Background
The problem of environmental pollution (white pollution) caused by the uncontrolled production and use of disposable/disposable petroleum-based plastics (e.g., packaging materials, agricultural films, edible cling films, disposable meal boxes, etc.) has attracted serious worldwide attention. Biodegradable polymers based on renewable resources, such as: polylactic acid (PLA), polyglycolic acid (PGA), polylactic-glycolic acid (PLGA), have been recognized by both domestic and foreign researchers as the most promising alternatives to petroleum-based plastics. Polyglycolic acid (PGA), which is a medical absorbable polymer material, has important applications in clinical and medical fields due to its good biodegradability and biocompatibility. As early as 30 years in the last century, Corothers synthesized polyglycolic acid, but the resulting polymer had poor mechanical properties due to its low molecular weight, and was not highly practical as a strength material. Since the 70 s, a large amount of polyglycolic acid was used for the preparation of absorbable sutures. In 1962, Cyanamid in the United states developed the first commercial surgical suture, which was sold under the name "Dexon". In 1975, absorbable suture prepared by copolymerizing glycolic acid and lactic acid at a molar ratio of 90:10 was made available under the trade designation "Vicryl" and the rate of polymer degradation was significantly increased by the addition of lactic acid. After that, the synthesis of medical biodegradable materials is more widely studied, and various glycolic acid copolymers are importantly applied to absorbable suture materials, tissue repair materials, genetic engineering, orthopedic fixation and drug controlled release systems. Currently, there are two main synthetic methods for polyglycolic acid: direct polycondensation and ring opening polymerization processes. The ring-opening polymerization method is also called a two-step method, namely, glycolide is prepared by the dehydration and depolymerization of glycolic acid, and then the polyglycolic acid is prepared by the ring-opening polymerization reaction of the glycolide under the action of a catalyst. Compared with the direct polycondensation method, the ring-opening polymerization method has mild reaction conditions and short reaction time, can controllably synthesize polyglycolic acid products with molecular weight of hundreds of thousands, and is a main preparation method of the commercialized polyglycolic acid. However, the purity and quality of the monomeric glycolide are extremely high.
The preparation method of glycolide is characterized in that glycolic acid serving as a reaction raw material forms an oligomer under the autocatalytic condition. The oligomer is depolymerized under the action of a catalyst to form crude glycolide. The crude glycolide is purified by adopting methods such as recrystallization, rectification, melt crystallization, water washing and the like to obtain polymer-grade glycolide.
(1) Recrystallization process
The recrystallization method is a method for separation and purification by utilizing the difference of the solubility of different substances in the same solvent at different temperatures. For example, Kang et al (Kang Lin, Newark Del, Glycolide purification Process, US5223630) first mix Glycolide with solvents such as acetone and stir, then recrystallize in ethyl acetate under low temperature conditions, the yield of refined Glycolide is 85% or more. Yamaoka et al (yamaoka et al, kakka, a method for producing and purifying a cyclic ester, CN100441575C) dissolving glycolide in a mixed solution of a lower ester or a lower alcohol and a lower ketone, and adding one or more low-boiling solvents such as methanol, ether, and dichloromethane to promote the formation of glycolide crystal nuclei in the supersaturated solution. Zhang Zheng (Zhang Zheng, Lishi Ying, Fengjun, a method for preparing glycolide, CN103242287B) and the like dissolve crude glycolide in one or more solutions of lower esters or lower alcohols, cool and crystallize after refluxing dissolution, filter crystals, and repeatedly recrystallize for purification. The method usually adopts an organic solvent, and is difficult to realize continuous operation, so the method is difficult to be applied to the commercial glycolide production.
(2) Rectification method
The rectification method is a process method for separating and purifying by utilizing the difference of the boiling points of each component in a crude product. For example, in Yamao et al (Yamao et al, Cizhi, Xiaochuan zhixing, a purification method of cyclic ester, CN101616907B), glycolide and a mixture of polyalkylene glycol ethers having a boiling point of 230-450 ℃ and a molecular weight of 150-450 are heated under normal pressure or reduced pressure to form a substantially homogeneous solution state, and the glycolide fraction is collected at an appropriate temperature.
The rectification method is one of the commonly used methods in the commercial glycolide production at present, but the method has the problems of complex process and high energy consumption. In addition, since the crude glycolide usually contains acidic impurities, the polymerization reaction of the glycolide can be caused during the rectification purification, and a polymer residue can be formed.
