CN108837847B - Catalyst and method for alcoholysis of polylactic acid - Google Patents
Catalyst and method for alcoholysis of polylactic acid Download PDFInfo
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Abstract
The invention belongs to the alcoholysis reaction of polylactic acid, and discloses a catalyst and a method for catalyzing alcoholysis of polylactic acid (PLA) to recover methyl lactate. The method uses a eutectic solvent ChCl-Zn (OAc)2As a catalyst, the reactant polylactic acid and alcohol are subjected to alcoholysis reaction to generate lactate. The catalyst and the method provided by the invention can effectively improve the alcoholysis rate of PLA and the yield of lactate, and meanwhile, the catalyst has the advantages of simple synthesis method, low cost and extremely small dosage, and is beneficial to industrial production.
Description
Technical Field
The invention relates to a catalyst and a method for alcoholysis of polylactic acid.
Background
Polylactic acid (PLA) is a polymer obtained by polymerizing lactic acid as a main raw material, and has sufficient raw material sources and can be regenerated. The production process of polylactic acid is pollution-free, and the product can be biodegraded, so that the polylactic acid can be recycled in nature, and is an ideal green high polymer material. With the widespread use of PLA, the amount of production and consumption thereof is sharply increased, eventually resulting in an increasingly expanded accumulation of waste polylactic acid. Although polylactic acid can be completely degraded in nature, the degradation period is too long, and carbon dioxide generated after degradation pollutes air and aggravates greenhouse effect, so that not only production research of PLA needs to be emphasized, but also degradation and recycling of PLA need to be emphasized.
The thermal cracking of polylactic acid is mostly carried out in a molten state, the reaction requires a high temperature, the chain scission of the high polymer is irregular, and a large amount of byproducts are generated and difficult to obtainTo a target product of high purity, the chemical depolymerization method is more practical than the thermal cracking method, wherein the alcoholysis method is one of the effective approaches, for example, S-nchez, etc. researches on the alcoholysis reaction of a mixture of PLA and polyethylene terephthalate (PET) catalyzed by zinc acetate, and experiments adopt infrared spectroscopy (FT-IR) and nuclear magnetic resonance hydrogen spectroscopy (PET)1H NMR and nuclear magnetic resonance carbon Spectroscopy: (13C NMR) and the like. The experiment shows that: compared with other solvents such as ethanol, glycol and the like, the mixture has better alcoholysis reaction in methanol, and the yield of the PLA alcoholysis product methyl lactate can reach 65% after reflux reaction for 15 hours under the standard atmospheric pressure, but PET hardly reacts, and under the condition, the aim of selectively recovering PLA from the waste plastic mixture is achieved. In the prior art, the ionic liquid is used as a catalyst to catalyze the alcoholysis reaction of PLA and methanol, the catalytic effect is obvious, although the ionic liquid as the catalyst can recover the catalyst, the ionic liquid is expensive and complex to prepare, and is not beneficial to industrial application, in addition, a large amount of volatile organic solvents are used in the synthesis, purification and recovery processes of the ionic liquid, and part of the ionic liquid as the catalyst or the solvent is lost to the environment, so that the environment is influenced to different degrees. Therefore, it is of great significance to find a new catalyst for catalyzing the alcoholysis reaction of polylactic acid, which has high catalytic efficiency, can be recycled, has low raw material price and is simple and convenient in synthesis process.
Disclosure of Invention
In order to solve the problems that the catalyst for alcoholysis of polylactic acid in the prior art is large in dosage, high in cost, incapable of biodegradation and still needs to improve the catalytic effect, the invention provides the catalyst for alcoholysis of polylactic acid.
