Process for the production of high purity 4,4'-difluorodiphenylmethane by melt crystallization

Abstract

A method for preparing high-purity 4,4'-difluorobenzophenone by melt crystallization relates to a method for preparing 4,4'-difluorobenzophenone. The method uses a crude product with a 4,4'-difluorobenzophenone content of 57% to 95% and high-boiling polymer impurities of no more than 18 wt% as raw material. The raw material is first melted and added to a crystallizer in liquid form, and the first-stage melt crystallization is completed through seed-assisted cooling crystallization and low-temperature sweating purification. Then, the material obtained by the first-stage crystallization is melted and subjected to secondary cooling crystallization and sweating purification to obtain high-purity 4,4'-difluorobenzophenone product. The method does not require organic solvents, has a relatively low operating temperature, has high tolerance to impurities in the raw material, can flexibly control the purity of the product by adjusting the sweating temperature, and can achieve a purity of 98.49% for the target component after single-stage purification of high-purity raw material and a purity of 97.43% for the product after two-stage crystallization purification of low-purity raw material. The equipment used is a conventional layer crystallizer, and the process flow is simple, facilitating scale-up and continuous implementation, and being suitable for industrial purification of 4,4'-difluorobenzophenone.

Description

Technical Field

[0001] This invention relates to a method for preparing 4,4'-difluorodiphenylmethane, specifically to a method for preparing high-purity 4,4'-difluorodiphenylmethane by melt crystallization. Background Technology

[0002] 4,4'-Difluorobenzophenone (4,4'-DFBP) is a core monomer for high-end high-temperature resistant specialty engineering plastics such as polyetheretherketone (PEEK) and polyetherketone (PEK), and is also widely used in the pharmaceutical, pesticide, and fine chemical industries, with market demand continuing to grow. The preparation routes for 4,4'-DFBP are divided into three categories: diazotization oxidation, Friedel-Crafts acylation, and fluorobenzene-formaldehyde condensation oxidation. Among these, the process of condensing fluorobenzene and formaldehyde under organic sulfonic acid catalysis to prepare difluorodiphenylmethane (DFDPM) intermediate, followed by nitric acid oxidation to obtain 4,4'-DFBP, has become a green synthesis route with great industrial potential in recent years due to its inexpensive raw materials, mild reaction conditions, and low emissions. A typical example is the preparation scheme disclosed in international patent WO2007 / 014692 A1.

[0003] During the condensation reaction of fluorobenzene and formaldehyde, two positional isomers are simultaneously generated: the target product 4,4'-difluorodiphenylmethane (4,4'-DFDPM) and the byproduct 2,4'-difluorodiphenylmethane (2,4'-DFDPM). In a conventional reaction system, their mass ratio is approximately 77:23, forming a mixture of DFDPM isomers. When this intermediate mixture is directly introduced into the nitric acid oxidation process, the two isomers are oxidized to generate 4,4'-DFBP and 2,4'-DFBP isomer impurities, respectively. 2,4'-DFBP significantly reduces the thermal stability, mechanical properties, and processing performance of PEEK polymers. High-end polymer-grade 4,4'-DFBP requires a 2,4'-DFBP impurity content of less than 0.1%. Efficiently separating the DFDPM mixed isomers before oxidation to obtain high-purity 4,4'-DFDPM raw material is beneficial for controlling isomer impurities in the final product from the source.

[0004] Existing technologies for separating DFDPM isomer mixtures have significant shortcomings. WO2007 / 014692A1 states that DFDPM mixed isomers can only be separated by vacuum distillation to remove highly polymerized impurities, but cannot separate the 4,4'-DFDPM and 2,4'-DFDPM isomers. Distillation can only purify the material and cannot separate isomers. After distillation, crude DFDPM still retains about 23% of 2,4'-DFDPM, making it unsuitable for direct use in the preparation of high-quality 4,4'-DFBP.

[0005] The industry primarily employs solvent recrystallization for the separation of difluorobenzophenone isomers, as illustrated in patent CN103804164A. This method typically involves multiple recrystallizations using an acetic acid-water mixed solvent after oxidation to obtain crude DFBP, which has several drawbacks: First, subsequent purification after oxidation consumes a large amount of nitric acid oxidant, generating substantial amounts of nitrogen oxides and increasing the load on tail gas recirculation and nitric acid consumption. Second, multiple solvent recrystallizations introduce large amounts of organic solvents, resulting in high energy consumption and significant equipment investment for mother liquor recovery and distillation, while solvent evaporation poses VOC environmental risks. Third, the single-stage recrystallization separation efficiency is limited, requiring more than three crystallizations to reduce isomer impurities to acceptable levels, leading to significant losses in overall product yield and high industrial production costs.

