A one-step lactic acid-lactide catalyst and its preparation method and application

Abstract

The present invention belongs to the field of lactide catalysts and relates to a one-step lactide-to-lactic acid catalyst, its preparation method, and its application. The preparation method comprises: placing a hydrogen-type molecular sieve in a lactic acid solution and heating it to dealuminate, thereby obtaining an internally dealuminated hydrogen-type molecular sieve; mixing the internally dealuminated hydrogen-type molecular sieve with a metal salt solution for reaction, and calcining it to obtain a lactide catalyst. The present invention effectively addresses the problem of reduced catalyst acidity due to dealumination by introducing a transition metal into a nanoporous aluminum silicate solid acid catalyst to replenish aluminum vacancies and restore the catalyst's acid content.

Description

Technical Field

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

[0002] Lactide plays a crucial role in the industrial production of polylactic acid (PLA). As a precursor for high-quality PLA, it is obtained by dimerization of lactic acid. Currently, lactide preparation processes are primarily divided into two-step and one-step methods. The two-step process requires polymerization of lactic acid into oligomers under low temperature and reduced pressure with the help of a catalyst. These oligomers then undergo ring-opening cleavage at high temperature and high vacuum. However, this method faces challenges such as high impurity levels, difficulty in product isolation 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, which then condenses these dimers into a cyclic structure. This process offers exceptionally high selectivity, suggesting significant development potential. However, the one-step method places extremely stringent requirements on the catalyst; an ideal catalyst must exhibit both high activity and high selectivity. Consequently, catalyst development has become a hot topic in recent years.

[0003] Nanoporous aluminosilicate solid acid catalysts, particularly molecular sieve-type catalysts, have been widely used in the petrochemical industry due to their unique shape selectivity and excellent control over product selectivity. Recent research has demonstrated that molecular sieve catalysts can directly catalyze the conversion of lactic acid to lactide. This process is not only cost-effective, but also offers easy byproduct recovery and a simple, straightforward reaction procedure, opening the way for the industrialized, one-step synthesis of lactide. However, it is noteworthy that catalysts undergo a series of complex chemical and physical changes when exposed to acidic environments and heated for extended periods, which pose significant challenges to their performance and stability. Specifically, hydrogen ions (H+) or other acidic groups in acidic systems, under sustained high temperatures, gradually penetrate and erode the catalyst's internal structure, particularly its skeleton. This erosion not only weakens the catalyst's overall structural strength but also promotes the removal of aluminum, a key element of the catalyst's skeleton. As aluminum is gradually lost, the number of acidic sites on the catalyst surface and within its interior decreases significantly. These acidic sites are key sites for catalytic reactions, and their reduction directly leads to a decrease in the catalyst's adsorption capacity for the target reactant and catalytic conversion efficiency. Therefore, catalysts heated in acidic systems for extended periods often lose their catalytic activity and selectivity, ultimately becoming inactivated, due to acid corrosion of the skeleton, aluminum removal, and the resulting loss of acidic sites. This phenomenon is undoubtedly a significant issue that requires significant attention and effort to address for catalyst applications in numerous fields, including chemical engineering, petroleum refining, and environmental protection.

[0004] In summary, the preparation of the catalyst and the impact of vacancies in the catalyst's acidic sites on the reaction are urgent issues that need to be addressed in the lactic acid-catalyzed lactide production process. These issues have greatly limited the development and application of the one-step lactide production process. Summary of the Invention

[0005] The present invention provides a one-step lactide production catalyst using lactic acid, a preparation method thereof, and an application thereof. The present invention mainly solves the problem of catalyst deactivation due to internal dealumination during the reaction of a nanoporous silica-alumina solid acid catalyst in the one-step lactide production process using lactic acid.

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

[0007] A method for preparing a lactide catalyst from lactic acid in one step comprises placing a hydrogen molecular sieve in a lactic acid solution and heating it to dealuminate, thereby obtaining an internally dealuminated hydrogen molecular sieve; and mixing the internally dealuminated hydrogen molecular sieve with a metal salt solution for reaction, thereby obtaining a lactide catalyst.

[0008] Furthermore, the hydrogen-type molecular sieve is a silicon-aluminum molecular sieve having a 12-membered ring channel, and the framework type of the hydrogen-type molecular sieve is at least one of BEA, FAU, and MOR.

[0009] Furthermore, the silicon-aluminum ratio of the hydrogen-type molecular sieve is 15-80.

[0010] Furthermore, the preparation method of the hydrogen molecular sieve includes: 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 thoroughly to obtain a gel precursor, and finally crystallizing the gel precursor at 120-160°C for 3-5 days, washing the crystallized mixed solution and centrifuging and drying it, and then calcining 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 under 70-90°C oil bath conditions, washing and centrifuging and drying it, and finally calcining it at 500-600°C for 4-8 hours to obtain a hydrogen molecular sieve.

