Process for the preparation of double alkali magnesium aluminate spinel catalysts and use thereof

Abstract

The application belongs to the technical field of fine chemical industry, and particularly relates to a preparation method and application of a double-alkali magnesium-aluminum spinel catalyst. Pseudo-boehmite and magnesium nitrate are mixed, then added into water and stirred, a dispersing agent and a precipitating agent are added and stirred, and then the mixture is aged, crystallized, centrifuged, washed, dried, calcined, and a spinel carrier MgAl2O4 is obtained. The spinel carrier MgAl2O4, a modifier and water are reacted, hydrothermally crystallized, centrifuged, washed, dried and calcined, and a treated spinel carrier MgAl2O4 is obtained. Lithium nitrate, calcium nitrate, a pre-precipitating agent and water are mixed and ultrasonically treated, concentrated and evaporated by heating, and the treated spinel carrier MgAl2O4 is impregnated in an equal volume of the mixture, hydrothermally reacted, centrifuged, washed, dried and calcined, and a Li2O-CaO / MgAl2O4 double-alkali magnesium-aluminum spinel catalyst is obtained. The application is simple in preparation, and the prepared catalyst is low in cost and high in catalytic activity.

Description

Technical Field

[0001] This invention belongs to the field of fine chemical technology, specifically relating to a method for preparing and applying a dual-alkali metal magnesium aluminum spinel catalyst. Background Technology

[0002] Salicylic acid, also known as o-hydroxybenzoic acid, is an organic acid with the chemical formula C7H6O3. It is a white crystalline powder, slightly soluble in cold water and readily soluble in hot water. Salicylic acid was one of the earliest drugs used by humans. It is widely distributed in the form of methyl esters in birch bark, holly leaves, and the roots of plants in the genus *Senecio scandens*.

[0003] Salicylic acid is a very important chemical raw material with a wide range of applications, most notably as a pharmaceutical intermediate in the medical industry. In drug production, salicylic acid is used not only in the production of commonly used antirheumatic, analgesic, and antipyretic drugs, but it is also a raw material for important drugs such as magnesium salicylate, salicylamine, methyl salicylate, bis(salicylic acid) esters, salicylamide, and aspirin (acetylsalicylic acid). In dye production, salicylic acid can be used to directly prepare mordant yellow and acid chrome yellow. In the smelting of some metals, salicylic acid can be used as an analytical reagent for metals such as iron and lead, and it can also be used as a disinfectant, preservative, and rubber vulcanization retardant.

[0004] The Kolbe-Schmitt process is currently the most widely used method for preparing salicylic acid. It involves the carboxylation of phenol with carbon dioxide. Other methods include the o-cresol method, the o-nitrotoluene method, the o-toluenesulfonic acid method, the air oxidation of anhydrous copper benzoate method, and the thermal decomposition of basic copper benzoate. While there are numerous processes for preparing salicylic acid, they all suffer from drawbacks such as high byproducts, low yields, difficult operation, and high costs. The Kolbe-Schmitt process, specifically the medium-pressure phenol method, first produces sodium phenolate from phenol, then undergoes carboxylation of sodium phenolate under medium pressure with carbon dioxide, and finally acidifies the resulting sodium salicylate to obtain the product. This method offers advantages such as low reaction cost, low investment, high single-pass conversion rate of phenol, and good product quality. Utilizing widely available and low-cost raw materials and developing environmentally friendly, mild-condition, and atom-economical synthetic routes are key to enhancing the competitiveness of the salicylic acid production industry.

[0005] Matter generally exists in four states: gas, liquid, solid, and supercritical. These states change with temperature and pressure. When the temperature and pressure of a fluid are above its critical temperature and critical pressure (i.e., both the relative temperature and relative pressure are greater than 1), the fluid is called a supercritical fluid. Supercritical fluids exist in a state that is neither liquid nor gaseous. Their physicochemical properties differ from those of normal fluids, and many of their properties lie between those of gases and liquids. Furthermore, near the critical point, small changes in temperature and pressure can lead to significant changes in fluid density, allowing for convenient control of properties such as viscosity, diffusion coefficient, and solubility by adjusting temperature and pressure.

