Method for preparing hindered phenolic antioxidant by using ionic liquid catalyst

Abstract

This invention discloses a method for preparing hindered phenolic antioxidants using ionic liquid catalysis, relating to the field of hindered phenolic antioxidant preparation technology. The method involves mixing an imidazolium zwitterionic precursor with a Brønsted proton donor to obtain an ionic liquid catalytic system. By combining the imidazolium nitrogen atom containing both ether oxygen substituents and sulfonate substituents with a substoichiometric Brønsted proton donor, and coordinating the pre-mixing of p-cresol with the ionic liquid catalytic system before isobutylene enters, it facilitates the continuous ortho-tert-butylation of p-cresol and inhibits non-target oligomerization of isobutylene in the later stages of the reaction. Simultaneously, the resulting ionic liquid catalytic phase can be separated and recycled, improving the selectivity of 2,6-di-tert-butyl-4-methylphenol and reducing the content of C8 and C12 isobutylene oligomers.

Description

Technical Field

[0001] This invention relates to the field of hindered phenolic antioxidant preparation technology, and in particular to a method for preparing hindered phenolic antioxidants by ionic liquid catalysis. Background Technology

[0002] 2,6-Di-tert-butyl-4-methylphenol (BHT) is a typical hindered phenolic antioxidant, widely used in polymer materials, lubricating oils, and fuel oils. In recent years, sulfonic acid-functionalized imidazolium salts, imidazolium hydrogen sulfates, and sulfonic acid-based zwitterionic ionic liquids have been reported for the tert-butylation reaction of p-cresol with isobutylene or tert-butanol. Researchers have explored various methods, such as adjusting the length of the cationic side chain, introducing sulfonate groups, changing the Brønsted acid equivalent, and loading or supporting, to improve catalytic activity, selectivity, and recyclability, and to reduce the oligomerization side reaction of isobutylene.

[0003] However, existing methods for preparing 2,6-di-tert-butyl-4-methylphenol by tert-butylation of p-cresol and isobutylene using ionic liquid catalysis typically employ a Brønsted acidic system. The acid center maintains catalytic activity towards isobutylene throughout the reaction. In the later stages of the reaction, as p-cresol and 2-tert-butyl-4-methylphenol gradually convert to the target product, the concentration of phenolic species capable of further ortho-tert-butylation decreases significantly, while isobutylene continues to be introduced. At this point, the non-target oligomerization catalysis of isobutylene by the acid center does not decrease synchronously, leading to a significant increase in C8 and C12 oligomers in the later stages of the reaction and a decrease in the selectivity of 2,6-di-tert-butyl-4-methylphenol. Summary of the Invention

[0004] In view of the aforementioned existing problems, the present invention is proposed.

[0005] This invention provides a method for preparing hindered phenolic antioxidants using ionic liquid catalysis, wherein the hindered phenolic antioxidant is 2,6-di-tert-butyl-4-methylphenol, characterized in that it comprises: The imidazolium zwitterion precursor was mixed with the Brønsted proton donor to obtain an ionic liquid catalytic system. One imidazolium nitrogen atom of the imidazolium zwitterionic precursor is attached to an ether-containing oxygen substituent, and the other imidazolium nitrogen atom is attached to a sulfonate substituent. Each mole of the imidazolium zwitterionic precursor contains one mole of sulfonate substituent. The amount of the Brønsted proton donor added, in acid equivalents, is 0.40 to 0.80 equivalents per mole of the imidazolium zwitterionic precursor; Before introducing isobutylene, p-cresol and the ionic liquid catalytic system are premixed to obtain a premixed reactant containing p-cresol and the ionic liquid catalytic system. Isobutylene is continuously introduced into the premixed reactants. No Brønsted proton donor is added during the reaction. This causes p-cresol to undergo a first ortho-tert-butylation to generate 2-tert-butyl-4-methylphenol. The resulting 2-tert-butyl-4-methylphenol then undergoes another ortho-tert-butylation to generate 2,6-di-tert-butyl-4-methylphenol. During the reaction, as the content of p-cresol and 2-tert-butyl-4-methylphenol decreases, the effective catalytic conversion degree of isobutylene participating in the target tert-butylation reaction of the reaction system decreases. Stop the flow of isobutylene, perform gas-liquid separation on the reaction material, and separate the liquid phase material into a crude product containing 2,6-di-tert-butyl-4-methylphenol and an ionic liquid catalytic phase.

[0006] Preferably, the ether-containing oxygen substituent is -(CH2)mOR 1 The sulfonate substituent is -(CH2)n-SO3 - Where m is 2–4, n is 3–4, and R 1 It is a C1 to C3 alkyl group.

[0007] Preferably, the imidazodium zwitterionic precursor is a 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazodium inner salt.

[0008] Preferably, the Brønsted proton donor is selected from one or two of sulfuric acid and methanesulfonic acid; The amount of the Brønsted proton donor added is 0.50 to 0.65 equivalents per mole of the imidazolonium zwitterionic precursor, in acid equivalents.

[0009] Preferably, the imidazolium zwitterionic precursor is mixed with the Brønsted proton donor at 50–80 °C for 0.5–2 h to obtain a homogeneous ionic liquid catalytic system.

[0010] Preferably, the amount of the ionic liquid catalytic system is 5% to 20% of the mass of p-cresol, and after the p-cresol and the ionic liquid catalytic system are pre-contacted at 50 to 65°C for 10 to 30 minutes, isobutylene is introduced into the resulting reaction system.

[0011] Preferably, the molar ratio of p-cresol to isobutylene is 1:2.0 to 2.3, the reaction temperature is 50 to 70°C, and the reaction gauge pressure is 0.05 to 0.30 MPa.

[0012] Preferably, the molar ratio of p-cresol to isobutylene is 1:2.05 to 2.20, the reaction temperature is 55 to 65°C, and the reaction gauge pressure is 0.10 to 0.25 MPa; After the reaction was completed, the crude product was purified by removing light radicals and crystallization to obtain 2,6-di-tert-butyl-4-methylphenol.

[0013] Preferably, the ionic liquid catalytic phase, after being subjected to depressurized devolatilization, is used as an ionic liquid catalytic system for the next batch of tert-butylation reaction of p-cresol and isobutylene.

[0014] The beneficial effects of this invention are as follows: This invention combines a zwitterionic precursor containing ether oxygen substituents and sulfonate substituents on the nitrogen atom of imidazolium with a substoichiometric Brønsted proton donor, and with the pre-mixing of p-cresol and the ionic liquid catalytic system before isobutylene enters, which is beneficial for the continuous ortho-tert-butylation of p-cresol and suppresses the non-target oligomerization of isobutylene in the later stage of the reaction; at the same time, the ionic liquid catalytic phase obtained after the reaction can be separated and recycled, which improves the selectivity of 2,6-di-tert-butyl-4-methylphenol and reduces the content of C8 and C12 isobutylene oligomers. Attached Figure Description

[0015] Figure 1 This is a flowchart of the method for preparing hindered phenolic antioxidants using ionic liquid catalysis in the examples.

[0016] Figure 2 The image shows the 1H NMR spectrum of the inner salt of 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazolium in the example.

[0017] Figure 3 The image shows the carbon NMR spectrum of the inner salt of 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazolium in the example.

[0018] Figure 4 The image shows the 1H NMR spectrum of the inner salt of 1-(3-ethoxypropyl)-3-(4-sulfonylbutyl)imidazolium in the example.

[0019] Figure 5 The image shows the carbon NMR spectrum of the inner salt of 1-(3-ethoxypropyl)-3-(4-sulfonylbutyl)imidazolium in the example.

[0020] Figure 6 The image shows the 1H NMR spectrum of the inner salt of 1-(4-propoxybutyl)-3-(3-sulfopropyl)imidazolium in the example.

[0021] Figure 7 The image shows the carbon NMR spectrum of the inner salt of 1-(4-propoxybutyl)-3-(3-sulfopropyl)imidazolium in the example.

[0022] Figure 8 The figure shows the reaction process curve in the example.

[0023] Figure 9This is a performance graph for cyclical use in the example. Detailed Implementation

[0024] To make the above-mentioned objectives, features and advantages of the present invention more readily understood, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0025] The attached diagram illustrates that... Figures 2-4 The images are overlaid; the top one shows the 13-carbon graph, and the bottom one shows the 1-hydrogen graph.

[0026] In the following preparation examples, embodiments, and comparative examples of this invention, the imidazole used has a purity of not less than 99.0%, the haloalkyl ethers and 1,3-propanesulfonyl lactone and 1,4-butanesulfonyl lactone have a purity of not less than 98.0%, the p-cresol has a purity of not less than 99.0%, the isobutylene has a purity of not less than 99.5%, the methanesulfonic acid has a purity of not less than 99.0%, the sulfuric acid has a mass fraction of 98%, and the organic solvents used are analytical grade or anhydrous reagents. Except for the imidazole-onium zwitterionic precursor prepared according to the preparation example and the structural control precursor prepared according to Comparative Example 4, all other raw materials are commercially available.

