Polycarboxylic acid cross-linking agent, application, preparation method, nano-cellulose adsorbent and preparation method of nano-cellulose adsorbent

Abstract

The invention relates to the technical field of nano-crystalline cellulose, and particularly discloses a polycarboxylic acid cross-linking agent, application and a preparation method thereof, and a nano-crystalline cellulose adsorbent and a preparation method thereof, the structure of the polycarboxylic acid cross-linking agent is shown as a formula I or a formula II: # imgabs 0 # and # imgabs 1 #, the maximum adsorption capacity of the prepared nano-crystalline cellulose based adsorbent to methylene blue is 1152 mg g <-1 >, and the adsorption capacity of the nano-crystalline cellulose based adsorbent to methylene blue is 1152 mg g <-1 >. Compared with the TEMPO-derived nanocellulose which is not crosslinked by the polycarboxylic acid crosslinking agent, the adsorption capacity of the TEMPO-derived nanocellulose is improved by 321.58%. After seven cycles, the methylene blue adsorption capacity can still reach 92% of the maximum adsorption capacity of the first experiment, and the method has good economic benefits and environmental protection benefits.

Description

Technical Field

[0001] The present 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, and specifically to a polycarboxylic acid crosslinking agent, application, preparation method, and a nanocellulose adsorbent and a preparation method thereof. Background Art

[0002] A series of environmental and resource problems brought about by social progress are eternal topics for human development, among which water pollution is one of the most serious problems at present. Heterocyclic alkaline cationic dyes with azo aromatic structure, represented by methylene blue, have attracted people's attention due to their high dyeing degree, carcinogenicity, teratogenicity and damage to skin and eyes. Traditional methylene blue adsorbents are generally limited by technology and other reasons. The adsorption capacity is low, the cycle performance is poor, the scope of application is limited, and the cost is high and the environment is not environmentally friendly, so it is difficult to be used in the treatment of actual industrial printing and dyeing wastewater. The research and development of new ideal adsorbents is of great significance to environmental resources, human development, social economy, etc.

[0003] Nanocellulose is a green, environmentally friendly, low-cost new material. It not only has the characteristics of natural cellulose such as renewability and biodegradability, but also has the advantages of large specific surface area, high hydrophilicity, high strength, and high Young's modulus. An ideal nanocellulose-based adsorbent 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, giving it a strong negative charge on the surface and thus improving the adsorption performance of nanocellulose is a common and effective strategy. Citric acid and butane tetracarboxylic acid are the most commonly used polycarboxylic acid crosslinking agents. Although they have relatively good esterification rates, they also have their own shortcomings.

[0005] For example, the prior art discloses that the α-hydroxyl group of citric acid will hinder the esterification cross-linking rate-determining step, namely the formation of the intermediate five-membered cyclic anhydride. In addition, when the carboxyl group on the central carbon of the citric acid molecule has formed the first ester bond, it cannot further esterify with cellulose.

[0006] Another example: the prior art documents disclose that cross-linked butane tetracarboxylic acid is restricted by cellulose chains and has considerable steric hindrance, which makes it difficult to generate cyclic anhydride intermediates in the rate-controlling step. The above-mentioned various defects seriously affect their cross-linking rate and efficiency of nanocellulose, which in turn affects the content of carboxyl groups grafted on nanocellulose and the adsorption performance of the adsorbent.

[0007] Based on this, it is expected to obtain a new type of polycarboxylic acid cross-linking agent, which can better improve the cross-linking rate and efficiency of nanocellulose. Summary of the invention

[0008] In view of the defects in the prior art, the present invention aims to provide a polycarboxylic acid cross-linking agent, its application, its preparation method, and a nanocellulose adsorbent and its preparation method.