[0006] Vacuum distillation is a conventional method for separating isomers, as illustrated in patent CN216497553U. However, 4,4'-DFDPM and 2,4'-DFDPM have very similar molecular weights and polarities, and their boiling points differ very little. Distillation separation requires the construction of a distillation unit with an ultra-high theoretical plate number, and the system requires strict vacuum control precision. Under high temperature and negative pressure conditions, DFDPM is prone to thermal decomposition and polymerization to generate heavy tar impurities. This not only results in extremely high separation energy consumption but also causes a large loss of intermediates, making large-scale production economically unfeasible and difficult to implement.

[0007] Laboratory separation methods such as column chromatography and adsorption separation are only suitable for the purification of small batches of samples. They have low adsorbent capacity, complex regeneration processes, and small processing capacity, making them unsuitable for continuous large-scale chemical production. Chemical resolution methods require the addition of additional resolution reagents, introducing new impurities, which are difficult to remove subsequently, increasing material and processing costs. They are not suitable for the industrial purification of DFDPM intermediates.

[0008] Existing processes all suffer from core pain points: either they cannot separate isomers at the DFDPM intermediate stage, forcing purification to be delayed until after oxidation, significantly increasing oxidant and solvent consumption and waste treatment costs; or they involve huge investments in separation equipment, high intermediate losses, and low overall yields, making it difficult to balance product purity, energy consumption, and environmental protection requirements. Currently, the industry lacks a solvent-free, low-energy-consumption, continuous, and highly efficient purification technology that directly separates DFDPM isomers in the pre-oxidation process.

[0009] Melt crystallization technology relies on the differences in melting point and crystallization rate of isomers to achieve solvent-free separation. It is widely used in the purification of xylene, dichlorobenzene, and aromatic ketone isomers. The entire process does not introduce organic solvents; separation is achieved solely through temperature-controlled crystallization, sweating, and melting discharge. It boasts advantages such as low energy consumption, no solvent pollution, high product recovery rate, and simple, easily sustainable equipment. However, existing published literature and patents do not yet present technical solutions for separating mixed isomers of 4,4'-DFDPM and 2,4'-DFDPM using melt crystallization, nor do they disclose cooling crystallization and sweating purification process parameters suitable for DFDPM systems.

[0010] In summary, the separation of 4,4′-difluorodiphenylmethane and 2,4′-difluorodiphenylmethane isomers in crude fluorobenzene-formaldehyde condensation is difficult, and traditional distillation processes suffer from high operating temperatures, easy material decomposition, high organic solvent consumption, large waste generation, and low product yield during solvent recrystallization. Existing DFDPM isomer separation technologies suffer from drawbacks such as the inability to perform pre-separation, high solvent consumption, high energy consumption, severe intermediate loss, and high equipment costs, hindering the industrial-scale promotion of the fluorobenzene-formaldehyde condensation oxidation route for preparing polymer-grade 4,4′-DFBP. Developing a solvent-free, low-loss, and highly efficient melt crystallization method for separating mixed DFDPM isomers before the oxidation process has significant industrial application value. Summary of the Invention

[0011] The purpose of this invention is to provide a method for preparing high-purity 4,4'-difluorodiphenylmethane by melt crystallization. This method uses the condensation of fluorobenzene and formaldehyde to obtain a mixture of 4,4'-DFDPM and 2,4'-DFDPM as raw materials. The difference in melting points between the two isomers is utilized to achieve efficient separation through melt crystallization, realizing solvent-free and highly efficient separation. High-purity 4,4'-difluorodiphenylmethane is directly obtained, reducing isomer impurities in the final product 4,4'-DFBP from the source. Simultaneously, it reduces nitric acid consumption, solvent recovery costs, and the amount of waste generated, improving the overall economic efficiency and environmental friendliness of the process.

