A method for preparing magnesium aluminum carbonate-type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursors

Abstract

This invention relates to the field of layered double hydroxide preparation technology, specifically disclosing a method for preparing magnesium aluminum carbonate type hydrotalcite based on a carbon dioxide-derived magnesium carbonate precursor. The method includes contacting MgO or Mg(OH)₂ with supercritical CO₂ in the presence of water to form a first precursor system containing MgCO₃·3H₂O; subjecting the first precursor system to hydrothermal aging in an aqueous phase to form a second precursor system containing Mg₅(CO₃)₄(OH)₂·4H₂O; and reacting the second precursor system with γ-Al₂O₃ under hydrothermal conditions to obtain Mg-Al-CO₃ layered double hydroxide. This invention uses CO₂ as the carbonate source and controls the hydrotalcite formation process through a two-step phase transformation of MgCO₃·3H₂O and a basic magnesium carbonate intermediate phase, avoiding the use of commercial basic magnesium carbonate or natural magnesite as a necessary magnesium source, and avoiding the use of Na₂O. + K + NH4 + Cl ‑ NO3 ‑ or SO4 2‑ The Mg-Al-CO3 hydrotalcite is produced by the introduction of foreign ions and the generation of saline wastewater. It can be used as a resin heat stabilizer, flame retardant, adsorbent material, or catalytic precursor.

Description

Technical Field

[0001] This invention relates to the field of layered double hydroxide preparation technology, specifically to a method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursor. Background Technology

[0002] Magnesium aluminum carbonate type hydrotalcite is a typical layered double hydroxide, whose layers are composed of Mg 2+ And Al 3+ Composed of the same structure, the interlayer typically contains CO3. 2- It contains water molecules. This type of material can be used as a resin heat stabilizer, flame retardant synergist, adsorbent, catalyst precursor, and functional filler.

[0003] Traditional Mg-Al-CO3 hydrotalcite is typically prepared by co-precipitation of magnesium salts, aluminum salts, alkalis, and carbonates, using raw materials such as MgCl2, Mg(NO3)2, AlCl3, Al(NO3)3, NaOH, and Na2CO3. This method is well-established, but it produces Na-containing compounds. + Cl - NO3 - or SO4 2- The mother liquor and washing wastewater from plasma treatment may contain foreign ions, which is not conducive to the preparation of low-salt, low-impurity materials.

[0004] Existing technologies also include methods for preparing hydrotalcite using CO2, magnesium carbonate, basic magnesium carbonate, or natural magnesite as carbonate or magnesium sources. However, some methods still require the addition of NaOH, KOH, ammonia, urea, aluminates, or soluble aluminum salts, making it difficult to avoid the introduction of foreign ions. Other methods use commercial basic magnesium carbonate or natural magnesite as raw materials, and the source of precursors, phase composition, and reactivity are limited by the raw materials themselves, making it difficult to simultaneously achieve in-situ CO2-derived precursors, avoid the introduction of foreign ions, and achieve γ-Al2O3 hydrothermal reconstruction.

[0005] Patent document CN116495761B discloses a method for preparing magnesium-aluminum hydrotalcite using magnesium chloride as the main magnesium source, aluminum hydroxide as the aluminum source, and adding magnesium carbonate and sodium hydroxide. This method can adjust the magnesium-aluminum ratio and supplement carbonate ions by using magnesium carbonate. However, due to the use of magnesium chloride and sodium hydroxide, Cl- introduced from the raw materials inevitably exists in the reaction liquid phase. - and Na +While the literature discloses the filtration and washing steps, it does not explain the closed-loop reuse method for the mother liquor and washing liquid, nor does it provide residual indicators of Na and Cl in the product. Therefore, in the preparation of low-sodium, low-chlorine hydrotalcite, it is still necessary to treat the mother liquor or washing liquid containing soluble ions. Furthermore, this method directly uses magnesium carbonate as an auxiliary raw material and does not involve the process of in-situ formation of different magnesium carbonate intermediate phases by CO2 and the phased control of these phases.

[0006] Patent document CN107416872B discloses a method for preparing magnesium aluminum carbonate type hydrotalcite by directly reacting magnesium oxide or magnesium hydroxide, an aluminum source, and carbon dioxide in water in the presence of catalytic amounts of sodium carbonate, sodium bicarbonate, or ammonium carbonate, and proposes recycling the filtrate. This method reduces the generation of stoichiometric salt byproducts in traditional co-precipitation processes; however, its reaction system still relies on added sodium- or ammonium-containing alkaline components, failing to completely avoid sodium from the raw material stage. + or NH4 + The introduction of .

