PH-responsive tannic acid / attapulgite composite microsphere and preparation method thereof

Abstract

The invention belongs to the field of inorganic mineral functional materials, and provides a pH-responsive tannic acid / attapulgite composite microsphere and a preparation method thereof. According to the preparation method, the design that epoxy silane grafted attapulgite and tannic acid are subjected to ring opening on epoxy groups through phenolic hydroxyl groups to form C-O-C covalent bond anchoring is adopted, and then spray drying and balling are performed to prepare the composite microspheres, so that low-viscosity atomized processing, controllable particle size balling and excellent mechanical stability and dry-wet cycle stability under the high-solid-content feeding condition are realized; the prepared inorganic mineral-polyphenol composite particles have high processability and mechanical stability and pH response water absorption performance, the problems that traditional inorganic mineral-polyphenol composite particles are difficult to give consideration to coupling contradiction between processability and mechanical stability and performance consistency is insufficient during metal coordination treatment are solved, and the inorganic mineral-polyphenol composite particles have wide application value in the fields of intelligent adsorption materials, environment response type functional fillers and the like.

Description

Technical Field

[0001] This invention relates to the field of inorganic mineral functional materials, specifically to a pH-responsive tannic acid / attapulgite composite microsphere and its preparation method. Background Technology

[0002] Attapulgite, a natural one-dimensional nanorod silicate mineral, possesses a unique fibrous crystal structure, abundant surface hydroxyl groups, and a large specific surface area, showing broad application prospects in drug carriers, adsorbent materials, and functional fillers. To meet practical application requirements, attapulgite-based functional materials need to possess good particle formability, controllable particle size distribution, excellent mechanical stability, and specific stimulus-response properties. Among these, pH-responsive performance is crucial for achieving targeted drug release at specific sites in the digestive tract, selective adsorption of pollutants under different pH environments, and the environmental adaptability regulation of smart coating materials. Furthermore, combining attapulgite with polyphenolic compounds can endow it with multiple functions such as antioxidant, antibacterial, and metal coordination, further expanding its application range. Therefore, developing attapulgite-based composite microspheres that combine pH-responsive performance, excellent mechanical stability, and scalable preparation characteristics is of great significance for promoting the industrial application of inorganic mineral functional materials.

[0003] Regarding the current development status of attapulgite-polyphenol composite materials, the following shortcomings and corresponding reasons exist. Chinese patent CN116616456A discloses a tannic acid-modified attapulgite composite material, but this method uses simple physical mixing or weak interaction bonding, leading to easy desorption of tannic acid in the aqueous phase, resulting in insufficient functional stability and water-wash resistance of the composite material. Chinese patent CN115322691A discloses a method for preparing attapulgite-based microspheres, but it does not involve surface chemical modification and polyphenol covalent anchoring, making it difficult to achieve precise control of pH-responsive water absorption performance. Furthermore, existing technologies for preparing attapulgite-based microspheres via spray drying often face problems such as excessively high slurry viscosity under high solids content feed conditions leading to atomization difficulties, uncontrollable particle size, low mechanical strength and easy breakage of microspheres, and poor structural stability after wet-dry cycles. Especially when metal coordination enhancement or functional regulation is introduced, the coordination effect is highly sensitive to the pH value and ionic environment of the system, which often results in large performance fluctuations and insufficient repeatability between batches, which seriously restricts the practical application of such materials. Summary of the Invention

[0004] The purpose of this invention is to provide a pH-responsive tannic acid / attapulgite composite microsphere and its preparation method, which solves the problem that current inorganic mineral-polyphenol composite particles are difficult to balance with low viscosity atomization processing window and controllable particle size, particle mechanical stability and water absorption swelling / wet-dry cycle stability under spray drying high solids feed conditions. It also addresses the problem of insufficient performance consistency and repeatability caused by the high sensitivity of coordination to pH and ionic environment when introducing metal coordination enhancement or functional regulation.

[0005] This invention constructs an active epoxy group platform by grafting epoxy silane onto the surface of attapulgite. Combined with the ring-opening covalent anchoring reaction of tannic acid polyphenolic hydroxyl groups on the epoxy groups, it achieves stable immobilization of tannic acid on the attapulgite surface, avoiding the desorption problems of functional components caused by traditional physical adsorption or weak interactions. The covalently anchored tannic acid not only endows the material with pH-responsive water absorption properties but also provides stable coordination sites for subsequent metal coordination through its abundant phenolic hydroxyl groups, solving the performance fluctuation problem during metal coordination treatment. Simultaneously, the introduction of tannic acid effectively reduces the viscosity of the spray-fed slurry, broadens the atomization processing window for high-solids-content feeds, and enhances the dry-wet cycle stability of the microspheres through covalent bonding, achieving synergistic optimization of processing and performance.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A pH-responsive tannic acid / attapulgite composite microsphere, wherein the composite microsphere is composed of attapulgite grafted with epoxy silane and covalently anchored to tannic acid;

[0008] The epoxy silane-grafted attapulgite is attapulgite grafted with γ-glycidyl oxypropyltrimethoxysilane.

[0009] Based on the total dry weight of the composite microspheres, the mass fraction of the epoxysilane-grafted attapulgite is 40wt%–90wt%, and the mass fraction of the tannic acid is 1wt%–20wt%.

[0010] The tannic acid is anchored to the surface of the epoxysilane-grafted attapulgite through a COC covalent bond formed by the ring-opening of the epoxy group by its phenolic hydroxyl group.

[0011] The tannic acid and the epoxysilane-grafted attapulgite are covalently anchored to form a tannic acid covalently anchored attapulgite composite intermediate, which is used as a raw material for preparing the composite microspheres.

[0012] Furthermore, the epoxysilane-grafted attapulgite is prepared through the following steps:

[0013] Preparation of A1 dispersion system: Attapulgite is added to a mixed solvent composed of ethanol and deionized water, wherein the volume ratio of ethanol to deionized water in the mixed solvent is 95:5 to 50:50, so that the solid content of attapulgite in the mixed solvent is 1wt% to 10wt%, based on the solid mass as a percentage of the total mass of the dispersion system.

[0014] A2 Hydrolysis and Grafting Reaction: Under the condition of stirring speed of 200 r / min to 1000 r / min, an aqueous solution of acetic acid with a mass fraction of 10% to 50% was added to the dispersion system of step A1. The pH value was monitored in real time and adjusted to 4.0 to 6.0. Then, γ-glycidoxypropyltrimethoxysilane was added. The mass ratio of γ-glycidoxypropyltrimethoxysilane to attapulgite was 0.02 to 0.20. The reaction was continued to be stirred at 40℃ to 70℃ for 1 h to 6 h.

[0015] A3 Separation and Drying: The reaction system is separated by centrifugation with a relative centrifugal force of 1000g to 10000g and a centrifugation time of 5min to 30min. It is washed 2 to 10 times with alternating ethanol and deionized water, with each washing solvent volume being 5 to 20 times the mass of the solid. It is dried at 40℃ to 120℃ to constant weight, wherein the constant weight is defined as the difference between two consecutive weighings being ≤0.5%, to obtain the epoxysilane-grafted attapulgite.

[0016] The amount of organosilane grafted onto the epoxysilane-grafted attapulgite described in A4 is 0.5wt% to 8.0wt%, and the amount of organosilane grafted onto the epoxysilane-grafted attapulgite is determined by thermogravimetric analysis based on the total dry weight of the epoxysilane-grafted attapulgite.

[0017] Furthermore, the tannic acid covalently anchored attapulgite composite intermediate is prepared through the following steps:

[0018] Preparation of B1 dispersion system: The epoxy silane-grafted attapulgite prepared according to the above method is dispersed in deionized water to make the solid content 1wt% to 15wt%, based on the solid mass as a percentage of the total mass of the dispersion system; tannic acid is added to the dispersion system, wherein the mass ratio of the tannic acid to the epoxy silane-grafted attapulgite is 0.15 to 0.50.

[0019] B2 covalent anchoring reaction: Under the condition of stirring speed of 200 r / min to 1000 r / min, add sodium hydroxide aqueous solution with a concentration of 0.1 mol / L to 5.0 mol / L, monitor the pH value in real time and adjust the pH value of the system to 8.0 to 10.0, and continue stirring the reaction at 20℃ to 60℃ for 0.5 h to 8 h;

[0020] B3 Separation and Drying: The reaction system is centrifuged at a relative centrifugal force of 1000g to 10000g for 5min to 30min. It is washed 2 to 10 times with deionized water, with each wash having a solvent volume of 5 to 20 times the solid mass. It is then dried at 40℃ to 120℃ to constant weight, where the constant weight is defined as a mass difference of ≤0.5% between two consecutive weighings, to obtain the tannic acid covalently anchored attapulgite composite intermediate.

[0021] The covalently bonded amount of tannic acid described in B4 is 1.0 wt% to 15.0 wt%. Based on the dry basis mass of epoxy silane-grafted attapulgite, the covalently bonded amount of tannic acid is calculated as follows: the covalently bonded amount of tannic acid (wt%) is equal to the dry basis mass of the tannic acid covalently anchored attapulgite composite intermediate minus the dry basis mass of epoxy silane-grafted attapulgite, then divided by the dry basis mass of epoxy silane-grafted attapulgite, and finally multiplied by 100%. The covalently bonded amount of tannic acid is determined by thermogravimetric analysis or ultraviolet spectrophotometry.

[0022] Furthermore, the composite microspheres undergo metal coordination treatment, which includes: contacting the composite microspheres with a ferric chloride aqueous solution for 0.5 min to 60 min at a stirring speed of 200 r / min to 1000 r / min, wherein the concentration of the ferric chloride is 0.001 mol / L to 0.100 mol / L, and the solid-liquid ratio of the composite microspheres to the ferric chloride aqueous solution is 1 g:5 mL to 1 g:200 mL (based on anhydrous ferric chloride), and the contact temperature is 10℃ to 60℃. During the contact process, no visible precipitate or visible stratification is observed in the solution system. Subsequently, the microspheres are washed 2 to 10 times with deionized water, with each wash solvent volume being 5 to 20 times the solid mass. The microspheres are then dried at 40℃ to 120℃ to constant weight, wherein the constant weight is defined as a mass difference of ≤0.5% between two consecutive weighings.

[0023] Furthermore, the particle size D50 of the composite microspheres is 80μm to 600μm, where D50 is the median diameter of the volume distribution measured by laser diffraction particle size analysis. The dispersion medium is deionized water, and the test temperature is 20℃ to 30℃.

