A method for preparing p-hydroxybenzaldehyde

Abstract

This invention belongs to the field of organic chemical intermediate preparation technology and discloses a method for preparing p-hydroxybenzaldehyde. The method includes: pre-activating p-methylphenol, methanol, sodium hydroxide, and a composite catalyst under methanol reflux at 65-70°C; feeding the resulting alkaline pre-activated solution into an oxidation reactor purged with nitrogen, and oxidizing the product by introducing oxygen at low pressure in three stages through an oxygen buffer tank, with the oxygenation endpoint determined by the residual p-methylphenol content and the oxygen absorption rate; subsequently, the product undergoes nitrogen purging, methanol recovery after solvent removal, crude product crystallization and centrifugation, first activated carbon decolorization, hydrochloric acid precipitation, second activated carbon decolorization, cooling crystallization, centrifugation, and drying to obtain p-hydroxybenzaldehyde. This invention improves the control stability and product quality consistency of the low-pressure oxidation process by combining composite catalyst pre-activation, staged oxygen supply, dynamic endpoint determination, and two-stage purification.

Description

Technical Field

[0001] This invention belongs to the field of organic chemical intermediate preparation technology, specifically relating to a method for preparing p-hydroxybenzaldehyde from p-methylphenol as a raw material through catalytic oxidation, solvent removal, acid precipitation and purification. Background Technology

[0002] p-Hydroxybenzaldehyde is an important aromatic aldehyde fine chemical intermediate used in pharmaceuticals, fragrances, pesticides, liquid crystal materials, and functional polymer materials. Among existing preparation routes, the production of p-hydroxybenzaldehyde via the catalytic oxidation of p-methylphenol in an alkaline alcohol solvent with oxygen-containing gas is one of the more common industrialization approaches. This route has relatively stable raw material sources and relatively direct reaction steps; however, it also presents challenges such as methyl oxidation, further oxidation of the aldehyde group, phenolate side reactions, and the formation of colored high-boiling substances, placing demands on the catalytic system, oxygenation method, temperature control, and post-treatment purification.

[0003] Chinese patent CN101462933B discloses a method and apparatus for the catalytic oxidation synthesis of p-hydroxybenzaldehyde. The method involves adding p-cresol (i.e., p-methylphenol), sodium hydroxide, cobalt acetate, and copper acetate to methanol to form a feed liquid, which is then fed into a rotating packed bed reactor at 55–75°C. This allows the oxidizing gas to react with the feed liquid to generate p-hydroxybenzaldehyde. This method enhances gas-liquid contact through a rotating packed bed, which helps shorten the reaction time and improve mass transfer efficiency. However, such methods typically rely on specific enhanced mass transfer equipment, which has high requirements for equipment structure and operation and maintenance. Furthermore, for conventional batch-type low-pressure oxidation systems, there is still room for improvement in how to match the oxygen supply to the reaction stage and control side reactions caused by excessive oxygen without using enhanced equipment such as rotating packed beds.

[0004] Furthermore, existing processes often use fixed oxygenation time or a single conversion rate as the reaction endpoint, which fails to adequately reflect changes in oxygen absorption rate, raw material residue, and side reaction trends across different batches. Regarding post-treatment, while conventional solvent removal, acid precipitation, and crystallization can yield the target product, the stability of removing colored impurities and subsequent oxidation byproducts is still affected by the composition of the reaction solution. Therefore, there is a need to provide a method suitable for low-pressure batch oxidation of p-hydroxybenzaldehyde, where catalyst pre-activation, segmented oxygen supply, dynamic endpoint determination, and two-stage purification processes are coordinated to improve the stability of reaction process control and the consistency of finished product quality. Summary of the Invention

[0005] The present invention aims to provide a method for preparing p-hydroxybenzaldehyde suitable for batch low-pressure oxidation systems, in order to solve the problems of crude oxygen supply control, single determination of reaction endpoint, and insufficient stability of by-products and color control in existing conventional cobalt-copper catalytic oxidation processes.

[0006] To achieve the above-mentioned technical objectives, the present invention provides the following technical solutions.

[0007] A method for preparing p-hydroxybenzaldehyde includes the following steps: S1. Pre-activation of raw materials: p-Cresol, methanol and sodium hydroxide are added to a mixing tank and stirred to dissolve. A composite catalyst composed of cobalt chloride, copper acetate and copper sulfate is added, wherein the mass ratio of cobalt chloride, copper acetate and copper sulfate is 1:(1.5~3.5):(0.3~1.2). The mixture is pre-activated at 65~70℃ under methanol reflux for 0.8~1.2h to obtain an alkaline pre-activated solution. S2, Replacement feeding: After replacing the oxidation reactor with nitrogen, the alkaline pre-activated feed liquid is fed into the oxidation reactor, and the external circulation is started; S3. Segmented low-pressure oxidation: After the liquid oxygen is oxidized, it is sent to the oxidation reactor through an oxygen buffer tank of 0.4-0.8MPa. Based on the benchmark oxygen feed flow rate calculated according to the set total oxygen supply and the planned oxygen supply time, oxygen is introduced in three stages according to the benchmark oxygen feed flow rates of 40%-70%, 90%-130%, and 20%-60% in sequence. In each stage, the temperature inside the oxidation reactor is controlled at 70-80℃ and the pressure is lower than 0.2MPa. S4. Endpoint determination and purging: When the residual amount of p-methylphenol drops below 1.0% and the oxygen absorption rate drops below 15% of the peak oxygen absorption rate for 30 consecutive minutes, stop the oxygen supply and purge with nitrogen. S5. Desolventizing and crude product separation: The reaction liquid is sent to the desolventizing kettle, where methanol is distilled off and recovered at 65-70°C. The obtained desolventized product is then crystallized and centrifuged to obtain crude p-hydroxybenzaldehyde solid. S6. First decolorization and acid precipitation: After dissolving the crude p-hydroxybenzaldehyde solid in water, the first activated carbon decolorization is carried out at 75-80℃. After filtration, the pH is adjusted to 2.0-2.5 with hydrochloric acid for acid precipitation crystallization. After centrifugation, the acid-precipitated p-hydroxybenzaldehyde solid is obtained. S7. Secondary decolorization and refined crystallization: The acid-precipitated p-hydroxybenzaldehyde solid is dissolved in water again, and then subjected to a second activated carbon decolorization at 78-82℃. After filtration, cooling to 25-35℃ for crystallization, centrifugation and drying are performed to obtain p-hydroxybenzaldehyde.

