Facilitated transport of CO2 through the transparent and flexible

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Facilitated transport of CO2 through the transparent and flexible cellulose membrane promoted by fixed-site carrier Xiong-Fei Zhang, Ting Hou, Jin Chen, Yi Feng, Bengang Li, Xiaoli Gu, He Ming, and Jianfeng Yao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07309 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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ACS Applied Materials & Interfaces

Facilitated transport of CO2 through the transparent and flexible cellulose membrane promoted by fixed-site carrier Xiong-Fei Zhanga, Ting Houb, Jin Chenb, Yi Fenga, Bengang Lib, Xiaoli Gua, Ming Heb,*, Jianfeng Yaoa,* a

College of Chemical Engineering, Jiangsu Key Lab for the Chemistry & Utilization

of Agr-Forest Biomass, Nanjing Forestry University, Nanjing 210037, China. Email: [email protected] b

College of Science, Nanjing Forestry University, Nanjing 210037, China. Email:

[email protected]

ABSTRACT Facilitated transport cellulose membranes with different zinc ion loadings are fabricated via a facile and green solvent system (zinc chloride/calcium chloride solution). Zn2+ ions lower the pristine hydrogen bonds that normally reinforce the cellulose chains, and Ca2+ ions facilitate interactions among the Zn-cellulose chains to form nanofibrils. The strategy provides an effective route to immobilize zinc species into membrane matrix and constructs facilitated transport pathway for CO2 molecules. The self-standing membranes are transparent, flexible and demonstrate ultra-selective CO2 permeation. The optimum separation performance is achieved over CM-0 with the highest zinc content (22.2 %) and it exhibits a CO2 permeability of 155.0 Barrer, with selectivity ratios of 27.2 (CO2/N2) and 100.6 (CO2/O2). The excellent separation performance is assigned to the π complexation mechanism between Zn2+ and CO2.

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Keywords: Cellulose membrane; Gas-separation; Zinc ions; π complexation; fixed-site carrier; hydrogen bonds.


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INTRODUCTION As a sustainable and environmental friendly material, cellulose forms a potential substitute for the petroleum-derived materials.1-3 Cellulose is a linear polysaccharide with β-1-4-linked D-glucopyranosyl as basic units. Such sturdy structures endow a compact network stabilized through powerful intra- and inter-chain hydrogen bonds.4 Because of its high extent of polymerization and crystalline index, cellulose is extremely difficult to dissolve in traditional solvents, which dramatically restricting commercial applications. The development of cellulose dissolution systems constitutes an active research field.

5-7

Several solvent systems have been developed,

such as tetramethylpiperidone oxide (TEMPO), N-methylmorpholine-N-oxide hydrate (NMMO), ion liquids, phosphoric acid and NaOH/urea.8-9 Problems associated with toxicity, limited solvency, difficult solvent recovery and high production costs are unfavorable to practical applications. 10

Janaswamy et al. embarked on a detailed research of solubilizing cellulose by inorganic molten salts and proved that zinc chloride/calcium chloride solution with specific ratios was effective to solubilize cellulose.10 Inorganic salt hydrates have been receiving particular interests due to the ease of recyclability. In this system, the zinc ions could break and weaken hydrogen bonds and the presence of Ca2+ ions cross-link the Zn-cellulose chains. The membranes prepared with this strategy exhibit excellent optical and mechanical properties. Cellulose membranes are promising alternatives to polymeric membranes such as Pebax, polyimide and polymers of intrinsic microporosity (PIMs). In particular, the zinc ions could be incorporated into 3 ACS Paragon Plus Environment


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the membrane matrix controllably. Sen et al. proposed that zinc chloride hydrate in the form of [Zn(OH2)6][ZnCl4] could be described as ionic liquid and its cellulose dissolution derived from strong hydrogen-bond-donating capacity. 11

More recently, metal ions-based facilitated transport membranes shows good prospects in separation applications.

