Method for preparing trans-1, 2-dichloroethylene by dehydrochlorination of 1, 1, 2-trichloroethane

Abstract

The invention provides a method for preparing trans-1, 2-dichloroethylene through dehydrochlorination of 1, 1, 2-trichloroethane, and belongs to the technical field of chlorinated olefin preparation. The preparation method provided by the invention comprises the following steps: mixing 1, 1, 2-trichloroethane with protective gas to a certain concentration, and carrying out HCl removal reaction under the action of a catalyst to generate trans-1, 2-dichloroethylene. According to the method for preparing trans-1, 2-dichloroethylene by removing HCl from 1, 1, 2-trichloroethane, provided by the invention, the raw materials are wide in source and cheap, the reaction is simple, and the operation is safe. By optimizing the surface properties and reaction conditions of the niobium oxyphosphate catalyst, the selectivity of t-DCE exceeds 40%, the yield of the target product is remarkably improved, and the requirements of industrial production for high-selectivity products are met.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of chloroolefins, and in particular to a method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane. Background Art

[0002] Trans-1,2-dichloroethylene (t-DCE) is a typical chloroolefin compound. Due to its low toxicity and high volatility, it has a wide range of applications in many fields, including as an organic solvent, a low-temperature extractant, for dissolving waxes, resins, varnishes and thermoplastic materials, and for the extraction of rubber and the separation of fish fat and oils. In addition, the ozone depletion potential of t-DCE is 0 and the global warming potential is 1, which is particularly suitable for the current trend of increasing environmental awareness. Therefore, the development of an efficient t-DCE production process not only has significant economic benefits but also important social value.

[0003] In the prior art, t-DCE is mainly prepared by the selective chlorination reaction of acetylene and chlorine. For example, in a number of patents such as CN200910264425.2, CN2019102054155, CN2021101826254, CN2021101799933, CN2022109894803, CN2023101163005, etc., synthetic processes and preparation systems for the production of t-DCE by acetylene chlorination have been invented. However, since acetylene and chlorine are flammable, explosive and highly toxic chemicals respectively, and the heat release of this reaction (△H = -223.1 kJ / mol) is extremely large, it is easy to cause a reaction runaway, and there are obvious potential safety hazards in the process. In addition, a small amount of t-DCE also comes from the by-product in the preparation of trichloroethylene by dehydrochlorination of tetrachloroethane. However, the selectivity of t-DCE in this process is less than 1%. Therefore, it is particularly important to develop a safer and more efficient new t-DCE production process.

[0004] A feasible alternative route is to prepare t-DCE by dehydrochlorination of 1,1,2-trichloroethane.

[0005]

[0006] As a stable and non-explosive chloroalkane, 1,1,2-trichloroethane has certain advantages. However, in its dehydrochlorination reaction, due to the electron-withdrawing inductive effect of chlorine atoms, the hydrogen atoms at the β-position (the carbon atom adjacent to the C-H bond and C-Cl bond) are active and prone to β-elimination reaction to obtain vinylidene chloride products. On the other hand, due to its lower symmetry and lower free energy of formation, the cis isomer is more stable under the reaction conditions. Thermodynamic driving makes the proportion of cis products higher than that of trans products, and the molar ratio of cis-1,2-dichloroethylene (abbreviated as c-DCE) to t-DCE in the product is usually greater than 3. At the same time, the dichloroethylene in the product is prone to polymerization at high temperature to form tetrachloro-2-butene and higher molecular weight compounds, thus reducing the selectivity of t-DCE. Under the influence of these factors, existing reports show that the selectivity of preparing t-DCE by the dehydrochlorination reaction of 1,1,2-trichloroethane is generally low, with a maximum of only about 20%. Therefore, developing a new process that can significantly improve the selectivity of t-DCE is of great significance for optimizing the existing process, reducing resource waste and environmental risks. Summary of the Invention

[0007] The object of the present invention is to provide a method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane to solve the technical problem of low selectivity of t-DCE in the prior art.

