A method for separating high-boiling components in the production of a high-carbon aldehyde

Abstract

The present application belongs to the field of chemical separation technology, and discloses a separation method of high-boiling-point components in high-carbon-aldehyde production. The method is characterized in that 0.05%-2.0% of functional ionic liquid is added in a rectification system, and the selective hydrogen bond effect of the anion thereof on the hydroxyl group / carbonyl group in impurities is utilized to directively inhibit the apparent vapor pressure of the impurities and significantly improve the relative volatility. The present application reconstructs the phase equilibrium relationship between high-carbon-aldehyde and high-boiling-point impurities from the thermodynamic essential level by introducing ionic liquid with molecular recognition function, breaks through the inherent limitation of traditional rectification which relies on weak physical property difference, realizes efficient, low-energy-consumption and high-stability purification of high-carbon-aldehyde product, and the ionic liquid can be recycled, has environmental friendliness and economy, and provides a novel-principle, simple-operation, efficient and energy-saving and industrial-feasible new method for high-carbon-aldehyde purification.

Description

Technical Field

[0001] This invention belongs to the field of chemical separation technology and relates to a method for separating high-boiling-point components in the production of high-carbon aldehydes. Background Technology

[0002] High carbon aldehydes are important intermediates in organic synthesis and are widely used in fragrances, pharmaceuticals and fine chemicals. Their purity is directly related to the reaction selectivity and final performance of downstream applications.

[0003] In industrial production, high-carbon aldehydes are typically obtained through aldol condensation, oxidation, or carbonylation. However, these side reactions inevitably generate a series of high-boiling-point impurities, particularly aldehyde dimers (such as aldol condensation products). These impurities, containing multiple polar functional groups (such as hydroxyl and carbonyl groups) in their molecular structure, not only exhibit poor thermal stability but also have boiling points extremely close to the target high-carbon aldehyde, often differing by only a few to a dozen degrees Celsius. This poses a significant challenge to traditional separation methods.

[0004] Distillation technology, with its mature operation and wide applicability, has become the main process for purifying aldehyde products. Conventional distillation relies on the difference in volatility between components to achieve separation, and its efficiency is highly dependent on the relative volatility. In high-carbon aldehyde systems, because the target product and high-boiling-point impurities (such as typical dimers like 3-hydroxy-2-methylpentanal) have similar molecular weights and van der Waals forces, their vapor-liquid equilibrium curves are extremely steep, and the relative volatility approaches 1, leading to a sharp increase in the required theoretical plate number.

[0005] To achieve acceptable separation results, industrial processes often resort to strategies such as high reflux ratios, multi-stage series operation, or reduced pressure operation. This not only significantly increases equipment investment and operational complexity but also leads to a substantial increase in energy consumption, severely limiting economic viability. Furthermore, under high-temperature or long-term residence conditions, some high-boiling-point impurities may undergo further condensation or decomposition, generating tar-like polymers, exacerbating the risk of coking and blockage within the tower, and consequently affecting the long-term stable operation of the unit. Summary of the Invention

[0006] To achieve the above-mentioned objectives, this invention provides a method for separating high-boiling-point components in the production of high-carbon aldehydes. The method involves introducing a functional ionic liquid with molecular recognition capabilities into a distillation system. By utilizing the selective hydrogen bonding between the liquid's anions and the hydroxyl or carbonyl groups in the high-boiling-point impurities, the apparent vapor pressure of the impurities is directionally suppressed, thereby significantly increasing the relative volatility between the target high-carbon aldehyde and the high-boiling-point impurities. This achieves efficient, low-energy-consumption, and high-stability purification of high-carbon aldehyde products under conventional distillation operating conditions.

[0007] The high-carbon aldehydes described in this invention refer to straight-chain or branched saturated or unsaturated aliphatic aldehydes with ≥8 carbon atoms, typically including but not limited to 2-ethylhexanal, n-octanal, n-nonanal, n-decanal, 2-methylheptanal, and their isomers. The high-boiling-point components mainly refer to dimer or polymeric impurities generated from the target high-carbon aldehyde via aldol condensation, whose molecular structures simultaneously contain at least one hydroxyl group and one carbonyl group. Typical examples include 3-hydroxy-2-methylpentanal and 5-hydroxy-2-ethylhexanal. The difference between their boiling points and those of the corresponding monomeric high-carbon aldehydes is typically less than 15°C, and they are mostly in the 180-250°C range under normal pressure.

