Preparation method, product and application of a MOF-based hollow carbon nanofiber material

Abstract

The present application relates to the technical field of pollution treatment, in particular to a preparation method, product and application of MOF-based hollow carbon nanofiber material. The preparation method comprises the following steps: adding polyacrylonitrile, a sacrificial pore-forming agent and ZIF-8 nanoparticles into an organic solvent to obtain a spinning solution; performing electrostatic spinning on the spinning solution, and then pre-oxidizing and carbonizing to obtain the MOF-based hollow carbon nanofiber material. By introducing the sacrificial pore-forming agent (PMMA) into the mixed PAN / ZIF-8 solution, the MOF-based hollow carbon nanofiber material prepared can obtain rich hierarchical pores and hollow structures, which is more conducive to the transmission of pollutants (VOCs in the air) into the MOF-based hollow carbon nanofiber material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pollutant treatment, in particular to a preparation method of MOF-based hollow carbon nanofiber material, product and application. BACKGROUND

[0002] Volatile organic compounds (VOCs) are one of the most common pollutants in indoor and outdoor air, which are diverse, including benzene, olefins and ketones, etc. When the concentration of VOCs exceeds a certain limit, it is not only harmful to human health, but also harmful to the ecological environment. A large number of studies have shown that VOCs in the air will undergo photochemical reactions under strong light irradiation, producing secondary pollutants such as ozone (O3) and aerosols, which can cause and exacerbate photochemical smog, haze and other air pollution problems. Therefore, effective removal of volatile organic compounds in the air has attracted more and more attention. At present, the main methods for removing VOCs include catalytic oxidation or combustion, biological treatment, separation and adsorption. Adsorption method is widely used for the removal of VOCs in practice due to its simplicity, low cost and almost no secondary pollutants.

[0003] In recent years, although various adsorbents such as zeolites, metal-organic frameworks (MOFs), biochar, etc. have been used to remove VOCs in the air, porous carbon materials are still the focus and hotspot of research, especially new raw materials for preparing carbon materials. In the past two decades, a large number of biological wastes with special internal structure and composition, such as organs of plants including roots, stems, leaves, flowers, fruits and shells, skeletons, scales of aquatic organisms, have been used as raw materials for producing porous carbon-based materials. Compared with complex biological wastes, MOFs with specific structure and composition as raw materials show the potential for controllable preparation of carbon-based materials.

[0004] Zeolitic imidazolate framework (ZIF-8) is one of the most popular MOFs, which is often used for preparing carbon-based materials due to its mild synthesis conditions, low raw material cost and batch production. So far, ZIF-8 derived carbon materials have been widely applied in the fields of adsorbents, supercapacitors, catalysis, etc. However, when the powdered ZIF-8 derived carbon materials are used as adsorbents, especially for volatile organic compounds, on the one hand, there is a problem of poor adsorption effect, on the other hand, there is a problem of difficult recycling and utilization.

[0005] In practical applications, powder carbon materials generally need to be processed into a certain shape. Electrospinning can simply and quickly assemble MOF particles into fibers, and finally prepare a self-supporting MOF-based carbon nanofiber felt, which avoids the secondary forming problem of powder MOF-derived carbon. At present, there are mainly five methods for preparing MOF-based nanofibers, including "direct electrospinning", "surface in-situ growth", "secondary growth", "layer-by-layer assembly" and "atomic layer deposition". Then, MOF-based carbon nanofibers are obtained through heat treatment. Among them, "direct electrospinning" is the most convenient, which can prepare MOF-based nanofibers by directly electrospinning a mixed solution of powder MOFs and polymer. Polyacrylonitrile (PAN) is often used as a blended polymer due to its high carbon yield and easy spinning. During pyrolysis, the PAN-derived carbon skeleton can support the MOF-based carbon nanofiber. However, the MOF-based carbon nanofiber prepared by this method will cause the MOF-derived porous carbon-based particles to be wrapped by the PAN-derived dense carbon layer, which not only limits the transmission of pollutants to the MOF-derived porous carbon-based particles, but also causes the specific surface area and pore volume of the obtained MOF-based carbon nanofiber to be low, further limiting their application in adsorption.

[0006] Therefore, it is of great significance to provide a MOF-based carbon nanofiber material with good VOCs adsorption performance in air for the field of pollutant treatment technology. SUMMARY

[0007] Based on the above, the present application provides a preparation method, product and application of a MOF-based hollow carbon nanofiber material, which has good adsorption performance for VOCs in air.

