Process for the preparation of 3,5,5-trimethylhexanoic acid and use thereof

Abstract

This invention discloses a method for preparing 3,5,5-trimethylhexanoic acid and its application, belonging to the technical field of 3,5,5-trimethylhexanoic acid preparation. The method for preparing 3,5,5-trimethylhexanoic acid includes the following steps: S1, dehydrating tert-butanol to prepare isobutylene; wherein the tert-butanol used contains <0.3% by mass of n-propanol, <0.3% by mass of isopropanol, <0.3% by mass of n-butanol, and <0.3% by mass of isobutanol; S2, preparing diisobutylene from the isobutylene obtained in S1; S3, preparing 3,5,5-trimethylhexanal from the diisobutylene obtained in S2 via hydroformylation; S4, preparing 3,5,5-trimethylhexanal from the 3,5,5-trimethylhexanal obtained in S3 via oxidation. The 3,5,5-trimethylhexanoic acid obtained by this invention has high purity.

Description

Technical Field

[0001] This invention belongs to the field of 3,5,5-trimethylhexanoic acid preparation technology, specifically relating to a method for preparing 3,5,5-trimethylhexanoic acid and its application. Background Technology

[0002] Air conditioning refrigeration lubricants are primarily synthesized through the esterification reaction of fatty acids and fatty alcohols, with 3,5,5-trimethylhexanoic acid being the most widely used fatty acid. Since refrigeration lubricants are used within air conditioning compressors, they are generally not replaceable. When problems arise with the refrigeration lubricant, the entire compressor typically needs to be replaced, which is costly and can lead to customer complaints and damage the product's brand image. Therefore, the long-term storage stability of refrigeration lubricants is crucial, generally requiring 8-10 years of continuous use without issues. This necessitates higher quality requirements for the reaction raw materials.

[0003] If there are many impurities in 3,5,5-trimethylhexanoic acid, the batch stability will be poor due to the repeated use during the synthesis of refrigeration lubricating oil, resulting in different service lives for the same product. This will affect the performance in the compressor and easily cause fluctuations in the product's lifespan. Summary of the Invention

[0004] Therefore, the present invention provides a method for producing 3,5,5-trimethylhexanoic acid and its application, which yields high-purity 3,5,5-trimethylhexanoic acid, facilitating the synthesis of high-quality refrigeration lubricating oil.

[0005] To this end, the present invention provides the following technical solution.

[0006] In a first aspect, the present invention provides a method for preparing 3,5,5-trimethylhexanoic acid, comprising the following steps:

[0007] S1. Isobutylene is prepared by dehydrating tert-butanol;

[0008] Of the tert-butanol used, the mass content of n-propanol is <0.3%, the mass content of isopropanol is <0.3%, the mass content of n-butanol is <0.3%, and the mass content of isobutanol is <0.3%; preferably, the mass content of n-propanol is <0.1%, the mass content of isopropanol is <0.1%, the mass content of n-butanol is <0.1%, and the mass content of isobutanol is <0.1%; the purity of tert-butanol is above 99.5%, preferably >99.5%;

[0009] S2. Prepare diisobutylene using the isobutylene obtained in S1;

[0010] S3. 3,5,5-Trimethylhexanol was prepared by hydroformylation of diisobutylene obtained by S2.

[0011] S4. 3,5,5-Trimethylhexanol obtained from S3 is oxidized to prepare 3,5,5-trimethylhexanoic acid.

[0012] In one possible implementation, S3 includes: mixing diisobutylene and a catalyst, carrying out a hydroformylation reaction under a syngas atmosphere, and then separating by distillation to obtain 3,5,5-trimethylhexanal.

[0013] Preferably, the catalyst is a metal salt, wherein the metal element in the metal salt is selected from one or more of rhodium, cobalt, and iridium;

[0014] Preferably, the catalyst is a rhodium salt, which includes one or more of rhodium acetate, rhodium octanoate, rhodium naphthenate, and rhodium acetylacetonate.

[0015] In one possible implementation, S3 satisfies at least one of the following conditions:

[0016] (1) The synthesis gas includes H2 and CO, and the molar ratio of the two is (0.6~2):1;

[0017] (2) The temperature of the hydroformylation reaction is 40–200 °C;

[0018] (3) The pressure of the synthesis gas is 0.05–30 MPaG;

[0019] (4) The purity of the obtained 3,5,5-trimethylhexanal is >99.9 wt%.

[0020] In one possible implementation, the hydroformylation reaction satisfies at least one of the following conditions:

[0021] (1) 0.5 ≤ y ≤ 1647 × e -0.029x Formula I

[0022] In Formula I, x is the temperature of the hydroformylation reaction, in °C;

[0023] The mass of the metal element in the catalyst is A mg, the mass of diisobutylene is B kg, and y = A / B;

[0024] (2) 0.0048×e 0.038x ≤z≤0.0059×e 0.058x Formula II

[0025] In Formula II, x is the temperature of the hydroformylation reaction, in °C; z is the pressure of the synthesis gas, in MPaG.

[0026] In one possible implementation, the hydroformylation reaction is carried out in the presence of a ligand;

[0027] Preferably, the ligand comprises At least one of them;

[0028] R1, R2, and R3 are each independently selected from C1 to C8 alkyl groups;

[0029] Preferably, R1, R2, and R3 are each independently selected from tert-butyl or octyl;

[0030] Preferably, R2 and R3 are the same group;

[0031] Preferably, the mass of the ligand is 0.02% to 0.5% of the mass of diisobutylene.

[0032] In one possible implementation, the hydroformylation reaction is carried out in the absence of ligands or in the presence of a medium.

[0033] In one possible implementation, S1 includes: tert-butanol undergoing catalytic dehydration via a first acidic catalyst and distillation separation to prepare isobutene;

[0034] Preferably, the first acidic catalyst is an acidic cation exchange resin;

[0035] Preferably, the temperature of the dehydration reaction is 60–120°C, and the pressure is 20–500 kPa.

[0036] Preferably, the liquid hourly space velocity (LISH) of the dehydration reaction is 2–10 h⁻¹. -1 ;

[0037] Preferably, the isobutylene obtained by S1 contains propylene content <300ppmw and n-butene content <500ppmw.

