Designing Supported Ionic Liquids (ILs) within Inorganic Nanosheets

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Designing Supported Ionic Liquids (ILs) within Inorganic Nanosheets for CO2 Capture Applications Yingjie Zhou, Jingjing Liu, Min Xiao, Yuezhong Meng, and Luyi Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11249 • Publication Date (Web): 03 Feb 2016 Downloaded from http://pubs.acs.org on February 9, 2016

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Designing Supported Ionic Liquids (ILs) within Inorganic Nanosheets for CO2 Capture Applications

Yingjie Zhou,1,2,3 Jingjing Liu,2 Min Xiao,3 Yuezhong Meng,3,* Luyi Sun2,*

1

School of Physics and Optoelectronic Engineering, Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing, Jiangsu, 210044, China 2

Department of Chemical & Biomolecular Engineering and Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States 3

The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong

Province/State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, China

*Authors to whom correspondence should be addressed: Dr. Luyi Sun, Tel: (860) 486-6895; Fax: (860) 486-4745; Email: [email protected] Dr. Yuezhong Meng, Tel: (0086) 20-8411-4113; Fax: (0086) 20-8411-4113; Email: [email protected]

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ABSTRACT A new methodology was developed for the immobilization of ionic liquids (ILs) on αzirconium phosphate (ZrP) and montmorillonite (MMT) single-layer nanosheets via a facile coassembly process. The co-assembled inorganic nanosheet/1-n-butyl-3-methylimidazolium chloride (BMIMCl) hybrids were systematically characterized. The results showed that the ILs were successfully assembled with ZrP or MMT single-layer nanosheets to form an intercalated structure. The inorganic nanosheet/IL hybrids can serve as efficient CO2 absorbents. The CO2 sorption of BMIMCl could be made up to 21 times more efficient due to the high exposure of the functional groups of BMIMCl in the co-assembled hybrids. CO2 was physically absorbed by the hybrids with a slow equilibrium time at lower temperatures, while higher temperatures allowed for faster diffusion and chemical absorption of CO2. The best CO2 capture capacities of the hybrids were 0.73 mmol/g at 60 °C for ZrP/BMIMCl and 0.42 mmol/g at 70 °C for MMT/BMIMCl.

Keywords: ionic liquids, α-zirconium phosphate, co-assembly, CO2 capture, immobilization

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1. Introduction Surging carbon dioxide (CO2) levels have pushed greenhouse gases to record highs.1 The high concentration of CO2 in the atmosphere is primarily due to the burning of fossil fuels..2-3 Therefore, the capture of CO2 from industrial flue gases is believed to be a direct solution to minimize the related environmental issues. In recent years, several new technologies and processes for CO2 capture, storage, and utilization have been developed. Among them, sorption-based technologies and processes have been the most widely researched.5 The wet scrubbing process using aqueous amines, which chemically and reversibly bind CO2 by carbamate formation, has already been widely adopted in CO2 capture facilities. However, this process has severe drawbacks that prohibit its large-scale application in industry, including high-energy penalty, amine loss and degradation, release of volatile organic compounds, and equipment corrosion,.6 In the past decade, numerous efforts have been devoted to exploring solid adsorbents for the capture of CO2, including activated carbonaceous materials, microporous/mesoporous silica and zeolites, carbonates, and polymeric resins loaded with or without nitrogen functionalities.

5, 7

Solid adsorbents could

produce certain technical and economic advantages compared to the conventional aqueous amine absorption, but they have their own limitations and challenges due to their insufficient CO2 adsorption capacity, low thermal stability, and poor recyclability. 7 Ionic liquids (ILs), especially the imidazolium-based ILs, have been widely explored as nonvolatile and reversible CO2 absorbents because of their high CO2 solubility.8 In addition, ILs can be designed for specific applications by varying the anion and/or cation.9-11 Functionalizing ILs with amine groups is the major approach to enhance the CO2 capture capacity. Up to date, a diverse range of ILs have been synthesized by combining imidazolium with various functional groups, and the related research has been reviewed recently.12-15 However, the high viscosity and the high cost of ILs prohibit their widespread application. One convenient approach to solve the problem is to immobilize ILs on a variety of supports,16-17 which leads to many benefits, including facilitating recycling, minimizing consumption, and increasing the efficiency of CO2 absorption.16 Various approaches have been developed for the immobilization of ILs through either covalent or 3