[0006] Supercritical carbon dioxide (SC-CO2) refers to a fluid containing carbon dioxide with both temperature and pressure above the critical point (31.1℃-7.39MPa). Supercritical fluids are high-density gases that do not liquefy and possess both gaseous and liquid properties. Supercritical CO2 not only has excellent solubility and mass transfer characteristics but is also an environmentally friendly, non-toxic solvent. It is widely used in extraction and separation, chemical reactions, replacing traditional process aids or solvents (in painting, printing and dyeing, cleaning, and foaming), and analytical testing (chromatography). In-depth research into its fundamental theories and continuous development of applied technologies will bring profound changes to traditional industries that rely on organic solvents, and promote the rational utilization of mineral resources such as petroleum and coal, as well as the effective development of renewable resources such as grains, oils, and traditional Chinese medicine. Its prospects are very promising. In the preparation of salicylic acid, using supercritical CO2 to replace traditional CO2 gas also plays an important role in improving reaction yield.

[0007] In recent years, research on the medium-pressure process for preparing salicylic acid from phenol has become a hot topic. The key to this process route lies in the development of suitable and efficient catalysts. The traditional Kolbe-Schmitt process suffers from problems such as low yield and environmental pollution, and the activity and stability of the catalyst cannot be guaranteed, which greatly restricts the development of the salicylic acid preparation industry.

[0008] Chinese patent CN105481685A discloses a method for one-step synthesis of salicylic acid from phenol and supercritical carbon dioxide. This method uses supported potassium carbonate as a catalyst to synthesize salicylic acid from phenol and supercritical carbon dioxide. The supported potassium carbonate catalyst is composed of the following raw materials in weight percentages: potassium carbonate 10%–25%, support 70%–89%, and binder 1%–5%. The support for the supported potassium carbonate catalyst is one or more of alumina, silica, activated carbon, or molecular sieves. In this patent, the active component of the catalyst is potassium carbonate. Small molecule water is generated during the preparation of salicylic acid from phenol, and potassium carbonate will dissolve to some extent during the catalytic process. This inevitably affects the catalytic effect and reduces the catalyst's cycle life. Summary of the Invention

[0009] The purpose of this invention is to provide a method for preparing a dual-alkali magnesium aluminum spinel catalyst, which is simple to prepare and produces a catalyst with low cost and high catalytic activity. This invention also provides applications of the dual-alkali magnesium aluminum spinel catalyst.

[0010] The preparation method of the dual-alkali metal magnesium aluminum spinel catalyst of the present invention includes the following steps:

[0011] (1) After mixing boehmite and magnesium nitrate, add water and mix well to obtain a first suspension; add dispersant and precipitant to the first suspension and stir, let stand and age to obtain a solid-liquid mixture; crystallize the solid-liquid mixture, centrifuge, wash the obtained solid, dry and calcine to obtain spinel support MgAl2O4;

[0012] (2) The spinel support MgAl2O4, the modifier and water are reacted to obtain a second suspension. The second suspension is subjected to hydrothermal crystallization, centrifuged, and the resulting solid is washed, dried and calcined to obtain the treated spinel support MgAl2O4.

[0013] (3) Lithium nitrate, calcium nitrate, preprecipitant and water are mixed and sonicated, heated and concentrated to evaporate to obtain impregnation solution. Spinel support MgAl2O4 after impregnation treatment with equal volume of impregnation solution is subjected to hydrothermal reaction after impregnation, centrifuged, the obtained solid is washed, dried and calcined to obtain Li2O-CaO / MgAl2O4 dual alkali metal magnesium aluminum spinel catalyst.

[0014] In step (1), the dispersant is polyvinylpyrrolidone, the precipitant is ammonia, the molar ratio of boehmite to magnesium nitrate is 1:0.84-1.02, the mass ratio of boehmite, water and dispersant is 1:10-25:0.08-0.2, the mass ratio of magnesium nitrate to precipitant is 1:2.6-9.8, and the mass concentration of precipitant is 25-26%.

[0015] In step (1), the stirring temperature is 60-80℃ and the stirring time is 10-15 min; the standing aging time is 35-90 min; the crystallization temperature is 100-120℃ and the crystallization time is 24-48 h; the drying temperature is 100-120℃ and the drying time is 6-24 h; the calcination temperature is 500-600℃ and the calcination time is 4-6 h.

