A Salen-titanium complex and its preparation method and application

Abstract

The present invention discloses a Salen-titanium complex and its preparation method and application, and relates to the technical field of polymer synthesis catalysis. The present invention adopts 2,5-furandicarboxaldehyde and 2-aminophenol as reaction raw materials, obtains a Salen ligand through a condensation reaction, and then mixes the Salen ligand with a titanium source for coordination reaction to obtain a Salen-titanium complex. The Salen-titanium complex can be used as a catalyst for polycondensation reaction to prepare polyvalerolactone, polyethylene 2,5-furandicarboxylate and polyethylene terephthalate-co-1,4-cyclohexanedimethanol. The Salen ligand prepared by the present invention has a unique spatial structure and electronic effect. After forming a complex with titanium, it also has a corresponding unique structure, which helps the complex to maintain a better dispersed state in the polymerization reaction system. At the same time, it also shows good hydrolysis resistance, thereby avoiding hydrolysis failure and ensuring that the polymerization reaction is effectively carried out.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer synthesis catalysis, in particular to a Salen-titanium complex and a preparation method and application thereof. Background Art

[0002] Salen ligands are formed by the condensation of salicylaldehyde (or its derivatives) and ethylenediamine (or other diamines). These ligands have a planar structure and can provide multiple coordinating atoms to form stable complexes with metal ions. These complexes exhibit unique catalytic properties in a variety of chemical reactions.

[0003] Polyester is a general term for polymers prepared by the polymerization reaction of polyols and polyacids, including polyvalerolactone (PVL), polyethylene terephthalate-co-1,4-cyclohexanedimethanol (PCTG), polyethylene 2,5-furandicarboxylate (PEF), polylactic acid (PLA), and polyethylene terephthalate (PET). Catalysts used in polyester synthesis include antimony, tin, germanium, and titanium. While antimony catalysts offer good catalytic activity and selectivity, trivalent antimony can be reduced to metallic antimony during the reaction, resulting in high levels of metallic antimony in the polymer and heavy metals in the polyester, which can affect its quality. Germanium catalysts have low catalytic activity and are expensive, making them unsuitable for large-scale industrial production. Furthermore, polyesters produced using germanium catalysts have a relatively high ether bond content, which results in a low melting point, affecting the polyester's heat resistance and processing properties. Tin-based catalysts offer high catalytic activity, good reaction selectivity, and mild reaction conditions. However, some tin-based catalysts are toxic, and the polyesters produced using these catalysts contain high levels of terminal carboxyl groups, which can affect the thermal stability of the polyester. Titanates, a commonly used titanium-based catalyst, are susceptible to hydrolysis in the presence of water. This hydrolysis process typically involves a nucleophilic substitution reaction between the alkoxy groups in the titanate and water, generating the corresponding alcohol and titanium hydroxide or oxide, which can affect the catalyst activity and the hue of the polyester.

[0004] In the prior art, CN115584018A discloses a method for preparing a polyester I-polyester II-polycarbonate triblock copolymer, wherein a bis-Schiff base tetravalent titanium is selected as a main catalyst for the copolymerization reaction. The structural formula of the bis-Schiff base tetravalent titanium is as follows:

[0005] , where R1 is a bridged alkyl segment, R2 is a tert-butyl group, a methyl group, a nitro group, or a chlorine atom, R3 is a tert-butyl group, a hydrogen atom, or a methoxy group, and X is a halide group, a nitrate group, or an acetate group. However, the primary function of the above catalysts is ring-opening copolymerization, and their applicability to polycondensation to prepare polyesters has not been explored.