[0008] Preferably, in step S1, the molar ratio of p-cresol to sodium hydroxide is 1:(2.0-3.5), the mass ratio of methanol to p-cresol is (0.02-0.10):1, and the total amount of the composite catalyst added is 0.1%-0.8% of the mass of p-cresol.

[0009] Preferably, in step S1, cobalt chloride, copper acetate, and copper sulfate are added in two separate steps. Cobalt chloride and copper acetate are added after sodium hydroxide is completely dissolved. Copper sulfate is added after a stable reflux liquid appears in the reflux condenser connected to the mixing vessel, and reflux pre-activation continues to disperse the composite catalyst in the alkaline methanol system.

[0010] Preferably, in step S1, the pH of the feed solution is controlled to be no less than 12 during the pre-activation process, and after the pre-activation is completed, the alkaline pre-activated feed solution is transferred into the oxidation reactor under the condition of heat preservation.

[0011] Preferably, in step S3, the total amount of oxygen introduced is determined according to the molar amount of p-methylphenol fed, and the total amount of oxygen introduced is the amount of oxygen molar amount corresponding to 1.05 to 1.40 times the molar amount of p-methylphenol fed, and is converted into the oxygen volume under standard conditions; the planned oxygenation time is 8 to 9 hours; and the reference oxygen feed flow rate is the oxygen volume flow rate obtained by dividing the total amount of oxygen introduced by the planned oxygenation time.

[0012] Preferably, in step S3, the first stage is from the start of oxygen supply until the cumulative oxygen supply reaches 15% to 25% of the set total oxygen supply; the second stage is from the cumulative oxygen supply increasing from 15% to 25% of the set total oxygen supply to 70% to 90%; and the third stage is from the cumulative oxygen supply exceeding 70% to 90% of the set total oxygen supply until the endpoint determination condition is met.

[0013] Preferably, in step S4, the residual amount of p-cresol is detected by liquid chromatography, wherein the residual amount of p-cresol is the mass percentage of p-cresol in the reaction solution relative to the total amount of organic components; the oxygen absorption rate is calculated based on the change in oxygen feed rate and tail gas oxygen content per unit time; and the peak oxygen absorption rate is the maximum oxygen absorption rate recorded in the same batch of oxidation process.

[0014] Preferably, in step S3, the oxidation vessel uses a circulating pump to circulate the reaction liquid between the bottom of the oxidation vessel, the circulating pipeline, and the inner cavity of the oxidation vessel. The volume of the circulating reaction liquid per hour is 2 to 8 times the volume of the reaction liquid inside the oxidation vessel. The temperature of the oxidation vessel is adjusted by circulating water or chilled water so that the temperature fluctuation inside the oxidation vessel relative to the set temperature does not exceed ±3℃.

[0015] Preferably, in step S5, the distilled methanol is condensed and then enters a methanol recovery tank, and after further distillation, it is returned to step S1 as recovered methanol for pre-activation of feed ingredients. The moisture content of the recovered methanol is controlled to be no higher than 1.0% before returning to step S1. In step S6, the holding time for the first activated carbon decolorization is 0.8 to 1.2 hours, the hydrochloric acid is 25% to 35% by mass, and the endpoint of acid precipitation is controlled by online pH detection. Preferably, in step S7, the heat preservation time for the second activated carbon decolorization is 1.5 to 2.5 hours, and at least part of the centrifuged mother liquor after the refined crystallization is returned to the crude p-hydroxybenzaldehyde solid water dissolution step or acid precipitation crystallization step in step S6 for recycling.

[0016] Preferably, in steps S3 and S4, the oxidation reactor is equipped with a remote detection unit for oxygen content, temperature, pressure, and stirring status; when the oxygen content reaches the preset high limit, the temperature reaches the preset high limit, the pressure reaches the preset high limit, or the stirring malfunctions, the oxygen inlet shut-off valve is cut off, the chilled water inlet / outlet shut-off valve and the nitrogen inlet shut-off valve are opened, and pressure relief protection is provided through a safety valve or rupture disc.

[0017] This invention combines a composite catalyst composed of cobalt chloride, copper acetate, and copper sulfate with alkaline methanol reflux pre-activation, ensuring that p-methylphenol, alkali, and catalyst are in a dispersed contact state before entering the oxidation reactor. During the oxidation stage, an oxygen buffer tank is used for oxygen supply, and a three-stage segmented low-pressure oxygen flow is employed. This ensures that the oxygen feed intensity in the initial, main, and supplementary oxidation stages is adapted to the reaction process. The oxygen flow endpoint is determined by the residual p-methylphenol content and oxygen absorption rate, reducing batch deviations caused by fixed-time reaction termination. In the post-treatment stage, a sequential combination of methanol recovery, first activated carbon decolorization, pH 2.0–2.5 acid precipitation, second activated carbon decolorization, and low-temperature crystallization reduces the impact of colored impurities and further oxidation byproducts on the finished product quality. Therefore, this invention enables the stable preparation of p-hydroxybenzaldehyde under low-pressure reactor conditions, while also considering raw material conversion, selectivity, finished product purity, color, and solvent recovery. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the overall process flow for the preparation method of p-hydroxybenzaldehyde according to the present invention.

