polyacrylonitrile composited nanofibers with enhanced adsorption performance of volatile organic compounds

Applied Surface Science 512 (2020) 145697

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Electrospun SiO2 aerogel/polyacrylonitrile composited nanofibers with enhanced adsorption performance of volatile organic compounds ⁎

Yuxi Yua, , Qingyan Maa, Ji-bin Zhangb, Guan-bin Liuc,

T

⁎

a

Fujian Key Laboratory of Advanced Materials, Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China Science and Technology on Reactor System Design Technology Laboratory, Nuclear Power Institute of China, Chengdu 610041, China c Clothing Research Institute, Xiamen University of Technology, Xiamen 361024, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Electrospinning SiO2 aerogel Composite nanofiber membranes Volatile organic compounds Thermal insulation Porous structure

Flexible polyacrylonitrile (PAN) composite nanofibers with high surface area functionalized by SiO2 aerogel were fabricated easily via the electrospinning technique for the first time. SiO2 aerogel with honeycomb porous structure assembled on the surface of PAN nanofibers as the solvent evaporated, which greatly improved the specific surface area of the resultant membranes. The high surface area played an important role in promoting the adsorption performance of membranes for volatile organic compounds (VOCs). The adsorption results showed chloroform was the most highly adsorbed among four VOCs (chloroform, xylene, formic acid and methanol). And the PAN membrane containing 100 wt% SiO2 aerogel (based on PAN) showed the highest VOCs adsorption capacity, which was 2.5–4.7 times (for four VOCs) greater than that of pristine PAN membrane and even higher than activated carbon. The unchanged VOCs adsorption capacity during 10 cycles demonstrated the resultant membranes had excellent regeneration. Furthermore, the porous structure of SiO2 aerogel/PAN composite membranes also enhanced their thermal insulation property. The membrane with 20 wt% SiO2 aerogel exhibited the lowest thermal conductivity among all samples. These excellent performances endow SiO2 aerogel/PAN composite membranes with the potential applications in VOCs adsorption and thermal insulation.

1. Introduction Electrospinning has become one of the main ways for the effective preparation of one dimensional (1D) nanofiber materials due to its manufacture simplicity, low cost, and process controllability and has been increasingly noticed in research and commercial applications over the past decade [1]. Various nanofibers can be fabricated by using electrospinning including organic fibers, inorganic fibers or organic/ inorganic composite fibers. The nanofibers can be deposited into different shapes of membranes by changing the received substrate and play an important role in various fields, such as filtration application, energy field, environmental engineering and protective clothing application [2,3]. The filtration application for removing harmful substances from air or water with various electrospun nanofibers has received widespread attention from many researchers in recent years due to their good adsorption performance and regeneration [4–6]. These filter media can protect people from both natural and man-made contaminants by filtering or adsorbing hazardous materials. Volatile organic compounds (VOCs) are organic chemicals with a low boiling point and high vapor pressure. They have been recognized

⁎

as one of the most important environmentally hazardous substances, arising from both anthropogenic emissions such as fossil fuels, chemical industries, automobile industries, paint industries and building materials and natural emissions [7,8]. Most VOCs are highly toxic and carcinogenic. Thus, the removal of VOCs emitted from diverse sources is desirable to decrease the negative effects on human health and the environment. Up to now, numerous VOCs treatment technologies have emerged, such as incineration, condensation, biological degradation, absorption, adsorption and catalysis oxidation [9]. Among these, adsorption technology has been recognized as an efficient and economical strategy because it has the potential to recover and reuse both adsorbent and adsorbate [10]. AC (activated carbon) is generally a reasonable choice for removing VOCs from the air as its high surface area and large pore volume allow it to adsorb the harmful gas at low concentrations [8,11]. However, AC has several disadvantages, including high-pressure drops on the adsorbent bed, declining adsorption capacity after initial adsorption and the difficulty of regeneration, which limited the application of AC [12]. Recently, many researchers focus on developing electrospun nanofiber membranes as an alternative to AC. Scholten et al.

Corresponding authors. E-mail addresses: [email protected] (Y. Yu), [email protected] (G.-b. Liu).

https://doi.org/10.1016/j.apsusc.2020.145697 Received 25 November 2019; Received in revised form 3 February 2020; Accepted 7 February 2020 Available online 07 February 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.

