A method for preparing mesoporous / macroporous magnesium oxide by using an organic acid-assisted dry gel

Abstract

The present invention relates to the technical field of inorganic material preparation, and specifically relates to a method for preparing mesoporous / macroporous magnesium oxide by using an organic acid-assisted dry gel. The method adopted is to first prepare a precursor dry gel, then carry out a heating reaction under the assistance of organic acid vapor, and finally calcine to obtain mesoporous / macroporous magnesium oxide. The mesoporous / macroporous magnesium oxide obtained by the present invention has a relatively fast adsorption rate and a relatively high adsorption capacity, and can achieve efficient and green treatment of organic dye wastewater or heavy metal wastewater.

Description

Technical Field

[0001] The present invention relates to the technical field of inorganic material preparation, and particularly to a method for preparing mesoporous / macroporous magnesium oxide by using an organic acid-assisted xerogel. Background Art

[0002] So far, magnesite has been mainly used in the preparation of refractory materials. Limited by the grade problem of magnesite, it is difficult to directly develop high-quality and high-value-added magnesium oxide products for the chemical industry.

[0003] In recent years, the water pollution problems caused by organic dye molecules and heavy metal ions have become increasingly serious. Congo red (CR) molecules and nickel ions (Ni), as typical representative pollutants, widely exist in industrial wastewater, seriously affecting the environment and human health. Currently, various methods such as ion exchange, chemical precipitation, reverse osmosis / electrodialysis, photocatalytic degradation, and adsorption have been developed to remove pollutants. Among them, adsorption is the most effective, widely used, economical, and environmentally friendly separation method for removing nickel ions and organic dyes from wastewater. However, due to high cost, low adsorption capacity, and limited recovery efficiency, the application of the adsorption method is severely restricted. Therefore, the development of effective adsorbents with low production cost, large adsorption capacity, and high recovery efficiency for adsorbing heavy metal ions and organic dyes is of great significance for wastewater treatment applications. Magnesium oxide has the characteristics of many active sites, high isoelectric point, and environmental friendliness, and is widely used in fields such as catalysis, adsorption, and antibacterial. The reported methods for preparing magnesium oxide adsorbents mainly include direct precipitation method, hydrothermal precipitation method, and homogeneous precipitation method, etc. Liu et al. used magnesium nitrate as the magnesium source, amphiphilic triblock copolymer Pluronic F127 as the soft template, and polystyrene colloidal crystal as the hard template, and successfully synthesized three-dimensional ordered macroporous magnesium oxide by the double-template method. The maximum adsorption capacity for removing Cd(II) in aqueous solution reached 2933 mg / g (Ceram. Int., 2021, 47(16): 22830-22838). CN106732325A discloses a method for preparing a super-high-capacity magnesium oxide adsorbent, by reacting Mg(NO 3 ) 2 and Na 2 CO 3 to prepare MgCO 3 ·xH 2O precursors are used to prepare high-capacity magnesium oxide adsorbents by calcination. The adsorption capacity of this adsorbent for congo red reaches 3100 mg / g. CN102908977A discloses a method for preparing hollow spherical magnesium oxide adsorbents. A solution prepared from magnesium nitrate, magnesium acetate, or a combination of magnesium nitrate and magnesium acetate is used as the raw material to prepare hollow spherical magnesium salts, and then hollow magnesium oxide is prepared by calcination. The prepared magnesium oxide has high adsorption performance for lead ions and cadmium ions, and the saturated adsorption capacity of lead ions is greater than 1400 mg / g. CN112875732A discloses a method for preparing nano-magnesium oxide. Using magnesium methoxide as the raw material, magnesium hydroxide sol is prepared, and magnesium hydroxide gel is further calcined to obtain nano-magnesium oxide. This product has good effects on degrading oxygen phosphorus and adsorbing heavy metal ions such as Pb(II). In summary, current research on magnesium oxide adsorbents mainly uses magnesium salt chemical reagents as raw materials, and the adsorption performance is limited. There is a lack of technology for directly preparing magnesium oxide adsorbents with higher adsorption performance using industrial light-burned magnesium oxide as the raw material. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a method for preparing mesoporous / macroporous magnesium oxide by an organic acid-assisted dry gel method. The obtained mesoporous / macroporous magnesium oxide has a hierarchical pore structure and better adsorption performance.

[0005] The technical solution adopted by the present invention is as follows:

[0006] A method for preparing mesoporous / macroporous magnesium oxide by an organic acid-assisted dry gel method, comprising the following steps:

[0007] 1) Mix an organic acid and light-burned magnesium oxide (added in portions. In the magnesium acetate system, acetic acid is added in portions, and in other organic acid systems, light-burned magnesium oxide is added in portions with a time interval of 0.5 - 2 h) in distilled water for reaction. After the reaction, centrifuge to obtain the supernatant, and the supernatant is freeze-dried or evaporated to crystallize to obtain the precursor dry gel;

[0008] 2) Carry out an organic acid vapor-assisted heating reaction on the precursor dry gel and the organic acid under separation conditions. After the reaction, cool to room temperature, and the product is freeze-dried to obtain a modified organic magnesium salt precursor dry gel;

[0009] 3) Calcine the modified organic magnesium salt precursor dry gel to obtain mesoporous / macroporous magnesium oxide.

