined a TFCM prepared using a

conventional polydimethylsiloxane (PDMS) gutter layer.19 Although the top selective layer of this assembly exhibited a CO2 permeance of 3,000 GPU and CO2/N2 ideal selectivity of 40~50, the overall TFCM CO2 permeance decreased to 1,100 GPU with CO2/N2 ideal selectivity of only


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20~40 – equating to a ~63% contribution by the PDMS gutter layer to the overall TFCM diffusion. The membrane intermediate gutter layer is therefore an attractive target for optimising the performance of TFCMs. Recently, our group reported a TFCM that replaced the polymeric gutter layer with a microporous layer made from metal-organic framework (MOF) particles to enhance overall gas permeance.18 The top selective layer of the TFCM was prepared by CAP on the rough MOF layer (CAP on MOF) where an Anodisc (𝛼-Al2O3) substrate was used as the porous support. With this CAP on MOF configuration, the porous MOF intermediate layer offered high stability and extremely high gas flux (ca. 45,000 GPU and 47,000 for CO2 and N2 respectively) compared with conventional PDMS-based gutter layers. Furthermore, the ultrathin (~30 nm), cross-linked CAP selective layer resulted in good CO2/N2 separation efficiency (CO2/N2 = 34). Following these promising results, we were motivated to extend our study into MOF-based membrane materials to examine their role as TFCM gutter layers. Initially, we considered physical flexibility of the final TFCM assembly to be a key requirement, leading to cost savings, mechanical robustness and ease of scale-up of flexible TCFMs compared with their rigid counterparts.25-26 For this reason, preparation of the MOF components as ultrathin nanosheets was desirable, in order to avoid the increased rigidity that may result from the inclusion of alternative MOF morphologies.25, 27-28 Herein, ultrathin MOF nanosheets-supported TFCMs were prepared on a flexible porous polymeric substrate for CO2 separation. The flexibility of these assemblies was a key consideration, ensuring better potential compatibility for scale-up and enhanced separation performance. In a two-step process, ultrathin MOF nanosheets were coated onto flexible polyacrylonitrile (PAN) substrates via vacuum filtration (Scheme 1b-i, ~25 nm, ca. ~15 nanosheets), then a selective polymer layer was spin-coated onto the MOF nanosheet layers to produce the target TFCMs


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(Scheme 1b-ii). During the vacuum filtration step, the MOF nanosheets stacked into a smooth layer, which provided the same function as a traditional polymeric gutter layer, but with significantly reduced gas transport resistance. The resultant flexible TFCMs remained intact even after significant bending (Scheme 1c). Under this configuration, the assembled TFCMs present the best CO2/N2 separation performance yet disclosed for a flexible composite membrane.

Scheme 1. The synthesis of ultrathin zinc(II) tetrakis(4-carboxy-phenyl)porphyrin) (ZnTCPP) MOF nanosheets, and their incorporation as gutter layer into flexible TFCMs. (a) Ultrathin ZnTCPP MOF nanosheets are prepared via a surfactant-assisted method, employing polyvinylpyrrolidone (PVP) as surfactant. PVP interacts with the open metal sites within the growing MOF, leading to anisotropic growth of the ZnTCPP MOFs and yielding ultrathin MOF nanosheets. (b) The assembly of ultrathin MOF nanosheets into a supported flexible TFCM: (i) MOF nanosheets are coated onto flexible polyacrylonitrile (PAN) support via vacuum filtration and (ii) the selective polymer layer is applied onto the MOF nanosheet layer by spin-coating. (c) Bending of the final TFCM, highlighting excellent flexibility. RESULTS AND DISCUSSION


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As a large and important MOF class, porphyrinic MOFs have been widely investigated due to their high thermal and chemical stabilities.29-30 In addition, porphyrinic MOFs incorporating pyrazine have shown potential for preparing well-defined, 2D nanosheet morphologies with facile metalation. However, the incorporation of pyrazine results in a significantly increased rigidity in the final MOF structure, which is undesirable in a flexible TFCM application. Recently, Zhao et al. reported a surfactant-assisted method to prepare ultrathin thin (~10 nm) MOF nanosheets in the presence of pyrazine.31 This result reveals that in order to obtain thinner, flexible MOF nanosheets, the growth of large crystal structures should be restrained. Inspired by Zhao’s earlier work on MOF nanosheets, ultrathin zinc(II) tetrakis(4-carboxy-phenyl)porphyrin) (ZnTCPP) MOF nanosheets were synthesized via a modified surfactant-assisted method in the absence of pyrazine (Scheme 1a). The polyvinylpyrrolidone (PVP) surfactant serves to attach to the open metal sites and restricts the growth of the ZnTCPP MOFs in the z-axis, leading to anisotropic growth and formation of ultrathin MOF nanosheets (Scheme 1a). The X-ray diffraction (XRD, Figure 1a), X-ray photoelectron spectroscopy (XPS, Figure 1b and c) and attenuated total reflectance Fouriertransform infrared spectroscopy (ATR-FTIR, Figure S4) confirm the crystalline structure of the ultrathin ZnTCPP nanosheets. The ultrathin ZnTCPP nanosheets displayed a sharp peak at 7.5° and a broad peak at 19.5°, representing the (002) and (004) crystallographic plane of their laminar structures, respectively. The C 1s peaks at 284.5, 286.6 and 288.4 eV correspond to the C–C, C– O and C=O bonds in the ZnTCPP MOF structures (Figure 1b).32 The Zn 2p spectra showed two characteristic peaks at 1022.2 and 1045.1 eV that can be attributed to 2p3/2 and 2p1/2 (Figure 1c). The red and blue regions in the ATR-FTIR spectra of the ZnTCPP nanosheets represent the characteristic C=O and C=N stretching vibrations of porphyrinic MOFs, respectively (Figure S4).31 Both scanning electron microscopy (SEM, Figure S5) and transmission electron microscopy


