Method for purifying trans-1, 4-cyclohexanedimethanol isomer

Abstract

The invention provides a method for purifying a trans-1, 4-cyclohexanedimethanol isomer, which comprises the following steps: dissolving a mixture of cis-trans isomers of 1, 4-cyclohexanedimethanol in a nitrile solvent at a certain temperature, and cooling and crystallizing to obtain the trans-1, 4-cyclohexanedimethanol isomer with higher purity. According to the method disclosed by the invention, the trans-1, 4-cyclohexanedimethanol can be prepared with high quality, and a good development prospect and huge economic benefits are undoubtedly brought to the downstream industry of the 1, 4-cyclohexanedimethanol.

Description

Technical Field

[0001] This invention belongs to the field of chemical engineering, and specifically relates to a method for purifying trans-1,4-cyclohexanediethanol isomer. Background Technology

[0002] 1,4-Cyclohexanediethanol (CHDM) is an important alicyclic diol monomer with wide applications in the chemical industry. It is not only a key raw material in the polyurethane, coatings, and adhesives industries, improving the hydrolysis resistance and flexibility of products, but also serves as a comonomer for synthesizing high-performance polyesters. In the polyester field, the content and spatial structure of CHDM alter material properties. The rigid cyclohexane structure effectively increases the glass transition temperature while reducing the regularity of the molecular chain. This combination endows copolyesters with excellent transparency, high impact resistance, and good processability. This provides irreplaceable advantages in high-value-added applications such as premium medical packaging, optical lenses, and specialty sheets.

[0003] This structural advantage has been fully demonstrated in the modification of polyethylene terephthalate (PET). Introducing CHDM as a third monomer into the PET backbone to replace some of the ethylene glycol effectively modulates the crystallization rate and crystallinity. This makes it possible to develop copolyesters with high transparency and impact resistance (such as PETG / PCTG), successfully expanding their applications to high-end sectors.

[0004] CHDM produced industrially via catalytic hydrogenation is a mixture of cis and trans isomers, typically in a ratio close to 1:3. Despite having the same chemical formula, they differ in spatial structure and physical properties. In trans CHDM, two hydroxymethyl groups occupy the para position relative to the cyclohexane ring, forming an extended chair conformation. This configuration exhibits high molecular symmetry and abundant lattice energy, resulting in a higher melting point. Conversely, in cis CHDM, the hydroxymethyl groups are located on the same side of the cyclohexane ring, adopting a twisted and compact conformation. This structure weakens intermolecular packing interactions, leading to a lower melting point. This structure-performance correlation is directly "inherited" into the properties of polyester materials. Trans CHDM promotes close packing and orderly arrangement of polyester chains, significantly increasing glass transition temperature, tensile strength, and modulus, making it suitable for applications requiring high heat resistance and extreme hardness. Conversely, cis CHDM disrupts the regularity of the segments and inhibits crystallization. While moderately sacrificing heat resistance and rigidity, the addition of trans CHDM effectively improves the toughness and impact resistance of the material, making it more suitable for high-transparency, high-toughness polyester systems. Therefore, precisely controlling the ratio of cis-trans isomers in copolyesters or obtaining high-purity monoisomers is a key approach to achieving targeted polyester performance design and high-end product development.

[0005] However, the physicochemical properties (such as boiling point and polarity) of cis / trans CHDM are very similar, making their efficient separation a long-standing industrial challenge. Currently, distillation methods suffer from high energy consumption and poor economic feasibility due to the small boiling point difference between the two isomers (e.g., US4999090). Catalytic isomerization is limited by thermodynamic equilibrium and catalyst efficiency, making it difficult to obtain high-purity products (e.g., US2917549). Among various methods, crystallization separation is promising due to its simplicity and lower cost. Exploration includes solvent systems such as glycol ether acetate (JP05186378), acetone (CN107200677), and ethyl acetate (Modification of poly(ethylene 2,5-furandicarboxylate)(PEF) with 1,4-cyclohexanedimethanol: Influence of stereochemistry of 1,4-cyclohexylene units). Polymer 2018, 137 (173-185), water (JP05058929), ethylene glycol, and butanediol (CN114436779). However, these solvents typically present inherent challenges in terms of low selectivity and balancing product yield with purity. This stems from an incomplete understanding of the differential interaction mechanisms between isomers and solvent molecules, hindering rational guidance for screening efficient solvent systems.

