- Email: [email protected]
Amine-impregnated porous nanofiber membranes for CO2 capture
Composites Communications 10 (2018) 45–51
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Amine-impregnated porous nanofiber membranes for CO2 capture a,c,1
Ghazala Zainab , Aijaz Ahmed Babar ⁎ Jianyong Yuc, Bin Dinga,b,c,
a,c,1
, Nousheen Iqbal
a,c,1
, Xianfeng Wang
a,b,c,⁎
T
,
a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, China c Innovation Center for Textile Science and Technology, Donghua University, Shanghai 200051, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrospinning Nanofiber membrane Amine impregnation CO2 adsorption
Global warming is a worldwide issue, mainly caused by excessive emission of carbon dioxide (CO2), thus, demands the development of robust and flexible sorbents with consider CO2 capturing capacity. Herein, we report an easy and facile two-step fabrication of hierarchically structured and highly porous polystyrene (PS)/polyurethane (PU) electrospun nanofiber membrane impregnated with various amines for post-combustion CO2 capture. Synthesized porous nanofiber membranes have highly flexible and strong mechanical characteristics. The comparative analysis of porous membranes functionalized with three functional amines shows that low molecular weight polyethyleneimine (PEIL) impregnated porous nanofiber membranes offer relatively higher CO2 adsorption (1.64 mmol/g) at 40 °C. In addition, optimized samples exhibit excellent regenerability, (≥ 90% of their original value) even after 19 adsorption-desorption cycles showing a stable and durable CO2 capture performance. This work paves the way for developing electrospun nanofiber membranes capable of efficient CO2 adsorption.
1. Introduction
electrospun nanofibers are gaining increasing attention of scientists in the recent years [7]. Moreover, in order to maximize their CCS capacity and selectivity, electrospun nanofibers can be functionalized with certain additives such as amines. Nowadays, amine functionalized porous materials (such as silica, carbon, nylon etc.) are being extensively examined to explore their potential capabilities for CCS owing to their effective CO2 capturing quality and easy application method [5,6,8]. A variety of various amine groups have been thoroughly examined for CO2 adsorption, however, polyethyleneimine (PEI) has been proved to be the most significant group of amine family because of its high number of functional sites result in outstanding affinity for CO2 [9]. Whereas, very little work has been reported about the use of TEA for CO2 adsorption [10]. These amines can essentially be applied by simply impregnating porous substrate in polymeric amine solutions, in-situ polymerization or grafting them on surface of the porous materials [11]. In the present work, amine functionalized polystyrene/polyurethane (PS/PU) nanofiber composite membrane has been developed. PS is commonly used polymer fiber with very high level of hydrophobicity and easily tailorable porous structure, thus, it has been chosen as primary substrate/support, because high porosity and water
Currently, global warming is the biggest alarming threat to environment and public health. One of the fundamental contributing source toward global warming is greenhouse effect. There are a number of gases, which cause greenhouse effect that leads to the problem of global warming, however, among them CO2 is the leading contributor. Massive CO2 emissions not only lead to global warming but also result in rise of sea level and environmental pollution [1,2]. That is why it is being highly demanded to take immediate efforts to reduce CO2 concentration in the atmosphere and limit further emission of CO2 in the atmosphere [3]. Therefore, new materials, techniques and technologies are being explored for developing effective, economic and scalable CO2 capture and storage (CCS) technologies [4]. Ideal materials for CCS need to have highly porous structure, good mechanical strength and flexibility, sustainable and environment friendly fabrication process, besides that they should have feasibility for easier surface functionalization [5]. In this regard, electrospun polymeric nanofibers successfully qualify most of the demands to be ideal materials for CCS [6]. Owing to their free-standing nature, tailorable physical characteristics, easy and economical fabrication process,
⁎
1
Corresponding authors at: Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, China. E-mail addresses: [email protected] (X. Wang), [email protected] (B. Ding). These author contributed equally to this work
https://doi.org/10.1016/j.coco.2018.06.005 Received 13 November 2017; Received in revised form 2 June 2018; Accepted 9 June 2018 2452-2139/ © 2018 Published by Elsevier Ltd.
Composites Communications 10 (2018) 45–51
G. Zainab et al.
Micromeritics Co., USA). XQ-1C tensile tester (Shanghai New Fiber Instrument Co., Ltd., China) was involved for examining the mechanical characteristics.
repellency are two primary demands for CO2 adsorption. PU is another commonly used elastomeric fiber with excellent mechanical characteristics, and has been employed to overcome poor mechanical strength of PS nanofibers in pristine state. As-synthesized composite membrane was functionalized with various amines by impregnation method, and effect of various amine groups on CO2 adsorption capacity of synthesized membrane was examined.
2.5. CO2 adsorption The CO2 capture performance, cycle performance and CO2/N2 selectivity of amine-impregnated nanofiber composite membranes was investigated by thermogravimetric analyzer (TGA, TA Instruments model Q600, USA). A flow of 200 mL/min for N2 as well as CO2 was maintained during the process. A sample weighing ~10 mg of composite membrane was placed on microbalance alumina sample cell. In order to remove any existing volatile impurities, sample was first exposed to 105 °C in N2 environment. The temperature was then adjusted to 40 °C and pure dry CO2 gas was introduced for 60 min to evaluate the CO2 adsorption performance. Additionally, the CO2 adsorption performance of optimized sample was also observed at 50 °C and 60 °C to examine influence of temperature on CO2 capture performance. CO2 desorption was carried under nitrogen environment for 40 min at 105 °C. CO2 adsorption capacity in mmol/g was calculated during adsorption process. Multiple adsorption/desorption cycles were carried out to evaluate recyclability of the optimized samples. Moreover, in order to investigate the feasibility of capturing CO2 in practical conditions, selectivity of optimized samples against CO2 and N2 was analyzed. Same procedure was repeated for N2 adsorption. The selectivity of CO2/N2 was defined as S= q1/q2, where q1 and q2 are the adsorption capacities of CO2 and N2 in the adsorption process, respectively.
2. Experimental section 2.1. Materials Polystyrene (PS, Mw = 350,000 g mol-1) was supplied from Wako Co., Ltd., Japan, and polyurethene (PU, Mw = 180,000 g mol-1) was procured from BASF Polyurethane Speciliaties Co., Ltd., China. Tetrahydrofuran (THF), dimethylformamide (DMF), ethanol, polyethyleneimine (PEI, Mn = 10,000 and 60,000) and triethylamine (TEA, Mn = 101.19) were obtained from Aladdin Chemical Co, Ltd., China. All chemicals were of analytical grade and used without further purification. 2.2. Solution preparation and membrane fabrication Homogenous solutions of PS (15 wt%) and PU (7 wt%) were prepared by dissolving them into a mixture of DMF/THF (1/1) individually under vigorous magnetic stirring overnight. Prepared solutions were then electrospun using DXES-3 electrospinning machine (SOF Nanotechnology Co., China) at the feeding rate of 1 mL/h under an applied voltage of 30 kV. Nanofiber membrane was collected on a metallic roller revolving at 20 cm distance from the feeding needles. Constant temperature (25 ± 2 °C) and relative humidity (45 ± 3%) were maintained throughout the fabrication process. Resultant nanofiber membranes were dried under vacuum for 4 h to remove any residual solvent.
