A method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate

Abstract

The present invention provides a method for preparing 2-hydroxy-4-methylthiobutyric acid isopropyl ester, using 2-hydroxy-4-methylthiobutyric acid and isopropanol as raw materials, and using a high-temperature electrodialysis device, under the action of an external current and an electrodialysis membrane, an esterification reaction can be completed without using an external catalyst to prepare 2-hydroxy-4-methylthiobutyric acid isopropyl ester. The present invention utilizes the selective permeability of high-temperature electrodialysis, and MHA slowly passes through the electrodialysis membrane from a light phase to a dense phase under the action of an external current, and at the same time, H + It plays a self-catalytic role. MHA reacts with a large amount of IPA instantly through the electrodialysis membrane, which improves the reaction rate. No external catalyst is needed, which reduces the generation of wastewater in the acid removal process, reduces production costs, and is clean and environmentally friendly. It belongs to the field of organic synthesis technology.

Description

Technical Field

[0001] The invention relates to the technical field of organic chemical industry, in particular to a method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate. Background Art

[0002] At present, there are mainly three methods for producing isopropyl 2-hydroxy-4-methylthiobutyrate:

[0003] First, 2-hydroxy-4-methylthiobutyric acid (MHA) and isopropanol (IPA) react under the catalysis of proton strong acid (sulfuric acid, hydrochloric acid, nitric acid) to produce 2-hydroxy-4-methylthiobutyric acid isopropyl ester (HMBI). The reaction process uses reactive distillation and other methods to improve the product yield. After the reaction is completed, alkali is added to adjust the pH of the tower bottom liquid to 8-10, and then solvent extraction is added. After the solvent is removed from the extract phase, HMBI is obtained by vacuum distillation. However, a large amount of inorganic salt wastewater is produced in the aqueous phase, and the acid used as a catalyst becomes a consumable after the proton acid is removed, resulting in increased production costs.

[0004] Second, MHA and IPA are reacted and distilled in an acid resin packed tower, and the tower bottom product is directly distilled under reduced pressure to obtain HMBI. However, there is no research on the decay time of acid resins and resin regeneration. Acid resins will face the problem of resin catalytic performance decay and deactivation. In addition, acid resins usually have weak high temperature resistance and are not suitable for use in high temperature environments.

[0005] Third, the invention patent with publication number CN102093271A discloses a method for preparing alkyl 2-hydroxy-4-methylthiobutyrate, using phosphoric acid as a catalyst, reacting MHA with IPA to generate HMBI, adding ammonia water to adjust the bottom liquid to pH 8-10 after the reaction, and then adding solvent extraction, drying, desolventizing, and distilling to obtain HMBI. However, the introduction of ammonia water to adjust the pH of the system will produce a large amount of ammonium phosphate wastewater, and the ammonium sulfate aqueous phase after separation will take away part of MHA and HMBI, resulting in a reaction yield of no more than 92%.

[0006] In summary, at present, in the production method of 2-hydroxy-4-methylthiobutyric acid isopropyl ester, traditional proton strong acid or phosphoric acid is used as a catalyst, and after the reaction, inorganic salt wastewater is produced during the acid removal process, resulting in high production costs; acid resin is used as a catalyst, which faces the problems of resin catalytic performance attenuation and deactivation, weak high temperature resistance of the catalyst, and inability to be recycled.

[0007] Based on the above situation, it is urgent to develop a new method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate. Summary of the invention

[0008] The object of the present invention is to provide a method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate, so as to solve the technical problems of high production cost and inability to recycle in the existing method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate.

[0009] To achieve the above object, the technical solution adopted by the present invention is: to provide a method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate, comprising the following steps:

[0010] (1) 2-hydroxy-4-methylthiobutyric acid is placed in the light phase of a high-temperature electrodialysis device, isopropanol is placed in the dense phase, a circulation pump is turned on, the system is heated during the circulation process, and after reaching the reaction temperature, an external current is turned on to react;

[0011] (2) After the reaction is completed, post-treatment is performed to obtain 2-hydroxy-4-methylthiobutyric acid isopropyl ester.