(3) Melt crystallization process
The melt crystallization method is a method for separating and purifying components in a crude product by utilizing the difference of melting points of the components. The melting point of the glycolide is 84-86 ℃, so that the reaction can be carried out at normal pressure and low temperature, and the operation is simple and safe. Compared with the recrystallization method using the solvent, the method does not need to use additional solvent, reduces the cost and environmental pollution, and has the energy consumption of only 1/3 of rectification. However, acidic impurities (glycolic acid and oligomers) contained in crude glycolide are highly required for the material of the equipment, and the crystallization efficiency is also lowered.
(4) Washing method
The water washing method is a method of separating and purifying the crude product by utilizing the solubility difference of each substance in water. The crude glycolide has small amount of glycolic acid, glycolic acid oligomer and water, and the solubility of acid in water is much higher than that of glycolide, so that the glycolide can be well separated. The presence of moisture in glycolide can also lead to its polymerization or degradation, so that timely drying is critical for purifying glycolide by water washing. The water washing method does not need to use an organic solvent, has low requirements on equipment conditions, is relatively more energy-saving and green, but has different water washing conditions of the glycolide with different qualities, has strict requirements on the subsequent drying process, and is rarely successfully applied in scientific research and industry at present.
Disclosure of Invention
In view of the above disadvantages of the prior art, the present invention aims to provide a method for preparing high purity glycolide, which is used to solve the problems of high glycolide synthesis cost, low production efficiency, high energy consumption, large amount of three wastes, inconvenience for industrialization, etc. in the prior art.
To achieve the above and other related objects, the present invention provides a method for preparing high-purity glycolide, comprising the steps of: heating glycolic acid or glycolate for dehydration or dealcoholization under the action of a catalyst to obtain glycolic acid oligomer, and then heating and carrying out reduced pressure cracking in a depolymerization kettle to obtain crude glycolide; recrystallizing the crude glycolide by using a solvent under the protection of protective gas, and separating a crystal by using melt crystallization after vacuum drying to obtain the polymer grade glycolide with the purity of more than or equal to 99.9%.
Further, the glycolate is selected from at least one of methyl glycolate, ethyl glycolate, propyl glycolate and butyl glycolate.
Further, the catalyst is at least one selected from tin compounds, antimony oxide and zinc compounds.
Optionally, the tin compound is selected from at least one of stannous chloride, stannous octoate, stannous chloride dihydrate, stannic lactate and stannous benzoate, and the zinc compound is selected from at least one of zinc chloride, zinc oxide, diethyl zinc, zinc acetate dihydrate and zinc lactate.
Further, the amount of the catalyst is 0.005-0.1% by weight of glycolic acid or glycolate.
Further, the temperature of the dehydration or dealcoholization reaction is normal temperature to 200 ℃, and the reaction pressure is 1 to 5 KPa.
Alternatively, after glycolic acid or glycolate is added into the reaction kettle, the temperature is increased from the normal temperature to 200 ℃ in a gradient manner, and dehydration or dealcoholization reaction is carried out.
Preferably, the glycolic acid used as the raw material is glycolic acid crystals, the glycolic acid crystals are firstly added into a reaction kettle, the temperature is increased from normal temperature to 90 ℃ in a gradient manner under the normal pressure condition, the glycolic acid crystals are melted, then a catalyst is added, the reaction kettle is vacuumized, the pressure is reduced to 1-5 KPa, the temperature is increased to 200 ℃ in a gradient manner, and dehydration or dealcoholization reaction is carried out; and/or taking glycolate as a raw material, adding the glycolate liquid and a catalyst into a reaction kettle, heating the glycolate liquid and the catalyst to 180 ℃ from normal temperature in a gradient manner under normal pressure, then vacuumizing to reduce the pressure of the reaction kettle to 1-5 KPa, heating the pressure to 200 ℃ in a gradient manner, and carrying out dehydration or dealcoholization reaction.
Further, after completely anhydrous or alcohol-free distillation, dehydration or dealcoholization reaction was completed to obtain a glycolic acid oligomer.
Further, the dehydration or dealcoholization reaction is carried out under the protection of protective gas.
Further, the reaction temperature in the depolymerization kettle is 200-260 ℃, and the reaction pressure is 500-1000 Pa.
Further, the solvent is at least one selected from ethyl acetate, acetone, methanol, ethanol, n-propanol and isopropanol.
Furthermore, the dosage of the solvent is 0.5-0.8 times of the mass of the crude glycolide.