[0010] In step 1), the molar ratio of the organic acid to the light-burned magnesium oxide is 3:1.5 - 3, where the mass fraction of the organic acid is 1 - 25%. During the reaction: the temperature is 55 - 90 °C, pH = 5.0 - 7.5; the organic acid is acetic acid, citric acid, lactic acid, malic acid, or tartaric acid.

[0011] The specific surface area of the light-burned magnesium oxide is about 12 m2 / g.

[0012] In the step 2), the solid-liquid ratio of the precursor xerogel to the organic acid is 1:5 to 10, the reaction temperature is 160 to 200 °C, and the reaction time is 6 to 24 h.

[0013] In the step 3), during calcination: the calcination temperature is 300 to 450 °C, the calcination time is 1 to 2 h, and the calcination atmosphere is air.

[0014] In the steps 1) and 2), during freeze-drying: the drying temperature is -100 to -80 °C, and the drying time is 10 to 36 h.

[0015] Mesoporous / macroporous magnesium oxide prepared by the above preparation method. Among them, the mesopores are 2 - 50 nm, and the macropores are greater than 50 nm.

[0016] Application of the mesoporous / macroporous magnesium oxide as an adsorbent.

[0017] Application of the mesoporous / macroporous magnesium oxide in adsorbing Congo red dye and Ni(II).

[0018] The morphology of the mesoporous / macroporous magnesium oxide is shown as a foamy structure, rod-shaped blocks, irregular blocks or granular.

[0019] Compared with the prior art, the beneficial technical effects of the present invention are:

[0020] The present invention first uses different organic acids and light-burned magnesium oxide as raw materials to prepare a series of organic acid magnesium precursor xerogels, and a bidentate bridging coordination is formed between the organic carboxylic acid and the metal atom. Since acetic acid contains one carboxyl group, lactic acid contains one carboxyl group and one hydroxyl group, malic acid contains two carboxyl groups and one hydroxyl group, tartaric acid contains two carboxyl groups and two hydroxyl groups, and citric acid contains three carboxyl groups and one hydroxyl group. When the number of functional groups of the organic acid increases, the corresponding ligand sites increase, and the coordination reaction is more likely to occur, which is more conducive to the formation of a three-dimensional network structure and the construction of hierarchical pores.

[0021] Furthermore, the present invention uses the organic acid vapor-assisted method to prepare a modified organic acid magnesium xerogel. During the organic acid vapor-assisted process, the organic acid vapor will further crosslink with the organic acid magnesium precursor, and the number of magnesium-organic acid connecting chains increases. On the one hand, the crosslinking degree of the magnesium-organic acid chain is enhanced, promoting the more perfect development of the organic acid magnesium precursor; on the other hand, some newly formed magnesium-organic acid chains crosslink with each other to form a new three-dimensional gel structure covering the surface of the organic acid magnesium aggregate particles.

[0022] The modified organic acid magnesium xerogel is calcined, and the organic groups are burned and decomposed into CO 2 and H 2O. The release of gas is beneficial to the formation of mesopores, and the xerogel with a complex network structure retains the network structure after calcination, which is beneficial to the formation of larger mesopores and macropores. Finally, mesoporous / macroporous magnesium oxide is obtained. The mesopores in this magnesium oxide have a pore diameter of 2 - 50 nm, and the macropores have a pore diameter of 50 - 160 nm. The mesopores can provide a large number of active sites, while the coexisting macropores provide short paths to enhance mass transfer and prevent pore blockage.

[0023] In the preparation process of the present invention, the raw materials are cheap and easily available, the production cost is low, and no additives need to be added. The prepared magnesium oxide has a strong adsorption capacity for congo red dye and Ni(II), has a fast adsorption rate and a high adsorption capacity, can achieve efficient and green treatment of organic dye wastewater, and has good industrial application prospects. Brief Description of the Drawings

[0024] Figure 1 XRD patterns of magnesium oxide prepared in Examples 1 - 4 and the change diagram of the removal rate of congo red with time;

[0025] Figure 2 N of magnesium oxide prepared in Example 4 2 Adsorption - desorption isothermal fitting curve, and the inset is the pore size distribution diagram;

[0026] Figure 3 Isothermal curve fitting of the adsorption of congo red by the magnesium oxide adsorbent prepared in Example 4;

[0027] Figure 4 XRD patterns of magnesium oxide prepared in Examples 5 - 8 and the change diagram of the removal rate of congo red with time;

[0028] Figure 5 Adsorption isothermal curve fitting of the adsorption of congo red by the magnesium oxide prepared in Example 8;

[0029] Figure 6 Adsorption isothermal curve fitting of the adsorption of Ni(II) by the magnesium oxide prepared in Example 8;

[0030] Figure 7 XRD patterns of magnesium oxide prepared in Examples 9 - 10 and the change diagram of the removal rate of congo red with time;

[0031] Figure 8 XRD patterns of magnesium oxide prepared in Examples 11 - 12 and the change diagram of the removal rate of congo red with time;

[0032] Figure 9 XRD patterns of magnesium oxide prepared in Examples 13 - 14 and the change diagram of the removal rate of congo red with time;

[0033] Figure 10SEM images of magnesium oxide prepared in Example 2 (left) and magnesium oxide prepared in Example 4 (right);

