A polycarboxylic acid crosslinking agent, applications, a preparation method, and a nanocellulose adsorbent and a preparation method thereof

Abstract

The application relates to the technical field of nanocellulose, and particularly discloses a polycarboxylic acid crosslinking agent, application, a preparation method, and a nanocellulose adsorbent and a preparation method thereof, wherein the polycarboxylic acid crosslinking agent has the following structure as shown in formula I or formula II: The maximum adsorption capacity of the nanocellulose-based adsorbent prepared by the application for methylene blue is 1152 mg g ‑1 , which is increased by 321.58% compared to the adsorption capacity of TEMPO-derived nanocellulose which is not crosslinked by the polycarboxylic acid crosslinking agent. After 7 cycles, the adsorption capacity for methylene blue can still reach 92% of the maximum adsorption capacity in the first experiment, and the nanocellulose-based adsorbent has good economic benefits and environmental protection benefits.

Description

Technical Field

[0001] This invention relates to the field of nanocellulose technology, and in particular to a synthesis process and application of a polycarboxylic acid crosslinking agent and a nanocellulose-based adsorbent. Specifically, it relates to a polycarboxylic acid crosslinking agent, its application, preparation method, and a nanocellulose adsorbent and its preparation method. Background Technology

[0002] Environmental and resource problems brought about by social progress are perpetual topics in human development, with water pollution being one of the most serious issues today. Heterocyclic basic cationic dyes with azo aromatic structures, such as methylene blue, have attracted attention due to their high staining power, carcinogenicity, teratogenicity, and harm to the skin and eyes. Traditional methylene blue adsorbents are generally limited by technology, exhibiting low adsorption capacity, poor circulation performance, and limited applicability. Furthermore, they are costly and not environmentally friendly enough, making them unsuitable for treating industrial dyeing and printing wastewater. The development of novel and ideal adsorbents is of great significance for environmental resources, human development, and socio-economic progress.

[0003] Nanocellulose is a green, environmentally friendly, and low-cost new material. It not only has the characteristics of natural cellulose such as being renewable and biodegradable, but also has the advantages of large specific surface area, high hydrophilicity, high strength, and high Young's modulus. Ideal nanocellulose-based adsorbents can be obtained through physical / chemical modification.

[0004] Using polycarboxylic acids containing three or more carboxyl groups as crosslinking agents to introduce a large number of carboxyl groups into nanocellulose through chemical crosslinking, thereby imparting a strong negative charge to the surface and improving the adsorption performance of nanocellulose, is a common and effective strategy. Citric acid and butanetetracarboxylic acid are the most commonly used polycarboxylic acid crosslinking agents. Although they have relatively good esterification rates, they also have their own drawbacks.

[0005] For example, existing technologies disclose that the α-hydroxyl group of citric acid hinders the rate-controlling step of esterification crosslinking, namely the formation of the intermediate five-membered cyclic anhydride. Furthermore, once the carboxyl group on the central carbon of the citric acid molecule has formed the first ester bond, it cannot undergo further esterification with cellulose.

[0006] For example, existing technical literature discloses that cross-linked butanetetracarboxylic acid is constrained by the cellulose chain, exhibiting considerable steric hindrance, which makes it difficult to form cyclic anhydride intermediates in the rate-controlling step. These aforementioned defects severely affect the cross-linking rate and efficiency of nanocellulose, consequently affecting the carboxyl group content grafted onto the nanocellulose and thus the adsorption performance of the adsorbent.

[0007] Therefore, it is desirable to obtain a novel polycarboxylic acid crosslinking agent that can better improve the crosslinking rate and crosslinking efficiency of nanocellulose. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a polycarboxylic acid crosslinking agent, its application, preparation method, and nanocellulose adsorbent and its preparation method.

[0009] To achieve the above objectives, the present invention proposes a polycarboxylic acid crosslinking agent, wherein the structure of the polycarboxylic acid crosslinking agent is as shown in Formula I or Formula II:

[0010]

