Liquefied straw polyol, coated controlled-release fertilizer and preparation method and application thereof

Abstract

The invention provides liquefied straw polyol as well as a preparation method and application thereof in a coated controlled-release fertilizer, and relates to the technical field of controlled-release fertilizer production. The preparation method of the liquefied straw polyol comprises the following steps: S1, a pretreatment stage: mixing a straw raw material and a deep eutectic solvent, reacting at 50-120 DEG C for 30-60 minutes, cooling, adding an iron-based MOF (Metal Organic Framework) and a persulfide, and filtering and drying to obtain pretreated straw; s2, a liquefaction stage: in an inert atmosphere, uniformly mixing the pretreated straw in the S1 with small molecular alcohol, an acid catalyst and a nickel-based metal catalyst, and reacting at 120-160 DEG C for 1-2 hours to obtain the liquefied straw polyol. According to the method disclosed by the invention, lignin in the straws can be effectively removed by coupling the deep eutectic solvent with the Fenton-like reagent, the purity and the stability of a liquefied product and the hydroxyl value of polyhydric alcohols are remarkably improved, and non-film-forming residues are reduced, so that the compactness and the nutrient controlled-release performance of a film material of the coated controlled-release fertilizer are improved.

Description

Technical Field

[0001] This invention relates to the field of controlled-release fertilizer production technology, and in particular to a liquefied straw polyol, a coated controlled-release fertilizer, its preparation method and application. Background Technology

[0002] With the deepening of the concept of sustainable agricultural development, the use of traditional fertilizers has gradually revealed its adverse environmental impacts, such as nutrient loss, soil degradation, and environmental pollution. To improve fertilizer utilization, reduce nutrient loss, and improve the soil environment, coated controlled-release fertilizers have emerged. Coated controlled-release fertilizers are fertilizers that release nutrients gradually or slowly by coating the fertilizer surface with a thin film material. Among them, bio-based polyurethane coating materials show promising application prospects due to their biodegradability, controllable fertilizer release rate and time, improved fertilizer utilization and crop yield, and the fact that some raw materials can be derived from renewable resources.

[0003] Bio-based polyurethane is a polymer material synthesized by the addition polymerization of bio-based polyols and isocyanates. Among them, liquefied straw polyols prepared from crop straw not only realize the resource utilization of agricultural waste, but also produce polyurethane materials with good biodegradability, effectively reducing the residual burden of coating materials in the soil. However, straw is mainly composed of cellulose, hemicellulose, and lignin. Lignin and hemicellulose are chemically bonded to cellulose molecules in the form of adhesives, making the entire lignocellulose structure very strong and possessing strong resistance to degradation. This results in low accessibility of the straw cellulose components to liquefying agents and acid catalysts during straw liquefaction, leading to incomplete liquefaction of the straw.

[0004] Meanwhile, the dense and robust structure of straw components, along with their complex composition, results in a complex composition of liquefaction products. These products contain not only a large amount of polyols and their derivatives, but also carbonyl compounds such as aldehydes, ketones, and acids. All of these factors negatively impact the structure and properties of polyurethane materials. Furthermore, the complex and stable three-dimensional network aromatic structure of lignin in straw makes it difficult to effectively depolymerize into the target small-molecule polyols under conventional liquefaction conditions. Instead, it consumes liquefaction reagents through side reactions, encapsulates cellulose / hemicellulose, hinders the reaction, and forms repolymerized products, thereby reducing liquefaction efficiency, increasing residue, and affecting the quality of the obtained polyols. Both of these factors ultimately lead to a porous and unstable structure in the prepared coating material, resulting in unstable controlled-release effects and a short controlled-release period.

[0005] Existing technology CN102795924A discloses a coated controlled-release fertilizer using crop straw as a biodegradable film and its production method. This method directly liquefies crop straw, resulting in biomass polyols with poor film density due to low liquefaction rates. Furthermore, side reactions during liquefaction generate numerous non-reactive or sterically hindered groups, leading to low cross-linking levels and a short nutrient release period. This method fails to address the problem of poor controlled-release effect found in the aforementioned coated controlled-release fertilizers.

[0006] Therefore, it is particularly important to find effective pretreatment methods for straw to ensure the high quality of bio-based polyols made from straw as raw material, thereby ensuring the stability of nutrient release in polyurethane-coated controlled-release fertilizers. Summary of the Invention

[0007] This invention addresses the shortcomings of current methods for preparing coated controlled-release fertilizers from biomass resources, which suffer from poor hydroxyl value and stability of liquefied straw polyols. It provides a method for preparing liquefied straw polyols by improving the structure of the straw through an efficient pretreatment process, thereby increasing liquefaction efficiency and yielding high-hydroxyl-value polyols. This method is more conducive to enhancing the slow-release effect and persistence of coated controlled-release fertilizers in field applications.