[0034] Figure 11 SEM images of magnesium oxide prepared in Example 6 (left) and magnesium oxide prepared in Example 8 (right);

[0035] Figure 12 SEM images of magnesium oxide prepared in Example 10 (a), Example 12 (b), and Example 14 (c);

[0036] Figure 13 N 2 adsorption - desorption isothermal fitting curve of magnesium oxide prepared in Example 8, and the inset is the pore size distribution diagram;

[0037] Figure 14 N 2 adsorption - desorption isothermal fitting curve of magnesium oxide prepared in Example 10, and the inset is the pore size distribution diagram;

[0038] Figure 15 N 2 adsorption - desorption isothermal fitting curve of magnesium oxide prepared in Example 12, and the inset is the pore size distribution diagram;

[0039] Figure 16 N 2 adsorption - desorption isothermal fitting curve of magnesium oxide prepared in Example 14, and the inset is the pore size distribution diagram. Detailed implementation manners

[0040] The following examples are used to illustrate the detailed implementation manners of the present invention. However, the following examples are only used to elaborate on the present invention in detail and do not limit the scope of the present invention in any way.

[0041] In the present invention, some experimental equipment is as follows: The Ultima_IV X - ray diffractometer produced by Rigaku Corporation of Japan is used for qualitative analysis of the phase composition of the product. In the experiment, the Auriga_FIB scanning electron microscope produced by Zeiss Company of the United States is used for morphological observation of the product, and the UV - 1600PC type ultraviolet - visible spectrophotometer produced by Xiangyi Instruments (Shanghai) Co., Ltd. is used to measure the absorbance of the solution. The specific surface area of the light - burned magnesium oxide used in the examples is about 12 m 2 / g, which is an industrial product produced by Puyang Pu High - Temperature Materials (Group) Co., Ltd. using cryptocrystalline magnesite as the raw material.

[0042] Example 1:

[0043] A method for preparing magnesium oxide:

[0044] 1) First, weigh 4 g of light-burned magnesia according to the molar ratio of light-burned magnesia to acetic acid of 1:2. Slowly add the weighed light-burned magnesia to 60 mL of deionized water, and stir evenly at room temperature using a magnetic stirrer to finally obtain a suspension. Weigh 11.44 mL of glacial acetic acid and pour it into the suspension in two portions (with a time difference of 1 h between the two times). Measure the pH = 6.60, heat and stir in a water bath at 90 °C, and keep warm for 3 h to allow the light-burned magnesia and acetic acid to react fully. Centrifuge the obtained sample to get a filtrate, and freeze-dry the filtrate at -80 °C for 36 h to obtain a precursor dry gel.

[0045] 2) Place the precursor dry gel obtained in step 1) in a crucible, put it into a box-type resistance furnace, and calcine it at 450 °C for 2 h (heating rate is 5 °C / min). The obtained sample is analyzed by XRD as the magnesia phase ( Figure 1 left).

[0046] Add 10 mg of the magnesia obtained in step 2) above to 50 mL of 100 mg / L congo red solution respectively, stir evenly at room temperature using a magnetic stirrer, and at 240 min, the adsorption rate of this magnesia to congo red reaches 100% ( Figure 1 right).

[0047] Example 2

[0048] A method for preparing magnesia:

[0049] 1) First, weigh 4 g of light-burned magnesia according to the molar ratio of light-burned magnesia to acetic acid of 1:2. Slowly add the weighed light-burned magnesia to 60 mL of deionized water, and stir evenly at room temperature using a magnetic stirrer to finally obtain a suspension. Weigh 11.44 mL of acetic acid and pour it into the suspension in two portions (with a time difference of 1 h between the two times, 5.72 mL each time). Measure the pH = 6.60, heat and stir in a water bath at 90 °C, and keep warm for 3 h to allow the light-burned magnesia and acetic acid to react fully. Centrifuge the obtained sample to get a filtrate, and evaporate and crystallize the filtrate at 105 °C to obtain a precursor dry gel.

[0050] 2) Place the obtained precursor dry gel in a crucible, put it into a box-type resistance furnace, and calcine it at 450 °C for 2 h (heating rate is 5 °C / min). The obtained sample is analyzed by XRD as the magnesia phase ( Figure 1 left).

[0051] Add 10 mg of the magnesia obtained in step 2) to 50 mL of 100 mg / L congo red solution respectively, stir evenly at room temperature using a magnetic stirrer, and at 240 min, the adsorption rate of this magnesia to congo red is measured to reach 72% ( Figure 1 right).

[0052] Example 3

[0053] Method for preparing mesoporous / macroporous magnesium oxide by using organic acid-assisted dry gel

[0054] 1) Weigh 1 g of the precursor dry gel prepared in step 1) of Example 1 and place it in a 10 mL small beaker. At the same time, measure 5 mL of acetic acid (1 mol / L) and pour it into another small beaker. Place the beakers containing acetic acid and the dry gel in the inner lining of a 100 mL stainless steel autoclave, seal it, and then place it in an intelligent electrothermal blast drying oven. Keep it at 180 °C for 12 h. After the reaction is completed, naturally cool it to room temperature. Place the product in a freeze dryer at -80 °C and freeze-dry it for 12 h to obtain a modified organic magnesium salt precursor dry gel.