[0027] The raw materials used to prepare imidazolium zwitterionic precursors and ionic liquid catalytic systems are dried or dehydrated before use. The resulting imidazolium zwitterionic precursors are dried under reduced pressure to constant weight and then sealed for storage. p-cresol is dehydrated under reduced pressure before use. Isobutylene is introduced into the reactor via a gas mass flow controller.

[0028] The acid equivalent referred to in this invention is the ratio of the amount of acid protons obtained by converting the Brønsted proton donor according to the nominal number of dissociable acid protons to the amount of imidazolium zwitterion precursor. Methanesulfonic acid is calculated as providing 1 acid equivalent per mole, and sulfuric acid is calculated as providing 2 acid equivalents per mole. When the two acids are used in combination, the values ​​are converted separately and summed, and the feed amount is corrected according to the actual mass fraction. This definition serves as a unified feed measurement standard.

[0029] The homogeneous ionic liquid catalytic system referred to in this invention refers to a system in which an imidazolium zwitterionic precursor is mixed with a specified acid equivalent of a Brønsted proton donor to form a continuous liquid phase without visible solid precipitation or macroscopic stratification. The phenolic species that can be further tert-butylated refer to p-cresol and 2-tert-butyl-4-methylphenol. The isobutylene oligomers refer to C8 and above components formed by non-target oligomerization of isobutylene, with C8 and C12 components being the main detection targets.

[0030] p-Cresol, 2-tert-butyl-4-methylphenol, and 2,6-di-tert-butyl-4-methylphenol were quantified using GC-FID. The chromatographic column was a 30m×0.25mm×0.25μm 5% phenyl-95% dimethyl polysiloxane capillary column. The injection port temperature was 250℃, and the detector temperature was 280℃. The column temperature was increased from 80℃ for 2 min, then increased to 180℃ at 10℃ / min, and then increased to 260℃ at 5℃ / min and held for 5 min. Each target component was quantified using the corresponding external standard, and the peak assignment was determined.

[0031] In this invention, the same sample processing method is used for sampling of reaction solutions involved in reaction progress detection, endpoint continuation reaction, reaction recovery by adding p-cresol, and recycling tests. Each time, about 0.20 g of reaction solution is taken and immediately added to 5 mL of pre-cooled acetonitrile solution containing 0.5 wt% triethylamine and thoroughly shaken to neutralize the acidic components in the sample and inhibit the acid-catalyzed reaction that continues after sampling. Then, the sample is filtered and chromatographically analyzed. The components of C8 and C12 oligomers are first identified by GC-MS, and then calibrated by GC-FID using diisobutylene and triisobutylene standards, respectively. The sum of the corrected contents of each isomer within the same carbon number range is taken as the content of this type of oligomer.

[0032] The conversion rate of p-cresol was calculated as the percentage of the difference between the initial amount of substance and the amount of substance remaining at detection, relative to the initial amount of substance. BHT selectivity was calculated as the percentage of BHT produced relative to the amount of p-cresol converted. The structure of the imidazolium zwitterionic precursor was confirmed using ¹H NMR, ¹³C NMR, and HRMS, and the water content was determined using the Karl Fischer method. Catalytic activity response was evaluated through continuous sampling, endpoint extension reaction, and p-cresol addition recovery reaction; specific conditions are described in the corresponding examples.

[0033] Preparation Example 1 In this preparation example, 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazolium inner salt was prepared. Under nitrogen protection, 4.40 g of sodium hydride (60% by mass) and 30 mL of anhydrous tetrahydrofuran were added to a 500 mL three-necked flask. 6.81 g (0.100 mol) of imidazole was dissolved in 40 mL of anhydrous tetrahydrofuran and slowly added dropwise to the flask at 0–5 °C. After the addition was complete, the mixture was stirred at 20–25 °C for 2 h. Then, 14.59 g (0.105 mol) of 1-bromo-2-methoxyethane was added, and the temperature was raised to 65–70 °C for 8 h. After the reaction was completed, the mixture was cooled to room temperature, the solid was removed by filtration, and the residue was washed twice with anhydrous tetrahydrofuran. The filtrates were combined, and the solvent was removed under reduced pressure. The crude product was then distilled under reduced pressure to obtain 4.56 g of 1-(2-methoxyethyl)imidazolium, with a yield of 36.1%.

[0034] Weigh 4.03 g (31.9 mmol) of the obtained 1-(2-methoxyethyl)imidazolium and add it to a 100 mL reaction flask. Add 40 mL of anhydrous ethyl acetate, and add 3.72 g (30.5 mmol) of 1,3-propanesulfonyl lactone under nitrogen protection and stirring. Reflux at 77–80 °C for 20 h. After the reaction is complete, cool to 20–25 °C and filter to collect the precipitate. Wash the obtained solid three times with 10 mL of ethyl acetate each time, and recrystallize with anhydrous ethanol. Dry under reduced pressure at 60 °C for 12 h to constant weight to obtain 5.58 g of 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazolium onium salt, with a yield of 73.7% based on 1,3-propanesulfonyl lactone.

[0035] The obtained 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazolium inner salt was structurally confirmed by ¹H NMR, ¹³C NMR and HRMS according to the aforementioned detection methods. The target structure was confirmed by the characteristic signals of the imidazolium ring proton, methoxy group, ether oxygen adjacent methylene group, imidazolium nitrogen adjacent methylene group and sulfonate adjacent methylene group, with one imidazolium nitrogen atom attached to 2-methoxyethyl and the other imidazolium nitrogen atom attached to 3-sulfopropyl.

[0036] Preparation Example 2 This preparation example prepares 1-(3-ethoxypropyl)-3-(4-sulfonylbutyl)imidazolium inner salt. The preparation method for the N-substituted imidazolium intermediate in Preparation Example 1 is followed, except that 1-bromo-2-methoxyethane is replaced with 17.54 g (0.105 mol) of 1-bromo-3-ethoxypropane, and the reaction time at 65–70 °C is adjusted to 10 h. After the same cooling, filtration, washing, solvent removal, and vacuum distillation, 7.42 g of 1-(3-ethoxypropyl)imidazolium is obtained, with a separation yield of 48.1%.

[0037] 4.63 g (30.0 mmol) of the obtained 1-(3-ethoxypropyl)imidazolium was weighed and added to a 100 mL reaction flask. 45 mL of anhydrous acetonitrile was added, and 3.95 g (29.0 mmol) of 1,4-butanesulfonyl lactone was added under nitrogen protection and stirring. The reaction was carried out at 80–82 °C for 24 h. After the reaction was complete, the acetonitrile was removed under reduced pressure. The residue was washed three times with 15 mL of ethyl acetate each time, then dissolved in anhydrous ethanol, and ethyl acetate was added to precipitate the product. After standing at 5 °C for 2 h, the product was filtered. The obtained solid was dried at 60 °C under reduced pressure for 12 h to constant weight, yielding 5.71 g of 1-(3-ethoxypropyl)-3-(4-sulfonyl)imidazolium onium salt, with a yield of 67.8% based on 1,4-butanesulfonyl lactone.

[0038] The obtained 1-(3-ethoxypropyl)-3-(4-sulfonylbutyl)imidazolium inner salt was structurally characterized according to the aforementioned detection method. Based on the NMR signals corresponding to the imidazolium ring, ethoxy group, ether-containing oxygen side chain composed of three methylene groups, and sulfonate-type side chain composed of four methylene groups, and the target molecular ion signal, the target zwitterion structure with m=3, n=4, and R¹=ethyl was confirmed.

[0039] Preparation Example 3 This preparation example prepares 1-(4-propoxybutyl)-3-(3-sulfopropyl)imidazolium inner salt. The preparation method for the N-substituted imidazolium intermediate in Preparation Example 1 is followed, except that 1-bromo-2-methoxyethane is replaced with 20.49 g (0.105 mol) of 1-bromo-4-propoxybutane, and the reaction time at 65–70 °C is adjusted to 12 h. After the same cooling, filtration, washing, solvent removal, and vacuum distillation, 9.34 g of 1-(4-propoxybutyl)imidazolium is obtained, with a separation yield of 51.2%.