[0008] To achieve the above-mentioned purpose, the present application provides the following solutions:

[0009] One of the technical solutions of the present application is a preparation method of a MOF-based hollow carbon nanofiber material, comprising the following steps:

[0010] Polyacrylonitrile, a sacrificial pore-forming agent and ZIF-8 nanoparticles are added to an organic solvent to obtain a spinning solution;

[0011] The spinning solution is electrospun, and then pre-oxidized and carbonized to obtain the MOF-based hollow carbon nanofiber material.

[0012] Further, the sacrificial pore-forming agent is polymethyl methacrylate (PMMA).

[0013] Further, the mass ratio of the polyacrylonitrile to the sacrificial pore-forming agent and the ZIF-8 nanoparticles is 1:1:1.

[0014] The mass ratio of polyacrylonitrile to the sacrificial pore-forming agent and ZIF-8 nanoparticles limits the above parameters: too much PAN results in a smaller specific surface area and porosity of the final obtained carbon nanofiber, too much PMMA results in a lower carbon yield during carbonization, and even complete burning; too much ZIF-8 nanoparticles results in difficulty in fiber formation.

[0015] The present application limits the MOF material to ZIF-8 nanoparticles because: the material is a regular polyhedron with a size of less than 100 nm, which is more conducive to the formation of uniform fibers; zinc salt is selected because zinc evaporates and disappears at a temperature higher than 900 DEG C, and the final formed carbon fiber does not contain metal; if it is another metal MOF, the carbon-based fiber obtained by carbonization below 1000 DEG C contains metal, so the specific surface area and pore volume are generally small. For example, ZIF67 in the same series, because the metal node is cobalt, the specific surface area and pore volume of the cobalt-containing carbon-based fiber obtained under the same preparation conditions are small, which is not conducive to the use of adsorbents.

[0016] Further, the organic solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide (DMAc), and N-dimethylpyrrolidone (NMP).

[0017] Further, the sum of the mass concentrations of polyacrylonitrile, the sacrificial pore-forming agent, and ZIF-8 nanoparticles in the spinning solution is 20-25%.

[0018] The sum of the mass concentrations of polyacrylonitrile, the sacrificial pore-forming agent, and ZIF-8 nanoparticles is limited to 20-25% because too low or too high concentrations are not conducive to the formation of uniform fibers.

[0019] Further, the pre-oxidation is specifically heating to 220-300 DEG C at a heating rate of 1-10 DEG C / min for 1-5 hours; and the carbonization is specifically heating to 900-1100 DEG C at a heating rate of 1-10 DEG C / min for 1-4 hours in an inert atmosphere.

[0020] The purpose of pre-oxidation is to obtain a stable nanofiber material.

[0021] Too high a pre-oxidation heating rate is not conducive to the formation of stable nanofibers, too low a pre-oxidation temperature does not reach the cyclization temperature of polyacrylonitrile, which is not conducive to the formation of stable carbon felt, too high a pre-oxidation temperature results in a large weight loss of the carbon material; too short a pre-oxidation time does not allow complete cyclization of polyacrylonitrile, and too long a pre-oxidation time causes decomposition of the material and waste of energy.

[0022] The carbonization temperature is too high, which can cause the decrease of the specific surface area and the pore volume of the prepared material; if the carbonization temperature is lower than 900 DEG C, the zinc metal cannot be volatilized, which can affect the specific surface area and the pore volume of the material, and is not conducive to the formation of the hollow structure; if the carbonization temperature is too high, the pore can be collapsed, which can decrease the specific surface area and the pore volume of the material, and the carbon yield is further decreased; if the carbonization time is too short, the carbonization is not complete; if the carbonization time is too long, the pore can be collapsed, and in both cases, the specific surface area and the pore volume can be decreased.

[0023] Further, the preparation method of the ZIF-8 nanoparticles comprises the following steps:

[0024] Zn(NO3)2.6H2O and 2-methylimidazole are respectively dissolved in an organic solvent, then the two solutions are mixed, stirring, centrifuging the obtained solid material and drying to obtain the ZIF-8 nanoparticles.

[0025] The second technical scheme of the application is a MOF-based hollow carbon nanofiber material prepared by the above preparation method.