[0038] In one possible implementation, S2 includes: performing a dimerization reaction on the isobutylene obtained in S1 and separating and preparing diisobutylene;

[0039] Preferably, the catalyst for the dimerization reaction is a second acidic catalyst; more preferably, the second acidic catalyst is an acidic cation exchange resin.

[0040] Preferably, the separation is a distillation separation;

[0041] Preferably, the dimerization reaction is carried out at a temperature of 60–180°C and a pressure of 100–600 kPa.

[0042] Preferably, the liquid hourly space velocity (LISH) of the dimerization reaction is 1–10 h⁻¹. -1 .

[0043] Preferably, the total content of 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene in the obtained diisobutylene is >99.9%.

[0044] In one possible implementation, the oxidation reaction in S4 is carried out in an air or oxygen atmosphere;

[0045] Preferably, the oxidation reaction is carried out at a temperature of 30–150°C, a pressure of 0–0.7 MPaG, and a time of 1–20 h.

[0046] In S4, after oxidation, the product is separated by distillation to obtain 3,5,5-trimethylhexanoic acid with a content >99.9%.

[0047] The dehydration and distillation processes described in step S1 can be carried out separately, with the reaction occurring first followed by distillation, or isobutylene can be obtained in one step using reactive distillation technology. When the dehydration and distillation processes described in step S1 are carried out separately, the dehydration reaction is carried out in a fixed-bed reactor.

[0048] Furthermore, the acidic cation exchange resin described in S1 is a macroporous strong acidic cation exchange resin with a specific surface area of ​​40–70 m². 2 / g, pore volume 0.2~0.5mL / g, H + The exchange capacity is ≥1 mmol / g, preferably a macroporous sulfonic acid type polystyrene resin, more preferably one of the resins of type A15, A35, A45, A70, D005, D009, and DNW-II.

[0049] The S2 dimerization reaction was carried out in a fixed-bed reactor.

[0050] The acidic cation exchange resin mentioned in S2 is a macroporous strong acidic cation exchange resin with a specific surface area of ​​40-70 m². 2 / g, pore volume 0.2~0.5mL / g, H + The exchange capacity is ≥1 mmol / g, preferably a macroporous sulfonic acid type polystyrene resin, more preferably one of the resins of type A15, A35, A45, A70, D005, D009, and DNW-II.

[0051] Solvents can be used in the S3 hydroformylation reaction. The solvent is preferably one or a mixture of several aromatic hydrocarbons, alkanes or cycloalkanes containing 6 to 12 carbon atoms, more preferably one or a mixture of several benzene, toluene, xylene, cyclohexane, methylcyclohexane, dimethylcyclohexane, hexane, heptane, octane, nonane, decane, undecane, and dodecane.

[0052] The S3 hydroformylation reaction can be carried out without a solvent, using diisobutylene as the solvent for the catalyst.

[0053] The reactor for S3 can be either a stirred tank or a bubble bed; the reaction can be carried out in a batch or continuous process, with a continuous process being preferred.

[0054] The reactor for S4 can be a stirred tank or a bubble bed; the reaction can be carried out in a batch or continuous process, with a continuous process being preferred.

[0055] Secondly, the present invention provides an application of 3,5,5-trimethylhexanoic acid prepared according to the above preparation method in the preparation of refrigeration lubricating oil.

[0056] Thirdly, the present invention provides an application of 3,5,5-trimethylhexanoic acid prepared according to the above preparation method in the preparation of cosmetics.

[0057] 3,5,5-Trimethylhexanoic acid, alone or together with other acids, undergoes esterification with alcohols to synthesize esters for use in refrigeration lubricants or cosmetics.

[0058] The technical solution of this invention has the following advantages:

[0059] 1. The method for preparing 3,5,5-trimethylhexanoic acid according to the present invention includes the following steps: S1, isobutylene is prepared by dehydration reaction of tert-butanol; wherein the mass content of n-propanol, isopropanol, n-butanol and isobutanol in the tert-butanol used is <0.1% by mass, <0.1% by mass, <0.1% by mass, and <0.1% by mass; S2, diisobutylene is prepared from the isobutylene obtained in S1; S3, 3,5,5-trimethylhexanal is prepared from the diisobutylene obtained in S2 by hydroformylation reaction; S4, 3,5,5-trimethylhexanal is prepared from the 3,5,5-trimethylhexanal obtained in S3 by oxidation reaction.

[0060] Currently, the industrialized 3,5,5-trimethylhexanoic acid products use commercially available diisobutylene as raw material. The total content of 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene generally does not exceed 99%, which can lead to batch stability issues when applied to the production of refrigeration lubricants.

[0061] This invention uses high-purity tert-butanol as a raw material to prepare 3,5,5-trimethylhexanoic acid, effectively reducing impurity content and ensuring product purity. It exhibits good batch stability in the synthesis of refrigeration lubricating oil base oils, resulting in refrigeration lubricating oils with a long service life.

[0062] 2. The hydroformylation reaction of this invention satisfies (1) 0.5 ≤ y ≤ 1647 × e -0.029x Formula I, where x is the temperature of the hydroformylation reaction; the mass of the metal element in the catalyst is A mg, the mass of diisobutylene is B kg, y = A / B; (2) 0.0048 × e 0.038x ≤z≤0.0059×e 0.058x Formula II, where x is the temperature (°C) of the hydroformylation reaction; and z is the pressure of the synthesis gas in MPaG.

[0063] The catalyst contains precious metal elements, which are expensive. The hydroformylation process of this invention satisfies the above formula, which can effectively balance the amount of precious metal elements used in hydroformylation with the matching of process conditions such as reaction temperature and pressure, thereby improving the utilization efficiency of the catalyst and reducing the amount of by-products generated.

[0064] Satisfying 0.5≤y≤1647×e -0.029x This avoids the problem of low reaction rates and poor reaction results due to insufficient catalyst dosage, which would require longer residence times and consequently larger reactor volumes to obtain the same amount of product. Furthermore, since multiple competing reactions exist during the process, and different reactions have varying sensitivities to catalyst concentration, the following condition must be met: 0.5 ≤ y ≤ 1647 × e -0.029x This can simultaneously avoid situations where the catalyst concentration is too high, causing the rate of increase in side reactions to exceed that of the main reaction, and the proportion of hydrogenation to produce alcohols and alkanes to increase significantly, thereby affecting economic efficiency.