[0040] 5.47 g (30.0 mmol) of the obtained 1-(4-propoxybutyl)imidazolium was weighed and added to a 100 mL reaction flask, followed by 40 mL of anhydrous acetonitrile. Under nitrogen protection and stirring, 3.54 g (29.0 mmol) of 1,3-propanesulfonyl lactone was added, and the reaction was carried out at 80–82 °C for 22 h. After the reaction was completed, the acetonitrile was removed under reduced pressure. The residue was washed three times with 15 mL of ethyl acetate each time, then dissolved in anhydrous ethanol and crystallized by adding ethyl acetate. After standing at 5 °C for 2 h, the residue was filtered. The obtained solid was dried at 60 °C under reduced pressure for 12 h to constant weight, yielding 6.16 g of 1-(4-propoxybutyl)-3-(3-sulfopropyl)imidazolium onium salt, with a yield of 69.8% based on 1,3-propanesulfonyl lactone.

[0041] The obtained 1-(4-propoxybutyl)-3-(3-sulfopropyl)imidazolium inner salt was structurally characterized according to the aforementioned detection method. Based on the NMR signals corresponding to the imidazolium ring, propoxy group, ether-containing oxygen side chain composed of four methylene groups, and sulfonate-type side chain composed of three methylene groups, and the target molecular ion signal, the target zwitterion structure with m=4, n=3, and R¹=propyl was confirmed.

[0042] Table 1. Preparation results of different imidazolium zwitterionic precursors

[0043] It should be noted that in general sulfonates or sulfonate-substituted alkyl compounds... 1 In the 1H NMR spectrum, CH2-SO3 directly linked to the sulfonate group - The signal is commonly found around δ 2.8–3.0. In Example 1 of this invention, the 2H signal at δ 2.39 was attributed to CH2-SO3.- This attribution is not determined solely based on a single chemical shift value, but rather by combining the complete molecular composition of the target compound, the integral relationships of each proton, the number of carbon atoms in the side chains, and so on. 13 The results of combined C NMR and HRMS analysis determined that the 3-sulfonic acid propyl side chain consists of N-CH2, CH2 in the chain, and CH2-SO3. - The N-CH2 signal, composed of three methylene groups directly linked to the imidazolium nitrogen, is contained within the 4H overlap signal at δ 4.38–4.28. The CH2 in the chain corresponds to the 2H signal at δ 2.08, and the remaining 2H signal at δ 2.39 is thus attributed to CH2-SO3. - The sum of the above signal and the integral of the 2-methoxyethyl side chain and the imidazolium ring proton is 16H, which, along with C9H, results in 16H. 16 The molecular composition of N2O4S is consistent.

[0044] CH2-SO3 - The actual chemical shift is influenced by factors such as solvent polarity, sample concentration, ion association state, adjacent ionic groups, and molecular conformation. Therefore, the signal in a specific compound can deviate from the common empirical range. The zwitterion obtained in this invention contains both imidazolium positively charged centers and sulfonate negatively charged centers. The local ionic environment formed in DMSO-d6 is different from that of ordinary neutral alkyl sulfonates. Therefore, δ2.8–3.0 is not used as the criterion for CH2-SO3. - The sole basis for signal attribution.

[0045] Further integration 13 C NMR showed that in Preparation Example 1, a carbon signal corresponding to the methylene group adjacent to the sulfonate group appeared at δ 47.3, and the resulting product... 13 Nine signals corresponding to the nine carbon atoms of the target molecule were observed by C NMR. Simultaneously, HRMS measured [M+H]⁺ at m / z 249.0906, consistent with the theoretical value of 249.0904. Therefore, even if CH₂-SO₃... - of 1 The difference in H chemical shifts from the empirical ranges of some common sulfonates does not affect the overall confirmation of the inner salt structure of 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazolium.

[0046] Example 1 4.324 g (17.41 mmol) of the 1-(2-methoxyethyl)-3-(3-sulfonic acid propyl)imidazolium inner salt obtained in Preparation Example 1 was weighed and placed in a reaction flask equipped with a mechanical stirrer and temperature control device. Under nitrogen protection, the temperature was raised to 50 °C, and 0.676 g of methanesulfonic acid with a purity of 99.0% was slowly added. According to the aforementioned acid equivalent calculation method, the amount of methanesulfonic acid added was 0.40 acid equivalents per mole of imidazolium zwitterionic precursor. After the addition was completed, the mixture was stirred at 50 °C for 0.5 h to obtain 5.00 g of an ionic liquid catalytic system in a continuous liquid phase without visible solid precipitation or macroscopic stratification.

[0047] 100.0 g of p-cresol and 5.00 g of the above-mentioned ionic liquid catalytic system were added to a 1 L pressure-resistant reactor equipped with a jacket, mechanical stirrer, gas mass flow controller, and back pressure regulating device. After purging the atmosphere inside the reactor three times with nitrogen, the temperature was raised to 50 °C under stirring at 600 r / min and pre-contacted for 10 min. Subsequently, 103.77 g (1.849 mol) of isobutylene was continuously introduced through the inlet pipe immersed in the liquid phase, so that the molar ratio of p-cresol to isobutylene was 1:2.00. The isobutylene feeding time was 5.0 h and the average feeding rate was 20.75 g / h. The reaction temperature was maintained at 50 ± 1 °C and the gauge pressure inside the reactor was maintained at 0.05 ± 0.01 MPa. No methanesulfonic acid or other Brønsted proton donors were added during the isobutylene introduction.

[0048] Feeding was stopped after a total of 103.77 g of isobutylene was added. Unreacted isobutylene in the reactor was transferred to a low-temperature recovery device and the pressure in the reactor was reduced to below 0.01 MPa. The resulting liquid phase was allowed to stand at 80°C for 25 min and then separated to obtain 187.9 g of hindered phenol crude product phase and 4.62 g of ionic liquid catalytic phase. The crude product phase was subjected to vacuum deionization at 2.5 kPa, with the temperature gradually increased from 90°C to 110°C for 45 min. The deionized material was then dissolved in anhydrous ethanol at 30% of its mass at 75°C. The material was cooled to 15°C at a cooling rate of 0.5°C / min and held at that temperature for 1 h. The resulting crystals were filtered, washed with anhydrous ethanol at 15°C, and then dried under reduced pressure at 45°C for 8 h.

[0049] Analyzing according to the aforementioned detection method, the conversion rate of p-cresol at the end of the reaction was 95.1%, the selectivity of 2,6-di-tert-butyl-4-methylphenol was 91.8%, the content of 2-tert-butyl-4-methylphenol in the organic components was 3.42 wt%, and the total content of C8 and C12 isobutylene oligomers was 1.36 wt%. After crystallization and drying, 174.6 g of 2,6-di-tert-butyl-4-methylphenol was obtained, with a product purity of 99.18%. Based on the theoretical mass of initial p-cresol completely converted to 2,6-di-tert-butyl-4-methylphenol and corrected for the purity of the obtained product, the separation yield was 85.0%.

[0050] Example 2 10.614 g (42.74 mmol) of the 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazolium inner salt obtained in Preparation Example 1 was weighed and placed in a reaction flask equipped with a mechanical stirrer and temperature control device. Under nitrogen protection, the temperature was raised to 50 °C. 1.245 g of 99.0% pure methanesulfonic acid and 0.642 g of 98% sulfuric acid were added slowly in sequence. The methanesulfonic acid provided 0.30 acid equivalents and the sulfuric acid provided 0.30 acid equivalents per mole (calculated as 2 acid equivalents per mole), so that the total amount of Brønsted proton donor added was 0.60 acid equivalents per mole of imidazolium zwitterionic precursor. After the addition was completed, the temperature was raised to 65 °C and stirred for 1.25 h to obtain 12.50 g of a homogeneous ionic liquid catalytic system.

[0051] Add 100.0g of p-cresol and 12.50g of the above-mentioned ionic liquid catalytic system to a 1L pressure-resistant reactor equipped with a jacket, mechanical stirrer, gas mass flow controller, sampling port and back pressure regulating device. Replace the atmosphere inside the reactor with nitrogen three times. Heat to 60℃ under stirring at 600r / min and maintain for 20min to ensure that p-cresol is fully pre-contacted with the ionic liquid catalytic system before isobutylene enters the reaction system.

[0052] After the pre-contact is completed, 111.55 g (1.988 mol) of isobutylene is continuously introduced through a gas mass flow controller to make the molar ratio of p-cresol to isobutylene 1:2.15. The isobutylene feeding time is 4.0 h and the average feeding rate is 27.89 g / h. The gauge pressure inside the reactor is maintained at 0.20 ± 0.01 MPa and the reaction temperature is maintained at 60 ± 1 °C through a back pressure regulating device. During the reaction, samples are taken every 30 min and the acid catalytic reaction is immediately terminated and chromatographic analysis is performed after sampling, in accordance with the aforementioned method. No methanesulfonic acid, sulfuric acid or other Brønsted proton donors are added during the entire isobutylene feeding stage.