[0012] The present invention adopts the following technical solution: A method for preparing high-purity 4,4′-difluorodiphenylmethane by secondary melt crystallization includes the following steps: The crude 4,4′-difluorodiphenylmethane solution was purified by two-stage static melt crystallization to obtain 4,4-difluorodiphenylmethane. The process included: S1, Primary Crystallization: Step 1) Liquid phase feeding: Before feeding the liquid crude product, trace amounts of light component fluorobenzene are removed by rotary evaporation. The resulting liquid difluorodiphenylmethane crude product is then fed into the melt crystallizer to the designed liquid level. The crude product contains ≥57% 4,4′-difluorodiphenylmethane by mass and ≤18 wt% total high-boiling-point polymeric impurities. Step 2) Cooling crystallization: Cool at a rate of 0.1-5 °C / min, and control the final cooling temperature between -15 °C and 0 °C. During the cooling process, when the material temperature drops to the range of -5 °C to 5 °C, add 0.3-0.8 wt% seed crystals. After the system reaches the set final temperature, maintain the temperature for 1-2 h to complete the crystallization. Then, discharge the mother liquor enriched with 2,4′-difluorodiphenylmethane and polymerization impurities.

[0013] Step 3) Purification by sweating: The crystals in the crystallizer are slowly heated to induce sweating at a rate of 0.1–5 ℃ / min; the final temperature of sweating is 0–15 ℃, and the sweating is maintained at this temperature for 0.5–2 h. The sweating liquid containing impurities is then discharged. Step 4) Melting and discharging: After sweating is completed, the crystal layer is heated to melt completely, and a first-grade purified 4,4′-difluorodiphenylmethane product is obtained; S2, secondary crystallization: Step 5) Melt the crystals obtained from the primary crystallization, cool them again at a rate of 0.1–5 °C / min, and repeat the crystallization and sweating process for secondary crystallization to obtain pure 4,4′-difluorodiphenylmethane product. The mother liquor generated in the secondary crystallization step is returned to the primary crystallization process for recycling, continuously enriching and recovering the target component.

[0014] Furthermore, in step 2), the cooling rate is 0.1 to 3 °C / min, the final cooling temperature is -15 to 0 °C, and 0.3 to 0.8 wt% of seed crystals are added when the material temperature drops to the range of -5 to 5 °C.

[0015] Furthermore, in step 3), the heating rate is 0.1–3 °C / min, and the final heating temperature is 0–15 °C.

[0016] Furthermore, in step 5), the cooling rate during the cooling crystallization process is 0.1–5 °C / min, the final cooling temperature is -10–5 °C, the heating rate is 0.1–3 °C / min, and the final heating temperature is 5–15 °C.

[0017] Advantages and beneficial effects of the present invention: This invention discloses a method for preparing high-purity 4,4′-difluorodiphenylmethane through two-stage melt crystallization. No organic solvents are added throughout the process. It relies on the difference in melting points between two isomers for layered crystallization and purification through sweating. This method is adaptable to DFDPM crude raw materials with varying impurity contents. The low-temperature operation prevents thermal decomposition of the material and directly improves the purity of 4,4′-difluorodiphenylmethane before oxidation. It controls the 2,4′-difluorodiphenyl ketone impurity in the final product, 4,4′-difluorodiphenyl ketone, from the source, while simultaneously reducing nitric acid consumption, solvent recovery energy consumption, and pollutant emissions. This method combines green environmental protection with industrial economics.

[0018] This invention has the following significant features: Significant purification effect: This invention, through staged cooling crystallization combined with controlled sweating process, can efficiently separate isomers with similar boiling points and high-boiling-point impurities, and can purify crude product with a purity of 68.58% to 97.43%, or increase it from 93.66% to 98.49%, significantly improving product quality.

[0019] 2. High process yield: This invention maintains a high yield while ensuring high purity, and reduces material loss.

[0020] 3. Strong adaptability to raw materials: This invention is applicable to a wide range of raw material purity, from 57.71% crude product to 93.66% refined material. Compared with distillation and solvent recrystallization processes, it has a higher tolerance for raw material purity and does not require pre-deep removal of heavy components.

[0021] 4. Green and pollution-free: This invention uses melt crystallization technology, which requires no organic solvents or additives throughout the process. It separates isomers before the oxidation process, reducing the amount of oxidant used. Low-temperature operation inhibits the thermal decomposition of materials, significantly reducing material loss compared to high-temperature distillation.