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noncovalent bonding with the supports, including simple physisorption, self-assembly, surface grafting, and polymerization.16, 18-20 The physisorption approach often results in the leaching of ILs from the supports during application.21 Polymerization and surface grafting typically require complex procedures, which are usually not cost effective.22 A self-assembly approach could be an optimum method because of its simple and economical process with a high IL utilization efficiency. Up to now, the main solid supports of choice are porous silica and zeolite.16 Due to the limited active sites on a regular solid support surface, the loading density of ILs is relatively low, which comprises their performance.16 Inorganic single-layer nanosheets exfoliated from layered compounds have attracted high interest, particularly as building blocks for the formation of functional nano-structured materials.23 Many layered materials (e.g., montmorillonite (MMT), laponite, etc.) can be exfoliated in polar solvents.24 Based on such single-layer nanosheets, various organic/inorganic layered hybrids with special functions have been explored via the co-assembly of the charged nanosheets with cationic/anionic counterparts.23, 25-26 α-Zirconium phosphate (ZrP, Zr(HPO4)2·H2O) is a synthetic layered compound with a well-defined molecular structure, which can be easily exfoliated into single-layer nanosheets.27 The crystallinity and lateral dimension of ZrP can be well controlled under different synthetic conditions.28-30 ZrP has a high cation exchange capability (CEC) of ca. 664 meq/100 g due to its high density of acidic and reactive surface –OH groups (ca. 4.2 –OH groups/nm2).31-33 By taking advantage of such acidic groups, guest species can be intercalated into ZrP layers31, 34 or react with the P-OH groups on the pre-exfoliated ZrP single-layer nanosheets,30,

35-46

forming various

nanostructured functional hybrids with diverse applications.47-48 Herein, a new group of layered functional materials via the co-assembly of ILs with ZrP or MMT nanosheets was developed. 1-n-Butyl-3-methylimidazolium chloride (BMIMCl) without any modification was used as the model IL, which was co-assembled with single-layer ZrP or MMT nanosheets based on their electrostatic interaction. The immobilized BMIMCl via this approach served as an excellent CO2 absorbent.

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2. Experimental 2.1 Materials Zirconyl chloride octahydrate (ZrOCl2·8H2O, 98%, Sigma-Aldrich, USA), phosphoric acid (85%, Sigma-Aldrich, USA), tetra-n-butylammonium hydroxide (TBA, Sigma-Aldrich, USA), montmorillonite (MMT, Cloisite® Na, CEC of 92.6 meq/100 g, BYK Additives Inc. , USA), 1-nbutyl-3-methylimidazolium chloride (BMIMCl) (>99.9%, BASF, USA) were used as received.

2.2 Preparation of ZrP and MMT nanosheets ZrP micro-crystals were synthesized using a previously reported hydrothermal method.28 Briefly, 6.0 g of ZrOCl2•8H2O and 60.0 mL of 6.0 M H3PO4 were reacted in a sealed Teflon-lined pressure vessel for 24 h at 200 °C. The obtained product was washed three times with deionized (DI) water through a centrifugation and re-dispersion process, dried at 65 °C overnight, and then gently ground into fine powders. To exfoliate ZrP, 3.3 mL of TBA (0.1 M) were added dropwise to 100 mg of ZrP micro-crystals dispersed in 6.7 mL of DI water, followed by ultrasonication in an ice bath for 3 h. The obtained ZrP nanosheets were further treated with 3.3 mL of 0.10 M HCl to re-generate the surface acidic hydroxyl groups.39, 49-50 MMT was directly exfoliated in water by ultra-sonicating 100 mg MMT in 30 mL DI water for 3 h.

2.3 Assembly of nanosheets and BMIMCl A pre-determined amount of BMIMCl was added to the exfoliated ZrP (or MMT) nanosheets at three ratios (1:0.5, 1:1.0, 1:2.0) based on their corresponding total CEC, and the samples were denoted as ZrP (or MMT)-BMIMCl-1-0.5 (or 1-1.0, 1-2.0), respectively. The mixture was then ultrasonicated for 3 h until reaching a uniform dispersion. The resulting dispersion was then cast on a glass slide and dried in an oven overnight at 85 °C. The assembled inorganic nanosheet /BMIMCl hybrids were then allowed to cool down at room temperature.