[0026] The third technical scheme of the application is the application of the above MOF-based hollow carbon nanofiber material in adsorbing volatile organic compounds in air.

[0027] The fourth technical scheme of the application is an adsorbent for adsorbing volatile organic compounds in air, and the raw material comprises the above MOF-based hollow carbon nanofiber material.

[0028] The fifth technical scheme of the application is a method for removing volatile organic compounds in air, wherein the above MOF-based hollow carbon nanofiber material is used as an adsorbent to remove volatile organic compounds in air by adsorption.

[0029] The application discloses the following technical effects:

[0030] The application introduces a sacrificial pore-forming agent (PMMA) into the mixed PAN / ZIF-8 solution, and the prepared MOF-based hollow carbon nanofiber material can obtain rich hierarchical pores and hollow structures, which is more conducive to the transmission of pollutants (VOCs in air) to the MOF-based hollow carbon nanofiber material.

[0031] The application obtains a new type of hierarchical porous MOF-based hollow carbon nanofiber material (CNFM), discusses the formation mechanism of the porous structure, selects benzene (a typical VOC) as a target pollutant to evaluate the adsorption performance of the obtained MOF-based hollow carbon nanofiber material, and proposes a possible adsorption mechanism.

[0032] The MOF-based hollow carbon nanofiber material prepared by the application has a large specific surface area, rich micropores and a graphite structure, is favorable for VOCs adsorption, and has important significance for studying ZIF-8 derived carbon material for VOCs adsorption and widening the application range of the ZIF-8 derived carbon material. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the application or the prior art, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0034] Figure 1 The synthesis process of the MOF-based hollow carbon nanofiber material of the application is shown in the figure.

[0035] Figure 2 The XRD pattern of the ZIF-8 nanoparticles of the application is shown in the figure.

[0036] Figure 3 The SEM (a) of the ZIF-8 nanoparticles, the SEM (b) of the Z-C prepared in Example 5, and the TEM (c) of the Z-C are shown in the figure.

[0037] Figure 4 The SEM of the P-NFM (a), the PP-NFM (b), the PZ-NFM (c) and the PPZ-NFM (d) prepared in Examples 1-4, and the TEM of the CNFM1 (a1), the CNFM2 (b1), the CNFM3 (c1) and the CNFM4 (d1) prepared in Examples 1-4 are shown in the figure.

[0038] Figure 5 The N2 adsorption / desorption isotherm (a) and the NLDFT pore size distribution (b) of the CNFM1, the CNFM2, the CNFM3 and the CNFM4 prepared in Examples 1-4 are shown in the figure.

[0039] Figure 6 The TGA curves of the PMMA powder, the PAN powder, the ZIF-8 nanoparticles and the NFM4 prepared in Example 4 after pre-oxidation are shown in the figure.

[0040] Figure 7 The adsorption isotherm (a) and (c) of the CNFM1, the CNFM2, the CNFM3 and the CNFM4 prepared in Examples 1-4 at 298K, the fitting curve of the two-site Langmuir-Freundlich model, the adsorption isotherm (b) of the CNFM4 prepared in Example 4 at different temperatures, and the fitting curve of the two-site Langmuir-Freundlich model (d) are shown in the figure.

[0041] Figure 8 Fitting curves of Langmuir model and Freundlich model at 298 K for CNFM1, CNFM2, CNFM3, CNFM4 prepared for Examples 1-4.

[0042] Figure 9 Fitting curves of Langmuir model and Freundlich model at 298 K for CNFM4 prepared for Example 4.

[0043] Figure 10 Breakthrough curves (a) and fitting curves of Yoon-Nelson model (b) for CNFM1, CNFM2, CNFM3, CNFM4 prepared for Examples 1-4, breakthrough curves (c) and fitting curves of Yoon-Nelson model (d) for CNFM4 prepared for Example 4 at different inlet concentrations, and breakthrough curves (e) and fitting curves of Yoon-Nelson model (f) for CNFM4 prepared for Example 4 at different relative humidity conditions for adsorption of benzene.

[0044] Figure 11 Breakthrough curves and fitting curves of Yoon-Nelson model (a) for CNFM1, CNFM2, CNFM3, CNFM4 prepared for Examples 1-4, breakthrough curves and fitting curves of Adams-Bohart model (b) for CNFM4 prepared for Example 4 at different inlet concentrations, and breakthrough curves and fitting curves of Adams-Bohart model (c) for CNFM4 prepared for Example 4 at different inlet humidity.