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

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

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

[0014] Furthermore, the internally dealuminated hydrogen molecular sieve is mixed with a metal salt solution and calcined at 500-600° C. for 4-8 hours.

[0015] Furthermore, the internally dealuminated hydrogen-type molecular sieve and the metal salt solution are mixed by at least one of co-precipitation, 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 use of the lactide catalyst in the preparation of lactide. The reaction temperature for preparing lactide is 120-160° C., and the reaction time is 3-7 hours.

[0018] The present invention provides a method for preparing a catalyst for producing lactide from lactic acid in a one-step process, comprising the following steps: placing a molecular sieve in a lactic acid solution and heating it for dealumination, thereby obtaining molecular sieve samples with varying degrees of dealumination at different times; mixing the dealuminated molecular sieve samples with a metal solution, and calcining the resulting metal-loaded molecular sieve samples. Since the molecular sieve produces a large number of vacancies after dealumination, resulting in a significant decrease in the acid content within the molecular sieve, metal loading can effectively fill the aluminum vacancies in the molecular sieve, thereby gradually restoring the acid content. The metals are selected based on their reactivity, with zinc, iron, tin, and lead being selected as supplementary metals. These metals all have a certain degree of activity in the reaction. Once incorporated into the molecular sieve, the metals not only restore the acid content but also stabilize the molecular sieve's framework, preventing it from being easily removed under acidic conditions and thus extending the life of the molecular sieve.

[0019] Beneficial effects

[0020] The present invention provides a method for introducing metal into a nanoporous aluminum silicate solid acid catalyst to supplement aluminum vacancies, thereby maintaining the acid content. This method effectively solves the problem of dealumination of the nanoporous aluminum silicate solid acid catalyst in an acidic system and can be used in the industrial production of a high-quality lactide catalyst prepared by a one-step method using lactic acid.

[0021] The method of the present invention effectively removes acidic sites from the catalyst during the lactic acid-catalyzed lactide production process. The nanoporous solid acid catalyst was added to a laboratory three-necked flask and the one-step lactide synthesis reaction from lactic acid was evaluated at atmospheric pressure and 140°C to examine the catalytic performance of the lactic acid-catalyzed lactide production process.

[0022] The present invention obtains a nanoporous solid acid catalyst by using a solid acid catalyst to supplement acid vacancies. The optimal catalytic result is 100% conversion rate and 99% lactide selectivity, which are much higher than those of the dealuminated nanoporous silica-alumina solid acid catalyst, achieving good technical effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The figure is a comparison of the X-ray diffraction spectra of H-β molecular sieve and H-β molecular sieve after dealuminization;

[0024] Figure 2 This is the NH3-TPD curve of H-β molecular sieve and H-β molecular sieve after metal supplementation. DETAILED DESCRIPTION

[0025] Example 1

[0026] A method for preparing a catalyst for converting lactide to lactide using a one-step process of lactic acid comprises the following steps: first, mixing l-lysine, tetraethylammonium hydroxide, and deionized water at room temperature and stirring until the solution becomes clear and transparent; then, adding potassium chloride and sodium metaaluminate and stirring until the solution becomes clear and transparent; then, slowly adding silica and stirring for 12 hours to obtain a gel precursor. The molar ratio of the raw materials for synthesizing the gel precursor is 1:0.45:0.066:0.04:10:SiO2:tetraethylammonium hydroxide (TEAOH):Al2O3:KCl:H2O. Finally, the gel precursor is transferred to a polytetrafluoroethylene-lined stainless steel reactor and crystallized at 140°C for 4 days. The crystallized mixed solution is washed, centrifuged, dried overnight, and then calcined in a muffle furnace at 550°C for 6 hours to obtain a beta molecular sieve.

[0027] The prepared molecular sieve sample was placed in a 1 M NH4NO3 solution and ion exchanged in an 80°C oil bath for 6 h. This was repeated twice, followed by washing, centrifugation, and overnight drying. Finally, it was calcined at 550°C for 6 h to obtain H-β molecular sieve (Si / Al = 15), which was designated as H-β-15, where 15 is the silicon-aluminum ratio after dealumination under these conditions.

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

[0029] Example 2

[0030] A method for preparing a catalyst for converting lactide to lactide using a one-step process using lactic acid comprises the following steps: first, heating 2 g of the H-β molecular sieve (Si / Al = 15) synthesized in Example 1 at 140°C in 40 mL of an 80% (wt%) lactic acid aqueous solution for 1 day. Subsequently, the mixture is washed with water and ethanol to a pH ≥ 6 and dried at 80°C. The resulting H-β molecular sieve, after internal dealumination, is designated H-β-20.4, where 20.4 represents the Si / Al ratio after dealumination under these conditions.

