Process for the preparation of a non-metallic catalyst for the two-step synthesis of lactide

Abstract

The application discloses a preparation method of a non-metal catalyst for synthesizing propiolactone by a two-step method. The method comprises the following steps: preparing a grafted monomer with a carboxylic acid group by a Friedel-Crafts acylation reaction of an acid anhydride and an aromatic hydrocarbon; preparing a PP matrix by a thermal-initiated grafting reaction of styrene to a PP molecular chain; and connecting the grafted monomer with the carboxylic acid group and the PP matrix by a Friedel-Crafts alkylation reaction by using dimethoxymethane as an alkylation reagent, so as to obtain a PP grafted copolymer catalyst which can be used for synthesizing propiolactone by a two-step method. In the preparation method, no peroxide initiator is used, and the prepared PP grafted copolymer has the advantages of large molecular weight and high carboxyl content, and can be used for catalyzing and synthesizing propiolactone.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of chemical production, and particularly relates to a preparation method of a non-metal catalyst for synthesizing lactide by a two-step method. BACKGROUND

[0002] Petroleum-based plastics are widely used due to their high quality and low price. However, they are difficult to be effectively recycled, resulting in a large number of plastic products being discarded or accumulated at will, causing an increasingly serious environmental pollution problem. In order to solve the problem of "white pollution", many biodegradable plastics have been developed. Among them, polylactic acid (PLA) has attracted widespread attention due to its wide raw material sources and excellent comprehensive performance, and researchers have developed PLA films, fibers, foams and other materials. There are two main routes for the preparation of PLA. One is direct polycondensation, but it is difficult to obtain high molecular weight PLA due to the difficulty in removing water in the later stage. The other is the ring-opening polymerization of lactide, that is, lactide is first prepared from lactic acid, and then PLA is prepared by ring-opening polymerization of lactide. This method can obtain PLA with high molecular weight and excellent mechanical properties, and is currently the main route for the synthesis of PLA. Therefore, in the lactide ring-opening polymerization route, the efficient synthesis of lactide is one of the key technologies restricting the development of the PLA industry.

[0003] There are one-step and two-step process routes for the synthesis of lactide. The one-step process directly converts lactic acid into lactide through dehydration and cyclization, but the preparation process generally involves the use of a large amount of organic solvent, which also brings great difficulty to the separation and purification of lactide. The current industrial method for producing lactide mainly uses the two-step process. In the two-step process, lactic acid is first polymerized into low molecular weight PLA under the action of a catalyst, and then PLA undergoes depolymerization to generate lactide under higher reaction temperature and vacuum degree. The catalysts for industrial production of lactide are mainly metal salt compounds such as stannous octoate, stannous chloride, stannous oxide, etc. The use of such catalysts on the one hand is easy to cause metal residues in the lactide product, and leads to the decrease of optical purity of lactide; on the other hand, the stannous catalysts are dispersed in the residual material after the reaction and are difficult to recycle. In addition to metals, inorganic acids and organic acids can also catalyze the synthesis of lactide. Inorganic strong acids such as sulfuric acid and hydrochloric acid can easily lead to many side reactions due to their strong acidity, which seriously affects the purity and yield of lactide, and also causes corrosion of equipment; small molecule organic strong acids can catalyze the synthesis of lactide well, but have certain volatility, which can pollute the product and affect the purity of the product. If a polymer containing carboxylic acid groups is used as a catalyst for the synthesis of lactide from lactic acid, the catalyst has a high molecular weight and is difficult to volatilize itself, so the synthesized lactide is not easy to be polluted. As a polymer, polypropylene (PP) has the advantages of low cost, easy availability, stable properties, non-toxicity, high molecular weight, and low volatile content. If carboxyl groups are introduced into the PP molecular chain through grafting reaction and a PP graft copolymer is prepared, an excellent catalyst for catalyzing the dehydration and cyclization of lactic acid to synthesize lactide can be obtained.