3. Results and discussions Scheme 1 illustrates the schematic demonstration of amine functionalized PS/PU nanofiber membrane development process. Development of composite nanofiber membrane was carried out in two critical steps: (1) Fabrication of porous PS/PU nanofiber composite membrane with rough surface morphology, (2) functionalization of synthesized nanofiber composite membrane with various amines. Porous PS/PU nanofiber composite membrane was fabricated via electrospinning technique using two solvents having different degree of volatility. Owing to the difference in volatile nature of the two solvents, solvent evaporation process intensified resulting in much faster evaporation of solvent and simultaneous interaction with external non-solvent species such as air and moisture resulted into the formation of porous fiber structure. Moreover, successful surface functionalization of the as-synthesized PS/PU nanofiber composite membrane was carried out in rotary evaporator using impregnation technique. Fig. 1a presents the morphological analysis of electrospun pristine PS/PU nanofiber membrane. It could be seen that fabricated membrane offered randomly oriented 3D fibrous structure. Since two polymers were electrospun side by side, therefore, two different ranges of fibers in the membrane were witnessed. PS fibers were highly porous and relatively thicker in diameter and ranged 2–3 μm. Whereas, PU nanofibers offered smooth and solid cylindrical shaped structure with average fiber diameter in the range of ~800–950 nm in the resultant PS/PU nanofiber membrane (A low magnification SEM image used for measuring fiber diameter has been provided in Fig. S1). This variance in physical structure and fiber diameter could be attributed to the difference in molecular weight of two polymers and use of two solvents with varied degree of volatile nature. Porosity and specific surface area are the two most important characteristic features, which determine the CO2 adsorption capacity of the substrate. Therefore, the hierarchical pore structure of membranes was examined via N2 adsorption-desorption isotherms at 77 K (Fig. 1b). Typical type IV isotherm with an apparent adsorption hysteresis loop as classified by IUPAC was seen [12]. It was observed that a series of typical physical adsorption behaviors including monolayer adsorption, multilayer adsorption and capillary condensation phenomenon
2.3. Impregnation of PS/PU composite membrane Amine solutions were prepared by dissolving PEI and TEA in ethanol individually under vigorous magnetic stirring until transparent solutions were achieved. Composite membranes were then immersed in transparent solutions for 1 h under continuous stirring. Later on, the solution along with membrane was transferred into a rotary evaporator (PV 10 Basic Plus D, Wilmington, NC) and ethanol was evaporated from the solution under vacuum at 60 °C for ~1 h and the resultant membrane was finally dried at 50 °C for 5 h under vacuum. Subsequent impregnated nanofiber membranes were tagged PS/PU-PEIL-Y PS/PUPEIH-Y and PS/PU-TEA-Y, where L and H represent low and high molecular weight of PEI, and Y corresponds to the amine weight ratio relative to the weight of pristine membrane. All the synthesized membranes were impregnated with four different weight ratios of amine polymers, weight ratios of amine to the weight of pristine membrane were 1/4, 1/2, 1/3 and 1/1. Subsequent PEIL, PEIH and TEA impregnated membranes were tagged as PS/PUPEIL-1, PS/PU-PEIL-2, PS/PU-PEIL-3, PS/PU-PEIL-4, PS/PU-PEIH-1, PS/ PU-PEIH-2, PS/PU-PEIH-3, PS/PU-PEIH-4, PS/PU-TEA-1, PS/PU-TEA-2, PS/PU-TEA-3, and PS/PU-TEA-4, respectively. 2.4. Structural characterization Fiber surface morphological analysis of synthesized composite membranes was carried out using scanning electron microscope (SEM, TESCAN VEGA 3, TESCAN Ltd., Czech Republic). Fourier transform infrared (FTIR) spectrometer (Nicolet iS10, USA) was employed to observe the presence of amine groups and CO2 molecules in the functionalized composite nanofiber membranes. Physical structure was examined by using an automatic adsorption system (ASAP 2020, 46
Composites Communications 10 (2018) 45–51
G. Zainab et al.
Scheme 1. Schematic illustration of the fabrication and amine impregnation of PS/PU nanofiber membrane.
Fig. 1. (a) SEM image (b) N2 adsorption/desorption isotherm of PS/PU nanofiber membrane. The inset shows the BJH pore size distribution curve.
Fig. 2. SEM images of PS/PU nanofiber membrane impregnated with (a) PEIL -1 (b) PEIL -2 (c) PEIL -3 and (d) PEIL -4.
47
Composites Communications 10 (2018) 45–51
G. Zainab et al.
Table 1 Textural properties of PS/PU composite membrane impregnated with various weight ratios of PEIL. Sample PS/PU PS/PU PS/PU PS/PU PS/PU a b c
-PEIL-PEIL-PEIL-PEIL-
1 2 3 4
SBETa (m2 g-1)
Vtotalb (cm3 g-1)
Smesoc (cm3 g-1)
62.27 10.84 8.85 6.81 4.04
0.0670 0.0356 0.0258 0.0187 0.0082
0.0664 0.0338 0.0258 0.0181 0.0082
Total surface area was calculated by the BET method. Total pore volume was calculated at P/P0 = 0.99. Smeso calculated by the BJH method.
weight PEI (PEIH), sticky fiber structure was seen for low concentration (Fig. S3a), however, with increase in PEIH concentration a relatively thick film leading to complete blockage of inter- and intra-fiber pores was observed (Fig. S3b-d). This relatively higher fiber stickiness and thicker film development could be attributed to the higher molecular weight of the PEIH. Moreover, no significant change in fiber diameter and membrane morphology was observed when pristine PS/PU nanofiber membrane was impregnated with TEA, which could be attributed to its very low molecular weight (Fig. S4a-d). Fig. 3 illustrates the FTIR analysis of the synthesized pristine and impregnated membranes. A broad band centered around ~3300 cm-1 was observed for all membranes impregnated with various amines, which could be attributed to N-H stretch. A linear increment in depth of this band was seen from TEA to PEIH and could be attributed to the enhanced number of amine sites of the three functional substrates. Furthermore, presence of the amine group on the impregnated membranes was also confirmed by peaks 1481 and 1532 cm-1, which represented symmetric and asymmetric bending of primary amines [14,15]. Physical structure of impregnated of samples was investigated by using using N2 adsorption-desorption technique. It was found that all PEIL (Fig. 4a) impregnated samples as well as PEIH and TEA impregnated (Fig. S5a) samples showed very similar characteristics, showing type IV isotherms with an obvious adsorption desorption hysteresis loop as shown for pristine membrane. Moreover, similar behavior was also witnessed by BJH model (Fig. 4b and Fig. S5b). Pore size of the impregnated membranes mainly ranged in between 25 to 50 nm which is still large enough for capturing CO2 molecules, thus, these membranes could promote CO2 adsorption and diffusion. The pore structure details of pure and amine functionalized PS/PU nanofiber composite membranes are listed in Table 1 and Table S1, certain decline in surface area (i.e. ~23.56-4.05 m2 g-1) for all impregnated
Fig. 3. FT-IR spectra of PS/PU, PS/PU-TEA, PS/PU-PEIL-2 and PS/PU-PEIH-1 nanofiber membranes.
