Use of a composite oxide catalyst in the selective gas phase oxidation of toluene

Abstract

The application provides a catalyst for selective gas-phase oxidation of toluene and a preparation method thereof, and relates to the field of heterogeneous catalysis. The application provides application of a composite oxide catalyst in selective gas-phase oxidation of toluene, and the MoVTeNbO x The morphology and size of the composite oxide are regulated, and a toluene selective gas-phase oxidation catalyst with excellent catalytic performance is obtained. x The composite oxide is used as a catalyst for catalyzing selective gas-phase oxidation of toluene, which is still the first time.

Description

Technical Field

[0001] This invention belongs to the field of heterogeneous catalysis applications, and particularly relates to a catalyst for the selective oxidation of toluene in the gas phase and its preparation method. Background Technology

[0002] Selective oxidation of hydrocarbon molecules to produce oxygen-containing organic products (epoxides, alcohols, aldehydes, ketones, acids, phenols, and esters, etc.) is an extremely important class of chemical reactions. Introducing oxygen into hydrocarbon compounds containing only carbon and hydrogen elements yields chemicals with higher value and wider applications, playing a crucial role in chemical industrial processes. Furthermore, the total production capacity of oxidation reactions is the second largest chemical industrial process (after polymerization). Unfortunately, due to the extreme difficulty of these reactions, research progress in this area over the past few decades has severely lagged behind the needs of societal development. Therefore, catalytic selective oxidation has always been one of the most important research directions in the field of catalysis. The molecular oxygen oxidation of toluene to benzaldehyde is a very typical example. Benzaldehyde, as the simplest and most important industrial aromatic aldehyde, is a very important fine chemical and organic synthesis intermediate, widely used in the fragrance, food, pharmaceutical, and pesticide industries. Currently, the main industrial processes for producing benzaldehyde are toluene chlorination hydrolysis and toluene homogeneous oxidation, but both suffer from serious corrosion and environmental pollution problems. Moreover, the produced benzaldehyde contains small amounts of halogens, hindering the high-end application of this compound in pharmaceuticals, fragrances, and food. The selective oxidation of toluene to benzaldehyde using air or oxygen as an oxidant under completely halogen-free conditions offers advantages such as high product purity, high atom utilization, and low emissions, making it a green and clean production method that fully meets the high-end requirements of industries like food and pharmaceuticals. Furthermore, toluene is abundant, thus the application of toluene as a raw material for benzaldehyde production shows promising prospects. However, despite over fifty years of research worldwide, this method has yet to be successful because the oxidation rate of benzaldehyde under oxygen-rich conditions is 4-5 orders of magnitude higher than that of toluene. This results in low product selectivity, hindering its industrialization and remaining a classic challenge in heterogeneous catalysis.

[0003] Maleic anhydride, scientifically known as maleic anhydride, is an important organic chemical raw material and fine chemical product. It is currently the world's third largest acid anhydride after phthalic anhydride and acetic anhydride. It is mainly used in the production of unsaturated polyester resins and alkyd resins, and is used in pesticides, pharmaceuticals, coatings, inks, lubricant additives, papermaking chemicals, textile finishing agents, food additives, and surfactants. Maleic anhydride is primarily produced by the partial oxidation of benzene or C4 hydrocarbons (such as n-butane) in the presence of an oxidizing catalyst. Industrially, maleic anhydride is mainly produced using the n-butane oxidation method. Currently, there are few reports on the selective oxidation of toluene to produce maleic anhydride.

[0004] CN101337185A relates to a catalyst for the gas-phase selective oxidation of toluene to benzaldehyde and benzoic acid, having the general formula Ag. x V y O z However, the high cost of raw materials limits its large-scale industrial application. Therefore, developing efficient and highly selective catalysts and mild, green catalytic processes is crucial. MoVTeNbO x Composite oxide catalysts are mainly used in reactions such as the oxidative dehydrogenation of ethane and propane to produce ethylene and propylene, and the selective (ammonia) oxidation of propane to produce acrylic acid and acrylonitrile (Bu Tingting et al. MoVTeNbO). x Research progress on catalysts applied to the oxidative dehydrogenation of ethane to ethylene [J]. Chemical Industry and Engineering Progress: 1-18.), while Mo and V-based catalysts are also commonly used catalysts for the gas-phase selective oxidation of toluene. Therefore, the study of MoVTeNbO x The application of composite oxides in the gas-phase selective oxidation of toluene is very important for the development of novel gas-phase selective oxidation catalysts for toluene. Summary of the Invention

[0005] The purpose of this invention is to provide an application of a composite oxide catalyst in the gas-phase selective oxidation of toluene, which has high activity in the gas-phase selective catalytic oxidation of toluene.

