Modified magnesium-aluminum layered double-metal hydroxide as well as preparation method and application thereof

Abstract

The invention is suitable for the technical field of solid catalysts, and provides a modified magnesium-aluminum layered double-metal hydroxide as well as a preparation method and application thereof. The modified magnesium-aluminum layered double hydroxide is rich in oxygen-containing functional groups, Al2O3 and MgO structures, and is used as a catalyst to catalyze glucose in water to prepare lactic acid. The catalyst takes low-cost aluminum-magnesium hydrotalcite (HT) as a carrier, is modified by a mechanical ball milling method, is free of organic solvent intervention in the whole process, is simple to operate and efficient in reaction, adopts easily available raw materials, and conforms to the green chemical engineering concept. The catalyst is rich in oxygen-containing functional groups, and contains multifunctional catalytic sites derived from Al2O3 and MgO structures and ionic liquid-based catalytic sites, so that stable existence of multiple active sites is guaranteed, and the thermal stability of the catalyst is improved. The catalyst can catalyze glucose to be converted into lactic acid in a water solution (the yield reaches 43.6%), and efficient and high-selectivity conversion from glucose to lactic acid is achieved.

Description

Technical Field

[0001] This invention belongs to the field of solid catalyst technology, and particularly relates to a modified magnesium-aluminum layered bimetallic hydroxide, its preparation method, and its application. Background Technology

[0002] Biomass, as the most abundant renewable natural polymer in nature, plays a crucial role in achieving carbon neutrality and green chemistry through its efficient conversion into high-value-added chemicals such as lactic acid. Lactic acid is not only a monomer of biodegradable polylactic acid (PLA) but also widely used in food, pharmaceuticals, and cosmetics, with continuously growing market demand. However, cellulose, a long-chain polymer composed of glucose units linked by β-1,4-glycosidic bonds, has extremely low solubility in aqueous phase, resulting in a very high activation energy barrier. Traditional acid-base or enzyme catalytic systems generally face bottlenecks such as harsh reaction conditions (e.g., strong acids, high temperatures), poor selectivity, severe equipment corrosion, difficulty in product separation, and high enzyme preparation costs. Therefore, developing solid catalysts that can directly and efficiently convert cellulose into lactic acid under mild aqueous conditions has become a core challenge urgently needing breakthroughs in the current biomass refining field.

[0003] Layered metal hydroxides (LDHs) are a collective term for hydrotalcite (HT) and hydrotalcite-like compounds (HTLCS), belonging to a class of inorganic materials with layered structures. Hydrotalcite (HT), as a specific prototype compound in the LDH family, is considered a highly promising solid base catalyst for cellulose conversion due to its advantages such as tunable layer elements, exchangeable interlayer anions, abundant surface hydroxyl groups, good thermal stability, and low preparation cost. The interlayers of hydrotalcite often contain carbonate ions (CO3-). 2- Carbonate ions, such as OH-, balance the charge, but under high-temperature calcination (e.g., 400-600℃), carbonate ions decompose into CO2 and are permanently lost. During rehydrolysis, the source of interlayer anions depends on the environment: in an ultrapure water system without CO2 interference, OH-... - It becomes the main equilibrium ion, ensuring structural recovery; if the environment contains CO2, it may introduce trace amounts of carbonate ions. However, the original magnesium aluminum hydrotalcite (MgAl-HT) still has significant defects: it can only provide lamellar hydroxyl groups and Al. 3+ / Mg 2+ The lack of Lewis acid sites and Brønsted acid sites that can effectively break β-1,4-glycosidic bonds makes it difficult to synergistically catalyze the cellulose hydrolysis-isomerization-dehydration tandem reaction. At the same time, the strong hydrophilicity of the HT surface makes it easy for it to aggregate in the aqueous phase, reducing the accessibility of cellulose macromolecules to the active sites. In addition, under long-term high-temperature hydrothermal reaction conditions, the HT plates are prone to recrystallization or dissolution, resulting in the loss of active sites and ultimately causing catalyst deactivation.

