Catalyst for preparing lactide from lactic acid through one-step method as well as preparation method and application of catalyst

Abstract

The invention belongs to the field of lactide catalysts, and relates to a lactide catalyst prepared from lactic acid through a one-step method and a preparation method and application thereof.The preparation method comprises the steps that a hydrogen type molecular sieve is placed in a lactic acid solution to be heated for dealumination, and an internally dealuminated hydrogen type molecular sieve is obtained; and mixing the hydrogen type molecular sieve with dealuminated inside with a metal salt solution for reaction, and roasting to obtain the lactide catalyst. The transition metal is introduced into the nanopore aluminum silicate solid acid catalyst to supplement holes after aluminum loss so as to recover the acid amount of the catalyst, so that the problem that the acid amount of the catalyst is reduced due to dealumination is effectively solved.

Description

Technical Field

[0001] The present invention belongs to the field of lactide catalysts, and relates to a catalyst for directly preparing lactide from lactic acid, a preparation method thereof, and an application thereof. Background Art

[0002] In the industrial production process of polylactic acid (PLA), lactide plays a crucial role. As a precursor for producing high-quality polylactic acid, it is obtained by the dimerization of lactic acid. Currently, the preparation process of lactide mainly includes a two-step method and a one-step method. The two-step method requires polymerizing lactic acid into oligomers with the help of a catalyst under low temperature and reduced pressure. Subsequently, these oligomers will undergo a ring-opening cracking process under high temperature and high vacuum conditions. However, this method faces problems such as high impurity content, difficult product separation and purification, and significant energy consumption. In contrast, the one-step method directly converts two lactic acid molecules into linear lactic acid dimers under the combined action of high temperature and a catalyst, and further promotes the condensation of these dimers into a cyclic structure. This process has extremely high selectivity, indicating its great development potential. However, the one-step method has extremely high requirements for the catalyst. An ideal catalyst must simultaneously exhibit high activity and high selectivity. Therefore, the research and development of catalysts has become a hot topic in the research field in recent years.

[0003] Nano-porous aluminosilicate solid acid catalysts, especially zeolite-type catalysts, with their unique advantages of shape selectivity, perform excellently in controlling product selectivity and have been widely used in the petrochemical field. The latest research shows that zeolite catalysts can directly catalyze the conversion of lactic acid to lactide. This process not only has a relatively low cost, but also the by-products are easy to recycle, and the reaction steps are simple and clear, providing the possibility for the industrial one-step synthesis of lactide. However, it should be noted that when the catalyst is exposed to an acidic environment for a long time and undergoes a heating process, it will face a series of complex chemical and physical changes, which pose severe challenges to its performance and stability. Specifically, hydrogen ions (H+) or other acidic groups in the acidic system, under the continuous action of high temperature, will gradually penetrate and erode the internal structure of the catalyst, especially its framework part. This erosion not only weakens the overall structural strength of the catalyst, but also promotes the removal of aluminum, one of the key elements constituting the catalyst framework. With the gradual loss of aluminum, the number of acidic sites on the surface and inside of the catalyst significantly decreases. These acidic sites are the key places where catalytic reactions occur, and their reduction directly leads to a decrease in the adsorption capacity of the catalyst for the target reactants and the catalytic conversion efficiency. Therefore, a catalyst heated in an acidic system for a long time often gradually loses its original catalytic activity and selectivity and finally becomes deactivated due to the acid erosion of the framework, the removal of aluminum, and the resulting loss of acidic sites. This phenomenon is undoubtedly a problem that needs to be highly emphasized and solved in the application of catalysts in many fields such as chemical engineering, petroleum refining, and environmental protection.

[0004] In summary, the preparation of the catalyst in the process of catalytic conversion of lactic acid to lactide and the influence of the vacancy of acidic sites inside the catalyst on the reaction are problems that need to be solved urgently. These problems have greatly restricted the development and application of the one-step method for synthesizing lactide from lactic acid. Summary of the Invention

[0005] The present invention provides a catalyst for the one-step method of synthesizing lactide from lactic acid, its preparation method and application. The main problem to be solved by the present invention is that in the process of the one-step method of synthesizing lactide from lactic acid, the nano-porous aluminosilicate solid acid catalyst is deactivated due to internal dealumination during the reaction.

[0006] To solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0007] A preparation method of a catalyst for the one-step method of synthesizing lactide from lactic acid, which comprises heating and dealuminating a hydrogen-type zeolite in a lactic acid solution to obtain a hydrogen-type zeolite with internal dealumination; and mixing and reacting the hydrogen-type zeolite with internal dealumination with a metal salt solution to obtain a lactide catalyst.

