Heteropolyacid catalyst regulation and control method and application of heteropolyacid catalyst in preparation of methacrylic acid

Abstract

The invention discloses a heteropolyacid catalyst regulation and control method and application of the heteropolyacid catalyst in preparation of methacrylic acid, the regulation and control method is characterized in that the structural general formula of the heteropolyacid catalyst is Cex (NH4) yH3-4x-yPMo12O40, and the value range of x is 0 lt; xlt; the value range of y is greater than or equal to 1.3 and less than or equal to 2.7. A cerium (Ce) element is introduced to replace a protonic acid site in the heteropolyacid catalyst, the roasting temperature of the catalyst is adjusted, and meanwhile, the concentration of the protonic acid site in the heteropolyacid catalyst and the oxidation-reduction performance are regulated and controlled, so that the catalytic activity and selectivity are improved. Compared with the performance of an unmodified heteropolyacid catalyst, the yield and selectivity of methacrylic acid can be respectively improved by 14% and 15%. The catalyst is simple and convenient in preparation method and excellent in performance, and has a good industrial application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of catalyst preparation and application, and particularly to a method for regulating a heteropolyacid catalyst and its application in the preparation of methacrylic acid. Background Art

[0002] The selective oxidation of methacrolein to methacrylic acid is a key step in the clean production process of methyl methacrylate. The core technology is to use a Mo-based heteropolyacid catalyst for catalytic oxidation reaction. Japan successfully developed a phosphomolybdic acid-based heteropolyacid catalyst in the 1980s, realizing the industrial production of methyl methacrylate. However, the existing catalysts still face problems such as low selectivity, low utilization rate of active components, and poor thermal stability.

[0003] To improve production efficiency, shorten the cycle, and improve process conditions, some patents have reported improvement methods for Mo-based heteropolyacid catalysts. CN109731592B discloses a heteropolyacid salt with molybdenum, phosphorus, vanadium, sodium, potassium, iron, and titanium as the main active components; CN1750878A discloses a heteropolyacid salt catalyst with molybdenum, phosphorus, vanadium, cesium, ammonium, and copper as the main active components, and the amount of water used in its preparation process has a significant impact on the catalyst performance; CN104001543A reports a synthesis method of a core-shell structured heteropolyacid salt, whose core structure is cesium phosphomolybdovanadate or ammonium phosphomolybdovanadate, the sub-outer layer is ammonium phosphomolybdovanadate or cesium phosphomolybdovanadate, and the outermost layer is a transition metal salt of phosphomolybdic acid. Although these catalysts overcome some defects of conventional heteropolyacid catalysts to a certain extent, their synthesis methods are still relatively cumbersome and the production conditions are relatively harsh.

[0004] Therefore, it is necessary to develop a new method for preparing a heteropolyacid catalyst to simplify the process, improve the service life of the catalyst, and further improve the yield and selectivity of methacrylic acid. Summary of the Invention

[0005] The present invention provides a method for regulating a heteropolyacid catalyst and its application in the preparation of methacrylic acid. By introducing Ce element and controlling the calcination conditions, the concentration of proton acid sites and the redox performance of the heteropolyacid catalyst can be effectively regulated, realizing the efficient selective oxidation of methacrolein to prepare methacrylic acid.

[0006] The object of the present invention is achieved by adopting the following technical solutions:

[0007] A method for regulating a heteropolyacid catalyst, the specific steps include:

[0008] 1) Preparing a precursor by the ionothermal method: Weighing the required mass of (NH4)6Mo7O 24, H3PO4 and the Ce source are dissolved in deionized water to form a mixed solution, and then the mixed solution is subjected to a reflux reaction for 5 hours. After the reaction is completed, the solid product is collected by filtration, and the filter cake is washed with deionized water, and then dried at 80 °C to obtain a pale yellow solid.

[0009] 2) Pulse program calcination treatment: The dried precursor is calcined in an air atmosphere for 6 hours, and after the calcination is completed, it is naturally cooled to room temperature to obtain the modified heteropolyacid catalyst.

[0010] Furthermore, the cerium source in step 1) is one or two of cerium nitrate, cerium chloride, cerium carbonate, cerium acetate or cerium fluoride; the content of Ce element is 0.007 - 3.0 wt%, the P / Mo molar ratio is 12, and the NH4 + content is 1.2 - 2.5 wt%; the treatment temperature of the ionic thermal method is controlled at 80 - 120 °C. The pulse program calcination temperature in step 2) is controlled at 320 - 380 °C.

