A guanidyl-functionalized polyacrylonitrile fiber, and a preparation method and application thereof

Abstract

The application belongs to the technical field of materials, and provides a guanidino group functionalized polyacrylonitrile fiber and a preparation method and application thereof. The application covalently introduces guanidino groups on the surface of the polyacrylonitrile fiber, constructs a heterogeneous catalyst with high alkalinity, excellent mechanical stability and convenient recyclability, utilizes intramolecular hydrogen bonds of a benzamide monohydroxymethyl product to promote spontaneous desorption of the product, realizes high-activity catalysis while avoiding catalyst poisoning, guarantees long-term cycle performance, and provides a new path for greenization and industrialization of guanidino catalysts. The application has the following advantages: using a cheap organic amine as a medium, covalently grafting guanidino groups for the first time under simple conditions, preparing a low-cost, mechanically stable alkaline heterogeneous catalyst; water-phase catalysis is consistent with green chemistry, precise control of selectivity and suppression of side reactions, the fiber is easy to filter and recycle, the structure and activity are stable in the cycle, the reaction is mild, the substrate has good adaptability, and the reaction is easy to scale up.

Description

Technical Field

[0001] This application belongs to the field of materials technology, and in particular relates to a guanidine-functionalized polyacrylonitrile fiber, its preparation method and application. Background Technology

[0002] Hydroxymethylation is one of the important functionalization reactions of amide compounds. By introducing a hydroxymethyl group into the amide molecule, its reactivity and molecular polarity can be significantly improved, providing key active sites for subsequent condensation, esterification, etherification, and other reactions. It has wide applications in the synthesis of pharmaceutical intermediates, the preparation of polymer monomers, and the production of fine chemicals. Traditional hydroxymethylation reactions mostly rely on small-molecule basic catalysts such as sodium hydroxide, potassium hydroxide, triethylamine, pyridine, and 1,5,7-triazabicyclo[4.4.0]decane (TBD). Although these catalysts can activate the amide carbonyl group and promote the nucleophilic attack of formaldehyde to generate hydroxymethylated products, they have inherent defects: First, these catalysts are mostly homogeneous systems, which are difficult to separate from the product after the reaction, resulting in catalyst waste, high product purification costs, and a serious environmental burden; Second, homogeneous basic systems are prone to over-hydroxymethylation or condensation side reactions, significantly reducing the selectivity of the target product.

[0003] To address the separation and recovery challenges in homogeneous catalysis, researchers have proposed immobilizing guanidine catalysts. Guanidine groups possess both strong basicity and hydrogen bond donor-acceptor properties, exhibiting excellent performance in organic base catalysis. In existing technologies, immobilized catalysts containing aliphatic guanidine groups have been used for silane polymer crosslinking and curing, while chiral guanidine catalysts with biaryl and quinine skeletons have been applied to asymmetric Aldol and Michael reactions, demonstrating the high catalytic efficiency and versatility of guanidine structures. However, these catalysts still exist in soluble small molecule form, belonging to homogeneous systems, and cannot be recycled. Future research will attempt to immobilize guanidine groups on solid supports to form heterogeneous catalysts, such as by silane coupling to silica gel surfaces, or by grafting onto supports such as resins and magnetic nanoparticles. Such immobilized catalysts have shown some reusability in reactions such as transesterification, biodiesel synthesis, and C–C bond construction, but there are still obvious shortcomings: the supports are mostly in powder or nanoparticle form, with low mechanical strength, and are easily lost during stirring and filtration. They also face problems such as separation difficulties and complex operation during industrial scale-up. Furthermore, the guanidine group is mostly physically adsorbed or weakly chemically bonded to the support, and the guanidine group is easily lost after repeated use, leading to a decline in catalyst activity.