[0007] Patent document WO1993022237A1 discloses a method for preparing hydrotalcite by first reacting magnesium oxide or magnesium hydroxide with carbon dioxide to form a hydromagnesite-like basic magnesium carbonate intermediate, and then reacting this intermediate with an aluminum source under hydrothermal conditions. The aluminum source used in this technology is Bayer liquid, alkali metal aluminate, or a combination of alkali metal aluminate and alumina trihydrate, with sodium aluminate being preferred. Therefore, the subsequent reaction still introduces soluble sodium components. This document does not disclose the continuous phase transformation process of first forming MgCO3·3H2O under supercritical carbon dioxide conditions, and then forming Mg5(CO3)4(OH)2·4H2O through independent hydrothermal aging, nor does it disclose the use of γ-Al2O3 as an aluminum source without added alkali metal for subsequent hydrothermal reconstruction.

[0008] In addition, existing studies on the MgO / Mg(OH)2-CO2-H2O system have mostly focused on CO2 mineralization or magnesium carbonate preparation, with less emphasis on using CO2-derived MgCO3·3H2O and its hydrothermal aging products as controllable precursors for the hydrothermal reconstruction of Mg-Al-CO3 layered double hydroxides. Therefore, it is necessary to provide a method for preparing Mg-Al-CO3 hydrotalcite that uses CO2-derived magnesium carbonate precursors as the core, reduces the introduction of foreign ions, and can control the evolution of the intermediate phase. Summary of the Invention

[0009] This invention aims to provide a method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursors. This method achieves a two-step phase transformation of MgO or Mg(OH)2 into Mg-Al-CO3 layered double hydroxides through three stages: supercritical CO2 carbonation, hydrothermal aging, and γ-Al2O3 hydrothermal reconstruction.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursor includes the following steps: (1) Contact MgO or Mg(OH)2 with supercritical CO2 in the presence of water to transform MgO or Mg(OH)2 into a first precursor system with magnesium-containing crystalline phase as the main phase, MgCO3·3H2O. (2) Before the wet heat aging, the first precursor system is subjected to solid-liquid separation, or CO2 is released to relieve pressure before the wet heat aging; then the first precursor system is subjected to wet heat aging in the aqueous phase, so that MgCO3·3H2O is converted into a second precursor system with Mg5(CO3)4(OH)2·4H2O as the main phase and containing magnesium crystal phase. (3) The second precursor system is reacted with γ-Al2O3 under hydrothermal conditions to obtain a product with Mg-Al-CO3 layered double hydroxide as the main phase.

[0011] As an improvement, the pressure of the supercritical CO2 in step (1) is 7.5-20 MPa, the temperature is 35-70℃, and the reaction time is 0.5-12 h.

[0012] Preferably, the supercritical CO2 pressure in step (1) is 8-12 MPa, the temperature is 40-55℃, and the reaction time is 2-8 h.

[0013] As an improvement, in step (1), when the magnesium source is MgO, the ratio of the volume of water to the mass of the magnesium source is 3 to 20 mL / g; when the magnesium source is Mg(OH)2, the mass of the magnesium source is calculated as the mass of equimolar MgO based on the Mg content therein.

[0014] Preferably, when the magnesium source is MgO, the ratio of water volume to magnesium source mass is 5–15 mL / g.

[0015] As an improvement, the wet heat aging temperature in step (2) is 60-110℃, the time is 0.5-12h, and the ratio of the volume of water to the solid mass of the first precursor system on a dry basis is 3-20mL / g.

[0016] Preferably, the damp heat aging temperature in step (2) is 75-95℃, the time is 1-8h, and it is carried out in a semi-open reflux, controllable exhaust or closed system with gas phase space.

[0017] As an improvement, the Mg / Al molar ratio in step (3) is 1.8-2.3, preferably 1.9-2.1.

[0018] As an improvement, the hydrothermal temperature of step (3) is 120-220℃, the reaction time is 2-24h, and the ratio of the volume of water to the solid mass of the first precursor system on a dry basis is 5-30mL / g.

[0019] As an improvement, the γ-Al2O3 is active alumina, or a transition alumina with γ-Al2O3 as the main phase, obtained by calcining Al(OH)3.