[0019] Figure 2 This is a schematic diagram of the segmented low-pressure oxidation control of the present invention, wherein... Figure 2 (a) is a schematic diagram of the device and piping relationships. Figure 2 (b) is a schematic diagram of the three-stage oxygen supply control logic. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of the present invention.

[0021] I. Raw materials, equipment and testing methods In this embodiment, unless otherwise specified, all percentage contents are mass percentage contents; all pressures are gauge pressures; and the oxygen volume under standard conditions is calculated at 0°C and 101.325 kPa, which is used to uniformly calculate and set the total amount of oxygen introduced and the baseline oxygen feed flow rate, and does not limit the oxygen to be under these temperature and pressure conditions during actual oxygen supply.

[0022] The p-methylphenol used is an industrial grade with a purity of not less than 98%; methanol has a purity of not less than 99.5%; sodium hydroxide is caustic soda flakes with a purity of not less than 96%; cobalt chloride, copper acetate, and copper sulfate are industrial or reagent-grade metal salts; hydrochloric acid has a mass fraction of 25%–35%; and activated carbon is powdered activated carbon suitable for decolorizing aromatic aldehyde intermediates. If cobalt chloride, copper acetate, or copper sulfate are in hydrated salt form, the amount added should be calculated based on their effective salt content.

[0023] The main equipment includes a batching kettle (connected to a reflux condenser), an oxidation kettle, an oxygen buffer tank, a flow meter, an oxygen feed regulating valve, a circulating pump, a solvent extraction kettle, a condenser, a methanol recovery tank, a crystallization kettle, a centrifuge, an activated carbon dissolving kettle, a decolorization kettle, a filter, an acid precipitation crystallization kettle, and drying equipment. The oxidation kettle is equipped with a temperature detection unit, a pressure detection unit, a tail gas oxygen content detection unit, a stirring status detection unit, an oxygen inlet shut-off valve, a nitrogen inlet shut-off valve, and a cooling medium shut-off valve; in case of abnormal pressure rise, pressure relief protection can be provided through safety valves or rupture discs.

[0024] The residues of p-methylphenol, p-hydroxybenzaldehyde, and major byproducts were determined by high-performance liquid chromatography (HPLC). The detection conditions were as follows: C18 column; mobile phase: methanol and 0.1% phosphoric acid aqueous solution (volume ratio 55:45); detection wavelength: 280 nm; column temperature: 30 °C; flow rate: 1.0 mL / min; injection volume: 10 μL. The sample was diluted with methanol and then filtered through a 0.45 μm filter before injection.

[0025] Methanol moisture content was determined using the Karl Fischer method. The color of the finished product was determined using the platinum-cobalt colorimetric method and expressed as APHA value. The purity of the finished product was calculated using the HPLC area normalization method or the external standard method. The yield was calculated based on the mass of the dried p-hydroxybenzaldehyde product and its HPLC purity.

[0026] The total amount of oxygen introduced is determined based on the molar amount of p-methylphenol fed, satisfying the following: In the formula, To determine the molar amount of oxygen corresponding to the total amount of oxygen introduced; The excess oxygen coefficient is taken as 1.05 to 1.40; This represents the molar amount of p-methylphenol added.

[0027] Under standard conditions, the total volume of oxygen introduced satisfies: In the formula, The set oxygen level under standard conditions is introduced into the total volume; This represents the molar volume of a gas under standard conditions. The meaning is the same as above.

[0028] The baseline oxygen feed flow rate meets the following requirements: In the formula, The baseline oxygen feed rate; The set oxygen level under standard conditions is introduced into the total volume; The planned oxygenation time is 8-9 hours.

[0029] The oxygen absorption rate can be calculated based on the changes in oxygen feed rate and tail gas oxygen content, satisfying the following: In the formula, For a moment oxygen uptake rate; For a moment oxygen feed volumetric flow rate; This represents the volume fraction of oxygen in the feed oxygen. This refers to the exhaust gas volumetric flow rate; This represents the volume fraction of oxygen in the exhaust gas. This represents the molar volume of the gas under standard conditions.

[0030] p-Cresol conversion rate satisfies: In the formula, The conversion rate of p-methylphenol; This represents the molar amount of p-methylphenol at the time of feeding; This represents the molar amount of p-methylphenol remaining during the reaction sampling.

[0031] The selectivity of p-hydroxybenzaldehyde refers to the proportion of p-methylphenol that has undergone the reaction that is converted into the target product p-hydroxybenzaldehyde, rather than into p-hydroxybenzoic acid, tar-like byproducts, high-boiling-point byproducts, or other substances. It must satisfy the following: In the formula, Selectivity for p-hydroxybenzaldehyde; This represents the molar amount of p-hydroxybenzaldehyde generated in the reaction solution.

[0032] The yield of p-hydroxybenzaldehyde product meets the following requirements: In the formula, The yield of p-hydroxybenzaldehyde product; The quality of the dried p-hydroxybenzaldehyde product; This represents the mass fraction of p-hydroxybenzaldehyde in the finished product. This represents the molar mass of p-hydroxybenzaldehyde; The meaning is the same as above.

[0033] To ensure comparability of test results, all embodiments and comparative examples were tested using the same feed scale, testing instruments, and calculation methods. During the oxidation reaction, a sample of the reaction solution was taken from the oxidation reactor every 30 minutes. After dilution with methanol and filtration, the residual amount of p-methylphenol, the amount of p-hydroxybenzaldehyde generated, and the peak area of ​​the main byproducts were detected by high-performance liquid chromatography (HPLC). The endpoint sample was the reaction solution after oxygen supply was stopped and nitrogen purging was completed. The finished product sample was taken from the finally dried p-hydroxybenzaldehyde product. After mixing each batch of finished product, samples were taken from no less than three sampling points and combined to form a representative sample for testing the purity, color, and calculating the yield of the finished product. Oxidation temperature fluctuation was represented by the maximum deviation of the oxidation reactor temperature record value from the start to the stop of oxygen supply during the oxidation reaction relative to the set temperature. All data were calculated as the average of parallel test results for the same batch; when the deviation of parallel test results exceeded the preset allowable range, resampling and retesting were performed.