[0004] Polymer fiber materials have attracted much attention in the fields of adsorption, separation, and catalysis due to their weaveability, high strength, and tunable surface chemistry. Polyacrylonitrile (PAN) fiber, as a common organic polymer material, contains abundant nitrile groups (–C≡N) in its molecular chain. It can be functionalized through reactions such as amination and amidation, and exhibits excellent thermal and chemical stability, maintaining structural integrity in aqueous or organic media. Its fiber morphology facilitates mechanical separation and recycling. Existing literature reports that guanidine salts or guanidine carbonates can react with PAN fibers to improve their thermal stability during carbonization. However, this process is only used for material pretreatment and does not possess organic catalytic function, nor does it involve research on immobilized guanidine groups for molecular reaction catalysis.

[0005] In summary, there is a lack of heterogeneous catalysts in the current technology that combine high catalytic activity, high selectivity, good mechanical stability and recyclability for the hydroxymethylation reaction of amide compounds. In particular, there are still technical bottlenecks in solving the problems of catalyst loss, product separation difficulties and side reaction control. Summary of the Invention

[0006] The purpose of this application is to provide a guanidine-functionalized polyacrylonitrile fiber and its preparation method and application, aiming to solve the problems of difficult separation of traditional homogeneous catalysts, insufficient mechanical strength of immobilized catalysts, easy loss of guanidine groups, and poor product selectivity, so as to achieve high efficiency, greenness and recyclability of the reaction.

[0007] The embodiments of this application are implemented as follows: a guanidine-functionalized polyacrylonitrile fiber, wherein the general structural formula of the guanidine-functionalized polyacrylonitrile fiber is: .

[0008] This application also provides a method for preparing the above-mentioned guanidine-functionalized polyacrylonitrile fiber, comprising: Polyacrylonitrile fibers were immersed in a mixture of N-methyl-1,2-ethylenediamine and distilled water. After being heated and refluxed, the fibers were washed and dried to obtain modified fibers. Under a nitrogen atmosphere, the modified fiber was heated and refluxed with dimethyl cyanamide, and then washed and dried to obtain guanidine-functionalized polyacrylonitrile fiber.

[0009] This application also provides an application of the above-mentioned guanidine-functionalized polyacrylonitrile fiber in the selective catalytic monohydroxymethylation reaction of benzamide.

[0010] This application also provides a method for preparing a benzamide monohydroxymethylated product, comprising the following steps: Benzamide, paraformaldehyde, the above-mentioned guanidine-functionalized polyacrylonitrile fiber and water were added sequentially to the reaction vessel, and the mixture was heated and stirred to carry out the benzamide monohydroxymethylation reaction. After the reaction was completed, the guanidine-functionalized polyacrylonitrile fiber was recovered by filtration and washed. The organic phase was extracted, concentrated under reduced pressure, and purified by column chromatography to obtain the benzamide monohydroxymethylated product.

[0011] This application successfully constructed a heterogeneous catalyst with high alkalinity, excellent mechanical stability, and convenient recyclability by covalently introducing guanidine groups onto the surface of polyacrylonitrile fibers. At the same time, the intramolecular hydrogen bonding of the benzamide monohydroxymethylation product generated in the reaction is used to promote the spontaneous desorption of the product from the fiber surface. This not only achieves high-activity catalysis of the benzamide hydroxymethylation reaction, but also effectively avoids catalyst poisoning caused by product adsorption, ensuring the long-term recyclability of the catalyst. This provides a new technical path for the green development and industrial application of guanidine catalysts.

[0012] This application offers significant advantages. It not only utilizes inexpensive organic amines as a medium to stably graft guanidine groups onto acrylonitrile fibers via covalent bonds for the first time under simple experimental conditions, thereby efficiently preparing a mechanically stable alkaline heterogeneous catalyst, reducing preparation costs and industrialization barriers, but also enables the catalyst to operate efficiently in an aqueous medium (meeting the requirements of green chemistry). It allows for precise control of reaction selectivity to specifically generate benzamide monohydroxymethylated products and suppress side reactions. Furthermore, the catalyst has a stable fiber morphology and can be recovered through simple filtration (with no significant attenuation of structure and activity during recycling). The reaction conditions are mild, it has good adaptability to benzamide substrates with different substituents, and the operation is simple and easy to scale up. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the selective catalytic benzamide monohydroxymethylation reaction mechanism of guanidine-functionalized polyacrylonitrile fibers provided in the embodiments of this application; Figure 2 The diagram shows the recycling performance of guanidine-functionalized polyacrylonitrile fibers provided in the embodiments of this application.