[0020] As an improvement, the first precursor system obtained in step (1) is separated into solid and liquid to obtain MgCO3·3H2O precursor before step (2), and the second precursor system obtained in step (2) is separated into solid and liquid to obtain basic magnesium carbonate precursor before step (3); or, steps (1), (2) and (3) are carried out continuously in the same reactor by programmed temperature control, pressure control, exhaust and water replenishment operations. After step (1) is completed, CO2 needs to be released to reduce pressure and exhaust until the system pressure is reduced to no more than 2MPa.

[0021] This invention also provides a method for preparing Mg-Al-CO3 layered double hydroxides based on a carbon dioxide-derived magnesium carbonate precursor.

[0022] The technical principle of this invention is as follows: In this invention, the goal of the first stage is not to directly form hydrotalcite, but to obtain a MgCO3·3H2O precursor system with controllable phase composition and high reactivity. When the magnesium source is MgO, it first undergoes hydration in the presence of water to generate Mg-OH surface species, and then is further carbonized by CO2 to form MgCO3·3H2O.

[0023] In the second stage, MgCO3·3H2O is transformed into a basic magnesium carbonate precursor system with Mg5(CO3)4(OH)2·4H2O as the main phase through a dissolution-reprecipitation and carbonate rearrangement process in the aqueous phase. This basic magnesium carbonate precursor system combines Mg source, OH structural unit, and CO3. 2- source.

[0024] In the third stage, the second precursor system reacts with γ-Al₂O₃ under hydrothermal conditions. γ-Al₂O₃ provides Al species through hydration and surface hydroxylation; the second precursor system provides Mg species, and the Al species provided by γ-Al₂O₃ are reconstructed to form a Mg-Al hydroxide layer. CO₃²⁻ 2- The positive charge enters the interlayer equilibrium plate, resulting in Mg-Al-CO3 layered double hydroxide.

[0025] Compared with the prior art, the present invention has the following advantages: First, CO2 should be used as the source of carbonate, avoiding the addition of carbonates such as Na2CO3 and NaHCO3. Second, commercial basic magnesium carbonate or natural magnesite should not be used as the necessary magnesium source, and the use of soluble magnesium salts, aluminum salts, and strong alkalis should be avoided to prevent the formation of Na+ ions. + K + NH4 + Cl - NO3 - or SO4 2- The method involves several key aspects: first, the introduction of foreign ions and the generation of saline wastewater; second, the reactivity of the precursor is regulated through the use of MgCO3·3H2O and a basic magnesium carbonate intermediate phase, resulting in a product primarily composed of Mg-Al-CO3 layered double hydroxides, which helps reduce the risk of residual secondary phases such as MgO, Mg(OH)2, MgCO3, AlOOH, or γ-Al2O3; third, experimental verification shows that the mass concentrations of Mg and Al in the mother liquor after the reaction are both below 10 mg / L, indicating high utilization of metal components; fourth, this method can be implemented in steps or continuously through a single reactor with segmented temperature and pressure control, venting, and water replenishment. The Mg-Al-CO3 hydrotalcite obtained by this invention can be used as a resin heat stabilizer, flame retardant additive, adsorbent material, or catalytic precursor. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of a two-step phase transformation process according to an embodiment of the present invention.

[0027] Figure 2 shows the XRD patterns of the Mg-Al-CO3 layered double hydroxides obtained in Examples 1, 2, and 3.

[0028] Figure 3 shows the FTIR spectra of the Mg-Al-CO3 layered double hydroxides obtained in Examples 1, 2, and 3.

[0029] Figure 4 shows the SEM morphology of the Mg-Al-CO3 layered double hydroxides obtained in Examples 1, 2 and 3.

[0030] Figure 5 shows the XRD pattern of the product prepared by one-pot method without the phase transformation of magnesium carbonate precursor (i.e., Comparative Example 1).

[0031] Figure 6 shows the XRD patterns of the products obtained in Comparative Example 2 and Example 3.

[0032] Figure 7 shows the XRD patterns of the products obtained in Comparative Example 3 and Comparative Example 4. Detailed Implementation

[0033] The following detailed description illustrates the specific implementation method: In this embodiment, deionized water and high-purity CO2 were used. MgO, Mg(OH)2, and γ-Al2O3 can be commercially available high-purity raw materials, preferably with a purity of 99.5% or higher. Parameters such as centrifugation speed, stirring speed, and drying time can be adjusted according to equipment specifications and sample quantity.

[0034] It should be noted that, for the convenience of intermediate product characterization, the first and second stage products in the following examples were centrifuged and dried; in actual industrial implementation, the slurry obtained in the first stage can be directly subjected to the wet heat aging stage through heating, depressurization, exhaust and water replenishment without drying, and the slurry obtained in the second stage can also be directly subjected to hydrothermal reconstruction by adding γ-Al2O3 without drying.