[0016] In step (2), the modifier is a mixture of triisopropanolamine and piperidine, with a mass ratio of triisopropanolamine to piperidine of 1:0.5-1.2. The mass ratio of spinel support MgAl2O4, modifier and water is 1:0.19-0.6:16-40. The reaction temperature is 60-75℃ and the reaction time is 10-20h. The hydrothermal crystallization temperature is 150-200℃ and the hydrothermal crystallization time is 24-96h. The drying temperature is 80-120℃ and the drying time is 12-48h. The calcination temperature is 500-600℃ and the calcination time is 4-6h.

[0017] In step (3), the preprecipitant is glycine, and the mass ratio of lithium nitrate, calcium nitrate, preprecipitant and treated spinel carrier MgAl2O4 is 0.6-2.5:0.6-2.1:3.4-21.8:5.

[0018] In step (3), the ultrasonic time is 10-30 min, the concentration and evaporation temperature is 80-90℃, the hydrothermal reaction temperature is 150-180℃, the hydrothermal reaction time is 24-48 h, the drying temperature is 80-100℃, the drying time is 12-24 h, the calcination temperature is 400-600℃, and the calcination time is 4-6 h.

[0019] The application of the dual-alkali magnesium-aluminum spinel catalyst prepared by the preparation method of the present invention is as follows: in a solvent, supercritical CO2, sodium phenolate and the dual-alkali magnesium-aluminum spinel catalyst undergo a carboxylation reaction to obtain a solid-liquid mixture. Water is added to the solid-liquid mixture and stirred. After standing and separating into layers, the lower aqueous phase is taken out and acidified to obtain a solution containing salicylic acid.

[0020] The solvent is toluene or xylene, the mass ratio of the dual-alkali metal magnesium aluminum spinel catalyst to sodium phenolate is 1:1-10, and the mass ratio of sodium phenolate to solvent is 1:5-20.

[0021] The carboxylation reaction temperature is 150-230℃, the carboxylation reaction time is 1-5h, and the carboxylation reaction pressure is 5-10MPa.

[0022] The volume ratio of the solid-liquid mixture to water is 1:0.5-1; acidification is carried out using dilute sulfuric acid with a mass concentration of 5-12% for 2-5 hours, and the acidification pH is 1-2.

[0023] The beneficial effects of this invention are as follows:

[0024] 1. Using PVP (polyvinylpyrrolidone) as a dispersant can increase the dispersibility of magnesium and aluminum components in the system and make the components more uniformly mixed; the crystallization treatment in step (1) can improve the crystallization degree of the spinel carrier and increase more active sites.

[0025] 2. The calcination in step (1) can decompose PVP, thus avoiding the negative impact of PVP on the carrier. At the same time, high-temperature calcination can improve the physical properties of the spinel carrier and increase its mechanical properties.

[0026] 3. A mixture of triisopropanolamine and piperidine is used as a modifier to treat the spinel support MgAl2O4. Triisopropanolamine, as a mild base, and piperidine, as a strong base, work synergistically to enhance the treatment effect, especially in reactions requiring strong alkaline conditions. Triisopropanolamine contains three -OH groups, which can act as hydrogen bond donors, while the nitrogen atom of piperidine can act as a hydrogen bond acceptor. The -OH groups of triisopropanolamine can form hydrogen bonds with the nitrogen atom of piperidine, enhancing the interaction between the two molecules. There is also a π-hydrogen interaction between triisopropanolamine and piperidine; the hydrogen atoms in the -CH or -OH groups of triisopropanolamine interact with the π-electron cloud of the piperidine ring. Van der Waals forces also exist between the triisopropanolamine and piperidine molecules, resulting from the instantaneous dipole interactions between atoms in the molecules. Van der Waals forces play an important role in intermolecular interactions, especially when the intermolecular distance is close. The use of triisopropanolamine and piperidine together can improve the pore-expansion effect of the catalyst, making the catalyst have more defects and improving the catalytic performance; at the same time, the modification of the triisopropanolamine and piperidine dual organic base can increase the base active sites on the support surface.