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reported that the polyurethane fibers have a similar sorption capacity to AC and have complete regeneration under ambient temperature and pressure conditions [12]. Other electrospun nanofibers modified by functional particles [4,11,13] also have been reported to possess VOCs adsorption performance to some degree. Nevertheless, the specific surface area of the above nanofiber membranes is only a dozen. Therefore, it is necessary to further increase the specific surface area of electrospun nanofiber membrane to promote its adsorption efficiency further. SiO2 aerogel is a class of typical mesoporous material with open foam-like structure distinguished for their high surface area, low density, high porosity and extremely low thermal conductivity, which makes it attractive for many applications, for instance, super thermal insulation, catalyst, adsorbent, drug delivery, and so on [14–17]. These outstanding features of SiO2 aerogel and the good regeneration make it very suitable for the removal of harmful substances from the environment [18,19]. However, the utilization of SiO2 aerogel has been hampered by their inherent fragility because of the weak pearl-necklace-like structure [20]. Currently, introducing fibers into the SiO2 aerogels by impregnation is one of the effective methods to improve aerogel brittleness to some degree, however, the complex preparation process and the harmful dust-releasing from the aerogel/fibers assemblies [21] are problems still. Electrospinning technology can fabricate composite nanofiber membranes with unique functionalities conveniently as mentioned above. In this paper, SiO2 aerogel and polyacrylonitrile (PAN) were successfully fabricated into a flexible nanofiber membrane with high surface area via electrospinning technology. To the best of our knowledge, the report is rare about SiO2 aerogel incorporated into the polymeric nanofiber by electrospinning technology directly, and no electrospun SiO2 aerogel/polymer composite membranes have been reported for VOCs adsorption. The form of membrane not only makes aerogel assembling in/on the fibers to address the problem of dust-releasing but is more convenient than powder or particle in practical application. We evaluated the ability of the PAN nanofiber membranes functionalized by SiO2 aerogel to adsorb VOCs from the air and investigated their thermal insulation performance at the same time. The combination of SiO2 aerogel and PAN fibers by electrospinning technology to fabricate a composite membrane with a high specific surface area presents a new dimension not only in the fields of environmental protection and energy conservation but in the fabrication of aerogel composites.

Table 1 Compositions of spinning solutions for nanofiber membranes. Samples PAN PAN PAN PAN PAN

+ + + + +

SA SA SA SA SA

0% 20% 50% 71.4% 100%

PAN (wt%)

SiO2 aerogel (wt%)

DMF (wt%)

12 10 8 7 6

0 2 4 5 6

88 88 88 88 88

12%. The mass ratios of SiO2 aerogel and PAN polymer in different spinning solutions were shown in Table 1. 2.3. Electrospinning Electrospinning was carried out by using the set-up shown in Fig. 1. As-prepared spinning solutions were loaded into a 5 ml syringe with a spinneret. Electrospinning parameters included 15 kV applied voltage, tip-to-collector distance of 10 cm and solution feed rate of 1.2 ml/h. The amount of spinning solution for each membrane was controlled to 22 ml ± 1 ml. After electrospinning, the electrospun fibrous membranes were dried for 12 h in an oven at 60 °C. The functionalized PAN nanofiber containing 0, 20, 50, 71.5 and 100 wt% of SiO2 aerogel were denoted as PAN + SA 0%, PAN + SA 20%, PAN + SA 50%, PAN + SA 71.4% and PAN + SA 100% respectively. 2.4. Characterization of electrospun composite nanofiber membranes The morphology of electrospun nanofiber membranes was observed by scanning electron microscopy (SEM) (SU-70, Hitachi) under an electron beam with an accelerating voltage of 5 kV and by high-resolution transmission electron microscopy (HR-TEM) (JEM-2100) with an acceleration voltage of 200 kV. The fiber diameter was measured from the SEM images by using SmileView software. 50 fibers were selected in total from top to bottom, from left to right in the SEM diagram to ensure the fibers in each part of the SEM diagram were selected. When the diameter of each fiber was measured, both the protruding part of the fiber caused by SiO2 aerogel and the rest of the fiber were measured. The resulting fiber diameter was the average of these measurements. Attenuated total reflection Fourier transform infrared (ATRFTIR) spectra were performed by using a Nicolet iS10. The surface area and pore size distribution of electrospun nanofiber membranes were analyzed with Brunauer-Emmett-Teller (BET) and Barrett-JoynerHalenda (BJH) methods from nitrogen gas adsorption and desorption at the temperature of liquid nitrogen (−196 °C) by using Micromeritics Tristar II 3020. Before the nitrogen adsorption, samples were degassed at 80 °C for 4 h. The thermogravimetric analysis (TGA) of electrospun nanofiber membranes was conducted by using a thermal analyzer (Netzsch STA449F3). The sample was maintained under an air atmosphere and was heated at 10 °C/min from room temperature to 800 °C. The thermal conductivity of electrospun nanofiber membranes was measured by using the transient plane heat source method (TCi, CTherm, Canada) at room temperature.

2. Materials and methods 2.1. Materials Polyacrylonitrile (PAN) and N, N dimethylformamide (DMF) were used as-received and purchased from Jilin Carbon Valley Carbon Fiber Co., China and Tianjin Datian Chemical Reagent Co., China respectively. Silica aerogel powder (hydrophilic) provided by Xiamen Nameite New Material Technology Co., China. Chloroform was purchased from Xilong Science Co., Ltd. Formic acid was purchased from Tianjin Damao Chemical Reagent. Methanol was purchased from Shantou Dahao Fine Chemical Co., Ltd. Xylene was purchased from Sinopharm Chemical Reagent Co., Ltd.

2.5. VOCs adsorption experiment VOCs adsorption was measured by using a homemade device [22]. In a typical chloroform adsorption experiment, a certain amount of sample was weighed and loaded into a glass vial, and then the glass vial containing the sample was weighed and hung in a reagent bottle saturated with the vapor of chloroform at 25 °C. After 24 h, the glass vial was taken out and weighed again. The adsorption capacity was calculated as C = (M1 − M0)/m × 1000 mg/g fiber, where M1 and M0 were the equilibrium and initial mass of the glass vial containing the sample respectively, m was the mass of the sample. Each sample was measured 5 times.

