A process for the synthesis of diethylene glycol bis(2-ethylhexanoate)

Abstract

The application discloses a method for synthesizing diethylene glycol bis(2-ethylhexanoate), and belongs to the technical field of fine chemical synthesis. The method uses diethylene glycol and 2-ethylhexanoic acid as raw materials, uses a composite solid acid of titanium-zirconium bimetallic oxide loaded with sulfate as a catalyst, adjusts the internal temperature of the reaction by using a feedback segmented temperature rising control strategy based on the real-time change of the cumulative water production of a water segregator, recovers the catalyst by adsorption under an applied magnetic field after the reaction is completed, and obtains the target product after neutralization by alkali liquor, water washing, decolorization by activated bleaching earth and filtration. The method has the characteristics of high acid site density of the catalyst, dynamic matching of the internal temperature of the reaction and the progress of the reaction, convenient magnetic separation and recovery of the catalyst, and good cyclic stability of the catalyst, and the obtained product has the characteristics of light color, low acid value and high purity, and is suitable for industrialized continuous production of diethylene glycol bis(2-ethylhexanoate).

Description

Technical Field

[0001] This invention belongs to the field of fine chemical synthesis technology, specifically relating to a method for synthesizing diethylene glycol bis(2-ethylhexanoic acid) ester. Background Technology

[0002] Diethylene glycol bis(2-ethylhexanoic acid) ester is a symmetrical aliphatic diester compound with a wide range of applications in the chemical industry. It can be used as a cold-resistant plasticizer, a component of synthetic rubber, a solvent for nitrocellulose coatings, and a base oil and additive for specialty lubricants. This compound is prepared by a bimolecular esterification reaction of diethylene glycol and 2-ethylhexanoic acid. The reaction process is a reversible, exothermic dehydration reaction. Timely removal of water from the product side and effective control of reaction equilibrium are the core aspects of the process design. Appropriate selection of the reaction temperature is a key factor in suppressing side reactions and raw material loss.

[0003] In existing industrial preparation technologies, homogeneous catalysis with concentrated sulfuric acid or traditional single zirconium-based solid superacid catalysis are the mainstream catalytic methods, and empirical isothermal heating esterification is the mainstream temperature control method. However, several technical problems have been exposed in long-term industrial practice. At the catalyst level, while homogeneous catalysis with concentrated sulfuric acid has the advantage of a faster reaction initiation rate, it suffers from difficulties in catalyst separation and recovery after the reaction, severe equipment corrosion, and a heavy burden of waste acid treatment. Furthermore, the strong acid medium easily triggers side reactions such as oxidation, dehydration, and polymerization in the final stage of the reaction, causing the product to darken in color and increase in acid value. Traditional single zirconium-based solid superacid catalysts, due to their relatively simple crystal structure, have surface sulfate ions that coordinate only with a single type of metal ion, resulting in limited anchoring strength. In high-temperature reaction solutions, they are rapidly lost due to hydration, leading to a significant decrease in acid site density during repeated cycles and unsatisfactory cycle stability. Moreover, the recovery of such catalysts relies on pressure filtration or vacuum filtration, and particle wear and blockage during the filtration process further depletes active centers. At the temperature control level, empirical isothermal esterification, because the internal reaction temperature is fixed to an empirical value, cannot be dynamically adjusted according to the reaction progress. In the early stages of the reaction, excessively high internal temperatures can lead to the distillation loss of 2-ethylhexanoic acid; in the middle stages, insufficient internal temperatures can slow the reaction progress; and in the final stages, maintaining high internal temperatures for extended periods can cause increased side reactions and product discoloration. These shortcomings at the catalyst level and temperature control level are coupled in the process, making it difficult for existing technologies to simultaneously achieve high conversion rates, superior product quality, and good catalyst cycle stability, thus limiting the product's application areas and downstream processing performance.

[0004] To address the above issues, there is an urgent need for a method for synthesizing diethylene glycol bis(2-ethylhexanoic acid) ester that can improve acid site density and cycle stability at the catalyst structure level, achieve dynamic matching between internal reaction temperature and reaction progress at the temperature control strategy level, and simplify the catalyst recovery process at the post-processing level. Summary of the Invention

[0005] To address the problems existing in the background art, the present invention provides a method for synthesizing diethylene glycol bis(2-ethylhexanoic acid) ester, comprising the following steps:

[0006] S1: Prepare a composite solid acid catalyst, wherein the composite solid acid catalyst has Fe3O4 particles as a magnetic core, titanium-zirconium bimetallic oxide as a porous shell, and surface-modified sulfate ions as active acid sites;

[0007] S2: Add diethylene glycol, 2-ethylhexanoic acid and the composite solid acid catalyst to the reactor and start stirring;

[0008] S3: Heating to carry out the esterification reaction. The instantaneous segmented water production rate is obtained based on the real-time change of the cumulative water production in the water separator. The deviation between the instantaneous segmented water production rate and the target water production rate is used as the feedback quantity. The reaction internal temperature is limited to the range from the set lower limit value to the upper limit value of the reaction internal temperature through proportional feedback adjustment.

[0009] S4: Based on the ratio of the cumulative water production to the theoretical water production in the water distributor, the reaction process is divided into multiple reaction stages, each corresponding to a different target water production rate, until the reaction is determined to be terminated.

[0010] S5: After stopping heating and cooling down, a magnetic field is applied to the outside of the reactor to adsorb and fix the composite solid acid catalyst onto the inner wall of the reactor, and the liquid phase crude ester is discharged.

[0011] S6: The crude liquid ester is subjected to alkali neutralization, water washing, decolorization and filtration in sequence to obtain diethylene glycol bis(2-ethylhexanoic acid) ester.

[0012] Furthermore, step S1 includes:

[0013] S11: Ferric chloride and ferric chloride were dissolved in deionized water deoxygenated by nitrogen at a molar ratio of 2:1. After stirring and heating, ammonia was added dropwise until the mixture became alkaline. After magnetic separation and washing with water, Fe3O4 magnetic nanoparticles were obtained.

[0014] S12: The Fe3O4 magnetic nanoparticles obtained in step S11 are dispersed in isopropanol, tetrabutyl titanate, zirconium oxynitrate and a soft template agent are added, and after stirring and hydrolysis, the mixture is aged, filtered, dried and calcined to obtain the core-shell precursor;

[0015] S13: The core-shell precursor obtained in step S12 is immersed in an aqueous solution of ammonium sulfate or sulfuric acid, filtered, dried and then calcined to obtain the composite solid acid catalyst.

[0016] Furthermore, it is characterized by:

[0017] In step S11, the mixture is stirred and heated to 60-80°C. The added ammonia water makes the pH value of the mixture reach 9-11, and the average particle size of the obtained Fe3O4 magnetic nanoparticles is 8-15 nm.

[0018] In step S12, the molar ratio of iron in the Fe3O4 magnetic nanoparticles, zirconium in the zirconium oxynitrate, and titanium in the tetrabutyl titanate is 1:(1-3):(0.5-1.5). The soft template agent is selected from polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer or hexadecyltrimethylammonium bromide. The hydrolysis temperature is 50-70℃, and the calcination temperature is 500-600℃.

[0019] In step S13, the concentration of the ammonium sulfate aqueous solution or sulfuric acid aqueous solution is 0.5-1.5 mol / L, and the calcination temperature is 540-560℃.

[0020] Further, in step S2, the molar ratio of 2-ethylhexanoic acid to diethylene glycol is (2.10-2.30):1, and the mass of the composite solid acid catalyst added accounts for 0.8-2.0% of the total mass of diethylene glycol and 2-ethylhexanoic acid.