[0008] In order to achieve the above-mentioned invention object, the present invention provides the following technical solutions:

[0009] The present invention provides a method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane, comprising the following steps:

[0010] Mix 1,1,2-trichloroethane with a protective gas to a certain concentration, and carry out dehydrochlorination reaction under the action of a catalyst to generate trans-1,2-dichloroethylene;

[0011] The catalyst is niobium phosphate catalyst.

[0012] Further, the preparation method of the niobium phosphate catalyst comprises the following steps:

[0013] 1) Mix a niobium source, phosphoric acid and water to obtain a mixed solution;

[0014] 2) Adjust the pH of the mixed solution to 3-5 with ammonia water, and precipitate a solid precursor after water bath reaction;

[0015] 3) Disperse the solid precursor in water, add an organic amine and phosphoric acid, adjust the pH to 3-5, and successively carry out a water bath reaction, a crystallization reaction and a calcination on the obtained dispersion to obtain a niobium phosphate catalyst.

[0016] Further, the concentration of 1,1,2-trichloroethane in the protective gas is 10-100%; the protective gas includes nitrogen, argon, helium or water vapor.

[0017] Further, the dehydrochlorination reaction is carried out in a fixed bed reactor, and the mass space velocity of the reaction is 10-100 h -1 , the absolute pressure of the reaction is 1.0-5.0 atm, and the reaction temperature is 200-400 °C.

[0018] Further, in the step 1), the niobium source is a water-soluble niobium compound, and the water-soluble niobium compound includes one or more of niobium oxalate, niobium tartrate, niobium citrate, niobium pentachloride and niobium malate.

[0019] Further, the concentration of niobium in the mixed solution is 0.1-0.5 mol / L, and the molar ratio of niobium to phosphorus is 1:1-3.

[0020] Further, in the step 3), the organic amine includes one or more of hexylamine, heptylamine, octylamine, nonylamine and decylamine.

[0021] Further, in the step 3), the solid content of the dispersion is 5-50 g / L, and the concentration of the organic amine in the dispersion is 0.1-1.0 mol / L.

[0022] Further, in the steps 2) and 3), the temperature of the water bath reaction is independently 30-60 °C, and the time of the water bath reaction is independently 1-4 h.

[0023] Further, in the step 3), the temperature of the crystallization reaction is 50-200 °C, and the time of the crystallization reaction is 12-48 h;

[0024] The temperature of the calcination is 300-600 °C, and the time of the calcination is 2-6 h.

[0025] Advantages of the present invention:

[0026] (1) The method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane provided by the present invention has wide and cheap raw material sources, simple reaction and safe operation. By optimizing the surface properties and reaction conditions of the niobium phosphate catalyst, the selectivity of t-DCE exceeds 40%, significantly improving the yield of the target product and meeting the requirements of industrial production for high-selectivity products.

[0027] (2) P-OH and Nb-OH on the surface of the niobium oxyphosphate catalyst have abundant Brønsted acid sites. Through protonation or hydrogen bonding, these active sites promote the removal of Cl atoms and the formation of double bonds in 1,1,2-trichloroethane molecules. On the one hand, it is beneficial for 1,1,2-trichloroethane to undergo anti-Zaitsev elimination reaction to obtain 1,2-dichloroethylene products; on the other hand, the optimized design of the catalyst makes the reaction path more inclined to generate dichloroethylene with a trans configuration, which is kinetically beneficial to increasing the formation rate of t-DCE, thereby improving the selectivity of t-DCE in the products.