[0053] After adding a total of 111.55 g of isobutylene, the feeding was stopped. The remaining isobutylene in the reactor was introduced into a low-temperature recovery device and the pressure was reduced to below 0.01 MPa. The resulting liquid phase was allowed to stand at 80°C for 30 min and then separated to obtain 203.8 g of hindered phenol crude product phase and 11.94 g of ionic liquid catalytic phase. The crude product phase was heated from 90°C to 112°C under reduced pressure for 50 min at 2.0 kPa to remove light components, resulting in 198.7 g of light-removed material. 59.6 g of anhydrous ethanol was added to the material and dissolved completely at 75°C. The material was cooled to 15°C at a cooling rate of 0.45°C / min and held at that temperature for 1.5 h. The resulting crystals were filtered, washed with anhydrous ethanol at 15°C, and then dried under reduced pressure at 45°C for 8 h.

[0054] Analyzing according to the aforementioned detection method, the conversion rate of p-cresol at the end of the reaction was 99.3%, the selectivity of 2,6-di-tert-butyl-4-methylphenol was 97.4%, the content of 2-tert-butyl-4-methylphenol in the organic components was 0.84 wt%, and the total content of C8 and C12 isobutylene oligomers was 0.58 wt%. After crystallization and drying, 194.2 g of 2,6-di-tert-butyl-4-methylphenol was obtained, with a product purity of 99.67%. Based on the theoretical mass of initial p-cresol completely converted to 2,6-di-tert-butyl-4-methylphenol and corrected for the purity of the obtained product, the separation yield was 95.0%.

[0055] Example 3 17.223 g (69.36 mmol) of the 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazolium inner salt obtained in Preparation Example 1 was weighed and placed in a reaction flask equipped with a mechanical stirrer and temperature control device. Under nitrogen protection, the temperature was raised to 60 °C, and 2.777 g of 98% sulfuric acid was slowly added. Based on the calculation that each mole of sulfuric acid provides 2 acid equivalents, the acid equivalent provided by the added sulfuric acid was 0.80 equivalents per mole of imidazolium zwitterionic precursor. After the addition was completed, the temperature was raised to 80 °C and stirred for 2 h to obtain 20.00 g of an ionic liquid catalytic system in a continuous liquid phase without visible solid precipitation or macroscopic stratification.

[0056] 100.0 g of p-cresol and 20.00 g of the above-mentioned ionic liquid catalytic system were added to a 1 L pressure-resistant reactor equipped with a jacket, mechanical stirrer, gas mass flow controller, and back pressure regulating device. The atmosphere inside the reactor was replaced with nitrogen three times. The temperature was raised to 65 °C under stirring at 600 r / min and pre-contacted for 30 min. Then, isobutylene was continuously introduced and the reaction temperature was adjusted to 70 °C. A total of 119.33 g (2.127 mol) of isobutylene was added, making the molar ratio of p-cresol to isobutylene 1:2.30. The isobutylene feeding time was 3.5 h and the average feeding rate was 34.09 g / h. The reaction temperature was maintained at 70 ± 1 °C and the gauge pressure inside the reactor was maintained at 0.30 ± 0.01 MPa. No sulfuric acid or other Brønsted proton donors were added during the isobutylene introduction.

[0057] After adding a total of 119.33 g of isobutylene, the feeding was stopped. The remaining isobutylene in the reactor was introduced into a low-temperature recovery device and the pressure was reduced to below 0.01 MPa. The resulting liquid phase was allowed to stand at 80 °C for 30 min and then separated to obtain 198.6 g of hindered phenol crude product phase and 19.10 g of ionic liquid catalytic phase. The crude product phase was heated from 92 °C to 115 °C under reduced pressure for 50 min at 2.5 kPa to remove light components, resulting in 191.8 g of light-removed material. 57.5 g of anhydrous ethanol was added to the material and dissolved completely at 75 °C. The material was cooled to 15 °C at a cooling rate of 0.5 °C / min and held at that temperature for 1 h. The resulting crystals were filtered and washed with anhydrous ethanol at 15 °C and then dried under reduced pressure at 45 °C for 8 h.

[0058] Analyzing according to the aforementioned detection method, the conversion rate of p-cresol at the end of the reaction was 98.9%, the selectivity of 2,6-di-tert-butyl-4-methylphenol was 93.9%, the content of 2-tert-butyl-4-methylphenol in the organic components was 0.61 wt%, and the total content of C8 and C12 isobutylene oligomers was 2.31 wt%. After crystallization and drying, 179.2 g of 2,6-di-tert-butyl-4-methylphenol was obtained, with a product purity of 99.12%. Based on the theoretical mass of initial p-cresol completely converted to 2,6-di-tert-butyl-4-methylphenol and corrected for the purity of the obtained product, the separation yield was 87.2%.

[0059] Table 2. Preparation results of 2,6-di-tert-butyl-4-methylphenol under different process conditions. project Example 1 Example 2 Example 3 imidazolium zwitterion precursor Preparation of the precursor obtained in Example 1 Preparation of the precursor obtained in Example 1 Preparation of the precursor obtained in Example 1 Brønsted proton donor mesylate Methanesulfonic acid + sulfuric acid sulfuric acid Acid equivalent 0.4 0.6 0.8 Precursor dosage / g 4.324 10.614 17.223 Mass of ionic liquid catalytic system (g) 5 12.5 20 Catalytic system preparation temperature / °C 50 65 80 Catalytic system preparation time / h 0.5 1.25 2 p-Cresol dosage / g 100 100 100 Catalytic system dosage / p-cresol mass / % 5 12.5 20 Pre-contact temperature / °C 50 60 65 Pre-contact time / min 10 20 30 p-Cresol:Isobutylene molar ratio 1∶2.00 1∶2.15 1∶2.30 Cumulative amount of isobutylene added / g 103.77 111.55 119.33 Isobutylene feed time / h 5 4 3.5 Average feed rate of isobutylene (g / h) 20.75 27.89 34.09 Reaction temperature / ℃ 50±1 60±1 70±1 Reaction gauge pressure / MPa 0.05±0.01 0.20±0.01 0.30±0.01 Brønsted proton donors were added during the reaction. no no no p-Cresol Conversion Rate / % 95.1 99.3 98.9 BHT selectivity / % 91.8 97.4 93.9 2-tert-butyl-4-methylphenol content / wt% 3.42 0.84 0.61 C8+C12 isobutylene oligomer content / wt% 1.36 0.58 2.31 hindered phenol crude product phase mass / g 187.9 203.8 198.6 Mass of the separated ionic liquid catalytic phase (g) 4.62 11.94 19.1 BHT product quality / g 174.6 194.2 179.2 BHT product purity / % 99.18 99.67 99.12 BHT separation yield / % 85 95 87.2 As shown in Table 2, when the acid equivalent, catalyst dosage, pre-contact conditions, molar ratio of p-cresol to isobutylene, reaction temperature, and reaction gauge pressure of the ionic liquid catalytic system are at the low, middle, and high values ​​of the set range, respectively, p-cresol can be further converted into 2,6-di-tert-butyl-4-methylphenol via 2-tert-butyl-4-methylphenol. Among them, the p-cresol conversion rate, BHT selectivity, and product purity obtained in Example 2 are high, and the content of C8 and C12 isobutylene oligomers is low, indicating that 0.60 acid equivalent and the corresponding intermediate reaction conditions have a good comprehensive reaction effect.

[0060] Activation response verification in Example 2 The catalytic activity response was verified according to the catalytic system composition and reaction conditions of Example 2. An ionic liquid catalytic system with an acid equivalent of 0.60 was prepared by using the 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazolium inner salt obtained in Preparation Example 1 and a Brønsted proton donor. 100.0 g (0.9247 mol) of p-cresol and 12.50 g of the ionic liquid catalytic system were added to a 1 L pressure-resistant reactor and pre-contaminated for 20 min at 60 °C and 600 r / min. Isobutylene was then continuously fed at an average feed rate of 27.89 g / h, and the reaction temperature was maintained at 60 ± 1 °C and the pressure inside the reactor was maintained at 0.20 ± 0.01 MPa. No methanesulfonic acid, sulfuric acid or other Brønsted proton donors were added during the entire verification process.

[0061] The reaction time was 0 min from the start of isobutylene introduction. During the reaction, samples were taken periodically through the sampling port of the reactor. At each time point, about 0.20 g of the reaction solution was taken and immediately added to a pre-cooled acetonitrile solution containing 0.5 wt% triethylamine according to the aforementioned unified sample processing method to neutralize the acidic components in the sample and inhibit the continued acid-catalyzed reaction after sampling. Chromatographic analysis was then performed.