[0008] The core of this invention lies in the functional ionic liquid employed. This ionic liquid is composed of an organic cation and anion containing hydrogen bond acceptor function. Its key feature is that the anion possesses strong hydrogen bond accepting ability and moderate steric hindrance, enabling it to selectively form OH···X type hydrogen bonds with hydroxyl groups in high-boiling-point impurities (where X is the electronegative atom in the anion), and also to form weak C=O···HC type secondary interactions with carbonyl oxygen groups in impurities. The organic cation primarily serves to balance charge and regulate solubility. Its structural design ensures that the ionic liquid has appropriate solubility in the high-carbon aldehyde bulk phase, allowing for uniform dispersion to fulfill its function while facilitating subsequent separation and recovery.

[0009] Specifically, the cation of the functional ionic liquid is selected from any of the following structures: 1-butyl-3-methylimidazolium, 1-ethyl-3-methylimidazolium, tetrabutylammonium, triethylmethylammonium, or N-methyl-N-propylpyrrolidineonium. The anion is selected from at least one of bis(trifluoromethanesulfonyl)imide, trifluoromethanesulfonate, p-toluenesulfonate, acetate, or lactate. Preferably, the ionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, with the chemical formula [C4mim][NTf2], wherein [NTf2]... - Both oxygen and nitrogen atoms in the anion have strong hydrogen bond acceptance capabilities, and the fluorine substituents endow it with excellent hydrophobicity and thermal stability.

[0010] In the separation method of this invention, the functional ionic liquid is added in trace amounts to the crude high-carbon aldehyde product to be separated, with the addition amount accounting for 0.05%-2.0% of the total mass of the crude product. When the addition amount is less than 0.05%, there are insufficient hydrogen bonding sites, which cannot effectively suppress the vapor pressure of impurities; when it is greater than 2.0%, although the separation effect is further improved, the ionic liquid itself may accumulate in the bottom of the column due to excessive concentration, increasing the recovery load and potentially affecting the color of the product. The optimal addition range is 0.1%-0.8%, within which the best balance between vapor pressure suppression efficiency and economy can be achieved.

[0011] The separation process is carried out in a conventional distillation column filled with structured or random packing material, with 10-40 theoretical plates. The operating pressure is atmospheric to 50 kPa absolute, and the top temperature is controlled within ±5°C of the target high-carbon aldehyde boiling point. The reflux ratio is set to 1:1-5:1. Before distillation begins, the metered functional ionic liquid is thoroughly mixed with the crude high-carbon aldehyde to form a homogeneous liquid feed. The feed location is in the middle of the distillation column, specifically determined based on the impurity content in the crude product and the purity requirements of the target product.

[0012] During distillation, functional ionic liquids flow downwards with the liquid phase and accumulate in the reboiler region. This is due to their significantly higher boiling points (typically above 300°C) and extremely low vapor pressures (<10). -4 The vapor pressure of the high-boiling-point impurity molecule (at 200℃) hardly rises with the gas phase, thus it does not contaminate the product at the top of the column. After contacting the anions of the ionic liquid in the descending liquid phase, the hydroxyl protons of the impurity molecule are strongly attracted by the anions, forming a stable hydrogen-bonded complex. This leads to a decrease in the effective free energy of the impurity molecule and a significant decrease in the apparent vapor pressure. Experimental measurements show that, with the addition of 0.5% [C4mim][NTf2], the vapor pressure of the typical impurity 3-hydroxy-2-methylpentanal at 180℃ decreases from 1.8 kPa to 0.6 kPa, a reduction of 66.7%, while the vapor pressure of the target product 2-ethylhexanal only decreases slightly from 2.5 kPa to 2.4 kPa, a negligible change. Therefore, the relative volatility α of both increases from 1.39 to 4.0, meeting the thermodynamic conditions for efficient separation.

[0013] In a preferred embodiment of the present invention, an ionic liquid recovery unit is installed in the reboiler of the distillation column. The heavy components discharged from the reboiler are cooled and then enter a settling separator, where they are allowed to stand at 80-120°C for 10-30 minutes. The density difference is used to achieve stratification of the ionic liquid phase and the high-boiling-point tar phase. The upper layer is a high-boiling-point substance containing trace amounts of ionic liquid, and the lower layer is enriched with functional ionic liquid. After removing solid particles through microfiltration, the lower ionic liquid can be directly recycled back to the feed system. A stability test after 100 hours of continuous operation shows that after five cycles, the hydrogen bond accepting capacity of the ionic liquid does not significantly decrease, and the separation efficiency remains stable.