[0004] To overcome the aforementioned shortcomings, researchers have attempted to functionalize HT through acid modification, calcination reconstruction, heteroatom doping, or loading acidic components. For example, intercalating HT with sulfuric acid or phosphoric acid can introduce Brønsted acid sites, but strong acid treatment often destroys the layered structure of HT, and residual free acid can exacerbate equipment corrosion. While loading with solid acids such as phosphotungstic acid and zirconium sulfate can increase the acid content, problems such as easy dissolution and uneven dispersion of active components exist. In addition, most modification methods still rely on hydrothermal or solvent reflux systems, requiring the use of large amounts of organic solvents or generating acid and alkali waste liquids, which contradicts the development concept of green chemical engineering. Therefore, there is an urgent need for a new green, efficient, and low-cost strategy that can precisely construct a multi-active-site system of metal-based active sites, oxygen-containing functional groups, and ionic liquid-based sites while maintaining the integrity of the HT layered framework, and improve its aqueous phase stability to achieve direct and highly selective conversion of cellulose to lactic acid. To this end, this invention proposes a modified magnesium-aluminum layered bimetallic hydroxide, its preparation method, and its application. Summary of the Invention

[0005] The purpose of this invention is to provide a modified magnesium-aluminum layered bimetallic hydroxide, its preparation method, and its application, in order to solve the problems mentioned in the background art.

[0006] The objective of this invention is achieved through the following technical solution: A method for preparing a modified magnesium-aluminum layered bimetallic hydroxide includes the following steps: Aluminum-magnesium hydrotalcite was pyrolyzed, then mixed with ultrapure water for further hydrolysis, followed by vacuum filtration and vacuum drying to obtain a magnesium-aluminum layered bimetallic hydroxide material with altered structure. The magnesium-aluminum layered bimetallic hydroxide material is mixed with an ionic liquid and then ball-milled. The ball-milled material was washed with deionized water, filtered, and then vacuum filtered and dried to obtain modified magnesium-aluminum layered bimetallic hydroxide.

[0007] Furthermore, the hydrotalcite pretreatment pyrolysis temperature is 400-600℃, and the pyrolysis time is 0.5-8h.

[0008] Furthermore, the hydrotalcite after pyrolysis is subjected to a re-hydrolysis treatment at a temperature of 40-80℃ for 1-4 hours.

[0009] Furthermore, the mass ratio of the magnesium-aluminum layered bimetallic hydroxide material to the ionic liquid is 6-1:0-6.

[0010] Furthermore, the ball milling process is carried out at a rotation speed of 5-20 Hz for 1-8 hours; the diameter of the grinding balls used in the ball milling process is 5-20 mm, and the ball-to-material mass ratio is 20-100:2-4.

[0011] Furthermore, the ionic liquid is a Brønsted acidic, neutral, or basic ionic liquid, specifically including: 1-butyl-3-methylimidazolium bromide ([Bmim]Br), 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), 1-butyl-3-methylimidazolium acetate ([Bmim][OAc]), 1-butyl-3-methylimidazolium hydroxide ([Bmim]OH), and 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4]).

[0012] A modified magnesium-aluminum layered bimetallic hydroxide prepared according to the above-described preparation method comprises a structurally modified magnesium-aluminum layered bimetallic hydroxide material and an ionic liquid monomer covalently grafted onto the magnesium-aluminum layered bimetallic hydroxide material; the modified magnesium-aluminum layered bimetallic hydroxide is rich in oxygen-containing functional groups and contains Al2O3, MgO structures and ionic liquid-based catalytic sites.

[0013] An application of the modified magnesium-aluminum layered bimetallic hydroxide described above as a catalyst for the catalytic preparation of lactic acid from glucose in water is described. The specific steps of the application are as follows: after mixing the modified magnesium-aluminum layered bimetallic hydroxide, glucose and water, nitrogen gas is introduced, and glucose reacts under the catalytic action of the catalyst to obtain lactic acid.