[0035] I. A general preparation method for magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursors, the steps of which are as follows: First stage: MgO or Mg(OH)2 is mixed with water to form a magnesium-containing slurry, which is placed in a high-pressure reactor. After the air inside the reactor is replaced with CO2, the mixture is reacted for 8 h at 45 ℃ and 9 MPa CO2 to form the first precursor system containing MgCO3·3H2O.

[0036] The second stage involves hygrothermal aging of the first precursor system at 85 °C for 6 h in the presence of water to form a second precursor system containing Mg5(CO3)4(OH)2·4H2O. Aging is carried out in a semi-open reflux system with controlled exhaust or a closed system with a gas phase space.

[0037] The third stage: The second precursor system was mixed with γ-Al2O3 at a Mg / Al molar ratio of 1.8-2.3, and deionized water was added to prepare a slurry. The mixture was then subjected to hydrothermal reaction at 120 °C for 24 h. After solid-liquid separation and drying, Mg-Al-CO3 layered double hydroxide was obtained.

[0038] This method does not involve the addition of any alkaline components containing Na, K, or NH4, alkali metal carbonates, or soluble magnesium or aluminum salts in any of its steps.

[0039] To verify the general preparation method of the present invention, the following typical examples 1-3 are listed as representatives.

[0040] Example 1: Preparation of nano-magnesium aluminum carbonate hydrotalcite using magnesium oxide as the magnesium source First stage: Weigh 0.57g of magnesium oxide and add 10mL of deionized water. Disperse the mixture by ultrasonic stirring for 30min to obtain a magnesium oxide slurry. Transfer the slurry to a high-pressure reactor. After sealing the reactor, replace the air inside with CO2 twice. Then, heat the reactor to 45℃, introduce CO2 until the pressure reaches 9MPa, and rotate the magnetic stirrer in the high-pressure reactor at 250rpm. React for 8h under stirring conditions. After the reaction is complete, cool the reactor to room temperature, slowly depressurize, remove the slurry, and centrifuge at 9500rpm for 1min to obtain the first-stage wet filter cake.

[0041] A small amount of the first-stage wet filter cake was dried at low temperature and then subjected to XRD analysis. The main product was MgCO3·3H2O, and the corresponding diffraction peaks were consistent with the characteristic peaks of MgCO3·3H2O. No obvious residual peaks of MgO or Mg(OH)2 were observed, indicating that MgO was carbonized by CO2 in the presence of water to form the MgCO3·3H2O precursor.

[0042] Second stage: The wet filter cake from the first stage was added to 17 mL of deionized water, ultrasonically stirred and dispersed for 30 min, and then aged at 85 °C for 6 h using a wet heat treatment system. The aging process was carried out in a semi-open reflux system, and the magnetic stirring speed was maintained at 100 rpm during the wet heat aging. After aging, the mixture was cooled to room temperature and centrifuged at 9500 rpm for 1 min to obtain the basic magnesium carbonate precursor precipitate, which was then vacuum dried at 50 °C for 12 h.

[0043] XRD analysis of the products from the second stage showed that the main product was Mg5(CO3)4(OH)2·4H2O, indicating that MgCO3·3H2O was converted into a magnesium carbonate precursor with Mg5(CO3)4(OH)2·4H2O as the main phase after wet heat aging.

[0044] Third stage: Weigh 0.44 g of the above-mentioned basic magnesium carbonate precursor and 0.12 g of γ-Al₂O₃ according to a Mg / Al molar ratio of 2, and add 10 mL of deionized water to prepare a slurry. After ultrasonic stirring and dispersion for 20 min, transfer to a polytetrafluoroethylene-lined stainless steel hydrothermal reactor, controlling the filling degree to 40%-80%. Then, perform a hydrothermal reaction at 120℃ for 24 h. After the reaction is completed, cool to room temperature, remove the product, centrifuge, and vacuum dry the obtained solid at 70℃ for 12 h to obtain magnesium aluminum carbonate type hydrotalcite.

