CN107151210B

CN107151210B - Method for preparing tung oil polyol under ultrasonic condition

Info

Publication number: CN107151210B
Application number: CN201610119880.3A
Authority: CN
Inventors: 李风华; 李澜鹏; 白富栋; 乔凯; 王领民
Original assignee: China Petroleum and Chemical Corp; Sinopec Fushun Research Institute of Petroleum and Petrochemicals
Current assignee: China Petroleum and Chemical Corp; Sinopec Fushun Research Institute of Petroleum and Petrochemicals
Priority date: 2016-03-03
Filing date: 2016-03-03
Publication date: 2020-07-07
Anticipated expiration: 2036-03-03
Also published as: CN107151210A

Abstract

The invention discloses a method for preparing tung oil polyalcohol by utilizing ultrasonic waves, which comprises the steps of adding tung oil, carboxylic acid, an inorganic acid catalyst and a hydroxylation reagent into an ultrasonic reactor in proportion, heating to 35-45 ℃, and starting the ultrasonic reactor; then under the condition of violent stirring, dropwise adding hydrogen peroxide, controlling the temperature of the reactor to be 45-65 ℃, after the dropwise adding is finished, continuously reacting for 1-3h, and finishing the reaction; and standing and layering the reaction system, separating out a water phase, and neutralizing, washing and distilling under reduced pressure to obtain the tung oil polyol. The method utilizes the characteristic that the conjugated double bond of the tung oil can improve the reaction activity of an epoxy group, adds the oil-soluble fatty acid during epoxidation, can effectively avoid the occurrence of a cross-linking side reaction, adopts an ultrasonic wave reinforced reaction process, obviously improves the reaction rate, reduces the decomposition of peroxycarboxylic acid, reduces the usage amount of hydrogen peroxide and carboxylic acid, and efficiently synthesizes the tung oil polyol product.

Description

Method for preparing tung oil polyol under ultrasonic condition

Technical Field

The invention belongs to the field of polyurethane materials, and particularly relates to a method for preparing tung oil polyol under the ultrasonic condition.

Background

Polyurethane materials have been widely used in industry and in people's daily life due to their good mechanical properties and easy moldability. The main raw materials for producing polyurethane comprise isocyanate, polyol and other additives, wherein the proportion of the polyol accounts for more than 50%. In the industrial production process, the polyol is mainly applied to the polyurethane field, so the influencing factors of the polyurethane industry are also the main influencing factors of the polyol market. The polyols can be classified into polyether polyols and polyester polyols according to their molecular structures, with polyether polyols dominating the market and occupying more than 70% of the total polyol demand.

Generally, polyols are prepared by extraction from petroleum. The scarcity of petroleum as an unrenewable resource causes the price to continuously rise, and the price of the main raw materials for producing the downstream products of polyhydric alcohols such as propylene oxide and ethylene oxide continuously rises, and the petroleum resource is consumed at all times according to the current consumption rate. Therefore, from the perspective of sustainable development and enterprise competitiveness, it is a strategic development task to find new materials and new processes that can replace petroleum-based polyethers.

The united states is the major world-wide soybean oil producing country, and in addition to food, research institutes in the united states are actively engaged in developing various chemical products using soybean oil as a raw material to replace petroleum-based chemicals. Recently, the method focuses on preparing soybean oil polyol by modifying double bonds through unsaturated bonds in soybean oil molecular chains to perform epoxidation and hydroxylation on vegetable oil. The method has the advantages of low reaction temperature (40-70 ℃), good product quality and color and luster, so the method is widely concerned.

Reacting peroxycarboxylic acid with soybean oil to prepare epoxidized soybean oil, wherein double bonds are converted into epoxy groups; then the epoxidized soybean oil and water and alcohol generate ring-opening reaction under the catalysis of a high-efficiency catalyst tetrafluoroboric acid to prepare the vegetable oil polyol containing hydroxyl, wherein the hydroxyl value of the polyol is 110-213mgKOH/g, the viscosity is 1000-7000 mPa.s, and the conversion rate can reach 85-95%.

US20070123725 provides a process for preparing a soybean oil polyol, comprising the epoxidation and hydroxylation process of an unsaturated vegetable oil to form a vegetable oil based polyether polyol. Firstly, unsaturated vegetable oil such as soybean oil or rapeseed oil reacts with organic acid and hydrogen peroxide solution to form epoxy vegetable oil, and then the epoxy vegetable oil is subjected to ring-opening reaction with mixed solution of methanol and water to generate vegetable oil polyalcohol.