In order to solve the technical problems, the invention adopts the following technical scheme:
the invention provides a catalyst for alcoholysis of polylactic acid, which is a eutectic solvent (DES) ChCl-Zn (OAc) prepared from choline chloride and zinc acetate2(ii) a The eutectic solvent ChCl-Zn (OAc)2The preparation method comprises the following steps: under the protection of nitrogen, choline chloride and zinc acetate with the molar ratio of 1:1 are heated and stirred to react for 2 hours at the temperature of 80-100 ℃,until the solid mixture becomes transparent and clear liquid, the eutectic solvent ChCl-Zn (OAc) is generated2。
At the beginning, the inventors tested various eutectic solvents as catalysts for the alcoholysis of PLA, but many of them had no catalytic effect on the alcoholysis of PLA, and even once the inventors abandoned the initial efforts for finding suitable catalysts for catalyzing the alcoholysis of PLA, however, as the tests were carried out, we surprisingly found that acidic eutectic solvents had little catalytic effect on the alcoholysis of PLA, while certain kinds of basic eutectic solvents, namely choline chloride eutectic solvents, ChCl-Zn (OAc)2The catalytic effect on the alcoholysis of PLA is probably because the alcoholysis reaction of PLA is a nucleophilic substitution reaction, and the stronger the alkalinity of the nucleophilic reagent is, the easier the nucleophilic substitution reaction is. The invention further explores ChCl-Zn (OAc) through infrared spectroscopy (IR) and semi-micro Gel Permeation Chromatography (GPC)2The reaction mechanism that catalyzes the alcoholysis of PLA, found by IR and GPC: as the conversion rate increased, the small molecular weight oligomers increased and the large molecular weight polymers decreased, and from this, it was presumed that the eutectic solvent ChCl-Zn (OAc)2Possible reaction mechanisms to catalyze the alcoholysis of PLA, ChCl-Zn (OAc)2The method has the advantages that the method forms hydrogen bonds with alcohol, so that the nucleophilicity of the alcohol is improved, the alcohol can attack a PLA chain to break the PLA chain, polymers with different molecular weights are formed, and the target product lactate is finally generated along with the continuous breaking of the PLA chain.
The invention also provides a method for alcoholysis of polylactic acid, which uses the eutectic solvent ChCl-Zn (OAc)2As a catalyst, the reactant polylactic acid and alcohol are subjected to alcoholysis reaction to generate lactate; the temperature of the alcoholysis reaction is 110-2The mass ratio of the polylactic acid to the polylactic acid is 0.06-0.14: 1.
Preferably, the temperature of the alcoholysis reaction is 125 ℃, the reaction time is 3h, the molar ratio of the polylactic acid and the alcohol serving as reactants is 1:7, and the catalyst eutectic solvent ChCl-Zn (OAc)2The mass ratio of the polylactic acid to the polylactic acid is 0.06: 1.
Preferably, the alcohol is selected from methanol, ethanol, propanol, isopropanol, n-butanol or isobutanol; the type of the organic alcohol is not particularly limited, and can be reasonably selected according to actual production needs, and the most common methanol for the alcoholysis reaction of the polylactic acid can be selected for alcoholysis.
The method for alcoholysis of polylactic acid also comprises a post-treatment step, wherein the alcoholysis reaction liquid is transferred into a container bottle together with alcohol, and then the liquid product lactate is obtained after atmospheric distillation and reduced pressure distillation, and the residue after distillation is recycled by using alcohol as a catalyst.
Compared with the catalyst commonly used in the prior art, the invention adopts green, environment-friendly, cheap and efficient ChCl-Zn (OAc)2The eutectic solvent is used for catalyzing the alcoholysis reaction of PLA, the choline chloride eutectic solvent has a good catalytic effect on the alcoholysis of PLA, and the catalyst is low in cost, simple to prepare, can be recycled, and can make up for the defects of the traditional catalyst; while ChCl-Zn (OAc)2The eutectic solvent has the properties of difficult volatilization, difficult combustion and no toxicity, and is biodegradable, so that the ionic liquid serving as a catalyst or a part of the solvent is lost into the environment and can not affect the environment to different degrees. Test results show that the catalyst and the method provided by the invention have the PLA alcoholysis rate basically reaching 93.5-98.9%, and the yield of lactate reaching 90.8-93.7%. According to the technical scheme, the invention adopts ChCl-Zn (OAc)2The eutectic solvent catalyzes the alcoholysis of PLA, so that the alcoholysis rate of PLA and the yield of lactate can be effectively improved, and meanwhile, the catalyst is simple in synthesis method, low in cost and extremely low in dosage, and is beneficial to industrial production.
