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2D layered double hydroxide membranes with intrinsic breathing effect toward CO2 for efficient carbon capture
Journal Pre-proof 2D layered double hydroxide membranes with intrinsic breathing effect toward CO2 for efficient carbon capture Yutao Liu, Hong Wu, Luofu Min, Shuqing Song, Leixin Yang, Yanxiong Ren, Yingzhen Wu, Rui Zhao, Hongjian Wang, Zhongyi Jiang PII:
S0376-7388(19)32816-9
DOI:
https://doi.org/10.1016/j.memsci.2019.117663
Reference:
MEMSCI 117663
To appear in:
Journal of Membrane Science
Received Date: 7 September 2019 Revised Date:
7 November 2019
Accepted Date: 11 November 2019
Please cite this article as: Y. Liu, H. Wu, L. Min, S. Song, L. Yang, Y. Ren, Y. Wu, R. Zhao, H. Wang, Z. Jiang, 2D layered double hydroxide membranes with intrinsic breathing effect toward CO2 for efficient carbon capture, Journal of Membrane Science (2019), doi: https://doi.org/10.1016/ j.memsci.2019.117663. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Graphical abstract:
2D layered double hydroxide membranes with intrinsic breathing effect toward CO2 for efficient carbon capture Yutao Liu,ab Hong Wu,∗abc Luofu Min,ab Shuqing Song,ab Leixin Yang,ab Yanxiong Ren,ab Yingzhen Wu,ab Rui Zhao,ab Hongjian Wangab and Zhongyi Jiang∗abd a
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical
Engineering and Technology, Tianjin University, Tianjin 300350, China. b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin, 300072,
China c
Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University,
Tianjin 300072, China d Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou, 350207, China
Abstract 2D material membranes with well-defined interlayer nanochannels hold great promise for precise molecular separation, where the size and surface chemical property of the channel determine the separation efficiency. The currently reported 2D material membranes for efficient CO2 separation are primarily built by introducing crosslinkers or intercalators into the interlayer channel. In this study, we demonstrate a novel 2D material membrane with inherent CO2-selective transport channels based on layered double hydroxide (LDH). The intrinsic breathing effect of LDH toward CO2 enables the spontaneous incorporation of CO2 molecules into the interlayer nanochannels and subsequent conversion/transport in the form of CO32- species which are released as CO2 at the downstream side of membrane. Moreover, the intercalated CO32- ions with an ionic radius of 0.136 nm narrow down the nanochannel size from 0.7 nm to 0.3 nm by electrostatic interaction with LDH layers, conferring the membrane a distinct molecular sieving ability for CO2/CH4 separation. The unique breathing effect and size-sieving effect jointly contribute to the high membrane separation performance with CO2 permeance of 150 GPU and CO2/CH4 selectivity of 33. This study is anticipated to extend the materials and strategies for design of the CO2 preferential transport channels in membranes. Key words: 2D LDH membrane, breathing effect, nanochannel, CO2 separation ∗
Corresponding author. Email address: [email protected]; [email protected]
1. Introduction The efficient capture of carbon dioxide from various energy-related industrial gases, such as biogas, natural gas and flue gas, evolves a major concern in energy and environmental issues [1, 2]. Membrane-based gas separation has shown great potential in carbon capture owing to the merits of high energy efficiency, small environmental footprint and low capital cost [3]. The current commercial polymeric CO2 separation membranes often suffer from the well-known trade-off relation between selectivity and permeability, restricting their practical application [4]. Development of novel membrane materials with a combined high permeability and high selectivity is urgently demanded [5, 6]. Recently, 2D materials with atomically precise structures have gained increasing attention in the fabrication of high performance membranes [7, 8]. The atomic-scale thickness of 2D materials renders 2D membranes ultrathin selective layers to realize high permeance. Furthermore, the well-defined and tunable interlayer nanochannels between adjacent 2D nanosheets provide a powerful platform for construction of efficient mass transfer channels [7, 8]. The currently reported 2D carbon capture membranes represented by graphene oxide membrane focus on the regulation and optimization of interlayer nanochannels [9-16]. The two commonly used regulation strategies include 1) crosslinking of the adjacent nanosheets to reduce the interlayer spacing down to a molecular sieving size and 2) functionalization of interlayer nanochannels to enhance physicochemical affinity or introduce facilitated transport carriers. Additional crosslinkers or intercalators are usually required to achieve the channel optimization. Herein, we present a new type of 2D material membranes with inherent CO2-selective transport nanochannels based on its intrinsic and unique breathing effect toward CO2 without any additional crosslinking or intercalating steps. Layered double hydroxide (LDH) comprises a stacking of positively charged metal hydroxide layers with anions and water molecules intercalated in the interlayer galleries. Among the anions, the carbonate ion is the most ubiquitous in both natural mineral and laboratory synthesized phases [17]. It has been found that the carbonate
ions intercalated in LDH interlayer could serve as mobile carriers for CO2 transport [18, 19]. Moreover, the dynamic exchange behavior between the interlayer CO32- and atmospheric CO2 has been discovered, in which the CO2 molecules transform into CO32- upon entering the LDH interlayer and drive the existing CO32- to release as CO2 molecules, thus achieving the carbon cycle, and this behavior is called the breathing effect of LDH toward CO2 [20, 21]. Featured by the above-mentioned physical structures and chemical characteristics, LDH is recognized as a potential 2D material for developing a novel 2D membrane with inherent CO2-selective nanochannels. So far, no report on 2D LDH membranes for CO2 separation has been found. Herein, 2D LDH membranes with inherent CO2-selective transport channels are first fabricated for efficient carbon capture. The specific breathing effect toward CO2 enables CO2 molecules to transport in the CO32- ions form through the interlayer channels, and the CO32- ions can regulate the channel to a sieving size for CO2/CH4, endowing the LDH membranes with inherent CO2-selective transport channels. The ultrathin LDH membranes with thickness of 30-70 nm exhibit superior CO2/CH4 separation performances which are comparable to state-of-the-art membranes.
2. Experimental 2.1 Materials All chemicals are of AR grade and used as received. Mg(NO3)2·6H2O and Al(NO3)3·9H2O were purchased from Aladdin Bio-chem Technology Co. Ltd. NaOH, Na2CO3, NaCl and formamide were purchased from Tianjin Li’anlongbohua Medical Chemistry Co. Ltd. 0.5 M HCl standard solution was purchased from Tianjin Jiangtian chemical Technology Co. Ltd. Polyacrylonitrile (PAN) ultrafiltration porous membrane with a molecular weight cut-off of 100,000 was purchased from Shanghai Mega Vision Membrane Engineering and Technology Co. Ltd. 2.2 Preparation of LDH nanosheets Carbonate-intercalating LDH with a Mg/Al mole ratio of 3 (MgAl-CO3-LDH) was prepared by coprecipitation-hydrothermal aging method [22]. Typically, 9 mmol
Mg(NO3)2·6H2O and 3 mmol Al(NO3)3·9H2O were mixed together and dissolved in 100 ml of deionized water to form a clear metal nitrite solution. Then the solution was titrated with a mixed basic solution of NaOH (1.6 M) and Na2CO3 (0.8 M) to pH=11 and then aged in autoclave at 180 °C for 48 h. The obtained LDH platelets were recovered by filtration or centrifugation, washed with deionized water and dried at 60 °C in a vacuum oven. For exfoliation, the prepared MgAl-CO3-LDH was converted to MgAl-NO3-LDH (NO3- intercalating MgAl-LDH) by a two-step anion exchange process [23, 24]. 0.5 g MgAl-CO3-LDH was dispersed in 500 ml aqueous solution containing 3.5 M NaCl and 3.3 mM HCl in a two-neck flask (using de-carbonate water and HCl standard solution). The reaction flask was shaken with N2-flow protection for 24 h at ambient temperature. The NaCl-HCl treated LDH sample (determined as MgAl-Cl-LDH) was isolated by centrifugation, washed with de-carbonate water and dried in the vacuum oven. The MgAl-Cl-LDH was dispersed into 2 M NaNO3 aqueous solution and shaken with N2-flow protection for 24 h at ambient temperature. The as-prepared MgAl-NO3-LDH product was isolated by the same procedure as described above. Then 0.1 g MgAl-NO3-LDH was mixed with 100 ml formamide, sealed after purging with N2 and shaken at a speed of 160 rpm for 2 days for exfoliation. The formed colloidal suspension was further treated by centrifugation at 2000 rpm for 10 min to remove unexfoliated particles. 2.3 Preparation of 2D LDH membranes 2D LDH membranes were prepared by a spin-casting method according to the preparation of highly interlocked GO laminar membranes [25]. To enhance the adhesion of LDH nanosheets on substrate surface during assembling process, PAN substrate was pre-modified with carboxyl groups through hydrolysis process in a basic solution to obtain HPAN [26]. The typical fabrication procedure was illustrated in Scheme 1. A certain amount of LDH nanosheet suspension was dropped on HPAN substrates and spun at a speed of 2500 rpm to assemble LDH laminar membranes. The coated membrane was dried in vacuum oven before use. To exclude the interference of substrate in some characterizations, free-standing LDH membrane was
prepared. LDH nanosheet suspension was vacuum-filtrated on anodic aluminum oxide (AAO) support to form thick separation layers and peeled off from the substrate.
