Used quaternary ammonium phosphotungstate catalyst as well as preparation method and application thereof

Abstract

The invention discloses an absent quaternary ammonium phosphotungstate catalyst as well as a preparation method and application thereof, and belongs to the field of catalytic synthesis of azelaic acid. The preparation method comprises the following steps: reacting phosphotungstic acid hydrate in an aqueous solution with the pH value of 4.0-5.0 adjusted by sodium bicarbonate at 40-60 DEG C for 3-6 hours to prepare vacancy sodium phosphotungstate, and carrying out ion exchange reaction on the vacancy sodium phosphotungstate and quaternary ammonium salt at 60-80 DEG C to obtain the vacancy quaternary ammonium phosphotungstate catalyst. The catalyst is used for catalyzing oxidation reaction of oleic acid and hydrogen peroxide, reaction is performed for 6-12 hours at 70-110 DEG C, and azelaic acid is efficiently prepared. The catalyst provided by the invention has vacancy active sites and quaternary ammonium salt modification, shows high catalytic activity and high selectivity for oleic acid double bond oxidation, has good stability, and is easy to recover and reuse through solid-liquid separation. The whole reaction system is mild in condition, does not need a strong corrosive oxidant or high-pressure equipment, and is safe and environment-friendly in process.

Description

Technical Field

[0001] This invention belongs to the field of catalyst preparation and organic synthesis technology, specifically relating to a vacant phosphotungstic acid quaternary ammonium salt catalyst and its preparation method, as well as the application of the catalyst in the catalytic oxidation of oleic acid to azelaic acid. Technical Background

[0002] Azelaic acid, a medium-chain organic dicarboxylic acid, is a key intermediate in the synthesis of high-performance nylon, advanced lubricants, plasticizers, and other fine chemicals, possessing significant industrial application value. Currently, azelaic acid is mainly prepared industrially through the oxidative cracking of oleic acid. Existing oxidation methods primarily include ozone oxidation, potassium permanganate oxidation, and hydrogen peroxide oxidation. Ozone oxidation suffers from high ozone toxicity, high energy consumption, and special requirements for reaction equipment. Potassium permanganate oxidation faces drawbacks such as poor selectivity and the generation of large amounts of manganese-containing waste, causing environmental pollution. In contrast, hydrogen peroxide, due to its strong oxidizing power, the fact that its only reaction byproduct is water, and its high safety, has become an environmentally friendly and ideal oxidant. In oleic acid oxidation systems using hydrogen peroxide as the oxidant, phosphotungstic acid is often used as a catalyst. However, phosphotungstic acid exhibits phase separation from the organic raw material oleic acid in homogeneous reaction systems, leading to low mass transfer efficiency and limited reaction rates. To address this issue, researchers have introduced phase transfer catalysts, forming a phosphotungstic acid quaternary ammonium salt catalytic system. This system improves mass transfer to some extent and facilitates catalyst separation and recovery. However, this existing technology still has significant shortcomings: firstly, under intense oxidation reaction conditions, the phosphotungstic acid anionic framework in the phosphotungstic acid quaternary ammonium salt may decompose, leading to the loss of active components and poor catalyst stability; secondly, some decomposition products of the catalyst may be mixed in with the product, increasing the difficulty of subsequent azelaic acid separation and purification. Therefore, developing a novel catalyst with high catalytic activity, high structural stability, and easy separation and recovery in the hydrogen peroxide oxidation system is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0003] Technical problem solved: Based on the shortcomings of existing phosphotungstic acid quaternary ammonium salt catalysts in the hydrogen peroxide oxidation system pointed out in the background art, this invention provides a vacant phosphotungstic acid quaternary ammonium salt catalyst, its preparation method and application, to overcome the problems of insufficient structural stability and easy loss of active components in the oxidation of oleic acid to azelaic acid by existing catalysts. At the same time, it realizes that the catalyst can be easily separated and reused while maintaining high catalytic activity and mass transfer efficiency. Finally, by improving the catalytic system, the process is simplified and the use of highly corrosive or hazardous reagents is avoided, thereby improving the safety and environmental friendliness of the entire production process.