Drawings
FIG. 1 is an IR spectrum of the PLA methanolysis product of example 5 with an IR spectrum of a methyl lactate standard;
FIG. 2 is a gas chromatogram of the PLA methanolysis reaction product of example 5;
FIG. 3 shows ChCl-Zn (OAc) in example 62A comparative diagram for the reuse performance investigation;
FIG. 4 depicts ChCl-Zn (OAc) before and after reuse in example 62An infrared spectrum of (1).
Detailed Description
The invention discloses a catalyst and a method for alcoholysis of polylactic acid. Those skilled in the art can modify the process parameters appropriately to achieve the desired results with reference to the disclosure herein. It is expressly intended that all such similar substitutes and modifications which would be obvious to those skilled in the art are deemed to be included in the invention. While the methods and applications of this invention have been described in terms of preferred embodiments, it will be apparent to those of ordinary skill in the art that variations and modifications in the methods and applications described herein, as well as other suitable variations and combinations, may be made to implement and use the techniques of this invention without departing from the spirit and scope of the invention.
The following detailed description of the invention refers to specific embodiments thereof for better understanding by those skilled in the art.
Example 1ChCl-Zn (OAc)2Preparation of eutectic solvent
Under the protection of nitrogen, choline chloride and zinc acetate in a molar ratio of 1:1 are heated and stirred to react for 2 hours at 90 ℃ until a solid mixture becomes transparent clear liquid to generate a eutectic solvent ChCl-Zn (OAc)2。
Example 2 selection of PLA alcoholysis reaction conditions
With ChCl-Zn (OAc)2Selecting the yield of methyl lactate as a standard for the catalyst, analyzing the influence of the reaction time, the reaction temperature, the catalyst dosage and the methanol dosage on the experiment by a response surface analysis method, and screening out better reaction conditions.
29 sets of experiments were performed with the three-level design according to the four experimental factors in Table 1, the results are shown in Table 2, the experimental data of Table 2 were analyzed for variance, and the results of regression model analysis of variance are shown in Table 3.
TABLE 1 response surface test factor level table
Table 2 response surface test protocol and results
TABLE 3 regression model analysis of variance
According to the regression coefficient estimation value, the contribution rate of each factor is as follows: b > C > A > D, namely the influence of four factors on the PLA methanol alcoholysis reaction is as follows: reaction time > catalyst amount > reaction temperature > methanol amount.
After single factor investigation, optimization yielded ChCl-Zn (OAc)2The preferable conditions for catalyzing the alcoholysis reaction of PLA methanol are as follows: t125 deg.C, T3 h, m (ChCl-Zn (OAc))2): m (pla) 0.06:1, n (pla) n (methanol) ═ 7: 1, the yield of the product methyl lactate under the condition can reach 93.7 percent at most.
Example 3PLA alcoholysis reaction
A polytetrafluoroethylene liner was prepared and charged with magnetons, 4g of PLA by mass, and 0.56g of ChCl-Zn (OAc) by mass2Finally, adding a certain amount of isopropanol, wherein n (isopropanol) ═ 7 (n) (PLA), placing the lining into a high-pressure reaction kettle with a magnetic stirring thermometer, reacting for 2 hours at 110 ℃, cooling to room temperature after the reaction is finished, transferring the reaction liquid into a single-mouth bottle after the kettle is opened, washing the lining with a small amount of isopropanol for multiple times, transferring the lining into the single-mouth bottle together, distilling unreacted isopropanol under normal pressure, and then carrying out reduced pressure distillation to collect a liquid product (the mass is 6.66g), wherein the product is characterized by isopropyl lactate by an infrared spectrogram; dissolving the still residue (the mass of the residue is 0.82g) after distillation with isopropanol, and directly recycling the residue as a catalyst; the alcoholysis rate of PLA is 93.5%, and the yield of isopropyl lactate is 90.8%.
Wherein M (PLA) represents the molar mass of the PLA repeat unit and M (lactate) represents the molar mass of the lactate.