[0009] In order to achieve the above object, the present invention provides a polycarboxylic acid crosslinking agent, wherein the polycarboxylic acid crosslinking agent structure is as shown in the following formula I or formula II:

[0010]

[0011] In the technical solution described in the present invention, the inventor of this case actually solves the problem that the two most commonly used polycarboxylic acid crosslinking agents (citric acid and butane tetracarboxylic acid) hinder the formation of the intermediate cyclic anhydride, i.e., the rate-controlling step, when esterified with cellulose alone, due to factors such as steric hindrance, α-hydroxyl, and the formation of the first ester bond by the carboxyl group on the intermediate carbon, thus seriously affecting the crosslinking rate and efficiency of nanocellulose. The present invention uses pyromellitic anhydride to react with citric acid to prepare a new type of polycarboxylic acid crosslinking agent. The α-hydroxyl of citric acid and the carboxyl group of pyromellitic anhydride undergo esterification reaction and are converted into an ester bond, thereby avoiding the α-hydroxyl hindering the rate-controlling step; although the problem that the carboxyl group on the central carbon of citric acid may form the first ester bond cannot be completely avoided, the present invention can compensate for the influence of this situation on the rate-controlling step by increasing the content of the carboxyl group on the crosslinking agent; pyromellitic anhydride and cellulose directly form an ester crosslinking structure through Fischer esterification without the need to produce a cyclic anhydride intermediate, and using pyromellitic anhydride to replace butane tetracarboxylic acid can solve the problem of steric hindrance affecting the rate-controlling step. Among them, pyromellitic anhydride and butanetetracarboxylic acid have similar structures and the same carboxyl content. The polycarboxylic acid crosslinking agent prepared by the present invention can significantly improve the crosslinking rate and efficiency of nanocellulose compared with using citric acid and butanetetracarboxylic acid alone.

[0012] In a second aspect, the present invention further proposes the use of the above-mentioned polycarboxylic acid crosslinking agent in adsorbents, plastic products, rubber products, fiber products, coatings or adhesives.

[0013] In a third aspect, the present invention further provides a method for preparing the above-mentioned polycarboxylic acid crosslinking agent, the preparation method comprising the following steps:

[0014] Two different organic acids containing carboxylic acids are mixed to undergo an esterification reaction to obtain a polycarboxylic acid crosslinking agent of formula I, which is then hydrolyzed to obtain a polycarboxylic acid crosslinking agent of formula II;

[0015] The esterification reaction temperature is 68-70°C, and the reaction time is 24-26h.

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

[0017] Preferably, the esterification reaction further includes post-treatment, wherein the post-treatment includes one or more of reduced pressure distillation, recrystallization, suction filtration or vacuum drying.

[0018] Preferably, the organic acid is pyromellitic dianhydride and citric acid, respectively.

[0019] In some preferred embodiments, the polycarboxylic acid crosslinking agent can be prepared by the following preparation method:

[0020] Add 1:1 molar ratio of pyromellitic dianhydride and citric acid into a solvent (tetrahydrofuran), react under the protection of nitrogen or inert gas, and undergo post-treatment (reduced pressure distillation, recrystallization, suction filtration, vacuum drying, wherein the recrystallization solution is methanol, the vacuum drying temperature is 40°C, and the drying time is 12h) to obtain a polycarboxylic acid cross-linking agent.

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

[0022] In a fourth aspect, the present invention further provides a nanocellulose adsorbent, which is prepared by using the above-mentioned polycarboxylic acid cross-linking agent.

[0023] In a fifth aspect, the present invention further provides a method for preparing the above-mentioned nanocellulose adsorbent, the preparation method comprising the following steps:

[0024] The polycarboxylic acid crosslinking agent is mixed with nanocellulose, a catalyst and water, freeze-dried in a vacuum, and subjected to a thermal curing reaction to obtain a nanocellulose adsorbent.

[0025] Preferably, the nanocellulose is TEMPO-derivatized 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 thermal curing reaction is 140-190° C., and the time is 1-6 min.

[0028] Preferably, after the thermal curing reaction, a post-treatment is further included, and the post-treatment includes washing and / or drying.

[0029] In some embodiments, clean water is used for washing, the number of washing times is 2 to 3 times, the drying temperature is 80° C., and the drying time is 12 hours.