2.2. Preparation of SiO2 aerogel/PAN blend solution Different amounts of SiO2 aerogel (0, 20, 50, 71.4, and 100 wt% based on PAN) were added to DMF solvent at room temperature by magnetic stirring for 2 h followed by further 10 min ultrasonication. Then, the corresponding mass of PAN powder was added to the above dispersion followed by magnetic stirring to form a uniform SiO2 aerogel/PAN blend solution finally at room temperature. The total weight percentage of SiO2 aerogel and PAN polymer in solutions was 2

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SiO2 aerogel

PAN

SiO2 aerogel PAN nanofiber

SiO2 aerogel/PAN blend solution

Fig. 1. The schematic for preparation of pure PAN and SiO2 aerogel/PAN composite nanofiber membranes via electrospinning technology.

fibers, exhibited that the honeycomb porous structure of SiO2 aerogel existed on the surface of the fiber, which is contributed to improving the specific surface area of electrospun nanofiber membranes. In addition, from a macro perspective, the SiO2 aerogel/PAN composite nanofiber membrane, taking PAN + SA 100% for instance, had good flexibility, as shown in Fig. 3(d). And according to the relevant study [21], it can be reasonably speculated that the SiO2 aerogel/PAN composite nanofiber membrane can address the problem of dust-releasing of aerogel composites effectively due to the interaction between SiO2 aerogel and PAN fiber and the entrapment of the aerogel by PAN fiber. To identify the existence of SiO2 aerogel, elemental analysis of composite nanofiber membranes, taking PAN + SA 100% as a test sample, was performed by SEM-EDS-elemental mapping techniques. The mapping images show the SiO2 aerogel was successfully dispersed inside the nanofibers and had a uniform distribution within the fibers (as shown in Fig. 3(c)). To investigate the porous nature of electrospun nanofiber membranes, N2 adsorption-desorption measurement was carried out (as shown in Fig. 4). In accordance with IUPAC classification, the N2 adsorption-desorption isotherms of all samples showed IV adsorption isotherm with H2-type hysteresis loop at different relative pressure ranges, which demonstrates there is a capillary condensation, confirming the presence of mesoporous structures [25]. The result was further proved by the pore size distribution calculated by using the BJH method from the desorption branches of the isotherms of electrospun nanofiber membranes, which were concentrated at ~12 nm as shown in Fig. 4(b). The corresponding surface area, pore volume and pore size of different samples were listed in Table 2. Compared with the pristine PAN nanofiber, the addition of SiO2 aerogel significantly increased the specific surface area and pore volume of these composite membranes. The specific surface area of PAN + SA 100% was maximum (289.20 m2/g), which was almost 16 times that of the pure PAN membrane (18.13 m2/g). However, the specific surface area of SiO2 aerogel was higher than that PAN + SA 100%, which may because SiO2 aerogel is only part of the composition of PAN + SA 100% and the PAN nanofiber skeleton with a surface area of only 18.13 m2/g [26]. Besides, the diffusion barrier for N2 gas induced by the polymer layer may be also a reason [27]. Generally speaking, either reducing fiber diameter [28] or alleviating the agglomeration phenomenon of nanoparticles is beneficial to improve the specific surface area of composite nanofibers [29]. Although the composite fiber diameter became larger than pure PAN fiber and the agglomeration of aerogel occurred, the specific surface area of SiO2 aerogel/PAN composite nanofiber membranes expanded still

To investigate the dynamic adsorption performance, the adsorption capacity of the sample at different time intervals was measured. To study the recyclability, the sample after adsorption was degassed in an oven under 35 °C and ambient pressure for 12 h to remove the adsorbate for the adsorption experiment next time. To further prove the stability of the sample, the adsorption capacities for four VOCs were measured after the sample was treated by heating at 5 °C/min from room temperature to 260 °C and soaking in hydrochloric acid (1 M) for 12 h. 3. Results and discussion 3.1. Characterization of electrospun nanofiber membranes The microscopic morphology and fiber diameter distribution of SiO2 aerogel/PAN nanofiber membranes with different SiO2 aerogel content are shown in Figs. 2 and S1. From SEM images (Fig. 2(a)–(e)) and Fig. S1, it could be observed that the changes in the fiber morphology and the fiber diameter distribution correlated closely with the amount of SiO2 aerogel. Pure PAN nanofibers had free beads, were nonporous, and had an average fiber diameter of ~580 nm with uniform diameter distribution. With the addition of SiO2 aerogel, the aerogel began to appear on the PAN fiber in the form of irregularities and nonuniformities similar to the blackberry-like structure. The number and size of the blackberry-like structure became even larger when the amount of SiO2 aerogel increased further. The ratio of PAN and SiO2 aerogel showed a crucial role in controlling the fiber diameter since the total solid content was kept constant at 12 wt% in all solutions. As displayed in Fig. S1 and Table 2, the fiber diameter increased as the amount of SiO2 aerogel increased. However, an adverse effect on the fiber diameter was shown with a further increase in the SiO2 aerogel content. The fiber diameter of PAN + SA 100% was the smallest in all composite nanofiber membranes (~713 nm). And the fiber diameter distribution got broadened with higher SiO2 aerogel concentration. The changes in the fiber diameter and the fiber diameter distribution may be attributed to the interaction between PAN chains and SiO2 aerogel [23,24] and the presence of blackberry-like structures on the surface of fibers. HR-TEM characterization was carried out to further investigate the fiber morphology, as shown in Fig. 3. Pristine PAN fibers (Fig. 3(a)) showed the cylindrical structure and smooth surface, and the composite fibers in PAN + SA 100% (Fig. 3(b)) showed SiO2 aerogel was distributed on the surface and inside of the fibers. The inset of Fig. 3(b), the TEM image of SiO2 aerogel aggregation on the surface of composite 3