[0004] Currently, the melt grafting method is usually used to graft carboxylic acid groups onto the PP molecular chain to prepare a PP graft copolymer, but this method uses peroxide initiators in the preparation process, which can easily cause β-chain scission reaction of the PP main chain, resulting in the decrease of the molecular weight of the PP and the generation of a large amount of small molecules, which increases the volatile content of the prepared PP graft copolymer. If the PP graft copolymer is used as a catalyst for the synthesis of lactide, it will also pollute the product lactide. In addition, the peroxide initiators also have certain risks in storage and use. SUMMARY

[0005] In view of the problem that the catalyst residues in the two-step process for preparing lactide can pollute the product, the present application provides a method for preparing a polypropylene (PP) graft copolymer based on Friedel-Crafts alkylation reaction. The method does not use peroxide initiators in the preparation process, and the prepared catalyst PP graft copolymer has the advantages of high molecular weight and high carboxyl content, and can be used for catalyzing the synthesis of lactide.

[0006] To achieve the above object, the present application adopts the following technical scheme:

[0007] A preparation method of a non-metal catalyst for synthesizing lactide by a two-step method, which is to graft carboxylic acid groups on the PP molecular main chain by using Friedel-Crafts alkylation reaction under the condition of not using peroxide initiator, to play a dual role of catalyzing lactic acid dehydration polymerization and low molecular weight PLA depolymerization, so as to obtain a PP grafted copolymer catalyst suitable for synthesizing lactide by a two-step method; it comprises the following steps:

[0008] (1) Preparation of grafted monomer: a grafted monomer with carboxylic acid groups is prepared by Friedel-Crafts acylation reaction of cyclic anhydride and aromatic hydrocarbon in the presence of Lewis acid; wherein the aromatic hydrocarbon plays a dual role of reactant and solvent in the reaction;

[0009] (2) Preparation of PP matrix: styrene is grafted to the PP molecular chain by thermal initiation to prepare a styrene grafted PP matrix;

[0010] (3) Preparation of grafted copolymer: dimethoxymethane is used as an alkylation reagent to connect the grafted monomer prepared in step (1) and the PP matrix prepared in step (2) by Friedel-Crafts alkylation reaction, so as to introduce carboxylic acid groups on the PP molecular main chain, and prepare a PP grafted copolymer.

[0011] Further, the cyclic anhydride in step (1) is one or more of succinic anhydride, glutaric anhydride, and maleic anhydride.

[0012] Further, the aromatic hydrocarbon in step (1) is one or more of benzene, naphthalene or its derivatives, such as toluene, xylene, etc. Preferably, an alkyl-substituted benzene ring derivative is used, wherein the alkyl group can be one or more methyl, ethyl, propyl, butyl, etc., and more preferably, a meta-disubstituted alkyl derivative is used. The introduced alkyl group is an electron-donating group, which can improve the reactivity of the benzene ring and is conducive to the grafting reaction; considering the positioning ability and steric effect of the alkyl substituent, the meta-disubstituted aromatic ring has the best reactivity.

[0013] Further, the Lewis acid in step (1) is one or more of ferric chloride and aluminum chloride.

[0014] Further, step (2) specifically uses an extruder to graft styrene to the PP molecular chain by melt grafting reaction, the extruder temperature is controlled at 160-250 ℃, the extruder speed is controlled at 100-400 rpm, the screw length-diameter ratio is 25-50, and the residence time is 30-60 s.

[0015] Further, step (3) specifically dissolves the PP matrix in a solvent, then adds dimethoxymethane and a catalyst, refluxes at 80-120 ℃ for 1-2 h, and then adds the grafted monomer, and continues to reflux at the same temperature for 2-4 h.

[0016] Further, the amount of dimethoxymethane used is 5-20% of the mass of the PP matrix, the amount of catalyst used is 0.5-2.0% of the mass of the PP matrix, and the amount of grafted monomer used is 5-20% of the mass of the PP matrix.

[0017] Further, the solvent is one or more of decalin and dichloromethane; the catalyst is one or more of aluminum trichloride and iron trichloride, and preferably aluminum trichloride.