[0006] Therefore, there is an urgent need in the field for a low-cost, simple, and highly selective method for the crystallization and separation of the isocis-trans 1,4-cyclohexanediethanol to prepare trans-1,4-cyclohexanediethanol with high purity. Summary of the Invention

[0007] This invention addresses the shortcomings of existing technologies, such as low content of the trans-1,4-cyclohexanediethanol isomer, low separation yield, and high cost. It argues that to achieve a breakthrough in separation technology, it is essential to elucidate the fundamental differences in the interactions between isomers and solvents at the molecular level. Density functional theory (DFT) analysis of weak intermolecular interactions, such as hydrogen bonds, enhances the applicant's understanding of molecular recognition and selectivity. In this work, DFT calculations are first used to screen potential solvents based on the binding energy between the solvent and the cis / trans CHDM isomers, as well as hydrogen bond geometry parameters, followed by experimental verification. Combining solvation free energy calculations with analyses of molecular electrostatic potential (MEP), reduced density gradient (RDG), and independent gradient model (IGMH), the selectivity mechanism is elucidated from the perspectives of solution thermodynamics and electronic structure. Subsequently, for the solvents selected by DFT, the effects of key process parameters—including feed / solvent mass ratio, crystallization temperature, cooling rate, stirring speed, and crystallization time—on product purity and yield are investigated.

[0008] This invention discloses a method for purifying trans-1,4-cyclohexanediethanol isomers, which involves dissolving a mixture of cis and trans-1,4-cyclohexanediethanol isomers in a nitrile solvent at a certain temperature, and obtaining trans-1,4-cyclohexanediethanol isomers with high purity by cooling and crystallization.

[0009] The nitrile solvent is one or a mixture of more than one of acetonitrile, propionitrile, butyronitrile, valerate, hexanoic acid, heptanoic acid, octanoic acid, acrylonitrile, isobutyronitrile, benzonitrile, and phenylacetonitrile.

[0010] The mass ratio of the raw material 1,4-cyclohexanediethanol cis-trans isomer mixture to the nitrile solvent is 4:1 to 1:4.

[0011] The mixture of cis-trans isomers of 1,4-cyclohexanediethanol is dissolved in a nitrile solvent at 40-60°C.

[0012] The cooling rate for the cooling crystallization is 0.2~1.5℃ / minute.

[0013] The endpoint temperature for the cooling crystallization is 6~20℃.

[0014] The stirring speed for the cooling crystallization is 100~400 rpm.

[0015] The crystallization time after the cooling crystallization reaches the endpoint temperature is 1~6 hours.

[0016] After crystallization, the product is filtered, washed, and dried under a vacuum of -0.8 to -0.9 MPa at 45 to 55°C for 2 to 15 hours to obtain the trans-1,4-cyclohexanediethanol product. The final trans-1,4-cyclohexanediethanol product is then obtained.

[0017] The beneficial effects of this invention are: by using nitrile solvents to separate the mixture of 1,4-cyclohexanediethanol isomers through cooling crystallization, a 1,4-cyclohexanediethanol product with a trans-CHDM content of over 91.8% is finally obtained. Detailed Implementation

[0018] The present invention will be further illustrated below through embodiments, the purpose of which is solely to provide a better understanding of the invention. Therefore, the examples given do not limit the scope of protection of the present invention. Example 1

[0019] 20g of CHDM (trans:cis = 75:25, mass ratio) and 20g of acetonitrile solvent were added to a 100ml jacketed reactor and mixed at a stirring speed of 150rpm. The temperature was raised to 50°C and held for 30 minutes using the temperature control program of a high-low temperature integrated reactor; subsequently, it was cooled to 20°C at a rate of 1°C / min and held for 3 hours. After crystallization, the product was filtered, washed, and dried to obtain the final product. Product quality indicators are shown in Table 1. Example 2

[0020] The solvent was changed to propionitrile, and other parameters remained the same as in Example 1. Product quality indicators are shown in Table 1. Example 3

[0021] The solvent was changed to nitrile butadiene, and other parameters remained the same as in Example 1. Product quality indicators are shown in Table 1. Example 4

[0022] The solvent was changed to hexanoic acid, and other parameters remained the same as in Example 1. Product quality indicators are shown in Table 1. Example 5

[0023] The solvent was changed to acrylonitrile, and other parameters remained the same as in Example 1. Product quality indicators are shown in Table 1. Example 6

[0024] The solvent was changed to benzonitrile, and other parameters remained the same as in Example 1. Product quality indicators are shown in Table 1. Example 7

[0025] The solvent was changed to a mixture of acetonitrile and butyronitrile, with an acetonitrile:butyronitrile ratio of 1:1 (wt), and other parameters remained the same as in Example 1. Product quality indicators are shown in Table 1. Example 8

[0026] The solvent was changed to a mixture of acetonitrile and phenylacetonitrile, with an acetonitrile:phenylacetonitrile ratio of 1:1 (wt), and other parameters remained the same as in Example 1. Product quality indicators are shown in Table 1. Example 9