[0045] Figure 12 Cyclic adsorption capacity of CNFM4 prepared for Example 4 for benzene. DETAILED DESCRIPTION

[0046] Various illustrative embodiments of the present application are now described in detail below. The following description includes specific details for the purpose of providing a thorough understanding of the present application. However, it will be apparent to those skilled in the art that the present application can be practiced without these specific details. In other instances, well-known structures and functions have not been described in detail in order to avoid obscuring the understanding of the present application.

[0047] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. Additionally, for a range of values of a parameter, unless otherwise indicated, each intervening value of the parameter is also specifically included within the scope of the present application. The intervening values of the parameter are combined with a stated value of the parameter in range form. These are only examples of the various values of the parameter, which are consistent with the present application. Other examples of the various values of the parameter, consistent with the present application, are also possible and encompassed within the scope of the various embodiments of the present application.

[0048] All technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains, unless indicated otherwise. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, the preferred methods and materials are described. All publications mentioned in this specification are herein incorporated by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. The citation of any reference is not an admission that it is prior art with respect to the present application. The description and examples are illustrative of the application and are not in limitation of the same.

[0049] Many modifications and variations of the present application described in the specification are possible without departing from the scope or spirit of the application. Other embodiments of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. The specification and examples are illustrative of the application and are not in limitation of the same.

[0050] As used herein, the terms "comprise", "comprising", "include", "including", "have", "having" or the like are open-ended and do not exclude additional, unrecited elements or method steps.

[0051] The raw materials used in the embodiments of the present application can be obtained through commercial channels unless otherwise specified.

[0052] The PMMA used in the embodiments of the present application has a M w = 150000.

[0053] The PAN used in the embodiments of the present application has a M w = 150000.

[0054] The ZIF-8 nanoparticles used in the embodiments of the present application are prepared by the following steps:

[0055] 0.003 mol Zn(NO3)2·6H2O and 0.06 mol 2-methylimidazole (C4H6N2) were dissolved in 60 mL of methanol, respectively, and then the two solutions were mixed and stirred at 1000 rpm for 1 hour; the final solid product was collected by centrifugation (10000 rpm, 5 min), washed with methanol three times, and dried at 100°C to obtain ZIF-8 nanoparticles (XRD, TEM images as shown in Figure 1 、 Figure 2 .

[0056] Figure 1 The synthesis process of the MOF-based hollow carbon nanofiber material of the present application is shown in the figure.

[0057] The detection method involved in the present application is as follows:

[0058] The Netzsch Tartus TG209F3 analyzer was used in N2 atmosphere (60 mL min-1 ) at 5 °C min -1 Thermal stability was investigated by thermogravimetric analysis (TGA) at a heating rate of 40 °C to 950 °C under the operating conditions;

[0059] Morphology and structural properties were examined by scanning electron microscopy (SEM) using a Hitachi S-4800 at 5 kV voltage and transmission electron microscopy (TEM) using a Hitachi H-7650 at 100 kV voltage;

[0060] Specific surface area and pore structure (Deltamax, Japan) were characterized by N2adsorption-desorption isotherms at 77 K;

[0061] Specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method and total pore volume was obtained from the adsorbed amount at a relative pressure of 0.99;

[0062] Pore size distribution was determined from non-local density functional theory (NLDFT) calculations based on N2adsorption data;

[0063] Raman spectra were performed using a LabRAM Aramis Raman spectrometer (Horiba Jobin Yvon, France) with a 785 nm laser excitation source;

[0064] Functional groups of CNFM4 were investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFT, VERTEX 70, Bruker Corporation);

[0065] Static adsorption experiments were performed using a Belsorp-Max type static gas adsorption instrument;

[0066] Dynamic adsorption experiments were performed at 298 K using a dynamic adsorption device, the inlet concentration of benzene (ppmv) during the dynamic adsorption process was adjusted by a mass flow meter, the dynamic adsorption device was composed of a gas distribution device, a constant temperature system (constant temperature circulating water bath), a fixed adsorption bed and a detection system (Fuli 9790 II gas chromatograph), during the experiment, when the outlet concentration of benzene reached 5% and 95% of the inlet concentration, it was the breakthrough point and the saturation point, respectively.

