CN112430909A

CN112430909A - Method for preparing flexible porous carbon fiber membrane by electrospinning rice straw source cellulose acetate, obtained flexible porous carbon fiber membrane and application thereof

Info

Publication number: CN112430909A
Application number: CN202011178048.3A
Authority: CN
Inventors: 闫俊涛; 刘金红; 宋光森; 王春蕾; 程群鹏; 范国枝; 柴波; 张顺喜; 方馨茹; 黄攀
Original assignee: Wuhan Polytechnic University
Current assignee: Wuhan Polytechnic University
Priority date: 2020-10-29
Filing date: 2020-10-29
Publication date: 2021-03-02
Anticipated expiration: 2040-10-29
Also published as: CN112430909B

Abstract

The invention belongs to the technical field of carbon fiber materials, and particularly relates to a method for preparing a flexible porous carbon fiber membrane based on rice straw source cellulose acetate electrospinning, an obtained flexible porous carbon fiber membrane and application thereof. The method comprises the following steps: 1) preparing a flexible composite fiber membrane of rice straw source cellulose acetate and polyvinylpyrrolidone by adopting an electrostatic spinning method; 2) washing the flexible composite fiber membrane obtained in the step 1) to remove polyvinylpyrrolidone in the flexible composite fiber membrane, and performing deacetylation to obtain a porous cellulose membrane; 3) calcining the porous cellulose membrane obtained in the step 2) to obtain the flexible porous carbon fiber membrane. The flexible porous carbon fiber membrane provided by the invention has the advantages of high specific surface area, high adsorption capacity and good cycle stability.

Description

Method for preparing flexible porous carbon fiber membrane by electrospinning rice straw source cellulose acetate, obtained flexible porous carbon fiber membrane and application thereof

Technical Field

The invention belongs to the technical field of carbon fiber materials, and particularly relates to a method for preparing a flexible porous carbon fiber membrane based on rice straw source cellulose acetate electrospinning, an obtained flexible porous carbon fiber membrane and application thereof.

Background

Nanofibers naturally have a high specific surface area, however, the efficient and orderly utilization of electrospinning techniques and the influence of electrospinning parameters remain to be optimized for the morphology and structure of nanofibers. The electrospun fiber has wide application in tissue engineering, wound dressing, enzyme immobilization and other aspects. Cellulose, being the most environmentally friendly of the available polymeric materials, is abundant in nature and is sufficient for electrostatic electrospinning. The use of cellulose has encountered a number of obstacles, due to the presence of intermolecular and intramolecular hydrogen bonds, resulting in limited solubility in common organic solvents. To overcome these obstacles, cellulose derivatives have been extensively studied. Among these derivatives, Cellulose Acetate (CA) is widely used for the preparation of electrospun fibers, which are then converted to cellulose fibers by deacetylation. CA is also receiving more and more attention because of its advantages such as better biocompatibility, degradability, insolubility in water, mechanical properties, non-toxicity, lower cost and excellent chemical resistance. CA has wide application prospect, such as antibacterial film, filamentation matrix, flexible film, biomedical nano composite material, affinity film, biomedical separation and the like. However, the CA electrospun material is still in need of further development. On the one hand, Polyacrylonitrile (PAN) is often used as a high molecular polymer for electrospinning to prepare carbon fibers, but PAN is expensive. On the other hand, the forest resources are deficient, and in order to save forest source cellulose, a substitute of the forest source cellulose needs to be developed. Cellulose is separated based on components of rice straws, rice straw source cellulose acetate suitable for electrostatic spinning is prepared through acetylation treatment, CA is used as a derivative product of natural high molecular cellulose, the cellulose acetate has the advantages of biodegradability, no pollution, no toxicity and the like, and is particularly suitable for being used as a high molecular polymer to prepare the cellulose acetate through electrostatic spinning, cellulose fibers are obtained through deacetylation treatment, and carbon fibers are prepared through preoxidation and high-temperature carbonization treatment. Therefore, the rice straw source cellulose acetate can replace polyacrylonitrile and forest resource cellulose acetate to be used for preparing carbon fibers by electrospinning. In addition, the common method for preparing the carbon material with high specific surface area usually uses strong alkali potassium hydroxide as an activating agent, and the existence of the potassium hydroxide seriously corrodes quartz tubes and equipment during the calcination in a tube furnace, so an advanced technical route needs to be explored, and the carbon material with high specific surface area can be prepared without using the activating agent potassium hydroxide.

Disclosure of Invention

In order to solve the defects of the prior art, the invention provides a method for preparing a flexible porous carbon fiber membrane based on rice straw source cellulose acetate electrospinning, an obtained flexible porous carbon fiber membrane and application thereof.

The technical scheme provided by the invention is as follows:

a method for preparing a flexible porous carbon fiber membrane based on rice straw source cellulose acetate electrospinning comprises the following steps:

1) preparing a flexible composite fiber membrane of rice straw source Cellulose Acetate (CA) and polyvinylpyrrolidone (PVP) by adopting an electrostatic spinning method;

2) washing the flexible composite fiber membrane obtained in the step 1) to remove polyvinylpyrrolidone in the flexible composite fiber membrane, and performing deacetylation to obtain a flexible porous cellulose membrane;

3) calcining the porous cellulose membrane obtained in the step 2) to obtain the flexible porous carbon fiber membrane.