[0014] Figure 3 This is a schematic diagram of a series of guanidine-functionalized fibers prepared from different organic amines provided in the embodiments of this application. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0016] This application provides a guanidine-functionalized polyacrylonitrile fiber, the general structural formula of which is: .

[0017] In this embodiment, the guanidine-functionalized polyacrylonitrile fiber is obtained by covalently grafting guanidine groups onto polyacrylonitrile fibers. The specific preparation method includes: Polyacrylonitrile fibers were immersed in a mixture of N-methyl-1,2-ethylenediamine and distilled water. After being heated and refluxed, the fibers were washed and dried to obtain modified fibers. Under a nitrogen atmosphere, the modified fiber was heated and refluxed with dimethyl cyanamide, and then washed and dried to obtain guanidine-functionalized polyacrylonitrile fiber.

[0018] Preferably, in the step of "immersing polyacrylonitrile fibers in a mixture of N-methyl-1,2-ethylenediamine and distilled water, followed by reflux heating, and then washing and drying to obtain modified fibers," the heating temperature needs to be controlled at 100-135℃, and the reaction time at 50-70 minutes. If the temperature is below 100℃ or above 135℃, the grafting reaction of the fiber surface groups will not be effectively completed, making it difficult to form the target modified structure. The optimal process parameters are a heating temperature of 130℃ and a reaction time of 60 minutes, under which efficient group grafting can be achieved, ensuring the structural and performance stability of the modified fibers.

[0019] Preferably, in the step of "heating and refluxing the modified fiber with dimethyl cyanamide under a nitrogen atmosphere, followed by washing and drying to obtain guanidine-functionalized polyacrylonitrile fiber", the heating temperature needs to be controlled at 95-110℃ and the reaction time at 9-12h. If the temperature is below 95℃, the reaction activity is insufficient, and the effective binding of guanidine groups to the fiber cannot be achieved. If the temperature is above 110℃, it is easy to cause the decomposition of the raw material or damage to the fiber structure, both of which will cause the grafting reaction to fail. The optimal process parameters are a heating temperature of 105℃ and a reaction time of 12h. Under these conditions, the guanidine groups can be efficiently and stably grafted onto the fiber surface, ensuring the catalytic performance and structural integrity of the final guanidine-functionalized polyacrylonitrile fiber.

[0020] Optionally, the step of immersing polyacrylonitrile fibers in a mixture of N-methyl-1,2-ethylenediamine and distilled water, reacting them under reflux, and then washing and drying them to obtain modified fibers includes: 2.00 g of polyacrylonitrile fiber was immersed in a mixture of 40.0 mL of N-methyl-1,2-ethylenediamine and 20.0 mL of distilled water. After reflux reaction at 130℃ for 60 min, the fiber was washed with distilled water at 60-70℃ and dried at 60℃ for 12 h to obtain the modified fiber.

[0021] Optionally, the step of heating and refluxing the modified fiber with dimethyl cyanamide under a nitrogen atmosphere, followed by washing and drying to obtain guanidine-functionalized polyacrylonitrile fiber includes: Under a nitrogen atmosphere, 1.0 g of the modified fiber was heated and refluxed with 25 mL of dimethyl cyanamide at 105 °C for 12 h, followed by washing with ethanol at 60-70 °C and drying at 60 °C for 12 h to obtain guanidine-functionalized polyacrylonitrile fiber.

[0022] This application also provides the application of the above-mentioned guanidine-functionalized polyacrylonitrile fibers, specifically in the selective catalytic reaction of benzamide monohydroxymethylation and the preparation of benzamide monohydroxymethylated products; the corresponding method for preparing benzamide monohydroxymethylated products includes the following steps: Benzamide, paraformaldehyde, the above-mentioned guanidine-functionalized polyacrylonitrile fiber and water were added sequentially to the reaction vessel, and the mixture was heated and stirred to carry out the benzamide monohydroxymethylation reaction. After the reaction was completed, the guanidine-functionalized polyacrylonitrile fiber was recovered by filtration and washed. The organic phase was extracted, concentrated under reduced pressure, and purified by column chromatography to obtain the benzamide monohydroxymethylated product.

