Balsam-Pear-Skin-Like Porous Polyacrylonitrile Nanofibrous

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Balsam pear skin-like porous polyacrylonitrile nanofibrous membranes grafted with polyethyleneimine for post-combustion CO2 capture Yufei Zhang, Jiming Guan, Xianfeng Wang, Jianyong Yu, and Bin Ding ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14635 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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ACS Applied Materials & Interfaces

Balsam pear skin-like porous polyacrylonitrile nanofibrous membranes grafted with polyethyleneimine for post-combustion CO2 capture Yufei Zhang,† Jiming Guan,† Xianfeng Wang,*,†,§ Jianyong Yu,§ and Bin Ding†,§

†

Key Laboratory of Textile Science & Technology, Ministry of Education, College of Textiles,

Donghua University, Shanghai 201620, China §

Innovation Center for Textile Science and Technology, Donghua University, Shanghai

200051, China * Corresponding author: Prof. Xianfeng Wang (E-mail: [email protected])

1

ACS Paragon Plus Environment


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ABSTRACT Amine-containing sorbents have been extensively studied for post-combustion carbon dioxide (CO2) capture due to their ability to chemisorb CO2 from the flue gas. However, most sorbents are in the form of powders currently, which is not the ideal configuration for the flue gas separation because of the fragile nature and poor mechanical properties, resulting in blocking of the flow pipes and difficult recycling. Herein, we present a novel approach for the facile fabrication of flexible, robust and polyethyleneimine (PEI)-grafted hydrolyzed porous PAN

nanofibrous

membranes

(HPPAN-PEI

NFM)

through

the

combination

of

electrospinning, pore-forming process, hydrolysis reaction and the subsequent grafting technique. Excitedly, we find that all the resultant porous PAN (PPAN) fibers exhibit a balsam pear skin-like porous structure due to the selective removal of poly(vinyl pyrrolidone) (PVP) from PAN/PVP fibers by water extraction. Significantly, HPPAN-PEI NFM retain their mesoporosity, as well as exhibit good thermal stability and prominent tensile strength (11.1 MPa) after grafting, guaranteeing their application in CO2 trapping from the flue gas. When expose to CO2 at 40 °C, the HPPAN-PEI NFM show an enhanced CO2 adsorption capacity of 1.23 mmol g-1 (based on the overall quantity of the sample) or 6.15 mmol g-1 (based on the quantity of grafted PEI). Moreover, the developed HPPAN-PEI NFM display significantly selective capture for CO2 over N2 and excellent recyclability. The CO2 capacity retain 92% of the initial capacity after 20 adsorption/desorption cycle tests, indicating that the resultant HPPAN-PEI NFM have good long-term stability. This work paves the way for fabricating NFM-based solid adsorption materials endowed porous structure applied to efficient post-combustion CO2 capture.

KEYWORDS: electrospinning, porous nanofibrous membranes, solid adsorption materials, grafting, CO2 capture, CO2/N2 selectivity 2


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1. INTRODUCTION Massive emission of carbon dioxide (CO2) worldwide has caused severe global warming, rising of sea level, and a series of environmental problems, which is an alarming threat to the environment and public health.1-5 It was reported that the concentration of CO2 in earth’s atmosphere has increased from a preindustrial value of ~280 ppm to 404 ppm in April 2016, the highest level in 400,000 years.6-8 Therefore, CO2 capture has been given tremendous attention and it is urgent necessary to develop the carbon capture and storage (CCS) technology to reduce the global excessive CO2 emission concentrated in the flue gas.9 In recent decades, various technologies such as amine solvents, solid sorbents, or membranes have been applied to CCS.10-12 Among these technologies, aqueous amine solution as a reasonable solvent for CO2 absorption has been intensively researched and used industrially.13 However, the amine scrubbing process tends to be energy-intensive, costly, as well as being toxic and corrosive in nature.11,14 To overcome these limitations, novel solid sorbent materials such as zeolites,15 metal-organic frameworks (MOFs),16 covalent organic polymers,17 porous polymer networks,18 and porous carbons,19 have been investigated as promising alternatives by virtue of their high porosity, non-corrosiveness, high thermal stability, and lower renewable energy consumption. In order to further improve CO2 adsorption capacity and selectivity for CO2 over N2, amine-functionalized porous materials have attracted wide attention due to their effective ability to chemisorb CO2. The sorbents can be prepared by impregnation with polymeric amines such as polyethyleneimine (PEI) into porous supports.20-22 Qi et al. reported a novel nanocomposite sorbent for CO2 capture based on PEI-impregnated mesoporous silica capsules, however, the specific surface area of the nanocomposite sorbent decrease dramatically from 1320 to 3.15 m2 g-1 when PEI loading is 75 wt%, decreasing by 99.8%.5 Li and co-workers developed a hybrid inorganic-organic magnetic nanosorbent by coating PEI 3