[0021] Furthermore, step S3 includes:

[0022] S31: Start heating and set the initial value of the internal reaction temperature to the lower limit value of the internal reaction temperature;

[0023] S32: By means of a liquid level detection device installed on the water distributor or a weighing device that cooperates with the water distributor, at a sampling cycle Periodically obtain the cumulative water production in the water distributor And calculate the instantaneous segmented water production rate according to the following formula. :

[0024] ;

[0025] In the formula, For the first Instantaneous segmented water production rate at each sampling time; For the first The cumulative water production in the water distributor at each sampling time; For the first The cumulative water production in the water distributor at each sampling time; The sampling period; It is an integer greater than or equal to 1;

[0026] S33: Based on the target water production rate corresponding to the current reaction stage. With the instantaneous segmented water production rate Calculate the set value of the reaction internal temperature at the next sampling time using the following formula. Based on this, the heating power is adjusted to keep the internal temperature of the reaction tracked in the next sampling period. :

[0027] ;

[0028] In the formula, For the first The set value of the internal temperature is reflected at each sampling time; For the first The measured values ​​of the internal temperature at each sampling time point; This is the proportionality coefficient; This represents the target water production rate corresponding to the current reaction stage. For the first Instantaneous segmented water production rate at each sampling time; This is the lower limit of the internal temperature of the reaction; This is the upper limit of the internal temperature of the reaction; Indicates taking and The larger of the two; Indicates taking and The smaller of the;

[0029] The sampling period The proportionality coefficient is 10-20 minutes. The reaction internal temperature limit is 4-8℃·min / mL. The upper limit of the internal temperature of the reaction is 114-118℃. The temperature is 128-132℃.

[0030] Furthermore, step S4 includes:

[0031] S41: The theoretical water production rate is calculated based on the amount of diethylene glycol fed using the following formula. :

[0032] ;

[0033] In the formula, This represents the theoretical water production rate. The amount of diethylene glycol added; The molar mass of water; The density of water is given at normal pressure and 25°C; the coefficient 2 indicates that 2 moles of water are produced when 1 mole of diethylene glycol is diesterized.

[0034] S42: Based on the cumulative water production in the water distributor With the theoretical water production ratio Based on the location, the target water production rate corresponding to the current reaction stage is determined in real time. :when This is the initial reaction stage. The flow rate is 0.30-0.50 mL / min; when The main reaction phase is during this time. The flow rate is 0.20-0.30 mL / min; when This is the final reaction stage. The flow rate is 0.05-0.15 mL / min.

[0035] S43: When the instantaneous segmented water production rate is no greater than 0.03 mL / min in two consecutive sampling periods, and the cumulative water production is... With theoretical water production When the ratio is not less than 95%, the reaction is considered terminated.

[0036] Furthermore, step S5 includes:

[0037] S51: Stop heating and allow the reaction solution to cool to 70-90℃;

[0038] S52: Apply a neodymium iron boron permanent magnet or electromagnet with a magnetic induction intensity of not less than 0.3T to the outside of the reactor, and maintain it for 3-7 minutes under the condition of stirring stopped or low speed stirring speed not greater than 100 rpm, so that the composite solid acid catalyst is adsorbed and fixed on the inner wall of the reactor.

[0039] S53: The liquid crude ester is discharged through the discharge valve at the bottom of the reactor. The composite solid acid catalyst is retained in the reactor and, after being washed and dried with anhydrous ethanol, is returned to step S2 as the catalyst for the next batch of esterification reaction.

[0040] Furthermore, after the composite solid acid catalyst is continuously recycled 7 times, the ratio of the diethylene glycol esterification conversion rate to the diethylene glycol esterification conversion rate at the time of the first use is not less than 90%.

[0041] Furthermore, step S6 includes:

[0042] S61: Wash the crude liquid phase with a saturated sodium carbonate aqueous solution preheated to 45-55℃ until the pH value of the aqueous phase reaches 7-8 to obtain a neutralized liquid phase;

[0043] S62: Wash the neutralized liquid phase with deionized water at 50-70℃ until the organic phase is neutral, to obtain the washed liquid phase.

[0044] S63: Add activated clay to the water-washed liquid phase at 80-95℃ and stir to decolorize, to obtain a decolorized liquid phase;

[0045] S64: Filter the decolorized liquid phase to obtain diethylene glycol bis(2-ethylhexanoic acid) ester.

[0046] The beneficial effects achieved by this invention are as follows:

[0047] This invention employs a three-layer composite solid acid catalyst. The added soft template agent decomposes during calcination to form mesoporous channels, resulting in a mesoporous shell structure. The resulting catalyst has a high specific surface area and numerous acid site loading sites, leading to a high acid site density per unit mass of catalyst and a faster intrinsic rate of esterification. In step S12, the introduction of titanium allows titanium and zirconium ions to coexist in a considerable proportion within the same oxide phase. The difference in radii between the two metal ions induces lattice distortion, providing more anchoring sites for surface-modified sulfate ions. The sulfate ions bond to the surface metal ions in a bidentate bridging or chelate bidentate coordination manner, exhibiting a higher anchoring strength than sulfate coordination on a single zirconium-based support. This results in lower sulfate ion loss during multiple cycles and a higher retention rate of cyclic activity.

[0048] Steps S3 and S4 jointly designed a feedback-based segmented temperature control strategy that uses the real-time change in the cumulative water production in the water separator as the feedback quantity, divides the reaction progress into stages, and corresponds to different target water production rates. This strategy overcomes the shortcomings of existing isothermal esterification methods where the internal reaction temperature is disconnected from the reaction progress. The proportional feedback control law in step S33, combined with the outer saturation limiting link, ensures that the internal reaction temperature is always constrained within a safe range between the lower and upper limits of the internal reaction temperature during the reaction process. This avoids the loss of raw material due to the internal reaction temperature exceeding the safe distillation threshold of 2-ethylhexanoic acid, and also avoids the difficulty in timely separation of water from the reaction system due to the internal reaction temperature falling below the atmospheric boiling point of water. Step S42 dynamically divides the reaction stages based on the ratio of cumulative water production to theoretical water production. This ensures a high target water production rate in the initial reaction stage to fully utilize the favorable conditions of high reactant concentration and strong forward driving force at the reaction initiation point. The target water production rate in the main reaction stage is moderate to balance the reverse hydrolysis tendency caused by reaction progression and product concentration increase. The target water production rate in the final reaction stage is low to match the target value of the feedback controller with the actual achievable water production rate, preventing the controller from pushing the internal temperature to near its upper limit to pursue an excessively high target value, thereby suppressing side reactions and product discoloration. Step S43 determines reaction termination based on a condition where both the water production rate and cumulative water production are satisfied. This avoids premature reaction termination leading to insufficient conversion and also avoids overheating leading to product quality degradation.

[0049] The composite solid acid catalyst structure in step S1 and the feedback-based segmented temperature control strategy in step S3 are not two independent technical approaches, but rather have a coupled and synergistic relationship at the reaction engineering level. The catalyst prepared in step S1, due to its high acid site density, can maintain a sufficient reaction rate even at a low internal reaction temperature, providing a large temperature adjustment margin for the feedback temperature controller in step S3. This allows the controller to maintain a low internal reaction temperature in the initial reaction stage to suppress feedstock evaporation, and to moderately increase the internal reaction temperature in the final reaction stage to overcome equilibrium limitations. The feedback temperature controller in step S3 dynamically optimizes the reaction path based on the actual activity of the catalyst, ensuring that the high activity advantage of the catalyst is fully released during the process, rather than being limited to a simple rate increase at a single internal reaction temperature. When the catalyst structure is downgraded to a traditional single zirconium-based solid superacid or the temperature control strategy is simplified to an empirical isothermal method, the above coupling mechanism fails, and the overall performance decline is greater than the sum of the declines corresponding to the two individual improvements. This phenomenon, conversely, confirms the synergistic effect produced by the combined use of the two innovations.

[0050] The catalyst magnetic separation and liquid-phase crude ester discharge method in step S5, and the liquid-phase crude ester refining process in step S6, jointly support the feasibility of the method for engineering and industrial scale-up. In step S5, the magnetic catalyst is fixed to the inner wall of the reactor by an external magnetic field before the liquid phase is discharged, allowing the catalyst to be directly retained in the reactor as feedstock for the next batch of reaction. This eliminates the need for a separate catalyst separation process and avoids filter media clogging and frequent replacement. The temperature range of the four steps in step S6—alkali neutralization, water washing, activated clay decolorization, and filtration—has been systematically optimized, resulting in a lower emulsification tendency during the phase separation process, improving the decolorization efficiency of the decolorizing agent, and achieving optimal levels of color, acid value, and purity in the final product. This invention improves upon existing technologies in several indicators, including reaction conversion rate, target product yield, total reaction time, product color, raw material distillation loss, and catalyst cycle stability, making it suitable for the industrial continuous production of diethylene glycol bis(2-ethylhexanoic acid) ester. Attached Figure Description

[0051] Figure 1 The graphs show a comparison of reaction process indicators between Examples 1, 2, and 3 and Comparative Examples 1, 2, and 3. (a) is a line graph showing the change of diethylene glycol conversion rate with reaction time in the six experiments, and (b) is a line graph showing the change of instantaneous fractional water production rate with reaction time in the six experiments.