Scheme 1 Schematic for the preparation of LDH membranes. 2.4 Characterizations The crystal structure and interlayer spacing of samples were characterized by X-ray diffraction (XRD) pattern using a D/MAX-2500 X-ray diffractometer with filtered Cu-Ka radiation. The chemical properties of samples were analyzed with Fourier transform infrared (FTIR) spectra (Bruker Vertex 70 FTIR spectrometer) and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI with monochromatized Al Kα radiation). The metal contents of LDH particles were measured after dissolving a certain amount of samples in aqueous HCl solution by inductively coupled plasma atomic emission spectroscopy (ICP-MS 7700x, Agilent). The morphology of LDH particles and membranes was observed by field emission scanning electron microscope (FESEM, Nanosem 430). SEM-elemental analysis was conducted by energy-dispersive X-ray spectroscopy (EDX) and elemental mapping on Elementar Vario Micro cube analyzer. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) and the corresponding Fast Fourier Transform (FFT) images of LDH nanosheets were obtained by Tecnai G2 20 S-TWIN TEM apparatus. The topographical measurement of LDH nanosheets was performed on an atomic
force microscope (AFM, NTEGRA Spectra) instrument in a tapping mode. Sample of nanosheets was prepared by immersing a pretreated Si wafer in nanosheet suspension [27]. The charge property and particle size distribution of LDH nanosheets in formamide suspension were determined by Malvern Zetasizer Nano ZS90 instrument. The surface potentials of LDH membranes and substrate were measured by solid surface zeta potential analyzer (Anton Paar GmbH, Austria). The adsorption isotherms of CO2 and CH4 at 30 °C on LDH membranes were measured by a gas adsorption instrument (BELSORP-max, MicrotracBEL, Japan). Thermal properties of LDH particles and membranes were measured using thermogravimetric analysis (TGA, Netzsch STA 449 F5) under a N2 atmosphere at a heating rate of 10 K/min. The membrane samples used for XRD, FTIR and TGA were thick freestanding LDH membranes. 2.5 Gas permeation tests The separation performance was investigated by a constant pressure-variable volume method on the homemade gas separation apparatus [11]. Typically, pure CO2, CH4 or their gas mixture (CO2/CH4 30:70) was employed as feed gas and N2 as sweep gas. For CO2/N2 mixture (CO2/N2 20:80) separation, CH4 was employed as sweep gas. The feed gas pressure was maintained at 2 bar and the flow rate of sweep gas was fixed at 30 ml/min. All gas permeation tests were conducted in humidified state at 30 °C. The feed gas and sweep gas were humidified in advance by passing through humidifying tanks. Before testing, the pristine LDH membrane was pretreated by placing in CO2 atmosphere for about half hour to exchange the interlayer nitrate ions with carbonate ions. After the gas permeance reached a steady state, the gas permeance P/l (1 GPU =1×10-6 cm3 (STP) cm-2 s-1 cmHg-1) was calculated using the equation: (P/l)i
=
Qi
/∆piA
(1) where Qi is the volumetric flow rate of gas “i” (cm3/s) (STP), ∆pi is the difference partial pressure across the membrane (cmHg), A is the effective membrane area (cm2).
The ideal gas selectivity αij was calculated by: αi/j
=
(P/l)i
/(P/l)j
(2) Since the permeate side is maintained at ambient pressure, the mixed-gas separation factor was also calculated by the eq (2). All gas permeation tests were repeated for at least three times and the average value was reported with statistical errors less than 5%.
3 Results and discussion 3.1 Characterization of LDH platelets and nanosheets The MgAl-LDH with a Mg/Al ratio of three was selected as the starting material since magnesium-rich LDH exhibits more rapid exchange of carbonate ions with atmospheric
CO2
[21].
MgAl-CO3-LDH
was
synthesized
with
coprecipitation-hydrothermal aging method. The XRD pattern of MgAl-CO3-LDH in Fig. 1a agrees well with previous reports [28], which proves the successful preparation of target product. The Mg/Al molar ratio of LDH samples was determined to be 3.1 by ICP, and further confirmed by EDX and XPS analysis (Fig. S1 and S2). In XRD pattern, the (003) peak at 2θ of 11.36° indicates a d-spacing value of 0.778 nm for MgAl-CO3-LDH [28]. The vibration band at 1369 cm-1 in FTIR spectrum (Fig. 1d and Fig. S3) provides evidence for the intercalated CO32- and the band at 1650 cm-1 is identified as the intercalated water molecules [27, 29, 30]. The morphology of MgAl-CO3-LDH is shown in Fig. 1b. As can be seen, the rounded-hexagonal pallets have lateral dimensional sizes from several hundred nanometers to micrometers. To achieve exfoliation in formamide, the MgAl-CO3-LDH was converted to MgAl-NO3-LDH by ion exchange. After the ion exchange, the (003) peak of MgAl-LDH shifts to a lower angle with an enlarged d-spacing of 0.835 nm, which indicates the replacement of CO32- with NO3- (Fig. 1a). The vibration band in FTIR spectra further confirms the conversion of CO32- (1369 cm-1) to NO3- (1383 cm-1) (Fig. 1d) [27, 29, 30]. The SEM images of MgAl-NO3-LDH (Fig. 1c) show no obvious
change in morphology compared with MgAl-CO3-LDH. The exfoliation was achieved by mechanically shaking MgAl-NO3-LDH in formamide. The total delamination of LDH particles yields a colloidal suspension of LDH nanosheets which shows a clear Tyndall effect under a light beam, as shown in the insert of Fig. 1e. The morphology of exfoliated LDH nanosheets was characterized by TEM and AFM. TEM image (Fig. 1e) presents a typical two-dimensional nanosheets with very thin and transparent morphology. HRTEM image and the corresponding FFT image (Fig. S4) reveal the hexagonal structure of LDH nanosheets, suggesting that the basic structure of the LDH basal plane remains unchanged after exfoliation [27, 29]. The AFM image in Fig. 1f reveals that the obtained LDH nanosheets have a lateral size of 0.5-1 µm. The height profile scan indicates the thickness of the nanosheet is 1-2 nm. Considering the literature value of monolayer LDH height is 0.8-1 nm which is regarded as the sum of crystallographic thickness of LDH basal layer (0.48 nm) and adsorbed layer of formamide molecules (∼0.3 nm) or counter anions [27-29], the obtained LDH nanosheets should be 1-2 layers. The equivalent size distribution of LDH nanosheets was measured of 0.22-1.7 µm by dynamic light scattering (DLS) (Fig. S5).