12-15

Facilitated transport membranes have been

used for diverse gas separation processes: 1) CO2 removal and capture; 2) O2 enrichment (O2/N2 separation) and 3) olefin/paraffin separation.13,

16-17

Enhanced

selectivity and permeance are obtained because the transport of the specific gas is accelerated through the reversible chemical process. Various metal species have been investigated as facilitated transport carriers, such as Hg2+, Ag+, Cd2+, Co2+, K+ and Zn2+.16,

18-19

Prior studies verified that positively polarized zinc ions displays a

significant effect on facilitating CO2 passage because of moderate interactions between them to ensure the bind-release balance. Chung and co-workers immobilized zinc ions into polyimide to fabricate composite membrane.15,

20-22

The CO2/CH4

selectivity of Zn2+ embedded membrane was 70% greater than that of metal-free membranes. Jiang and co-workers fabricated facilitated transport membrane by embedding zinc ions into poly(ether-block-amide) matrix for CO2/CH4 separation.12 The composite membrane exhibited the highest CO2 permeability of 137.9 Barrer. However, to disperse and embed metal carriers with high and controllable loadings into membrane matrix is still a big problem. Herein, we found that self-standing cellulose membranes prepared from a green zinc chloride/calcium chloride system exhibit ultra-selective CO2 permeation and a reversible π complexation mechanism is 4 ACS Paragon Plus Environment

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proposed to explain the facilitated transport behavior for CO2 molecules. This strategy provides opportunities to address the issues about carriers (zinc ions) stability.

EXPERIMENTAL SECTION Materials. α-cellulose (in short α-C, 0.05 mol of anhydrous glucose unit) was purchased from Aladdin Chemical Reagent Company. Zinc chloride (ZnCl2, 99.0%) and calcium chloride (CaCl2, 99.0%) were sponsored by Sinopharm Chemical Reagent Company. N2, CO2 and O2 with purity higher than 99.9% were purchased from Nanjing Special Gases Company.

Preparation of the self-standing cellulose membranes. Initially, α-C (1.05 g) and DI water (3.63 g) were mixed into a 25 mL glass beaker to obtain a cellulose paste. In a separate flask, calculated amounts of ZnCl2 (9.87 g) and CaCl2 (0.25 g) were dissolved in 3.38 g DI water and then equilibrated at 65 ◦C in an oil bath for 30 min. The obtained ZnCl2/CaCl2 solution was mixed with the cellulose paste thoroughly for another 30 min. The bubbles were removed from the Ca-Zn-cellulose solution via a vacuum drying oven (DZF-6050). The membranes were casted by a casting knife (400 µm) on a glass plate. Afterwards, the glass plate and the membrane were soaked in 500 mL ethanol. The membrane was coagulated for 30 min until divorcing completely from the glass plate, then immobilized on a metallic frame and dried at 25 ◦

C. The acquired membranes were immersed in a 500 mL water bath for different

times (0, 10, 20, 30 and 240 min) to partially or completely remove Zn2+ ions. Finally, the membranes were dried for further characterization and denoted as CM-x (x stands



for the water immersion time). The films were shaped into rectangular strips (8x5 cm2), and stored at 25 ◦C and 30% relative humidity (RH) for further use. Characterization. X-ray diffraction (XRD, Rigaku Smartlab) experiments were performed to investigate the crystal patterns. The chemical structures of the samples were explored by a Fourier transform infrared spectra spectrophotometer (FT-IR, Thermo Electron Nicolet-360, USA). Scanning electron microscopy (SEM, JSM-7600F, JEOL, Japan) and Transmission electron microscopy (TEM, JEM-1400, JEOL, Japan) were used to explore the morphologies of the membranes. Atomic force microscopy (AFM) was recorded to reveal the surface morphology of the membranes by utilizing Dimension Edge (Bruker, Germany). The samples (0.1 wt%) were coated on a mica substrate and dried for 12 h. The thermo-gravimetric (TG) data were obtained via a Pyris 6 thermo-gravimetric analyzer (PerkinElmer, USA) in N2 atmosphere. Tensile tests were conducted by using a tensile tester (Shimadzu EZ-TEST, Japan). The membrane thickness (µm) was obtained from a micrometer and the experimental errors of membrane thickness were within ±3%. The solution viscosity was determined by a viscometer at 65 ◦C (Cannon-Fenske Routine Instrument , USA).

Gas permeation tests. The gas-separation behavior of the cellulose membranes was evaluated from the permeances of O2, N2 and CO2, using a standard apparatus with a flat-sheet cell (Figure S1) at room temperature (25 ºC) and 2 bar. 23-24 The tests were carried out under the dry condition. Before tests, the gas existed in the pipeline was exhausted by vacuum pump. The permeability, Pi (Barrer) was calculated as Eq. 1.


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Pi=Ni l/(A ∆pi)

Eq.1

Where Ni (cm3•s-1), l (cm), A (cm2) and ∆pi (cmHg) corresponding to the permeate flow speed of gas i, the membrane thickness, the membrane area, and the pressure difference, respectively.