US20060041157 describes a process for making a soy oil polyol comprising reacting a partially epoxidized vegetable oil under catalyst conditions with a ring-opening reagent comprising a small molecule polyol, vegetable oil polyol or other polyol to form an oligomeric vegetable oil-based polyol. The functionality of the oligomeric vegetable oil polyol is 1 to 6 and the hydroxyl value is 20 to 300 mgKOH/g.

Tung oil is an important industrial raw material and a traditional export commodity. At present, the annual output of China tung oil reaches more than 10 ten thousand tons, which accounts for about 35 percent of the world tung oil output. Therefore, it is of particular importance to study tung oil and industrialize more products related to tung oil. However, the tung oil is used for preparing the high-quality vegetable oil polyol, because the tung oil has high unsaturation degree and is the only unsaturated vegetable oil with conjugated double bonds in nature, the iodine value reaches more than 170, wherein more than 85% of unsaturated bonds are carbon-carbon conjugated triene bonds, epoxy groups have high reaction activity and poor selectivity and are easy to generate side reactions due to the existence of the conjugated double bonds in the epoxidation process of the tung oil for preparing the polyol, so that a macromolecular cross-linked product is generated, the viscosity is increased sharply, and the tung oil is usually solid at room temperature and cannot be used for further synthesizing polyurethane materials. Research literature (such as Epoxidation of natural triglyceridees with ethylene oxidation, Journal of the American oil chemists' Society, 1996, 73: 461-.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a method for preparing tung oil polyol under the ultrasonic condition. The invention utilizes the characteristic that the conjugated double bond of the tung oil can improve the reaction activity of an epoxy group, adds the oil-soluble fatty acid while epoxidizing, and utilizes ultrasonic waves to strengthen the reaction process, thereby effectively avoiding the occurrence of cross-linking side reaction, improving the two-phase reaction rate, shortening the reaction time, reducing the decomposition of peroxycarboxylic acid, reducing the use amount of hydrogen peroxide and carboxylic acid and saving the production cost.

The preparation method of the tung oil polyol comprises the following steps: adding tung oil, carboxylic acid, an inorganic acid catalyst and a hydroxylation reagent into an ultrasonic reactor in proportion, heating to 35-45 ℃, and starting the ultrasonic reactor; then dropwise adding a hydrogen peroxide solution under vigorous stirring, controlling the dropwise adding speed to keep the reaction temperature at 45-65 ℃, and after the dropwise adding is finished, continuing to react for 1-3h to finish the reaction; and standing and layering the reaction system, separating out a water phase, and neutralizing, washing and distilling under reduced pressure to obtain the tung oil polyol.

The ultrasonic reactor can enable the reaction process to be carried out efficiently under the ultrasonic condition, and the ultrasonic power is 30-200W. The ultrasonic reactor is adopted, so that the reaction system is under the ultrasonic radiation condition, a large amount of cavitation bubbles are formed in the reaction system under the ultrasonic cavitation effect, the cavitation bubbles rapidly form, grow and collapse, so that liquid around the cavitation bubbles is flushed into the centers of the cavitation bubbles at a high speed, a phase interface is disturbed, two-phase mass transfer can be effectively enhanced, the reaction rate of two-phase reaction is improved, and the reaction time is shortened; and the decomposition of peroxycarboxylic acid can be reduced, the use amount of hydrogen peroxide and carboxylic acid can be reduced, and the production cost can be saved.

The carboxylic acid of the present invention may be formic acid or acetic acid, preferably formic acid. The carboxylic acid can react with the hydrogen peroxide solution to generate peroxycarboxylic acid, the peroxycarboxylic acid converts conjugated double bonds in the tung oil into epoxy bonds and releases the carboxylic acid, so that the carboxylic acid is not consumed while the epoxy bonds are generated, but the reaction rate of the system is slowed down due to too small content of the carboxylic acid, and the mass ratio of the carboxylic acid to the tung oil is controlled to be 0.03:1-0.3: 1.

The inorganic acid catalyst can be one or more of sulfuric acid, phosphoric acid, hydrochloric acid and the like, and is preferably sulfuric acid; the dosage of the inorganic acid catalyst is 0.01-1.0 percent of the mass of the tung oil.