[0008] Further, the hydrogen-type molecular sieve is a silica-alumina molecular sieve with 12-membered ring channels, and the framework type of the hydrogen-type molecular sieve is at least one of BEA, FAU, and MOR.

[0009] Further, the silica-alumina ratio of the hydrogen-type molecular sieve is 15 - 80.

[0010] Further, the preparation method of the hydrogen-type molecular sieve includes: mixing and stirring l-lysine, tetraethylammonium hydroxide, and deionized water at room temperature, then adding potassium chloride and sodium metaaluminate and continuing to stir, and then adding silica white and stirring thoroughly to obtain a gel precursor. Finally, the gel precursor is crystallized at 120 - 160 °C for 3 - 5 days. After washing, centrifuging, and drying the crystallized mixed solution, it is calcined in a muffle furnace at 530 - 580 °C for 4 - 8 h to obtain β molecular sieve. The β molecular sieve is added to the NH 4 NO 3 solution and ion-exchanged under an oil bath condition at 70 - 90 °C, washed, centrifuged, and dried, and finally calcined at 500 - 600 °C for 4 - 8 h to obtain the hydrogen-type molecular sieve.

[0011] Further, the temperature of the thermal dealumination is 120 - 160 °C, and the time is 1 - 7 days.

[0012] Further, the metal salt is selected from at least one of zinc salts, iron salts, tin salts, and lead salts.

[0013] Further, the mass ratio of metal ions in the metal salt solution to the internally dealuminated hydrogen-type molecular sieve is 0.5 - 20:100.

[0014] Further, after mixing the internally dealuminated hydrogen-type molecular sieve with the metal salt solution, it is calcined at 500 - 600 °C for 4 - 8 h.

[0015] Further, the mixing method of the internally dealuminated hydrogen-type molecular sieve and the metal salt solution is at least one of coprecipitation, impregnation, grinding, and ball milling.

[0016] The present invention also provides a lactide catalyst prepared by the above method.

[0017] The present invention also provides the application of the above lactide catalyst in the preparation of lactide. The reaction temperature for preparing lactide is 120 - 160 °C, and the reaction time is 3 - 7 h.

[0018] The present invention provides a preparation method of a catalyst for preparing lactide by one-step method from lactic acid, comprising the following steps: placing molecular sieve in lactic acid solution for heating and dealumination to obtain molecular sieve samples with different degrees of dealumination at different times. Mixing the dealuminated molecular sieve samples with metal solution and obtaining the metal-loaded molecular sieve samples after calcination. Since a large number of cavities will be generated in the molecular sieve after dealumination, resulting in a significant decrease in the acid amount inside the molecular sieve, this way of metal loading can effectively fill the aluminum vacancies in the molecular sieve, thereby gradually restoring the acid amount. In the selection of metals, following the metal activity series, zinc, iron, tin and lead are selected as supplementary metals, and these metals all have certain activity for this reaction. After the metals enter the molecular sieve, in addition to restoring the acid amount, they also have a certain stabilizing effect on the framework of the molecular sieve and will not be easily removed under acidic conditions, thus prolonging the service life of the molecular sieve.

[0019] Beneficial effects

[0020] The present invention provides a method for introducing metals into a nano-porous aluminosilicate solid acid catalyst to supplement the vacancy of aluminum, thereby maintaining the acid amount, effectively solving the problem of dealumination of the nano-porous aluminosilicate solid acid catalyst in an acidic system, and can be used in the industrial production of catalysts for preparing high-quality lactide by one-step method from lactic acid.

[0021] The method of the present invention effectively solves the removal of acidic sites inside the catalyst during the catalytic reaction of lactic acid to lactide. Adding the nano-porous solid acid catalyst into a laboratory three-necked flask, evaluating the reaction of one-step synthesis of lactide from lactic acid under normal pressure and at 140 °C, and investigating the catalytic situation of the catalytic reaction of lactic acid to lactide.