[0014] Furthermore, the reaction temperature is 80-120℃, the time is 1-24h, and the pressure is 0.1-1MPa.

[0015] Furthermore, the mass concentration of glucose in the mixture obtained after mixing the catalyst, glucose, and water is 1-5 wt%; the mass ratio of the catalyst to glucose is 0-0.09:0.036-0.9.

[0016] Compared with the prior art, the beneficial effects of the present invention are: The modified magnesium-aluminum layered bimetallic hydroxide prepared by this invention uses low-cost aluminum-magnesium hydrotalcite as a carrier and is functionally modified by mechanical ball milling. The entire preparation process does not involve organic solvents, avoiding the problems of using organic solvents or generating acid and alkali waste liquids in traditional methods. At the same time, the preparation method has simple operation steps, high reaction efficiency, and readily available raw materials, which is in line with the concept of green chemical industry. In terms of catalyst performance, its advantages are particularly prominent: the catalyst is rich in oxygen-containing functional groups and contains multifunctional catalytic sites derived from Al2O3 and MgO structures, as well as ionic liquid-based catalytic sites. The ionic liquid, through the strong tensile and shear forces generated by ball milling, covalently grafts with oxygen-containing functional groups (such as -OH) in the hydrotalcite, thereby forming ionic liquid-based catalytic sites. This design not only ensures the stable existence of multiple active sites but also improves the thermal stability of the catalyst, solving the defects of traditional hydrotalcite in the easy recrystallization and dissolution of the layers under high temperature hydrothermal conditions. In terms of application, the catalyst can directly catalyze the conversion of glucose to lactic acid in aqueous solution. The ionic liquid-based catalytic sites can help glucose transfer and work synergistically with the magnesium-based and aluminum-based multifunctional catalytic sites on the hydrotalcite to effectively promote lactic acid formation, with a lactic acid yield of up to 43.6% and a glyceraldehyde yield of up to 42.3%. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the pretreatment of magnesium aluminum hydrotalcite.

[0018] Figure 2 This is a schematic diagram of the structure of the ionic liquid used.

[0019] Figure 3 The images show the XRD patterns of the catalysts prepared in Examples 1-4.

[0020] Figure 4 The conversion rate and yield of the products obtained by the catalysts prepared in Examples 1-4 in aqueous solvent for the catalytic conversion of glucose are shown. Detailed Implementation

[0021] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0022] Figure 1 This paper presents the pretreatment process of magnesium aluminum hydrotalcite, covering the structural changes caused by calcination and the structural restoration through rehydration. Pyrolysis decomposes the original interlayer carbonate ions into CO2, which is then lost. During rehydration, the ultrapure water environment primarily promotes the formation of OH groups in the interlayer. - Anions balance the charge on the laminates, ensuring structural recovery. Although trace amounts of CO2 in the environment may partially introduce carbonate ions, this invention focuses on OH-. -The functional groups play a dominant role in enhancing catalytic activity. This provides a basic material processing approach for the preparation and performance research of subsequent catalysts. As the treatment method of hydrotalcite changes from simple pyrolysis to re-hydrolysis, and then to re-hydrolysis combined with ionic liquids, the selectivity of the catalyst for glucose conversion gradually shifts from producing fructose to producing lactic acid and glyceraldehyde.

[0023] from Figure 3 The untreated hydrotalcite spectrum shown exhibits obvious characteristic diffraction peaks of hydrotalcite (HT), with sharp peaks and high intensity, indicating that the untreated hydrotalcite has a complete and ordered layered crystal structure without phase decomposition or structural damage. This serves as the original reference standard for subsequent treatments.

[0024] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0025] Example 1: A method for preparing pyrolyzed hydrotalcite, the specific steps of which are as follows: Magnesium aluminum hydrotalcite was heated to 500ºC in a nitrogen atmosphere at a heating rate of 5℃ / min and held at 500℃ for 1 hour.