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onto magnetite nanoparticles via sonication and evaporation methods,23 similarly, the specific surface area of the magnetic nanosorbent significantly decrease (from 39.6 to 3.6 m2 g-1) after loading with 17 wt% PEI, decreasing by 91%. The above studies imply that pores will be plugged by impregnation method after loading large amounts of PEI on the surface of sorbents, thus it is pretty difficult for CO2 molecules to diffuse into the interior of PEI multilayers and porous sorbents, limiting its enhancement in adsorption capacity. Grafting of amine-containing functional groups based on chemical reactions can ease pores plugging problems of impregnation method, as well as provide highly dispersed and stable CO2-adsorption sites.24,25 These amine-containing functional groups are covalent bonded to the surface of the porous giving rise to amine porous nanofibers with high selectivity towards CO2. For example, Chai et al. reported the amine functionalization of mesoporous carbon via chemical grafting process, the special surface area decrease by 60%.25 It suggests that chemical grafting is a better way to reduce the decline range of the specific surface area compared with impregnation method. Therefore, the amine-grafted solid sorbents have shown great potential to meet the desired CO2 adsorption capacity, as well as good cycle stability. However, the above-mentioned solid sorbent materials are mainly in the form of particles or powders, which suffer from several drawbacks such as fragile nature and low mechanical strength, causing issues such as blocking of the pipelines and recycling problems during the flue gas separation.26,27 To solve these problems, it is essential to develop flexible, continuous, and robust solid sorbent materials with porous structures for post-combustion CO2 capture. Recently, electrospinning is believed to be the most versatile and scalable nanofabrication technique, which is capable of fabricating a variety of continuous and structure-changeable nanofibrous membranes (NFM).28-33 NFM would be a good candidate as solid sorbent materials for post-combustion CO2 capture due to their outstanding features such as low cost, large surface area, easy-to-design pore structure and high pore volume, good continuity and 4


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flexibility, as well as easy to be functionalized, etc.7,34 However, NFM also have some drawbacks, such as the strength of the NFM obtained directly by electrospinning should be further increased to meet practical requirements, and the preparation rate also need to be further improved for increasing production. In this contribution, we demonstrate a novel strategy for creating flexible and robust PEI-grafted hydrolyzed porous PAN (HPPAN-PEI) NFM with a balsam pear skin-like nanoporous structure through the combination of electrospinning method, pore-forming process, hydrolysis reaction and the subsequent grafting technique. The as-prepared HPPAN-PEI NFM with porous structure are excellent sorbent materials for post-combustion CO2 capture. To the best of our knowledge, there are no reports on porous PAN (PPAN) NFM grafted with PEI for CO2 capture up to now. This study aims at generating PEI-grafted NFM-based sorbents with high surface area, tailorable porous structure, enhanced CO2 capacity, excellent CO2/N2 selectivity and recyclability, as well as prominent tensile strength and good thermal stability. Thus, our work has revealed, for the first time, the developed HPPAN-PEI NFM can be potentially applied to the efficient post-combustion CO2 capture.

2. EXPERIMENTAL SECTION 2.1. Materials. Polyacrylonitrile (PAN, Mw=90 000) was purchased from Kaneka Co., Ltd., Japan. Poly(vinyl pyrrolidone) (PVP, Mw=1 300 000), polyethyleneimine (PEI, Mw=600, 99%), absolute ethanol (EtOH, 99.5%), and hydrochloric acid (HCl, 37 wt%) were all bought from Shanghai Aladdin Reagent Co., Ltd., China. N,N-Dimethylfomamide (DMF) was supplied by Macklin Biochemical Co., Ltd., China. Sodium hydroxide (NaOH) was provided by Macklin Biochemical Co., Ltd., China. CO2 (≥ 99.99%) and N2 (≥ 99.999%) used for thermogravimetric analysis (TGA) were obtained through Shanghai Cheng Gong Gas Industry Co., Ltd., China. All chemicals were employed without further processing. Deionized (DI) water produced by a Water Purification System (UPT-11-20T) was used 5



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throughout this work. 2.2. Preparation of PAN/PVP NFM. PAN/PVP NFM were fabricated by the electrospinning technique as follows: briefly, PAN powder was firstly dried at 70 °C in vacuum for 2 h to remove water,35,36 then 8 wt% of PAN powder was added into DMF slowly with vigorous stirring for 30 min. Subsequently, PVP powder was dissolved in the above PAN/DMF solution with various PAN/PVP weight ratios of 8/7.5, 8/10, 8/12.5, and 8/15, respectively. The blended solution (50 g of the total weight) was vigorously stirred for 12 h. The PAN/PVP NFM were prepared under a constant spinning environment (ambient temperature 23 ± 2 °C and relative humidity 45 ± 3%) utilizing a DXES-3 electrospinning machine (SOF Nanotechnology Co., Ltd., China). Firstly, the as-prepared homogeneous PAN/PVP solution was transferred into a plastic syringe (10 mL) and kept a tip-to-collector distance of 20 cm. A high voltage of 30 kV and feed rate of 1.5 mL h-1 were applied. The obtained NFM were collected on nonwoven fabric covered grounded metallic rotating roller. Finally, the resulting free-standing PAN/PVP NFM were dried at 70 °C for removing the residual solvent. The obtained white membranes with various weight ratios of PAN/PVP: 8/7.5, 8/10, 8/12.5, and 8/15 were denoted as PAN/PVP-1, PAN/PVP-2, PAN/PVP-3, and PAN/PVP-4 NFM, respectively. 2.3. Preparation of PPAN NFM. The as-synthesized PAN/PVP NFM were added into the appropriate amount of DI water, which was placed in a Teflon-lined stainless steel autoclave, tightly capped, then followed by heating at 100 °C for 8 h to remove PVP from the hybrid fibers. After cooling to room temperature naturally, the liquor was decanted and the obtained white membranes were washed four times with DI water to remove the residual extracted PVP. Finally, the as-synthesized PPAN NFM were achieved by drying the samples in an oven at 100 °C for 2 h. The prepared white NFM from various weight ratios of the original PAN/PVP (8/7.5, 8/10, 8/12.5, and 8/15) were denoted as PPAN-1, PPAN-2, PPAN-3, and 6