[0027] 4. Glycine is used as a pre-precipitant during the impregnation process. This precipitates the added lithium nitrate and calcium nitrate as hydroxides. Compared to the traditional pre-precipitant urea, glycine decomposes relatively slowly, thus slowing down the metal precipitation rate. This results in finer and denser calcium hydroxide and lithium hydroxide crystals, leading to more uniform dispersion of the treated oxide particles and a more uniform active site on the catalyst. Calcium nitrate has a low decomposition temperature. When undergoing a hydrothermal reaction at 150-180℃, the low decomposition temperature of calcium nitrate allows for the instantaneous release of energy, rapidly increasing the temperature around the molecules. Glycine dispersed in the system also begins to decompose during this rapid temperature increase. The synergistic heating effect allows for complete decomposition of calcium nitrate in a short time. Furthermore, the NH3 and CO2 released by glycine simultaneously contribute to the abundance of mesopores in the catalyst.

[0028] 5. This invention does not employ direct equal-volume impregnation. Instead, it uses a method of first heating and concentrating the impregnation solution to obtain an impregnation liquid, followed by equal-volume impregnation. This heating and concentration method allows for a more uniform mixing of glycine and the two nitrates. As the temperature increases, the solubility of most solid solutes in water increases. This means that during the heating process, the solubility of glycine and nitrates also increases, allowing them to dissolve more completely in the water. Once these substances are completely dissolved, their molecules or ions in the solution can move and mix more freely. Increased temperature leads to intensified thermal motion of molecules in the solution. The faster the molecules move, the more frequent their collisions become, which helps to break up any potential aggregation or precipitation, allowing the molecules or ions of glycine and nitrates to be more evenly distributed throughout the solution. Heating and concentration help increase the solubility of the solute, intensify molecular motion, and reduce the amount of solvent through the concentration effect. These factors work together to ensure a more uniform mixing of glycine and the two nitrates. The uniform dispersion of glycine and the two nitrates ensures that each part of the impregnated support can fully contact the impregnation solution, improving the overall impregnation efficiency, avoiding local over- or under-impregnation, and exposing more active sites in subsequent reactions, thereby improving catalytic efficiency.

[0029] 6. Lithium has an electronegativity of 0.98, while calcium has an electronegativity of 1.00. This slight difference in electronegativity can lead to a slight transfer of electrons from lithium to calcium. This electron transfer between lithium and calcium can modulate the electron density of catalytically active sites. This electronic effect can alter the binding strength of adsorbed species, thereby increasing catalytic performance. Lithium oxide and calcium oxide, as alkali metal oxides, possess abundant basic active sites, enabling them to adsorb reactants and activate sodium phenolate molecules, making them more readily reactable. Furthermore, lithium oxide and calcium oxide can guide the reaction towards the formation of sodium salicylate, reducing the formation of byproducts. Detailed Implementation

[0030] The present invention will be further described below with reference to embodiments.

[0031] Example 1

[0032] The preparation method of the dual-alkali metal magnesium aluminum spinel catalyst includes the following steps:

[0033] (1) 5g of boehmite and 7.8965g of magnesium nitrate were mixed in a ball mill for 30min, and then 50g of water was added and mixed to obtain the first suspension. 0.4g of polyvinylpyrrolidone was added to the first suspension, and 29.304g of ammonia water with a mass concentration of 26% was continuously added to maintain pH=7-10. The mixture was stirred at 60℃ for 12min and allowed to stand for 35min to age to obtain a solid-liquid mixture. The solid-liquid mixture was placed in a hydrothermal reactor and crystallized at 100℃ for 24h. After centrifugation and washing the obtained solid to neutrality, it was dried at 105℃ for 12h and calcined in a muffle furnace at 500℃ for 4h to obtain spinel support MgAl2O4.

[0034] (2) 5g spinel support MgAl2O4, 0.57g triisopropanolamine, 0.428g piperidine and 80g water were added to a flask and reacted at 60°C for 10h to obtain a second suspension. The second suspension was transferred to a hydrothermal reactor and hydrothermally crystallized at 150°C for 24h. The solid was centrifuged and washed. It was dried at 100°C for 12h and calcined at 500°C in a muffle furnace for 4h to obtain the treated spinel support MgAl2O4.

