A method for synthesizing lactide by dehydration cyclization of lactic acid

Abstract

The present invention relates to a method for synthesizing lactide by dehydration cyclization of lactic acid, which comprises mixing a lactic acid solution with an organic solvent, adding a Sn-SPP molecular sieve catalyst, carrying out a stirring reflux reaction at normal pressure and a certain temperature, adding water after the reaction is completed to separate the two phases, and distilling the organic phase under reduced pressure to obtain a lactide product; in the Sn-SPP molecular sieve catalyst, Sn atoms are components of the molecular sieve framework, the Si / Sn atomic ratio is 60-500, and the SnO2 content is 0.5%-4.0%. The present invention adopts heteroatom Sn-SPP molecular sieve as a catalyst for synthesizing lactide by dehydration cyclization of lactic acid, and has the advantages of simple process flow, mild reaction conditions, and high optical purity of lactide.

Description

Technical Field

[0001] The invention belongs to the field of biodegradable material synthesis, and particularly relates to a method for synthesizing lactide by dehydration cyclization of lactic acid. Background Art

[0002] Polylactic acid is a new type of biodegradable material with good biodegradability. After use, it can be completely degraded by microorganisms in nature, and finally produces carbon dioxide and water, which does not pollute the environment. This is very beneficial to environmental protection and is a recognized environmentally friendly material. Commercial high molecular weight polylactic acid is generally prepared by catalytic ring-opening polymerization using lactide as a monomer, so the synthesis of lactide is of great significance to the production of polylactic acid.

[0003] At present, the synthesis of lactide is mainly carried out through a two-step method: in the first step, the lactic acid molecules are first dehydrated to lactic acid oligomers with a certain molecular weight; in the second step, the lactic acid oligomers are cracked under a depolymerization catalyst, high temperature, and high vacuum conditions to obtain lactide. CN102675277A discloses a method for preparing lactide by reduced pressure catalysis, comprising: (1) first removing free water from lactic acid at 75-90°C and 0.08-0.09MPa vacuum for 1-2 hours; (2) adding a catalyst and a surfactant, wherein the catalyst is zinc oxide with a percentage of 0.5-1% of the reactant, stannous octoate with a volume ratio of 0.5-1% of the reactant, and the surfactant is Tween 80 with a volume ratio of 0.5-1% of the reactant or sodium dodecyl sulfate with a mass ratio of 0.5-1% of the reactant; (3) then keeping the temperature at 0.08-0.09MPa vacuum and 125-135°C to remove bound water for 5 hours; (4) stirring and heating to 165-200°C, distilling and collecting lactide, with a crude yield of 83-91%. The two-step lactide synthesis process has the advantages of high lactide product yield and high process maturity, but it also has disadvantages such as lengthy process route, relatively harsh reaction conditions (high temperature, high vacuum), low raw material utilization rate, and low product optical purity.

[0004] In recent years, the process of preparing lactide by direct dehydration and cyclization of lactic acid has attracted wide attention from researchers. CN108610323A discloses a method for preparing lactide: lactic acid is heated and reacted under reduced pressure in the presence of H-Beta molecular sieve to obtain lactide; the reaction pressure is 50-1500Pa, and the reaction temperature is 120-150°C. This invention directly completes the synthesis of lactide from lactic acid to obtain lactide with high optical purity (>96%), overcoming the shortcomings of the two-step process with long process flow and high reaction temperature. However, there are also problems such as the reaction process requires high vacuum conditions, the one-time yield of lactide is low, and the aluminum element in the framework of the silicon-aluminum H-Beta molecular sieve is easily eroded by the lactic acid raw material, causing the molecular sieve framework structure to collapse.