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(TEM, Figure 1d) revealed the flexible nature of the ZnTCPP nanosheets, which allows them to cover the rough surface of the porous polymeric supports when assembled into a TFCM. The transparent ZnTCPP nanosheet solution showed a typical Tyndall effect with laser light scattering (Figure 1d inset), and atomic force microscopy (AFM) measurements reveal the ZnTCPP nanosheet thickness to be around 3~4 nm (Figure 1e and f). To the best of our knowledge, this is the thinnest porphyrinic MOFs that has been synthesized via a bottom-up method.33 Importantly, the pore size of the ZnTCPP nanosheets was determined as ~1.27 nm, which can effectively prevent the dilute polymer solutions from penetrating into the porous support during membrane assembly, without restricting gas diffusion (Figure S6). Overall, we hypothesized that these features should enable the successful replacement of a conventional TFCM polymeric gutter layer with a flexible and highly permeable MOF nanosheet layer.

Figure 1. Examination of ultrathin MOF nanosheets. (a) XRD patterns of the ZnTCPP nanosheets (red) and the bulk ZnTCPP (black) MOFs. (b) C 1s and (c) Zn 2p high-resolution XPS spectra of


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the ZnTCPP nanosheets. (d) TEM image of the ZnTCPP nanosheets. Inset: image showing the Tyndall effect of the ZnTCPP nanosheets in ethanol. (e) AFM image and (f) the corresponding height profile of the ZnTCPP nanosheets.

Figure 2. Surface analysis of the assembled TFCM incorporating three commercial polymeric materials as selective layer. (a) ATR-FTIR of the three TFCMs. The blue and pink regions in (a) are attributed to C=O and C–O stretching vibrations of the three membranes, respectively. The green and purple regions represent the N–H and C–N stretching vibrations of Pebax®1657 and Pebax®2533 membranes, respectively. C 1s XPS spectra of the (b) Polyactive®, (c) Pebax®2533 and (d) Pebax®1657 TFCMs.


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To examine this, the ZnTCPP MOF nanosheets were coated onto a flexible PAN support using a straightforward vacuum filtration method (Scheme 1a). The filtration approach used in this work has the advantage of ensuring the ultrathin MOF nanosheets are uniformly stacked in a preferred orientation.34 Upon coating with MOF nanosheets, the surface of the PAN substrate became smoother (Figure S7a and b). This smooth MOF nanosheet layer provided a suitable surface for the coating of a thin, polymeric selective layer. To illustrate this concept, three commercially available polymeric membrane materials: Polyactive®, Pebax®1657 and Pebax®2533, were each spin-coated as selective layer onto the MOF layer to investigate CO2/N2 separation performance (Figure S1-S3). ATR-FTIR and XPS results confirmed the successful coating of each selective layer onto the MOF layer (Figure 2). Due to the presence of aromatic carbonyl groups (–Ar–C=O), Polyactive® showed the highest C=O vibration intensity. Pebax®1657 had higher N–H and C–N vibration intensities than those of Pebax®2533 owing to the high proportions of N–H and C–N repeat units of Pebax®1657 within the same chain length (Figure S2). On the contrary, Pebax®2533 exhibited a higher C–O vibration intensity (Figure S3). The C 1s XPS spectra of all the three TFCMs presented the characteristic C–C, C–O and C=O bonds. Due to the presence of the conjugated benzene ring, the incorporated Polyactive® showed higher C=O binding energy than Pebax®1657 and Pebax®2533. In order to assess the composite characteristics of the TFCMs, the Polyactive®-based TFCM was cut using a focused ion beam (FIB) and the cross-section observed by TEM (Figure 3b). Total thickness of the ZnTCPP MOF nanosheet gutter layer and Polyactive® selective layer was determined to be ~100 nm. This ultrathin layer formation can be attributed to the lower surface affinity of the selective polymer solution to the MOF layer during the spin-coating process, as compared with conventional polymeric gutter layers. The thickness of the MOF layer was


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measured at ~25 nm by scanning transmission electron microscopy (STEM, Figure 2c) and energy-dispersive X-ray (EDX) spectroscopy mapping (Figure 2d and Figure S8). To our best knowledge, this is the thinnest gutter layer reported for a TFCM in the open literature. Furthermore, using the straightforward vacuum filtration method means that the thickness of the MOF layer can easily be tuned by changing the relative amounts of MOF nanosheets in the coating suspension. An upper thickness limit was noted for the MOF layer of around 600 nm, whereby cracks were

Figure 3. Examination of the ZnTCPP MOF nanosheet-based composite membranes. (a) SEM image (top view) of a PAN support coated with ultrathin ZnTCPP MOF nanosheets. (b) TEM and (c) STEM images (cross section) of the Polyactive®-based TFCM. (d) EDX mapping image of the red region in (c), highlighting the ultrathin MOF layer.