[0028] (3) The specific surface area of the niobium oxyphosphate catalyst synthesized by the template method is 200 - 500 m 2 / g, and the average pore diameter is 1.0 nm. This specific pore size and shape not only provide abundant active sites for the reaction but also play a screening role in the diffusion behavior of reaction intermediates and products. It selectively generates t-DCE, effectively inhibits the polymerization reaction between dichloroethylene molecules, avoids the formation of high-molecular by-products, prevents the loss of target products, and prevents catalyst deactivation. Description of the Drawings

[0029] Figure 1 It is the N2 adsorption-desorption curve graph of the catalyst prepared in Example 4 of the present invention;

[0030] Figure 2 It is the pyridine infrared analysis result graph of the catalyst prepared in Example 4 of the present invention. Detailed Embodiments

[0031] The present invention provides a method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane, which includes the following steps:

[0032] Mix 1,1,2-trichloroethane with a protective gas to a certain concentration, and carry out the de-HCl reaction under the action of a catalyst to generate trans-1,2-dichloroethylene;

[0033] The catalyst is a niobium oxyphosphate catalyst.

[0034] In the present invention, the preparation method of the niobium oxyphosphate catalyst includes the following steps:

[0035] 1) Mix a niobium source, phosphoric acid, and water to obtain a mixed solution;

[0036] 2) Adjust the pH of the mixed solution to 3 - 5 with ammonia water, and precipitate a solid precursor after a water bath reaction;

[0037] 3) Disperse the solid precursor in water, add organic amine and phosphoric acid, adjust the pH to 3 - 5, and successively carry out a water bath reaction, a crystallization reaction, and a calcination on the obtained dispersion to obtain a niobium oxophosphate catalyst.

[0038] In the present invention, the concentration of 1,1,2 - trichloroethane in the protective gas is 10 - 100%, preferably 20 - 80%, and more preferably 30 - 60%; the protective gas includes nitrogen, argon, helium, or water vapor, and is preferably nitrogen.

[0039] In the present invention, the de - HCl reaction is carried out in a fixed - bed reactor, and the mass space velocity of the reaction is 10 - 100 h -1 , preferably 30 - 80 h -1 , more preferably 50 - 70 h -1 ; the absolute pressure of the reaction is 1.0 - 5.0 atm, preferably 2.0 - 4.0 atm, and more preferably 3.0 atm; the reaction temperature is 200 - 400 °C, preferably 220 - 360 °C, and more preferably 240 - 320 °C.

[0040] In the present invention, in step 1), the niobium source is a water - soluble niobium compound, and the water - soluble niobium compound includes one or more of niobium oxalate, niobium tartrate, niobium citrate, niobium pentachloride, and niobium malate, and is preferably one or more of niobium oxalate, niobium tartrate, and niobium citrate.

[0041] In the present invention, the concentration of niobium in the mixed solution is 0.1 - 0.5 mol / L, preferably 0.2 - 0.4 mol / L, and more preferably 0.3 mol / L; the molar ratio of niobium to phosphorus is 1:1 - 3, and is preferably 1:2.

[0042] In the present invention, in step 3), the organic amine includes one or more of hexylamine, heptylamine, octylamine, nonylamine, and decylamine, and is preferably one or more of hexylamine, heptylamine, and octylamine.

[0043] In the present invention, in step 3), the solid content of the dispersion is 5 - 50 g / L, preferably 10 - 40 g / L, and more preferably 20 - 30 g / L; the concentration of the organic amine in the dispersion is 0.1 - 1.0 mol / L, preferably 0.2 - 0.8 mol / L, and more preferably 0.3 - 0.6 mol / L.

[0044] In the present invention, in steps 2) and 3), the temperature of the water bath reaction is independently 30 - 60 °C, preferably 40 - 50 °C, and more preferably 45 °C; the time of the water bath reaction is independently 1 - 4 h, preferably 2 - 3 h.

[0045] In the present invention, in step 3), the temperature of the crystallization reaction is 50 - 200 °C, preferably 80 - 180 °C, and more preferably 100 - 150 °C; the time of the crystallization reaction is 12 - 48 h, preferably 16 - 40 h, and more preferably 20 - 30 h.

[0046] In the present invention, in step 3), the temperature of the calcination is 300 - 600 °C, preferably 350 - 550 °C, and more preferably 400 - 500 °C; the time of the calcination is 2 - 6 h, preferably 3 - 5 h, and more preferably 4 h.