[0008] Another object of the present invention is to provide a liquefied straw polyol.

[0009] Another object of the present invention is to provide an application of liquefied straw polyol in the preparation of coated controlled-release fertilizer.

[0010] Another object of the present invention is to provide a coated controlled-release fertilizer.

[0011] In a first aspect, the present invention provides a method for preparing liquefied straw polyol, comprising the following steps: S1. Pretreatment stage: Mix straw raw material and deep eutectic solvent at a mass ratio of (10~30): (75~95), react at 50~120℃ for 30~60min, add iron-based MOF and persulfide and react at room temperature for 10~14h to obtain pretreated straw; S2. Liquefaction stage: Under an inert atmosphere, the pretreated straw in S1 is mixed with small molecule alcohol, acid catalyst and nickel-based metal catalyst in a mass ratio of (10-20):(50-100):(0.1-5):(0.1-5), and reacted at 120-160℃ for 1-2 hours to obtain liquefied straw polyol.

[0012] According to the method for preparing liquefied straw polyol provided by the present invention, preferably, the mass ratio of hydrogen donor, hydrogen acceptor and water in the deep eutectic solvent in S1 is (5-25):(5-25):(50-90).

[0013] According to the method for preparing liquefied straw polyol provided by the present invention, preferably, the inert gas is selected from nitrogen, argon, etc., and preferably nitrogen.

[0014] According to the method for preparing liquefied straw polyol provided by the present invention, preferably, the iron-based MOF in S1 is prepared by the following method: S11. Prepare a mixed solution of ferric chloride and terephthalic acid, using N,N-dimethylformamide solution as the solvent; S12. The mixture is reacted at 120~130℃ for 10~14h to obtain iron-based MOF.

[0015] According to the method for preparing liquefied straw polyol provided by the present invention, preferably, the ratio of ferric chloride, terephthalic acid and N,N-dimethylformamide solution is (0.15-0.18) g : (0.15-0.18) g : (40-50) mL; The preferred N,N-dimethylformamide solution is prepared by mixing N,N-dimethylformamide, water and ethanol in a volume ratio of (30-40):(4-6):(4-6).

[0016] According to the method for preparing liquefied straw polyol provided by the present invention, preferably, the persulfide in S1 is selected from one or more of sodium persulfate, potassium persulfate, and ammonium persulfate.

[0017] According to the method for preparing liquefied straw polyol provided by the present invention, preferably, the small molecule alcohol in S2 is selected from one or more of ethylene glycol, diethylene glycol, propylene glycol, and butanediol; And / or, the acid catalyst described in S2 is selected from one or more of sulfuric acid, phosphoric acid, or oxalic acid; And / or, the nickel-based metal catalyst described in S2 is selected from one or more of nickel chloride, Raney nickel, and NiMOF.

[0018] Secondly, the present invention also provides a method for preparing liquefied straw polyols to obtain liquefied straw polyols.

[0019] The liquefied straw polyol provided by the present invention preferably has a hydroxyl value of 300-900 mgKOH / g.

[0020] Thirdly, the present invention also provides the application of liquefied straw polyol in the preparation of coated controlled-release fertilizer.

[0021] Fourthly, the present invention also provides a coated controlled-release fertilizer, prepared by the following method: Preheat the fertilizer granules to 50~80℃, then mix the liquefied straw polyol, polyester polyol and curing agent in a mass ratio of (20~50):(20~50):(40~70), and spray them onto the surface of the fertilizer granules. The amount of spraying is 2~5% of the mass of the fertilizer granules. After the coating material is completely cured, the coated controlled-release fertilizer is obtained.

[0022] In the coated controlled-release fertilizer provided by the present invention, preferably, the polyester polyol has a hydroxyl value of 50-250 mgKOH / g and a number-average molecular weight of 500-2000.

[0023] In the coated controlled-release fertilizer provided by the present invention, preferably, the curing agent is a prepolymerized isocyanate with an NCO content of 20-35%.

[0024] Beneficial effects: 1. This invention uses straw as raw material. First, the straw is pretreated with a deep eutectic solvent, an iron-based MOF, and persulfate. This process removes lignin from the straw under the synergistic effect of Fenton oxidation and the deep eutectic solvent, yielding pretreated straw powder. Then, the pretreated straw powder, small molecule alcohol, acid catalyst, and nickel-based metal catalyst are mixed and liquefied to produce liquefied straw polyol. This invention utilizes a deep eutectic solvent coupled with a Fenton-like reagent to effectively remove lignin from straw, ensuring the quality of the obtained liquefied straw polyol and ultimately extending the controlled-release period of polyurethane-coated controlled-release fertilizer.