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observed in the material after overnight drying under vacuum overnight (Figure S9). This may be attributed to the shrinking effects between the highly packed nanosheet layers during desiccation. Analysis of the MOF nanosheets by thermogravimetric analysis (TGA) revealed good thermal stability, with a decomposition temperature of 300°C (Figure S10). However, the quantification of the MOF nanosheets in the TFCMs was too low to be accurately identified by TGA. Overall, we propose that the flexibility, versatility and cost of this TFCM technology positions it well for potential, larger-scale commercial applications. We next sought to examine the CO2/N2 separation performance of the prepared ZnTCPP MOF nanosheet-based TFCM. To this end, gas separation efficiency was measured using an in-house constant pressure, variable volume (CPVV) apparatus. The CO2 and N2 permeance of the ZnTCPP MOF nanosheet layer on the PAN substrate (before coating of the selective polymer layer) was 18,600 and 19,000 GPU, respectively, even though the ZnTCPP nanosheets exhibited a higher adsorption of CO2 than N2 (Figure S11). Such an ultrathin MOF nanosheet layer, unlike its conventional polymeric counterparts, have significantly reduced gas diffusion resistance (greater than 6 times more permeable than PDMS), an important finding for the future design of TFCMs. It was noted that as the volume of MOF nanosheet coating suspension increased from 2.0 mL to 10.0 mL, the resulting thicker ZnTCPP MOF nanosheet layer caused a sharp decrease in CO2 and N2 permeance (1,800 and 2,400 GPU, respectively). We believe that the decreased gas permeance may be the result of the random stacking of the ultrathin MOF nanosheets, decreasing the actual diffusion pore size. Despite this, the H2 permeance of the thicker MOF layer remained relatively high at 11,000 GPU. The permeation difference between H2, CO2 and N2 in these systems can be attributed to the combination of Knudsen diffusion and molecular sieving effects.35 Therefore,


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including a MOF layer in the design of TFCMs seems to be ideal for optimal gas permeation properties. To benchmark the CO2/N2 separation performance of the TFCMs in this study, we compared our MOF-based TFCMs with other, state-of-the-art flexible composite membranes reported in the literature (Figure 4, Table 1). Using PDMS or PTMSP as gutter layers, conventional Pebax-based composite membranes (Entry 1–8, black squares) reported previously are located outside of the target area for viable post-combustion CO2 capture inferred from Merkel et al.3 After substituting the conventional polymeric gutter layer with a porous ZnTCPP MOF nanosheet layer (Figure 4, Entries 15 and 16) in this work, a dramatical increase in CO2 permeance without loss of selectivity was observed and the corresponding performance moved into the aforementioned designated target area. Directly comparing our ZnTCPP MOF nanosheet-based TFCM incorporating Pebax®1657 as selective layer with the equivalent PDMS-based TFCM (Table 1, Entry 15 vs. Entry 2) highlights a 47% increase in permeance when moving to the MOF-based system. The excellent performance of the ZnTCPP MOF nanosheet-based TFCMs results from combining the high separation efficiency of the thin selective layer with the high gas-permeability of the MOF nanosheet layer. Examining the total assembled thickness of our TFCMs reveals a slightly higher thickness for the Pebax®-based composites (Entries 15 and 16, Table 1) over the Polyactive®-based systems (Entry 17, Table 1). We attribute this to the lower solubility of Pebax-type copolymers in the casting solvents, resulting in a more viscous casting solution which yields a thicker selective layer when applied by spin-coating. Despite this, the CO2 permeability of the two Pebax®-based selective layers examined here were observed at 200 and 280 Barrer (for Entry 15 and 16, respectively), similar to previous reports.36-37 The highest CO2 permeance (~2,100 GPU) was


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obtained for the TFCM incorporating Polyactive® as the selective layer (Entry 17, Table 1). This result reflects a ~30% increase in gas permeance compared with the equivalent composite membrane employing a PDMS gutter layer (Entry 13, Table 1). Furthermore, according to the well-established resistance model of gas permeation through a composite membrane, the ultrathin ZnTCPP MOF nanosheet layer only accounted for ~10% of the total membrane resistance for CO2

Figure 4. CO2/N2 ideal selectivity versus CO2 permeance plot comparing the performance of ultrathin MOF nanosheet supported TFCMs with the state-of-the-art flexible composite membranes reported in the literature.14, 16, 20, 23-24, 36-43 All the presented data of this work were the average values collected at least three membranes. The region of the target performance area for post-combustion CO2 capture is inferred from Merkel et al.3 Squares, circles, triangles and pentagons denote composite membranes using the Pebax, PEG, Polyactive and other polymers as selective layers, respectively. To classify the gutter layers, entry 1-14 denote composite membranes using conventional PDMS or PTMSP as gutter layers and Entries 15-19 denote composite membranes using MOF as gutter layers (35 °C and 1.0 bar, this work). Specific details have been summarized in Table 1.