[0045] The magnesium aluminum carbonate root-type hydrotalcite prepared in Example 1 was analyzed by XRD, as follows: Figure 2As shown, characteristic diffraction peaks of Mg-Al-CO3 layered double hydroxides appear in the low-angle region, and crystal plane-related peaks such as (003), (006), (012), (015), (018), (110), and (113) can be observed. Figure 2 In the low-angle region, a strong characteristic diffraction peak appears at 2θ≈11.5°, which belongs to the (003) crystal plane of Mg-Al-CO3 layered double hydroxide (LDH). At 2θ≈23° and 2θ≈34.5°, diffraction peaks belonging to the (006) and (012) crystal planes are clearly observed, respectively. The peak positions are basically consistent with the LDH standard spectrum. No strong diffraction peaks of obvious MgO, Mg(OH)2, MgCO3 or γ-Al2O3 secondary phases are seen in the spectrum, indicating that the obtained product is mainly composed of Mg-Al-CO3 layered double hydroxide. According to the ICP and thermogravimetric results, the above Mg-Al-CO3 layered double hydroxide structure can be approximately represented as Mg 0.68 Al 0.33 (OH)2(CO3) 0.17 0.5H2O.

[0046] FTIR spectrum as shown Figure 3 As shown, at 3455cm -1 Absorption peaks for the OH stretching vibrations of interlayer water and hydroxyl groups appear nearby, at 1633 cm⁻¹. -1 A bending vibration peak of interlayer water was observed nearby, at 1355 cm⁻¹. -1 CO3 was detected nearby 2- The characteristic absorption peak is at 553 cm⁻¹. -1 and 448cm -1 The presence of Mg-O and Al-O lattice vibration absorption peaks nearby indicates the presence of hydroxylated Mg-Al metal layers, interlayer water, and carbonate anions in the product. The infrared and XRD results corroborate each other, supporting the formation of the Mg-Al-CO3 layered double hydroxide.

[0047] SEM topography image as follows Figure 4 As shown, the product mainly consists of irregular polygonal and some near-hexagonal lamellae. The lamellae surfaces are relatively flat with clear edges. As can be seen from the scale bars in the figure, the lateral dimensions of the lamellae range from submicron to micron, with some variation in size distribution. The lamellae aggregate in a face-to-face stacking, edge-to-face interlacing, and different orientation interpenetration manner, locally forming loose micron-scale secondary aggregates, exhibiting a distinct layered lamellae morphology. Combined with XRD and FTIR characterization results, this further supports the conclusion that the obtained product is a material with Mg-Al-CO3 layered double hydroxide as the main phase.

[0048] Further ICP-OES analysis of the mother liquor from Example 1 showed that the mass concentrations of Mg and Al in the mother liquor were both below 10 mg / L, and Na... + K + and Cl - NO3 - SO4 2- The absence of detected foreign ions indicates that Mg and Al components mainly enter the solid phase product, and this method can avoid the large-scale generation of soluble salt byproducts in traditional coprecipitation methods.

[0049] Example 2: Preparation of nano-magnesium aluminum carbonate hydrotalcite using magnesium hydroxide as the magnesium source First stage: Weigh 0.83g of magnesium hydroxide and add 10mL of deionized water. Disperse the mixture by ultrasonic stirring for 30min to obtain a magnesium hydroxide slurry. The amount of magnesium hydroxide used is approximately the same as the 0.57g magnesium oxide in Example 1, containing the same molar amount of Mg. Transfer the slurry to a high-pressure reactor, seal it, and replace the air inside the reactor twice with CO2. Then, raise the temperature to 45℃, introduce CO2 until the pressure reaches 9MPa, and maintain the magnetic stirrer rotation speed of the high-pressure reactor at 250rpm. React for 8h under stirring conditions. After the reaction is complete, cool the reactor to room temperature, slowly depressurize, remove the slurry, and centrifuge at 9500rpm for 1min to obtain the first-stage wet filter cake.

[0050] A small amount of the first-stage wet filter cake was dried at low temperature and then subjected to XRD analysis. The main product was MgCO3·3H2O, and the corresponding diffraction peaks were consistent with the characteristic peaks of MgCO3·3H2O. No obvious residual peaks of Mg(OH)2 were observed, indicating that Mg(OH)2 can be carbonized by CO2 in the presence of water to form the MgCO3·3H2O precursor.

[0051] Second stage: 17 mL of deionized water was added to the wet filter cake obtained in the first stage of Example 2, and the mixture was ultrasonically stirred and dispersed for 30 min. Then, it was hygrothermally aged at 85 °C for 6 h. The aging process was carried out in a semi-open reflux system, and the magnetic stirring speed was maintained at 100 rpm during the hygrothermally aged process. After aging, the mixture was cooled to room temperature and centrifuged at 9500 rpm for 1 min to obtain the basic magnesium carbonate precursor precipitate, which was then vacuum dried at 50 °C for 12 h.

