Acid-base synergistic solid catalyst, synthesis method thereof and application of solid catalyst in preparation of cellulose nanosheets

Abstract

The invention belongs to the technical field of conversion and utilization of biomass materials, and relates to an acid-base synergetic solid catalyst, a synthesis method thereof and application of the acid-base synergetic solid catalyst in preparation of cellulose nanosheets. The solid catalyst provided by the invention has ortho-position weakly acidic and weakly alkaline sites, so that interlayer hydrogen bonds of cellulose can be selectively broken to prepare cellulose nanosheets. The synergistic effect of the zwitterions simulates the function of the ionic liquid, but the ionic strength of the ionic liquid is only suitable for preparation of nanosheets. A hydrolysis reaction is carried out under a microwave condition, and cellulose can be hydrolyzed into 2-D nanosheets within a short time. The method for preparing the cellulose nanosheets by synergistically destroying hydrogen bonds through the solid acid catalyst provides a new technical route for value-added utilization of cellulose.

Description

Technical Field

[0001] The present invention belongs to the technical field of conversion and utilization of biomass materials, and relates to an acid-base synergistic solid catalyst, a synthesis method thereof, and application in the preparation of cellulose nanosheets. Background Art

[0002] Cellulose, composed of linear polymers of glucose units linked by β-1,4-glycosidic bonds, is one of the world's most abundant, environmentally friendly, and biocompatible materials, and the most abundant polysaccharide on Earth. Cellulose molecular chains are arranged in parallel within a plane, forming a two-dimensional plane, relying on hydrogen bonds. This exhibits high rigidity and stability, making these regions difficult to dissociate. Interlayers are primarily held together by van der Waals forces and weak hydrogen bonds, forming a layered stacked structure. Materials based on cellulose and its derivatives have been used for over 150 years in various applications, including food, papermaking, biomaterials, and pharmaceuticals.

[0003] The binding energy within a cellulose layer is eight times greater than that between layers. When subjected to external forces, the interlayer bonds are more easily broken. Currently, many methods exist for degrading and preparing nanocellulose, such as acid hydrolysis, enzymatic hydrolysis, Creath oxidation, mechanical methods, and mechanochemical methods, but all have limitations. Meanwhile, various ionic liquids (ILs) have been found to be effective solvents for cellulose. Due to their acid-base coordination properties, ionic liquids are often referred to as "designable solvents." Many hydrogen bonds in cellulose can be disrupted or reduced due to strong interactions with the cations and anions of the ionic liquid, thereby weakening the solid crystal structure. For example, Chinese invention patent CN108250460A discloses the use of ionic liquids to dissolve cellulose. However, ionic liquids are corrosive and toxic, resulting in excessive degradation of the product, difficulty in separation, and easy contamination. Solid acids have been extensively studied due to their environmentally friendly and recyclable properties. For example, Chinese invention patent CN117960153A uses a boron-containing bifunctional solid acid to hydrolyze cellulose, and Chinese invention patent CN116239104A uses polyhydroxy carbon materials to promote cellulose hydrolysis. Although solid acid can avoid liquid contamination, it is not selective for the hydrolysis of cellulose. Its acidic sites preferentially attack glycosidic bonds rather than interlayer forces, resulting in the degradation product being mainly CNC.

[0004] Cellulose nanocrystals can be degraded and exfoliated to produce 2-D cellulose nanosheets. These 2-D nanosheets combine the properties of nanomaterials with the key characteristics of cellulose, such as high specific strength and modulus, optical transparency, hydrophobicity, barrier properties, and broad chemical modifiability. They are widely used in films, coatings, nanopaper, fillers, and biomedical materials. However, when cellulose is degraded to produce nanosheets, if the degradation conditions are too strong, the reaction becomes uncontrollable, and cellulose hydrolysis proceeds unrestrictedly to produce monomeric glucose and byproducts such as levulinic acid and 5-HMF, without forming sheets. If the hydrolysis conditions are too weak, it becomes difficult to break the hydrogen bonds between cellulose sheets, and the cellulose retains its blocky structure. Therefore, the ability of a catalyst to disrupt cellulose hydrogen bonds is crucial for the preparation of 2-D nanosheets. Currently, no green methods have been reported that can selectively hydrolyze cellulose to produce 2-D nanosheets, specifically by breaking hydrogen bonds between sheets. Summary of the Invention

[0005] In order to solve the above problems existing in the prior art, the purpose of the present invention is to provide an acid-base synergistic solid catalyst and its synthesis method and application in the preparation of cellulose nanosheets, so as to overcome the shortcomings of the prior art.

[0006] The first object of the present invention is achieved through the following technical solutions:

[0007] An acid-base synergistic solid catalyst, the structural formula of which is shown below:

[0008]

[0009] Here, m is 10 to 2000.

[0010] The acid-base synergistic solid catalyst is a polymer formed by connecting phenylboronic acid and pyrrole through methylene groups.