Specifically, the step 1) specifically comprises the following steps:

1a) obtaining a mixed solvent composed of dichloromethane and glacial acetic acid, and adding a straw-derived cellulose acetate and polyvinylpyrrolidone into the mixed solvent to obtain a spinning solution, wherein the volume ratio of dichloromethane to glacial acetic acid is (1.8-2.2): 1, the weight ratio of the straw-derived cellulose acetate to polyvinylpyrrolidone is (9-7): 1-3), and the solid content in the spinning solution is 11-13 wt%;

1b) carrying out electrostatic spinning on the spinning solution obtained in the step 1a), wherein the spinning parameters are respectively set as follows: spinning negative voltage of-4.5 to-5.5 kV and spinning positive voltage of 9 to 11kV, the receiving distance of 14 to 16cm, the spinning temperature of 25 to 27 ℃, the humidity of about 38 to 42 percent and the flow rate of spinning solution of 0.35 to 0.45 mu l/min, and then drying the obtained composite membrane to obtain the flexible composite fiber membrane.

Specifically, the step 2) specifically comprises the following steps: placing the flexible composite fiber membrane obtained in the step 1) in 0.09-0.11 mol/L NaOH solution, standing for 20-28 h, washing and filtering to be neutral so as to dissolve PVP in the composite membrane, and performing deacetylation treatment on CA to obtain the porous cellulose membrane.

Specifically, the step 3) specifically comprises the following steps: placing the porous cellulose membrane obtained in the step 2) in a tube furnace, heating to 220-260 ℃ at a heating rate of 0.9-1.1 ℃/min, and preserving heat for 1.5-2.5 hours to obtain pre-oxidized carbon fibers; then argon is used as protective gas, the temperature is heated to 500-1000 ℃ at the heating rate of 2.7-3.3 ℃/min, and the temperature is kept for 2.5-3.5 h. Finally obtaining the flexible porous carbon fiber.

Preferably, when the weight ratio of the CA to the PVP is 8:2 and the calcining temperature is 900 ℃, the carbon fiber has the maximum specific surface area of 1427.6m²The maximum adsorption capacity of the solution to TC is 672mg/g, and after the solution is recycled for 5 times, the adsorption capacity is balancedThe amount is still as high as 601 mg/g.

Specifically, the preparation method of the rice straw source cellulose acetate comprises the following steps:

a) cleaning and drying the rice straws by water, crushing by a crusher, and sieving to obtain 50-100 meshes of rice straw scraps for later use;

b) pouring the rice straw scraps obtained in the step a) and an ethanol aqueous solution into a reaction kettle, sealing, heating for reaction, then carrying out suction filtration to obtain rice straw scraps residues, and drying to obtain pretreated rice straw scraps, wherein the mass concentration of ethanol in the ethanol aqueous solution is 60-70 wt%, the heating reaction temperature is 160-240 ℃, and the reaction time is 2-3 hours;

c) adding every 2g of the pretreated rice straw scraps obtained in the step b) and 35-45 mL of potassium hydroxide solution into a flask, stirring and reacting at 85-95 ℃, filtering after the reaction is finished, and then adding 180-220 mL of 2 wt% H₂O₂Heating and stirring the solution at 65-75 ℃ for 1.5-2.5 h to obtain the rice straw source cellulose, wherein the concentration of the potassium hydroxide solution is 4-6 wt%, and the stirring reaction time is 1.5-2.5 h;

d) adding 7-9 mL of glacial acetic acid into every 0.5g of the rice straw source cellulose obtained in the step c), adding concentrated sulfuric acid and acetic anhydride to perform acetylation reaction, adding distilled water into the acetylated solution after the reaction time is over to precipitate, performing vacuum filtration, washing the filtrate with deionized water until the filtrate is neutral, and finally freeze-drying the sample to obtain the rice straw source cellulose acetate, wherein the mass ratio of the rice straw source cellulose to the acetic anhydride is 1 (3-5).

Has the advantages that: the composite fiber membrane is prepared by skillfully introducing high polymer material polyvinylpyrrolidone as a pore-forming agent into electrostatic spinning and blending the pore-forming agent polyvinylpyrrolidone with rice stem source cellulose acetate, and when the cellulose acetate is deacetylated by aqueous alkali solution treatment, the pore-forming agent polyvinylpyrrolidone is synchronously dissolved and removed by water, so that the flexible porous carbon fiber membrane is prepared by pre-oxidation and calcination treatment. Because the conversion of cellulose to carbon fiber is superior to the conversion of cellulose acetate to carbon fiber in terms of carbon content, quality and performance, cellulose acetate needs to be deacetylated to convert it to cellulose and then converted to carbon fiber.

The invention also provides the flexible porous carbon fiber membrane prepared by the method.

The invention also provides application of the flexible porous carbon fiber as a flexible adsorption membrane material.

The porous carbon fiber provided by the invention has the advantages of high specific surface area, high adsorption capacity and good cycle stability.