[0006] The specific solution of the present invention is as follows:

[0007] The application of a composite oxide catalyst in the catalytic preparation of benzaldehyde or maleic anhydride from toluene, wherein the catalyst has the general formula MoVTeNbO x Where x is an indeterminate constant;

[0008] The molar ratio of Mo, V, Te, and Nb in the catalyst is 1:(0.1-0.3):(0.2-0.3):(0.1-0.2). The catalyst is applied to the gas-phase selective oxidation or liquid-phase selective oxidation of toluene to prepare benzaldehyde or maleic anhydride.

[0009] The gas-phase selective oxidation method is carried out by feeding toluene, air and catalyst in a reaction ratio of toluene (μL): air (mL): catalyst (g) = 1:(2-50):(0.1-0.6) under the conditions of reaction temperature of 200-400℃ and reaction pressure of 0.05MPa-0.15MPa.

[0010] The preparation steps of the catalyst are as follows:

[0011] (1) Dissolve molybdenum salt, vanadium salt and telluric acid in distilled water, and control the temperature of the aqueous solution to 60-90℃;

[0012] (2) Dissolve the niobium salt in distilled water and control the temperature of the aqueous solution to 60-90℃;

[0013] (3) Add the aqueous solution obtained in step (2) to the aqueous solution in step (1) and stir continuously;

[0014] (4) After the mixture obtained in step (3) has cooled, sodium citrate is added while stirring continuously;

[0015] (5) Transfer the mixture obtained in step (4) to a reaction vessel and hydrothermally heat it at 170-190°C for 36-60 hours;

[0016] (6) The precipitate obtained in step (5) is centrifuged and washed, dried at 60-90°C, and the resulting solid is heated from room temperature to 200-300°C at a heating rate of 2-10°C / min and calcined in air for 1-3 hours. Then, it is heated from room temperature to 550-650°C at a heating rate of 2-10°C / min and calcined in argon for 1-3 hours.

[0017] (7) Add the powder from step (6) to a hydrogen peroxide aqueous solution with a mass fraction of 5-10%, stir at 60-70°C for 3-6 hours, filter, wash and dry to obtain the final catalyst;

[0018] Preferably, the molybdenum salt, vanadium salt, telluric acid, and niobium salt are (NH4)6Mo7O, respectively. 24 •4H2O, VOSO4·xH2O, H6TeO6, and C4H4NNbO9·xH2O. In VOSO4·xH2O and C4H4NNbO9·xH2O, x represents an indeterminate constant.

[0019] Preferably, molybdenum salt, vanadium salt, telluric acid, niobium salt and sodium citrate are added in a molar ratio of Mo, V, Te, Nb and sodium citrate of 1:(0.1~0.3):(0.2~0.3):(0.1~0.2):(0.1~0.2).

[0020] This invention regulates the production of MoVTeNbO by controlling the hydrothermal synthesis volume. x The morphology and size of the composite oxide were determined, resulting in a toluene gas-phase selective oxidation catalyst with excellent catalytic performance. Specifically, in the embodiments of the present invention, the reaction vessel for preparing the catalyst was a hydrothermal reactor with a polytetrafluoroethylene (PTFE) liner volume of 100–200 mL; when the PTFE liner volume used in the catalyst preparation process was 100 mL, the final catalyst was named MoVTeNbO. x -100; When the volume of the polytetrafluoroethylene liner used in the catalyst preparation process is 200 mL, the final catalyst is named MoVTeNbO x -200.

[0021] In the embodiments provided by the present invention, the catalyst is pressed into particles with a diameter of 0.25 to 0.4 mm, placed in a quartz glass U-shaped tube, and subjected to a gas-phase selective oxidation reaction at a temperature range of 200 to 400°C, at atmospheric pressure, and under continuous air and toluene vapor conditions to obtain benzaldehyde or maleic anhydride; wherein the air flow rate is 20 to 30 mL / min, and the toluene feed rate is 1 to 2 μL / min.