[0062] Using 0 min, 30 min, 60 min, 120 min, 180 min, and 240 min as representative detection nodes of the reaction process, the p-cresol, 2-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-methylphenol, and C8 and C12 isobutylene oligomers in the samples at each node were quantitatively analyzed.

[0063] Based on the initial addition of 0.9247 mol of p-cresol, the material conversions showed that at 0 min, 30 min, 60 min, 120 min, 180 min, and 240 min, the p-cresol concentrations were 924.7 mmol, 708.4 mmol, 472.9 mmol, 191.7 mmol, 42.8 mmol, and 6.5 mmol, respectively; the 2-tert-butyl-4-methylphenol concentrations were 0 mmol, 161.8 mmol, 272.6 mmol, 226.5 mmol, 92.6 mmol, and 10.3 mmol, respectively; and the 2,6-di-tert-butyl-4-methylphenol concentrations were 0 mmol, 52.5 mmol, 171.4 mmol, 493.9 mmol, 775.4 mmol, and 894.4 mmol, respectively. The amounts of C8 isobutylene oligomers were 0 g, 0.07 g, 0.18 g, 0.39 g, 0.62 g and 0.74 g, respectively, and the amounts of C12 isobutylene oligomers were 0 g, 0.02 g, 0.05 g, 0.16 g, 0.31 g and 0.44 g, respectively.

[0064] In the above reaction process, p-cresol gradually decreased as isobutylene was continuously introduced. 2-tert-butyl-4-methylphenol was generated in the early stage of the reaction and reached 272.6 mmol at 60 min before gradually decreasing. 2,6-di-tert-butyl-4-methylphenol continued to increase to 894.4 mmol at 240 min. This indicates that p-cresol first undergoes an ortho-tert-butylation to form 2-tert-butyl-4-methylphenol. The resulting intermediate then undergoes another ortho-tert-butylation to form 2,6-di-tert-butyl-4-methylphenol, gradually reducing the amount of p-cresol and 2-tert-butyl-4-methylphenol in the system that can continue to undergo tert-butylation.

[0065] At 240 min, the total amount of p-cresol and 2-tert-butyl-4-methylphenol decreased to 16.8 mmol, accounting for 1.82% of the initial amount of p-cresol. At this time, isobutylene was continuously supplied and the reaction was continued at an average feed rate of 27.89 g / h at 60±1℃, 0.20±0.01 MPa, and 30 min. During this 30 min, a total of 13.95 g of isobutylene was input, and the unreacted gas discharged through the back pressure regulating device was introduced into the low temperature recovery device.

[0066] When the reaction was continued for 270 min, 4.8 mmol of p-cresol, 5.6 mmol of 2-tert-butyl-4-methylphenol, and 900.0 mmol of 2,6-di-tert-butyl-4-methylphenol were detected. At the same time, 0.78 g of C8 isobutylene oligomer and 0.46 g of C12 isobutylene oligomer were detected. Compared with 240 min, the increase of 2,6-di-tert-butyl-4-methylphenol was only 5.6 mmol and the combined increase of C8 and C12 isobutylene oligomers was only 0.06 g. This indicates that when the temperature, pressure, and isobutylene supply conditions remain unchanged and the level of phenolic species that can be further tert-butylated has been reduced to a low level, the degree of chemical conversion of isobutylene in the system is significantly reduced.

[0067] At 270 min, without separating the ionic liquid catalytic system and other materials in the reactor, 5.00 g (46.24 mmol) of p-cresol was directly added to the same reaction system, increasing the p-cresol content from 4.8 mmol to approximately 51.0 mmol. The reaction was then maintained at 60 ± 1 °C, 0.20 ± 0.01 MPa, and an average isobutylene feed rate of 27.89 g / h for another 30 min.

[0068] At the end of the reaction 30 minutes after the addition of p-cresol, 10.4 mmol of p-cresol, 22.7 mmol of 2-tert-butyl-4-methylphenol, and 919.4 mmol of 2,6-di-tert-butyl-4-methylphenol were detected. At the same time, 0.82 g of C8 isobutylene oligomer and 0.49 g of C12 isobutylene oligomer were also detected. Compared with before the addition of p-cresol, the levels of 2-tert-butyl-4-methylphenol and 2,6-di-tert-butyl-4-methylphenol in the system increased. This indicates that the ability of isobutylene to participate in the tert-butylation reaction was significantly restored after the addition of p-cresol, which can continue tert-butylation, without re-preparing the ionic liquid catalytic system and without adding Brønsted proton donors.

[0069] The above reaction time series, the response results after continued isobutylene supply following the endpoint, and the response results after the addition of p-cresol to the same system collectively indicate that the ionic liquid catalytic system can maintain isobutylene participation in the target tert-butylation reaction in the presence of p-cresol and 2-tert-butyl-4-methylphenol. As the two tert-butylation phenolic species are converted to 2,6-di-tert-butyl-4-methylphenol and reduced to a lower level, the catalytic conversion degree of isobutylene in the system decreases when the temperature, pressure, and isobutylene feed conditions remain unchanged. After the addition of p-cresol, the formation of 2-tert-butyl-4-methylphenol and 2,6-di-tert-butyl-4-methylphenol is enhanced again. This indicates that the observed change in the effective catalytic conversion degree of isobutylene participation in the target tert-butylation reaction is related to the change in the content of tert-butylation phenolic species in the system, and is not caused by the permanent deactivation of the ionic liquid catalytic system.

[0070] Comparative Example 1 A control experiment was conducted according to the method of Example 2. 10.614 g (42.74 mmol) of the 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazolium inner salt obtained in Preparation Example 1 was used. The total amount of Brønsted proton donor was increased from 0.60 acid equivalents to 1.00 acid equivalents. 2.075 g of methanesulfonic acid with a purity of 99.0% and 1.070 g of sulfuric acid with a mass fraction of 98% were added, each providing 0.50 acid equivalents. The mixture was stirred at 65°C for 1.25 h to obtain the catalytic system.

[0071] Except for the amount of acid added and the resulting changes in the mass of the catalytic system, the amount of p-cresol added, the pre-contact at 60℃ for 20 min, the amount of isobutylene added (111.55 g), the feeding time (4.0 h), the reaction temperature (60 ± 1℃), the gauge pressure (0.20 ± 0.01 MPa), and the stirring conditions were all the same as in Example 2. After 4.0 h of isobutylene feeding, i.e., when the reaction reached 240 min, the p-cresol conversion rate was 99.6%, the BHT selectivity was 92.7%, the 2-tert-butyl-4-methylphenol content was 0.47 wt%, and the total C8 and C12 oligomers were 2.68 wt%. After continuing to introduce isobutylene for 30 min, the oligomers increased by 0.83 g. Subsequently, without separating the reaction system, 5.00 g of p-cresol was added and the reaction was continued for 30 min, resulting in an increase of 18.8 mmol of BHT. The results show that although increasing the acid equivalent maintained a high conversion rate, the BHT selectivity decreased and the isobutylene oligomerization was significantly enhanced in the later stages of the reaction.

[0072] Comparative Example 2 The same ionic liquid catalytic system as in Example 2 was used, except that the step of pre-contacting p-cresol and the ionic liquid catalytic system for 20 min before introducing isobutylene was omitted. 100.0 g of p-cresol was added to a 1 L pressure vessel, purged three times with nitrogen, and heated to 60 °C with stirring at 600 rpm; 12.50 g of the ionic liquid catalytic system was maintained at 60 °C. Starting at the beginning of isobutylene feeding was designated as 0 min. Simultaneously with the start of isobutylene feeding, the catalytic system was added to the vessel within 1 min, ensuring that p-cresol, the catalytic system, and isobutylene began contacting from 0 min, preventing the formation of premixed materials before isobutylene introduction. A total of 111.55 g of isobutylene was added over 4.0 h, with an average feeding rate of 27.89 g / h. The temperature was maintained at 60 ± 1 °C and the gauge pressure at 0.20 ± 0.01 MPa. No Brønsted proton donor was added during the feeding process. After 240 min of reaction, the conversion rate of p-cresol was 97.7%, the selectivity of BHT was 93.2%, the content of 2-tert-butyl-4-methylphenol was 2.64 wt%, and the total content of C8 and C12 oligomers was 1.71 wt%. Subsequent 30 min of isobutylene introduction resulted in an increase of 0.39 g of oligomers. These results indicate that the removal of premixing increased intermediate residues and oligomerization side reactions, which can be used to evaluate the effect of the "phenol-before-olefin" reaction on the two consecutive ortho-tert-butylation reactions and the changes in activity in the later stages of the reaction.