[0022] By using the method of supplementing acid vacancies with a solid acid catalyst, the present invention obtains a nano-porous solid acid catalyst, and the optimal catalytic result thereof is a conversion rate of 100% and a lactide selectivity of 99%, which is much higher than that of the dealuminated nano-porous aluminosilicate solid acid catalyst, achieving good technical effects. Description of the drawings

[0023] Figure 1 It is a comparative X-ray diffraction spectrum diagram of H-β molecular sieve and dealuminated H-β molecular sieve;

[0024] Figure 2 It is the NH 3 -TPD curve graph of H-β molecular sieve and H-β molecular sieve after supplementing metals. Detailed implementation manners

[0025] Example 1

[0026] A preparation method of a catalyst for the one-step preparation of lactide from lactic acid, comprising the following steps: First, mix L-lysine, tetraethylammonium hydroxide and deionized water at room temperature and stir until the solution is clear and transparent. Then add potassium chloride and sodium aluminate and stir until the solution is clear and transparent. After that, slowly add white carbon black and stir for 12 h to obtain a gel precursor. The raw materials for synthesizing the gel precursor, SiO 2 : tetraethylammonium hydroxide (TEAOH): Al 2 O 3 : KCl: H 2 O have a molar ratio of 1:0.45:0.066:0.04:10. Finally, transfer the gel precursor to a stainless steel reactor lined with polytetrafluoroethylene and crystallize at 140 °C for 4 days. Wash, centrifuge and dry the crystallized mixed solution overnight, and then calcine in a muffle furnace at 550 °C for 6 h to obtain β-zeolite.

[0027] Place the prepared zeolite sample in 1 M NH 4 NO 3 solution and carry out ion exchange at 80 °C in an oil bath for 6 h. After repeating 2 times, wash, centrifuge and dry overnight. Finally, calcine at 550 °C for 6 h to obtain H-β zeolite (Si / Al = 15), denoted as H-β-15, where 15 is the silicon-aluminum ratio after dealumination under this condition.

[0028] The reaction conditions for the catalytic preparation of lactide from lactic acid are as follows: The raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalysis is carried out in a three-necked flask for 5 h.

[0029] Example 2

[0030] A preparation method of a catalyst for the one-step preparation of lactide from lactic acid, comprising the following steps: First, heat 2 g of the H-β zeolite (Si / Al = 15) synthesized in Example 1 at 140 °C in 40 mL of an 80% (wt%) aqueous lactic acid solution for 1 day. Subsequently, wash with water and ethanol until the pH ≥ 6 and dry at 80 °C. Obtain the internally dealuminated H-β zeolite, denoted as H-β-20.4, where 20.4 is the silicon-aluminum ratio after dealumination under this condition.

[0031] The reaction conditions for the catalytic preparation of lactide from lactic acid are as follows: The raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalysis is carried out in a three-necked flask for 5 h.

[0032] Example 3

[0033] A preparation method of a catalyst for preparing lactide by one-step method from lactic acid, comprising the following steps: Take stannic chloride pentahydrate and add it to deionized water, and prepare a stannic chloride solution according to the ratio that the mass ratio of tin ions to molecular sieve is 1%; Add this solution to H-β-20.4 synthesized in Example 2, and impregnate it with an equal volume to load a tin solution with a mass fraction of 1% Sn, and calcine it in a muffle furnace at 550 °C for 6 h to obtain H-β-20.4 with a mass fraction of 1% Sn, denoted as 1% Sn-H-β-20.4.

[0034] The reaction conditions for the catalytic conversion of lactic acid to lactide are as follows: The raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalytic reaction is carried out in a three-necked flask for 5 h.

[0035] Example 4

[0036] A preparation method of a catalyst for preparing lactide by one-step method from lactic acid, comprising the following steps: Take zinc nitrate and add it to deionized water, and prepare a zinc nitrate solution according to the ratio that the mass ratio of zinc ions to molecular sieve is 1%; Add this solution to H-β-20.4 synthesized in Example 2, and impregnate it with an equal volume to load a zinc solution with a mass fraction of 1% Zn, and calcine it in a muffle furnace at 550 °C for 6 h to obtain H-β-20.4 with a mass fraction of 1% Zn, denoted as 1% Zn-H-β-20.4.

[0037] The reaction conditions for the catalytic conversion of lactic acid to lactide are as follows: The raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalytic reaction is carried out in a three-necked flask for 5 h.

[0038] Example 5

[0039] A preparation method of a catalyst for preparing lactide by one-step method from lactic acid, comprising the following steps: Take ferric nitrate and add it to deionized water, and prepare a ferric nitrate solution according to the ratio that the mass ratio of iron ions to molecular sieve is 1%; Add this solution to H-β-20.4 synthesized in Example 2, and impregnate it with an equal volume to load an iron solution with a mass fraction of 1% Fe, and calcine it in a muffle furnace at 550 °C for 6 h to obtain H-β-20.4 with a mass fraction of 1% Fe, denoted as 1% Fe-H-β-20.4.

[0040] The reaction conditions for the catalytic conversion of lactic acid to lactide are as follows: The raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalytic reaction is carried out in a three-necked flask for 5 h.