Further, the temperature of solid-liquid separation after recrystallization is 5 to 10 ℃.
Further, the crude glycolide is solid, the crude glycolide is mixed with a solvent, the mixture is heated, the temperature is kept for 30-60min after the crude glycolide is completely dissolved, then the temperature is reduced for crystallization (namely, recrystallization process), and the solid is obtained by suction filtration at 5-10 ℃, namely, the crystal.
Further, the vacuum drying temperature is 50-60 ℃, and the vacuum drying time is not less than 4 h.
Further, the melting crystallization operation comprises film hanging, cooling crystallization, temperature rise and sweating, product collection and mother liquor discharge, wherein glycolide is heated to 85-90 ℃ for melting, and then the temperature is reduced and crystallized at 0.1-0.5 ℃/min, and the temperature difference of temperature reduction is controlled to be 15-30 ℃; then heating up and sweating at the speed of 0.1-0.5 ℃/min, controlling the temperature difference of heating up to 5-20 ℃, and removing the sweating; finally heating and melting to collect glycolide products.
Further, the residue in the depolymerization kettle is mechanically applied to the next batch of cracking reaction.
Further, the recrystallization and melt crystallization mother liquor is mechanically applied to the next batch of dehydration or dealcoholization reaction.
In the present invention, the shielding gas is at least one of the shielding gases commonly used in the art, such as nitrogen, argon, helium, and the like.
As described above, the method for preparing high-purity glycolide according to the present invention has the following advantageous effects:
the method purifies the crude glycolide by using a recrystallization and melt crystallization coupled method, can realize the recycling of materials by mechanically applying mother liquor, does not generate waste, and simultaneously prepares a high-purity (more than or equal to 99.9%) glycolide product with high yield. The invention is a high-efficiency, environment-friendly and high-purity glycolide synthesis method, and is suitable for industrial production.
Detailed Description
The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention.
The invention provides a method for preparing high-purity glycolide, which comprises the following steps:
heating glycolic acid or glycolate for dehydration or dealcoholization under the action of a catalyst to obtain glycolic acid oligomer, and then heating and carrying out reduced pressure cracking in a depolymerization kettle to obtain crude glycolide; recrystallizing the crude glycolide by using a solvent under the protection of protective gas, and separating a crystal by using melt crystallization after vacuum drying to obtain the polymer grade glycolide with the purity of more than or equal to 99.9%.
Further, the glycolate is selected from at least one of methyl glycolate, ethyl glycolate, propyl glycolate and butyl glycolate.
Further, the catalyst is at least one selected from tin compounds, antimony oxide and zinc compounds. Optionally, the tin compound is selected from at least one of stannous chloride, stannous octoate, stannous chloride dihydrate, stannic lactate and stannous benzoate, and the zinc compound is selected from at least one of zinc chloride, diethyl zinc, zinc acetate dihydrate and zinc lactate.
Further, the amount of the catalyst is 0.005-0.1% by weight of glycolic acid or glycolate.
Further, the temperature of the dehydration or dealcoholization reaction is normal temperature-200 ℃, and the reaction pressure is 1-5 KPa.
Alternatively, after glycolic acid or glycolate is added into the reaction kettle, the temperature is increased from the normal temperature to 200 ℃ in a gradient manner, and dehydration or dealcoholization reaction is carried out. Preferably, the glycolic acid used as the raw material is glycolic acid crystals, the glycolic acid crystals are firstly added into a reaction kettle, the temperature is increased from normal temperature to 90 ℃ in a gradient manner under the normal pressure condition, the glycolic acid crystals are melted, then a catalyst is added, the reaction kettle is vacuumized, the pressure is reduced to 1-5 KPa, the temperature is increased to 200 ℃ in a gradient manner, and dehydration or dealcoholization reaction is carried out; the method comprises the steps of adding glycollate liquid and a catalyst into a reaction kettle, heating the glycollate liquid and the catalyst to 180 ℃ from normal temperature in a gradient manner under normal pressure, then vacuumizing to reduce the pressure of the reaction kettle to 1-5 KPa, heating the pressure to 200 ℃ in a gradient manner, and carrying out dehydration or dealcoholization reaction.
Further, after completely anhydrous or alcohol-free distillation, dehydration or dealcoholization reaction is completed to obtain glycolic acid oligomer.
Further, the dehydration or dealcoholization reaction is carried out under the protection of protective gas.
Further, the reaction temperature in the depolymerization kettle is 200-260 ℃, and the reaction pressure is 500-1000 Pa.