[0006] Therefore, it is necessary to develop a new type of efficient and environmentally friendly catalyst for polyester production, which can effectively alleviate the problem of catalyst hydrolysis and reduce the heavy metal content in polyester. At the same time, it can also avoid the production of harmful by-products and broaden the application scope of polyester in biology, medicine, food and other fields. Summary of the Invention

[0007] In view of the above-mentioned prior art, the purpose of the present invention is to provide a Salen-titanium complex and its preparation method and application. The present invention uses 2,5-furandicarboxaldehyde and 2-aminophenol as reaction raw materials, obtains a Salen ligand through a condensation reaction, and then mixes the Salen ligand with a titanium source and undergoes a coordination reaction to obtain a Salen-titanium complex. The obtained Salen-titanium complex can be used as a catalyst for polycondensation reaction to prepare polyvalerolactone, polyethylene furandicarboxylate, and polyethylene terephthalate-co-1,4-cyclohexanedimethanol. The Salen ligand prepared by the present invention has a unique spatial structure and electronic effect. After forming a complex with titanium, it also has a corresponding unique structure. This structure helps the complex maintain a good dispersion state in the polymerization reaction system. At the same time, it also exhibits good hydrolysis resistance, thereby avoiding hydrolysis failure and ensuring the effective progress of the polymerization reaction.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] The first aspect of the present invention provides a Salen-titanium complex, the structural formula of which is shown in formula (I):

[0010]

[0011] The second aspect of the present invention provides a method for preparing the above-mentioned Salen-titanium complex, comprising the following steps:

[0012] (1) 2,5-furandicarboxaldehyde and 2-aminophenol are mixed and placed in toluene for condensation reaction. After the reaction is completed, the mixture is filtered, and the filtered solid is collected and washed to obtain a Salen ligand;

[0013] (2) Salen ligand and titanium source are used as reactants, and toluene is used as reaction solvent. After coordination reaction, purification is performed to obtain a Salen-titanium complex.

[0014] Preferably, in step (1), the molar ratio of 2,5-furandicarboxaldehyde to 2-aminophenol is 1:(2.1-2.2).

[0015] Preferably, in step (1), the mass ratio of the total mass of 2,5-furandicarboxaldehyde and 2-aminophenol to toluene is 1:(40-100).

[0016] Preferably, in step (1), the condensation reaction temperature is 120-140° C., and the condensation reaction time is 8-12 h.

[0017] Preferably, in step (2), the titanium source is one of tetrabutyl titanate, titanium tetrachloride, and titanium bromide.

[0018] Preferably, in step (2), the molar ratio of the Salen ligand to the titanium source is 1:(1-1.1).

[0019] Preferably, in step (2), the mass ratio of the Salen-ligand to the reaction solvent is 1:(40-100).

[0020] Preferably, in step (2), the coordination reaction temperature is 130-140° C., and the coordination reaction time is 12-15 h.

[0021] Preferably, in step (2), acetonitrile, N,N-dimethylformamide or acetone is used for recrystallization.

[0022] The third aspect of the present invention provides the use of the above-mentioned Salen-titanium complex in any one of the following 1)-3):

[0023] 1) Catalytic preparation of polyvalerolactone;

[0024] 2) catalyzing a polycondensation reaction to prepare polyethylene 2,5-furandicarboxylate;

[0025] 3) Catalytic polycondensation reaction to prepare polyethylene terephthalate-co-1,4-cyclohexanedimethanol ester.

[0026] Preferably, the specific process of preparing polyvalerolactone (PVL) by catalytic ring-opening polymerization is:

[0027] Salen-titanium complex is added as a catalyst to dehydrated delta-valerolactone, and then an initiator is added to carry out a ring-opening polymerization reaction at a reaction temperature of 120-150° C. and a vacuum degree of 20-50 Pa for 3-10 hours to prepare polyvalerolactone.

[0028] More preferably, the dehydrated δ-valerolactone is prepared by the following method: dehydrating δ-valerolactone under vacuum conditions for 2-8 hours to obtain the dehydrated δ-valerolactone.

[0029] Further preferably, the molar ratio of the Salen-titanium complex and the dehydrated δ-valerolactone is 1:(100-500); the initiator is benzyl alcohol, and the molar ratio of the Salen-titanium complex and the initiator is (1-1.5):1.

[0030] More preferably, the ring-opening polymerization reaction conditions are: a reaction temperature of 120-150° C., a vacuum degree of 20-50 Pa, and a ring-opening polymerization reaction time of 3-10 h.