[0022] Beneficial effects:

[0023] This invention provides an application of a composite oxide catalyst in the gas-phase selective oxidation of toluene, by controlling the hydrothermal synthesis volume to regulate the MoVTeNbO catalyst. x The morphology and size of the composite oxide were determined, leading to the acquisition of a toluene gas-phase selective oxidation catalyst with excellent catalytic performance. This catalyst has a simple preparation process, good reproducibility, and promising application prospects. (The text then mentions MoVTeNbO, but the connection to the catalyst is unclear and seems unrelated.) x This is the first time that composite oxides have been used as catalysts for the selective oxidation of toluene in the gas phase. Attached Figure Description

[0024] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein:

[0025] Figure 1 This is the MoVTeNbO prepared in Example 1 of the present invention. x Scanning electron microscope image at -100°.

[0026] Figure 2 This is the MoVTeNbO prepared in Example 1 of the present invention. x X-ray powder diffraction pattern at -100°.

[0027] Figure 3 This is the MoVTeNbO prepared in Example 1 of the present invention. x Results of selective oxidation activity of toluene in the gas phase at -100°C.

[0028] Figure 4 This is the MoVTeNbO prepared in Example 2 of the present invention. x -200 scanning electron microscope image.

[0029] Figure 5 This is the MoVTeNbO prepared in Example 2 of the present invention. x X-ray powder diffraction pattern at -200°.

[0030] Figure 6 This is the MoVTeNbO prepared in Example 2 of the present invention. x Results of selective oxidation activity of toluene in the gas phase at -200°C.

[0031] Figure 7 This is the MoVTeNbO prepared in Example 3 of the present invention. x Scanning electron microscope image at -100°.

[0032] Figure 8 This is the MoVTeNbO prepared in Example 3 of the present invention. x X-ray powder diffraction pattern at -100°.

[0033] Figure 9 This is the MoVTeNbO prepared in Example 3 of the present invention. x Results of selective oxidation activity of toluene in the gas phase at -100°C. Detailed Implementation

[0034] The invention can be further illustrated by the following examples, which are for illustrative purposes only and not for limiting the invention. Any person skilled in the art will understand that these examples do not limit the invention in any way, and that appropriate modifications and data transformations can be made thereto without departing from the spirit and scope of the invention. Unless otherwise stated, all chemicals were purchased from commercially available products.

[0035] Example 1

[0036] Step 1: Weigh out 0.005 mol, 0.00875 mol, 0.00805 mol, and 0.0042 mol of (NH4)6Mo7O according to the molar ratio of Mo, V, Te, and Nb of 1:0.25:0.23:0.12. 24 ·4H2O, VOSO4·xH2O, H6TeO6, C4H4NNbO9·xH2O. Dissolve the first three in 45mL of distilled water and heat to 80℃ with constant stirring. This solution is called solution A.

[0037] Step 2: Dissolve C4H4NNbO9·xH2O in 20mL of distilled water and heat to 80℃ with constant stirring; this is called solution B.

[0038] Step 3: Once the temperatures of solutions A and B have both dropped to 40°C, slowly add solution B to solution A while stirring continuously for 0.5 hours.

[0039] Step 4: After the mixed emulsion obtained in Step 3 has cooled, add 5 mL of 0.84 mol / L sodium citrate aqueous solution and stir until homogeneous.

[0040] Step 5: Transfer the mixture obtained in Step 4 to a 100 mL hydrothermal reactor and hydrothermally heat it at 175 °C for 48 h.

[0041] Step six: Wash the mixture obtained in step five by centrifugation three times with distilled water, then by centrifugation once with ethanol, and dry it at 70°C.

[0042] Step 7: The solid obtained in Step 6 is heated from room temperature to 250°C at a heating rate of 5°C / min and calcined in air for 2 hours. After it is cooled to room temperature, it is then heated from room temperature to 600°C at a heating rate of 5°C / min and calcined in argon for 2 hours to obtain a black powder.

[0043] Step 8: The black powder obtained in Step 7 is added to a 7.5% (w / w) aqueous solution of hydrogen peroxide, stirred at 60°C for 3 hours, filtered, washed, and dried to obtain the final catalyst, named MoVTeNbO. x -100.