[0022] 5. Simple equipment: No high theoretical plate vacuum distillation equipment is required; conventional tubular layer crystallizers can complete the separation. The process temperature control scheme is simple, the materials can be recycled, and the construction investment and operating energy consumption are lower than those of distillation and solvent recrystallization processes. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the process flow of the present invention. Detailed Implementation

[0024] The following are specific examples of embodiments of the melt crystallization method for 4,4′-difluorodiphenylmethane, but they should not be construed as limiting the scope of the above-mentioned subject matter of this invention to the following embodiments. All technologies implemented based on the above content of this invention fall within the scope of this invention.

[0025] Experimental instruments and testing conditions for this invention: A static tubular crystallizer was used to conduct layer-by-layer melt crystallization experiments. The crude product was placed inside the tube for the experiment, and the outer jacket was connected to a constant-temperature bath to control the temperature during the crystallization and sweating stages. The inner diameter of the inner tube of the crystallizer was 15 mm, the outer diameter of the outer tube was 40 mm, and the effective crystallization section length was 250 mm.

[0026] The content of isomers in the material was quantitatively analyzed using a Shimadzu GC-2014 gas chromatograph with a flame ionization detector (FID). An SK-5 capillary column with dimensions of 0.25 mm × 0.25 μm × 30 m was used. The column temperature program was set as follows: 70℃ for 0.5 min, increased to 100℃ at 10℃ / min and held for 14 min, then increased to 250℃ at 20℃ / min and held for 10 min. The vaporization chamber temperature was 230℃, the FID detector temperature was 270℃, and the split ratio was 20:1. The hydrogen flow rate was 30 mL / min, the air flow rate was 400 mL / min, and the nitrogen carrier gas flow rate was 30 mL / min. Ethyl acetate was used as the sample dilution solvent.

[0027] In the following examples, the yield of primary crystallization and the yield of secondary crystallization are both total yields, calculated based on the mass of 4,4′-difluorodiphenylmethane in the original crude product. Example 1

[0028] 240.61 g of crude product was placed inside the inner tube, containing 57.71% 4,4′-difluorodiphenylmethane, 24.58% 2,4′-difluorodiphenylmethane, and 17.71% high-boiling-point polymeric impurities. The product was cooled at a rate of 1 °C / min, with 0.8 wt% seed crystals added. The final cooling temperature was -10 °C, and the mixture was held at this temperature for 2 hours. The mother liquor containing enriched impurities was then discharged. The material was then heated to 0 °C at a rate of 0.5 °C / min for a sweating treatment. The sweating temperature was maintained for 1 hour, and the sweating liquid containing isomers and heavy components was discharged. After melting, the product was discharged, yielding 125.32 g of primary product with a 4,4′-difluorodiphenylmethane purity of 87.59%. The yield of 4,4′-difluorodiphenylmethane from a single crystallization was 79.05%.

[0029] The crystals obtained from the first crystallization were remelted, and the crystallization and sweating processes were repeated for secondary crystallization. The material was cooled at a rate of 2℃ / min, with 0.8 wt% seed crystals added. The final cooling temperature was -5℃, and the mixture was held at this temperature for 2 hours. The mother liquor enriched with impurities was then discharged. The material was then heated to 5℃ at a rate of 1℃ / min for sweating treatment, maintaining this temperature for 1 hour. The sweating liquid was then discharged. After melting, the product was discharged, yielding 114.20 g of 4,4′-difluorodiphenylmethane with a purity of 97.36%. Based on the total amount of 4,4′-difluorodiphenylmethane in the initial crude raw material, the overall yield of the two-stage melt crystallization was 75.13%. If the secondary mother liquor is returned to the primary crystallization process for recycling, the target component can be continuously enriched and recovered. Under steady-state continuous operation conditions, the overall process yield can be increased to 78.28%.

[0030] Example 2 16 g of crude product was placed inside the inner tube, containing 68.58% 4,4′-difluorodiphenylmethane, 29.58% 2,4′-difluorodiphenylmethane, and 1.84% high-boiling-point polymeric impurities. The product was cooled at a rate of 0.5 °C / min, and 0.31 wt% seed crystals were added. The final cooling temperature was -15 °C, and the mixture was held at this temperature for 2 hours. The mother liquor enriched with impurities was then discharged. The material was then heated to 3 °C at a rate of 0.5 °C / min for a sweating treatment, which was maintained at this temperature for 1 hour. The sweating liquid was then discharged. After melting, the product was discharged, yielding 9.49 g of primary crystallized product with a 4,4′-difluorodiphenylmethane purity of 90.71% and a primary crystallization yield of 78.45%.