[0018] The non-metallic catalyst prepared above can be used to catalyze the two-step synthesis of lactide, and the specific operation is as follows:

[0019] (1) Preparation of low-molecular-weight polylactic acid (oPLA) by pre-polymerization of lactic acid: 100 parts of lactic acid and 0-10 parts of the PP grafted copolymer catalyst (preferably 3-5 parts) are added to a reactor, stirring is started, the stirring speed is 200-800 rpm, the vacuum degree is 0.05-0.1 mPa, the temperature is 120-150°C, and the reaction time is 4-8 h; after the pre-polymerization is completed, the molecular weight of the obtained pre-polymer is determined by a titration method to be 400-4000. During the pre-polymerization of lactic acid, the carboxylic acid groups on the molecular chain of the catalyst PP grafted copolymer catalyze the esterification reaction between lactic acid molecules, and the high vacuum degree ensures that the water produced in the reaction is removed from the system, constantly driving the reaction to move in the direction of polymerization.

[0020] (2) Preparation of lactide by depolymerization of oPLA: the oPLA prepared in step (1) is used as a reactant to synthesize lactide by depolymerization; the depolymerization temperature is 200-220°C, the vacuum degree is 0.095-0.1 mPa, and the reaction time is 0.5-1 h. During the depolymerization, the oPLA is catalyzed by the carboxylic acid groups on the molecular chain of the catalyst PP grafted copolymer to undergo a depolymerization reaction to form lactide, and the high vacuum degree causes the product to be removed from the system, further driving the depolymerization reaction to proceed.

[0021] The Friedel-Crafts alkylation reaction is a reaction in which an alkylating agent and hydrogen on an aromatic ring undergo a substitution reaction under the catalysis of a Lewis acid to generate an alkyl-substituted aromatic hydrocarbon. Therefore, the grafting monomer containing an aromatic ring and the PP matrix modified with an aromatic ring are prepared, and dimethoxymethane is used as an alkylating agent to respectively undergo an alkylation reaction with the grafting monomer containing an aromatic ring and the PP matrix modified with an aromatic ring under the catalysis of a Lewis acid, so that the grafting monomer is grafted to the PP matrix to prepare the PP grafted copolymer catalyst. This method avoids the use of a peroxide initiator in the conventional PP melt grafting process, and the prepared PP grafted copolymer catalyst has the advantages of high molecular weight, high grafting rate, low volatile content, and no pollution to the product lactide, and can be used to catalyze the synthesis of lactide by dehydration and cyclization of lactic acid.

[0022] The significant advantages of the present application are:

[0023] (1) The preparation process of the present application uses styrene modified PP as the matrix, and then the carboxylic acid monomer containing aromatic ring is grafted onto the aromatic group of the modified PP through Friedel-Crafts alkylation reaction. The catalyst preparation process does not use dangerous peroxide as initiator, avoiding the safety hidden danger of peroxide in storage and use, and at the same time, the molecular weight of the catalyst is higher, the residual of unreacted carboxylic acid monomer and other volatile components is lower, which can effectively avoid the pollution to lactide product.

[0024] (2) The catalyst prepared by the method has the advantages of high grafting rate, large molecular weight, weak corrosion to equipment at high temperature, etc., and the use of weak acid instead of traditional metal catalyst as active center can make the obtained lactide have high optical purity. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 FTIR spectrum of MAH-Xylene prepared for Example 1.

[0026] Figure 2 FTIR spectrum of the product prepared for Examples 1-3. DETAILED DESCRIPTION

[0027] A preparation method of a non-metallic catalyst for two-step synthesis of lactide, which specifically comprises the following steps:

[0028] (1) Preparation of grafting monomer: cyclic anhydride and aromatic hydrocarbon are added into a reactor at a molar ratio of 1:4-1:10, and stirred while heating to disperse, then Lewis acid is added in portions at 25-50℃, the molar ratio of cyclic anhydride to Lewis acid is 1:2-1:4, reflux for 1-2 h, then stop the reaction; after the reaction is completed, the reaction liquid is poured into ice hydrochloric acid to remove the Lewis acid, and the crystallization is obtained after cooling, then the grafting monomer is obtained by recrystallization;