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PPAN-4 NFM, respectively. 2.4. Preparation of HPPAN-PEI NFM. PEI-grafted PPAN NFM were fabricated using two-step process by combining the hydrolysis reaction with the subsequent grafting technique, which were described briefly as below. Firstly, 0.1 g of PPAN NFM with the highest surface area and optimized pore structure in all samples were immersed into a mixed solution containing 28 wt% DI water, 2 wt% NaOH, and 70 wt% EtOH in a flask at 80 °C for 6 min, and then rinsed with DI water for four times. Subsequently, the hydrolyzed PPAN (denoted as HPPAN) NFM were prepared by dipping in an appropriate amount of HCl (0.1 mol mL-1) for 2-3 min, rinsing with DI water repeatedly until the rinsed solution became neutral, and drying in an oven at 100 °C for 2 h for further modification. The as-prepared pale yellow HPPAN NFM were weighed and recorded as Wo. The preparation procedure of the chemical grafting of PEI onto HPPAN NFM as follows: in a typical preparation, the 50 wt% PEI solution was prepared by dissolving PEI into DI water with vigorous stirring. Subsequently, HPPAN NFM were added to PEI solution (20 g), and then the grafting reaction was performed in a water-bath at 90 °C for 6 h. After reaction, the PEI-grafted NFM were washed with DI water thoroughly until reaching a neutral pH, then dried at 100 °C for 2 h to get light yellow PEI-grafted HPPAN (named as HPPAN-PEI) NFM. After drying, weighed and recorded as Wg. The grafting ratio was obtained by the weight variation of membranes before and after grafting, and then calculated according to the following formula:

D g (% ) =

Wg − Wo × 100 Wo

(1)

2.5. Preparation of PEI-impregnated PPAN-2 NFM. PEI-impregnated PPAN-2 NFM were fabricated using a well-studied wet impregnation method.37 Briefly, 10 mg of PEI was dissolved in 50 mL of EtOH under stirring for 30 min. After that, 30 mg of PPAN-2 NFM were immersed into transparent solution for 30 min under continuously stirring, and then the impregnation process was carried out in the rotary evaporator (PV 10 Basic Plus D, 7



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Wilmington, NC) at 60 °C for 2 h under vacuum. Subsequently, the NFM were dried in vacuum at 50 °C for 6 h. PEI-impregnated PPAN-2 NFM with ~20 wt% loading of immobilized PEI were fabricated. The loading amount of PEI on the PPAN-2 NFM was obtained by comparing the weight changes of the PPAN-2 NFM before and after impregnation. 2.6. Characterization. The surface and cross-sectional morphologies of the NFM were observed through S-4800 field emission scanning electron microscopy (FE-SEM). In particular, the cross-sectional FE-SEM samples were prepared as follows: a small piece of dried fibrous membrane was clamped with a tweezers and immersed in liquid nitrogen for ~10 seconds, then the fibrous membrane was immediately removed from liquid nitrogen after breaking with tweezers, and finally, the sample was dried in a vacuum oven at room temperature for 1 hour. N2 adsorption-desorption isotherms were evaluated at 77 K using an physisorption analyzer ASAP 2020, and on the basis of which, the specific surface area and pore size distribution were performed using Brunauer-Emmett-Teller (BET) and Barrett-Joynes-Halenda (BJH) models, respectively. Fourier transform infrared (FT-IR) spectra was conducted on Nicolet iS10 spectrometer in the range of 3700-900 cm-1. TGA of HPPAN-PEI NFM was tested by the thermogravimetric analyzer (SDT Q600) from room temperature to 800 °C at a heating rate of 10 °C min-1 under N2 flow. Mechanical properties of the NFM were tested by using the XQ-1A tensile testing machine according to the international standard (ISO 1798:2008). 2.7. CO2 adsorption measurements. The CO2 adsorption performance, cycle performance, and CO2/N2 selectivity of HPPAN-PEI NFM were studied using TGA. The setup of CO2 adsorption performance testing was shown in Figure S1 (Supporting Information). A flow 8


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rate of 200 mL min-1 was applied for CO2 and N2 gases. Similar to our previous protocols,38,39 approximately 4 mg of dried HPPAN-PEI NFM were cut into small pieces, and then placed into the TGA microbalance alumina sample cell. Subsequently, the program was set as follows: the sample was heated to 105 °C under N2 atmosphere for 60 min to remove any absorbed CO2 and moisture, after that, the temperature was reduced to 40 °C (or 25, 60, 80 °C for investigating the effect of adsorption temperature on the adsorption process), pure dry CO2 was introduced for 70 min at 40 °C. The desorption process was performed by switching to pure N2 to desorb CO2 for 40 min at 105 °C. The weight variation of the samples was recorded and adsorption capacity (based on overall quantity of the sample or based on the quantity of grafted PEI) in mmol g-1 was calculated during the CO2 adsorption processes. The CO2 adsorption capacity based on the quantity of grafted PEI (mmol g-1) was calculated as follows: (adsorption capacity based on overall quantity of the sample)/(PEI grafting ratio). In order to evaluate the recyclability of the samples, after the first adsorption, the samples were regenerated by heating under N2 at 105 °C for 40 min, followed by introducing CO2 at 40 °C for another adsorption process. The adsorption/desorption process was continued for 20 cycles. Adsorption testing of N2 was carried out similarly. 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.23,40,41

3. RESULTS AND DISCUSSION As schematically shown in Figure 1, we designed the HPPAN-PEI NFM based on two crucial processes: (1) balsam pear skin-like PPAN NFM were obtained by water extraction, and (2) HPPAN NFM must be grafted firmly with PEI to improve CO2 adsorption capacity and

9



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CO2/N2 selectivity. The first procedure was satisfied using electrospinning method and pore-forming process, the formation of nanoporous structure was due to the selective removal of PVP from electrospun PAN/PVP fibers, which based on a way that phase separation taken place during water extraction. To satisfy the second procedure, we adopted a scalable and readily accessible method. Before grafting, the PPAN NFM are used as support, and are directly hydrolyzed to produce a large number of carboxyl groups on the surface for fully reacting with amino groups in PEI. Finally, the PEI was successfully grafted onto balsam pear skin-like porous PAN NFM for post-combustion CO2 capture.