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Table 1. Comparison of the state-of-art flexible composite membranes for CO2/N2 separation. The facilitated transport membranes were excluded. Single gas operation conditions

(1)

Membrane configuration Selective layer Gutter Total layer thickness (nm)* Pebax®1657/P1 PDMS 675

35 °C, 3.5 bar

916

(2)

Pebax®1657

PTMSP

910

35 °C, 3.5 bar

(3)

Pebax®2533

PDMS

900

(4)

Pebax®2533/P21-1

PDMS

(5) (6)

Entry

CO2 permeance (GPU)

CO2/N2 ideal selectivity 33

Ref.

1160

20

23

35 °C, 3.5 bar

417

22

37

810

35 °C, 3.5 bar

1030

21

37

Pebax®2533/P21-4.3 PDMS

910

35 °C, 3.5 bar

1330

18

38

PDMS

600

35 °C, 3.5 bar

1070

22

20

PDMS

750

35 °C, 3.4 bar

728

20

39

PDMS

550

35 °C, 3.4 bar

1000

20

40

(9)

Pebax®2533/ HMA-PEO Pebax®2533/(PEG -b-PDMS) Pebax®2533/ SNP1 PEG/FeDA

PDMS

450

35 °C, 3.4 bar

1140

44

41

(10)

PEG/PEI-SiO2

PDMS

350

35 °C, 3.4 bar

1300

27

42

(11)

PEG

PDMS

> 240

35 °C, 3.5 bar

1260

40

16

(12)

PDMS10-PI26DMS10 Polyactive®

PDMS

> 240

35 °C, 2.0 bar

550

17

43

PDMS

130

30 °C

1590

50

14

PDMS

> 300

35 °C, 3.0 bar

1260

22

24

(15)

Polyactive®/ P@MOF Pebax®1657

ZnTCPP

130

35 °C, 1.0 bar

1710

34

(16)

Pebax®2533

ZnTCPP

160

35 °C, 1.0 bar

1820

27

(17)

Polyactive®

ZnTCPP

100

(18)

Polyactive®

CuBDC

150

35 °C, 1.0 bar 35 °C, 3.5 bar 35 °C, 5.0 bar 35 °C, 1.0 bar

2070 2160 2100 1600

33 31 32 28

(19)

Polyactive®

CoTCPP

600

35 °C, 1.0 bar

900

46

(7) (8)

(13) (14)

36

This work

* The total thickness represents the thickness of selective layer and gutter layer.


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transport, which is much lower than the contribution of a PDMS gutter layer (~63%, Entry 11, Table 1). A lower CO2/N2 ideal selectivity was observed for our ZnTCPP MOF nanosheet-based TFCM employing a Polyactive® selective layer (CO2/N2 = ~30; Entry 17, Table 1), compared with the equivalent PDMS-based system reported previously (CO2/N2 = 50; Entry 13, Table 1).14 This decrease in ideal selectivity may be attributed to the spin-coating technique, in which the dilute Polyactive® solution may entrain some non-selective free volume defects in the ultrathin top layer.24 We also examined the PAN/ZnTCPP/Polyactive® membrane performance in the presence of water vapor (100% relative humidity). As expected, we observed decrease in either CO2 permeance (25%) and CO2/N2 selectivity (35%) due to the competitive sorption effect (Figure S13).44 However, the membrane material can recover its separation performance after dried in vacuum, highlighting good stability towards water vapor. The MOF-based TFCMs were also examined for their gas separation efficiency in a mixed gas separation (CO2/N2=10/90) scenario. Under these conditions, the CO2 permeance (1,460 GPU) and CO2/N2 separation factor (25) were both lower than observed in the single gas tests, which may be expected because of the lower CO2 concentration and diffusion. Importantly, due to the excellent mechanical strength of the porous PAN substrate (Young’s modulus (EPAN) = 894 MPa),16 the TFCMs described here could be operated at elevated pressures (6.0 bar) over prolonged time periods. Over 80% of the original membrane permeance and selectivity could be maintained when operated over 30 days, highlighting the good stability of the MOF nanosheet-based TFCMs (Figure S14). The slight decrease in permeance and selectivity may be attributed to the aging or plasticization. A significant advantage of the TFCMs reported here is that they maintain structural integrity and consistent performance despite being physically bent for storage over 7 days, or even after being completely rolled back on themselves (Figure S15). To