[0029] (2) Preparation of PP matrix: 0.5-20 % of styrene by mass is added to polypropylene (PP) resin, and after mixing uniformly, it is extruded and pelletized by an extruder, the extruder temperature is controlled at 160-250℃, the extruder speed is controlled at 100-400 rpm, the screw length-diameter ratio is 25-50, and the residence time is 30-60 s; the PP pellets after extrusion are refined by dissolving with xylene and precipitating with acetone, and the unreacted styrene is removed, and the PP matrix is obtained;

[0030] (3) Preparation of the graft copolymer: the PP matrix is dissolved in a solvent, then 5-20% of dimethoxymethane by mass of the PP matrix is added, after mixing, 0.5-2.0% of a catalyst by mass of the PP matrix is added, reflux is carried out at 80-120°C for 1-2 h, dimethoxymethane is reacted with the benzene ring of the PP matrix, then 5-20% of a graft monomer by mass of the PP matrix is added, and reflux is continued at the same temperature for 2-4 h, so as to introduce carboxylic acid on the PP molecular main chain; after the reaction is completed, the reaction liquid is poured into a large amount of acetone, the product is precipitated, and after being extracted, dried, and then the crude product is obtained after the removal of dimethoxymethane and the graft monomer; then the crude product is redissolved in dimethylbenzene, after being cooled to 75-90°C, dilute hydrochloric acid is added to reflux for 1 h to remove unreacted substances, after the reflux is completed, the solution is poured into acetone, after the product is precipitated, it is extracted, dried, and then the PP graft copolymer catalyst for the synthesis of lactide is prepared.

[0031] In the step (1), the cyclic anhydride is one or more of succinic anhydride, glutaric anhydride, and maleic anhydride. The aromatic hydrocarbon is one or more of benzene, naphthalene, or derivatives thereof. The Lewis acid is one or more of ferric chloride and aluminum chloride.

[0032] In the step (3), the solvent is one or more of decaline and dichloromethane; and the catalyst is one or more of aluminum chloride and ferric chloride.

[0033] In order to make the content of the application more convenient to understand, the technical solutions of the application are further described below in combination with specific embodiments, but the application is not limited thereto.

[0034] The grafting rate of the catalyst PP graft copolymer prepared in the application is determined by the acid-base titration method, and the specific operation steps are as follows:

[0035] (1) Preparation and calibration of KOH-ethanol standard solution.

[0036] 1.2 g of KOH is dissolved in 2 mL of deionized water, and after dissolution, it is diluted to 1000 mL with anhydrous ethanol, and then it is left to stand for 24 h. The prepared KOH-ethanol solution is injected into a previously rinsed alkaline burette with the solution, the tip of the burette is adjusted to be full of the solution without air bubbles, then the liquid level in the burette is adjusted, and the accurate scale is recorded. 0.0400 g of oven-dried potassium hydrogen phthalate is accurately weighed into a conical flask, 20 mL of deionized water is added to dissolve it, and 2-5 drops of phenolphthalein indicator are added dropwise. The conical flask is placed below the burette, and titration is started. After the last drop of liquid is added, the solution in the conical flask changes from colorless to pink, and at this time, the accurate scale of the burette is recorded. The above operation is repeated twice, and the concentration of the KOH-ethanol solution is calculated according to formula (1).

[0037] Formula (1)

[0038] In the formula: C KOH — — the concentration of KOH-ethanol solution, mol / L;

[0039] m — the mass of potassium hydrogen phthalate, g;

[0040] V — the volume of KOH-ethanol standard solution consumed, mL.

[0041] (2) Determination of the grafting rate of PP graft copolymer.

[0042] About 1 g of purified PP graft copolymer was weighed into a three-necked flask together with 50 mL of xylene, the temperature was set to 120°C, the magnetic stirring speed was 200 rpm, and the sample was heated to reflux for 30 min to completely dissolve the sample. After the sample was completely dissolved, the heating was stopped, the solution was cooled to 75°C, and then the heating was restarted. After the sample temperature was stabilized by refluxing for a period of time, an appropriate amount of calibrated KOH-ethanol standard solution was injected into the basic burette, and the initial scale of the burette was adjusted and recorded. The three-necked flask was placed below the basic burette, and after 2-5 drops of phenolphthalein indicator were added to the solution, the titration was started, and the solution temperature was kept stable at 75°C during the entire titration process. After the last drop of liquid was added, the solution in the conical flask changed from colorless to pink, and the accurate scale of the burette was recorded. The grafting rate of the PP graft copolymer sample was calculated using Formula (2).