Figure 1. Schematic illustration of the synthesis of PEI-grafted balsam pear skin-like porous PAN NFM. Figure 2 presented FE-SEM images of the NFM before and after removal of PVP by water treatment, which revealed that all of the fibers (i.e. PAN/PVP and PPAN fibers) were randomly oriented and continuous as three-dimensional (3D) open-cell nonwoven geometry with a uniform fiber diameter distribution. Typically, as shown in Figure 2a-d, the PAN/PVP NFM with various weight ratios of PAN/PVP (8/7.5, 8/10, 8/12.5, and 8/15) exhibited an 10


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obvious smooth surface morphology, simultaneously these fibers were highly interlaced each other. Additionally, the fiber diameter distributions demonstrated that the average diameter increased gradually with increased weight concentration of PVP, ranging from 470 to 1100 nm (Figure S2a-d). Importantly, the PAN/PVP NFM could be scaled up to 60 × 70 cm2 (Figure S3) by electrospinning, which is of great significance for practical application. Then, after removing PVP, we were excited to find that uniform rough structure appeared on the surface of each fiber (Figure 2e-h), and the average diameter also increased gradually with increased weight concentration of removed PVP (Figure S2e-h), ranging from 381 to 731 nm. Significantly, high-magnification FE-SEM images (inset of Figure 2e-h) clearly showed that all the resultant PPAN fibers have a balsam pear skin-like porous structure due to the phase separation resulting from the selective removal of PVP from PAN/PVP fibers during water extraction process. This selective removal of PVP owing to its solubility in water resulted into the pores were uniformly distributed on each fiber featured with hierarchical roughness and nanotexture. The balsam pear skin-like porous structure not only exists on the surface of the resulting fibers, but also in the interior. Furthermore, it could be speculated that the formation of wrinkled fiber surface may be due to the residual PVP caused aggregation owing to the hydrogen bonds in PVP after water extraction. Evidence for the removal of PVP also came from the FT-IR spectral analysis. To give further insight into the internal porous structure of single fiber after removing PVP, the cross-sectional structure of PPAN NFM were investigated (Figure 2i-l), indicating that the interior of the PPAN fibers retained a highly porous structure throughout. Based on these observations, it could be concluded that the versatile nanoporous structure of PPAN NFM-based solid sorbents would facilitate the gas diffusion into and out of the porous NFM and PEI grafting, which played an important role in improving CO2 adsorption performance. Besides, the electrospun fibers exhibited a large number of porous structures among the fibers, which may significantly promote the 11



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contact between the sorbents and CO2, thus helping the adsorption process.

Figure 2. FE-SEM images of (a) PAN/PVP-1, (b) PAN/PVP-2, (c) PAN/PVP-3, (d) PAN/PVP-4, (e) PPAN-1, (f) PPAN-2, (g) PPAN-3, and (h) PPAN-4 NFM. Insets show the corresponding high-magnification FE-SEM images. The inset photograph of (f) shows the as-prepared PPAN-2 NFM with flexibility. Cross-sectional FE-SEM images of (i) PPAN-1, (j) PPAN-2, (k) PPAN-3, and (l) PPAN-4 NFM. To evaluate the influence of pore-forming on the electrospun PAN/PVP NFM, the hierarchical pore structure was analyzed adopting N2 adsorption-desorption isotherms at 77 K. As displayed in Figure 3a, all curves presented type IV characteristics with an obvious adsorption hysteresis loop according to the IUPAC classification.42,43 It could be observed that a series of typical physical adsorption behaviors including monolayer adsorption, multilayer adsorption and capillary condensation phenomenon, demonstrating the characteristics of mesopores within the as-prepared PPAN NFM.44,45 Significantly, when p/p0 > 0.8, the capillary condensation of N2 occurred in mesopores, resulting in an abrupt rise in N2 adsorption.44 And the narrow H1 hysteresis loop during the overall pressure region suggested 12