Further, the solvent is at least one selected from ethyl acetate, acetone, methanol, ethanol, n-propanol and isopropanol.
Furthermore, the dosage of the solvent is 0.5-0.8 time of the mass of the crude glycolide.
Further, the temperature of solid-liquid separation after recrystallization is 5 to 10 ℃. Specifically, the crude glycolide is a solid, the crude glycolide is mixed with a solvent, the mixture is heated, after the crude glycolide is completely dissolved, the temperature is kept for 30-60min, then the mixture is cooled and crystallized, and the crystallization is carried out at 5-10 ℃, so that the obtained solid is a crystal.
Further, the vacuum drying temperature is 50-60 ℃, and the vacuum drying time is not less than 4 h.
Further, the melt crystallization operation comprises the steps of film hanging, cooling crystallization, heating and sweating, product collection and mother liquor discharge, and the steps are as follows: heating glycolide to 85-90 ℃ for melting, and then cooling and crystallizing at 0.1-0.5 ℃/min, wherein the temperature difference of cooling is controlled at 15-30 ℃; then heating up and sweating at the speed of 0.1-0.5 ℃/min, controlling the temperature difference of heating up to 5-20 ℃, and removing the sweating; finally heating and melting to collect glycolide products.
Further, the residue in the depolymerization kettle is mechanically applied to the next batch of cracking reaction.
Further, the mother liquor of recrystallization and melt crystallization is applied to the next batch of dehydration or dealcoholization reaction.
The protective gas used in the present invention is at least one of the protective gases commonly used in the art, such as nitrogen, argon, helium, etc.
The invention is further illustrated by the following specific examples.
Example 1
The method for preparing high-purity glycolide in the embodiment comprises the following steps:
adding 305g of glycolic acid crystals into a four-neck flask, heating to 90 ℃ under the normal pressure under the protection of nitrogen, adding 0.03g of stannous chloride after the solid is completely melted, then vacuumizing to reduce the system pressure to 3KPa, heating to 200 ℃ in a gradient manner, and obtaining glycolic acid oligomer after complete anhydrous evaporation. Regulating the system pressure to 800Pa, carrying out cracking reaction at the temperature of 200-260 ℃, continuously distilling out light yellow liquid, and cooling to obtain light yellow solid, namely the crude glycolide with the yield of 92%.
Heating and dissolving crude glycolide (214g) and 150g ethyl acetate, preserving heat for 30min after complete dissolution, then cooling and crystallizing, carrying out suction filtration at 10 ℃, carrying out vacuum drying on the solid to obtain a white product, and mechanically applying the mother liquor to the next batch of experiments after removing the solvent. Purifying a recrystallized product through a melt crystallizer, wherein the main operations comprise film hanging, cooling crystallization, heating and sweating, product collection, mother liquor discharge and the like, and the specific process comprises the following steps: firstly heating glycolide to 90 ℃ for melting, then cooling and crystallizing at the speed of 0.2 ℃/min, and controlling the temperature difference of cooling at 20 ℃; then heating and sweating are carried out at the speed of 0.2 ℃/min, the temperature difference of heating is controlled to be 20 ℃, and the sweating liquid is removed; finally heating up and melting to collect glycolide products, and mechanically applying mother liquor to the next batch of reaction. After two times of purification, 171.2g of white glycolide product with the purity of 99.91 percent, the purification yield of 80 percent and the total yield of 73.6 percent is obtained.
Example 2
The method for preparing high-purity glycolide in the embodiment comprises the following steps:
adding 305g of glycolic acid crystals into a four-neck flask, heating to 90 ℃ under the normal pressure under the protection of nitrogen, adding 0.03g of stannous chloride after the solid is completely melted, then vacuumizing to reduce the system pressure to 1KPa, heating to 200 ℃ in a gradient manner, and obtaining glycolic acid oligomer after complete anhydrous evaporation. Regulating the system pressure to 500Pa, carrying out cracking reaction at the temperature of 200-260 ℃, continuously distilling out light yellow liquid, and cooling to obtain light yellow solid, namely the crude glycolide with the yield of 94%.