[0044] Test Example 1: Physicochemical Properties of the Catalyst Prepared in Example 1

[0045] 1. Characterization Test

[0046] The catalyst prepared in Example 1 had a specific surface area of ​​14.5 m², as determined by testing. 2 / g (The specific surface area of ​​the catalyst is tested using the BET method. The principle is to determine the absolute amount of nitrogen adsorbed by the sample under different partial pressures, and then calculate the monolayer adsorption amount through BET theory, thereby determining the specific surface area).

[0047] The morphology of the catalyst was characterized by scanning electron microscopy, such as... Figure 1 As shown, it appears as broken brick pieces.

[0048] The prepared catalyst was analyzed by XRD, and the results are as follows: Figure 2 As shown, its structure is mainly in the M1 phase.

[0049] 2. Catalytic performance test

[0050] The obtained catalyst was pressed into particles with a diameter of 0.25–0.4 mm. 0.3 g of the catalyst was placed in a quartz glass U-tube and the catalytic reaction was carried out in the range of 200–400 °C, at atmospheric pressure, and under continuous air and toluene vapor conditions. The liquid toluene injection rate was 1 μL / min and the air flow rate was 25 mL / min.

[0051] Qualitative and quantitative analysis of the product was performed using gas chromatography, and its specific catalytic activity was as follows: Figure 3 As shown, the main product of the catalytic reaction is benzaldehyde. At 400℃, the toluene conversion rate is 23%, the benzaldehyde selectivity is 68%, and the selectivity of other products is less than 20%.

[0052] Example 2

[0053] Step 1: Weigh out 0.01 mol, 0.0175 mol, 0.0161 mol, and 0.0084 mol of (NH4)6Mo7O according to the molar ratio of Mo, V, Te, and Nb of 1:0.25:0.23:0.12. 24 ·4H2O, VOSO4·xH2O, H6TeO6, C4H4NNbO9·xH2O. Dissolve the first three in 90mL of distilled water and heat to 80℃ with constant stirring. This solution is called solution A.

[0054] Step 2: Dissolve C4H4NNbO9·xH2O in 40mL of distilled water and heat to 80℃ with constant stirring; this is called solution B.

[0055] Step 3: Once the temperatures of solutions A and B have both dropped to 40°C, slowly add solution B to solution A while stirring continuously for 0.5 hours.

[0056] Step 4: After the mixed emulsion obtained in Step 3 has cooled, add 10 mL of 0.84 mol / L sodium citrate aqueous solution and stir until homogeneous.

[0057] Step 5: Transfer the mixture obtained in Step 4 to a 200 mL hydrothermal reactor and hydrothermally heat it at 175 °C for 48 h.

[0058] Step six: Wash the mixture obtained in step five by centrifugation three times with distilled water, then by centrifugation once with ethanol, and dry it at 70°C.

[0059] Step 7: The solid obtained in Step 6 is heated from room temperature to 250°C at a heating rate of 5°C / min and calcined in air for 2 hours. After it is cooled to room temperature, it is then heated from room temperature to 600°C at a heating rate of 5°C / min and calcined in argon for 2 hours to obtain a black powder.

[0060] Step 8: The black powder obtained in Step 7 is added to a 7.5% (w / w) aqueous solution of hydrogen peroxide, stirred at 60°C for 3 hours, filtered, washed, and dried to obtain the final catalyst, named MoVTeNbO. x -200.

[0061] Test Example 2: Physicochemical Properties of the Catalyst Prepared in Example 2

[0062] 1. Characterization Test

[0063] The catalyst prepared in Example 2 was found to have a specific surface area of ​​20.2 m². 2 / g (The specific surface area of ​​the catalyst is tested using the BET method. The principle is to determine the absolute amount of nitrogen adsorbed by the sample under different partial pressures, and then calculate the monolayer adsorption amount through BET theory, thereby determining the specific surface area).

[0064] The morphology of the catalyst was characterized by scanning electron microscopy, such as... Figure 4 As shown, it resembles firewood, indicating that changes in the volume of the hydrothermal reactor affect the MoVTeNbO content. x Appearance has a significant impact.

[0065] The prepared catalyst was analyzed by XRD, and the results are as follows: Figure 5 As shown, its structure is mainly M1 phase, indicating that the change in the volume of the hydrothermal reactor affects the structure of MoVTeNbO. x The crystal structure has a relatively small impact.