[0011] In the technical solution described in this invention, the inventors address the problem that when the two most commonly used polycarboxylic acid crosslinking agents (citric acid and butanetetracarboxylic acid) are esterified with cellulose alone, steric hindrance, α-hydroxyl groups, and the formation of a first ester bond by the carboxyl group on the intermediate carbon hinder the rate-controlling step, namely the formation of the intermediate cyclic anhydride, thus severely affecting the crosslinking rate and efficiency of nanocellulose. This invention utilizes the reaction of pyromellitic dianhydride with citric acid to prepare a novel polycarboxylic acid crosslinking agent. The α-hydroxyl group of citric acid undergoes an esterification reaction with the carboxyl group of pyromellitic dianhydride, transforming into an ester bond, thus avoiding the hindrance of the rate-controlling step by the α-hydroxyl group. While the potential formation of a first ester bond by the carboxyl group on the central carbon of citric acid cannot be completely avoided, this invention compensates for this by increasing the carboxyl group content on the crosslinking agent. Pyromellitic dianhydride and cellulose directly form an ester crosslinking structure through the Fischer esterification reaction without the need for the formation of a cyclic anhydride intermediate. Replacing butanetetracarboxylic acid with pyromellitic dianhydride solves the problem of steric hindrance affecting the rate-controlling step. Among them, pyromellitic dianhydride and butanetetracarboxylic acid have similar structures and the same carboxyl group content. The polycarboxylic acid crosslinking agent prepared in this invention can significantly improve the crosslinking rate and crosslinking efficiency of nanocellulose compared with the use of citric acid and butanetetracarboxylic acid alone.

[0012] Secondly, the present invention also proposes the application of the above-mentioned polycarboxylic acid crosslinking agent in adsorbents, plastic products, rubber products, fiber products, coatings or adhesives.

[0013] Thirdly, the present invention also provides a method for preparing the above-mentioned polycarboxylic acid crosslinking agent, the method comprising the following steps:

[0014] Two different organic acids containing carboxylic acids are mixed and subjected to esterification to obtain the polycarboxylic acid crosslinking agent shown in Formula I, which is then hydrolyzed to obtain the polycarboxylic acid crosslinking agent shown in Formula II.

[0015] The esterification reaction temperature is 68–70℃, and the reaction time is 24–26 h.

[0016] Preferably, the esterification reaction is specifically performed by adding the two different organic acids to a solvent and reacting them under the protection of nitrogen or an inert gas.

[0017] Preferably, the esterification reaction further includes a post-treatment process, which includes one or more of vacuum distillation, recrystallization, filtration, or vacuum drying.

[0018] Preferably, the organic acids are pyromellitic dianhydride and citric acid.

[0019] In some preferred embodiments, the polycarboxylic acid crosslinking agent can be prepared using the following methods:

[0020] Pyromellitic dianhydride and citric acid in a molar ratio of 1:1 are added to a solvent (tetrahydrofuran) and reacted under nitrogen or inert gas protection. After post-treatment (reduced pressure distillation, recrystallization, filtration, and vacuum drying, wherein the recrystallization solution is methanol, the vacuum drying temperature is 40℃, and the drying time is 12h), a polycarboxylic acid crosslinking agent is obtained.

[0021] In some further preferred embodiments, the molar volume ratio of pyromellitic dianhydride to solvent is 0.1 mol: 100 mL.

[0022] Fourthly, the present invention also proposes a nanocellulose adsorbent, which is prepared using the above-mentioned polycarboxylic acid crosslinking agent.

[0023] Fifthly, the present invention also provides a method for preparing the above-mentioned nanocellulose adsorbent, the preparation method comprising the following steps:

[0024] The above-mentioned polycarboxylic acid crosslinking agent was mixed with nanocellulose, catalyst and water, freeze-dried under vacuum and then thermosetting to obtain nanocellulose adsorbent.

[0025] Preferably, the nanocellulose is TEMPO-derived nanocellulose;

[0026] And / or, the catalyst is sodium hypophosphite.

[0027] Preferably, the mass ratio of the polycarboxylic acid crosslinking agent, nanocellulose, and catalyst is (1-6):10:(0.5-3); the temperature of the thermosetting reaction is 140-190°C, and the time is 1-6 min.

[0028] Preferably, the thermosetting reaction is followed by a post-treatment, which includes washing and / or drying.

[0029] In some implementations, the washing process uses clean water, involves washing 2 to 3 times, and the drying temperature is 80°C for 12 hours.

[0030] Preferably, when mixing the polycarboxylic acid crosslinking agent, nanocellulose, catalyst and water, magnetic stirring can be used for a stirring time of 6 hours.

[0031] It should be noted that there are many catalysts that can catalyze the esterification reaction between nanocellulose and polycarboxylic acid crosslinking agents. Sodium hypophosphite used in this invention is one of the most commonly used and efficient catalysts. It can also act as an effective reducing agent to prevent nanocellulose from being oxidized at high temperatures. Therefore, sodium hypophosphite is preferred as the catalyst in this invention.