[0026] The obtained samples were analyzed using XRD, and the results were... Figure 3 As shown in the spectrum, characteristic peaks of MgO and Al2O3 were observed compared to the untreated hydrotalcite sample, indicating that the pyrolysis treatment altered the structure of the hydrotalcite.

[0027] Example 2: A method for preparing hydrotalcite through rehydrolysis, the specific steps of which are as follows: ① Take magnesium aluminum hydrotalcite and heat it to 500ºC in a nitrogen atmosphere at a heating rate of 5℃ / min, and keep it at 500℃ for 2h.

[0028] ② Mix 2g of hydrotalcite obtained from pyrolysis at 500ºC with 200mL of ultrapure water and maintain the mixture at 50ºC for 3 hours. Further hydrolysis reconstructs the pyrolysis products into a layered hydrotalcite precursor containing surface hydroxyl groups (-OH), with OH groups naturally forming between the layers. - To balance the charge on the plates.

[0029] ③ After hydrolysis, the hydrotalcite is vacuum filtered and vacuum dried (to prevent the treated hydrotalcite from coming into contact with air).

[0030] The obtained samples were analyzed using XRD, and the results were... Figure 3 As shown in the spectrum. It can be observed from the spectra that, compared to the untreated hydrotalcite sample and the sample obtained in Example 1, the peak shape of the sample obtained in Example 2 is similar to that of the untreated hydrotalcite sample, but the intensity changes, indicating that the layered structure is restored after re-hydrolysis, with OH groups forming the interlayer structure. -The dominant anion is OH. The XRD pattern did not show obvious carbonate characteristic peaks, further confirming that the carbonate decomposition caused by pyrolysis is irreversible, and the structural recovery mainly depends on OH. - Reinsertion. This regenerated hydrotalcite is rich in OH... - The functional groups further suggest that this "regenerated" hydrotalcite often exhibits superior properties compared to the original hydrotalcite. This indicates that under suitable conditions, pyrolyzed hydrotalcite can partially or completely recover its original layered structure through rehydrolysis.

[0031] Example 3: A method for preparing an ionic liquid-hydrolyzed hydrotalcite composite material, the specific steps of which are as follows: ① Take 2g of the material from Example 2, and the ionic liquid [Bmim]OH (the structure of the ionic liquid is shown in the figure). Figure 2 Mix 2g thoroughly and transfer to a 50mL ball mill jar.

[0032] ② Add 30g of steel grinding balls with a diameter of 5mm to the grinding jar and perform mechanical ball milling. The ball milling conditions are: rotation speed of 10Hz and time of 4h.

[0033] ③ After ball milling, wash with 40 mL of deionized water, filter, and repeat 6 times. Then vacuum filter and vacuum dry.

[0034] The obtained samples were analyzed using XRD, and the results were... Figure 3 As shown in the spectrum, it can be observed that the peak shape of the sample obtained in Example 3 is shifted compared to that of the sample obtained in Example 2, indicating that the treatment with the ionic liquid [Bmim]OH has affected the structure of the re-hydrolyzed hydrotalcite.

[0035] Application Example 1: A method for catalytically preparing fructose from glucose in an aqueous liquid phase, comprising the following steps: Take 0.036 g of glucose and 2 mL of water, mix well, and then transfer to a reaction vessel. Stir magnetically at 500 rpm, apply 0.1 MPa nitrogen gas, and maintain a temperature of 100°C. o The reaction was carried out at C for 2 hours. After the reaction, the reaction mixture was quickly rinsed with water to cool it. The resulting mixture was filtered through a filter membrane, and the supernatant was used for liquid chromatography to determine the product.

[0036] Calculate the conversion rate of glucose, the yield of fructose, the yield of lactic acid, and the yield of glyceraldehyde. The formulas for calculating the yields are as follows: ; ; The conditions for liquid chromatography were: mobile phase 0.05 M H₂SO₄, column temperature 50 °C. o C, flow rate 0.5 mL / min, differential detector RI and PAD detector.