Fig. 1 (a) XRD patterns of MgAl-CO3-LDH and MgAl-NO3-LDH. (b) SEM image of
MgAl-CO3-LDH. (c) SEM image of MgAl-NO3-LDH. (d) FTIR spectra of MgAl-CO3-LDH and MgAl-NO3-LDH. (e) TEM image of LDH nanosheets inserted with Tyndall effect in colloidal suspensions. (f) AFM image of LDH nanosheets. The height profile of the nanosheet corresponds to the blue dashed line. 3.2 Characterization of 2D LDH membranes
Fig. 2 SEM images of (a-c) cross-sectional LDH composite membranes with accumulated LDH stacked layers of 30 and 70 nm and (d-f) top view of the membrane surfaces: PAN substrate, coated with LDH layers of 30 and 70 nm. EDX mapping of C, Mg and Al elements in (g) surface and (h) cross section of LDH composite membrane. Spin-coating method was adopted for 2D LDH membrane preparation, since it is more favorable for horizontal alignments of 2D nanosheets onto support surface to get homogenous membrane [31]. Fig. 2 exhibits the SEM images of membrane surfaces and cross sections. The ultrathin LDH layers are tightly and uniformly adhered onto
the HPAN substrate (Fig. 2a). The typical laminate structure of LDH layers can be clearly seen in a thicker coated membrane (Fig. 2b and c), which indicates the successful preparation of well-stacked LDH laminar membranes. The membrane thickness can be easily adjusted in the range of 8-70 nm by varying the dosage of LDH nanosheets during spin-coating procedure and the relationship between membrane thickness and LDH amount was plotted in Fig. S6. From the top-view, LDH membrane shows a fairly smooth surface compared with the GO membrane with many wrinkles (Fig. 2e and f) which are usually considered as structural defects that will degrade the separation factor [31]. The smooth membrane surface is attributed to the rigid host layer of LDH and the centrifugal force of spin-coating method which results in more ordered alignment of nanosheets than the sole vertical pulling force of vacuum filtration. When the thickness of LDH membrane is reduced to 8 nm (Fig. S7), there are some pores and defects exposing on the membrane surface, while with assembly of more LDH layers (with thickness more than 15nm), the substrates are entirely covered by continuous LDH layers without visible pinholes or defects, forming a highly smooth membrane surface as shown in Fig. 2. The EDX mapping of the selected elements in Fig. 2g-h show a homogenous distribution in both membrane surface and cross section. The cross-sectional SEM image of thick freestanding LDH membrane displays a clear laminate structure as shown in Fig. S8, further indicating the well-stacked interlayer channels.
Fig. 3 (a) XRD patterns and (b) FTIR spectra of LDH composite membrane and
substrates. Table 1 Zeta potential of the LDH colloidal suspensions, PAN substrates and LDH composite membranes with different thicknesses. Sample
LDH nanosheets
Zeta potential (mV)
18.9
LDH membranes with different thickness PAN HPAN 8 nm 30 nm 70 nm 100 nm -40.72
-77.96
-64.39
-41.69
-11.02
24.86
The membrane structure was further characterized by XRD. Fig. 3a depicts XRD patterns of LDH membrane and substrate. In addition to the major diffractions of the substrate, the small peak at 2θ of 7.46° is the diffraction of (003) peak of LDH membrane which indicates a d-spacing of 1.18 nm based on Bragg’s law. Considering that the d-spacing value includes the LDH monolayer thickness of 0.48 nm [27-29], the free interlayer spacing of LDH membrane is estimated to be 0.7 nm. The enlarged interlayer spacing of LDH membrane compared with that of MgAl-NO3-LDH platelets is due to the accompany of water and residual formamide molecules with the intercalated NO3- [30]. In addition, in contrast with the broad diffraction peaks of GO membranes [10, 11], the sharp peak of LDH membrane at 2θ of 6-9° signifies a narrow interlayer spacing distribution and further confirms the more ordered stacking of LDH nanosheets in the laminar membrane. The hydrolyzed PAN (HPAN) has rich carboxyl groups on surface, which shows negatively charged property [32]. Therefore, to enhance the interfacial adhesion of selective layer on substrate, the negatively charged HPAN was used as substrate for deposition of positive charged LDH nanosheets during the assembly process. Fig. 3b displays the FTIR spectra of LDH membrane and substrates, the new infrared peaks at around 1570 cm-1 and 1405 cm-1 of HPAN prove the formation of carboxyl groups on substrate surface [26]. For LDH membranes, bands below 800 cm-1 are associated with the stretching and bending modes of metal-oxygen (M-O) [27, 33]. Due to the interference of the HPAN substrate reflection, the infrared peak of intercalating NO3in LDH membrane was not observed while a sharp peak at 1690 cm-1 of the residual formamide molecules was shown [34]. Table 1 shows the zeta potential of LDH
nanosheets suspension and the surface potential of LDH membranes and substrates. The hydrolyzed PAN exhibits a negative surface of -77.96 mV, with the assembly of positively charged LDH layers with a zeta potential of 18.90 mV, the membrane surface charge gradually turns to be positive. Consequently, the electrostatic interaction and hydrogen-bond interaction with HPAN substrate reinforce the interface interaction and further protect the LDH selective layer from delamination. The thermal behaviors of LDH platelet and membrane were analyzed by TG-DSC (Fig. S9). The lamellar membranes consisted of exfoliated LDH nanosheets have weaker interactions with interlayer water or formamide molecules than LDH platelet, leading an earlier loss of interlayer molecules. The same phenomenon happened in the second weight loss stage of the interlayer ions loss and dehydroxylation of LDH layers.
Fig. 4 (a) FTIR spectra, (b) XRD patterns and (c-f) XPS wide spectra and high-resolution spectra of C 1s, Mg 2p and Al 2p for LDH membranes. LDH(NO3-) mem represents the pristine LDH membrane; LDH(CO32-) mem represents the LDH membrane treated with CO2. Since the breathing effect of LDH toward CO2 actually refers to the dynamic exchange behavior between the interlayer CO32- and atmospheric CO2, the intercalated NO3- in LDH membranes should be exchanged to CO32- to make breathing effect work in CO2 separation process. In consideration of the strong
chemisorption of LDH host layers to CO2 (adsorption enthalpy of MgAl-LDH toward CO2 reaches 92.04 KJ/mol) [35] and the instability of intercalated NO3- which can be easily replaced by other ions (especially by CO32- under even trace CO2 in atmospheric environment) [23], the pristine LDH membrane was pretreated by CO2 for exchange of nitrate ions with carbonate ions. And a series of characterizations were further carried out to reveal the physiochemical properties of LDH membrane nanochannels as shown in Fig. 4. To eliminate the interference of substrate, free standing LDH membranes were used as test samples. The intercalated ion species of LDH membranes were analyzed using FTIR (Fig. 4a). FTIR spectrum of the pristine LDH membrane (LDH(NO3-) membrane) shows that the NO3- and residual formamide molecules intercalate in the nanochannels of LDH stack layers. The alteration of vibration peaks of the corresponding intercalated anions for LDH membrane after CO2 treatment (LDH(CO32-) membrane) demonstrates that the CO2 successfully entered into the LDH interlayer channels, converted to CO32- under the action of water molecules and replaced the NO3- and formamide molecules. Therefore, the CO32initiator of breathing effect toward CO2 has been constructed in interlayer nanochannels. Interlayer nanochannel size is another dominant parameter that governs the separation efficiency [36]. A uniform channel size that falls between the sizes of molecule pair or ion pair would realize size sieving [37]. The interlayer spacing of CO2 treated LDH membrane was determined by XRD. In Fig. 4b, the in-plane reflections disappeared, suggesting the highly horizontal orientation of LDH nanosheets in the membrane. The main diffractions from left to right are assigned to (003), (006) and (009) peaks according to the d-spacing relation (interlayer spacing = d003 = 2d006 = 3d009) of LDH [38]. The XRD pattern of LDH(NO3-) membrane shows the (003) peak at 2θ of 7.46° suggesting that the free interlayer spacing of LDH membrane is 0.7 nm which is identical to the membrane with substrate (Fig. 3). Since the interlayer channel is oversized for the small gas molecules, the sieving mechanism cannot work. However, it is interesting to note that the interlayer spacing of
LDH(CO32-) membrane measured by XRD is decreased to 0.3 nm. This reduced interlayer channel size is attributed to a smaller CO32- with an ionic radius of 0.136 nm and the stronger interaction between CO32- and LDH host layer which pulls the adjacent LDH layers closer. The intercalating and interaction of CO32- with LDH host layer were further confirmed by XPS (Fig. 4c-f). C 2p spectrum shows the increase of CO32- in LDH(CO32-) membrane, proving the carbonate intercalating. The Mg 2p spectrum of LDH(CO32-) membrane exhibits a shift toward higher position compared with LDH(NO3-) membrane, which means the decrease of electron cloud density of Mg atoms [39]. This is because the CO32- with higher electronegativity than NO3- has stronger interaction with Mg adsorption sites on LDH host layer [40, 41]. And this also explains the size shrinkage of interlayer nanochannels in LDH membranes after CO2 treatment. The same case is for Al 2p spectrum with a slight shift toward higher position. For clear understanding, schematic of the membrane structures corresponding to the evolution of the XRD and FTIR results was sketched in Scheme 2.