[0024] Furthermore, the water absorption ratio of the composite microspheres in a buffer aqueous solution with a pH of 5.0–6.0 is higher than that in a buffer aqueous solution with a pH of 7.5–9.0, and the ratio of the former to the latter is 1.2–5.0. The water absorption ratio is calculated by soaking the composite microspheres dried to constant weight in a buffer aqueous solution of the corresponding pH value, subtracting the dry weight before soaking from the weight after soaking, and then dividing by the dry weight before soaking. The constant weight is defined as a weight difference of ≤0.5% between two consecutive weighings. The soaking time is 30 min–24 h, the soaking temperature is 20 ° C–30 ° C, and the liquid-to-solid ratio is 200 mL / g–500 mL / g.

[0025] As a concept of this invention, the design of grafting γ-glycidoxypropyltrimethoxysilane onto the surface of attapulgite is mainly used to enhance the surface activity and subsequent functionalization capabilities of the material. The abundant silanol groups on the attapulgite surface undergo hydrolysis and condensation reactions with the methoxy groups of the silane coupling agent, forming a stable Si-O-Si covalent bond layer on its surface, while retaining the active epoxy groups facing the solution side. This epoxy platform provides reactive sites for the covalent anchoring of tannic acid, avoiding the desorption problem of polyphenol components in the aqueous phase under traditional physical adsorption methods. The grafting reaction is carried out under weakly acidic conditions, with the pH controlled within the range of 4.0–6.0, which promotes silane hydrolysis while avoiding gelation caused by excessive condensation, ensuring the uniformity of the grafted layer and the retention rate of active epoxy groups. The reaction temperature is set between 40℃ and 70℃, within which the silane hydrolysis rate and condensation reaction rate reach equilibrium, which is beneficial for forming a dense and uniform grafted layer. The grafting amount was controlled within the range of 0.5wt% to 8.0wt%. A lower grafting amount maintained the inherent structural characteristics and dispersibility of attapulgite, while a higher grafting amount provided more epoxy group reaction sites, offering sufficient functional groups for subsequent tannic acid anchoring and metal coordination. Tannic acid achieved chemical anchoring on the attapulgite surface through ring-opening addition reactions between multiple phenolic hydroxyl groups within its molecule and epoxy groups, forming stable COC covalent bonds. This covalent anchoring method not only ensured the stable fixation of tannic acid but also endowed the material with pH-responsive water absorption properties and metal coordination ability through the large number of residual phenolic hydroxyl groups in the tannic acid molecule, demonstrating the synergistic effect of epoxy silane grafting and tannic acid covalent anchoring.

[0026] This invention also discloses a method for preparing the pH-responsive tannic acid / attapulgite composite microspheres, comprising the following steps:

[0027] Preparation of S1 epoxysilane-grafted attapulgite: Preparation of epoxysilane-grafted attapulgite;

[0028] Preparation of S2 tannic acid covalently anchored attapulgite composite intermediate: Preparation of tannic acid covalently anchored attapulgite composite intermediate;

[0029] S3 Spray Drying to Form Microspheres: The tannic acid covalently anchored attapulgite composite intermediate obtained in step S2 is added to deionized water and stirred at a stirring speed of 200 r / min to 1000 r / min to obtain a spray-feed slurry. The solid content of the spray-feed slurry is 10 wt% to 40 wt%, based on the solid mass as a percentage of the total slurry mass. The spray-feed slurry is then spray-dried at an inlet temperature of 130℃ to 200℃ and an outlet temperature of 60℃ to 110℃. The spray drying is performed using a two-fluid nozzle atomization or centrifugal atomization, with an atomization pressure of 0.2 MPa to 0.8 MPa or an atomization speed of 5000 r / min to 20000 r / min and a feed rate of 0.2 L / h to 5.0 L / h to obtain the composite microspheres.

[0030] Furthermore, before step S1, a pretreatment step for attapulgite is included: attapulgite is placed in a hydrochloric acid aqueous solution for acid activation treatment, wherein the concentration of the hydrochloric acid is 0.1 mol / L to 2.0 mol / L, the treatment temperature is 20℃ to 80℃, and the treatment time is 0.5 h to 6 h. Subsequently, solid-liquid separation is performed, and the attapulgite is washed 2 to 10 times with deionized water, each time with a solvent volume of 5 to 20 times the mass of the solid. The washing continues until the pH value of the filtrate is 6 to 8 after two consecutive measurements. The filtrate is then dried at 50℃ to 120℃ to constant weight, wherein the constant weight is defined as a mass difference of ≤0.5% between two consecutive weighings.

[0031] In step S1, the volume ratio of ethanol to deionized water in the mixed solvent is 95:5 to 50:50.

[0032] Further, in step S3, when preparing the spray-fed slurry, polyvinylpyrrolidone with a weight-average molecular weight of 10,000 to 1,300,000 is added, wherein the amount of polyvinylpyrrolidone added is 0.05 wt% to 2.00 wt% of the total solids of the spray-fed slurry.

[0033] Further, after step S3, a metal coordination treatment step is included: the composite microspheres obtained in step S3 are contacted with a ferric chloride aqueous solution for 0.5 min to 60 min at a stirring speed of 200 r / min to 1000 r / min. The concentration of the ferric chloride is 0.001 mol / L to 0.100 mol / L. Based on anhydrous ferric chloride, the solid-liquid ratio of the composite microspheres to the ferric chloride aqueous solution is 1 g: 5 mL to 1 g: 200 mL. The contact temperature is 10℃ to 60℃. During the contact process, no visible precipitate or visible stratification is observed in the solution system. Subsequently, the microspheres are washed 2 to 10 times with deionized water, with each wash solvent volume being 5 to 20 times the solid mass. The microspheres are then dried at 40℃ to 120℃ to constant weight. The constant weight is defined as the difference between two consecutive weighings being ≤0.5%.

[0034] Furthermore, the ethanol in the mixed solvent mentioned in step A1 is anhydrous ethanol or ethanol with a volume fraction ≥95%.

[0035] Further, in step A1, after adding attapulgite to the mixed solvent, the mixture is stirred for 10 min to 60 min under magnetic or mechanical stirring conditions, with a stirring speed of 200 r / min to 1000 r / min and a stirring temperature of 15℃ to 35℃.

[0036] Furthermore, the γ-glycidyl etheroxypropyltrimethoxysilane mentioned in step A2 is added either all at once or in batches.

[0037] Furthermore, in step B1, when dispersing epoxysilane-grafted attapulgite in deionized water, the mixture is stirred for 10 to 60 minutes under magnetic or mechanical stirring conditions, with a stirring speed of 200 to 1000 r / min and a stirring temperature of 15 to 35°C.

[0038] Furthermore, in step S3, the stirring time for preparing the spray-feed slurry is 10 min to 60 min, and the stirring temperature is 15℃ to 35℃.

[0039] Furthermore, the pH value of the ferric chloride aqueous solution in the metal coordination treatment is 1.5 to 3.5.

[0040] Furthermore, the composite microspheres possess pH-responsive water absorption properties.

[0041] As another concept of this invention, the design of using spray drying technology to prepare composite microspheres is mainly used to enhance the formability, controllability, and scalability of the material. After the tannic acid covalently anchors the attapulgite composite intermediate to form a stable dispersion system in the aqueous phase, it is rapidly dehydrated and sphericalized through spray drying, achieving a morphological transformation from nanorod-shaped particles to micron-sized spherical particles. The solid content of the spray-fed slurry is controlled within the range of 10wt% to 40wt%. Lower solid content helps reduce slurry viscosity and improve atomization, while higher solid content can improve production efficiency and reduce energy consumption. The covalent anchoring of tannic acid not only endows the material with functionality but also effectively reduces the viscosity of high-solids slurries through the flexibility and hydrophilicity of its molecular chains, broadening the atomization processing window and solving the problem of difficult atomization of traditional inorganic mineral slurries under high solids conditions. The spray drying inlet temperature is set at 130℃ to 200℃, and the outlet temperature is controlled at 60℃ to 110℃. Rapid water evaporation causes the particle surface to solidify quickly, forming a microsphere structure with a certain mechanical strength. Atomization is achieved using a two-fluid nozzle or centrifugal atomization. Adjustment of atomization pressure or rotation speed allows for precise control of the microsphere particle size, resulting in composite microspheres with a D50 range of 80μm to 600μm. Adding an appropriate amount of polyvinylpyrrolidone (PVP) as a pelletizing aid, its polymer chains act as a binder and structural stabilizer during drying, further enhancing the mechanical strength and wet-dry cycle stability of the microspheres. The metal coordination treatment step is performed after microsphere molding, utilizing the residual phenolic hydroxyl groups on the tannic acid surface to react with Fe... 3+ By forming coordination bonds, the material is endowed with metal coordination function without destroying the microsphere structure. Furthermore, since tannic acid is stably anchored through covalent bonds, the repeatability and consistency of the coordination treatment are significantly improved.

[0042] The synergistic mechanism of epoxysilane-grafted attapulgite and tannic acid in this invention is manifested in a multi-level coupling of structure, function, and performance. Epoxysilane-grafted attapulgite, as an inorganic framework, provides structural support and mechanical strength to the material, while the active epoxy groups on its surface provide a reaction platform for tannic acid anchoring. Tannic acid, as a functional component, achieves covalent anchoring through the ring-opening reaction of polyphenolic hydroxyl groups with epoxy groups. This not only avoids the desorption problems of traditional physical adsorption methods but also endows the material with pH-responsive water absorption properties and metal coordination capabilities through the large number of residual phenolic hydroxyl groups. During the spray drying and pelletizing process, the nanorod-like structure of epoxysilane-grafted attapulgite provides the physical basis for pelleting, while the flexible molecular chains and hydrophilicity of tannic acid effectively reduce slurry viscosity and improve atomization. The synergistic effect of both components enables atomized processing under high-solids-content feed conditions. In terms of pH response performance, the phenolic hydroxyl groups in tannic acid molecules exhibit different protonation states under different pH environments. At pH 5.0–6.0, the phenolic hydroxyl groups are mainly in the protonated form, which easily forms a more stable hydrogen bond-solventization network with water molecules and inorganic surfaces, making the microsphere structure more relaxed and enhancing swelling and water absorption. At pH 7.5–9.0, the phenolic hydroxyl groups are partially deprotonated to form phenolic anions. Under the action of buffer salt / ionic strength, the charge is more easily shielded, and at the same time, it is easier to generate cohesive effects (such as ion bridging / coordination cohesion) with inorganic surfaces, making the structure more compact, thereby inhibiting swelling and water absorption. The hydrophobicity of the epoxy silane graft layer further inhibits water absorption under neutral to alkaline conditions. The two work together to achieve a pH response water absorption ratio of 1.2–5.0. In terms of mechanical stability, the epoxy silane graft layer firmly anchors tannic acid to the surface of attapulgite through Si-O-Si covalent bonds, avoiding the loss of functional components and structural collapse during wet-dry cycles. The hydrogen bonds and coordination between tannic acid molecules further enhance the cohesion of the microspheres. The two work together to ensure the long-term stability of the material.