Example 4PLA alcoholysis reaction
A polytetrafluoroethylene liner was prepared and charged with magnetons, 4g by mass of PLA was added, and 0.24g by mass of ChCl-Zn (OAc) was added2Finally, adding a certain amount of ethanol n (PLA), wherein n (ethanol) is 2; putting the lining into a high-pressure reaction kettle with a magnetic stirring thermometer, reacting for 2.5h at 120 ℃, cooling to room temperature after the reaction is finished, opening the kettle, transferring reaction liquid into a single-mouth bottle, washing the lining for many times by using a small amount of ethanol, transferring the lining into the single-mouth bottle together, distilling unreacted ethanol under normal pressure, carrying out reduced pressure distillation, and representing a distilled liquid product (with the mass of 6g) as ethyl lactate by using an infrared spectrogram; dissolving the residual still residue (with the mass of 0.45g) after distillation with ethanol, and directly recycling the residual still residue as a catalyst; the alcoholysis rate of PLA is 94.8%, and the yield of ethyl lactate is 91.5%.
Example 5PLA alcoholysis reaction
A polytetrafluoroethylene liner was prepared and charged with magnetons, 4g by mass of PLA was added, and 0.24g by mass of ChCl-Zn (OAc) was added2Finally adding a certain amount of methanol, n (PLA) 1: 7; putting the lining into a high-pressure reaction kettle with a magnetic stirring thermometer, reacting for 3 hours at 125 ℃, cooling to room temperature after the reaction is finished, opening the kettle, transferring the reaction liquid into a single-mouth bottle, washing the lining with a small amount of methanol for multiple times, transferring the lining into the single-mouth bottle together, distilling unreacted methanol under normal pressure, and carrying out reduced pressure distillation to obtain a liquid product (the mass is 5.43g), wherein an infrared spectrogram of the liquid product and an infrared spectrogram of a methyl lactate standard sample are shown in figure 1, and a gas chromatogram of a PLA methanol alcoholysis reaction product; dissolving the distilled kettle residue (the mass is 0.3g) with methanol, and directly recycling the kettle residue as a catalyst; the PLA alcoholysis rate was 98.5%, and the yield of BPA was 93.7%.
In FIG. 1, a is an infrared spectrum of a standard sample, b is an infrared spectrum of an alcoholysis product, 3445cm-12986cm below the peak of the absorption of the stretching vibration attributed to O-H-1、2957cm-1Telescopic vibration absorption peak of belonged C-H, 1743cm-1Is a peak generated by stretching vibration of C ═ O, 1372cm-1is-CH31223cm of bending vibration absorption peak of-1、1132cm-1The product is proved to be methyl lactate by infrared analysis, and the product is compared with an infrared spectrogram of a methyl lactate standard sample, and two curves are basically completely matched, so that the PLA methanol alcoholysis product is proved to be methyl lactate.
The data in fig. 2 show that a is the gas chromatogram of the standard sample, b is the gas chromatogram of the product, and under the same detection conditions, compared with the gas chromatogram of the methyl lactate standard sample, the peak position and retention time of the product are basically consistent with those of the standard sample, so that the reaction product can be determined to be methyl lactate. The purity of methyl lactate is 100% according to the spectrogram.
Example 6 catalyst ChCl-Zn (OAc)2Is repeatedly used
Para catalyst ChCl-Zn (OAc)2The cyclic utilization was carried out, the reaction conditions of the cyclic utilization were the same as those of example 5, and the repeatability of the catalyst-catalyzed PLA methanolysis experiment was evaluated in turn, with the results shown in table 4.
TABLE 4 repeat experiments under preferred reaction conditions
The data in Table 4 show that the catalyst ChCl-Zn (OAc) obtained by the preparation method provided by the invention2The repeatability of the catalytic PLA methanolysis experiment is good.
Para ChCl-Zn (OAc)2The recycling performance of the alloy is examined, and the result is shown in figure 3. From FIG. 3, ChCl-Zn (OAc)2The first time of recycling, the conversion rate of PLA is 98.3%, the yield of methyl lactate is 93.5%, and the change is very small compared with that before recycling; after 5 times of recycling, the conversion rate of PLA and the yield of methyl lactateThe rate can also be maintained at a higher value, indicating that ChCl-Zn (OAc)2Has good recycling performance in the PLA methanol alcoholysis reaction, and before and after the recycling, ChCl-Zn (OAc)2The catalytic activity of (a) is substantially unchanged.