[0032] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0033] (1) The present invention utilizes the reaction of pyromellitic dianhydride and citric acid to prepare a novel polycarboxylic acid crosslinking agent, which can significantly improve the crosslinking rate and crosslinking efficiency of nanocellulose compared with the two most commonly used polycarboxylic acids (citric acid and butanetetracarboxylic acid).

[0034] (2) On the one hand, the present invention introduces a large number of carboxyl groups into the cellulose chain through a polycarboxylic acid crosslinking agent, giving it strong electronegativity on the surface, which significantly improves the adsorption performance of the nanocellulose-based adsorbent for methylene blue; on the other hand, the flexible crosslinking bridges formed between the cellulose chains by the polycarboxylic acid crosslinking agent and the carboxyl functional groups lead to more hydrogen bond space, making the structure of nanocellulose more stable, thereby enhancing the cycle performance of the nanocellulose-based adsorbent.

[0035] (3) The maximum adsorption capacity of the nanocellulose-based adsorbent prepared in this invention for methylene blue is 1152 mg g. -1 The adsorption capacity of TEMPO-derived nanocellulose was increased by 321.58% compared to that of TEMPO-derived nanocellulose without cross-linking by polycarboxylic acid cross-linking agent. After 7 cycles, the adsorption capacity for methylene blue could still reach 92% of the maximum adsorption capacity in the first experiment, which has good economic and environmental benefits. Attached Figure Description

[0036] Figure 1 This is the synthetic route for polycarboxylic acid crosslinking agents;

[0037] Figure 2 This is the synthetic route for nanocellulose-based adsorbents;

[0038] Figure 3 This is a scanning electron microscope image of the polycarboxylic acid crosslinking agent in Example 1;

[0039] Figure 4 This is a scanning electron microscope image of the nanocellulose-based adsorbent in Example 1;

[0040] Figure 5 These are the Fourier transform infrared spectroscopy test results of Example 1;

[0041] Figure 6 This is a graph showing the change in the adsorption capacity of the nanocellulose-based adsorbent for methylene blue as a function of pH in Example 1.

[0042] Figure 7 This is a graph showing the change in the adsorption capacity of the nanocellulose-based adsorbent for methylene blue as a function of the initial concentration in Example 1.

[0043] Figure 8 This is a graph showing the change in the adsorption capacity of the nanocellulose-based adsorbent for methylene blue over adsorption time in Example 1.

[0044] Figure 9 This is a graph showing the change in the adsorption capacity of the nanocellulose-based adsorbent for methylene blue as a function of the number of cycles in Example 1. Detailed Implementation

[0045] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0046] The present invention will be further described below with reference to the embodiments:

[0047] Figure 1 This is the synthetic route for polycarboxylic acid crosslinking agents;

[0048] Figure 2 This is the synthetic route for nanocellulose-based adsorbents.

[0049] Combination Figure 1 and Figure 2 As can be seen, in the technical solution described in this invention, a novel polycarboxylic acid crosslinking agent is prepared by esterification using a mixed polycarboxylic acid system of pyromellitic dianhydride and citric acid, with tetrahydrofuran as the solvent. Based on this, a nanocellulose-based adsorbent is prepared by esterification crosslinking of the polycarboxylic acid crosslinking agent with TEMPO-derived nanocellulose using sodium hypophosphite as a catalyst.

[0050] In some specific embodiments, the actual process route is as follows: Pyromellitic dianhydride and citric acid in a molar ratio of 1:1 are added to a solvent (tetrahydrofuran), and the reaction is carried out under nitrogen or inert gas protection. After post-treatment (reduced pressure distillation, recrystallization, filtration, and vacuum drying, wherein the recrystallization solution is methanol, the vacuum drying temperature is 40°C, and the drying time is 12 h), a polycarboxylic acid crosslinking agent is obtained. The mass ratio of the polycarboxylic acid crosslinking agent, nanocellulose, and catalyst is 1–6:10:0.5–3; the thermosetting reaction temperature is 140–190°C, and the time is 1–6 min. After post-treatment (washing and drying, wherein the washing solution is water, the washing frequency is 3 times, the drying temperature is 80°C, and the drying time is 12 h), a nanocellulose-based adsorbent is obtained. The catalyst is used to catalyze the esterification reaction between the polycarboxylic acid crosslinking agent and nanocellulose during the thermosetting reaction.