[0005] CN112028869A discloses a method for synthesizing lactide in one step: lactate is used as raw material, tin-containing silicon-aluminum molecular sieve is used as catalyst, and lactide is synthesized in one step in gas phase at normal pressure. The lactate conversion rate can reach more than 98% in the reaction, and the lactide selectivity is greater than 92%. However, the lactate condensation process and the lactide depolymerization process are mutually reversible reactions, and there is an equilibrium limit of 50% conversion rate for lactate. The patent does not mention what method is used to solve the thermodynamic equilibrium limit problem in the lactate condensation reaction process to generate lactide. Moreover, since lactide generally undergoes severe chiral transformation after exceeding 200°C, there is a problem of lactide racemization at higher temperatures. In addition, due to the use of silicon-aluminum molecular sieve catalysts, there is also the problem of acid corrosion in lactic acid raw materials. Summary of the invention

[0006] In view of the shortcomings of the existing one-step method for synthesizing lactide, the present invention provides a method for synthesizing lactide by direct dehydration and cyclization of lactic acid. The present invention uses heteroatom Sn-SPP molecular sieve as a catalyst for synthesizing lactide by dehydration and cyclization of lactic acid, which has the advantages of simple process flow, mild reaction conditions, and high optical purity of lactide.

[0007] The method for synthesizing lactide by dehydration cyclization of lactic acid provided by the present invention comprises the following contents:

[0008] The lactic acid solution is mixed with a benzene organic solvent, and a Sn-SPP molecular sieve catalyst is added to carry out a stirring reflux reaction at normal pressure and a certain temperature. After the reaction is completed, water is added to separate the two phases, and the organic phase is subjected to reduced pressure distillation to obtain a lactide product. In the Sn-SPP molecular sieve catalyst, Sn atoms are components of the molecular sieve skeleton, the Si / Sn atomic ratio is 60-500, and the SnO2 content is 0.5%-4.0% based on the catalyst mass.

[0009] In the present invention, the preparation method of the Sn-SPP molecular sieve is: silicon source, tetrabutylphosphine hydroxide, ethanol, water and tin source are mixed in proportion, standing and crystallizing at a certain temperature, and the product is separated, washed, dried and calcined to obtain the Sn-SPP molecular sieve.

[0010] In the above catalyst preparation method, the silicon source is at least one of ethyl orthosilicate, methyl orthosilicate, etc., preferably ethyl orthosilicate.

[0011] In the above catalyst preparation method, the tin source is at least one of tin tetrachloride pentahydrate, stannous octoate, etc., preferably tin tetrachloride pentahydrate.

[0012] In the catalyst preparation method, the molar ratio of the silicon source, tetrabutylphosphine hydroxide, ethanol, water and tin source is 1:0.2-0.4:3-5:10-30:0.004-0.024.

[0013] In the above catalyst preparation method, the mixed materials are placed at 130-150° C. for crystallization for 2-5 days. The product can be separated by centrifugation, filtration, etc., washed with water until neutral, dried at 80-100° C. for 2-12 hours, and calcined at 500-550° C. for 3-5 hours to obtain Sn-SPP molecular sieve.

[0014] In the above catalyst preparation method, the prepared Sn-SPP molecular sieve has an MFI type topological structure, the particle size of the Sn-SPP molecular sieve is 200-500nm, and the BET specific surface area is 300-500m 2 / g, pore volume 0.25-0.4 cm 3 / g, and the average pore size is 2.5-7nm.

[0015] In the synthesis method of the present invention, the lactic acid is at least one of L-lactic acid, D-lactic acid, etc., the optical purity of the lactic acid is ≥99.5%, and the mass concentration of the lactic acid solution is 20%-80%.

[0016] In the synthesis method of the present invention, the benzene series organic solvent can be at least one of benzene, toluene, ethylbenzene and xylene, preferably toluene.

[0017] In the synthesis method of the present invention, the mass ratio of the catalyst to lactic acid is 1:0.5-5, preferably 1:2-4.

[0018] In the synthesis method of the present invention, the solid-liquid ratio of the catalyst to the organic solvent is 1g:5-100mL, preferably 1g:10-30mL.

[0019] In the synthesis method of the present invention, the reaction temperature is 100-170°C, the stirring speed is 100-1000r / min, and the reaction time is 2-6h. During the reflux reaction, free water in the lactic acid solution and water molecules generated by the condensation reaction of lactic acid molecules in the reaction system are continuously removed.

[0020] In the synthesis method of the present invention, after the reaction is completed, water is added to the reaction system to separate the aqueous phase and the organic phase, wherein the volume ratio of water to the organic solvent is 1-5:1, preferably 1-2:1.

[0021] In the synthesis method of the present invention, the organic phase obtained after the two phases are separated is subjected to reduced pressure distillation under the conditions of 100-120° C. and 1-5 kPa absolute pressure to obtain a lactide product, and the organic solvent is recovered for repeated use.