demonstrated the characteristics of mesopores within as-prepared PS/ PU nanofiber composite membrane [13]. Narrow hysteresis loops during overall pressure region indicate that the pores are open, the phenomenon is also validated by Fig. 1a. Additionally, porous structure of the as-synthesized PS/PU nanofiber composite membranes was further analyzed using typical Barret-Joyner-Halenda (BJH) model (Fig. 1b inset). Since we need high surface area and relatively larger pore sizes compared to kinetic CO2 gas molecules for successful CO2 adsorption, it could be observed that membrane offered high Brunauer-Emmett-Teller (BET) specific area (62.27 m2 g-1), and typical poly-disperse mesopores characteristic. Additionally, pore size mainly centered at ~25–50 nm was large enough to capture CO2 gas molecules, thus, the resultant optimized membranes could promote CO2 adsorption and diffusion. Moreover, membrane was mechanically strong enough to withstand external stresses (Fig. S2). Fig. 2a-d demonstrates the fiber surface morphology of PS/PU impregnated membrane. When the as-synthesized PS/PU nanofiber composite membrane was impregnated with low molecular weight PEI (PEIL), an obvious increment in the average fiber diameter was observed (Fig. 2a-b) indicating successful deposition of PEIL on PS/PU nanofiber composite membranes. It is apparent from Fig. 2c-d that when PEIL concentration increased beyond a critical limit, an obvious thick polymer film covered most of fiber surface and blocked inter- and intra-polymer gaps. On the other hand, when the porous PS/PU nanofiber composite membrane was impregnated with high molecular
Fig. 4. (a) N2 adsorption/desorption isotherms and (b) BJH pore size distribution curves of pristine and amine impregnated PS/PU nanofiber membranes. 48
Composites Communications 10 (2018) 45–51
G. Zainab et al.
Fig. 5. CO2 adsorption isotherms of PS/PU nanofibers membrane impregnated with various amine groups (a) PS/PU-PEIL (b) PS/PU-PEIH and (c) PS/PU-TEA nanofiber membranes. (d) Comparison of CO2 adsorption behavior of PS/PU nanofiber membrane impregnated with various amines.
(~1.37–1.64 mmol g-1) could be observed with increasing PEIL concentration which ceased when the weight ratio of PEIL exceeds 50% of weight of pristine membrane (Fig. 5a). This enhanced CO2 adsorption for PS/PU-PEIL-2 could be credited collectively to the inter-fiber pores, high number CO2 capturing sites offered by PEIL deposition and the critical balance between fiber porosity and PEIL deposition. However, when the PEIL weight ratio surpassed the critical limit (i.e. 50% of weight of pristine membrane), the balance between porous structure and CO2 capturing sites was disturbed, therefore, as a result low space was left to hold CO2 molecules which may be attributed for decrease in the CO2 adsorption performance. A consistent decrement in CO2 adsorption (~0.95-0.48 mmol g-1) for membranes impregnated with PEIH is very obvious from Fig. 5b. This consistent decline for same weight ratios compared to PEIL may be ascribed to the higher molecular weight of PEIH and blockage of fiber pores owing to opaque film formation on the fiber surface. Fig. 5c demonstrates the CO2 performance for the samples treated with TEA and shows trend similar to the membranes treated with PEIL (Fig. 5a). Steadiness in CO2 adsorption (~0.61–0.88 mmol g-1) with enhancing TEA weight ratio could be viewed from PS/PU-TEA-1 to PS/PU-TEA-3, however, no improvement in the CO2 adsorption performance was observed when membranes were subjected to 100% TEA concentration (Fig. 5c). This trend of CO2 adsorption performance may be ascribed to the relatively lower molecular weight of TEA and balance between TEA critical concentration and fiber porosity. Low CO2 adsorption for TEA compared to PEIL or PEIH may also be accredited to the very low number of amines present in TEA structure and its structural design, which as a result produced less number of CO2 capturing sites compared PEI functionalized samples. Fig. 5d compares the CO2 adsorption
Table 2 CO2 adsorption of PS/PU composite membrane impregnated with various weight ratios of PEIL. Concentration (%)
PS/PU-PEIL CO2 capacity (mmol g-1)
PS/PU-PEIH CO2 capacity (mmol g-1)
PS/PU-TEA CO2 capacity (mmol g-1)
0 1:4 1:2 1:3 1:1
0.33 1.37 1.64 1.04 0.71
0.33 0.95 0.80 0.63 0.48
0.33 0.61 0.77 0.88 0.42
membranes compared to pure PS/PU nanofiber membrane (62.27 m2 g1 ) was noticed which may be ascribed to increase in fiber diameter and film deposition. Since surface area and porous structure are the major characteristics which ensure large number of active sites for CO2 capture. Therefore, only a balance between active sites and porous structure of substrate would result into maximum CO2 adsorption capacity. Fig. 5a-d illustrates the CO2 performance of the pure and impregnated PS/PU nanofiber composite membranes, functionalized with various amines, captured at 40 °C, results are also summarized in Table 2. PEIL impregnated PS/PU nanofiber composite membranes offered higher CO2 adsorption capacity (~0.42–1.64 mmol g-1) for all the concentrations compared to pure PS/PU nanofiber membrane (~0.33 mmol g-1). Although, all the PEIL impregnated samples were relatively less porous and have lower surface area compared pristine membrane but their high affinity for CO2 molecules leaded to high CO2 adsorption. Additionally, linear increment in CO2 adsorption
49
Composites Communications 10 (2018) 45–51
G. Zainab et al.
Fig. 6. (a) Effect of temperature on CO2 adsorption, (b) CO2 adsorption/desorption cycles, (c) cyclic adsorption capacity and (d) selective adsorption of CO2 over N2 at 40 °C of optimized membrane.
CO2 capturing capacity, and excellent regenerative ability, membranes need to be highly selective towards CO2 gas molecules in the flue gas. Since CO2 and N2 are two major components of flue gas, therefore, designed sorbents must have high affinity towards CO2 in mixed gas environments. Fig. 6d demonstrates the adsorption kinetics of pre-optimized sample for CO2 and N2 under same experimental conditions. It could be seen that membrane offered over 20 times higher adsorption capacity for CO2 (~1.64 mmol/g) compared to N2 (~0.06 mmol/g) showing that membrane had very high selectivity (S = 26) for CO2. CO2 adsorption mechanisms in different materials may be broadly categorized into two major categories, i.e. chemisorption and physisorption. Chemisorption mechanism holds CO2 molecules firmly compared to physisorption mechanism which binds CO2 molecules weakly. Thus, chemisorption mechanism is believed to be more effective than physisorption [17]. Additionally, the CO2 adsorption value in porous materials also mainly depends on surface active (functional) sites and porosity of the substrate used as the support. Since our materials are meso-porous and functionalized with amine group, thus, both porosity of materials and functional -NH2 sites contributed in effective CO2 capture. Therefore, when CO2 molecules came into contact with porous materials having active functional sites, they were seized in the membrane pores via physicochemical reaction. Moreover, amine group present in synthesized membrane also reacted with CO2 molecules to form various CO2 amine complexes, Eqs. (1–3) demonstrate the possible reaction between amine group and CO2 molecules [18].