[0031] The crystals obtained from the first crystallization were melted again, and the crystallization and sweating processes were repeated for secondary crystallization: the temperature was lowered at a rate of 1℃ / min, 0.32 wt% of seed crystals were added, the final temperature was -8℃, and the temperature was maintained for 2 h, after which the mother liquor enriched with impurities was discharged; then the material was heated to 7℃ at a rate of 0.5℃ / min for sweating treatment, and the final temperature was maintained for 1 h, after which the sweating liquid was discharged; after heating and melting, the material was discharged, yielding 8.67 g of the secondary crystallized product with a 4,4′-difluorodiphenylmethane purity of 98.36%. Based on the total amount of 4,4′-difluorodiphenylmethane in the initial crude raw material, the single-pass total yield of the two-stage melt crystallization was 76.13%.

[0032] If the secondary mother liquor is returned to the primary crystallization process for recycling, the target component can be continuously enriched and recovered. Under steady-state continuous operation conditions, the overall process yield can be increased to 79.91%.

[0033] Example 3 16 g of crude product was placed inside the inner tube, containing 68.58% 4,4′-difluorodiphenylmethane, 29.58% 2,4′-difluorodiphenylmethane, and 1.84% high-boiling-point polymeric impurities. The product was cooled at a rate of 0.3℃ / min, and 0.31 wt% seed crystals were added. The final cooling temperature was -15℃, and the mixture was held at this temperature for 2 hours. The mother liquor enriched with impurities was then discharged. The material was then heated to 4℃ at a rate of 0.5℃ / min for a sweating treatment, which was maintained at this temperature for 1 hour. The sweating liquid was then discharged. After melting, the product was discharged, yielding 9.55 g of primary crystallized product with a 4,4′-difluorodiphenylmethane purity of 91.07% and a primary crystallization yield of 79.26%.

[0034] The crystals obtained from the first crystallization were melted again, and the crystallization and sweating processes were repeated for secondary crystallization: the temperature was lowered at a rate of 0.3℃ / min, 0.31 wt% of seed crystals were added, the final temperature was -8℃, and the temperature was held for 2 h, after which the mother liquor enriched with impurities was discharged; then the material was heated to 11℃ at a rate of 0.5℃ / min for sweating treatment, and the final temperature was held constant for 1 h, after which the sweating liquid was discharged; after heating and melting, the material was discharged, yielding 8.75 g of the secondary crystallized product with a 4,4′-difluorodiphenylmethane purity of 98.73%. Based on the total amount of 4,4′-difluorodiphenylmethane in the initial crude raw material, the single-pass total yield of the two-stage melt crystallization was 77.69%.

[0035] If the secondary mother liquor is returned to the primary crystallization process for recycling, the target component can be continuously enriched and recovered. Under steady-state continuous operation conditions, the overall process yield can be increased to 79.11%.

[0036] Example 4 30.55 g of crude product was placed inside the inner tube, containing 66.45% 4,4′-difluorodiphenylmethane, 28.81% 2,4′-difluorodiphenylmethane, and 4.74% high-boiling-point polymeric impurities. The product was cooled at a rate of 5 °C / min, with 0.33 wt% seed crystals added. The final cooling temperature was 0 °C, and the mixture was held at this temperature for 2 hours. The mother liquor enriched with impurities was then discharged. The material was then heated to 10 °C at a rate of 5 °C / min for a sweating treatment. The sweating temperature was maintained for 1 hour, and the sweating liquid was discharged. After melting, the product was discharged, yielding 16.54 g of the product with a 4,4′-difluorodiphenylmethane purity of 88.56% and a primary crystallization yield of 72.15%.