[0067] Example 1

[0068] Step 1, PAN powder was added into DMF (N,N-dimethylformamide) and continuously stirred at 60 °C to form a homogeneous PAN solution with a mass concentration of 10%. Subsequently, the solution was transferred into a 20 mL syringe and connected to the electrospinning equipment. The high voltage was 20 kV, the needle-collector distance was 15 cm and the flow rate was 1 mL·h -1electrospinning under the electrospinning conditions of a high voltage of 20 kV, a needle-collector distance of 15 cm, and a feeding rate of 1 mL-h

[0069] Step 2, the NFM2 prepared in Step 1 was first pre-oxidized (heated to 250℃ at a heating rate of 10℃ / min in an oven for 1 h), and then transferred to a tube furnace to be heated to 950℃ at a heating rate of 5℃ / min under N2 atmosphere for 1 h to obtain a PAN / PMMA derived carbon nanofiber material (CNFM2).

[0070] Example 2

[0071] Step 1, a PAN and PMMA (polymethyl methacrylate) blend with a total mass concentration of 20% (w:w, 1:1) was added into DMF to form a uniform solution under continuous stirring at 60℃. Subsequently, the solution was transferred to a 20 mL syringe and connected to an electrospinning device. Electrospinning was performed under the electrospinning conditions of a high voltage of 20 kV, a needle-collector distance of 15 cm, and a feeding rate of 1 mL-h -1 electrospinning under the electrospinning conditions of a high voltage of 20 kV, a needle-collector distance of 15 cm, and a feeding rate of 1 mL-h

[0072] Step 2, the NFM2 prepared in Step 1 was first pre-oxidized (heated to 250℃ at a heating rate of 10℃ / min in an oven for 1 h), and then transferred to a tube furnace to be heated to 950℃ at a heating rate of 5℃ / min under N2 atmosphere for 1 h to obtain a PAN / PMMA derived carbon nanofiber material (CNFM2).

[0073] Example 3

[0074] Step 1, a PAN and ZIF-8 blend with a total mass concentration of 20% (w:w, 1:1) was added into DMF to form a uniform solution under continuous stirring at 60℃. Subsequently, the solution was transferred to a 20 mL syringe and connected to an electrospinning device. Electrospinning was performed under the electrospinning conditions of a high voltage of 20 kV, a needle-collector distance of 15 cm, and a feeding rate of 1 mL-h -1 electrospinning under the electrospinning conditions of a high voltage of 20 kV, a needle-collector distance of 15 cm, and a feeding rate of 1 mL-h

[0075] Step 2, the NFM3 prepared in step 1 was first pre-oxidized (heated to 250℃ at a heating rate of 10℃ / min in an oven for 1h), and then transferred to a tube furnace and heated to 950℃ at a heating rate of 5℃ / min under N2 atmosphere for 1h, to obtain a PAN / ZIF-8 derived carbon nanofiber material (CNFM3).

[0076] Example 4

[0077] Step 1, a PAN, PMMA and ZIF-8 blend with a total mass ratio of 20% (w:w:w, 1:1:1) was added into DMF and continuously stirred at 60℃ to form a uniform solution. Subsequently, the solution was transferred to a 20mL syringe and connected to the electrospinning equipment. Electrospinning was carried out under the electrospinning conditions of a high voltage of 20kV, a needle-collector distance of 15cm and a feeding rate of 1mL·h -1 -1, and then dried in a vacuum oven at 60℃ for 24h to remove residual solvent; to obtain a PAN / PMMA / ZIF-8 nanofiber material (NFM4).

[0078] Step 2, the NFM4 prepared in step 1 was first pre-oxidized (heated to 250℃ at a heating rate of 10℃ / min in an oven for 1h), and then transferred to a tube furnace and heated to 950℃ at a heating rate of 5℃ / min under N2 atmosphere for 1h, to obtain a PAN / PMMA / ZIF-8 derived carbon nanofiber material, i.e. a MOF-based hollow carbon nanofiber material (labeled as CNFM4).

[0079] Example 5

[0080] The ZIF-8 nanoparticles were first pre-oxidized (heated to 250℃ at a heating rate of 10℃ / min in an oven for 1h), and then transferred to a tube furnace and heated to 950℃ at a heating rate of 5℃ / min under N2 atmosphere for 1h, to obtain a ZIF-8-based porous carbon (Z-C).