[0012] Preferably, in step (1), the molar ratio of 2-hydroxy-4-methylthiobutyric acid to isopropanol is 1:2-10.

[0013] Preferably, in step (1), the solvent in the light phase includes water; a small amount of 2-hydroxy-4-methylthiobutyric acid is added to the dense phase to increase the initial conductivity of the dense phase, and the amount of 2-hydroxy-4-methylthiobutyric acid added is 0.3-1% of the amount of isopropanol added.

[0014] Preferably, in step (1), the reaction temperature is 60-80°C.

[0015] Preferably, in step (1), the applied current is turned on and the voltage is set to 100-120V.

[0016] Preferably, in step (1), the reaction ends when the current reaches 1.8-2.4 A.

[0017] Preferably, in step (2), the post-treatment includes post-treatment of the dense phase, specifically: discharging the dense phase liquid for adiabatic flash evaporation, flashing off the unreacted isopropanol to obtain a finished liquid, and subjecting the finished liquid to vacuum distillation to obtain 2-hydroxy-4-methylthiobutyric acid isopropyl ester.

[0018] Preferably, in step (2), the pressure of the adiabatic flash evaporation is 8-12 KPa, the temperature of the vacuum distillation is 150-170° C., and the pressure is 0.5-5 KPa.

[0019] Preferably, in step (2), the post-treatment also includes post-treatment of the light phase, specifically: discharging the light phase liquid for adiabatic flash evaporation, combining the light phase liquid with the bottom liquid after vacuum distillation after flash evaporation to remove water, and recycling it into the light phase of the high-temperature electrodialysis device for recycling.

[0020] Preferably, in step (2), the pressure of the adiabatic flash evaporation is 8-12 KPa.

[0021] The invention provides a method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate. 2-hydroxy-4-methylthiobutyric acid and isopropanol are used as raw materials. An esterification reaction can be completed by using a high-temperature electrodialysis device under the action of an external current and an electrodialysis membrane without using an external catalyst to prepare isopropyl 2-hydroxy-4-methylthiobutyrate.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) The present invention utilizes the selective permeability of high-temperature electrodialysis. Under the action of an external current, MHA slowly passes through the electrodialysis membrane from the light phase to the concentrated phase. At the same time, H + It plays a self-catalytic role. MHA reacts with a large amount of IPA instantly through the electrodialysis membrane, which improves the reaction rate. No external catalyst is needed, which reduces the generation of wastewater in the acid removal process, reduces production costs, and is clean and environmentally friendly. In addition, the HMBI polymers and MHA polymers after adiabatic flash evaporation of the dense phase liquid and the light phase liquid can be returned to the light phase of the high-temperature electrodialysis device for recycling, which reduces costs and significantly improves the reaction yield of HMBI, with a yield of more than 99%.

[0024] (2) Since 2-hydroxy-4-methylthiobutyric acid is a mixture containing 68% MHA monomers and about 20% MHA polymers, the MHA polymers have a small charge-to-mass ratio and a slow migration rate. They are gradually hydrolyzed into MHA monomers in the dilute phase circulation, and then enter the concentrated phase through the electrodialysis membrane for reaction, which improves the selectivity of HMBI production and significantly improves the reaction yield. In the entire process, based on the newly added MHA, the yield of HMBI is as high as over 99%, and the purity is also as high as over 99%.

[0025] The present invention is an optimized route that is clean and environmentally friendly, has lower production cost, higher selectivity, higher reaction yield, and can be recycled, and has high feasibility for industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 This is a reaction process diagram of the method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate of the present application;

[0028] Figure 2 This is the LC-MS spectrum of the product prepared in Example 2 of the present application. DETAILED DESCRIPTION

[0029] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application clearer, the present application is further described in detail below in conjunction with the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present application and are not intended to limit the present application. The raw materials and devices used in the present invention are conventional commercial products unless otherwise specified. The methods used in the present invention are conventional methods unless otherwise specified.