[0011] Meanwhile, the present invention discloses a method for catalytically oxidizing methacrolein to methacrylic acid by using the heteropolyacid, and the steps include:

[0012] (1) The heteropolyacid catalyst needs to be crushed into solid particles of 20 - 40 mesh before use,

[0013] (2) A certain proportion of air and methacrolein are mixed (the partial pressure ratio of methacrolein to oxygen is 3) and then introduced into a fixed-bed reactor, the reaction temperature is 270 - 350 °C, and the space velocity is 1200 h -1 .

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] By introducing Ce element and controlling the calcination conditions, the concentration of proton acid sites and the redox performance of the heteropolyacid catalyst are regulated, thereby improving the catalytic activity and selectivity. At the same time, the catalyst preparation method is simple, the cost is low, and the stability is good, which is conducive to industrial production applications. Specific Embodiments

[0016] The technical solutions of the present invention will be further described below through specific embodiments.

[0017] Determination of the concentration of proton acid sites: The concentration of proton acid sites in the catalyst was determined by back titration. The specific steps were as follows: Weigh 0.5 g of the dried sample and place it in a 150 mL conical flask. Add 30 mL of deionized water and a few drops of phenolphthalein solution. After shaking well, add 0.1 mol / L standard NaOH solution dropwise until the mixed solution turns red. Subsequently, slowly add 0.1 mol / L standard hydrochloric acid solution until the solution returns to colorless and remains unchanged for a period of time. Record the amounts of the NaOH solution and the hydrochloric acid solution used. Finally, calculate the concentration of proton acid sites in the sample according to the acid-base titration formula. The determination results of the concentration of proton acid sites and the amount of NH3-TPD acid in the Ce element-modified catalyst used in the examples are shown in Table 1.

[0018] Example 1

[0019] Cerium nitrate was used as the Ce source to prepare the heteropolyacid precursor, and a Ce 0.005 (NH4) x H 2.8- x PMo 12 O 40 heteropolyacid catalyst was prepared by the precipitation method. The specific operation steps were as follows: At room temperature, add 29.96 g of (NH4)6Mo7O 24 ·4H2O, 30.5 mg of Ce(NO3)3·6H2O, and 1.61 g of 85% H3PO4 into 300 mL of deionized water. After stirring evenly, heat up to 100 °C and react under stirring and reflux for 5 hours. After the reaction, let it stand and filter, wash the filter cake with deionized water, and dry it at 80 °C to obtain a light yellow solid. The obtained solid was calcined in an air atmosphere at 320 °C, 340 °C, 360 °C, 370 °C, and 380 °C for 6 hours respectively to obtain the catalyst, denoted as Cat 1.

[0020] Catalyst testing: The performance evaluation of the catalyst was carried out in a fixed-bed reactor. The inner diameter of the reaction tube was 22 mm, the catalyst loading was 0.75 g, and the reaction space velocity was 1200 h -1 . The reaction temperature was investigated at 310 °C under atmospheric pressure. After stable operation for 3 h, samples of the liquid product and the tail gas were taken for analysis every 15 min. The experimental repeatability and stability were good, and the mass difference before and after the reaction was within 1%. The liquid product was analyzed by FID gas chromatography, and the gas product was analyzed by TCD gas chromatography. The evaluation results are shown in Table 2.

[0021] Example 2

[0022] Cerium chloride was used as the Ce source to prepare the heteropolyacid precursor, and a Ce 0.005 (NH4) x H 2.8- x PMo 12 O40 Heteropoly acid catalyst. The specific operation steps are as follows: At room temperature, 29.96 g of (NH4)6Mo7O 24 ·4H2O, 17.2 mg of CeCl3 and 1.61 g of 85% H3PO4 are added to 300 mL of deionized water. After stirring evenly, the temperature is raised to 100 °C, and the reaction is refluxed with stirring for 5 hours. After the reaction is completed, it is allowed to stand and filtered, and the filter cake is washed with deionized water and dried at 80 °C to obtain a pale yellow solid. The obtained solid is calcined in an air atmosphere at 320 °C, 340 °C, 360 °C, 370 °C and 380 °C for 6 hours respectively to obtain the catalyst, denoted as Cat 2.