results of all amine functionalized samples of PS/PU nanofiber composite membrane. It was observed that PEIL functionalized samples offered relatively much higher CO2 adsorption performance (1.64 mmol/g), which is comparatively higher than many of the polymers based reported works till to date (Table S2), and the probable reason behind this phenomenon has already been discussed in the analysis of Fig. 5a. Therefore, samples treated with PEIL-2 were chosen for further investigation. Fig. 6a presents the CO2 adsorption performance against various temperatures for the optimized samples. A consistent decline in the CO2 adsorption was found for rising temperatures, this decrement in the CO2 adsorption may be associated to the weakening of the interaction between PEI and CO2 molecules with rising temperature [16]. Since high adsorption and good regeneration ability are extremely important features required for commercial production. Therefore, PS/PU-PEI-2 was chosen as optimized sample for further examination. As the CO2 desorption is carried out at high temperature (i.e. 105 °C), and stability amine group in high temperature conditions is believed to be challenging task. Thus, in order to determine CO2 adsorption stability of the optimized sample, optimized membrane was exposed to up to 19 CO2 adsorption-desorption cycles (Fig. 6b), where CO2 adsorption occurred at 40 °C and for desorption process sample was exposed to pure N2 for 40 min at 105 °C. It was further observed that the resultant PS/PU-PEI-2 nanofiber membrane could retain over 90% of initial value even after 19 CO2 adsorption-desorption cycles, offering a stable enough CO2 adsorption performance and sufficient regenerative ability (Fig. 6b-c). Additionally, Thermal stability of the PEI-2 (amine group source polymer) was also confirmed via TGA analysis (Fig. S6), which showed there was no significant weight loss up to ~280 °C. Furthermore, for commercial applications besides high and stable 50
CO2 + 2RNH2 ⇄ RNHCOO− + RNH3+
(1)
CO2 + 2R2 NH ⇄ R2 NH2+ + R2 NCOO−
(2)
CO2 + 2R3 N ⇄ R 4 N+ + R2 NCOO−
(3)
Composites Communications 10 (2018) 45–51
G. Zainab et al.
References
In our work, since the main source of amine group was PEI comprising of primary, secondary and tertiary amines, which regulated the reaction of each mole of CO2 with two mole of amines under dry conditions. Therefore, after the surface functional sites were fully occupied then CO2 molecules diffused into the pores of the synthesized membrane until adsorption reached saturation eventually. Thus, it was determined that both active functional sites as well as porous structure play key role in regulating the CO2 adsorption performance of the membrane.
[1] N. Du, H.B. Park, M.M. Dal-Cin, M.D. Guiver, Advances in high permeability polymeric membrane materials for CO2 separations, Energy Environ. Sci. 5 (6) (2012) 7306–7322. [2] G. Qi, Y. Wang, L. Estevez, X. Duan, N. Anako, A.H.A. Park, W. Li, C.W. Jones, E.P. Giannelis, High efficiency nanocomposite sorbents for CO2 capture based on amine-functionalized mesoporous capsules, Energy Environ. Sci. 4 (2) (2011) 444–452. [3] A.L. Dzubak, L.C. Lin, J. Kim, J.A. Swisher, R. Poloni, S.N. Maximoff, B. Smit, L. Gagliardi, Ab initio carbon capture in open-site metal–organic frameworks, Nat. Chem. 4 (10) (2012) 810–816. [4] B. Li, Y. Duan, D. Luebke, B. Morreale, Advances in CO2 capture technology: a patent review, Appl. Energy. 102 (2013) 1439–1447. [5] N. Iqbal, X. Wang, J. Yu, B. Ding, Robust and flexible carbon nanofibers doped with amine functionalized carbon nanotubes for efficient CO2 capture, Adv. Sustain. Syst. 1 (3-4) (2017) 1700855. [6] G. Zainab, N. Iqbal, A.A. Babar, C. Huang, X. Wang, J. Yu, B. Ding, Free-standing, spider-web-like polyamide/carbon nanotube composite nanofibrous membrane impregnated with polyethyleneimine for CO2 capture, Compos. Commun. 6 (2017) 41–47. [7] A.A. Babar, X. Wang, N. Iqbal, J. Yu, B. Ding, Tailoring differential moisture transfer performance of nonwoven/polyacrylonitrile-SiO2 nanofiber composite membranes, Adv. Mater. Interfaces 4 (15) (2017) 1700062. [8] D. Shimon, C.-H. Chen, J.J. Lee, S.A. Didas, C. Sievers, C.W. Jones, S.E. Hayes, 15N Solid state NMR spectroscopic study of surface amine groups for carbon capture: 3aminopropylsilyl grafted to SBA-15 mesoporous silica, Environ. Sci. Technol. 52 (3) (2018) 1488–1495. [9] W. Li, J. Wu, S.S. Lee, J.D. Fortner, Surface tunable magnetic nano-sorbents for carbon dioxide sorption and separation, Chem. Eng. J. 313 (2017) 1160–1167. [10] S. Rinprasertmeechai, S. Chavadej, P. Rangsunvigit, S. Kulprathipanja, Carbon dioxide removal from flue gas using amine-based hybrid solvent absorption, Int. J. Chem. Biol. Eng. 6 (2012) 296–300. [11] S.H. Chai, Z.M. Liu, K. Huang, S. Tan, S. Dai, Amine functionalization of microsized and nanosized mesoporous carbons for carbon dioxide capture, Ind. Eng. Chem. Res. 55 (27) (2016) 7355–7361. [12] X. Wang, Y. Li, X. Li, J. Yu, S.S. Al-Deyab, B. Ding, Equipment-free chromatic determination of formaldehyde by utilizing pararosaniline-functionalized cellulose nanofibrous membranes, Sens. Actuators B Chem. 203 (2014) 333–339. [13] Y. Si, X. Wang, Y. Li, K. Chen, J. Wang, J. Yu, H. Wang, B. Ding, Optimized colorimetric sensor strip for mercury (II) assay using hierarchical nanostructured conjugated polymers, J. Mater. Chem. A 2 (3) (2014) 645–652. [14] X. Wang, V. Schwartz, J.C. Clark, X. Ma, S.H. Overbury, X. Xu, C. Song, Infrared study of CO2 sorption over “molecular basket” sorbent consisting of polyethylenimine-modified mesoporous molecular sieve, J. Phys. Chem. C 113 (17) (2009) 7260–7268. [15] J. Charles, G. Ramkumaar, S. Azhagiri, S. Gunasekaran, FTIR and thermal studies on nylon-66 and 30% glass fibre reinforced nylon-66, Eur. -J. Chem. 6 (1) (2009) 23–33. [16] H.B. Wang, P.G. Jessop, G. Liu, Support-free porous polyamine particles for CO2 capture, ACS Macro Lett. 1 (8) (2012) 944–948. [17] P. Bollini, S.A. Didas, C.W. Jones, Amine-oxide hybrid materials for acid gas separations, J. Mater. Chem. 21 (39) (2011) 15100–15120. [18] Y. Zhang, J. Guan, X. Wang, J. Yu, B. Ding, Balsam-pear-skin-like porous polyacrylonitrile nanofibrous membranes grafted with polyethyleneimine for postcombustion CO2 capture, ACS Appl. Mater. Interfaces 9 (46) (2017) 41087–41098.
4. Conclusion In summary, we have successfully developed a free-standing PS/PU nanofiber membrane with hierarchical porous structure and strong mechanical characteristics. Synthesized membranes were impregnated with various amines (PEIL, PEIH and TEA). Certain decrease in the pore volume and surface area of impregnated membranes was observed when compared with pristine PS/PU nanofiber membrane. The critical analysis showed that amongst optimized samples functionalized with three different amines, PEIL impregnated samples offered relatively higher CO2 adsorption (1.64 mmol/g) compared to the samples functionalized with PEIH (0.95 mmol/g) and TEA (0.88 mmol/g). Moreover, easy regeneration at ~105 °C, excellent durability, high performance stability and decent retention of over 90% of initial value even after 19 adsorption-desorption cycles suggested PEIL impregnated membranes to be potential candidate for CO2 adsorption application. Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51503028 and 51673037), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. TP2016019), the Fundamental Research Funds for the Central Universities (No. 2232016A3-03), the Shanghai RisingStar Program (No. 16QA1400200), the Shanghai Committee of Science and Technology (No. 15JC1400500), and the National Key R&D Program of China (No. 2016YFB0303200). Conflict of interest The authors declare no competing financial interest. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.coco.2018.06.005.
