Fluorinated high-crystallinity polyheptylimide and preparation method thereof

Abstract

The invention relates to a method for preparing fluorinated high-crystallinity polyheptylimide (F-PHI) through cooperation of fluoride and metal salt, and belongs to the field of structural design and synthesis of efficient photocatalytic materials. The crystal structure and the atomic structure of graphite phase carbon nitride (g-C3N4) are regulated and controlled by using a one-step method, and the problems of difficult carrier migration of g-C3N4, unsatisfactory photocatalytic effect, complex process and the like at present are solved. A nitrogen-containing precursor, a metal salt and fluoride in a proper proportion are selected and subjected to mixed calcination treatment and metal salt medium induction to achieve polyheptylimide (PHI) growth, PHI polycondensation is more perfect through the fluoride, the obtained F-PHI is more perfect in-plane, interlayer stacking is more compact, the orderliness of atom arrangement is enhanced, and crystallinity is improved. Meanwhile, a C-F bond is introduced into PHI through fluorination treatment, and effective modification of a PHI atomic structure is achieved. The F-PHI overcomes the defects that g-C3N4 is weak in light absorption, low in specific surface area, low in photon-generated carrier separation efficiency and the like, and excellent photocatalytic performance is obtained. The preparation method of the material is simple, easy to operate, low in cost and easy for large-scale preparation and industrial production, and has a wide application prospect in the fields of photocatalysis or electrocatalysis and the like.

Description

Technical Field

[0001] The invention belongs to the field of photocatalytic materials, and specifically relates to a method for preparing fluorinated highly crystalline polyheptazine imide by thermal polycondensation of a nitrogen-rich precursor, a metal salt and a fluoride. Background Art

[0002] Photocatalyst graphite carbon nitride (g-C3N4) has good photocatalytic application prospects due to its advantages such as no metal elements, easy synthesis, good optical and electronic properties, high thermal stability and chemical stability. g-C3N4 is a semiconductor with a direct band gap of 2.7 eV. Its CB is mainly composed of the Pz orbital of C atoms, and VB is mainly composed of the Pz orbital of N atoms. Compared with the normal H2 electrode (NHE) reduction potential, the energy band positions of CB and VB are -1.3 eV and +1.4 eV, respectively (Muhammad H. et al, An overview of the recent progress in polymeric carbon nitride based photocatalysis, Chemical Record, 2021, 21(7): 1181), which can drive most photocatalytic reactions, such as photocatalytic hydrogen production, CO2 reduction, and degradation of pollutants in water. g-C3N4 can be directly synthesized by high-temperature polycondensation of nitrogen-rich precursors such as urea, melamine, dicyandiamide, and cyanamide, and the resulting product is a melon structure (Wang XC et al, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nature Materials, 2009, 8, 76-80). Due to the limitations of mass transfer and heat transfer processes, the melon structure is usually in a low crystalline or amorphous state, and the high-density defects lead to low carrier mobility and high recombination probability, which is not conducive to the photocatalytic reaction.

[0003] At present, there are methods such as morphology regulation, element doping, construction of heterojunction composite materials and crystallinity regulation to improve the photocatalytic performance of g-C3N4. Element doping can increase the electron or hole density, adjust the band gap structure, expand light absorption, reduce the probability of carrier recombination and create more active sites by adding a small amount of foreign atoms to the regular lattice of the semiconductor. It is an effective method to improve the photocatalytic performance of materials. For example, Wu et al. designed a surface fluorination treatment method to inhibit the accumulation of C=O by forming CF, and by optimizing the oxygen evolution reaction path of adjacent N atoms, the fluorinated g-C3N4 catalyst has an overall water decomposition ability under visible light, and the photocatalytic hydrogen production rate is increased by an order of magnitude compared with the original g-C3N4 (Wu J. et al, Breaking through water-splitting bottlenecks over carbon nitride with fluorination, Nature Communications, 2022, 13, (1): 6999). Compared with melon, polyheptazine imide (PHI) has a more ordered structure, a higher degree of polymerization, and a more extended conjugated surface, which is conducive to promoting the migration of carriers. Therefore, PHI usually has higher photocatalytic performance than melon. Further heteroatom (fluorine) modification of the structure of PHI is expected to obtain a better photocatalytic effect. Therefore, it is of great research and application value to explore the influence of fluorination treatment technology on the structural characteristics of PHI and further improve its photocatalytic performance. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI), using fluoride and metal salt to assist the heat treatment process to achieve the regulation of the crystal structure and atomic structure of g-C3N4, and solve the problems faced by g-C3N4 as a functional material, such as low absorbance, narrow absorption range, difficult carrier migration, and insufficient photocatalytic activity.