2.4 Characterization 5


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The microstructure of the exfoliated ZrP and MMT nanosheets was characterized by transmission electron microscope (TEM) using afield-emission TEM (JEOL 2010F, Japan) with an accelerating voltage of 200 kV. Surface morphology of the samples was recorded on an FEI Helios NanoLabTM 400 DualBeamTM (USA) scanning electron microscope (SEM) operated at 15 kV. The thermal properties of the samples were characterized by a thermogravimetric analyzer (TGA, TA Instruments model Q500, UK) under an air atmosphere (40 mL/min) at a heating rate of 10 °C/min. All samples were pretreated in an air flow at 100 °C for 30 min to remove absorbed moisture prior to each test. Fourier transform infrared (FTIR) spectra of the samples were recorded on an Analect RFX-65A (USA) type FTIR spectrometer using KBr pellet samples. The KBr pellets were prepared by grinding spectroscopic grade KBr with the samples (0.1 wt %). The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 diffractometer (Germany) with BraggBrentano θ-2θ geometry (40 kV and 30 mA), using graphite monochromatized Cu Kα radiation.

2.5 CO2 capture The CO2 capture analysis was carried out on a thermogravimetric analyzer (TGA, TA Instruments model Q500, UK). An appropriate amount of sample was loaded into a platinum pan and dried for 1 h at 100 °C before testing to remove trace amounts of moisture and volatile impurities. Subsequently, a flow of CO2 at a rate of 100 mL/min was introduced to the cell. The weight of the samples as a function of temperature (heating rate: 10 °C/min) was recorded. In addition, the CO2 capture measurements were performed at specific constant temperatures (30, 40, 50, 60, 70, and 80 °C) as a function of reaction time. The instant CO2 capture amount was continuously recorded with temperature at a rising rate of 10 °C/min.

3. Results and discussion ZrP can be easily exfoliated into single-layer nanosheets using TBA,27 while MMT can spontaneously delaminate into individual nanosheets in an aqueous dispersion with the assistance of ultrasonication.51 TEM images of single-layer ZrP and MMT nanosheets are shown in Figure 1. 6



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To avoid re-stacking, the samples were prepared by depositing a drop of dilute suspension of the exfoliated nanosheets (ca. 0.06 wt%) on a TEM carbon grid directly. The size distributions (analyzed using software Nano Measurer, V1.2) of ZrP and MMT single-layer nanosheets are shown in Figure 1(c) and 1(d). The average lateral dimensions of ZrP and MMT single-layer nanosheets are ca. 230±50nm and ca. 300±100 nm, respectively. Considering one of the key tasks is to support ILs within nanosheets by re-stacking exfoliated single layer nanosheets with ILs, herein the pure nanosheets were also re-stacked and characterized. Figure 1(e) and 1(f) show the SEM images of the cross-section of the restacked ZrP and MMT single-layer nanosheets, respectively. For effective re-stacking, a relatively high concentration of ZrP (or MMT) nanosheet suspension (0.3 wt%) was cast on a silicon wafer and then dried at room temperature. From the SEM images, one can observe that the single-layer MMT nanosheets were re-stacked at a higher degree of order than ZrP nanosheets. This is believed owing to strong hydrogen bonding between ZrP nanosheets because of their surface hydroxyl groups, which lead to the formation of gels during drying process.

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0.20

0.30

(c) ZrP Size distribution

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0.15 0.10 0.05 0.00 180

(d) MMT Size distribution

0.20 Fraction

Fraction

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0.15 0.10 0.05

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Size (nm)

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Figure 1. TEM images of (a) single-layer ZrP nanosheets and (b) single-layer MMT nanosheets; lateral size distribution of (c) ZrP and (d) MMT single-layer nanosheets; SEM images of the cross section of (e) restacked ZrP single-layer nanosheets and (f) MMT single-layer nanosheets.