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that the pores are open, consisting with FE-SEM images (Figure 2i-l). The pore structure parameters of PPAN NFM were listed in Table 1. Clearly, the BET surface area of PPAN NFM slightly increased from 43.22 to 50.53 m2 g-1 with the weight concentration of removed PVP increasing from 7.5 to 10 wt%. However, the BET surface area of PPAN-3 (42.83 m2 g-1) and PPAN-4 (33.14 m2 g-1) NFM decreased slightly due to the increased fiber diameter and packing density. Furthermore, considering that the pore-forming process is important for the generation of high surface area as well as hierarchical pore structure of fibers, thus the PAN/PVP NFM were investigated by N2 adsorption-desorption method (Figure S4). Thus, we compared the average fiber diameter (Figure S2) of PAN/PVP NFM to PPAN-2 NFM, confirming PAN/PVP-1 NFM with a diameter of 470 nm as the contrast sample due to its average fiber diameter was close to that of PPAN-2 NFM. Here, the curve also exhibited an isotherm of type IV, the pore size distribution, the BET surface area, and the pore structure parameters of PAN/PVP-1 NFM were displayed in the insets of Figure S4. It was worth noting that the BET surface area was as low as 5.09 m2 g-1, which was about ten times smaller than that (50.53 m2 g-1) of PPAN-2 NFM. Hence, this result further proved that the BET surface area and pore volume were indeed increased efficiently after removing PVP. Meanwhile, as displayed in Figure 3b, the typical BJH model was applied to further explore the porous structure of NFM.46 The PPAN NFM exhibited a typical polydisperse mesoporous feature and the mesopore sizes were chiefly centered at around 20-24 nm, which was much larger than the kinetic diameter of CO2 gas molecule and thus could promote CO2 adsorption and diffusion. Combining total pore volume with the cumulative mesopore volume of BJH model, the mesopore volume fraction could be obtained, which was 93.6%, 94.7%, 94.0%, and 94.1%, respectively (Table 1). These results also suggested that the nanoporous structure and high surface area could efficiently provide a large numbers of active sites for grafting PEI or trapping CO2. Therefore, the PPAN NFM are potential 13



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candidates for efficient post-combustion CO2 adsorption application. Ultimately, the PPAN-2 NFM were selected as supported sample for further experiments due to their porous structure and the largest BET surface area in all PPAN NFM, which could be beneficial for grafting of PEI and CO2 diffusion.47,48

Figure 3. (a) N2 adsorption-desorption isotherms and (b) BJH pore size distribution curves of PPAN NFM.

Table 1 Pore structure parameters of PPAN NFM. Sample

SBETa (m2 g-1)

Vtotalb (cm3 g-1)

Vmesoc (cm3 g-1)

PVFmesod (%)

PPAN-1

43.22

0.2035

0.1904

93.6

PPAN-2

50.53

0.2277

0.2156

94.7

PPAN-3

42.83

0.1952

0.1834

94.0

PPAN-4

33.14

0.1326

0.1248

94.1

Note: a BET specific surface area. b Total pore volume was calculated at P/P0 = 0.99. c Vmeso was calculated by the BJH method. d PVFmeso denotes the mesopore volume fraction. PPAN NFM were grafted with PEI combining the hydrolysis reaction with the subsequent grafting technique for improving the CO2 adsorption performance. As displayed in the inset of Figure 4a, the HPPAN-PEI NFM still remained flexible nature. Notably, it could be seen that the overall parts of the obtained fibers still remained good fibrous and balsam pear 14


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skin-like porous morphologies (Figure 4a), indicating that the grafting reaction did not substantially destroy the original morphology. At the same time, it clearly showed that the surface of HPPAN-PEI NFM was also featured with nanotextures and hierarchical roughness. A careful examination of the cross-sectional FE-SEM image (Figure 4b) revealed that the NFM retained their interior porous structure after grafting PEI. Moreover, there was no core-shell structure for HPPAN-PEI fibers, which suggested that the interaction between PEI and PPAN NFM was linked by the chemical grafting reaction rather than physical coating. Besides, we compared the FE-SEM image of PEI-impregnated PPAN-2 NFM (Figure S5) to PEI-grafted NFM (Figure 4a), confirming the pores of PEI-impregnated PPAN-2 NFM were plugged after loading large amounts of PEI on the surface of NFM. To prove the pores plugging problems of impregnation method exactly, we further investigated the PEI-impregnated PPAN-2 NFM using N2 adsorption-desorption method (Figure S6). Here, the curve also exhibited an isotherm of type IV, the pore size distribution, the BET surface area, and the pore structure parameters of PEI-impregnated PPAN-2 NFM were displayed in the insets of Figure S6. Predictably, the BET surface area and the total pore volume were as low as 7.22 m2 g-1 and 0.0408 cm3 g-1, respectively, which were lower than that (16.79 m2 g-1 and 0.0859 cm3 g-1) of HPPAN-PEI NFM. Hence, this result further proved that pores were indeed plugged by impregnation method after loading large amounts of PEI on the surface of sorbents, and grafting of amine-containing functional groups based on chemical reactions can ease pores plugging problems of impregnation method. The inset of Figure 4b exhibited the model of single fiber for the as-prepared HPPAN-PEI NFM. More comprehensive evidence for the PEI was successfully grafted onto the PPAN NFM also came from the FT-IR spectral 15



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analysis. Although PEI was already grafted onto the PPAN NFM, the porous characteristics throughout the NFM was another vital factor that would bring great improvement on CO2 adsorption performance. Therefore, we next focused attention on determining whether the grafting process affected the structure of pores throughout the fibers. The N2 adsorption-desorption isotherm (Figure 4c) of HPPAN-PEI NFM was collected at 77 K to evaluate the BET surface area and pore structure. The curve exhibited type IV isotherm characteristic. As shown in the insets of Figure 4c, the corresponding BET surface area and BJH adsorption cumulative mesopore volume were calculated to be 16.79 m2 g-1 and 0.0768 cm3 g-1, respectively. The mesopore size chiefly centered at around 21 nm was close to that of PPAN NFM (20-24nm). However, it could be observed that the BET surface area and pore volume decreased in a certain extent after PEI grafting onto PPAN NFM, which could possibly be credited to the uniform loading of PEI throughout fibers. Additionally, minor increase in average fiber diameter from 461 to 489 nm (Figure 4d) owing to the deposition of PEI was observed which also played a certain role in reducing the BET surface area and pore volume.

