Polyacrylonitrile Derived Robust and Flexible Poly(ionic liquid)s Nanoﬁber Membrane as Catalyst Supporter

: Poly(ionic liquid)s nanoﬁber (PIL NF) membrane was derived from polyacrylonitrile by converting its cyano groups to imidazoline moieties via cyclization with ethylenediamine, followed by quaternization with 1-bromobutane. The novel PIL NF is further decorated with photocatalyst phosphotungstic acid PW12 via anion exchanging to give PW-PIL. The degradation rate of the novel supported photocatalyst towards methyl orange irradiated under visible light was found to be 98%. In addition, the nanoﬁber membrane morphology is beneﬁcial for easy recycling, and 98% of original degradation rate was maintained after 5 cycles of photocatalysis degradation. This robust, efﬁcient, and recyclable material offers a new approach for serving as catalyst supporter. The photocatalyst PW-PIL is reported for the ﬁrst time. The inexpensive functional membrane is used to exploit the sun as a cheap and clean source of light.


Introduction
Polyacrylonitrile nanofiber (PAN NF) membrane has quite a few advantages including low density, good flexibility, low cost, and is applied in various areas such as the textile industry, construction materials and precursor of carbon fiber [1][2][3][4]. Besides, application of PAN could also be extended to other fields including adsorption of metal-ions, catalysis or catalyst supporter, by converting chemically active cyano groups in PAN into other kinds of functional groups such as amino [5], carboxyl [6], and imidazoline [7]. These achievements inspire us to further develop novel functional materials from PAN NF membrane, by utilization of their abundant cyano groups [8].
Poly(ionic liquid)s (PILs) have gained great interest in material research due to their favorable properties of both ionic liquid and conventional polyelectrolyte, especially for serving as a supporter of functional materials via simple anion exchanging. PILs are traditionally prepared via classic free radical polymerization of ionic liquid monomers with vinyl groups [9]. This method is widely employed and easy to handle, but its applicability is often limited by the lower degree of polymerization [10]. Another way to obtain PILs is by modifying the nitrogen-containing side groups (e.g., imidazole, pyridine) existing in synthesized polymer. Since PAN could be converted to poly(2-vinylimidazoline) via cyclization reaction with ethylenediamine, we can expect that the PAN-based PILs can be derived via ionization of the imidazoline groups while maintaining the degree of polymerization and nanofiber membrane morphology of PAN.
Recently, handling wastewater containing organic dyes has been an emerging topic for researchers [11]. At present, several methods such as membrane separation, adsorption and biochemical degradation technology have been developed [12], but their applications are limited by their relatively low removal rate of pollutants, production of secondary pollution or harmful intermediates and so forth [13][14][15][16]. Recently, the photocatalytic degradation of organic dyes in wastewater, especially by irradiation with visible light, has attracted much attention from scientists [17][18][19], because photocatalysis is an effective way to consume renewable energy, which is more environmentally friendly than other catalytic systems. Polyoxometalates (POMs) are a class of stable transition metal oxides with excellent photochemical properties similar to semiconductors [20,21], but the separation and recycling of water-soluble POMs is still a challenge. Loading POMs on supporters is considered to be an effective method to solve the problem [22][23][24], and developing inexpensive, accessible and stable supporters with high immobilization capacity are required. Table 1 shows the development of photocatalytic membrane materials in recent years. We compare and analyze the research status of photocatalysts from the following aspects: the raw materials, preparation method, degradation materials, preparation cost, reusability, and visible light response. The conclusion being that the preparation of a visible light responsive, low cost, easy recovery, good flexibility, insoluble in water, simple preparation method of polymer catalyst is an effective way to degrade dye wastewater. No response [39] In the present work, low-cost and flexible PAN NF membranes are modified to PILs and furthermore decorated with photocatalytic active POM phosphotungstic acid (PW12). The photocatalytic properties of the as-obtained PW-PIL composite membrane were in-vestigated for degradation of methyl orange (MO) under visible light, and this kind of composite photocatalytic material with high molecular weight and nanofiber membrane morphology exhibited good catalytic ability and recyclability.

