A catalyst for preparing lactide from lactic acid, its preparation method and application

Abstract

The present invention belongs to the field of lactide catalysts and relates to a catalyst for preparing lactide from lactic acid, its preparation method, and its application. The preparation method comprises the following steps: mixing a silanization agent and an organic solvent, adding a nanoporous aluminosilicate solid acid catalyst, heating and refluxing to obtain a nanoporous solid acid catalyst with a dehydroxylated outer surface; and calcining the dehydroxylated outer surface nanoporous solid acid catalyst to obtain an inert outer surface catalyst that does not induce secondary reactions of lactide. The present invention chemically associates silane with acidic species on the outer surface of the nanoporous aluminosilicate solid acid catalyst, thereby eliminating the outer surface acidity, effectively addressing the problem of secondary reactions of lactide on the outer surface of the catalyst.

Description

Technical Field

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

[0002] Polylactic acid (PLA), one of the major synthetic bioplastics on the market, has attracted considerable attention for its wide applications and potential to effectively mitigate marine plastic pollution. In the industrial production of PLA, lactide is a precursor for high-quality PLA synthesis and is obtained by dimerization of lactic acid. Currently, there are two main methods for preparing lactide: a two-step process and a one-step process. The two-step process involves first polymerizing lactic acid into oligomers at low temperature, reduced pressure, and a catalyst, followed by ring-opening cleavage of the oligomers at high temperature and high vacuum. However, this method suffers from high impurity levels, difficulty in separation and purification, and high energy consumption. The one-step process directly converts two lactic acid molecules into linear lactic acid dimers at high temperature and a catalyst, which are then condensed into cyclic dimers. This reaction is highly selective and holds great promise for future development. However, this process places extremely high demands on the catalyst, requiring both high activity and selectivity. Therefore, catalyst preparation has become a research hotspot in recent years. Nanoporous aluminosilicate solid acid catalysts (such as molecular sieves) have been widely used in the petrochemical industry due to their shape-selective properties, which effectively control product selectivity. Recent studies have demonstrated the ability to directly synthesize lactide from lactic acid using molecular sieve-catalyzed reactions. This approach offers advantages such as low cost, easy by-product recovery, and a simple reaction, making industrial one-step lactide synthesis feasible. However, acidic species on the catalyst's surface can cause the generated lactide to undergo secondary reactions, leading to ring-opening polymerization and the formation of by-products such as oligomeric lactic acid. These by-products can reduce the quality of the resulting lactide, thereby affecting the quality of the subsequent polylactic acid product.

[0003] In summary, the preparation of the catalyst and the influence of acidic species on the catalyst surface on the reaction in the process of lactic acid-catalyzed lactide production are urgent issues that need to be addressed. These issues have greatly limited the development and application of the one-step lactide production process using lactic acid. Summary of the Invention

[0004] To address the problem in the existing one-step process for preparing lactide from lactic acid that acidic species present on the outer surface of the catalyst cause the generated lactide to undergo secondary reactions, resulting in the production of by-products such as polymers and a reduction in the quality of the lactide product, the present invention provides a catalyst for preparing lactide from lactic acid that does not induce secondary reactions of lactide and has an inert outer surface, as well as a preparation method and application thereof.

[0005] The technical solution adopted in the present invention is as follows:

[0006] A method for preparing a catalyst for preparing lactide from lactic acid comprises the following steps: mixing a silanization agent and an organic solvent, adding a nanoporous silica-alumina solid acid catalyst, heating and refluxing to obtain a nanoporous solid acid catalyst with a dehydroxylated outer surface; and calcining the nanoporous solid acid catalyst with a dehydroxylated outer surface to obtain the catalyst for preparing lactide from lactic acid.

[0007] Furthermore, the nanoporous silica-alumina solid acid catalyst is an activated molecular sieve, the molecular sieve is a silica-alumina molecular sieve with 12-membered ring channels, and the molecular sieve framework type is at least one of BEA, FAU, and MOR.

[0008] Furthermore, the preparation method of the activated molecular sieve is to heat and activate the H-β molecular sieve at 130-170° C. for 2-4 hours.

[0009] Furthermore, the preparation method of the H-β molecular sieve includes: fully stirring l-lysine, tetraethylammonium hydroxide and deionized water at room temperature, then adding potassium chloride and sodium aluminate and fully stirring, then adding white carbon black and fully stirring to obtain a gel precursor, and finally crystallizing the gel precursor at 130~150°C for 3~6 days, washing the crystallized mixed solution and centrifuging and drying it, and then calcining it in a muffle furnace at 500~600°C for 4~8h to obtain a molecular sieve; placing the molecular sieve in an NH4NO3 solution and ion exchanging it in an oil bath at 70~100°C for 4~8h, washing, centrifuging and drying overnight, and finally calcining it at 480~620°C for 3~8h to obtain an H-β molecular sieve.