16


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Figure 4. (a) The surface and (b) the cross-sectional FE-SEM images of HPPAN-PEI NFM. The insets of (a) and (b) show the as-prepared HPPAN-PEI NFM with flexibility and the model of its single fiber, respectively. (c) N2 adsorption-desorption isotherm of HPPAN-PEI NFM, the insets show the BJH pore size distribution curve and pore structure parameters of HPPAN-PEI NFM, respectively. (d) The diameter distribution of HPPAN-PEI NFM. As shown in Figure 5, FT-IR analysis was further carried out to verify the conversion of characteristic groups in the reaction processes. All spectra displayed a main band at 2245 cm-1, which belonged to C≡N stretching vibration, indicating the presence of PAN. The band located at 1663 cm-1 was attributed to the C=O stretching vibration, the band located at 1450 cm-1 was assigned to the –CH2 bending vibration, and the band located at 1285 cm-1 was attributed to the stretching vibration of C–N, these three characteristic peaks proved that the existence of PVP. After removing PVP, it was found that the strength of these characteristic absorption peaks of PVP were weakened obviously, revealing that most of PVP was removed 17



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and a little bit of PVP was retained in PPAN NFM. The reason might be that PVP and PAN were partially miscible and the entangled polymer chains formed a semi-interpenetrating polymer networks resulting in PVP was embedded and cannot be removed by hot water.49 HPPAN NFM showed high pronounced peaks at 1667 cm-1 (C=O) and 1409 cm-1 (–C(=O)– O), which confirmed that the PPAN NFM were hydrolyzed successfully. The FT-IR spectra of HPPAN-PEI NFM displayed the presence of the broad band located around 3300 cm-1, which was assigned to N–H stretching vibration of primary amines (R–NH2).50 The peaks found in the region from 1200 to 1700 cm-1 were relevant because they provided important evidence for grafting of amine and the formation of amide bond. The bands appeared at 1563 and 1485 cm-1, investigated after grafting PEI, were ascribed to the presence of asymmetric and symmetric bending N–H primary amines.51 The characteristic peak around 1654 cm-1 was assigned to the stretching vibration of C=O, and the peak at 1260 cm-1 belonged to the C–N stretching vibration. The –NH2 group and –NH group showed the presence of primary amine and secondary amine in the HPPAN-PEI NFM, respectively. These changes described above all revealed that PEI was successfully grafted onto the HPPAN NFM.

Figure 5. FT-IR spectra of PAN/PVP-2, PPAN-2, HPPAN, and HPPAN-PEI NFM. 18


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We next investigated the CO2 adsorption/desorption behaviors of the HPPAN-PEI NFM through TGA method. To simulate the temperature of the flue gas, we adjusted the adsorption temperature to 40 °C and pure dry CO2 was introduced for 70 min. As shown in Figure 6a, the CO2 adsorption capacity at 40 °C reached 1.23 mmol g-1 (based on the overall quantity of the sample) or 6.15 mmol g-1 (based on the quantity of grafted PEI) in 70 min. The desorption process was performed by switching to pure N2 to desorb CO2 for 40 min at 105 °C. A CO2 adsorption capacity of >1 mmol/ (g sorbent) was indicated to potentially reduce the cost of CO2 sequestration.52 Furthermore, to evaluate the effect of the presence of mesopores inside the support on CO2 capture, the CO2 adsorption capacity of PEI-grafted PAN/PVP-1 (named as PAN/PVP-1-PEI) and HPPAN-PEI NFM (both were treated with the same grafting method) at 40 °C was investigated (Figure S7). The capacity of PAN/PVP-1-PEI NFM reached 0.55 mmol g-1, which obviously lower than that of HPPAN-PEI NFM, revealing that the importance of hierarchical pore structure on enhancing the CO2 diffusion to the inside of NFM. In addition, we were pleased to find that a significant increase in tensile strength after forming pores or grafting PEI (Figure S8a). In particular, the tensile strength of the resulting HPPAN-PEI NFM increased to 11.1 MPa, because the amine groups in PEI could react with the carboxyl groups produced by hydrolysis as an effective curing agent that could increase the interfacial crosslinking density and strengthen interfacial adhesion, which were in favor of the stress transfer between fibers.53,54 Meanwhile, because of the hyperbranched structure, PEI could crosslink fibers via physical twining. Moreover, the surface of porous fibers were rough, which may create a mechanical interlocking effect to prevent the pull out of fibers.55 Thus, the mechanical strength of HPPAN-PEI NFM was enhanced remarkably, which realized the preparation of flexible and robust NFM, contributing to resist the flue gas shock during post-combustion CO2 capture. It was known that the CO2 adsorption capacity varied with temperature, mainly due to the 19