[0055] 2) Place the precursor dry gel obtained in step 1) in a crucible, put it into a box-type resistance furnace, and calcine it at 450 °C for 2 h (heating rate: 5 °C / min) to obtain mesoporous / macroporous magnesium oxide. The mesoporous / macroporous magnesium oxide sample is analyzed by XRD as the magnesium oxide phase ( Figure 1 left).

[0056] Add 10 mg of the above-mentioned mesoporous / macroporous magnesium oxide to 50 mL of 100 mg / L congo red solution respectively, stir evenly with a magnetic stirrer at room temperature. At 180 min, the adsorption rate of this magnesium oxide to congo red reaches 100%. The experimental results show that after being treated with acetic acid vapor-assisted dry gel, the prepared magnesium oxide has improved adsorption performance ( Figure 1 ), and can adsorb all congo red at 180 min Complete .

[0057] Example 4

[0058] Method for preparing mesoporous / macroporous magnesium oxide by using organic acid-assisted dry gel

[0059] 1) Weigh 1 g of the precursor dry gel prepared in step 1) of Example 2 and place it in a 10 mL small beaker. At the same time, measure 5 mL of acetic acid (1_mol / L) and pour it into another small beaker. Place the beakers containing acetic acid and the dry gel in the inner lining of a 100 mL stainless steel autoclave, seal it, and then place it in an intelligent electrothermal blast drying oven. Keep it at 180 °C for 12 h. After the reaction is completed, naturally cool it to room temperature. Place the product in a freeze dryer at -80 °C and freeze-dry it for 12 h to obtain a modified organic magnesium salt precursor dry gel.

[0060] 2) Place the precursor dry gel obtained in step 1) in a crucible, put it into a box-type resistance furnace, and calcine it at 450 °C for 2 h (heating rate: 5 °C / min) to obtain mesoporous / macroporous magnesium oxide. The mesoporous / macroporous magnesium oxide sample is analyzed by XRD as the magnesium oxide phase ( Figure 1 left).

[0061] Add 10 mg of the above mesoporous / macroporous magnesium oxide to 50 mL of a 100 mg / L congo red solution, stir evenly with a magnetic stirrer at room temperature, and measure that the adsorption rate of the mesoporous / macroporous magnesium oxide to congo red reaches 100% at 10 min.

[0062] Prepare 50 mL of congo red solutions with concentrations of 600 mg / L, 800 mg / L, 1000 mg / L, 1100 mg / L, 1200 mg / L, 1300 mg / L, 1400 mg / L, and 1500 mg / L respectively. Under the condition of stirring at room temperature, add 20 mg of the magnesium oxide adsorbent prepared in Example 4 respectively. After reaching the adsorption equilibrium, sample and measure the concentration of congo red in the filtrate, and draw the adsorption isotherm. The adsorption of the magnesium oxide prepared in Example 4 to congo red more conforms to the Langmuir isothermal adsorption model, indicating that the adsorption of the magnesium oxide prepared in Example 4 to congo red is monolayer adsorption. The calculated maximum adsorption capacity is 3077 mg / g( Figure 3 ).

[0063] Example 5

[0064] A method for preparing magnesium oxide:

[0065] 1) First, weigh 9.6 g of citric acid monohydrate according to the molar ratio of light-burned magnesium oxide to citric acid of 5:3. Slowly add the weighed citric acid monohydrate to 60 mL of deionized water to finally obtain a citric acid solution. Heat the citric acid solution to 70 °C, weigh 3 g of light-burned magnesium oxide and add it to the citric acid solution in two portions (the time interval between the two additions is 1 h, and 1.5 g is added each time). Measure that the pH = 6.50. Stir with a water bath at 70 °C for 6 h to fully react the light-burned magnesium oxide and citric acid. Centrifuge the obtained sample to obtain a filtrate, and freeze-dry the filtrate at -80 °C for 36 h to obtain a dry gel.

[0066] 2) Place the dry gel obtained in step 1) in a crucible, put it into a box-type resistance furnace, and calcine it at 450 °C for 2 h (the heating rate is 5 °C / min). The obtained sample is analyzed by XRD as the magnesium oxide phase( Figure 4 left).

[0067] Add 10 mg of the above adsorbent to 50 mL of a 100 mg / L congo red solution, stir evenly with a magnetic stirrer at room temperature, and the adsorption rate of the magnesium oxide to congo red reaches 100% at 180 min( Figure 4 right).

[0068] Example 6

[0069] A method for preparing magnesium oxide:

[0070] 1) First, weigh 9.6 g of citric acid monohydrate according to the molar ratio of light-burned magnesium oxide to citric acid of 5:3. Slowly add the weighed citric acid monohydrate to 60 mL of deionized water to finally obtain a citric acid solution. Heat the citric acid solution to 70 °C, weigh 3 g of light-burned magnesium oxide and add it to the citric acid solution in two portions (with a time difference of 1 h between the front and back, and 1.5 g added each time). Measure the pH = 6.50, heat and stir in a water bath at 70 °C, and keep warm for 6 h to allow the light-burned magnesium oxide and acetic acid to react fully. Centrifuge the obtained sample to get a filtrate, and evaporate and crystallize the filtrate at 105 °C to obtain a dry gel.