The ideal selectivity is obtained from Eq. 2. The mean value is got from five measurements. Sij=Pi/Pj

Eq. 2

Where (Sij), Pi and Pj corresponding to the ideal selectivity, the permeability of single gases of i and j, respectively.

RESULTS AND DISCUSSIONS Characterization of the Ca-Zn-cellulose solution. To determine the final sizes of cellulose nanofibrils in the Ca-Zn-cellulose system (3% cellulose, 68% ZnCl2, 2% CaCl2 and 27% water), TEM and AFM images were recorded. As illustrated in Figure 1, individual nanofibrils are clearly visible without significant aggregation, which have mostly uniform number-average widths of 5-8 nm and lengths of about 500 nm (calculated from 50 fibrils). The relative high aspect ratio of the CNFs (nearly 100) is promising to enhance nano-reinforcing effects for membranes. Detailed experiments revealed that 68% ZnCl2 solution affords the best solubilization capacity, which is in line with the literature.10 Lower concentrations of ZnCl2 cannot fully dissolve the cellulose and result sedimented particles. While higher concentrations led to a thick opaque paste. Scheme 1 depicts the interactions between a pair of cellulose chains (a



nonpolar polymer with extensive intra- and intermolecular hydrogen bonding).

The difficulty for dissolving cellulose is because of the prevailing hydrogen bonds in cellulose chains.25 Upon Zn2+ ions addition (Stage II), the divalent Zn2+ ions permeate through the cellulose nanosheets and break the pristine hydrogen bonds. Zn2+ ions compete with the protons for the hydroxyl groups. 11 The intrinsic hydrogen bonds are weakened gradually, leading to cellulose solubility. Sen et al. suggested that ZnCl2 hydrate is a kind of ionic liquid with the structure of [Zn(OH2)6][ZnCl4]. 11 The hydrate has a relative high hydrogen bond donating ability and is promising for dissolving cellulose.

Figure 1. TEM image (a) and AFM image (b) of cellulose nanofibrils in Ca-Zn-cellulose solution (3% cellulose, 68% ZnCl2, 2% CaCl2).

The addition of suitable amount of Ca2+ (molar ratio, Ca:Zn =0.03:1) increased the viscosity of ZnCl2 solution from 1500 cps to 3200 cps and yielded transparent and homogeneous gelling solution (Figure 2a). In the presence of Ca2+ ions, the Zn2+ ions and cellulose chains show different behaviors. It is proposed that Ca2+ ions cross-link adjacent Zn-cellulose junction zones and lead to the formation of nanofibrils (Scheme 8 ACS Paragon Plus Environment

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1).

26

In summary, Zn2+ plays the role of solubilizer and Ca2+ acted as gelling agent

in the Ca-Zn-cellulose system. Membrane characterizations. Figure 2a displays the state of Ca-Zn-cellulose gelling solution with a viscosity of 3200 cps. This homogeneous solution is convenient for membrane casting. The light transmittance spectrum and digital photograph of CM-0 are shown in Figure 2b. The light transmittances of all membranes are higher than 89% (600 nm). 27-28 The diameters of cellulose nanofibrils are smaller than the light wavelength, thus restricting light scattering.29 The high transparency indicates good dispersibility of both cellulose nanofibrils and metal ions in the membrane matrix. In addition, the optical observation of the CM-0 membrane demonstrates smooth and glossy appearance.



Scheme 1. Schematic illustration of the interactions among Ca-Zn-cellulose system.

SEM images of surface and cross-section of CM-0 membrane are displayed in Figure 2c and Figure 2d. CM-0 exhibits a flat and homogeneous surface without obvious agglomeration and cracks, demonstrating that the Zn species were highly dispersed. The smooth and compact surface could be further verified by AFM observations (Figure S2), which illustrates an uniformly surface heights and in good


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agreement with TEM measurements. The cross-section displays a fairly regular lamellar structure with a thickness of ca. 41 µm. This is due to the nanofibrils are oriented parallel to the membrane surface and firmly cross-linked with each other during the drying process.

Figure 2. (a) Photograph of the dispersion state of cellulose suspension in the Ca-Zn-cellulose system; (b) Light transmittance spectra of the self-standing CM-0 membrane (the inset photographs show the optical and bendable behaviors of CM-0); (c) SEM surface image of CM-0 membrane and (d) fracture surface of CM-0 membrane.