[0022] In the synthesis method of the present invention, the aqueous phase obtained after the two phases are separated is stirred and reacted at 100-150° C. for 3-6 hours, and the stirring speed is 100-1000 r / min. The Sn-SPP molecular sieve is filtered and separated to be recovered, and the filtrate containing lactic acid is recovered and reused; the recovered catalyst is washed with water for multiple times, dried at 80-100° C. for 2-12 hours, and then reused.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) Sn-SPP heteroatom molecular sieve is used as a catalyst for the direct dehydration cyclization of lactic acid to prepare lactide. Lactide can be synthesized in one step at normal pressure and low temperature, effectively avoiding the racemization of lactide at high reaction temperature. The product has high optical purity. It also avoids high vacuum operating conditions and reduces operating costs. It has the advantages of simple process flow, mild reaction conditions, high optical purity of lactide, and good selectivity.

[0025] (2) The use of Sn-SPP molecular sieve avoids the corrosion and removal of aluminum elements in the silicon-aluminum molecular sieve framework by lactic acid raw materials at high reaction temperatures, improves the stability of the molecular sieve framework structure, avoids the loss of metal elements, and ensures the reuse of the catalyst.

[0026] (3) Taking advantage of the difference in solubility of lactic acid, lactic acid oligomers and lactide in organic solvents, a benzene-based solvent is selected to dissolve the lactide generated by the reaction and then separate it. On the one hand, it can effectively avoid side reactions such as hydrolysis and deep condensation of lactide in the reaction system. On the other hand, removing lactide from the reaction system can effectively break the chemical equilibrium of the reversible reactions of lactic acid condensation and lactide hydrolysis, promote the occurrence of lactic acid condensation reaction, and effectively solve the problem of thermodynamic equilibrium limitation.

[0027] (4) The Sn-SPP heteroatom molecular sieve is synthesized by in-situ hydrothermal crystallization, making Sn a solid component of the molecular sieve skeleton. This avoids the metal particle shedding that may occur during the use of metal-loaded molecular sieves prepared by the traditional impregnation method, thereby ensuring the reusability and high efficiency of the molecular sieve catalyst.

[0028] (5) The organic solvent, molecular sieve catalyst, etc. of the present invention are easy to recycle and reuse, the utilization rate of raw materials is high, the equipment is simple and the cost is low, and it has good prospects for industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the X-ray diffraction spectrum (XRD) of the Sn-SPP molecular sieve synthesized in Example 1. DETAILED DESCRIPTION

[0030] The method and effects of the present invention are further described in detail below in conjunction with the examples. The examples are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes, but the protection scope of the present invention is not limited to the following examples.

[0031] The experimental methods in the following examples, unless otherwise specified, are all conventional methods in the art. The experimental materials used in the following examples, unless otherwise specified, were purchased from conventional biochemical reagent stores.

[0032] The catalyst composition of the present invention is detected by ICPS-7510 plasma emission spectrometer; the lactic acid and lactide contents in the reaction product are detected by Agilent 1260 liquid chromatography external standard method, and the optical purity of lactide is determined by AutopolVI polarimeter.

[0033] Example 1

[0034] (1) Tetraethyl orthosilicate, tetrabutylphosphine hydroxide, ethanol, water and tin tetrachloride pentahydrate were mixed in a molar ratio of 1:0.3:4:20:0.015, and then allowed to stand for crystallization at 140°C for 3 days. The product particles were centrifuged at 8000 r / min for 10 min, washed with water until neutral, dried at 100°C for 12 h, and calcined at 550°C for 3 h to obtain Sn-SPP molecular sieve. Figure 1 The XRD spectrum of the Sn-SPP molecular sieve shows a typical MFI molecular sieve framework structure, and the diffraction peak of SnO2 particles does not appear in the spectrum, indicating that Sn exists in the Sn-SPP molecular sieve in the form of a framework. According to the test, the particle size of the Sn-SPP molecular sieve is about 300nm, the SnO2 content in the Sn-SPP molecular sieve is 2.51%, the Si / Sn atomic ratio is 97, and the BET specific surface area is 320 m 2 / g, pore volume 0.28 cm 3 / g, and the average pore size is 2.9nm.