[0043] Beneficial technical effects

[0044] Stable covalent anchoring structure: An active epoxy group platform is constructed by grafting epoxy silane onto the surface of attapulgite. Combined with the ring-opening covalent anchoring reaction of tannic acid polyphenol hydroxyl groups on epoxy groups, a stable COC covalent bond is formed. This avoids the problem of polyphenol components desorption in the aqueous phase caused by traditional physical adsorption or weak interactions, ensuring the long-term stability of functional components and the structural integrity of the material under dry and wet cycling conditions, and providing a reliable chemical structural basis for the practical application of the material.

[0045] Excellent pH-responsive water absorption performance: After tannic acid is covalently anchored on the surface of attapulgite, the large number of residual phenolic hydroxyl groups exhibit differentiated protonation states and hydrogen bonding modes under different pH environments. Under weakly acidic conditions of pH 5.0 to 6.0, swelling and water absorption are enhanced through hydrogen bond networks. Under neutral to alkaline conditions of pH 7.5 to 9.0, water absorption is inhibited due to weakened electrostatic repulsion and enhanced hydrophobicity. This achieves significant pH-responsive characteristics with a water absorption ratio of 1.2 to 5.0, providing precise environmental response control capabilities for applications such as pH-sensitive drug controlled release and selective adsorption of pollutants.

[0046] Expanded high-solids-content atomizable processing window: The covalent anchoring of tannic acid not only endows the material with functionality, but its molecular chain flexibility and hydrophilicity effectively reduce the viscosity of the spray-fed slurry, achieving low-viscosity atomizable processing within the solids content range of 10wt% to 40wt%. This solves the problems of atomization difficulties and uncontrollable pellet size caused by the excessively high viscosity of traditional inorganic mineral slurries under high solids content conditions, significantly improving the production efficiency and economy of spray drying preparation, and reducing energy consumption and manufacturing costs.

[0047] Enhanced mechanical stability and wet-dry cycle stability: The epoxy silane graft layer firmly anchors tannic acid to the attapulgite surface through Si-O-Si covalent bonds, preventing the loss of functional components and the collapse of the microsphere structure during wet-dry cycles. At the same time, the hydrogen bonding between tannic acid molecules and optional metal coordination crosslinking further enhance the cohesion and mechanical strength of the microspheres, ensuring the structural stability of the material under repeated water absorption-dehydration cycles and mechanical stress conditions, thus extending the service life and reliability of the material.

[0048] Stable metal coordination function and performance consistency: After tannic acid is stably anchored on the surface of attapulgite through covalent bonds, its residual phenolic hydroxyl groups provide a stable number and uniformly distributed coordination sites for subsequent metal coordination. When in contact with metal salt solutions such as ferric chloride, stable metal-phenolic hydroxyl coordination bonds are formed. Due to the chemical immobilization of coordination sites, the coordination instability caused by changes in pH and ionic environment in traditional physical adsorption systems is avoided. This significantly improves the batch-to-batch performance consistency and repeatability of metal coordination treatment, providing a guarantee for the functional regulation and large-scale application of materials. Attached Figure Description

[0049] Figure 1 The images show the XRD crystal structure analysis diagrams of Example 1 and Comparative Example 7.

[0050] Figure 2 The images show the FTIR chemical bond characterization diagrams for Example 1 and Comparative Example 7.

[0051] Figure 3 The pH response water absorption ratio test graphs for Example 1, Comparative Example 7, and Comparative Example 3 are shown.

[0052] Figure 4 The figures show the Zeta potential pH titration curves for Example 1 and Comparative Example 7.

[0053] Figure 5 Viscosity-shear rate diagrams of the slurry rheological curves for Example 1 and Comparative Example 8.

[0054] Figure 6 The particle size distribution curves are for Example 1 and Comparative Example 8. Detailed Implementation

[0055] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0056] Example 1

[0057] The pH-responsive tannic acid / attapulgite composite microspheres prepared in this embodiment were obtained by spray drying attapulgite covalently anchored to tannic acid via epoxy silane grafting. The epoxy silane-grafted attapulgite in this embodiment was attapulgite grafted with γ-glycidoxypropyltrimethoxysilane. The tannic acid in this embodiment is anchored to the surface of the epoxy silane-grafted attapulgite via a COC covalent bond formed by the ring-opening of the epoxy group by its phenolic hydroxyl group. The tannic acid and the epoxy silane-grafted attapulgite in this embodiment are covalently anchored to form a tannic acid covalently anchored attapulgite composite intermediate, which serves as the raw material for preparing the composite microspheres of this embodiment.

[0058] The preparation method of this embodiment includes the following steps:

[0059] Attapulgite pretreatment: 100g of attapulgite was placed in 1000mL of 1.0mol / L hydrochloric acid aqueous solution for acid activation treatment at 50℃ for 3h. Then, solid-liquid separation was performed, and the attapulgite was washed 5 times with deionized water. The volume of the solvent was 10 times the mass of the solid each time. The washing was continued until the pH value of the filtrate was measured to be 6 to 8 on two consecutive occasions. The filtrate was dried at 80℃ to constant weight. The constant weight was obtained when the difference between two consecutive weighings was less than or equal to 0.5%. The pretreated attapulgite was obtained.

[0060] Preparation of A1 dispersion system: 50g of pretreated attapulgite was added to a mixed solvent consisting of 700mL of anhydrous ethanol and 300mL of deionized water. In this embodiment, the volume ratio of ethanol to deionized water in the mixed solvent was 70:30, so that the solid content of attapulgite in the mixed solvent of this embodiment was 5wt%. Based on the solid mass as a percentage of the total mass of the dispersion system, the mixture was stirred under magnetic stirring for 30min at a stirring speed of 600r / min and a stirring temperature of 25℃.

[0061] A2 Hydrolysis and Grafting Reaction: Under the condition of stirring speed of 600 r / min, a 30% (w / w) aqueous solution of acetic acid was added to the dispersion system of step A1. The pH value was monitored in real time and adjusted to 5.0. Then, 5.0 g of γ-glycidoxypropyltrimethoxysilane was added. In this embodiment, the mass ratio of γ-glycidoxypropyltrimethoxysilane to attapulgite was 0.10. In this embodiment, γ-glycidoxypropyltrimethoxysilane was added in one go. The reaction was continued to be stirred at 55°C for 3.5 h.

[0062] A3 Separation and Drying: The reaction system was centrifuged at a relative centrifugal force of 5000g for 15min. It was washed 6 times with alternating anhydrous ethanol and deionized water, with each washing solvent volume being 12 times the mass of the solid. It was dried at 80℃ to constant weight, where the constant weight was defined as the difference between two consecutive weighings being less than or equal to 0.5%, thus obtaining the epoxysilane-grafted attapulgite of this embodiment.

[0063] In this embodiment, the amount of organosilane grafted onto the epoxysilane-grafted attapulgite is 4.0 wt%. Based on the total dry weight of the epoxysilane-grafted attapulgite in this embodiment, the amount of organosilane grafted onto the attapulgite is determined by thermogravimetric analysis.

[0064] Preparation of dispersion system B1: 40g of epoxysilane-grafted attapulgite prepared according to the aforementioned method was dispersed in 460mL of deionized water to achieve a solid content of 8wt%. Based on the solid mass as a percentage of the total mass of the dispersion system, the mixture was stirred magnetically for 30min at a speed of 600r / min and a temperature of 25℃. 12.8g of tannic acid was added to the dispersion system. In this embodiment, the mass ratio of tannic acid to epoxysilane-grafted attapulgite was 0.32.

[0065] B2 covalent anchoring reaction: Under the condition of stirring speed of 600 r / min, a sodium hydroxide aqueous solution with a concentration of 1.0 mol / L was added, the pH value was monitored in real time and the pH value of the system was adjusted to 9.0, and the reaction was continued to be stirred at 40℃ for 4 h.

[0066] B3 Separation and Drying: The reaction system was centrifuged at a relative centrifugal force of 5000g for 15min. It was washed 6 times with deionized water, with each wash solvent volume being 12 times the solid mass. It was dried at 80℃ to constant weight, where the constant weight was defined as the difference between two consecutive weighings being less than or equal to 0.5%. This yielded the tannic acid covalently anchored attapulgite composite intermediate of this embodiment.

[0067] In this embodiment, the covalently bonded amount of tannic acid is 6.5 wt%. Based on the dry basis mass of epoxy silane-grafted attapulgite, the covalently bonded amount of tannic acid (wt%) is equal to the dry basis mass of the tannic acid covalently anchored attapulgite composite intermediate minus the dry basis mass of epoxy silane-grafted attapulgite, then divided by the dry basis mass of epoxy silane-grafted attapulgite, and finally multiplied by 100%. The covalently bonded amount of tannic acid in this embodiment was determined by thermogravimetric analysis.

[0068] S3 Spray Drying to Form Microspheres: 100g of the tannic acid covalently anchored attapulgite composite intermediate obtained in step B3 was added to 300mL of deionized water and stirred for 30min at a stirring speed of 600r / min to obtain a spray-feed slurry. The stirring temperature was 25℃. The solid content of the spray-feed slurry in this embodiment was 25wt%, based on the solid mass as a percentage of the total slurry mass. The spray-feed slurry of this embodiment was spray-dried at an inlet temperature of 165℃ and an outlet temperature of 85℃. The spray drying in this embodiment used a two-fluid nozzle atomization with an atomization pressure of 0.5MPa and a feed rate of 2.6L / h to obtain the composite microspheres of this embodiment.

[0069] The composite microspheres in this embodiment have a particle size D50 of 340 μm. The particle size D50 in this embodiment is the median diameter of the volume distribution measured by laser diffraction particle size analysis. The dispersion medium is deionized water, and the test temperature is 25℃.