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

[0032] Example 3

[0033] A method for preparing a catalyst for preparing lactide from lactic acid by a one-step process comprises the following steps: adding tin tetrachloride pentahydrate to deionized water to prepare a tin tetrachloride solution in which the mass ratio of tin ions to molecular sieve is 1%; adding the solution to H-β-20.4 synthesized in Example 2, impregnating an equal volume of the H-β-20.4 with a tin solution having a loading mass fraction of 1% Sn; and calcining the H-β-20.4 in a muffle furnace at 550° C. for 6 h to obtain H-β-20.4 having a mass fraction of 1% Sn, which is recorded as 1%Sn-H-β-20.4.

[0034] The reaction conditions for lactide catalyzed by lactic acid are as follows: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140°C, and catalytic reaction in a three-necked flask for 5 h.

[0035] Example 4

[0036] A method for preparing a catalyst for preparing lactide from lactic acid in a one-step process comprises the following steps: adding zinc nitrate to deionized water to prepare a zinc nitrate solution in which the weight ratio of zinc ions to molecular sieve is 1%; adding the solution to H-β-20.4 synthesized in Example 2, impregnating an equal volume of the H-β-20.4 with the zinc solution having a Zn loading mass fraction of 1%; and calcining the H-β-20.4 at 550° C. for 6 h in a muffle furnace to obtain H-β-20.4 having a Zn mass fraction of 1%, which is recorded as 1%Zn-H-β-20.4.

[0037] The reaction conditions for lactide catalyzed by lactic acid are as follows: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140°C, and catalytic reaction in a three-necked flask for 5 h.

[0038] Example 5

[0039] A method for preparing a catalyst for preparing lactide from lactic acid by a one-step process comprises the following steps: adding ferric nitrate to deionized water to prepare a ferric nitrate solution in which the mass ratio of iron ions to molecular sieve is 1%; adding the solution to H-β-20.4 synthesized in Example 2, impregnating an equal volume of the H-β-20.4 with an iron solution having a loading mass fraction of 1% Fe; and calcining the H-β-20.4 in a muffle furnace at 550° C. for 6 h to obtain H-β-20.4 having a mass fraction of 1% Fe, which is recorded as 1%Fe-H-β-20.4.

[0040] The reaction conditions for lactide catalyzed by lactic acid are as follows: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140°C, and catalytic reaction in a three-necked flask for 5 h.

[0041] Example 6

[0042] A method for preparing a catalyst for preparing lactide from lactic acid by a one-step process comprises the following steps: adding lead nitrate to deionized water to prepare a lead nitrate solution in which the lead ion accounts for 1% by mass of the molecular sieve; adding the solution to H-β-20.4 synthesized in Example 2, impregnating an equal volume of the H-β-20.4 with a lead solution having a Pb loading fraction of 1% by mass; and calcining the H-β-20.4 in a muffle furnace at 550° C. for 6 h to obtain H-β-20.4 having a Pb mass fraction of 1%, which is recorded as 1%Pb-H-β-20.4.

[0043] The reaction conditions for lactide catalyzed by lactic acid are as follows: using 80% (wt%) lactic acid aqueous solution as raw material, toluene as solvent, reaction temperature at 140°C, and catalytic reaction in a three-necked flask for 5 h.

[0044] The methods used in Examples 7 to 30 were the same as those in Examples 3 to 6, except that the catalyst type was changed. The catalyst used was H-β-x with dealumination heating times of 2 d, 3 d, 4 d, 5 d, 6 d, and 7 d, respectively.

[0045] The methods used in Examples 31 to 50 were the same as those in Examples 19 to 22, except that the metal loading was changed. The metal loadings (wt %) used were 0.5, 2, 5, 10, 15, and 20, respectively.

[0046] It is denoted as n%MH-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 were identical to those in Example 2, except for the dealumination heating time. The heating times used were 2 days, 3 days, 4 days, 5 days, 6 days, and 7 days, respectively, and are denoted as H-β-x, where x is the silicon-to-aluminum ratio after the acid treatment.

[0048] Comparative Example 7

[0049] First, l-lysine, tetraethylammonium hydroxide, and deionized water were mixed and stirred at room temperature until the solution was clear. Potassium chloride and sodium metaaluminate were then added and stirred until the solution was clear. Silica was then slowly added and stirred for 12 hours. Finally, the gel precursor was transferred to a polytetrafluoroethylene-lined stainless steel reactor to synthesize the gel precursor. The molar ratio of the gel precursor raw materials was SiO₂:TEAOH:Al₂O₃:KCl:H₂O (1:0.45:0.025:0.04:10). Crystallization was performed at 140°C for 4 days. The crystallized mixture was washed, centrifuged, dried overnight, and calcined in a muffle furnace at 550°C for 6 hours to obtain β molecular sieve.