The co-assembly of single-layer nanosheets with BMIMCl was achieved by the ion-ion electrostatic interactions between the nanosheets and BMIM+, and the entire process is briefly illustrated in Figure 2 using ZrP nanosheets as an example. First, the pristine ZrP micro-crystals were exfoliated into single-layer nanosheets. Then the nanosheets were acid treated to recover the acidic hydroxyl groups. The treated ZrP single-layer nanosheets were subsequently mixed with BMIMCl under ultrasonication. The final homogeneous mixture was cast on a glass slide and dried in an oven to form ZrP/BMIMCl intercalated hybrids. MMT/BMIMCl intercalated hybrids were similarly prepared, except that MMT was exfoliated in water without any subsequent treatment. 8



TBA ZrP TBA Exfoliation OH OH

OH

OH OH OH

OH

OH

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BMIM+

Assembly

Intercalated hybrids

Figure 2. Schematic of the preparation of ZrP/BMIMCl intercalated hybrid thin films.

The thermal stability of pristine ZrP, MMT, BMIMCl, and the co-assembled hybrids were investigated by TGA, as shown in Figures 3 and 4. Two major weight losses in the ranges of 100170 and 420-600 °C were observed from the TGA thermogram of the pristine ZrP (Figure 3), corresponding to the loss of hydration water and loss of condensation water, respectively.9 MMT went through one major degradation process at around 600 °C (Figure 4), which is due to the loss of the condensation water. The ZrP/BMIMCl hybrids experienced two main weight losses in the range of 210-320 °C and 320-430 °C. The first step of weight loss agreed well with the decomposition of BMIMCl, but it was slightly delayed. This step of weight loss was associated with the decomposition of the BMIMCl packed between ZrP nanosheets but without direct interaction with the nanosheets. The second step of weight loss could be attributed to the BMIM+ cations interacting with the surface hydroxyl groups on ZrP nanosheets. Overall, the increasing amount of BMIMCl in the hybrids was consistent with the formulation. The MMT/BMIMCl intercalated hybrids presented a similar degradation process, but there was a third step of the 9


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decomposition of condensation water.52 100

(a) ZrP

Weight Remaining (%)

80

60

(c) ZrP-BMIMCl-1-0.5

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(d) ZrP-BMIMCl-1-1.0 20

(e) ZrP-BMIMCl-1-2.0 (b) BMIMCl

0 200

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800

Temperature (゜C)

Figure 3. TGA thermograms of the pristine ZrP, BMIMCl, and co-assembled ZrP/BMIMCl intercalated hybrids. 100

(a) MMT

80

Weight Remaining (%)

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(b) BMIMCl

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Temperature (゜C)

Figure 4. TGA thermograms of the pristine MMT, BMIMCl, and co-assembled MMT/BMIMCl intercalated hybrids. 10



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The XRD patterns of the co-assembled ZrP/BMIMCl intercalated hybrids are presented in Figure 5. The ZrP micro-crystals (Figure 5(a)) displayed a high degree of crystallinity with an interlayer distance of 7.6 Å.27-28 The ZrP micro-crystals were first exfoliated by TBA to form single-layer nanosheets, and the nanosheets were subsequently treated by HCl to regenerate the surface hydroxyl groups. The protonated ZrP single-layer nanosheets tended to form a gel due to the strong hydrogen bonding between the sheets and water molecules.27 After drying, the nanosheets restacked. The XRD pattern of the restacked ZrP nanosheets (Figure 5(b)) exhibited a new broad peak centered at ca. 10.2°, while the original peak disappeared; this suggests that a new layered structure with an interlayer distance of ca. 8.7 Å formed, but the new layered structure was not as ordered as the original ZrP layered crystals. These could also be observed from its SEM image presented in Figure 1(e). The XRD patterns of the co-assembled ZrP/BMIMCl hybrids with various ratios are presented in Figure 5(c)-(e). The basal diffraction peak of each hybrid shifted to a lower value compared to the protonated ZrP, indicating the intercalation of BMIM+ cations into the layers. The interlayer distance increased and the layered structure became less ordered with an increasing amount of BMIMCl. When the CEC ratio of BMIMCl to ZrP was increased to 2.0:1, the system restacked to a loose layered structure with an interlayer distance of 17.7 Å. The lowered order of stacking is believed to be due to the excessive BMIMCl adsorbed on the surfaces of individual ZrP nanosheets, which prohibited an ordered assembly of ZrP nanosheets.