To further reveal the properties of as-prepared membranes, XRD analysis is carried out to compare the crystallinity changes (Figure 3a). The crystal structure of the α-C was exactly in line with the literature, preserving the pristine structure of native wood



cellulose.

28, 30-31

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The distinct peaks centered at 15.2o and 23.1o are ascribed to the

(101) and (002) planes of a cellulose I type crystal structure. However, the XRD patterns of the membranes indicate obvious diffraction peaks of cellulose II as a dominant component with representative peaks at approximately 12.0°, 20.2° and 22.5°.

28, 32

The cellulose II type crystallites with longitudinal direction is in-plane

oriented, providing evidence for the well-stacked layers observed in the SEM cross-section. The crystalline transformation is ascribed to the Zn ions enter the crystalline regions of the cellulose and break the pristine hydrogen bonding network. No peaks associated with Zn species are detected, verifying that the uniform dispersion of Zn element in the membrane and good compatibility between cellulose matrix and metal ions. CM-30 exerted the highest crystallinity compared with other membranes, which might be assigned to the strongest interactions of the hydroxyls. The interaction between Zn2+ ions and the hydroxyl groups of cellulose was verified by FT-IR spectroscopy. As displayed in Figure 3b, α-C has characteristic absorption bands at 3324, 1317 and 897 cm-1, corresponding to the O-H stretching, H-C-H wagging vibration, and β-glycosidic linkage, respectively.

33-35

The band

centered at 1047 cm-1 in free-standing cellulose membranes is assigned to the C-O bond valence vibration. The distinct shrinkage of peak at 3324 cm-1 in membranes is attributed to the notable loss of inter and intra hydrogen bonds cause by metal addition.

EDX analysis was recorded to investigate the membrane composition (Table 1) and the metal contents decrease following the order of CM-0 > CM-10 > CM-20 > CM-30. 12 ACS Paragon Plus Environment

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The Zn contents decrease gradually with the prolonged treatment in water bath. The Zn content in CM-20 (3.17%) is close to CM-30 (2.92%), implying that the Zn element almost remains stable after immersion in water for 20 min. To provide more evidence, the metal contents are further explored by TG experiments (Figure 3c). The weight loss before 300 oC is mainly due to the loss of water of hydration from coordinated zinc chloride.

11

While the notable weight loss around 300-350 oC

corresponds to the decomposition of C6 structures. TG curve of α-C possesses the lowest amount of residual ash (7.79%) upon decomposition than other films. The residual components in metal-containing membranes mainly are zinc oxide, hydroxide and chloride and the residual weight is in good agreement with EDX results. Table 1. Element contents in different membranes calculated from EDX analyis. Sample Zn (%) Ca (%) Cl (%) C (%) O (%) CM-0

22.23

2.38

17.46

24.08

33.85

CM-10

13.51

1.69

7.02

30.07

47.71

CM-20

3.17

0.21

1.84

33.46

61.32

CM-30

2.92

0.15

1.56

33.72

61.65

As shown in Figure 3d, all membranes display high Young’s moduli, high tensile strength (23.65-33.21 MPa), and strain-to-failure values of 6.53-13.25%. The introduction of Ca2+ ions facilitates the nanofibrils formation, which is responsible for the higher tensile strength of films. The membrane mechanical property is highly correlated with the immersion time in water. The CM-0 membrane without soaking in water had the lowest tensile strength (23.65 MPa) and highest strain-to-failure



(13.25%). 36 The mechanical property is associated with the Zn content in the films. A higher Zn content in CM-0 destroys the hydrogen bond network and thus is detrimental to the membrane strength. 37

Figure 3. (a) XRD patterns, (b) FT-IR spectra and (c) TG curves (N2 atmosphere) of pristine α-C and as-prepared membranes; (d) Tensile strength and strain-to-failure propertities of the membranes. Gas separation performance. Single gas permeabilities (CO2, N2 and O2) across the membranes with different Zn contents are summarized in Figure 4a. All membranes have the parallel permeabilities of N2 and O2. In other words, the barrier capacities of self-standing cellulose membranes to N2 and O2 have no connection with Zn2+ ions. Cellulose-based materials were reported as excellent candidates for O2-barrier.38-39 In the case of CO2, the permeability is highly dependent on the Zn