[0081] Effect verification example 1

[0082] The materials prepared in examples 1-5 were detected, and the results were as follows:

[0083] 1, the XRD pattern of the ZIF-8 nanoparticles of the application is as shown in Figure 2 ; the SEM (a), the SEM pattern (b) of the Z-C prepared in example 5 and the TEM pattern (c) of the ZIF-8 nanoparticles of the application are as shown in Figure 3 . Figure 2 , Figure 3It can be seen that the synthesized ZIF-8 nanoparticles have a uniform size in the range of 50-100 nm. After carbonization, the dense internal structure of ZIF-8 is transformed into a hollow spherical structure.

[0084] 2. The CNFM1 prepared in Example 1 exhibited a rough surface and a dense internal structure after carbonization. Figure 4 In (a), (a1)), and CNFM2 prepared in Example 2 ( Figure 4 Images (b) and (b1) show a hollow structure along the fiber length direction. CNFM3 prepared in Example 3 ( Figure 4 Images (c) and (c1) show a dense and uniform hollow spherical structure derived from ZIF-8 nanoparticles. Furthermore, by introducing PMMA and ZIF-8 into the PAN spinning solution, a PAN / PMMA / ZIF-8 derived carbon nanofiber material (CNFM4) was finally prepared. Figure 4 CNFM4 (d1) not only exhibits a similar particle distribution and uniform hollow spherical structure on its fiber surface as CNFM3, but also possesses striped grooves on its fiber surface and hollow tubular structures inside the carbon nanofibers, similar to CNFM2. CNFM4 simultaneously possesses both types of hollow structures, which is beneficial for the mass transfer of pollutants within it.

[0085] 3. For example Figure 5 As shown, the type I nitrogen adsorption isotherm of CNFM1 indicates that PAN-derived CNFM is a typical microporous material. The other three carbon nanofiber materials (CNFM2, CNFM3, and CNFM4) also exhibit abundant micropores, as their adsorption isotherms show a high steep slope when P / P0 < 0.1. Furthermore, the adsorption-desorption isotherms of CNFM2, CNFM3, and CNFM4 all show hysteresis loops, indicating that these materials also possess mesopores. Table 1 summarizes the parameters of all prepared materials based on the analysis and calculation of the isotherm data. The specific surface area (S) of CNFM1, CNFM2, CNFM3, and CNFM4 is also shown in Table 1. BET The values ​​are 53.0, 583.6, 538.1, and 935.5m respectively. 2 g -1 Their total pore volume (V total The values ​​were 0.03, 0.45, 0.50, and 0.69 cm, respectively. 3 g -1 CNFM2 and CNFM3 showed significantly larger S values ​​than CNFM1. BET and V total This can be attributed to the complete decomposition of PMMA during heat treatment and the porous, hollow structure derived from ZIF-8 nanoparticles. Therefore, CNFM4 exhibits the highest S0 under the combined effect of ZIF-8 and PMMA. BET and Vtotal It is about 20 times that of CNFM1, comparable to porous carbon fibers prepared by activation process, and has a hollow structure that cannot be obtained by activation process.

[0086] Based on the nonlocal density functional theory (NLDFT) model, the pore size distribution (PSD) of the materials was obtained by calculating the N2 adsorption isotherm. The PSD of CNFM1 is concentrated in the microporous region, while abundant mesopores were observed in CNFM2, CNFM3, and CNFM4. The hierarchical pore structure of CNFM4 will greatly promote the mass transfer of pollutants to carbon nanofibers, thereby further improving the adsorption rate.

[0087] Table 1

[0088]

[0089] 4. The role of raw materials depends on their thermal stability. Therefore, studying the thermal stability of raw materials can reveal the formation mechanism of the porous hollow structure of the prepared MOF-based hollow carbon nanofiber materials. TGA was performed on pre-oxidized (heated to 250°C in an oven at a heating rate of 10°C / min and held for 1 hour) PMMA powder, PAN powder, ZIF-8 nanoparticles, and pre-oxidized NFM4. Figure 6 As shown, after heating to 950℃ under a N2 atmosphere, the pre-oxidized PAN powder and pre-oxidized ZIF-8 particles decreased by 41.48% and 43.77%, respectively, indicating good thermal stability. At approximately 400℃, PMMA completely lost its original weight, revealing the role of PMMA as a pore-forming agent. Thermogravimetric analysis of pre-oxidized NFM4 showed a slight weight decrease at 950℃, retaining approximately 29.85% of its original weight. The weight of pre-oxidized NFM4 also decreased sharply at ~400℃, similar to that of pre-oxidized PMMA powder. In summary, microphase separation occurred between PAN and PMMA, with PMMA transforming into a gaseous substance, forming a hollow tubular structure. ZIF-8 served as a template for obtaining hollow spherical carbon, and PAN was derived into thermally stable carbon to support the framework of porous MOF-based hollow carbon nanofiber materials (CNFM).