[0065] The reaction rate of diisobutylene hydroformylation is directly related to the reaction pressure. When the reaction pressure is below 0.0048 × e 0.038x At this pressure, the syngas pressure is insufficient to support the rapid formation of the catalytic center and the required reaction rate, resulting in a low reaction rate and poor reaction performance. To obtain the same amount of product, a longer residence time is required, necessitating a larger reactor volume. When the reaction pressure exceeds 0.0059 × e 0.058t Subsequently, due to the excessively high pressure of the syngas, the pressure of carbon monoxide, one of the components of the syngas, also increases. Under these conditions, carbon monoxide will firmly bind to the catalytic center. Under this pressure, there is no space left for the catalytic center to bind with diisobutylene. Diisobutylene is difficult to bind with the catalytic center, resulting in a decrease in the reaction rate and a waste of catalyst.

[0066] 3. The hydroformylation reaction is carried out in the presence of a ligand; preferably, the ligand comprises... At least one of them; R1, R2, and R3 are each independently selected from alkyl groups from C1 to C8.

[0067] The unique chemical structures of these two types of ligands determine the resulting chemical electronic effects and steric hindrance effects. Benzotriazole's special electron-donating properties provide an electron-rich atmosphere for the catalytic center, which, in synergy with the steric hindrance effect, allows the double bond of diisobutylene to bind to the catalytic center from the terminal position. This then completes the insertion of carbon monoxide and the addition of hydrogen, ultimately achieving a ratio of over 100:1 between the target product 3,5,5-trimethylhexanal and the byproduct 2,2,4,4-tetramethyl-1-pentanal. This effectively reduces the number of isomers in the 3,5,5-trimethylhexanal product. Since aldehyde oxidation yields a carboxylic acid with a corresponding structure, the resulting 3,5,5-trimethylhexanoic acid has high purity and low isomer content.

[0068] In the 3,5,5-trimethylhexanoic acid synthesized by the method described in this invention, the mass content of tert-butanol is <0.000005%. Attached Figure Description

[0069] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0070] Figure 1 The gas chromatograms are of 3,5,5-trimethylhexanoic acid synthesized in Example 1 and Comparative Examples 1-3. Detailed Implementation

[0071] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0072] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0073] The reagent raw materials used in the embodiments and comparative examples of this invention are sourced from the following:

[0074] The tert-butanol in Examples 1-3 was obtained from Wanhua Chemical.

[0075] The reagent-grade tert-butanol used in Comparative Example 1 was purchased from Beijing Innovent.

[0076] A15, A35, and A70 resins were purchased from Dow Chemical (Shanghai) Co., Ltd.

[0077] DNW-II type resin was purchased from Dandong Mingzhu Special Resin Co., Ltd.

[0078] Rhodium acetylacetone, cobalt acetate, rhodium acetate, and triphenylphosphine were purchased from Aladdin Reagent Co., Ltd.

[0079] Potassium isononate was prepared in-house by reacting 3,5,5-trimethylhexanoic acid and potassium hydroxide in an equimolar ratio, and was purchased from Latin Reagent Co., Ltd.

[0080] The mixed C4, hydrogen, carbon monoxide, and air were purchased from Yantai Development Zone Shuangfeng Industrial Co., Ltd.

[0081] 2-(2'-hydroxy-5'-tert-octylphenyl)benzotriazole and 2,2'-methylenebis(4-tert-octyl-6-benzotriazolephenol) were purchased from Tianjin Lianlong New Materials Co., Ltd.

[0082] 6-Chlorodibenzo-[d,f][1,3,2]phosphine dioxin and 6-chlorodibenzo[D,F][1,3,2]phosphine dioxin were purchased from Beijing Inokai Technology Co., Ltd.

[0083] Unless otherwise specified, all other reagents and raw materials are commercially available products.

[0084] The conversion rate of reactants and the selectivity of the product were determined by gas chromatography using the internal standard method. The conversion rate was calculated as: (moles of reactant - moles of residue after reaction) / moles of reactant × 100%. The product selectivity was calculated as: (moles of product after reaction) / (moles of reactant - moles of residue after reaction) × 100%. The gas chromatograph was an Agilent Technologies GC-7820, using a 0.25 mm × 30 m DB-5 capillary column. The column oven temperature was programmed to increase from 40 °C to 280 °C at a rate of 15 °C / min and maintained for 5 min. An FID detector was used. The vaporization chamber temperature was 280 °C, the detector temperature was 300 °C, the argon flow rate was 2.1 mL / min, the hydrogen flow rate was 30 mL / min, the air flow rate was 400 mL / min, and the injection volume was 1.0 μL.

[0085] Unless otherwise specified, the reaction conversion rate and selectivity percentages used in the embodiments and comparative examples of this invention are all molar percentages, and the purity content of substances are all mass percentages.

[0086] Example 1

[0087] This embodiment provides a method for preparing 3,5,5-trimethylhexanoic acid, including the following steps:

[0088] (1) Isobutylene was prepared by dehydration reaction of tert-butanol;

[0089] The dehydration reaction was carried out using a 100 mL jacketed fixed-bed reactor, packed with 80 mL of A15 resin catalyst, under a nitrogen atmosphere, at a reaction pressure of 200 kPaA and a temperature of 80 °C. Tert-butanol (containing 0.09% n-propanol, 0.04% isopropanol, 0.02% n-butanol, and 0.01% isobutanol by mass) was added at a liquid hourly space velocity (LHSV) of 2.0 h⁻¹. -1 The flow rate was from bottom to top through the fixed bed. After 10 hours of continuous reaction, samples were taken at the outlet of the fixed bed for analysis. The conversion rate of tert-butanol was 21%, and the selectivity of isobutene was 99.9%.

[0090] The reaction liquid from the fixed bed outlet is fed into a continuous distillation column with an inner diameter of 25 mm. The column is packed with 3 mm * 3 mm triangular spiral packing about 90 cm high. The feed position is 50 cm from the top of the packing. The pressure at the top of the column is 0.4 MPaG. An isobutylene stream at 40 °C is collected. The isobutylene purity is 99.93%, propylene is 132 ppmw, and n-butene content is 112 ppmw.

