Catalyst for preparing lactide from lactic acid as well as preparation method and application of catalyst

Abstract

The invention belongs to the field of lactide catalysts, and relates to a catalyst for preparing lactide from lactic acid as well as a preparation method and application thereof, the preparation method comprises the following steps: mixing a silanization reagent and an organic solvent, adding a nanopore silicon-aluminum solid acid catalyst, heating and refluxing to obtain a nanopore solid acid catalyst of which the outer surface is dehydroxylated; and roasting the nanopore solid acid catalyst with the dehydroxylated outer surface to obtain the catalyst which has an inert outer surface and does not induce secondary reaction of lactide. According to the method, silane is associated with acidic species on the outer surface of the nanoporous aluminum silicate solid acid catalyst by adopting a chemical method, so that the acidity of the outer surface is eliminated, and the problem of secondary reaction of lactide on the outer surface of the catalyst is effectively solved.

Description

Technical Field

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

[0002] Polylactic acid (PLA), as one of the main synthetic bio-based plastics on the market at present, has attracted much attention due to its wide application and effective reduction of marine plastic pollution. In the industrial production of polylactic acid, lactide is a precursor for the synthesis of high-quality polylactic acid and is obtained by the dimerization of lactic acid. There are mainly two methods for the current preparation of lactide: the two-step method and the one-step method. The two-step method refers to first polymerizing lactic acid into oligomers under low temperature, reduced pressure and the action of a catalyst, and then subjecting the oligomers to ring-opening cleavage under high temperature and high vacuum conditions. However, this method has problems such as high impurity content, difficult separation and purification, and high energy consumption. The one-step method directly converts two lactic acid molecules into linear lactic acid dimers under high temperature and the action of a catalyst, and then condenses them into cyclic dimers. This reaction has high selectivity and extremely broad development prospects. However, this process has extremely high requirements for the catalyst, and the catalyst needs to have both high activity and selectivity at the same time. Therefore, the preparation of the catalyst has become a research hotspot in recent years. Nano-porous aluminosilicate solid acid catalysts (such as molecular sieves) can effectively control the selectivity of products due to their shape-selective characteristics and have been widely used in the petrochemical industry. In recent years, studies have shown that molecular sieves can be used to catalyze the reaction to directly synthesize lactide from lactic acid, which has the advantages of low cost, easy recovery of by-products, and simple reaction, making it possible to synthesize lactide by the industrial one-step method. However, the acidic species on the outer surface of the catalyst will cause the generated lactide to undergo a secondary reaction, thereby ring-opening polymerization to form by-products such as oligolactic acid. These by-products will reduce the quality of the generated lactide, thereby affecting the quality of the subsequent polylactic acid products obtained by polymerization.

[0003] In summary, the preparation of the catalyst and the influence of the acidic species on the catalyst surface on the reaction during the catalytic preparation of lactide from lactic acid are problems that need to be solved urgently. These problems have greatly restricted the development and application of the one-step method for preparing lactide from lactic acid. Summary of the Invention

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

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

[0006] A preparation method of a catalyst for preparing lactide from lactic acid, comprising the following steps: mixing a silylating reagent and an organic solvent, adding a nanoporous silica-alumina solid acid catalyst, and obtaining a nanoporous solid acid catalyst with dehydroxylated outer surface after heating under reflux; calcining the nanoporous solid acid catalyst with dehydroxylated outer surface to obtain a catalyst for preparing lactide from lactic acid.

[0007] Further, 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 framework type of the molecular sieve is at least one of BEA, FAU, and MOR.

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

[0009] Further, 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, and then adding white carbon black and fully stirring to obtain a gel precursor. Finally, the gel precursor is crystallized at 130-150 °C for 3-6 days. After washing, centrifuging and drying the crystallized mixed solution, it is calcined in a muffle furnace at 500-600 °C for 4-8 h to obtain a molecular sieve; the molecular sieve is placed in NH 4 NO 3 solution and ion-exchanged at 70-100 °C in an oil bath for 4-8 h, washed, centrifuged, dried overnight, and finally calcined at 480-620 °C for 3-8 h to obtain H-β molecular sieve.

[0010] Further, the silylating reagent is selected from at least one of tetraethyl orthosilicate, trimethoxysilane, triethoxysilane, butyltrichlorosilane, hexamethyldisilazane, trifluoromethyltrimethylsilane, and diphenyldifluorosilane.