[0011] Preferably, the acid-base synergistic solid catalyst is a polymer formed by connecting phenylboronic acid and pyrrole through methylene groups and supporting a heteropoly acid.

[0012] Its structural formula is shown below:

[0013]

[0014] The second object of the present invention is achieved through the following technical solutions:

[0015] A method for synthesizing an acid-base synergistic solid catalyst comprises the following steps:

[0016] Phenylboronic acid and pyrrole are mixed, and then a weak / non-polar solvent, a Lewis acid catalyst, and acetal are added. After the reaction, the mixture is filtered and dried.

[0017] Preferably, the weak / non-polar solvent is one or more of dichloroethane, dichloromethane, p-dichlorobenzene, chlorobenzene, CS2 (carbon disulfide), carbon tetrachloride (CCl4), o-dichlorobenzene, m-dichlorobenzene, 1,2,4-trichlorobenzene, o-xylene, p-xylene, nitrobenzene, 1,3,5-tris(bromomethyl)benzene, and 1,4-bis(chloromethyl)benzene.

[0018] Preferably, the Lewis acid is one or more of FeCl3 (ferric chloride), AlCl3 (aluminum chloride), SnCl4 (tin tetrachloride), BF3·OEt3 (boron difluoride ether complex), ZnCl2 (zinc chloride), TiCl4, SbCl5, GaCl3, InCl3, and Sc(OTf)3.

[0019] Preferably, the acetal is one or more of dimethylol formal, diethanol formal, and acetaldehyde dimethyl acetal.

[0020] Preferably, the molar proportions of phenylboric acid, pyrrole, weak / non-polar solvent, Lewis acid catalyst, and acetal are 1 part, 0.8-1.2 parts, 15-50 parts, 1-6 parts, and 1-10 parts, respectively.

[0021] Furthermore, the molar fractions of phenylboric acid, pyrrole, weak / non-polar solvent, Lewis acid catalyst, and acetal are 1 part, 0.9-1.1 parts, 20-40 parts, 1-4 parts, and 2-6 parts, respectively.

[0022] Preferably, the mixing of phenylboric acid and pyrrole is carried out at 10-40° C. for 1-5 hours.

[0023] Preferably, the reaction temperature is 80-120° C., and the reaction time is 10-30 h.

[0024] Preferably, the synthesis method comprises the following steps:

[0025] Phenylboric acid and pyrrole are mixed, and then a weak / non-polar solvent, a Lewis acid catalyst, and acetal are added. After reaction, the mixture is filtered and dried. A heteropoly acid is then added to the dried mixture, stirred in a polar solvent, filtered, and dried to obtain a solid catalyst. The solid catalyst of the present invention is loaded with a heteropoly acid to control acidity and alkalinity, promote the hydrolysis of the amorphous region of cellulose, and improve the conversion rate of cellulose.

[0026] Preferably, the heteropoly acid is one or more of phosphotungstic acid, silicotungstic acid, borotungstic acid, aluminotungstic acid, germanotungstic acid, phosphomolybdic acid, silicomolybdic acid, boromolybdic acid, germanomolybdic acid, arsenotungstic acid, arsenomolybdic acid, cobaltmolybdic acid, aluminomolybdic acid, chromomolybdic acid, hexamolybdic acid, hexatungstic acid, silver-substituted phosphotungstic acid, cesium-substituted phosphotungstic acid, and potassium-substituted phosphotungstic acid.

[0027] Preferably, the mass ratio of the heteropoly acid to the dry matter is 1:0.5-6.

[0028] Preferably, stirring is carried out in a polar solvent at 10-40° C. for 15-50 h.

[0029] Examples of the polar solvent include water, ethanol, methanol, isopropanol, acetonitrile, dimethyl sulfoxide, N,N-dimethylformamide, and N-methylpyrrolidone, and acetonitrile is preferred.

[0030] The third object of the present invention is achieved through the following technical solutions:

[0031] A method for preparing cellulose nanosheets comprises the following steps:

[0032] S1. Add cellulose to a pretreatment solution, heat and stir, and then refrigerate at 0-8°C; add the refrigerated cellulose solution to ethanol and / or water at 0-10°C, stir to precipitate solids, dilute and wash the solution multiple times until the pH of the washing solution reaches 7±0.1, and filter to obtain the pretreated cellulose;

[0033] S2, mixing the solid catalyst with pretreated cellulose, and hydrolyzing the cellulose in a microwave environment to obtain stacked cellulose nanosheets;

[0034] S3. Add the stacked cellulose nanosheets into a polar solvent, stir and swell, and obtain single cellulose nanosheets through ultrasonication, filtration, and drying.

[0035] In step S1:

[0036] Preferably, the pretreatment solution is one of H3PO4 solution, zinc chloride solution, a mixed solution of zinc chloride and aluminum chloride, a mixed solution of NaOH and urea, a mixed solution of LiCl and DMAc, and ionic liquids BmimCl, BmimAc, EmimOAc, and AmimCl.