Characterization of PW-PIL NF Membrane
The PAN, PVIm, Br-PIL, PW-PIL NF membranes and PW12 were characterized by FT-IR, and the spectra were collected in Figure 1a. The peak at 1451 cm −1 in the spectrum of PAN represents the in-plane vibrations of C-H. The peaks at 2246 and 1074 cm −1 are appointed as the stretching and bending vibrations of C≡N [40][41][42]. In the spectrum of PVIm, the characteristic peak at 2246 cm −1 associated with C≡N disappears, while the new peaks at 1673 and 1389 cm −1 are assigned to the stretching vibration of C=N and C-N in imidazoline ring, suggesting the successful conversion of the C≡N group to imidazoline unit via cyclization with ethylenediamine [43,44]. Furthermore, in the PW12 spectrum, the peaks at 1074, 998, 892 and 813 cm −1 are assigned as the vibration of P-O, W-O, W-O-W corner and W-O-W edge , respectively. In the (PW-PIL) IR spectrum, the peaks at 1074 and 998 cm −1 related to the vibration of the P-O group and the W-O resonance appeared, while the peaks at 1673 and 1074 cm −1 related to imidazoline can be seen [45], implying successful production of desired PW-PIL composite. XPS analysis is conducted to investigate the chemical composition of the surface and the elemental chemical states of the as-prepared samples. The elements of C, O and N can be detected on the surface of PAN. The C and N elements result from PAN molecules, while the elements of P and W are assigned to PW12 (Figure 2a). The W content is 5.14%, suggesting that the anion exchange process was successful. High-resolution XPS was employed to monitor the N element Binding Energy (Figure 2b). The N1S peak at 400.7 eV (N1) and 402.3 eV (N2) belong to the imidazolinium. In Figure 2c, the XPS spectrum indicates that the intensity ratio of C1S. C-C has the highest peak, and the remaining two peaks were from C-N and C=N, respectively.  Figure 1b shows the SEM images of PAN and PW-PIL NF membranes. Compared to the PAN, the surface of PW-PIL is rougher, which is mostly attributed to uniformly distributed POM on the PW-PIL. In addition, honeycomb like cavity can be observed on the surface of PW-PIL NF, which may increase surface area of the membrane as well as adsorption capacity [46] and could be anticipated as an ideal polymer-supporter.
XPS analysis is conducted to investigate the chemical composition of the surface and the elemental chemical states of the as-prepared samples. The elements of C, O and N can be detected on the surface of PAN. The C and N elements result from PAN molecules, while the elements of P and W are assigned to PW12 (Figure 2a). The W content is 5.14%, suggesting that the anion exchange process was successful. High-resolution XPS was employed to monitor the N element Binding Energy (Figure 2b). The N 1S peak at 400.7 eV (N 1 ) and 402.3 eV (N 2 ) belong to the imidazolinium. In Figure 2c, the XPS spectrum indicates that the intensity ratio of C 1S . C-C has the highest peak, and the remaining two peaks were from C-N and C=N, respectively.
suggesting that the anion exchange process was successful. High-resolution XPS was e ployed to monitor the N element Binding Energy (Figure 2b). The N1S peak at 400.7 (N1) and 402.3 eV (N2) belong to the imidazolinium. In Figure 2c, the XPS spectrum in cates that the intensity ratio of C1S. C-C has the highest peak, and the remaining two pe were from C-N and C=N, respectively. In order to inspect the light absorption features of the raw and modified materi UV-visible diffuse reflectance spectra (DRS) of the PAN, PW12, PVIm, Br-PIL and PW-NF membranes were measured, and the results are shown in Figure 3a. As expected, significant absorption could be observed for PAN in both the UV and visible region. S ilarly, pure PW12 exhibited little absorption in the visible area (>400 nm). Neverthel PW-PIL exhibits significant absorption in the UV and visible area, indicating that visib light absorption of PW12 photocatalyst could be effectively improved by the cha In order to inspect the light absorption features of the raw and modified materials, UV-visible diffuse reflectance spectra (DRS) of the PAN, PW12, PVIm, Br-PIL and PW-PIL NF membranes were measured, and the results are shown in Figure 3a. As expected, no significant absorption could be observed for PAN in both the UV and visible region. Similarly, pure PW12 exhibited little absorption in the visible area (>400 nm). Nevertheless, PW-PIL exhibits significant absorption in the UV and visible area, indicating that visiblelight absorption of PW12 photocatalyst could be effectively improved by the charge mobility of the PIL [47]. The bandgap energy between energy bands of PW-PIL is 3.09 eV (Kubelka-Munk theory) according to the UV-vis diffuse reflectance spectra.
Catalysts 2022, 12, x FOR PEER REVIEW 5 o mobility of the PIL [47]. The bandgap energy between energy bands of PW-PIL is 3.09 (Kubelka-Munk theory) according to the UV-vis diffuse reflectance spectra. X-ray diffraction measurements on the sample PAN, PW12, PVIm, Br-PIL and P PIL were performed for confirming their crystal structure, and the relevant results w collected in the Figure 3b. For PAN NF membrane, a diffraction peak could be obser at the range of 16 to 17 of 2θ, which should be attributed to the hexagonal (100) cry plane [48]. Another weaker crystal diffraction peak (110) is present at a 2θ value of ab 28 to 29. In the Figure 3a of PVIm, the decrease in the 2θ value of 17 and the chang others in the (100) diffraction peak can be attributed to the production of an imidazo group. In the sample of PW12, the 2θ values are at 10, 20, 23, 25, 28, and 31 correspond to the (110, 200, 220, 310, 222, 400) crystal form of PW12, respectively, as reported in reference [49]. This assignment is advocated by correlation with the XRD pattern of PW in which all peaks are observed on the JCPDS number 50-0657 card. In the case of Brthe 2θ value of 17 is equal to that in the PVIm, and the 2θ value of 29 is significantly w ened [50]. The comparison of the Br-PIL and PVIm patterns demonstrate that the ion tion does not significantly the crystal form of PVIm. In the diffraction pattern of PWno impurity peaks can be observed, and comparing with PAN, the peaks associated w 2θ values of 17, 29 disappear, while the two novel peaks appear at the 2θ values of 8 correspond to the PW12 (200) and (001) crystal form, further suggesting successful thesis of PW-PIL.