[0004] Technical solution: A method for preparing a vacant phosphotungstic acid quaternary ammonium salt catalyst includes the following steps: (1) dissolving phosphotungstic acid hydrate in water, adding sodium bicarbonate to adjust the pH of the system to 4.0-5.0, and reacting at 40℃-60℃ for 3-6 hours to obtain a vacant phosphotungstic acid sodium salt aqueous solution; (2) dissolving quaternary ammonium salt in water to obtain a quaternary ammonium salt aqueous solution, adding the quaternary ammonium salt aqueous solution dropwise to the vacant phosphotungstic acid sodium salt aqueous solution obtained in step (1), and reacting at 60℃-80℃ for 0.5-2 hours; (3) after the reaction is completed, allowing the mixture to stand, filtering and separating to obtain a solid, and drying to obtain the vacant phosphotungstic acid quaternary ammonium salt catalyst.

[0005] In step (2), the quaternary ammonium salt is trioctylmethylammonium chloride, trioctylmethylammonium bromide or tetraoctylammonium chloride.

[0006] In step (2), the molar ratio of the vacant sodium phosphotungstate salt to the quaternary ammonium salt is (0.5-1.5):1.

[0007] The vacant phosphotungstic acid quaternary ammonium salt catalyst prepared by the above method.

[0008] The above-mentioned missing phosphotungstic acid quaternary ammonium salt catalyst is used in the catalytic oxidation of oleic acid to prepare azelaic acid.

[0009] A method for preparing azelaic acid by oxidizing oleic acid includes the following steps: mixing oleic acid, a catalyst and hydrogen peroxide, and carrying out an oxidation reaction under heating conditions to obtain azelaic acid; wherein the catalyst is the above-mentioned vacant phosphotungstic acid quaternary ammonium salt catalyst.

[0010] The mass ratio of oleic acid, hydrogen peroxide and the catalyst is 1:(1-8):(0.03-0.20); wherein the hydrogen peroxide is calculated as a pure substance.

[0011] The temperature for the above oxidation reaction is 70℃~110℃.

[0012] The oxidation reaction takes 6 to 12 hours.

[0013] After the reaction is complete, the catalyst is separated by centrifugation. The separated catalyst can be reused after drying.

[0014] Beneficial Effects: The vacant phosphotungstic acid quaternary ammonium salt catalyst prepared in this invention utilizes the coordinated unsaturated sites in the vacant phosphotungstic acid as strong Lewis acid sites, effectively activating the double bonds of the oxidant hydrogen peroxide and oleic acid, promoting efficient oxidation reactions. Simultaneously, the selective oxidation of the oleic acid double bonds suppresses side reactions, thereby significantly improving the yield and purity of azelaic acid. Example data shows that using this catalyst, the oleic acid conversion rate can reach 96.5%, and the azelaic acid selectivity can reach 94.8%. Furthermore, the vacant phosphotungstic acid framework of this catalyst remains stable under oxidation conditions and is not easily decomposed, reducing the loss of active components. Moreover, thanks to the modification with quaternary ammonium salt, the catalyst can be uniformly dispersed in the organic reaction system, improving the mass transfer efficiency with oleic acid and accelerating the reaction rate. The catalyst's stability and easy separation characteristics have been experimentally confirmed; after three repeated uses, the catalytic activity did not show a significant decrease. Finally, the catalytic reaction system provided by this invention has mild conditions and can achieve efficient conversion under normal pressure. It avoids the use of highly corrosive or hazardous reagents in traditional nitric acid oxidation or ozonation methods, and does not require high-pressure equipment. This not only reduces the cost of separation and purification, but also shows significant advantages in terms of production safety and environmental friendliness. Attached Figure Description

[0015] Figure 1 Infrared spectra of vacant phosphotungstic acid and the vacant phosphotungstic acid quaternary ammonium salt prepared in Example 1.

[0016] Figure 2 XRD patterns of phosphotungstic acid, vacant phosphotungstic acid, and the vacant phosphotungstic acid quaternary ammonium salt prepared in Example 1.

[0017] Figure 3 The ultraviolet absorption spectra of phosphotungstic acid, vacant phosphotungstic acid, and the vacant phosphotungstic acid quaternary ammonium salt prepared in Example 1 are shown.

[0018] Figure 4 This is a SEM image of the vacant phosphotungstic acid quaternary ammonium salt prepared in Example 1.

[0019] Figure 5 The image shows the XRD pattern of the vacant phosphotungstic acid quaternary ammonium salt prepared in Example 1 after three uses.