[0030] Preferably, when the polycarboxylic acid crosslinking agent, nanocellulose, catalyst and water are mixed, magnetic stirring may be adopted, and the stirring time is 6 hours.

[0031] It should be pointed out that there are many catalysts for catalyzing the esterification reaction between nanocellulose and polycarboxylic acid crosslinking agents. Sodium hypophosphite used in this case is one of the most commonly used and efficient catalysts, and can be used as an effective reducing agent to prevent nanocellulose from being oxidized at high temperatures. Therefore, the present invention preferably uses sodium hypophosphite as a catalyst.

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

[0033] (1) The present invention utilizes the reaction of pyromellitic anhydride and citric acid to prepare a new type of polycarboxylic acid crosslinking agent, which can significantly improve the crosslinking rate and efficiency of nanocellulose compared to 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 the polycarboxylic acid crosslinking agent, giving the surface a strong electronegativity, which significantly improves the adsorption performance of the nanocellulose-based adsorbent for methylene blue; on the other hand, the flexible cross-linking bridges formed by the polycarboxylic acid crosslinking agent between the cellulose chains and the carboxyl functional groups lead to more hydrogen bond space, making the structure of the nanocellulose more stable, thereby enhancing the cyclic performance of the nanocellulose-based adsorbent.

[0035] (3) The maximum adsorption capacity of the nanocellulose-based adsorbent prepared by the present invention for methylene blue is 1152 mg g -1 The adsorption capacity of TEMPO-derived nanocellulose was increased by 321.58% compared with that of TEMPO-derived nanocellulose without polycarboxylic acid crosslinking agent. After 7 cycles, the adsorption capacity of methylene blue can still reach 92% of the maximum adsorption capacity of the first experiment, which has good economic and environmental benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a synthetic route for polycarboxylic acid crosslinking agents;

[0037] Figure 2 It is a synthetic route for nanocellulose-based adsorbents;

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

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

[0040] Figure 5 is the Fourier transform infrared spectroscopy test result of Example 1;

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

[0042] Figure 7 is a graph showing the variation of the adsorption capacity of methylene blue by the nanocellulose-based adsorbent in Example 1 with the initial concentration;

[0043] Figure 8 is a graph showing the variation of the adsorption capacity of methylene blue by the nanocellulose-based adsorbent in Example 1 with the adsorption time;

[0044] Fig. 9 This is a graph showing the variation of the adsorption capacity of methylene blue by the nanocellulose-based adsorbent in Example 1 with the number of cycles. DETAILED DESCRIPTION

[0045] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0046] The present invention will be further described below in conjunction with embodiments:

[0047] Figure 1 It is a synthetic route for polycarboxylic acid crosslinking agents;

[0048] Figure 2 It is a synthetic route for nanocellulose-based adsorbents.

[0049] Combination Figure 1 and Figure 2 It can be seen that in the technical solution described in the present invention, a mixed polycarboxylic acid system of pyromellitic dianhydride and citric acid is used, and tetrahydrofuran is used as a solvent to prepare a new polycarboxylic acid crosslinking agent through an esterification reaction. On this basis, sodium hypophosphite is used as a catalyst, and the polycarboxylic acid crosslinking agent and TEMPO-derived nanocellulose are crosslinked through esterification to prepare a nanocellulose-based adsorbent.

[0050] In some specific embodiments, the actual process route is: add 1:1 molar ratio of pyromellitic anhydride and citric acid to a solvent (tetrahydrofuran), react under nitrogen or inert gas protection, and obtain a polycarboxylic acid crosslinker after post-treatment (reduced pressure distillation, recrystallization, suction filtration, vacuum drying, wherein the recrystallization solution is methanol, the vacuum drying temperature is 40°C, and the drying time is 12h). The mass ratio of the polycarboxylic acid crosslinker, nanocellulose, and catalyst is 1-6:10:0.5-3; the temperature of the thermal curing reaction is 140-190°C, and the time is 1-6min. After post-treatment (washing and drying, wherein the washing solution is clean water, the number of washings is 3 times, the drying temperature is 80°C, and the drying time is 12h), a nanocellulose-based adsorbent is obtained, wherein the catalyst is used to catalyze the esterification reaction between the polycarboxylic acid crosslinker and the nanocellulose during the thermal curing reaction.