Scheme 2 Structures of LDH membranes before and after CO2 treatment. Specific gas adsorption of CO2 and CH4 on LDH(NO3-) membrane and LDH(CO32-) membrane was measured (Fig.5). Both the LDH(NO3-) membrane and the LDH(CO32-) membrane behave a preferential adsorption of CO2 compared to CH4. The CO2 adsorption capacity of LDH(CO32-) membrane is slightly higher than that of LDH(NO3-) membrane. And different from LDH(CO32-) membrane, a two-step adsorption occurred in CO2 adsorption of LDH(NO3-) membrane, which might be induced by the transformation of LDH(NO3-) membrane to LDH(CO32-) membrane. The adsorption platform was considered as the complete replacement of interlayer
NO3- with CO32-. After that, the amount of CO2 adsorption continues to increase to near that of LDH(CO32-) membrane.
Fig. 5 Comparison of gas adsorption on (a) LDH(NO3-) membrane and (b) LDH(CO32-) membrane. 3.3 Gas separation performance of 2D LDH membranes The CO2/CH4 separation performances of LDH membranes with adjustable thickness were evaluated under humidified condition. Fig. 6a shows the mixed gas permeation of LDH(CO32-) membranes with various membrane thickness. It can be seen the ultrathin LDH membrane with 8 nm-thick selective layer exhibits a CO2 permeance of 202 GPU with a CO2/CH4 selectivity of 7. With increasing the membrane thickness to 30~70 nm, the CO2/CH4 selectivity increased to 33~37 with a CO2 permeance of 152~106 GPU since the minor defects of thinner LDH membrane were concealed by thick stacked LDH layers. The high selectivity of LDH membranes can be explained by the synergy of breathing effect to CO2 and size sieving mechanism. On one hand, the breathing effect toward CO2 by LDH renders the feed gas CO2 transfer in the form of CO32- across the membrane. In detail, CO2 molecules are incorporated into LDH interlayer, then react with interlayer water forming 2H+ and CO32-, replace the initially intercalated CO32- and drive it to react with 2H+ to release CO2 and water, thus completing the carbon cycle between LDH and environmental CO2 [20, 21]. On the other hand, the interlayer channel size of 2D LDH membrane is regulated to 0.3 nm by the intercalated CO32-, which retards the
transport of CH4 molecules with a dynamic diameter of 0.38 nm. Although the small channel would also retard the CO2 molecules (0.33 nm), there is another way for CO2 molecules to pass through the LDH laminar channel, namely transfer as form of CO32with an ionic radius of 0.136 nm which does not suffer from the sieving. Therefore, the distinct size sieving also works between CO2/CH4 pairs.
Fig. 6 (a) Mixed CO2/CH4 separation performances of LDH(CO32-) membranes with different thickness. (b) Single CO2/CH4 permeation performances of LDH(CO32-) membranes. (c) CO2/CH4 long-term operation test of 30-nm-thick LDH(CO32-) membrane. (d) CO2/CH4 separation performance of LDH(CO32-) membranes compared with state-of-the-art CO2 separation membranes. Single gas permeation test of LDH(CO32-) membranes was conducted for comparison (Fig. 6b). For most membranes, the separation factor of mixed gas is typically lower than the ideal selectivity of single gas due to the competitive sorption effects or CO2 plasticization [42], but the LDH membranes instead exhibit slightly higher gas selectivity for mixed gas separation. This phenomenon was mostly reported in hydrocarbon/light gas separation, where the condensable gas with strong adsorption interaction would occupy the adsorption site of membrane and retard the
passage of light gas [43, 44]. However, this seldom happened for CO2/gas separation [45]. The higher selectivity in CO2/CH4 mixture of LDH membranes is attributed to the intrinsic breathing effect of LDH material toward CO2 which renders the preferential adsorption and transportation of CO2, thereby hindering the transport of another component in gas mixtures. And this unusual behavior also indicates the critical role of breathing effect of LDH toward CO2 on CO2-selective transport. To verify the size sieving effect, LDH(NO3-) membrane with interlayer spacing of 0.7nm was also tested for single gas permeation for comparison and the results were shown in Fig. S10-S11. Compared with LDH(CO32-) membranes, the ideal CO2/CH4 selectivities of LDH(NO3-) membranes show a dramatic reduction. For example, the CO2/CH4 selectivity of 30 nm-thick membrane reduced from 32.3 (mixed gas) and 29.9 (single gas) of LDH(CO32-) membrane to 18.9 (single gas) of LDH(NO3-) membrane. For single CO2 gas permeance, there is no difference in membrane structure and transport mechanism with LDH(CO32-) membrane since the LDH(NO3-) membrane would be transformed to LDH(CO32-) membrane after CO2 permeated. The case does not fit the single CH4 gas permeance, because the LDH(NO3-) membrane without CO2 treatment does not exhibit a shrinkage of nanochannel size. The structure of CH4 permeated membranes were also characterized by XRD, FTIR, and XPS as shown in Fig. S12-14. The results show that the CH4 permeated LDH membranes have same interlayer nanochannel structures with the LDH(NO3-) membrane with a channel size of 0.7 nm, thus the sieving effect does not work. The schematic transportation mechanisms of mixed gas and single gas in LDH(CO32-) membrane and LDH(NO3-) membrane were sketched in Fig. S15. The long-term operation test of 2D LDH membrane was conducted (Fig. 6c). The membrane maintained the high CO2/CH4 separation performance during the 144 h operation test, exhibiting an excellent permeance stability. We further compared the CO2/CH4 separation performance of LDH membrane with state-of-the-art CO2 separation membranes including polymer TFC membranes [46-49], MOF membranes [50-52], zeolite [53] and 2D membranes [10-14, 25]. As shown in Fig. 6d, the
performance of our LDH membranes are comparable to the current advanced membranes and transcend the limits of MOF membrane performance [12, 54]. To further explore the versatile potential of LDH membrane in carbon capture, the separation performance of LDH membranes for CO2/N2 mixture was tested. As shown in Fig. 7a, the LDH membrane also exhibits superior CO2/N2 separation performance. The 30nm-thick LDH membrane exhibits CO2 permeance of 152 GPU and CO2/N2 selectivity of 36, and with the increase of membrane thickness to 70nm, the CO2/N2 selectivity increases up to 44. In addition, the membrane displays good long-term operation stability (Fig. 7b).
Fig. 7 (a) CO2/N2 mixture separation performances of LDH(CO32-) membranes with different thickness. (b) CO2/N2 long-term operation test of 30-nm-thick LDH(CO32-) membrane.
4. Conclusions We demonstrated for the first time the construction of 2D LDH membranes with inherent CO2-selective transport nanochannels for CO2 separation. Ultrathin LDH membranes with a tunable thickness from 30-70 nm are fabricated by the assembly of exfoliated 2D LDH nanosheets on porous substrates. With the intrinsic breathing effect of LDH toward CO2, the CO2 molecules can transfer in the form of CO32through the interlayer nanochannels of membranes. Furthermore, the interlayer CO32regulates the size of membrane nanochannel from 0.7 nm to 0.3 nm, conferring a distinct sieving effect for CO2 and CH4 molecules. Featuring with the inherent
physicochemical CO2-selective nanochannels, the 2D LDH membranes exhibit high CO2/CH4 separation performance compared with state-of-the-art membranes. The exploration in 2D LDH membranes for efficient carbon capture contributes to the rational design and sustainable development of 2D material membranes. The inherent selective transport nanochannels of anion intercalation LDHs may inspire the development of other kinds of intercalation materials for constructing precise molecular or ionic separation membranes.
Acknowledgements This study was supported from the National Natural Science Foundation of China (Nos. 21838008, 21621004, 21878215 and 21576189), National Key R&D Program of China (No. 2017YFB0603400), State Key Laboratory of Petroleum Pollution Control (No. PPC2017014), State Key Laboratory of Separation Membranes and Membrane Processes and Tianjin Polytechnic University (No. M1-201701), National Key Laboratory of United Laboratory for Chemical Engineering (SKL-ChE-17B01).
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Highlights: 1. 2D LDH membrane for carbon capture was prepared for the first time. 2. Inherent CO2-selective transport nanochannels were built in membrane. 3. Intrinsic breathing effect of LDH to CO2 render CO2 transport in CO32- form. 4. CO32- regulates the nanochannel size to 0.3nm, for sieving CO2/CH4 pair. 5. Breathing effect and size-sieving effect contribute to efficient CO2 separation.