[0010] Furthermore, the silanization agent is selected from at least one of tetraethyl silicate, trimethoxysilane, triethoxysilane, butyltrichlorosilane, hexamethyldisilazane, trifluoromethyltrimethylsilane, and diphenyldifluorosilane.

[0011] Furthermore, the organic solvent is selected from at least one of benzene, toluene, xylene, trimethylbenzene, linear alkanes, and cycloalkanes.

[0012] Furthermore, the heating reflux temperature is 60°C to 120°C.

[0013] Furthermore, the nanoporous solid acid catalyst with dehydroxylated outer surface is calcined at 520-580° C. for 4-8 hours to obtain a catalyst for preparing lactide from lactic acid.

[0014] The present invention also provides a lactide catalyst prepared by the above method.

[0015] Furthermore, the reaction temperature for preparing lactide is 120-160° C., and the reaction time is 3-7 hours.

[0016] The present invention provides a method for preparing a catalyst for producing lactide from lactic acid, comprising the following steps: mixing a silanization agent (solution A) and an organic solvent (solution B), adding nanoporous solid acid catalyst powder, and heating under reflux for a predetermined time to obtain a nanoporous solid acid catalyst with its outer surface dehydroxylated. The principle is that silane associates with hydroxyl groups on the surface of the nanoporous solid acid catalyst, eliminating the catalyst's surface acidity and inhibiting secondary reactions of the generated lactide, thereby reducing the production of byproducts such as polymers. The number and length of the carbon chains of the silanization agent both affect the surface modification results. If the carbon chain length is too short (n < 2), the silanization agent will clog the internal pores of the nanoporous solid acid catalyst, reducing catalytic activity. If the carbon chain length is too long (n > 4), the silanization agent's surface association hinders lactic acid molecules from entering the pores, thereby affecting catalytic performance and reducing conversion. Only within the moderate carbon chain length range (n = 2-4) will the silanization agent associate with the hydroxyl groups on the surface of the nanoporous solid acid catalyst without clogging the internal pores, thereby improving the lactide selectivity of the reaction. Therefore, the catalytic results show that as the carbon chain length of the silanizing agent increases, lactic acid conversion initially decreases, then increases and remains constant, while lactide selectivity initially increases and then decreases. With the addition of branching, lactic acid conversion initially increases and then decreases, while lactide selectivity initially increases and then decreases. Furthermore, incorporating heteroatoms, such as fluorine atoms, into the silanizing agent also yields excellent catalytic performance. This is due to the excellent hydrophobicity of fluorosilanes, but their catalytic ability is still affected by the carbon chain length and branching.

[0017] Beneficial effects

[0018] The present invention provides a method for eliminating the acidic species on the outer surface of a porous catalyst by associating silane with the acidic species on the outer surface, effectively solving the problem of secondary reaction of lactide on the outer surface of the catalyst. The method can be used in the industrial production of high-quality lactide catalysts prepared by a one-step method using lactic acid.

[0019] The method described in this invention effectively eliminates acidic species on the catalyst's outer surface during the lactic acid-catalyzed production of lactide. The deacidified 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 lactide production using lactic acid.

[0020] In the present invention, a silane group is associated with the hydroxyl group on the outer surface of the solid acid catalyst by using a method of surface silanization of the solid acid catalyst to obtain a nanoporous solid acid catalyst with dehydroxylation on the outer surface. The optimal catalytic result is 100% conversion rate and 99% lactide selectivity, which are much higher than those of nanoporous silica-alumina solid acid catalysts without surface modification, achieving good technical effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1The figure is a comparison of the X-ray diffraction spectra of H-β molecular sieve and H-β molecular sieve after silanization;

[0022] Figure 2 The nitrogen adsorption and desorption curves of H-β molecular sieve and H-β molecular sieve after silanization are shown;

[0023] Figure 3 The pyridine infrared spectra of H-β molecular sieve and H-β molecular sieve after silanization. DETAILED DESCRIPTION

[0024] Example 1

[0025] A method for preparing a catalyst for producing lactide from 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 the gel precursor is SiO2:tetraethylammonium hydroxide (TEAOH):l-lysine (L-lysine):Al2O3:KCl:H2O:10. 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.