[0052] After drying, the product obtained in the second stage of Example 2 was analyzed by XRD. It was found that the main component was Mg5(CO3)4(OH)2·4H2O, indicating that MgCO3·3H2O was transformed into a magnesium carbonate precursor with Mg5(CO3)4(OH)2·4H2O as the main phase after wet heat aging.

[0053] Third stage: Weigh 0.44 g of the above-mentioned basic magnesium carbonate precursor and 0.12 g of γ-Al₂O₃ according to a Mg / Al molar ratio of 2, and add 10 mL of deionized water to prepare a slurry. After ultrasonic stirring and dispersion for 20 min, transfer to a polytetrafluoroethylene-lined stainless steel hydrothermal reactor, controlling the filling degree to 40%-80%. Then, perform a hydrothermal reaction at 120℃ for 24 h. After the reaction is completed, cool to room temperature, remove the product, centrifuge, and vacuum dry the obtained solid at 70℃ for 12 h to obtain magnesium aluminum carbonate type hydrotalcite.

[0054] The magnesium aluminum carbonate root-type hydrotalcite prepared in Example 2 was analyzed by XRD, as follows: Figure 2 As shown, characteristic diffraction peaks of Mg-Al-CO3 layered double hydroxides are observed, and no strong diffraction peaks of obvious Mg(OH)2, MgCO3, or γ-Al2O3 precursor phases are seen. This indicates that Mg-Al-CO3 layered double hydroxides can also be prepared via the CO2-derived magnesium carbonate precursor route using Mg(OH)2 as the magnesium source. Based on ICP and thermogravimetric results, its structural formula can be approximately represented as Mg... 0.67 Al 0.33 (OH)2(CO3) 0.17 0.5H2O.

[0055] FTIR spectrum as shown Figure 3 As shown, at 3460 cm -1 Absorption peaks for the OH stretching vibrations of interlayer water and hydroxyl groups appear nearby, at 1628 cm⁻¹. -1 A bending vibration peak of interlayer water was observed nearby, at 1359 cm⁻¹. -1 CO3 was detected nearby 2- The characteristic absorption peak is at 548 cm⁻¹. -1 and 444cm -1 The presence of Mg-O and Al-O lattice vibration absorption peaks nearby indicates the presence of hydroxylated Mg-Al metal layers, interlayer water, and carbonate anions in the product.

[0056] SEM morphology Figure 4 As shown, the sample also consists mainly of plate-like crystals, which are irregular polygonal or nearly hexagonal in shape, with relatively smooth surfaces and clear edges. Larger and smaller plate-like crystals interweave, and some plate-like crystals are distributed vertically or at an angle, forming secondary aggregates through face-to-face stacking and edge-to-face interlacing. Compared with Example 1, the outlines and edges of some plate-like crystals in Example 2 are more obvious, but both exhibit the overall morphology of interlaced layered plate-like crystals. Combined with the XRD and FTIR characterization results, it is shown that by using Mg(OH)2 as the magnesium source, through CO2-derived magnesium carbonate precursor, and further hydrothermal reconstruction with γ-Al2O3, a product with Mg-Al-CO3 layered double hydroxide as the main phase can be prepared.

[0057] Further ICP-OES and ion chromatography tests were performed on the final reaction mother liquor of Example 2. The results showed that the mass concentrations of Mg and Al in the mother liquor were both below 10 mg / L, and Na... + K + and Cl - NO3 - SO4 2- The absence of detected foreign ions indicates that the method of the present invention has a high utilization rate of metal components and can reduce the generation of saline waste liquid.

[0058] Example 3: Preparation of magnesium aluminum carbonate rhizotype hydrotalcite by a single-stage continuous phase transformation Weigh 0.57 g of magnesium oxide, add 10 mL of deionized water, and ultrasonically disperse for 30 min to obtain magnesium oxide slurry. Transfer the slurry to a high-pressure reactor equipped with a CO2 inlet / outlet, pressure gauge, exhaust valve, stirring device, and temperature control device, and replace the air in the reactor twice with CO2.

[0059] In the first stage, the reactor was heated to 45°C, CO2 was introduced to 9 MPa, and the reaction was carried out for 8 hours under stirring at 250 rpm, so that MgO was converted into the first precursor system containing MgCO3·3H2O through hydration and carbonation in the presence of water.

[0060] Second stage: After the first stage, without removing the reactants, the system is cooled to a safe operating temperature, and some CO2 is slowly released under safe operating conditions to reduce the system pressure to a low pressure of 0.8 MPa. Then the system is heated to 85°C and wet-heat aged for 6 hours under stirring at 100 rpm to further convert MgCO3·3H2O into a second precursor system containing Mg5(CO3)4(OH)2·4H2O.