[0091] (2) Diisobutylene is produced by dimerization of isobutylene:

[0092] The dimerization reaction was carried out using a 100 mL fixed-bed reactor with an external jacket, packed with 80 mL of DNW-II resin catalyst, under a nitrogen atmosphere, at a reaction pressure of 300 kPaA and a temperature of 120 °C. The isobutylene obtained in step (1) was produced at a liquid hourly space velocity of 1.5 h⁻¹. -1 The flow rate was from bottom to top through the fixed bed. After 10 hours of continuous reaction, a sample was taken from the outlet of the fixed bed for analysis. The composition of the stream was 0.36% isobutylene, 4.81% tert-butanol, 0.01% C6 olefins, 0.01% C7 olefins, 75.42% diisobutylene, and 19.39% C9 and above olefins.

[0093] The reaction liquid from the fixed bed outlet is fed into a batch distillation column with an inner diameter of 25 mm, packed with 3 mm * 3 mm θ ring packing about 70 cm high. The fraction collected at a top pressure of 0.1 MPaG and a temperature of 99–104 °C is diisobutylene, which contains 77.31% 2,4,4-trimethyl-1-pentene, 22.63% 2,4,4-trimethyl-2-pentene, 0.03% C8 olefins, 0.01% C6 olefins, and 0.02% C7 olefins.

[0094] (3) Hydroformylation of diisobutylene to 3,5,5-trimethylhexanal:

[0095] Synthesis of ligand A: 5.0 g of 6-chlorodibenzo-[d,f][1,3,2]phosphine dioxane was dissolved in 100 mL of dichloromethane and cooled to -40 °C. A dichloromethane solution of 2,2'-methylenebis(4-tert-octyl-6-benzotriazolephenol) (6.6 g of 2,2'-methylenebis(4-tert-octyl-6-benzotriazolephenol), 50 mL of triethylamine, and 20 mL of dichloromethane) was slowly added dropwise while maintaining the reaction solution temperature at -40 °C. After the addition was complete, the temperature was raised to 25 °C and stirred for 12 h. Then, 150 mL of water was added and stirred for 0.5 h. After standing and separating into layers, the lower oil phase was collected. The dichloromethane was distilled off under reduced pressure at 50 °C, and the solution was recrystallized from n-hexane and dried to obtain ligand A. 8.3g, yield 76.4%, HPLC purity 99%, ICP chloride content <0.01%.

[0096] 100 mg of rhodium acetylacetone, 1.0 g of ligand A, and 500 g of diisobutylene obtained in step (2) were added to a 1 L pressure-resistant stirred reactor. The air in the reactor was replaced with syngas three times. After stirring and heating to 110 °C, syngas with a H2:CO molar ratio of 1:1 was introduced into the reactor until the relative pressure of the reactor was 3.4 MPaG. When the pressure in the reactor decreased, syngas was added until the system pressure was 3.4 MPaG. The temperature and pressure in the reactor were kept constant. After 10 h of reaction, samples were taken for analysis. The reaction conversion rate was 96.8%, and the selectivity of 3,5,5-trimethylhexanal was 95.4%.

[0097] The resulting hydroformylation reaction solution was fed into a batch distillation column with an inner diameter of 25 mm and packed with 3 mm * 3 mm θ ring packing about 90 cm high. The fraction collected at the top of the column at a pressure of 0.1 MPaG and a temperature of 165–168 °C was 3,5,5-trimethylhexanal with a purity of 99.94%.

[0098] (4) Preparation of 3,5,5-trimethylhexanol by oxidation:

[0099] Add 302.6 g of 3,5,5-trimethylhexanal and 3.0 g of potassium isononate catalyst from step (3) to a 0.5 L pressure-resistant stirred reactor, and stir to raise the temperature to 50 °C. Introduce compressed air into the reactor until the relative pressure of the reactor is 0.4 MPaG. When the pressure in the reactor drops, replenish the air until the system pressure is 0.4 MPaG. Keep the temperature and pressure in the reactor constant. After 6 h of reaction, take a sample for analysis. The conversion rate of 3,5,5-trimethylhexanal is 99.8%, and the selectivity of 3,5,5-trimethylhexanoic acid is 98.9%.

[0100] The resulting oxidation reaction solution was fed into a batch distillation column with an inner diameter of 25 mm and packed with 3 mm * 3 mm θ ring packing about 120 cm high. The fraction collected at the top of the column at a pressure of 5 kPaA and a temperature of 145–148 °C was 3,5,5-trimethylhexanoic acid with a content of 99.96%, of which the tert-butanol content was <0.000001%.

[0101] This embodiment also provides a method for synthesizing a base oil for refrigeration lubricating oil, including:

[0102] Steps (1) to (4) were repeated in multiple batches to obtain 30 kg of 3,5,5-trimethylhexanoic acid with a purity of 99.97%. 1186.9 g of the self-made 3,5,5-trimethylhexanoic acid and 254.3 g of pentaerythritol were added to a 2 L three-necked flask. The acid-to-alcohol molar ratio in the raw materials was 7.5:1.0, with an acid excess of 25%. A condenser and a water separator were connected to the flask. The system was purged with nitrogen until the oxygen content was below 200 ppm (volume ratio). The system temperature was raised to 250 °C and maintained at 250 °C. The water and some acid generated in the reaction were distilled out of the three-necked flask as gases, condensed into liquid by the condenser, and then entered the water separator. The acid in the water separator was refluxed back into the three-necked flask to continue the reaction, while the water in the water separator was discharged from the system. After 10 hours of reaction, samples were taken every hour to test the hydroxyl value of the product. The esterification reaction was considered complete when the hydroxyl value of the system was less than 3 mg KOH / g. At 12 hours, the hydroxyl value of the sampled system was 2.63 mg KOH / g. The acid reflux of the separator was turned off, and the vacuum of the system was gradually reduced to <50 kPaA to distill off unreacted 3,5,5-trimethylhexanoic acid, which was discharged from the separator and collected. The system was sampled and analyzed for acid value every 30 minutes. Acid removal was considered complete when the system acid value was less than 0.05 mg KOH / g. After 90 minutes, the system acid value was 0.011 mg KOH / g, indicating complete acid removal. The system temperature was lowered to below 30°C, and nitrogen was introduced to restore atmospheric pressure. 1090.8 g of refrigeration lubricating oil base oil was obtained in the flask. 232.4 g of unreacted 3,5,5-trimethylhexanoic acid, with a purity of 99.94%, was discharged from the separator. This portion of 3,5,5-trimethylhexanoic acid was mixed with fresh 3,5,5-trimethylhexanoic acid and used for the synthesis of the next batch of refrigeration lubricating oil base oil.