[0041] Example 6

[0042] A preparation method of a catalyst for preparing lactide by one-step method from lactic acid, comprising the following steps: adding lead nitrate into deionized water, preparing a lead nitrate solution according to the ratio that the mass ratio of lead ions to molecular sieve is 1%; adding this solution into H-β-20.4 synthesized in Example 2, impregnating with an equal volume of a lead solution with a mass fraction of 1% Pb, and calcining in a muffle furnace at 550 °C for 6 h to obtain H-β-20.4 with a mass fraction of 1% Pb, denoted as 1% Pb-H-β-20.4.

[0043] The reaction conditions for the catalytic preparation of lactide from lactic acid are as follows: the raw material used is an 80% (wt%) aqueous lactic acid solution, the solvent is toluene, the reaction temperature is 140 °C, and the catalytic reaction is carried out in a three-necked flask for 5 h.

[0044] The methods used in Examples 7 to 30 are the same as those in Examples 3 to 6, only the types of catalysts used are changed. The catalysts used are H-β-x with the dealumination heating times of 2 d, 3 d, 4 d, 5 d, 6 d, and 7 d in sequence.

[0045] The methods used in Examples 31 to 50 are the same as those in Examples 19 to 22, only the metal loadings are changed. The metal loadings (wt%) used are 0.5, 2, 5, 10, 15, and 20 in sequence.

[0046] Denoted as n% M-H-β-x, where n is the metal loading (wt%), M is the metal type, and x is the silicon-aluminum ratio of the catalyst after dealumination.

[0047] The methods used in Comparative Examples 1 to 6 are the same as those in Example 2, only the dealumination heating times are changed. The heating times used are 2 d, 3 d, 4 d, 5 d, 6 d, and 7 d in sequence, denoted as H-β-x, where x is the silicon-aluminum ratio after acid treatment.

[0048] Comparative Example 7

[0049] First, mix L-lysine, tetraethylammonium hydroxide, and deionized water at room temperature and stir until the solution is clear and transparent, then add potassium chloride and sodium aluminate and stir until the solution is clear and transparent, then slowly add white carbon black and stir for 12 h, and finally transfer the gel precursor to a stainless steel reaction kettle lined with polytetrafluoroethylene to synthesize the gel precursor. The raw material ratio of the gel precursor is: SiO 2 : TEAOH: Al 2 O 3 : KCl: H 2 O has a molar ratio of 1:0.45:0.025:0.04:10. Crystallize at 140 °C for 4 days. Wash, centrifuge, and dry the crystallized mixed solution overnight, and then calcine in a muffle furnace at 550 °C for 6 h to obtain β zeolite.

[0050] The prepared molecular sieve sample was placed in 1 M NH 4 NO 3 solution and ion-exchanged for 6 h under the condition of an 80 °C oil bath. After repeating this process twice, it was washed, centrifuged, and dried overnight. Finally, it was calcined at 550 °C for 6 h to obtain H-β molecular sieve (Si / Al = 40).

[0051] The reaction conditions for the catalytic conversion of lactic acid to lactide were as follows: the raw material used was an 80% (wt%) aqueous lactic acid solution, the solvent was toluene, the reaction temperature was 140 °C, and the reaction was catalyzed in a three-necked flask for 5 h.

[0052] Comparative Example 8

[0053] First, L-lysine, tetraethylammonium hydroxide, and deionized water were mixed and stirred at room temperature until the solution became clear and transparent. Then, potassium chloride and sodium aluminate were added and stirred until the solution became clear and transparent. After that, silica white was slowly added and stirred for 12 h. Finally, the gel precursor was transferred to a stainless-steel autoclave lined with polytetrafluoroethylene. The raw material ratio for synthesizing the gel precursor was as follows: the molar ratio of SiO 2 : TEAOH: Al 2 O 3 : KCl: H 2 O was 1:0.45:0.0125:0.04:10. It was crystallized at 140 °C for 4 days. After the crystallized mixed solution was washed, centrifuged, and dried overnight, it was calcined in a muffle furnace at 550 °C for 6 h to obtain β molecular sieve.

[0054] The prepared molecular sieve sample was placed in 1 M NH 4 NO 3 solution and ion-exchanged for 6 h under the condition of an 80 °C oil bath. After repeating this process twice, it was washed, centrifuged, and dried overnight. Finally, it was calcined at 550 °C for 6 h to obtain H-β molecular sieve (Si / Al = 80).

[0055] The reaction conditions for the catalytic conversion of lactic acid to lactide were as follows: the raw material used was an 80% (wt%) aqueous lactic acid solution, the solvent was toluene, the reaction temperature was 140 °C, and the reaction was catalyzed in a three-necked flask for 5 h.