[0014] In another preferred embodiment of the present invention, the functional ionic liquid is pre-loaded onto a porous support to form a supported ionic liquid adsorbent, which is then packed into a specific packing section of a distillation column. The porous support has a specific surface area greater than 200 m². 2The packing material consists of silica gel, activated carbon, or mesoporous silica, with an ionic liquid loading of 20%-50% of the carrier mass. In this mode, the crude high-carbon aldehyde vapor comes into contact with the immobilized ionic liquid during its ascent. Impurities are selectively adsorbed and retained on the packing surface, while the target aldehyde passes through smoothly. This method avoids direct mixing of the ionic liquid and the product, simplifying subsequent separation steps and making it suitable for scenarios requiring extremely high product purity (e.g., pharmaceutical grade, purity >99.9%). The packing material, after adsorption saturation, can be regenerated by purging with an inert gas (e.g., nitrogen) at 150°C for 30 minutes, without loss of the ionic liquid.

[0015] The method described in this invention has broad applicability to the initial concentration of high-boiling-point impurities in the feed, ranging from 0.5% to 10% (mass fraction). When the impurity concentration is below 0.5%, conventional distillation is sufficient, and this technology is unnecessary. When the concentration is above 10%, it is recommended to first remove most of the heavy components through pre-flash evaporation or thin-film evaporation before using the method of this invention for deep purification to avoid excessive load on the reboiler.

[0016] Regarding operating parameters, the preferred operating pressure for the distillation column is 20-40 kPa absolute. Reduced pressure operation not only lowers the system's boiling point and reduces the risk of decomposition of heat-sensitive substances, but also amplifies the relative inhibitory effect of the ionic liquid on the vapor pressure of impurities. The liquid phase residence time within the column is controlled between 5-20 minutes to ensure sufficient contact time between impurity molecules and the ionic liquid to complete hydrogen bonding, while avoiding prolonged high temperatures that could lead to side reactions.

[0017] The technical solution of this invention has high compatibility in engineering implementation. Existing high-carbon aldehyde distillation units only require the addition of an ionic liquid metering and dispensing system and a column bottom recovery unit, without structural modifications to the main equipment. The ionic liquid dispensing system includes a storage tank, a metering pump, a static mixer, and an online concentration monitor, enabling precise, continuous, and automated dosing control. Concentration monitoring employs near-infrared spectroscopy, using the intensity of characteristic absorption peaks to provide real-time feedback on the ionic liquid concentration, and using closed-loop regulation of the dispensing rate.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] 1. Significantly improves separation efficiency. By selectively suppressing the vapor pressure of high-boiling-point impurities through molecular recognition mechanisms, the relative volatility of the target high-carbon aldehyde and impurities is increased by 2 to 3 times or more. Under the same separation requirements, the number of theoretical plates required is reduced, or the product purity is improved under the same tower equipment conditions.

[0020] 2. Significantly reduce energy consumption. Due to the increased relative volatility, the reflux ratio can be reduced from 8:1 to 15:1 in the traditional process to 2:1 to 4:1. The overhead condensation load and the reboiler load decrease simultaneously, resulting in a reduction in overall energy consumption.

[0021] 3. Enhanced process stability. The presence of ionic liquids inhibits further condensation reactions of high-boiling-point impurities in the high-temperature zone, reduces the amount of tar generated in the reboiler, and significantly reduces the number of unplanned shutdowns.

[0022] 4. Improved product quality. The content of high-boiling-point impurities in the top product can be stably controlled below 500 ppm, meeting the standards for electronic or pharmaceutical applications, and there are no foreign entrainer residues, avoiding solvent contamination problems in traditional extractive distillation;

[0023] 5. Balancing environmental friendliness and economic efficiency. The ionic liquid used is chemically stable, non-volatile, non-flammable, and recyclable, with a loss rate of less than 0.1% per batch. The total life cycle cost is lower than that of traditional high reflux ratio distillation or azeotropic distillation processes.

[0024] 6. By introducing ionic liquids with molecular recognition capabilities, the phase equilibrium relationship between high-carbon aldehydes and high-boiling-point impurities is reconstructed from a thermodynamic perspective. This breaks through the inherent limitation of traditional distillation relying on slight differences in physical properties, and provides a novel, easy-to-operate, efficient, energy-saving, and industrially feasible method for purifying high-carbon aldehydes. Detailed Implementation

[0025] This invention provides a method for separating high-boiling-point components in the production of high-carbon aldehydes. The core of this method lies in introducing a functional ionic liquid with molecular recognition capabilities into the distillation system. By utilizing the selective hydrogen bonding between the anion of this ionic liquid and the hydroxyl or carbonyl groups in the high-boiling-point impurities, the apparent vapor pressure of the impurities is directionally suppressed, thereby significantly increasing the relative volatility between the target high-carbon aldehyde and the high-boiling-point impurities. This achieves efficient, low-energy-consumption, and high-stability purification of high-carbon aldehyde products under conventional distillation operating conditions.