[0037] The crystals obtained from the first crystallization were remelted, and the crystallization and sweating processes were repeated for secondary crystallization. The material was cooled at a rate of 3℃ / min, with 0.30 wt% seed crystals added. The final cooling temperature was 5℃, and the mixture was held at this temperature for 2 hours. The mother liquor enriched with impurities was then discharged. The material was then heated to 15℃ at a rate of 3℃ / min for sweating treatment. The final sweating temperature was held constant for 2 hours, and the sweating liquid was discharged. After melting, the material was discharged, yielding 13.57 g of the secondary crystallized product with a 4,4′-difluorodiphenylmethane purity of 97.77%. Based on the total amount of 4,4′-difluorodiphenylmethane in the initial crude raw material, the single-pass overall yield of the two-stage melt crystallization was 62.01%.

[0038] If the secondary mother liquor is returned to the primary crystallization process for recycling, the target component can be continuously enriched and recovered. Under steady-state continuous operation conditions, the overall process yield can be increased to 71.23%.

[0039] Example 5 11.96 g of crude product was placed inside the inner tube. The crude product contained 93.66% 4,4′-difluorodiphenylmethane, 6.81% 2,4′-difluorodiphenylmethane, and 0.53% high-boiling-point polymeric impurities. The product was cooled at a rate of 1 °C / min, and 0.42 wt% seed crystals were added. The final cooling temperature was -13 °C, and the mixture was held at this temperature for 2 hours. The mother liquor enriched with impurities was then discharged. The material was then heated to 8 °C at a rate of 0.5 °C / min for a sweating treatment. The sweating temperature was maintained for 1 hour, and the sweating liquid was discharged. After melting, the product was discharged, yielding 11.07 g of the product with a 4,4′-difluorodiphenylmethane purity of 98.49% and a primary crystallization yield of 97.33%.

Claims

1. A method for preparing high-purity 4,4'-difluorodiphenylmethane by melt crystallization, characterized in that, The method includes the following preparation steps: S1, Primary Crystallization: Step 1) Liquid phase feeding: Before feeding the liquid crude product, trace amounts of light component fluorobenzene are removed by rotary evaporation. The resulting liquid difluorodiphenylmethane crude product is then fed into the melt crystallizer to the designed liquid level. The crude product contains ≥57% 4,4′-difluorodiphenylmethane by mass and ≤18 wt% total high-boiling-point polymeric impurities. Step 2) Cooling crystallization: Cool at a rate of 0.1-5 °C / min, and control the final cooling temperature between -15 °C and 0 °C; during the cooling process, add 0.3-0.8 wt% seed crystals when the material temperature drops to the range of -5-5 °C; after the system reaches the set final temperature, maintain the temperature for 1-2 h to complete the crystallization, and then discharge the mother liquor enriched with 2,4′-difluorodiphenylmethane and polymerization impurities. Step 3) Purification by sweating: The crystals in the crystallizer are slowly heated to induce sweating at a rate of 0.1–5 ℃ / min; the final temperature of sweating is 0–15 ℃, and the sweating is maintained at this temperature for 0.5–2 h. The sweating liquid containing impurities is then discharged. Step 4) Melting and discharging: After sweating is completed, the crystal layer is heated to melt completely, and a first-grade purified 4,4′-difluorodiphenylmethane product is obtained; S2, secondary crystallization: Step 5) Melt the crystals obtained from the primary crystallization, cool them again at a rate of 0.1–5 °C / min, and repeat the crystallization and sweating process to perform secondary crystallization, thereby obtaining a pure 4,4′-difluorodiphenylmethane product. The mother liquor produced in the secondary crystallization step is returned to the primary crystallization process for recycling, continuously enriching and recovering the target components.

2. The method for preparing high-purity 4,4'-difluorodiphenylmethane by melt crystallization according to claim 1, characterized in that, In step 2), the cooling rate is 0.1 to 3℃ / min, the final cooling temperature is -15 to 0℃, and 0.3-0.8 wt% of seed crystals are added when the material temperature drops to the range of -5 to 5℃.

3. The method for preparing high-purity 4,4'-difluorodiphenylmethane by melt crystallization according to claim 1, characterized in that, In step 3), the heating rate is 0.1–3 °C / min, and the final heating temperature is 0–15 °C.

4. The method for preparing high-purity 4,4'-difluorodiphenylmethane by melt crystallization according to claim 1, characterized in that, In step 5), the cooling rate during the cooling crystallization process is 0.1–3℃ / min, the final cooling temperature is -10–5℃, the heating rate is 0.1–3℃ / min, and the final heating temperature is 5–15℃.

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