[0005] The technical solution of the present invention is:

[0006] The preparation method of fluorinated highly crystalline polyheptazine imide (F-PHI) is to uniformly mix a nitrogen-rich precursor, a fluoride and a metal salt and then calcine the mixture. The product is then washed and dried to remove the metal salt, thereby obtaining a fluorinated crystalline polyheptazine imide (F-PHI). The fluoride and the metal salt affect the polycondensation process of the nitrogen-rich precursor at high temperature, thereby improving the crystallinity of the polyheptazine imide and adsorbing fluorine atoms on the surface of g-C3N4 to form CF bonds, thereby improving the crystallinity, light absorption, carrier behavior and photocatalytic performance.

[0007] In the method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI), the nitrogen-rich precursor is one or a mixture of dicyandiamide, melamine or urea.

[0008] In the method for preparing fluorinated highly crystalline polyheptazinimide (F-PHI), the metal salt is a monosalt of potassium chloride, a monosalt of sodium chloride, or a binary salt of potassium chloride and sodium chloride. The preferred binary salt has a mass ratio of potassium chloride to sodium chloride of 1 to 2.

[0009] The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI), wherein the fluoride is sodium fluoride 、 Potassium fluoride or lithium fluoride.

[0010] In the method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI), the mass ratio of the nitrogen-rich precursor to the metal salt is 1:2 to 1:2.5, and the mass ratio of the fluoride to the nitrogen-rich precursor is 2% to 10%.

[0011] The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI) has a calcination temperature of 520° C. to 560° C., a heating rate of 2 to 10° C. / min, and a calcination time of 2 to 4 hours.

[0012] In the method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI), the calcination atmosphere is air, nitrogen and argon.

[0013] The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI) comprises repeatedly washing the obtained solid sample with deionized water at a temperature of 20 to 80° C. after the calcination reaction is completed to remove the remaining metal salt, and drying the solid sample in an oven at 50 to 80° C.

[0014] The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI) utilizes fluoride and metal salt to affect the precursor condensation process at high temperature to achieve changes in the crystal structure and electronic structure of g-C3N4, and simultaneously adsorbs fluorine atoms on the surface of g-C3N4 to form CF bonds, thereby further improving carrier behavior and photocatalytic activity.

[0015] The advantages and beneficial effects of the present invention are:

[0016] The preparation method of fluorinated highly crystalline polyheptazine imide (F-PHI) provided by the present invention uses metal salt as a template and medium to guide the formation of PHI structure during heat treatment to achieve structural regulation of g-C3N4. The additionally added fluoride makes the calcination polycondensation process more complete, enhances the orderliness of the atomic arrangement in the plane and interlayer of fluorinated highly crystalline polyheptazine imide (F-PHI), improves the deficiency of PHI crystallinity regulation, and at the same time, the fluorination process helps g-C3N4 to closely combine with fluorine atoms to form CF bonds, thereby improving the photocatalytic hydrogen production performance. The material price of the present invention is low, the process is simple, easy to operate, and easy to prepare on a large scale and industrialized production. The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI) provided by the present invention can effectively regulate the crystal structure and atomic structure of fluorinated polyheptazine imide (F-PHI), enhance light absorption, improve g-C3N4 carrier phase migration and surface transfer process, and can be applied to fields such as photocatalysis or electrocatalysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Is the X-ray diffraction pattern of the sample, wherein the abscissa 2Theta is the diffraction angle (degree), the ordinate Intensity is the intensity (au), (a) is PHI, and (b) is F-PHI.