[0037] When the pretreatment solution is H3PO4 solution, zinc chloride solution, aluminum chloride solution, a mixed solution of zinc chloride and aluminum chloride, a mixed solution of NaOH and urea, or a mixed solution of LiCl and DMAc, 85-100 wt% ethanol aqueous solution is added for precipitation; when the pretreatment solution is the ionic liquid BmimCl, BmimAc, EmimOAc, or AmimCl, water is added for precipitation.

[0038] Preferably, the mass ratio of cellulose to pretreatment solution is 1:2-20.

[0039] The H3PO4 solution is formed by dissolving H3PO4 in water, and the H3PO4 concentration is preferably 70-90 wt%. The zinc chloride solution is formed by dissolving zinc chloride in water, and the zinc chloride concentration is 65-90 wt%. The aluminum chloride solution is formed by dissolving aluminum chloride in water, and the aluminum chloride concentration is 65-90 wt%. The mixed solution of zinc chloride and aluminum chloride is formed by dissolving zinc chloride and aluminum chloride in water, and the zinc chloride concentration is 30-50 wt%, and the aluminum chloride concentration is 30-50 wt%. The mixed solution of NaOH and urea is formed by dissolving NaOH and urea in water, and the NaOH concentration is 4-8 wt%, and the urea concentration is 7-12 wt%. The mixed solution of LiCl and DMAc is formed by dissolving LiCl in DMAc, and the LiCl concentration is 0.1-5 mol / L.

[0040] Examples of the cellulose include one or more of corncob cellulose, straw cellulose, cotton cellulose, wood cellulose, hemp cellulose, and bamboo cellulose.

[0041] Preferably, the heating and stirring temperature is 40 to 55° C., and the stirring time is 1 to 8 hours.

[0042] Add ethanol and / or water at 0-10°C to the refrigerated fiber. Sufficient ethanol and / or water is required to precipitate the fiber. For example, the weight ratio of the refrigerated fiber to ethanol and / or water is 1:1-50. Preferably, stirring is performed to precipitate the solid at a temperature of 10-40°C for 10-20 hours.

[0043] In step S2:

[0044] Preferably, the mass ratio of the solid catalyst to the pretreated cellulose is 1:0.8-1.5.

[0045] The solid catalyst and the pretreated cellulose are mixed in water, with the mass ratio of the solid catalyst to the water being 1:100-500.

[0046] Preferably, the microwave hydrolysis temperature is 150-200° C., the hydrolysis time is 5-40 min, and the hydrolysis is preferably performed in a microwave synthesizer.

[0047] In step S3:

[0048] Preferably, the mass ratio of cellulose hydrolyzate to polar solvent is 1:1-10.

[0049] The polar solvent in step S3 can be water, ethanol, methanol, isopropanol, acetonitrile, dimethyl sulfoxide, N,N-dimethylformamide, N-methylpyrrolidone, etc., preferably dimethyl sulfoxide.

[0050] Preferably, the stirring and swelling is carried out at 10-40° C., and the stirring time is 2-10 h.

[0051] Preferably, the sonication step comprises a first sonication, dilution, and a second sonication.

[0052] The first ultrasonic time is 0.5 to 5 hours, the dilution ratio is 1:5 to 20, and the second ultrasonic time is 1 to 10 minutes.

[0053] The drying here is preferably freeze-drying.

[0054] The fourth object of the present invention is achieved through the following technical solutions:

[0055] A cellulose nanosheet is prepared by the method.

[0056] Preferably, the cellulose nanosheets have a length of 1 to 60 μm and a thickness of 5 to 80 nm. More preferably, the cellulose nanosheets have a length of 1 to 40 μm and a thickness of 10 to 40 nm.

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] 1. The present invention provides a novel solid catalyst. The solid catalyst uses phenylboronic acid and pyrrole as raw materials. Under catalytic conditions, a weak acid-base synergistic solid acid is synthesized through a Friedel-Crafts "knitting" polymerization reaction. The acid and basic sites of the solid acid are forcibly loosely bound by a methylene bond (-CH2-), and are further loaded with a heteropolyacid to regulate the acidity and alkalinity.

[0059] 2. The solid catalyst of the present invention uses pyrrole and phenylboronic acid as the alkaline and acidic groups, respectively. Pyrrole and phenylboronic acid are both weakly acidic and alkaline. Therefore, due to the presence of ortho-positioned weakly acidic and weakly basic sites, the solid catalyst can selectively break the hydrogen bonds between cellulose layers to produce cellulose nanosheets. This synergistic effect of anions and cations mimics the function of ionic liquids, but their ionic strength is only suitable for the production of nanosheets. Therefore, the solid catalyst of the present invention has an appropriate ionic strength to selectively break the hydrogen bonds between cellulose layers, selectively hydrolyze the amorphous regions of cellulose, and retain the crystalline nanosheets, thereby exfoliating the nanosheets. Furthermore, the catalyst is environmentally friendly and easily recyclable.