Heating and dissolving crude glycolide (219g) and 153g ethyl acetate, preserving heat for 30min after complete dissolution, then cooling and crystallizing, carrying out suction filtration at 5 ℃, carrying out vacuum drying on the solid to obtain a white product, and mechanically applying mother liquor after solvent removal to the next batch of experiments; purifying a recrystallized product through a melt crystallizer, wherein the main operations comprise film hanging, cooling crystallization, heating and sweating, product collection, mother liquor discharge and the like, and the specific process comprises the following steps: firstly heating glycolide to 85 ℃ for melting, then cooling and crystallizing at 0.3 ℃/min, and controlling the temperature difference of cooling at 25 ℃; then heating and sweating are carried out at the speed of 0.4 ℃/min, the temperature difference of heating is controlled to be 10 ℃, and the sweating liquid is removed; finally heating up and melting to collect glycolide products, and mechanically applying mother liquor to the next batch of reaction. After two times of purification, 179.6g of white glycolide product is obtained, the purity is 99.93 percent, the purification yield is 82 percent, and the total yield is 77.08 percent.
Example 3
The method for preparing high-purity glycolide in the embodiment comprises the following steps:
adding 305g of glycolic acid crystals into a four-neck flask, heating to 90 ℃ under the normal pressure under the protection of nitrogen, adding 0.03g of stannous chloride after the solid is completely melted, then starting to vacuumize and reduce to 2KPa, heating to 200 ℃ in a gradient manner, and obtaining glycolic acid oligomer after complete anhydrous evaporation. Regulating the system pressure to 600Pa, carrying out cracking reaction at the temperature of 200-260 ℃, continuously distilling out light yellow liquid, and cooling to obtain light yellow solid, namely the crude glycolide with the yield of 94%.
Heating and dissolving crude glycolide (219g) and 131g ethyl acetate, preserving heat for 30min after complete dissolution, then cooling and crystallizing, carrying out suction filtration at 6 ℃, carrying out vacuum drying on the solid to obtain a white product, and mechanically applying mother liquor to the next batch of experiments after removing a solvent; purifying a recrystallized product through a melt crystallizer, wherein the main operations comprise film hanging, cooling crystallization, heating and sweating, product collection, mother liquor discharge and the like, and the specific process comprises the following steps: firstly heating glycolide to 88 ℃ for melting, then cooling and crystallizing at 0.4 ℃/min, and controlling the temperature difference of cooling at 20 ℃; then heating and sweating are carried out at the speed of 0.4 ℃/min, the temperature difference of heating is controlled to be 15 ℃, and the sweating liquid is removed; finally heating up and melting to collect glycolide products, and mechanically applying mother liquor to the next batch of reaction. After twice purification, 188.3g of white glycolide product with the purity of 99.92 percent, the purification yield of 86 percent and the total yield of 80.8 percent are obtained.
Example 4
The method for preparing high-purity glycolide in the embodiment comprises the following steps:
adding 305g of glycolic acid crystals into a four-neck flask, heating to 90 ℃ under the normal pressure under the protection of nitrogen, adding 0.03g of stannous octoate after the solid is completely melted, then vacuumizing to reduce the system pressure to 5KPa, heating to 200 ℃ in a gradient manner, and obtaining glycolic acid oligomer after complete anhydrous evaporation. Regulating the system pressure to 1000Pa, carrying out cracking reaction at the temperature of 200-260 ℃, continuously distilling out light yellow liquid, and cooling to obtain light yellow solid, namely the crude glycolide with the yield of 94.5%.
Heating and dissolving crude glycolide (220g) and ethyl acetate (131 g), preserving heat for 30min after complete dissolution, then cooling and crystallizing, carrying out suction filtration at 8 ℃, carrying out vacuum drying on the solid to obtain a white product, and mechanically applying mother liquor to the next batch of experiments after removing a solvent; purifying a recrystallized product through a melt crystallizer, wherein the main operations comprise film hanging, cooling crystallization, heating and sweating, product collection, mother liquor discharge and the like, and the specific process comprises the following steps: firstly heating glycolide to 87 ℃ for melting, then cooling and crystallizing at 0.5 ℃/min, and controlling the temperature difference of cooling at 30 ℃; then heating and sweating are carried out at the speed of 0.5 ℃/min, the temperature difference of heating is controlled to be 20 ℃, and the sweating liquid is removed; finally heating up and melting to collect glycolide products, and mechanically applying mother liquor to the next batch of reaction. After twice purification, 188.1g of white glycolide product with the purity of 99.94%, the purification yield of 85.5% and the total yield of 80.7% are obtained.