[0043] Formula (2)

[0044] In the formula: GY MAH — the grafting rate of P-g-(St-MX), i.e. the content of acid centers in the catalyst, wt%;

[0045] c KOH — — the concentration of KOH-ethanol solution, mol / L;

[0046] m— — the mass of catalyst, g;

[0047] V— — the volume of KOH-ethanol standard solution consumed, mL.

[0048] Example 1

[0049] The grafting monomer was prepared using m-xylene as the aromatic hydrocarbon and maleic anhydride as the cyclic anhydride, and the reaction formula is as follows:

[0050] .

[0051] The specific preparation process is as follows: 8.5 g of MAH and 50 ml of m-xylene are added to a reactor, heated to 50°C, and then 11 g of aluminum chloride is slowly added to the reactor. After 2 hours of refluxing, the reaction is stopped, and the reaction liquid in the reactor is slowly poured into ice hydrochloric acid to hydrolyze the aluminum chloride. After hydrolysis, stand for 30 min, the reaction liquid is layered, and the collected oil phase is cooled in an ice water bath and waited for 1-2 h to precipitate the crude product. After suction filtration and drying, the obtained crude product and 25 mL of benzene are added to a flask, heated to 50 ℃ to dissolve the sample, then 1 mL of petroleum ether is added, and the product is recrystallized after standing at room temperature for one night. After suction filtration and drying, the refined product is obtained, named MAH-Xylene. The FTIR spectrum of the product MAH-Xylene is shown in Figure 1 .

[0052] It can be seen from Figure 1 that 1421 cm -1 is the OH bending vibration peak, corresponding to the hydroxyl group on the carboxyl group of the product; 1701 cm -1 and 1666 cm -1 are the absorption peaks of C=O, and due to the conjugation effect of the benzene ring and the carbon-carbon double bond, the absorption peak of C=O is red-shifted; 1600 cm -1 is the stretching vibration of benzene ring C=C. The above results show that MAH and m-xylene are successfully connected together.

[0053] Example 2

[0054] 10 kg of PP powder and 200 g of styrene are mixed in a high-speed mixer for 30 min; after mixing, they are added to a twin-screw extruder for extrusion and granulation, and the temperature of each zone of the twin-screw extruder is set to 140, 160, 180, 200, 200, 200, 200, 200, 200, 200, 200, 190 ℃ (die), and the rotation speed is 250 rpm. After extrusion, an appropriate amount of granules is dissolved in xylene, and then poured into a beaker containing a large amount of acetone. The white flocculent product precipitated after suction filtration and drying is removed to obtain PP-g-St, which is used for the preparation of graft copolymer. The purified PP-g-St is pressed into a transparent film with a thickness of about 100 μm using a hot press, and FTIR test is performed, and the results are shown in Figure 2 .

[0055] Example 3-10

[0056] Examples 3-10 were prepared by using Friedel-Crafts alkylation reaction to connect PP-g-St and MAH-Xylene according to the specific reaction conditions listed in Table 1; the reaction equation is as follows:

[0057] .

[0058] The specific preparation steps are as follows: PP-g-St prepared in Example 2 is added to a reactor containing 50 ml of solvent, heated to 130 ℃ under reflux to dissolve, then slowly cooled to 80 ℃, after the temperature is stable, dimethoxymethane is added to the system, stirred for 10 min to make it fully dispersed, then 0.5% of the catalyst based on the mass of the PP matrix is added under reflux for 1 h; after the reflux is completed, MAH-Xylene prepared in Example 1 is added to the system in portions, and the system is maintained at 80 ℃ for 4 h. After the reaction is completed, the reaction liquid is poured into a large amount of acetone, the product is precipitated, filtered and dried to obtain a crude product with unreacted MAH-Xylene removed; the crude product is dissolved in xylene and cooled to 75 ℃, then dilute hydrochloric acid is added under reflux for 1 h to hydrolyze aluminum trichloride (or ferric trichloride) in the crude product, after the reflux is completed, the solution is poured into acetone, after the product is precipitated, it is filtered and dried to obtain a PP graft copolymer catalyst, which is named as PP-g-(St-MX).