[0060] 3. This invention uses a novel solid catalyst to hydrolyze cellulose under microwave conditions, quickly converting cellulose into 2D nanosheets. The production of cellulose nanosheets through the synergistic disruption of hydrogen bonds by a solid acid catalyst provides a new technological path for the value-added utilization of cellulose.

[0061] 4. During the preparation of the cellulose nanosheets of the present invention, the cellulose is first pretreated, which is beneficial for the solid catalyst to catalyze the hydrolysis of cellulose to generate 2-D nanosheets.

[0062] 5. The present invention converts cellulose into high-value nanosheets, which is beneficial to the recycling of waste cellulose.

[0063] 6. The catalyst of the present invention produces a large amount of oligosaccharides and glucose while selectively preparing nanosheets.

[0064] 7. The preparation method of the cellulose nanosheets of the present invention is simple and efficient, the entire process is green and environmentally friendly, the equipment is simple, and it is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 is the structural formula of the solid acid catalyst prepared in Example 1;

[0066] Figure 2 FTIR image of the solid acid catalyst prepared in Example 1

[0067] Figure 3 This is a SEM image of the solid acid catalyst prepared in Example 1;

[0068] Figure 4 The POM (left) and optical microscope morphology (right) of the cellulose solution after phosphoric acid treatment in Example 8;

[0069] Figure 5 The SEM images of the cellulose after pretreatment (left) and cellulose after hydrolysis (right) in Example 8 are shown;

[0070] Figure 6 SEM (left middle picture) and TEM (right picture) of the cellulose nanosheets prepared in Example 8. DETAILED DESCRIPTION

[0071] Below by specific embodiment and accompanying drawing, technical scheme of the present invention is further described explanation, it should be understood that specific embodiment described herein is only for helping to understand the present invention, is not used for specific limitation of the present invention.And accompanying drawing used herein, is only for better illustrating the disclosure of the present invention, does not have limiting effect on protection scope.If no special instructions, the raw materials adopted in the embodiment of the present invention are all raw materials commonly used in this area, and the method adopted in the embodiment is all conventional method in this area.

[0072] Example 1

[0073] The synthesis method of the acid-base synergistic solid catalyst provided in this embodiment is as follows:

[0074] 0.04 mol of phenylboronic acid and 0.04 mol of pyrrole were stirred at room temperature (25°C in this article and will not be repeated below) for 3 hours. 100 mL of 1,2-dichloroethane, 0.08 mol of FeCl3 catalyst, and 0.16 mol of dimethylformal were added, and the mixture was stirred at 80°C for 16 hours. The catalyst was obtained by vacuum filtration, washing, and drying. Phosphotungstic acid (PTTA:catalyst mass ratio of 1:3) was added to the catalyst and stirred in acetonitrile at room temperature for 24 hours. The excess acetonitrile was removed by filtration and washing, and then dried to obtain the HPW-loaded solid acid catalyst.

[0075] Figure 1 is the structural formula of the solid acid catalyst prepared in Example 1; phenylboric acid and pyrrole are connected through the methylene bond of dimethanol formal under catalytic conditions to form a polymer, and HPW is loaded on the polymer. Figure 2 This is the infrared spectrum of the solid acid catalyst prepared in Example 1.

[0076] Figure 3 This is the SEM image of the solid acid catalyst prepared in Example 1.

[0077] Example 2

[0078] The synthesis method of the acid-base synergistic solid catalyst provided in this embodiment is as follows:

[0079] Mix 0.04 mol of phenylboric acid and 0.04 mol of pyrrole by stirring at room temperature for 3 hours. Add 100 mL of 1,2-dichloroethane, 0.08 mol of FeCl3 catalyst, and 0.16 mol of dimethylformal. Stir and react at 80°C for 16 hours. Vacuum filter, wash, and dry to obtain a solid acid catalyst unloaded with HPW.

[0080] Example 3

[0081] The synthesis method of the acid-base synergistic solid catalyst provided in this embodiment is as follows:

[0082] 0.04 mol of phenylboric acid and 0.04 mol of pyrrole were stirred at room temperature for 3 hours. 100 mL of p-dichlorobenzene, 0.06 mol of FeCl3 catalyst, and 0.12 mol of dimethylformal were added and stirred at 80°C for 20 hours. The catalyst was obtained by vacuum filtration, washing, and drying. Phosphotungstic acid (phosphotungstic acid:catalyst mass ratio of 1:4) was added to the catalyst and stirred in acetonitrile solution at room temperature for 20 hours. The excess acetonitrile was removed by filtration and washing, and then dried to obtain a solid acid catalyst loaded with HPW.

[0083] Example 4

[0084] The synthesis method of the acid-base synergistic solid catalyst provided in this embodiment is as follows:

[0085] Mix 0.04 mol of phenylboric acid and 0.04 mol of pyrrole by stirring at room temperature for 2 hours. Add 120 mL of carbon tetrachloride, 0.10 mol of AlCl3 catalyst, and 0.13 mol of dimethylformal, and stir at 80°C for 24 hours. Vacuum filter, wash, and dry to obtain the catalyst. Add phosphotungstic acid (phosphotungstic acid:catalyst mass ratio of 1:2) to the catalyst and stir in acetonitrile at room temperature for 28 hours. Filter, wash, and remove excess acetonitrile, then dry to obtain the HPW-loaded solid acid catalyst.