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further investigate the impact of the MOF composition and thickness on the integrity and flexibility of the MOF gutter layers, copper 1,4-benzenedicarboxylate (CuBDC) MOF nanosheets (thickness ~10 nm) and cobalt tetrakis(4-carboxy-phenyl)porphyrin) (CoTCPP) MOF nanosheets ( thickness ~80 nm) were also synthesized and subsequently applied as gutter layers in alternative MOF nanosheet-based TFCMs (Figure S16 and S17). Firstly, the CoTCPP nanosheets were observed to be thicker than the ZnTCPP nanosheets. This may be attributed to the difference in coordination numbers of the Co2+ and Zn2+ metal nodes. The six available coordination sites of Co2+ leads to a preference for MOF growth along the c-axis. Whereas for the ZnTCPP nanosheets, the five coordination sites of Zn2+ makes it easier for the PVP surfactant to control the anisotropic growth of the ZnTCPP MOFs.45-46 Although it is difficult to quantify the flexibility of the prepared MOF nanosheets, SEM and TEM analysis of the three MOF nanosheets indicated decreasing flexibility in the order of ZnTCPP > CuBDC > CoTCPP (where ZnTCPP is the most flexible) (Figure 1d, S5, S16 and S17). However, the relative order of CO2 and N2 permeance of the corresponding gutter layers is reversed, which may again be due to the presence of non-selective pores, demonstrating the relatively poorer stacking abilities of the CuBDC and CoTCPP nanosheets (Table S1). The CuBDC nanosheet-supported TFCMs (Entry 18, Table 1) showed decreases to both CO2 permeance (1,600 GPU) and CO2/N2 ideal selectivity (28), simultaneously. This is attributed to the lower flexibility of the CuBDC nanosheets generating large free voids during stacking. Such voids permit the selective layer polymer solution to penetrate into the gutter layer during membrane assembly, effectively increasing the gas diffusion length and resulting in lower permeance compared with the ZnTCPP systems (Figure S18a and b). Although this performance is still located in the target area, it should be noted that this TFCM leaked after being slightly bent. The CoTCPP nanosheets exhibited the lowest flexibility, and the corresponding TFCMs showed


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the lowest CO2 permeance (900 GPU, Entry 19, Table 1). Again, we suggest that the impaired stacking of the CoTCPP nanosheets allows the diluted Polyactive® solution to penetrate into the PAN support, increasing the average thickness of the selective layer (Figure S18c). The combined thickness of the gutter and selective layers of this TFCM was around 600 nm (Figure S19b), with an observed permeability around 540 Barrer, which is much higher than the theoretical value. This may be attributed to the formation of mixed matrix layers as the selective polymer solution penetrates into the CoTCPP MOF layer. We therefore conclude that flexibility in the MOF nanosheets is a key criterion for optimising MOF-based TFCM design, with the most flexible MOF nanosheets yielding increased gas separation performance due to the inclusion of less defects and voids in the gutter layer. CONCLUSIONS In conclusion, MOF nanosheets were developed as an ultrathin gutter layer (~25 nm, ca. ~15 nanosheets) for the fabrication of highly permeable, flexible, gas separation composite membranes. The presented porous MOF nanosheet gutter layer is able to significantly reduced gas transport resistance, hence maximizing the gas flux of the resultant composite membrane. An optimised TFCM showed the best separation performance observed to date for a flexible composite membrane, with a CO2 permeance of ~2,100 GPU and a CO2/N2 ideal selectivity of ~30, satisfying the desired properties for an economically viable post-combustion CO2 capture membrane. The outstanding gas separation performance, paired with the excellent flexibility and stability of the described ultrathin MOF-based membranes, suggests a powerful strategy for the fabrication of high-performance composite membranes for gas separation. METHODS


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Synthesis of ZnTCPP nanosheets. ZnTCPP nanosheets were synthesized according to a modified surfactant-assisted method.31 Specifically, Zn(NO3)2·6H2O (2.3 mg) and PVP (10.0 mg) were dissolved into 12.0 mL of the mixture of DMF and ethanol (V: V = 3: 1). Then TCPP (2.0 mg) was dissolved into 4.0 mL of the mixture of DMF and ethanol (V: V = 3: 1) and added to the above solution drop-wise. The mixed solution was transferred into a stainless-steel autoclave lined with polytetrafluoroethylene to crystallize at 80 °C for 20 h under autogeneous pressure. Finally, the resultant ZnTCPP nanosheets were washed with 30.0 mL of ethanol three times and collected by centrifuging (7,800 r.p.m). For comparison, bulk ZnTCPP was synthesized under a similar procedure except for the absence of PVP. Synthesis of CoTCPP nanosheets. CoTCPP nanosheets were synthesized according to the same method as ZnTCPP nanosheets. Specifically, Co(NO3)2·6H2O (2.2 mg) and PVP (15.0 mg) were dissolved into 12.0 mL of the mixture of DMF and ethanol (V: V = 3: 1). Then TCPP (2.0 mg) was dissolved into 5.0 mL of the mixture of DMF and ethanol (V: V = 3: 1) and added to the above solution drop-wise. The mixed solution was transferred into a stainless-steel autoclave lined with polytetrafluoroethylene to crystallize at 80 °C for 24 h under autogeneous pressure. Finally, the resultant CoTCPP nanosheets were washed with 30.0 mL of ethanol three times and collected by centrifuging (7,800 r.p.m). Synthesis of CuBDC nanosheets. CuBDC nanosheets were synthesized following the bottom-up synthesis strategy described by Rodenas et al.47 Typically, H2BDC (30.0 mg) was dissolved in a mixture solution (2.0 mL of DMF and 1.0 mL of CH3CN) and poured to the bottom of a 17.0 mL vial. To this solution, a mixed solution (1.0 mL of DMF and 1.0 mL of CH3CN) was carefully added as the transitional layer. Finally, the top layer (1.0 mL of DMF and 2.0 mL of CH3CN) containing 30.0 mg of Cu(NO3)2·2.5H2O was added above the transitional layer. The vial was