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reaction process between the amine groups of PEI and CO2 molecules belonged to chemical adsorption.39 Figure 6a presented the CO2 adsorption/desorption performance of HPPAN-PEI NFM at different adsorption temperatures (25, 40, 60, and 80 °C), the results were summarized in Table 2. Evidently, the CO2 adsorption capacity decreased as the adsorption temperature increased. The capacity reached the highest value of 1.50 mmol g-1 (based on the overall quantity of the sample) or 7.50 mmol g-1 (based on the quantity of grafted PEI) at 25 °C. Thermodynamically, as the temperature increased, the chemical reaction between the amine groups of PEI and CO2 molecules was weakened, and the interaction between fibers and CO2 was also weakened.38 The observed temperature dependence could have resulted from an interplay between sorption kinetics and thermodynamics. While a higher temperature reduced the CO2 binding constant, it increased the accessibility of the amine groups.56 To prove that CO2 was successfully adsorbed into the HPPAN-PEI NFM, we further investigated the FT-IR spectra of HPPAN-PEI NFM after CO2 adsorption (denoted as HPPAN-PEI-CO2 NFM) but not desorption (inset of Figure 6d). Peak at 2369 cm-1 could be ascribed to the stretching vibration of CO2. Additionally, the –NH2, –NH, and –N groups in the HPPAN-PEI NFM will also react with CO2, and have a potential of forming carbamate (inserted equation of Figure 7). The spectrum suggested the generation of carbamate species according to characteristic of chemical adsorption, judging from the peaks observed in the range of 1200 to 1700 cm-1.57,58 For practical application, in addition to a high CO2 adsorption capacity, HPPAN-PEI NFM need to satisfy a good selectivity towards CO2 from gas mixtures. As a typical example, N2 is the major component in the flue gas. Figure 6b displayed the adsorption kinetics of CO2 and N2 over the HPPAN-PEI NFM using TGA at simulated the flue gas temperature of 40 °C, suggesting that CO2 capture at high adsorption rates, but N2 adsorption occurred at extremely slow rates. It could be seen that the CO2 and N2 adsorption capacity reached 1.23 and 0.046 20


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mmol g-1 (based on the overall quantity of the sample), respectively, this result was interesting because of the evidence that high CO2/N2 selectivity (S = 27) could be obtained. Additionally, the simulate curve of CO2/N2 selectivity during adsorption process shown in the inset of Figure 6b. It was found that higher selectivity could be achieved in a short adsorption time, evidently, the kinetic selectivity reached as high as 46 when the adsorption time was ~8 min. Thus, optimized membrane offered considerable level of selectivity indicating potential possibility of capturing CO2 from flue gas at short time cycles. From an economic point of view, a superior regenerative ability of solid sorbents is very crucial for reducing the cost for practical CO2 capture application. Thus, the cyclic stability of HPPAN-PEI NFM was assessed using TGA (Figure 6c). It could be calculated that the CO2 capacity retained 92% (1.13 mmol g-1) of the initial capacity (1.23 mmol g-1) after 20 cycles (Figure 6d), even at such a high desorption temperature (105 °C) for 40 min in each cycle. The excellent recyclability of HPPAN-PEI NFM may be attributed to the improved thermal stability of PEI (higher than desorption temperature of 105 °C) and the good chemical reaction reversibility between CO2 molecules and the amine groups of PEI. The TG curve of HPPAN-PEI NFM was presented in Figure S8b, it could be observed that HPPAN-PEI NFM have an onset thermal decomposition temperature of about 290 °C, revealing reasonably excellent thermal stability. Whereas, the adsorption capacity loss slightly may be due to the weight loss of grafted PEI during the temperature-swing regeneration process.59 Particularly, note that the amine molecules would react with –COOH group produced by hydrolysis reaction to form hydrogen bond, promoting PEI adhere to HPPAN NFM stably. To further evaluate the influence of grafting and impregnation methods on CO2 capture, the CO2 adsorption performance of PEI-impregnated PPAN-2 NFM at 40 °C was investigated (Figure S7). The capacity of PEI-impregnated PPAN-2 NFM reached 0.37 mmol g-1, which obviously lower than those of HPPAN-PEI and PAN/PVP-1 NFM obtained from grafting 21



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method, indicating that pores may be plugged by loading large amounts of PEI on the surface of PPAN NFM via impregnation method, limiting the CO2 diffusion into the interior of PEI multilayers and porous fibers, therefore reducing its adsorption capacity.

Figure 6. (a) CO2 adsorption/desorption of HPPAN-PEI NFM at different adsorption temperatures. (b) Selective adsorption of CO2 over N2 at 40 °C for HPPAN-PEI NFM. The inset simulates the curve of CO2/N2 selectivity during adsorption process. (c) Cycles of CO2 adsorption/desorption of HPPAN-PEI NFM. Each cycle consisted of flowing CO2 for 70 min at 40 °C and then flowing N2 for 40 min at 105 °C. (d) CO2 adsorption vs. cycle time. The inset shows FT-IR spectra of HPPAN-PEI-CO2 NFM.

Table 2 CO2 adsorption of HPPAN-PEI and PAN/PVP-1-PEI NFM. CO2 capacity (mmol g-1) Sample

Grafting ratio (%)

Adsorption temperature (°C)

Based on the overall quantity of the sample

22


Based on the quantity of grafted PEI

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HPPAN-PEI

20%

PAN/PVP-1-PEI

25

1.50

7.50

40

1.23

6.15

60

1.08

5.40

80

1.03

5.15

40

0.55

2.75

In order to further evaluate the advantages of the HPPAN-PEI NFM for CO2 capture, we quantitatively compared this work with other studies previously reported using the solid sorbents. As summarized in Table 3, the performances of solid sorbents were located in the unattractive level due to the low CO2 capacity and selectivity. On the contrary, the present HPPAN-PEI NFM clearly transcended the tradeoff between CO2 capacity and selectivity and exhibited a good CO2 adsorption capacity and much higher CO2/N2 selectivity. Additionally, the CO2 capacity and CO2/N2 selectivity of the HPPAN-PEI NFM are able to meet practical application and easy to operate at room temperature, outperforming previous solid sorbent materials in the form of particles. Table 3 Comparison of the structure parameters and properties of solid CO2 sorbents between this work and the previous reports.