[0071] 2) Place the dry gel obtained in step 1) in a crucible, put it into a box-type resistance furnace, and calcine it at 450 °C for 2 h (heating rate is 5 °C / min). The obtained sample is analyzed by XRD as magnesium oxide phase ( Figure 4 left).

[0072] Add 10 mg of the above adsorbent to 50 mL of 100 mg / L congo red solution, stir evenly with a magnetic stirrer at room temperature, and the adsorption rate of this magnesium oxide to congo red reaches 100% at 180 min ( Figure 4 right).

[0073] Example 7

[0074] A method for preparing mesoporous / macroporous magnesium oxide assisted by organic acid dry gel:

[0075] 1) Weigh the dry gel prepared in step 2) of Example 5 and place it in a 10 mL small beaker. At the same time, measure 5 mL of citric acid (1 mol / L) and pour it into another small beaker. Place the small beakers containing citric acid and the dry gel in a 100 mL stainless steel autoclave liner, seal it, and place it in an intelligent electrothermal blast drying oven. Keep warm at 180 °C for 12 h. After the reaction ends, naturally cool to room temperature, and place the product in a freeze dryer at -80 °C for freeze drying for 12 h to obtain a modified organic magnesium salt precursor dry gel.

[0076] 2) Place the precursor dry gel obtained in step 1) in a crucible, put it into a box-type resistance furnace, and calcine it at 450 °C for 2 h (heating rate is 5 °C / min) to obtain mesoporous / macroporous magnesium oxide. The mesoporous / macroporous magnesium oxide sample is analyzed by XRD as magnesium oxide phase ( Figure 4 left).

[0077] Add 10 mg of the above mesoporous / macroporous magnesium oxide as an adsorbent to 50 mL of 100 mg / L congo red solution, stir evenly with a magnetic stirrer at room temperature, and the adsorption rate of this magnesium oxide to congo red reaches 100% at 120 min ( Figure 4 right).

[0078] Example 8

[0079] Method for preparing mesoporous / macroporous magnesium oxide assisted by organic acid dry gel:

[0080] 1) Weigh the dry gel prepared in step 2) of Example 6 and place it in a 10 mL small beaker. At the same time, measure 5 mL of citric acid (1 mol / L) and pour it into another small beaker. Place the small beakers containing citric acid and the dry gel in the inner lining of a 100 mL stainless steel autoclave, seal it, and then place it in an intelligent electrothermal blast drying oven. Keep it at 180 °C for 12 h. After the reaction is completed, let it cool naturally to room temperature. Place the product in a freeze dryer at -80 °C and freeze-dry it for 12 h to obtain a modified organic magnesium salt precursor dry gel.

[0081] 2) Place the precursor dry gel obtained in step 1) in a crucible, put it into a box-type resistance furnace, and calcine it at 450 °C for 2 h (heating rate: 5 °C / min) to obtain mesoporous / macroporous magnesium oxide. The mesoporous / macroporous magnesium oxide sample is analyzed by XRD as the magnesium oxide phase ( Figure 4 left).

[0082] Add 10 mg of the above-mentioned mesoporous / macroporous magnesium oxide as an adsorbent to 50 mL of a 100 mg / L congo red solution, stir evenly with a magnetic stirrer at room temperature, and the adsorption rate of the magnesium oxide to congo red reaches 100% at 30 min ( Figure 4 right).

[0083] Prepare 50 mL of congo red solutions with concentrations of 600 mg / L, 800 mg / L, 1000 mg / L, 1100 mg / L, 1200 mg / L, 1300 mg / L, 1400 mg / L, and 1500 mg / L respectively. Under the condition of stirring at room temperature, add 20 mg of the magnesium oxide adsorbent prepared in Example 8 respectively. After reaching the adsorption equilibrium, take samples and measure the concentration of congo red in the filtrate, and draw the adsorption isotherm. The adsorption of congo red by the magnesium oxide prepared in Example 8 better conforms to the Langmuir isothermal adsorption model, indicating that the adsorption of congo red by the magnesium oxide prepared in Example 8 is monolayer adsorption. The calculated maximum adsorption capacity is 4041 mg / g ( Figure 5 ).

[0084] Prepare 50 mL of Ni(II) dye solutions with concentrations between 400 mg / L, 500 mg / L, 600 mg / L, 700 mg / L, 800 mg / L, 900 mg / L, and 1000 mg / L respectively, and add 20 mg (0.4 g / L) of the adsorbent to each solution. Stir at room temperature and stir until the adsorption equilibrium is reached under the condition of pH = 6. The adsorption of Ni(II) by the magnesium oxide adsorbent prepared in Example 8 better conforms to the Langmuir isothermal adsorption model, indicating that the adsorption of Ni(II) by the magnesium oxide is monolayer adsorption. The calculated maximum adsorption capacity is 1089 mg / gFigure 6 )。

[0085] Example 9

[0086] A method for preparing magnesium oxide:

[0087] Take 4.66 mL of lactic acid according to the molar ratio of light-burned magnesium oxide to lactic acid = 1:2. Slowly add the weighed lactic acid into a beaker containing 60 mL of deionized water to finally obtain a lactic acid solution, and heat the lactic acid solution to 70 °C. Weigh 1 g of light-burned magnesium oxide and add it to the lactic acid solution in two portions (with a time difference of 1 h between the two additions, 0.5 g each time). Measure the pH = 5.07, and stir with a water bath at 70 °C for 6 h to allow the light-burned magnesium oxide and lactic acid to react fully. Centrifuge the obtained sample to get a filtrate, and evaporate and crystallize the filtrate at 105 °C to obtain a dry gel. Place the obtained dry gel in a crucible, put it into a box-type resistance furnace, and calcine it at 450 °C for 2 h (heating rate is 5 °C / min). The obtained sample is analyzed by XRD as the magnesium oxide phase ( Figure 7 left).