[0059] The grafting rate of the prepared catalyst is determined by titration, and the results are listed in Table 1.

[0060] Table 1 Reaction conditions and grafting rate test results of Examples 3-10

[0061]

[0062] The FTIR spectrum of the catalyst PP-g-(St-MX) prepared in Example 3 is shown in Figure 2 . It can be seen from Figure 2 that compared with PP and PP-g-St, PP-g-(St-MX) has new absorption peaks at 1701 cm -1 and 1666 cm -1 , indicating that the acidic groups are successfully grafted onto the PP backbone; in addition, compared with PP-g-St, the benzene ring C=C stretching vibration peak at 1600 cm -1 and the out-of-plane bending deformation absorption peak of Ph-H at 700 cm -1 of PP-g-(St-MX) are enhanced, which also indicates the successful grafting of MAH-Xylene.

[0063] Application Example

[0064] The catalyst PP-g-(St-MX) prepared in Example 3 and Example 10 was used to synthesize lactide. Specifically, 100 parts of lactic acid and 5 parts of the catalyst were added to a reactor, stirring was started at a speed of 400 rpm, the vacuum degree was 0.09 mPa, the temperature was 150°C, and the reaction time was 4 h. After the prepolymerization was completed, the prepared oPLA was used as a reactant to synthesize lactide through depolymerization, the depolymerization temperature was 220°C, the vacuum degree was 0.099 mPa, and the reaction time was 1 h.

[0065] The test results of the yield and purity of the obtained lactide are shown in Table 2. The molecular weight of the prepolymer and the depolymerization residue prepared by catalytic preparation in Example 3 was determined by titration, and the results are shown in Table 3.

[0066] Table 2 Yield and purity of lactide synthesized by catalytic preparation of PP-g-(St-MX) prepared in Example 3 and Example 10

[0067]

[0068] Table 3 Titration results of the molecular weight of the prepolymer and the depolymerization residue prepared by catalytic preparation in Example 3

[0069]

[0070] As shown in Table 2, the PP graft copolymer prepared by the method of the present application can be used as a catalyst for the two-step synthesis of lactide, and the yield of lactide can be as high as 83%, and the optical purity is greater than 99%, meeting the requirements of the ring-opening method for preparing PLA. As shown in Table 3, the molecular weight of the prepolymer obtained by catalytic preparation in Example 3 is 705, and the molecular weight of the depolymerization residue is 1157, indicating that the PP-g-(St-MX) prepared in Example 3 has the activity of catalyzing the esterification of lactic acid to form oPLA.

[0071] Comparative Example 1

[0072] In Comparative Example 1, stannous chloride was used as a catalyst to synthesize lactide. Specifically, 10 g of 80% lactic acid was added with 1 wt% stannous chloride, and the same reaction conditions as in the application examples were used to synthesize lactide. The yield of the prepared lactide was 81%, and the optical purity was 94.1%. The synthesized lactide was placed in a muffle furnace at 800°C for calcination for 4 h to obtain a light yellow solid, and the content of residual Sn in the lactide was calculated to be 42 ppm, indicating that the lactide prepared by using a metal catalyst has the problem of residual metal ions in the product.

[0073] Comparative Example 2

[0074] Comparative Example 2: Synthesis of lactide using sulfuric acid as catalyst. 10 g of 80% lactic acid was added with 1 wt% of concentrated sulfuric acid, and the same reaction conditions as in the application examples were used to synthesize lactide. The yield of the lactide product was only 28%, and the optical purity was 90%, indicating that a large amount of side reactions occurred during the synthesis of lactide using sulfuric acid as catalyst. In addition, after calcining the prepared lactide in a muffle furnace, 50 ppm of iron was detected in the product by ICP, indicating that sulfuric acid corroded the reactor during the reaction. The catalyst prepared by the present application uses carboxyl groups for catalysis, which has weak acidity, high catalytic activity, high yield and optical purity of lactide, and no corrosion to the reactor.