Sorbent

SBET (m2 g-1)

Adsorption temperature (°C)

Nitrogen-doped porous carbon nanofiber webs

63-67

45

1.13-1.23

22

7

Amine-grafted polysuccinimide (PSI-100% MEA-EDA)

2.9667

40

0.93

-

58

Amine functionalized silica (HMS-F-50PEI)

17

25

0.87

-

60

Porous carbon (CP-2-600)

1700

25

3.9

5.3

41

Nitrogen doped porous carbon (NDPC-2-600)

1211

25

4.3

15.2

61

Supramolecular organic framework (SOF-7)

21

25

1.49

9.1

62

Amine-grafted carbon nanotubes

198

25

0.93

-

63

CO2 capacity S (CO2/N2) (mmol g-1)

23


Ref.


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(APTS-grafted CNTs) NH2-MCM-36

145

25

1.2

5.45

64

HPPAN-PEI NFM

16.79

25

1.50

33

This work

HPPAN-PEI NFM

16.79

40

1.23

27

This work

The mechanism of CO2 adsorption process from the flue gas by HPPAN-PEI NFM (Figure 7) elaborated as below: firstly, the CO2 molecules were rapidly trapped by reacting with the amino groups grafted on the surface of the fibers, so that which were rapidly adsorbed on the surface of the fibers. The CO2 adsorption caused by grafting of PEI was mainly attributed to the chemical reactions between CO2 molecules and the amine groups of PEI, which could form carbamate and other CO2-amine complexes, and the three equations of the chemical adsorption reaction were presented in the inset of Figure 7. Pure PEI molecules are composed of primary, secondary and tertiary amines, triggering reactions between 2 mol of amine and 1 mol of CO2 under dry conditions. After the adsorption sites on the surface of the fibers were fully occupied by CO2 molecules, the unreacted CO2 molecules on the surface gradually diffused into the fibers through its balsam pear skin-like porous structure on the surface and internal porous structure throughout until the adsorption eventually reached saturation. It revealed that the importance of high surface area and hierarchical pore structure on improving CO2 adsorption performance. Therefore, the PEI-grafted NFM would exhibit great potential to meet the desired CO2 adsorption capacity, excellent selectivity for CO2 over N2, and good cycle stability through chemical reactions between CO2 and the amine groups of PEI. It could be concluded that both porous structure throughout the fibers and the grafting of PEI for HPPAN-PEI NFM played an important role in determining the CO2 capacity, CO2/N2 selectivity, and recyclability. Importantly, the mechanism of CO2 adsorption on NFM including physical and chemical adsorption, showing HPPAN-PEI NFM are potential solid adsorption materials for post-combustion CO2 capture.

24


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Figure 7. Schematic illustration of CO2 adsorption process from the flue gas by HPPAN-PEI NFM.

4. CONCLUSION In summary, we developed a novel strategy for constructing flexible, robust and PEI-grafted PAN NFM with a balsam pear skin-like nanoporous structure for CO2 capture, which through the combination of electrospinning, pore-forming process, hydrolysis reaction and the subsequent grafting technique. Owing to the hierarchical pore structure and grafting of amine groups, HPPAN-PEI NFM were endowed with CO2 capacity of 1.23 mmol g-1 (based on the overall quantity of the sample) or 6.15 mmol g-1 (based on the quantity of grafted PEI) at 40 °C, excellent CO2/N2 selectivity (27), recyclability, good thermal stability and prominent tensile strength (11.1 MPa), indicating that HPPAN-PEI NFM are potential solid adsorption materials for post-combustion CO2 capture. The results pave the way for keeping on the study work on high-performance solid CO2 adsorption materials with the aim of achieving higher capacity and excellent cycle stability by applying electrospun NFM.

ASSOCIATED CONTENT Supporting Information. The setup of CO2 adsorption performance testing (Figure S1). The 25



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diameter distributions of PAN/PVP and PPAN NFM (Figure S2). Photograph of the large-scale (60 × 70 cm2) PAN/PVP NFM (Figure S3). N2 adsorption-desorption isotherm of PAN/PVP-1 NFM, and the insets show the BJH pore size distribution curve and pore structure parameters of PAN/PVP-1 NFM, respectively (Figure S4). FE-SEM image of PEI-impregnated PPAN-2 NFM (Figure S5). N2 adsorption-desorption isotherm of PEI-impregnated PPAN-2 NFM, and the insets show the BJH pore size distribution curve and pore structure parameters of PEI-impregnated PPAN-2 NFM, respectively (Figure S6). CO2 adsorption of HPPAN-PEI, PAN/PVP-1-PEI, and PEI-impregnated PPAN-2 NFM (Figure S7). Tensile strength of PAN/PVP-2, PPAN-2, and HPPAN-PEI NFM, TGA curve of HPPAN-PEI NFM (Figure S8). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (Nos. 51503028 and 51673037), the Shanghai Committee of Science and Technology (No. 15JC1400500), the Shanghai Rising-Star Program (No. 16QA1400200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. TP2016019), 26


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the National Key R&D Program of China (No. 2016YFB0303200), and the Fundamental Research Funds for the Central Universities.

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(27) Zhang, Y.; Yuan, S.; Feng, X.; Li, H.; Zhou, J.; Wang, B., Preparation of Nanofibrous Metal-Organic Framework Filters for Efficient Air Pollution Control. J. Am. Chem. Soc. 2016, 138, 5785-5788. (28) Si, Y.; Wang, L.; Wang, X.; Tang, N.; Yu, J.; Ding, B., Ultrahigh-Water-Content, Superelastic,

and

Shape-Memory

Nanofiber-Assembled

Hydrogels

Exhibiting

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338x154mm (300 x 300 DPI)


Balsam-Pear-Skin-Like Porous Polyacrylonitrile Nanofibrous

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