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(a)

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(d)

Average Diameter:731nm

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Population(%)

Population(%)

30

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Average Diameter:890nm

60

40 30 20

40 30 20 10

10 0 400

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Fiber Diameter(nm)

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Fiber Diameter(nm) 100

Population(%)

(e)

Average Diameter:713nm

80 60 40 20 0 0

600

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Fiber Diameter(nm)

Fig. 2. FE-SEM images and fiber diameter distributions of (a) PAN + SA 0%, (b) PAN + SA 20%, (c) PAN + SA 50%, (d) PAN + SA 71.4% and (e) PAN + SA 100%

nanofibers, the peak at around 2243 cm−1 can be assigned to eC^N stretching vibration, and the absorption peaks of 2937 cm−1 and 1452 cm−1 belong to the stretching vibration and bending vibration of eCH2e respectively [31]. The ATR-FTIR spectra of SiO2 aerogel/PAN composite nanofiber membranes presented the existence of characteristic absorption peaks of both SiO2 and PAN molecules, which demonstrated that SieO groups were successfully introduced into the fiber structure. Due to the inductive effect of nitrile groups, the absorption peak that belongs to the asymmetric stretching vibration of SieOeSi bond in composite membranes was to be found shifted to 1081 cm−1 compared to neat SiO2 aerogel. In addition, the reduction of the peak at 2243 cm−1 for the group eC^N means that part of nitrile groups on the surface of the fibers are diluted. To evaluate the thermal stability of electrospun nanofiber membranes, the thermal gravimetric analysis was conducted in an air atmosphere from room temperature to 800 °C. The TG graphs of SiO2 aerogel and electrospun nanofiber membranes are exhibited in Fig. 6(a). The TG curve of SiO2 aerogel exhibited a weight loss of about 8% from room to 800 °C ascribed to the desorption of residual solvent and the thermal decomposition of organic component [32], which confirmed that SiO2 aerogel had excellent thermal stability up to

gradually as the aerogel content increased. It is speculated that common nanoparticles can slightly enhance the specific surface area of composites by decreasing fiber diameter and increasing surface roughness, however, since the SiO2 aerogel itself has a porous structure and high specific surface area, the growth in the specific surface area and pore volume of composite membranes is mainly owing to the contribution of aerogel porous structure itself. In addition, the porosities of nanofiber membranes (the method of characterization was in Supplementary information), which are derived from both the interlacing of fibers and the porous SiO2 aerogel on the surface of fibers, were about 90% as shown in Table 2. Fiber diameter, pore size, BET surface area and porosity are important factors that directly affect the adsorption of electrospun nanofiber membranes for harmful gases due to the more available sites provided by the large surface area for adsorption processes [13]. Fig. 5 shows the ATR-FTIR spectra of SiO2 aerogel, pure PAN nanofiber and SiO2 aerogel/PAN composite nanofiber membranes. The characteristic absorption peaks of SiO2 aerogel were observed to be around 1051 cm−1, 800 cm−1 and 960 cm−1, which can be attributed to the asymmetric and symmetric stretching vibration of SieOeSi and the bending vibration of SieOH [30]. In the spectrum of pure PAN

Table 2 Physical property of electrospun nanofiber membranes, SiO2 aerogel and activated carbon. Samples

Fiber diameter (nm)

Density (g/cm3)

BET surface area (m2/g)

Pore size (nm)

Pore volume (cm3/g)

Porosity (%)

PAN + SA 0% PAN + SA 20% PAN + SA 50% PAN + SA 71.4% PAN + SA 100% Activated carbon SiO2 aerogel

580 990 890 731 713 – –

0.098 0.112 0.121 0.132 0.151 – –

18.13 106.94 183.91 213.67 289.20 1248.50 815.38

9.12 13.11 11.35 12.69 12.47 3.46 15.46

0.069 0.448 0.607 0.786 1.037 0.810 2.975

92.0 86.6 89.3 89.8 88.6 – –

4

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(a)

(b)

(d)

(c)

(e)

O

(f) Si Fig. 3. HRTEM images of (a) PAN + SA 0% and (b) PAN + SA 100% (upper inset is TEM image of SiO2 aerogel aggregation on surface of fibers). (c) EDX mapping images of Si and O and (d) flexible display picture of PAN + SA 100%. Optical images of (e) PAN + SA 0% and (f) PAN + SA 20%.

shifted toward a higher temperature region. Moreover, When the temperature further was heated up to 800 °C, the residual amount raised with the increase of SiO2 aerogel content, as shown in Fig. 6(b). In conclusion, the results indicated that the thermal stability of electrospun nanofiber membranes was enhanced because of the addition of SiO2 aerogel, which might lie in the strong interaction between eCN groups in the PAN chains and eOH groups in SiO2 aerogel.

