Temperature-resistant and salt-resistant polyanionic cellulose and preparation method thereof

Abstract

The application discloses a kind of temperature-resistant salt-tolerant polyanionic cellulose and preparation method thereof, belong to polyanionic cellulose preparation technical field.The method is first bleached wood pulp or cotton pulp cellulose is activated by low concentration NaOH, then grafted with γ-aminoethyl-γ-aminopropyl trimethoxysilane (KH-792) in alcohol-water system, and amino-functionalized cellulose is obtained;Subsequently, amino-functionalized cellulose is added to UiO-66-NH2 precursor sol, and MOF shell is grown in situ by microwave, forming core-shell structure;Finally, freeze-dried to build multi-level hole aerogel, and the finished product is obtained after crushing.And further ion exchange can be carried out by CaCl2 solution, and Ca-UiO-66-NH2@Cellulose is obtained.Product 1 wt% addition in saturated brine Apparent viscosity ≥65 mPa·s, viscosity retention rate ≥90% after 180℃ / 16 h aging, API filtration loss ≤6 mL, suitable for deep well high calcium drilling fluid filtration loss and viscosity, temperature-resistant, salt-tolerant, and environmental performance is superior to commercially available PAC.

Description

Technical Field

[0001] This invention belongs to the field of polyanionic cellulose preparation technology, specifically relating to a temperature- and salt-resistant polyanionic cellulose and its preparation method. Background Technology

[0002] Polyanionic cellulose (PAC), a water-soluble cellulose ether derivative, is widely used in drilling fluid systems to improve viscosity, reduce filtration loss, and stabilize the wellbore. However, traditional PAC is prone to molecular chain degradation, conformational collapse, and insufficient ion exchange capacity under high-temperature (>150℃) and high-salt (especially high calcium and magnesium ion) environments, leading to significant viscosity decreases and a sharp increase in filtration loss, making it difficult to meet the requirements of deep wells, ultra-deep wells, and drilling in complex formations. This is mainly due to its single polymer structure lacking a heat-resistant framework, underdeveloped pore structure, and weak resistance to ion interference. Although existing technologies attempt to improve PAC performance by introducing inorganic nanomaterials or cross-linking modification, they often face problems such as complex preparation processes, uneven loading, and poor controllability of pore structure, making it difficult to achieve long-term stability under high-temperature and high-salt environments. Therefore, developing a polyanionic cellulose with high specific surface area, hierarchical pore structure, high ion exchange capacity, and excellent thermal stability has become an urgent need in the field of drilling fluid materials.

[0003] Metal-organic frameworks (MOFs) are crystalline porous materials formed by the self-assembly of metal ions or clusters with organic ligands through coordination bonds. They possess advantages such as high specific surface area, tunable pore size, and structural designability. Among them, UiO-66-NH2 has attracted much attention due to its excellent thermal and chemical stability. Introducing it into the PAC system to construct a robust organic-inorganic hybrid structure holds promise for utilizing the rigid channels and thermal stability of MOFs to protect the PAC molecular chains, thereby significantly improving their temperature and salt resistance. However, achieving robust and uniform growth of MOFs on cellulose substrates and constructing a synergistic hierarchical porous structure remains a pressing technical challenge in this field. Summary of the Invention

[0004] To address the common technical challenge of rapid performance degradation of polyanionic cellulose under high-temperature and high-salt environments in existing technologies, this invention aims to provide a temperature- and salt-resistant polyanionic cellulose and its preparation method. This method utilizes innovative MOF (UiO-66-NH2) in-situ hybridization and aerogel construction technology to significantly improve the viscosity stability and filtration loss control of the product under harsh conditions such as temperatures above 180°C and saturated salt water. To solve the aforementioned technical problems, this invention adopts the following technical solution: This invention provides a method for preparing heat- and salt-resistant polyanionic cellulose, comprising the following steps: S1: Activated cellulose is obtained by activating bleached wood pulp or cotton pulp cellulose with 1–3 wt% NaOH at room temperature for 1–3 h, followed by acid neutralization, alcohol replacement, and drying. S2: Activated cellulose is dispersed in an alcohol-water mixed solvent with an alcohol-water mass ratio of 85:15. The pH is adjusted to 4.5–5.5 with acetic acid. γ-aminoethyl-γ-aminopropyltrimethoxysilane KH-792 is added dropwise under nitrogen protection at an amount of 25–35% of the dry weight of cellulose. The mixture is refluxed at 45–55 °C for 3–5 h to obtain aminated cellulose. S3: Immediately add aminated cellulose to the MOF precursor sol. The sol composition is: 0.05–0.15 mol / L ZrCl4, 0.05–0.15 mol / L 2-aminoterephthalic acid, 0.5–1.0 mol / L acetic acid, DMF / H2O volume ratio 7:3, and cellulose to sol mass ratio 1:20–1:30. S4: In a closed microwave reactor, the temperature was increased to 120 °C at 2.45 GHz and 400–600 W, held at the temperature for 15–30 min, cooled and filtered, DMF was recovered, and washed with ethanol to obtain UiO-66-NH2@Cellulose wet gel. S5: The wet gel was pre-frozen at -40 ℃ for 12 h, and then freeze-dried for 48 h to obtain hierarchical porous UiO-66-NH2@Cellulose aerogel; S6: Pulverize the aerogel to ≤0.15 mm to obtain the heat- and salt-resistant polyanionic cellulose product.