[0086] Example 5

[0087] The synthesis method of the acid-base synergistic solid catalyst provided in this embodiment is as follows:

[0088] 0.04 mol of phenylboric acid and 0.04 mol of pyrrole were stirred at room temperature for 4 hours. 110 mL of 1,2-dichloroethane, 0.12 mol of ZnCl2 catalyst, and 0.15 mol of dimethylformal were added and stirred at 120°C for 16 hours. The catalyst was obtained by vacuum filtration, washing, and drying. Phosphotungstic acid (PTTA:catalyst mass ratio:1:1) was added to the catalyst and stirred in acetonitrile at room temperature for 30 hours. The excess acetonitrile was removed by filtration and washing, and then dried to obtain a solid acid catalyst loaded with HPW.

[0089] Example 6

[0090] The synthesis method of the acid-base synergistic solid catalyst provided in this embodiment is as follows:

[0091] Mix 0.04 mol of phenylboronic acid and 0.04 mol of pyrrole by stirring at room temperature for 4 hours. Add 130 mL of 1,2-dichloroethane, 0.09 mol of FeCl3 catalyst, and 0.18 mol of dimethylformal, and stir at 120°C for 20 hours. Vacuum filter, wash, and dry to obtain the catalyst. Add silicotungstic acid (mass ratio of silicotungstic acid to catalyst is 1:2) to the catalyst and stir in acetonitrile solution at room temperature for 22 hours. Filter and wash to remove excess acetonitrile, and dry to obtain a solid acid catalyst loaded with silicotungstic acid.

[0092] Example 7

[0093] The synthesis method of the acid-base synergistic solid catalyst provided in this embodiment is as follows:

[0094] 0.04 mol of phenylboric acid and 0.04 mol of pyrrole were stirred at room temperature for 2 hours. 100 mL of 1,2-dichloroethane, 0.08 mol of FeCl3 catalyst, and 0.20 mol of dimethylformal were added and stirred at 120°C for 24 hours. The catalyst was obtained by vacuum filtration, washing, and drying. Phosphomolybdic acid (phosphomolybdic acid to catalyst mass ratio of 1:3) was added to the catalyst and stirred in acetonitrile at room temperature for 32 hours. The excess acetonitrile was removed by filtration and washing, and then dried to obtain a solid acid catalyst loaded with phosphomolybdic acid.

[0095] Example 8

[0096] The preparation method of the cellulose nanosheets provided in this embodiment is as follows:

[0097] S1. Add 30 g of corncob cellulose to 200 mL of 85% H₃PO₄ solution, stir at 40°C for 2 h, then refrigerate at 4°C overnight. Simultaneously, refrigerate 800 mL of anhydrous ethanol. Pour the refrigerated anhydrous ethanol into the refrigerated cellulose solution and stir at room temperature for 14 h to precipitate solids. Repeat dilution and deionized water washing until the washing solution reaches a pH of 7. Filter to obtain the pretreated cellulose.

[0098] S2. Add 0.05 g of the catalyst obtained in Example 1 and 0.05 g of the pretreated cellulose obtained in step S1 to 20 mL of deionized water and hydrolyze in a microwave reactor at 180° C. for 15 min to obtain a cellulose hydrolyzate.

[0099] S3. Add the cellulose hydrolyzate to dimethyl sulfoxide at a mass ratio of 4:9, stir and swell at room temperature for 6 hours, sonicate at 40 kHz for 1 hour, dilute at a ratio of 1:10, sonicate again for 5 minutes, filter, wash, and freeze-dry to obtain cellulose nanosheets.

[0100] Figure 4 The POM (left) and optical microscope morphology (right) of the cellulose solution after phosphoric acid treatment in Example 8. Figure 5 These are SEM images of the cellulose (left) and cellulose hydrolyzate (right) after pretreatment in Example 8. It can be seen that the cellulose hydrolyzate is composed of stacked nanosheets. Figure 6 The SEM (left middle picture) and TEM (right picture) of the cellulose nanosheets prepared in Example 8. After ultrasonic treatment, the cellulose hydrolyzate forms single nanosheets with a length of approximately 1-60 μm and a thickness of approximately 5-80 nm.

[0101] The cellulose conversion rate in Example 8 was 67.4%.

[0102] Example 9

[0103] The difference between the preparation method of the cellulose nanosheets provided in this embodiment and that in embodiment 8 is that the catalyst of embodiment 2 is used in embodiment 9, and the rest is the same as that in embodiment 8.

[0104] The cellulose conversion rate in Example 9 was 49.0%.