[0011] Further, the organic solvent is selected from at least one of benzene, toluene, xylene, mesitylene, straight-chain alkane, and cycloalkane.

[0012] Further, the temperature of heating under reflux is 60 °C to 120 °C.

[0013] Further, the nanoporous solid acid catalyst with dehydroxylated outer surface is calcined at 520-580 °C for 4-8 h 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] Further, the reaction temperature for preparing lactide is 120-160 °C, and the reaction time is 3-7 h.

[0016] The present invention provides a method for preparing a catalyst for preparing lactide from lactic acid, comprising the following steps: mixing a silylating reagent (solution A) and an organic solvent (solution B), adding a nanoporous solid acid catalyst powder, and heating under reflux for a certain time to obtain a nanoporous solid acid catalyst with dehydroxylated outer surface. The principle lies in that silane associates with the hydroxyl groups on the surface of the nanoporous solid acid catalyst, eliminating the surface acidity of the catalyst and inhibiting the secondary reaction of the formed lactide, thereby reducing the generation of by-products such as polymers. Both the number and length of the carbon chains of the silylating reagent will affect the result of surface modification. If the carbon chain length is too short (n < 2), the silylating reagent will block the internal pores of the nanoporous solid acid catalyst, thus reducing the catalytic activity; if the carbon chain length is too long (n > 4), the association of the silylating reagent on the surface hinders the entry of lactic acid molecules into the pores, thus affecting the catalytic performance and reducing the conversion rate. Only in the region with a moderate carbon chain length (n = 2 - 4), the silylating reagent associates with the hydroxyl groups on the surface of the nanoporous solid acid catalyst and does not block the internal pores, improving the selectivity of lactide in the reaction. Therefore, the catalytic result shows that as the carbon chain of the silylating reagent increases, the lactic acid conversion rate first decreases, then increases, and then remains unchanged, and the lactide selectivity first increases and then decreases; as the number of branches increases, the lactic acid conversion rate first increases and then decreases, and the lactide selectivity first increases and then decreases. In addition, incorporating heteroatoms, such as fluorine atoms, into the silane reagent will also result in excellent catalytic performance. This is because fluorosilane has excellent hydrophobic ability, but its catalytic ability is still affected by the carbon chain length and the number of branches.

[0017] Beneficial effects

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

[0019] The method of the present invention effectively solves the elimination of acidic species on the outer surface of the catalyst during the catalytic preparation of lactide from lactic acid. Adding the nanoporous solid acid catalyst with deacidified outer surface to 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 preparation of lactide from lactic acid.

[0020] In the present invention, by using the method of surface silylation of a solid acid catalyst, a silyl group is associated with the hydroxyl groups on the outer surface of the solid acid catalyst to obtain a nanoporous solid acid catalyst with dehydroxylated outer surface. The optimal catalytic result shows a conversion rate of 100% and a lactide selectivity of 99%, which is much higher than that of the unmodified nanoporous silica-alumina solid acid catalyst, achieving good technical effects. Brief description of the drawings

[0021] Figure 1X-ray diffraction pattern comparison diagram of H-β molecular sieve and silylated H-β molecular sieve;

[0022] Figure 2 Nitrogen adsorption and desorption curve diagram of H-β molecular sieve and silylated H-β molecular sieve;

[0023] Figure 3 Pyridine infrared spectrum diagram of H-β molecular sieve and silylated H-β molecular sieve. Detailed implementation method

[0024] Example 1

[0025] A preparation method of a catalyst for preparing 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, and then slowly add white carbon black and stir for 12 h to obtain a gel precursor. The molar ratio of the raw materials of the gel precursor SiO 2 : tetraethylammonium hydroxide (TEAOH): L-lysine (L-lysine): Al 2 O 3 : KCl: H 2 O is 1:0.45:0.3:0.03:0.04:10. Finally, transfer the gel precursor to a stainless steel reaction kettle 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 molecular sieve.

[0026] Place the prepared molecular sieve sample in 1 M NH 4 NO 3 solution and ion exchange at 80 °C in an oil bath for 6 h. After repeating 2 times, wash, centrifuge and dry overnight, and finally calcine at 550 °C for 6 h to obtain H-β molecular sieve (Si / Al = 15).

[0027] The reaction conditions for 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.