[0031] Preferably, the specific process of preparing polyethylene 2,5-furandicarboxylate (PEF) by catalytic polycondensation reaction is:

[0032] After mixing 2,5-furandicarboxylic acid, ethylene glycol and Salen-titanium complex, an esterification reaction is carried out at 0.20-0.30 MPa and 180-230°C. When the mass of water generated during the esterification reaction reaches 90% of the theoretical mass of water generated, the reaction conditions are changed to 30-60 Pa and 190-250°C, and a condensation reaction is carried out for 3-5 hours to obtain polyethylene furandicarboxylate.

[0033] More preferably, the molar ratio of 2,5-furandicarboxylic acid to ethylene glycol is 1:(1.4-1.8), and the molar ratio of Salen-titanium complex to 2,5-furandicarboxylic acid is 1:(500-1000).

[0034] Preferably, the specific operation of preparing polyethylene terephthalate-co-1,4-cyclohexanedimethanol ester (PCTG) by catalytic polycondensation reaction is:

[0035] After purified terephthalic acid, 1,4-cyclohexanedimethanol, ethylene glycol and Salen-titanium complex are mixed, an esterification reaction is carried out at 0.20-0.30 MPa and 200-230°C. When the mass of water generated during the esterification reaction reaches 90% of the theoretical mass of water generated, the reaction conditions are changed to 30-60 Pa and 210-260°C, and a condensation reaction is carried out for 3-6 hours. After the reaction is completed, polyethylene terephthalate-co-1,4-cyclohexanedimethanol ester is obtained.

[0036] More preferably, the molar ratio of purified terephthalic acid, 1,4-cyclohexanedimethanol, and ethylene glycol is 6:(4-5):(3-4), and the molar ratio of the Salen-titanium complex to purified terephthalic acid is 1:(500-1000).

[0037] Beneficial effects of the present invention:

[0038] The present invention uses 2,5-furandicarboxaldehyde and 2-aminophenol as reaction raw materials to produce a salen ligand through a condensation reaction. The salen ligand is then mixed with a titanium source and subjected to a coordination reaction to produce a salen-titanium complex. The resulting salen-titanium complex can be used as a catalyst in a polycondensation reaction to prepare polyvalerolactone, polyethylene furandicarboxylate, and polyethylene terephthalate-co-1,4-cyclohexanedimethanol.

[0039] The Salen ligand prepared by the present invention has a unique spatial structure and electronic effect. After forming a complex with titanium, the resulting Salen-titanium complex also has a corresponding unique structure, which helps the complex maintain a better dispersion state in the polymerization reaction system. At the same time, there is a strong coordination effect between the titanium metal center in the Salen-titanium complex and the Salen ligand, which greatly improves the hydrolysis resistance compared to the traditional tetrabutyl titanate catalyst, thereby avoiding hydrolysis failure and ensuring the effective polymerization reaction. It can be seen that using the Salen-titanium complex prepared by the present invention to replace traditional tin, antimony and other catalysts for the polycondensation reaction to prepare polyester can effectively prevent the catalyst from hydrolysis and reduction, thereby reducing the heavy metal content in the polyester, ensuring the quality of the polyester, and expanding the application of polyester in the fields of food, biology, medicine, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 : X-ray single crystal diffraction characterization pattern of the Salen-ligand prepared in Example 1;

[0041] Figure 2 : Infrared spectrum of the Salen-titanium complex prepared in Example 1;

[0042] Figure 3 : The polyvalerolactone obtained in Example 2 1 H NMR nuclear magnetic characterization diagram;

[0043] Figure 4 : Polyethylene 2,5-furandicarboxylate obtained in Example 3 1 H NMR nuclear magnetic characterization diagram;

[0044] Figure 5 : Infrared spectrum of polyethylene terephthalate-co-1,4-cyclohexanedimethanol ester prepared in Example 4;

[0045] Figure 6 : Polyethylene terephthalate-co-1,4-cyclohexanedimethanol ester prepared in Example 4 1 H NMR nuclear magnetic characterization diagram. DETAILED DESCRIPTION

[0046] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0047] In the prior art, traditional catalysts such as tin and antimony, while exhibiting some catalytic activity when used in polymerization reactions to produce polyester, suffer from several significant drawbacks. For example, heavy metals like tin and antimony are toxic, and residual heavy metals in polymers pose potential hazards to the environment and human health, limiting their application in safety-critical applications such as food, biology, and medicine. Furthermore, titanates, commonly used titanium-based catalysts, are susceptible to hydrolysis in the presence of water, leading to catalyst deactivation.