Conflict of Interest The authors declare no conflict of interest
S0376-7388(19)32816-9
DOI:
https://doi.org/10.1016/j.memsci.2019.117663
Reference:
MEMSCI 117663
To appear in:
Journal of Membrane Science
Received Date: 7 September 2019 Revised Date:
7 November 2019
Accepted Date: 11 November 2019
Please cite this article as: Y. Liu, H. Wu, L. Min, S. Song, L. Yang, Y. Ren, Y. Wu, R. Zhao, H. Wang, Z. Jiang, 2D layered double hydroxide membranes with intrinsic breathing effect toward CO2 for efficient carbon capture, Journal of Membrane Science (2019), doi: https://doi.org/10.1016/ j.memsci.2019.117663. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Graphical abstract:
2D layered double hydroxide membranes with intrinsic breathing effect toward CO2 for efficient carbon capture Yutao Liu,ab Hong Wu,∗abc Luofu Min,ab Shuqing Song,ab Leixin Yang,ab Yanxiong Ren,ab Yingzhen Wu,ab Rui Zhao,ab Hongjian Wangab and Zhongyi Jiang∗abd a
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical
Engineering and Technology, Tianjin University, Tianjin 300350, China. b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin, 300072,
China c
Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University,
Tianjin 300072, China d Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou, 350207, China
Abstract 2D material membranes with well-defined interlayer nanochannels hold great promise for precise molecular separation, where the size and surface chemical property of the channel determine the separation efficiency. The currently reported 2D material membranes for efficient CO2 separation are primarily built by introducing crosslinkers or intercalators into the interlayer channel. In this study, we demonstrate a novel 2D material membrane with inherent CO2-selective transport channels based on layered double hydroxide (LDH). The intrinsic breathing effect of LDH toward CO2 enables the spontaneous incorporation of CO2 molecules into the interlayer nanochannels and subsequent conversion/transport in the form of CO32- species which are released as CO2 at the downstream side of membrane. Moreover, the intercalated CO32- ions with an ionic radius of 0.136 nm narrow down the nanochannel size from 0.7 nm to 0.3 nm by electrostatic interaction with LDH layers, conferring the membrane a distinct molecular sieving ability for CO2/CH4 separation. The unique breathing effect and size-sieving effect jointly contribute to the high membrane separation performance with CO2 permeance of 150 GPU and CO2/CH4 selectivity of 33. This study is anticipated to extend the materials and strategies for design of the CO2 preferential transport channels in membranes. Key words: 2D LDH membrane, breathing effect, nanochannel, CO2 separation ∗
Corresponding author. Email address: [email protected]; [email protected]
1. Introduction The efficient capture of carbon dioxide from various energy-related industrial gases, such as biogas, natural gas and flue gas, evolves a major concern in energy and environmental issues [1, 2]. Membrane-based gas separation has shown great potential in carbon capture owing to the merits of high energy efficiency, small environmental footprint and low capital cost [3]. The current commercial polymeric CO2 separation membranes often suffer from the well-known trade-off relation between selectivity and permeability, restricting their practical application [4]. Development of novel membrane materials with a combined high permeability and high selectivity is urgently demanded [5, 6]. Recently, 2D materials with atomically precise structures have gained increasing attention in the fabrication of high performance membranes [7, 8]. The atomic-scale thickness of 2D materials renders 2D membranes ultrathin selective layers to realize high permeance. Furthermore, the well-defined and tunable interlayer nanochannels between adjacent 2D nanosheets provide a powerful platform for construction of efficient mass transfer channels [7, 8]. The currently reported 2D carbon capture membranes represented by graphene oxide membrane focus on the regulation and optimization of interlayer nanochannels [9-16]. The two commonly used regulation strategies include 1) crosslinking of the adjacent nanosheets to reduce the interlayer spacing down to a molecular sieving size and 2) functionalization of interlayer nanochannels to enhance physicochemical affinity or introduce facilitated transport carriers. Additional crosslinkers or intercalators are usually required to achieve the channel optimization. Herein, we present a new type of 2D material membranes with inherent CO2-selective transport nanochannels based on its intrinsic and unique breathing effect toward CO2 without any additional crosslinking or intercalating steps. Layered double hydroxide (LDH) comprises a stacking of positively charged metal hydroxide layers with anions and water molecules intercalated in the interlayer galleries. Among the anions, the carbonate ion is the most ubiquitous in both natural mineral and laboratory synthesized phases [17]. It has been found that the carbonate
ions intercalated in LDH interlayer could serve as mobile carriers for CO2 transport [18, 19]. Moreover, the dynamic exchange behavior between the interlayer CO32- and atmospheric CO2 has been discovered, in which the CO2 molecules transform into CO32- upon entering the LDH interlayer and drive the existing CO32- to release as CO2 molecules, thus achieving the carbon cycle, and this behavior is called the breathing effect of LDH toward CO2 [20, 21]. Featured by the above-mentioned physical structures and chemical characteristics, LDH is recognized as a potential 2D material for developing a novel 2D membrane with inherent CO2-selective nanochannels. So far, no report on 2D LDH membranes for CO2 separation has been found. Herein, 2D LDH membranes with inherent CO2-selective transport channels are first fabricated for efficient carbon capture. The specific breathing effect toward CO2 enables CO2 molecules to transport in the CO32- ions form through the interlayer channels, and the CO32- ions can regulate the channel to a sieving size for CO2/CH4, endowing the LDH membranes with inherent CO2-selective transport channels. The ultrathin LDH membranes with thickness of 30-70 nm exhibit superior CO2/CH4 separation performances which are comparable to state-of-the-art membranes.
2. Experimental 2.1 Materials All chemicals are of AR grade and used as received. Mg(NO3)2·6H2O and Al(NO3)3·9H2O were purchased from Aladdin Bio-chem Technology Co. Ltd. NaOH, Na2CO3, NaCl and formamide were purchased from Tianjin Li’anlongbohua Medical Chemistry Co. Ltd. 0.5 M HCl standard solution was purchased from Tianjin Jiangtian chemical Technology Co. Ltd. Polyacrylonitrile (PAN) ultrafiltration porous membrane with a molecular weight cut-off of 100,000 was purchased from Shanghai Mega Vision Membrane Engineering and Technology Co. Ltd. 2.2 Preparation of LDH nanosheets Carbonate-intercalating LDH with a Mg/Al mole ratio of 3 (MgAl-CO3-LDH) was prepared by coprecipitation-hydrothermal aging method [22]. Typically, 9 mmol
Mg(NO3)2·6H2O and 3 mmol Al(NO3)3·9H2O were mixed together and dissolved in 100 ml of deionized water to form a clear metal nitrite solution. Then the solution was titrated with a mixed basic solution of NaOH (1.6 M) and Na2CO3 (0.8 M) to pH=11 and then aged in autoclave at 180 °C for 48 h. The obtained LDH platelets were recovered by filtration or centrifugation, washed with deionized water and dried at 60 °C in a vacuum oven. For exfoliation, the prepared MgAl-CO3-LDH was converted to MgAl-NO3-LDH (NO3- intercalating MgAl-LDH) by a two-step anion exchange process [23, 24]. 0.5 g MgAl-CO3-LDH was dispersed in 500 ml aqueous solution containing 3.5 M NaCl and 3.3 mM HCl in a two-neck flask (using de-carbonate water and HCl standard solution). The reaction flask was shaken with N2-flow protection for 24 h at ambient temperature. The NaCl-HCl treated LDH sample (determined as MgAl-Cl-LDH) was isolated by centrifugation, washed with de-carbonate water and dried in the vacuum oven. The MgAl-Cl-LDH was dispersed into 2 M NaNO3 aqueous solution and shaken with N2-flow protection for 24 h at ambient temperature. The as-prepared MgAl-NO3-LDH product was isolated by the same procedure as described above. Then 0.1 g MgAl-NO3-LDH was mixed with 100 ml formamide, sealed after purging with N2 and shaken at a speed of 160 rpm for 2 days for exfoliation. The formed colloidal suspension was further treated by centrifugation at 2000 rpm for 10 min to remove unexfoliated particles. 2.3 Preparation of 2D LDH membranes 2D LDH membranes were prepared by a spin-casting method according to the preparation of highly interlocked GO laminar membranes [25]. To enhance the adhesion of LDH nanosheets on substrate surface during assembling process, PAN substrate was pre-modified with carboxyl groups through hydrolysis process in a basic solution to obtain HPAN [26]. The typical fabrication procedure was illustrated in Scheme 1. A certain amount of LDH nanosheet suspension was dropped on HPAN substrates and spun at a speed of 2500 rpm to assemble LDH laminar membranes. The coated membrane was dried in vacuum oven before use. To exclude the interference of substrate in some characterizations, free-standing LDH membrane was
prepared. LDH nanosheet suspension was vacuum-filtrated on anodic aluminum oxide (AAO) support to form thick separation layers and peeled off from the substrate.