[0052] Figure 2The charts show the key performance indicators of Examples 1, 2, and 3 compared with Comparative Examples 1, 2, and 3. Among them, (a) is a bar chart of diethylene glycol conversion rate of the 6 groups of experiments, (b) is a bar chart of target product yield of the 6 groups of experiments, (c) is a bar chart of total reaction time of the 6 groups of experiments, and (d) is a bar chart of Hazen color number of the 6 groups of experiments.

[0053] Figure 3 This is a line graph comparing the internal reaction temperature of Examples 1, 2, and 3 with that of Comparative Examples 1, 2, and 3 over reaction time. There is no distinction between (a) and (b).

[0054] Figure 4 The graph shows the stability of the catalysts in Example 1, Example 2, Example 3 and Comparative Example 3 in terms of cyclic use. (a) is a line graph showing the change of diethylene glycol conversion rate of the four catalysts with the number of cycles, and (b) is a line graph showing the change of sulfate retention rate of the four catalysts with the number of cycles.

[0055] Figure 5 This is a flowchart of the method for synthesizing diethylene glycol bis(2-ethylhexanoic acid) ester according to the present invention. Detailed Implementation

[0056] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. In addition, the forms of the various structures described in the following embodiments are merely illustrative. The present invention is not limited to the structures described in the following embodiments. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0057] This invention relates to a method for synthesizing diethylene glycol bis(2-ethylhexanoic acid) ester based on the bimolecular esterification reaction of diethylene glycol and 2-ethylhexanoic acid. The target product, diethylene glycol bis(2-ethylhexanoic acid) ester, is a symmetrical diester and can be used as a cold-resistant plasticizer, in synthetic rubber, nitrocellulose coatings, specialty lubricant base oils, and additives. The esterification reaction is a reversible exothermic dehydration reaction. Under conventional catalytic conditions, the yield is limited by the equilibrium constant. Furthermore, at excessively high reaction temperatures, the boiling point of 2-ethylhexanoic acid (228°C) easily evaporates with the generated water; at excessively low temperatures, both the reaction rate and the water evaporation rate are insufficient. (Refer to...) Figure 5 The method of the present invention includes six main steps, from step S1 to step S6, and the specific implementation of each step is described below.

[0058] Step S1 involves the preparation of the composite solid acid catalyst. The prepared composite solid acid catalyst uses Fe3O4 particles as a magnetic core, a titanium-zirconium bimetallic oxide as a porous shell, and surface-modified sulfate ions as active acid sites. The design concept of this core-shell-modified three-layer structure is to impart magnetic response to the catalyst with the ferromagnetic core, allowing the catalyst to be separated and recovered from the liquid-phase products under the action of an external magnetic field after the reaction. The porous shell of the bimetallic oxide serves as the supporting substrate for sulfate ions, and the anchoring strength of sulfate ions is improved through lattice distortion and surface defects caused by the difference in radii of the two metal ions. The surface-chemically bonded sulfate ions serve as acid sites for catalyzing the esterification reaction. Step S1 is composed of sub-steps S11, S12, and S13 sequentially.

[0059] S11 is the preparation of the Fe3O4 magnetic core. Ferric chloride and ferric chloride are dissolved in deionized water deoxygenated with nitrogen at a molar ratio of 2:1. The 2:1 molar ratio corresponds to the Fe content in Fe3O4. 3+ with Fe 2+ The natural stoichiometry ensures high phase purity of the coprecipitate. Nitrogen deoxygenation aims to remove dissolved oxygen from the water and prevent Fe... 2+ Oxidized to Fe 3+ This disrupts the stoichiometric ratio. After stirring and heating, ammonia is added dropwise. The ammonia acts as a weak base and reacts with the Fe in the iron salt solution. 3+ with Fe 2+ The precipitate is co-precipitated as hydroxide and converted to Fe3O4 under continuous stirring. The preferred temperature for stirring and heating is 60-80℃, and the pH of the mixture is adjusted to 9-11 by adding ammonia water. Within this pH range, the precipitation conversion rate and grain size are both suitable. The average particle size of the obtained Fe3O4 magnetic nanoparticles is preferably 8-15 nm. Within this particle size range, the particles have a large specific surface area without exhibiting superparamagnetism due to excessively small size, thus ensuring sufficient magnetic response during subsequent magnetic separation. After magnetic separation and water washing, the desired Fe3O4 magnetic core for S12 is obtained.

[0060] S12 is used for the preparation of the core-shell precursor. Fe3O4 magnetic nanoparticles obtained in S11 are dispersed in isopropanol, and tetrabutyl titanate (Ti(OC4H9)4), zirconium oxynitrate (ZrO(NO3)2), and a soft template agent are added. The preferred molar ratio of iron in the Fe3O4 magnetic nanoparticles, zirconium oxynitrate, and titanium in the tetrabutyl titanate is 1:1-3:0.5-1.5. This ratio allows Ti... 4+ With Zr 4+ They coexist in a considerable proportion in the same oxide phase, forming a Ti-O-Zr bridging structure. 4+ With Zr 4+Differences in ionic radii induce lattice distortion, providing more anchoring sites for subsequently loaded sulfate ions. The soft template agent is selected from polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer or hexadecyltrimethylammonium bromide, and its function is to provide a micellar template during hydrolysis-polymerization. After calcination and decomposition, it leaves mesoporous channels, giving the resulting shell a mesoporous structure. The hydrolysis temperature is preferably 50-70℃. During hydrolysis, tetrabutyl titanate and zirconium oxynitrate hydrolyze into hydrated oxide sols. After aging, filtration, and drying, it is calcined at 500-600℃. Calcination dehydrates and partially crystallizes the hydrated oxides, while the template agent decomposes and is removed, yielding a core-shell precursor. The BET specific surface area of ​​the obtained core-shell precursor is preferably not less than 110 m² / g to provide sufficient acid site loading substrate.

[0061] S13 involves sulfate site modification. The core-shell precursor obtained in S12 is impregnated in an aqueous solution of ammonium sulfate or sulfuric acid, preferably with a concentration of 0.5-1.5 mol / L. Within this concentration range, sulfate ions can be uniformly distributed on the surface of the metal oxide, avoiding both insufficient acid site density due to excessively low concentration and sulfate aggregation leading to inactivation due to excessively high concentration. After impregnation, the solution is filtered, dried, and calcined at 540-560℃. Within this calcination temperature range, stable bidentate bridging or chelate bidentate coordination bonds are formed between sulfate ions and surface metal ions, resulting in an acid site structure where Lewis acid sites and Brønsted acid sites coexist. If the calcination temperature is too low, sulfate ions exist only as surface salts, resulting in weak acid strength; if the calcination temperature is too high, sulfate ions decompose and are lost, leading to a decrease in acid site density. After S13, the composite solid acid catalyst is obtained.

[0062] Step S2 involves loading the reactor. Diethylene glycol, 2-ethylhexanoic acid, and the composite solid acid catalyst obtained in S1 are added to the reactor, and stirring is started. The preferred molar ratio of 2-ethylhexanoic acid to diethylene glycol is 2.10-2.30:1, i.e., 5%-15% excess 2-ethylhexanoic acid. The introduction of excess 2-ethylhexanoic acid serves two purposes: it shifts the esterification equilibrium in the subsequent S3 stage towards the product side, increasing the conversion rate of diethylene glycol; and the excess acid keeps the reaction liquid liquid even when the temperature drops to 70-90°C in the S5 stage, facilitating discharge. The preferred mass of the composite solid acid catalyst is 0.8%-2.0% of the total mass of diethylene glycol and 2-ethylhexanoic acid, balancing catalytic efficiency and catalyst dosage per unit feed.

[0063] Step S3 is the heating esterification and reaction internal temperature feedback regulation, which consists of sub-steps S31, S32 and S33 in sequence. The core approach is to use the rate of change of the cumulative water production in the water separator as the feedback quantity to adjust the reaction internal temperature in real time, so that the reaction temperature tracks the reaction progress.

[0064] S31 is the initial heating stage. Heating is initiated, and the initial internal reaction temperature is set to the lower limit of the internal reaction temperature. . The preferred temperature is 114-118℃, which is higher than the boiling point of water under normal pressure, so that the generated water can be continuously evaporated and collected by the water separator, while it is much lower than the boiling point of 2-ethylhexanoic acid, 228℃, so that the evaporation loss of the raw material 2-ethylhexanoic acid is suppressed.