[0020] Figure 6 This is the mass spectrum of the product, azelaic acid.

[0021] Figure 7 The image shows the 1H NMR spectrum of the product, azelaic acid. Detailed Implementation

[0022] The following embodiments further illustrate the content of the present invention, but should not be construed as limiting the present invention. Modifications and substitutions made to the methods, steps, or conditions of the present invention without departing from the spirit and essence of the invention are all within the scope of the present invention. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0023] Example 1

[0024] 9 mmol of phosphotungstic acid hydrate was weighed, dissolved in 90 mL of water, and transferred to a 250 mL round-bottom flask. NaHCO3 was added to adjust the pH to 5, and the mixture was heated in a water bath to 60 °C for 3 h to form an aqueous solution of vacant sodium phosphotungstic acid, which was used for further synthesis of vacant quaternary ammonium phosphotungstic acid. The aqueous solution of vacant sodium phosphotungstic acid was removed by rotary evaporation and dried in a vacuum drying oven at 80 °C to obtain vacant sodium phosphotungstic acid for structural characterization.

[0025] Example 2

[0026] Aqueous solutions of vacant sodium phosphotungstenate were synthesized according to the method in Example 1. 9 mmol of trioctylmethylammonium chloride quaternary ammonium salt was dissolved in 10 mL of water to obtain an aqueous solution of the quaternary ammonium salt. This aqueous solution was then added dropwise to the aqueous solution of vacant sodium phosphotungstenate. The reaction was carried out at 80°C for 1 h. After the reaction was completed, the mixture was allowed to stand, filtered, and the solid reactant was obtained. After drying, the vacant sodium phosphotungstenate quaternary ammonium salt catalyst was obtained.

[0027] Example 3

[0028] Aqueous solutions of vacant sodium phosphotungstenate were synthesized according to the method in Example 1. 9 mmol of trioctylmethylammonium bromide quaternary ammonium salt was dissolved in 10 mL of water to obtain an aqueous solution of the quaternary ammonium salt. This aqueous solution was then added dropwise to the aqueous solution of vacant sodium phosphotungstenate. The reaction was carried out at 80°C for 1 h. After the reaction was completed, the mixture was allowed to stand, filtered, and the solid reactant was obtained. After drying, the vacant sodium phosphotungstenate quaternary ammonium salt catalyst was obtained.

[0029] Example 4

[0030] Aqueous solutions of vacant sodium phosphotungstenate were synthesized according to the method in Example 1. 9 mmol of tetraoctylammonium chloride quaternary ammonium salt was dissolved in 10 mL of water to obtain an aqueous solution of the quaternary ammonium salt. This aqueous solution was then added dropwise to the aqueous solution of vacant sodium phosphotungstenate. The reaction was carried out at 80°C for 1 h. After the reaction was completed, the mixture was allowed to stand, filtered, and the solid reactant was obtained. After drying, the vacant sodium phosphotungstenate quaternary ammonium salt catalyst was obtained.

[0031] Example 5

[0032] The vacant sodium phosphotungstic acid salt and the vacant phosphotungstic acid quaternary ammonium salt prepared in Example 2 were systematically characterized.

[0033] Figure 1 The infrared spectra of phosphotungstic acid, sodium phosphotungstic acid (without phosphotungstic acid), and quaternary ammonium phosphotungstic acid (without phosphotungstic acid) are presented. The characteristic absorption bands of phosphotungstic acid are located at 1060, 990, 892, and 782 cm⁻¹, which are attributed to PO₄²⁻¹, respectively a W=O d WO b -W and W–O c –W stretching vibrations. In contrast, the absorption peak of the P-Oa bond in the missing phosphotungstic acid sodium salt at 1060 cm⁻¹ is split into two characteristic peaks at approximately 1037 and 1093 cm⁻¹, confirming the formation of the mono-missing Keggin structure and its reduced symmetry. In the spectrum of the missing phosphotungstic acid quaternary ammonium salt, the sharp absorption peaks at 2927 cm⁻¹ and 2857 cm⁻¹ are attributed to the CH stretching vibrations of the quaternary ammonium cation, indicating the successful introduction of the organic quaternary ammonium salt.