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heated at 40 °C for 24 h under static conditions. The resultant CuBDC nanosheets were washed with DMF and CHCl3, collected by centrifuging (7,800 r.p.m). For comparison, bulk CuBDC particles were synthesized following the method described in reference 48. Specifically, 105.3 g of Cu(NO3)2·2.5H2O and 72.4 mg of BDC were dissolved in 43.0 mL of DMF. The solution was then transferred into a stainless-steel autoclave lined with polytetrafluoroethylene to crystallize at 40 °C for 24 h under autogeneous pressure. The resulting powders were collected and washed using the same method described above. Preparation of PAN/MOF membranes. (i) PAN/ZnTCPP. The pristine ZnTCPP nanosheets were dispersed in CH2Cl2 and refluxed at 50 °C for 24 h to remove the PVP and DMF. Then, the ZnTCPP nanosheets were collected by centrifuging (7,800 r.p.m) and dried at 150 °C in vacuum for 12 h. 3.0 mg of the dried MOF nanosheets were dispersed in 100.0 mL of ethanol by sonication. The colloidal suspension of the ZnTCPP nanosheets was obtained by centrifuging (7,800 r.p.m). Then, 2.0 mL of the colloidal suspension was filtered onto PAN support (Ø: 2.5 cm) by vacuum. The PAN/MOF membrane was dried in vacuum overnight before coating selective layers. (ii) PAN/CoTCPP (or CuBDC). In this case, the MOF nanosheets were dispersed in CH2Cl2, refluxed at 50 °C for 24 h, collected by centrifuging (7,800 r.p.m) and dried at 150 °C in vacuum for 12 h. 3.0 mg of the dried MOF nanosheets were dispersed in 100.0 mL of ethanol by sonication. Then, 1.0 mL of the colloidal suspension was filtered on a PAN support (Ø: 2.5 cm) by vacuum. Preparation of PAN/MOF/Polyactive® membranes. 2 wt% of Polyactive® solution was prepared by dispersing 2.0 g of Polyactive® solid in 100.0 g of dehydrated THF and refluxing at 75 °C overnight. Then, 0.4 mL of Polyactive® (2 wt%) was coated on the PAN/MOF by spincoating (1,700 r.p.m). The resultant PAN/MOF/PolyActive® membranes were put in vacuum overnight before testing for CO2/N2 separation.


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Preparation of PAN/MOF/Pebax®1657 membranes. 2 wt% of Pebax 1657® membrane solution was prepared by dispersing 1.0 g of Pebax1657® solid in 50.0 g of ethanol/water (V: V=7:3) and refluxing at 80 °C overnight. Then, 0.3 mL of Pebax®1657 solution was coated on the PAN/MOFs by spin-coating (1,700 r.p.m). The resultant PAN/MOF/ Pebax®1657 membranes were put in vacuum overnight before testing for CO2/N2 separation. Preparation of PAN/MOF/ Pebax®2533 membranes. 2 wt% of Pebax®2533 membrane solution was prepared by dispersing 1.0 g of Pebax®2533 solid in 50.0 g of i-propanol/n-butanol (V: V=3:1) and refluxing at 80 °C overnight. Then, 0.3 mL of Pebax®2533 solution was coated on the PAN/MOFs by spin-coating (1,700 r.p.m). The resultant PAN/MOF/ Pebax®2533 membranes were put in vacuum overnight before testing for CO2/N2 separation. Single gas permeation tests. The permeance of pure gas through the prepared membranes were tested by an in-house built constant pressure variable volume (CPVV) apparatus at 35 °C under dry and humid conditions. Pure gas permeance (Pi, unit GPU, 1 GPU = 3.3 × 10-1 mol m-2 s-1 Pa1))

of the prepared membranes is calculated on the basis of the following equation:

𝑃𝑖 =

𝐽𝑖

(1)

𝛥𝑝 × 𝐴

where 𝐽𝑖 is the gas flux of penetrant component 𝑖, 𝛥𝑝 is the partial pressure difference across the membrane and A is the effective area of the membrane. The ideal selectivity (𝛼𝑖𝑗∗ ) of gas pairs (𝑖 and 𝑗) is defined as:

𝛼𝑖𝑗∗ =

𝑃𝑖

(2)