[0088] Add 10 mg of the above adsorbent to 50 mL of a 300 mg / L congo red solution, stir evenly with a magnetic stirrer at room temperature, and the adsorption rate of this magnesium oxide to congo red reaches 100% at 120 min ( Figure 7 right).

[0089] Example 10

[0090] A method for preparing mesoporous / macroporous magnesium oxide assisted by organic acid dry gel:

[0091] Weigh 1 g of the dry gel prepared in Example 9 and place it in a 10 mL small beaker. At the same time, measure 5 mL of lactic acid (1 mol / L) and put it into another beaker. Place the small beakers containing lactic acid and the dry gel in a 100 mL stainless steel autoclave liner respectively, seal it and place it in an intelligent electrothermal blast drying oven, keep it at 180 °C for 12 h. After the reaction, cool it naturally to room temperature, place the product in a freeze dryer at -80 °C for freeze drying for 12 h, and then calcine it at 450 °C for 2 h (heating rate is 5 °C / min) to obtain mesoporous / macroporous magnesium oxide. The obtained sample is analyzed by XRD as the magnesium oxide phase ( Figure 7 left).

[0092] Add 10 mg of the above mesoporous / macroporous magnesium oxide as an adsorbent to 50 mL of a 300 mg / L congo red solution, stir evenly with a magnetic stirrer at room temperature, and the adsorption rate of this magnesium oxide to congo red reaches 100% at 60 min ( Figure 7 right).

[0093] Example 11

[0094] A method for preparing magnesium oxide:

[0095] Weigh 10.0568 g of malic acid according to the molar ratio of light-burned magnesium oxide to malic acid = 1:1. Slowly add the weighed malic acid into a beaker containing 60 mL of deionized water to finally obtain a malic acid solution. Measure the pH = 5.55, and heat the malic acid solution to 70 °C. Weigh 3 g of light-burned magnesium oxide and add it to the malic acid solution in two portions (with a time difference of 1 h between the two additions, 1.5 g each time). Stir the mixture in a water bath at 70 °C for 6 h to allow the light-burned magnesium oxide and malic acid to react fully. Centrifuge the obtained sample to get a filtrate, and evaporate and crystallize the filtrate at 105 °C to obtain a dry gel. Place the obtained dry gel in a crucible, put it into a box-type resistance furnace, and calcine it at 450 °C for 2 h (heating rate: 5 °C / min). The obtained sample is analyzed by XRD to be the magnesium oxide phase ( Figure 8 left).

[0096] Add 10 mg of the above adsorbent to 50 mL of a 200 mg / L congo red solution, stir evenly with a magnetic stirrer at room temperature. At 120 min, the adsorption rate of this magnesium oxide to congo red reaches 100% ( Figure 8 right).

[0097] Example 12

[0098] A method for preparing mesoporous / macroporous magnesium oxide assisted by organic acid dry gel:

[0099] Weigh 1 g of the dry gel prepared in Example 11 and place it in a 10 mL small beaker. At the same time, measure 5 mL of malic acid (1 mol / L) and pour it into another beaker. Place the small beakers containing malic acid and the dry gel together in the inner lining of a 100 mL stainless steel autoclave, seal it, and place it in an intelligent electrothermal blast drying oven. Keep it at 180 °C for 12 h. After the reaction, let it cool naturally to room temperature. Place the product in a freeze dryer at -80 °C and freeze dry it for 12 h. Then calcine it at 450 °C for 2 h (heating rate: 5 °C / min) to obtain mesoporous / macroporous magnesium oxide. The obtained sample is analyzed by XRD to be the magnesium oxide phase ( Figure 8 left).

[0100] Add 10 mg of the above adsorbent to 50 mL of a 200 mg / L congo red solution, stir evenly with a magnetic stirrer at room temperature. At 60 min, the adsorption rate of this magnesium oxide to congo red reaches 100% ( Figure 8 right).

[0101] Example 13

[0102] A method for preparing magnesium oxide:

[0103] Weigh 1.1257 g of tartaric acid according to the molar ratio of light-burned magnesia to tartaric acid = 1:1. Slowly add the weighed tartaric acid to a beaker containing 60 mL of deionized water to finally obtain a tartaric acid solution. Measure the pH = 5.86, and heat the tartaric acid solution to 70 °C. Weigh 0.3 g of light-burned magnesia and add it to the tartaric acid solution in two portions (with a time difference of 1 h between the two additions, 0.15 g each time). Heat and stir in a water bath at 70 °C and keep warm for 6 h to allow the light-burned magnesia and tartaric acid to react fully. Centrifuge the obtained sample to get a filtrate, and evaporate and crystallize the filtrate at 105 °C to obtain magnesium tartrate xerogel. Place the obtained xerogel in a crucible, put it into a box-type resistance furnace, and calcine it at 450 °C for 2 h (heating rate is 5 °C / min). The obtained sample is analyzed by XRD as the magnesia phase ( Figure 9 left).