[0047] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0048] Examples 1 - 7

[0049] Examples 1 - 7 relate to a preparation method of a microporous NbOPO4 catalyst, including the following steps:

[0050] A certain amount of niobium pentachloride is dissolved in 100 mL of an aqueous phosphoric acid solution with a concentration of 0.2 mol / L, and the molar concentration of niobium is controlled to be 0.2 mol / L. Then the pH value of the solution is adjusted to 4 with ammonia water, and a precipitation reaction is carried out with stirring at 30 °C for 2 h. The solid precipitate is filtered and washed. Then the solid precipitate is dispersed in water, and a certain amount of decylamine is added thereto. The solid concentration in the dispersion is controlled to be 20 g / L, and the decylamine concentration is 0.5 mol / L. Then the dispersion is transferred to a crystallization kettle with a polytetrafluoroethylene inner lining, crystallized at 100 °C for 24 h, then washed, dried, and calcined at 400 °C for 4 h to obtain a microporous NbOPO4 catalyst.

[0051] Among them, catalyst A is obtained in Example 1, catalyst B is obtained in Example 2, catalyst C is obtained in Example 3, catalyst D is obtained in Example 4, catalyst E is obtained in Example 5, catalyst F is obtained in Example 6, and catalyst G is obtained in Example 7.

[0052] The Nb and phosphoric acid concentrations, precipitation reaction pH, solid concentration in the dispersion, type and concentration of the organic amine, as well as the crystallization reaction and calcination reaction temperatures in each example are shown in Table 1.

[0053] Table 1. Nb concentration, n(Nb) / n(P) molar ratio, precipitation reaction pH, solid concentration in the dispersion, type and concentration of the organic amine, as well as the crystallization reaction and calcination reaction temperatures in each example

[0054]

[0055] Examples 8 - 15

[0056] Examples 8 to 15 relate to a production process of t-DCE, which specifically includes the following steps:

[0057] Use a metering pump to pump trichloroethane into a vaporization chamber at 200 °C for vaporization, then mix it with a certain amount of nitrogen and enter a catalyst bed at a certain temperature for catalytic reaction. The catalyst is the above-mentioned NbOPO4. The volume fraction of trichloroethane, reaction temperature, pressure, mass space velocity and catalyst type in the raw material gas are shown in Table 2.

[0058] Table 2. Volume fraction of trichloroethane, reaction temperature, pressure, space velocity and catalyst type in each example

[0059]

[0060]

[0061] Comparative Example 1

[0062] This comparative example relates to a production process of preparing t-DCE by dehydrochlorination of 1,1,2-trichloroethane. The difference between this comparative example and Example 10 is that no catalyst is added in this comparative example.

[0063] Comparative Example 2

[0064] This comparative example relates to a production process of preparing t-DCE by dehydrochlorination of 1,1,2-trichloroethane. The difference between this comparative example and Example 10 is that the reaction temperature in this comparative example is 150 °C.

[0065] Comparative Example 3

[0066] This comparative example relates to a production process of preparing t-DCE by dehydrochlorination of 1,1,2-trichloroethane. The difference between this comparative example and Example 10 is that the reaction temperature in this comparative example is 450 °C.

[0067] Comparative Examples 4 to 6

[0068] This comparative example relates to a production process of preparing t-DCE by dehydrochlorination of 1,1,2-trichloroethane. The difference between this comparative example and Example 10 is the catalyst. The specific catalysts for each comparative example are shown in Table 3.

[0069] Table 3. Catalyst types for each comparative example

[0070]

[0071]

[0072] Test Example

[0073] The catalysts in Examples 1 - 7 were collected and tested for specific surface area and surface acid amount. The specific surface area test was carried out on a ASAP 2020 physical adsorption instrument produced by Micromeritics, USA. Before the experiment, the sample was pretreated under vacuum at 200 °C for 480 min, and then the N2 adsorption - desorption experiment was carried out at - 196 °C to obtain the N2 adsorption - desorption curve. The BET (Brunauer - Emmett - Teller) method was used to calculate its BET specific surface area.