[0070] The composite microspheres of this embodiment exhibit a higher water absorption ratio in a buffer solution with a pH of 5.5 than in a buffer solution with a pH of 8.0, with a ratio of 3.1. The water absorption ratio in this embodiment is calculated by immersing the composite microspheres, dried to constant weight, in a buffer solution of the corresponding pH value. The weight after immersion is calculated by subtracting the initial dry weight from the final dry weight, and then dividing by the initial dry weight. Constant weight is defined as a difference of less than or equal to 0.5% between two consecutive weighings. The immersion time is 12 hours, the immersion temperature is 25°C, and the liquid-to-solid ratio is 350 mL / g.

[0071] Features of Example 1: This example uses moderate parameter configurations: epoxy silane-grafted attapulgite content is 65wt%, tannic acid content is 10wt%, organosilane grafting amount is 4.0wt%, tannic acid covalent binding amount is 6.5wt%, composite microsphere particle size D50 is 340μm, and pH-responsive water absorption ratio is 3.1. The process parameters selected in this example are robust, the preparation process is stable and reliable, and the product performance is balanced. It is suitable for general pH-responsive water-absorbing material applications, including soil water retention, controlled-release carriers, and smart switch-type adsorbents.

[0072] Example 2

[0073] The pH-responsive tannic acid / attapulgite composite microspheres prepared in this embodiment were obtained by spray drying attapulgite covalently anchored to tannic acid via epoxy silane grafting. Polyvinylpyrrolidone was added during the preparation of the spray-feed slurry. The epoxy silane-grafted attapulgite in this embodiment was attapulgite grafted with γ-glycidoxypropyltrimethoxysilane. The tannic acid in this embodiment was anchored to the surface of the epoxy silane-grafted attapulgite via a COC covalent bond formed by the ring-opening of the epoxy groups by its phenolic hydroxyl groups. The tannic acid and the epoxy silane-grafted attapulgite in this embodiment were covalently anchored to form a tannic acid covalently anchored attapulgite composite intermediate, which served as the raw material for preparing the composite microspheres of this embodiment.

[0074] The preparation method of this embodiment includes the following steps:

[0075] Attapulgite pretreatment: 100g of attapulgite was placed in 1200mL of 1.5mol / L hydrochloric acid aqueous solution for acid activation treatment at 65℃ for 4h. Then, solid-liquid separation was performed, and the attapulgite was washed 7 times with deionized water. The volume of the solvent was 15 times the mass of the solid each time. The washing was continued until the pH value of the filtrate was measured to be 6 to 8 twice. The filtrate was dried at 100℃ to constant weight. The constant weight was obtained when the difference between two consecutive weighings was less than or equal to 0.5%.

[0076] Preparation of A1 dispersion system: 60g of pretreated attapulgite was added to a mixed solvent consisting of 570mL of 95% ethanol and 380mL of deionized water. In this embodiment, the volume ratio of ethanol to deionized water in the mixed solvent was 60:40, so that the solid content of attapulgite in the mixed solvent of this embodiment was 6wt%. Based on the solid mass as a percentage of the total mass of the dispersion system, the mixture was stirred for 40min under mechanical stirring conditions at a stirring speed of 700r / min and a stirring temperature of 30℃.

[0077] A2 Hydrolysis and Grafting Reaction: Under the condition of stirring speed of 700 r / min, 35% acetic acid aqueous solution was added to the dispersion system of step A1. The pH value was monitored in real time and adjusted to 5.5. Then, 7.8 g of γ-glycidoxypropyltrimethoxysilane was added in three batches. In this example, the mass ratio of γ-glycidoxypropyltrimethoxysilane to attapulgite was 0.13. The reaction was continued to be stirred at 60°C for 4.5 h.

[0078] A3 Separation and Drying: The reaction system was centrifuged at a relative centrifugal force of 6500g for 20min. It was washed 7 times alternately with 95% ethanol and deionized water, with each washing solvent volume being 15 times the mass of the solid. It was dried at 95℃ to constant weight, where the constant weight was defined as the difference between two consecutive weighings being less than or equal to 0.5%, thus obtaining the epoxysilane-grafted attapulgite of this embodiment.

[0079] In this embodiment, the amount of organosilane grafted onto the epoxysilane-grafted attapulgite is 5.5 wt%. Based on the total dry weight of the epoxysilane-grafted attapulgite in this embodiment, the amount of organosilane grafted onto the attapulgite is determined by thermogravimetric analysis.

[0080] Preparation of dispersion system B1: 50g of epoxysilane-grafted attapulgite prepared according to the aforementioned method was dispersed in 450mL of deionized water to achieve a solid content of 10wt%. Based on the solid mass as a percentage of the total mass of the dispersion system, the mixture was stirred for 40min at a stirring speed of 700r / min and a stirring temperature of 30℃. 8.5g of tannic acid was added to the dispersion system. In this embodiment, the mass ratio of tannic acid to epoxysilane-grafted attapulgite was 0.17.

[0081] B2 covalent anchoring reaction: Under the condition of stirring speed of 700 r / min, a sodium hydroxide aqueous solution with a concentration of 2.0 mol / L was added, the pH value was monitored in real time and the pH value of the system was adjusted to 9.5, and the reaction was continued to be stirred at 50℃ for 5 h.

[0082] B3 Separation and Drying: The reaction system was centrifuged at a relative centrifugal force of 6500g for 20min. It was washed 7 times with deionized water, with each wash solvent volume being 15 times the solid mass. It was dried at 95℃ to constant weight, where the constant weight was defined as the difference between two consecutive weighings being less than or equal to 0.5%. This yielded the tannic acid covalently anchored attapulgite composite intermediate of this embodiment.

[0083] In this embodiment, the covalently bonded amount of tannic acid is 3.5 wt%. Based on the dry basis mass of epoxy silane-grafted attapulgite, the covalently bonded amount of tannic acid (wt%) is equal to the dry basis mass of the tannic acid covalently anchored attapulgite composite intermediate minus the dry basis mass of epoxy silane-grafted attapulgite, then divided by the dry basis mass of epoxy silane-grafted attapulgite, and finally multiplied by 100%. The covalently bonded amount of tannic acid in this embodiment was determined by thermogravimetric analysis.

[0084] S3 Spray Drying to Form Microspheres: 100g of the tannic acid covalently anchored attapulgite composite intermediate obtained in step B3 was added to 230mL of deionized water and stirred for 40min at a stirring speed of 700r / min and a stirring temperature of 30℃. 1.0g of polyvinylpyrrolidone with a weight average molecular weight of 500,000 was added to the slurry, and stirring was continued for 20min to obtain a spray-fed slurry. In this embodiment, the amount of polyvinylpyrrolidone added was 1.00wt% of the total solids of the spray-fed slurry, and the solid content of the spray-fed slurry in this embodiment was 30wt%, based on the solid mass as a percentage of the total slurry mass. The spray-fed slurry of this embodiment was spray-dried at an inlet temperature of 180℃ and an outlet temperature of 95℃. The spray drying in this embodiment used centrifugal atomization at a speed of 15000r / min and a feed rate of 3.5L / h to obtain the composite microspheres of this embodiment.

[0085] S4 Metal Coordination Treatment: Under a stirring speed of 700 r / min, 50 g of the composite microspheres obtained in step S3 were contacted with 5000 mL of 0.050 mol / L ferric chloride aqueous solution for 30 min. Based on anhydrous ferric chloride, the solid-liquid ratio of the composite microspheres to the ferric chloride aqueous solution was 1 g: 100 mL. The contact temperature was 35 °C. During the contact process in this embodiment, no visible precipitate or visible stratification was observed in the solution system. The pH value of the ferric chloride aqueous solution in this embodiment was 2.5. Subsequently, the microspheres were washed 7 times with deionized water, with each washing solvent volume being 15 times the solid mass. The microspheres were dried at 95 °C to constant weight. Constant weight was defined as the difference between two consecutive weighings being less than or equal to 0.5%. The composite microspheres of this embodiment, after metal coordination treatment, were obtained.

[0086] The composite microspheres in this embodiment have a particle size D50 of 450 μm. The particle size D50 in this embodiment is the median diameter of the volume distribution measured by laser diffraction particle size analysis. The dispersion medium is deionized water, and the test temperature is 25℃.

[0087] The composite microspheres in this embodiment exhibit a higher water absorption ratio in a buffer solution with a pH of 5.2 than in a buffer solution with a pH of 8.5, with a ratio of 2.5. The water absorption ratio in this embodiment is calculated by immersing the composite microspheres, dried to constant weight, in a buffer solution of the corresponding pH value. The weight after immersion is calculated by subtracting the initial dry weight from the final dry weight, and then dividing by the initial dry weight. Constant weight is defined as a difference of less than or equal to 0.5% between two consecutive weighings. The immersion time is 18 hours, the immersion temperature is 25°C, and the liquid-to-solid ratio is 400 mL / g.

[0088] Features of Example 2: This example uses a formulation with a high epoxy silane-grafted attapulgite content of 75 wt% and a low tannic acid content of 5 wt%. The organosilane grafting amount is 5.5 wt%, the tannic acid covalent binding amount is 3.5 wt%, and 1 wt% polyvinylpyrrolidone is added, followed by metal coordination treatment. The composite microspheres have a particle size D50 of 450 μm and a pH-responsive water absorption ratio of 2.5. The composite microspheres prepared in this example have high mechanical strength and good dispersibility, making them suitable for applications requiring strong structural stability and flowability, including functional fillers, composite material reinforcing phases, and fluidized bed adsorbents.

[0089] Example 3

[0090] The pH-responsive tannic acid / attapulgite composite microspheres prepared in this embodiment were obtained by spray drying attapulgite covalently anchored to tannic acid via epoxy silane grafting. Polyvinylpyrrolidone was added during the preparation of the spray-feed slurry. The epoxy silane-grafted attapulgite in this embodiment was attapulgite grafted with γ-glycidoxypropyltrimethoxysilane. The tannic acid in this embodiment was anchored to the surface of the epoxy silane-grafted attapulgite via a COC covalent bond formed by the ring-opening of the epoxy groups by its phenolic hydroxyl groups. The tannic acid and the epoxy silane-grafted attapulgite in this embodiment were covalently anchored to form a tannic acid covalently anchored attapulgite composite intermediate, which served as the raw material for preparing the composite microspheres of this embodiment.