51
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Amine-impregnated porous nanofiber membranes for CO2 capture a,c,1
Ghazala Zainab , Aijaz Ahmed Babar ⁎ Jianyong Yuc, Bin Dinga,b,c,
a,c,1
, Nousheen Iqbal
a,c,1
, Xianfeng Wang
a,b,c,⁎
T
,
a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, China c Innovation Center for Textile Science and Technology, Donghua University, Shanghai 200051, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrospinning Nanofiber membrane Amine impregnation CO2 adsorption
Global warming is a worldwide issue, mainly caused by excessive emission of carbon dioxide (CO2), thus, demands the development of robust and flexible sorbents with consider CO2 capturing capacity. Herein, we report an easy and facile two-step fabrication of hierarchically structured and highly porous polystyrene (PS)/polyurethane (PU) electrospun nanofiber membrane impregnated with various amines for post-combustion CO2 capture. Synthesized porous nanofiber membranes have highly flexible and strong mechanical characteristics. The comparative analysis of porous membranes functionalized with three functional amines shows that low molecular weight polyethyleneimine (PEIL) impregnated porous nanofiber membranes offer relatively higher CO2 adsorption (1.64 mmol/g) at 40 °C. In addition, optimized samples exhibit excellent regenerability, (≥ 90% of their original value) even after 19 adsorption-desorption cycles showing a stable and durable CO2 capture performance. This work paves the way for developing electrospun nanofiber membranes capable of efficient CO2 adsorption.
1. Introduction
electrospun nanofibers are gaining increasing attention of scientists in the recent years [7]. Moreover, in order to maximize their CCS capacity and selectivity, electrospun nanofibers can be functionalized with certain additives such as amines. Nowadays, amine functionalized porous materials (such as silica, carbon, nylon etc.) are being extensively examined to explore their potential capabilities for CCS owing to their effective CO2 capturing quality and easy application method [5,6,8]. A variety of various amine groups have been thoroughly examined for CO2 adsorption, however, polyethyleneimine (PEI) has been proved to be the most significant group of amine family because of its high number of functional sites result in outstanding affinity for CO2 [9]. Whereas, very little work has been reported about the use of TEA for CO2 adsorption [10]. These amines can essentially be applied by simply impregnating porous substrate in polymeric amine solutions, in-situ polymerization or grafting them on surface of the porous materials [11]. In the present work, amine functionalized polystyrene/polyurethane (PS/PU) nanofiber composite membrane has been developed. PS is commonly used polymer fiber with very high level of hydrophobicity and easily tailorable porous structure, thus, it has been chosen as primary substrate/support, because high porosity and water
Currently, global warming is the biggest alarming threat to environment and public health. One of the fundamental contributing source toward global warming is greenhouse effect. There are a number of gases, which cause greenhouse effect that leads to the problem of global warming, however, among them CO2 is the leading contributor. Massive CO2 emissions not only lead to global warming but also result in rise of sea level and environmental pollution [1,2]. That is why it is being highly demanded to take immediate efforts to reduce CO2 concentration in the atmosphere and limit further emission of CO2 in the atmosphere [3]. Therefore, new materials, techniques and technologies are being explored for developing effective, economic and scalable CO2 capture and storage (CCS) technologies [4]. Ideal materials for CCS need to have highly porous structure, good mechanical strength and flexibility, sustainable and environment friendly fabrication process, besides that they should have feasibility for easier surface functionalization [5]. In this regard, electrospun polymeric nanofibers successfully qualify most of the demands to be ideal materials for CCS [6]. Owing to their free-standing nature, tailorable physical characteristics, easy and economical fabrication process,
⁎
1
Corresponding authors at: Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles, Donghua University, Shanghai 201620, China. E-mail addresses: [email protected] (X. Wang), [email protected] (B. Ding). These author contributed equally to this work
https://doi.org/10.1016/j.coco.2018.06.005 Received 13 November 2017; Received in revised form 2 June 2018; Accepted 9 June 2018 2452-2139/ © 2018 Published by Elsevier Ltd.
Composites Communications 10 (2018) 45–51
G. Zainab et al.
Micromeritics Co., USA). XQ-1C tensile tester (Shanghai New Fiber Instrument Co., Ltd., China) was involved for examining the mechanical characteristics.
repellency are two primary demands for CO2 adsorption. PU is another commonly used elastomeric fiber with excellent mechanical characteristics, and has been employed to overcome poor mechanical strength of PS nanofibers in pristine state. As-synthesized composite membrane was functionalized with various amines by impregnation method, and effect of various amine groups on CO2 adsorption capacity of synthesized membrane was examined.
2.5. CO2 adsorption The CO2 capture performance, cycle performance and CO2/N2 selectivity of amine-impregnated nanofiber composite membranes was investigated by thermogravimetric analyzer (TGA, TA Instruments model Q600, USA). A flow of 200 mL/min for N2 as well as CO2 was maintained during the process. A sample weighing ~10 mg of composite membrane was placed on microbalance alumina sample cell. In order to remove any existing volatile impurities, sample was first exposed to 105 °C in N2 environment. The temperature was then adjusted to 40 °C and pure dry CO2 gas was introduced for 60 min to evaluate the CO2 adsorption performance. Additionally, the CO2 adsorption performance of optimized sample was also observed at 50 °C and 60 °C to examine influence of temperature on CO2 capture performance. CO2 desorption was carried under nitrogen environment for 40 min at 105 °C. CO2 adsorption capacity in mmol/g was calculated during adsorption process. Multiple adsorption/desorption cycles were carried out to evaluate recyclability of the optimized samples. Moreover, in order to investigate the feasibility of capturing CO2 in practical conditions, selectivity of optimized samples against CO2 and N2 was analyzed. Same procedure was repeated for N2 adsorption. The selectivity of CO2/N2 was defined as S= q1/q2, where q1 and q2 are the adsorption capacities of CO2 and N2 in the adsorption process, respectively.
2. Experimental section 2.1. Materials Polystyrene (PS, Mw = 350,000 g mol-1) was supplied from Wako Co., Ltd., Japan, and polyurethene (PU, Mw = 180,000 g mol-1) was procured from BASF Polyurethane Speciliaties Co., Ltd., China. Tetrahydrofuran (THF), dimethylformamide (DMF), ethanol, polyethyleneimine (PEI, Mn = 10,000 and 60,000) and triethylamine (TEA, Mn = 101.19) were obtained from Aladdin Chemical Co, Ltd., China. All chemicals were of analytical grade and used without further purification. 2.2. Solution preparation and membrane fabrication Homogenous solutions of PS (15 wt%) and PU (7 wt%) were prepared by dissolving them into a mixture of DMF/THF (1/1) individually under vigorous magnetic stirring overnight. Prepared solutions were then electrospun using DXES-3 electrospinning machine (SOF Nanotechnology Co., China) at the feeding rate of 1 mL/h under an applied voltage of 30 kV. Nanofiber membrane was collected on a metallic roller revolving at 20 cm distance from the feeding needles. Constant temperature (25 ± 2 °C) and relative humidity (45 ± 3%) were maintained throughout the fabrication process. Resultant nanofiber membranes were dried under vacuum for 4 h to remove any residual solvent.