[0005] Furthermore, in step S5, after freeze-drying, the specific surface area increases to 700–1000 m² / g, the pore volume is 0.25–0.35 cm³ / g, and the loading is 15–30 wt%.

[0006] Furthermore, step S5 freeze-drying forms a three-dimensional hierarchical porous structure with a pore size distribution of 2–50 nm.

[0007] Furthermore, after step S6, the process includes ion exchange with 0.3–0.8 mol / L CaCl2 solution at 25–50 °C for 30–120 min to obtain Ca-UiO-66-NH2@Cellulose aerogel.

[0008] Furthermore, the apparent viscosity of 1 wt% heat- and salt-resistant polyanionic cellulose product in a saturated salt aqueous solution is ≥65 mPa·s (170 s⁻¹, 25 °C). Viscosity retention rate ≥90% after aging at 180 ℃ for 16 h; API filtration loss ≤6 mL (180 ℃, 3.5 MPa, 1 wt% dosage).

[0009] Furthermore, the heat- and salt-resistant polyanionic cellulose is suitable for use as a filtration reducer and viscosity enhancer in deep well drilling fluids.

[0010] The temperature- and salt-resistant polyanionic cellulose prepared in this invention is produced through a multi-step continuous interface engineering process. A covalently anchored metal-organic framework (MOF) reinforcing phase is constructed on a cellulose matrix, forming a stable organic-inorganic hybrid aerogel structure with hierarchical pores. This process begins with the hydrolytic condensation reaction of hydroxyl groups on the cellulose surface with the methoxy groups of the silane coupling agent KH-792 under acidic catalysis. This step successfully introduces primary amino functional groups with strong coordination ability onto the cellulose chains. These amino groups not only alter the surface properties of cellulose but also serve as nucleation sites for subsequent MOF growth. When the aminated cellulose is placed in a precursor sol containing zirconium ions and 2-aminoterephthalic acid ligands, the amino groups on the cellulose surface can effectively lower the nucleation barrier of the MOF (UiO-66-NH2) through interaction with zirconium clusters or organic ligands. This guides the MOF crystals to undergo in-situ, uniform epitaxial growth on the cellulose fiber surface and in the interstices, rather than spontaneous homogeneous nucleation in solution. In this process, acetic acid acts as a modulator, controlling the formation rate of zirconium clusters through competitive coordination, thereby regulating the crystal size and defect density of the MOF. Finally, through freeze-drying, the solvent in the wet gel system is directly sublimated, maximizing the preservation of the three-dimensional hierarchical pore structure composed of cellulose fiber networks and MOF crystal stacking, which includes micropores (from the MOF itself) and mesopores (from the interface between the two), rather than causing pore collapse due to liquid surface tension.