[0025] 2. Regarding straw pretreatment, this invention utilizes a Fenton-like system formed by persulfate and iron-based MOF to generate sulfate free radicals (SO4· - This solvent, characterized by its long lifespan, high oxidizing capacity, and high selectivity, can efficiently degrade lignin in straw, significantly improving the exposure and reactivity of cellulose / hemicellulose. Simultaneously, the swelling effect of the eutectic solvent synergistically combines with the oxidation effect of sulfate radicals to disrupt the dense structure of straw. This not only efficiently removes lignin but also increases porosity and specific surface area, thereby increasing the contact between straw and the liquefied solvent and ultimately improving the subsequent liquefaction rate and conversion efficiency.

[0026] 3. The present invention introduces a nickel-based metal catalyst during the liquefaction process, which can selectively reduce aldehydes, ketones, acids and esters in the liquefaction byproducts to alcohols, greatly improving the hydroxyl value and stability of liquefied straw polyols, making liquefied straw polyols more suitable for reaction with isocyanates.

[0027] 4. The polyurethane film produced by the reaction of high hydroxyl value liquefied straw polyol and isocyanate obtained by this invention has high crosslinking degree and strong density, and its mechanical properties and water resistance are significantly improved. The coated controlled-release fertilizer has better slow-release effect and persistence in field application.

[0028] 5. The liquefied straw polyol obtained by this invention can not only be used in the preparation of polyurethane-coated controlled-release fertilizers, but also be extended to the fields of bio-based polyurethane coatings and biodegradable polymer materials, and has high promotional value. Attached Figure Description

[0029] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0030] Figure 1 The diagram shows the composition of straw after pretreatment in Example 1 and Comparative Examples 1-3.

[0031] Figure 2 The graph shows the residue rate determination results for the preparation of liquefied straw polyols in Examples 1 and Comparative Examples 1-3.

[0032] Figure 3 The graph shows the hydroxyl value determination results of liquefied straw polyols in Example 1 and Comparative Examples 1-3.

[0033] Figure 4 The diagram shows the composition of the liquefaction products of the liquefied straw polyols prepared in Example 1 and Comparative Examples 1-3.

[0034] Figure 5 The graph shows the results of the sustained-release effect test of the coated controlled-release fertilizer in Example 1 and Comparative Examples 1-3. Detailed Implementation

[0035] The following examples are for illustrative purposes only and are not intended to limit the scope of the invention. Where specific techniques or conditions are not specified in the examples, they should be performed according to the techniques or conditions described in the literature in this field, or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased from legitimate channels.

[0036] The iron-based MOF mentioned in this invention is prepared by the following method: 0.32 g of ferric chloride (FeCl3) and 0.32 g of terephthalic acid were added to a mixed solvent consisting of 70 mL of N,N-dimethylformamide, 10 mL of ultrapure water and 10 mL of ethanol, stirred and ultrasonically dispersed. The resulting mixture was transferred to a polytetrafluoroethylene-lined reactor and reacted at 125 °C for 12 h. After the reaction was completed, the mixture was naturally cooled to room temperature, and the resulting solid product was collected. After centrifugation and washing, the product was dried at 60 °C for 12 h to obtain Fe-MOF material.

[0037] The polyester polyol mentioned in this invention is polycaprolactone with a hydroxyl value of 204 mgKOH / g and a number-average molecular weight of 500, and was purchased from Xuzhou Yihuiyang New Material Co., Ltd.

[0038] The curing agent mentioned in this invention is a prepolymer isocyanate with an NCO content of 30.5-32.0%, purchased from Yantai Wanhua Chemical Group Co., Ltd.

[0039] In a specific embodiment, the present invention provides a method for preparing liquefied straw polyol, comprising the following steps: S1. Pretreatment stage: Mix straw raw material and deep eutectic solvent at a mass ratio of (10~30): (75~95), react at 50~120℃ for 30~60min, add iron-based MOF and persulfide and react at room temperature for 10~14h to obtain pretreated straw; S2. Liquefaction stage: Under an inert atmosphere, the pretreated straw in S1 is mixed with small molecule alcohol, acid catalyst and nickel-based metal catalyst in a mass ratio of (10-20):(50-100):(0.1-5):(0.1-5), and reacted at 120-160℃ for 1-2 hours to obtain liquefied straw polyol.

[0040] It should be noted that: Straw is mainly composed of cellulose, hemicellulose, and lignin. Lignin and hemicellulose are linked to cellulose molecules by chemical bonds in the form of adhesives, making the entire lignocellulose structure very strong and resistant to degradation. This results in low accessibility of the straw cellulose components to liquefying agents and acid catalysts during straw liquefaction, leading to incomplete liquefaction of the straw.