800 °C. The profiles of PAN nanofibers with and without SiO2 aerogel presented that there were three stages of pyrolysis roughly. Specifically, the phase that all membranes showed a slight weight loss of about 2% mainly owing to small molecule evaporation before ca.180 °C was considered as stage I. Stage II was up to about 300 °C corresponding to the stabilization process of PAN, including cyclization, dehydrogenation, crosslinking, etc. The last part is related to the decomposition reaction of PAN [33]. Additionally, the weight loss reached 10% at ca.343 °C for the pure PAN membrane. In contrast, the membranes with SiO2 aerogel decomposed more slowly and the weight loss reached 10% at ca. 346 °C, 369 °C, 356 °C, and 371 °C for PAN + SA 20%, PAN + SA 50%, PAN + SA 71.4%, and PAN + SA 100% respectively. It was clearly shown that the temperature at which composite nanofiber membranes with the addition SiO2 aerogel were decomposed slightly

3.2. Adsorption performance of electrospun nanofiber membranes 3.2.1. Adsorption capacities of nanofiber membranes The high surface area, high porosity and high pore connectivity of SiO2 aerogel make it very suitable for the removal of VOCs from the air. The adsorption capacities of SiO2 aerogel/PAN nanofiber membranes

(b)

700

dV/dlog(D) Pore Volume (cm³/g)

Quantity Adsorbed (cm³/g STP)

(a) PAN+SA 0% PAN+SA 20% PAN+SA 50% PAN+SA 71.4% PAN+SA 100%

600 500 400 300 200 100 0 0.0

0.2

0.4

0.6

0.8

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Relative Pressure (P/Po)

2.5

PAN+SA 0% PAN+SA 20% PAN+SA 50% PAN+SA 71.4% PAN+SA 100%

2.0 1.5 1.0 0.5 0.0 0

20

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60

Fig. 4. (a) N2 adsorption–desorption isotherm; (b) BJH pore size distribution curves. 5

80

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Applied Surface Science 512 (2020) 145697

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Y. Yu, et al.

1051

SiO2 aerogel

Intensity (a.u.)

PAN+SA 100% PAN+SA 71.4% PAN+SA 50% PAN+SA 20% 1081

PAN+SA 0% 2937

4000

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2243

3000

2500

960 800

1452

2000

1500

1000

500

PAN+SA 0% PAN+SA 20% PAN+SA 50% PAN+SA 71.4% PAN+SA 100% activated carbon

1600 1200 800 400 0

formic acid chloroform

xylene

methanol

Fig. 7. VOCs adsorption capacity of PAN + SA 0%, PAN + SA 20%, PAN + SA 50%, PAN + SA 71.4% and PAN + SA 100%.

Wavenumber(cm-1) Fig. 5. ATR-FTIR spectra of PAN + SA 0%, PAN + SA 20%, PAN + SA 50%, PAN + SA 71.4%, PAN + SA 100% and SiO2 aerogel.

Table 3 The VOCs adsorption capacity of electrospun nanofiber membranes, SiO2 aerogel and activated carbon.

with different SiO2 aerogel content for chloroform, formic acid, methanol and xylene are displayed in Fig. 7 and Table 3. It was observed that these electrospun nanofiber membranes had adsorption capacity for VOCs, including the pure PAN membranes with no functionalized by SiO2 aerogel. The VOCs adsorption capacity of electrospun nanofiber membranes enhanced as the amount of SiO2 aerogel increased. PAN + SA 100% had the maximum VOCs adsorption capacity compared with the other membranes. This is because the existence of maximum SiO2 aerogel within the composite membrane results in its excellent specific surface area, maximum pore volume, appropriate pore size and porosity (as shown in Table 2), which make for the promotion of the adsorption capacity of membranes for VOCs [8,13]. Moreover, the surface chemical functional groups of composite membranes also have an impact on their VOCs adsorption capacity, especially for polar adsorbate [9]. The eC^N, SieO and SieOH groups enable composite membranes to possess a higher affinity for polar organic molecules and provide more active sites for them. Also, SiO2 aerogel had higher VOCs adsorption capacity than PAN + SA 100% due to the better micro performance. However, the high price and poor flexibility of SiO2 aerogel have an adverse impact on its wide application in adsorption. On the other hand, the VOCs adsorption capacity trend of different membranes toward different hydrocarbons was similar, with the order of chloroform > formic acid > methanol > xylene. Some characteristics of the VOCs themselves should be responsible for their different adsorption by electrospun nanofiber membranes, such as their

Samples

VOCs

PAN + SA 0% PAN + SA 20% PAN + SA 50% PAN + SA 71.4% PAN + SA 100% Activated carbon SiO2 aerogel

Formic acid (mg/g)

Chloroform (mg/g)

Xylene (mg/g)

Methanol (mg/g)