The hydroxylation reagent is oil-soluble fatty acid, and the dosage of the hydroxylation reagent is 0.1-0.5 time of the mass of the tung oil. The oil-soluble fatty acid may be selected from C₆-C₁₂One or more of straight chain or branched chain saturated fatty acids, such as n-hexanoic acid, n-heptanoic acid, n-octanoic acid, isocaproic acid, isoheptanoic acid, isocaprylic acid, etc. Compared with small molecular alcohol reagents, the reagent has the advantages that the reagent adopts oil-soluble fatty acidThe following advantages are provided: (1) the plant oil epoxidation reaction is a violent exothermic reaction, so that the reaction speed is accelerated under the ultrasonic condition, temperature overshoot is more likely to occur, and the small molecular alcohol reagent serving as a hydroxylation reagent has poor reaction selectivity at a higher temperature, so that a cross-linking side reaction cannot be avoided, and the viscosity of a product is increased rapidly; the hydrogen of the fatty acid is easier to ionize, and the reaction activity is obviously higher than that of the alcoholic hydroxyl group formed by the adjacent vegetable oil molecular chain, so that the fatty acid still has good reaction selectivity at higher reaction temperature, avoids the cross-linking side reaction among the vegetable oil molecular chains, has wider reaction temperature window, and can be better suitable for an ultrasonic reactor; (2) the oil-soluble fatty acid can be dissolved in the oil phase in the reaction system, so that the problem of reduced hydroxylation ring-opening reaction rate caused by migration between oil and water phases is avoided, and the reaction selectivity is improved; (3) the oil-soluble fatty acid and the formic acid as the oxygen-carrying agent do not have chemical reaction in the system, so that the concentration of a reaction substrate is reduced, and the epoxidation and in-situ ring-opening reaction effects are influenced; (4) the use amounts of hydroxylation reagent, formic acid and hydrogen peroxide are reduced, and deionized water is not added in the initial stage of the reaction system, so that the production raw material cost and the treatment capacity of industrial wastewater are reduced.

The hydrogen peroxide solution of the present invention acts as an oxidizing agent to oxidize carboxylic acids to peroxycarboxylic acids. The higher the concentration of the hydrogen peroxide is, the more violent the reaction is, the serious the heat release of the system is, and the side reaction is easy to occur, so the concentration of the hydrogen peroxide solution is selected to be 20wt% -60wt%, and the dosage of the hydrogen peroxide solution is 0.4-0.8 times of the quality of the tung oil.

The neutralization reaction of the present invention may be carried out by using ammonia, sodium carbonate or sodium bicarbonate, preferably sodium bicarbonate solution having a concentration of 5wt% to 20 wt%. The washing temperature is 50-80 ℃ to prevent the system from emulsifying. The reduced pressure distillation is to remove the hydroxylation reagent and residual moisture in the system under the conditions of the pressure of 1000-3000Pa and the temperature of 60-120 ℃ so as to ensure that the moisture content of the product is less than 0.1 wt%.

The tung oil polyol of the present invention is prepared by the above-described process of the present invention. The hydroxyl value of the prepared tung oil polyol is 120-270mgKOH/g, the acid value is lower than 1.0mgKOH/g, the water content is lower than 0.1wt%, the yield is higher than 93%, and the tung oil polyol is suitable for preparing polyurethane materials.

More than 85% of unsaturated bonds in tung oil molecules are carbon-carbon conjugated triene bonds, so that epoxy groups have high reaction activity, poor selectivity and easy side reaction in the process of preparing the polyol, and consequently, macromolecular cross-linked products are generated, the viscosity is increased sharply, and the polyurethane material cannot be further synthesized. According to the invention, by utilizing the characteristic that the conjugated double bond of the tung oil can improve the reaction activity of an epoxy group, the tung oil polyol is prepared by adding the oil-soluble fatty acid while epoxidizing and under the ultrasonic condition, the occurrence of a cross-linking side reaction can be effectively avoided, the reaction rate of a two-phase reaction is improved, and the tung oil polyol product is efficiently synthesized. Compared with petroleum-based polyol, the prepared tung oil polyol has the advantages of renewable raw materials, no toxicity, good biodegradability and the like, and belongs to environment-friendly bio-based polyol.