[0090] 5. Figure 7Adsorption isotherms of CNFMs for benzene at 298 K are shown in Figure (a). All the curves are type-IV isotherms, reflecting a typical physical adsorption process. At low absolute pressure, the adsorption capacity increases rapidly, which can be attributed to the strong π-π interactions between benzene rings and the graphitic structure of CNFMs (referring to CNFM1, CNFM2, CNFM3 and CNFM4). With the increase of absolute pressure, the adsorption capacity increases slightly, then a second sharp increase is observed, which is due to the gradual formation of multilayer adsorption until the saturation vapor pressure is reached. The multilayer adsorption at high pressure is mainly attributed to the condensation of pores. The adsorption capacity of CNFM1 for benzene is very low, while that of CNFM2, CNFM3 and CNFM4 increases significantly, and CNFM4 has the highest adsorption capacity for benzene at low and high pressures, which are 3.86 and 12.13 mmol g -1 At low benzene vapor pressure, the adsorption capacity of CNFM2 is slightly higher than that of CNFM3, while at high pressure, the adsorption capacity of CNFM2 is lower than that of CNFM3. These results can be attributed to the positive correlation between the adsorption capacity at saturation vapor pressure and the V total of CNFMs, while the adsorption capacity at low pressure is positively correlated with the S BET and V micro of CNFMs. Figure 7 Figure (b) shows that at the same absolute pressure, the adsorption capacity decreases with the increase of temperature, indicating that the adsorption process is exothermic.

[0091] Three models, Langmuir, Freundlich and bi-site Langmuir-Freundlich, were used to describe the adsorption isotherm data. As shown in Figures Figure 8 and Figure 9 , the fitted curves of Langmuir and Freundlich models deviate significantly from the experimental data, while the bi-site Langmuir-Freundlich can better describe the experimental data (R 2 > 0.99)( Figure 7 Figures (c)-(d), Table S1). Based on the bi-site Langmuir-Freundlich model, the adsorption isotherm can be divided into two regions, type-I and type-III isotherms. The first region at low pressure can be accurately described by Langmuir, indicating the monolayer adsorption of benzene on the surface of CNFMs, and the second region indicates the weak interaction between benzene and adsorbate, and with the increase of absolute pressure, the increase of benzene-benzene interaction leads to the formation of benzene multilayer adsorption.

[0092] 6、The dynamic benzene adsorption behavior of CNFM1, CNFM2, CNFM3 and CNFM4 at 298 K was studied, and the initial concentration of benzene was 50 ppmv, and the flow rate was 50 mL min -1 . AsFigure 10 The breakthrough and saturation adsorption sequence of benzene is CNFM4 > CNFM3 ≈ CNFM2 > CNFM1, which is consistent with the static adsorption behavior of these materials in the low pressure region. CNFM4 has the largest saturation and breakthrough adsorption capacity, 0.5714 and 0.4902 mmol g -1 , respectively, which is only 11.7% higher than the breakthrough adsorption capacity, indicating that the equilibrium is reached quickly. In addition, in order to study the effect of inlet concentration, three different concentrations were chosen. As the inlet concentration increased from 20 to 100 ppm, the breakthrough time (t b ) and saturation time (t s ) decreased from 260 to 95 minutes and 190 to 60 minutes, respectively, which can be attributed to the fact that higher concentration of benzene molecules can occupy the binding sites on CNFM4 faster. Higher pollutant inlet concentration leads to higher driving force for benzene mass transfer, thus faster adsorption breakthrough and saturation. It is worth noting that the saturation adsorption capacity increases with the increase of inlet concentration. However, the breakthrough adsorption (q b ) at 100 ppm inlet benzene concentration is lower than the other two lower inlet concentrations, indicating that the adsorbent has a poor purification ability for high concentration benzene. In addition, in actual working conditions, water vapor is often the key factor affecting the adsorption capacity of the adsorbent. Figure 10 (c) shows that the breakthrough and saturation times of benzene vapor on CNFM4 both become shorter with the increase of humidity, which means that the adsorption capacity decreases with the increase of humidity. However, even at 60 RH%, the breakthrough and saturation adsorption can still maintain 0.2620 and 0.3642 mmol g -1 , indicating its potential application in actual environment.