Photodegradation of Methyl Orange under Visible Light
The visible light-based photocatalytic degradation reaction of MO was carried ou water. Methyl orange is an azo dye whose chromophores are mainly azo fragments. main degradation method of azo dyes is chemical oxidation, in the presence of oxid such as POM or ozone, azo fragments can be oxidized. Thus, degrading MO is use verify the experiment. The results are shown in Figure 4a [51]. In comparison, contro periments were also performed by conducting the photocatalytic degradation in the sence of PW-PIL or simply blending PAN with PW12 under the same experimental dition. The concentration change of MO during catalysis process was monitored by m uring UV-visible spectra of samples separated from the solution at a given interval. results demonstrate the degradation rate under dark conditions is only 1%, but 98% X-ray diffraction measurements on the sample PAN, PW12, PVIm, Br-PIL and PW-PIL were performed for confirming their crystal structure, and the relevant results were collected in the Figure 3b. For PAN NF membrane, a diffraction peak could be observed at the range of 16 to 17 of 2θ, which should be attributed to the hexagonal (100) crystal plane [48]. Another weaker crystal diffraction peak (110) is present at a 2θ value of about 28 to 29. In the Figure 3a of PVIm, the decrease in the 2θ value of 17 and the change of others in the (100) diffraction peak can be attributed to the production of an imidazoline group.
In the sample of PW12, the 2θ values are at 10, 20, 23, 25, 28, and 31 corresponding to the (110, 200, 220, 310, 222, 400) crystal form of PW12, respectively, as reported in the reference [49]. This assignment is advocated by correlation with the XRD pattern of PW12, in which all peaks are observed on the JCPDS number 50-0657 card. In the case of Br-PIL, the 2θ value of 17 is equal to that in the PVIm, and the 2θ value of 29 is significantly weakened [50]. The comparison of the Br-PIL and PVIm patterns demonstrate that the ionization does not significantly the crystal form of PVIm. In the diffraction pattern of PW-PIL, no impurity peaks can be observed, and comparing with PAN, the peaks associated with 2θ values of 17, 29 disappear, while the two novel peaks appear at the 2θ values of 8, 23 correspond to the PW12 (200) and (001) crystal form, further suggesting successful synthesis of PW-PIL.