[0026] The high-carbon aldehydes targeted in this invention are straight-chain or branched saturated or unsaturated aliphatic aldehydes with ≥8 carbon atoms, typically including 2-ethylhexanal, n-octanal, n-nonanal, n-decanal, 2-methylheptanal, and their structural isomers. The high-boiling-point components mainly originate from the aldol condensation side reaction that occurs during the synthesis of the target high-carbon aldehyde, generating dimer or polymeric impurities. These impurities contain at least one hydroxyl group and one carbonyl functional group in their molecular structure. Typical examples include 3-hydroxy-2-methylpentanal and 5-hydroxy-2-ethylhexanal, whose atmospheric boiling points are typically in the range of 180-250℃, and the boiling point difference between these impurities and the corresponding monomeric high-carbon aldehyde is less than 15℃, making effective separation difficult with traditional distillation.

[0027] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples, so as to ensure that those skilled in the art can fully understand and implement the present invention.

[0028] Example 1: The functional ionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (addition amount 0.5%); the high carbon aldehyde is 2-ethylhexanal (crude product high boiling impurities 5%); the distillation pressure is 30 kPa absolute pressure, the reflux ratio is 3:1, and the number of theoretical plates is 25; the ionic liquid is recycled.

[0029] Separation process: Ionic liquid and crude product are mixed → feed to distillation column → distillation separation → product is collected from the top of the column → heavy components settle in the bottom of the column → ionic liquid is filtered and recovered → recycled.

[0030] Example 2: The amount of ionic liquid added was 0.1%, and the rest of the formulation and process were the same as in Example 1;

[0031] Separation process: Same as in Example 1.

[0032] Example 3: The amount of ionic liquid added was 0.8%, and the rest of the formulation and process were the same as in Example 1;

[0033] Separation process: Same as in Example 1.

[0034] Example 4: The ionic liquid is 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (addition amount 0.5%), and the rest of the formulation and process are the same as in Example 1;

[0035] Separation process: Same as in Example 1.

[0036] Example 5: The ionic liquid is tetrabutylammonium p-toluenesulfonate (addition amount 0.5%), and the rest of the formulation and process are the same as in Example 1;

[0037] Separation process: Same as in Example 1.

[0038] Example 6: Distillation reflux ratio 2:1, other formulations and processes are the same as in Example 1;

[0039] Separation process: Same as in Example 1.

[0040] Example 7: Distillation reflux ratio 4:1, other formulations and processes are the same as in Example 1;

[0041] Separation process: Same as in Example 1.

[0042] Example 8: Ionic liquid was immobilized on mesoporous silica (30% loading) and packed into the packing section of a distillation column. The remaining formulation and process were the same as in Example 1.

[0043] Separation process: Immobilized ionic liquid loading → crude product feed → distillation separation → product collection → adsorbent regeneration → recycling.

[0044] Comparative Example 1: Non-functional ionic liquid, with the remaining formulation and process the same as in Example 1;

[0045] Separation process: Crude product directly fed → distillation separation → product collection.

[0046] Comparative Example 2: Non-ionic liquid, distillation reflux ratio 12:1, theoretical plate number 40, other formulations and processes are the same as in Example 1;

[0047] Separation process: Same as Comparative Example 1.

[0048] Test method:

[0049] Separation performance testing: Gas chromatography was used to determine the purity of the top product and the amount of high-boiling impurities; relative volatility was calculated to evaluate the separation efficiency.

[0050] Energy consumption and stability testing: statistical analysis of energy consumption in the distillation process (condensation and reboiling loads); monitoring of the recycling performance of ionic liquids; observation of coking in the reboiler.

[0051] Ionic liquid recovery test: Determine the recovery rate of ionic liquid after sedimentation and separation; detect the structural integrity of ionic liquid after recycling.

[0052] The test data comparisons are shown in Table 1 and Table 2.

[0053] Table 1 Comparison of Product Purity, High-Boiling Impurity Residue, and Relative Volatility

[0054] ;

[0055] Table 2 Comparison of Energy Consumption Reduction Rate and Cyclic Stability of Ionic Liquids

[0056] ;

[0057] Examples 1-8 show product purity ≥99.5% and energy consumption reduction ≥30%, which is far superior to the comparative examples. Comparative example 1 has extremely low separation efficiency due to the absence of ionic liquid, and comparative example 2 requires a high reflux ratio to approach the target purity, resulting in a sharp increase in energy consumption. This confirms that the directional control of ionic liquid is the key to efficient separation.

