A modified magnesium-aluminum layered double hydroxide and a preparation method and application thereof

Abstract

The application belongs to the technical field of solid catalysts, and provides a modified magnesium-aluminum layered double hydroxide and 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 applied to catalyze the preparation of lactic acid from glucose in water as a catalyst. The catalyst uses low-cost aluminum-magnesium hydrotalcite (HT) as a carrier, is modified by a mechanical ball milling method, has no organic solvent intervention in the whole process, is simple to operate, efficient in reaction and easy to obtain raw materials, and meets the green chemical concept. The catalyst is rich in oxygen-containing functional groups, contains multifunctional catalytic sites derived from Al2O3 and MgO structures and ionic liquid-based catalytic sites, guarantees the stable existence of multiple active sites, and improves the thermal stability of the catalyst. The catalyst can catalyze the conversion of glucose into lactic acid (the yield reaches 43.6%) in an aqueous solution, and realizes the efficient and high-selectivity conversion of glucose into lactic acid.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of solid catalysts, and particularly relates to a modified magnesium-aluminum layered double hydroxide as well as a preparation method and application thereof. BACKGROUND

[0002] Biomass, as the most abundant renewable natural polymer in nature, has important significance for realizing carbon neutralization and green chemical industry through its efficient conversion to prepare high-value-added chemicals such as lactic acid. Lactic acid is not only the monomer of degradable polylactic acid (PLA), but also widely used in food, medicine, cosmetics and other fields, and the market demand continues to grow. However, cellulose is a long-chain polymer connected by glucose units through β-1,4-glucosidic bonds, which has extremely low solubility in aqueous phase, resulting in extremely high activation energy barrier. Traditional acid, base or enzyme catalytic systems generally face the bottlenecks of harsh reaction conditions (such as strong acid, high temperature), poor selectivity, serious equipment corrosion, difficult product separation and high cost of enzyme preparation. Therefore, it has become a core problem to be urgently broken through in the field of biomass refining to develop a solid catalyst that can directly catalyze the efficient and high-selectivity conversion of cellulose to lactic acid under mild aqueous conditions.

[0003] Layered double hydroxide (LDH) is the general term of hydrotalcite (HT) and hydrotalcite-like compounds (HTLCS), which belongs to a kind of inorganic material with layered structure. Among them, hydrotalcite (HT) as a specific prototype compound in the LDH family, with the advantages of adjustable layer plate elements, exchangeable interlayer anions, abundant surface hydroxyl groups, good thermal stability and low preparation cost, is considered as a potential solid base catalyst for cellulose conversion. The interlayer of hydrotalcite often contains anions such as carbonate (CO3 2- ) to balance the charge, but under high temperature calcination (such as 400-600℃), carbonate will decompose into CO2 and permanently lost. During the rehydration process, the source of interlayer anions depends on the environment: in the ultrapure water system without CO2 interference, OH - becomes the main counterion to ensure the structure recovery; if the environment contains CO2, a small amount of carbonate may be introduced. However, the original magnesium-aluminum hydrotalcite (MgAl-HT) still has significant defects: it can only provide layer plate hydroxyl groups and Al 3+ / Mg 2+ Lewis acid sites, lacks Brønsted acid sites that can effectively break β-1,4-glucosidic bonds, and is difficult to synergistically catalyze the cellulose hydrolysis-isomerization-dehydration series reaction; at the same time, the strong hydrophilicity of HT surface makes it easy to aggregate in aqueous phase, reducing the contact accessibility of cellulose macromolecules to active sites; in addition, under long-term high-temperature hydrothermal reaction conditions, the HT layer plate is easy to recrystallize or dissolve, resulting in the loss of active sites and eventually leading to catalyst deactivation.

[0004] To overcome the above-mentioned defects, researchers have tried to functionalize HT by acid modification, calcination reconstruction, heteroatom doping or loading acidic components, etc. For example, the intercalation of sulfuric acid or phosphoric acid into HT can introduce Bronsted acid sites, but strong acid treatment often destroys the layered structure of HT, and the residual free acid can aggravate equipment corrosion; although solid acid loading such as phosphotungstic acid and zirconium sulfate can increase the acid amount, there are problems such as easy dissolution of active components and uneven dispersion. In addition, most modification methods still rely on hydrothermal or solvent reflux system, which requires a large amount of organic solvent or generates acid and alkali waste liquid, which is contrary to the development concept of green chemical industry. Therefore, a new strategy of green, efficient and low cost is urgently needed to precisely construct a multi-active site system of metal-based active sites-oxygen-containing functional groups-ionic liquid-based sites while maintaining the integrity of the HT layered skeleton, and to improve its water stability, so as to realize the direct and high-selectivity conversion of cellulose to lactic acid. Therefore, the present application proposes a modified magnesium-aluminum layered double hydroxide and a preparation method and application thereof. SUMMARY

[0005] The purpose of the present application is to provide a modified magnesium-aluminum layered double hydroxide and a preparation method and application thereof, aiming to solve the problems raised in the above background.