[0023] Catalyst testing: The performance evaluation of the catalyst is carried out in a fixed-bed reactor. The inner diameter of the reaction tube is 22 mm, the catalyst loading is 0.75 g, and the reaction space velocity is 1200 h -1 . The reaction temperature is investigated at 310 °C under atmospheric pressure operation. After stable operation for 3 h, samples of the liquid product and the tail gas are taken for analysis every 15 min. The experimental repeatability and stability are good, and the mass difference before and after the reaction is within 1%. The liquid product is analyzed by FID gas chromatography, and the gas product is analyzed by TCD gas chromatography. The evaluation results are shown in Table 2.

[0024] Example 3

[0025] Cerium carbonate is used as the Ce source to prepare the heteropoly acid precursor, and Ce 0.005 (NH4) x H 2.8- x PMo 12 O 40 Heteropoly acid catalyst. The specific operation steps are as follows: At room temperature, 29.96 g of (NH4)6Mo7O 24 ·4H2O, 29.4 mg of Ce2(CO3)3 and 1.61 g of 85% H3PO4 are added to 300 mL of deionized water. After stirring evenly, the temperature is raised to 100 °C, and the reaction is refluxed with stirring for 5 hours. After the reaction is completed, it is allowed to stand and filtered, and the filter cake is washed with deionized water and dried at 80 °C to obtain a pale yellow solid. The obtained solid is calcined in an air atmosphere at 320 °C, 340 °C, 360 °C, 370 °C and 380 °C for 6 hours respectively to obtain the catalyst, denoted as Cat 3.

[0026] Catalyst testing: The performance evaluation of the catalyst is carried out in a fixed-bed reactor. The inner diameter of the reaction tube is 22 mm, the catalyst loading is 0.75 g, and the reaction space velocity is 1200 h -1The reaction temperature was investigated at 310 °C under atmospheric pressure. After stable operation for 3 h, samples of the liquid product and the tail gas were taken for analysis every 15 min. The experimental repeatability and stability were good, and the mass difference before and after the reaction was within 1%. The liquid product was analyzed by FID gas chromatography, and the gas product was analyzed by TCD gas chromatography. The evaluation results are shown in Table 2.

[0027] Example 4

[0028] Cerium fluoride was used as the Ce source to prepare the heteropolyacid precursor, and Ce 0.005 (NH4) x H 2.8- x PMo 12 O 40 heteropolyacid catalyst. The specific operation steps are as follows: At room temperature, 29.96 g of (NH4)6Mo7O 24 ·4H2O, 13.8 mg of CeF3, and 1.61 g of 85% H3PO4 were added to 300 mL of deionized water. After stirring evenly, the temperature was raised to 100 °C, and the reaction was refluxed with stirring for 5 hours. After the reaction was completed, it was allowed to stand and filtered. The filter cake was washed with deionized water and dried at 80 °C to obtain a pale yellow solid. The obtained solid was calcined in an air atmosphere at 320 °C, 340 °C, 360 °C, 370 °C, and 380 °C for 6 hours to obtain the catalyst, denoted as Cat 4.

[0029] Catalyst test: The performance evaluation of the catalyst was carried out in a fixed-bed reactor. The inner diameter of the reaction tube was 22 mm, the catalyst loading was 0.75 g, and the reaction space velocity was 1200 h -1 。The reaction temperature was investigated at 310 °C under atmospheric pressure. After stable operation for 3 h, samples of the liquid product and the tail gas were taken for analysis every 15 min. The experimental repeatability and stability were good, and the mass difference before and after the reaction was within 1%. The liquid product was analyzed by FID gas chromatography, and the gas product was analyzed by TCD gas chromatography. The evaluation results are shown in Table 2.

[0030] Example 5

[0031] Cerium acetate was used as the Ce source to prepare the heteropolyacid precursor, and Ce 0.005 (NH4) x H 2.8- x PMo 12 O 40 heteropolyacid catalyst. The specific operation steps are as follows: At room temperature, 29.96 g of (NH4)6Mo7O 24·4H2O, 22.2 mg Ce(CH3COO)3 and 1.61 g of 85% H3PO4 were added to 300 mL of deionized water. After stirring evenly, the temperature was raised to 100 °C, and the reaction was refluxed with stirring for 5 hours. After the reaction ended, it was allowed to stand and filtered. The filter cake was washed with deionized water and dried at 80 °C to obtain a pale yellow solid. The obtained solid was calcined in an air atmosphere at 320 °C, 340 °C, 360 °C, 370 °C and 380 °C for 6 hours respectively to obtain the catalyst, denoted as Cat 5.