[0073] Comparative Example 3 1-Butyl-3-methylimidazolium hydrogen sulfate was used as a continuous acidic ionic liquid control. To ensure that the nominal acid proton mass was close to the 25.64 mmol acid equivalent of the Brønsted proton donor used in Example 2, 6.06 g of the ionic liquid was weighed and added to 100.0 g of p-cresol. The mixture was pre-contaminated at 60 °C for 20 min, followed by continuous introduction of 111.55 g of isobutylene at a rate of 27.89 g / h for 4.0 h. The reaction temperature was maintained at 60 ± 1 °C and the gauge pressure was maintained at 0.20 ± 0.01 MPa. No additional acidic substances were added during the reaction. The other stirring, sampling, and detection conditions were the same as in Example 2. After 240 min of reaction, the conversion rate of p-cresol was 99.4%, the selectivity of BHT was 90.3%, the content of 2-tert-butyl-4-methylphenol was 0.56 wt%, and the total content of C8 and C12 oligomers was 3.46 wt%. After further introducing isobutylene for 30 min, the oligomer content increased by 1.08 g. Subsequently, without separating the system, 5.00 g of p-cresol was added and the reaction was continued for another 30 min, resulting in an increase of 16.9 mmol of BHT. The results indicate that in the continuous acidic ionic liquid control system used, although it can promote the tert-butylation of p-cresol, there is still a significant oligomerization behavior of isobutylene after the decrease in the number of phenolic species that can be further tert-butylated. This comparative example is used to provide a performance reference for different continuous acidic ionic liquid systems under similar reaction conditions.

[0074] Comparative Example 4 A 1-propyl-3-(3-sulfopropyl)imidazolium inner salt without ether oxygen was prepared as a structural control precursor. The preparation method was as follows: under nitrogen protection, 4.40 g of sodium hydride (60% by mass) and 30 mL of anhydrous tetrahydrofuran were added to a 500 mL three-necked flask. 6.81 g of imidazole (0.100 mol) was dissolved in 40 mL of anhydrous tetrahydrofuran and slowly added dropwise at 0–5 °C. After the addition was completed, the mixture was stirred at 20–25 °C for 2 h. Then, 12.91 g of 1-bromopropane (0.105 mol) was added and the temperature was raised to 65–70 °C for 8 h.

[0075] After the reaction was completed, the mixture was cooled to room temperature, filtered to remove the solid, and the residue was washed twice with anhydrous tetrahydrofuran. The filtrates were combined and the solvent was removed under reduced pressure. The crude product was then distilled under reduced pressure to obtain 1-propylimidazolium.

[0076] Weigh 3.52 g (31.9 mmol) of the obtained 1-propylimidazole and add it to a 100 mL reaction flask. Add 40 mL of anhydrous ethyl acetate and add 3.72 g (30.5 mmol) of 1,3-propanesulfonyl lactone under nitrogen protection and stirring. Reflux at 77–80 °C for 20 h.

[0077] After the reaction was completed, the mixture was cooled to 20-25°C and the precipitate was collected by filtration. The obtained solid was washed three times with 10 mL of ethyl acetate each time, and then recrystallized with anhydrous ethanol. The solid was dried under reduced pressure at 60°C for 12 h to constant weight to obtain 1-propyl-3-(3-sulfopropyl)imidazolium inner salt.

[0078] The products were prepared in parallel and combined according to the above method to obtain the amount required for subsequent control experiments.

[0079] It should be noted that the obtained structural control precursor was structurally confirmed using ¹H NMR, ¹³C NMR, and HRMS according to the aforementioned detection methods. Its structure was confirmed by the corresponding characteristic signals of the imidazolium ring proton, the n-propyl side chain, the imidazolium nitrogen-adjacent methylene group, and the 3-sulfonic acid propyl side chain. Compared with the 1-(2-methoxyethyl)-3-(3-sulfonic acid propyl)imidazolium inner salt obtained in Preparation Example 1, the obtained structural control precursor retains the 3-sulfonic acid propyl substituent. The only difference is that the 2-methoxyethyl substituent containing ether oxygen is replaced with the n-propyl substituent.

[0080] To ensure that the amount of precursor material and the Brønsted proton donor acid equivalent were consistent with those in Example 2, 9.929 g (42.74 mmol) of the precursor was weighed and mixed with 1.245 g of 99.0% pure methanesulfonic acid and 0.642 g of 98% sulfuric acid. Each acid provided 0.30 acid equivalent, and the total acid equivalent was 0.60. The mixture was stirred at 65°C for 1.25 h to obtain the catalytic system. Subsequently, 100.0 g of p-cresol was pre-contaminated at 60°C for 20 min, followed by a continuous feed of 111.55 g of isobutylene at 27.89 g / h for 4.0 h. The reaction temperature was maintained at 60 ± 1°C and the gauge pressure at 0.20 ± 0.01 MPa. No Brønsted proton donor was added during the feed, and the other conditions were the same as in Example 2. After 240 min of reaction, the p-cresol conversion was 98.3%, the BHT selectivity was 94.1%, the 2-tert-butyl-4-methylphenol content was 1.46 wt%, and the total C8 and C12 oligomers were 1.37 wt%. After continuing to feed isobutylene for 30 min, the oligomers increased by 0.33 g. Subsequently, 5.00 g of p-cresol was added and the reaction was continued for 30 min, resulting in an increase of 15.6 mmol of BHT.

[0081] Compared with Example 2, the target selectivity and the oligomerization inhibition effect in the later stage of the reaction were weakened after removing the side chain ether oxygen.

[0082] Table 3 Comparison of BHT preparation performance and catalytic activity response under different catalytic systems and process conditions index Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 p-Cresol Conversion Rate / % 99.3 99.6 97.7 99.4 98.3 BHT selectivity / % 97.4 92.7 93.2 90.3 94.1 2-tert-butyl-4-methylphenol content in organic components / wt% 0.84 0.47 2.64 0.56 1.46 Content of C8+C12 isobutylene oligomers in organic components / wt% 0.58 2.68 1.71 3.46 1.37 After reacting for 240 min, isobutylene was continuously introduced for 30 min, resulting in an increase in C8+C12 oligomer content per g. 0.06 0.83 0.39 1.08 0.33 BHT production after re-addition of p-cresol / mmol 19.4 18.8 — 16.9 15.6 As shown in Table 3, in Example 2, while the conversion rate of p-cresol reached 99.3%, the selectivity of BHT was 97.4%. After 240 min of reaction, the total content of C8 and C12 isobutylene oligomers was only 0.58 wt%. After continuing to introduce isobutylene for 30 min, the oligomer content increased by only 0.06 g. After adding p-cresol, the BHT content increased by 19.4 mmol.

[0083] In Comparative Example 1, simply increasing the Brønsted proton donor from 0.60 acid equivalent to 1.00 acid equivalent slightly improved the p-cresol conversion, but reduced the BHT selectivity to 92.7%, increased the oligomer content to 2.68 wt%, and resulted in an oligomer increment of 0.83 g in the endpoint continuation reaction. This indicates that the substoichiometric proton supply is not simply a matter of reducing the amount of acid, but is related to the inhibition of non-target isobutylene conversion in the later stages of the reaction.

[0084] In Comparative Example 2, after removing isobutylene from the previous premixing step, the conversion rate of p-cresol and the selectivity of BHT decreased to 97.7% and 93.2%, respectively, while the content of monotert-butyl intermediates and oligomers increased. This indicates that "phenol first, then olefin" is beneficial for two consecutive ortho-tert-butylation and reduces side reactions in the later stages of the reaction.

[0085] When using a continuous acidic ionic liquid as the catalytic system in Comparative Example 3, after 4.0 h of isobutylene feeding, isobutylene was continued to be introduced for 30 min, and the total amount of C8 and C12 oligomers increased by 1.08 g, indicating that the continuous acidic ionic liquid control system still has a significant tendency for isobutylene oligomerization in the later stage of the reaction.

[0086] Comparative Example 4, while maintaining the same amount of precursor, acid equivalent, and main reaction conditions, only removed the ether-containing oxygen side chain. The BHT selectivity decreased to 94.1%, the oligomer increased by 0.33 g after the endpoint, and the amount of BHT generated after adding p-cresol was lower than that in Example 2. This indicates that the homomolecular combination of the ether oxygen substituent and the sulfonate substituent is related to the response behavior.

[0087] The four control groups collectively demonstrate that the precursor structure, substoichiometric proton supply, and premixing steps are not independent arbitrary choices, but rather collectively influence the selectivity of the target tert-butylation and the isobutylene conversion behavior in the later stages of the reaction.