Photodegradation of Methyl Orange under Visible Light
The visible light-based photocatalytic degradation reaction of MO was carried out in water. Methyl orange is an azo dye whose chromophores are mainly azo fragments. The main degradation method of azo dyes is chemical oxidation, in the presence of oxidants such as POM or ozone, azo fragments can be oxidized. Thus, degrading MO is used to verify the experiment. The results are shown in Figure 4a [51]. In comparison, control experiments were also performed by conducting the photocatalytic degradation in the absence of PW-PIL or simply blending PAN with PW12 under the same experimental condition. The concentration change of MO during catalysis process was monitored by measuring UV-visible spectra of samples separated from the solution at a given interval. The results demonstrate the degradation rate under dark conditions is only 1%, but 98% of MO is rapidly degraded in the presence of PW-PIL after the equal period under visible light irradiation, and the colour change of MO aqueous solution in the course of the photocatalytic degradation process is shown in Figure 4c. These results indicate that PW-PIL is an efficient photocatalyst for the degradation of MO [52,53]. In addition, from Figure 4a. B we can find that simply blending the PAN and the PW12 can only obtain c.a. 20% photocatalytic degradation under the same conditions. 4a. B we can find that simply blending the PAN and the PW12 can only obtain c.a. 20% photocatalytic degradation under the same conditions. For investigating the recyclability of the supported catalyst, PW-PIL NF membrane was separated from the photocatalytic reaction system and subsequently washed with deionized water and ethanol three times, respectively, followed by drying in a vacuum oven at 60 °C for 8 h to perform the new cycle of photocatalysis experiments under the identical condition. The results shown in Figure 4b indicate that after five cycles, 98% of the initial degradation rate of MO could be maintained, suggesting that PW-PIL NF membrane is an effective photocatalyst and convenient for recyclization since the membrane morphology can avoid the additional centrifugation process which is usually necessary for recovery of the powder like catalyst supports, and the strong Coulombic force between the negative charged PW12 and the positive charged imidazolinium group allows higher immobilization stability of the photocatalyst PW12. A potential mechanism from aspects of molecule energy and structure is given in Figures 5 and 6. The bandgap energy between energy bands of PW-PIL is 3.09 eV (Kubelka-Munk theory) according to the UV-vis diffuse reflectance spectra, indicating that PW-PIL can effectively absorb visible light [54]. As the imidazolinium groups with the For investigating the recyclability of the supported catalyst, PW-PIL NF membrane was separated from the photocatalytic reaction system and subsequently washed with deionized water and ethanol three times, respectively, followed by drying in a vacuum oven at 60 • C for 8 h to perform the new cycle of photocatalysis experiments under the identical condition. The results shown in Figure 4b indicate that after five cycles, 98% of the initial degradation rate of MO could be maintained, suggesting that PW-PIL NF membrane is an effective photocatalyst and convenient for recyclization since the membrane morphology can avoid the additional centrifugation process which is usually necessary for recovery of the powder like catalyst supports, and the strong Coulombic force between the negative charged PW12 and the positive charged imidazolinium group allows higher immobilization stability of the photocatalyst PW12.
A potential mechanism from aspects of molecule energy and structure is given in Figures 5 and 6. The bandgap energy between energy bands of PW-PIL is 3.09 eV (Kubelka-Munk theory) according to the UV-vis diffuse reflectance spectra, indicating that PW-PIL can effectively absorb visible light [54]. As the imidazolinium groups with the positive charge can seize the negatively charged PW12 via coulombic interaction, a built-in field could be formed between PW12 and PIL chain, which would enhance the transfer of photo generated electrons. It is known that POM can be considered as a low dimensional semiconductor structure [55]. Under visible light irradiation, electrons of PW12 excite from valence band (VB) energy level to conduction band (CB) energy level, and the photogenerated electrons entering CB energy level react with O 2 on surface to form O 2− , while the photogenerated holes of VB level react with OH − to form ·OH, which further degrades MO. H 2 O 2 acts as an initiator to form ·OH with O 2− [56][57][58].