The micromolecular alcohol reagent is added in the epoxidation reaction process of the tung oil, so that the cross-linking side reaction among tung oil molecular chains can be avoided under certain conditions, and a tung oil polyalcohol product is synthesized. However, in the epoxidation reaction process, the small molecular alcohol reagent is easy to perform esterification reaction with formic acid serving as an oxygen-carrying agent, so that the concentration of the small molecular alcohol reagent and the formic acid in a reaction system is obviously reduced, the epoxidation and in-situ ring-opening reaction rate is slow, and the problems of poor crosslinking side reaction effect and the like are solved. Therefore, the reaction system must avoid the problems by increasing the feeding amount of small molecular alcohol reagents, formic acid and hydrogen peroxide and adding deionized water, and the production raw material cost and the treatment amount of industrial wastewater are greatly increased. In addition, since the small molecular alcohol reagent has a problem of poor high-temperature reaction selectivity under the condition of an acid catalyst, and the defects of reduction of hydroxyl value, increase of viscosity and the like of a product easily occur when the reaction temperature is high, the reaction temperature must be strictly controlled, and the construction cost of a production device is increased. In addition, in the epoxidation reaction process of tung oil, oil and water phases exist in the system, the generated peroxycarboxylic acid is in the water phase, the vegetable oil belongs to the oil phase, the epoxidation reaction is only carried out on the phase interface of the oil phase and the water phase, so that the reaction rate is reduced, the peroxycarboxylic acid is easy to decompose after being formed, and the consumption of an oxidant and the cost of raw materials are increased. The invention adopts oil-soluble fatty acid as a hydroxylation reagent, so that the epoxy bond in-situ ring-opening reaction has the characteristics of higher selectivity, wider reaction temperature window and high reaction rate under the ultrasonic condition. And the problems that the concentration of the small molecular alcohol reagent and the formic acid in a reaction system is obviously reduced, the epoxidation and in-situ ring-opening reaction rate is low, the effect of preventing a crosslinking side reaction is not good and the like due to the fact that the small molecular alcohol reagent and the formic acid are easy to perform an esterification reaction in the epoxidation reaction process are solved, the use amounts of the hydroxylation reagent, the formic acid and hydrogen peroxide are effectively reduced, deionized water is not required to be added in the initial stage of the reaction system, and the production raw material cost and the treatment amount of industrial wastewater are reduced.

In addition, in the preparation process of the tung oil polyol, because oil and water phases exist in the system in the epoxidation reaction process, the generated peroxycarboxylic acid is in the water phase, and the tung oil is in the oil phase, the reaction rate is reduced, the peroxycarboxylic acid is easy to decompose, and the consumption is increased. The invention adopts the ultrasonic reactor, greatly strengthens the micro mixing and mass transfer process of a two-phase reaction system, can effectively improve the reaction rate of the two-phase reaction, shortens the reaction time, can reduce the decomposition of peroxycarboxylic acid, reduces the use amount of hydrogen peroxide solution and carboxylic acid, and saves the production cost.

Detailed Description

The present invention will be further described with reference to the following examples. In the present invention, wt% means mass fraction.

The hydroxyl value of the tung oil polyol product prepared by the invention is measured according to a phthalic anhydride esterification method in GB/T12008.3-2009, the acid value is measured according to a GB/T12008.5-2010 method, and the viscosity is measured according to a rotary viscometer method in GB/T12008.7-2010.

Example 1

Adding 500g of tung oil, 80g of formic acid (85 wt% solution), 0.5g of concentrated sulfuric acid and 150g of n-hexanoic acid into an ultrasonic reactor, heating to 40 ℃, and setting the ultrasonic power to be 120W; dropping 325g of 30wt% hydrogen peroxide solution under vigorous stirring, controlling the reaction temperature to be 50 ℃, continuing to react for 2h after the dropping is finished, and finishing the reaction; the reaction system was allowed to stand for delamination, the aqueous phase was removed, the oil phase was washed with 10wt% aqueous sodium bicarbonate solution to neutrality, and the product was then washed with hot water at 70 ℃ 3 times. Distilling the washed product for 2h under the conditions of 2000Pa and 80 ℃ to obtain the tung oil polyalcohol product. Distilling the washed product for 2h under the conditions of 2000Pa and 80 ℃ to obtain the tung oil polyol. The hydroxyl value is 241mgKOH/g, the viscosity is 8117 mPa.s, the acid value is 0.75mgKOH/g, the water content is lower than 0.1wt%, the yield is 94.3%, and the polyurethane rigid foam can be prepared.