[0030] The present invention is described below in conjunction with Examples 1-13.

[0031] (I) Method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate

[0032] Example 1

[0033] A method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate comprises the following steps:

[0034] (1) 260.03 g (1.73 mol) of 2-hydroxy-4-methylthiobutyric acid (MHA) was placed in the light phase of a high-temperature electrodialysis device, the solvent in the light phase being water, and then 207.95 g (3.46 mol) of isopropyl alcohol (IPA) was placed in the dense phase, and 1 g of MHA was added to the dense phase to increase the initial conductivity of the dense phase; the circulation pump was turned on, and a heat exchanger was provided on the circulation pipeline to heat the system during the circulation process. When the system temperature reached 80°C, the system was kept warm, an external current was turned on, and the voltage was set to 100 V to carry out the reaction; the reaction was terminated when the current reached 2.0 A and remained stable.

[0035] (2) After the reaction is completed, the external voltage is turned off, the concentrated phase liquid is discharged, and adiabatic flash evaporation is performed at 10 KPa to flash off the unreacted IPA to obtain a finished liquid. The finished liquid is subjected to vacuum distillation at 3 KPa and 160°C to obtain 279.70 g of 2-hydroxy-4-methylthiobutyric acid isopropyl ester (HMBI) at the top of the tower. HMBI is detected by liquid chromatography, and its liquid phase purity is measured to be 99.50% and the yield is 95.50; HMBI polymers are obtained at the bottom of the tower;

[0036] The light phase liquid was discharged and adiabatic flash evaporation was performed at 10 KPa to flash off the solvent water. After dehydration, it was combined with the bottom liquid after vacuum distillation to obtain 20.1 g of liquid, the main components of which were HMBI polymers and MHA polymers. The liquid was then recycled into the light phase of the high temperature electrodialysis device for recycling.

[0037] The above reaction process is as follows Figure 1 shown.

[0038] The reaction equation is as follows:

[0039] .

[0040] Example 2

[0041] The difference between this embodiment and Example 1 is that in step (1), the amount of IPA added is 260.01 g (4.33 mol), and the remaining operations are the same, to obtain 283.07 g of HMBI, the liquid phase purity of which is 99.53%, and the yield is 96.65%.

[0042] Example 3

[0043] The difference between this embodiment and Example 1 is that in step (1), the amount of IPA added is 415.89 g (6.92 mol), and the remaining operations are the same, to obtain 283.27 g of HMBI, the liquid phase purity of which is 99.62%, and the yield is 96.72%.

[0044] Example 4

[0045] The difference between this embodiment and Example 1 is that in step (1), the amount of IPA added is 623.84 g (10.38 mol), and the remaining operations are the same, to obtain 283.80 g of HMBI, the liquid phase purity of which is 99.55%, and the yield is 96.90%.

[0046] Example 5

[0047] The difference between this embodiment and Example 1 is that in step (1), the amount of IPA added is 831.78 g (13.84 mol), and the remaining operations are the same, to obtain 283.92 g of HMBI, the liquid phase purity of which is 99.56%, and the yield is 96.94%.

[0048] Example 6

[0049] The difference between this embodiment and Example 1 is that in step (1), the amount of IPA added is 1039.73 g (17.30 mol), and the remaining operations are the same, to obtain 284.07 g of HMBI, the liquid phase purity of which is 99.65%, and the yield is 96.99%.

[0050] Example 7

[0051] The difference between this embodiment and Example 1 is that in step (1), when the system temperature reaches 60°C, the system is kept warm. The remaining operations are the same, and 276.77 g of HMBI is obtained, the liquid phase purity is 99.63%, and the yield is 94.50%.

[0052] Example 8

[0053] The difference between this embodiment and Example 1 is that in step (1), when the system temperature reaches 70°C, the system is kept warm. The remaining operations are the same, and 278.30 g of HMBI is obtained, the liquid phase purity is 99.52%, and the yield is 95.02%.