[0065] S32 is for collecting water production data and calculating the instantaneous segmented water production rate. This is achieved through a liquid level detection device installed on the distributor or a weighing device used in conjunction with the distributor, with a sampling cycle... Periodically obtain the cumulative water production in the water distributor . The optimal value is 10-20 min, which balances feedback response speed and sampling noise smoothing. Instantaneous segmented water production rate Calculated using the following formula.

[0066] ;

[0067] In the formula, For the first Instantaneous segmented water production rate (mL / min) at each sampling time; For the first The cumulative water production (mL) in the water distributor at each sampling time; For the first The cumulative water production (mL) in the water distributor at each sampling time; The sampling period (min); It is an integer greater than or equal to 1. This formula is the first-order backward difference of a discrete signal. Its physical meaning is the ratio of the increase in the cumulative water production of the water distributor between two adjacent sampling times to the sampling period. It reflects the dehydration rate of the reaction system in each sampling period, and also indirectly reflects the instantaneous rate of the esterification reaction in that period.

[0068] S33 is for internal reaction temperature feedback regulation. The target water production rate corresponds to the current reaction stage. The instantaneous segmented water production rate is determined by S42 in step S4. and The deviation between the two values ​​is amplified by a proportional element and used to update the setpoint of the reaction internal temperature. The setpoint of the reaction internal temperature at the next sampling time. Calculated using the following formula.

[0069] ;

[0070] In the formula, For the first The set value (°C) of the internal temperature at each sampling time; For the first The measured internal temperature (°C) at each sampling time; This is a proportionality constant (°C·min / mL); This represents the target water production rate (mL / min) corresponding to the current reaction stage. For the first Instantaneous segmented water production rate (mL / min) at each sampling time; This represents the lower limit of the internal reaction temperature (°C). This represents the upper limit of the internal temperature of the reaction (°C). Indicates taking and The larger of the two; Indicates taking and The smaller of the two. This formula consists of a nested discrete proportional control law and a saturation limiting stage. Intermediate term. and Multiply and The algebraic sum of the products of the differences is the standard discrete proportional control law, which acts when... Below When the deviation is positive, this item is relatively... The increase causes the internal temperature to rise in the next cycle, accelerating the reaction rate and increasing the water production rate; when Higher than When the deviation is negative, this item is more... The decrease in temperature leads to a drop in internal temperature, slows the reaction rate, and reduces the water production rate. Proportionality coefficient The physical meaning is the temperature adjustment range corresponding to a unit deviation in water production rate. The preferred value is 4-8℃·min / mL. This value ensures a moderate temperature adjustment step size within a unit deviation of the water production rate, avoiding both slow response and overshoot. (Outer layer) and Nested as a saturation limiting stage, first with Cut off the control law output to no more than The range, then by Cut it to at least The scope, combined effect is to Limited to closed intervals Inside. The preferred temperature range is 128-132℃, with this upper limit preventing the internal temperature from exceeding the safe evaporation threshold of 2-ethylhexanoic acid. The heating device is based on... Adjust the heating power to track the internal temperature of the reaction during the next sampling period. .

[0071] Step S4 is the dynamic division of the reaction stage and the determination of reaction termination, which consists of sub-steps S41, S42 and S43.

[0072] S41 represents the theoretical water production capacity. Calculation of theoretical water production. Calculated using the following formula.

[0073] ;

[0074] In the formula, The theoretical water production (mL); This refers to the amount of diethylene glycol added (in moles). The molar mass of water is 18 (g / mol); The density of water at normal pressure and 25°C is taken as 1.0 (g / mL); the coefficient 2 indicates that 2 moles of water are generated for every mole of diethylene glycol diesterization. This formula is derived from stoichiometry, and the derivation process is as follows: One molecule of diethylene glycol contains two hydroxyl groups. In the bimolecular esterification with 2-ethylhexanoic acid, for every molecule of the target diester product generated, two molecules of water are also generated. Therefore, the amount of diethylene glycol added... The ratio of the amount of substance of water produced to the amount of substance produced is 1:2, and the total mass of water produced is 2 multiplied by 1. Multiply by Divide by This yields the theoretically generated volume of water. This volume is calculated based on the density of water at normal pressure and 25°C, and is under calibration conditions closely matching the actual water volume collected by the distributor. The ratio has a clear physical meaning.

[0075] S42 represents the dynamic division of reaction stages based on reaction progress and the target water production rate. Real-time switching. The cumulative water production in the distributor... With the theoretical water production ratio This is an indicator reflecting the overall progress. When At this stage, the reaction is in its initial phase, with a large amount of raw materials yet to participate in the reaction. Equilibrium has not yet been established, the driving force for forward esterification is strong, and the target water production rate is high. Take the larger value, preferably 0.30-0.50 mL / min. When At this point, the reaction enters the main reaction stage. The forward reaction still dominates, but the product concentration is already high, and the tendency for reverse hydrolysis increases. Therefore, the target water production rate needs to be slightly reduced so that the feedback controller can operate within a more stable internal temperature range. The preferred flow rate is 0.20-0.30 mL / min. When At this point, the reaction enters the final stage, and the concentrations of both diethylene glycol and 2-ethylhexanoic acid are already low. The equilibrium limitation of the reaction becomes more apparent. The preferred value is 0.05-0.15 mL / min. This value matches the target value of the feedback controller with the actual achievable permeate rate of the reaction, preventing the controller from pushing the internal temperature to its upper limit in pursuit of an excessively high target value. Nearby, thus suppressing side reactions and product discoloration.

[0076] S43 is the reaction termination criterion. This refers to the instantaneous segmented water production rate within two consecutive sampling periods. All values ​​were no greater than 0.03 mL / min, and the cumulative water production was... With theoretical water production When the product ratio is not less than 95%, the reaction is considered terminated and heating is stopped. Under these conditions, the reaction has reached a high degree of transformation, and the marginal benefit of continuing heating is small. Excessive heating may cause discoloration of the product and side reactions, thus terminating the reaction.

[0077] Step S5 is catalyst magnetic separation and liquid-phase crude ester discharge, which consists of sub-steps S51, S52 and S53.

[0078] S51 is for cooling the reaction solution. After the reaction is terminated, heating is stopped, and the reaction solution is cooled to 70-90℃. Within this cooling temperature range, the reaction solution remains in a flowing liquid state, which facilitates subsequent discharge. At the same time, the temperature has decreased compared to the reaction temperature, which can reduce the tendency of the product to oxidize and discolor when it comes into contact with air at high temperatures.

[0079] S52 involves magnetic adsorption of the catalyst. A magnetic field is applied to the outside of the reactor. The magnetic field source is preferably a neodymium iron boron permanent magnet or an electromagnet with a magnetic induction intensity of not less than 0.3T. A magnetic induction intensity of 0.3T is sufficient to magnetize the Fe3O4 particles, overcoming their diffusion resistance in the liquid phase. This allows the composite solid acid catalyst to migrate and be adsorbed and fixed along the magnetic field gradient towards the inner wall of the reactor under the influence of the magnetic field. S52 can be performed under conditions where stirring is stopped or at a low stirring speed not exceeding 100 r / min. When stirring is stopped, the catalyst is directly adsorbed by the magnetic field. During low-speed stirring, the particles are continuously agitated in the reaction liquid and brought closer to the inner wall of the reactor, which helps accelerate catalyst migration in reaction liquids with higher viscosity. Therefore, the stirring method can be flexibly selected for systems of different viscosities. The holding time is preferably 3-7 minutes, which is sufficient for most of the magnetic catalyst particles to migrate to the inner wall of the reactor.

[0080] S53 involves the discharge of the liquid-phase crude ester and catalyst recovery. The liquid-phase crude ester is discharged through the discharge valve at the bottom of the reactor, while the composite solid acid catalyst remains on the inner wall of the reactor. Subsequently, the catalyst is washed and dried with anhydrous ethanol and returned to step S2 as the catalyst for the next batch of esterification reaction. The purpose of washing with anhydrous ethanol is to dissolve the small amount of 2-ethylhexanoic acid and target product molecules adhering to the catalyst surface, preventing their introduction as trace impurities in the next batch. Anhydrous ethanol itself does not introduce water, thus protecting the sulfate acid sites from hydration deactivation. After the composite solid acid catalyst is continuously recycled 7 times, the ratio of the diethylene glycol esterification conversion rate to the diethylene glycol esterification conversion rate at the time of the first use can be maintained above 90%, reflecting the inhibitory effect of the core-shell-modified three-layer structure on sulfate loss.