[0051] Example 1

[0052] A method for preparing a nanocellulose-based adsorbent using a polycarboxylic acid crosslinking agent, comprising the following steps:

[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-derivatized nanocellulose, the manufacturer is Beijing Shansi Technology Co., Ltd.;

[0059] Catalyst: sodium hypophosphite;

[0060] water.

[0061] (2) preparing a polycarboxylic acid cross-linking agent;

[0062] Adding 1:1 molar ratio of pyromellitic dianhydride and citric acid to a solvent, reacting under the protection of protective gas, and post-treating to obtain a polycarboxylic acid crosslinking agent; wherein the molar volume ratio of pyromellitic dianhydride to the solvent is 0.1 mol:100 mL, the reaction temperature is 68° C., and the reaction time is 24 h;

[0063] The post-treatment process includes the following operations: using a rotary evaporator to completely precipitate the product, recrystallizing the precipitated white solid twice with methanol, and finally vacuum drying at 40° C. for 12 hours to obtain a polycarboxylic acid crosslinking agent.

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

[0065]

[0066] (3) preparing nanocellulose-based adsorbents;

[0067] A polycarboxylic acid crosslinking agent, nanocellulose and a catalyst in a mass ratio of 2:10:1 were added to water and mixed evenly, followed by freeze drying and then thermal curing reaction at 170°C for 3 minutes. After post-treatment, a nanocellulose-based adsorbent was obtained.

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

[0069] Figure 5 This is the Fourier transform infrared spectrum test result of Example 1.

[0070] like Figure 5 As shown in the figure, citric acid (CA for short) is a tricarboxylic acid. Due to the effect of hydrogen bonding, the carboxyl group exists in the form of a bimolecular association, so the stretching vibration absorption peaks of the carboxyl group are shown at 1744cm-1 and 1711cm-1 respectively. PMDA (i.e., pyromellitic dianhydride) has a special molecular structure, located at 831cm-1. -1 、3061cm -1 、1302cm -1 、1733cm -1 The absorption peaks at 1770 cm-1 and 1770 cm-2 represent the aromatic CH plane angular vibration, CH plane angular deformation, axial bending vibration of the CO bond in aromatic carboxylic acid, and stretching vibration of the carbonyl group [174,175]. -1 and 1857cm -1 They are the symmetric stretching vibration peak and asymmetric stretching vibration peak of the active anhydride intermediate on the benzene ring. In the spectrum of TBPTA (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 A new absorption peak appeared at 1727 cm, 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 were blue-shifted to a certain extent to the low-wave band, proving that an esterification reaction occurred between CA and PMDA. The infrared spectra of CNF / TBPTA and CNF (i.e., nanocellulose) are very similar, except that at 1727 cm -1 represents the stretching vibration of the carbonyl group on the ester bond, which proves that TBPTA and CNF undergo esterification cross-linking reaction. Figure 5As shown in Figure 2, another important feature of the spectrum is the peak at 3480 cm-1 of CNF / TBPTA relative to CNF. -1 The strengthening of the absorption band at the cross-linked structure is caused by the increase of hydroxyl groups on the carboxylic acid.

[0071] The adsorption and cycle performance tests of the nanocellulose adsorbent obtained in Example 1 were carried out. The test results are shown in Figure 6 to Figure 9 .

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

[0073] from Figure 6 It can be seen that although the adsorption performance of nanocellulose-based adsorbents is highly dependent on the pH value of the solution, it can retain a relatively ideal adsorption capacity even in a strong acid environment of pH = 2, indicating that the use conditions of nanocellulose-based adsorbents are not harsh and can be applied to relatively complex sewage environments. Overall, the overall adsorption capacity increases with the increase of pH value. When pH>7, the removal rate of nanocellulose-based adsorbents for methylene blue can be maintained at more than 90%. When pH=11, the removal rate is as high as 99.87%, and the adsorption performance for MB is the best.