Scheme 1 Schematic for the preparation of LDH membranes. 2.4 Characterizations The crystal structure and interlayer spacing of samples were characterized by X-ray diffraction (XRD) pattern using a D/MAX-2500 X-ray diffractometer with filtered Cu-Ka radiation. The chemical properties of samples were analyzed with Fourier transform infrared (FTIR) spectra (Bruker Vertex 70 FTIR spectrometer) and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI with monochromatized Al Kα radiation). The metal contents of LDH particles were measured after dissolving a certain amount of samples in aqueous HCl solution by inductively coupled plasma atomic emission spectroscopy (ICP-MS 7700x, Agilent). The morphology of LDH particles and membranes was observed by field emission scanning electron microscope (FESEM, Nanosem 430). SEM-elemental analysis was conducted by energy-dispersive X-ray spectroscopy (EDX) and elemental mapping on Elementar Vario Micro cube analyzer. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) and the corresponding Fast Fourier Transform (FFT) images of LDH nanosheets were obtained by Tecnai G2 20 S-TWIN TEM apparatus. The topographical measurement of LDH nanosheets was performed on an atomic
force microscope (AFM, NTEGRA Spectra) instrument in a tapping mode. Sample of nanosheets was prepared by immersing a pretreated Si wafer in nanosheet suspension [27]. The charge property and particle size distribution of LDH nanosheets in formamide suspension were determined by Malvern Zetasizer Nano ZS90 instrument. The surface potentials of LDH membranes and substrate were measured by solid surface zeta potential analyzer (Anton Paar GmbH, Austria). The adsorption isotherms of CO2 and CH4 at 30 °C on LDH membranes were measured by a gas adsorption instrument (BELSORP-max, MicrotracBEL, Japan). Thermal properties of LDH particles and membranes were measured using thermogravimetric analysis (TGA, Netzsch STA 449 F5) under a N2 atmosphere at a heating rate of 10 K/min. The membrane samples used for XRD, FTIR and TGA were thick freestanding LDH membranes. 2.5 Gas permeation tests The separation performance was investigated by a constant pressure-variable volume method on the homemade gas separation apparatus [11]. Typically, pure CO2, CH4 or their gas mixture (CO2/CH4 30:70) was employed as feed gas and N2 as sweep gas. For CO2/N2 mixture (CO2/N2 20:80) separation, CH4 was employed as sweep gas. The feed gas pressure was maintained at 2 bar and the flow rate of sweep gas was fixed at 30 ml/min. All gas permeation tests were conducted in humidified state at 30 °C. The feed gas and sweep gas were humidified in advance by passing through humidifying tanks. Before testing, the pristine LDH membrane was pretreated by placing in CO2 atmosphere for about half hour to exchange the interlayer nitrate ions with carbonate ions. After the gas permeance reached a steady state, the gas permeance P/l (1 GPU =1×10-6 cm3 (STP) cm-2 s-1 cmHg-1) was calculated using the equation: (P/l)i
=
Qi
/∆piA
(1) where Qi is the volumetric flow rate of gas “i” (cm3/s) (STP), ∆pi is the difference partial pressure across the membrane (cmHg), A is the effective membrane area (cm2).
The ideal gas selectivity αij was calculated by: αi/j
=
(P/l)i
/(P/l)j
(2) Since the permeate side is maintained at ambient pressure, the mixed-gas separation factor was also calculated by the eq (2). All gas permeation tests were repeated for at least three times and the average value was reported with statistical errors less than 5%.
3 Results and discussion 3.1 Characterization of LDH platelets and nanosheets The MgAl-LDH with a Mg/Al ratio of three was selected as the starting material since magnesium-rich LDH exhibits more rapid exchange of carbonate ions with atmospheric
CO2
[21].
MgAl-CO3-LDH
was
synthesized
with
coprecipitation-hydrothermal aging method. The XRD pattern of MgAl-CO3-LDH in Fig. 1a agrees well with previous reports [28], which proves the successful preparation of target product. The Mg/Al molar ratio of LDH samples was determined to be 3.1 by ICP, and further confirmed by EDX and XPS analysis (Fig. S1 and S2). In XRD pattern, the (003) peak at 2θ of 11.36° indicates a d-spacing value of 0.778 nm for MgAl-CO3-LDH [28]. The vibration band at 1369 cm-1 in FTIR spectrum (Fig. 1d and Fig. S3) provides evidence for the intercalated CO32- and the band at 1650 cm-1 is identified as the intercalated water molecules [27, 29, 30]. The morphology of MgAl-CO3-LDH is shown in Fig. 1b. As can be seen, the rounded-hexagonal pallets have lateral dimensional sizes from several hundred nanometers to micrometers. To achieve exfoliation in formamide, the MgAl-CO3-LDH was converted to MgAl-NO3-LDH by ion exchange. After the ion exchange, the (003) peak of MgAl-LDH shifts to a lower angle with an enlarged d-spacing of 0.835 nm, which indicates the replacement of CO32- with NO3- (Fig. 1a). The vibration band in FTIR spectra further confirms the conversion of CO32- (1369 cm-1) to NO3- (1383 cm-1) (Fig. 1d) [27, 29, 30]. The SEM images of MgAl-NO3-LDH (Fig. 1c) show no obvious
change in morphology compared with MgAl-CO3-LDH. The exfoliation was achieved by mechanically shaking MgAl-NO3-LDH in formamide. The total delamination of LDH particles yields a colloidal suspension of LDH nanosheets which shows a clear Tyndall effect under a light beam, as shown in the insert of Fig. 1e. The morphology of exfoliated LDH nanosheets was characterized by TEM and AFM. TEM image (Fig. 1e) presents a typical two-dimensional nanosheets with very thin and transparent morphology. HRTEM image and the corresponding FFT image (Fig. S4) reveal the hexagonal structure of LDH nanosheets, suggesting that the basic structure of the LDH basal plane remains unchanged after exfoliation [27, 29]. The AFM image in Fig. 1f reveals that the obtained LDH nanosheets have a lateral size of 0.5-1 µm. The height profile scan indicates the thickness of the nanosheet is 1-2 nm. Considering the literature value of monolayer LDH height is 0.8-1 nm which is regarded as the sum of crystallographic thickness of LDH basal layer (0.48 nm) and adsorbed layer of formamide molecules (∼0.3 nm) or counter anions [27-29], the obtained LDH nanosheets should be 1-2 layers. The equivalent size distribution of LDH nanosheets was measured of 0.22-1.7 µm by dynamic light scattering (DLS) (Fig. S5).