[0081] Step S6 is the purification of crude ester in the liquid phase, which consists of sub-steps S61, S62, S63 and S64.

[0082] S61 involves alkaline neutralization. The crude ester in the liquid phase is washed with a saturated sodium carbonate aqueous solution preheated to 45-55°C until the pH of the aqueous phase reaches 7-8. Preheating aims to maintain the crude ester at a low viscosity, facilitating sufficient contact with the aqueous solution and preventing emulsification caused by high interfacial tension between the two phases in a cold state. Saturated sodium carbonate is a weak base and reacts with residual 2-ethylhexanoic acid to form water-soluble sodium 2-ethylhexanoate, which enters the aqueous phase. Simultaneously, it neutralizes trace amounts of sulfate ions lost from the catalyst surface. A pH of 7-8 indicates that the acidic substances have been fully neutralized, yielding the neutralized liquid phase.

[0083] S62 is a water wash. The neutralized liquid phase is washed with deionized water at 50-70°C until the organic phase is neutral. The purpose of this step is to remove residual water-soluble salts such as sodium carbonate and sodium 2-ethylhexanoate, as well as trace amounts of water-extractable small molecule impurities. The water washing temperature of 50-70°C is also to avoid emulsification, resulting in a water-washed liquid phase.

[0084] S63 involves decolorizing with activated clay. Activated clay is added to the water-washed liquid phase at 80-95℃ and stirred for decolorization. Activated clay is an acid-activated montmorillonite-type clay mineral with a high specific surface area and certain adsorption selectivity. It can adsorb coloring species represented by trace oxidation byproducts, thermal degradation byproducts, and conjugated polyene structures in the product. At 80-95℃, the product viscosity is low and the clay dispersion is good, which is beneficial to improving the decolorization efficiency per unit mass of clay, resulting in a decolorized liquid phase.

[0085] S64 is filtration. The decolorized liquid phase is filtered to separate the activated clay from the product, yielding the colorless to light yellow target product, diethylene glycol bis(2-ethylhexanoic acid) ester.

[0086] After implementing steps S1 to S6 above, the quality level of the target product obtained by this method can reach a Hazen color number of no more than 50, an acid value of no more than 0.05 mg KOH / g, and a purity of no less than 99.0%. From a process perspective, this method combines a magnetic core-shell structure with sulfate acid site modification, giving the catalyst a high acid site density. Furthermore, magnetic separation and recovery avoids the wear and clogging associated with traditional filtration and recovery, and the activity retention rate remains high even after multiple cycles. Using the real-time change in the cumulative water production of the water separator as feedback, the reaction progress is divided into stages corresponding to different target water production rates. This allows the internal temperature regulation to dynamically switch with the reaction stage, preventing 2-ethylhexanoic acid distillation loss in the early stage, overcoming equilibrium limitations in the middle stage, and suppressing side reactions and color changes caused by overheating in the final stage. The temperature range settings for the three post-treatment steps result in a lower emulsification tendency during phase separation, improving purification efficiency. Those skilled in the art can select and adjust the specific parameter values ​​for each step based on the above specific implementation method, according to the reaction scale, equipment characteristics, and raw material quality, to implement the method.

[0087] Example 1: This Example 1 is a representative and complete implementation. The specific implementation process is as follows:

[0088] S1 is a composite solid acid catalyst prepared by sequentially comprising S11, S12, and S13. In S11, 19.46 g of ferric chloride and 11.93 g of ferric chloride were weighed and added to 100 mL of nitrogen-deoxygenated deionized water. The mixture was stirred and heated to 70 °C. 25% ammonia solution was slowly added dropwise until the pH of the mixture reached 10. The mixture was aged at 70 °C for 30 min. After magnetic separation and washing three times with deionized water, approximately 13.9 g of Fe3O4 magnetic nanoparticles with an average particle size of 12 nm were obtained. In S12, the Fe3O4 obtained in S11 was dispersed in 200 mL of isopropanol. 88.7 g of zirconium oxynitrate, 61.2 g of tetrabutyl titanate, and 8 g of polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer were added as a soft template agent at a molar ratio of Fe to Zr to Ti of 1:2:1. The mixture was stirred and hydrolyzed at 60 °C for 4 h, aged for 12 h, filtered, dried at 110 °C for 10 h, and calcined at 550 °C for 3 h to obtain the core-shell precursor, which had a BET specific surface area of ​​128 m² / g. In S13, 50 g of the core-shell precursor was impregnated in 500 mL of a 1.0 mol / L ammonium sulfate aqueous solution at room temperature for 12 h, filtered, dried at 110 °C for 8 h, and calcined at 550 °C for 3 h to obtain the composite solid acid catalyst of this embodiment, denoted as catalyst A.

[0089] S2 is the reactor loading. In a 500mL three-necked flask equipped with a mechanical stirrer, a Pt-100 temperature sensor, a Dean-Stark water separator, a reflux condenser, and an external magnetic separation unit, 95g of diethylene glycol and 284g of 2-ethylhexanoic acid are added to make the molar ratio of 2-ethylhexanoic acid to diethylene glycol 2.20:1. Catalyst A 5.69g is added, accounting for 1.5% of the total mass of diethylene glycol and 2-ethylhexanoic acid. Stirring is started at a speed of 400r / min.

[0090] S3 is for heating esterification and feedback regulation of the reaction internal temperature, and consists of S31, S32, and S33 sequentially. Heating is initiated in S31, setting the initial value of the reaction internal temperature. Set as the lower limit of the reaction internal temperature =116℃. In S32, an electronic level gauge located at the bottom of the distributor is used to measure the sampling period. =Accumulated water production is obtained periodically every 15 minutes. , and according to Calculate the instantaneous segmented water production rate. In S33, press... The reaction internal temperature setpoint is calculated for the next sampling time. In this embodiment, a proportionality coefficient is used. =6℃·min / mL, upper limit of internal reaction temperature =130℃, the heating device adjusts the heating power accordingly to track the internal temperature of the reaction in the next sampling cycle. .

[0091] S4 is the dynamic division of reaction stages and the determination of reaction termination, consisting of S41, S42, and S43 in sequence. Within S41, according to... Calculate the theoretical water production in this embodiment. =0.895mol =18g / mol =1.0g / mL, therefore =32.22 mL. In S42, the ratio of cumulative water production to theoretical water production... Real-time switching of target water production rate within the specified range When 0≤ When <30% is the initial reaction stage, and =0.40mL / min, when 30%≤ When <75% is the main reaction stage, and =0.25mL / min, when 75%≤ When <95% is the final reaction stage, and =0.10 mL / min. In S43, when two consecutive sampling periods... All are not greater than 0.03 mL / min, and The reaction is terminated when the concentration is not less than 95%, and the total reaction time is recorded. =3.8h.

[0092] S5, consisting of S51, S52, and S53, is a catalyst magnetic separation and liquid-phase crude ester discharge process. In S51, heating is stopped, and the reaction solution is cooled to 80°C. In S52, a 0.4T NdFeB permanent magnet is applied to the outside of the reactor, and the mixture is stirred at a low speed of 50 r / min for 5 minutes to adsorb and fix catalyst A onto the inner wall of the reactor. In S53, the liquid-phase crude ester is discharged through the discharge valve at the bottom of the reactor. The catalyst A remaining in the reactor is washed twice with anhydrous ethanol and dried at 80°C for 2 hours before being used in the next batch of esterification reactions.

[0093] S6 is the liquid-phase crude ester purification process, consisting of S61, S62, S63, and S64 in sequence. In S61, the liquid-phase crude ester is placed in a separatory funnel, and 100 mL of saturated sodium carbonate aqueous solution preheated to 50 °C is added. The mixture is washed thoroughly in three batches until the pH of the aqueous phase reaches 8. In S62, the mixture is washed twice with deionized water at 60 °C until the organic phase is neutral. In S63, 3 g of activated clay is added, and the mixture is stirred at 90 °C for 30 min to decolorize. In S64, the mixture is filtered while hot to obtain approximately 307 g of a colorless to pale yellow transparent liquid diethylene glycol bis(2-ethylhexanoic acid) ester product.