[0034] II. Overall Implementation of the Method of the Invention like Figure 1 As shown, the present invention provides a method for preparing p-hydroxybenzaldehyde, including S1 pre-activation of raw materials, S2 displacement feeding, S3 segmented low-pressure oxidation, S4 endpoint determination and purging, S5 solvent removal and crude product separation, S6 primary decolorization and acid precipitation, and S7 secondary decolorization and refined crystallization.

[0035] In the S1 pre-activation step, p-methylphenol, methanol, and sodium hydroxide are added to the mixing vessel. Stirring is started to gradually dissolve the sodium hydroxide, forming an alkaline methanol system. Subsequently, a composite catalyst composed of cobalt chloride, copper acetate, and copper sulfate is added. The mass ratio of cobalt chloride, copper acetate, and copper sulfate in the composite catalyst is 1:(1.5–3.5):(0.3–1.2). The mixing vessel is heated to 65–70°C and maintained at this temperature under methanol reflux for 0.8–1.2 hours to obtain an alkaline pre-activated feed solution.

[0036] In one embodiment, cobalt chloride and copper acetate are added after sodium hydroxide is completely dissolved. Copper sulfate is added after a stable reflux liquid appears in the reflux condenser connected to the batching vessel, and reflux pre-activation continues to disperse the composite catalyst in the alkaline methanol system. During pre-activation, the pH of the feed solution is controlled to be no lower than 12. After pre-activation, the alkaline pre-activated feed solution is transferred to the oxidation vessel under heat preservation conditions.

[0037] In the S2 replacement feeding step, the oxidation reactor is purged with nitrogen. After purging, the oxygen content inside the oxidation reactor is checked and found to meet the feeding requirements before the alkaline pre-activated feed solution is fed into the oxidation reactor. After feeding is completed, external circulation is started, allowing the reaction solution to circulate between the bottom of the oxidation reactor, the circulation pipeline, and the inner cavity of the oxidation reactor. The volume of the circulating reaction solution per hour can be 2 to 8 times the volume of the reaction solution inside the oxidation reactor. External circulation is used to reduce local differences in material temperature, catalyst dispersion state, and reactant concentration within the reactor.

[0038] External circulation refers to the process of drawing the reaction liquid from the bottom or lower outlet of the oxidation vessel, pumping it through a circulation pump to an external circulation pipeline, and then returning it to the oxidation vessel from the top, side, or inlet, creating a continuous circulation flow between the oxidation vessel and the external circulation pipeline. The external circulation pipeline can be equipped with heat exchange sections, sampling ports, or mixing sections to assist in regulating the reaction liquid temperature, improving the mixing state of the materials, and enhancing the uniformity of oxygen contact with the reaction liquid. In this invention, after activating external circulation, the alkaline pre-activated feed liquid remains in circulation before entering the oxidation stage. This helps reduce local temperature differences, catalyst dispersion differences, and reactant concentration differences within the oxidation vessel, providing more stable initial conditions for the subsequent staged oxygenation oxidation process.

[0039] like Figure 2 As shown, in the S3 segmented low-pressure oxidation step, refer to Figure 2 (a) Liquid oxygen is vaporized and then enters an oxygen buffer tank, the pressure of which is controlled at 0.4–0.8 MPa. Oxygen then enters the oxidation reactor from the oxygen buffer tank via a flow meter and a regulating valve. During the oxidation process, the temperature inside the oxidation reactor is controlled at 70–80°C, and the pressure is below 0.2 MPa.

[0040] refer to Figure 2 (b) In this embodiment, the oxygen flow rate in the three stages can be controlled as follows: the oxygen flow rate in the first oxygenation stage is 40% to 70% of the baseline oxygen feed flow rate; the oxygen flow rate in the second oxygenation stage is 90% to 130% of the baseline oxygen feed flow rate; and the oxygen flow rate in the third oxygenation stage is 20% to 60% of the baseline oxygen feed flow rate. The first oxygenation stage is from the start of oxygenation until the cumulative oxygen intake reaches 15% to 25% of the set total oxygen intake; the second oxygenation stage is from the cumulative oxygen intake increasing from 15% to 25% of the set total oxygen intake to 70% to 90%; and the third oxygenation stage is from the cumulative oxygen intake exceeding 70% to 90% of the set total oxygen intake until the endpoint determination condition is met.

[0041] The first oxygenation stage uses a low oxygen feed rate, lower than or close to the baseline oxygen feed rate, to gradually transition the system from anoxic or hypoxic state to an oxidizing state, thereby reducing temperature fluctuations at the start of the reaction. The second oxygenation stage uses an oxygen feed rate close to or slightly higher than the baseline oxygen feed rate to meet the higher oxygen consumption demands of the main oxidation stage. The third oxygenation stage uses a low-flow supplemental oxygen supply to reduce residual p-methylphenol and minimize side reactions caused by excess oxygen in the later stages of the reaction.

[0042] In the S4 endpoint determination and purging step, a dual endpoint determination is performed using both the residual amount of p-cresol and the oxygen absorption rate. When the residual amount of p-cresol drops below 1.0% and the oxygen absorption rate continuously decreases to below 15% of the peak oxygen absorption rate for 30 minutes, oxygen supply is stopped, and the mixture is purged with nitrogen. The peak oxygen absorption rate is the maximum oxygen absorption rate recorded during the same batch of oxidation. Using this dual endpoint condition avoids batch deviations caused by terminating the reaction at a fixed time.

[0043] In the S5 desolventizing and crude product separation step, the oxidation reaction solution is sent to the desolventizing vessel under nitrogen pressure, where methanol is distilled off at 65–70°C. The distilled methanol is condensed and then enters the methanol recovery tank, where it is further refined by distillation and returned to the pre-activation step of the feedstock as recovered methanol. Before returning the recovered methanol for use, the moisture content is controlled to be no higher than 1.0%. The desolventized material is then subjected to crude product crystallization and centrifugation to obtain crude p-hydroxybenzaldehyde solid.