[0006] The purpose of the present application is realized by the following technical solutions:

[0007] A preparation method of a modified magnesium-aluminum layered double hydroxide, comprising the following steps:

[0008] The aluminum-magnesium hydrotalcite is subjected to pyrolysis treatment, and then mixed with ultrapure water for re-hydrolysis treatment, followed by vacuum filtration and vacuum drying to obtain a magnesium-aluminum layered double hydroxide material with changed structure;

[0009] The magnesium-aluminum layered double hydroxide material is mixed with an ionic liquid and subjected to ball milling treatment;

[0010] The material after ball milling is washed with deionized water, filtered, and then subjected to vacuum filtration and vacuum drying to obtain a modified magnesium-aluminum layered double hydroxide.

[0011] Further, the hydrotalcite pre-treatment pyrolysis temperature is 400-600℃, and the pyrolysis time is 0.5-8h.

[0012] Further, the hydrotalcite after pyrolysis treatment is subjected to re-hydrolysis treatment at a temperature of 40-80℃ for 1-4h.

[0013] Further, the mass ratio of the magnesium-aluminum layered double hydroxide material to the ionic liquid is 6-1:0-6.

[0014] Further, the rotation speed of the ball milling treatment is 5-20 Hz, and the time is 1-8 h; the diameter of the ball milling beads used in the ball milling treatment is 5-20 mm, and the mass ratio of the ball milling beads to the material is 20-100:2-4.

[0015] Further, the ionic liquid is a Bronsted acidic, neutral or basic ionic liquid, and specifically includes 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]).

[0016] A modified magnesium-aluminum layered double hydroxide prepared by the preparation method described above, comprising a magnesium-aluminum layered double hydroxide material with a changed structure and ionic liquid monomers grafted on the magnesium-aluminum layered double hydroxide material in a covalent form; the modified magnesium-aluminum layered double hydroxide is rich in oxygen-containing functional groups and simultaneously contains Al2O3, MgO structures and ionic liquid-based catalytic sites.

[0017] An application of the modified magnesium-aluminum layered double hydroxide described above as a catalyst for catalyzing the preparation of lactic acid from glucose in water, and the specific steps of the application are as follows: after mixing the modified magnesium-aluminum layered double hydroxide, glucose and water, nitrogen is introduced, and the glucose reacts under the catalysis of the catalyst to obtain lactic acid.

[0018] Further, the temperature of the reaction is 80-120 DEG C, the time is 1-24 h, and the pressure is 0.1-1 MPa.

[0019] Further, the mass concentration of the glucose in the mixed solution obtained after mixing the catalyst, glucose and water is 1-5 wt%, and the mass ratio of the catalyst to the glucose is 0-0.09:0.036-0.9.

[0020] Compared with the prior art, the application has the following beneficial effects:

[0021] The modified magnesium-aluminum layered double hydroxide prepared in the application takes low-cost aluminum-magnesium hydrotalcite as a carrier and adopts mechanical ball milling method for functional modification, the whole preparation process does not involve organic solvent, avoiding the problems of organic solvent use or acid and alkali waste liquid generation in the traditional method; meanwhile, the preparation method has simple operation steps, high reaction efficiency and easy-to-obtain raw materials, conforming to the concept of green chemical industry. In terms of catalyst performance, the advantages are particularly prominent: 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, wherein the ionic liquid is grafted with the oxygen-containing functional groups (such as -OH) in the hydrotalcite in the form of covalent bond through the strong stretching and shearing force generated by ball milling, and then the ionic liquid-based catalytic sites are formed, which not only guarantees the stable existence of multiple active sites, but also improves the thermal stability of the catalyst, solving the defects that the traditional hydrotalcite is easy to recrystallize and dissolve under high-temperature hydrothermal conditions; in application, the catalyst can directly catalyze the conversion of glucose into lactic acid in aqueous solution, the ionic liquid-based catalytic sites can assist the transfer of glucose, and synergize with the magnesium-based and aluminum-based multifunctional catalytic sites on the hydrotalcite to effectively promote the formation of lactic acid, and the lactic acid yield can reach 43.6%, and the glycerol yield can reach 42.3%. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 Pretreatment diagram of magnesium-aluminum hydrotalcite.