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7.6 Å (a) ZrP 8.7Å (b) ZrP-protonated

Intensity (a.u.)

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13.5 Å (c) ZrP-BMIMCl-1-0.5 16.4 Å (d) ZrP-BMIMCl-1-1.0 17.7Å

2

4

6

8

(e) ZrP-BMIMCl-1-2.0 10 12 14 16 18 20 22 24 26 28 30 32 34

2degrees

Figure 5. XRD patterns of the co-assembled ZrP/BMIMCl intercalated hybrids.

Figure 6 presents the XRD patterns of MMT and the assembled MMT/BMIMCl intercalated hybrids. The neat MMT possesses an interlayer distance of 11.7 Å. After the MMT was exfoliated in water and restacked, the interlayer distance was increased to 12.1 Å and the restacked MMT nanosheets remained a highly ordered structure as evidenced by its narrow diffraction peak. This is consistent with the SEM characterization presented in Figure 1(f). The increased interlayer distance is due to the embedded water molecules, which were hydrogen bonded with the edge/surface hydroxyl groups on MMT.53 After BMIMCl was assembled with the exfoliated MMT nanosheets, the resultant intercalated hybrids displayed a similar well-ordered layered structure as MMT, and the interlayer distance only slightly increased. The more BMIMCl in the hybrid, the larger the interlayer distance. It is believed that MMT/BMIMCl intercalated hybrids exhibited a more ordered layered structure than ZrP/BMIMCl because there was a lower amount of BMIMCl within MMT layers due to the significantly lower CEC of MMT than that of ZrP. The excessive intercalants inevitably disrupted the layered structure, as shown in Figure 5 (especially in ZrP/BMIMCl-1-2.0). A more ordered intercalated structure should be beneficial for 12



the capture of CO2 as BMIMCl is more uniformly distributed, which will be discussed in detail below. 11.7 Å (a) MMT powder 12.1Å

Intensity (a.u.)

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(b) Restacked MMT nanosheets

12.7 Å (c) MMT-BMIMCl-1-0.5 13.0 Å (d) MMT-BMIMCl-1-1.0 13.3 Å (e) MMT-BMIMCl-1-2.0 5

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15

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35

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2degrees

Figure 6. XRD patterns of the co-assembled MMT/BMIMCl intercalated hybrids.

The intercalated hybrids prepared via the co-assembly of BMIMCl with ZrP (or MMT) single-layer nanosheets were evaluated for CO2 capture. In order to investigate the sorption state of BMIMCl with CO2, CO2 was bubbled at a flow rate of 100 mL/min through BMIMCl in a glass bubbler at 30 °C and 50 °C for 30 min right before FTIR characterization. The results are presented in Figure 7. Before the experiment, the BMIMCl was carefully dried at 120 °C, and cooled down to the desired temperatures. Compared with that of the pure BMIMCl, new peaks appeared on the FTIR spectra of the BMIMCl sample bubbled with CO2. For the adsorption reaction at 30 °C, a new peak showed up at 2347 cm-1, corresponding to free CO2.54 This new peak indicates a physical adsorption of CO2 by BMIMCl at lower temperatures. When the experimental temperature was raised to 50 °C, the peak of the free CO2 disappeared. Instead, a new peak located at 1704 cm-1 belong to carboxylate55-56 emerged; this suggests the chemisorption of CO2 by BMIMCl, which was mainly attributed to the interaction between the absorbed CO2 and the imidazolium groups of 13


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the BMIMCl.55, 57

(a) BMIMCl (b) BMIMCl+CO2-30 oC

Transmittance

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(c) BMIMCl+CO2-50 oC

N+

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+CO2

N+

N

R

Imidazolium group O-

O

4000

3500

3000

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Wavenumbers (cm-1)

Figure 7. FTIR spectra of (a) BMIMCl, and BMIMCl bubbled with CO2 at (b) 30 °C and (c) 50 °C. The insert scheme shows the interaction between CO2 and an imidazolium group.