[0011] The unique hybrid structure and reaction pathway described above together determine the product's excellent temperature and salt resistance. Its mechanism of action can be attributed to the synergistic effect of "interface stability" and "pore confinement." At high temperatures, the firmly covalently grafted UiO-66-NH2MOF, acting as a rigid, highly thermally stable nano-reinforcement, plays the role of a physical cross-linking point, effectively inhibiting the thermal motion, coiling, and degradation of cellulose molecular chains, thus macroscopically exhibiting extremely high high-temperature viscosity retention. In high-salt environments, traditional polyanionic cellulose fails due to the compression of the electric double layer, while the hybrid aerogel of this product, relying on its abundant micropores and mesopores, generates a strong "confinement" and "anchoring" effect on free water through capillary forces and surface tension, forming a stable bound water layer. Simultaneously, the negatively charged interface generated by the anionic properties of the MOF framework and cellulose itself forms a dynamic equilibrium with the cations in the salt water, jointly maintaining the hydrated lubricating film of the system under high pressure. This mechanism, which combines physical sieving with chemical stabilization, allows the material to maintain excellent thickening properties and extremely low filtration loss even under extreme conditions such as high temperature of 180°C and saturated brine.

[0012] The beneficial effects of this invention are: 1. A uniform UiO-66-NH2 core-shell structure was constructed on the surface of cellulose by silane grafting and microwave in-situ growth of MOF. The rigid channels of MOF and the three-dimensional network of cellulose formed a stable hierarchical pore structure, which significantly improved the specific surface area and thermal stability. 2. The freeze-drying process avoids pore collapse, imparts high pore volume and narrow distribution of mesopores to the material, and enhances ion buffering capacity and polymer chain anchoring effect; 3. Optional Ca 2+ Ion exchange further optimizes performance in high-calcium environments, resulting in lower filtration loss. 4. The product exhibits high viscosity retention and low filtration loss under high temperature and high salinity conditions, demonstrating significantly superior performance compared to commercially available PACs and meeting the requirements for deep well drilling. Attached Figure Description

[0013] Figure 1 Example 1: 77 K nitrogen adsorption-desorption isotherm of the finished product; Figure 2 Example 1: BJH pore size distribution diagram of the finished product Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only a part of the embodiments of this invention, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0015] The following examples illustrate the present invention's method for preparing a temperature- and salt-resistant polyanionic cellulose.

[0016] Example 1: (1) Cellulose activation 20g of bleached softwood pulp was weighed and immersed in 400mL of 2wt% NaOH solution. The pulp was magnetically stirred at 25℃ for 2h to remove hemicellulose and residual lignin. It was then filtered and washed with deionized water until neutral. The pulp was then acidified with 0.1mol / L HCl for 10min to protonate the surface hydroxyl groups. The pulp was washed with water until pH≈6. The water was replaced with ethanol three times. The pulp was then vacuum dried at 60℃ for 12h to obtain 18.3g of activated cellulose (AC).

[0017] (2) Silane grafting covalent anchoring 10 g of AC was dispersed in 200 mL of an ethanol / water (85:15, w / w) mixed solvent. The pH was adjusted to 5.0 with acetic acid, and dissolved oxygen was removed by purging with nitrogen for 30 min. 3 g of γ-aminoethyl-γ-aminopropyltrimethoxysilane (KH-792) (30% of the dry weight of AC) was added dropwise under N2 protection at 50 °C, and the mixture was refluxed for 4 h. After the reaction was complete, the mixture was cooled, filtered, washed three times with ethanol, and dried under vacuum at 80 °C for 8 h to obtain 11.4 g of aminated cellulose (Cell-NH2) with a nitrogen content of 2.2 wt% (elemental analysis), indicating successful silane grafting.

[0018] (3) Microwave in-situ growth of UiO-66-NH2 core-shell structure Preparation of MOF precursor sol: ZrCl4 3.5g, 2-aminoterephthalic acid 2.7g, and acetic acid 6.0g were dissolved sequentially in a mixed solvent of DMF 70mL and deionized water 30mL, and stirred at 25℃ until clear (molar ratio Zr:ligand:acetic acid ≈ 1:1:10). Cell-NH2 2.0g was added to 40mL of the above sol, pre-adsorbed for 30min, and then transferred to a 100mL PTFE-lined microwave reactor. The reactor was heated to 120℃ at 500W for 5min, held at that temperature for 20min, and then allowed to cool naturally. The product was filtered, DMF was recovered and recycled, and the product was washed with ethanol until colorless to obtain UiO-66-NH2@Cellulose wet gel.

[0019] (4) Freeze-drying to construct hierarchical porous aerogels The wet gel was pre-frozen at -40°C for 12 hours, and then transferred to a freeze dryer (cold trap -40°C, vacuum <10Pa) for 48 hours to obtain 2.60 g of white fluffy aerogel with a loading of 25 wt%.