Drawings

Fig. 1 is a schematic view of a preparation process of the membrane-shaped porous carbon nanofiber.

FIG. 2 shows IR spectra (a) CA/PVP (b) PVP (c) straw source CA (d) CA membrane (e) RC membrane.

FIG. 3 is an XRD pattern (a) C-600(b) C-750(C) C-900.

FIG. 4 shows a Raman spectrum (a) C-600(b) C-750(C) C-900.

FIG. 5 is an SEM image of (a-b) CA/PVP membranes (c-d) RC membranes (e-f) CA membranes.

FIG. 6 shows SEM pictures of (a) C-CA/PVP (b) C-CA (C) C-RC.

FIG. 7 is an SEM image of (a) C-1(b) C-2(C) C-3.

FIG. 8 is an SEM image of (a) C-600(b) C-750(C) C-900.

FIG. 9 shows the nitrogen adsorption and desorption curves (a) C-CA/PVP (b) C-RC.

FIG. 10 shows the pore size distribution of (a-b) C-600; (C-d) C-750; (e-f) C-900.

FIG. 11 shows pore size distribution diagrams of (a), (c) and (e) nitrogen desorption curves (b), (d) and (f): (a-b) C-1(C-d) C-2(e-f) C-3.

FIG. 12 is an adsorption analysis chart in which: (a) adsorption equilibrium curve diagrams of carbon fibers under different TC concentrations; (b) adsorption kinetics curves of carbon fibers prepared at different temperatures (a) C-900(b) C-CA/PVP (C) C-750(d) C-600; (c) adsorption kinetics curves of carbon fibers prepared by different CA/PVP ratios are (a) C-2(b) C-3(C) C-1(d) C-CA/PVP; (d) the carbon fiber absorbs TC solution repeatedly.

Fig. 13 is an optical photograph of a flexible porous carbon fiber membrane made by the present technology, the carbon fibers having a fluffy and black appearance.

Detailed Description

The principles and features of this invention are described below in conjunction with examples which are set forth to illustrate, but are not to be construed to limit the scope of the invention.

Examples

1. Preparation of CA/PVP composite fiber membrane by electrostatic spinning method

A certain volume of dichloromethane and glacial acetic acid (2/1v/v) are uniformly stirred to obtain a mixed solvent. Weighing a certain mass of CA and PVP, adding into the mixed solvent, magnetically stirring for 6h to obtain a CA/PVP (w: w 9:1, 8:2, 7:3) spinning solution with a certain mass concentration, and controlling the content of CA/PVP in the spinning solution to be 12 wt%.

Respectively carrying out electrostatic spinning on the prepared CA/PVP spinning solution, wherein the spinning parameter settings are respectively as follows: negative spinning voltage of-5 kV and positive spinning voltage of 10kV, receiving distance of 15cm, spinning temperature of 25-27 ℃, humidity of about 40% and spinning solution flow rate of 0.4 mul/min, placing the obtained CA/PVP composite membrane in an oven at 80 ℃, and drying for later use. The prepared CA/PVP composite nanofiber membrane is white and has better flexibility.

2. Preparation of porous carbon fiber

The obtained CA/PVP (w: w ═ 9:1, 8:2, 7:3) composite membrane was placed in a 0.1mol/L NaOH solution, left to stand for 24 hours, washed and filtered to neutrality. Dissolving PVP in the composite membrane, and performing deacetylation treatment on CA to obtain porous cellulose membranes RC (RC-1, RC-2 and RC-3) respectively.

Placing porous cellulose membranes RC-1, RC-2 and RC-3 in a tube furnace, pre-oxidizing, heating to 240 ℃ at the heating rate of 1 ℃/min, and preserving heat for 2 hours to obtain pre-oxidized carbon fibers. Then argon is used as protective gas, the temperature is heated to 900 ℃ at the heating rate of 3 ℃/min, and the temperature is kept for 3 h. Finally obtaining the porous carbon fibers (C-1, C-2 and C-3).

Independently placing the porous cellulose membrane RC-2 in a tube furnace, pre-oxidizing, heating to 240 ℃ at the heating rate of 1 ℃/min, and preserving heat for 2 hours to obtain the pre-oxidized carbon fiber. Then argon is used as protective gas, the temperature is respectively heated to 600 ℃, 750 ℃ and 900 ℃ at the heating rate of 3 ℃/min, and the temperature is kept for 3 h. Finally obtaining the porous carbon fibers (C-600, C-750 and C-900) with different calcination temperatures. The specific experimental process is shown in figure 1.

Preparation of rice straw source cellulose acetate

Extraction of rice straw cellulose

(1) Raw material treatment

Washing rice straw with water, oven drying, pulverizing with pulverizer, sieving, and collecting 50-100 mesh rice straw scraps.