[0066] 2. Catalytic performance test

[0067] The obtained catalyst was pressed into particles with a diameter of 0.25–0.4 mm. 0.3 g of the catalyst was placed in a quartz glass U-tube and the catalytic reaction was carried out in the range of 200–400 °C, at atmospheric pressure, and under continuous air and toluene vapor conditions. The liquid toluene injection rate was 1 μL / min and the air flow rate was 25 mL / min.

[0068] Qualitative and quantitative analysis of the product was performed using gas chromatography, and its specific catalytic activity was as follows: Figure 6 As shown, the main product of the catalytic reaction is maleic anhydride. At 400℃, the toluene conversion rate is 99.9%, the selectivity of maleic anhydride is 64%, and the selectivity of other products is less than 30%.

[0069] Example 3

[0070] Step 1: Weigh out 0.005 mol, 0.00875 mol, 0.004025 mol, and 0.0042 mol of (NH4)6Mo7O according to the molar ratio of Mo, V, Te, and Nb of 1:0.25:0.115:0.12. 24 ·4H2O, VOSO4·xH2O, H6TeO6, C4H4NNbO9·xH2O. Dissolve the first three in 45mL of distilled water and heat to 80℃ with constant stirring. This solution is called solution A.

[0071] Step 2: Dissolve C4H4NNbO9·xH2O in 20mL of distilled water and heat to 80℃ with constant stirring; this is called solution B.

[0072] Step 3: Once the temperatures of solutions A and B have both dropped to 40°C, slowly add solution B to solution A while stirring continuously for 0.5 hours.

[0073] Step 4: After the mixed emulsion obtained in Step 3 has cooled, add 5 mL of 0.97 mol / L sodium citrate aqueous solution and stir until homogeneous.

[0074] Step 5: Transfer the mixture obtained in Step 4 to a 100 mL hydrothermal reactor and hydrothermally heat it at 175 °C for 60 h.

[0075] Step six: Wash the mixture obtained in step five by centrifugation three times with distilled water, then by centrifugation once with ethanol, and dry it at 70°C.

[0076] Step 7: The solid obtained in Step 6 is heated from room temperature to 250°C at a heating rate of 5°C / min and calcined in air for 2 hours. After it is cooled to room temperature, it is then heated from room temperature to 600°C at a heating rate of 5°C / min and calcined in argon for 2 hours to obtain a black powder.

[0077] Step 8: The black powder obtained in Step 7 is added to a 7.5% (w / w) aqueous solution of hydrogen peroxide, stirred at 60°C for 3 hours, filtered, washed, and dried to obtain the final catalyst, named MoVTeNbO. x -100.

[0078] Test Example 3: Physicochemical Properties of the Catalyst Prepared in Example 3

[0079] 1. Characterization Test

[0080] Its specific surface area was measured to be 17.8 m². 2 / g (The specific surface area of ​​the catalyst is tested using the BET method. The principle is to determine the absolute amount of nitrogen adsorbed by the sample under different partial pressures, and then calculate the monolayer adsorption amount through BET theory, thereby determining the specific surface area).

[0081] The morphology of the catalyst was characterized by scanning electron microscopy, such as... Figure 7 As shown, it appears in fragmented strips.

[0082] The prepared catalyst was analyzed by XRD, and the results are as follows: Figure 8 As shown, although its structure is also mainly in the M1 phase, its crystallinity decreases, indicating that appropriately adjusting the addition amount of different components affects the crystallinity of MoVTeNbO. x Crystal structure has a certain influence.

[0083] 2. Catalytic performance test

[0084] The obtained catalyst was pressed into particles with a diameter of 0.25–0.4 mm. 0.3 g of the catalyst was placed in a quartz glass U-tube and the catalytic reaction was carried out in the range of 200–400 °C, at atmospheric pressure, and under continuous air and toluene vapor conditions. The liquid toluene injection rate was 1 μL / min and the air flow rate was 25 mL / min.

[0085] Qualitative and quantitative analysis of the product was performed using gas chromatography, and its specific catalytic activity was as follows: Figure 9As shown, the main product of the catalytic reaction is benzaldehyde. At 400℃, the toluene conversion rate is 31.3%, the benzaldehyde selectivity is 53.8%, and the selectivity of other products is less than 30%.