[0034] Figure 2 The X-ray diffraction patterns of the above samples are shown. Compared with phosphotungstic acid, the diffraction peak intensity of the missing sodium phosphotungstic acid salt is weakened, and its crystallinity is reduced. This can be attributed to the exchange of hydrogen protons by sodium ions and the change in the number of hydrated water molecules. Its characteristic diffraction peaks at 2θ = 8.5°, 10.0°, 12.5°, 15.0°, 17.5°, 20.0°, and 22.5° are basically consistent with those of phosphotungstic acid, indicating that its main framework still maintains the Keggin structure. For the missing quaternary ammonium phosphotungstic acid salt, its crystallinity is further reduced, and some diffraction peak positions are shifted, confirming that the introduction of organic quaternary ammonium cations has a fine-tuning effect on the lattice parameters.

[0035] Figure 3 The images show the UV-Vis absorption spectra of the three samples. All three samples exhibit a strong absorption band around 260 nm, which is attributed to O in Keggin-type heteropoly anions. b / O c The charge transfer transition →W indicates that the construction of the vacancy structure and the introduction of the quaternary ammonium salt did not alter the Keggin framework of the catalyst. Compared to phosphotungstic acid, the absorption peak intensity of the vacancy-deficient sodium phosphotungstic acid salt is slightly reduced at this point, which is due to the weakening of vibrational intensity caused by the vacancy structure. The vacancy-deficient quaternary ammonium phosphotungstic acid salt, however, exhibits stronger absorption with a slight red shift, possibly due to the electrostatic interaction between the vacancy-deficient phosphotungstic acid anion and the quaternary ammonium cation.

[0036] Figure 4 This is a scanning electron microscope (SEM) image of a quaternary ammonium phosphotungstic acid salt with missing pores. The sample exhibits a three-dimensional, interconnected porous network framework structure with irregular pore morphology and a pore size distribution ranging from approximately 0.5 to 2 μm. The pore walls are cross-linked to form a continuous network structure, which endows the material with excellent mass transfer properties and load-bearing capacity.

[0037] Example 6

[0038] In comparison, phosphotungstic acid quaternary ammonium salt catalysts were synthesized using unmodified phosphotungstic acid. The specific synthesis method is as follows: 9 mmol of phosphotungstic acid hydrate and 9 mmol of trioctylmethylammonium chloride quaternary ammonium salt were dissolved separately in 100 mL of anhydrous ethanol to form homogeneous solutions. Under magnetic stirring, the trioctylmethylammonium chloride quaternary ammonium salt ethanol solution was slowly added dropwise to the phosphotungstic acid ethanol solution. After the addition was complete, stirring was continued for 30 min, followed by aging at room temperature for 60 min. The supernatant was removed by centrifugation, and the precipitate was washed three times with anhydrous ethanol. The precipitate was then vacuum dried for 12 h to obtain phosphotungstic acid quaternary ammonium salt.

[0039] Example 7

[0040] The vacant phosphotungstic acid quaternary ammonium salt and phosphotungstic acid quaternary ammonium salt catalysts prepared in Examples 2-5 were applied to the selective oxidation of oleic acid to azelaic acid. The conversion rate of oleic acid and the selectivity of azelaic acid were characterized, and the results are listed in Table 1 for comparison.

[0041] The specific method for preparing azelaic acid by catalytic oxidation of oleic acid with missing phosphotungstic acid quaternary ammonium salt is as follows: Weigh 5g of oleic acid, 0.05g of missing phosphotungstic acid quaternary ammonium salt, and 14g of H2O2 (30%) into a 100mL three-necked flask. Place the flask in a water bath equipped with a stirrer, install a condenser, and react at 80℃ for 4h to obtain azelaic acid. After the reaction is complete, centrifuge to separate the catalyst, transfer the supernatant to a 100mL separatory funnel, extract three times with ethyl acetate, combine the organic phases, and rotary evaporate. Combine the supernatant organic phases and rotary evaporate. Take 1g of the rotary evaporated product into a pressure-resistant tube, add methanol and concentrated sulfuric acid for methyl esterification, extract the reaction solution with isooctane, rotary evaporate, remove water with anhydrous sodium sulfate, and then analyze by gas chromatography. As a comparison, the phosphotungstic acid quaternary ammonium salt synthesized in Example 5 was used instead of the missing phosphotungstic acid quaternary ammonium salt, and the reaction was carried out according to the above method. It can be seen that the missing phosphotungstic acid quaternary ammonium salt catalyst has better catalytic activity and can significantly improve the conversion rate of oleic acid and the selectivity of azelaic acid.