[0054] Example 9

[0055] The difference between the present embodiment and Example 1 is that in step (1), the applied current is turned on and the voltage is set to 110 V. The remaining operations are the same, and 279.99 g of HMBI is obtained, the liquid phase purity of which is 99.50%, and the yield is 95.60%.

[0056] Example 10

[0057] The difference between the present embodiment and Example 1 is that in step (1), the applied current is turned on and the voltage is set to 120 V. The remaining operations are the same, and 280.14 g of HMBI is obtained, the liquid phase purity of which is 99.51%, and the yield is 95.65%.

[0058] Embodiment 11

[0059] The difference between the present embodiment and Example 1 is that in step (1), the reaction is terminated until the current reaches 1.8 A. The remaining operations are the same, and 279.70 g of HMBI is obtained, the liquid phase purity of which is 99.55%, and the yield is 95.50%.

[0060] Example 12

[0061] The difference between the present embodiment and Example 1 is that in step (1), the reaction is terminated until the current reaches 2.4 A. The remaining operations are the same, and 279.44 g of HMBI is obtained, with a liquid phase purity of 99.58% and a yield of 95.41%.

[0062] It should be noted that: in the above-mentioned embodiments 1-12, in step (1), the solvent in the light phase is water, which is only a preferred embodiment. In actual production, the solvent in the light phase can be other solvents, which can be adaptively adjusted according to specific circumstances. In step (1), 1g of MHA is added to the dense phase to increase the initial conductivity of the dense phase. This is only a preferred embodiment. In actual production, the amount of MHA added to the dense phase can be 0.3-1% of the amount of IPA added, which can be adaptively adjusted according to specific circumstances. In step (1), the high-temperature electrodialysis device can be an electrodialyzer or other types of high-temperature electrodialysis devices; the heating device can be a heat exchanger or other types of heating devices; the adiabatic flash evaporation can use an adiabatic flash evaporation separator or other types of adiabatic flash evaporation devices; the above-mentioned equipment can be adaptively adjusted according to specific circumstances. In step (2), the dense phase liquid is adiabatically flashed at 10KPa, and the finished liquid is subjected to vacuum distillation at 3KPa and 160°C. This is only a preferred embodiment. In actual production, the pressure of the adiabatic flash can be 8-12KPa, the temperature of the vacuum distillation can be 150-170°C, and the pressure can be 0.5-5KPa, which can be adaptively adjusted according to specific circumstances. In step (2), the light phase liquid is adiabatically flashed at 10KPa. This is only a preferred embodiment. In actual production, the pressure of the adiabatic flash can be 8-12KPa, which can be adaptively adjusted according to specific circumstances.

[0063] In order to more intuitively compare the effects of the process parameters of Examples 1-12 on the yield of HMBI, the following Table 1 is formed.

[0064] Table 1 Effect of process parameters of Examples 1-12 on the yield of HMBI

[0065]

[0066] From Table 1, we can see that:

[0067] Comparing Examples 1-6, it can be seen that as the molar ratio of MHA to IPA increases from 1:2 to 1:10, the reaction yield of the product HMBI gradually increases, but the increase is not obvious. Therefore, considering that the energy consumption of the later processing will increase with the increase in the amount of IPA, the molar ratio of MHA to IPA is preferably 1:2.5.

[0068] Comparing Examples 1, 7-8, it can be seen that as the reaction temperature increases from 60°C to 80°C, the reaction yield of the product HMBI gradually increases. Therefore, the reaction temperature is preferably 80°C.

[0069] Comparing Examples 1, 9-10, it can be seen that as the voltage increases from 100 V to 120 V, the reaction yield of the product HMBI gradually increases, but the increase is not obvious. Therefore, from an economic point of view, the applied voltage is preferably 100 V.

[0070] Comparing Examples 1 and 11-12, it can be seen that as the current increases from 1.8 A to 2.4 A, the reaction yield of the product HMBI gradually decreases, but the decrease is not obvious. Therefore, considering that the energy consumption of the later processing will increase with the increase of the current, the current is preferably 2.0 A after comprehensive consideration.