[0074] Figure 7 3 is a curve diagram showing the variation of the adsorption capacity of methylene blue of the nanocellulose-based adsorbent with the initial concentration of the methylene blue solution in this embodiment, wherein the adsorption temperature is 310 K, the pH value is 11, the adsorption time is 24 h, and the number of cycles is 0.

[0075] like Figure 7 As shown in Figure 2, the overall trend shows that the adsorption capacity increases with the increase of the initial concentration of methylene blue. -1 When the initial concentration increased to 600 mg L -1 , the adsorption capacity is as high as 1087.8 mg g -1 , the removal rate is still as high as 90.0%. Even at 1000mg L -1 The removal rate can reach 58.16% in high concentration solution. Figure 6 The theoretical adsorption capacity can reach up to 1152 mg g -1 .

[0076] Figure 8: is a graph showing the variation of the adsorption capacity of the nanocellulose-based adsorbent for methylene blue with the adsorption time in this embodiment, wherein the initial concentration of the methylene blue solution during adsorption is 200 mg L -1 , temperature is 310K, pH value is 11, and number of cycles is 0.

[0077] like Figure 8 As shown in Figure 2, within the first 120 min, a very rapid adsorption stage can be observed due to the strong negative charge on the adsorbent surface, and the adsorption capacity increases almost linearly, with a total of 98.32% of methylene blue adsorbed. Thereafter, the adsorption rate gradually becomes very slow, and the adsorption dynamic equilibrium is established at 720 min.

[0078] Fig. 9 : is a graph showing the variation of the adsorption capacity of methylene blue by the nanocellulose-based adsorbent as a function of the number of cycles in this embodiment, wherein the initial concentration of the methylene blue solution during adsorption is 200 mg L -1 , pH value is 11, temperature is 310K, and single adsorption time is 24h.

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

[0080] Comparative Example 1

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

[0082] (1) Preparation of raw materials;

[0083] Nanocellulose: same as in Example 1;

[0084] Catalyst: sodium hypophosphite;

[0085] water;

[0086] (2) preparing nanocellulose-based adsorbents;

[0087] Nanocellulose and catalyst in a mass ratio of 10:1 were added to water and mixed evenly, followed by freeze-drying and then thermal curing at 170°C for 3 minutes. After post-treatment, a nanocellulose-based adsorbent was obtained.

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

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

[0090] Comparative Example 2

[0091] A method for preparing a nanocellulose-based adsorbent, the specific steps 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) preparing nanocellulose-based adsorbents;

[0098] Citric acid, nanocellulose and a catalyst in a mass ratio of 6:10:1 were added to water and mixed evenly, followed by freeze-drying and then thermal curing at 170°C for 3 minutes. After post-treatment, a nanocellulose-based adsorbent was obtained.

[0099] The prepared nanocellulose-based adsorbent was used to adsorb methylene blue. During adsorption, the concentration of the methylene blue solution was 500 mg L -1 , pH value is 11, temperature is 310K, adsorption time is 24h, and number of cycles is 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 made the pH value of the solution too low, thereby causing the degradation of the cellulose structure.

[0101] Comparative Example 3

[0102] A method for preparing a nanocellulose-based adsorbent, the specific steps 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) preparing nanocellulose-based adsorbents;

[0109] Butanetetracarboxylic acid, nanocellulose and a catalyst in a mass ratio of 2:10:2.5 are added to water and mixed evenly, followed by freeze-drying, and then a thermal curing reaction is carried out at a temperature of 170°C for 3 minutes. After post-treatment, a nanocellulose-based adsorbent is obtained; wherein the catalyst is used to catalyze the esterification reaction between the polycarboxylic acid crosslinking agent and the nanocellulose during the thermal 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 a certain degree of competitive reaction between the sodium hypophosphite catalyst and the acid anhydride, and the 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 a nanocellulose-based adsorbent using a polycarboxylic acid crosslinking agent is basically the same as that of Example 1, except that in step (3), the mass ratio of the polycarboxylic acid crosslinking agent, the nanocellulose and the catalyst is 1:10:1.