[0051] Example 1

[0052] A method for preparing nanocellulose-based adsorbents using polycarboxylic acid crosslinking agents, the specific steps of which are as follows:

[0053] (1) Preparation of raw materials;

[0054] Pyromellitic dianhydride;

[0055] Citric acid;

[0056] Solvent: Tetrahydrofuran;

[0057] Protective gas: nitrogen or inert gas;

[0058] Nanocellulose: TEMPO-derived nanocellulose, manufactured by Beijing Shansheng Technology Co., Ltd.

[0059] Catalyst: Sodium hypophosphite;

[0060] water.

[0061] (2) Preparation of polycarboxylic acid crosslinking agents;

[0062] A pyromellitic dianhydride and citric acid in a molar ratio of 1:1 were added to a solvent and reacted under a protective gas atmosphere. After post-treatment, a polycarboxylic acid crosslinking agent was obtained. The molar volume ratio of pyromellitic dianhydride to solvent was 0.1 mol: 100 mL, the reaction temperature was 68 °C, and the reaction time was 24 h.

[0063] The post-processing includes the following steps: the product is completely precipitated using a rotary evaporator, and the precipitated white solid is recrystallized twice with methanol. Finally, it is vacuum dried at 40°C for 12 hours to obtain the polycarboxylic acid crosslinking agent.

[0064] Based on literature and experimental results, the final structural formula of the prepared polycarboxylic acid crosslinking agent is as follows:

[0065]

[0066] (3) Preparation of nanocellulose-based adsorbents;

[0067] After mixing the polycarboxylic acid crosslinking agent, nanocellulose, and catalyst in water at a mass ratio of 2:10:1, the mixture was first freeze-dried and then thermo-cured at 170°C for 3 minutes. After post-treatment, the nanocellulose-based adsorbent was obtained.

[0068] The polycarboxylic acid crosslinking agent and nanocellulose adsorbent obtained in Example 1 can be found in [reference needed]. Figures 3-5 ,in, Figure 3 This is a scanning electron microscope image of the polycarboxylic acid crosslinking agent in Example 1; Figure 4 This is a scanning electron microscope image of the nanocellulose-based adsorbent in Example 1.

[0069] Figure 5 The results are Fourier transform infrared spectroscopy test results of Example 1.

[0070] like Figure 5 As shown, citric acid (CA) is a tricarboxylic acid. Due to hydrogen bonding, the carboxyl group exists in a bimolecular associated form, thus exhibiting stretching vibration absorption peaks at 1744 cm⁻¹ and 1711 cm⁻¹. PMDA (pyromellitic dianhydride) has a unique molecular structure, with absorption peaks located at 831 cm⁻¹. -1 3061cm -1 1302cm -1 1733cm -1 The absorption peaks at 1770 cm⁻¹ represent the CH plane angular vibration, CH plane angular deformation, axial bending vibration of the CO bond in aromatic carboxylic acids, and stretching vibration of the carbonyl group, respectively [174, 175]. -1 and 1857cm -1 These represent the symmetric and asymmetric stretching vibration peaks of the active anhydride intermediate on the benzene ring, respectively. In the spectrum of TPTTA (i.e., the polycarboxylic acid crosslinking agent in this case), the absorption peak representing the anhydride on PMDA completely disappears, and the peak at 1633 cm⁻¹... -1 A new absorption peak appeared at 1727 cm⁻¹, representing the stretching vibration peak of C=O on the aromatic ester bond. The absorption peaks belonging to the carboxyl groups of CA and PMDA both showed a certain degree of blue shift to lower wavelengths, proving that an esterification reaction occurred between CA and PMDA. The infrared spectra of CNF / TBPTA and CNF (i.e., nanocellulose) are very similar, the difference being at 1727 cm⁻¹. -1 The position represents the stretching vibration of the carbonyl group on the ester bond, which proves that TPTTA and CNF underwent an esterification crosslinking reaction. Figure 5As shown, another important feature of the spectrum is that CNF / TBPTA is relative to CNF at 3480 cm⁻¹. -1 The enhancement of the absorption band is due to the increase of hydroxyl groups on the carboxylic acid in the cross-linked structure.

[0071] The adsorption and cycling performance of the nanocellulose adsorbent obtained in Example 1 were tested, and the test results are shown in [Figure 1]. Figures 6-9 .

[0072] Figure 6 This is a graph showing the adsorption capacity of the nanocellulose-based adsorbent for methylene blue as a function of pH in this embodiment, wherein the initial concentration of the methylene blue solution during adsorption is 200 mg / L. -1 The temperature was 310K, the adsorption time was 24h, and the number of cycles was 0.