[0058] Increasing the amount of ionic liquid added (Examples 2→1→3) improves separation purity and reduces impurity residue; immobilized ionic liquid (Example 8) simplifies the recovery process and has excellent regeneration performance; the reflux ratio in the range of 2:1-4:1 can balance purity and energy consumption.

[0059] The embodiments are compatible with existing distillation equipment and require no major modifications; the ionic liquid has a high recycling rate and low loss; the risk of coking in the column bottom is reduced and the continuous operation cycle is extended; the product purity meets electronic / pharmaceutical grade requirements.

[0060] Compared to traditional non-ionized liquid distillation (Comparative Example 1), the purity of the example was increased by 1.5%, the impurity residue was reduced by 98%, and the energy consumption was reduced by 42%. Compared to the high reflux ratio process (Comparative Example 2), the energy consumption was reduced by 67%, the equipment load was significantly reduced, and the industry problem of separating high carbon aldehydes from high boiling impurities was solved.

[0061] The method described in this invention uses functional ionic liquids to regulate phase equilibrium, and different parameter combinations can achieve efficient and low-consumption separation, making it suitable for deep purification in the large-scale production of high-carbon aldehydes.

[0062] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for separating high-boiling-point components in the production of high-carbon aldehydes, characterized in that, The method includes mixing a functional ionic liquid with crude high-carbon aldehyde to form a homogeneous liquid phase, which is then fed into a distillation column for separation. The high-carbon aldehydes are 2-ethylhexanal, n-octanal, n-nonanal, n-decanal, and 2-methylheptanal; The high-boiling-point component is a dimer or polymeric impurity generated by the aldol condensation side reaction of the high-carbon aldehyde, which contains at least one hydroxyl group and one carbonyl group in its molecular structure. The functional ionic liquid is composed of an organic cation and an anion containing hydrogen bond acceptor function, and its addition amount accounts for 0.05%-2.0% of the total mass of the crude high-carbon aldehyde; the organic cation is selected from 1-butyl-3-methylimidazolium, 1-ethyl-3-methylimidazolium, tetrabutylammonium, triethylmethylammonium or N-methyl-N-propylpyrrolidineonium; the anion is selected from at least one of bis(trifluoromethanesulfonyl)imide, trifluoromethanesulfonate, p-toluenesulfonate, acetate or lactate. During the distillation process, the functional ionic liquid forms OH···X type hydrogen bonds with the hydroxyl groups in the high-boiling-point component through its anions, and forms C=O···HC type secondary interactions with the carbonyl oxygen, selectively suppressing the apparent vapor pressure of the high-boiling-point component, thereby increasing the relative volatility between the high carbon aldehyde and the high-boiling-point component and achieving efficient separation.

2. The method for separating high-boiling-point components in the production of high-carbon aldehydes according to claim 1, characterized in that, The amount of the functional ionic liquid added is 0.1%-0.8% of the total mass of the crude high-carbon aldehyde.

3. The method for separating high-boiling-point components in the production of high-carbon aldehydes according to claim 2, characterized in that, The functional ionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.

4. The method for separating high-boiling-point components in the production of high-carbon aldehydes according to claim 1, characterized in that, The distillation column operates at an absolute pressure of 20-40 kPa, with the top temperature controlled within ±5°C of the target high-carbon aldehyde boiling point. The reflux ratio is 2:1-4:1, the number of theoretical plates is 10-40, and the feed position is located at the 15th to 25th theoretical plate.

5. The method for separating high-boiling-point components in the production of high-carbon aldehydes according to claim 1, characterized in that, The crude high-carbon aldehyde contains 0.5%-10% by mass of high-boiling-point components and less than 50 ppm of moisture.

6. The method for separating high-boiling-point components in the production of high-carbon aldehydes according to claim 1, characterized in that, It also includes sedimentation separation of the heavy components discharged from the column bottom to recover the functional ionic liquid; the sedimentation separation is carried out at 80-120℃ for 10-30 minutes to separate the system into an upper high-boiling phase and a lower enriched ionic liquid phase; the lower enriched ionic liquid phase is filtered through micropores and then recycled back to the feed system.

7. The method for separating high-boiling-point components in the production of high-carbon aldehydes according to claim 6, characterized in that, The density of the functional ionic liquid is greater than that of high-boiling tar, forming a clear phase interface; the microporous filtration uses a 5μm stainless steel sintered filter element.

8. The method for separating high-boiling-point components in the production of high-carbon aldehydes according to claim 1, characterized in that, The functional ionic liquid is preloaded onto a porous support to form a fixed ionic liquid adsorbent. The porous support is silica gel, activated carbon, or mesoporous silica, and the ionic liquid loading is 20%-50% of the support mass.