[0037] The glucose conversion rate was calculated to be 50.1%, and the fructose yield was 27.5% (see [reference]). Figure 4 ).

[0038] Application Example 2: A method for catalytically preparing fructose from glucose in an aqueous liquid phase, comprising the following steps: Take 0.036 g of untreated hydrotalcite, 0.036 g of glucose, and 2 mL of water, mix them thoroughly, and then transfer them to a reaction vessel. Stir magnetically at 500 rpm under 0.1 MPa nitrogen atmosphere at 100°C. o The reaction was carried out at C for 2 hours. After the reaction, the reaction mixture was quickly rinsed with water to cool it. The resulting mixture was filtered through a filter membrane, and the supernatant was used for liquid chromatography to determine the product.

[0039] The calculated conversion rates were: glucose 60.6%, fructose 10.7%, glyceraldehyde 0.7%, and lactic acid 1% (see [reference]). Figure 4 ).

[0040] Application Example 3: A method for catalyzing the production of fructose, lactic acid, and glyceraldehyde from glucose in water using the catalyst of Example 1, comprising the following steps: Take 0.036 g of the hydrotalcite catalyst obtained in Example 1, 0.036 g of glucose, and 2 mL of water, mix them thoroughly, and then transfer them to a reaction vessel. Stir the mixture magnetically at 500 rpm under 0.1 MPa nitrogen atmosphere at 100°C. o The reaction was carried out at C for 2 hours. After the reaction, the reaction mixture was quickly rinsed with water to cool it. The resulting mixture was filtered through a filter membrane, and the supernatant was used for liquid chromatography to determine the product.

[0041] The conversion rates were calculated as follows: glucose 99.4%, fructose 3.3%, lactic acid 9.2%, and glyceraldehyde 16.1% (see [reference]). Figure 4 ).

[0042] Application Example 4: A method for catalyzing the production of fructose, lactic acid, and glyceraldehyde from glucose in water using the catalyst of Example 2, comprising the following steps: Take 0.036 g of the hydrotalcite catalyst obtained in Example 2, 0.036 g of glucose, and 2 mL of water, mix them thoroughly, and then transfer them to a reaction vessel. Stir the mixture magnetically at 500 rpm under 0.1 MPa nitrogen atmosphere at 100°C. o The reaction was carried out at C for 2 hours. After the reaction, the reaction mixture was quickly rinsed with water to cool it. The resulting mixture was filtered through a filter membrane, and the supernatant was used for liquid chromatography to determine the product.

[0043] The calculated conversion rates were: glucose 99.7%, fructose 1.0%, lactic acid 8.9%, and glyceraldehyde 24.2% (see [reference]). Figure 4 ).

[0044] Application Example 5: A method for catalyzing the production of fructose, lactic acid, and glyceraldehyde from glucose in water using the catalyst of Example 2 and the ionic liquid [Bmim]OH, comprising the following steps: Take 0.036 g of the hydrotalcite catalyst obtained in Example 2, 0.036 g of the ionic liquid [Bmim]OH, 0.036 g of glucose, and 2 mL of water, mix them thoroughly, and then transfer them to a reaction vessel. Stir the mixture magnetically at 500 rpm under 0.1 MPa nitrogen atmosphere at 100°C. o The reaction was carried out at C for 2 hours. After the reaction, the reaction mixture was quickly rinsed with water to cool it. The resulting mixture was filtered through a filter membrane, and the supernatant was used for liquid chromatography to determine the product.

[0045] The conversion rate of glucose was calculated to be 100%, the fructose yield to be 0%, the lactic acid yield to be 42.3%, and the glyceraldehyde yield to be 24.8% (see [reference]). Figure 4 ).