[0105] When cellulose was hydrolyzed using a catalyst without HPW loading, SEM observation revealed that the cellulose hydrolysis product had a nanosheet structure. However, the cellulose conversion rate was lower than that of Example 8.

[0106] Example 10

[0107] The preparation method of the cellulose nanosheets provided in this embodiment is as follows:

[0108] S1. Add 30 g of corncob cellulose to 250 mL of a 90% H₃PO₄ solution, stir at 45°C for 3 h, then refrigerate overnight at 4°C. Simultaneously, refrigerate 850 mL of anhydrous ethanol. Pour the refrigerated anhydrous ethanol into the refrigerated cellulose solution and stir at room temperature for 16 h to precipitate solids. Repeat dilution and deionized water washing until the washing solution reaches a pH of 7. Filter to obtain the pretreated cellulose.

[0109] S2. Add 0.05 g of the catalyst obtained in Example 3 and 0.05 g of the pretreated cellulose obtained in step S1 to 25 mL of deionized water and hydrolyze in a microwave reactor at 180° C. for 15 minutes to obtain a cellulose hydrolyzate.

[0110] S3. Add the cellulose hydrolyzate to dimethyl sulfoxide at a mass ratio of 4:11, stir and swell at room temperature for 7 hours, sonicate at 40 kHz for 1.5 hours, dilute at a ratio of 1:12, sonicate again for 3 minutes, filter, wash, and freeze-dry to obtain cellulose nanosheets.

[0111] Example 11

[0112] The preparation method of the cellulose nanosheets provided in this embodiment is as follows:

[0113] S1. Add 30 g of corncob cellulose to 350 mL of 85% H₃PO₄ solution, stir at 48°C for 2 h, then refrigerate at 4°C overnight. Simultaneously, refrigerate 900 mL of anhydrous ethanol. Pour the refrigerated anhydrous ethanol into the refrigerated cellulose solution and stir at room temperature for 15 h to precipitate solids. Repeat dilution and deionized water washing until the washing solution reaches a pH of 7. Filter to obtain the pretreated cellulose.

[0114] S2. Add 0.05 g of the catalyst obtained in Example 4 and 0.05 g of the pretreated cellulose obtained in step S1 to 18 mL of deionized water and hydrolyze in a microwave reactor at 190° C. for 15 min to obtain a cellulose hydrolyzate.

[0115] S3. Same as Example 8.

[0116] Example 12

[0117] The preparation method of the cellulose nanosheets provided in this embodiment is as follows:

[0118] S1. Add 30 g of corncob cellulose to 220 mL of 85% H₃PO₄ solution, stir at 52°C for 3 h, then refrigerate at 4°C overnight. Simultaneously, refrigerate 820 mL of anhydrous ethanol. Pour the refrigerated anhydrous ethanol into the refrigerated cellulose solution and stir at room temperature for 17 h to precipitate solids. Repeat dilution and deionized water washing until the washing solution reaches a pH of 7. Filter to obtain the pretreated cellulose.

[0119] S2. Add 0.05 g of the catalyst obtained in Example 5 and 0.05 g of the pretreated cellulose obtained in step S1 to 20 mL of deionized water and hydrolyze in a microwave reactor at 180° C. for 5 minutes to obtain a cellulose hydrolyzate.

[0120] S3. Same as Example 8.

[0121] Example 13

[0122] The preparation method of the cellulose nanosheets provided in this embodiment is as follows:

[0123] S1. Add 30 g of corncob cellulose to 280 mL of a 75% zinc chloride aqueous solution, stir at 43°C for 4 hours, then refrigerate at 4°C overnight. Simultaneously, refrigerate 850 mL of a 90 wt% ethanol aqueous solution. Pour the refrigerated ethanol aqueous solution into the refrigerated cellulose solution and stir at room temperature for 18 hours to precipitate solids. Repeat dilution and deionized water washing until the washing solution reaches a pH of 7, and filter to obtain the pretreated cellulose.

[0124] S2. Add 0.05 g of the catalyst obtained in Example 6 and 0.05 g of the pretreated cellulose obtained in step S1 to 18 mL of deionized water and hydrolyze in a microwave reactor at 180° C. for 25 min to obtain a cellulose hydrolyzate.

[0125] S3. Same as Example 10.

[0126] Comparative Example 1

[0127] The synthesis method of the solid catalyst provided in Comparative Example 1 is as follows:

[0128] To 0.08 mol of phenylboronic acid, add 100 mL of 1,2-dichloroethane, 0.08 mol of FeCl3 catalyst, and 0.16 mol of dimethylformal. Stir and react at 80°C for 16 hours. Vacuum filter, wash, and dry to obtain the catalyst. Add phosphotungstic acid (phosphotungstic acid:catalyst mass ratio of 1:3) to the catalyst and stir in acetonitrile at room temperature for 24 hours. Filter, wash, and remove excess acetonitrile, then dry to obtain a solid catalyst.

[0129] The preparation method of the cellulose product provided in Comparative Example 1 is as follows:

[0130] S1. Same as step S1 in embodiment 8.