[0028] Example 2

[0029] A preparation method of a catalyst for preparing lactide from lactic acid, comprising the following steps: First, heat and activate 1 g of the H-β zeolite (Si / Al = 15) synthesized in Example 1 at 150 °C for 3 h. Then, prepare a mixed solution of tetraethyl orthosilicate and toluene, disperse 0.001 g of tetraethyl orthosilicate evenly in 40 mL of toluene, slowly add it to the activated H-β zeolite, and stir and reflux at 70 °C for 3 h. Subsequently, wash with toluene and ethanol and dry at 80 °C. Finally, calcine the obtained solid powder in a muffle furnace at 550 °C for 6 h to obtain 0.1% TEOS-H-β zeolite. Here, 0.1% is the mass ratio of tetraethyl orthosilicate to H-β zeolite.

[0030] The reaction conditions for 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.

[0031] Example 3

[0032] A preparative method of a catalyst for preparing lactide from lactic acid, comprising the following steps: First, heat and activate 1 g of the H-β zeolite (Si / Al = 15) synthesized in Example 1 at 150 °C for 3 h. Then, prepare a mixed solution of tetraethyl orthosilicate and toluene, disperse 0.005 g of tetraethyl orthosilicate evenly in 40 mL of toluene, slowly add it to the activated H-β zeolite, and stir and reflux at 70 °C for 3 h. Subsequently, wash with toluene and ethanol and dry at 80 °C. Finally, calcine the obtained solid powder in a muffle furnace at 550 °C for 6 h to obtain 0.5% TEOS-H-β zeolite.

[0033] The reaction conditions for 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.

[0034] Example 4

[0035] A preparation method of a catalyst for preparing lactide from lactic acid, comprising the following steps: First, heat and activate 1 g of the H-β zeolite (Si / Al = 15) synthesized in Example 1 at 150 °C for 3 h. Then, prepare a mixed solution of tetraethyl orthosilicate and toluene, disperse 0.01 g of tetraethyl orthosilicate evenly in 40 mL of toluene, slowly add it to the activated H-β zeolite, and stir and reflux at 70 °C for 3 h. Subsequently, wash with toluene and ethanol and dry at 80 °C. Finally, calcine the obtained solid powder in a muffle furnace at 550 °C for 6 h to obtain 1% TEOS-H-β zeolite.

[0036] The reaction conditions for the preparation of lactide by lactic acid catalysis 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.

[0037] The methods used in Examples 5 to 31 are the same as those in Examples 2 to 4, only the silanization reagents and their usage amounts are changed. The silanization reagents used are trimethoxysilane, triethoxysilane, hexamethyldisilazane, tetraphenylsilane, octadecyl dimethyl methoxysilane, hexamethyldisilazane, butyltrichlorosilane, trifluoromethyltrimethylsilane, and diphenyldifluorosilane, and the usage amounts are 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. Then, potassium chloride and sodium metaaluminate were added and stirred until the solution was clear and transparent. After that, silica white was slowly added and stirred for 12 h to obtain a gel precursor. The raw materials of the gel precursor, SiO 2 : tetraethylammonium hydroxide (TEAOH): l-lysine (L-lysine): Al 2 O 3 : KCl: H 2 O have a molar ratio of 1:0.45:0.0125:0.03:0.04:10. Finally, the gel precursor was transferred to a stainless-steel autoclave lined with polytetrafluoroethylene and crystallized at 140 °C for 4 days. The crystallized mixed solution was washed, centrifuged, and dried overnight, and then calcined in a muffle furnace at 550 °C for 6 h to obtain β zeolite.

[0040] The prepared zeolite sample was placed in a 1 M NH 4 NO 3 solution and ion-exchanged at 80 °C in an oil bath for 6 h. After repeating 2 times, it was washed, centrifuged, and dried overnight. Finally, it was calcined at 550 °C for 6 h to obtain H-β zeolite (Si / Al = 40).

[0041] The reaction conditions for the preparation of lactide by lactic acid catalysis 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.

[0042] Comparative Example 2

[0043] 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 metaaluminate were added and stirred until the solution was clear and transparent. After that, silica white was slowly added and stirred for 12 h to obtain a gel precursor. The raw materials of the gel precursor SiO 2 : tetraethylammonium hydroxide (TEAOH): l-lysine (L-lysine): Al 2 O 3 : KCl: H 2 O had a molar ratio of 1:0.45:0.3:0.00625:0.04:10. Finally, the gel precursor was transferred to a stainless-steel autoclave lined with polytetrafluoroethylene and crystallized at 140 °C for 4 days. The crystallized mixed solution was washed, centrifuged, dried overnight, and then calcined in a muffle furnace at 550 °C for 6 h to obtain β-zeolite.