[0073] from Figure 6 As can be seen, although the adsorption performance of the nanocellulose-based adsorbent is highly dependent on the pH value of the solution, it can still maintain a relatively ideal adsorption capacity even in a strongly acidic environment with pH=2. This indicates that the application conditions of the nanocellulose-based adsorbent are not harsh and it can be applied to relatively complex wastewater environments. Overall, the overall adsorption capacity increases with increasing pH value. When pH>7, the removal rate of methylene blue by the nanocellulose-based adsorbent can remain above 90%, and when pH=11, the removal rate reaches as high as 99.87%, showing the best adsorption performance for MB.

[0074] Figure 7 This is a graph showing the change in the adsorption capacity of the nanocellulose-based adsorbent for methylene blue as a function of the initial concentration of the methylene blue solution in this embodiment. The adsorption temperature was 310K, the pH value was 11, the adsorption time was 24h, and the number of cycles was 0.

[0075] like Figure 7 As shown, the overall trend indicates that the adsorption capacity increases with increasing initial methylene blue concentration. When the initial concentration is below 200 mg / L... -1 At that time, the removal rate of methylene blue remained above 99.5%, almost completely removed. When the initial concentration was increased to 600 mg / L... -1 The adsorption capacity is as high as 1087.8 mg g. -1 The removal rate remains as high as 90.0%. Even at 1000 mg / L... -1 Even in high-concentration solutions, the removal rate can reach 58.16%. This was verified using the Langmuir adsorption isotherm model. Figure 6 By fitting the data, the theoretical adsorption capacity can reach up to 1152 mg g. -1 .

[0076] Figure 8This is a graph showing the change in the adsorption capacity of the nanocellulose-based adsorbent for methylene blue over adsorption time in this embodiment. The initial concentration of the methylene blue solution during adsorption was 200 mg / L. -1 The temperature was 310K, the pH was 11, and the number of cycles was 0.

[0077] like Figure 8 As shown, a very rapid adsorption phase can be observed in the first 120 minutes due to the strong negative charge on the adsorbent surface, with the adsorption capacity increasing almost linearly, resulting in the adsorption of 98.32% of methylene blue. Thereafter, the adsorption rate gradually slows down, and an adsorption dynamic equilibrium is established at 720 minutes.

[0078] Figure 9 This is a graph showing the change in the adsorption capacity of the nanocellulose-based adsorbent for methylene blue as a function of the number of cycles in this embodiment. The initial concentration of the methylene blue solution during adsorption was 200 mg / L. -1 The pH value was 11, the temperature was 310K, and the single adsorption time was 24h.

[0079] like Figure 9 As shown, after 7 cycles, the adsorption capacity did not decrease significantly and could still reach 92% of the maximum adsorption capacity in the first experiment. The experimental data demonstrate that the nanocellulose-based adsorbent has excellent regeneration performance, which can reduce production costs and save resources.

[0080] Comparative Example 1

[0081] A method for preparing a nanocellulose-based adsorbent, the specific steps of which are as follows:

[0082] (1) Preparation of raw materials;

[0083] Nanocellulose: Same as in Example 1;

[0084] Catalyst: Sodium hypophosphite;

[0085] water;

[0086] (2) Preparation of nanocellulose-based adsorbents;

[0087] Nanocellulose and catalyst in a mass ratio of 10:1 were added to water and mixed evenly. The mixture was first freeze-dried and then thermo-cured at 170°C for 3 minutes. After post-treatment, nanocellulose-based adsorbent was obtained.

[0088] The prepared nano-cellulose-based adsorbent was used to adsorb methylene blue at a concentration of 500 mg / L. -1 The pH value was 11, the temperature was 310K, the adsorption time was 24h, and the number of cycles was 0.

[0089] Compared with Comparative Example 1, the adsorption capacity of the nanocellulose-based adsorbent prepared in Example 1 for methylene blue is 321.58% higher than that in Comparative Example 1. This is because the polycarboxylic acid crosslinking agent introduces a large number of carboxyl functional groups into cellulose, thereby endowing the surface with a strong negative charge.