[0020] (5) Crushing and Finished Product Preparation The UiO-66-NH2@Cellulose aerogel obtained in step (4) was placed in a high-speed pulverizer and run at 15,000 r / min for 30 s. After passing through a 100-mesh sieve, a white, fluffy powder with a particle size ≤0.15 mm was obtained, which is the finished product of heat- and salt-resistant polyanionic cellulose. This powder can be directly used in deep well high-calcium drilling fluid systems without pre-swelling.

[0021] like Figure 1As shown, the sample exhibits a typical H3-type hysteresis loop: the adsorption branch rises steeply in the low-pressure region (p / p0 < 0.1), indicating abundant micropores; in the medium-to-high-pressure region (0.1–0.8), it rises continuously without a plateau, corresponding to the slit-like mesopores formed by the stacking of plate-like particles. The desorption branch lies above the adsorption branch throughout, with the hysteresis loop opening to the high-pressure end (p / p0 ≈ 0.45–0.85) and closing at p / p0 = 1, resulting in a total pore volume of 0.38 cm³ / g (STP). Calculations using the 0.05–0.30 linear segment yielded a BET specific surface area of ​​905 m² / g and a BJH peak pore size of approximately 4 nm, confirming the introduction of a hierarchical porous structure.

[0022] Figure 2 The BJH pore size distribution (desorption branch) of UiO-66-NH2@Cellulose is presented. A sharp main peak appears at 4.0 nm, corresponding to the lattice pores of UiO-66 and the secondary pores at the cellulose-MOF interface; the tail signal extends to 50 nm, reflecting the narrow slit mesopores formed during freeze-drying. The full width at half maximum (FWHM) of the main peak is approximately 1.2 nm, and the peak area accounts for more than 50% of the cumulative pore volume, confirming that the sample possesses a narrowly distributed, high-capacity mesoporous structure, which is beneficial for ion buffering and polymer chain anchoring.

[0023] Saturated brine-based slurry (26 wt% NaCl) was prepared according to API 13A standard, and the results were determined with a 1 wt% sample addition: Apparent viscosity: 65 mPa·s (25℃, 170 s⁻¹); Viscosity after hot rolling at 180℃ for 16 hours: 59 mPa·s, retention rate 91%; API filtration loss: 5.8 mL (180℃, 3.5 MPa); All indicators are significantly better than commercially available high-viscosity PAC.

[0024] Example 2: (1) Preparation of precursor Use 2.0g of UiO-66-NH2@Cellulose aerogel obtained by following steps (1)-(4) of Example 1 for later use.

[0025] (2) Ca²⁺ ion exchange The aerogel was immersed in 50 mL of 0.5 mol / L CaCl2 solution and stirred in a constant temperature water bath at 30 °C for 1 h. It was then vacuum filtered and repeatedly washed with deionized water until no Cl⁻ was found in the filtrate (no white precipitate was observed upon titration with AgNO3). The resulting wet material was pre-frozen again at -40 °C for 12 h and freeze-dried for 48 h (-40 °C, <10 Pa) to obtain 1.95 g of a light gray, fluffy solid, namely the final product Ca-UiO-66-NH2@Cellulose. 2+ Exchange capacity: 450 mg / g (EDTA titration).

[0026] (3) Crushing and Finished Product Preparation The Ca-UiO-66-NH2@Cellulose aerogel obtained in step (2) was placed in a high-speed pulverizer and run at 15,000 r / min for 30 s. After passing through a 100-mesh sieve, a white fluffy powder with a particle size ≤0.15 mm was obtained, which is the finished product of heat-resistant and salt-resistant polyanionic cellulose.

[0027] Prepare a saturated brine-based slurry (26 wt% NaCl) according to API 13A, and determine its composition at a 1 wt% addition: Apparent viscosity at 25℃: 68 mPa·s (170 s⁻¹); Viscosity after hot rolling at 180℃ for 16 hours: 62 mPa·s, retention rate 91%; API filtration loss: 5.5 mL (180℃, 3.5 MPa), a further decrease of 0.3 mL compared to Na⁺ type final product; Apparent viscosity in high-calcium environment (25wt%CaCl2): 72mPa·s, filtration loss: 6.0mL.

[0028] Conclusion: Ca-UiO-66-NH2@Cellulose combines high specific surface area, high Ca²⁺ exchange capacity, and excellent temperature and salt resistance, making it particularly suitable for deep well high-calcium drilling fluid systems.