[0103] The synthesis process of the above-mentioned refrigeration lubricating oil base oil was repeated 19 times. In each batch, the 3,5,5-trimethylhexanoic acid removed in the previous batch was reacted with fresh 3,5,5-trimethylhexanoic acid. The final batch of refrigeration lubricating oil base oil product had an acid value of 0.012 mg KOH / g, and the purity of the 3,5,5-trimethylhexanoic acid discharged during the deacidification stage was 99.92%.

[0104] Application evaluation of refrigeration lubricants:

[0105] The base oil of the 20th batch of synthesized refrigeration lubricating oil was mixed with 0.5% glycidyl tert-butylbenzoate (0.5% is based on the mass of the refrigeration lubricating oil base oil used, the same below), 0.3% triphenyl thiophosphate, 0.1% di-tert-butyl-p-cresol and 50 ppm benzotriazole and then used as refrigeration lubricating oil for application performance evaluation. The following three groups of experiments were mainly carried out.

[0106] 1) High-temperature chemical stability test

[0107] 50g of refrigeration lubricating oil and 5g of refrigerant R134a were packaged in a 250ml autoclave, heated to 175℃, and the system pressure was 5MPa for 14 days. The evaluation results are shown in Table 1.

[0108] 2) Solution viscosity measurement experiment

[0109] 100g of refrigerant lubricating oil was added to a 250ml autoclave equipped with an online viscometer. The reactor was evacuated to a pressure <0.1kPaA and maintained for 30min. Then, the autoclave temperature was lowered to -10 to -5℃, and refrigerant R134a was injected until the system pressure reached 0.2MPaG. The system was then heated to 100℃ and the solubility viscosity was measured. The evaluation results are shown in Table 1.

[0110] 3) Bench accelerated life test

[0111] Accelerated life durability tests were conducted on refrigeration lubricating oil samples using a household air conditioner compressor model PA290G2CS-4MU1 (manufactured by Midea, 3P model). The test duration was 1000 hours. After the test, the refrigeration lubricating oil before and after the test was compared and analyzed. The compressor was disassembled and observed after the test, and the wear degree of the core components was measured. The evaluation results are shown in Table 1.

[0112] Example 2

[0113] (1) Isobutylene was prepared by dehydration reaction of tert-butanol;

[0114] The dehydration reaction was carried out using a 100 mL jacketed fixed-bed reactor, packed with 80 mL of LD005 resin catalyst, under a nitrogen atmosphere, at a reaction pressure of 30 kPaA and a temperature of 60 °C. Tert-butanol (containing 0.09% n-propanol, 0.04% isopropanol, 0.02% n-butanol, and 0.01% isobutanol by mass) was added at a liquid hourly space velocity (LHSV) of 2.0 h⁻¹. -1 The flow rate was from bottom to top through the fixed bed. After 10 hours of continuous reaction, samples were taken at the outlet of the fixed bed for analysis. The conversion rate of tert-butanol was 16%, and the selectivity of isobutene was 99.9%.

[0115] The reaction liquid from the fixed bed outlet is fed into a continuous distillation column with an inner diameter of 25 mm. The column is packed with 3 mm * 3 mm triangular spiral packing about 90 cm high. The feed position is 50 cm from the top of the packing. The pressure at the top of the column is 0.4 MPaG. An isobutylene stream at 40 °C is collected. The isobutylene purity is 99.93%, propylene is 113 ppmw, and n-butene content is 97 ppmw.

[0116] (2) Diisobutylene isomerization reaction to produce diisobutylene:

[0117] The dimerization reaction was carried out using a 100 mL fixed-bed reactor with an outer jacket, filled with 80 mL of LD009 resin catalyst, under a nitrogen atmosphere, at a reaction pressure of 580 kPaA and a temperature of 170 °C. The isobutylene obtained in step (1) was released at a liquid hourly space velocity of 9 h⁻¹. -1 The flow rate was from bottom to top through the fixed bed. After 10 hours of continuous reaction, a sample was taken from the outlet of the fixed bed for analysis. The composition of the stream was 0.12% isobutylene, 4.45% tert-butanol, 0.01% C6 olefins, 0.01% C7 olefins, 71.46% diisobutylene, and 23.95% C9 and above olefins.

[0118] The reaction liquid from the fixed bed outlet is fed into a batch distillation column with an inner diameter of 25 mm, packed with 3 mm * 3 mm θ ring packing about 70 cm high. The fraction collected at a top pressure of 0.1 MPaG and a temperature of 99–104 °C is diisobutylene, which contains 74.23% 2,4,4-trimethyl-1-pentene, 25.72% 2,4,4-trimethyl-2-pentene, 0.02% C8 olefins, 0.01% C6 olefins, and 0.02% C7 olefins.

[0119] (3) Hydroformylation of diisobutylene to 3,5,5-trimethylhexanal:

[0120] Synthesis of ligand B: 5.0 g of 6-chlorodibenzo-[d,f][1,3,2]phosphorus dioxane was dissolved in 100 mL of dichloromethane and cooled to -40 °C. A dichloromethane solution of 2,2'-methylenebis(4-tert-butyl-6-benzotriazolephenol) (5.5 g of 2,2'-methylenebis(4-tert-butyl-6-benzotriazolephenol), 50 mL of triethylamine, and 20 mL of dichloromethane) was slowly added dropwise while maintaining the reaction solution temperature at -40 °C. After the addition was complete, the temperature was raised to 25 °C and stirred for 12 h. Then, 150 mL of water was added and stirred for 0.5 h. After standing and separating into layers, the lower oil phase was collected. The dichloromethane was distilled off under reduced pressure at 50 °C, and the solution was recrystallized from n-hexane and dried to obtain ligand B. 7.2g, yield 73.8%, HPLC purity 99%, ICP chloride content <0.01%.