[0041] In the preparation method of liquefied straw polyols provided by this invention, in the pretreatment stage of step S1, the straw undergoes lignin removal through the synergistic effect of Fenton oxidation and deep eutectic solvent, and the pretreatment is carried out using a persulfate and Fe-based MOF-type Fenton system, generating SO4· - Free radicals have long lifespans, strong oxidizing power, and high selectivity. They have a significant effect on lignin degradation but less damage to cellulose. Combined with the swelling effect of deep eutectic solvents, they can effectively destroy the dense structure of straw, significantly improving the exposure and reactivity of cellulose / hemicellulose.

[0042] Furthermore, the synergistic pretreatment of persulfate and Fe-based MOF-type Fenton systems also has a structural improvement effect, and the swelling effect of the deep eutectic solvent is related to SO4· - The synergistic effect of oxidation disrupts the dense structure of straw, resulting in more thorough lignin removal and a significant increase in porosity and specific surface area. This provides more reaction sites for the liquefaction reaction, allowing for more thorough contact between the straw and the liquefaction solvent, thereby improving the liquefaction rate and conversion efficiency.

[0043] In the preparation method of liquefied straw polyols of this invention, the aforementioned special and efficient pretreatment deeply removes lignin, improving the exposure and reactivity of cellulose / hemicellulose. Based on this raw material treatment, the invention further incorporates a specific liquefaction system, introducing a nickel-based metal catalyst during the liquefaction process. This selectively reduces aldehydes, ketones, acids, and esters in the byproducts to alcohols, thereby reducing the accumulation of undesirable byproducts, improving the purity and stability of the liquefied product, and significantly increasing the hydroxyl value of the polyol, making the product more suitable for reaction with isocyanates. Therefore, its application in coated controlled-release fertilizers results in superior film-forming performance and better sustained-release stability.

[0044] Therefore, by pretreating straw with a Fenton-like reagent coupled with a deep eutectic solvent, and simultaneously using a metal catalyst to improve quality and efficiency during the liquefaction stage, it is possible to promote the complete liquefaction of biomass, increase the hydroxyl value of polyols, and reduce non-film-forming residues, thereby improving the density and nutrient release performance of the coated controlled-release fertilizer film.

[0045] The liquefaction stage of this invention is carried out under an inert gas atmosphere at atmospheric pressure, and its main function is to prevent oxidation and remove volatile byproducts generated during the reaction. The gas flow rate used in the 0.5-2L reactor ranges from 50-300 ml / min.

[0046] In some specific embodiments, in order to achieve better pretreatment effect, effectively remove lignin, enhance the swelling effect of the deep eutectic solvent, destroy the dense structure of straw raw materials, and improve the reaction efficiency with liquefied solvent, in the preparation method of liquefied straw polyol mentioned in this invention, the mass ratio of hydrogen donor, hydrogen acceptor and water in the deep eutectic solvent in S1 is preferably (5-25):(5-25):(50-90).

[0047] For example, in some specific embodiments, the mass fraction of the hydrogen donor can be 5 parts, 8 parts, 10 parts, 12 parts, 15 parts, 18 parts, 20 parts, 22 parts, 25 parts, or any range of values.

[0048] For example, in some specific embodiments, the mass fraction of hydrogen acceptor can be a point value or any range of values ​​such as 5 parts, 8 parts, 10 parts, 12 parts, 15 parts, 18 parts, 20 parts, 22 parts, or 25 parts.

[0049] For example, in some specific embodiments, the mass fraction of water can be a point value or any range of values ​​such as 50 parts, 55 parts, 60 parts, 65 parts, 70 parts, 75 parts, 80 parts, 85 parts, or 90 parts.

[0050] This invention does not specifically limit the types of hydrogen donors and acceptors mentioned. For example, in some specific embodiments, the hydrogen donor mentioned in this invention can be selected from at least one of organic acids, amino acids, ureas, or polyols, more preferably oxalic acid, citric acid, glycerol, etc. Glycerol, citric acid, and oxalic acid are not only inexpensive and environmentally friendly, but also contribute to obtaining high-quality liquefied straw polyols.

[0051] For example, in some specific embodiments, the hydrogen acceptor mentioned in this invention may be selected from at least one of choline chloride, betaine chloride, and metal salts (e.g., aluminum chloride AlCl3, ferric chloride FeCl3, cuprous chloride CuCl2).

[0052] In some specific embodiments, the iron-based MOF mentioned in this invention is preferably prepared by the following method: S11. Prepare a mixed solution of ferric chloride and terephthalic acid, using N,N-dimethylformamide solution as the solvent; S12. The mixture is reacted at 120~130℃ for 10~14h to obtain iron-based MOF.