[0090] Comparative Example 2

[0091] A method for preparing a nanocellulose-based adsorbent, the specific steps of which are as follows:

[0092] (1) Preparation of raw materials;

[0093] Citric acid;

[0094] Nanocellulose: Same as in Example 1;

[0095] Catalyst: Sodium hypophosphite;

[0096] water;

[0097] (2) Preparation of nanocellulose-based adsorbents;

[0098] Citric acid, nanocellulose, and catalyst in a mass ratio of 6:10:1 were added to water and mixed evenly. The mixture was first freeze-dried and then thermo-cured at 170°C for 3 minutes. After post-treatment, nanocellulose-based adsorbent was obtained.

[0099] The prepared nano-cellulose-based adsorbent was used to adsorb methylene blue at a concentration of 500 mg / L. -1 The pH value was 11, the temperature was 310K, the adsorption time was 24h, and the number of cycles was 0.

[0100] Compared with Example 1, the adsorption capacity of the adsorbent prepared in Comparative Example 2 decreased by 24.61%. This is because the excessive introduction of carboxyl groups caused the pH of the solution to be too low, which led to the degradation of the cellulose structure.

[0101] Comparative Example 3

[0102] A method for preparing a nanocellulose-based adsorbent, the specific steps of which are as follows:

[0103] (1) Preparation of raw materials;

[0104] Butanetetracarboxylic acid;

[0105] Nanocellulose: Same as in Example 1;

[0106] Catalyst: Sodium hypophosphite;

[0107] water;

[0108] (2) Preparation of nanocellulose-based adsorbents;

[0109] Butanetetracarboxylic acid, nanocellulose, and catalyst in a mass ratio of 2:10:2.5 were added to water and mixed evenly. The mixture was first freeze-dried and then thermo-cured at 170°C for 3 minutes. After post-treatment, nanocellulose-based adsorbent was obtained. The catalyst was used to catalyze the esterification reaction between the polycarboxylic acid crosslinking agent and nanocellulose during the thermo-curing reaction.

[0110] The prepared nanocellulose-based adsorbent was used to adsorb methylene blue.

[0111] Compared with Example 1, the adsorption capacity of the adsorbent prepared in Comparative Example 3 decreased by 4.98%. This is because there is also a certain degree of competitive reaction between the sodium hypophosphite catalyst and the acid anhydride. Excess sodium hypophosphite catalyst forms a stable acylphosphonate, which may limit the esterification reaction between cellulose and the crosslinking agent.

[0112] Example 2

[0113] A method for preparing nanocellulose-based adsorbents using a polycarboxylic acid crosslinking agent is basically the same as in Example 1, except that the mass ratio of the polycarboxylic acid crosslinking agent, nanocellulose, and catalyst in step (3) is 1:10:1.

[0114] An application of a nanocellulose-based adsorbent was described, in which the nanocellulose-based adsorbent prepared in this embodiment was used to adsorb methylene blue. The initial concentration of the methylene blue solution during adsorption was 500 mg / L. -1 With a pH of 11, a temperature of 310 K, an adsorption time of 24 h, and 0 cycles, the final adsorption capacity of the nanocellulose-based adsorbent was 940.21 mg g. -1 .

[0115] Example 3

[0116] A method for preparing nanocellulose-based adsorbents using a polycarboxylic acid crosslinking agent is basically the same as in Example 1, except that the mass ratio of the polycarboxylic acid crosslinking agent, nanocellulose, and catalyst in step (3) is 3:10:1.

[0117] An application of a nanocellulose-based adsorbent was described, in which the nanocellulose-based adsorbent prepared in this embodiment was used to adsorb methylene blue. The initial concentration of the methylene blue solution during adsorption was 500 mg / L. -1 With a pH of 11, a temperature of 310 K, an adsorption time of 24 h, and 0 cycles, the final adsorption capacity of the nanocellulose-based adsorbent was 835.46 mg g. -1 .

[0118] Example 4

[0119] A method for preparing nanocellulose-based adsorbents using a polycarboxylic acid crosslinking agent is basically the same as in Example 1, except that the mass ratio of the polycarboxylic acid crosslinking agent, nanocellulose, and catalyst in step (3) is 2:10:0.5.

[0120] An application of a nanocellulose-based adsorbent was described, in which the nanocellulose-based adsorbent prepared in this embodiment was used to adsorb methylene blue. The initial concentration of the methylene blue solution during adsorption was 500 mg / L. -1 With a pH of 11, a temperature of 310 K, an adsorption time of 24 h, and 0 cycles, the final adsorption capacity of the nanocellulose-based adsorbent was 928.52 mg g. -1 .