[0044] In the S6 decolorization and acid precipitation step, the crude p-hydroxybenzaldehyde solid is dissolved in water, activated carbon is added, and the first activated carbon decolorization is carried out at 75–80℃ for 0.8–1.2 hours. The decolorized material is filtered and then fed into an acid precipitation crystallization kettle. The pH is adjusted to 2.0–2.5 with 25%–35% hydrochloric acid for acid precipitation crystallization. The acid-precipitated material is centrifuged to obtain acid-precipitated p-hydroxybenzaldehyde solid.

[0045] In the S7 secondary decolorization and refining crystallization step, the acid-precipitated p-hydroxybenzaldehyde solid is dissolved again in water, activated carbon is added, and a second activated carbon decolorization is carried out at 78–82℃ for 1.5–2.5 hours. The decolorized material is filtered and then fed into a refining crystallization kettle, cooled to 25–35℃ for crystallization, centrifuged, and dried to obtain p-hydroxybenzaldehyde. At least a portion of the centrifugation mother liquor after refining crystallization can be recycled back to the crude p-hydroxybenzaldehyde solid water dissolution step or the acid precipitation crystallization step.

[0046] III. Examples The following examples illustrate a kilogram-scale verification test based on a 1kg feed of p-methylphenol. In actual industrial production, the scale can be scaled up or adjusted proportionally according to equipment volume, heat transfer conditions, and test results.

[0047] 1. Example 1 Weigh 1000g of p-methylphenol, with a purity of 98.2%; weigh 906g of sodium hydroxide; weigh 60g of methanol. Weigh 3.5g of a composite catalyst, in which cobalt chloride, copper acetate, and copper sulfate are prepared at an effective salt mass ratio of 1:2.5:0.7.

[0048] Add p-methylphenol and methanol to the mixing vessel, start stirring, and add sodium hydroxide in batches. After the sodium hydroxide dissolves, add cobalt chloride and copper acetate. Heat the mixing vessel to 68°C, and after a stable reflux liquid appears in the reflux condenser connected to the mixing vessel, add copper sulfate. Continue reflux pre-activation at 68°C for 1.0 h to obtain an alkaline pre-activated solution. At the end of pre-activation, the pH of the solution is 12.8.

[0049] The oxidation reactor was purged with nitrogen, and then the alkaline pre-activated feed solution was transferred into the oxidation reactor. The agitator and external circulation were started, with the circulation pump set to circulate a reaction liquid volume five times the volume inside the reactor per hour. The oxidation reactor temperature was set to 75℃, and the pressure was controlled to be below 0.2 MPa.

[0050] Based on the molar amount of p-methylphenol fed, the excess oxygen coefficient is calculated. The value is set to 1.20. The planned oxygenation time is 8.5 hours. The oxygen flow rate for the first oxygenation stage is set to 60% of the baseline oxygen feed flow rate until the cumulative oxygen flow reaches 20% of the set total oxygen flow. The oxygen flow rate for the second oxygenation stage is set to 115% of the baseline oxygen feed flow rate until the cumulative oxygen flow reaches 80% of the set total oxygen flow. The oxygen flow rate for the third oxygenation stage is set to 40% of the baseline oxygen feed flow rate until the endpoint determination criteria are met.

[0051] During the oxidation process, samples were taken every 30 minutes for HPLC analysis, and the oxygen feed rate, tail gas oxygen content, reactor temperature, and reactor pressure were recorded simultaneously. When the residual p-cresol content reached 0.62% and the oxygen absorption rate continuously decreased to 12.4% of the peak oxygen absorption rate for 30 minutes, oxygen supply was stopped and nitrogen purging was performed. The reactor temperature fluctuated within ±2.1℃ during the oxidation process, and the reactor pressure ranged from 0.12 to 0.18 MPa.

[0052] The reaction mixture was pumped to a desolventizing vessel, where methanol was distilled off at 68°C. The methanol was collected after condensation and entered the recovery system. The desolventized product was subjected to crude crystallization and centrifugation to obtain crude p-hydroxybenzaldehyde solid. The crude p-hydroxybenzaldehyde solid was dissolved in water, and activated carbon equivalent to 1.0% of the crude product mass was added. The mixture was kept at 78°C for 1.0 h for decolorization. After filtration, the pH was adjusted to 2.3 with 30% hydrochloric acid, followed by acid precipitation crystallization and centrifugation to obtain acid-precipitated p-hydroxybenzaldehyde solid. The acid-precipitated p-hydroxybenzaldehyde solid was dissolved in water again, and activated carbon equivalent to 0.8% of the acid-precipitated solid mass was added. The mixture was kept at 80°C for 2.0 h for decolorization. After filtration, the mixture was cooled to 30°C for crystallization, centrifuged, and dried to obtain the finished p-hydroxybenzaldehyde product.

[0053] According to the test results, the conversion rate of p-methylphenol in this embodiment was 99.3%, the selectivity of p-hydroxybenzaldehyde was 92.4%, the HPLC purity of the finished product was 99.2%, the yield of the finished product was 84.6%, and the color of the finished product was APHA28.

[0054] 2. Example 2 The difference between this embodiment and Example 1 is as follows: the total amount of composite catalyst added is 0.2% of the mass of p-methylphenol; cobalt chloride, copper acetate, and copper sulfate are prepared at an effective salt mass ratio of 1:1.5:0.3; the pre-activation temperature is 65℃, and the pre-activation time is 0.8h; the oxygen excess coefficient is... The oxygen flow rate is set to 1.05; the planned oxygenation time is 8.0 hours; in the first oxygenation stage, the oxygen flow rate is 50% of the baseline oxygen feed flow rate, and the cumulative oxygen flow reaches 15% of the set total oxygen supply; in the second oxygenation stage, the oxygen flow rate is 100% of the baseline oxygen feed flow rate, and the cumulative oxygen flow reaches 75% of the set total oxygen supply; in the third oxygenation stage, the oxygen flow rate is 30% of the baseline oxygen feed flow rate, until the endpoint determination condition is met. The remaining steps are the same as in Example 1.