[0026] 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).

[0027] The reaction conditions for preparing 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 catalysis in a three-necked flask for 5 h.

[0028] Example 2

[0029] A method for preparing a catalyst for producing lactide from lactic acid comprises the following steps: first, heating 1 g of the H-β molecular sieve (Si / Al = 15) synthesized in Example 1 at 150°C for 3 hours to activate it. Then, a mixed solution of tetraethyl silicate and toluene is prepared: 0.001 g of tetraethyl silicate is evenly dispersed in 40 mL of toluene, slowly added to the activated H-β molecular sieve, and stirred and refluxed at 70°C for 3 hours. The mixture is then washed with toluene and ethanol and dried at 80°C. Finally, the resulting solid powder is calcined in a muffle furnace at 550°C for 6 hours to obtain a 0.1% TEOS-H-β molecular sieve. The mass ratio of tetraethyl silicate to H-β molecular sieve is 0.1%, where 0.1% is the mass ratio of tetraethyl silicate to H-β molecular sieve.

[0030] The reaction conditions for preparing lactide by lactic acid catalysis 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.

[0031] Example 3

[0032] A method for preparing a catalyst for producing lactide from lactic acid comprises the following steps: first, heating 1 g of the H-β molecular sieve (Si / Al = 15) synthesized in Example 1 at 150°C for 3 hours to activate it. Then, a mixed solution of tetraethyl silicate and toluene is prepared: 0.005 g of tetraethyl silicate is evenly dispersed in 40 mL of toluene, slowly added to the activated H-β molecular sieve, and stirred and refluxed at 70°C for 3 hours. The mixture is then washed with toluene and ethanol and dried at 80°C. Finally, the resulting solid powder is calcined in a muffle furnace at 550°C for 6 hours to obtain a 0.5% TEOS-H-β molecular sieve.

[0033] The reaction conditions for preparing lactide by lactic acid catalysis 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.

[0034] Example 4

[0035] A method for preparing a catalyst for producing lactide from lactic acid comprises the following steps: first, heating 1 g of the H-β molecular sieve (Si / Al = 15) synthesized in Example 1 at 150°C for 3 hours to activate it. Then, a mixed solution of tetraethyl silicate and toluene is prepared: 0.01 g of tetraethyl silicate is evenly dispersed in 40 mL of toluene, slowly added to the activated H-β molecular sieve, and stirred and refluxed at 70°C for 3 hours. The mixture is then washed with toluene and ethanol and dried at 80°C. Finally, the resulting solid powder is calcined in a muffle furnace at 550°C for 6 hours to obtain a 1% TEOS-H-β molecular sieve.

[0036] The reaction conditions for preparing lactide by lactic acid catalysis 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.

[0037] The methods used in Examples 5 to 31 were the same as those in Examples 2 to 4, except that the silanization reagents and amounts were changed. The silanization reagents used were trimethoxysilane, triethoxysilane, hexamethyldisilazane, tetraphenylsilane, octadecyldimethylmethoxysilane, hexamethyldisilazane, butyltrichlorosilane, trifluoromethyltrimethylsilane, and diphenyldifluorosilane, in the amounts of 0.01 g, 0.05 g, and 0.1 g, respectively.

[0038] Comparative Example 1

[0039] First, l-lysine, tetraethylammonium hydroxide, and deionized water were mixed and stirred at room temperature until the solution was clear and transparent. Potassium chloride and sodium metaaluminate were then added and stirred until the solution was clear and transparent. Then, silica was slowly added and stirred for 12 hours to obtain a gel precursor. The molar ratio of the raw materials for the gel precursor was 1:0.45:0.0125:0.03:0.04:10:1 ...

[0040] 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).

[0041] The reaction conditions for preparing 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 catalysis in a three-necked flask for 5 h.

[0042] Comparative Example 2

[0043] First, l-lysine, tetraethylammonium hydroxide, and deionized water were mixed and stirred at room temperature until the solution was clear and transparent. Potassium chloride and sodium metaaluminate were then added and stirred until the solution was clear and transparent. Then, silica was slowly added and stirred for 12 hours to obtain a gel precursor. The molar ratio of the raw materials for the gel precursor was 1:0.45:0.3:0.00625:0.04:10:10:1. Finally, the gel precursor was transferred to a stainless steel reactor lined with polytetrafluoroethylene and crystallized 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.

[0044] 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).

[0045] The reaction conditions for preparing 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 catalysis in a three-necked flask for 5 h.