[0121] Example 5

[0122] A method for preparing nanocellulose-based adsorbents using a polycarboxylic acid crosslinking agent is basically the same as in Example 1, except that the mass ratio of the polycarboxylic acid crosslinking agent, nanocellulose, and catalyst in step (3) is 2:10:2.

[0123] An application of a nanocellulose-based adsorbent was described, in which the nanocellulose-based adsorbent prepared in this embodiment was used to adsorb methylene blue. The initial concentration of the methylene blue solution during adsorption was 500 mg / L. -1 With a pH of 11, a temperature of 310 K, an adsorption time of 24 h, and 0 cycles, the final adsorption capacity of the nanocellulose-based adsorbent was 935.76 mg g. -1 .

[0124] Example 6

[0125] A method for preparing nanocellulose-based adsorbents using a polycarboxylic acid crosslinking agent is basically the same as in Example 1, except that the mass ratio of the polycarboxylic acid crosslinking agent, nanocellulose, and catalyst in step (3) is 2:10:3.

[0126] An application of a nanocellulose-based adsorbent was described, in which the nanocellulose-based adsorbent prepared in this embodiment was used to adsorb methylene blue. The initial concentration of the methylene blue solution during adsorption was 500 mg / L. -1 With a pH of 11, a temperature of 310 K, an adsorption time of 24 h, and 0 cycles, the final adsorption capacity of the nanocellulose-based adsorbent was 905.69 mg g. -1 .

[0127] Example 7

[0128] A method for preparing nanocellulose-based adsorbents using a polycarboxylic acid crosslinking agent is basically the same as in Example 1, except that the curing temperature in step (3) is 140°C.

[0129] An application of a nanocellulose-based adsorbent was described, in which the nanocellulose-based adsorbent prepared in this embodiment was used to adsorb methylene blue. The initial concentration of the methylene blue solution during adsorption was 500 mg / L. -1 With a pH of 11, a temperature of 310 K, an adsorption time of 24 h, and 0 cycles, the final adsorption capacity of the nanocellulose-based adsorbent was 911.89 mg g. -1 .

[0130] Example 8

[0131] A method for preparing nanocellulose-based adsorbents using a polycarboxylic acid crosslinking agent is basically the same as in Example 1, except that the curing temperature in step (3) is 190°C.

[0132] An application of a nanocellulose-based adsorbent was described, in which the nanocellulose-based adsorbent prepared in this embodiment was used to adsorb methylene blue. The initial concentration of the methylene blue solution during adsorption was 500 mg / L. -1 With a pH of 11, a temperature of 310 K, an adsorption time of 24 h, and 0 cycles, the final adsorption capacity of the nanocellulose-based adsorbent was 833.75 mg g. -1 .

[0133] Example 9

[0134] A method for preparing nanocellulose-based adsorbents using a polycarboxylic acid crosslinking agent is basically the same as in Example 1, except that the curing time in step (3) is 1 min.

[0135] An application of a nanocellulose-based adsorbent was described, in which the nanocellulose-based adsorbent prepared in this embodiment was used to adsorb methylene blue. The initial concentration of the methylene blue solution during adsorption was 500 mg / L. -1 With a pH of 11, a temperature of 310 K, an adsorption time of 24 h, and 0 cycles, the final adsorption capacity of the nanocellulose-based adsorbent was 857.61 mg g. -1 .

[0136] Example 10

[0137] A method for preparing nanocellulose-based adsorbents using a polycarboxylic acid crosslinking agent is basically the same as in Example 1, except that the curing time in step (3) is 6 min.

[0138] An application of a nanocellulose-based adsorbent was described, in which the nanocellulose-based adsorbent prepared in this embodiment was used to adsorb methylene blue. The initial concentration of the methylene blue solution during adsorption was 500 mg / L. -1 With a pH of 11, a temperature of 310 K, an adsorption time of 24 h, and 0 cycles, the final adsorption capacity of the nanocellulose-based adsorbent was 980.11 mg g.-1 .

[0139] Examples 1-10 illustrate the adsorption performance testing of nanocellulose-based adsorbents:

[0140] a. Prepare an aqueous solution of methylene blue (manufacturer: Maclean Biotech, Inc., CAS No. 61-73-4) with a concentration of 50–1000 mg / g. -1 pH = 2-11.

[0141] b. Adsorption performance test:

[0142] Add 10 mg of the nanocellulose-based adsorbent to 20 mL of methylene blue solution, and shake in a constant-temperature shaker at 200 rpm and 310 K. Take the supernatant and measure the absorbance at 665 nm using a UV spectrophotometer.