[0023] Preferably, the molar ratio of benzamide to paraformaldehyde is 1:(3-5); the amount of guanidine-functionalized polyacrylonitrile fiber is 10 mol% of the amount of benzamide; and the molar-volume ratio of benzamide to water is 1 mmol:2 mL.

[0024] Preferably, the reaction temperature is 100℃, the reaction time is 12h, and the stirring rate is 200-300r / min.

[0025] Preferably, the guanidine-functionalized polyacrylonitrile fibers recovered by filtration are washed with dichloromethane; the extraction solvent is dichloromethane, the extraction is performed 3 times, and the amount of solvent used for each extraction is 10 mL.

[0026] Preferably, the eluent for column chromatography is a mixture of dichloromethane and methanol in a volume ratio of 10:1.

[0027] The mechanism of the selective catalytic benzamide monohydroxymethylation reaction of the above-mentioned guanidinium-functionalized polyacrylonitrile fibers is as follows: the guanidinium groups on the fiber surface activate the carbonyl sites of benzamide through hydrogen bonding, lowering the reaction energy barrier to promote nucleophilic attack and ensure efficient initiation of the catalytic reaction; simultaneously, the benzamide monohydroxymethylation product generated in the reaction forms stable intramolecular hydrogen bonds, which significantly weakens the interaction between the product and the fiber support surface, thereby automatically desorbing and releasing it from the catalytic microenvironment. This process not only avoids catalyst poisoning caused by product adsorption but also provides key support for the multiple recycling of the fiber catalyst. The specific mechanism can be seen in Figure 1.

[0028] The following detailed description of guanidine-functionalized polyacrylonitrile fibers, their preparation methods, and applications is provided with specific examples, as shown below. Unless otherwise specified, the experimental methods used in the following examples are conventional methods; the materials and reagents used are commercially available unless otherwise specified.

[0029] Example 1: Preparation of guanidine-functionalized polyacrylonitrile fibers under optimal reaction conditions In a 100 mL round-bottom flask equipped with a reflux condenser, add 40.0 mL of N-methyl-1,2-ethylenediamine and 20.0 mL of distilled water. Then immerse 2.00 g of dried PANF in the solution and reflux the mixture at 130 °C for 60 min. After the reaction is complete, remove the modified fiber, wash thoroughly with distilled water (60-70 °C) to remove any unreacted residue, and then dry in an oven at 60 °C for 12 hours to obtain the modified PAN fiber. A F.

[0030] In a 50 mL round-bottom flask under a nitrogen atmosphere, add 25 mL of dimethyl cyanamide and 1.0 g of dried PAN. A F. The mixture was then refluxed at 105°C for 12 h. After completion, the fibers were collected, thoroughly washed with ethanol (60-70°C) to remove any unreacted residues, and then dried in an oven at 60°C for 12 h to obtain guanidine-functionalized polyacrylonitrile fibers (PAN). G F. The synthesis route is shown below: Example 2: Guanidinyl-functionalized polyacrylonitrile fibers for selective catalytic benzamide monohydroxymethylation reaction Add benzamide (1 mmol), paraformaldehyde (4 mmol), and 10 mol% PAN to a 15 mL dry pressure-resistant tube. GF (guanidinyl-functionalized polyacrylonitrile fiber prepared in Example 1) and water (2 mL). The mixture was heated to 100 °C and stirred for 12 hours. After the reaction was complete, the fiber was recovered by filtration and washed with dichloromethane. The organic phase was extracted with dichloromethane (10 mL × 3) and concentrated under reduced pressure. The crude product was purified by column chromatography (dichloride:methanol = 10:1) to obtain the desired benzamide monohydroxymethylated target product in a separate yield of 94.9%. The synthetic route is shown below: As shown in Figure 2, the guanidine-functionalized polyacrylonitrile fiber (PAN) prepared in Example 1... G F) In the selective catalytic reaction of benzamide monohydroxymethylation, after the catalyst is repeatedly recycled and used six times, the yield of the target product, benzamide monohydroxymethylation product, can still be maintained at 90.6%. The catalytic activity and its own structure are basically unchanged, and the fiber morphology remains stable. It can be separated and recovered in a simple way.