To further verify that DES has good recycling performance, 5 recycles of ChCl-Zn (OAc)2And fresh ChCl-Zn (OAc)2Performing structural characterization, wherein the characterization structure is shown in FIG. 4, a is an infrared spectrogram before recycling, b is an infrared spectrogram after recycling for 5 times, and the infrared spectrogram is obtained by recycling for 5 times ChCl-Zn (OAc)2With fresh ChCl-Zn (OAc)2By comparison of the IR spectra, ChCl-Zn (OAc) was found to be reused 5 times2With fresh ChCl-Zn (OAc)2The main characteristic absorption peaks of (A) are basically consistent, which indicates before and after the reuse, ChCl-Zn (OAc)2No changes in structure and composition, further elucidating ChCl-Zn (OAc)2Has good recycling performance.
Comparative example 1 selection of catalyst types
The catalysts of example 5 were sequentially modified to the catalysts shown in the table in the same reaction conditions as in example 5, and the catalytic effects of the respective catalysts are shown in table 5. Among them, other kinds of the choline chloride DES in table 5 were prepared in the same manner as in example 1.
TABLE 5 Effect of different Choline eutectic solvents on the alcoholysis reaction of PLA
The data in table 5 show that different catalysts have different catalytic effects on PLA degradation under the same reaction conditions. ChCl-C2H2O4·2H2O, ChCl-2 acetamide, ChCl-PTSA, ChCl-succinic acid, ChCl-adipic acid and ChCl-benzoic acid have no catalytic effect on PLA; and ChCl-2ZnCl2、ChCl-2Urea、ChCl-2CuCl2·2H2O、ChCl-Zn(OAc)2、ChCl-2MnCl2·4H2O has a certain catalytic effect on the degradation of polylactic acid, wherein ChCl-2CuCl is particularly used2·2H2O and ChCl-Zn (OAc)2The catalytic effect is better, but when the catalyst is recycled, the eutectic solvent ChCl-2CuCl2·2H2O generates a large amount of flocculent precipitates after being added with water, and the structure of the catalyst is changed and cannot be recycled, so that the ChCl-Zn (OAc) provided by the invention2The catalyst has the best catalytic effect when used for catalyzing PLA. And the invention employs ChCl-Zn (OAc)2Simple preparation method, low cost, eutectic solvent ChCl-Zn (OAc)2Is biodegradable, and can avoid the influence of ionic liquid as catalyst or solvent on environment in different degrees.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (6)
1. The method for depolymerizing the lactic acid by alcohol is characterized in that a catalyst used in the method is a eutectic solvent ChCl-Zn (OAc) prepared from choline chloride and zinc acetate2(ii) a Using said eutectic solvent ChCl-Zn (OAc)2As a catalyst, the reactant polylactic acid and alcohol are subjected to alcoholysis reaction to generate lactate.
2. The method of claim 1, wherein the eutectic solvent is ChCl-zn (oac)2The preparation method comprises the following steps: under the protection of nitrogen, choline chloride and zinc acetate in a molar ratio of 1:1 are heated and stirred to react for 2 hours at the temperature of 80-100 ℃ until a solid mixture becomes transparent clear liquid to generate a eutectic solvent ChCl-Zn (OAc)2。
3. The method of claim 1, wherein the temperature of the alcoholysis reaction is 110-The time of (1) is 1-3h, the molar ratio of the reactants polylactic acid and alcohol is 1:2-7, and the catalyst is ChCl-Zn (OAc)2The mass ratio of the polylactic acid to the polylactic acid is 0.06-0.14: 1.
4. The method of claim 3, wherein the alcoholysis reaction is carried out at 125 ℃ for 3h, the molar ratio of polylactic acid and alcohol is 1:7, and the catalyst eutectic solvent is ChCl-Zn (OAc)2The mass ratio of the polylactic acid to the polylactic acid is 0.06: 1.
5. The method of claim 1 or 3 or 4, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol, isopropanol, n-butanol, and isobutanol.
6. The method of claim 1, further comprising a post-treatment step, wherein the alcoholysis reaction solution is transferred into a container bottle together with alcohol, and then the liquid product lactate is obtained after atmospheric distillation and reduced pressure distillation, and the residue after distillation is dissolved with alcohol and used as a catalyst for recycling.
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