[0035] (3) The water absorption rate of the treated spinel carrier MgAl2O4 was measured to be 80.65%. The amount of water used was calculated based on the water absorption rate. The amount of water used = the mass of the treated spinel carrier MgAl2O4 to be impregnated × the water absorption rate × 2. The amount of water used is 5 × 80.65% × 2 = 8.065g. Mix 8.065g of water, 0.745g of lithium nitrate, 0.615g of calcium nitrate and 6.56g of glycine and sonicate for 10min. Heat up The water was concentrated and evaporated at 80℃ until the mass of the mixture was 11.95g. Evaporation was then stopped to obtain an impregnation solution. 5g of the treated spinel support MgAl2O4 was impregnated with the impregnation solution for 24h. After impregnation, the mixture was hydrothermally reacted at 150℃ for 24h in a hydrothermal reactor. The solid was centrifuged, washed, dried at 80℃ for 12h, and calcined at 500℃ for 4h to obtain the Li2O-CaO / MgAl2O4 dual-alkali metal magnesium aluminum spinel catalyst.

[0036] The application of dual-alkali metal magnesium-aluminum spinel catalysts includes the following steps:

[0037] Weigh 1g of sodium phenolate and 20g of toluene and put them into a reactor. Then add 0.5g of Li2O-CaO / MgAl2O4 dual-alkali metal magnesium aluminum spinel catalyst. Replace the air in the reactor with CO2 8 times. After liquefying the carbon dioxide using a condenser, add the liquefied CO2 into the reactor using a plunger pump until the initial pressure is 8MPa. Turn on the stirring device at 800r / min and heat to 180℃ for 3h. After natural cooling, vent the reactor to remove excess CO2 and obtain a solid-liquid mixture. Add deionized water to the solid-liquid mixture and stir to obtain a mixture. The volume ratio of solid-liquid mixture to deionized water is 1:0.8. Wash the reactor with 15ml of deionized water and add the washing liquid to the above mixture. Let it stand to separate into layers. Take the lower aqueous phase and add 10% dilute sulfuric acid to acidify for 3h. Adjust the pH to 2 to obtain a solution containing salicylic acid.

[0038] The solution containing salicylic acid was detected by liquid chromatography. The results showed that the conversion rate of sodium phenolate was 91.29% and the selectivity of salicylic acid was 93.01%.

[0039] Example 2

[0040] The preparation method of the dual-alkali metal magnesium aluminum spinel catalyst includes the following steps:

[0041] (1) 5g of boehmite and 7.7545g of magnesium nitrate were mixed in a ball mill for 30min, and then 125g of water was added and mixed to obtain the first suspension. 1g of polyvinylpyrrolidone was added to the first suspension, and 75.4g of ammonia water with a mass concentration of 26% was continuously added to maintain pH=7-10. The mixture was stirred at 80℃ for 15min and allowed to stand for 90min to age to obtain a solid-liquid mixture. The solid-liquid mixture was placed in a hydrothermal reactor and crystallized at 120℃ for 48h. After centrifugation and washing the obtained solid to neutrality, it was dried at 120℃ for 6h and calcined in a muffle furnace at 600℃ for 6h to obtain spinel support MgAl2O4.

[0042] (2) 5g spinel support MgAl2O4, 1.4g triisopropanolamine, 1.6g piperidine and 200g water were added to a flask and reacted at 75°C for 20h to obtain a second suspension. The second suspension was transferred to a hydrothermal reactor and hydrothermally crystallized at 180°C for 96h. The solid was centrifuged and washed. It was dried at 120°C for 48h and calcined at 600°C in a muffle furnace for 6h to obtain the treated spinel support MgAl2O4.

[0043] (3) The water absorption rate of the treated spinel carrier MgAl2O4 was measured to be 76.29%. Based on the water absorption rate, the amount of water needed was calculated as follows: water amount = mass of the treated spinel carrier MgAl2O4 to be impregnated × water absorption rate × 2. The amount of water needed was 5 × 76.29% × 2 = 7.629 g. 7.629 g of water, 2.485 g of lithium nitrate, 2.05 g of calcium nitrate and 21.8 g of glycine were mixed and sonicated for 30 min. The temperature was then raised to [temperature missing]. The water was concentrated and evaporated at 90℃ until the mass of the mixture was 30.15g. Evaporation was then stopped to obtain an impregnation solution. 5g of the treated spinel support MgAl2O4 was impregnated with the impregnation solution for 24h. After impregnation, the mixture was hydrothermally reacted at 180℃ for 48h in a hydrothermal reactor. The solid was centrifuged, washed, dried at 100℃ for 24h, and calcined at 600℃ for 5h to obtain the Li2O-CaO / MgAl2O4 dual alkali metal magnesium aluminum spinel catalyst.