[0091] The preparation method of this embodiment includes the following steps:

[0092] Attapulgite pretreatment: 100g of attapulgite was placed in 1500mL of 0.5mol / L hydrochloric acid aqueous solution for acid activation treatment at 35℃ for 2h. Then, solid-liquid separation was performed, and the attapulgite was washed 4 times with deionized water. The volume of the solvent was 8 times the mass of the solid each time. The washing was continued until the pH value of the filtrate was measured to be 6 to 8 twice. The filtrate was dried at 65℃ to constant weight. The constant weight was obtained when the difference between two consecutive weighings was less than or equal to 0.5%.

[0093] Preparation of dispersion system A1: 35g of pretreated attapulgite was added to a mixed solvent consisting of 332mL of anhydrous ethanol and 568mL of deionized water. In this embodiment, the volume ratio of ethanol to deionized water in the mixed solvent was 63:37, so that the solid content of attapulgite in the mixed solvent of this embodiment was 3.5wt%. Based on the solid mass as a percentage of the total mass of the dispersion system, the mixture was stirred under magnetic stirring for 25min at a stirring speed of 400r / min and a stirring temperature of 20℃.

[0094] A2 Hydrolysis and Grafting Reaction: Under the condition of stirring speed of 400 r / min, a 20% (w / w) aqueous solution of acetic acid was added to the dispersion system of step A1. The pH value was monitored in real time and adjusted to 4.5. Then, 2.1 g of γ-glycidoxypropyltrimethoxysilane was added. In this embodiment, the mass ratio of γ-glycidoxypropyltrimethoxysilane to attapulgite was 0.06. In this embodiment, γ-glycidoxypropyltrimethoxysilane was added in one go. The reaction was continued to be stirred at 48°C for 2.5 h.

[0095] A3 Separation and Drying: The reaction system was centrifuged at a relative centrifugal force of 3500g for 12min. It was washed four times with alternating layers of anhydrous ethanol and deionized water, with each wash having a solvent volume eight times the mass of the solid. The system was dried at 65°C to constant weight, where the difference between two consecutive weighings was less than or equal to 0.5%. This yielded the epoxysilane-grafted attapulgite of this embodiment.

[0096] In this embodiment, the amount of organosilane grafted onto the epoxysilane-grafted attapulgite is 2.5 wt%. Based on the total dry weight of the epoxysilane-grafted attapulgite in this embodiment, the amount of organosilane grafted onto the attapulgite is determined by thermogravimetric analysis.

[0097] Preparation of dispersion system B1: 35g of epoxysilane-grafted attapulgite prepared according to the aforementioned method was dispersed in 315mL of deionized water to achieve a solid content of 10wt%. The mixture was stirred for 25min under magnetic stirring at a speed of 400r / min and a temperature of 20℃, based on the solid mass as a percentage of the total mass of the dispersion system. 16.0g of tannic acid was added to the dispersion system. In this embodiment, the mass ratio of tannic acid to epoxysilane-grafted attapulgite was 0.46.

[0098] B2 covalent anchoring reaction: Under the condition of stirring speed of 400 r / min, a sodium hydroxide aqueous solution with a concentration of 0.5 mol / L was added, the pH value was monitored in real time and the pH value of the system was adjusted to 8.5, and the reaction was continued to be stirred at 30℃ for 2.5 h.

[0099] B3 Separation and Drying: The reaction system was centrifuged at a relative centrifugal force of 3500g for 12min. It was washed four times with deionized water, with each wash having a solvent volume eight times the solid mass. It was then dried at 65℃ to constant weight, where the constant weight was defined as a mass difference of less than or equal to 0.5% between two consecutive weighings. This yielded the tannic acid covalently anchored attapulgite composite intermediate of this embodiment.

[0100] B4 In this embodiment, the covalently bound amount of tannic acid is 11.5 wt%. Based on the dry basis mass of epoxy silane-grafted attapulgite, the covalently bound amount of tannic acid (wt%) is equal to the dry basis mass of the tannic acid covalently anchored attapulgite composite intermediate minus the dry basis mass of epoxy silane-grafted attapulgite, then divided by the dry basis mass of epoxy silane-grafted attapulgite, and finally multiplied by 100%. The covalently bound amount of tannic acid in this embodiment was determined by ultraviolet spectrophotometry.

[0101] S3 Spray Drying to Form Microspheres: 100g of the tannic acid covalently anchored attapulgite composite intermediate obtained in step B3 was added to 650mL of deionized water and stirred for 25min at a stirring speed of 400r / min and a stirring temperature of 20℃. 2.0g of polyvinylpyrrolidone with a weight average molecular weight of 1,000,000 was added to the slurry, and stirring was continued for 20min to obtain a spray-fed slurry. In this embodiment, the amount of polyvinylpyrrolidone added was 2.00wt% of the total solids in the spray-fed slurry, and the solids content of the spray-fed slurry in this embodiment was 13wt%, based on the solids mass as a percentage of the total slurry mass. The spray-fed slurry of this embodiment was spray-dried at an inlet temperature of 145℃ and an outlet temperature of 72℃. The spray drying in this embodiment used a two-fluid nozzle atomization with an atomization pressure of 0.35MPa and a feed rate of 1.5L / h to obtain the composite microspheres of this embodiment.

[0102] The particle size D50 of the composite microspheres in this embodiment is 220 μm. The particle size D50 in this embodiment is the median diameter of the volume distribution measured by laser diffraction particle size analysis. The dispersion medium is deionized water and the test temperature is 25℃.

[0103] The composite microspheres of this embodiment exhibit a higher water absorption ratio in a buffer solution with a pH of 5.8 than in a buffer solution with a pH of 7.8, with a ratio of 4.2. The water absorption ratio in this embodiment is calculated by immersing the composite microspheres, dried to constant weight, in a buffer solution of the corresponding pH value. The weight after immersion is calculated by subtracting the initial dry weight from the final dry weight, and then dividing by the initial dry weight. Constant weight is defined as a difference of less than or equal to 0.5% between two consecutive weighings. The immersion time is 8 hours, the immersion temperature is 25°C, and the liquid-to-solid ratio is 300 mL / g.

[0104] Example 3 Features: This example uses a formulation with a low epoxy silane-grafted attapulgite content of 50 wt% and a high tannic acid content of 15 wt%. The organosilane grafting amount is 2.5 wt%, the tannic acid covalent binding amount is 11.5 wt%, and 2 wt% polyvinylpyrrolidone is added. The composite microsphere particle size D50 is 220 μm, and the pH-responsive water absorption ratio is 4.2. The composite microspheres prepared in this example have significant pH-responsive performance and a high density of tannic acid functional groups, making them suitable for applications requiring strong pH sensitivity and bioactivity, including drug controlled-release carriers, pH-responsive adsorbents, biomedical materials, and smart packaging materials.

[0105] Example 4

[0106] The pH-responsive tannic acid / attapulgite composite microspheres prepared in this embodiment were obtained by spray drying attapulgite covalently anchored to tannic acid via epoxy silane grafting. Polyvinylpyrrolidone was added during the preparation of the spray-feed slurry. The epoxy silane-grafted attapulgite in this embodiment was attapulgite grafted with γ-glycidoxypropyltrimethoxysilane. The tannic acid in this embodiment was anchored to the surface of the epoxy silane-grafted attapulgite via a COC covalent bond formed by the ring-opening of the epoxy groups by its phenolic hydroxyl groups. The tannic acid and the epoxy silane-grafted attapulgite in this embodiment were covalently anchored to form a tannic acid covalently anchored attapulgite composite intermediate, which served as the raw material for preparing the composite microspheres of this embodiment.

[0107] The preparation method of this embodiment includes the following steps:

[0108] Attapulgite pretreatment: 100g of attapulgite was placed in 2000mL of 1.8mol / L hydrochloric acid aqueous solution for acid activation treatment at 75℃ for 5.5h. Subsequently, solid-liquid separation was performed, and the attapulgite was washed 9 times with deionized water. The volume of the solvent in each wash was 18 times the mass of the solid. The washing continued until the pH value of the filtrate was measured to be 6 to 8 on two consecutive occasions. The filtrate was dried at 110℃ to constant weight. The constant weight was obtained when the difference between two consecutive weighings was less than or equal to 0.5%.

[0109] Preparation of A1 dispersion system: 70g of pretreated attapulgite was added to a mixed solvent consisting of 665mL of anhydrous ethanol and 95mL of deionized water. In this embodiment, the volume ratio of ethanol to deionized water in the mixed solvent was 88:12, so that the solid content of attapulgite in the mixed solvent of this embodiment was 8.5wt%. Based on the solid mass as a percentage of the total mass of the dispersion system, the mixture was stirred for 50min under mechanical stirring conditions at a stirring speed of 900r / min and a stirring temperature of 32℃.

[0110] A2 Hydrolysis and Grafting Reaction: Under the condition of stirring speed of 900 r / min, 45% acetic acid aqueous solution was added to the dispersion system of step A1. The pH value was monitored in real time and adjusted to 5.8. Then, 12.6 g of γ-glycidoxypropyltrimethoxysilane was added in two batches. In this example, the mass ratio of γ-glycidoxypropyltrimethoxysilane to attapulgite was 0.18. The reaction was continued to be stirred at 68°C for 5.5 h.

[0111] A3 Separation and Drying: The reaction system was centrifuged at a relative centrifugal force of 9000g for 28min. It was washed 9 times alternately with 95% ethanol and deionized water, with each washing solvent volume being 18 times the mass of the solid. It was dried at 110℃ to constant weight, where the constant weight was defined as the difference between two consecutive weighings being less than or equal to 0.5%, thus obtaining the epoxysilane-grafted attapulgite of this embodiment.

[0112] In this embodiment, the amount of organosilane grafted onto the epoxysilane-grafted attapulgite is 7.2 wt%. Based on the total dry weight of the epoxysilane-grafted attapulgite in this embodiment, the amount of organosilane grafted onto the attapulgite is determined by thermogravimetric analysis.

[0113] Preparation of Dispersion System B1: 65g of epoxysilane-grafted attapulgite prepared according to the aforementioned method was dispersed in 360mL of deionized water to achieve a solid content of 15wt%. Based on the solid mass as a percentage of the total mass of the dispersion system, the mixture was stirred for 50min at a stirring speed of 900r / min and a stirring temperature of 32℃. 10.0g of tannic acid was added to the dispersion system. In this embodiment, the mass ratio of tannic acid to epoxysilane-grafted attapulgite was 0.15.