[0131] S2. Add 0.05 g of the solid catalyst obtained in Comparative Example 1 and 0.05 g of the pretreated cellulose obtained in step S1 to 20 mL of deionized water and hydrolyze in a microwave reactor at 180° C. for 15 minutes to obtain a cellulose hydrolyzate.

[0132] S3. Add the cellulose hydrolyzate to dimethyl sulfoxide at a mass ratio of 4:9, stir and swell at room temperature for 6 hours, sonicate for 1 hour, dilute at a ratio of 1:10, sonicate for another 5 minutes, filter, wash, and freeze-dry to obtain the product.

[0133] After hydrolysis, SEM observation revealed that the cellulose hydrolysis products under the action of the catalyst were not nanosheet structures.

[0134] Comparative Example 2

[0135] The synthesis method of the solid catalyst provided in Comparative Example 2 is as follows:

[0136] To 0.08 mol of pyrrole, add 100 mL of 1,2-dichloroethane, 0.08 mol of FeCl3 catalyst, and 0.16 mol of dimethylformal. Stir and react at 80°C for 16 hours. Vacuum filter, wash, and dry to obtain a dry product. Add phosphotungstic acid (phosphotungstic acid: dry product mass ratio is 1:3) to the dry product. Stir in acetonitrile solution at room temperature for 24 hours. Filter, wash, remove excess acetonitrile, and dry to obtain a solid catalyst.

[0137] The preparation method of the cellulose product provided in Comparative Example 2 is as follows:

[0138] S1. Same as step S1 in embodiment 8.

[0139] S2. Add 0.05 g of the solid catalyst obtained in Comparative Example 2 and 0.05 g of the pretreated cellulose obtained in Step S1 to 20 mL of deionized water and hydrolyze in a microwave reactor at 180° C. for 15 minutes to obtain a cellulose hydrolyzate.

[0140] S3, same as step S3 of Comparative Example 1.

[0141] After hydrolysis, SEM observation revealed that the cellulose hydrolysis products under the action of the catalyst were not nanosheet structures.

[0142] Comparative Example 3

[0143] The preparation method of the cellulose product provided in Comparative Example 3 is as follows:

[0144] S1. Same as step S1 in embodiment 8.

[0145] S2. Add 0.05 g of the pretreated cellulose obtained in step S1 to 20 mL of deionized water and hydrolyze in a microwave reactor at 180° C. for 15 minutes to obtain a cellulose hydrolyzate.

[0146] S3, same as step S3 of Comparative Example 1.

[0147] After hydrolysis, SEM observation revealed that the cellulose hydrolysis products in the absence of catalyst were not nanosheet structures.

[0148] Comparative Example 4

[0149] The preparation method of the cellulose product provided in Comparative Example 4 is as follows:

[0150] S1. Add 30 g of corncob cellulose to 200 mL of water, stir at 40°C for 2 h, then refrigerate at 4°C overnight. Simultaneously, refrigerate 800 mL of anhydrous ethanol. Pour the refrigerated anhydrous ethanol into the refrigerated cellulose solution and stir at room temperature for 14 h to precipitate solids. Repeat dilution and deionized water washing until the washing solution reaches a pH of 7. Filter to obtain the pretreated cellulose.

[0151] S2. Add 0.05 g of the catalyst obtained in Example 1 and 0.05 g of the pretreated cellulose obtained in step S1 to 20 mL of deionized water and hydrolyze in a microwave reactor at 180° C. for 15 min to obtain a cellulose hydrolyzate.

[0152] S3. Add the cellulose hydrolyzate to dimethyl sulfoxide at a mass ratio of 4:9, stir and swell at room temperature for 6 hours, sonicate for 1 hour, dilute at a ratio of 1:10, sonicate for another 5 minutes, filter, wash, and freeze-dry to obtain the product.

[0153] After hydrolysis, SEM observation revealed that the cellulose hydrolysis products under the action of the catalyst were not nanosheet structures.

[0154] The various aspects, embodiments, and features of the present invention should be considered in all respects as illustrative and not limiting, the scope of which is defined solely by the claims. Other embodiments, modifications, and uses will be apparent to those skilled in the art without departing from the spirit and scope of the invention as claimed.

[0155] In the preparation method of the present invention, the order of the steps is not limited to the order listed. Persons skilled in the art will appreciate that variations in the order of the steps are within the scope of the present invention without inventive effort. Furthermore, two or more steps or actions may be performed simultaneously.

[0156] Finally, it should be noted that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit its implementation. Persons skilled in the art may make various modifications, additions, or substitute similar methods for the described specific embodiments. It is not necessary and impossible to provide comprehensive examples of all implementations here. However, obvious variations or modifications arising from the essential spirit of the present invention remain within the scope of protection of the present invention, and interpreting them as any additional limitations would be contrary to the spirit of the present invention.

Claims

1. An acid-base synergistic solid catalyst, characterized in that: Its structural formula is shown below: Here, m is 10 to 2000.