(2) Pretreatment of rice straw

5g of the rice straw scraps and a certain amount of ethanol-water solution are poured into a reaction kettle, and the reaction kettle is quickly sealed and heated. After reacting for a certain time, respectively obtaining the rice straw residue and the black liquor by suction filtration. Immediately using the solution with the same concentration to clean the rice straws, and washing until the filtrate is clear (collecting the washing liquid). Comparing the change of the rice straws before and after pretreatment; measuring the pH value of the black liquor after suction filtration; and respectively adding three times of deionized water into the black liquor and the washing liquid obtained after the suction filtration, standing for a period of time, then carrying out suction filtration and drying to obtain the lignin directly dissolved out and the lignin adsorbed on the rice straws, and weighing. And (4) weighing the dried rice straw residues to obtain the mass of the sample, and calculating the yield.

(3) Alkali treatment and hydrogen peroxide bleaching

2g of the pretreated rice straw and 40mL of potassium hydroxide solution with a certain concentration are added into a flask and stirred for 2 hours at 90 ℃. After the reaction was complete, the mixture was filtered and 200mL of 2 wt% H was added₂O₂The solution was heated and stirred at 70 ℃ for 2h to obtain cellulose.

Acetylation modification of rice straw cellulose

0.5g of rice straw cellulose is weighed and placed in a three-neck flask, 8mL of glacial acetic acid is added, and a certain amount of concentrated sulfuric acid and acetic anhydride are added for acetylation reaction at a certain temperature. After the reaction time is over, adding distilled water into the acetylated solution for precipitation, then carrying out vacuum filtration, washing the solution with deionized water until the filtrate is neutral, and finally freeze-drying the sample to obtain the cellulose acetate

3. Adsorption test

In the adsorption test, tetracycline hydrochloride (TC) is used as an adsorbate, the prepared porous carbon fiber is used as an adsorbent, and an adsorption equilibrium curve, an adsorption kinetics curve and a cyclic adsorption performance are respectively obtained through tests, so that the adsorption performance of the activated carbon fiber is evaluated.

3.1 adsorption equilibrium experiment

Preparing a series of TC solutions (100mg/L, 200mg/L, 300mg/L and 400 mg/L) with different concentrations, respectively putting 50mL of TC solution into a 150mL conical flask, and then weighing 5mg of carbon fiber membrane to add. Sealing the conical bottle mouth with tin foil paper, placing in a constant temperature oscillator at 25 ℃, oscillating for 12h, taking 2mL of supernatant, and obtaining the concentration of the TC solution before and after adsorption through an ultraviolet-visible spectrophotometer. And calculating the maximum adsorption capacity of the carbon fiber on the TC solution according to the formula 3-1.

Wherein: q_e-equilibrium adsorption capacity of carbon fibers (mg/g);

C₀-initial concentration of TC solution (mg/L);

C_e-post-adsorption TC solution equilibrium concentration (mg/L);

volume of V-TC solution (mL);

mass of m-carbon fiber (mg);

3.2 adsorption kinetics experiment

50mL of 300mg/L TC solution was added to a 150mL Erlenmeyer flask, and 5mg of carbon fiber was accurately weighed and placed in the Erlenmeyer flask. Placing the conical flask in a constant temperature oscillator at 25 deg.C, shaking, and timing to obtain 2mL of supernatant at 10min, 30min, 60min, 120min, 300min, and 420 min. And measuring the concentration of the TC solution at different adsorption times by an ultraviolet-visible spectrophotometer, and then calculating the adsorption quantity of the carbon fiber to the TC at different times according to a formula (3-2).

Wherein Q_t-the amount of carbon fibre adsorbed at tmin (mg/g);

C₀-initial concentration of TC solution (mg/L);

C_t-concentration of TC solution (mg/L) at adsorption of tmin;

volume of V-TC solution (mL);

mass of m-carbon fiber (mg);

3.3 Cyclic adsorption experiment

In order to research whether the recycling performance of the porous carbon fiber is stable or not, a cyclic adsorption experiment is carried out on the carbon fiber, and the method comprises the following specific steps: performing adsorption experiment with carbon fiber pair, obtaining supernatant through a rubber head dropper when the adsorption is balanced, determining the concentration of TC solution to obtain the adsorption capacity of the carbon fiber, and centrifuging to separate out the adsorbent. And (3) washing the carbon fiber for multiple times by using ethanol and deionized water, drying in an oven, and repeating the step for 5 times to obtain the adsorption capacity of the carbon fiber after circulation for different times, so as to evaluate the repeated adsorption capacity of the carbon fiber on TC.