[0114] B2 covalent anchoring reaction: Under the condition of stirring speed of 900 r / min, a sodium hydroxide aqueous solution with a concentration of 4.5 mol / L was added, the pH value was monitored in real time and the pH value of the system was adjusted to 9.8, and the reaction was continued to be stirred at 55℃ for 7 h.

[0115] B3 Separation and Drying: The reaction system was centrifuged at a relative centrifugal force of 9000g for 28min. It was washed 9 times with deionized water, with each wash solvent volume being 18 times the solid mass. It was dried at 110℃ to constant weight, where the constant weight was defined as a mass difference of less than or equal to 0.5% between two consecutive weighings. This yielded the tannic acid covalently anchored attapulgite composite intermediate of this embodiment.

[0116] In this embodiment, the covalently bonded amount of tannic acid is 1.8 wt%. Based on the dry basis mass of epoxy silane-grafted attapulgite, the covalently bonded amount of tannic acid (wt%) is equal to the dry basis mass of the tannic acid covalently anchored attapulgite composite intermediate minus the dry basis mass of epoxy silane-grafted attapulgite, then divided by the dry basis mass of epoxy silane-grafted attapulgite, and finally multiplied by 100%. The covalently bonded amount of tannic acid in this embodiment was determined by thermogravimetric analysis.

[0117] S3 Spray Drying to Form Microspheres: 100g of the tannic acid covalently anchored attapulgite composite intermediate obtained in step B3 was added to 150mL of deionized water and stirred for 50min at a stirring speed of 900r / min and a stirring temperature of 32℃. 0.5g of polyvinylpyrrolidone with a weight average molecular weight of 60,000 was added to the slurry, and stirring was continued for 20min to obtain a spray-fed slurry. In this embodiment, the amount of polyvinylpyrrolidone added was 0.50wt% of the total solids in the spray-fed slurry, and the solids content of the spray-fed slurry in this embodiment was 40wt%, based on the solids mass as a percentage of the total slurry mass. The spray-fed slurry of this embodiment was spray-dried at an inlet temperature of 192℃ and an outlet temperature of 102℃. The spray drying in this embodiment used centrifugal atomization at a speed of 18000r / min and a feed rate of 4.5L / h to obtain the composite microspheres of this embodiment.

[0118] S4 Metal Coordination Treatment: Under a stirring speed of 900 r / min, 60 g of the composite microspheres obtained in step S3 were contacted with 10000 mL of 0.008 mol / L ferric chloride aqueous solution for 45 min. Based on anhydrous ferric chloride, the solid-liquid ratio of the composite microspheres to the ferric chloride aqueous solution was 1 g: 167 mL. The contact temperature was 52 °C. During the contact process in this embodiment, no visible precipitate or visible stratification was observed in the solution system. The pH value of the ferric chloride aqueous solution in this embodiment was 3.2. Subsequently, the microspheres were washed 9 times with deionized water, with each washing solvent volume being 18 times the solid mass. The microspheres were dried at 110 °C to constant weight. Constant weight was defined as the difference between two consecutive weighings being less than or equal to 0.5%. The composite microspheres of this embodiment, after metal coordination treatment, were obtained.

[0119] The particle size D50 of the composite microspheres in this embodiment is 550 μm. The particle size D50 in this embodiment is the median diameter of the volume distribution measured by laser diffraction particle size analysis. The dispersion medium is deionized water and the test temperature is 28℃.

[0120] The composite microspheres of this embodiment exhibit a higher water absorption ratio in a buffer solution with a pH of 5.3 than in a buffer solution with a pH of 8.8, with a ratio of 1.8. The water absorption ratio in this embodiment is calculated by immersing the composite microspheres, dried to constant weight, in a buffer solution of the corresponding pH value. The weight after immersion is calculated by subtracting the initial dry weight from the final dry weight, and then dividing by the initial dry weight. Constant weight is defined as a difference of less than or equal to 0.5% between two consecutive weighings. The immersion time is 20 hours, the immersion temperature is 28°C, and the liquid-to-solid ratio is 480 mL / g.

[0121] Example 4 Features: This example uses an epoxy silane-grafted attapulgite content of 85 wt% (close to the upper limit) and a tannic acid content of 3 wt% (close to the lower limit), with an organosilane grafting amount of 7.2 wt% and a tannic acid covalent binding amount of 1.8 wt%. A small amount of polyvinylpyrrolidone (0.5 wt%) is added, and metal coordination treatment is performed. The composite microspheres have a particle size D50 of 550 μm and a pH-responsive water absorption ratio of 1.8. This example verifies the feasibility of the claims. The prepared composite microspheres have high attapulgite content, high silane grafting amount, and large particle size, making them suitable for applications requiring high mechanical strength, excellent wear resistance, and stable structure, including wear-resistant coating fillers, high-strength composite materials, heavy-duty adsorbents, and filter media.

[0122] Comparative Example 1: Basically the same as Example 1, except that the mass fraction of epoxysilane-grafted attapulgite is 35wt%, the mass fraction of tannic acid is adjusted to 12wt% to maintain the covalent anchoring ratio, and the amounts of other components and preparation conditions remain unchanged.

[0123] Comparative Example 2: Basically the same as Example 1, except that the mass fraction of epoxysilane-grafted attapulgite is 95wt%, the mass fraction of tannic acid is adjusted to 2wt% to maintain the covalent anchoring ratio, and the amounts of other components and preparation conditions remain unchanged.

[0124] Comparative Example 3: Basically the same as Example 1, except that the mass fraction of tannic acid is 0.5 wt%, the mass fraction of epoxysilane-grafted attapulgite is adjusted to 70 wt%, and the amounts of other components and preparation conditions remain unchanged.

[0125] Comparative Example 4: Basically the same as Example 1, except that the mass fraction of tannic acid is 25 wt%, the mass fraction of epoxysilane-grafted attapulgite is adjusted to 45 wt%, and the amounts of other components and preparation conditions remain unchanged.

[0126] Comparative Example 5: Basically the same as Example 1, except that in step A2, the mass ratio of γ-glycidoxypropyltrimethoxysilane to attapulgite is 0.01, which reduces the amount of epoxysilane grafting to 0.4 wt%, while other preparation conditions remain unchanged.

[0127] Comparative Example 6: Basically the same as Example 1, except that the mass ratio of tannic acid to epoxysilane-grafted attapulgite in step B1 is 0.60, which increases the covalent bonding amount of tannic acid to 16.5 wt%, while other preparation conditions remain unchanged.

[0128] Comparative Example 7: Basically the same as Example 1, except that attapulgite without γ-glycidoxypropyltrimethoxysilane grafting was used. The attapulgite was directly physically mixed with tannic acid and then spray-dried into spheres without covalent anchoring. Other preparation conditions remained unchanged.

[0129] Comparative Example 8: Basically the same as Example 1, except that the solid content of the spray-feed slurry in step S3 is 5wt%, the spray drying inlet temperature is adjusted to 140℃, the outlet temperature is adjusted to 70℃, and other preparation conditions remain unchanged.

[0130] Performance testing:

[0131] pH response water absorption ratio test

[0132] Test Subject: Composite microsphere samples dried to constant weight. Test Objective: To evaluate the water absorption and swelling properties and pH response characteristics of composite microspheres under different pH conditions. Test Principle: The phenolic hydroxyl groups of tannic acid ionize to different degrees at different pH levels. Under acidic conditions, the phenolic hydroxyl groups are more protonated, which is more conducive to the formation of hydrogen bond-solventization networks and structural relaxation, thereby enhancing swelling and water absorption. Under alkaline conditions, after the phenolic hydroxyl groups are deprotonated, the structure tends to be denser under the influence of ion shielding and possible ion bridging / cohesion, thus reducing water absorption and producing pH-responsive water absorption differences. Experimental Method: Weigh 0.5 g of composite microspheres dried to constant weight and place them in 200 mL of phosphate buffer solutions at pH 5.5 and pH 8.0, respectively. Immerse at 25℃ for 12 h, gently shaking every 2 h. After immersion, remove the microspheres, blot dry with filter paper, and weigh immediately. Key parameters: Test temperature 25±2℃, liquid-to-solid ratio 400mL / g, soaking time 12h, pH buffering capacity ≥0.05mol / L. Data processing: Water absorption ratio = (mass after soaking - dry mass before soaking) / dry mass before soaking, pH response ratio = water absorption ratio at pH 5.5 / water absorption ratio at pH 8.0, n≥3 for each test group, results are taken as mean ± standard deviation.

[0133] Composite microsphere particle size distribution test

[0134] Test Subject: Composite microsphere samples prepared by spray drying. Test Objective: To characterize the particle size distribution and median diameter (D50) of the composite microspheres and evaluate the controllability of the microsphere formation process. Test Principle: Based on the principle of laser diffraction, the scattering angle of the laser beam by the particles is related to the particle size. The volume distribution is calculated using Mie scattering theory. Experimental Method: A laser particle size analyzer was used. 0.1 g of composite microspheres were dispersed in 50 mL of deionized water and ultrasonically dispersed for 3 min. The dispersed microspheres were then added to the dispersion cell for testing. The laser wavelength was 632.8 nm, the test range was 0.1-3000 μm, and the shading rate was controlled at 10-20%. Three consecutive tests were performed, and the average value was taken. Key Parameters: Test temperature 25±2℃, ultrasonic power 100W, dispersion medium was deionized water, and refractive index was 1.55. Data Processing: D10, D50, D90, and span (Span = (D90-D10) / D50) were recorded. For each sample, n≥3 tests were performed. The results were taken as the mean ± standard deviation.

[0135] Slurry viscosity and atomization performance test

[0136] Test Subjects: Spray-feed slurries with different solid contents. Test Objective: To evaluate the rheological properties and atomization processing window of the slurries, addressing the core challenge of atomizing low-viscosity slurries under high-solids-content feed conditions. Test Principle: Slurry viscosity determines atomization ease. Viscosity curves at different shear rates are measured using a rotational rheometer to assess the slurry's atomization adaptability. Experimental Method: A rotational rheometer with a plate-to-plate clamp (1mm spacing) is used. The test temperature is 25℃, and the shear rate range is 1-1000s. -1 The viscosity-shear rate curves were recorded. Atomization performance was evaluated using a two-fluid nozzle under atomization pressures of 0.3-0.7 MPa, observing the atomization cone angle and droplet uniformity. Key parameters: Test temperature 25±1℃, solid content range 10-40wt%, shear rate 1-1000s. -1 Atomization pressure: 0.5 MPa. Data processing: Record apparent viscosity and thixotropic index. Atomization performance is characterized by atomization cone angle (°) and Sauter average diameter (μm), with n≥3 for each group.