[0088] Test case The 11.94 g of ionic liquid catalyst phase obtained after the reaction in Example 2 was used for a recycling test. The catalyst phase was placed in a vacuum devolatilization vessel equipped with mechanical stirring and temperature control, and stirred and devolatilized for 40 min at 60 °C and an absolute pressure of 5.0 ± 0.5 kPa to remove dissolved or entrained isobutylene and other volatile components. No imidazolium zwitterionic precursor, methanesulfonic acid, sulfuric acid or other Brønsted proton donors were added during the devolatilization process. After the devolatilization was completed, 11.82 g of ionic liquid catalyst phase in a continuous liquid phase with no visible solid precipitation or macroscopic stratification was obtained. The obtained catalyst phase was used entirely for the next batch of tert-butylation reaction of p-cresol and isobutylene.

[0089] The first cycle was conducted according to the p-cresol feed amount, pre-contact conditions, isobutylene addition amount and feeding method, reaction temperature, reaction gauge pressure, and stirring conditions as described in Example 2. No Brønsted proton donor was added during the reaction. After the reaction, gas-liquid separation was performed according to the method in Example 2, and the separated ionic liquid catalyst phase was allowed to stand at 80°C for 30 min. Then, vacuum devolatilization was performed at 60°C, absolute pressure of 5.0±0.5 kPa, and for 40 min, and all of it was used for the next batch of reaction. The second to fifth cycles were conducted in the same manner, and the mass loss of the catalyst phase generated during phase separation, transfer, sampling, and devolatilization was not compensated. The mass of the catalyst phase obtained after devolatilization before the start of each cycle was 11.82 g, 11.63 g, 11.43 g, 11.27 g, and 11.09 g, respectively. After the fifth cycle, the same separation and devolatilization treatment was performed to obtain 10.92 g of catalyst phase. Based on the 11.94 g of catalyst phase obtained in the first separation, the cumulative apparent mass loss was calculated to be 8.54%.

[0090] The aforementioned quality degradation refers to the apparent quality change of the catalyst phase after cyclic separation and vacuum devolatilization. This may originate from a small amount of catalyst phase being carried away and transferred with the product phase during phase separation, as well as operational losses during sampling, and the removal of dissolved or entrained volatile components during vacuum devolatilization. Therefore, it is not possible to determine whether the imidazolium zwitterionic precursor or Brønsted acidic component has been lost proportionally based solely on the change in catalyst phase quality. Since all the actual recovered and devolatilized catalyst phase is used in the next batch in each cycle, and the aforementioned apparent quality loss is not compensated, the actual catalyst phase quality input in each batch gradually decreases. The resulting cycle data reflects the overall recycling performance, including actual phase separation, transfer, and devolatilization losses.

[0091] At the end of each cycle, p-cresol, 2-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-methylphenol, and C8 and C12 isobutylene oligomers were determined according to the aforementioned detection method. The p-cresol conversion rate and BHT selectivity were calculated according to the aforementioned method. At the same time, the mass percentage of 2-tert-butyl-4-methylphenol and C8 and C12 isobutylene oligomers in the organic components were recorded, so that the cycle test used the same analytical method and evaluation criteria as Examples 1-3 and the comparative examples.

[0092] To evaluate the retention of catalytic activity after recycling without altering the composition of the main catalytic phase responsible for the continuous cyclic reaction and to avoid the impact of endpoint feed continuation and p-cresol addition on subsequent cycles, three parallel cyclic experiments with the same catalytic system composition and operating conditions as the main cycle were set up and run to the 1st, 3rd, and 5th cycles, respectively. After the corresponding cyclic reaction reached 240 min, the isobutylene feed rate of 60±1℃, 0.20±0.01MPa, and 27.89 g / h was maintained for another 30 min. The total increments of C8 and C12 oligomers were measured to be 0.07 g, 0.10 g, and 0.11 g, respectively. Subsequently, without separating the catalytic system or adding Brønsted proton donors, 5.00 g (46.24 mmol) of p-cresol was added, and the reaction was continued for another 30 min. The resulting increases in BHT were 18.3 mmol, 17.7 mmol, and 17.2 mmol, respectively. The catalytic activity response of the catalytic system after different cycles was evaluated under two conditions: a decrease and a resurgence of tert-butylated phenolic species.

[0093] Table 4 Results of Recycling of Ionic Liquid Catalytic Systems Loop count p-Cresol Conversion Rate / % BHT selectivity / % 2-tert-butyl-4-methylphenol content / wt% C8+C12 isobutylene oligomer content / wt% Target reaction recovery retention rate after p-cresol supplementation / % 1 99 97 0.92 0.63 94.3 2 98.7 96.8 1.06 0.67 — 3 98.9 96.5 1.14 0.7 91.2 4 98.4 96.3 1.31 0.76 — 5 98.5 95.9 1.39 0.81 88.7 The target reaction recovery retention rate mentioned in Table 4 after adding p-cresol is based on the 19.4 mmol of BHT increase obtained in the catalytic activity response verification of Example 2 after adding 5.00 g of p-cresol for 270 min and continuing the reaction for 30 min, as 100%. It is calculated according to the ratio of the BHT increase obtained in parallel experiments with the same number of cycles after adding the same mass of p-cresol for 30 min to 19.4 mmol. The target reaction recovery retention rates obtained in the 1st, 3rd and 5th cycles are 94.3%, 91.2% and 88.7%, respectively.

[0094] As shown in Table 4, the ionic liquid catalytic phase obtained in Example 2, after vacuum devolatilization, can be continuously used in subsequent batches of p-cresol and isobutylene tert-butylation reaction without the addition of imidazolium zwitterionic precursor and Brønsted proton donor. The p-cresol conversion rate remained at 98.4%–99.0% and the BHT selectivity remained at 95.9%–97.0% in the first to fifth cycles. The contents of 2-tert-butyl-4-methylphenol and C8 and C12 isobutylene oligomers changed only slightly. In the first, third and fifth parallel response tests, after the phenolic species that could continue to be tert-butylated decreased to a low level, the oligomer increment obtained by maintaining the same temperature, pressure and isobutylene supply conditions remained at a low level. After the addition of p-cresol, the BHT generation was enhanced again. This indicates that even without compensating for the apparent mass loss of the catalytic phase during the cycle operation, the ionic liquid catalytic phase can still maintain high target tert-butylation reaction performance after 5 separations, vacuum devolatilization and recycling.

[0095] The above-mentioned recycling method refers to the reuse of the catalytic phase between reaction batches. In actual scale-up applications, the ionic liquid catalytic phase obtained after separation after the reaction can be returned to subsequent batches of reaction after vacuum devolatilization, and the recovery quality of the catalytic phase during phase separation, transfer and devolatilization processes can be calculated by material balance. The recycling test of this invention was conducted without compensating for the apparent mass loss of the catalytic phase in the above operation process. After the 5th cycle, it still maintained a high p-cresol conversion rate and BHT selectivity, indicating that the catalytic phase is feasible for multiple batches of reuse.

[0096] Experimental results Preparation Examples 1-3 yielded 1-(2-methoxyethyl)-3-(3-sulfonylpropyl)imidazolium inner salts, 1-(3-ethoxypropyl)-3-(4-sulfonylbutyl)imidazolium inner salts, and 1-(4-propoxybutyl)-3-(3-sulfonylpropyl)imidazolium inner salts, respectively. The structures of the obtained precursors were confirmed by ¹H NMR, ¹³C NMR, and HRMS. Specific embodiments for synthesis of raw materials, reaction conditions, separation and purification, and structural characterization are given, respectively, with m = 2-4, n = 3-4, and R¹ = C1-C3 alkyl groups. This provides a basis for the preparation and structural confirmation of the structural range of ether-containing oxygen substituents and sulfonate-type substituents simultaneously connected to the same imidazolium skeleton, as summarized in this invention.

[0097] Examples 1-3 were carried out at different positions within the ranges defined by acid equivalents of 0.40, 0.60, and 0.80, as well as the amount of catalyst system, pre-contact conditions, molar ratio of p-cresol to isobutylene, reaction temperature, and reaction gauge pressure. All three examples were able to further convert p-cresol to 2,6-di-tert-butyl-4-methylphenol via 2-tert-butyl-4-methylphenol, and obtained the corresponding p-cresol conversion rate, BHT selectivity, intermediate content, isobutylene oligomer content, product purity, and separation yield, respectively. This demonstrates that the overall process parameters of the present invention have specific implementation support at different positions within the corresponding ranges. Among them, Example 2 further verified the implementation effect under the combination of 0.60 acid equivalent and the corresponding preferred process conditions.