[0056] The results of the specific examples and comparative examples are shown in the following table:

[0057]

[0058]

[0059] The above results show that in the present invention, by using the method of supplementing acid vacancies with solid acid catalysts, the obtained nanoporous solid acid catalyst has the best catalytic results with a conversion rate of 100% and a lactide selectivity of 99%, which is much higher than that of the dealuminated nanoporous silicoaluminate solid acid catalyst, achieving good technical effects. Among them, the metal is Sn or Pb, the mass percentage of the metal ion in the internally dealuminated nanoporous solid acid catalyst is 5-10%, and the effect is the best when the dealumination time is 5d. This is because after the metal enters the molecular sieve, in addition to restoring the acid amount, it also has a certain stabilizing effect on the framework of the molecular sieve and will not be easily removed under acidic conditions, thus prolonging the life of the molecular sieve.

[0060] In Figure 1 , by comparing the X-ray diffraction pattern before and after silylation treatment, the position of the diffraction peak of the H-β molecular sieve did not change, but the peak intensity decreased significantly, indicating that although the overall structure of the molecular sieve remained stable during long-term heating in an acidic system, the pore channels gradually collapsed.

[0061] In Figure 2 , by comparing the NH 3 -TPD curve before and after metal supplementation, it was found that with the increase of the heating time, the total acid amount of the H-β molecular sieve gradually decreased. After loading the metal, the total acid amount of the H-β molecular sieve gradually increased, indicating that the loading of the metal gradually supplemented the acidic sites of the H-β molecular sieve and restored the total acid amount.

Claims

1. A method for preparing a catalyst for preparing lactide from lactic acid in one step, comprising the following steps: The hydrogen-type molecular sieve is placed in a lactic acid solution and heated to dealuminate to obtain an internally dealuminated hydrogen-type molecular sieve; the internally dealuminated hydrogen-type molecular sieve is mixed with a metal salt solution for reaction, and a lactide catalyst is obtained after calcination.

2. The method for preparing a catalyst for preparing lactide from lactic acid by one-step process according to claim 1, characterized in that: The hydrogen-type molecular sieve is a silicon-aluminum molecular sieve with 12-membered ring channels, and the framework type of the hydrogen-type molecular sieve is at least one of BEA, FAU, and MOR.

3. The method for preparing a catalyst for preparing lactide from lactic acid by one-step process according to claim 1, characterized in that: The silicon-aluminum ratio of the hydrogen-type molecular sieve is 15-80.

4. The method for preparing a catalyst for preparing lactide from lactic acid in one step according to claim 1, characterized in that: The preparation method of the hydrogen molecular sieve comprises: mixing l-lysine, tetraethylammonium hydroxide and deionized water at room temperature, then adding potassium chloride and sodium aluminate and continuing to stir, then adding white carbon black and stirring sufficiently to obtain a gel precursor, finally crystallizing the gel precursor at 120-160° C. for 3-5 days, washing the crystallized mixed solution and centrifugally drying it, and then roasting it in a muffle furnace at 530-580° C. for 4-8 hours to obtain a β molecular sieve, adding the β molecular sieve to an NH4NO3 solution and performing ion exchange at 70-90° C. oil bath conditions, washing, centrifugally drying it, and finally roasting it at 500-600° C. for 4-8 hours to obtain a hydrogen molecular sieve.

5. The method for preparing a catalyst for preparing lactide from lactic acid in one step according to claim 1, characterized in that: The temperature of the heating dealumination is 120-160° C., and the time is 1-7 days.

6. The method for preparing a catalyst for preparing lactide from lactic acid in one step according to claim 1, characterized in that: The metal salt is selected from at least one of zinc salt, iron salt, tin salt and lead salt.

7. The method for preparing a catalyst for preparing lactide from lactic acid in one step according to claim 1, characterized in that: The mass ratio of metal ions in the metal salt solution to the internally dealuminated hydrogen-type molecular sieve is 0.5-20:

100.

8. The method for preparing a catalyst for preparing lactide from lactic acid in one step according to claim 1, characterized in that: The internally dealuminated hydrogen-type molecular sieve is mixed with a metal salt solution and calcined at 500-600°C for 4-8 hours.

9. A lactide catalyst, characterized in that The method is prepared by any one of claims 1 to 8.

10. Use of the lactide catalyst according to claim 9 in the preparation of lactide, characterized in that: The reaction temperature for preparing lactide is 120-160°C, and the reaction time is 3-7h.

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