[0137] Mechanical strength test of composite microspheres

[0138] Test Object: Composite microsphere samples dried to constant weight. Test Purpose: To evaluate the mechanical stability of the composite microspheres and ensure they are not easily broken during storage, transportation, and application. Test Principle: The breaking force of the microspheres is determined through a single-particle compression test, reflecting the internal structural strength and interfacial bonding force of the particles. Experimental Method: Using a texture analyzer or universal testing machine, single microsphere particles with a diameter of 300-400 μm are selected and placed between a flat indenter. Compression is performed at a constant rate of 0.5 mm / min until the particles break, and the peak breaking force is recorded. Key Parameters: Compression rate 0.5 mm / min, test temperature 25 ± 2℃, particles with a sphericity ≥ 0.9, and particle size deviation ≤ 10%. Data Processing: The breaking force (N) is taken as the average ± standard deviation of at least 30 particles in each group. The breaking stress σ = F / (πd² / 4) is calculated, where F is the breaking force and d is the particle diameter.

[0139] Dry and wet cycle stability test

[0140] Test Subject: Composite microsphere samples dried to constant weight. Test Objective: To evaluate the structural stability and performance retention rate of composite microspheres during repeated water absorption-drying cycles, addressing the core challenge of stability during water absorption swelling / wet-drying cycles. Test Principle: Simulating pH changes and alternating wet-drying conditions in real-world applications, the structural integrity and functional reversibility of the microspheres are examined through cyclic testing. Experimental Method: 1.0 g of composite microspheres is weighed and placed in 100 mL of pH 5.5 buffer solution, soaked at 25℃ for 6 h until water saturation, then removed and dried at 60℃ for 4 h to constant weight, constituting one cycle. Five consecutive cycles are performed, and particle size distribution, water absorption ratio, and breakage rate are measured after each cycle. Key Parameters: Number of cycles: 5; soaking temperature: 25±2℃; drying temperature: 60±5℃; liquid-to-solid ratio: 100 mL / g. Data Processing: Performance retention rate = (performance in Nth cycle / initial performance) × 100%; breakage rate = (mass of broken particles / total mass) × 100%; n ≥ 3 for each group.

[0141] XRD structural characterization

[0142] Test Subjects: Epoxysilane-grafted attapulgite, tannic acid covalently anchored attapulgite composite intermediates, and composite microsphere powder samples. Test Objective: To characterize the changes in attapulgite crystal structure and the influence of tannic acid covalent anchoring on the crystal phase, demonstrating covalent bonding. Test Principle: X-ray diffraction was used to detect the crystal structure. The characteristic peaks of attapulgite were located at 2θ=8.4°(110), 19.8°(200), and 26.7°(040). The changes in peak intensity and position after grafting and anchoring reflected the structural evolution. Experimental Method: An X-ray diffractometer was used with Cu Kα radiation (λ=1.5406Å), tube voltage 40kV, tube current 40mA, scanning range 5-60°, step size 0.02°, and scanning rate 2° / min. Samples were ground to 200 mesh and tested in pellet form. Key parameters: Scan range 5-60°, step size 0.02°, radiation source Cu Kα, test temperature 25±2℃. Data processing: Export raw XRD spectrum data (2θ-intensity) to CSV format, and use Jade or Origin software for peak position calibration, peak intensity calculation, and crystal plane index assignment. n≥2 samples were tested for each sample.

[0143] Figure 1 shows the XRD crystal structure analysis diagrams of Example 1 and Comparative Example 7. The parameters were fixed as follows: all samples were dry powders ground to 200 mesh and then pressed into tablets for testing; the test temperature was 25±2℃; the X-ray radiation source was Cu Kα with a wavelength of 1.5406 Å; the tube voltage was 40 kV; the tube current was 40 mA; the scanning range was 5° to 60°; the step size was 0.02°; and the scanning rate was 2°·min. -1 The parameter change was that the sample system was changed from the epoxy silane-grafted attapulgite and tannic acid covalently anchored system of Example 1 to the physically mixed system of attapulgite and tannic acid of Comparative Example 7. Example 1 maintained the characteristic peak positions of attapulgite at approximately 8.4°, 19.8°, and 26.7° at 2θ. Simultaneously, the peak intensity and peak shape showed consistent changes compared to Comparative Example 7, accompanied by enhanced background scattering. This indicates that the stable combination of organic components introduced an amorphous contribution without disrupting the main crystal structure of attapulgite, supporting the rationality and stability of the system construction at the crystal structure level.

[0144] Figure 2 The images show the FTIR chemical bond characterization of Example 1 and Comparative Example 7, with fixed parameters: all samples were dry powders, and the test wavenumber range was 4000 cm⁻¹. -1 Up to 400 cm -1 The resolution and number of scans were determined using standard infrared testing conditions, and baseline consistency was achieved. The parameter change was that the sample system was changed from the epoxy silane-grafted attapulgite and tannic acid covalently anchored system of Example 1 to the physical mixture system of Comparative Example 7. The characteristic absorption peak of the epoxy group in Example 1 was 910 cm⁻¹. -1 Relatively weakened and at 1050cm-1 Up to 1150 cm -1 The absorption of C–O–C related stretching vibrations in the region is enhanced, while at 3300 cm⁻¹ -1 Up to 3500 cm -1 The identifiable shift in the position and shape of the broad peak of the phenolic hydroxyl group indicates that the phenolic hydroxyl group of tannic acid participates in the reaction and forms a more stable bonding environment, verifying the construction logic of the system and the effectiveness of the introduction of functional groups at the chemical bond level.

[0145] Figure 3 The graph shows the pH response water absorption ratio test results for Example 1, Comparative Example 7, and Comparative Example 3. The parameters were fixed as follows: sample dried to constant weight, sample weight 0.5 g, soaking temperature 25±2℃, and liquid-to-solid ratio 400 mL·g. -1 Soaking time: 12 h; buffer solution pH: 5.5 or 8.0, with a buffer capacity of not less than 0.05 mol·L⁻¹. -1 Each group has n≥3 and is expressed as mean ± standard deviation. The varying parameters are the sample changing from Example 1 to Comparative Example 7 and Comparative Example 3, and the solution pH switching between 5.5 and 8.0. Example 1 showed a water absorption ratio of 5.8±0.3 at pH 5.5, which was significantly higher than 1.9±0.2 at pH 8.0, resulting in a pH response ratio of approximately 3.1. In contrast, Comparative Example 7 and Comparative Example 3 both had ratios of approximately 1.1. This indicates that significant pH-selective swelling and water absorption behavior can only be obtained when the functional component is effectively introduced and at an appropriate level. From a performance perspective, this demonstrates that the synergistic design can produce a stable and quantifiable pH response.

[0146] Figure 4 The figures show the Zeta potential pH titration curves for Example 1 and Comparative Example 7. Fixed parameters included a test pH range of 3 to 11 with equally spaced points, consistent ionic strength and temperature of the dispersion medium, and repeated measurements at each pH point, expressed as mean ± standard deviation. Changing parameters included the sample changing from Example 1 to Comparative Example 7 and a gradual increase in system pH. Example 1 showed a relatively positive shift in the Zeta potential in the acidic range and a significant negative shift in the alkaline range, exhibiting a reversible charge regulation trend with pH changes. Comparative Example 7 showed a smaller overall change, indicating that Example 1 had a phenolic hydroxyl-related charge contribution that could be regulated by protonation and deprotonation on its surface, further supporting the consistency between its pH response mechanism and surface chemical design.

[0147] Figure 5 The viscosity-shear rate diagrams of the slurry rheological curves for Example 1 and Comparative Example 8 are shown below. The parameters were fixed as follows: rotational rheometer plate-to-plate clamp spacing 1 mm, test temperature 25°C, and shear rate range 1 s. -1 up to 1000 s -1The viscosity-shear rate curves were recorded and expressed as mean ± standard deviation. The atomization method was kept consistent with the spray drying equipment, except that the solid content of the spray feed slurry was changed from 25 wt% in Example 1 to 5 wt% in Comparative Example 8. Example 1 maintained a moderate apparent viscosity plateau at medium-high shear rates and remained stable for 100 s. -1 The viscosity of the sample reached 285±15 mPa·s, which is within the viscosity window required for both shear thinning and atomization. In contrast, the viscosity of the control sample 8 was only about 28±5 mPa·s under the same conditions. This shows that atomization is not necessarily better with lower viscosity, but rather there is a suitable viscosity window. If the viscosity is too low, it will lead to insufficient droplet stability and an increased risk of poor atomization cone morphology. From the perspective of the processing, this demonstrates that optimizing the solid content can achieve a more reasonable balance between atomization and stable pellet formation.

[0148] Figure 6 The particle size distribution curves for Example 1 and Comparative Example 8 are shown. The fixed parameters were: laser diffraction particle size analysis, deionized water as the dispersion medium, test temperature 25±2℃, sample pre-dispersion and ultrasonication for 3 min, and shading rate controlled between 10% and 20%. Each sample was repeatedly tested, and the average value was taken, with both differential and cumulative distribution curves output simultaneously. The parameters were varied by changing the sample from Example 1 to Comparative Example 8 and corresponding to different spray feed solid content conditions. The median diameter (D50) of the cumulative distribution in Example 1 was 340±18 μm, and the differential distribution showed a single peak with a reasonable distribution width, indicating that the pellet size was controllable and batch-to-batch stable. In contrast, the D50 of Comparative Example 8 was 85±8 μm, showing a significant downward shift in particle size and a change in distribution morphology. This indicates that excessively low solid content leads to changes in atomization and drying shrinkage behavior, resulting in smaller pellet size and poorer morphological stability. From the perspective of particle size distribution evidence, this demonstrates that the process window setting plays a decisive role in the controllability of pellet formation.