[0055] Oxygenation was stopped when the residual amount of p-methylphenol was 0.93% and the oxygen absorption rate dropped to 14.2% of the peak oxygen absorption rate for 30 consecutive minutes. Testing showed that the conversion rate of p-methylphenol was 98.7%, the selectivity of p-hydroxybenzaldehyde was 90.8%, the HPLC purity of the finished product was 98.8%, the yield was 82.1%, and the color of the finished product was APHA42.

[0056] 3. Example 3 The difference between this embodiment and Example 1 is as follows: the total amount of composite catalyst added is 0.6% of the mass of p-methylphenol; cobalt chloride, copper acetate, and copper sulfate are prepared at an effective salt mass ratio of 1:3.5:1.2; the pre-activation temperature is 70°C, and the pre-activation time is 1.2 h; the oxygen excess coefficient is... The value is 1.35; the planned oxygenation time is 9.0 hours; the oxygen flow rate in the first oxygenation stage is 70% of the baseline oxygen feed flow rate, and the cumulative oxygen flow reaches 25% of the set total oxygen supply; the oxygen flow rate in the second oxygenation stage is 125% of the baseline oxygen feed flow rate, and the cumulative oxygen flow reaches 90% of the set total oxygen supply; the oxygen flow rate in the third oxygenation stage is 50% of the baseline oxygen feed flow rate, until the endpoint determination condition is met. The remaining steps are the same as in Example 1.

[0057] Oxygenation was stopped when the residual amount of p-methylphenol was 0.48% and the oxygen absorption rate dropped to 10.8% of the peak oxygen absorption rate for 30 consecutive minutes. Testing showed that the p-methylphenol conversion rate was 99.5%, the selectivity for p-hydroxybenzaldehyde was 91.6%, the HPLC purity of the finished product was 99.1%, the yield was 83.8%, and the color of the finished product was APHA34.

[0058] IV. Comparative Examples 1. Comparative Example 1: Existing Conventional Cobalt-Copper Catalytic Oxidation Method This comparative example is used to simulate the existing conventional cobalt-copper catalytic oxidation method for preparing p-hydroxybenzaldehyde. The feed scale of this comparative example is the same as that of Example 1, except that: the catalyst used is cobalt acetate and copper acetate, added at a mass ratio of 1:2.5, without adding cobalt chloride and copper sulfate; pre-activation with methanol reflux at 65–70°C is not performed, and the mixture is only transferred to the oxidation reactor after complete dissolution and homogenization of sodium hydroxide; a constant oxygen feed flow rate is used, without segmented oxygen supply according to the first, second, and third oxygen supply stages; the reaction endpoint is controlled by a fixed oxygen supply time of 8.5 hours; and the post-treatment involves solvent removal, acid precipitation crystallization, and a single recrystallization, without the two-stage decolorization process of "first activated carbon decolorization—acid precipitation—second activated carbon decolorization".

[0059] The results showed that the conversion rate of p-methylphenol was 97.2%, the selectivity of p-hydroxybenzaldehyde was 84.1%, the HPLC purity of the finished product was 97.0%, the yield of the finished product was 73.6%, the color of the finished product was APHA148, and the temperature fluctuation of the reactor during the oxidation process was ±7.2℃.

[0060] 2. Comparative Example 2: No copper sulfate added This comparative example is basically the same as Example 1, except that copper sulfate is not added to the composite catalyst; only cobalt chloride and copper acetate are used in a mass ratio of 1:2.5, and the total amount of catalyst added is kept the same as in Example 1. The remaining pre-activation, staged oxygenation, endpoint determination, and post-treatment steps are the same as in Example 1.

[0061] The results showed that the conversion rate of p-methylphenol was 97.8%, the selectivity of p-hydroxybenzaldehyde was 86.5%, the HPLC purity of the finished product was 97.9%, the yield of the finished product was 76.4%, the color of the finished product was APHA95, and the temperature fluctuation during the oxidation process was ±3.6℃.

[0062] 3. Comparative Example 3: No methanol reflux pre-activation. This comparative example is basically the same as Example 1, except that: after mixing p-methylphenol, methanol, sodium hydroxide, and the composite catalyst until the sodium hydroxide dissolves, the mixture is directly transferred to the oxidation reactor for oxidation without pre-activation by methanol reflux at 65-70°C. The remaining steps of staged oxygenation, endpoint determination, and post-treatment are the same as in Example 1.

[0063] The results showed that the conversion rate of p-methylphenol was 96.9%, the selectivity of p-hydroxybenzaldehyde was 85.2%, the HPLC purity of the finished product was 97.5%, the yield of the finished product was 74.9%, the color of the finished product was APHA108, and the temperature fluctuation of the reactor during the oxidation process was ±5.8℃.

[0064] 4. Comparative Example 4: Using constant flow rate oxygenation This comparative example is basically the same as Example 1, except that it does not use a three-stage segmented oxygenation process. Instead, oxygen is introduced at a constant baseline oxygen feed rate throughout the entire oxidation process. The total oxygen supply, planned oxygenation time, and endpoint determination criteria are the same as in Example 1. The remaining steps are the same as in Example 1.

[0065] The results showed that the conversion rate of p-methylphenol was 99.1%, the selectivity of p-hydroxybenzaldehyde was 87.3%, the HPLC purity of the finished product was 98.2%, the yield of the finished product was 78.2%, the color of the finished product was APHA112, and the temperature fluctuation of the reactor during the oxidation process was ±6.9℃.