[0104] Add 10 mg of the above adsorbent to 50 mL of 200 mg / L congo red solution, stir evenly with a magnetic stirrer at room temperature. At 180 min, the adsorption rate of this magnesia to congo red reaches 100% ( Figure 9 right).

[0105] Example 14

[0106] A method for preparing mesoporous / macroporous magnesia by using an organic acid-assisted xerogel:

[0107] Weigh 1 g of the xerogel prepared in Example 13 and place it in a 10 mL small beaker. At the same time, measure 5 mL of tartaric acid (1 mol / L) and pour it into another beaker. Place the small beakers containing tartaric acid and the xerogel together in a 100 mL stainless steel autoclave liner, seal it, and place it in an intelligent electrothermal blast drying oven. Keep warm at 180 °C for 12 h. After the reaction, naturally cool to room temperature. Place the product in a freeze dryer at -80 °C for freeze drying for 12 h, and calcine it at 450 °C for 2 h (heating rate is 5 °C / min) to obtain mesoporous / macroporous magnesia. The obtained sample is analyzed by XRD as the magnesia phase ( Figure 9 left).

[0108] Add 10 mg of the above adsorbent to 50 mL of 200 mg / L congo red solution, stir evenly with a magnetic stirrer at room temperature. At 60 min, the adsorption rate of this magnesia to congo red reaches 100% ( Figure 9 right).

[0109] From the data analysis of the above examples, it can be seen that:

[0110] From Figures 10 - 12It can be seen from the SEM images that the magnesium oxides prepared in Examples 4, 6, 8, 10, 12, and 14 all have mesoporous and macroporous structures. The magnesium oxide prepared in Example 2 agglomerates severely, with some aggregating into irregular blocks, while the magnesium oxide prepared in Example 4 with the assistance of acetic acid vapor has better dispersion and obvious pore structures.

[0111] It can be seen Figure 11 that the magnesium oxides prepared in Examples 6 and 8 are irregular blocks, but the magnesium oxide prepared in Example 8 has more distinct edges and corners, and there are more large aggregates and small particles deposited on the surface. This is because during the process assisted by citric acid vapor, the precursor xerogel is fully aged, and the particles interact with each other, causing the particles to approach further, thereby shrinking the gel skeleton. The citric acid vapor-assisted process forms a new three-dimensional gel structure covering the surface of the magnesium organic acid aggregate particles, and more particles are deposited on the surface after calcination.

[0112] It can be seen Figure 12 that the magnesium oxide - magnesium citrate - evaporation crystallization - vapor-assisted obtained in Example 8 is an irregular block with distinct edges and corners, and a layer of nanoscale particles is evenly distributed on the surface of the aggregate particles; the magnesium oxide - magnesium lactate - evaporation crystallization - vapor-assisted obtained in Example 10 is a foam-like structure with a thinner pore wall; the magnesium oxide - magnesium malate - evaporation crystallization - vapor-assisted obtained in Example 12 is an irregular block with vertical stripes on the surface; the magnesium oxide - magnesium tartrate - evaporation crystallization - vapor-assisted obtained in Example 14 is a rod-shaped block composed of parallel arrangement of magnesium oxide microspheres, and there are oblique stripes on the surface of the rod-shaped block.

[0113] It can be seen Figure 2 from / 13 / 14 / 15 / 16 that the magnesium oxides prepared in Examples 4, 8, 10, 12, and 14 have a hierarchical structure composed of mesopores and macropores, where the mesopores are 2 - 20 nm and the macropores are 50 - 160 nm. According to the trend of the gas adsorption isotherm, it can be determined as a Type IV curve, indicating that there are a certain number of mesopores in the magnesium oxide. From the pore size distribution diagram in the inset, it can be seen that the pore size of the magnesium oxide sample prepared in Example 4 is concentrated around 16 nm, mainly mesopores, and there are also a small number of macropores. The pore size of the magnesium oxide sample prepared in Example 8 is concentrated around 5 nm, mainly mesopores, and there are also a small number of macropores. The pore size of the magnesium oxide sample prepared in Example 10 is concentrated around 10 nm, mainly mesopores, and there are also a small number of macropores. The pore size of the magnesium oxide sample prepared in Example 12 is concentrated around 7 nm, mainly mesopores, and there are also a small number of macropores. The mesopore / macropore pore size distribution range of the magnesium oxide sample prepared in Example 14 becomes wider, and the number of macropores increases significantly.

[0114] As can be seen by comparing Example 1 and Example 3, after the precursor xerogel was modified with acetic acid in a forced-air drying oven, the adsorption time of the finally prepared magnesium oxide for all Congo red was shortened from 240 min required before modification to reach 100% to 180 min, indicating that the adsorption performance of the obtained mesoporous / macroporous magnesium oxide was significantly improved.

[0115] As can be seen by comparing Example 2 and Example 4, after the precursor xerogel was modified with acetic acid in a forced-air drying oven, the adsorption rate of the finally prepared magnesium oxide for all Congo red increased from 72% to 100%, and the adsorption performance was improved.