3. Results and discussions Scheme 1 illustrates the schematic demonstration of amine functionalized PS/PU nanofiber membrane development process. Development of composite nanofiber membrane was carried out in two critical steps: (1) Fabrication of porous PS/PU nanofiber composite membrane with rough surface morphology, (2) functionalization of synthesized nanofiber composite membrane with various amines. Porous PS/PU nanofiber composite membrane was fabricated via electrospinning technique using two solvents having different degree of volatility. Owing to the difference in volatile nature of the two solvents, solvent evaporation process intensified resulting in much faster evaporation of solvent and simultaneous interaction with external non-solvent species such as air and moisture resulted into the formation of porous fiber structure. Moreover, successful surface functionalization of the as-synthesized PS/PU nanofiber composite membrane was carried out in rotary evaporator using impregnation technique. Fig. 1a presents the morphological analysis of electrospun pristine PS/PU nanofiber membrane. It could be seen that fabricated membrane offered randomly oriented 3D fibrous structure. Since two polymers were electrospun side by side, therefore, two different ranges of fibers in the membrane were witnessed. PS fibers were highly porous and relatively thicker in diameter and ranged 2–3 μm. Whereas, PU nanofibers offered smooth and solid cylindrical shaped structure with average fiber diameter in the range of ~800–950 nm in the resultant PS/PU nanofiber membrane (A low magnification SEM image used for measuring fiber diameter has been provided in Fig. S1). This variance in physical structure and fiber diameter could be attributed to the difference in molecular weight of two polymers and use of two solvents with varied degree of volatile nature. Porosity and specific surface area are the two most important characteristic features, which determine the CO2 adsorption capacity of the substrate. Therefore, the hierarchical pore structure of membranes was examined via N2 adsorption-desorption isotherms at 77 K (Fig. 1b). Typical type IV isotherm with an apparent adsorption hysteresis loop as classified by IUPAC was seen [12]. It was observed that a series of typical physical adsorption behaviors including monolayer adsorption, multilayer adsorption and capillary condensation phenomenon
2.3. Impregnation of PS/PU composite membrane Amine solutions were prepared by dissolving PEI and TEA in ethanol individually under vigorous magnetic stirring until transparent solutions were achieved. Composite membranes were then immersed in transparent solutions for 1 h under continuous stirring. Later on, the solution along with membrane was transferred into a rotary evaporator (PV 10 Basic Plus D, Wilmington, NC) and ethanol was evaporated from the solution under vacuum at 60 °C for ~1 h and the resultant membrane was finally dried at 50 °C for 5 h under vacuum. Subsequent impregnated nanofiber membranes were tagged PS/PU-PEIL-Y PS/PUPEIH-Y and PS/PU-TEA-Y, where L and H represent low and high molecular weight of PEI, and Y corresponds to the amine weight ratio relative to the weight of pristine membrane. All the synthesized membranes were impregnated with four different weight ratios of amine polymers, weight ratios of amine to the weight of pristine membrane were 1/4, 1/2, 1/3 and 1/1. Subsequent PEIL, PEIH and TEA impregnated membranes were tagged as PS/PUPEIL-1, PS/PU-PEIL-2, PS/PU-PEIL-3, PS/PU-PEIL-4, PS/PU-PEIH-1, PS/ PU-PEIH-2, PS/PU-PEIH-3, PS/PU-PEIH-4, PS/PU-TEA-1, PS/PU-TEA-2, PS/PU-TEA-3, and PS/PU-TEA-4, respectively. 2.4. Structural characterization Fiber surface morphological analysis of synthesized composite membranes was carried out using scanning electron microscope (SEM, TESCAN VEGA 3, TESCAN Ltd., Czech Republic). Fourier transform infrared (FTIR) spectrometer (Nicolet iS10, USA) was employed to observe the presence of amine groups and CO2 molecules in the functionalized composite nanofiber membranes. Physical structure was examined by using an automatic adsorption system (ASAP 2020, 46
Composites Communications 10 (2018) 45–51
G. Zainab et al.
Scheme 1. Schematic illustration of the fabrication and amine impregnation of PS/PU nanofiber membrane.
Fig. 1. (a) SEM image (b) N2 adsorption/desorption isotherm of PS/PU nanofiber membrane. The inset shows the BJH pore size distribution curve.
Fig. 2. SEM images of PS/PU nanofiber membrane impregnated with (a) PEIL -1 (b) PEIL -2 (c) PEIL -3 and (d) PEIL -4.
47
Composites Communications 10 (2018) 45–51
G. Zainab et al.
Table 1 Textural properties of PS/PU composite membrane impregnated with various weight ratios of PEIL. Sample PS/PU PS/PU PS/PU PS/PU PS/PU a b c
-PEIL-PEIL-PEIL-PEIL-
1 2 3 4
SBETa (m2 g-1)
Vtotalb (cm3 g-1)
Smesoc (cm3 g-1)
62.27 10.84 8.85 6.81 4.04
0.0670 0.0356 0.0258 0.0187 0.0082
0.0664 0.0338 0.0258 0.0181 0.0082
Total surface area was calculated by the BET method. Total pore volume was calculated at P/P0 = 0.99. Smeso calculated by the BJH method.
weight PEI (PEIH), sticky fiber structure was seen for low concentration (Fig. S3a), however, with increase in PEIH concentration a relatively thick film leading to complete blockage of inter- and intra-fiber pores was observed (Fig. S3b-d). This relatively higher fiber stickiness and thicker film development could be attributed to the higher molecular weight of the PEIH. Moreover, no significant change in fiber diameter and membrane morphology was observed when pristine PS/PU nanofiber membrane was impregnated with TEA, which could be attributed to its very low molecular weight (Fig. S4a-d). Fig. 3 illustrates the FTIR analysis of the synthesized pristine and impregnated membranes. A broad band centered around ~3300 cm-1 was observed for all membranes impregnated with various amines, which could be attributed to N-H stretch. A linear increment in depth of this band was seen from TEA to PEIH and could be attributed to the enhanced number of amine sites of the three functional substrates. Furthermore, presence of the amine group on the impregnated membranes was also confirmed by peaks 1481 and 1532 cm-1, which represented symmetric and asymmetric bending of primary amines [14,15]. Physical structure of impregnated of samples was investigated by using using N2 adsorption-desorption technique. It was found that all PEIL (Fig. 4a) impregnated samples as well as PEIH and TEA impregnated (Fig. S5a) samples showed very similar characteristics, showing type IV isotherms with an obvious adsorption desorption hysteresis loop as shown for pristine membrane. Moreover, similar behavior was also witnessed by BJH model (Fig. 4b and Fig. S5b). Pore size of the impregnated membranes mainly ranged in between 25 to 50 nm which is still large enough for capturing CO2 molecules, thus, these membranes could promote CO2 adsorption and diffusion. The pore structure details of pure and amine functionalized PS/PU nanofiber composite membranes are listed in Table 1 and Table S1, certain decline in surface area (i.e. ~23.56-4.05 m2 g-1) for all impregnated
Fig. 3. FT-IR spectra of PS/PU, PS/PU-TEA, PS/PU-PEIL-2 and PS/PU-PEIH-1 nanofiber membranes.