Example 2

Adding 500g of tung oil, 100g of formic acid (85 wt% solution), 0.25g of concentrated sulfuric acid and 250g of n-hexanoic acid into an ultrasonic reactor, heating to 45 ℃, and setting the ultrasonic power to be 180W; dropwise adding 380g of 30wt% hydrogen peroxide solution under vigorous stirring, controlling the temperature of the reactor to be 65 ℃, and continuing to react for 1.5h after the dropwise adding is finished, thus finishing the reaction; the reaction system was allowed to stand for delamination, the aqueous phase was removed, the oil phase was washed with 10wt% aqueous sodium bicarbonate solution to neutrality, and the product was then washed with hot water at 70 ℃ 3 times. Distilling the washed product for 2h under the conditions of 2000Pa and 80 ℃ to obtain the tung oil polyol. The hydroxyl value is 255mgKOH/g, the viscosity is 8220mPa & s, the acid value is 0.76mgKOH/g, the water content is lower than 0.1wt%, the yield is 94.5%, and the polyurethane rigid foam can be prepared.

Example 3

Adding 500g of tung oil, 60g of formic acid (85 wt% solution), 4g of concentrated sulfuric acid and 120g of n-hexanoic acid into an ultrasonic reactor, heating to 35 ℃, and setting the ultrasonic power to 80W; dropwise adding 280g of 30wt% hydrogen peroxide solution under vigorous stirring, controlling the temperature of the reactor to be 45 ℃, and continuing to react for 2.5h after the dropwise adding is finished, thus finishing the reaction; the reaction system was allowed to stand for delamination, the aqueous phase was removed, the oil phase was washed with 10wt% aqueous sodium bicarbonate solution to neutrality, and the product was then washed with hot water at 70 ℃ 3 times. Distilling the washed product for 2h under the conditions of 2000Pa and 80 ℃ to obtain the tung oil polyol. The hydroxyl value is 177mgKOH/g, the viscosity is 12440 mPa.s, the acid value is 0.88mgKOH/g, the water content is lower than 0.1wt%, the yield is 93.0%, and the polyurethane can be prepared.

Example 4

The same process conditions as in example 1 were used except that the carboxylic acid added was acetic acid. The obtained tung oil polyol has a hydroxyl value of 155mgKOH/g, viscosity of 3130mPa · s, an acid value of 0.51mgKOH/g, water content of less than 0.1wt%, and a yield of 93.1%, and can be used for preparing polyurethane products.

Example 5

The same treatment conditions as in example 1 were used except that the inorganic acid catalyst added was phosphoric acid. The prepared tung oil polyol has a hydroxyl value of 226mgKOH/g, a viscosity of 7820mPa & s, an acid value of 0.72mgKOH/g, a water content of less than 0.1wt%, and a yield of 93.9%, and can be used for preparing polyurethane rigid foam products.

Example 6

The same process conditions as in example 1 were used except that the hydroxylating agent added was n-heptanoic acid. The hydroxyl value of the prepared tung oil polyol is 233mgKOH/g, the viscosity is 8440 mPa.s, the acid value is 0.77mgKOH/g, the water content is lower than 0.1wt%, the yield is higher than 94.1%, and the tung oil polyol can be used for preparing polyurethane rigid foam products.

Example 7

The same process conditions as in example 1 were used except that the hydroxylating agent added was isoheptanoic acid. The hydroxyl value of the prepared tung oil polyol is 208mgKOH/g, the viscosity is 8950 mPa.s, the acid value is 0.82mgKOH/g, the water content is lower than 0.1wt%, the yield is higher than 93.8%, and the tung oil polyol can be used for preparing polyurethane rigid foam products.

Example 8

The same process conditions as in example 1 were used except that the hydroxylating agent added was n-octanoic acid. The hydroxyl value of the prepared tung oil polyol is 224mgKOH/g, the viscosity is 8640mPa & s, the acid value is 0.79mgKOH/g, the water content is lower than 0.1wt%, the yield is higher than 94.0%, and the tung oil polyol can be used for preparing polyurethane rigid foam products.