433.6 625.9 834.5 1006.6 1084.7 838.0 2458.3

619.4 1213.2 1394.0 1708.1 1841.1 1134.5 5910.0

117.2 372.5 488.7 586.7 623.7 530.4 1850.4

177.3 526.3 640.0 790.9 835.0 527.3 2196.8

molecular polarity, vapor pressure, molecular weight, and so on. Firstly, for chloroform, electrospun nanofiber membranes showed the strongest adsorption capacity. Chloroform is a polar solvent [12] and can form dipole-dipole interactions, halogen bond and van der Waals force with membranes, which is contributed to the high adsorption capacity of these membranes for chloroform. And the large molecular weight and high vapor pressure (as shown in Table S2) of chloroform may be also associated with the above result [34,35]. Compared with chloroform, the adsorption capacities for formic acid and methanol were lower, which may be ascribed to their lower molecular weight and lower vapor pressure. The membranes possessed a higher adsorption capacity for formic acid compared with methanol because formic acid molecule contains two polar functional groups, an aldehyde group and

(b) Mass residual content (%)

(a) 100 80

Weight (%)

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PAN+SA 0% PAN+SA 20% PAN+SA 50% PAN+SA 71.4% PAN+SA 100% SiO2 aerogel

0 100 200 300 400 500 600 700 800 900

Temperature (°C)

50 40 30 20 10 0 0

20

40

60

80

100

SiO2 aerogel addition content (%)

Fig. 6. (a) thermal analysis curves under air and (b) mass residual content of PAN + SA 0%, PAN + SA 20%, PAN + SA 50%, PAN + SA 71.4% and PAN + SA 100%. 6

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fit than the PFO with higher R2 values and the lower deviation between experimental and calculated adsorbed amounts, qexp and qcal, respectively. Thus, the PSO model had a better description of the adsorption process for four volatile organic compounds than PFO. That is to say, the adsorption is mainly controlled by chemical adsorption, which is depending on the electron sharing or electron exchange between adsorbent and adsorbate [5]. However, the amounts of experimental and calculated adsorption capacities were not exactly matched especially for xylene. The intraparticle diffusion step is likely to be involved in the adsorption process which mainly depends on either surface or pore diffusion [38]. From Fig. 8(c) and Table 5, it could be seen that the lower R2 value of chloroform pointed to poor data correlation by this model compared with the other three VOCs, which reveals that the intraparticle diffusion isn't suitable to the chloroform adsorption process. For the other three VOCs adsorption processes, two steps could be observed. The first stage of adsorption happened quickly, indicating the VOCs diffuse to the external surface of the composite membranes to be adsorbed. The second stage corresponds to the intra-particle diffusion [5]. Besides, the plots didn't pass through the origin, and this indicates that the resistance of the boundary layer cannot be ignored and intra-particle diffusion can control the adsorption rate but is not the sole rate-limiting process [39].

a hydroxyl group, and methanol only contains hydroxyl groups, which makes nanofiber membranes have a higher affinity for formic acid. As for xylene, the adsorption capacity of nanofiber membranes was smallest, which is related to not only the lowest vapor pressure of xylene among four VOCs but also the chemical characteristics of xylene. Xylene is a hydrophobic compound and is apt to be adsorbed by a hydrophobic (nonpolar) surface. Therefore, the presence of oxygencontaining groups and other polar groups in electrospun nanofiber membranes will be unfavorable for the adsorption capacity of xylene by nanofiber membranes [8,36]. In brief, several factors may jointly influence the overall VOCs adsorption performance of electrospun nanofiber membranes, not only the physicochemical properties of adsorbent and adsorbate but also the interactions between them should be taken into consideration and analyzed synthetically [13]. 3.2.2. The comparison of VOCs adsorption between PAN + SA 100% membrane and activated carbon Fig. 7 and Table 3 also show that the adsorption capacity of AC (the BET surface area: 1248.50 m2/g) was lower than that of PAN + SA 100% when they were exposed to VOCs although the surface area of PAN + SA 100% (289.3 m2/g) was about 4.3-fold smaller than that of AC. This indicated the different adsorption mechanisms between AC and SiO2 aerogel/PAN composite membranes. The tentative hypothesis of this is the VOCs not only are adsorbed on the surface of SiO2 aerogel/ PAN composite membranes but also can enter into the inner of membranes [12], that is, the inner layer of the membranes also can be regarded as adsorption surface. Furthermore, the pore size and the pore volume of PAN + SA 100% were larger than that of AC (as shown in Table 2), which makes a certain effect on VOCs adsorption capacity. This implies that the highest surface area does not always mean the best adsorption ability for organic compounds [37], therefore, the performances of adsorbent (specific surface area, pore volume, pore size, etc.) should be considered and evaluated comprehensively in the process of adsorption VOCs.