[0035] (2) A 50% mass concentration L-lactic acid solution (optical purity 99.5%) and toluene were mixed, and the Sn-SPP molecular sieve catalyst prepared in step (1) was added, wherein the mass ratio of the catalyst to lactic acid was 1:2, and the solid-liquid ratio of the catalyst to toluene was 1 g:20 mL. The mixture was stirred and refluxed at 140°C and 500 r / min for 3 h. During the reaction, free water and reaction water in the lactic acid solution were removed. After the reaction was completed, water was added at a volume ratio of water to organic solvent of 1:1 to separate the two phases. The two phases were separated by a separatory funnel and chromatographic analysis was performed respectively. The toluene phase was distilled under reduced pressure at 100°C and 3 kPa absolute pressure to obtain the L-lactide product, and toluene was recovered at the same time. The aqueous phase was treated at 120°C and 300 r / min stirring conditions for 4 h, and then the catalyst and lactic acid recovery liquid were separated by filtration. The recovered catalyst was washed with water several times, dried at 100°C for 8 h, and then used for standby. The synthesis effect of lactide is shown in Table 1.

[0036] Example 2

[0037] (1) Ethyl orthosilicate, tetrabutylphosphine hydroxide, ethanol, water and tin tetrachloride pentahydrate were mixed in a molar ratio of 1:0.2:3:10:0.006, and then allowed to stand for crystallization at 130°C for 2 days. The product particles were centrifuged at 10,000 r / min for 10 min, washed with water until neutral, dried at 100°C for 12 h, and calcined at 550°C for 3 h to obtain Sn-SPP molecular sieve. The obtained Sn-SPP molecular sieve has a particle size of about 200 nm, and no diffraction peak of SnO2 particles appears in the XRD spectrum, indicating that Sn exists in the Sn-SPP molecular sieve in the form of a skeleton composition. The SnO2 content in the Sn-SPP molecular sieve was detected to be 0.82%, and the Si / Sn atomic ratio was 302; the BET specific surface area of ​​the Sn-SPP molecular sieve was 345 m 2 / g, pore volume 0.30 cm 3 / g, and the average pore size is 3.1nm.

[0038] (2) An 80% L-lactic acid solution (optical purity 99.5%) and toluene were mixed, and the Sn-SPP molecular sieve catalyst prepared in step (1) was added, wherein the mass ratio of the catalyst to the lactic acid was 1:1, and the solid-liquid ratio of the catalyst to toluene was 1 g:10 mL. The mixture was stirred and refluxed at 110°C and 100 r / min for 6 h, and free water and reaction water in the lactic acid solution were removed during the reaction. After the reaction was completed, water was added at a volume ratio of water to the organic solvent of 1:1 to separate the two phases, and the two phases were separated by a separatory funnel and chromatographic analysis was performed respectively. The toluene phase was distilled under reduced pressure at 100°C and 3 kPa absolute pressure to obtain the L-lactide product, and toluene was recovered at the same time; the aqueous phase was treated at 100°C and 100 r / min for 3 h, and then the catalyst and lactic acid recovery liquid were separated and recovered by filtration. The synthesis effect of lactide is shown in Table 1.

[0039] Example 3

[0040] (1) Ethyl orthosilicate, tetrabutylphosphine hydroxide, ethanol, water and tin tetrachloride pentahydrate were mixed in a molar ratio of 1:0.4:5:30:0.02, and then allowed to stand for crystallization at 150°C for 5 days. The product particles were centrifuged at 5000r / min for 10min, washed with water until neutral, dried at 100°C for 12h, and calcined at 550°C for 3h to obtain Sn-SPP molecular sieve. The particle size of the obtained Sn-SPP molecular sieve was about 500nm, and the diffraction peak of SnO2 particles did not appear in the XRD spectrum, indicating that Sn existed in the Sn-SPP molecular sieve in the form of a skeleton composition. The SnO2 content in the Sn-SPP molecular sieve was detected to be 3.74%, and the Si / Sn atomic ratio was 64. The BET specific surface area of ​​the Sn-SPP molecular sieve was 305m 2 / g, pore volume 0.26 cm 3 / g, and the average pore size is 2.6nm.