[0048] Based on this, the present invention provides a Salen-titanium complex and its preparation method and application. The structural formula of the Salen-titanium complex is The preparation method comprises: using 2,5-furandicarboxaldehyde and 2-aminophenol as reaction raw materials, preparing a salen ligand through a condensation reaction, and then mixing the salen ligand with a titanium source and performing a coordination reaction to prepare a salen-titanium complex. The salen-titanium complex prepared in the present invention can be used as a catalyst in a polycondensation reaction to prepare polyvalerolactone, polyethylene furandicarboxylate, and polyethylene terephthalate-co-1,4-cyclohexanedimethanol.

[0049] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0050] The experimental materials used in the examples of the present invention are all conventional experimental materials in the art and can be purchased through commercial channels.

[0051] Example 1: Preparation of Salen-Titanium Complex

[0052] (1) 2,5-furandicarboxaldehyde and 2-aminophenol were mixed in a molar ratio of 1:2.1, placed in toluene, and subjected to condensation reaction at 130°C for 10 hours. After the reaction was completed, the mixture was filtered, and the filtered solid was collected and washed with acetone to obtain a brown solid, which was the Salen ligand; wherein the mass ratio of the total mass of 2,5-furandicarboxaldehyde and 2-aminophenol to toluene was 1:70;

[0053] (2) Salen ligand and titanium tetrachloride were mixed in a molar ratio of 1:1.05 as reactants, and toluene was used as a reaction solvent, wherein the mass ratio of the Salen ligand to the reaction solvent was 1:70. The coordination reaction was carried out at 135° C. for 14 hours. After removing the toluene solvent under vacuum conditions, recrystallization was carried out using 20 mL of acetonitrile to obtain a Salen-titanium complex.

[0054] The structural formula of the Salen-titanium complex prepared in this embodiment is shown in formula (I):

[0055] Abbreviated as [TiCl2L].

[0056] The structure of the Salen-titanium complex prepared in this example was characterized. Figure 1-Figure 2 shown.

[0057] Depend on Figure 1 It can be seen that the Salen ligand contains a typical [O, N, N, O] structure. The aldehyde group of salicylaldehyde and the amino group of ethylenediamine form an imine bond (C=N) through a condensation reaction. The resulting chelate structure can form a stable complex with metal ions, which is the key to the role of the Salen ligand in catalytic reactions.

[0058] Depend on Figure 2 It can be seen that 2955cm -1 The peaks at 1618 and 1636 cm-1 are attributed to the skeletal stretching vibration of CH on the benzene ring. -1 It is the -CN- stretching vibration peak in the ligand. When coordinated with titanium metal atom, 798cm -1 The characteristic absorption of titanium coordination bond appears at , indicating that the ligand coordinates with titanium to form a complex.

[0059] Example 2: Preparation of polyvalerolactone (PVL) by ring-opening polymerization catalyzed by salen-titanium complex

[0060] Dehydrating δ-valerolactone under vacuum conditions for 2 hours to completely dehydrate the δ-valerolactone to obtain dehydrated δ-valerolactone; adding the Salen-titanium complex prepared in Example 1 as a catalyst to the dehydrated δ-valerolactone, and then adding benzyl alcohol as an initiator, wherein the molar ratio of the Salen-titanium complex to the δ-valerolactone is 1:500, and the molar ratio of the Salen-titanium complex to the initiator is 1:1, and conducting a ring-opening polymerization reaction at a temperature of 130° C. and a vacuum degree of 35 Pa for 8 hours;

[0061] The reaction system was poured into 200 mL of a cold mixed solution, wherein the mixed solution was prepared by mixing methanol and acetic acid at a mass ratio of 200:1, thereby obtaining polyvalerolactone.

[0062] Depend on Figure 3 It can be seen that almost all the methylene peaks appear at 4.11 ppm, which indicates that the conversion rate of the ring-opening polymerization of valerolactone is close to 100%.