[0074] The acid amount in the catalyst was measured on an iS50 infrared spectrometer of ThermoFisher, USA. The acid amount and acid strength of the catalyst were determined by pyridine temperature - programmed adsorption - desorption test. First, 20 mg of the catalyst was weighed and pressed into a thin slice. Then the sample was placed in a sample cell made of a quartz tube. The sample cell was evacuated to below 25 Pa and heated to 400 °C at a rate of 10 °C / min and held for 60 min, and then cooled to room temperature. Then, the sample cell was placed in the infrared spectrometer, and the infrared data of the sample was scanned and recorded as the background peak. Then, a certain amount of pyridine vapor was adsorbed on the surface of the catalyst. After that, pyridine was closed, and the sample cell was evacuated to below 25 Pa again and heated to 100 °C for the first desorption, and held for 2 h until the internal pressure no longer changed. The sample cell was placed in the infrared spectrum scanner for the first desorption data scan, using the spectral file scanned in the previous step as the background. After the scan, the sample cell was continuously evacuated and heated to 200 °C to desorb pyridine and held for 2 h until the pressure no longer changed, and then placed in the infrared instrument for the second data scan. The above steps were repeated for pyridine desorption and spectral scanning at 400 °C. After the scan, the Omnic integration software for infrared was used to calculate the integrated area of the peak at 1540 cm -1 in the spectral data. Combining with the mass of the sample slice, the pyridine - adsorbed acid amount at the pyridine desorption temperature of 200 - 400 °C was calculated, which was the B - acid amount of medium strength in the catalyst.

[0075] The present invention Figure 1 shows the N2 adsorption - desorption curve of Example 4, Figure 2 shows the pyridine infrared spectrum of Example 4.

[0076] The products of each example and the comparative example were collected. After the reaction, the mixed gas directly entered the online gas chromatography analysis through a quantitative loop for quantitative analysis. The gas chromatography was equipped with an FID detector and an HP - 5 capillary column. The product composition was qualitatively analyzed on a gas chromatography - mass spectrometry (GC - MS) instrument, and then each product was quantitatively analyzed on the gas chromatography to calculate the selectivity of each product. The products in each example included c - DCE, t - DCE, vinylidene chloride, etc. The conversion rate and selectivity calculation formulas in each example are as follows, where n is the amount of substance, and the data results are recorded in Table 4.

[0077]

[0078]

[0079] Table 4. Specific surface area and acid amount of catalysts in each example

[0080] <![CDATA[Specific surface area (m 2 / g)]]> Amount of B acid (μmol / g) Example 1 287 183 Example 2 362 256 Example 3 318 317 Example 4 439 208 Example 5 335 294 Example 6 408 247 Example 7 379 305

[0081] Table 5. Catalyst performance in each example

[0082]

[0083]

[0084] According to the comparison of Examples 8 - 15, under the action of niobium phosphate catalyst, the conversion rate of trichloroethane exceeds 90%, and the selectivity of t-DCE in the product exceeds 40%. Among them, the volume fraction of trichloroethane, reaction temperature, pressure, space velocity and catalyst type will also affect the yield and recovery rate of dichloroethylene.

[0085] In Comparative Example 1, the conversion rate of trichloroethane is only 4%, and no t-DCE product is detected. While the conversion rate of trichloroethane and the selectivity of t-DCE in Examples 8 - 15 are 90 - 98% and 40 - 60% respectively, indicating that when no catalyst is used, dichloroethylene product cannot be obtained.

[0086] In Comparative Example 2, the conversion rate of trichloroethane is only 5%, and no t-DCE product is detected. While the conversion rate of trichloroethane and the selectivity of t-DCE in Examples 8 - 15 are 90 - 98% and 40 - 60% respectively, indicating that when the reaction temperature is lower than 200 °C, the activity of the catalyst is low and dichloroethylene product cannot be obtained.

[0087] In the product composition of Comparative Example 3, the selectivity of t-DCE is only 4%, indicating that when the reaction temperature is higher than 400 °C, the product selectivity will change significantly and t-DCE cannot be obtained with high selectivity.