Figure 8 shows the instant CO2 capture amounts by BMIMCl and the co-assembled intercalated hybrids as a function of temperature. To be noted, the instant CO2 capture amount is the sum of the absorption and desorption, which is different from the equilibrium CO2 capacity at a constant temperature. The BMIMCl sample showed a virtually constant CO2 capture amount of ca. 0.03 mmol/g as the temperature increased. The instant CO2 capture amount of both the MMT/BMIMCl and ZrP/BMIMCl intercalated hybrids went through two peaks with an increasing temperature, indicating the existence of two different CO2 absorption mechanisms in the samples. From the FTIR characterization presented in Figure 7, the CO2 was physisorbed by the BMIMCl at 30 °C, and the CO2 was totally chemisorbed when the temperature increased up to 50 °C. The physical and chemical sorption should coexist between 30 and 50 °C. Therefore, the first peak at ca. 40 °C is assigned to the coexistence of the physical and chemical sorption of CO2, and the second peak of the hybrids (ca. 60 °C for ZrP/BMIMCl and ca. 70 °C for MMT/BMIMCl) is 14



attributed to the chemical absorption of CO2. Increasing temperature in the low temperature regime is kinetically favorable for CO2 diffusion in the ILs due to its lower viscosity, but not favorable for physisorption. The low rate of mass transfer at low temperatures is the reason of a lower instant CO2 capture amount. Moreover, the second peak showed the highest instant CO2 capture amount, making it more ideal for CO2 capture. Compared with the ZrP/BMIMCl hybrid, the MMT/BMIMCl intercalated hybrid presented a broader absorption curve up to higher temperatures. This might be owing to the fact that the BMIMCl was better protected in the MMT/BMIMCl hybrids due to the larger lateral dimensional of MMT (Figure 1) and the lower interlayer distance (Figures 5 and 6), which slows down the desorption of CO2.

1.2

CO2 Capture amount (mmol/g)

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1.0

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0.4

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(a) BMIMCl 0.0 30

40

50

60

70

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90

100

110

120

Temperature (゜C)

Figure 8. Instant CO2 capture amount of (a) BMIMCl, (b) ZrP-BMIMCl-1-2.0, and (c) MMTBMIMCl-1-2.0 as a function of temperature.

The CO2 capture capacity of BMIMCl, the two host materials (ZrP and MMT), and the coassembled intercalated hybrids during a 180 min exposure period under a flow of CO2 at a rate of 100 mL/min at 30 ºC are displayed in Figure 9. The equilibrium CO2 capture amounts are summarized in Table 1. From Figure 9, one can observe that ZrP can barely capture CO2, which is 15


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expected as the surface P-OH groups on ZrP are acidic. The CO2 capture capacity of MMT is rather limited, which is consistent with the data in the literature.58 BMIMCl also exhibited a very low CO2 capture capacity and a slow equilibrium, which is believed to be due to its severe agglomeration during its solid state and, thus, limited surface area.6 After BMIMCl was assembled with MMT or ZrP nanosheets, the formed hybrids exhibited a significantly enhanced CO2 capture capacity. Since neither ZrP nor MMT possesses a high CO2 capture capacity, the results suggest that CO2 was mainly captured by the BMIMCl in the layered hybrids. The equilibrium CO2 capture amount per mole of BMIMCl for free BMIMCl, ZrP-BMIMCl-1-2.0, and MMT-BMIMCl-1-2.0 are 12.93, 77.60, and 282.76 mmol, respectively (Table 1). Formation of intercalated hybrids could enhance the CO2 capture capacity of BMIMCl by up to ca. 21 times compared to the pure BMIMCl. The low CO2 capture capacity of the pure BMIMCl is mainly owing to the slow diffusion of CO2 inside BMIMCl.4 Pure BMIMCl has a high viscosity and agglomerates severely. Thus, CO2 was only captured on the surface layer. The remarkably enhanced CO2 capture capacity and the fast equilibrium of the assembled hybrids should be owing to the high accessibility of the BMIMCl in the assembled hybrids. To be noted, the MMT/BMIMCl hybrid exhibited a higher equilibrium CO2 capture amount and a faster equilibrium than ZrP/BMIMCl hybrid, although the ZrP/BMIMCl hybrid contained a higher concentration of BMIMCl. This is likely because of two reasons: (1) the surface acidic hydroxyl groups of ZrP have a strong interaction with BMIMCl, preventing its interaction with CO2; (2) a larger amount of BMIMCl was trapped within the ZrP layers because of the higher CEC of ZrP, but the overall efficiency is lower because of the thicker layer of BMIMCl (similar to that of neat BMIMCl). In brief, the results showed that supporting BMIMCl on nanosheets could effectively improve its CO2 capture capacity.