[0053] In S11, ferric chloride (FeCl3) and terephthalic acid are added to a mixed solvent and dispersed evenly to obtain a mixed solution, for example, by stirring and ultrasonic dispersion.

[0054] After the reaction in S12 is completed, the mixture is naturally cooled to room temperature. The resulting solid product is collected, centrifuged, washed, and dried to obtain Fe-MOF material.

[0055] In some specific embodiments, the ratio of ferric chloride, terephthalic acid and N,N-dimethylformamide solution mentioned in this invention is (0.15-0.18) g : (0.15-0.18) g : (40-50) mL.

[0056] In some specific embodiments, the N,N-dimethylformamide solution mentioned in this invention comprises 30-40 mL of N,N-dimethylformamide, 4-6 mL of ultrapure water, and 4-6 mL of ethanol.

[0057] In some specific embodiments, the uniform dispersion mentioned in this invention can be achieved through stirring and ultrasonic treatment, as follows: The stirring time is 15–25 min, and the ultrasonic time is 15–25 min.

[0058] In some specific embodiments, the solid product processing conditions mentioned in this invention are as follows: Centrifugation conditions: 2500–5000 rpm, time: 4–6 min; The washing process involves repeated washing with ethanol 3 to 5 times; The drying process is carried out at 50–70 °C for 10–14 h.

[0059] In this invention, the specific types of sulfides mentioned are not specifically limited. For example, in some specific embodiments, the sulfides mentioned in this invention may be selected from one or more of sodium persulfate, potassium persulfate, and ammonium persulfate.

[0060] In some specific embodiments, the small molecule alcohols mentioned in S2 of the present invention may be selected from one or more of ethylene glycol, diethylene glycol, propylene glycol, and butanediol.

[0061] In some specific embodiments, the acid catalyst mentioned in S2 of the present invention may be selected from one or more of sulfuric acid, phosphoric acid, or oxalic acid; In some specific embodiments, the nickel-based metal catalyst mentioned in S2 of the present invention may be selected from one or more of nickel chloride, Raney nickel, and NiMOF.

[0062] In specific embodiments, the types of straw raw materials mentioned in this invention are not specifically limited. They can be commonly used biomass straw raw materials for producing liquefied straw polyols, such as one or more crop straws such as rice straw, wheat straw, corn straw, and soybean straw.

[0063] In a specific embodiment, the present invention also provides a method for preparing liquefied straw polyols to obtain liquefied straw polyols.

[0064] In some specific embodiments, the hydroxyl value of the liquefied straw polyol mentioned in this invention is 300–900 mgKOH / g. For example, it can be a point value such as 300 mgKOH / g, 400 mgKOH / g, 500 mgKOH / g, 600 mgKOH / g, 700 mgKOH / g, 800 mgKOH / g, 900 mgKOH / g, or any range of values.

[0065] The polyurethane film generated by the reaction of high hydroxyl polyol and isocyanate provided by this invention has high crosslinking degree and strong density, and its mechanical properties and water resistance are significantly improved. The coated controlled-release fertilizer has a better slow-release effect and durability in field applications.

[0066] Furthermore, the high-performance liquefied straw polyol provided by this invention can not only be used in coated controlled-release fertilizers, but also extended to fields such as bio-based polyurethane coatings and biodegradable polymer materials, and has high promotional value.

[0067] In a specific embodiment, the present invention also provides an application of liquefied straw polyol in the preparation of coated controlled-release fertilizer.

[0068] In a specific embodiment, the present invention also provides a coated controlled-release fertilizer, which is prepared by the following method: Preheat the fertilizer granules to 50~80℃, then mix the liquefied straw polyol, polyester polyol and curing agent in a mass ratio of (20~50):(20~50):(40~70), and spray them onto the surface of the fertilizer granules. The amount of spraying is 2~5% of the mass of the fertilizer granules. After the coating material is completely cured, the coated controlled-release fertilizer is obtained.

[0069] In some specific embodiments, the amount of liquefied straw polyol can be 20 parts, 30 parts, 40 parts, 50 parts, or any range thereof; the amount of polyester polyol can be 20 parts, 30 parts, 40 parts, 50 parts, or any range thereof; and the amount of curing agent can be 40 parts, 50 parts, 60 parts, 70 parts, or any range thereof.

[0070] In some specific embodiments, in order to better and faster preheat the fertilizer granules, and to allow the fertilizer granules to fully react with the liquefied straw polyol, polyester polyol and curing agent, thereby improving the reaction quality and efficiency, the fertilizer granules used in this invention are preferably fertilizer granules with an average particle size of 2 to 6 mm.

[0071] In some specific embodiments, the polyester polyol mentioned in this invention has a hydroxyl value of 50-250 mgKOH / g and a number-average molecular weight of 500-2000. For example, it can be selected from one or more oligomers of polycaprolactone, polyethylene glycol, polypropylene glycol, polybutylene glycol, polyethylene adipate, and polyethylene terephthalate.