Claims

1. Use of a composite oxide catalyst in the selective oxidation of toluene, characterized in that, The general formula of the catalyst is MoVTeNbOx; The molar ratio of Mo, V, Te and Nb is 1: (0.1-0.3): (0.2-0.3): (0.1-0.2), and the catalyst is applied to the preparation of benzaldehyde or maleic anhydride by the toluene gas phase selective oxidation method or the liquid phase selective oxidation method. Step one, prepare an aqueous solution of molybdenum salt, vanadium salt and telluric acid to obtain solution A; Step two, prepare an aqueous solution of niobium salt to obtain solution B; Step three, mix solution B with solution A; Step four, add sodium citrate to the mixed solution obtained in step three; Step five, place the mixture obtained in step four in a 100ml or 200ml hydrothermal kettle to obtain benzaldehyde or maleic anhydride, respectively.

2. Use according to claim 1, characterized in that, The toluene gas phase selective oxidation method for preparing benzaldehyde is as follows: under the conditions of a reaction temperature of 200-400℃ and a reaction pressure of 0.05MPa-0.15MPa, toluene, air and the catalyst are fed according to toluene (μL): air (mL): catalyst (g) = 1: (2-50): (0.1-0.6), and benzaldehyde is prepared after the reaction.

3. Use according to claim 1, characterized in that, The molar ratio of Mo, V, Te and Nb is 1:0.25:0.23:0.12 or 1:0.25:0.115:0.

12.

4. Use according to claim 1, characterized in that, The toluene gas phase selective oxidation method for preparing maleic anhydride is as follows: under the conditions of a reaction temperature of 250-400℃ and a reaction pressure of 0.05MPa-0.15MPa, toluene, air and the catalyst are fed according to toluene (μL): air (mL): catalyst (g) = 1: (2-50): (0.1-0.6), and the volume of the hydrothermal synthesis of the raw material is adjusted, and maleic anhydride is prepared after the reaction.

5. Use according to any one of claims 1 to 4, characterized in that, The preparation steps of the catalyst are as follows: Step one, prepare an aqueous solution of molybdenum salt, vanadium salt and telluric acid, and control the solution temperature at 60-90℃ to obtain solution A; Step two, prepare an aqueous solution of niobium salt, and control the solution temperature at 60-90℃ to obtain solution B; Step three, mix solution B with solution A under the condition of 30-50℃; Step four, add sodium citrate to the mixed solution obtained in step three; Step five, place the mixture obtained in step four in a reaction container, and perform the reaction under the condition of 170-190℃; Step six, perform water washing, alcohol washing and drying treatment on the mixture obtained in step five; Step seven, sequentially perform air calcination treatment and argon atmosphere calcination treatment on the solid obtained in step six; Step eight, place the solid obtained in step seven in hydrogen peroxide aqueous solution for post-treatment, and then perform separation, washing and drying treatment to obtain the final catalyst.

6. Use according to claim 5, characterized in that, The molybdenum salt, vanadium salt, telluric acid and niobium salt are (NH4)6Mo7O24·4H2O, VOSO4·xH2O, H6TeO6 and C4H4NNbO9·xH2O, respectively. The molar ratio of Mo, V, Te, Nb and sodium citrate is 1: (0.1-0.3): (0.2-0.3): (0.1-0.2): (0.1-0.2), and the molybdenum salt, vanadium salt, telluric acid, niobium salt and sodium citrate are fed.

7. Use according to claim 5, characterized in that, ​ ​ 8. Use according to claim 5, characterized in that, The air roasting treatment of step seven is to roast the raw material to be treated in air at a temperature increasing rate of 4-8℃ / min from room temperature to 240-280℃ for 2-4h; the argon atmosphere roasting treatment of step seven is to roast the raw material to be treated at a temperature increasing rate of 4-8℃ / min from room temperature to 500-600℃ for 2-4h.

9. Use according to claim 5, characterized in that, The post-treatment of step eight is to put the raw material to be treated in a 6%-8% hydrogen peroxide aqueous solution and mix for 2-4h at a temperature of 50-70℃. The post-treatment of step eight is to put the raw material to be treated in a 6%-8% hydrogen peroxide aqueous solution and mix for 2-4h at a temperature of 50-70℃.

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