[0029] Comparative Example 1: Referring to API 13A standards, commercially available high-viscosity anionic cellulose (PAC-HV, unhybridized) was directly selected as a blank control without further processing. Performance tests were conducted using the same base pulp formulation (26 wt% NaCl saturated brine) and dosage (1 wt%) as in Example 1, and the results are as follows: Apparent viscosity at 25 ℃: 35 mPa·s (170 s⁻¹); Viscosity after hot rolling at 180 ℃ for 16 h: 19 mPa·s, retention rate 54%; API filtration loss: 14 mL (180 ℃, 3.5 MPa); Apparent viscosity in high-calcium environment (25 wt% CaCl2): 28 mPa·s, filtration loss: 12 mL.

[0030] Compared with Example 1, pure PAC-HV exhibits significant viscosity decay and high filtration loss under high temperature and high salt conditions, and does not possess the temperature and salt resistance required for deep well applications.

[0031] Finally, it should be noted that the above specific 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 examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing heat- and salt-resistant polyanionic cellulose, characterized in that, In order, they include: S1: Activated cellulose is obtained by activating bleached wood pulp or cotton pulp cellulose with 1–3 wt% NaOH at room temperature for 1–3 h, followed by acid neutralization, alcohol replacement, and drying. S2: Activated cellulose is dispersed in an alcohol-water mixed solvent with an alcohol-water mass ratio of 85:

15. The pH is adjusted to 4.5–5.5 with acetic acid. γ-aminoethyl-γ-aminopropyltrimethoxysilane KH-792 is added dropwise under nitrogen protection at an amount of 25–35% of the dry weight of cellulose. The mixture is refluxed at 45–55 °C for 3–5 h to obtain aminated cellulose. S3: Immediately add aminated cellulose to the MOF precursor sol. The sol composition is: 0.05–0.15 mol / L ZrCl4, 0.05–0.15 mol / L 2-aminoterephthalic acid, 0.5–1.0 mol / L acetic acid, DMF / H2O volume ratio 7:3, and cellulose to sol mass ratio 1:20–1:

30. S4: In a closed microwave reactor, the temperature was increased to 120 °C at 2.45 GHz and 400–600 W, held at the temperature for 15–30 min, cooled and filtered, DMF was recovered, and washed with ethanol to obtain UiO-66-NH2@Cellulose wet gel. S5: The wet gel was pre-frozen at -40 ℃ for 12 h, and then freeze-dried for 48 h to obtain hierarchical porous UiO-66-NH2@Cellulose aerogel; S6: Pulverize the aerogel to ≤0.15 mm to obtain the heat-resistant and salt-resistant polyanionic cellulose product.

2. The method for preparing a temperature- and salt-resistant polyanionic cellulose according to claim 1, characterized in that, In step S5, after freeze-drying, the specific surface area increases to 700–1000 m². 2 / g, pore volume 0.25–0.35 cm³ 3 / g, with a loading of 15–30 wt%.

3. The method for preparing a temperature- and salt-resistant polyanionic cellulose according to claim 1, characterized in that, In step S5, freeze-drying forms a three-dimensional hierarchical porous structure with a pore size distribution of 2–50 nm.

4. The method for preparing a temperature- and salt-resistant polyanionic cellulose according to claim 1, characterized in that, Step S5 includes adding UiO-66-NH2@Cellulose aerogel to 0.3–0.8 mol / L CaCl2 solution for ion exchange at 25–50℃ for 30–120 min to obtain Ca-UiO-66-NH2@Cellulose aerogel.

5. A temperature- and salt-resistant polyanionic cellulose prepared according to any one of claims 1-4, characterized in that, 1 wt% of the heat- and salt-resistant polyanionic cellulose product has an apparent viscosity ≥65 mPa·s in a saturated salt aqueous solution, and its test conditions are 170 s. -1 25 ℃; Viscosity retention rate ≥90% after aging at 180 ℃ for 16 h; API filtration loss ≤6 mL, the test conditions are 180 ℃, 3.5 MPa.

6. The temperature- and salt-resistant polyanionic cellulose according to claim 5, characterized in that, The temperature- and salt-resistant polyanionic cellulose is suitable for use as a filtration reducer and viscosity enhancer in deep well drilling fluids.

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