2. A method for synthesizing an acid-base synergistic solid catalyst, characterized in that: The following steps are involved: Phenylboronic acid and pyrrole are mixed, and then a weak / non-polar solvent, a Lewis acid catalyst, and acetal are added. After the reaction, the mixture is filtered and dried.

3. The synthesis method according to claim 2, characterized in that The weak / non-polar solvent is one or more of dichloroethane, dichloromethane, p-dichlorobenzene, chlorobenzene, CS2, carbon tetrachloride, o-dichlorobenzene, m-dichlorobenzene, 1,2,4-trichlorobenzene, o-xylene, p-xylene, nitrobenzene, 1,3,5-tris(bromomethyl)benzene, and 1,4-bis(chloromethyl)benzene; and / or, the Lewis acid is one or more of FeCl3, AlCl3, SnCl4, BF3·OEt3, ZnCl2, TiCl4, SbCl5, GaCl3, InCl3, Sc(OTf)3; and / or, the acetal is one or more of dimethylol formal, diethanol formal, and acetaldehyde dimethyl acetal; and / or, the molar proportions of phenylboric acid, pyrrole, weak / non-polar solvent, Lewis acid catalyst, and acetal are 1 part, 0.8-1.2 parts, 15-50 parts, 1-6 parts, and 1-10 parts, respectively; and / or, mixing phenylboric acid and pyrrole at 10-40° C. for 1-5 h; And / or, the reaction temperature is 80-120° C., and the reaction time is 10-30 h.

4. The synthesis method according to claim 2, characterized in that The synthesis method comprises the following steps: Phenylboric acid and pyrrole are mixed, and then a weak / non-polar solvent, a Lewis acid catalyst, and acetal are added. After the reaction, the mixture is filtered and dried. Then, a heteropoly acid is added to the dried product, and the mixture is stirred in a polar solvent, filtered, and dried to obtain a solid catalyst.

5. The synthesis method according to claim 4, characterized in that The heteropoly acid is one or more of phosphotungstic acid, silicotungstic acid, borotungstic acid, aluminotungstic acid, germanotungstic acid, phosphomolybdic acid, silicomolybdic acid, boromolybdic acid, germanomolybdic acid, arsenotungstic acid, arsenomolybdic acid, cobaltmolybdic acid, aluminomolybdic acid, chromomolybdic acid, hexamolybdic acid, hexatungstic acid, silver-substituted phosphotungstic acid, cesium-substituted phosphotungstic acid, and potassium-substituted phosphotungstic acid; and / or, the mass ratio of the heteropoly acid to the dry matter is 1:0.5-6; and / or, stirring in a polar solvent at 10-40° C. for 15-50 hours.

6. A method for preparing cellulose nanosheets, characterized in that: The following steps are involved: S1. Add cellulose to a pretreatment solution, heat and stir, and then refrigerate at 0-8°C; add the refrigerated cellulose solution to ethanol and / or water at 0-10°C, stir to precipitate solids, dilute and wash the solution multiple times until the pH of the washing solution reaches 7±0.1, and filter to obtain the pretreated cellulose; S2. Mixing the solid catalyst according to any one of claims 2 to 5 with pretreated cellulose, and hydrolyzing the mixture in a microwave environment to obtain stacked cellulose nanosheets. S3. Add the stacked cellulose nanosheets into a polar solvent, stir and swell, and obtain single cellulose nanosheets through ultrasonication, filtration, and drying.

7. The preparation method according to claim 6, characterized in that In step S1, the pretreatment solution is one of H3PO4 solution, zinc chloride solution, a mixed solution of zinc chloride and aluminum chloride, a mixed solution of NaOH and urea, a mixed solution of LiCl and DMAc, and ionic liquids BmimCl, BmimAc, EmimOAc, and AmimCl; And / or, in step S1, the heating and stirring temperature is 40-55° C., and the stirring time is 1-8 hours; And / or, in step S1, stirring is performed to precipitate solid, and the stirring is performed at 10 to 40° C. for 10 to 20 hours.

8. The preparation method according to claim 6, characterized in that In step S2, the mass ratio of the solid catalyst to the pretreated cellulose is 1:0.8-1.5; And / or, in step S2, the microwave hydrolysis temperature is 150-200° C., and the hydrolysis time is 5-40 min.

9. The preparation method according to claim 6, characterized in that In step S3, the mass ratio of cellulose hydrolyzate to polar solvent is 1:1-10; and / or, the stirring and swelling is carried out at 10 to 40° C. for 2 to 10 hours; And / or, the ultrasonic step includes a first ultrasonication, dilution, and a second ultrasonication; the first ultrasonication time is 0.5 to 5 hours, the dilution ratio is 1:5 to 20, and the second ultrasonication time is 1 to 10 minutes.

10. A cellulose nanosheet, characterized in that: It is prepared by the preparation method according to any one of claims 6 to 9; the cellulose nanosheet has a length of 1 to 60 μm and a thickness of 5 to 80 nm.

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