[0072] In the coated controlled-release fertilizer provided by the present invention, preferably, the curing agent is a prepolymerized isocyanate with an NCO content of 20-35%. For example, it can be a point value of 20%, 25%, 28%, 29%, 30%, 31%, 32%, 35%, or any range of values.

[0073] The method for determining the NCO content of the curing agent in the coated controlled-release fertilizer mentioned in this invention is as follows: GB / T29493.6-2013 Determination of harmful substances in textile dyeing and finishing auxiliaries—Part 6: Determination of isocyanate group content in polyurethane prepolymers. Example 1 A method for preparing liquefied straw polyol includes the following steps: S1. Pretreatment stage: Glycerol, choline chloride and water are mixed in a mass ratio of 10:10:60 to obtain a eutectic solvent premix. After mixing the crushed corn stalks with the eutectic solvent, the mixture was placed at 80°C for 60 min and cooled. Iron-based MOF and sodium persulfate were then added to the reaction system. The mass ratio of corn stalks, eutectic solvent, iron-based MOF and sodium persulfate was 20:90:0.5:0.1. The mixture was shaken at room temperature for 12 h. After the reaction was completed, the mixture was filtered and dried to obtain the pretreated stalks. S2. Liquefaction stage: Under a nitrogen atmosphere, pretreated straw, diethylene glycol, 1,2-propanediol, 98% sulfuric acid, and nickel chloride are mixed evenly in a mass ratio of 14:60:10:1.4:0.56 and reacted at 140℃ for 2 hours to obtain liquefied straw polyol.

[0074] Example 2 A method for preparing liquefied straw polyol is basically the same as that in Example 1, except that in the S1 pretreatment stage, corn straw, deep eutectic solvent, iron-based MOF and sodium persulfate are in a mass ratio of 10:85:0.5:1, and the nickel-based catalyst used in the S2 liquefaction stage is Raney nickel.

[0075] Example 3 A method for preparing liquefied straw polyol is basically the same as that in Example 1, except that the mass ratio of corn straw, deep eutectic solvent, iron-based MOF and sodium persulfate in the S1 pretreatment stage is 30:100:0.5:1, and the nickel-based catalyst used in the S2 liquefaction stage is NiMOF.

[0076] Example 4 A method for preparing liquefied straw polyol is basically the same as that in Example 1, except that the acid catalyst used in the S2 liquefaction stage is oxalic acid and the nickel catalyst is NiMOF.

[0077] Example 5 A coated controlled-release fertilizer is prepared by the following method: Urea particles with an average particle size of 5 mm were preheated to 65°C in a rotating drum. Then, the liquefied straw polyol prepared in Example 1 was mixed with polycaprolactone and isocyanate (MDI) at a mass ratio of 30:10:60 and used as a coating material. The mixture was sprayed onto the surface of the urea. Each spraying amount was 1% of the urea mass. The coating material was sprayed repeatedly until the coating material accounted for 4% of the total mass of the coated controlled-release fertilizer, thus obtaining the coated controlled-release fertilizer.

[0078] The liquefied straw polyols from Examples 2-4 were used to prepare corresponding coated controlled-release fertilizers according to the above method.

[0079] Comparative Example 1 A method for preparing liquefied straw polyol includes the following steps: Under a nitrogen atmosphere, rice straw powder, diethylene glycol, 1,2-propanediol, 98% sulfuric acid, and nickel chloride were mixed evenly in a mass ratio of 14:60:10:1.4:0.56 and reacted at 140°C for 2 hours to obtain liquefied straw polyol.

[0080] The difference between Comparative Example 1 and Example 1 is that there is no pretreatment step.

[0081] Comparative Example 2 A method for preparing liquefied straw polyol includes the following steps: S1. Straw Pretreatment Glycerol, choline chloride, and water were mixed in a mass ratio of 10:10:60 to obtain a eutectic solvent premix; crushed straw was added and reacted at 80 °C for 60 min. The mass ratio of straw to eutectic solvent premix was 20:80. After cooling, the mixture was filtered and dried to obtain pretreated straw. S2. Preparation of liquefied straw polyols Under a nitrogen atmosphere, pretreated straw, diethylene glycol, 1,2-propanediol, 98% sulfuric acid, and nickel chloride were mixed evenly in a mass ratio of 14:60:10:1.4:0.56 and reacted at 140°C for 2 hours to obtain liquefied straw polyol.

[0082] Pretreatment without Fenton reagent.