3.3. Regeneration and recyclability of electrospun nanofiber membranes The VOCs desorption behaviors of PAN + SA 100% and AC were performed by measuring the residual content of VOCs adsorbed by PAN + SA 100% and AC at different time intervals under 35 °C and ambient pressure. The results are shown in Fig. 9. As the contact time increased, the residual content of VOCs adsorbed by PAN + SA 100% and AC decreased. Compared with AC, the VOCs desorption rate of PAN + SA 100% was faster and four hours were enough to achieve 100% desorption for four VOCs, while AC can't be regenerated simply. This indicated that SiO2 aerogel/PAN composite nanofiber membranes could be regenerated in a simple and mild way. The practical applications of composite membranes mainly depend upon the membranes’ reusability. Therefore, we carried out the repeated VOCs adsorption experiments on PAN + SA 100%. As shown in Fig. 10(a), the composite membranes had nearly equal adsorption capacity up to 10 cycles. The measurement demonstrated that SiO2 aerogel/PAN composite nanofiber membranes had completely reversible adsorption and desorption behavior. Besides, to further prove the stability of SiO2 aerogel/PAN composite nanofiber membranes, the adsorption capacities for four VOCs were measured after PAN + SA 100% was treated by soaking in hydrochloric acid and heating respectively. The VOCs adsorption capacity of the treated membrane didn't decrease as shown in Fig. 10(b), indicating the excellent thermal stability and acid resistance of composite membranes. This implies the composite membranes have the potential to be applied in complicated environments that may contain high-temperature and acid ambient.

3.2.3. Adsorption kinetics The effect of contact time on the adsorption for VOCs by PAN + SA 100% composite nanofiber membranes is shown in Fig. 8(a). It was observed that initially VOCs were adsorbed rapidly and with time the adsorption rate was almost constant. The adsorption processes for four VOCs by PAN + SA 100% could reach equilibrium within 500 min. The high rate of adsorption during initial times results from the availability of sufficient unsaturated active sites on the surface of the membranes for adsorbing VOCs, while prolonging the reaction time, the active sites are occupied, thus, the adsorption rate decreases until it gradually approaches a plateau. To understand the adsorption mechanism better, the linear forms of pseudo-first-order (PFO), pseudo-second-order (PSO) models and the intraparticle diffusion equation were used to describe the adsorption kinetics of VOCs by SiO2 aerogel/PAN nanofiber composite membranes. The linear forms of these models [38] are expressed as:

Pseudo-first-order kinetics: Pseudo-second-order kinetics:

Intraparticle diffusion:

ln(qe − qt ) = lnqe − k1 t t 1 t = + qt qe k2 qe2 1

qt = kp t 2 + C

(1)

3.4. Thermal insulation performance of electrospun nanofiber membranes (2)

Fig. 11 presents the thermal conductivities of pure PAN and SiO2 aerogel/PAN composite nanofiber membranes at room temperature. The thermal conductivity of PAN + SA 20% was ~39 mW·m−1·k−1, which was the lowest in all membranes. The thermal conduction mechanism is consisted of thermal transportation by the gas phase, by the solid phase, and by radiation [16]. Fig. 11 shows that the pure PAN membrane itself had thermal insulation because electrospun pure PAN nanofibers had a fluffy structure (Fig. 3(e)) and high porosity (Table 2), which can entrap large volume of gas phase in the inter-fiber spaces [40]. So, the convection thermal transfer of gas in the inter-fiber spaces is the main factor of thermal conduction in pure PAN membranes. After

(3)

whereqe (mg/g) and qt (mg/g) are the amounts of VOCs adsorbed at equilibrium and at time t (min) respectively; k1 (min−1), k2 (g·mg−1·min−1) and kp (mg/g/min1/2) are the rate constant of adsorption for PFO, PSO and intraparticle diffusion respectively. The intraparticle diffusion constant C reflects the boundary layer effect. Three linear fits for the adsorption kinetics of VOCs onto PAN + SA 100% are shown in Fig. 8(b)–(d) and their parameters are listed in Tables 4 and 5. It could be observed that the PSO model yielded a better 7

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(b)

2500

Chloroform Xylene

2000

Formic acid Methanol

10

1500 1000

6 4

500

2

0

0 0 100 200 300 400 500 600 700 800

Chloroform Formic acid Xylene Methanol

1.0 0.8

t/qt

1500

Time(min)

(d) 1.2

Chloroform Formic acid Xylene Methanol

2000

qt(mg/g)

0 100 200 300 400 500 600 700 800

Time(min)

(c) 2500

Chloroform Formic acid Xylene Methanol

8

ln(qe-qt)

Adsorption capacity (mg/g)

(a)

1000

0.6 0.4

500

0.2

0

0.0

0

5

10

15

t1/2(min1/2)

20

25

0 100 200 300 400 500 600 700 800

30

Time(min)

Fig. 8. Adsorption kinetics of four VOCs by PAN + SA 100% (a), fitting by pseudo-first-order (b), the intraparticle diffusion (c) and pseudo-second-order (d) models.

the addition of SiO2 aerogel, the density of composite membranes increased and the fluffy property declined (Fig. 3(f) and Table 2), which is useful for decreasing the thermal transfer of gas phase. At the same time, the contact between fibers is replaced by SiO2 aerogel, and the porous structure of SiO2 aerogel on composite nanofibers surface and the interface between SiO2 aerogel and nanofibers can increase the heat transfer path and strengthen phonon scattering on the boundaries of the solid backbone [41,42], which is beneficial for suppressing the energy transport and decreasing solid heat transfer. In addition, the higher surface area and density of composite membranes are also helpful for weakening the radiation heat transfer [16,43]. Hence, When the content of SiO2 aerogel was 20 wt%, the thermal conductivity of the composite membrane was decreased effectively. With the further increase of SiO2 aerogel, the agglomeration phenomenon occurs. SiO2 aerogel cannot unevenly disperse on the surface of composite nanofibers, and part of the heat can be transferred from the nanofibers with little aerogel. Therefore, this results in a decline in thermal insulation performance.