Materials and Methods
Polyacrylonitrile nanofiber (PAN NF, Mw = 150,000) electrospinning membrane was purchased from Fushun Petrochemical Company. N, N-dimethylformamide (DMF), hydrogen peroxide, ethylene diamine, toluene, ethanol, phosphotungstic acid, 1-bromobutane, methyl orange, ethylene glycol and acetone were purchased from Aladdin Reagent co. as analytical pure agents and used without further purification. All the reagents used in the article are analytical pure.

Synthesis of Imidazolin Group-Containing Polymers
PAN NFs membrane (200 mg) was cut and immersed in a mixture of ethylene glycol (2 mL), ethylenediamine (2 mL) and DMF (20 mL) in H 2 O (20 mL) and the mixture was heated to 80 • C and maintained for 4 h under N 2 atmosphere. After the mixture was naturally cooled down the modified membrane was washed with deionized water and acetone for three times, respectively, and then dried in vacuum oven at 60 • C for 8 h, to obtain an imidazoline group-containing poly(2-vinylimidazolinium) membrane, which was designated as PVIm [7].

Synthesis of Br-Based Poly(ionic liquid)s NF Membrane
The PVIm NFs membrane (100 mg) and 1-bromobutane (1 mL) were added in DMF (40 mL) and then the temperature was elevated to 80 • C to conduct the quaternization reaction for 24 h. After cooling down, the obtained light-yellow membrane was washed with water and ethanol for three times, respectively, and then dried in vacuum oven at 60 • C for 8 h, to obtain a PIL membrane containing Br-anion, poly(1-butyl-2-vinylimidazolinium) bromide, which was designated as Br-PIL.

Synthesis of PW 12 O 40 3− Based Poly(ionic liquid)s NF Membrane
Phosphotungstic acid (100 mg) and Br-PIL (20 mg) were added in water (40 mL) and stirred at 60 • C for 4 h to perform ion exchange reaction. The obtained greyishgreen membrane was washed with deionized water for three times, and then dried in a vacuum oven at 60 • C for 8 h to obtain a PIL membrane with PW 12 O 40 3− as counter anion, poly(1-butyl-2-vinylimidazolinium) phosphotungstate, which was designated as PW-PIL (Scheme 1).
Catalysts 2022, 12, x FOR PEER REVIEW 8 of 12 reaction for 24 h. After cooling down, the obtained light-yellow membrane was washed with water and ethanol for three times, respectively, and then dried in vacuum oven at 60 °C for 8 h, to obtain a PIL membrane containing Br-anion, poly(1-butyl-2-vinylimidazolinium) bromide, which was designated as Br-PIL.

Synthesis of PW12O40 3− Based Poly(Ionic Liquid)s NF Membrane
Phosphotungstic acid (100 mg) and Br-PIL (20 mg) were added in water (40 mL) and stirred at 60 °C for 4 h to perform ion exchange reaction. The obtained greyish-green membrane was washed with deionized water for three times, and then dried in a vacuum oven at 60 °C for 8 h to obtain a PIL membrane with PW12O40 3− as counter anion, poly(1-butyl-2-vinylimidazolinium) phosphotungstate, which was designated as PW-PIL (Scheme 1). Scheme 1. Synthetic route of PVIm, Br-PIL and PW-PIL.

Preparation of PW-PIL NF Membranes
The synthetic route of PW-PIL NF membrane is shown in Scheme 1. Briefly, the PAN NF membrane was fabricated using an electrospinning method, and their cyano groups were converted to imidazoline moieties via cyclization with ethylenediamine. The obtained poly(2-vinylimidazoline) (denominated as PVIm) was subsequently ionized by 1bromobutane to give Br--based poly(ionic liquid) (Br-PIL), which was further anion exchanged with phosphotungstate acid PW12 to obtain the PW-PIL NF membrane.