[0063] Example 3: Preparation of polyethylene 2,5-furandicarboxylate (PEF) by polycondensation catalyzed by a salen-titanium complex

[0064] After mixing 2,5-furandicarboxylic acid, ethylene glycol and the Salen-titanium complex prepared in Example 1, an esterification reaction was carried out at a pressure of 0.28 MPa and a temperature of 200° C. to obtain ethylene glycol furandicarboxylate, which was then dehydrated.

[0065] The molar ratio of 2,5-furandicarboxylic acid to ethylene glycol is 1:1.6, and the molar ratio of Salen-titanium complex to 2,5-furandicarboxylic acid is 1:500.

[0066] When the mass of water generated during the esterification reaction reaches 90% of the theoretical mass of water generated, the reaction conditions are adjusted to a pressure of 60 Pa and a temperature of 250° C. and the polycondensation reaction is carried out for 5 hours. After the reaction is completed, the mixture is water-cooled and pelletized to obtain polyethylene furandicarboxylate.

[0067] The PEF prepared in this embodiment was characterized by structure. Figure 4 shown.

[0068] like Figure 4 As shown, the chemical shift absorption band at 7.48 ppm corresponds to the absorption peak of the four hydrogen atoms on the furan ring; the absorption peak at 4.89 ppm corresponds to the absorption peak of the four hydrogen atoms on the methylene group of the ethylene glycol group; and the proton peak of the methylene connected to the terminal hydroxyl group at 4.11 ppm.

[0069] Example 4: Preparation of polyethylene terephthalate-co-1,4-cyclohexanedimethanol (PCTG) by polycondensation catalyzed by salen-titanium complex

[0070] Purified terephthalic acid, 1,4-cyclohexanedimethanol, ethylene glycol and Salen-titanium complex were mixed at a ratio of 6 mol: 3.2 mol: 2.4 mol, and esterification reaction was carried out at a pressure of 0.30 MPa and a temperature of 220°C.

[0071] When the mass of water generated during the esterification reaction reaches 90% of the theoretical mass of water generated, the reaction conditions are changed to a pressure of 60 Pa and a condensation reaction at 260°C for 4 hours. After the reaction is completed, polyethylene terephthalate-co-1,4-cyclohexanedimethanol is obtained by water cooling and pelletizing.

[0072] The PCTG prepared in this embodiment was subjected to structural characterization, and the results were as follows: Figure 5-Figure 6 shown.

[0073] like Figure 5As shown, the absorption band at 8.10 ppm corresponds to the resonance absorption peak of the four hydrogen atoms on the benzene ring; the absorption peaks at 4.70 ppm and 2.06 ppm correspond to the resonance absorption of the four hydrogen atoms on the methylene group of the ethylene glycol group; the absorption bands at 4.30 ppm and 4.19 ppm correspond to the resonance absorption of the four hydrogen atoms on the methylene group adjacent to the ester bond in the trans and cis conformations of the six-membered cyclohexane ring. Because the cyclohexylene group has two hydrogen environments, while the ethylene group has only one hydrogen environment, which is different from the first two, they can be separated in the spectrum due to different chemical shifts; some peaks are split, while others are not. The absorption band in the δ1.16-1.82 range corresponds to the resonance absorption of the eight hydrogen atoms in the cyclohexyl group and is composed of four small peaks.

[0074] like Figure 6 The infrared spectrum of 1,4-cyclohexanedimethanol modified ethylene terephthalate copolyester is shown, where the wave number is 1712 cm -1 The peak corresponds to the stretching vibration peak of the carbon-oxygen double bond, with a wave number greater than 2800 cm -1 The peaks are the symmetric stretching vibration and antisymmetric stretching vibration peaks of methylene, with a wave number of 960 cm -1 1238cm -1 , 1377cm -1 The absorption peak corresponding to the methylene group on the six-membered ring of cyclohexane proves the successful polymerization of 1,4-cyclohexanedimethanol.