[0071] In summary, the preferred reaction conditions for HMBI are: the molar ratio of MHA to IPA is 1:2.5, the reaction temperature is 80°C, the applied voltage is 100 V, and the current is 2.0 A. Under the preferred reaction conditions, the yield of HMBI is as high as 96.65%.

[0072] The product prepared in Example 2 was tested and its LC-MS spectrum was obtained as follows: Figure 2 shown.

[0073] Depend on Figure 2 It can be seen that the formula of the peak at 193.08853 is C 8 H 17 O 3 S+, Exact Mass is 192.0820, Mol.Wt. is 192.2758, m / e is 192.0820 (100.0%), 193.0854 (8.9%), 194.0778 (4.4%), which is consistent with the structure of 2-hydroxy-4-methylthiobutyric acid isopropyl ester. Therefore, the product prepared in Example 2 is 2-hydroxy-4-methylthiobutyric acid isopropyl ester.

[0074] (II) Recycling performance of the method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate

[0075] Embodiment 13

[0076] (1) 20.1 g of the liquid obtained after adiabatic flash evaporation in Example 2 was returned to the light phase of the high-temperature electrodialysis device, and then 260.03 g (1.73 mol) of 2-hydroxy-4-methylthiobutyric acid (MHA) was placed in the light phase, the solvent in the light phase was water, and then 260.01 g (4.33 mol) of isopropyl alcohol (IPA) was placed in the dense phase, and 1 g of MHA was added to the dense phase to increase the initial conductivity of the dense phase; the circulation pump was turned on, and a heat exchanger was provided on the circulation pipeline to heat the system during the circulation process. When the system temperature reached 80°C, the system was kept warm, an external current was turned on, and the voltage was set to 100 V to react; until the current reached 2.0 A and remained stable, the reaction was terminated.

[0077] (2) After the reaction is completed, the external voltage is turned off, the concentrated phase liquid is discharged, and an adiabatic flash is performed at 10 KPa to flash off the unreacted IPA to obtain a finished liquid. The finished liquid is subjected to vacuum distillation at 3 KPa and 160°C to obtain 2-hydroxy-4-methylthiobutyric acid isopropyl ester (HMBI) at the top of the tower. HMBI is detected by liquid chromatography to measure its liquid phase purity and yield (based on the newly added MHA); HMBI polymers are obtained at the bottom of the tower;

[0078] The light phase liquid is discharged and adiabatically flashed at 10Kpa to flash off the solvent water. After water removal, it is combined with the bottom liquid after vacuum distillation to obtain a liquid whose main components are HMBI polymers and MHA polymers. The liquid is then recycled into the light phase of the high temperature electrodialysis device for recycling.

[0079] The liquid obtained after adiabatic flash evaporation was recovered and reused 12 times according to the above steps. Each time, MHA and IPA were newly added according to the molar ratio of MHA to IPA of 1:2.5. The yield of HMBI in each cycle (based on the newly added MHA) is shown in Table 2.

[0080] Table 2 Effect of the number of cycles on the yield of HMBI

[0081]

[0082] It can be seen from Table 2 that after the liquid obtained after adiabatic flash evaporation (light phase concentrated liquid + bottom liquid, the main components are HMBI polymer and MHA polymer) is recycled, the reaction yield of the product HMBI is significantly increased, and the yield is as high as more than 99%. It can be seen that the liquid obtained after adiabatic flash evaporation can be returned to the light phase of the high-temperature electrodialysis device to achieve recycling. With the increase in the number of cycles, the reaction yield of the product HMBI is basically stable, indicating the stability of the HMBI preparation method of the present invention.

[0083] It should be noted that Table 2 above only shows the quality, purity and yield data of HMBI when it is recycled 12 times, and the actual number of cycles is greater than 12 times.