[0042] Table 1. Effect of different catalysts on the selective oxidation of oleic acid to azelaic acid

[0043]

[0044] Example 8

[0045] The quaternary ammonium phosphotungstic acid obtained in Example 1 was used as a catalyst to prepare azelaic acid according to the method in Example 4. The reusability stability of the catalyst was tested. After the reaction, the solution was transferred to a centrifuge tube, saturated sodium chloride solution and ethyl acetate were added, and solid-liquid separation was achieved by centrifugation. The obtained catalyst was dried at 60°C and used as a catalyst for the preparation of azelaic acid. The quaternary ammonium phosphotungstic acid catalyst was reused three times. After each reaction, the conversion rate of oleic acid and the selectivity of azelaic acid were analyzed by gas chromatography. The results are shown in Table 2. After repeated use, it was found that the catalyst conversion rate and selectivity only decreased slightly, indicating that there was no leakage of active species. The XRD pattern of the catalyst recovered after three cycles was similar to (…). Figure 5 Fresh catalysts are similar ( Figure 2 This indicates that the structure of the recovered catalyst was preserved. Simultaneously, the structure of the product, azelaic acid, was identified. Figure 5 This is the mass spectrum of azelaic acid. The peak at m / z = 189 is the [M+ H] peak of azelaic acid, and the peak at m / z = 211 is the [M+ Na] peak of azelaic acid. Figure 6 This is the 1H NMR spectrum of azelaic acid. Comparison with the mass spectrum confirms that the product is azelaic acid.

[0046] Table 2 Reusability of Defective Phosphotungstic Acid Quaternary Ammonium Salts

[0047]

Claims

1. A method for preparing a vacant phosphotungstic acid quaternary ammonium salt catalyst, characterized in that, The process includes the following steps: (1) dissolving phosphotungstic acid hydrate in water, adding sodium bicarbonate to adjust the pH of the system to 4.0-5.0, and reacting at 40℃-60℃ for 3-6 hours to obtain an aqueous solution of vacant sodium phosphotungstic acid salt; (2) dissolving quaternary ammonium salt in water to obtain an aqueous solution of quaternary ammonium salt, adding the aqueous solution of quaternary ammonium salt to the aqueous solution of vacant sodium phosphotungstic acid salt obtained in step (1), and reacting at 60℃-80℃ for 0.5-2 hours; (3) after the reaction is completed, allowing the mixture to stand, filtering and separating to obtain a solid, and drying to obtain the vacant phosphotungstic acid quaternary ammonium salt catalyst.

2. The preparation method according to claim 1, characterized in that, In step (2), the quaternary ammonium salt is trioctylmethylammonium chloride, trioctylmethylammonium bromide or tetraoctylammonium chloride.

3. The preparation method according to claim 1, characterized in that, In step (2), the molar ratio of the vacant sodium phosphotungstate salt to the quaternary ammonium salt is (0.5-1.5):

1.

4. The vacant phosphotungstic acid quaternary ammonium salt catalyst prepared by any of the preparation methods described in claims 1-3.

5. The application of the vacant phosphotungstic acid quaternary ammonium salt catalyst according to claim 4 in the catalytic oxidation of oleic acid to azelaic acid.

6. A method for preparing azelaic acid by oxidizing oleic acid, characterized in that, The process includes the following steps: mixing oleic acid, a catalyst, and hydrogen peroxide, and then performing an oxidation reaction under heating conditions to obtain azelaic acid; the catalyst is the vacant phosphotungstic acid quaternary ammonium salt catalyst as described in claim 4.

7. The method according to claim 6, characterized in that, The mass ratio of oleic acid, hydrogen peroxide and catalyst is 1:(1-8):(0.03-0.20); wherein the hydrogen peroxide is calculated as a pure substance.

8. The method according to claim 6, characterized in that, The oxidation reaction is carried out at a temperature of 70℃ to 110℃.

9. The method according to claim 6, characterized in that, The oxidation reaction takes 6 to 12 hours.

10. The method according to claim 6, characterized in that, After the reaction is complete, the catalyst is separated by centrifugation, and the separated catalyst can be reused after drying.

Images