4. Analysis of results

4.1 IR

FIG. 2(a-e) are respectively infrared spectra of electrospun CA/PVP membrane, PVP, rice straw source CA, CA/PVP membrane soaked in water, and RC membrane soaked in CA/PVP membrane alkali. From curve (b), PVP at 1291cm^-1Has a characteristic peak nearby, which is generated by-C-N-stretching vibration of PVP molecules and is 1641cm^-1The characteristic peak at (a) is generated by the vibrational transition of-C ═ O. 3370cm^-1The left and right positions are typical O-H stretching vibration absorption peaks in water. Curve (c) is the infrared spectrum of the straw-derived cellulose acetate, 1751cm^-1The absorption peak at (B) corresponds to the stretching vibration of C ═ O, 1376cm^-1The absorption peak is formed by-O (C ═ O) -CH₃Caused by stretching vibration of C-H bond in group, 1234cm^-1The nearby absorption peak corresponds to the C-O stretching vibration in the acetyl group. As can be seen from the curve (a), the electrospun CA/PVP nanofiber has all characteristic peaks of PVP and CA, which proves that the CA/PVP composite fiber membrane is successfully prepared by electrospinning with the straw source CA and PVP as raw materials. As can be seen from the curve (d), the sample obtained by soaking the CA/PVP nano-fiber in water has no characteristic peak of PVP and retains all the characteristic peaks of CA,this shows that the pore-forming agent PVP in the CA/PVP nanofiber can be completely dissolved out through the water soaking treatment, and the deacetylation of CA can not be realized, so that the CA nanofiber is finally obtained. From curve (e), the sample obtained by soaking the CA/PVP nanofibers in sodium hydroxide has no characteristic peak of the pore-forming agent PVP, and C ═ O at 1751cm^-1The absorption peak is obviously weakened, which shows that the pore-forming agent PVP is dissolved and removed after the alkali treatment, and meanwhile, CA is deacetylated and is converted from cellulose acetate into cellulose, so that the porous cellulose fiber membrane RC is obtained.

4.2 XRD

XRD spectral scans were performed on carbon fibers treated at different calcination temperatures as shown in fig. 3. All samples showed two diffraction peaks at 23 ° and 43 °, and the distinct diffraction peak at 2 θ ═ 23 ° represented the {002} crystal plane of the graphitic structure, which is an amorphous structure of carbon. The diffraction peak appearing at 2 θ ═ 43 ° represents the {100} crystal plane of the graphite structure. An XRD map shows that the carbon fiber prepared by electrospinning is amorphous carbon with certain graphite microcrystals. a. b and c respectively correspond to the calcining temperatures of 600, 750 and 900 ℃.

4.3 Raman

The porous carbon fibers obtained by treatment at different calcination temperatures have different surface structures. a. And b and c are respectively scanned by Raman spectrum corresponding to the carbon fibers with the calcining temperature of 600 ℃, 750 ℃ and 900 ℃ (as shown in figure 4). As can be seen from FIG. 4, the Raman spectrum of the carbon fiber showed two distinct peaks, each at 1343cm^-1Nearby D peak and 1584cm^-1Nearby G peak. It is customary to determine the ratio of the intensity of the D peak due to disordered structure to the intensity of the G peak due to graphite crystals (R ═ I)_D/I_G) The graphitization degree of the carbon fiber is characterized, and the smaller the R value is, the larger the graphitization degree of the carbon fiber is, and the lower the defect rate is. The R values of the C-600, C-750 and C-900 fibers are 1.2746, 1.0092 and 0.8369 respectively. It can be seen that the R value of the carbon fiber is smaller and smaller with the increase of the calcination temperature, which indicates that the calcination temperature has a great influence on the graphitization degree of the carbon fiber, the higher the temperature is, the microcrystalline structure of the carbon fiber becomes complete gradually, and the higher the graphitization degree is, the fewer defects are contained. When the calcination temperature is 9At 00 ℃, the R value is 0.8369 and is less than 1, which shows that the graphitized structure of the C-900 is greatly increased and the defects are obviously reduced.

4.4 SEM

By subjecting the electrospun CA/PVP membrane (W: W8: 2) to alkali treatment or water treatment, respectively, nanofiber membranes with different micro-morphologies can be obtained as shown in FIG. 5. FIGS. 5(a-b), (c-d), and (e-f) are SEM images of CA/PVP films, cellulose RC films, and CA fiber films, respectively. As can be seen from FIGS. 5(a-b), the electrospun CA/PVP film consisted of nanofibers of uniform diameter, with a smoother surface and no beading present, distributed uniformly, and was ideal nanofibers. It can be seen from FIGS. 5(e-f) that the CA/PVP films were soaked in water and still retained the fiber structure, and the fiber diameters were relatively uniform. At the same time, some cross-linking fusion occurs between fibers, which may be caused by PVP dissolution to crack the structure of part of the fibers, even the fibers are fused together with nearby fibers to form a structure similar to a thin film. It can be seen from fig. 5(c-d) that after the CA/PVP film is subjected to alkali soaking, it can be seen that although the RC film still retains the fiber morphology, the fibers are significantly changed. As can be seen from fig. 5(c), the morphology of the fibers is somewhat blurred, and a large number of thin films appear between the fibers. The PVP is dissolved out after alkali soaking, and meanwhile, the surface of CA can be deacetylated under the action of alkali, so that the appearance of the fibers is changed to a certain extent, part of the fibers are dissolved under the action of weak alkali, fusion crosslinking occurs among the fibers, and the fibers are swelled due to long-time soaking.