Fig. 1 (a) XRD patterns of MgAl-CO3-LDH and MgAl-NO3-LDH. (b) SEM image of
MgAl-CO3-LDH. (c) SEM image of MgAl-NO3-LDH. (d) FTIR spectra of MgAl-CO3-LDH and MgAl-NO3-LDH. (e) TEM image of LDH nanosheets inserted with Tyndall effect in colloidal suspensions. (f) AFM image of LDH nanosheets. The height profile of the nanosheet corresponds to the blue dashed line. 3.2 Characterization of 2D LDH membranes
Fig. 2 SEM images of (a-c) cross-sectional LDH composite membranes with accumulated LDH stacked layers of 30 and 70 nm and (d-f) top view of the membrane surfaces: PAN substrate, coated with LDH layers of 30 and 70 nm. EDX mapping of C, Mg and Al elements in (g) surface and (h) cross section of LDH composite membrane. Spin-coating method was adopted for 2D LDH membrane preparation, since it is more favorable for horizontal alignments of 2D nanosheets onto support surface to get homogenous membrane [31]. Fig. 2 exhibits the SEM images of membrane surfaces and cross sections. The ultrathin LDH layers are tightly and uniformly adhered onto
the HPAN substrate (Fig. 2a). The typical laminate structure of LDH layers can be clearly seen in a thicker coated membrane (Fig. 2b and c), which indicates the successful preparation of well-stacked LDH laminar membranes. The membrane thickness can be easily adjusted in the range of 8-70 nm by varying the dosage of LDH nanosheets during spin-coating procedure and the relationship between membrane thickness and LDH amount was plotted in Fig. S6. From the top-view, LDH membrane shows a fairly smooth surface compared with the GO membrane with many wrinkles (Fig. 2e and f) which are usually considered as structural defects that will degrade the separation factor [31]. The smooth membrane surface is attributed to the rigid host layer of LDH and the centrifugal force of spin-coating method which results in more ordered alignment of nanosheets than the sole vertical pulling force of vacuum filtration. When the thickness of LDH membrane is reduced to 8 nm (Fig. S7), there are some pores and defects exposing on the membrane surface, while with assembly of more LDH layers (with thickness more than 15nm), the substrates are entirely covered by continuous LDH layers without visible pinholes or defects, forming a highly smooth membrane surface as shown in Fig. 2. The EDX mapping of the selected elements in Fig. 2g-h show a homogenous distribution in both membrane surface and cross section. The cross-sectional SEM image of thick freestanding LDH membrane displays a clear laminate structure as shown in Fig. S8, further indicating the well-stacked interlayer channels.
Fig. 3 (a) XRD patterns and (b) FTIR spectra of LDH composite membrane and
substrates. Table 1 Zeta potential of the LDH colloidal suspensions, PAN substrates and LDH composite membranes with different thicknesses. Sample
LDH nanosheets
Zeta potential (mV)
18.9
LDH membranes with different thickness PAN HPAN 8 nm 30 nm 70 nm 100 nm -40.72
-77.96
-64.39
-41.69
-11.02
24.86
The membrane structure was further characterized by XRD. Fig. 3a depicts XRD patterns of LDH membrane and substrate. In addition to the major diffractions of the substrate, the small peak at 2θ of 7.46° is the diffraction of (003) peak of LDH membrane which indicates a d-spacing of 1.18 nm based on Bragg’s law. Considering that the d-spacing value includes the LDH monolayer thickness of 0.48 nm [27-29], the free interlayer spacing of LDH membrane is estimated to be 0.7 nm. The enlarged interlayer spacing of LDH membrane compared with that of MgAl-NO3-LDH platelets is due to the accompany of water and residual formamide molecules with the intercalated NO3- [30]. In addition, in contrast with the broad diffraction peaks of GO membranes [10, 11], the sharp peak of LDH membrane at 2θ of 6-9° signifies a narrow interlayer spacing distribution and further confirms the more ordered stacking of LDH nanosheets in the laminar membrane. The hydrolyzed PAN (HPAN) has rich carboxyl groups on surface, which shows negatively charged property [32]. Therefore, to enhance the interfacial adhesion of selective layer on substrate, the negatively charged HPAN was used as substrate for deposition of positive charged LDH nanosheets during the assembly process. Fig. 3b displays the FTIR spectra of LDH membrane and substrates, the new infrared peaks at around 1570 cm-1 and 1405 cm-1 of HPAN prove the formation of carboxyl groups on substrate surface [26]. For LDH membranes, bands below 800 cm-1 are associated with the stretching and bending modes of metal-oxygen (M-O) [27, 33]. Due to the interference of the HPAN substrate reflection, the infrared peak of intercalating NO3in LDH membrane was not observed while a sharp peak at 1690 cm-1 of the residual formamide molecules was shown [34]. Table 1 shows the zeta potential of LDH
nanosheets suspension and the surface potential of LDH membranes and substrates. The hydrolyzed PAN exhibits a negative surface of -77.96 mV, with the assembly of positively charged LDH layers with a zeta potential of 18.90 mV, the membrane surface charge gradually turns to be positive. Consequently, the electrostatic interaction and hydrogen-bond interaction with HPAN substrate reinforce the interface interaction and further protect the LDH selective layer from delamination. The thermal behaviors of LDH platelet and membrane were analyzed by TG-DSC (Fig. S9). The lamellar membranes consisted of exfoliated LDH nanosheets have weaker interactions with interlayer water or formamide molecules than LDH platelet, leading an earlier loss of interlayer molecules. The same phenomenon happened in the second weight loss stage of the interlayer ions loss and dehydroxylation of LDH layers.
Fig. 4 (a) FTIR spectra, (b) XRD patterns and (c-f) XPS wide spectra and high-resolution spectra of C 1s, Mg 2p and Al 2p for LDH membranes. LDH(NO3-) mem represents the pristine LDH membrane; LDH(CO32-) mem represents the LDH membrane treated with CO2. Since the breathing effect of LDH toward CO2 actually refers to the dynamic exchange behavior between the interlayer CO32- and atmospheric CO2, the intercalated NO3- in LDH membranes should be exchanged to CO32- to make breathing effect work in CO2 separation process. In consideration of the strong
chemisorption of LDH host layers to CO2 (adsorption enthalpy of MgAl-LDH toward CO2 reaches 92.04 KJ/mol) [35] and the instability of intercalated NO3- which can be easily replaced by other ions (especially by CO32- under even trace CO2 in atmospheric environment) [23], the pristine LDH membrane was pretreated by CO2 for exchange of nitrate ions with carbonate ions. And a series of characterizations were further carried out to reveal the physiochemical properties of LDH membrane nanochannels as shown in Fig. 4. To eliminate the interference of substrate, free standing LDH membranes were used as test samples. The intercalated ion species of LDH membranes were analyzed using FTIR (Fig. 4a). FTIR spectrum of the pristine LDH membrane (LDH(NO3-) membrane) shows that the NO3- and residual formamide molecules intercalate in the nanochannels of LDH stack layers. The alteration of vibration peaks of the corresponding intercalated anions for LDH membrane after CO2 treatment (LDH(CO32-) membrane) demonstrates that the CO2 successfully entered into the LDH interlayer channels, converted to CO32- under the action of water molecules and replaced the NO3- and formamide molecules. Therefore, the CO32initiator of breathing effect toward CO2 has been constructed in interlayer nanochannels. Interlayer nanochannel size is another dominant parameter that governs the separation efficiency [36]. A uniform channel size that falls between the sizes of molecule pair or ion pair would realize size sieving [37]. The interlayer spacing of CO2 treated LDH membrane was determined by XRD. In Fig. 4b, the in-plane reflections disappeared, suggesting the highly horizontal orientation of LDH nanosheets in the membrane. The main diffractions from left to right are assigned to (003), (006) and (009) peaks according to the d-spacing relation (interlayer spacing = d003 = 2d006 = 3d009) of LDH [38]. The XRD pattern of LDH(NO3-) membrane shows the (003) peak at 2θ of 7.46° suggesting that the free interlayer spacing of LDH membrane is 0.7 nm which is identical to the membrane with substrate (Fig. 3). Since the interlayer channel is oversized for the small gas molecules, the sieving mechanism cannot work. However, it is interesting to note that the interlayer spacing of
LDH(CO32-) membrane measured by XRD is decreased to 0.3 nm. This reduced interlayer channel size is attributed to a smaller CO32- with an ionic radius of 0.136 nm and the stronger interaction between CO32- and LDH host layer which pulls the adjacent LDH layers closer. The intercalating and interaction of CO32- with LDH host layer were further confirmed by XPS (Fig. 4c-f). C 2p spectrum shows the increase of CO32- in LDH(CO32-) membrane, proving the carbonate intercalating. The Mg 2p spectrum of LDH(CO32-) membrane exhibits a shift toward higher position compared with LDH(NO3-) membrane, which means the decrease of electron cloud density of Mg atoms [39]. This is because the CO32- with higher electronegativity than NO3- has stronger interaction with Mg adsorption sites on LDH host layer [40, 41]. And this also explains the size shrinkage of interlayer nanochannels in LDH membranes after CO2 treatment. The same case is for Al 2p spectrum with a slight shift toward higher position. For clear understanding, schematic of the membrane structures corresponding to the evolution of the XRD and FTIR results was sketched in Scheme 2.