𝑃𝑗


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Mixed gas permeation tests. The mixed gas separation performance was measured with helium as the sweeping gas on the permeate side at 35 °C and the gas pressure drop was kept at 1 bar. The gas composition was 10/90% (CO2/N2) in volume. The gas flow rate of the permeate side was determined by a digital flowmeter (ADM2000, Agilent Technologies). The composition of the permeate side was analyzed by a gas scanning electron microscope chromatography (GC 490 micro, Agilent Technologies). Resistance model. The well-established resistance model is used to evaluate the single layer performance of an asymmetric membrane with distinct layer thickness in single gas penetration measurements, and the total resistance (𝑅𝑡) can be expressed as:16, 20 𝑅 𝑡 = 𝑅𝑠 + 𝑅𝑖 + 𝑅𝑝

(3)

where 𝑅𝑠, 𝑅𝑖 and 𝑅𝑝 are the resistances of selective layer, intermediate gutter layer and porous support layer, respectively. The resistance arising from the porous support (𝑅𝑝) can be ignored. Hence, the total resistance can be written as a product of thickness over permeability of selective layer and gutter layer:

𝑅𝑡 = 𝑅𝑠 + 𝑅𝑖 =

𝑙𝑠 𝑃𝑠

+

𝑙𝑖

(4)

𝑃𝑖

where 𝑙 and 𝑃 are the thickness and permeability of the corresponding layers, respectively. ASSOCIATED CONTENT Supporting Information Available: The materials, characterization, ATR-FTIR, CO2 and N2 adsorption isotherms, XRD, XPS, SEM, TGA, TEM and AFM of MOF nanosheets, TGA, EDX


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ACS Nano

mapping and cross-section SEM, stability results of the membranes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR CONTRIBUTIONS The manuscript was written with contributions from all authors. QF, PW and GQ conceived the project. ML designed the experiments and wrote the manuscript. PG made the gas separation rig. KX, MN and ST contributed to the preparation and characterizations of materials. ACKNOWLEDGMENTS This work was performed in part at the Materials Characterization and Fabrication Platform (MCFP) at the University of Melbourne and the Victorian Node of the Australian National Fabrication Facility (ANFF). The authors appreciate the Bio21 Advanced Microscopy Facility for the assistance with material characterization, Dr. Alex Duan for assistance with the AAS measurement, Mr. Xin Fang and Ms. Jianhua Zhao for the gas adsorption test, Dr. Hiep Lu for the mixed gas separation test and Dr. Meiting Zhao for the constructive comments on the synthesis of MOF nanosheets. Min Liu acknowledges the support from China Scholarship Council - University of Melbourne Research Scholarship (No. 201606260063). Qiang Fu acknowledges the Australian Research Council under the Future Fellowship (FT180100312). REFERENCES 1. Seoane, B.; Coronas, J.; Gascon, I.; Etxeberria Benavides, M.; Karvan, O.; Caro, J.; Kapteijn, F.; Gascon, J. Metal-Organic Framework Based Mixed Matrix Membranes: A Solution for Highly Efficient CO2 Capture? Chem. Soc. Rev. 2015, 44, 2421-2454.


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26. Varoon, K.; Zhang, X.; Elyassi, B.; Brewer, D. D.; Gettel, M.; Kumar, S.; Lee, J. A.; Maheshwari, S.; Mittal, A.; Sung, C.-Y. Dispersible Exfoliated Zeolite Nanosheets and Their Application as A Selective Membrane. Science 2011, 334, 72-75. 27. Peng, Y.; Li, Y.; Ban, Y.; Yang, W. Two-Dimensional Metal-Organic Framework Nanosheets for Membrane-Based Gas Separation. Angew. Chem. Int. Ed. 2017, 56, 9757-9761. 28. Wang, X.; Chi, C.; Zhang, K.; Qian, Y.; Gupta, K. M.; Kang, Z.; Jiang, J.; Zhao, D. Reversed Thermo-Switchable Molecular Sieving Membranes Composed of Two-Dimensional MetalOrganic Nanosheets for Gas Separation. Nat. Commun. 2017, 8, 14460. 29. Zou, C.; Wu, C. D. Functional Porphyrinic Metal-Organic Frameworks: Crystal Engineering and Applications. Dalton Trans. 2012, 41, 3879-3888. 30. Burnett, B. J.; Barron, P. M.; Choe, W. Recent Advances in Porphyrinic Metal–Organic Frameworks: Materials Design, Synthetic Strategies, and Emerging Applications. CrystEngComm 2012, 14, 3839-3846. 31. Zhao, M.; Wang, Y.; Ma, Q.; Huang, Y.; Zhang, X.; Ping, J.; Zhang, Z.; Lu, Q.; Yu, Y.; Xu, H.; Zhao, Y.; Zhang, H. Ultrathin 2D Metal-Organic Framework Nanosheets. Adv. Mater. 2015, 27, 7372-7378. 32. Cao, F.; Zhao, M.; Yu, Y.; Chen, B.; Huang, Y.; Yang, J.; Cao, X.; Lu, Q.; Zhang, X.; Zhang, Z.; Tan, C.; Zhang, H. Synthesis of Two-Dimensional CoS1.097/Nitrogen-Doped Carbon Nanocomposites Using Metal-Organic Framework Nanosheets as Precursors for Supercapacitor Application. J. Am. Chem. Soc. 2016, 138, 6924-6927. 33. Ding, Y.; Chen, Y. P.; Zhang, X.; Chen, L.; Dong, Z.; Jiang, H. L.; Xu, H.; Zhou, H. C. Controlled Intercalation and Chemical Exfoliation of Layered Metal-Organic Frameworks Using A Chemically Labile Intercalating Agent. J. Am. Chem. Soc. 2017, 139, 9136-9139.