[0023] Figure 2 Structure diagram of the ionic liquid used.

[0024] Figure 3 XRD spectrum of the catalyst prepared in Examples 1-4.

[0025] Figure 4 Product conversion rate and product yield of the catalyst prepared in Examples 1-4 in the catalytic conversion of glucose in aqueous solution. DETAILED DESCRIPTION

[0026] In order to have a clearer understanding of the technical features, objectives and beneficial effects of the present application, the technical solutions of the present application are described in detail as follows, but cannot be understood as limiting the implementable scope of the present application.

[0027] Figure 1 The pretreatment process of magnesium-aluminum hydrotalcite is presented, which covers the structure change caused by calcination treatment and the structure recovery in the rehydration process. The original interlayer carbonate is decomposed into CO2 and lost in the pyrolysis process; in the rehydration process, the ultrapure water environment mainly promotes the formation of OH - anions to balance the layer charge and ensure the structure recovery. Although trace CO2 in the environment may partially introduce carbonate, the present application focuses on OH -The dominant role of the functional groups is to enhance the catalytic activity. This provides a processing approach for the subsequent catalyst preparation and performance research. With the change of the hydrotalcite processing method from simple pyrolysis to re-hydrolysis, and then to ionic liquid combined with re-hydrolysis, the selectivity of the catalyst to glucose conversion gradually changes from fructose to lactic acid and glycerol.

[0028] From Figure 3 As can be observed from the spectrum of the untreated hydrotalcite, the characteristic diffraction peak of the hydrotalcite (HT) is obvious, the peak shape is sharp and the intensity is high, indicating that the untreated hydrotalcite has a complete and ordered layered crystal structure, and no phase decomposition or structure damage occurs, which is the original reference benchmark for subsequent various treatments.

[0029] The specific implementation of the present application is described in detail below in combination with specific examples.

[0030] Example 1: A preparation method of pyrolysis-treated hydrotalcite, the specific steps are as follows:

[0031] Take the magnesium aluminum hydrotalcite and heat it to 500ºC at a temperature rising rate of 5ºC / min in a nitrogen atmosphere, and keep it at 500ºC for 1h.

[0032] The obtained sample is analyzed by XRD, and the results are shown in the following spectrum: Figure 3 As can be observed from the spectrum, compared with the spectrum of the untreated hydrotalcite sample, the characteristic peaks of MgO and Al2O3 appear, indicating that the pyrolysis treatment changes the structure of the hydrotalcite.

[0033] Example 2: A preparation method of re-hydrolysis-treated hydrotalcite, the specific steps are as follows:

[0034] ①Take the magnesium aluminum hydrotalcite and heat it to 500ºC at a temperature rising rate of 5ºC / min in a nitrogen atmosphere, and keep it at 500ºC for 2h.

[0035] ②Take 2g of the hydrotalcite obtained by pyrolysis at 500ºC and mix it with 200mL of ultrapure water, and keep it at 50ºC for 3h. Through re-hydrolysis, the pyrolysis product is restructured into a layered hydrotalcite precursor containing surface hydroxyl groups (-OH), and OH - is naturally formed between the layers to balance the charge of the layer plate.

[0036] ③After hydrolysis, the hydrotalcite is subjected to vacuum filtration and vacuum drying (to avoid contact of the treated hydrotalcite with air).

[0037] The obtained sample is analyzed by XRD, and the results are shown in the following spectrum: Figure 3 As can be observed from the spectrum, compared with the spectrum of the untreated hydrotalcite sample and the sample obtained in Example 1, it is found that 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, and OH -The main anion is carbonate. The XRD spectrum does not show obvious carbonate characteristic peaks, further confirming that the pyrolysis-induced carbonate decomposition is irreversible, and the structure recovery mainly depends on the reinsertion of OH - groups. This regenerated hydrotalcite is rich in OH - groups, and it is further speculated that this "regenerated" hydrotalcite often exhibits better performance than the original hydrotalcite. It is shown that the pyrolyzed hydrotalcite can partially or completely restore its original layered structure under suitable conditions through rehydration.

[0038] Example 3: A method for preparing an ionic liquid-hydrolysis treated hydrotalcite composite material, the specific steps are as follows:

[0039] ①Take 2g of the material of Example 2, 2g of ionic liquid [Bmim]OH (the structure of the ionic liquid is shown in Figure 2 ), mix well, and transfer to a 50mL ball mill jar.