Scheme 2 Structures of LDH membranes before and after CO2 treatment. Specific gas adsorption of CO2 and CH4 on LDH(NO3-) membrane and LDH(CO32-) membrane was measured (Fig.5). Both the LDH(NO3-) membrane and the LDH(CO32-) membrane behave a preferential adsorption of CO2 compared to CH4. The CO2 adsorption capacity of LDH(CO32-) membrane is slightly higher than that of LDH(NO3-) membrane. And different from LDH(CO32-) membrane, a two-step adsorption occurred in CO2 adsorption of LDH(NO3-) membrane, which might be induced by the transformation of LDH(NO3-) membrane to LDH(CO32-) membrane. The adsorption platform was considered as the complete replacement of interlayer
NO3- with CO32-. After that, the amount of CO2 adsorption continues to increase to near that of LDH(CO32-) membrane.
Fig. 5 Comparison of gas adsorption on (a) LDH(NO3-) membrane and (b) LDH(CO32-) membrane. 3.3 Gas separation performance of 2D LDH membranes The CO2/CH4 separation performances of LDH membranes with adjustable thickness were evaluated under humidified condition. Fig. 6a shows the mixed gas permeation of LDH(CO32-) membranes with various membrane thickness. It can be seen the ultrathin LDH membrane with 8 nm-thick selective layer exhibits a CO2 permeance of 202 GPU with a CO2/CH4 selectivity of 7. With increasing the membrane thickness to 30~70 nm, the CO2/CH4 selectivity increased to 33~37 with a CO2 permeance of 152~106 GPU since the minor defects of thinner LDH membrane were concealed by thick stacked LDH layers. The high selectivity of LDH membranes can be explained by the synergy of breathing effect to CO2 and size sieving mechanism. On one hand, the breathing effect toward CO2 by LDH renders the feed gas CO2 transfer in the form of CO32- across the membrane. In detail, CO2 molecules are incorporated into LDH interlayer, then react with interlayer water forming 2H+ and CO32-, replace the initially intercalated CO32- and drive it to react with 2H+ to release CO2 and water, thus completing the carbon cycle between LDH and environmental CO2 [20, 21]. On the other hand, the interlayer channel size of 2D LDH membrane is regulated to 0.3 nm by the intercalated CO32-, which retards the
transport of CH4 molecules with a dynamic diameter of 0.38 nm. Although the small channel would also retard the CO2 molecules (0.33 nm), there is another way for CO2 molecules to pass through the LDH laminar channel, namely transfer as form of CO32with an ionic radius of 0.136 nm which does not suffer from the sieving. Therefore, the distinct size sieving also works between CO2/CH4 pairs.
Fig. 6 (a) Mixed CO2/CH4 separation performances of LDH(CO32-) membranes with different thickness. (b) Single CO2/CH4 permeation performances of LDH(CO32-) membranes. (c) CO2/CH4 long-term operation test of 30-nm-thick LDH(CO32-) membrane. (d) CO2/CH4 separation performance of LDH(CO32-) membranes compared with state-of-the-art CO2 separation membranes. Single gas permeation test of LDH(CO32-) membranes was conducted for comparison (Fig. 6b). For most membranes, the separation factor of mixed gas is typically lower than the ideal selectivity of single gas due to the competitive sorption effects or CO2 plasticization [42], but the LDH membranes instead exhibit slightly higher gas selectivity for mixed gas separation. This phenomenon was mostly reported in hydrocarbon/light gas separation, where the condensable gas with strong adsorption interaction would occupy the adsorption site of membrane and retard the
passage of light gas [43, 44]. However, this seldom happened for CO2/gas separation [45]. The higher selectivity in CO2/CH4 mixture of LDH membranes is attributed to the intrinsic breathing effect of LDH material toward CO2 which renders the preferential adsorption and transportation of CO2, thereby hindering the transport of another component in gas mixtures. And this unusual behavior also indicates the critical role of breathing effect of LDH toward CO2 on CO2-selective transport. To verify the size sieving effect, LDH(NO3-) membrane with interlayer spacing of 0.7nm was also tested for single gas permeation for comparison and the results were shown in Fig. S10-S11. Compared with LDH(CO32-) membranes, the ideal CO2/CH4 selectivities of LDH(NO3-) membranes show a dramatic reduction. For example, the CO2/CH4 selectivity of 30 nm-thick membrane reduced from 32.3 (mixed gas) and 29.9 (single gas) of LDH(CO32-) membrane to 18.9 (single gas) of LDH(NO3-) membrane. For single CO2 gas permeance, there is no difference in membrane structure and transport mechanism with LDH(CO32-) membrane since the LDH(NO3-) membrane would be transformed to LDH(CO32-) membrane after CO2 permeated. The case does not fit the single CH4 gas permeance, because the LDH(NO3-) membrane without CO2 treatment does not exhibit a shrinkage of nanochannel size. The structure of CH4 permeated membranes were also characterized by XRD, FTIR, and XPS as shown in Fig. S12-14. The results show that the CH4 permeated LDH membranes have same interlayer nanochannel structures with the LDH(NO3-) membrane with a channel size of 0.7 nm, thus the sieving effect does not work. The schematic transportation mechanisms of mixed gas and single gas in LDH(CO32-) membrane and LDH(NO3-) membrane were sketched in Fig. S15. The long-term operation test of 2D LDH membrane was conducted (Fig. 6c). The membrane maintained the high CO2/CH4 separation performance during the 144 h operation test, exhibiting an excellent permeance stability. We further compared the CO2/CH4 separation performance of LDH membrane with state-of-the-art CO2 separation membranes including polymer TFC membranes [46-49], MOF membranes [50-52], zeolite [53] and 2D membranes [10-14, 25]. As shown in Fig. 6d, the
performance of our LDH membranes are comparable to the current advanced membranes and transcend the limits of MOF membrane performance [12, 54]. To further explore the versatile potential of LDH membrane in carbon capture, the separation performance of LDH membranes for CO2/N2 mixture was tested. As shown in Fig. 7a, the LDH membrane also exhibits superior CO2/N2 separation performance. The 30nm-thick LDH membrane exhibits CO2 permeance of 152 GPU and CO2/N2 selectivity of 36, and with the increase of membrane thickness to 70nm, the CO2/N2 selectivity increases up to 44. In addition, the membrane displays good long-term operation stability (Fig. 7b).
Fig. 7 (a) CO2/N2 mixture separation performances of LDH(CO32-) membranes with different thickness. (b) CO2/N2 long-term operation test of 30-nm-thick LDH(CO32-) membrane.
4. Conclusions We demonstrated for the first time the construction of 2D LDH membranes with inherent CO2-selective transport nanochannels for CO2 separation. Ultrathin LDH membranes with a tunable thickness from 30-70 nm are fabricated by the assembly of exfoliated 2D LDH nanosheets on porous substrates. With the intrinsic breathing effect of LDH toward CO2, the CO2 molecules can transfer in the form of CO32through the interlayer nanochannels of membranes. Furthermore, the interlayer CO32regulates the size of membrane nanochannel from 0.7 nm to 0.3 nm, conferring a distinct sieving effect for CO2 and CH4 molecules. Featuring with the inherent
physicochemical CO2-selective nanochannels, the 2D LDH membranes exhibit high CO2/CH4 separation performance compared with state-of-the-art membranes. The exploration in 2D LDH membranes for efficient carbon capture contributes to the rational design and sustainable development of 2D material membranes. The inherent selective transport nanochannels of anion intercalation LDHs may inspire the development of other kinds of intercalation materials for constructing precise molecular or ionic separation membranes.
Acknowledgements This study was supported from the National Natural Science Foundation of China (Nos. 21838008, 21621004, 21878215 and 21576189), National Key R&D Program of China (No. 2017YFB0603400), State Key Laboratory of Petroleum Pollution Control (No. PPC2017014), State Key Laboratory of Separation Membranes and Membrane Processes and Tianjin Polytechnic University (No. M1-201701), National Key Laboratory of United Laboratory for Chemical Engineering (SKL-ChE-17B01).
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Highlights: 1. 2D LDH membrane for carbon capture was prepared for the first time. 2. Inherent CO2-selective transport nanochannels were built in membrane. 3. Intrinsic breathing effect of LDH to CO2 render CO2 transport in CO32- form. 4. CO32- regulates the nanochannel size to 0.3nm, for sieving CO2/CH4 pair. 5. Breathing effect and size-sieving effect contribute to efficient CO2 separation.
Conflict of Interest The authors declare no conflict of interest