[0094] Example 2 differs from Example 1 in that the amount of 2-ethylhexanoic acid added in S2 is 271g, making the molar ratio of 2-ethylhexanoic acid to diethylene glycol 2.10:1, and the amount of catalyst A added is 2.93g, accounting for 0.8% of the total mass. The proportioning coefficient in S3... =4℃·min / mL, lower limit of reaction internal temperature =114℃, upper limit of internal reaction temperature =128℃. Target permeate rates for the initial reaction stage, main reaction stage, and final reaction stage in S4. The flow rates were 0.30 mL / min, 0.20 mL / min, and 0.05 mL / min, respectively. The remaining steps were the same as in Example 1, and the total reaction time was... =4.5h.

[0095] Example 3 differs from Example 1 in that the amount of 2-ethylhexanoic acid added in S2 is 297g, making the molar ratio of 2-ethylhexanoic acid to diethylene glycol 2.30:1, and the amount of catalyst A added is 7.84g, accounting for 2.0% of the total mass. The proportioning coefficient in S3... =8℃·min / mL, lower limit of reaction internal temperature =118℃, upper limit of internal reaction temperature =132℃. Target permeate rates for the initial reaction stage, main reaction stage, and final reaction stage in S4. The flow rates were 0.50 mL / min, 0.30 mL / min, and 0.15 mL / min, respectively. The remaining steps were the same as in Example 1, and the total reaction time was... =3.5h.

[0096] Comparative Example 1, representing the existing technology baseline, employs homogeneous catalysis with concentrated sulfuric acid and empirical isothermal esterification. The differences from Example 1 are as follows: Step S1 is omitted; in S2, 5.69 g of 98% concentrated sulfuric acid replaces catalyst A; in S3, feedback-based segmented temperature control is not used, but constant heating is employed to stabilize the internal reaction temperature at 125°C; in S4, there are no stage divisions, and the cumulative water production of the separator is continuously monitored; the reaction is terminated when no water droplets are generated in the separator for 2 consecutive hours; in S5, magnetic separation is not performed, and the reaction proceeds directly to S6 after termination. The remaining steps are the same as in Example 1, with the total reaction time being... =5.5h.

[0097] Comparative Example 2 differs from Example 1 in that the same catalyst A is used in S2, but feedback-based segmented temperature control is not employed in S3; instead, constant heating is used to stabilize the internal reaction temperature at 125°C. In S4, there is no stage division, and the method for determining reaction termination is the same as in Comparative Example 1. The remaining steps are the same as in Example 1, and the total reaction time is... =5.0h.

[0098] Comparative Example 3 differs from Example 1 in that S1 does not use a core-shell structure catalyst A, but instead uses conventional SO4. 2- The ZrO2 solid superacid catalyst, designated as catalyst B, is prepared by hydrolyzing ZrOCl2·8H2O to obtain Zr(OH)4 precipitate. After filtration and drying at 110°C, the precipitate is impregnated in a 1.0 mol / L ammonium sulfate aqueous solution for 12 h, followed by filtration, drying at 110°C, and calcination at 550°C for 3 h. Since catalyst B does not exhibit magnetic response, vacuum filtration is used instead of magnetic separation to recover the catalyst in step S5. The remaining steps, including the feedback-based segmented temperature control strategy in step S3, are the same as in Example 1. Because the acid site density of catalyst B is lower than that of catalyst A, under the action of the feedback-based segmented temperature control law, the internal temperature of the reaction in this comparative example initially decreases from... =116℃ rapidly rose to =130℃ and saturated for a long time, total reaction time =4.5h.

[0099] Experiment 1: Comparison of reaction performance and product indicators; One batch of reaction was conducted using the methods of Examples 1, 2, 3, Comparative Examples 1, 2, and 3. Samples were taken at 15-minute intervals during the reaction, and the diethylene glycol content in the reaction solution was determined by gas chromatography to calculate the diethylene glycol conversion rate. After the reaction was terminated, product samples were taken to determine relevant indicators, and the total reaction time for each group was recorded. The 2-ethylhexanoic acid distillation loss rate is calculated as the ratio of the total amount of 2-ethylhexanoic acid contained in the aqueous phase of the separator to the initial amount of acid added, based on the time from reaction initiation to the determination of reaction termination in step S43 of this invention. Key performance indicators for each experimental group are shown in Table 1. During the reaction process of each group... and The curve of change with reaction time is as follows: Figure 1 As shown, the key performance indicators for each group are as follows: Figure 2 As shown.

[0100] Table 1. Overall performance data of each embodiment and comparative example

[0101]

[0102] from Figure 1 As can be seen from (a), the three embodiments The curve exhibits a segmented, broken line shape, with a steeper slope in the initial reaction stage, a moderate slope in the main reaction stage, and a decreasing slope in the final reaction stage, corresponding to the phased decrease in the target water production rate defined in step S42 of this invention. (Comparative Examples 1 and 2) The curve exhibits a typical exponential saturation shape, with a steep initial slope that monotonically decreases as the reaction progresses. (Comparative Example 3) The curve exhibits an S-shape, with a slow initial rise, a steeper slope in the main reaction phase, and a gradual flattening in the final stage. This reflects the relatively low acid site density of catalyst B, resulting in a weaker reaction rate in the initial stage, until the internal reaction temperature approaches a certain level under feedback temperature control. Only then does it enter a relatively rapid reaction phase. From Figure 1 As can be seen in (b), the three embodiments The curve exhibits a stepped decline in more than three stages, with the values ​​highest in the initial reaction stage, moderate in the main reaction stage, and lowest in the final reaction stage. This corresponds to the phased switching design. Comparative Example 1 and Comparative Example 2... The curve exhibits a single-peak decay, with the peak appearing near the reaction initiation point. This is because the internal temperature of the reaction remains at a relatively high level from the reaction initiation point under a constant temperature of 125℃; (Comparative Example 3) The curve exhibits a bell-shaped single peak, with the peak occurring in the middle of the reaction, reflecting the change in internal temperature of the reaction under the feedback temperature control of catalyst B. Rise to The transition process causes the peak water production rate to lag behind the reaction initiation point.

[0103] from Figure 2It can be seen that, in terms of both diethylene glycol conversion and target product yield, the three examples are superior to the three comparative examples, while in terms of total reaction time and Hazen color number, the three examples are inferior to the three comparative examples. Combining the data in Table 1, Example 1, compared to Comparative Example 1... The efficiency was improved by 4.0 percentage points, the total reaction time was shortened by 1.7 hours, the Hazen color number decreased by 75 color number units, and the 2-ethylhexanoic acid distillation loss rate decreased by 0.9 percentage points. Both product quality and production efficiency were superior. From a comparative perspective, Comparative Example 2, compared to Comparative Example 1, replaced the catalyst from homogeneous concentrated sulfuric acid catalysis with catalyst A of this invention. An increase of 1.4 percentage points; Comparative Example 3, compared to Comparative Example 1, changed the control strategy from isothermal to feedback-based segmented heating. The improvement was 0.6 percentage points; the sum of the contributions of the two independent improvements was 2.0 percentage points, which was less than the overall improvement of 4.0 percentage points of Example 1 compared with Comparative Example 1. The difference of 2.0 percentage points reflects that there is a positive synergistic relationship between the catalyst structure improvement and the temperature control strategy improvement, that is, the effect produced by the combined use of the two technologies is greater than the sum of the effects of their individual use.

[0104] Examples 1, 2, and 3 all achieve the desired results even with different values ​​for the S2 feeding ratio and the S3 feedback control parameter. The composite solid acid catalyst A prepared in step S1 has a high acid site density due to its core-shell structure and the mesoporous channels formed by the soft template, achieving a purity of not less than 97% and a purity of not less than 99.1%. This allows it to maintain a sufficient reaction rate even at a relatively low internal reaction temperature. The feedback temperature controller in step S3 has a large temperature adjustment margin, which can maintain a low internal reaction temperature in the initial reaction stage to suppress the distillation of 2-ethylhexanoic acid, and appropriately increase the internal reaction temperature in the final reaction stage to overcome the equilibrium limitation. If the catalyst structure is downgraded to that of traditional SO4... 2- If the temperature control strategy for / ZrO2 is simplified to isothermal, then the coupling mechanism between the two in reaction engineering will fail.