[0044] The application of dual-alkali metal magnesium-aluminum spinel catalysts includes the following steps:

[0045] Weigh 0.5g sodium phenolate and 5g toluene and add them to the reactor. Then add 0.5g Li2O-CaO / MgAl2O4 dual-alkali metal magnesium aluminum spinel catalyst. Replace the air in the reactor with CO2 8 times. After liquefying the carbon dioxide using a condenser, add the liquefied CO2 to the reactor using a plunger pump until the initial pressure is 5MPa. Turn on the stirring device at 800r / min and heat to 150℃ for 5h. After natural cooling, vent the reactor to remove excess CO2 and obtain a solid-liquid mixture. Add deionized water to the solid-liquid mixture and stir to obtain a mixture. The volume ratio of solid-liquid mixture to deionized water is 1:0.5. Wash the reactor with 15ml of deionized water and add the washing liquid to the above mixture. Let it stand to separate into layers. Take the lower aqueous phase and add 5% dilute sulfuric acid to acidify for 5h. Adjust the pH to 1.5 to obtain a solution containing salicylic acid.

[0046] The solution containing salicylic acid was detected by liquid chromatography. The results showed that the conversion rate of sodium phenolate was 94.27% and the selectivity of salicylic acid was 92.73%.

[0047] Example 3

[0048] The preparation method of the dual-alkali metal magnesium aluminum spinel catalyst includes the following steps:

[0049] (1) 5g of boehmite and 9.29g of magnesium nitrate were mixed in a ball mill for 30min, and then 100g of water was added and mixed to obtain the first suspension. 0.6g of polyvinylpyrrolidone was added to the first suspension, and 24.42g of ammonia water with a mass concentration of 26% was continuously added dropwise to maintain pH=7-10. The mixture was stirred at 65℃ for 10min and allowed to stand for 60min to age to obtain a solid-liquid mixture. The solid-liquid mixture was placed in a hydrothermal reactor and crystallized at 110℃ for 35h. After filtration, the obtained solid was washed until neutral and dried at 100℃ for 24h. It was then calcined in a muffle furnace at 550℃ for 5h to obtain spinel support MgAl2O4.

[0050] (2) 5g spinel support MgAl2O4, 1g triisopropanolamine, 0.5g piperidine and 150g water were added to a flask and reacted at 70°C for 15h to obtain a second suspension. The second suspension was transferred to a hydrothermal reactor and hydrothermally crystallized at 200°C for 48h. After centrifugation, the obtained solid was washed, dried at 80°C for 40h, and calcined in a muffle furnace at 550°C for 5h to obtain the treated spinel support MgAl2O4.

[0051] (3) The water absorption rate of the treated spinel carrier MgAl2O4 was measured to be 78.35%. Based on the water absorption rate, the amount of water needed was calculated as follows: water amount = mass of the treated spinel carrier MgAl2O4 to be impregnated × water absorption rate × 2. The amount of water needed was 5 × 78.35% × 2 = 7.835 g. 7.835 g of water, 0.621 g of lithium nitrate, 1.476 g of calcium nitrate, and 3.434 g of glycine were mixed and sonicated for 20 min. The water was concentrated and evaporated at 85℃ until the mass of the mixture was 9.45g. Evaporation was then stopped to obtain an impregnation solution. 5g of the treated spinel support MgAl2O4 was impregnated with the impregnation solution for 24h. After impregnation, the mixture was hydrothermally reacted at 170℃ for 30h in a hydrothermal reactor. The solid was centrifuged, washed, dried at 90℃ for 20h, and calcined at 400℃ for 6h to obtain the Li2O-CaO / MgAl2O4 dual-alkali metal magnesium aluminum spinel catalyst.