[0032] Catalyst testing: The performance evaluation of the catalyst was carried out in a fixed-bed reactor. The inner diameter of the reaction tube was 22 mm, the catalyst loading was 0.75 g, and the reaction space velocity was 1200 h -1 . The reaction temperature was investigated at 310 °C under atmospheric pressure. After stable operation for 3 h, samples were taken every 15 min to analyze the liquid-phase products and tail gas. The experimental repeatability and stability were good, and the mass difference before and after the reaction was within 1%. The liquid-phase products were analyzed by FID gas chromatography, and the gas products were analyzed by TCD gas chromatography. The evaluation results are shown in Table 2.

[0033] Comparative Example 1

[0034] (NH4) x H 3-x PMo 12 O 40 heteropolyacid catalyst was prepared by precipitation method. The specific operation steps are as follows: At room temperature, 29.96 g of (NH4)6Mo7O 24 ·4H2O and 1.61 g of 85% H3PO4 were added to 300 mL of deionized water. After stirring evenly, the temperature was raised to 100 °C, and the reaction was refluxed with stirring for 5 hours. After the reaction ended, it was allowed to stand and filtered. The filter cake was washed with deionized water and dried at 80 °C to obtain a white solid. The obtained solid was calcined in an air atmosphere at 320 °C, 340 °C, 360 °C, 370 °C and 380 °C for 6 hours respectively to obtain the catalyst, denoted as Cat 6.

[0035] Catalyst testing: The performance evaluation of the catalyst was carried out in a fixed-bed reactor. The inner diameter of the reaction tube was 22 mm, the catalyst loading was 0.75 g, and the reaction space velocity was 1200 h -1 . The reaction temperature was investigated at 310 °C under atmospheric pressure. After stable operation for 3 h, samples were taken every 15 min to analyze the liquid-phase products and tail gas. The experimental repeatability and stability were good, and the mass difference before and after the reaction was within 1%. The liquid-phase products were analyzed by FID gas chromatography, and the gas products were analyzed by TCD gas chromatography. The evaluation results are shown in Table 2.

[0036] Table 1 Proton acid content in Example 1

[0037]

[0038]

[0039] Table 2 Catalyst Activity Evaluation Results

[0040]

[0041]

Claims

1. A method for regulating a heteropolyacid catalyst and its application in the preparation of methacrylic acid, characterized in that: After doping with cerium (Ce) element, the obtained heteropolyacid catalyst still has the Keggin structure. By changing the calcination temperature, the concentration of proton acid sites and the redox performance of the catalyst can be regulated. The chemical general formula of the heteropolyacid catalyst is Ce x (NH4) y H 3-4x-y PMo 12 O 40 , where the value range of x is 0 < x < 0.5, and the value range of y is 1.3 ≤ y ≤ 2.

7. The obtained catalyst is applied to the selective oxidation of methacrolein to prepare methacrylic acid.

2. The method for controlling a heteropolyacid catalyst according to claim 1, comprising the following steps: a) Ion thermal method to prepare precursor: Add appropriate amount of (NH4)6Mo7O 24 , H3PO4, Ce source and deionized water were added to a three-necked flask in appropriate proportions, and then the flask was placed in an oil bath at a set temperature, the required temperature of the mixed solution was maintained, and stirring was continued for 5 hours. After the reaction reflux was completed, the solid product was collected by filtration and dried at 80°C for 10 hours to obtain a catalyst precursor; b) Pulse program calcination treatment: Place an appropriate amount of precursor in a porcelain boat, use pulse program calcination technology, adjust the frequency of the pulse signal through a controller, heat to the required temperature at a specific frequency and maintain for 6 hours, and naturally cool to room temperature after calcination to finally obtain a Ce-modified heteropolyacid catalyst.

3. The method for controlling the heteropolyacid catalyst according to claim 2, characterized in that: The Ce source is one or two of cerium nitrate, cerium chloride, cerium carbonate, cerium acetate or cerium fluoride.

4. The method for controlling the heteropolyacid catalyst according to claim 2, characterized in that: The content of Ce element is 0.007-3.0wt%, the P / Mo molar ratio is 12, and the NH4 + The content is 1.2-2.5wt%.

5. The method for controlling the heteropolyacid catalyst according to claim 2, characterized in that: The treatment temperature of the ion thermal method is controlled at 80-120°C, and the pulse program calcination temperature is controlled at 320-380°C.

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