[0093] Table 2

[0094]

[0095] The experimental data was fitted using the Adams-Bohart model and the Yoon-Nelson model. Figure 11 It is shown that the Adams-Bohart model cannot match the experimental data, while the fitting curve of the Yoon-Nelson model can accurately describe the experimental data, with a higher R 2 (Table 3, Figure 10 (b), (d), (f)). The τ YN calculated by the Yoon-Nelson model fitting curve is very close to the experimental value of τ exp , further confirming the accuracy of the Yoon-Nelson model. Comparing the rate constants (k YN ) of different materials, CNFM1 has the highest k YNThis is due to the rapid adsorption of benzene on the surface of PAN-derived carbon nanofibers. The k YN of CNFM2 is higher than that of CNFM3, indicating that the mass transfer of benzene in the hollow tubular structure inside CNFM2 carbon nanofibers is faster than that in the hollow spherical structure inside CNFM3 carbon nanofibers. Compared with CNFM2 and CNFM3, CNFM4 has a higher k YN , indicating that both hollow structures promote the adsorption rate of benzene on CNFM4. In addition, the higher the inlet concentration of benzene, the larger the k YN , and the increase in gas humidity will lead to the decrease of k YN .

[0096] Table 3

[0097]

[0098]

[0099] 7、The stability of the adsorbate is an important factor for evaluating its practical application value. The adsorption saturated material is desorbed in a vacuum environment at 200℃ for 2h, and then the next adsorption experiment is carried out. It can be seen from Figure 12 that the static adsorption of CNFM4 prepared in Example 4 does not decrease after 5 adsorption and desorption cycles, showing good adsorption cycle performance.

[0100] The present application successfully prepared a super-porous MOF-based hollow carbon nanofiber material (CNFM4) with two hollow structures by simple electrospinning and carbonization. The MOF-based hollow carbon nanofiber material has a large specific surface area and pore volume. The high specific surface area and abundant micropores are beneficial to the adsorption of benzene at low pressure, and the total pore volume plays a key role in the adsorption of benzene at high pressure. The breakthrough adsorption experiment shows that the adsorbate not only can purify low concentration benzene under high humidity conditions, but also has a high adsorption rate due to its hierarchical pore structure as a molecular channel. The high adsorption of low concentration benzene, good water resistance and excellent reusability of the MOF-based hollow carbon nanofiber material show the potential value of removing benzene in practical applications.

[0101] The above-described examples are only to describe the preferred modes of the present application, and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements of the technical solutions of the present application made by those skilled in the art shall fall within the protection scope determined by the claims of the present application.

Claims

1. Use of a MOF-based hollow carbon nanofiber material for adsorbing volatile organic compounds in air, characterized in that, The preparation method of the MOF-based hollow carbon nanofiber material comprises the following steps: polyacrylonitrile, a sacrificial pore-forming agent and ZIF-8 nanoparticles are added to an organic solvent to obtain a spinning solution; electrospinning is performed on the spinning solution, followed by pre-oxidation and carbonization to obtain the MOF-based hollow carbon nanofiber material; the sacrificial pore-forming agent is polymethyl methacrylate; the mass ratio of the polyacrylonitrile to the sacrificial pore-forming agent and ZIF-8 nanoparticles is 1:1:1; the organic solvent is one or more of N,N-dimethylformamide, N,N-dimethylacetamide and N-dimethylpyrrolidone; the sum of the mass concentrations of polyacrylonitrile, the sacrificial pore-forming agent and ZIF-8 nanoparticles in the spinning solution is 20-25%; the pre-oxidation specifically comprises heating at a heating rate of 1-10℃ / min to 220-300℃ and holding for 1-5h; and the carbonization specifically comprises heating at a heating rate of 5℃ / min to 950℃ in an inert atmosphere and holding for 1h.

2. A method for removing volatile organic compounds from air, characterized in that, The MOF-based hollow carbon nanofiber material used in the application of claim 1 is used as an adsorbent to remove volatile organic compounds in air by an adsorption method.

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