[0075] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A Salen-titanium complex, characterized in that Its structural formula is shown in formula (I), 2. The method for preparing the Salen-titanium complex according to claim 1, characterized in that: The following steps are involved: (1) 2,5-furandicarboxaldehyde and 2-aminophenol are mixed and placed in toluene for condensation reaction. After the reaction is completed, the mixture is filtered, and the filtered solid is collected and washed to obtain a Salen ligand; (2) Salen ligand and titanium source are used as reactants, and toluene is used as reaction solvent. After coordination reaction, purification is performed to obtain a Salen-titanium complex.

3. The method for preparing the Salen-titanium complex according to claim 2, wherein: In step (1), the molar ratio of 2,5-furandicarboxaldehyde and 2-aminophenol is 1:(2.1-2.2); the mass ratio of the total mass of 2,5-furandicarboxaldehyde and 2-aminophenol to toluene is 1:(40-100).

4. The method for preparing the Salen-titanium complex according to claim 2, wherein: In step (1), the condensation reaction temperature is 120-140° C., and the condensation reaction time is 8-12 h.

5. The method for preparing the Salen-titanium complex according to claim 2, wherein: In step (2), the titanium source is one of tetrabutyl titanate, titanium tetrachloride, and titanium bromide; the molar ratio of the Salen ligand to the titanium source is 1:(1-1.1); and the mass ratio of the Salen ligand to the reaction solvent is 1:(40-100).

6. The method for preparing the Salen-titanium complex according to claim 2, wherein: In step (2), the coordination reaction temperature is 130-140° C., and the coordination reaction time is 12-15 h.

7. Use of the Salen-titanium complex according to claim 1 in any one of the following 1) to 3): 1) Catalytic preparation of polyvalerolactone; 2) catalyzing a polycondensation reaction to prepare polyethylene 2,5-furandicarboxylate; 3) Catalytic polycondensation reaction to prepare polyethylene terephthalate-co-1,4-cyclohexanedimethanol ester.

8. The use according to claim 7, characterized in that The specific process of catalytic preparation of polyvalerolactone is: Adding a Salen-titanium complex as a catalyst to the dehydrated δ-valerolactone, and then adding an initiator, and performing a ring-opening polymerization reaction at a reaction temperature of 120-150° C. and a vacuum degree of 20-50 Pa for 3-10 hours to obtain polyvalerolactone; The molar ratio of the Salen-titanium complex and the dehydrated delta-valerolactone is 1:(100-500); the initiator is benzyl alcohol, and the molar ratio of the Salen-titanium complex and the initiator is (1-1.5):

1.

9. The use according to claim 7, characterized in that The specific process of preparing polyethylene 2,5-furandicarboxylate by catalytic polycondensation reaction is as follows: After mixing 2,5-furandicarboxylic acid, ethylene glycol and Salen-titanium complex, carry out esterification reaction at 0.20-0.30 MPa and 180-230° C., change the reaction conditions to 30-60 Pa and 190-250° C., and carry out polycondensation reaction for 3-5 hours to obtain polyethylene furandicarboxylate; The molar ratio of 2,5-furandicarboxylic acid to ethylene glycol is 1:(1.4-1.8), and the molar ratio of Salen-titanium complex to 2,5-furandicarboxylic acid is 1:(500-1000).

10. The use according to claim 7, characterized in that The specific operation of preparing polyethylene terephthalate-co-1,4-cyclohexanedimethanol by catalytic polycondensation reaction is as follows: After mixing purified terephthalic acid, 1,4-cyclohexanedimethanol, ethylene glycol and Salen-titanium complex, an esterification reaction is carried out at 0.20-0.30 MPa and 200-230° C., and when the mass of water generated during the esterification reaction reaches 90% of the mass of the theoretically generated water, the reaction conditions are changed to 30-60 Pa and 210-260° C., and a polycondensation reaction is carried out for 3-6 hours. After the reaction is completed, polyethylene terephthalate-co-1,4-cyclohexanedimethanol ester is obtained; The molar ratio of purified terephthalic acid, 1,4-cyclohexanedimethanol and ethylene glycol is 6:(4-5):(3-4), and the molar ratio of the Salen-titanium complex and purified terephthalic acid is 1:(500-1000).

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