[0041] A 20% L-lactic acid solution (optical purity 99.5%) and toluene were mixed, and the Sn-SPP molecular sieve catalyst prepared in step (1) was added, wherein the mass ratio of the catalyst to the lactic acid was 1:3, and the solid-liquid ratio of the catalyst to toluene was 1 g:30 mL. The mixture was stirred and refluxed at 160°C and 1000 r / min for 2 h, and free water and reaction water in the lactic acid solution were removed during the reaction. After the reaction was completed, water was added at a volume ratio of water to organic solvent of 2:1 to separate the two phases, and the two phases were separated by a separatory funnel and chromatographic analysis was performed respectively. The toluene phase was distilled under reduced pressure at 100°C and 3 kPa absolute pressure to obtain the L-lactide product, and toluene was recovered at the same time; the aqueous phase was treated at 150°C and 1000 r / min for 6 h, and then the catalyst and lactic acid recovery liquid were filtered and separated. The synthesis effect of lactide is shown in Table 1.

[0042] Example 4

[0043] The reaction process of preparing lactide by dehydrating lactic acid is the same as that in Example 1, except that the lactic acid in step (2) is D-lactic acid, the mass concentration of the D-lactic acid solution is 80%, and the optical purity is 99.5%. The synthetic effect of lactide is shown in Table 1.

[0044] Example 5

[0045] The reaction process of preparing lactide by dehydrating lactic acid is the same as that in Example 1, except that the organic solvent in step (2) is benzene. The synthetic effect of lactide is shown in Table 1.

[0046] Example 6

[0047] The reaction process of preparing lactide by dehydrating lactic acid is the same as that in Example 1, except that the organic solvent in step (2) is ethylbenzene. The synthetic effect of lactide is shown in Table 1.

[0048] Example 7

[0049] The reaction process of preparing lactide by dehydrating lactic acid is the same as that in Example 1, except that the organic solvent in step (2) is p-xylene. The synthetic effect of lactide is shown in Table 1.

[0050] Example 8

[0051] The reaction process of preparing lactide by dehydrating lactic acid is the same as that in Example 1, except that the organic solvent in step (2) is recycled toluene. The synthetic effect of lactide is shown in Table 1.

[0052] Example 9

[0053] The reaction process of preparing lactide by dehydrating lactic acid is the same as that in Example 1, except that the L-lactic acid raw material described in step (2) is the L-lactic acid recovery liquid obtained in Example 1. The synthesis effect of lactide is shown in Table 1.

[0054] Comparative Example 1

[0055] The same as Example 1, except that: H-Beta molecular sieve (silicon to aluminum ratio 25, Al content 6.37% based on metal oxide mass) was used instead of Sn-SPP molecular sieve in Example 1 as catalyst. The synthesis effect of lactide is shown in Table 1.

[0056] Comparative Example 2

[0057] The same as Example 1, except that: Sn-SPP molecular sieve (SnO2 content 2.44% by mass) prepared by traditional liquid phase impregnation method is used to replace the Sn-SPP molecular sieve catalyst in Example 1. The synthesis effect of lactide is shown in Table 1.

[0058] Comparative Example 3

[0059] The same as Example 1, except that the reaction temperature of the lactic acid dehydration process for preparing lactide in step (2) is 200°C. The synthetic effect of lactide is shown in Table 1.

[0060] Comparative Example 4

[0061] Same as Example 1, except that cyclohexane is used instead of toluene in step (2). The synthesis effect of lactide is shown in Table 1.

[0062] Table 1 Test results of embodiments and comparative examples

[0063]

[0064] It can be seen from the results in the above-mentioned embodiments and comparative examples that the Sn-SPP molecular sieve of the present invention is used as a catalyst for the direct dehydration cyclization of lactic acid to prepare lactide, which effectively avoids the corrosion of the lactic acid raw material to the skeleton aluminum in the traditional silicon-aluminum molecular sieve catalyst under high temperature conditions, and improves the stability of the molecular sieve catalyst. In addition, the present invention uses a benzene organic solvent as a protective agent for lactide, which reduces side reactions such as lactide hydrolysis and deep condensation on the one hand, and on the other hand, the real-time removal of lactide effectively solves the thermodynamic equilibrium limitation problem in the process of preparing lactide by lactic acid condensation. At the same time, the lower reaction temperature of the present invention effectively alleviates the racemization problem of the lactide product. In the comparative example that does not adopt the complete technical solution of the present invention, the various beneficial effects described in this application cannot be achieved.