[0044] The prepared zeolite sample was placed in 1 M NH 4 NO 3 solution and ion-exchanged at 80 °C in an oil bath for 6 h. After repeating this process twice, it was washed, centrifuged, dried overnight, and finally calcined at 550 °C for 6 h to obtain H-β zeolite (Si / Al = 80).

[0045] The reaction conditions for the preparation of lactide by lactic acid catalysis were as follows: an 80% (wt%) aqueous lactic acid solution was used as the raw material, toluene was used as the solvent, the reaction temperature was 140 °C, and the catalysis was carried out in a three-necked flask for 5 h.

[0046] The results of the specific examples and the comparative examples are shown in the following table:

[0047]

[0048]

[0049] The above results show that in the present invention, by using the method of surface silanization of a solid acid catalyst, silane groups are associated with the hydroxyl groups on the outer surface of the solid acid catalyst to obtain a nanoporous solid acid catalyst with dehydroxylated outer surface. The best catalytic result is a conversion rate of 100% and a lactide selectivity of 99%, which is much higher than that of the nanoporous silica-alumina solid acid catalyst without surface modification, achieving good technical effects. Among them, the silanization reagent used is diphenyldifluorosilane, and the effect is the best when the percentage of the silanization reagent to H-β zeolite is 0.5 - 1%.

[0050] In Figure 1 by comparing the X-ray diffraction spectra before and after silanization treatment, the positions of the diffraction peaks of H-β zeolite did not change, and no other diffraction peaks appeared, indicating that this method does not damage the carrier itself.

[0051] In Figure 2 , by comparing the nitrogen adsorption and desorption isotherm curves before and after the silylation treatment, it was found that after the silylation treatment, the specific surface area of the H-β zeolite increased significantly, but the pore size distribution remained unchanged, indicating that during the silylation process, the silylating reagent did not enter the zeolite pores and only associated with the surface hydroxyl groups of the zeolite.

[0052] In Figure 3 , by comparing the pyridine infrared spectra before and after the silylation treatment, smaller pyridine molecules can enter the zeolite pores and be adsorbed by the acid sites inside and outside the zeolite, resulting in a larger amount of L acid. Using larger 2,6-dimethylpyridine molecules for pyridine adsorption and desorption, this pyridine molecule cannot enter the pore interior due to its large size and is thus often used to measure the acid amount on the outer surface of the zeolite. By comparison, after the silylation treatment, the acid amount on the outer surface of the zeolite decreased significantly, especially the amount of L acid, which represents that the hydroxyl groups on the zeolite surface were largely covered, proving that the zeolite modification strategy effectively reduced the outer 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 dehydroxylated outer surface; and calcining the nanoporous solid acid catalyst with dehydroxylated outer surface to obtain a catalyst for preparing lactide from lactic acid.

2. The method for preparing a catalyst for preparing lactide from lactic acid according to claim 1, characterized in that: 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.

3. The method for preparing a catalyst for preparing lactide from lactic acid according to claim 2, characterized in that: 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.

4. The method for preparing a catalyst for preparing lactide from lactic acid according to claim 3, characterized in that: The preparation method of the H-β molecular sieve comprises: 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, finally crystallizing the gel precursor at 130-150° C. for 3-6 days, washing and centrifugally drying the crystallized mixed solution, and then calcining in a muffle furnace at 500-600° C. for 4-8 hours to obtain a molecular sieve; placing the molecular sieve in an NH4NO3 solution and performing ion exchange at 70-100° C. oil bath conditions for 4-8 hours, washing, centrifuging and drying overnight, and finally calcining at 480-620° C. for 3-8 hours to obtain an H-β molecular sieve.

5. The method for preparing a catalyst for preparing lactide from lactic acid according to claim 1, characterized in that: The silanization agent is selected from at least one of tetraethyl silicate, trimethoxysilane, triethoxysilane, butyltrichlorosilane, hexamethyldisilazane, trifluoromethyltrimethylsilane and diphenyldifluorosilane.

6. The method for preparing a catalyst for preparing lactide from lactic acid according to claim 1, characterized in that: The organic solvent is selected from at least one of benzene, toluene, xylene, trimethylbenzene, straight-chain alkanes and cycloalkanes.

7. The method for preparing a catalyst for preparing lactide from lactic acid according to claim 1, characterized in that: The heating reflux temperature is 60°C~120°C.

8. 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.

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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