16



0.5

CO2 capture amount (mmol/g )

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(e) MMT-BMIMCl-1-2.0

0.4

(d) ZrP-BMIMCl-1-2.0 0.3

0.2

0.1

(a) BMIMCl (c) MMT

(b) ZrP

0.0 0

30

60

90

120

150

180

Time (min)

Figure 9. CO2 capture amount of BMIMCl, host materials, and the co-assembled layered hybrids as a function of time at 30 °C.

Table 1. Equilibrium CO2 capture amount of various samples at 30 °C. Sample BMIMCl ZrP MMT ZrP-BMIMCl-1-2.0 MMT-BMIMCl-1-2.0

Mass of layered host/gram sample (g) 0 1.00 1.00 0.30 0.76

Mass of BMIMCl/gram sample (g)

Equilibrium CO2 capture amount per gram sample (mmol)

1.00 0 0 0.70 0.24

0.07 0.01 0.06 0.31 0.40

Equilibrium CO2 capture amount per mol BMIMCl (mmol) 12.93 77.60 282.76

The CO2 capture behavior of the ZrP/BMIMCl and MMT/BMIMCl hybrids under a flow of CO2 (100 mL/min) at various isothermal temperatures are displayed in Figures 10 and 11, respectively. For the ZrP/BMIMCl hybrid, the CO2 absorption equilibrium capacity generally first increased and then decreased with an increasing isothermal temperature, except at 50 °C. The highest CO2 capacity is 0.73 mmol/g at 60 °C. On the opposite, the CO2 absorption equilibrium capacity of MMT/BMIMCl hybrid first decreased and then increased with an increasing isothermal 17


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temperature, and the highest CO2 capacity reached 0.42 mmol/g at 70 °C. Both hybrids exhibited a quicker CO2 absorption equilibrium with an increasing isothermal temperature, which is owing to a quicker diffusion of CO2 in the samples at higher temperatures. The most ideal isothermal temperatures are consistent with the instant CO2 capture test results for the two hybrids as shown in Figure 8. The valley between the first and the second peak at ca. 50 °C should be attributed to the significant desorption of the physically absorbed CO2 and the start of the chemical absorption. At higher isothermal temperatures, the CO2 absorption was mainly owing to chemical bonding, thus a higher concentration of BMIMCl in the hybrid will result in a higher equilibrium CO2 capacity. Because of the higher CEC of ZrP (664.0 meq/100 g) compared with MMT (92.6 meq/100g), there is more BMIMCl in the same amount of ZrPBMIMCl-1-2.0 than in MMT-BMIMCl-1-2.0 (as shown in Figures 3 and 4, as well as in Table 1).This should be the reason that ZrP-BMIMCl-1-2.0 showed a higher CO2 capture capacity than MMT-BMIMCl-1-2.0 at their respective ideal isothermal temperatures. At isothermal temperatures below 50 ºC, the physical absorption dominated, and the absorbed amount was first increased and then decreased with an increasing temperature. There are two main factors that affect the CO2 capture capacity below 50 ºC in the physical absorption range: (1) the inherent CO2 capture capacity of each component in the intercalated hybrids, and (2) temperature. According to Figure 8, instant physical absorption increased with temperature up to 40 ºC, then started to decrease. In general, a low temperature would be favored for physical absorption as the physically adsorbed CO2 would be desorbed more significantly at higher temperatures. The viscosity of BMIMCl became lower with an increasing isothermal temperature,59 leading to less inhibition of CO2 diffusion; this is the reason why there was an initial increase in CO2 capture capacity. But with further increase in temperature, the improved diffusion could not compensate for the resulting desorption of CO2, so the CO2 capture capacity decreased. As shown in Figure 9, the physical adsorption capacity of MMT and BMIMCl are comparable and much higher than that of ZrP. This should be the reason that ZrP-BMIMCl-1-2.0 exhibited a lower CO2 capture capacity than MMTBMIMCl-1-2.0 at an isothermal temperature of 30 °C. However, with a larger amount of BMIMCl 18