[0083] Comparative Example 3 A method for preparing liquefied straw polyol includes the following steps: S1. Straw Pretreatment After crushing, rice straw was mixed with Fe-MOF, sodium persulfate and water in a mass ratio of 10:0.5:0.1:60 and shaken at room temperature for 12 hours. The mixture was then filtered and dried to obtain pretreated straw. S2. Preparation of liquefied straw polyols Under a nitrogen atmosphere, pretreated straw, diethylene glycol, 1,2-propanediol, 98% sulfuric acid, and nickel chloride were mixed evenly in a mass ratio of 14:60:10:1.4:0.56 and reacted at 140°C for 2 hours to obtain liquefied straw polyol.

[0084] Pretreatment without deep eutectic solvent.

[0085] Comparative Example 4 A method for preparing liquefied straw polyol includes steps that are basically the same as those in Example 1, except that a traditional Fenton reagent (ferrous sulfate + hydrogen peroxide) is used.

[0086] Comparative Example 5 A coated controlled-release fertilizer is prepared by the following method: Urea particles with an average particle size of 5 mm were preheated to 65°C in a rotating drum. Then, the liquefied straw polyol prepared in Comparative Example 1 was mixed with polycaprolactone and isocyanate (MDI) in a mass ratio of 30:10:60 and used as a coating material. The mixture was sprayed onto the surface of the urea. Each spraying amount was 1% of the urea mass. The coating material was sprayed repeatedly until the coating material accounted for 4% of the total mass of the coated controlled-release fertilizer, thus obtaining the coated controlled-release fertilizer.

[0087] The liquefied straw polyols of Comparative Examples 2-4 were prepared into corresponding coated controlled-release fertilizers according to the above method.

[0088] Result detection (1) Performance testing of liquefied straw polyols The composition of the straw prepared in the examples and comparative examples, as well as the residue rate, hydroxyl value, and composition of the liquefaction products of the liquefied straw polyols, were tested: 1. Straw composition: The contents of cellulose, hemicellulose and lignin in straw were determined using the Van Soest method.

[0089] 2. Residue rate: Weigh 2.0 g of liquefied product (accurate to 0.0001 g), and then weigh 20 mL of a mixed solution of 1,4-dioxane and water (volume ratio 4:1) using a graduated cylinder. Place both solutions in a beaker and stir in an 80°C water bath for 30 min. Filter while hot, and wash the residue with dioxane until the filtrate is colorless.

[0090] Place the filter paper with residue into a 120℃ drying oven and dry for 4 hours. After drying, weigh the paper.

[0091] The formula for calculating the residue ratio is as follows: Residue rate (%) = (m3-m2) / m1×100; Where m1 represents the mass of the liquefied product; m2 represents the mass of the filter paper; and m3 represents the mass of the residue and the filter paper.

[0092] 3. Hydroxyl value: The determination method is in accordance with GB / T 12008.3-2009.

[0093] 4. Composition of Liquefaction Products: Qualitative and quantitative analysis of the liquefaction products was performed using gas chromatography-mass spectrometry (GC-MS, MSQ8100, Shanghai). Samples were filtered using a 0.22 μm aqueous filter before testing. An Innowax capillary column (30 m × 0.32 mm × 0.25 μm) was used, with helium (99.999%) as the carrier gas. The GC-MS instrument was equipped with an EI ion source with an electron energy of 70 eV and an ion source temperature of 280 °C. MS (EI) measurements were performed in full scan mode, and the corresponding spectra were retrieved using the standard NIST spectral library. The column temperature program was as follows: increasing from 50 °C to 250 °C at a rate of 10 °C / min and holding for 10 min. The injection port temperature was 250 °C, the injection volume was 0.1 μL, and the split ratio was 50:1.

[0094] Depend on Figure 1 It can be seen that the lignin content of the pretreated straw used in Example 1 of this invention is significantly reduced. Figures 2-4 It can be seen that the straw polyol prepared in Example 1 of this invention has the lowest residue rate, the highest hydroxyl value, the highest liquefaction efficiency, and the lowest content of byproducts such as aldehydes, ketones, and esters. This indicates that the present invention uses a green, natural, and pollution-free eutectic solvent coupled with a Fenton-like reagent to pretreat straw, which significantly improves the quality of the prepared straw polyol.

[0095] The specific test data is shown in Table 1 below.

[0096] Table 1. Serial Number Residue rate / % Hydroxyl value / mgKOH / g Example 1 3.46 693 Example 2 2.98 604 Example 3 3.88 591 Example 4 3.75 481 Comparative Example 1 8.27 566 Comparative Example 2 5.09 585 Comparative Example 3 6.51 574 Comparative Example 4 4.78 597 (2) Performance testing of coated controlled-release fertilizer According to the national standard GB / T 23348-2009 for slow-release fertilizers, the nitrogen release rate of the coated controlled-release fertilizers prepared using the examples and comparative examples was determined, and the time required for the cumulative nutrient release rate to reach 80% was recorded as the controlled-release period.