[0052] The application of dual-alkali metal magnesium-aluminum spinel catalysts includes the following steps:

[0053] Weigh 5g of sodium phenolate and 25g of toluene and put them into a reactor. Then add 0.5g of Li2O-CaO / MgAl2O4 dual-alkali metal magnesium aluminum spinel catalyst. Replace the air in the reactor with CO2 8 times. After liquefying the carbon dioxide using a condenser, add the liquefied CO2 into the reactor using a plunger pump until the initial pressure is 10MPa. Turn on the stirring device at 800r / min and heat to 230℃ for 1h. After natural cooling, vent the reactor to remove excess CO2 and obtain a solid-liquid mixture. Add deionized water to the solid-liquid mixture and stir to obtain a mixture with a volume ratio of solid-liquid mixture to deionized water of 1:1. Add the washing liquid obtained after washing the reactor with 15ml of deionized water to the above mixture. Let it stand to separate into layers. Take the lower aqueous phase and add 12% dilute sulfuric acid to acidify for 2h. Adjust the pH to 1 to obtain a solution containing salicylic acid.

[0054] The solution containing salicylic acid was detected by liquid chromatography. The results showed that the conversion rate of sodium phenolate was 94.93% and the selectivity of salicylic acid was 93.55%.

[0055] Comparative Example 1

[0056] Without adding polyvinylpyrrolidone, the other steps are the same as in Example 1.

[0057] The solution containing salicylic acid was detected by liquid chromatography. The results showed that the conversion rate of sodium phenolate was 78.493% and the selectivity of salicylic acid was 85.148%.

[0058] As can be seen from the comparison between Example 1 and Comparative Example 1, the use of polyvinylpyrrolidone in the preparation process of the present invention can increase the dispersibility of each phase, make the crystal precipitation more uniform, and thus make the catalyst more effective.

[0059] Comparative Example 2

[0060] Replace piperidine with an equal mass of triisopropanolamine, i.e., use only a single triisopropanolamine modifier, and follow the same steps as in Example 1.

[0061] The solution containing salicylic acid was detected by liquid chromatography. The results showed that the conversion rate of sodium phenolate was 79.264% and the selectivity of salicylic acid was 82.375%.

[0062] Comparative Example 3

[0063] Replace triisopropanolamine with an equal mass of piperidine, i.e., use only a single piperidine modifier, and follow the same steps as in Example 1.

[0064] The solution containing salicylic acid was detected by liquid chromatography. The results showed that the conversion rate of sodium phenolate was 86.409% and the selectivity of salicylic acid was 84.519%.

[0065] Comparative Example 4

[0066] Replace glycine with an equal molar amount of urea, and follow the same steps as in Example 1.

[0067] The solution containing salicylic acid was detected by liquid chromatography. The results showed that the conversion rate of sodium phenolate was 87.337% and the selectivity of salicylic acid was 88.029%.

[0068] Comparative Example 5

[0069] Instead of using heated concentration and evaporation, the same volume is directly impregnated, and the other steps are the same as in Example 1.

[0070] The solution containing salicylic acid was detected by liquid chromatography. The results showed that the conversion rate of sodium phenolate was 88.707% and the selectivity of salicylic acid was 90.061%.

[0071] Comparative Example 6

[0072] Replace lithium nitrate with an equal molar amount of calcium nitrate, i.e. do not use lithium nitrate, and follow the same steps as in Example 1.

[0073] The solution containing salicylic acid was detected by liquid chromatography. The results showed that the conversion rate of sodium phenolate was 85.792% and the selectivity of salicylic acid was 86.691%.

[0074] Comparative Example 7

[0075] Replace calcium nitrate with an equal molar amount of lithium nitrate, i.e. do not use calcium nitrate, and follow the same steps as in Example 1.

[0076] The solution containing salicylic acid was detected by liquid chromatography. The results showed that the conversion rate of sodium phenolate was 80.101% and the selectivity of salicylic acid was 88.903%.