[0066] 5. Comparative Example 5: The reaction was terminated only at a fixed time. This comparative example is basically the same as Example 1, except that the dual endpoint determination of residual p-methylphenol and oxygen absorption rate is not used. Instead, the reaction is stopped after 8.5 hours of oxygenation. The remaining staged oxygenation and post-treatment steps are the same as in Example 1.

[0067] The results showed that the conversion rate of p-methylphenol was 99.4%, the selectivity of p-hydroxybenzaldehyde was 88.3%, the HPLC purity of the finished product was 98.4%, the yield was 79.1%, and the color of the finished product was APHA86. In the HPLC analysis, the peak area ratio of p-hydroxybenzoic acid and high-boiling-point byproducts increased compared to Example 1.

[0068] 6. Comparative Example 6: Decolorization with activated carbon only once. This comparative example is basically the same as Example 1, except that: the acid-precipitated p-hydroxybenzaldehyde solid is not subjected to a second activated carbon decolorization; after acid precipitation and centrifugation, it is directly crystallized, centrifuged, and dried. The remaining steps are the same as in Example 1.

[0069] The test results showed that the conversion rate of p-methylphenol was 99.3%, the selectivity of p-hydroxybenzaldehyde was 92.1%, the HPLC purity of the finished product was 98.2%, the yield of the finished product was 83.7%, and the color of the finished product was APHA176.

[0070] V. Test Results The test results are shown in Table 1.

[0071] Table 1. Preparation process and finished product test results of p-hydroxybenzaldehyde under different preparation conditions. As shown in Table 1, under the conditions of composite catalyst ratio, pre-activation temperature, pre-activation time, segmented low-pressure oxygenation, dual endpoint determination, and two-stage decolorization and acid precipitation purification specified in this invention, Examples 1-3 achieved a p-methylphenol conversion rate of 98.7%-99.5%, a p-hydroxybenzaldehyde selectivity of 90.8%-92.4%, a product HPLC purity of 98.8%-99.2%, a product yield of 82.1%-84.6%, and a product color of APHA 28-42.

[0072] Comparative Example 1 is a conventional cobalt-copper catalytic oxidation method, but its catalytic system, oxygenation method, endpoint control, and post-treatment process do not employ the combined control method of this invention. Compared with Example 1, Comparative Example 1 shows lower selectivity, product purity, product yield, and color, and larger fluctuations in oxidation temperature. These results demonstrate that this invention does not merely adjust a single parameter in a conventional preparation method, but rather achieves a coordinated reaction process and post-treatment process through a combination of composite catalysts, pre-activation, staged low-pressure oxidation, dual endpoint determination, and two-stage decolorization and acid precipitation purification.

[0073] Comparative Example 2, without the addition of copper sulfate, showed lower selectivity and yield than Example 1 while maintaining the same total catalyst amount. This result indicates that copper sulfate has an impact on the oxidation reaction process in this composite catalyst system.

[0074] Comparative Example 3, which did not undergo methanol reflux pre-activation, showed increased temperature fluctuations during oxidation, resulting in decreased conversion, selectivity, and yield. These results indicate that ensuring sufficient contact between p-methylphenol, sodium hydroxide, and the composite catalyst under methanol reflux conditions before oxygen introduction is beneficial in reducing the heterogeneity of the initial oxidation stage.

[0075] Comparative Example 4 used a constant flow rate of oxygen. Although the conversion rate of p-methylphenol was close to that of Example 1, the selectivity, product yield, and color were all lower than those of Example 1, and the temperature fluctuations were larger. These results indicate that a constant flow rate of oxygen cannot adapt to the changes in oxygen consumption at different stages of the reaction, while segmented oxygen supply can control the oxygen supply intensity at the initial stage, the main oxidation stage, and the supplementary oxidation stage.

[0076] Comparative Example 5, which terminated the reaction at a fixed time, achieved a higher conversion rate, but its selectivity and color were lower than those of Example 1, and the proportion of byproducts increased. This result indicates that a fixed time endpoint is insufficient to accommodate variations in oxidation rates across different batches; using a dual endpoint of residual p-methylphenol and oxygen uptake rate can achieve more stable process results in terms of both feedstock conversion and side reaction control.

[0077] Comparative Example 6 underwent only one activated carbon decolorization process, and its oxidation stage data were close to those of Example 1, but the purity and color of the finished product were lower than those of Example 1. This result indicates that the second activated carbon decolorization and refined crystallization are mainly used to improve the purity and appearance of the finished product, and together with the upstream oxidation control, constitute a complete preparation method.

[0078] In summary, the composite catalyst, pre-activation, segmented low-pressure oxygenation, dual endpoint determination, and two-stage decolorization and acid precipitation purification steps all have an impact on the reaction process control or product quality. Furthermore, the combination of these steps can maintain good conversion rate, selectivity, yield, color, and temperature control stability in the preparation process of p-hydroxybenzaldehyde.

[0079] The foregoing description of embodiments of the present invention, through which those skilled in the art are able to implement or use the present invention, will be readily apparent to those skilled in the art. Various modifications to these embodiments will be readily apparent to those skilled in the art. The general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novelty disclosed herein.