[0121] Add 0.26g rhodium acetate, 2.5g ligand B, and 500g diisobutylene obtained in step (2) to a 1L pressure-resistant stirred reactor. After purging with syngas three times, stir and heat to 70℃. Introduce syngas with a H2:CO molar ratio of 2:1 into the reactor until the relative pressure is 0.1MPaG. When the pressure in the reactor drops, replenish the syngas until the system pressure is 0.1MPaG. Keep the temperature and pressure in the reactor constant. After 20 hours of reaction, take a sample for analysis. The reaction conversion rate is 87.1%, and the selectivity of 3,5,5-trimethylhexanal is 97.1%.

[0122] The resulting hydroformylation reaction solution was fed into a batch distillation column with an inner diameter of 25 mm and packed with 3 mm * 3 mm θ ring packing about 90 cm high. The fraction collected at the top of the column at a pressure of 0.1 MPaG and a temperature of 165–168 °C was 3,5,5-trimethylhexanal with a purity of 99.96%.

[0123] (4) Preparation of 3,5,5-trimethylhexanol by oxidation:

[0124] Add 299.8 g of 3,5,5-trimethylhexanal and 3.0 g of potassium isononate from step (3) to a 0.5 L pressure-resistant stirred reactor, and stir to raise the temperature to 90 °C. Introduce compressed air into the reactor until the relative pressure of the reactor is 0.2 MPaG. When the pressure in the reactor drops, replenish the air until the system pressure is 0.2 MPaG. Keep the temperature and pressure in the reactor constant. After 6 h of reaction, take a sample for analysis. The conversion rate of 3,5,5-trimethylhexanal is 99.9%, and the selectivity of 3,5,5-trimethylhexanoic acid is 98.4%.

[0125] The resulting oxidation reaction solution was fed into a batch distillation column with an inner diameter of 25 mm and packed with 3 mm * 3 mm θ ring packing about 120 cm high. The fraction collected at the top of the column at a pressure of 5 kPaA and a temperature of 145–148 °C was 3,5,5-trimethylhexanoic acid with a content of 99.97%, of which the tert-butanol content was <0.000001%.

[0126] Example 3

[0127] (1) Isobutylene was prepared by dehydration reaction of tert-butanol;

[0128] The dehydration reaction was carried out using a 100 mL jacketed fixed-bed reactor, packed with 80 mL of A45 resin catalyst, under a nitrogen atmosphere, at a reaction pressure of 500 kPaA and a temperature of 120 °C. Tert-butanol (containing 0.09% n-propanol, 0.04% isopropanol, 0.02% n-butanol, and 0.01% isobutanol by mass) was added at a liquid hourly space velocity (LHSV) of 9.0 h⁻¹. -1The flow rate was from bottom to top through the fixed bed. After 10 hours of continuous reaction, samples were taken at the outlet of the fixed bed for analysis. The conversion rate of tert-butanol was 31%, and the selectivity of isobutene was 99.7%.

[0129] The reaction liquid from the fixed bed outlet is fed into a continuous distillation column with an inner diameter of 25 mm. The column is packed with 3 mm * 3 mm triangular spiral packing about 90 cm high. The feed position is 50 cm from the top of the packing. The pressure at the top of the column is 0.4 MPaG. An isobutylene stream at 40 °C is collected. The isobutylene purity is 99.93%, propylene is 123 ppmw, and n-butene content is 67 ppmw.

[0130] (2) Diisobutylene isomerization reaction to produce diisobutylene:

[0131] The dimerization reaction was carried out using a 100 mL fixed-bed reactor with an outer jacket, filled with 80 mL of D009 resin catalyst, under a nitrogen atmosphere, at a reaction pressure of 100 kPaA and a temperature of 80 °C. The isobutylene obtained in step (1) was released at a liquid hourly space velocity of 10 h⁻¹. -1 The flow rate was from bottom to top through the fixed bed. After 10 hours of continuous reaction, a sample was taken from the outlet of the fixed bed for analysis. The composition of the stream was 1.32% isobutylene, 4.88% tert-butanol, 0.01% C6 olefins, 0.01% C7 olefins, 82.54% diisobutylene, and 11.24% C9 and above olefins.

[0132] The reaction liquid from the fixed bed outlet is fed into a batch distillation column with an inner diameter of 25 mm, packed with 3 mm * 3 mm θ ring packing about 70 cm high. The fraction collected at a top pressure of 0.1 MPaG and a temperature of 99–104 °C is diisobutylene, which contains 79.11% 2,4,4-trimethyl-1-pentene, 20.85% 2,4,4-trimethyl-2-pentene, and 0.02% C8 olefins, 0.01% C6 olefins, and 0.01% C7 olefins.

[0133] (3) Hydroformylation of diisobutylene to 3,5,5-trimethylhexanal:

[0134] Synthesis of ligand C: 2.5 g of 6-chlorodibenzo[D,F][1,3,2]dioxophospholipid was dissolved in 100 mL of dichloromethane and cooled to -40 °C. A dichloromethane solution of 2-(2'-hydroxy-5'-tert-octylphenyl)benzotriazole (3.3 g of 2-(2'-hydroxy-5'-tert-octylphenyl)benzotriazole, 50 mL of triethylamine, and 20 mL of dichloromethane) was slowly added dropwise while maintaining the reaction solution temperature at -40 °C. After the addition was complete, the temperature was raised to 25 °C and stirred for 12 h. Then, 150 mL of water was added and stirred for 0.5 h. After standing and separating into layers, the lower oil phase was collected. The dichloromethane was distilled off under reduced pressure at 50 °C, and the solution was recrystallized from n-hexane and dried to obtain ligand C. 3.6g, yield 66.9%, HPLC purity 98%, ICP chloride content <0.01%.

[0135] Add 0.008g rhodium octanoate, 0.2g ligand C, and 500g diisobutylene obtained in step (2) to a 1L pressure-resistant stirred reactor. After purging with syngas three times, stir and heat to 180℃. Then, introduce syngas with a H2:CO molar ratio of 0.7:1 into the reactor until the relative pressure of the reactor is 20MPaG. When the pressure in the reactor drops, replenish the syngas until the system pressure is 20MPaG. Keep the temperature and pressure in the reactor constant. After 5 hours of reaction, take a sample for analysis. The reaction conversion rate is 97.7%, and the selectivity of 3,5,5-trimethylhexanal is 96.4%.