[0098] Based on the implementation support obtained within the aforementioned process scope, each comparative example set up controls focusing on the Brønsted proton donor acid equivalent, the pre-contact method before isobutylene enters the reaction system, the type of catalytic system, and the ether oxygen structure in the imidazolium side chain. In Comparative Example 1, increasing the Brønsted proton donor to 1.00 acid equivalent resulted in decreased BHT selectivity and increased isobutylene oligomerization in the later stage of the reaction; in Comparative Example 2, eliminating the premixing before isobutylene entry resulted in increased content of mono-tert-butyl intermediates and oligomers; in Comparative Example 4, using a control precursor without ether oxygen structure, resulted in decreased target tert-butylation selectivity and reduced oligomerization inhibition in the later stage of the reaction.

[0099] The above comparisons support the relationship between substoichiometric Brønsted proton supply, pre-contact before isobutylene entry, and the presence of ether-containing oxygen side chains, and the reaction performance obtained in this invention. Comparative Example 3 uses different sustained acidic ionic liquid catalytic systems to provide a performance reference for different catalytic systems under similar reaction conditions.

[0100] Example 2 and Comparative Example 3, which uses a continuous acidic ionic liquid, exhibited different oligomerization behaviors of isobutylene in the later stage of the reaction under similar main operating conditions such as the amount of p-cresol fed, the amount of isobutylene added, the reaction temperature, the reaction gauge pressure, and the reaction time. Specifically, in Example 2, after 4.0 h of isobutylene feeding, when isobutylene was continued to be fed under the same conditions for 30 min, the total increase of C8 and C12 oligomers was only 0.06 g, while in Comparative Example 3, the total increase of oligomers in the corresponding stage was 1.08 g.

[0101] Since the chemical compositions of the catalytic systems used in Example 2 and Comparative Example 3 are different, this comparison is mainly used to illustrate that the two types of catalytic systems exhibit different oligomerization behaviors in the later stages of the reaction under similar reaction conditions. This difference is not attributed to a single structure or a single acid factor. The response results of continuing to supply isobutylene and re-adding p-cresol in the same catalytic system in Example 2 are used to further determine the relationship between the weakening of the target tert-butylation reaction in the later stages of the reaction and the change in the content of phenolic species that can continue to be tert-butylated in the system.

[0102] The reaction time series of Example 2 further shows that p-cresol gradually decreases as the reaction proceeds, 2-tert-butyl-4-methylphenol is formed in the early stage of the reaction and continues to decrease after reaching a high content, while BHT continuously increases throughout the feeding process. After the combined stoichiometry of p-cresol and 2-tert-butyl-4-methylphenol decreases to a low level, the effective catalytic conversion of isobutylene by the system is significantly reduced when the temperature, pressure and isobutylene supply conditions are maintained to continue the reaction. After p-cresol is replenished in the same reaction system, the formation of 2-tert-butyl-4-methylphenol and BHT is enhanced again. This proves that the observed changes in catalytic activity are related to the presence of phenolic species that can continue to be tert-butylated, and indicates that the decrease in the catalytic conversion degree in the later stage of the reaction is not due to permanent deactivation of the ionic liquid catalytic system.

[0103] Further recycling experiments showed that the ionic liquid catalytic phase obtained after reaction, after vacuum devolatilization at 60℃ and an absolute pressure of 5.0±0.5kPa, could be directly used in subsequent batches of reaction. It could be continuously recycled 5 times without the addition of imidazolium zwitterionic precursor and Brønsted proton donor. After the 5th cycle, it still maintained a high p-cresol conversion rate and BHT selectivity, as well as a low isobutylene oligomerization level. Parallel response experiments corresponding to the 1st, 3rd, and 5th cycles still showed that the increase in isobutylene oligomerization was small after the reduction of tert-butylated phenolic species and the BHT generation was enhanced after the addition of p-cresol. Thus, the two technical effects of catalytic phase recycling and catalytic activity changing with the composition of reactants were supported by corresponding experiments in the same process system.

[0104] Based on the above preparation examples, embodiments, comparative examples, catalytic activity response verification, and recycling test results, this invention employs an amphoteric precursor with one imidazolium nitrogen atom linked to an ether oxygen substituent and another imidazolium nitrogen atom linked to a sulfonate substituent. Combined with a Brønsted proton donor of 0.40–0.80 acid equivalents per mole of precursor, pre-contact of p-cresol with the ionic liquid catalytic system before isobutylene enters the reaction system, and no further addition of Brønsted proton donors during isobutylene feeding, high BHT selectivity and low isobutylene oligomer content were achieved under the aforementioned conditions.

[0105] The reaction process, endpoint continuation reaction, and p-cresol addition test in Example 2 further showed that when p-cresol and 2-tert-butyl-4-methylphenol were reduced to a lower level, the target tert-butylation reaction was weakened accordingly, while the target tert-butylation reaction was enhanced again after p-cresol was added back.

[0106] The recycling test also showed that the ionic liquid catalytic phase obtained after the reaction can be used for subsequent batch reactions after vacuum devolatilization.

[0107] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing hindered phenolic antioxidants via ionic liquid catalysis, wherein the hindered phenolic antioxidant is 2,6-di-tert-butyl-4-methylphenol, characterized in that... include: The imidazolium zwitterion precursor was mixed with the Brønsted proton donor to obtain an ionic liquid catalytic system. One imidazolium nitrogen atom of the imidazolium zwitterionic precursor is attached to an ether-containing oxygen substituent, and the other imidazolium nitrogen atom is attached to a sulfonate substituent. Each mole of the imidazolium zwitterionic precursor contains one mole of sulfonate substituent. The amount of the Brønsted proton donor added, in acid equivalents, is 0.40 to 0.80 equivalents per mole of the imidazolium zwitterionic precursor; Before introducing isobutylene, p-cresol and the ionic liquid catalytic system are premixed to obtain a premixed reactant containing p-cresol and the ionic liquid catalytic system. Isobutylene is continuously introduced into the premixed reactants. No Brønsted proton donor is added during the reaction. This causes p-cresol to undergo a first ortho-tert-butylation to generate 2-tert-butyl-4-methylphenol. The resulting 2-tert-butyl-4-methylphenol then undergoes another ortho-tert-butylation to generate 2,6-di-tert-butyl-4-methylphenol. Stop the flow of isobutylene, perform gas-liquid separation on the reaction product, and separate the liquid phase into a crude product containing 2,6-di-tert-butyl-4-methylphenol and an ionic liquid catalytic phase.

2. The method for preparing hindered phenolic antioxidants by ionic liquid catalysis as described in claim 1, characterized in that, The ether-containing oxygen substituent is -(CH2)mOR 1 The sulfonate substituent is -(CH2)n-SO3 - Where m is 2–4, n is 3–4, and R 1 It is a C1 to C3 alkyl group.

3. The method for preparing hindered phenolic antioxidants by ionic liquid catalysis as described in claim 2, characterized in that, The zwitterion precursor of the imidazodium is the inner salt of 1-(2-methoxyethyl)-3-(3-sulfopropyl)imidazodium.

4. The method for preparing hindered phenolic antioxidants by ionic liquid catalysis as described in claim 3, characterized in that, The Brønsted proton donor is selected from one or both of sulfuric acid and methanesulfonic acid; The amount of the Brønsted proton donor added is 0.50 to 0.65 equivalents per mole of the imidazolonium zwitterionic precursor, in acid equivalents.

5. The method for preparing hindered phenolic antioxidants by ionic liquid catalysis as described in claim 4, characterized in that, The imidazolium zwitterionic precursor was mixed with Brønsted proton donor at 50–80 °C for 0.5–2 h to obtain a homogeneous ionic liquid catalytic system.

6. The method for preparing hindered phenolic antioxidants by ionic liquid catalysis as described in claim 5, characterized in that, The amount of the ionic liquid catalytic system is 5% to 20% of the mass of p-cresol. After the p-cresol and the ionic liquid catalytic system are pre-contacted at 50 to 65°C for 10 to 30 minutes, isobutylene is introduced into the resulting reaction system.

7. The method for preparing hindered phenolic antioxidants by ionic liquid catalysis as described in claim 6, characterized in that, The molar ratio of p-cresol to isobutylene is 1:2.0 to 2.3, the reaction temperature is 50 to 70℃, and the reaction gauge pressure is 0.05 to 0.30 MPa.

8. The method for preparing hindered phenolic antioxidants by ionic liquid catalysis as described in claim 7, characterized in that, The molar ratio of p-cresol to isobutylene is 1:2.05–2.20, the reaction temperature is 55–65℃, and the reaction gauge pressure is 0.10–0.25 MPa. After the reaction was completed, the crude product was purified by removing light radicals and crystallization to obtain 2,6-di-tert-butyl-4-methylphenol.

9. The method for preparing hindered phenolic antioxidants by ionic liquid catalysis as described in claim 8, characterized in that, After being subjected to depressurized devolatilization, the ionic liquid catalytic phase is used as the ionic liquid catalytic system for the next batch of tert-butylation reaction of p-cresol and isobutylene.

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