[0018] Figure 2 It is the UV-visible absorption spectrum of the sample; wherein, the abscissa Wavelength is the wavelength (nm), the ordinate Abs is the absorbance (au), (a) is PHI, and (b) is F-PHI.

[0019] Figure 3 It is a comparison chart of the photocatalytic performance of the samples; wherein, the horizontal axis Time is time (min), the vertical axis is Yield of H2 (μmol / g), (a) is PHI, and (b) is F-PHI. DETAILED DESCRIPTION

[0020] The present invention is further described in detail below through examples and drawings.

[0021] Example 1

[0022] (1) Commercial melamine, potassium chloride, sodium chloride and sodium fluoride are used as raw materials.

[0023] (2) Weigh 5 g of melamine, 4.4 g of sodium chloride, 5.6 g of potassium chloride and 0.5 g of sodium fluoride and grind them for 30 minutes to achieve uniform mixing.

[0024] (3) Add the sample collected in (2) into a crucible, place it in the center of a muffle furnace, and heat it in an air atmosphere at a heating rate of 5°C / min. The target temperature of the muffle furnace is set to 550°C. The holding time is 4 h, and the crucible is cooled to room temperature.

[0025] (4) The sample collected in (3) was repeatedly washed and precipitated with deionized water at 25°C to remove the metal salt in the sample until no white precipitate was produced when the supernatant was titrated with silver nitrate. The precipitate was dried in an oven at 70°C.

[0026] Example 2

[0027] (1) Commercial urea, potassium chloride, sodium chloride and lithium fluoride are selected as raw materials.

[0028] (2) Weigh 4 g urea, 4.4 g sodium chloride, 5.6 g potassium chloride and 0.8 g lithium fluoride, grind them for 30 minutes to achieve uniform mixing.

[0029] (3) Add the sample collected in (2) into a crucible, place it in the center of a muffle furnace, and heat it in an air atmosphere at a heating rate of 2°C / min. The target temperature of the muffle furnace is set to 520°C. The holding time is 4 h, and the crucible is cooled to room temperature.

[0030] (4) The sample collected in (3) was repeatedly washed and precipitated with deionized water at 60°C to remove the metal salt in the sample until no white precipitate was produced when the supernatant was titrated with silver nitrate. The precipitate was dried in an oven at 75°C.

[0031] Example 3

[0032] (1) Commercial melamine, potassium chloride and potassium fluoride are used as raw materials.

[0033] (2) Weigh 5 g of melamine, 10 g of potassium chloride and 0.5 g of potassium fluoride, and grind them for 30 minutes to achieve uniform mixing.

[0034] (3) The samples collected in (2) were added to crucibles and placed in the center of a muffle furnace. They were heated in a nitrogen atmosphere at a heating rate of 8°C / min. The target temperature of the muffle furnace was set to 560°C. The holding time was 2 h. The samples were then cooled to room temperature in the furnace.

[0035] (4) The sample collected in (3) was repeatedly washed and precipitated with deionized water at a temperature of 80°C to remove the metal salt in the sample until no white precipitate was produced when the supernatant was titrated with silver nitrate. The precipitate was dried in an oven at 80°C.

[0036] Structural characterization:

[0037] Figure 1 and Figure 2 The materials shown in are the characterization of the materials obtained in Example 1. Figure 1 In the figure, a is polyheptazine imide (PHI) obtained by the co-condensation of nitrogen-rich precursor melamine and metal salt, and b is fluorinated highly crystalline polyheptazine imide (F-PHI) obtained by the co-condensation of nitrogen-rich precursor, metal salt and fluoride. By comparison, it can be seen that the characteristic structure of the 8° and 28° diffraction peaks of PHI is still maintained after the addition of fluoride. The 8° peak of F-PHI corresponds to the enhancement of the (100) crystal plane, indicating that the in-plane structure is more perfect; the (002) crystal plane corresponding to the 28° peak is shifted to a larger angle, indicating that the interlayer stacking is more compact. The improvement of crystallinity is conducive to the transfer of electrons from the inside to the surface, reducing the non-radiative recombination at the recombination centers such as defect sites, and improving the photocatalytic performance. Figure 2 The UV-visible absorption spectra of the samples. Both PHI and F-PHI show typical semiconductor absorption. The absorbance of F-PHI is significantly higher than that of PHI, indicating that the addition of fluorine has a significant effect on improving the light absorption of the system.