[0046] The specific examples and comparative examples are shown in the table below:

[0047]

[0048]

[0049] These results demonstrate that the present invention, through the use of a solid acid catalyst surface silanization method to associate silane groups with hydroxyl groups on the solid acid catalyst's outer surface, produces a nanoporous solid acid catalyst with dehydroxylated outer surface. The catalyst achieves optimal catalytic results, demonstrating a 100% conversion rate and 99% lactide selectivity, significantly exceeding those achieved with unmodified nanoporous silica-alumina solid acid catalysts. The optimal catalytic effect is achieved when the silanization agent used is diphenyldifluorosilane and the ratio of the silanization agent to the H-β molecular sieve is 0.5-1%.

[0050] exist Figure 1 By comparing the X-ray diffraction spectra before and after silanization treatment, the position of the H-β molecular sieve diffraction peak did not change, and no other diffraction peaks appeared, indicating that this method would not damage the carrier itself.

[0051] exist Figure 2By comparing the nitrogen adsorption-desorption curves before and after silanization treatment, it was found that the specific surface area of H-β molecular sieve increased significantly after silanization treatment, but the pore size distribution did not change, indicating that during the silanization process, the silanization agent did not enter the molecular sieve pores, but only associated with the hydroxyl groups on the surface of the molecular sieve.

[0052] exist Figure 3 In the experiment, by comparing the infrared spectra of pyridine before and after silanization treatment, smaller pyridine molecules can enter the pores of the molecular sieve and be adsorbed by the acid points inside and outside the molecular sieve, resulting in a larger amount of L-acid. Larger 2,6-dimethylpyridine molecules are used for pyridine adsorption and desorption. Due to their large size, the pyridine molecules cannot enter the pores and are often used to measure the amount of acid on the external surface of the molecular sieve. By comparison, after silanization treatment, the amount of acid on the external surface of the molecular sieve is significantly reduced, especially the amount of L-acid. This means that the hydroxyl groups on the surface of the molecular sieve are covered in large quantities, proving that the molecular sieve modification strategy effectively reduces the external surface acid sites.

Claims

1. A method for preparing a catalyst for preparing lactide from lactic acid, characterized in that: The method comprises the following steps: mixing a silanization agent and an organic solvent, adding a nanoporous silica-alumina solid acid catalyst, heating and refluxing to obtain a nanoporous solid acid catalyst with a dehydroxylated outer surface; calcining the nanoporous solid acid catalyst with a dehydroxylated outer surface to obtain a catalyst for preparing lactide from lactic acid; the nanoporous silica-alumina solid acid catalyst is an activated molecular sieve, and the silanization agent is diphenyldifluorosilane; The preparation method of the activated molecular sieve is to heat and activate the H-β molecular sieve at 130-170°C for 2-4 hours; the preparation method of the H-β molecular sieve includes: fully stirring l-lysine, tetraethylammonium hydroxide and deionized water at room temperature, then adding potassium chloride and sodium metaaluminate and fully stirring, then adding white carbon black and fully stirring to obtain a gel precursor, finally crystallizing the gel precursor at 130-150°C for 3-6 days, washing and centrifuging the crystallized mixed solution, and then calcining it in a muffle furnace at 500-600°C for 4-8 hours to obtain the molecular sieve; placing the molecular sieve in an NH4NO3 solution and performing ion exchange in an oil bath at 70-100°C for 4-8 hours, washing, centrifuging and drying overnight, and finally calcining it at 480-620°C for 3-8 hours to obtain the H-β molecular sieve.

2. The method for preparing a catalyst for preparing lactide from lactic acid according to claim 1, wherein: The molecular sieve is a silica-alumina molecular sieve with 12-membered ring channels, and the molecular sieve framework type is at least one of BEA, FAU, and MOR.

3. The method for preparing a catalyst for preparing lactide from lactic acid according to claim 1, wherein: The organic solvent is selected from at least one of benzene, toluene, xylene, trimethylbenzene, linear alkanes, and cycloalkanes.

4. The method for preparing a catalyst for preparing lactide from lactic acid according to claim 1, wherein: The heating reflux temperature is 60℃~120℃.

5. The method for preparing a catalyst for preparing lactide from lactic acid according to claim 1, characterized in that: The nanoporous solid acid catalyst with dehydroxylated outer surface is calcined at 520-580° C. for 4-8 hours to obtain a catalyst for preparing lactide from lactic acid.

6. A lactide catalyst, characterized in that The method according to any one of claims 1 to 5 is used for preparation.

7. Use of the lactide catalyst according to claim 6 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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