[0061] Third stage: After the second stage, the system was cooled to room temperature, 0.36 g of γ-Al₂O₃ was added, along with 5 mL of deionized water. After stirring for 20 min, the reactor was resealed and hydrothermally reacted at 120 °C for 24 h in the same pressure-resistant reactor. After the reaction, the mixture was cooled, centrifuged, and dried to obtain magnesium aluminum carbonate type hydrotalcite.

[0062] A small sample was taken after the first stage for XRD analysis, and the main product was MgCO3·3H2O. A small sample was taken after the second stage for XRD analysis, and the main product was Mg5(CO3)4(OH)2·4H2O. The final product, after XRD and FTIR analysis, showed the characteristic structure of a Mg-Al-CO3 layered double hydroxide. The SEM morphology of the final product is shown below. Figure 4As shown, it is mainly composed of irregular polygonal lamellae, with the lateral dimensions of the lamellae ranging from submicron to micron scale. Some larger lamellae coexist with smaller lamellae. The lamellae aggregate in a face-to-face stacking and edge-to-face staggered manner, with vertical or tilted lamellae visible locally, forming a secondary agglomeration structure with a certain degree of porosity. Its overall morphology is similar to that of Examples 1 and 2, all exhibiting obvious layered lamellae characteristics. The above results indicate that the method of the present invention can achieve continuous phase transformation of magnesium carbonate precursors using a one-pot segmented control method, and obtain a product with Mg-Al-CO3 layered double hydroxide as the main phase.

[0063] Comparative Example 1: One-pot preparation without magnesium carbonate precursor phase inversion 0.57 g of magnesium oxide and 0.12 g of γ-Al₂O₃ were weighed and added to 10 mL of deionized water. The mixture was ultrasonically stirred and dispersed for 30 min to obtain a precursor slurry. The precursor slurry was transferred to a high-pressure reactor, sealed, and the air inside the reactor was replaced twice with CO₂. The temperature was then raised to 120 °C, and CO₂ was introduced until the pressure reached 0.1 MPa gauge pressure. The reaction was carried out at 250 rpm for 24 h. After the reaction was completed, the mixture was cooled to room temperature, the pressure was slowly released, the slurry was removed, centrifuged, and vacuum dried at 50 °C for 12 h to obtain the comparative product. The difference in this comparative example is that the hydrotalcite synthesis process was carried out in a one-pot method, without the staged phase transformation of MgCO₃·3H₂O and basic magnesium carbonate precursor.

[0064] The obtained product was analyzed by XRD, such as Figure 5 As shown, some characteristic diffraction peaks of Mg-Al-CO3 layered double hydroxides can be observed, but the residual peaks of MgCO3 and AlOOH are relatively strong, indicating that under normal pressure carbon dioxide atmosphere, the one-pot method without the phase transformation of MgCO3·3H2O and basic magnesium carbonate precursors is difficult to effectively suppress the formation of magnesium carbonate and aluminum oxide hydroxyl side phases.

[0065] Comparative Example 2: Preparation of magnesium aluminum carbonate rhizotype hydrotalcite by single-stage continuous phase inversion without pressure reduction The specific implementation steps are as described in Example 3. The difference from Example 3 is that: after the first stage, no pressure reduction and exhaust operation is performed (in Example 3, after the first stage, CO2 needs to be released to reduce the system pressure to 0.8MPa), but the system enters the subsequent wet heat aging stage by maintaining a high CO2 pressure state.

[0066] The product obtained in Comparative Example 2 was analyzed by XRD, such as... Figure 6As shown, no obvious characteristic peaks of Mg-Al-CO3 layered double hydroxides were observed. The main characteristic peaks observed were those related to Mg5(CO3)4(OH)2.5H2O and Al2O3. This indicates that under the condition of continuous high CO2 partial pressure, the system tends to form basic magnesium carbonate and inhibit the participation of Al source in the reconstruction of Mg-Al layers, making it difficult for Mg-Al-CO3 layered double hydroxides to be effectively generated.

[0067] Comparative Example 3: Preparation using commercial basic magnesium carbonate as the magnesium source The specific steps of the third stage of Example 1 were carried out directly, except that commercial basic magnesium carbonate was used instead of the basic magnesium carbonate precursor derived from CO2 and subjected to wet heat aging in Example 1. Therefore, Comparative Example 3 does not include the first and second stage operation steps.