[0105] Experimental Example 2: This experiment used catalyst A prepared in Example 1 and catalyst B used in Comparative Example 3, and performed multiple physicochemical characterizations on them. Specific surface area and pore size were determined using a specific surface area and pore size analyzer via nitrogen physical adsorption; average pore size and pore volume were calculated using the BJH method. Surface acid site density was obtained by integrating ammonia temperature-programmed desorption using a chemisorption analyzer within the range of 100-600℃. The contents of Fe, Zr, and Ti were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES); the S content was determined using an elemental analyzer; and SO4 content was determined. 2-The loading was obtained by multiplying the sulfur content by 96 / 32; the saturation magnetization was measured at room temperature using a vibrating sample magnetometer; the average particle size was obtained by the arithmetic mean of the particle sizes of more than 100 particles in a transmission electron microscope image. The characterization data are shown in Table 2.

[0106] Table 2 Comparison of physicochemical characterization of catalyst A in Example 1 and catalyst B in Comparative Example 3

[0107]

[0108] Table 2 shows that the BET specific surface area of ​​catalyst A is approximately 1.97 times that of catalyst B, and its average pore size and total pore volume are also significantly larger. This difference originates from the mesoporous channels formed by the decomposition of the soft template agent added in step S12 of this invention during calcination. The increase in NH3-TPD acid site density matches the increase in specific surface area, reflecting a higher number of active acid sites per unit mass of catalyst. The SO42- content of catalysts A and B... 2- The loadings are similar, but catalyst A has a saturation magnetization of 28 A·m² / kg, while catalyst B has no magnetic response.

[0109] It can be seen that the mesoporous structure and higher acid site density directly correspond to the superior reactivity of catalyst A compared to catalyst B in Experiment Example 1. These two factors form a causal chain: the soft template agent regulates the formation of mesoporous channels, the mesoporous structure provides a higher specific surface area, the higher specific surface area supports more acid sites, and the increased acid site density accelerates the esterification reaction rate per unit time. This explains why, in Experiment Example 3, catalyst B, under feedback temperature control, experienced a sustained instantaneous segmented water production rate lower than the target water production rate, causing the internal reaction temperature to be pushed up to [a certain value]. The reason for the saturation phenomenon; the existence of saturation magnetization allows catalyst A to be separated from the liquid phase product under an applied magnetic field, providing a physical basis for the magnetic separation operation in step S5 of this invention; the introduction of Ti element in catalyst A makes Ti 4+ With Zr 4+ Ti coexists in a considerable proportion in the same oxide phase 4+ With Zr 4+ Differences in ionic radii induce lattice distortion, which is beneficial for surface-modified SO4. 2- Provide more anchor points.

[0110] In Experiment 3, one batch of reaction was carried out using the methods of Examples 1, 2, 3, Comparative Examples 1, 2, and 3. During the reaction, the internal temperature measured by the Pt-100 temperature sensor was recorded at 30-second intervals. In the embodiments and Comparative Example 3, the heating power is adjusted in real time according to the feedback control law of step S33 of the present invention, while Comparative Examples 1 and 2 maintain the internal temperature of the reaction at a stable 125°C according to an empirical constant temperature control method. (Each group...) Curves Figure 3 As shown. Since both Comparative Example 1 and Comparative Example 2 use constant temperature control and have the same internal reaction temperature setpoint, their... The trajectories completely overlap, in Figure 3 Different shapes of symbols are used to mark and distinguish them at different times.

[0111] from Figure 3 It can be seen that the three embodiments The curve from the initial stage of the reaction The value slowly rises to the median plateau value, remaining stable during the main reaction phase, until it rises significantly again after the reaction enters the final stage. Nearby, it presents an overall shape with two leaps; Examples 2 and 3 are due to and The values ​​are the lower and upper limits, respectively, and the curve is shifted overall but retains the same shape. Comparative Example 1 and Comparative Example 2 The curve represents the 125℃ horizontal line, reflecting the characteristics of constant temperature control; the two trajectories completely overlap. (Comparative Example 3) The curve changes from the initial stage of the reaction. Rapidly rise to =130℃ and maintained for a long time, exhibiting saturation lock-in characteristics.

[0112] Implementation Examples The two jumps in the curve correspond to the phased switching of the target permeate rate in step S42 and the saturation limiting step in S33 of the present invention. In the initial stage of the reaction, the feedback controller needs a higher internal temperature to make the instantaneous segmented permeate rate move to a higher initial stage. Approaching, main reaction phase The decrease causes the controller to tend to maintain the internal temperature of the reaction, thus stabilizing the curve. Although the final stage... Taking a lower value, but the depletion of reactants causes the instantaneous fractional water production rate to be lower even at higher reaction internal temperatures. The controller continuously outputs a positive deviation, pushing the internal temperature of the reaction to [a certain level]. Nearby, the effectiveness of the outer layer saturation limiting mechanism was verified. (Comparative Example 3) The curve remained stationary for a long time in the early stages of the reaction. Nearby, under the same feedback temperature control strategy, insufficient activity of catalyst B resulted in an instantaneous segmented water production rate consistently lower than [previous value]. The output of the feedback controller is continuously fed The saturation is consistent with the characterization data in Experimental Example 2, where the acid site density of catalyst B is only 0.59 times that of catalyst A. This establishes a one-to-one correspondence between the catalyst acid site density and the feedback temperature control response, reinforcing the factual basis for the synergy between the catalyst structure in step S1 and the temperature control strategy in step S3 of this invention from the perspective of the temperature control process. (Comparative Example 1 and Comparative Example 2) Although the curves completely overlap at 125℃, the two are different in Table 1. The differences of 1.4 percentage points between 94.2% and 95.6% reflect the independent contribution of catalyst structure differences to reaction conversion under the same internal reaction temperature.

[0113] Example 4: Catalyst Cycling Stability Experiment. In this example, after the reaction batches of Examples 1, 2, and 3 were completed, catalyst A, recovered by magnetic separation in step S5, was washed with anhydrous ethanol, dried at 80°C for 2 hours, and directly used in the next batch of reaction under unchanged reaction conditions, for a total of 10 cycles. Comparative Example 3 used catalyst B, which was recovered by vacuum filtration and washed and dried in the same manner, also for 10 cycles. The diethylene glycol conversion rate in each cycle is shown in the figure. Following the method described in Experimental Example 1, after each cycle, a small sample of the catalyst was taken and the content of sulfur (S) and SO4 was determined using an elemental analyzer. 2- Retention rate is based on the amount of SO4 in the catalyst after this cycle. 2- Content of SO4 in the catalyst before first use 2- The content ratio is expressed. Data from 10 cycles are shown in Table 3, including the cyclic activity curves and SO4 content curves for Examples 1, 2, 3, and Comparative Example 3. 2- Retention rate curve as shown Figure 4 As shown.

[0114] Table 3. Data from 10 cycles of Examples 1, 2, 3 and Comparative Example 3

[0115]

[0116] from Figure 4 As can be seen from (a), the three embodiments The curve declines gently in the first 7 cycles, and the curves in Examples 1, 2, and 3 show a gradual decline after the 7th cycle. The percentages decreased from 98.2% to 95.0%, from 97.0% to 92.8%, and from 98.5% to 96.2%, respectively. and The activity retention rates calculated by the ratios were 96.7%, 95.7%, and 97.7%, respectively, all not less than 90%; Comparative Example 3... The curve drops significantly in the first 7 cycles, and at the 7th cycle... The activity retention rate decreased from 94.8% to 75.0%, with a retention rate of 79.1%, far below the requirement of at least 90% activity retention rate after seven consecutive cycles of catalyst use as described in this invention. Figure 4 As can be seen from (b), the SO4 in the three embodiments 2- The retention rates remained above 89%, 86%, and 92% respectively within 7 cycles, while the SO4 in Comparative Example 3... 2- The retention rate was only 58% after 7 cycles, indicating the limitation of the cycling activity of this invention, achieved through SO4. 2- The synchronous correspondence between retention rate and activity retention rate reveals the following reason for the influence of catalyst structure on cycle stability: the activity retention rate of all three examples is not less than 90%, indicating that the activity retention rate of the composite solid acid catalyst in the method described in this invention is not less than 90% after continuous cycling for 7 times; SO4 2- Retention rate and There is a positive correlation between retention rates, i.e., SO4 2- The higher the retention rate, the better. A higher retention rate indicates that the main reason for the decrease in catalyst activity is the loss of sulfate sites on the surface, rather than the destruction of the support structure. Combining the characterization data from Experimental Example 2, the Ti in catalyst A... 4+ With Zr 4+ When two types of metal ions coexist in the same oxide phase at a molar ratio of 0.5-1.5:(1-3), the difference in radii between the two types of metal ions induces lattice distortion, causing SO42- on the surface. 2- Can be used with Ti 4+ Zr 4+ Anchoring via double-tooth bridging or chelate double-tooth coordination provides stronger anchoring strength than SO4 on a single zirconium-based carrier. 2- The higher coordination and the mesoporous structure of the core-shell precursor further reduce the hydration effect of water on sulfate ions in the reaction solution, jointly inhibiting sulfate ion loss. This mechanism is the microscopic basis for the good cycle stability of the composite solid acid catalyst prepared in step S1 of the present invention. At the same time, it supports the synergistic value of the magnetic separation and recovery method in step S5 of the present invention and the catalyst structure. That is, the magnetic separation operation avoids particle wear and mechanical loss of active centers caused by filtration and recovery.