[0040] ②Add 30g of steel ball milling beads with a diameter of 5mm to the ball mill jar, and perform mechanical ball milling under the following conditions: rotation speed of 10Hz and time of 4h.

[0041] ③After ball milling, wash with deionized water, the amount of deionized water is 40mL, and perform suction filtration, which is repeated 6 times. Then perform vacuum suction filtration and vacuum drying.

[0042] The obtained sample is analyzed by XRD, and the results are shown in Figure 3 . From the spectrum, it can be observed that the peak shape of the sample obtained in Example 3 is shifted compared to the spectrum of the sample obtained in Example 2, which indicates that the ionic liquid [Bmim]OH treatment has an effect on the structure of the rehydrated hydrotalcite.

[0043] Application Example 1: A method for catalyzing glucose to prepare fructose in an aqueous liquid phase, the steps are as follows:

[0044] Take 0.036g of glucose and 2mL of water, mix well, and then transfer to a reaction kettle, the magnetic stirring speed is 500rmp, 0.1 MPa nitrogen, and the reaction is carried out at 100 o C for 2h. After the reaction, the reaction mixture is quickly washed and cooled with water, the obtained mixture is filtered through a filter membrane, and the supernatant is subjected to liquid chromatography to determine the product.

[0045] The conversion rate of glucose, the yield of fructose, the yield of lactic acid, and the yield of glyceraldehyde are calculated. The yield calculation formula is as follows:

[0046] ;

[0047] ;

[0048] The conditions of liquid chromatography are as follows: mobile phase 0.05M H2SO4, column temperature 50 o C, flow rate 0.5mL / min, differential detector RI and PAD detector.

[0049] The conversion rate of glucose is 50.1%, and the yield of fructose is 27.5% (see Figure 4 ).

[0050] Application Example 2: A method for catalyzing glucose to prepare fructose in aqueous liquid phase, the steps are as follows:

[0051] Take 0.036g of untreated hydrotalcite, 0.036g of glucose, and 2mL of water, mix uniformly, then transfer to the reaction kettle, the magnetic stirring speed is 500rmp, 0.1 MPa nitrogen, under the condition of 100 o C for 2h, after reaction, quickly rinse the cooled reaction mixture with water, filter the obtained mixture with filter membrane, take the supernatant for liquid chromatography to determine the product.

[0052] The conversion rate of glucose is 60.6%, and the yield of fructose is 10.7%, the yield of glyceraldehyde is 0.7%, and the yield of lactic acid is 1% (see Figure 4 ).

[0053] Application Example 3: A method for catalyzing glucose to generate fructose, lactic acid and glyceraldehyde in water by using the catalyst of Example 1, the steps are as follows:

[0054] Take 0.036g of hydrotalcite catalyst obtained in Example 1, 0.036g of glucose, and 2mL of water, mix uniformly, then transfer to the reaction kettle, the magnetic stirring speed is 500rmp, 0.1 MPa nitrogen, under the condition of 100 o C for 2h, after reaction, quickly rinse the cooled reaction mixture with water, filter the obtained mixture with filter membrane, take the supernatant for liquid chromatography to determine the product.

[0055] The conversion rate of glucose is 99.4%, and the yield of fructose is 3.3%, the yield of lactic acid is 9.2%, and the yield of glyceraldehyde is 16.1% (see Figure 4 ).

[0056] Application Example 4: A method for catalyzing glucose to generate fructose, lactic acid and glyceraldehyde in water by using the catalyst of Example 2, the steps are as follows:

[0057] Take 0.036g of hydrotalcite catalyst obtained in Example 2, 0.036g of glucose, and 2mL of water, mix uniformly, then transfer to the reaction kettle, the magnetic stirring speed is 500rmp, 0.1 MPa nitrogen, under the condition of 100 oThe reaction mixture was washed with water immediately after the reaction, and the obtained mixture was filtered through a membrane filter, and the supernatant was subjected to liquid chromatography to determine the product.

[0058] The conversion rate of glucose was calculated to be 99.7%, the yield of fructose was 1.0%, the yield of lactic acid was 8.9%, and the yield of glyceraldehyde was 24.2% (see Table 1). Figure 4

[0059] Application Example 5: A method for catalyzing glucose to generate fructose, lactic acid and glyceraldehyde in water by using the catalyst of Example 2 and ionic liquid [Bmim]OH, the steps are as follows:

[0060] The hydrotalcite catalyst obtained in Example 2 was taken as 0.036 g, 0.036 g of ionic liquid [Bmim]OH, 0.036 g of glucose, and 2 mL of water, which were mixed uniformly and then transferred to a reaction kettle, the magnetic stirring speed was 500 rpm, 0.1 MPa nitrogen, and the reaction was carried out at 100 o The reaction mixture was washed with water immediately after the reaction, and the obtained mixture was filtered through a membrane filter, and the supernatant was subjected to liquid chromatography to determine the product.