[0136] The resulting hydroformylation reaction solution was fed into a batch distillation column with an inner diameter of 25 mm and packed with 3 mm * 3 mm θ ring packing about 90 cm high. The fraction collected at the top of the column at a pressure of 0.1 MPaG and a temperature of 165–168 °C was 3,5,5-trimethylhexanal with a purity of 99.96%.

[0137] (4) Preparation of 3,5,5-trimethylhexanol by oxidation:

[0138] Add 300.1 g of 3,5,5-trimethylhexanal and 3.0 g of potassium isononate from step (3) to a 0.5 L pressure-resistant stirred reactor, and stir to raise the temperature to 150 °C. Introduce compressed air into the reactor until the relative pressure of the reactor is 0.7 MPaG. When the pressure in the reactor drops, replenish the air until the system pressure is 0.7 MPaG. Keep the temperature and pressure in the reactor constant. After reacting for 1 hour, take a sample for analysis. The conversion rate of 3,5,5-trimethylhexanal is 99.9%, and the selectivity of 3,5,5-trimethylhexanoic acid is 97.2%.

[0139] The resulting oxidation reaction solution was fed into a batch distillation column with an inner diameter of 25 mm and packed with 3 mm * 3 mm θ ring packing about 120 cm high. The fraction collected at the top of the column at a pressure of 5 kPaA and a temperature of 145–148 °C was 3,5,5-trimethylhexanoic acid with a content of 99.97%, of which the tert-butanol content was <0.000001%.

[0140] Comparative Example 1

[0141] The experimental procedure for this comparative example is the same as that for Example 1, except that the tert-butanol used is reagent-grade tert-butanol, with a content of 0.31% tert-butanol, 0.02% isopropanol, 0.04% n-butanol, and 0.41% isobutanol.

[0142] The 3,5,5-trimethylhexanoic acid synthesized in step (4) has a purity of 99.06%.

[0143] The base oil of the synthetic refrigeration lubricating oil has an acid value of 0.024 mg KOH / g, and the purity of the 3,5,5-trimethylhexanoic acid discharged during the deacidification stage is 97.84%.

[0144] The same evaluation of refrigeration lubricating oil was conducted, and the results are shown in Table 1.

[0145] Comparative Example 2

[0146] This comparative example provides a method for preparing 3,5,5-trimethylhexanoic acid, comprising the following steps:

[0147] (1) Diisobutylene produced by mixing C4 dimerization

[0148] The dimerization reaction was carried out using a 100 mL jacketed fixed-bed reactor, packed with 80 mL of DNW-II resin catalyst, under a nitrogen atmosphere, at a reaction pressure of 300 kPa and a temperature of 120 °C. A mixture of C4 molecules (composition: 12.9% 1-butene, 16.5% 2-butene, 11.0% isobutene, 0.1% propylene, 0.2% propane, 23.0% isobutane, 35.2% n-butane, 0.2% butadiene, and 0.9% C5 components) was prepared at a liquid hourly space velocity (LHSV) of 1.5 h⁻¹. -1 The flow rate was from bottom to top through the fixed bed. After 10 hours of continuous reaction, samples were taken from the fixed bed outlet for analysis. The stream composition was as follows: 1-butene 8.9%, 2-butene 17.3%, isobutene 0.7%, propylene 0.1%, propane 0.2%, isobutane 22.4%, n-butane 34.1%, butadiene 0.2%, C5 components 0.9%, C6 olefins 0.01%, C7 olefins 0.05%, C8 olefins 13.64%, and C9 and above olefins 1.5%; among which, the C8 olefins contained approximately 71.6% diisobutene.

[0149] The reaction liquid from the fixed bed outlet is fed into a batch distillation column with an inner diameter of 25 mm, packed with 3 mm * 3 mm θ ring packing about 70 cm high. The fraction collected at a top pressure of 0.1 MPaG and a temperature of 99–104 °C is diisobutylene, which contains 75.46% 2,4,4-trimethyl-1-pentene, 20.76% 2,4,4-trimethyl-2-pentene, 3.76% C8 olefins, 0.01% C6 olefins, and 0.01% C7 olefins.

[0150] (2) Preparation of 3,5,5-trimethylhexanoic acid from diisobutylene

[0151] The synthesis of 3,5,5-trimethylhexanoic acid was carried out according to steps (3) to (4) of Example 1, the only difference being that the diisobutylene used was the diisobutylene obtained in step (1) of Comparative Example 2. The obtained 3,5,5-trimethylhexanoic acid content was 98.95%.

[0152] The synthesis and application evaluation methods of the comparative example refrigeration lubricating oil base oil are the same as those in Example 1. The synthesized refrigeration lubricating oil base oil has an acid value of 0.029 mg KOH / g, and the purity of 3,5,5-trimethylhexanoic acid discharged during the deacidification stage is 95.47%. The evaluation results of the refrigeration lubricating oil are shown in Table 1.

[0153] Comparative Example 3

[0154] This comparative example provides a method for preparing 3,5,5-trimethylhexanoic acid, comprising the following steps:

[0155] (1) Methyl tert-butyl ether is methanol-dehydrogenated to isobutylene

[0156] The dehydration reaction was carried out using a 100 mL fixed-bed reactor with an external jacket, packed with 80 mL of LA15 resin catalyst, under a nitrogen atmosphere, at a reaction pressure of 0.5 MPaG and a temperature of 90 °C. Methyl tert-butyl ether (containing 0.32% methyl isobutyl ether) was added at a liquid hourly space velocity (LHSV) of 2.0 h⁻¹. -1 The flow rate was from bottom to top through the fixed bed. After 10 hours of continuous reaction, samples were taken at the outlet of the fixed bed for analysis. The conversion rate of methyl tert-butyl ether was 21%, and the selectivity of isobutylene was 99.8%.

[0157] The reaction liquid from the fixed bed outlet is fed into a continuous distillation column with an inner diameter of 25 mm. The column is packed with 3 mm * 3 mm triangular spiral packing about 90 cm high. The feed position is 50 cm from the top of the packing. The pressure at the top of the column is 0.4 MPaG. An isobutylene stream at 40 °C is collected. The isobutylene purity is 99.79%, and the propylene purity is 0.21%.

[0158] The subsequent synthesis and application evaluation of 3,5,5-trimethylhexanoic acid and refrigeration lubricating oil base oil were the same as in Example 1, except that the isobutylene was sourced from step (1) of Comparative Example 3.