[0117] The above description is merely a preferred embodiment of the present invention and is 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 synthesizing diethylene glycol bis(2-ethylhexanoic acid) ester, characterized in that, Includes the following steps: S1: Preparation of composite solid acid catalysts; S2: Add diethylene glycol, 2-ethylhexanoic acid and the composite solid acid catalyst to the reactor and start stirring; S3: Heating to carry out the esterification reaction. The instantaneous segmented water production rate is obtained based on the real-time change of the cumulative water production in the water separator. The deviation between the instantaneous segmented water production rate and the target water production rate is used as the feedback quantity. The reaction internal temperature is limited to the range from the set lower limit value to the upper limit value of the reaction internal temperature through proportional feedback adjustment. S4: Based on the ratio of the cumulative water production to the theoretical water production in the water distributor, the reaction process is divided into multiple reaction stages, each corresponding to a different target water production rate, until the reaction is determined to be terminated. S5: After stopping heating and cooling down, a magnetic field is applied to the outside of the reactor to adsorb and fix the composite solid acid catalyst onto the inner wall of the reactor, and the liquid phase crude ester is discharged. S6: The crude liquid ester is subjected to alkali neutralization, water washing, decolorization and filtration in sequence to obtain diethylene glycol bis(2-ethylhexanoic acid) ester.

2. The method according to claim 1, characterized in that, Step S1 includes: S11: Ferric chloride and ferric chloride were dissolved in deionized water deoxygenated by nitrogen at a molar ratio of 2:

1. After stirring and heating, ammonia was added dropwise until the mixture became alkaline. After magnetic separation and washing with water, Fe3O4 magnetic nanoparticles were obtained. S12: The Fe3O4 magnetic nanoparticles obtained in step S11 are dispersed in isopropanol, tetrabutyl titanate, zirconium oxynitrate and a soft template agent are added, and after stirring and hydrolysis, the mixture is aged, filtered, dried and calcined to obtain the core-shell precursor; S13: The core-shell precursor obtained in step S12 is immersed in an aqueous solution of ammonium sulfate or sulfuric acid, filtered, dried and then calcined to obtain the composite solid acid catalyst.

3. The method according to claim 2, characterized in that: In step S11, the mixture is stirred and heated to 60-80°C. The added ammonia water makes the pH value of the mixture reach 9-11, and the average particle size of the obtained Fe3O4 magnetic nanoparticles is 8-15 nm. In step S12, the molar ratio of iron in the Fe3O4 magnetic nanoparticles, zirconium in the zirconium oxynitrate, and titanium in the tetrabutyl titanate is 1:(1-3):(0.5-1.5). The soft template agent is selected from polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer or hexadecyltrimethylammonium bromide. The hydrolysis temperature is 50-70℃, and the calcination temperature is 500-600℃. In step S13, the concentration of the ammonium sulfate aqueous solution or sulfuric acid aqueous solution is 0.5-1.5 mol / L, and the calcination temperature is 540-560℃.

4. The method according to claim 1, characterized in that, In step S2, the molar ratio of 2-ethylhexanoic acid to diethylene glycol is (2.10-2.30):1, and the mass of the composite solid acid catalyst added accounts for 0.8-2.0% of the total mass of diethylene glycol and 2-ethylhexanoic acid.

5. The method according to claim 1, characterized in that, Step S3 includes: S31: Start heating and set the initial value of the internal reaction temperature to the lower limit value of the internal reaction temperature; S32: By means of a liquid level detection device installed on the distributor or a weighing device that cooperates with the distributor, at a sampling cycle Periodically obtain the cumulative water production in the water distributor And calculate the instantaneous segmented water production rate according to the following formula. : ； In the formula, For the first Instantaneous segmented water production rate at each sampling moment; For the first The cumulative water production in the water distributor at each sampling time; For the first The cumulative water production in the water distributor at each sampling time; The sampling period; It is an integer greater than or equal to 1; S33: Based on the target water production rate corresponding to the current reaction stage. With the instantaneous segmented water production rate Calculate the set value of the reaction internal temperature at the next sampling time using the following formula. Based on this, the heating power is adjusted to keep the internal temperature of the reaction tracked in the next sampling period. : ； In the formula, For the first The set value of the internal temperature is reflected at each sampling time; For the first The measured values ​​of the internal temperature at each sampling time point; This is the proportionality coefficient; This represents the target water production rate corresponding to the current reaction stage. For the first Instantaneous segmented water production rate at each sampling moment; This is the lower limit of the internal temperature of the reaction; This is the upper limit of the internal temperature of the reaction; Indicates taking and The larger of the two; Indicates taking and The smaller of the; The sampling period The time is 10-20 minutes, and the proportionality coefficient is... The reaction internal temperature limit is 4-8℃·min / mL. The upper limit of the internal temperature of the reaction is 114-118℃. The temperature is 128-132℃.

6. The method according to claim 1, characterized in that, Step S4 includes: S41: The theoretical water production rate is calculated based on the amount of diethylene glycol fed using the following formula. : ； In the formula, This represents the theoretical water production rate. This refers to the amount of diethylene glycol added. The molar mass of water; The density of water is given at normal pressure and 25°C; the coefficient 2 indicates that 2 moles of water are produced when 1 mole of diethylene glycol is diesterized. S42: Based on the cumulative water production in the water distributor With the theoretical water production ratio Based on the location, the target water production rate corresponding to the current reaction stage is determined in real time. :when This is the initial reaction stage. The flow rate is 0.30-0.50 mL / min; when The main reaction phase is during this time. The flow rate is 0.20-0.30 mL / min; when This is the final reaction stage. The flow rate is 0.05-0.15 mL / min. S43: When the instantaneous segmented water production rate is no greater than 0.03 mL / min in two consecutive sampling periods, and the cumulative water production is... With theoretical water production When the ratio is not less than 95%, the reaction is considered terminated.

7. The method according to claim 1, characterized in that, Step S5 includes: S51: Stop heating and allow the reaction solution to cool to 70-90℃; S52: Apply a neodymium iron boron permanent magnet or electromagnet with a magnetic induction intensity of not less than 0.3T to the outside of the reactor, and maintain it for 3-7 minutes under the condition of stirring stopped or low speed stirring speed not greater than 100 rpm, so that the composite solid acid catalyst is adsorbed and fixed on the inner wall of the reactor. S53: The liquid crude ester is discharged through the discharge valve at the bottom of the reactor. The composite solid acid catalyst is retained in the reactor and, after being washed and dried with anhydrous ethanol, is returned to step S2 as the catalyst for the next batch of esterification reaction.

8. The method according to claim 7, characterized in that, After the composite solid acid catalyst is continuously recycled 7 times, the ratio of the diethylene glycol esterification conversion rate to the diethylene glycol esterification conversion rate at the time of the first use is not less than 90%.

9. The method according to claim 1, characterized in that, Step S6 includes: S61: Wash the crude liquid phase with a saturated sodium carbonate aqueous solution preheated to 45-55℃ until the pH value of the aqueous phase reaches 7-8 to obtain a neutralized liquid phase; S62: Wash the neutralized liquid phase with deionized water at 50-70℃ until the organic phase is neutral, to obtain the washed liquid phase. S63: Add activated clay to the water-washed liquid phase at 80-95℃ and stir to decolorize, to obtain a decolorized liquid phase; S64: Filter the decolorized liquid phase to obtain diethylene glycol bis(2-ethylhexanoic acid) ester.

Images