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content and nearly one order of magnitude larger than that of N2 and O2. The efficient CO2/O2 separation is important in applications such as food-packing, fuel cells and efficient flue gas cleanup.4,17 The influence of Zn content on the final gas separation performance (permeability and ideal selectivity) was tested. Metal-free cellulose membrane (CM-240) shows a CO2 permeability value of 23.5 Barrer with CO2/N2 and CO2/O2 ideal selectivities of 4.6 and 21.3, respectively. As shown in Figure 4a and Figure 4b, CM-30 has a CO2 permeability of 39.3 Barrer with a CO2/N2 ideal selectivity of 8.0 and a CO2/O2 ideal selectivity of 38.1. By contrast, CM-0 possesses a CO2 permeability of 155.0 Barrer with a corresponding CO2/N2 ideal selectivity of 27.2 and a CO2/O2 ideal selectivity of 100.6. The impacts of pressure differences on performance were explored by increasing the pressure from 2 to 4 bar. The CO2 permeability, CO2/O2 and CO2/N2 selectivity of all membranes increase with feeding pressure (Table S1). Slight improvement in the CO2 permeability with elevating pressure is ascribed to the interaction and affinity between CO2 molecules and zinc ions located on cellulose chains.

24, 40

The cellulose membranes are advantageous for

the diffusion of CO2 and the CO2 selectivity is beyond most of the state-of-the-art of polymer-based membranes (Table S2).41,42



Figure 4. Gas permeation performances (a) and ideal selectivity (b) for different membranes. Facilitated CO2 transport mechanism. Fixed-site carrier membranes have been attracting increasing attention in the field of CO2 separation and many mechanisms have been proposed to the separation process.27,

43-44

Among them, the hopping

mechanism is the dominant one for metal incorporated membrane materials.43 As illustrated in Scheme 2a, a possible process is proposed to explain the facilitated transport of CO2 molecules. The Zn2+ ions bound to cellulose nanofibrils chains and embedded in cellulose membrane play the role of CO2 carriers. They cannot migrate freely and only vibrate within a confined nanospace, which is defined as diverse fixed-site carrier membrane. In particular, CO2 molecules donate the π electrons to the Zn2+ ions’ empty s orbital to form σ-bonds.

12, 45

While the empty π* anti-bonding

orbitals of CO2 molecules receive d electrons from Zn2+ ions and lead to the formation of π-bonds. The Zn-CO2 complex is thus constructed (Scheme 2b). This unstable compound is apt to be resolved to release CO2.

46-47

Hence, the reversible interaction

between the Zn2+ ions and CO2 facilitates the fast transport of CO2. Zn2+ ions are ideal CO2 carriers because of their moderate CO2 binding ability, which combine with CO2 16 ACS Paragon Plus Environment

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quickly and the bonds could be broken readily.

The transfer channels for CO2 are abundant enough in CM-0 to achieve connected network in the membrane. For CM-20 and CM-30 with relatively low metal loadings, the transfer channels are discontinuous. Therefore, the hopping platforms are insufficient and the transport capacity of membrane is severely restricted. Such complexation interactions do not exist between Zn element and N2 and O2. The lower penetration rate of O2 than that of N2 was assigned to the special barrier property of intra-molecular hydrogen bonds to O2. 48

Scheme 2. Mechanism illustration of the transport of CO2: (a) high-speed CO2 transport channels and (b) reversible Zn- CO2 orbit interaction. CONCLUSION In this study, cellulose is dissolved rapidly in CaCl2/ZnCl2 aqueous solution at 65 ◦C, which is a green and simple process. This strategy provides an effective route to immobilize and disperse zinc species into membrane matrix. The as-prepared membranes possessed a homogenous structure, excellent optical transmittance, as well as good tensile strength. Particularly, the membranes exhibit substantially CO2 separation performance. The optimum separation performance was achieved over 17 ACS Paragon Plus Environment


CM-0 with the highest zinc content, which exhibited a CO2 permeability of 155.0 Barrer (CO2/N2 and CO2/O2 ideal selectivities of 27.2 and 100.6, respectively). The excellent separation performance is assigned to the π complexation mechanism between the zinc-CO2 interaction.

ACKNOWLEDGMENTS We acknowledge the funding from the China Postdoctoral Science Foundation (2017M611824), the National Natural Science Foundation of China (NNSFC: 21774059), the Natural Science Key Project of the Jiangsu Higher Education Institutions (15KJA220001), and Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. X. Zhang and T. Hou contribute equally to this work.

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Facilitated transport of CO2 through the transparent and flexible

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