The CA/PVP membrane, the porous CA fiber membrane and the porous cellulose RC membrane are calcined under the same condition to obtain carbon with different forms, which are respectively marked as C-CA/PVP, C-CA and C-RC, and the influence of water treatment and alkali treatment on the preparation of carbon fibers is discussed. FIGS. 6(a), (b), and (C) are SEM images of C-CA/PVP, C-CA, and C-RC, respectively. As shown in FIG. 6(a), C-CA/PVP has a fiber structure, the fibers cross each other to form a network structure, the fiber diameter is not very uniform, and the surface has no porous structure. This shows that the carbon fiber can be obtained by calcining the CA/PVP membrane, but the C-CA/PVP has no porous structure. As can be seen from FIG. 6(b), C-CA shows a blocky morphology with some holes of different sizes on the surface. This is mainly due to the low thermal stability of CA, which leads to the collapse of the fibers during calcination of the CA film due to the high temperature melting, and finally to the formation of a massive char, leaving pores of varying sizes on the surface. As can be seen from FIG. 6(C), C-RC has fibrous morphology, uniform fiber diameter and uniform distribution, and micropores with uniform size are formed on the surface of the fiber. This shows that after the CA/PVP membrane is subjected to alkali treatment, the pore-forming agent PVP in the CA/PVP fiber can be dissolved out, holes are left on the surface of the fiber, the RC is obtained after deacetylation, the thermal stability of the RC is improved, and the morphology of the fiber can still be kept after calcination. Therefore, the CA/PVP membrane is subjected to alkali treatment to obtain a porous cellulose fiber membrane, and then the porous C-RC fiber is obtained by calcining.

The CA/PVP films with different amounts of the pore-foaming agent PVP are subjected to alkali treatment and then calcined at 900 ℃ to obtain C-1, C-2 and C-3, and the appearance of the carbon fiber can be influenced by the amount of the PVP. FIG. 7 is a scanning electron micrograph of C-1, C-2, and C-3, respectively. As can be seen from FIG. 7(a), the C-1 fiber obtained when the ratio of CA/PVP is 9:1(w/w) has uniform diameter, smooth surface and no obvious holes are observed, which may be caused by too low content of PVP in the fiber and no obvious influence of PVP dissolution on the morphology of the fiber. The content of PVP was increased so that the ratio of CA/PVP was 8:2, yielding C-2. As can be seen from 7(b), C-2 has a fibrous morphology, uniform fiber diameter and uniform distribution, and micropores with uniform sizes are formed on the surface of the fiber. And continuously increasing the content of PVP to ensure that the ratio of CA/PVP reaches 7:3, obtaining C-3. As can be seen from 7(b), C-3 still has a fibrous morphology, but the surface of the fiber has only a few large pores, and no more numerous micropores exist. This is probably because as the content of PVP increases, the CA/PVP spin solution concentration becomes larger, the PVP is not dispersed uniformly enough, and finally the resulting C-3 fiber has a reduced surface porosity and some relatively large pores are left.

The calcination temperature is also an important factor influencing the carbon fiber, and the porous cellulose RC membrane is calcined at 600 ℃, 700 ℃ and 900 ℃ respectively to obtain C-600, C-750 and C-900 respectively. FIG. 8 is a scanning electron micrograph of C-600, C-750, and C-900, respectively. As can be seen from FIG. 8(a), when the calcination temperature was 600 ℃, the obtained C-600 fiber had a good one-dimensional structure, non-uniform fiber diameter, smooth surface, and no porous structure. The calcination temperature is increased to 750 ℃ to obtain the C-750 fiber, and as can be seen from FIG. 6(b), the C-750 fiber still has a good one-dimensional structure, and the fiber surface is relatively smooth. The calcination temperature is continuously increased to 900 ℃ to obtain the C-900 fiber, and as can be seen from FIG. 8(C), the C-900 fiber has a good one-dimensional structure, uniform fiber diameter and uniform distribution, and pores with uniform sizes are formed on the surface of the fiber. This is mainly because as the temperature is increased, more and more volatile components in the fiber are lost, and pores are left on the surface of the fiber, so that the porous carbon fiber such as C-900 is formed.

4.5 BET

The CA/PVP raw membrane and the porous cellulose RC membrane are calcined under the same condition to obtain different carbon fibers C-CA/PVP and C-RC, nitrogen adsorption and desorption tests are carried out on the obtained carbon fibers, and the difference of the specific surface areas of the carbon fibers prepared by calcining the fiber membranes with different components is examined, as shown in figure 9. According to the BET test result, the specific surface area of the C-CA/PVP carbon fiber is 729.0m²(ii)/g, pore volume 0.174 cm 3/g. The specific surface area of the C-RC carbon fiber is 1427.6m²The pore volume is 0.819cm 3/g. This shows that the specific surface area of the C-RC carbon fiber obtained by calcining RC is greatly improved compared with the specific surface area of the C-CA/PVP carbon fiber obtained by directly calcining the CA/PVP raw film. The reason is that after the CA/PVP raw membrane is treated by the alkali solution, the pore-forming agent PVP is dissolved out, deacetylation of CA is realized to obtain a porous cellulose membrane, and then the porous cellulose membrane is calcined to obtain a porous carbon fiber RC membrane, and the thermal stability of the RC membrane is higher, so that the C-RC carbon fiber obtained by calcining at 900 ℃ has excellent specific surface area. In addition, the carbonization performance of the cellulose is better than that of CA/PVP.