Example 9

The same process conditions as in example 1 were used except that the hydroxylating agent added was isooctanoic acid. The hydroxyl value of the prepared tung oil polyol is 201mgKOH/g, the viscosity is 9210mPa & s, the acid value is 0.85mgKOH/g, the water content is lower than 0.1wt%, the yield is higher than 93.5%, and the tung oil polyol can be used for preparing polyurethane rigid foam products.

Comparative example 1

The same process conditions as in example 1 were used except that no hydroxylating agent was added. The viscosity of the product, 452000 mPas, was too high to be dissolved and could not be used for further hydroxylation reaction, so that it could not be used for the production of polyurethane products.

Comparative example 2

The same processing conditions as in example 1 were used except that soybean oil was used to prepare the polyol. The hydroxyl value of the product is 6.4mgKOH/g, and the product cannot be used for preparing polyurethane products because the product does not undergo hydroxylation reaction and has too low hydroxyl value.

Comparative example 3

The same process conditions and raw material amounts as in example 1 were used, except that a common synthesis reaction vessel was used to prepare tung oil polyol. The hydroxyl value of the product is 197mgKOH/g which is obviously lower than the hydroxyl value of 241mgKOH/g of the product in example 1, which indicates that the utilization rate of formic acid and hydrogen peroxide solution is lower by using a common synthesis reaction kettle, the high hydroxyl value product can not be prepared under the condition of the same dosage, and the production cost is increased.

Comparative example 4

The same process conditions as in example 1 were used except that methanol was used as the hydroxylating agent. The hydroxyl value of the product is 108mgKOH/g, the viscosity is 94500 mPa.s, the actual concentration of methanol in a reaction system is low due to the fact that the methanol is easy to perform esterification reaction with formic acid in the reaction system, the reaction heat release is aggravated due to the fact that an ultrasonic reactor is adopted, the epoxidation reaction rate is high, the reaction selectivity of the methanol serving as a hydroxylation reagent is poor when the temperature is high, the cross-linking side reaction of tung oil epoxidation cannot be avoided by alcohol reagents such as the methanol due to the above factors, the viscosity of the product is increased rapidly, and the polyurethane product cannot be further synthesized.

Claims

1. A method for preparing tung oil polyalcohol by using ultrasonic waves is characterized by comprising the following steps: adding tung oil, carboxylic acid, an inorganic acid catalyst and a hydroxylation reagent into an ultrasonic reactor in proportion, heating to 35-45 ℃, and starting the ultrasonic reactor; then dropwise adding a hydrogen peroxide solution under vigorous stirring, controlling the dropwise adding speed to keep the reaction temperature at 45-65 ℃, and after the dropwise adding is finished, continuing to react for 1-3h to finish the reaction; standing and layering the reaction system, separating out a water phase, and neutralizing, washing and distilling under reduced pressure to obtain tung oil polyol; the hydroxylation reagent is oil-soluble fatty acid, and the oil-soluble fatty acid is selected from one or more of linear chain or branched chain saturated fatty acids of C6-C12.

2. The method of claim 1, wherein: the ultrasonic power of the ultrasonic reactor is 30-200W.

3. The method of claim 1, wherein: the carboxylic acid is formic acid or acetic acid, and the mass ratio of the carboxylic acid to the tung oil is controlled to be 0.03:1-0.3: 1.

4. The method of claim 1, wherein: the inorganic acid catalyst is one or more of sulfuric acid, phosphoric acid or hydrochloric acid, and the dosage of the inorganic acid catalyst is 0.01-1.0% of the mass of the tung oil.

5. The method of claim 1, wherein: the dosage of the oil-soluble fatty acid is 0.1-0.5 times of the mass of the tung oil.

6. The method according to claim 1 or 5, characterized in that: the oil-soluble fatty acid is n-hexanoic acid, n-heptanoic acid, n-octanoic acid, isocaproic acid, isoheptanoic acid or isooctanoic acid.

7. The method of claim 1, wherein: the concentration of the hydrogen peroxide solution is 20-60 wt%, and the dosage is 0.4-0.8 time of the quality of the tung oil.

8. The method of claim 1, wherein: the neutralization reaction is carried out by using a sodium bicarbonate solution with the concentration of 5wt% -20 wt%; the washing temperature is 50-80 ℃; the reduced pressure distillation is to remove the hydroxylation reagent and residual moisture in the system under the conditions of the pressure of 1000-3000Pa and the temperature of 60-120 ℃ so as to ensure that the moisture content of the product is less than 0.1 wt%.