in the intercalated hybrids, the inhibition for CO2 diffusion will be more significant due to the viscosity and aggregation of BMIMCl; this should be the other reason that ZrP-BMIMCl-1-2.0 exhibited poor CO2 capture capacity at an isothermal temperature of 30 °C. All the information explains why the CO2 capture capacity for ZrP-BMIMCl-1-2.0 first increased and then decreased with an increasing isothermal temperature below 50 °C. However, the viscosity effect is very limited for MMT-BMIMCl-1-2.0 because of its much lower BMIMCl concentration (24 wt% BMIMCl in MMT-BMIMCl-1-2.0 vs. 70 wt% BMIMCl in ZrP-BMIMCl-1-2.0, see Table 1), and thus its CO2 capture capacity continuously decreased with an increasing temperature below 50 °C. In addition to the IL loading and the viscosity, a more complex process involving the interactions of the BMIMCl with the inorganic nanosheets should also have some effects on the CO2 capture capacity. As the isothermal temperature continuously increased, chemical absorption became a major function for CO2 capture. The initially increased amount of CO2 capture was attributed to the larger amount of chemically bonded CO2, but further increasing temperatures eventually led to desorption.

0.8

CO2 capture amount (mmol/g )

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(A)

o

(c) 60 C

0.7 0.6 0.5 o

0.4

(b) 40 C

0.3

(a) 30 C o (d) 50 C

0.2

(e) 70 C

0.1

(f) 80 C

o

o

o

0.0 0

20

40

60

80

100

120

140

Time (min)

19


160

180

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0.8

(B)

CO2 capture amount (mmol/g)

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 30

40

50

60

Temperature

70

80

(oC)

Figure 10. (A) Kinetic data of CO2 capture of ZrP-BMIMCl-1-2.0 at various isothermal temperatures; (B) CO2 capture amount of ZrP-BMIMCl-1-2.0 after equilibrium for 150 min at various temperatures.

0.5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(A) (e) 70 oC

0.4

(a) 30 oC

(f) 80 oC

(d) 60 oC

0.3

(b) 40 oC

0.2

(c) 50 oC

0.1

0.0 0

50

100

Time (min)

20


150


0.45

(B)

0.40


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0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 30

40

50

60

70

80

Temperature (oC)

Figure 11. (A) Kinetic data of CO2 capture of MMT-BMIMCl-1-2.0 at various isothermal temperatures; (B) CO2 capture amount of MMT-BMIMCl-1-2.0 after equilibrium for 150 min at various temperatures.

While this research only focuses on CO2 capture, it should be noted that many ILs (including BMIMCl) can serve as a catalyst for various reactions, including the coupling of CO2 with propylene oxide,9 methanol,60 and hydrogen.61 Based on the above results, it is envisioned that the co-assembled BMIMCl with nanosheets would exhibit excellent catalytic activities in the conversion of CO2 into various chemical products, such as carbonates,9 methanol,62 syngas,63 and many other fine/pharmaceutical chemicals.64

4. Conclusions The co-assembly of BMIMCl with ZrP (or MMT) nanosheets via a facile approach was successfully achieved. The co-assembled layered hybrids could serve as highly efficient CO2 absorbents. The CO2 capture capacity of BMIMCl was made up to 21 times more efficient, which is due to its highly accessible functional sites in the intercalated hybrids. CO2 was physically 21


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absorbed on the hybrids with a slow equilibrium at lower temperatures, while higher temperatures allowed for faster diffusion and chemical absorption of CO2. The highest CO2 capture capacity achieved was 0.73 mmol/g at 60 °C for ZrP/BMIMCl and 0.42 mmol/g at 70 °C for MMT/BMIMCl. It is believed that many other ILs could be similarly immobilized within inorganic nanosheets via this facile co-assembly process for CO2 capture and other applications.

Acknowledgement This research is sponsored by the National Science Foundation (DMR-1205670) and the Key Laboratory for Ultrafine Materials of the Ministry of Education at East China University of Science and Technology. Y. Z. thanks the support by the startup foundation for introducing the talent of Nanjing University of Information Science & Technology (NUIST) (2241131301102).

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