[0068] The product obtained in Comparative Example 3 was analyzed by XRD, such as... Figure 7 As shown, characteristic diffraction peaks of Mg-Al-CO3 layered double hydroxides can be observed; however, compared with Example 1, the intensity of the LDH characteristic peak of this sample is weaker, indicating that although commercial basic magnesium carbonate can participate in the reaction as a magnesium source, its phase composition and reactivity are not as good as the magnesium carbonate precursor of Example 1 of this invention, which is derived from CO2 and controlled by wet heat aging.

[0069] Comparative Example 4: Preparation of hydrotalcite following preparation of magnesium carbonate precursor under non-supercritical CO2 conditions The specific method is the same as in Example 1. The difference between Example 1 and Example 2 is that CO2 is introduced into the first stage until the pressure is 5 MPa (in Example 1, CO2 is introduced into the first stage until the pressure is 9 MPa), while the other conditions for wet heat aging and hydrothermal reconstruction remain unchanged.

[0070] The product obtained in Comparative Example 4 was analyzed by XRD, such as Figure 7 As shown, characteristic diffraction peaks of Mg-Al-CO3 layered double hydroxides can be observed; however, compared with Example 1, the intensity of the LDH characteristic peaks in this sample is weaker, indicating that the magnesium carbonate precursor formed under non-supercritical CO2 conditions is less reactive in terms of phase composition than the precursor formed under supercritical CO2 conditions.

[0071] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursor, characterized in that, Includes the following steps: (1) Contact MgO or Mg(OH)2 with supercritical CO2 in the presence of water to transform MgO or Mg(OH)2 into a first precursor system with magnesium-containing crystalline phase as the main phase, MgCO3·3H2O. (2) Before the wet heat aging, the first precursor system is subjected to solid-liquid separation, or CO2 is released to relieve pressure before the wet heat aging; then the first precursor system is subjected to wet heat aging in the aqueous phase, so that MgCO3·3H2O is converted into a second precursor system with Mg5(CO3)4(OH)2·4H2O as the main phase and containing magnesium crystal phase. (3) The second precursor system is reacted with γ-Al2O3 under hydrothermal conditions to obtain a product with Mg-Al-CO3 layered double hydroxide as the main phase.

2. The method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursor according to claim 1, characterized in that: The pressure of the supercritical CO2 in step (1) is 7.5-20 MPa, the temperature is 35-70℃, and the reaction time is 0.5-12 h.

3. The method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursor according to claim 2, characterized in that: The supercritical CO2 pressure in step (1) is 8-12 MPa, the temperature is 40-55℃, and the reaction time is 2-8 h.

4. The method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursor according to claim 1, characterized in that: In step (1), when the magnesium source is MgO, the ratio of the volume of water to the mass of the magnesium source is 3 to 20 mL / g; when the magnesium source is Mg(OH)2, the mass of the magnesium source is calculated as the mass of equimolar MgO based on the Mg content therein.

5. The method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursor according to claim 1, characterized in that: The wet heat aging temperature in step (2) is 60-110℃, the time is 0.5-12h, and the ratio of the volume of water to the solid mass of the first precursor system on a dry basis is 3-20mL / g.

6. The method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursor according to claim 1, characterized in that: In step (3), the Mg / Al molar ratio is 1.8-2.

3.

7. The method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursor according to claim 1, characterized in that: The hydrothermal temperature of step (3) is 120-220℃, the reaction time is 2-24h, and the ratio of the volume of water to the mass of the first precursor system solid on a dry basis is 5-30mL / g.

8. The method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursor according to claim 1, characterized in that: The γ-Al2O3 is active Al2O3, or a transitional alumina with γ-Al2O3 as the main phase, obtained by calcining Al(OH)3.

9. The method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursor according to claim 1, characterized in that: The first precursor system obtained in step (1) is subjected to solid-liquid separation to obtain MgCO3·3H2O precursor before step (2) is performed. The second precursor system obtained in step (2) is subjected to solid-liquid separation to obtain basic magnesium carbonate precursor before step (3) is performed. Alternatively, steps (1), (2), and (3) can be carried out continuously in the same reactor through programmed temperature control, pressure control, venting, and water replenishment. After step (1) is completed, CO2 needs to be released to reduce pressure and vent until the system pressure is reduced to no more than 2 MPa.

10. The Mg-Al-CO3 layered double hydroxide prepared by the method for preparing magnesium aluminum carbonate type hydrotalcite based on carbon dioxide-derived magnesium carbonate precursors according to any one of claims 1 to 9.

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