[0084] The invention provides a method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate. 2-hydroxy-4-methylthiobutyric acid and isopropanol are used as raw materials. An esterification reaction can be completed by using a high-temperature electrodialysis device under the action of an external current and an electrodialysis membrane without using an external catalyst to prepare isopropyl 2-hydroxy-4-methylthiobutyrate.

[0085] Compared with the prior art, (1) the present invention utilizes the selective permeability of high-temperature electrodialysis. Under the action of an external current, MHA slowly passes through the electrodialysis membrane from the light phase to the concentrated phase. At the same time, H+ It plays a self-catalytic role. MHA reacts with a large amount of IPA instantly through the electrodialysis membrane, which increases the reaction rate. No external catalyst is needed, which reduces the generation of wastewater in the deacidification process, reduces production costs, and is clean and environmentally friendly. In addition, the HMBI polymer and MHA polymer after adiabatic flash evaporation of the dense phase liquid and the light phase liquid can be returned to the light phase of the high-temperature electrodialysis device for recycling, which reduces costs and significantly increases the reaction yield of HMBI, with a yield of more than 99%. (2) Since 2-hydroxy-4-methylthiobutyric acid is a mixture containing 68% MHA monomer and about 20% MHA polymer, the MHA polymer has a small charge-to-mass ratio and a slow migration rate. It is gradually hydrolyzed into MHA monomer in the light phase cycle, and then enters the dense phase through the electrodialysis membrane for reaction, which improves the selectivity of HMBI and significantly improves the reaction yield. In the whole process, based on the newly added MHA, the yield of HMBI is as high as more than 99%, and the purity is also as high as more than 99%. The present invention is an optimized route that is clean and environmentally friendly, has lower production cost, higher selectivity, higher reaction yield, and can be recycled, and has high feasibility for industrial application.

[0086] The embodiments described above are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate, characterized in that: The following steps are involved: (1) 2-hydroxy-4-methylthiobutyric acid is placed in the light phase of a high-temperature electrodialysis device, isopropanol is placed in the dense phase, a circulation pump is turned on, the system is heated during the circulation process, and after reaching the reaction temperature, an external current is turned on to react; The reaction temperature is 60-80°C, the applied current is turned on and the voltage is set to 100-120V; (2) After the reaction is completed, post-treatment is performed to obtain 2-hydroxy-4-methylthiobutyric acid isopropyl ester; The post-treatment includes post-treatment of the dense phase, specifically: discharging the dense phase liquid for adiabatic flash evaporation, flashing out the unreacted isopropanol to obtain a finished liquid, and performing vacuum distillation on the finished liquid to obtain 2-hydroxy-4-methylthiobutyric acid isopropyl ester; Post-treatment also includes post-treatment of the light phase, specifically: discharging the light phase liquid for adiabatic flash evaporation, combining it with the bottom liquid after vacuum distillation after flash evaporation to remove water, and returning it to the light phase of the high-temperature electrodialysis device for recycling.

2. The method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate according to claim 1, characterized in that: In step (1), the molar ratio of 2-hydroxy-4-methylthiobutyric acid to isopropanol is 1:2-10.

3. The method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate according to claim 1, characterized in that: In step (1), the solvent in the light phase includes water; a small amount of 2-hydroxy-4-methylthiobutyric acid is added to the concentrated phase to increase the initial conductivity of the concentrated phase, and the amount of 2-hydroxy-4-methylthiobutyric acid added is 0.3-1% of the amount of isopropanol added.

4. The method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate according to claim 1, characterized in that: In step (1), when the current reaches 1.8-2.4A, the reaction ends.

5. The method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate according to claim 1, characterized in that: In step (2), the pressure for adiabatic flash evaporation of the dense phase liquid is 8-12 KPa, and the temperature for vacuum distillation of the completed liquid is 150-170°C and the pressure is 0.5-5 KPa.

6. The method for preparing isopropyl 2-hydroxy-4-methylthiobutyrate according to claim 1, characterized in that: In step (2), the pressure for adiabatic flash evaporation of the light phase liquid is 8-12 KPa.

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