demonstrated the characteristics of mesopores within as-prepared PS/ PU nanofiber composite membrane [13]. Narrow hysteresis loops during overall pressure region indicate that the pores are open, the phenomenon is also validated by Fig. 1a. Additionally, porous structure of the as-synthesized PS/PU nanofiber composite membranes was further analyzed using typical Barret-Joyner-Halenda (BJH) model (Fig. 1b inset). Since we need high surface area and relatively larger pore sizes compared to kinetic CO2 gas molecules for successful CO2 adsorption, it could be observed that membrane offered high Brunauer-Emmett-Teller (BET) specific area (62.27 m2 g-1), and typical poly-disperse mesopores characteristic. Additionally, pore size mainly centered at ~25–50 nm was large enough to capture CO2 gas molecules, thus, the resultant optimized membranes could promote CO2 adsorption and diffusion. Moreover, membrane was mechanically strong enough to withstand external stresses (Fig. S2). Fig. 2a-d demonstrates the fiber surface morphology of PS/PU impregnated membrane. When the as-synthesized PS/PU nanofiber composite membrane was impregnated with low molecular weight PEI (PEIL), an obvious increment in the average fiber diameter was observed (Fig. 2a-b) indicating successful deposition of PEIL on PS/PU nanofiber composite membranes. It is apparent from Fig. 2c-d that when PEIL concentration increased beyond a critical limit, an obvious thick polymer film covered most of fiber surface and blocked inter- and intra-polymer gaps. On the other hand, when the porous PS/PU nanofiber composite membrane was impregnated with high molecular
Fig. 4. (a) N2 adsorption/desorption isotherms and (b) BJH pore size distribution curves of pristine and amine impregnated PS/PU nanofiber membranes. 48
Composites Communications 10 (2018) 45–51
G. Zainab et al.
Fig. 5. CO2 adsorption isotherms of PS/PU nanofibers membrane impregnated with various amine groups (a) PS/PU-PEIL (b) PS/PU-PEIH and (c) PS/PU-TEA nanofiber membranes. (d) Comparison of CO2 adsorption behavior of PS/PU nanofiber membrane impregnated with various amines.
(~1.37–1.64 mmol g-1) could be observed with increasing PEIL concentration which ceased when the weight ratio of PEIL exceeds 50% of weight of pristine membrane (Fig. 5a). This enhanced CO2 adsorption for PS/PU-PEIL-2 could be credited collectively to the inter-fiber pores, high number CO2 capturing sites offered by PEIL deposition and the critical balance between fiber porosity and PEIL deposition. However, when the PEIL weight ratio surpassed the critical limit (i.e. 50% of weight of pristine membrane), the balance between porous structure and CO2 capturing sites was disturbed, therefore, as a result low space was left to hold CO2 molecules which may be attributed for decrease in the CO2 adsorption performance. A consistent decrement in CO2 adsorption (~0.95-0.48 mmol g-1) for membranes impregnated with PEIH is very obvious from Fig. 5b. This consistent decline for same weight ratios compared to PEIL may be ascribed to the higher molecular weight of PEIH and blockage of fiber pores owing to opaque film formation on the fiber surface. Fig. 5c demonstrates the CO2 performance for the samples treated with TEA and shows trend similar to the membranes treated with PEIL (Fig. 5a). Steadiness in CO2 adsorption (~0.61–0.88 mmol g-1) with enhancing TEA weight ratio could be viewed from PS/PU-TEA-1 to PS/PU-TEA-3, however, no improvement in the CO2 adsorption performance was observed when membranes were subjected to 100% TEA concentration (Fig. 5c). This trend of CO2 adsorption performance may be ascribed to the relatively lower molecular weight of TEA and balance between TEA critical concentration and fiber porosity. Low CO2 adsorption for TEA compared to PEIL or PEIH may also be accredited to the very low number of amines present in TEA structure and its structural design, which as a result produced less number of CO2 capturing sites compared PEI functionalized samples. Fig. 5d compares the CO2 adsorption
Table 2 CO2 adsorption of PS/PU composite membrane impregnated with various weight ratios of PEIL. Concentration (%)
PS/PU-PEIL CO2 capacity (mmol g-1)
PS/PU-PEIH CO2 capacity (mmol g-1)
PS/PU-TEA CO2 capacity (mmol g-1)
0 1:4 1:2 1:3 1:1
0.33 1.37 1.64 1.04 0.71
0.33 0.95 0.80 0.63 0.48
0.33 0.61 0.77 0.88 0.42
membranes compared to pure PS/PU nanofiber membrane (62.27 m2 g1 ) was noticed which may be ascribed to increase in fiber diameter and film deposition. Since surface area and porous structure are the major characteristics which ensure large number of active sites for CO2 capture. Therefore, only a balance between active sites and porous structure of substrate would result into maximum CO2 adsorption capacity. Fig. 5a-d illustrates the CO2 performance of the pure and impregnated PS/PU nanofiber composite membranes, functionalized with various amines, captured at 40 °C, results are also summarized in Table 2. PEIL impregnated PS/PU nanofiber composite membranes offered higher CO2 adsorption capacity (~0.42–1.64 mmol g-1) for all the concentrations compared to pure PS/PU nanofiber membrane (~0.33 mmol g-1). Although, all the PEIL impregnated samples were relatively less porous and have lower surface area compared pristine membrane but their high affinity for CO2 molecules leaded to high CO2 adsorption. Additionally, linear increment in CO2 adsorption
49
Composites Communications 10 (2018) 45–51
G. Zainab et al.
Fig. 6. (a) Effect of temperature on CO2 adsorption, (b) CO2 adsorption/desorption cycles, (c) cyclic adsorption capacity and (d) selective adsorption of CO2 over N2 at 40 °C of optimized membrane.
CO2 capturing capacity, and excellent regenerative ability, membranes need to be highly selective towards CO2 gas molecules in the flue gas. Since CO2 and N2 are two major components of flue gas, therefore, designed sorbents must have high affinity towards CO2 in mixed gas environments. Fig. 6d demonstrates the adsorption kinetics of pre-optimized sample for CO2 and N2 under same experimental conditions. It could be seen that membrane offered over 20 times higher adsorption capacity for CO2 (~1.64 mmol/g) compared to N2 (~0.06 mmol/g) showing that membrane had very high selectivity (S = 26) for CO2. CO2 adsorption mechanisms in different materials may be broadly categorized into two major categories, i.e. chemisorption and physisorption. Chemisorption mechanism holds CO2 molecules firmly compared to physisorption mechanism which binds CO2 molecules weakly. Thus, chemisorption mechanism is believed to be more effective than physisorption [17]. Additionally, the CO2 adsorption value in porous materials also mainly depends on surface active (functional) sites and porosity of the substrate used as the support. Since our materials are meso-porous and functionalized with amine group, thus, both porosity of materials and functional -NH2 sites contributed in effective CO2 capture. Therefore, when CO2 molecules came into contact with porous materials having active functional sites, they were seized in the membrane pores via physicochemical reaction. Moreover, amine group present in synthesized membrane also reacted with CO2 molecules to form various CO2 amine complexes, Eqs. (1–3) demonstrate the possible reaction between amine group and CO2 molecules [18].