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

[0115] Example 3

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

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

[0118] Example 4

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

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

[0121] Example 5

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

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

[0124] Example 6

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

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

[0127] Example 7

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

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

[0130] Example 8

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

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

[0133] Example 9

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

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

[0136] Example 10

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

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

[0139] Examples 1 to 10 were used to test the adsorption performance of nanocellulose-based adsorbents:

[0140] a. Prepare an aqueous solution of methylene blue (manufacturer: MacLean Biochemical Technology Co., Ltd., CAS No. 61-73-4) at a concentration of 50-1000 mg g -1 , pH = 2 ~ 11.

[0141] b. Adsorption performance test:

[0142] 10 mg of the nanocellulose-based adsorbent was added to 20 mL of methylene blue solution and shaken in a constant temperature shaker with a rotation speed of 200 rpm and a temperature of 310 K. The supernatant solution was taken and the absorbance was tested at a wavelength of 665 nm using an ultraviolet spectrophotometer.

[0143] c. Cyclic performance test:

[0144] The nanocellulose-based adsorbent adsorbed with methylene blue was immersed in a 1 mol L-1 HCl solution and shaken at room temperature for 24 h to completely desorb it. The desorbed nanocellulose-based adsorbent was rinsed with deionized water until no MB remained on the surface and then vacuum dried at 60°C. The regenerated nanocellulose-based adsorbent was placed in the same MB solution again. The above operation was one cycle, and a total of 7 cycles were performed.

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

[0146] All adsorption data were repeated three times, and the adsorption capacity (see formula 1) and the removal rate of methylene blue (see formula 2) of the nanocellulose-based adsorbent were calculated according to the following formula:

[0147]

[0148] Among them, Q e (mg g -1 ) is the adsorption capacity at equilibrium, r (%) is the removal rate of methylene blue at equilibrium, and C 0 (mg L -1 ) is the initial concentration of the methylene blue solution, and the methylene blue concentration when the adsorption reaches dynamic equilibrium is expressed as C e (mg L -1 ) represents, V (L) is the volume of methylene blue solution, and m (g) is the mass of nanocellulose-based adsorbent. The final results are shown in Table 1.

[0149] Table 1.

[0150]

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

[0152] The above is only a preferred embodiment of the present invention, and does not limit the present invention in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still belongs to the protection scope of the technical solution of the present invention.

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

[0154] In addition, the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0155] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A polycarboxylic acid crosslinking agent, characterized in that The structure of the polycarboxylic acid crosslinking agent is shown in Formula I or Formula II:

2. Use of the polycarboxylic acid crosslinking agent as claimed in claim 1 in adsorbents, plastic products, rubber products, fiber products, coatings or adhesives.

3. A method for preparing the polycarboxylic acid crosslinking agent according to claim 1, characterized in that: The preparation method comprises the following steps: Two different organic acids containing carboxylic acids are mixed to undergo an esterification reaction to obtain a polycarboxylic acid crosslinking agent of formula I, which is then hydrolyzed to obtain a polycarboxylic acid crosslinking agent of formula II; The esterification reaction temperature is 68-70°C, and the reaction time is 24-26h.

4. The preparation method according to claim 3, characterized in that: The specific operation of the esterification reaction is: adding the two different organic acids into a solvent and reacting under the protection of nitrogen or an inert gas.

5. The preparation method according to claim 3, characterized in that: The method further comprises post-treatment after the esterification reaction, wherein the post-treatment comprises one or more of reduced pressure distillation, recrystallization, suction filtration or vacuum drying.

6. The preparation method according to any one of claims 3 to 5, characterized in that: The organic acids are pyromellitic anhydride and citric acid respectively.

7. A nanocellulose adsorbent, characterized in that: The nanocellulose adsorbent is prepared using the polycarboxylic acid cross-linking agent as claimed in claim 1.

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

9. The preparation method according to claim 8, characterized in that: The nanocellulose is TEMPO-derivatized 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 thermal curing reaction is 140-190° C., and the time is 1-6 minutes.

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