[0149] As can be seen from the performance of the examples and comparative examples in Table 1, the composite microspheres of the present invention exhibit excellent performance in pH response, mechanical stability, and dry-wet cycling stability. The pH response ratios of Examples 1-4 are 1.8-4.2, significantly higher than the 1.1-2.8 of Comparative Examples 1-8, demonstrating that the synergistic design of epoxy silane grafting and tannic acid covalent anchoring effectively enhances pH sensitivity. The breaking force of the examples is 9.8-18.2 N, generally higher than most samples in the 3.5-22.5 N range of the comparative examples. The breaking force of Comparative Examples 1, 4, 5, 6, 7, and 8 is significantly lower, indicating that the lack of key components or parameter imbalance leads to a decrease in mechanical properties. After 5 cycles, the performance retention rate of the examples is 89%-96%, generally higher than the 45%-62% of Comparative Examples 4, 6, 7, and 8, and higher than the 68%-72% of Comparative Examples 1 and 5 (with the exception of 97% for Comparative Example 2), verifying the advantages of the dry-wet cycling stability design of the present invention. Comparative Example 1 suffered from insufficient covalent anchoring points due to excessively low attapulgite content in epoxy silane grafting, resulting in a double decrease in pH responsiveness and mechanical strength. Comparative Examples 4 and 6, due to excessively high tannin content or overloaded anchoring, exhibited loose internal structures in the microspheres, leading to significant deterioration in crushing force and cycling stability. Comparative Example 7, without epoxy silane grafting treatment, only underwent physical adsorption of tannins, resulting in a pH response ratio of only 1.1 and poor cycling stability. Comparative Example 8 suffered from excessively low spray solids content, small particle size, and excessively low slurry viscosity, leading to uneven pellet formation and insufficient mechanical properties. Although Examples 2 and 4 were slightly superior to Examples 1 and 3 in certain indicators such as crushing force, considering pH responsiveness, particle size controllability, and processing window, Examples 1-4 exhibited balanced and stable overall performance, adaptable to different application requirements.

[0150] Table 1 Performance Comparison Summary Table

[0151]

[0152] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A pH responsive tannin / attapulgite composite microsphere, characterized in that, The composite microspheres are composed of tannic acid covalently anchored epoxy silane grafted palygorskite; The epoxy silane grafted palygorskite is palygorskite grafted by γ-glycidoxypropyltrimethoxysilane; The mass fraction of the epoxy silane grafted palygorskite is 40wt%-90wt% and the mass fraction of the tannic acid is 1wt%-20wt% based on the total dry mass of the composite microspheres; The tannic acid is covalently anchored on the surface of the epoxy silane grafted palygorskite through C-O-C covalent bond formed by ring-opening of the epoxy group by the phenolic hydroxyl group of the tannic acid; The tannic acid and the epoxy silane grafted palygorskite are covalently anchored to form a tannic acid covalently anchored palygorskite composite intermediate, which is used as a raw material for preparing the composite microspheres.

2. The composite microspheres of claim 1, wherein, The epoxy silane grafted palygorskite is prepared by the following steps: A1 dispersion system preparation: palygorskite is added into a mixed solvent composed of ethanol and deionized water, the volume ratio of ethanol to deionized water in the mixed solvent is 95:5-50:50, and the solid content of palygorskite in the mixed solvent is 1wt%-10wt% based on the total mass of the dispersion system; A2 hydrolysis and grafting reaction: under the condition that the stirring speed is 200r / min-1000r / min, 10%-50% acetic acid aqueous solution is added into the dispersion system of step A1, the pH value is monitored in real time and adjusted to 4.0-6.0, then γ-glycidoxypropyltrimethoxysilane is added, the mass ratio of γ-glycidoxypropyltrimethoxysilane to palygorskite is 0.02-0.20, and the reaction is continued to stir at 40℃-70℃ for 1h-6h; A3 separation and drying: the reaction system is centrifuged at a relative centrifugal force of 1000g-10000g for 5min-30min, washed with ethanol and deionized water alternately for 2-10 times, the volume of each washing solvent is 5-20 times the solid mass, and dried at 40℃-120℃ to constant weight, the difference between the mass of two consecutive measurements is ≤0.5%, to obtain the epoxy silane grafted palygorskite; The organic silane grafting amount of the epoxy silane grafted palygorskite is 0.5wt%-8.0wt% based on the total dry mass of the epoxy silane grafted palygorskite, and the organic silane grafting amount is determined by thermogravimetric analysis.

3. The composite microspheres of claim 1, wherein, The tannic acid covalently anchored palygorskite composite intermediate is prepared by the following steps: B1 dispersion system preparation: the epoxy silane grafted palygorskite prepared by the foregoing method is dispersed in deionized water, the solid content is 1wt%-15wt% based on the total mass of the dispersion system; tannic acid is added into the dispersion system, the mass ratio of tannic acid to the epoxy silane grafted palygorskite is 0.15-0.50; B2 covalent anchoring reaction: under the condition of stirring speed of 200 r / min-1000 r / min, adding sodium hydroxide aqueous solution with concentration of 0.1 mol / L-5.0 mol / L, monitoring pH value in real time and adjusting pH value of the system to 8.0-10.0, and then continuing stirring reaction at 20℃-60℃ for 0.5 h-8 h; B3 separation and drying: centrifuging the reaction system, centrifugal relative centrifugal force being 1000 g-10000 g, centrifuging time being 5 min-30 min, washing with deionized water for 2-10 times, each time washing solvent volume being 5 times-20 times of solid mass, and drying at 40℃-120℃ until constant weight, the constant weight being that the difference between two consecutive mass measurements is ≤0.5%, to obtain the tannic acid covalently anchored attapulgite composite intermediate; B4 the tannic acid covalent binding amount being 1.0 wt%-15.0 wt%, based on the dry mass of the epoxy silane grafted attapulgite, the tannic acid covalent binding amount being calculated by the following method: tannic acid covalent binding amount wt% being equal to the dry mass of the tannic acid covalently anchored attapulgite composite intermediate minus the dry mass of the epoxy silane grafted attapulgite, divided by the dry mass of the epoxy silane grafted attapulgite, and then multiplied by 100%, the tannic acid covalent binding amount being determined by thermogravimetric analysis method or ultraviolet spectrophotometry.

4. The composite microspheres of claim 1, wherein, The composite microspheres are subjected to metal coordination treatment, which comprises: contacting the composite microspheres with ferric chloride aqueous solution under the condition of stirring speed of 200 r / min-1000 r / min for 0.5 min-60 min, the concentration of ferric chloride being 0.001 mol / L-0.100 mol / L, based on anhydrous ferric chloride, the solid-liquid ratio of the composite microspheres to the ferric chloride aqueous solution being 1 g:5 mL-1 g:200 mL, and the contacting temperature being 10℃-60℃, and there is no visible precipitation and no visible stratification in the solution system during the contacting process, followed by washing with deionized water for 2-10 times, each time washing solvent volume being 5 times-20 times of solid mass, and drying at 40℃-120℃ until constant weight, the constant weight being that the difference between two consecutive mass measurements is ≤0.5%.

5. The composite microspheres of claim 1, wherein, The particle size D50 of the composite microspheres is 80 μm-600 μm, the particle size D50 being the median diameter in volume distribution measured by laser diffraction particle size analysis method, and the dispersion medium being deionized water, and the test temperature being 20℃-30℃.

6. The composite microspheres of claim 1, wherein, The water absorption ratio of the composite microspheres in a buffer aqueous solution with pH value of 5.0-6.0 is higher than that in a buffer aqueous solution with pH value of 7.5-9.0, and the ratio of the former to the latter is 1.2-5.0; the water absorption ratio being the mass ratio calculated by the mass of the composite microspheres dried to constant weight after being soaked in the buffer aqueous solution corresponding to the pH value, minus the dry mass before soaking, divided by the dry mass before soaking, the constant weight being that the difference between two consecutive mass measurements is ≤0.5%, the soaking time being 30 min-24 h, the soaking temperature being 20℃-30℃, and the liquid-solid ratio being 200 mL / g-500 mL / g.

7. A method for preparing the pH-responsive tannin / attapulgite composite microspheres according to any one of claims 1-6, characterized in that, The method comprises the following steps: Preparation of S1 epoxy silane grafted palygorskite: prepare epoxy silane grafted palygorskite; Preparation of S2 tannin covalently anchored palygorskite composite intermediate: prepare tannin covalently anchored palygorskite composite intermediate; S3 spray drying into spheres: add the tannin covalently anchored palygorskite composite intermediate obtained in step S2 into deionized water, and stir at a stirring speed of 200 r / min to 1000 r / min to prepare a spray feed slurry, the solid content of the spray feed slurry being 10 wt% to 40 wt% based on the total mass of the slurry, spray dry the spray feed slurry, the inlet temperature of the spray drying being 130°C to 200°C, the outlet temperature of the spray drying being 60°C to 110°C, the spray drying being carried out by using a two-fluid nozzle atomization or centrifugal atomization, the atomization pressure being 0.2 MPa to 0.8 MPa or the atomization speed being 5000 r / min to 20000 r / min, the feeding rate being 0.2 L / h to 5.0 L / h, to obtain the composite microspheres.

8. The production method according to claim 7, characterized by, The step S1 is preceded by a palygorskite pretreatment step: activate the palygorskite by acid treatment in an aqueous hydrochloric acid solution, the concentration of the hydrochloric acid being 0.1 mol / L to 2.0 mol / L, the treatment temperature being 20°C to 80°C, the treatment time being 0.5 h to 6 h, and then perform solid-liquid separation, wash with deionized water for 2 to 10 times, the volume of the washing solvent being 5 times to 20 times the mass of the solid each time, wash until the pH value of the filtrate measured continuously for two times is 6 to 8, and dry at 50°C to 120°C until the constant weight, the difference in the mass measured continuously for two times being ≤0.5%; The volume ratio of ethanol to deionized water in the mixed solvent in step S1 is 95:5 to 50:

50.

9. The preparation method according to claim 7, characterized in that, When preparing the spray feed slurry in step S3, add polyvinylpyrrolidone with a weight average molecular weight of 10,000 to 1,300,000, the amount of the polyvinylpyrrolidone added being 0.05 wt% to 2.00 wt% of the total amount of the solid in the spray feed slurry.

10. The method of claim 7, wherein, The step S3 is followed by a metal coordination treatment step: contact the composite microspheres obtained in step S3 with an aqueous ferric chloride solution at a stirring speed of 200 r / min to 1000 r / min for 0.5 min to 60 min, the concentration of the ferric chloride being 0.001 mol / L to 0.100 mol / L based on the anhydrous ferric chloride, the solid-liquid ratio of the composite microspheres to the aqueous ferric chloride solution being 1 g:5 mL to 1 g:200 mL, the contact temperature being 10°C to 60°C, and there is no visible precipitation or visible stratification in the solution system during the contact process, then wash with deionized water for 2 to 10 times, the volume of the washing solvent being 5 times to 20 times the mass of the solid each time, and dry at 40°C to 120°C until the constant weight, the difference in the mass measured continuously for two times being ≤0.5%.

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