results of all amine functionalized samples of PS/PU nanofiber composite membrane. It was observed that PEIL functionalized samples offered relatively much higher CO2 adsorption performance (1.64 mmol/g), which is comparatively higher than many of the polymers based reported works till to date (Table S2), and the probable reason behind this phenomenon has already been discussed in the analysis of Fig. 5a. Therefore, samples treated with PEIL-2 were chosen for further investigation. Fig. 6a presents the CO2 adsorption performance against various temperatures for the optimized samples. A consistent decline in the CO2 adsorption was found for rising temperatures, this decrement in the CO2 adsorption may be associated to the weakening of the interaction between PEI and CO2 molecules with rising temperature [16]. Since high adsorption and good regeneration ability are extremely important features required for commercial production. Therefore, PS/PU-PEI-2 was chosen as optimized sample for further examination. As the CO2 desorption is carried out at high temperature (i.e. 105 °C), and stability amine group in high temperature conditions is believed to be challenging task. Thus, in order to determine CO2 adsorption stability of the optimized sample, optimized membrane was exposed to up to 19 CO2 adsorption-desorption cycles (Fig. 6b), where CO2 adsorption occurred at 40 °C and for desorption process sample was exposed to pure N2 for 40 min at 105 °C. It was further observed that the resultant PS/PU-PEI-2 nanofiber membrane could retain over 90% of initial value even after 19 CO2 adsorption-desorption cycles, offering a stable enough CO2 adsorption performance and sufficient regenerative ability (Fig. 6b-c). Additionally, Thermal stability of the PEI-2 (amine group source polymer) was also confirmed via TGA analysis (Fig. S6), which showed there was no significant weight loss up to ~280 °C. Furthermore, for commercial applications besides high and stable 50
CO2 + 2RNH2 ⇄ RNHCOO− + RNH3+
(1)
CO2 + 2R2 NH ⇄ R2 NH2+ + R2 NCOO−
(2)
CO2 + 2R3 N ⇄ R 4 N+ + R2 NCOO−
(3)
Composites Communications 10 (2018) 45–51
G. Zainab et al.
References
In our work, since the main source of amine group was PEI comprising of primary, secondary and tertiary amines, which regulated the reaction of each mole of CO2 with two mole of amines under dry conditions. Therefore, after the surface functional sites were fully occupied then CO2 molecules diffused into the pores of the synthesized membrane until adsorption reached saturation eventually. Thus, it was determined that both active functional sites as well as porous structure play key role in regulating the CO2 adsorption performance of the membrane.
[1] N. Du, H.B. Park, M.M. Dal-Cin, M.D. Guiver, Advances in high permeability polymeric membrane materials for CO2 separations, Energy Environ. Sci. 5 (6) (2012) 7306–7322. [2] G. Qi, Y. Wang, L. Estevez, X. Duan, N. Anako, A.H.A. Park, W. Li, C.W. Jones, E.P. Giannelis, High efficiency nanocomposite sorbents for CO2 capture based on amine-functionalized mesoporous capsules, Energy Environ. Sci. 4 (2) (2011) 444–452. [3] A.L. Dzubak, L.C. Lin, J. Kim, J.A. Swisher, R. Poloni, S.N. Maximoff, B. Smit, L. Gagliardi, Ab initio carbon capture in open-site metal–organic frameworks, Nat. Chem. 4 (10) (2012) 810–816. [4] B. Li, Y. Duan, D. Luebke, B. Morreale, Advances in CO2 capture technology: a patent review, Appl. Energy. 102 (2013) 1439–1447. [5] N. Iqbal, X. Wang, J. Yu, B. Ding, Robust and flexible carbon nanofibers doped with amine functionalized carbon nanotubes for efficient CO2 capture, Adv. Sustain. Syst. 1 (3-4) (2017) 1700855. [6] G. Zainab, N. Iqbal, A.A. Babar, C. Huang, X. Wang, J. Yu, B. Ding, Free-standing, spider-web-like polyamide/carbon nanotube composite nanofibrous membrane impregnated with polyethyleneimine for CO2 capture, Compos. Commun. 6 (2017) 41–47. [7] A.A. Babar, X. Wang, N. Iqbal, J. Yu, B. Ding, Tailoring differential moisture transfer performance of nonwoven/polyacrylonitrile-SiO2 nanofiber composite membranes, Adv. Mater. Interfaces 4 (15) (2017) 1700062. [8] D. Shimon, C.-H. Chen, J.J. Lee, S.A. Didas, C. Sievers, C.W. Jones, S.E. Hayes, 15N Solid state NMR spectroscopic study of surface amine groups for carbon capture: 3aminopropylsilyl grafted to SBA-15 mesoporous silica, Environ. Sci. Technol. 52 (3) (2018) 1488–1495. [9] W. Li, J. Wu, S.S. Lee, J.D. Fortner, Surface tunable magnetic nano-sorbents for carbon dioxide sorption and separation, Chem. Eng. J. 313 (2017) 1160–1167. [10] S. Rinprasertmeechai, S. Chavadej, P. Rangsunvigit, S. Kulprathipanja, Carbon dioxide removal from flue gas using amine-based hybrid solvent absorption, Int. J. Chem. Biol. Eng. 6 (2012) 296–300. [11] S.H. Chai, Z.M. Liu, K. Huang, S. Tan, S. Dai, Amine functionalization of microsized and nanosized mesoporous carbons for carbon dioxide capture, Ind. Eng. Chem. Res. 55 (27) (2016) 7355–7361. [12] X. Wang, Y. Li, X. Li, J. Yu, S.S. Al-Deyab, B. Ding, Equipment-free chromatic determination of formaldehyde by utilizing pararosaniline-functionalized cellulose nanofibrous membranes, Sens. Actuators B Chem. 203 (2014) 333–339. [13] Y. Si, X. Wang, Y. Li, K. Chen, J. Wang, J. Yu, H. Wang, B. Ding, Optimized colorimetric sensor strip for mercury (II) assay using hierarchical nanostructured conjugated polymers, J. Mater. Chem. A 2 (3) (2014) 645–652. [14] X. Wang, V. Schwartz, J.C. Clark, X. Ma, S.H. Overbury, X. Xu, C. Song, Infrared study of CO2 sorption over “molecular basket” sorbent consisting of polyethylenimine-modified mesoporous molecular sieve, J. Phys. Chem. C 113 (17) (2009) 7260–7268. [15] J. Charles, G. Ramkumaar, S. Azhagiri, S. Gunasekaran, FTIR and thermal studies on nylon-66 and 30% glass fibre reinforced nylon-66, Eur. -J. Chem. 6 (1) (2009) 23–33. [16] H.B. Wang, P.G. Jessop, G. Liu, Support-free porous polyamine particles for CO2 capture, ACS Macro Lett. 1 (8) (2012) 944–948. [17] P. Bollini, S.A. Didas, C.W. Jones, Amine-oxide hybrid materials for acid gas separations, J. Mater. Chem. 21 (39) (2011) 15100–15120. [18] Y. Zhang, J. Guan, X. Wang, J. Yu, B. Ding, Balsam-pear-skin-like porous polyacrylonitrile nanofibrous membranes grafted with polyethyleneimine for postcombustion CO2 capture, ACS Appl. Mater. Interfaces 9 (46) (2017) 41087–41098.
4. Conclusion In summary, we have successfully developed a free-standing PS/PU nanofiber membrane with hierarchical porous structure and strong mechanical characteristics. Synthesized membranes were impregnated with various amines (PEIL, PEIH and TEA). Certain decrease in the pore volume and surface area of impregnated membranes was observed when compared with pristine PS/PU nanofiber membrane. The critical analysis showed that amongst optimized samples functionalized with three different amines, PEIL impregnated samples offered relatively higher CO2 adsorption (1.64 mmol/g) compared to the samples functionalized with PEIH (0.95 mmol/g) and TEA (0.88 mmol/g). Moreover, easy regeneration at ~105 °C, excellent durability, high performance stability and decent retention of over 90% of initial value even after 19 adsorption-desorption cycles suggested PEIL impregnated membranes to be potential candidate for CO2 adsorption application. Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51503028 and 51673037), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. TP2016019), the Fundamental Research Funds for the Central Universities (No. 2232016A3-03), the Shanghai RisingStar Program (No. 16QA1400200), the Shanghai Committee of Science and Technology (No. 15JC1400500), and the National Key R&D Program of China (No. 2016YFB0303200). Conflict of interest The authors declare no competing financial interest. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.coco.2018.06.005.
51