[0050] The prepared molecular sieve sample was placed in a 1 M NH4NO3 solution and ion exchanged in an 80°C oil bath for 6 h. This was repeated twice, followed by washing, centrifugation, and overnight drying. Finally, it was calcined at 550°C for 6 h to obtain H-β molecular sieve (Si / Al = 40).

[0051] The reaction conditions for lactic acid-catalyzed lactide production are as follows: using 80% (wt%) lactic acid aqueous solution as the raw material, toluene as the solvent, the reaction temperature is 140°C, and the catalysis is carried out in a three-necked flask for 5 hours.

[0052] Comparative Example 8

[0053] First, l-lysine, tetraethylammonium hydroxide, and deionized water were mixed and stirred at room temperature until the solution was clear. Potassium chloride and sodium metaaluminate were then added and stirred until the solution was clear. Silica was then slowly added and stirred for 12 hours. Finally, the gel precursor was transferred to a stainless steel reactor lined with polytetrafluoroethylene. The raw material ratio for the gel precursor was 1:0.45:0.0125:0.04:10. Crystallization was performed at 140°C for 4 days. The crystallized mixture was washed, centrifuged, dried overnight, and calcined in a muffle furnace at 550°C for 6 hours to produce β molecular sieve.

[0054] The prepared molecular sieve sample was placed in a 1 M NH4NO3 solution and ion exchanged in an 80°C oil bath for 6 h. This was repeated twice, followed by washing, centrifugation, and overnight drying. Finally, it was calcined at 550°C for 6 h to obtain H-β molecular sieve (Si / Al = 80).

[0055] The reaction conditions for lactic acid-catalyzed lactide production are as follows: using 80% (wt%) lactic acid aqueous solution as the raw material, toluene as the solvent, the reaction temperature is 140°C, and the catalysis is carried out in a three-necked flask for 5 hours.

[0056] The specific examples and comparative examples are shown in the table below:

[0057]

[0058]

[0059]

[0060] The above results show that the nanoporous solid acid catalyst obtained by the method of using a solid acid catalyst to supplement acid vacancies in the present invention has the best catalytic results of 100% conversion and 99% lactide selectivity, which is much higher than the nanoporous silica-alumina solid acid catalyst after dealumination, and has achieved good technical effects. Among them, the metal is Sn or Pb, the mass percentage of metal ions to internal dealumination nanoporous solid acid catalyst is 5-10%, and the best effect is achieved when the dealumination time is 5 days. This is because after the metal enters the molecular sieve, in addition to restoring the acid content, it also has a certain stabilizing effect on the molecular sieve framework, and it will not be easily removed under acidic conditions, thereby extending the life of the molecular sieve.

[0061] exist Figure 1 By comparing the X-ray diffraction spectra before and after silanization treatment, the diffraction peak position of H-β molecular sieve did not change, but the peak intensity was greatly reduced, indicating that although the overall structure of the molecular sieve remained stable when heated in an acidic system for a long time, the pores gradually collapsed.

[0062] exist Figure 2 By comparing the NH3-TPD curves before and after metal addition, it was found that the total acid content of H-β molecular sieve gradually decreased with the increase of heating time. After metal loading, the total acid content of H-β molecular sieve gradually increased, indicating that the metal loading gradually replenished the acid sites of H-β molecular sieve, causing the total acid content to gradually recover.

Claims

1. A method for preparing a catalyst for producing 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 for dealumination to obtain an internally dealuminated hydrogen-type molecular sieve; the internally dealuminated hydrogen-type molecular sieve is mixed with a lead nitrate solution for reaction, and then calcined to obtain a lactide catalyst; the mass ratio of lead ions to the internally dealuminated hydrogen-type molecular sieve is 10%; The preparation method of the hydrogen molecular sieve includes: mixing l-lysine, tetraethylammonium hydroxide and deionized water at room temperature, then adding potassium chloride and sodium metaaluminate and continuing to stir, then adding white carbon black and stirring thoroughly to obtain a gel precursor, finally crystallizing the gel precursor at 120-160° C. for 3-5 days, washing the crystallized mixed solution, centrifugally drying it, and then calcining 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 in an oil bath at 70-90° C., washing, centrifugally drying it, and finally calcining it at 500-600° C. for 4-8 hours to obtain a hydrogen molecular sieve.

2. The method for preparing a catalyst for preparing lactide from lactic acid by one-step method according to claim 1, wherein: The hydrogen-type molecular sieve is a silicon-aluminum molecular sieve having a 12-membered ring channel, 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 method 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, wherein: The temperature of the heating dealumination is 120-160° C., and the time is 1-7 days.

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

100.

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

7. A lactide catalyst, characterized in that The method according to any one of claims 1 to 6 is used for preparation.

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

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