Claims

1. A method for synthesizing lactide by dehydration cyclization of lactic acid, characterized in that The method comprises the following contents: mixing a lactic acid solution with a benzene organic solvent, adding a Sn-SPP molecular sieve catalyst, carrying out a stirring reflux reaction at normal pressure and a certain temperature, adding water after the reaction is completed to separate the two phases, and obtaining a lactide product by vacuum distillation of the organic phase; in the Sn-SPP molecular sieve catalyst, Sn atoms are components of the molecular sieve framework, the Si / Sn atomic ratio is 60-500, and the SnO2 content is 0.5%-4.0% based on the catalyst mass; The Sn-SPP molecular sieve preparation method comprises: mixing a silicon source, tetrabutylphosphine hydroxide, ethanol, water and a tin source in proportion, standing and crystallizing at a certain temperature, and separating, washing, drying and roasting the product to obtain the Sn-SPP molecular sieve; the molar ratio of the silicon source, tetrabutylphosphine hydroxide, ethanol, water and the tin source is 1:0.2-0.4:3-5:10-30:0.004-0.024; The prepared Sn-SPP molecular sieve has an MFI type topological structure, a particle size of 200-500nm, and a BET specific surface area of ​​300-500m 2 / g, pore volume 0.25-0.4 cm 3 / g, average pore size is 2.5-7nm; The reaction temperature for synthesizing lactide is 100-170° C., the stirring speed is 100-1000 r / min, and the reaction time is 2-6 h.

2. The method according to claim 1, characterized in that: The silicon source is at least one of ethyl orthosilicate and methyl orthosilicate.

3. The method according to claim 2, characterized in that: The silicon source is tetraethyl orthosilicate.

4. The method according to claim 1, characterized in that: The tin source is at least one of tin tetrachloride pentahydrate and stannous octoate.

5. The method according to claim 4, characterized in that: The tin source is tin tetrachloride pentahydrate.

6. The method according to claim 1, characterized in that: The mixed materials are placed at 130-150°C for crystallization for 2-5 days.

7. The method according to claim 1, characterized in that: The product is separated by centrifugation and filtration, washed with water until neutral, dried at 80-100° C. for 2-12 h, and calcined at 500-550° C. for 3-5 h to obtain Sn-SPP molecular sieve.

8. The method according to claim 1, characterized in that: The lactic acid is at least one of L-lactic acid and D-lactic acid, the optical purity of the lactic acid is ≥99.5%, and the mass concentration of the lactic acid solution is 20%-80%.

9. The method according to claim 1, characterized in that: The benzene series organic solvent is at least one of benzene, toluene, ethylbenzene and xylene.

10. The method according to claim 9, characterized in that: The benzene series organic solvent is toluene.

11. The method according to claim 1, characterized in that: The mass ratio of the catalyst to lactic acid is 1:0.5-5.

12. The method according to claim 11, characterized in that: The mass ratio of the catalyst to lactic acid is 1:2-4.

13. The method according to claim 1, characterized in that: The solid-liquid ratio of the catalyst to the organic solvent is 1g:5-100mL.

14. The method according to claim 13, characterized in that: The solid-liquid ratio of the catalyst to the organic solvent is 1g:10-30mL.

15. The method according to claim 1, characterized in that: During the reflux reaction, free water in the lactic acid solution and water molecules generated by the condensation reaction of lactic acid molecules in the reaction system are continuously removed.

16. The method according to claim 1, characterized in that: After the reaction is completed, water is added to the reaction system to separate the aqueous phase and the organic phase, wherein the volume ratio of water to the organic solvent is 1-5:

1.

17. The method according to claim 16, characterized in that: The volume ratio of water to organic solvent is 1-2:

1.

18. The method according to claim 1, characterized in that: The organic phase obtained after the two phases are separated is subjected to reduced pressure distillation under the conditions of 100-120° C. and 1-5 kPa absolute pressure to obtain a lactide product, and the organic solvent is recovered for repeated use.

19. The method according to claim 1 or 18, characterized in that: The aqueous phase obtained after the two phases are separated is stirred and reacted at 100-150° C. for 3-6 hours at a stirring speed of 100-1000 r / min.

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