[0116] As can be seen by comparing Example 3 and Example 4, the adsorption effect of using the evaporation crystallization method to prepare the precursor xerogel is better than that of the freeze-drying method. The adsorption time of the finally prepared magnesium oxide for all Congo red was shortened from 180 min to 10 min. This is because during the evaporation crystallization process, the sol evaporates and crystallizes at a lower temperature, promoting the transformation of the sol into a gel. The complexation reaction between the organic acid and magnesium ions continues, the solution viscosity increases continuously, and the complex molecules approach each other to form hydrogen bonds, finally obtaining a cross-linked network of magnesium organic acid. Therefore, the evaporation crystallization method can better improve the cross-linking degree of the gel network. The gel obtained after acetic acid vapor assistance is a wet gel. Freeze-drying the fully gelled wet gel can better preserve the original network structure, effectively inhibiting particle aggregation, and the prepared product has better adsorption performance.

[0117] As can be seen by comparing Example 5 and Example 7, after the precursor xerogel was modified with citric acid in a forced-air drying oven, the adsorption time of the finally prepared magnesium oxide for all Congo red was shortened from 180 min required before modification to reach 100% to 120 min, indicating that the adsorption performance of the obtained mesoporous / macroporous magnesium oxide was significantly improved.

[0118] As can be seen by comparing Example 6 and Example 8, after the precursor xerogel obtained by the evaporation crystallization method was modified with citric acid in a forced-air drying oven, the adsorption time of the finally prepared magnesium oxide for all Congo red was shortened from 180 min required before modification to reach 100% to 30 min, indicating that the adsorption performance of the obtained mesoporous / macroporous magnesium oxide was significantly improved.

[0119] When comparing Example 9 and Example 10, after the precursor xerogel was modified with lactic acid in a forced-air drying oven, the adsorption time of the finally prepared magnesium oxide for all Congo red was shortened from 120 min required before modification to reach 100% to 60 min, indicating that the adsorption performance of the obtained mesoporous / macroporous magnesium oxide was significantly improved.

[0120] Compared with Example 12, after the precursor xerogel was modified with malic acid in a forced-air drying oven, the adsorption time of the finally obtained magnesium oxide for all Congo red was shortened from 120 min required before modification to reach 100% to 60 min, indicating that the adsorption performance of the obtained mesoporous / macroporous magnesium oxide was significantly improved.

[0121] Compared with Example 14, after the precursor xerogel was modified with tartaric acid in a forced-air drying oven, the adsorption time of the finally obtained magnesium oxide for all Congo red was shortened from 180 min required before modification to reach 100% to 60 min, indicating that the adsorption performance of the obtained mesoporous / macroporous magnesium oxide was significantly improved.

[0122] In summary, regardless of whether the precursor xerogel is obtained by freeze-drying or evaporation crystallization, the adsorption effect is effectively improved after being modified by organic acid vapor assistance. During the vapor assistance process, the organic acid vapor and the magnesium organic acid precursor will further crosslink, and the number of magnesium-organic acid linkages increases, promoting the more perfect development of the magnesium organic acid precursor; and some newly formed magnesium-organic acid chains crosslink with each other to form a new three-dimensional gel structure covering the surface of the magnesium organic acid aggregate particles. This is of great significance for the morphological diversity and pore size distribution regulation of the magnesium oxide obtained after calcination, and finally mesoporous / macroporous magnesium oxide is obtained. It has been verified that mesoporous / macroporous magnesium oxide exhibits excellent performance in the adsorption of Congo red and Ni(II).

[0123] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Any other modifications or equivalent replacements made by those of ordinary skill in the art to the technical solutions of the present invention shall be covered by the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing mesoporous / macroporous magnesium oxide by organic acid-assisted dry gel, characterized in that The following steps are involved: 1) Mixing an organic acid and light-burned magnesium oxide in distilled water for reaction, centrifuging to obtain a supernatant after the reaction is completed, and freeze-drying or evaporating and crystallizing the supernatant to obtain a precursor xerogel; 2) The precursor xerogel and the organic acid are subjected to an organic acid steam-assisted heating reaction under separation conditions, the solid-liquid ratio of the precursor xerogel and the organic acid is 1:5-10, the reaction temperature is 160-200 °C, the reaction time is 6-24 h, and after the reaction is completed, the reaction is cooled to room temperature, and the product is freeze-dried to obtain a modified organic magnesium salt precursor xerogel; 3) Calcine the modified organic magnesium salt precursor dry gel to obtain mesoporous / macroporous magnesium oxide.

2. The method according to claim 1, characterized in that: In the step 1), the molar ratio of the organic acid to the light-burned magnesium oxide is 3:1.5-3, wherein the mass fraction of the organic acid is 1-25%. During the reaction, the temperature is 55-90°C and the pH is 5.0-7.

5. The organic acid is acetic acid, citric acid, lactic acid, malic acid or tartaric acid.

3. The method according to claim 1, characterized in that: In the step 3), during calcination: the calcination temperature is 300-450°C, the calcination time is 1-2 hours, and the calcination atmosphere is air.

4. The method according to claim 1, characterized in that: During freeze drying in step 1) and step 2), the drying temperature is -100 to -80°C, and the drying time is 10 to 36 hours.

Images