Preparation of PW-PIL NF Membranes
The synthetic route of PW-PIL NF membrane is shown in Scheme 1. Briefly, the PAN NF membrane was fabricated using an electrospinning method, and their cyano groups were converted to imidazoline moieties via cyclization with ethylenediamine. The obtained poly(2-vinylimidazoline) (denominated as PVIm) was subsequently ionized by 1-bromobutane to give Br-based poly(ionic liquid) (Br-PIL), which was further anion exchanged with phosphotungstate acid PW12 to obtain the PW-PIL NF membrane.

Photocatalytic Degradation of Methyl Orange (MO) and Recyclization
The photocatalytic performance of the sample was determined using a 500-watt xenon lamp (CHFXQ500W, Beijing Integrity Technology Co., Ltd. (Beijing, China)) as the light source to control the wavelength of light with a 400 nm filter plate. The radiation intensity was 100 mW/cm 2 . The visible light-based photocatalytic degradation reaction of MO was carried out in water. Briefly, PW-PIL (20 mg) and hydrogen peroxide (10 µL) were added to a 50 ppm aqueous solution (20 mL) of MO, and the mixture was initially stirred (with magneton for magnetic stirring) in dark for 120 min. Afterwards photocatalytic degradation of MO was conducted under irradiation of a 500 W xenon lamp (using a 400 nm optical filter to filter off ultraviolet light) for 2 h at room temperature. For comparison, control experiments were also performed by conducting the photocatalytic degradation without photocatalyst or in the presence of blending of PAN with PW12 under the same experimental condition. The concentration change of MO during catalysis process was monitored by measuring UV-visible spectra of samples separated from the solution at an interval of 20 min. For investigating the recyclability of the supported catalyst, PW-PIL NF membrane was separated from the photocatalytic reaction system and subsequently washed with deionized water and ethanol for three times, respectively, followed by drying in a vacuum oven at 60 • C for 8 h to perform the new cycle of photocatalysis experiments under the identical condition.

Characterization
The morphology of the sample was observed by scanning electron microscopy (SEM) (Hitachi S-4800, operating at 15 kV). The UV-visible spectra of liquid samples were measured with an ultraviolet spectrophotometer (TU 1901). UV-visible diffuse reflectance spectra (DRS) of solid samples were performed using a Shimadzu-2550 UV system with a reference to BaSO 4 in the range of 200-800 nm. Fourier transferred Infrared (FT-IR) spectra were recorded using a Perkin Elmer 1600 infrared spectrophotometer. X-ray diffraction spectra of the samples in the range of 5-90 • were characterized using a Rigaku D/max-2500 diffractometer operating at 40 kV and 150 mA (λ = 0.15406 nm). The degree of anion exchange was determined using X-ray photoelectron spectroscopy (XPS) (ESCALAB MK II). Photoluminescence of a 150 W ozone-free Xe source was recorded on a fluorescence spectrophotometer (FS5-TCSPC Edinburgh Instruments, Livingston, UK).

Conclusions
In conclusion, a robust, flexible, and recyclable PIL NF membrane decorated with photocatalyst POM is prepared by modification of low-cost raw material PAN NF membrane through cyclization reaction, ionization and anion exchanging process, and then employed for photo degradation of MO under irradiation of visible light. The degradation rate was found to be around 98% and can be well maintained after 5 cycles. From the perspective of economic benefit, the market price of PILs is c.a. $150 thousand dollars per kg, while the price of PAN is only one million of PILs. In addition, the photocatalyst is mostly in powder form, meaning that the additional centrifugation process is required for recycling, while the membrane photocatalyst is much easier to recycle. The photocatalyst PW-PIL is reported for the first time. The inexpensive functional PIL NF membrane could be in principle produced on a large scale and give rise to more economical photodegradation approaches. A sustainable process can be obtained when a photocatalytic membrane utilises the sun as a cheap and clean source of light. On this aspect, the development of new photocatalysts with high activity under visible light and their application in the various research fields is a mandatory task.