[0097] Depend on Figure 5It can be seen that the controlled-release fertilizer prepared using the liquefied straw polyol prepared in Example 1 of this invention has the longest controlled-release period, which is 60 days; the controlled-release fertilizer prepared using the liquefied straw polyol prepared in Example 2 has a controlled-release period of 58 days; the controlled-release fertilizer prepared using the liquefied straw polyol prepared in Example 3 has a controlled-release period of 52 days; the controlled-release fertilizer prepared using the liquefied straw polyol prepared in Example 4 has a controlled-release period of 42 days; and the controlled-release fertilizer prepared using the liquefied straw polyol prepared in Comparative Example 1 has a controlled-release period of 42 days. The controlled-release fertilizer had a controlled-release period of 16 days; the controlled-release fertilizer prepared using the liquefied straw polyol prepared in Comparative Example 2 had a controlled-release period of 21 days; the controlled-release fertilizer prepared using the liquefied straw polyol prepared in Comparative Example 3 had a controlled-release period of 28 days; and the controlled-release fertilizer prepared using the liquefied straw polyol prepared in Comparative Example 4 had a controlled-release period of 30 days. This indicates that the straw polyol prepared by the coupled pretreatment of the eutectic solvent and Fenton-like reagent in this invention has a synergistic effect on improving the controlled-release period of the coated controlled-release fertilizer.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing liquefied straw polyol, characterized in that, Includes the following steps: S1. Pretreatment stage: Mix straw raw material and deep eutectic solvent at a mass ratio of (10~30): (75~95), react at 50~120℃ for 30~60min, add iron-based MOF and persulfide and react at room temperature for 10~14h to obtain pretreated straw; S2. Liquefaction stage: Under an inert atmosphere, the pretreated straw in S1 is mixed with small molecule alcohol, acid catalyst and nickel-based metal catalyst in a mass ratio of (10-20):(50-100):(0.1-5):(0.1-5), and reacted at 120-160℃ for 1-2 hours to obtain liquefied straw polyol.

2. The method for preparing liquefied straw polyol according to claim 1, characterized in that, The mass ratio of hydrogen donor, hydrogen acceptor and water in the deep eutectic solvent described in S1 is (5-25):(5-25):(50-90).

3. The method for preparing liquefied straw polyol according to claim 1 or 2, characterized in that, The iron-based MOF described in S1 is prepared by the following method: S11. Prepare a mixed solution of ferric chloride and terephthalic acid, using N,N-dimethylformamide solution as the solvent; S12. The mixture is reacted at 120~130℃ for 10~14h to obtain iron-based MOF.

4. The method for preparing liquefied straw polyol according to claim 3, characterized in that, The ratio of ferric chloride, terephthalic acid, and N,N-dimethylformamide solution is (0.15-0.18) g : (0.15-0.18) g : (40-50) mL; The preferred N,N-dimethylformamide solution is prepared by mixing N,N-dimethylformamide, water and ethanol in a volume ratio of (30-40):(4-6):(4-6).

5. The method for preparing liquefied straw polyol according to any one of claims 1 to 4, characterized in that, The persulfide in S1 is selected from one or more of sodium persulfate, potassium persulfate, and ammonium persulfate; And / or, the small molecule alcohol in S2 is selected from one or more of ethylene glycol, diethylene glycol, propylene glycol, and butanediol; And / or, the acid catalyst described in S2 is selected from one or more of sulfuric acid, phosphoric acid, or oxalic acid; And / or, the nickel-based metal catalyst described in S2 is selected from one or more of nickel chloride, Raney nickel, and NiMOF.

6. A liquefied straw polyol prepared by the method of any one of claims 1 to 5.

7. The liquefied straw polyol according to claim 6, characterized in that, The hydroxyl value of the liquefied straw polyol is 300–900 mgKOH / g.

8. The application of the liquefied straw polyol of claim 6 or 7 in the preparation of coated controlled-release fertilizer.

9. A coated controlled-release fertilizer, characterized in that, Prepared by the following method: Preheat the fertilizer granules to 50~80℃, then mix the liquefied straw polyol, polyester polyol and curing agent in a mass ratio of (20~50):(20~50):(40~70), and spray them onto the surface of the fertilizer granules. The amount of spraying is 2~5% of the mass of the fertilizer granules. After the coating material is completely cured, the coated controlled-release fertilizer is obtained.

10. The coated controlled-release fertilizer according to claim 9, characterized in that, The hydroxyl value of the polyester polyol is 50-250 mgKOH / g, and the number average molecular weight is 500-2000; And / or, the curing agent is a prepolymerized isocyanate with an NCO content of 20-35%.

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