[0075] Comparative Example 3

[0076] Comparative Example 3: Synthesis of lactide using maleic anhydride as catalyst. 10 g of 80% lactic acid was added with 1 wt% of maleic anhydride, and the same reaction conditions as in the application examples were used to synthesize lactide. The yield of the lactide product was 78.8%, and the optical purity was 84%. However, due to the low molecular weight of maleic anhydride itself, it is easily volatilized from the system under the influence of high temperature and high vacuum degree during the preparation of lactide, causing product contamination and making the prepared lactide have a certain irritating odor. Acid-base titration test showed that there were 400 ppm of maleic anhydride residues in the product.

[0077] Comparative Example 4

[0078] 10 kg of PP powder, 50 g of initiator L101 and 150 g of MAH were mixed by a high-speed mixer for 30 min, and then PP-g-MAH was prepared by the extrusion process described in Example 2. The grafting rate of PP-g-MAH was determined by titration, and the molecular weight change before and after extrusion was determined by GPC. The results are shown in Table 4.

[0079] Table 4: Molecular weight, grafting rate and volatile content test results of samples of Example 3 and Comparative Example 4

[0080]

[0081] As can be seen from Table 4, the molecular weight and grafting rate of the PP graft copolymer catalyst prepared by grafting the PP matrix with styrene using a grafting monomer with carboxyl groups are superior to those prepared by melt grafting method, and the volatile content is much lower than that prepared by melt grafting method, which is close to the original PP. This is because the present method does not use peroxide during the preparation process, and no free radicals are involved in the reaction, which does not cause degradation of PP, and has the advantages of high molecular weight and low volatile content.

[0082] The prepared PP-g-MAH was used as catalyst to synthesize lactide, specifically, 10 g of 80% lactic acid was added with 5 wt% of PP-g-MAH, and lactide was synthesized under the same reaction conditions as in the application examples. The yield of the prepared lactide product was 65.8%, and the optical purity was 98%.

[0083] Comparative Example 5

[0084] MAH was used as grafting monomer, and PP-g-St prepared in Example 2 was used to prepare catalyst for lactide dehydration and cyclization to synthesize lactide by the same method as in Example 3, and was used for the synthesis of lactide, the grafting rate and the purity and yield of the synthesized lactide are shown in Table 5.

[0085] Comparative Example 6

[0086] MAH-Xylene prepared in Example 1 was used as grafting monomer, and PP was used as matrix to prepare catalyst for lactide dehydration and cyclization to synthesize lactide by the same method as in Example 3, and was used for the synthesis of lactide, the grafting rate and the purity and yield of the synthesized lactide are shown in Table 5.

[0087] Comparative Example 7

[0088] Benzoic acid was used as grafting monomer, and PP-g-St prepared in Example 2 was used to prepare catalyst for lactide dehydration and cyclization to synthesize lactide by the same method as in Example 3, and was used for the synthesis of lactide, the grafting rate and the purity and yield of the synthesized lactide are shown in Table 5.

[0089] Example 11

[0090] MAH and mesitylene were used to prepare grafting monomers by the same method as in Example 1, and then they were used to prepare catalyst for lactide dehydration and cyclization to synthesize lactide by the same method as in Example 3, and were used for the synthesis of lactide, the grafting rate and the purity and yield of the synthesized lactide are shown in Table 5.

[0091] Example 12

[0092] MAH and naphthalene were used to prepare grafting monomers by the same method as in Example 1, and then they were used to prepare catalyst for lactide dehydration and cyclization to synthesize lactide by the same method as in Example 3, and were used for the synthesis of lactide, the grafting rate and the purity and yield of the synthesized lactide are shown in Table 5.