[0031] To further verify the practical application durability of this functionalized fiber, the original polyacrylonitrile fiber (PANF) and the amination-modified fiber (PANF) in Example 1 were compared. A F), Guanidine functionalized fibers (PAN) G F), and guanidine functionalized fibers (PAN) after 1 and 5 cycles of use. G F-1, PAN G Mechanical strength tests were conducted on F-5 (breaking strength BS reflects the absolute tensile strength of the fiber, and breaking strength retention rate RBS measures the proportion of the original strength retained after chemical modification; both are key indicators for evaluating the actual service durability of the fiber). The test results are shown in Table 1: PAN obtained after amino-modification A F, the fracture strength decreased to 8.86 cN, but the fracture strength retention rate reached 94%, and the strength only decreased slightly; PAN was further prepared by guanidine modification. G The tensile strength of PAN decreased slightly to 8.13 cN, but the tensile strength retention rate remained at 86%. Of particular note is the PAN... G After one and five cycles of use, F still maintains good mechanical properties, with breaking strengths of 8.11 cN and 7.92 cN, respectively, corresponding to breaking strength retention rates of 85% and 84% (see items 4 and 5 in Table 1), which fully demonstrates that the guanidine functionalized polyacrylonitrile fiber has the durability to meet the needs of practical applications.

[0032] Table 1 In the preliminary experiments, this application systematically studied the effects of different reaction times, different amine types, and different reaction temperatures on the preparation of guanidine-functionalized polyacrylonitrile fibers, as detailed below: (1) Preparation of guanidine-functionalized polyacrylonitrile fibers with different reaction times In a 100 mL round-bottom flask equipped with a reflux condenser, add 40.0 mL of N-methyl-1,2-ethylenediamine and 20.0 mL of distilled water. Then immerse 2.00 g of dried PANF in the solution and reflux the mixture at 130 °C for 60 min. After the reaction is complete, remove the modified fiber, wash thoroughly with distilled water (60-70 °C) to remove any unreacted residue, and then dry in an oven at 60 °C for 12 hours to obtain the modified PAN fiber. A F.

[0033] In a 50 mL round-bottom flask under a nitrogen atmosphere, add 25 mL of dimethyl cyanamide and 1.0 g of dried PAN. A F. The mixture was then refluxed at 105°C for different reaction hours (6-30 h). After completion, the fibers were collected, thoroughly washed with ethanol (60-70°C) to remove any unreacted residues, and then dried in an oven at 60°C for 12 h to obtain guanidine-functionalized polyacrylonitrile fibers (PAN). G F.

[0034] High-functionality catalysts possess dense networks of active sites, promoting substrate adsorption and enhancing catalytic efficiency. For PAN... G The degree of functionalization of F is highly time-dependent. To optimize its active site density, the effect of reaction time on fiber modification was systematically studied, as shown in Table 2.

[0035] Table 2 (2) Preparation of guanidine-functionalized polyacrylonitrile fibers with different amine types, the synthetic route is shown in Figure 3. The amine reagents used in this study included N-methyl-1,2-ethylenediamine, N-methyl-1,3-propanediamine, N-methyl-1,4-butanediamine, and N-methyl-1,6-hexanediamine. Functionality studies determined the optimal reaction times for the four amines to be 1 h, 2 h, 4 h, and 6 h, respectively. The corresponding amination-modified fibers prepared were named PAN. A F-1 to PAN A F-4 (PAN) A F-1 corresponds to N-methyl-1,2-ethylenediamine, PAN AF-2 corresponds to N-methyl-1,3-propanediamine, PANAF-3 corresponds to N-methyl-1,4-butanediamine, and PANAF-4 corresponds to N-methyl-1,6-hexanediamine. The guanidine-functionalized polyacrylonitrile fibers further prepared are named PAN, respectively. G F-1* to PAN G F-4*.