Example 5
The method for preparing high-purity glycolide in the embodiment comprises the following steps:
adding 360g of methyl glycolate and 0.4g of stannous octoate into a four-neck flask, stirring under normal pressure, reacting, heating to 180 ℃, then vacuumizing, reducing the system pressure to 3KPa, heating to 200 ℃, and obtaining the glycolic acid oligomer after no fraction (namely methanol) is evaporated. Regulating the system pressure to 500Pa, carrying out cracking reaction at the temperature of 200-260 ℃, continuously distilling out light yellow liquid, and cooling to obtain light yellow solid, thus obtaining the crude glycolide with the yield of 92.5%.
Heating and dissolving crude glycolide (215g) and 129g ethyl acetate, preserving heat for 30min after complete dissolution, then cooling and crystallizing, carrying out suction filtration at 10 ℃, carrying out vacuum drying on the solid to obtain a white product, and mechanically applying mother liquor after removing a solvent to the next batch of experiments; purifying a recrystallized product through a melt crystallizer, wherein the main operations comprise film hanging, cooling crystallization, heating and sweating, product collection, mother liquor discharge and the like, and the specific process comprises the following steps: firstly heating glycolide to 90 ℃ for melting, then cooling and crystallizing at the speed of 0.1 ℃/min, and controlling the temperature difference of cooling at 15 ℃; then heating and sweating are carried out at the speed of 0.1 ℃/min, the temperature difference of heating is controlled to be 5 ℃, and the sweating liquid is removed; finally heating up and melting to collect glycolide products, and mechanically applying mother liquor to the next batch of reaction. After twice purification, 178.5g of white product is obtained, the purity is 99.94%, the purification yield is 83%, and the total yield is 76.8%.
Comparative example 1
The process steps for preparing high purity glycolide in this comparative example were as follows:
adding 305g of glycolic acid crystals into a four-neck flask, heating to 90 ℃ under the normal pressure under the protection of nitrogen, adding 0.03g of stannous chloride after the solid is completely melted, then vacuumizing to reduce the system pressure to 3KPa, heating to 200 ℃ in a gradient manner, and obtaining glycolic acid oligomer after complete anhydrous evaporation. Regulating the system pressure to 800Pa, carrying out cracking reaction at the temperature of 200-260 ℃, continuously distilling out light yellow liquid, and cooling to obtain light yellow solid, thus obtaining the crude glycolide with the yield of 93%.
Heating and dissolving crude glycolide (216g) and 130g ethyl acetate, preserving heat for 30min after complete dissolution, then cooling and crystallizing, carrying out suction filtration at 10 ℃, carrying out vacuum drying on the solid to obtain a white product, and mechanically applying mother liquor to the next batch of experiments after removing a solvent; after purification, 186.2g of white glycolide product is obtained, the purity is 98.5%, the purification yield is 86%, and the total yield is 80.0%.
Comparative example 2
The process steps for preparing high purity glycolide in this comparative example were as follows:
adding 305g of glycolic acid crystals into a four-neck flask, heating to 90 ℃ under the normal pressure under the protection of nitrogen, adding 0.03g of stannous chloride after the solid is completely melted, then vacuumizing to reduce the system pressure to 3KPa, heating to 200 ℃ in a gradient manner, and obtaining glycolic acid oligomer after complete anhydrous evaporation. Regulating the system pressure to 800Pa, carrying out cracking reaction at the temperature of 200-260 ℃, continuously distilling out light yellow liquid, and cooling to obtain light yellow solid, thus obtaining the crude glycolide with the yield of 93.5%.
Purifying crude glycolide by a melt crystallizer, wherein the main operations comprise film hanging, cooling crystallization, heating and sweating, product collection, mother liquor discharge and the like; controlling the crystallization temperature difference to be 15-30 ℃, sweating and heating to be 5-20 ℃, and mechanically applying the mother liquor to the next batch of reaction. After two times of purification, 189.3g of white glycolide product is obtained, the purity is 99.05%, the purification yield is 86.8%, and the total yield is 81.2%.
As can be seen from the above, the purity of the glycolide products of examples 1 to 5 was not less than 99.9%, which was higher than that of comparative examples 1 to 2. The above results show that the purification of crude glycolide by the method of the invention using rectification coupled with melt crystallization can further improve the quality of the product compared with the product purified by a single recrystallization or melt crystallization method.
The foregoing embodiments are merely illustrative of the principles and utilities of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or change the above-mentioned embodiments without departing from the spirit and scope of the present invention. Accordingly, it is intended that all equivalent modifications or changes which can be made by those skilled in the art without departing from the spirit and technical spirit of the present invention be covered by the claims of the present invention.