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34. Tsapatsis, M. 2-Dimensional Zeolites. AlChE J. 2014, 60, 2374-2381. 35. Li, C.; Meckler, S. M.; Smith, Z. P.; Bachman, J. E.; Maserati, L.; Long, J. R.; Helms, B. A. Engineered Transport in Microporous Materials and Membranes for Clean Energy Technologies. Adv. Mater. 2018, 30, 1704953. 36. Scofield, J. M. P.; Gurr, P. A.; Kim, J.; Fu, Q.; Kentish, S. E.; Qiao, G. G. Blends of Fluorinated Additives with Highly Selective Thin-Film Composite Membranes to Increase CO2 Permeability for CO2/N2 Gas Separation Applications. Ind. Eng. Chem. Res. 2016, 55, 83648372. 37. Scofield, J. M. P.; Gurr, P. A.; Kim, J.; Fu, Q.; Halim, A.; Kentish, S. E.; Qiao, G. G. HighPerformance Thin Film Composite Membranes with Well-Defined Poly(Dimethylsiloxane)-BPoly(Ethylene Glycol) Copolymer Additives for CO2 Separation. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1500-1511. 38. Scofield, J. M. P.; Gurr, P. A.; Kim, J.; Fu, Q.; Kentish, S. E.; Qiao, G. G. Development of Novel Fluorinated Additives for High Performance CO2 Separation Thin-Film Composite Membranes. J. Membr. Sci. 2016, 499, 191-200. 39. Halim, A.; Fu, Q.; Yong, Q.; Gurr, P. A.; Kentish, S. E.; Qiao, G. G. Soft Polymeric Nanoparticle Additives for Next Generation Gas Separation Membranes. J. Mater. Chem. 2014, 2, 4999-5009. 40. Tan, S.; Fu, Q.; Scofield, J. M. P.; Kim, J.; Gurr, P. A.; Ladewig, K.; Blencowe, A.; Qiao, G. G. Cyclodextrin-Based Supramolecular Polymeric Nanoparticles for Next Generation Gas Separation Membranes. J. Mater. Chem. 2015, 3, 14876-14886.


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41. Kim, J.; Fu, Q.; Scofield, J. M.; Kentish, S. E.; Qiao, G. G. Ultra-Thin Film Composite Mixed Matrix Membranes Incorporating Iron(Iii)-Dopamine Nanoparticles for CO2 Separation. Nanoscale 2016, 8, 8312-23. 42. Kim, J.; Fu, Q.; Xie, K.; Scofield, J. M. P.; Kentish, S. E.; Qiao, G. G. CO2 Separation Using Surface-Functionalized SiO2 Nanoparticles Incorporated Ultra-Thin Film Composite Mixed Matrix Membranes for Post-Combustion Carbon Capture. J. Membr. Sci. 2016, 515, 54-62. 43. Gurr, P. A.; Scofield, J. M. P.; Kim, J.; Fu, Q.; Kentish, S. E.; Qiao, G. G. Polyimide Polydimethylsiloxane Triblock Copolymers for Thin Film Composite Gas Separation Membranes. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3372-3382. 44. Chen, G. Q.; Scholes, C. A.; Qiao, G. G.; Kentish, S. E. Water Vapor Permeation in Polyimide Membranes. J. Membr. Sci. 2011, 379, 479-487. 45. Choi, E. Y.; Barron, P. M.; Novotny, R. W.; Son, H. T.; Hu, C. H.; Choe, W. Pillared Porphyrin Homologous Series: Intergrowth in Metal-Organic Frameworks. Inorg. Chem. 2009, 48, 426-428. 46. Choi, E. Y.; Wray, C. A.; Hu, C. H.; Choe, W. Highly Tunable Metal-Organic Frameworks with Open Metal Centers. CrystEngComm 2009, 11, 553-555. 47. Rodenas, T.; Luz, I.; Prieto, G.; Seoane, B.; Miro, H.; Corma, A.; Kapteijn, F.; Llabres i Xamena, F. X.; Gascon, J. Metal-Organic Framework Nanosheets in Polymer Composite Materials for Gas Separation. Nat. Mater. 2015, 14, 48-55. 48. Carson, C. G.; Hardcastle, K.; Schwartz, J.; Liu, X.; Hoffmann, C.; Gerhardt, R. A.; Tannenbaum, R. Synthesis and Structure Characterization of Copper Terephthalate MetalOrganic Frameworks. Eur. J. Inorg. Chem. 2009, 2009, 2338-2343.


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For TOC only


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Ultrathin MetalâOrganic Framework Nanosheets as a Gutter Layer for

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