[0036] The specific preparation process is as follows: Amine-modified fibers (PAN) A F-1 to PAN A F-4) Preparation: In a 100 mL round-bottom flask equipped with a reflux condenser, add 40.0 mL of the specified amine reagent and 20.0 mL of distilled water, mix well, and then immerse 2.00 g of dried polyacrylonitrile fiber (PANF). Heat the mixture to 130 °C and reflux for the optimal reaction time (1-6 h) for the corresponding amine. After the reaction is complete, remove the fiber and wash it thoroughly with distilled water at 60-70 °C to remove unreacted residues. Then dry it in an oven at 60 °C for 12 hours to obtain PAN. A F-1 to PAN A F-4.

[0037] Guanidoyl functionalized polyacrylonitrile fiber (PAN) G F-1* to PAN G Preparation of F-4*): In a 50 mL round-bottom flask under nitrogen atmosphere, add 25 mL of dimethyl cyanamide and 1.0 g of dried amination-modified fibers (PANAF-1 to PANAF-4) as described above; heat the mixture to 105 °C and reflux for 1 hour; after the reaction is complete, collect the fibers and wash them extensively with ethanol at 60-70 °C to remove unreacted residues, then dry them in an oven at 60 °C for 12 hours to obtain PAN. G F-1* to PAN G F-4*.

[0038] For the prepared PAN A F-1 to PAN A F-4, PAN G F-1* to PAN G For F-4*, the performance was tested according to the method in Example 2, "Guidinyl functionalized polyacrylonitrile fiber for selective catalytic benzamide monohydroxymethylation reaction". Some test conditions and target product yield results are shown in Table 3.

[0039] Table 3 Compared to existing technologies, the core advantages of this application lie in the synergistic optimization of selectivity control, structural stability, recyclability, and green processes: by fixing guanidine groups to the surface of the fiber support in the form of covalent bonds, the key problems of easy loss of active components and insufficient mechanical strength of traditional immobilized catalysts are fundamentally solved, thereby achieving highly efficient catalysis under mild aqueous conditions—the yield of the target product, benzamide monohydroxymethylation, can reach up to 94.9%; at the same time, the monohydroxymethylation structure is stabilized by intramolecular hydrogen bonding of the product, effectively suppressing side reactions such as over-hydroxymethylation and condensation, and significantly improving product selectivity; in addition, the fiber-shaped catalyst can be directly adapted to fixed-bed or continuous-flow reaction systems, providing a practical technical solution for industrial production.

[0040] In summary, this application is not a simple replacement or conventional improvement of existing technologies, but rather a breakthrough in the comprehensive performance of catalytic reactions in terms of "high efficiency, high selectivity, structural stability, greenness, and recyclability" through multi-dimensional technological innovation (covalent grafting process, intramolecular hydrogen bond regulation, and fiber support selection). It combines academic innovation with industrial application potential, and successfully overcomes the technical bottlenecks in existing technologies such as catalyst loss, poor product selectivity, complex process operation, and difficulty in industrialization.

[0041] This application clarifies a method for preparing guanidine-functionalized polyacrylonitrile fibers and achieves the monohydroxymethylation conversion of benzamide based on a unique selective reaction mechanism. Those skilled in the art can refer to the disclosure herein and, without departing from the core concept of this application, make appropriate adjustments to the reaction conditions, process parameters, structural design, and other aspects to achieve the technical solution of this application. The method is summarized through examples of preferred effects, and relevant operators can reproduce the technical effects based on the disclosure. All similar substitutions and variations based on the ideas of this application are obvious to those skilled in the art and should be included within the scope of protection of this application.