Table 5 The intraparticle diffusion model parameters for VOCs adsorption by PAN + SA 100%. Compound

Chloroform Formic acid Xylene Methanol

Step1

Step2

C1

Kp1

R2p1

287.72 109.89 19.24 14.48

150.69 69.06 28.15 39.33

0.9274 0.9806 0.9982 0.9969

C2

Kp2

R2p2

1469.96 722.00 231.57 643.95

17.41 21.89 17.78 7.59

0.7629 0.9435 0.9808 0.9726

SiO2 aerogel dispersed in the polymer matrix at various concentrations and was stacked into a blackberry-like structure on the PAN fiber at higher content. The problem of dust-releasing of aerogel composites could be addressed effectively due to the interaction between SiO2 aerogel and PAN fiber and the entrapment of the aerogel by PAN fiber. The BET surface area and thermal stability of composite nanofiber membranes were improved greatly with the addition of SiO2 aerogel owing to the porous structure and the high temperature resistance nature of the SiO2 aerogel itself. The VOCs adsorption capacity of electrospun nanofiber membranes enlarged with the increase of SiO2 aerogel content. PAN + SA 100% displayed the highest VOCs adsorption capacity even higher than AC. Moreover, the SiO2 aerogel/PAN

4. Conclusion In summary, we developed a novel and flexible SiO2 aerogel/PAN composite nanofiber membranes via a simple and one-step electrospinning technique. From the electron micrograph, it could be seen that

Table 4 Kinetic parameters of pseudo-first-order and pseudo-second-order models for VOCs adsorption by PAN + SA 100%. Compound

Chloroform Formic acid Xylene Methanol

PSO

PFO

qe,exp

qe,cal

K2

1841.1 1084.7 623.7 835.0

1907.97 1308.66 775.19 943.40

3.43 1.86 1.12 1.13

R × × × ×

10−5 10−5 10−5 10−5

2

0.9989 0.9929 0.9625 0.9795

8

qe,cal

K1

868.9712 960.9958 810.3611 977.7656

3.75 6.94 6.62 7.92

R2 × × × ×

10−3 10−3 10−3 10−3

0.75765 0.99152 0.79851 0.89955

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(b)

(a) PAN+SA 100% Activated carbon

80

Residual content (%)

Residual content (%)

100

60 40 20

100 80 60 40 20

0

0 0

100 200 300 400 500 600

0

Time (min)

(c) 100

Time (min)

100

PAN+SA 100% Activated carbon

80

100 200 300 400 500 600

(d) Residual content (%)

Residual content (%)

PAN+SA 100% Activated carbon

60 40 20

PAN+SA 100% Activated carbon

80 60 40 20 0

0 0

50

100

150

200

250

0

300

50

100

150

200

250

300

Time (min)

Time (min)

Fig. 9. The desorption behaviors of (a) chloroform, (b) formic acid, (c) methanol and (d) xylene by PAN + SA 100% and activated carbon.

review & editing. Qingyan Ma: Formal analysis, Investigation, Writing - original draft. Ji-bin Zhang: Writing - review & editing. Guan-bin Liu: Writing - review & editing, Supervision, Methodology.

composite nanofiber membranes could be regenerated under 35 °C and ambient pressure conditions within four hours, showing a completely reversible adsorption and desorption. Also, PAN + SA 20% was found to possess better thermal insulation compared to pure PAN membrane, suggesting that the composite nanofiber membrane is a good candidate as heat insulation material at the same time. However, it was undeniable that although the BET surface area of the composite membrane was almost 16 times that of the pure PAN membrane, it was lower than that of the pure aerogel, which may also because the porous structure of the SiO2 aerogel is destroyed to some extent during the spinning and dosing process besides the discussed reasons above. Hence, it is crucial to explore new strategies to reduce the damage of aerogel structures in electrospun nanofiber membrane in future studies.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Natural Science Foundation of China, China (No. 51675452), the Fund of Nuclear Power Institute of China for Innovation, China (No. HDLCXZX-2019-HD-025), Joint Fund for Equipment Pre-research and Aerospace Science and Technology, China (No. 6141B061012).

CRediT authorship contribution statement Yuxi Yu: Conceptualization, Methodology, Resources, Writing -

(b)

2000 1600

1 4 7 10

1200

2 5 8

3 6 9

chloroform

Adsorption capacity (mg/g)

Adsorption capacity (mg/g)

(a)

Methanol Xylene

800

Formic acid

400

volatile organic compound

2400

Xylene Methanol

2000

Formic acid Chloroform

1600 1200 800 400 0

OS

ATS

TTS

Fig. 10. (a) Cyclic adsorption behaviors of four VOCs on PAN + SA 100%; (b) The VOCs adsorption capacity contrast of PAN + SA 100% original sample (OS), PAN + SA 100% treated by 1 M hydrochloric acid (ATS) and treated by heating to 260 ℃ (TTS). 9

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43 42 41 40 39 38

0

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40

60

80

100

SiO2 aerogel content (%) Fig. 11. Thermal conductivity of SiO2 aerogel/PAN composite nanofiber membranes with different SiO2 aerogel content.

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11