[0038] Photocatalytic testing

[0039] The photocatalytic performance of the material was evaluated by taking hydrogen production as an example: 50 mg of the photocatalyst of Example 1 was added to 45 ml of deionized water, 5 ml of triethanolamine and 4 ml of 1 mol / L chloroplatinic acid solution. The solution was irradiated with a Xe lamp (power: 300 W) with an ultraviolet cutoff filter (λ>420 nm), and the gas chromatography was connected to test the production of hydrogen produced at the same time. It can be seen that the photocatalytic performance of F-PHI is about 10 times that of PHI.

[0040] The results of the examples show that the present invention utilizes the metal salt template effect to solve the current problems of difficult modification, low crystallinity and complex process of g-C3N4 through fluorination treatment. The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI) provided by the present invention can significantly regulate the crystal structure of polyheptazine imide (PHI), and adsorb fluorine atoms on the surface of g-C3N4 to form CF bonds, thereby improving the crystallinity of g-C3N4, enhancing light absorption, and improving the phase migration and surface transfer process of g-C3N4 carriers. The catalyst can be applied to the fields of photocatalysis or electrocatalysis.

Claims

1. A method for preparing a fluorinated highly crystalline polyheptazine imide, characterized in that: The nitrogen-rich precursor, metal salt and fluoride are ground and mixed uniformly and then calcined. The product is then washed and dried to remove the metal salt, thereby obtaining fluorinated highly crystalline polyheptazine imide (F-PHI).

2. The method for preparing the fluorinated highly crystalline polyheptazine imide (F-PHI) according to claim 1, characterized in that: The nitrogen-rich precursor is one or a mixture of dicyandiamide, melamine and urea.

3. The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI) according to claim 1, characterized in that: The metal salt is a monobasic salt of potassium chloride, a monobasic salt of sodium chloride, or a binary salt of potassium chloride and sodium chloride.

4. The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI) according to claim 3, characterized in that: The mass ratio of potassium chloride to sodium chloride in binary salt is 1 to 2.

5. The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI) according to claim 1, characterized in that: The fluoride is sodium fluoride, potassium fluoride or lithium fluoride.

6. The method for preparing the fluorinated highly crystalline polyheptazine imide (F-PHI) according to claim 1, 2, 3, 4 or 5, characterized in that: The mass ratio of the nitrogen-rich precursor to the metal salt is 1:2 to 1:2.5, and the mass ratio of the added amount of fluoride to the nitrogen-rich precursor is 2% to 10%.

7. The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI) according to claim 1, 2, 3, 4 or 5, characterized in that: The nitrogen-rich precursor, metal salt and fluoride need to be ground finely and evenly in a mortar.

8. The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI) according to claim 1, 2, 3, 4 or 5, characterized in that: The calcination atmosphere is air, nitrogen or argon, the calcination temperature is 520°C to 560°C, the heating rate is 2 to 10°C / min, and the calcination time is 2 to 4 hours.

9. The method for preparing fluorinated highly crystalline polyheptazine imide (F-PHI) according to claim 1, 2, 3, 4 or 5, characterized in that: After the calcination reaction is completed, the obtained solid sample is repeatedly washed with deionized water at a temperature of 20 to 80° C. to remove the remaining metal salt, and then dried in an oven at 50 to 80° C.

10. A fluorinated highly crystalline polyheptazine imide composite, characterized in that: It is prepared by the preparation method described in any one of claims 1 to 9.

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