[0061] The conversion rate of glucose was calculated to be 100%, the yield of fructose was 0%, the yield of lactic acid was 42.3%, and the yield of glyceraldehyde was 24.8% (see Table 1). Figure 4

[0062] Application Example 6: A method for catalyzing glucose to generate fructose, lactic acid and glyceraldehyde in water by using the catalyst of Example 3, the steps are as follows:

[0063] The hydrotalcite catalyst obtained in Example 3 was taken as 0.036 g, 0.036 g of glucose, and 2 mL of water, which were mixed uniformly and then transferred to a reaction kettle, the magnetic stirring speed was 500 rpm, 0.1 MPa nitrogen, and the reaction was carried out at 100 o The reaction mixture was washed with water immediately after the reaction, and the obtained mixture was filtered through a membrane filter, and the supernatant was subjected to liquid chromatography to determine the product.

[0064] The conversion rate of glucose was calculated to be 100%, the yield of fructose was 0%, the yield of lactic acid was 43.6%, and the yield of glyceraldehyde was 42.3% (see Table 1). Figure 4

[0065] The above is only a preferred embodiment of the present application, it should be noted that for those skilled in the art, without departing from the concept of the present application, several modifications and improvements can be made, which should be considered as the protection scope of the present application, which will not affect the effect and practicality of the patent.​​​

Claims

1. Use of a modified magnesium-aluminum layered double hydroxide 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 the modified magnesium-aluminum layered double hydroxide, glucose and water are mixed, nitrogen is introduced, the glucose reacts under the catalysis of the catalyst to obtain lactic acid; The preparation method of the modified magnesium-aluminum layered double hydroxide comprises the following steps: The aluminum-magnesium hydrotalcite is subjected to pyrolysis treatment, and then is mixed with ultrapure water to perform re-hydrolysis treatment, followed by vacuum filtration and vacuum drying to obtain a magnesium-aluminum layered double hydroxide material with changed structure; The magnesium-aluminum layered double hydroxide material is mixed with an ionic liquid and subjected to ball milling treatment; The material after the ball milling treatment is cleaned with deionized water, filtered, and then subjected to vacuum filtration and vacuum drying to obtain the modified magnesium-aluminum layered double hydroxide; The pyrolysis temperature of the hydrotalcite before the pretreatment is 400-600 DEG C, and the pyrolysis time is 0.5-8 h; The temperature of the re-hydrolysis treatment of the hydrotalcite after the pyrolysis treatment is 40-80 DEG C, and the time is 1-4 h; The mass ratio of the magnesium-aluminum layered double hydroxide material to the ionic liquid is 6-1:0-6, and the mass of the ionic liquid is not 0; The rotation speed of the ball milling treatment is 5-20 Hz, and the time is 1-8 h; the diameter of the ball milling beads is 5-20 mm, and the mass ratio of the ball milling beads to the material is 20-100:2-4; The ionic liquid is a Bronsted acid, neutral or basic ionic liquid, and specifically includes 1-butyl-3-methylimidazole bromide, 1-butyl-3-methylimidazole chloride, 1-butyl-3-methylimidazole acetate, 1-butyl-3-methylimidazole hydroxide and 1-butyl-3-methylimidazole bisulfate; The modified magnesium-aluminum layered double hydroxide prepared by the preparation method comprises a magnesium-aluminum layered double hydroxide material with changed structure and an ionic liquid monomer grafted on the magnesium-aluminum layered double hydroxide material in a covalent form; the modified magnesium-aluminum layered double hydroxide is rich in oxygen-containing functional groups, and simultaneously contains Al2O3, MgO structures and ionic liquid-based catalytic sites.

2. Use according to claim 1, characterized in that, The temperature of the reaction is 80-120 DEG C, the time is 1-24 h, and the pressure is 0.1-1 MPa.

3. Use according to claim 1, characterized in that, The mass concentration of the glucose in the mixed solution obtained after the catalyst, the glucose and the water are mixed is 1-5 wt%; the mass ratio of the catalyst to the glucose is 0-0.09:0.036-0.9, and the mass of the catalyst is not 0.

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