[0093] Table 5 Grafting rate of catalyst prepared in Comparative Examples 5-7 and Examples 11-12, and yield and purity of lactide synthesized by the catalyst

[0094]

[0095] As shown in Table 5, the reaction in Example 3 uses m-xylene and MAH to prepare the grafting monomer, two methyl groups are introduced on the benzene ring to improve the reactivity of the benzene ring, and the prepared catalyst PP-g-(St-MX) has a grafting rate of 2.2%, and the catalytic synthesis of lactide has good yield and optical purity. In contrast, the grafting monomer used in Comparative Example 5 does not contain an aromatic ring and cannot undergo a Friedel-Crafts alkylation reaction with the alkylating agent dimethoxymethane, so it cannot be grafted onto the PP matrix, and thus the prepared catalyst has a grafting rate of 0 and cannot catalyze the dehydration and cyclization of lactic acid to produce lactide; when PP is used as the matrix in Comparative Example 6, the molecular chain does not contain an aromatic ring and cannot undergo a Friedel-Crafts alkylation reaction with the grafting monomer A, and thus the prepared catalyst has a grafting rate of 0 and cannot catalyze the synthesis of lactic acid to produce lactide; when benzoic acid is used as the grafting monomer in Comparative Example 7, the presence of an electron-withdrawing carboxyl group on the benzene ring makes it less active, and the prepared catalyst has a grafting rate of only 0.1%, and the yield and purity of the catalytically synthesized lactide are also lower.

[0096] The above description is only the preferred embodiment of the present application, and any equivalent changes and modifications made within the scope of the patent application of the present application shall be included in the scope of the present application.

Claims

1. A method for the preparation of a non-metallic catalyst for the two-step synthesis of lactide, characterized in that, The application discloses a method for preparing a PP grafted copolymer which is suitable for a two-step synthesis of lactide by using a Friedel-Crafts alkylation reaction to graft carboxylic acid groups on a PP molecular main chain without using a peroxide initiator. The method specifically comprises the following steps: (1) preparation of a grafted monomer: a grafted monomer with carboxylic acid groups is prepared by a Friedel-Crafts acylation reaction of a cyclic anhydride and an aromatic hydrocarbon in the presence of a Lewis acid; (2) preparation of a PP matrix: styrene is grafted to a PP molecular chain by thermal initiation to prepare a styrene-grafted PP matrix; (3) preparation of a grafted copolymer: a Friedel-Crafts alkylation reaction is used to connect the grafted monomer prepared in the step (1) and the PP matrix prepared in the step (2) by using dimethoxymethane as an alkylation reagent, so that carboxylic acid groups are introduced on the PP molecular main chain to prepare the PP grafted copolymer.

2. The method of claim 1, wherein the non-metallic catalyst is prepared by a process comprising: In the step (1), the cyclic anhydride is one or more of succinic anhydride, glutaric anhydride and maleic anhydride; the aromatic hydrocarbon is one or more of benzene, naphthalene or a derivative thereof; and the Lewis acid is one or more of ferric chloride and aluminum chloride.

3. The method of preparing a non-metallic catalyst according to claim 1, characterized in that, In the step (2), the PP resin is uniformly mixed with styrene, and then is extruded and pelletized by using an extruder to graft the styrene to the PP molecular chain by a melt grafting reaction; the temperature of the extruder is controlled to be 160-250 DEG C, the rotating speed of the extruder is controlled to be 100-400 rpm, the length-diameter ratio of the screw is 25-50, and the residence time is 30-60 s.

4. The method of preparing a non-metallic catalyst according to claim 1, characterized in that, In the step (3), the PP matrix is dissolved in a solvent, then dimethoxymethane and a catalyst are added, the mixture is refluxed at 80-120 DEG C for 1-2 h, and then the grafted monomer is added, and the mixture is continuously refluxed at the same temperature for 2-4 h.

5. The method of claim 4, wherein the non-metallic catalyst is prepared by a process comprising: The amount of the dimethoxymethane is 5-20% of the mass of the PP matrix, the amount of the catalyst is 0.5-2.0% of the mass of the PP matrix, and the amount of the grafted monomer is 5-20% of the mass of the PP matrix.

6. The method of claim 4, wherein the non-metallic catalyst is prepared by the steps of: The solvent is one or more of decalin and dichloromethane; and the catalyst is one or more of aluminum chloride and ferric chloride.

7. Application of a non-metallic catalyst prepared by any one of the methods in claims 1-6 to a two-step synthesis of lactide.

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