[0159] The purity of the 3,5,5-trimethylhexanoic acid synthesized in this comparative example is 98.95%.

[0160] The base oil of the refrigeration lubricating oil synthesized in this comparative example has an acid value of 0.036 mg KOH / g, and the purity of the 3,5,5-trimethylhexanoic acid discharged during the deacidification stage is 96.37%.

[0161] Gas chromatograms of 3,5,5-trimethylhexanoic acid synthesized in Example 1 and Comparative Examples 1-3 are compared. Figure 1 As shown. By Figure 1 It can be seen that the 3,5,5-trimethylhexanoic acid obtained by this invention has high purity.

[0162] The same comparative example was used to evaluate the refrigeration lubricant, and the evaluation results are shown in Table 1.

[0163] Table 1 Synthesis and Application Evaluation of Base Oils for Refrigeration Lubricating Oils

[0164]

[0165] As can be seen from the examples and comparative examples, the synthesis method of 3,5,5-trimethylhexanoic acid described in this invention can effectively control the impurity content and synthesize high-purity 3,5,5-trimethylhexanoic acid. Even after multiple reuses during the synthesis of refrigeration lubricating oil base oil, the recovered 3,5,5-trimethylhexanoic acid still maintains high purity. The synthesized refrigeration lubricating oil base oil has a low acid value and high product quality. Refrigeration lubricating oil formulated with this base oil exhibits stable properties, good chemical stability, and excellent lubrication performance. Long-term bench tests show minimal impact on the compressor, effectively extending the compressor's service life. It possesses significant application advantages and has industrial application value.

[0166] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing 3,5,5-trimethylhexanoic acid, characterized in that, Includes the following steps: S1. Isobutylene is prepared by dehydrating tert-butanol; Of the tert-butanol used, the mass content of n-propanol is <0.3%, the mass content of isopropanol is <0.3%, the mass content of n-butanol is <0.3%, and the mass content of isobutanol is <0.3%. S2. Prepare diisobutylene using the isobutylene obtained in S1; S3. 3,5,5-Trimethylhexanol was prepared by hydroformylation of diisobutylene obtained by S2. S4. 3,5,5-Trimethylhexanol obtained from S3 is oxidized to prepare 3,5,5-trimethylhexanoic acid.

2. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 1, characterized in that, S3 comprises: mixing diisobutylene and a catalyst, carrying out a hydroformylation reaction under a syngas atmosphere, and then separating by distillation to obtain 3,5,5-trimethylhexanal.

3. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 2, characterized in that, The catalyst is a metal salt, and the metal element in the metal salt is selected from one or more of rhodium, cobalt and iridium.

4. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 3, characterized in that, The catalyst is a rhodium salt, which includes one or more of rhodium acetate, rhodium octanoate, rhodium naphthenate, and rhodium acetylacetonate.

5. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 2, characterized in that, S3 satisfies at least one of the following conditions: (1) The synthesis gas includes H2 and CO, and the molar ratio of the two is (0.6~2):1; (2) The temperature of the hydroformylation reaction is 40–200 °C; (3) The pressure of the synthesis gas is 0.05~30 MPaG; (4) The purity of the obtained 3,5,5-trimethylhexanal is >99.9wt%.

6. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 2, characterized in that, The hydroformylation reaction satisfies at least one of the following conditions: (1) 0.5 ≤ y ≤ 1647 × e -0.029x Equation I In Formula I, x is the temperature of the hydroformylation reaction, in °C; The mass of the metal element in the catalyst is A mg, the mass of diisobutylene is B kg, and y = A / B. (2) 0.0048×e 0.038x ≤z≤0.0059×e 0.058x Formula II In Formula II, x is the temperature of the hydroformylation reaction, in °C; z is the pressure of the synthesis gas, in MPaG.

7. The method for preparing 3,5,5-trimethylhexanoic acid according to any one of claims 1-6, characterized in that, The hydroformylation reaction is carried out in the presence of a ligand.

8. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 7, characterized in that, The ligands include and At least one of them; R1, R2, and R3 are each independently selected from alkyl groups from C1 to C8.

9. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 8, characterized in that, R1, R2, and R3 are each independently selected from tert-butyl or octyl.

10. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 8, characterized in that, R2 and R3 are the same group.

11. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 8, characterized in that, The mass of the ligand is 0.02% to 0.5% of the mass of diisobutylene.

12. The method for preparing 3,5,5-trimethylhexanoic acid according to any one of claims 1-6, characterized in that, S1 comprises: isobutylene prepared by catalytic dehydration of tert-butanol through a first acidic catalyst and distillation separation.

13. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 12, characterized in that, The first acidic catalyst is an acidic cation exchange resin.

14. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 12, characterized in that, The dehydration reaction is carried out at a temperature of 60–120°C and a pressure of 20–500 kPa.

15. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 12, characterized in that, The liquid hourly space velocity (LHSV) of the dehydration reaction is 2–10 h⁻¹. -1 .

16. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 12, characterized in that, The isobutylene produced by S1 contains propylene content <300ppmw and n-butene content <500ppmw.

17. The method for preparing 3,5,5-trimethylhexanoic acid according to any one of claims 1-6, characterized in that, S2 includes: performing a dimerization reaction on isobutylene obtained from S1 and separating and preparing diisobutylene.

18. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 17, characterized in that, The catalyst for the dimerization reaction is a second acidic catalyst.

19. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 18, characterized in that, The second acidic catalyst is an acidic cation exchange resin.

20. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 17, characterized in that, The separation is a distillation separation.

21. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 17, characterized in that, The dimerization reaction is carried out at a temperature of 60–180°C and a pressure of 100–600 kPa.

22. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 17, characterized in that, The liquid hourly space velocity (LHSV) of the dimerization reaction is 1–10 h⁻¹. -1 .

23. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 17, characterized in that, The total content of 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene in the prepared diisobutylene is >99.9%.

24. The method for preparing 3,5,5-trimethylhexanoic acid according to any one of claims 1-6, characterized in that, In step S4, the oxidation reaction is carried out in an air or oxygen atmosphere.

25. The method for preparing 3,5,5-trimethylhexanoic acid according to claim 24, characterized in that, The oxidation reaction is carried out at a temperature of 30–150°C, a pressure of 0–0.7 MPaG, and a time of 1–20 h.

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