Claims

1. The application of guanidine-functionalized polyacrylonitrile fiber in the selective catalytic monohydroxymethylation reaction of benzamide, characterized in that, Benzamide, paraformaldehyde, guanidine-functionalized polyacrylonitrile fiber and water were added sequentially to the reaction vessel, and the mixture was heated and stirred to carry out the monohydroxymethylation reaction of benzamide. After the reaction was completed, the guanidine-functionalized polyacrylonitrile fiber was recovered by filtration and washed. The organic phase was extracted, concentrated under reduced pressure, and purified by column chromatography to obtain the benzamide monohydroxymethylated product. The molar ratio of benzamide to paraformaldehyde is 1:(3-5); the amount of guanidine-functionalized polyacrylonitrile fiber used is 10 mol% of the amount of benzamide; the molar-volume ratio of benzamide to water is 1 mmol:2 mL; the reaction temperature is 100℃, the reaction time is 12 h; the stirring rate is 200-300 r / min; the guanidine-functionalized polyacrylonitrile fiber recovered by filtration is washed with dichloromethane; the extraction solvent is dichloromethane, the extraction is performed 3 times, and the amount of solvent used for each extraction is 10 mL; the eluent for column chromatography is a mixture of dichloromethane and methanol, with a volume ratio of 10:

1. The general structural formula of the guanidine-functionalized polyacrylonitrile fiber is: ； The preparation method of the guanidine-functionalized polyacrylonitrile fiber includes: Polyacrylonitrile fibers were immersed in a mixture of N-methyl-1,2-ethylenediamine and distilled water. After being heated and refluxed, the fibers were washed and dried to obtain modified fibers. The heating temperature is 100-135℃, and the reaction time is 50-70 min; Under a nitrogen atmosphere, the modified fiber was heated and refluxed with dimethyl cyanamide, followed by washing and drying to obtain guanidine-functionalized polyacrylonitrile fiber; the heating temperature was 95-110℃ and the reaction time was 9-12h.

2. The application of guanidine-functionalized polyacrylonitrile fiber according to claim 1 in the selective catalytic monohydroxymethylation reaction of benzamide, characterized in that, In the step of immersing polyacrylonitrile fibers in a mixture of N-methyl-1,2-ethylenediamine and distilled water, followed by heating and reflux reaction, and then washing and drying to obtain modified fibers, the heating temperature is 130℃ and the reaction time is 60min.

3. The application of the guanidine-functionalized polyacrylonitrile fiber according to claim 1 in the selective catalytic monohydroxymethylation reaction of benzamide, characterized in that, In the step of heating and refluxing the modified fiber with dimethyl cyanamide under a nitrogen atmosphere, followed by washing and drying to obtain guanidine-functionalized polyacrylonitrile fiber, the heating temperature is 105°C and the reaction time is 12 hours.

4. The application of the guanidine-functionalized polyacrylonitrile fiber according to claim 1 in the selective catalytic monohydroxymethylation reaction of benzamide, characterized in that, The step of immersing polyacrylonitrile fibers in a mixture of N-methyl-1,2-ethylenediamine and distilled water, reacting them under reflux, and then washing and drying them to obtain modified fibers includes: 2.00 g of polyacrylonitrile fiber was immersed in a mixture of 40.0 mL of N-methyl-1,2-ethylenediamine and 20.0 mL of distilled water. After reflux reaction at 130℃ for 60 min, the fiber was washed with distilled water at 60-70℃ and dried at 60℃ for 12 h to obtain the modified fiber.

5. The application of the guanidine-functionalized polyacrylonitrile fiber according to claim 1 in the selective catalytic monohydroxymethylation reaction of benzamide, characterized in that, The step of reacting the modified fiber with dimethyl cyanamide under a nitrogen atmosphere by heating and reflux, followed by washing and drying to obtain guanidine-functionalized polyacrylonitrile fiber includes: Under a nitrogen atmosphere, 1.0 g of the modified fiber was heated and refluxed with 25 mL of dimethyl cyanamide at 105 °C for 12 h, followed by washing with ethanol at 60-70 °C and drying at 60 °C for 12 h to obtain guanidine-functionalized polyacrylonitrile fiber.

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