[0027] Solvent: 30g nitrile; crystallization temperature: 20℃; other parameters are the same as in Example 1. Product quality indicators are shown in Table 1. Example 10

[0028] Solvent: 20g nitrile; crystallization temperature: 16℃; other parameters are the same as in Example 1. Product quality indicators are shown in Table 1. Example 11

[0029] Solvent: 13.3 g nitrile; crystallization temperature: 16 °C; other parameters are the same as in Example 1. Product quality indicators are shown in Table 1. Example 12

[0030] Solvent: 8.5g nitrile; crystallization temperature: 16℃; other parameters are the same as in Example 1. Product quality indicators are shown in Table 1. Example 13

[0031] Solvent: 5g of nitrile butadiene; crystallization temperature: 16℃; other parameters are the same as in Example 1. Product quality indicators are shown in Table 1. Example 14

[0032] Solvent: 46.7 g of nitrile; crystallization temperature: 16 °C; other parameters are the same as in Example 1. Product quality indicators are shown in Table 1. Example 15

[0033] The stirring speed was changed to 200 rpm, and other parameters remained the same as in Example 1. Product quality indicators are shown in Table 1. Example 16

[0034] The crystallization time was changed to 5 hours, and other parameters remained the same as in Example 1. Product quality indicators are shown in Table 1. Example 17

[0035] The cooling rate was changed to 0.3℃ / min, and other parameters remained the same as in Example 1. Product quality indicators are shown in Table 1.

[0036] Table 1 Parameter number Solvent type Mass ratio of raw materials to solvent Stirring speed, rpm Cooling rate, ℃ / min Crystallization temperature, ℃ Crystallization time, h Trans CHDM content, % Example 1 Acetonitrile 5:5 150 1 20 3 91.8 Example 2 Propylon 5:5 150 1 20 3 93.5 Example 3 Nitrile 5:5 150 1 20 3 93.7 Example 4 Hexanonitrile 5:5 150 1 20 3 95.6 Example 5 Acrylonitrile 5:5 150 1 20 3 95.8 Example 6 Benzonitrile 5:5 150 1 20 3 95.0 Example 7 Acetonitrile / butyronitrile = 1:1 5:5 150 1 20 3 92.6 Example 8 Acetonitrile:Phenylacetonitrile = 1:1 5:5 150 1 20 3 93.8 Example 9 Nitrile 4:6 150 1 20 3 94.8 Example 10 Nitrile 5:5 150 1 16 3 95.6 Example 11 Nitrile 6:4 150 1 16 3 94.1 Example 12 Nitrile 7:3 150 1 16 3 93.8 Example 13 Nitrile 8:2 150 1 16 3 93.6 Example 14 Nitrile 3:7 150 1 16 3 95.2 Example 15 Nitrile 5:5 200 1 20 3 94.7 Example 16 Nitrile 5:5 150 1 20 5 95.4 Example 17 Nitrile 5:5 150 0.3 20 3 94.6

Claims

1. A method for purifying trans-1,4-cyclohexanediethanol isomer, characterized in that, A mixture of cis and trans isomers of 1,4-cyclohexanediethanol was dissolved in a nitrile solvent at a certain temperature, and the trans-1,4-cyclohexanediethanol isomer with high purity was obtained by cooling and crystallization.

2. The preparation method according to claim 1, characterized in that, The nitrile solvent is one or a mixture of more than one of acetonitrile, propionitrile, butyronitrile, valerate, hexanoic acid, heptanoic acid, octanoic acid, acrylonitrile, isobutyronitrile, benzonitrile, and phenylacetonitrile.

3. The preparation method according to claims 1 and 2, characterized in that, The mass ratio of the mixture of cis-trans isomers of 1,4-cyclohexanediethanol to nitrile solvent is 4:1 to 1:

4.

4. The preparation method according to claim 1, characterized in that, A mixture of cis- and trans isomers of 1,4-cyclohexanediethanol is dissolved in a nitrile solvent at 40–60 °C.

5. The preparation method according to claim 1, characterized in that, The cooling rate for cooling crystallization is 0.2~1.5℃ / minute.

6. The preparation method according to claim 1, characterized in that, The final temperature for cooling crystallization is 6~20℃.

7. The preparation method according to claim 1, characterized in that, The stirring speed for cooling and crystallization is 100~400 rpm.

8. The preparation method according to claim 1, characterized in that, The crystallization time after cooling and crystallization reaches the endpoint temperature is 1~6 hours.

9. The preparation method according to claim 1, characterized in that, After crystallization, the product was filtered, washed, and dried at a vacuum of -0.8 to -0.9 MPa and a temperature of 45 to 55 °C for 2 to 15 hours to obtain trans-1,4-cyclohexanediethanol.