Claims

1. A method for preparing a dual-alkali metal magnesium aluminum spinel catalyst, characterized in that... Includes the following steps: (1) After mixing boehmite and magnesium nitrate, add water and mix well to obtain a first suspension; add dispersant and precipitant to the first suspension and stir, let stand and age to obtain a solid-liquid mixture; crystallize the solid-liquid mixture, centrifuge, wash the obtained solid, dry and calcine to obtain spinel support MgAl2O4; (2) The spinel support MgAl2O4, the modifier and water are reacted to obtain a second suspension. The second suspension is subjected to hydrothermal crystallization, centrifuged, and the resulting solid is washed, dried and calcined to obtain the treated spinel support MgAl2O4. (3) Lithium nitrate, calcium nitrate, preprecipitant and water are mixed and sonicated, heated and concentrated to evaporate to obtain impregnation liquid. Spinel support MgAl2O4 after impregnation with the same volume of impregnation liquid is subjected to hydrothermal reaction after impregnation with the same volume. After centrifugation, the obtained solid is washed, dried and calcined to obtain Li2O-CaO / MgAl2O4 dual alkali metal magnesium aluminum spinel catalyst. The dispersant in step (1) is polyvinylpyrrolidone; In step (2), the modifier is a mixture of triisopropanolamine and piperidine; In step (3), the pre-precipitant is glycine, and the concentration and evaporation temperature is 80-90℃.

2. The method for preparing the dual-alkali metal magnesium aluminum spinel catalyst according to claim 1, characterized in that... In step (1), the precipitant is ammonia water, the molar ratio of boehmite to magnesium nitrate is 1:0.84-1.02, the mass ratio of boehmite, water and dispersant is 1:10-25:0.08-0.2, and the mass ratio of magnesium nitrate to precipitant is 1:2.6-9.

8.

3. The method for preparing the dual-alkali metal magnesium aluminum spinel catalyst according to claim 1, characterized in that... In step (1), the stirring temperature is 60-80℃ and the stirring time is 10-15 min; the standing aging time is 35-90 min; the crystallization temperature is 100-120℃ and the crystallization time is 24-48 h; the drying temperature is 100-120℃ and the drying time is 6-24 h; the calcination temperature is 500-600℃ and the calcination time is 4-6 h.

4. The method for preparing the dual-alkali metal magnesium aluminum spinel catalyst according to claim 1, characterized in that... In step (2), the mass ratio of triisopropanolamine to piperidine is 1:0.5-1.2, the mass ratio of spinel support MgAl2O4, modifier and water is 1:0.19-0.6:16-40; the reaction temperature is 60-75℃, the reaction time is 10-20h; the hydrothermal crystallization temperature is 150-200℃, the hydrothermal crystallization time is 24-96h; the drying temperature is 80-120℃, the drying time is 12-48h; the calcination temperature is 500-600℃, the calcination time is 4-6h.

5. The method for preparing the dual-alkali metal magnesium aluminum spinel catalyst according to claim 1, characterized in that... In step (3), the mass ratio of lithium nitrate, calcium nitrate, preprecipitant and treated spinel carrier MgAl2O4 is 0.6-2.5:0.6-2.1:3.4-21.8:

5.

6. The method for preparing the dual-alkali metal magnesium aluminum spinel catalyst according to claim 1, characterized in that... In step (3), the ultrasonic time is 10-30 min; the hydrothermal reaction temperature is 150-180℃ and the hydrothermal reaction time is 24-48 h; the drying temperature is 80-100℃ and the drying time is 12-24 h; the calcination temperature is 400-600℃ and the calcination time is 4-6 h.

7. An application of a dual-alkali metal magnesium aluminum spinel catalyst prepared using the preparation method of any one of claims 1-6, characterized in that... In a solvent, supercritical CO2, sodium phenolate, and a dual-alkali metal magnesium aluminum spinel catalyst undergo a carboxylation reaction to obtain a solid-liquid mixture. Water is added to the solid-liquid mixture and stirred. After standing and separating into layers, the lower aqueous layer is taken out and acidified to obtain a solution containing salicylic acid.

8. The application according to claim 7, characterized in that... The solvent is toluene or xylene, the mass ratio of the dual-alkali metal magnesium aluminum spinel catalyst to sodium phenolate is 1:1-10, and the mass ratio of sodium phenolate to solvent is 1:5-20.

9. The application according to claim 7, characterized in that... The carboxylation reaction temperature is 150-230℃, the carboxylation reaction time is 1-5h, and the carboxylation reaction pressure is 5-10MPa.

10. The application according to claim 7, characterized in that... The volume ratio of the solid-liquid mixture to water is 1:0.5-1; acidification is carried out using dilute sulfuric acid with a mass concentration of 5-12% for 2-5 hours, and the acidification pH is 1-2.