[0046] Application Example 6: A method for catalyzing the production of fructose, lactic acid, and glyceraldehyde from glucose in water using the catalyst of Example 3, comprising the following steps: Take 0.036 g of the hydrotalcite catalyst obtained in Example 3, 0.036 g of glucose, and 2 mL of water, mix them thoroughly, and then transfer them to a reaction vessel. Stir the mixture magnetically at 500 rpm under 0.1 MPa nitrogen atmosphere at 100°C. o The reaction was carried out at C for 2 hours. After the reaction, the reaction mixture was quickly rinsed with water to cool it. The resulting mixture was filtered through a filter membrane, and the supernatant was used for liquid chromatography to determine the product.

[0047] The conversion rate of glucose was calculated to be 100%, the fructose yield to be 0%, the lactic acid yield to be 43.6%, and the glyceraldehyde yield to be 42.3% (see [reference]). Figure 4 ).

[0048] The above are merely preferred embodiments of the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept 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.

Claims

1. A method for preparing a modified magnesium-aluminum layered bimetallic hydroxide, characterized in that, Includes the following steps: Aluminum-magnesium hydrotalcite was pyrolyzed, then mixed with ultrapure water for further hydrolysis, followed by vacuum filtration and vacuum drying to obtain a magnesium-aluminum layered bimetallic hydroxide material with altered structure. The magnesium-aluminum layered bimetallic hydroxide material is mixed with an ionic liquid and then ball-milled. The ball-milled material was washed with deionized water, filtered, and then vacuum filtered and dried to obtain modified magnesium-aluminum layered bimetallic hydroxide.

2. The preparation method according to claim 1, characterized in that, The hydrotalcite pretreatment pyrolysis temperature is 400-600℃, and the pyrolysis time is 0.5-8h.

3. The preparation method according to claim 1, characterized in that, The hydrotalcite after pyrolysis is subjected to a re-hydrolysis treatment at a temperature of 40-80℃ for 1-4 hours.

4. The preparation method according to claim 1, characterized in that, The mass ratio of the magnesium-aluminum layered bimetallic hydroxide material to the ionic liquid is 6-1:0-6.

5. The preparation method according to claim 1, characterized in that, The ball milling process is performed at a rotation speed of 5-20 Hz for 1-8 hours; the diameter of the grinding balls used in the ball milling process is 5-20 mm, and the ball-to-material mass ratio is 20-100:2-4.

6. The preparation method according to claim 1, characterized in that, The ionic liquid is a Brønsted acidic, neutral, or basic ionic liquid, specifically including: 1-butyl-3-methylimidazolium bromide, 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium hydroxide, and 1-butyl-3-methylimidazolium hydrogen sulfate.

7. A modified magnesium-aluminum layered bimetallic hydroxide prepared by the preparation method according to any one of claims 1-6, characterized in that, The invention comprises a structurally modified magnesium-aluminum layered bimetallic hydroxide material and an ionic liquid monomer covalently grafted onto the magnesium-aluminum layered bimetallic hydroxide material; the modified magnesium-aluminum layered bimetallic hydroxide is rich in oxygen-containing functional groups and contains Al2O3, MgO structures and ionic liquid-based catalytic sites.

8. The application of the modified magnesium-aluminum layered bimetallic hydroxide according to claim 7 as a catalyst for the catalytic preparation of lactic acid from glucose in water, characterized in that, The specific steps of the application are as follows: After mixing modified magnesium-aluminum layered bimetallic hydroxide, glucose and water, nitrogen gas is introduced. Under the catalytic action of the catalyst, glucose reacts to produce lactic acid.

9. The application according to claim 8, characterized in that, The reaction is carried out at a temperature of 80-120℃ for 1-24 hours and at a pressure of 0.1-1 MPa.

10. The application according to claim 8, characterized in that, The mass concentration of glucose in the mixture obtained by mixing the catalyst, glucose and water is 1-5 wt%; the mass ratio of the catalyst to glucose is 0-0.09:0.036-0.9.

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