FIG. 10 shows N in C-600, C-750, and C-900, respectively₂Adsorption and desorption curves and pore size distribution maps. The graph shows that the adsorption and desorption curves of the carbon fibers belong to IV-type isotherms, the curves are smooth, and hysteresis loops appear, which indicates that the prepared carbon fibers are typical mesoporous materials. FIG. 10(b), (d)And (f) it is found that the pore size distributions of these three types of carbon fibers are relatively narrow, mainly centered around 20 nm. As can be seen from FIG. 10(a), the hysteresis loop of the nitrogen adsorption desorption curve at low pressure of the C-600 carbon fiber obtained by calcination at 600 ℃ is not closed. This is probably because the carbon fibers contain an irregular nanoporous structure. When the calcining temperature is 600 ℃, the specific surface area of the C-600 carbon fiber is 403.4m²Per g, pore volume of 0.176cm³(ii) in terms of/g. When the calcining temperature is 750 ℃, the specific surface area of the C-750 carbon fiber is 632.5m²Per g, pore volume of 0.342 cm³(ii) in terms of/g. When the calcining temperature is 900 ℃, the specific surface area of the C-900 carbon fiber is 1427.6m²Per g, pore volume of 0.819cm³(ii) in terms of/g. This shows that the specific surface area of the carbon fiber is larger and larger as the calcination temperature is increased, and the pore volume is also increased, mainly because more and more volatile components in the fiber are lost as the temperature is increased, and the nano pores are left on the surface of the fiber, so that the specific surface area and the pore volume of the carbon fiber are increased, which is consistent with the result of SEM pictures, therefore, the calcination at 900 ℃ is selected to prepare the carbon fiber, and the large specific surface area is helpful for adsorbing more tetracycline pollutants in the wastewater.

FIG. 11 shows N in C-1, C-2 and C-3₂Adsorption and desorption curves and pore size distribution maps. As can be seen from the figure, the adsorption and desorption curves of the C-1, C-2 and C-3 carbon fibers all belong to IV-type isotherms, the curves are smooth, and hysteresis loops appear, which indicates that the prepared carbon fibers are typical mesoporous materials. As is clear from FIG. 11(a), the specific surface area of the C-1 carbon fiber obtained when the ratio of CA/PVP was 9:1(w/w) was 1156.3 m²(ii)/g, pore volume 0.292cm 3/g. The content of PVP is increased, and when the ratio of CA to PVP is 8:2, the specific surface area of the prepared C-2 carbon fiber is 1427.6m²The pore volume is 0.819cm 3/g. The content of PVP is continuously increased, and when the ratio of CA/PVP reaches 7:3, the specific surface area of the prepared C-3 carbon fiber is 1177.0m²(ii)/g, pore volume 0.446cm 3/g. From the above, it can be seen that, as the content of PVP in the spinning solution increases, the specific surface area of the obtained carbon fiber increases first and then decreases, and when the ratio of CA to PVP is 8:2, the specific surface area of the obtained C-2 fiber is the largest, which is the best ratio of CA to PVP. As can be seen from FIGS. 11(b), (d) and (f),the pore size distribution of the three carbon fibers is narrow and mainly concentrated at about 19 nm.

4.6 study of adsorption Properties of Flexible porous carbon fiber Membrane

FIG. 12(a) is a graph showing the adsorption equilibrium of the C-900 fibers at various concentrations of TC, and the equilibrium adsorption amount of the C-900 fibers is gradually increased as the initial concentration of TC is increased, mainly because the contact ratio between TC molecules and carbon fibers is increased and the effective number of collisions is increased as the concentration of TC is increased. When the initial concentration of TC is 300mg/L, the equilibrium adsorption capacity also reaches the maximum value of 680mg/g, and the equilibrium adsorption capacity is almost unchanged when the initial concentration of TC is continuously increased. This is because when the amount of adsorbed TC reaches a certain amount, the adsorption is in dynamic equilibrium, and the TC molecules already present on the carbon fiber are repelled from the TC molecules in the solution, so that the residual active sites on the surface of the carbon fiber cannot act, and the adsorption is saturated.

In order to test the adsorption performance of the carbon fibers obtained at different calcination temperatures on the TC solution, adsorption kinetics experiments were performed on the TC solution with an initial concentration of 300 mg/L. As shown in fig. 12(b), the total amount of adsorption of the TC solution by the obtained carbon fiber increases as the calcination temperature increases. This is because the increase of the calcination temperature increases pores on the carbon fiber, increases the specific surface area, increases the pore volume, and facilitates the adsorption of tetracycline hydrochloride by the carbon fiber. This is also consistent with the nitrogen adsorption and desorption test results. When the calcination temperature is 900 ℃, the equilibrium adsorption capacity of the obtained C-900 carbon fiber to the tetracycline hydrochloride is 680mg/g, which is 3.5 times and 1.6 times of the equilibrium adsorption capacity of the C-600 carbon fiber and the C-750 carbon fiber respectively. Meanwhile, the equilibrium adsorption capacity of the C-CA/PVP carbon fiber to the tetracycline hydrochloride is 480mg/g, which is far less than the equilibrium adsorption capacity of the C-900 carbon fiber to the tetracycline hydrochloride. The method shows that the removal of PVP pore-forming agent in the electrospun CA/PVP membrane and the deacetylation treatment of CA have very important effect on the preparation of carbon fiber with large specific surface area, the soaking in alkali liquor has the double effects of removing the PVP pore-forming agent and promoting the deacetylation of CA, the removal of PVP leaves holes, the deacetylation of CA is converted into cellulose, the high-temperature carbonization and thermal stability effects of the cellulose are superior to those of CA and PVP, and the porous carbon fiber is formed by calcination, so that the porous carbon fiber has better adsorption performance on tetracycline hydrochloride.