[0088] In the product compositions of Comparative Examples 4 - 6, the selectivity of t-DCE does not exceed 5%, indicating that when other common metal catalysts are selected, t-DCE cannot be obtained with high selectivity.

[0089] As can be seen from the above embodiments, the present invention provides a method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane. The method of the present invention uses niobium phosphate with a microporous structure as a catalyst. The abundant Bronsted acid centers on its surface contribute to the highly selective dehydrochlorination reaction of 1,1,2-trichloroethane to produce the target product, trans-1,2-dichloroethylene. At the same time, the suitable pore structure of the catalyst can effectively inhibit the problems of reduced selectivity caused by the polymerization reaction of dichloroethylene and catalyst deactivation. By developing a niobium phosphate catalyst with both abundant Bronsted acid centers and a reasonable pore structure, and combining with optimized reaction conditions, this technology realizes the highly selective dehydrochlorination reaction of 1,1,2-trichloroethane, and the selectivity of trans-1,2-dichloroethylene exceeds 40%. The process of the present invention not only significantly improves the yield of the target product, but also effectively overcomes the problems of side reactions and catalyst deactivation, showing a chemical application prospect with both high efficiency and greenness.

[0090] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane, characterized in that, It includes the following steps: Mix 1,1,2-trichloroethane with a protective gas to a certain concentration, and carry out a dehydrochlorination reaction under the action of a catalyst to produce trans-1,2-dichloroethylene; The catalyst is a niobium phosphate catalyst.

2. The method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane according to claim 1, wherein the preparation method of the niobium phosphate catalyst includes the following steps: 1) Mix a niobium source, phosphoric acid and water to obtain a mixed solution; 2) Adjust the pH of the mixed solution to 3-5 with ammonia water, and precipitate a solid precursor after a water bath reaction; 3) Disperse the solid precursor in water, add an organic amine and phosphoric acid, adjust the pH to 3-5, and sequentially carry out a water bath reaction, a crystallization reaction and a calcination on the obtained dispersion to obtain a niobium phosphate catalyst.

3. The method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane according to claim 1, characterized in that, The concentration of 1,1,2-trichloroethane in the protective gas is 10-100%; the protective gas includes nitrogen, argon, helium or water vapor.

4. The method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane according to claim 1 or 3, characterized in that, The dehydrochlorination reaction is carried out in a fixed-bed reactor, and the mass space velocity of the reaction is 10-100 h -1 , the absolute pressure of the reaction is 1.0-5.0 atm, and the temperature of the reaction is 200-400 °C.

5. The method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane according to claim 2, characterized in that, In step 1), the niobium source is a water-soluble niobium compound, and the water-soluble niobium compound includes one or more of niobium oxalate, niobium tartrate, niobium citrate, niobium pentachloride and niobium malate.

6. The method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane according to claim 2 or 5, characterized in that, The concentration of niobium in the mixed solution is 0.1-0.5 mol / L, and the molar ratio of niobium to phosphorus is 1:1-3.

7. The method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane according to claim 6, characterized in that, In step 3), the organic amine includes one or more of hexylamine, heptylamine, octylamine, nonylamine and decylamine.

8. The method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane according to claim 2 or 5 or 7, characterized in that, In step 3), the solid content of the dispersion is 5-50 g / L, and the concentration of the organic amine in the dispersion is 0.1-1.0 mol / L.

9. The method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane according to claim 8, characterized in that, In steps 2) and 3), the temperature of the water bath reaction is independently 30-60 °C, and the time of the water bath reaction is independently 1-4 h.

10. The method for preparing trans-1,2-dichloroethylene by dehydrochlorination of 1,1,2-trichloroethane according to claim 9, characterized in that, In step 3), the temperature of the crystallization reaction is 50-200 °C, and the time of the crystallization reaction is 12-48 h; The temperature of the calcination is 300-600 °C, and the time of the calcination is 2-6 h.

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