Claims

1. A process for the preparation of p-hydroxybenzaldehyde, characterized in that, Includes the following steps: S1. Add p-methylphenol, methanol and sodium hydroxide to a mixing tank and stir to dissolve. Add a composite catalyst composed of cobalt chloride, copper acetate and copper sulfate, wherein the mass ratio of cobalt chloride, copper acetate and copper sulfate is 1:(1.5~3.5):(0.3~1.2). Pre-activate under methanol reflux at 65~70℃ for 0.8~1.2h to obtain an alkaline pre-activated solution. S2. After purging the oxidation reactor with nitrogen, the alkaline pre-activated liquid is sent into the oxidation reactor, and the external circulation is started. S3. After the liquid oxygen is oxidized, it is sent to the oxidation reactor through an oxygen buffer tank of 0.4-0.8MPa. Based on the benchmark oxygen feed flow rate calculated according to the set total oxygen supply and the planned oxygen supply time, oxygen is introduced in three stages in sequence according to the benchmark oxygen feed flow rates of 40%-70%, 90%-130%, and 20%-60%. In each stage, the temperature inside the oxidation reactor is controlled at 70-80℃ and the pressure is lower than 0.2MPa. S4. When the residual amount of p-methylphenol drops below 1.0% and the oxygen absorption rate drops below 15% of the peak oxygen absorption rate for 30 consecutive minutes, stop the oxygen supply and purge with nitrogen. S5. The reaction liquid is sent to the desolventizing vessel, where it is distilled off and methanol is recovered at 65-70°C. The desolventized product is then crystallized and centrifuged to obtain crude p-hydroxybenzaldehyde solid. S6. After dissolving the crude p-hydroxybenzaldehyde solid in water, the first activated carbon decolorization was carried out at 75-80℃. After filtration, the pH was adjusted to 2.0-2.5 with hydrochloric acid for acid precipitation crystallization. After centrifugation, the acid-precipitated p-hydroxybenzaldehyde solid was obtained. S7. After the acid-precipitated p-hydroxybenzaldehyde solid is dissolved in water again, it is decolorized a second time with activated carbon at 78-82℃, filtered, cooled to 25-35℃ for crystallization, centrifuged and dried to obtain p-hydroxybenzaldehyde.

2. The method for preparing p-hydroxybenzaldehyde according to claim 1, characterized in that, In step S1, the molar ratio of p-cresol to sodium hydroxide is 1:(2.0-3.5), the mass ratio of methanol to p-cresol is (0.02-0.10):1, and the total amount of the composite catalyst added is 0.1%-0.8% of the mass of p-cresol.

3. The method for preparing p-hydroxybenzaldehyde according to claim 1, characterized in that, In step S1, cobalt chloride, copper acetate, and copper sulfate are added in two separate steps. Cobalt chloride and copper acetate are added after sodium hydroxide is completely dissolved. Copper sulfate is added after a stable reflux liquid appears in the reflux condenser connected to the mixing vessel, and reflux pre-activation continues to disperse the composite catalyst in the alkaline methanol system.

4. The method for preparing p-hydroxybenzaldehyde according to claim 1, characterized in that, In step S1, the pH of the feed solution is controlled to be no less than 12 during the pre-activation process. After the pre-activation is completed, the alkaline pre-activated feed solution is transferred into the oxidation reactor under the condition of heat preservation.

5. The method for preparing p-hydroxybenzaldehyde according to claim 1, characterized in that, In step S3, the total amount of oxygen introduced is determined according to the molar amount of p-methylphenol fed. The total amount of oxygen introduced is the amount of oxygen corresponding to 1.05 to 1.40 times the molar amount of p-methylphenol fed, and is converted into the oxygen volume under standard conditions. The planned oxygenation time is 8 to 9 hours. The reference oxygen feed flow rate is the oxygen volume flow rate obtained by dividing the total amount of oxygen introduced by the planned oxygenation time.

6. The method for preparing p-hydroxybenzaldehyde according to claim 5, characterized in that, In step S3, the first stage is from the start of oxygen supply until the cumulative oxygen supply reaches 15% to 25% of the set total oxygen supply; the second stage is from the cumulative oxygen supply increasing from 15% to 25% of the set total oxygen supply to 70% to 90%; and the third stage is from the cumulative oxygen supply exceeding 70% to 90% of the set total oxygen supply until the endpoint determination condition is met.

7. The method for preparing p-hydroxybenzaldehyde according to claim 1, characterized in that, In step S4, the residual amount of p-cresol is detected by liquid chromatography. The residual amount of p-cresol is the mass percentage of p-cresol in the reaction solution relative to the total amount of organic components. The oxygen absorption rate is calculated based on the change in oxygen feed rate and tail gas oxygen content per unit time. The peak oxygen absorption rate is the maximum oxygen absorption rate recorded in the same batch of oxidation process.

8. The method for preparing p-hydroxybenzaldehyde according to claim 1, characterized in that, In step S3, the oxidation vessel uses a circulating pump to circulate the reaction liquid between the bottom of the oxidation vessel, the circulation pipeline, and the inner cavity of the oxidation vessel. The volume of the circulating reaction liquid per hour is 2 to 8 times the volume of the reaction liquid inside the oxidation vessel. The temperature of the oxidation vessel is adjusted by circulating water or chilled water so that the temperature fluctuation inside the oxidation vessel relative to the set temperature does not exceed ±3℃.

9. The method for preparing p-hydroxybenzaldehyde according to claim 1, characterized in that, In step S5, the distilled methanol is condensed and then enters the methanol recovery tank. After further distillation, it is returned to step S1 as recovered methanol for pre-activation of feed ingredients. The moisture content of the recovered methanol is controlled to be no higher than 1.0% before returning to step S1. In step S6, the holding time for the first activated carbon decolorization is 0.8 to 1.2 hours, the hydrochloric acid is 25% to 35% by mass, and the acid precipitation endpoint is controlled by online pH detection. And / or, in step S7, the holding time for the second activated carbon decolorization is 1.5 to 2.5 hours, and at least part of the centrifuged mother liquor after the refined crystallization is returned to the crude p-hydroxybenzaldehyde solid water dissolution step or acid precipitation crystallization step in step S6 for recycling.

10. The method for preparing p-hydroxybenzaldehyde according to any one of claims 1 to 9, characterized in that, In steps S3 and S4, the oxidation reactor is equipped with a remote detection unit for oxygen content, temperature, pressure, and stirring status. When the oxygen content reaches the preset high limit, the temperature reaches the preset high limit, the pressure reaches the preset high limit, or the stirring malfunctions, the oxygen inlet shut-off valve is shut off, the chilled water inlet / outlet shut-off valve and the nitrogen inlet shut-off valve are opened, and pressure relief protection is provided through a safety valve or rupture disc.

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