In order to test the adsorption performance of carbon fibers obtained by different CA/PVP ratios on TC solution, an adsorption kinetic experiment is carried out on the TC solution with the initial concentration of 300 mg/L. As can be seen from FIG. 12(C), the maximum adsorption capacity of C-1 carbon fiber to tetracycline hydrochloride was 518 mg/g at a CA/PVP ratio of 9: 1. The PVP content is increased, when the ratio of CA/PVP is 8:2, the maximum adsorption capacity of the C-2 carbon fiber to tetracycline hydrochloride is 680mg/g, and the adsorption capacity reaches the maximum. The PVP content is continuously increased, the ratio of CA to PVP is 7:3, the maximum adsorption capacity of the C-3 fiber to the tetracycline hydrochloride is 620mg/g, and the adsorption capacity is reduced. The experimental results are consistent with SEM analysis and BET test data. When the ratio of CA/PVP is 8:2, the obtained electro-spinning CA/PVP composite membrane is subjected to alkali treatment and then is calcined at 900 ℃, the obtained carbon fiber has the largest surface area and pore volume and the best adsorption performance on tetracycline hydrochloride, the adsorption balance is achieved for about 300min on TC solution with the initial concentration of 300mg/L, and the balance adsorption capacity is 680 mg/g.

In order to test whether the adsorbent can be used in real life, a cyclic adsorption test must be performed on the porous carbon fiber. In order to completely separate the carbon fiber with saturated adsorption from the adsorbate, the used carbon fiber needs to be washed with ethanol and water for multiple times and then dried to obtain the desorbed carbon fiber. Then the carbon fiber is repeatedly subjected to the adsorption experiment to test the equilibrium adsorption capacity of the carbon fiber, and then is subjected to desorption according to the desorption method, and the cycle experiment is carried out for 5 times. From 12(d), after the carbon fiber is recycled, the equilibrium adsorption capacity of each time is slightly reduced, but the equilibrium adsorption capacity is still large, and finally after 5 times of recycling, the equilibrium adsorption capacity is 601mg/g and is reduced by only 10.6%, which shows that the porous carbon fiber prepared by electrospinning has better recycling performance. In addition, the porous carbon fiber membrane has certain flexibility, is convenient for separation and recovery, and is beneficial to the application in industry.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims

1. A method for preparing a flexible porous carbon fiber membrane based on rice straw source cellulose acetate electrospinning is characterized by comprising the following steps:

1) preparing a flexible composite fiber membrane of rice straw source cellulose acetate and polyvinylpyrrolidone by adopting an electrostatic spinning method;

2) washing the flexible composite fiber membrane obtained in the step 1) to remove polyvinylpyrrolidone in the flexible composite fiber membrane, and performing deacetylation to obtain a porous cellulose membrane;

2. The method for preparing the flexible porous carbon fiber membrane based on the rice straw-derived cellulose acetate electrospinning according to claim 1, wherein the step 1) specifically comprises the following steps:

3. The method for preparing the flexible porous carbon fiber membrane based on the rice straw-derived cellulose acetate electrospinning according to claim 1, wherein the step 2) specifically comprises the following steps: placing the flexible composite fiber membrane obtained in the step 1) in 0.09-0.11 mol/L NaOH solution, standing for 8-28 h, then washing to be neutral, deacetylating cellulose acetate in the flexible composite fiber membrane while dissolving out polyvinylpyrrolidone in the flexible composite fiber membrane, and filtering to obtain the flexible porous cellulose membrane.

4. The method for preparing the flexible porous carbon fiber membrane based on the rice straw-derived cellulose acetate electrospinning according to claim 1, wherein the step 3) specifically comprises the following steps: placing the porous cellulose membrane obtained in the step 2) in a tube furnace, heating to 220-260 ℃ at a heating rate of 0.9-1.1 ℃/min, and preserving heat for 1.5-2.5 hours to obtain pre-oxidized carbon fibers; and then argon is used as protective gas, the temperature is heated to 500-1000 ℃ at the heating rate of 2.7-3.3 ℃/min, and the temperature is kept for 2.5-3.5 hours, so that the flexible porous carbon fiber membrane is finally obtained.

5. The method for preparing the flexible porous carbon fiber membrane based on the electrospinning of the rice straw-derived cellulose acetate according to any one of claims 1 to 4, characterized in that the method for preparing the rice straw-derived cellulose acetate comprises the steps of:

6. A flexible porous carbon fiber membrane prepared by the method for preparing the flexible porous carbon fiber membrane based on the rice straw-derived cellulose acetate electrospinning according to any one of claims 1 to 5.

7. Use of a flexible porous carbon fibre membrane according to claim 6, characterised in that: as a flexible adsorption film material.