[0143] c. Cyclic performance testing:

[0144] The nanocellulose-based adsorbent containing methylene blue was immersed in a 1 mol L⁻¹ HCl solution and shaken at room temperature for 24 h to allow complete desorption. The desorbed nanocellulose-based adsorbent was then rinsed with deionized water until no MB residue remained on the surface and vacuum dried at 60 °C. The regenerated nanocellulose-based adsorbent was then placed back into the same MB solution. This process constituted one cycle, and a total of seven cycles were performed.

[0145] Calculation of the adsorption capacity of nanocellulose-based adsorbents:

[0146] All adsorption data were repeated three times. The adsorption capacity of the nanocellulose-based adsorbent (see Equation 1) and the removal rate of methylene blue (see Equation 2) were calculated using the following formulas:

[0147]

[0148] Among them, Q e (mg g -1 ) represents the adsorption capacity at equilibrium, r (%) represents the removal rate of methylene blue at equilibrium, and C0 (mg / L) represents the adsorption capacity at equilibrium. -1 Let C be the initial concentration of the methylene blue solution. The concentration of methylene blue at which adsorption reaches dynamic equilibrium is denoted by Cm. e (mg L -1 () indicates that V(L) is the volume of the methylene blue solution and m(g) is the mass of the nanocellulose-based adsorbent. The final results are shown in Table 1.

[0149] Table 1.

[0150]

[0151] As shown in Table 1, the nanocellulose-based adsorbent provided by this invention can be effectively used for the adsorption of methylene blue in wastewater. Under appropriate preparation conditions, the adsorption capacity and removal rate of the nanocellulose-based adsorbent for methylene blue can be improved. The nanocellulose-based adsorbent obtained in Example 1 has the best adsorption performance, with an adsorption capacity increased by 321.58% compared to the nanocellulose obtained in Comparative Example 1 that was not cross-linked by the polycarboxylic acid cross-linking agent.

[0152] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

[0153] It should be noted that the scope of protection of the prior art in this invention is not limited to the embodiments given in this application. All prior art that does not contradict the solution of this invention, including but not limited to prior patent documents, prior publications, prior public uses, etc., can be included in the scope of protection of this invention.

[0154] Furthermore, the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0155] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A polycarboxylic acid crosslinking agent, characterized in that, The structure of the polycarboxylic acid crosslinking agent is shown in Formula II below: 。 2. The application of the polycarboxylic acid crosslinking agent as described in claim 1 in the preparation of adsorbents or adhesives.

3. The application of the polycarboxylic acid crosslinking agent as described in claim 1 in the preparation of plastic products, rubber products, fiber products or coatings.

4. A method for preparing the polycarboxylic acid crosslinking agent as described in claim 1, characterized in that, The preparation method includes the following steps: Pyromellitic dianhydride and citric acid are mixed and subjected to an esterification reaction to obtain the polycarboxylic acid crosslinking agent shown in Formula I, which is then hydrolyzed to obtain the polycarboxylic acid crosslinking agent shown in Formula II. The esterification reaction temperature is 68–70℃, and the reaction time is 24–26 h.

5. The preparation method according to claim 4, characterized in that, The esterification reaction is specifically performed by adding pyromellitic dianhydride and citric acid to a solvent and reacting under nitrogen or inert gas protection.

6. The preparation method according to claim 4, characterized in that, The esterification reaction is followed by a post-treatment process, which includes one or more of the following: vacuum distillation, recrystallization, filtration, or vacuum drying.

7. A nanocellulose adsorbent, characterized in that, The nanocellulose adsorbent was prepared using the polycarboxylic acid crosslinking agent as described in claim 1.

8. A method for preparing the nanocellulose adsorbent as described in claim 7, characterized in that, The preparation method includes the following steps: The polycarboxylic acid crosslinking agent as described in claim 1 is mixed with nanocellulose, a catalyst, and water, then freeze-dried under vacuum and subjected to a thermosetting reaction to obtain a nanocellulose adsorbent.

9. The preparation method according to claim 8, characterized in that, The nanocellulose is TEMPO-derived nanocellulose; And / or, the catalyst is sodium hypophosphite.

10. The preparation method according to claim 8, characterized in that, The mass ratio of the polycarboxylic acid crosslinking agent, nanocellulose, and catalyst is (1-6):10:(0.5-3); the temperature of the thermosetting reaction is 140-190℃, and the time is 1-6 min.

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