A continuous synthesis process for preparing salicylaldehyde by directional oxidation of o-cresol

Abstract

The application relates to the technical field of organic synthesis, and discloses a continuous synthesis process for preparing salicylaldehyde through directional oxidation of o-cresol, which continuously mixes o-cresol, a reaction solvent and a weak alkaline buffer, so that the o-cresol is in a partial phenolate state and enters a reaction system; then, a gas-liquid micro-dispersion unit and oxygen which is introduced in sections form a gas-liquid micro-dispersion flow, and the o-cresol is continuously passed through a multi-temperature-zone fixed bed reactor filled with copper-vanadium-cerium composite oxides / silicon carbide honeycomb catalysts, so that the side chain methyl of the o-cresol is selectively oxidized into an aldehyde group; after the reaction effluent is rapidly cooled, the effluent enters a bisulfite aldehyde group capturing unit, deep oxidation of salicylaldehyde is inhibited, and then, the salicylaldehyde product is obtained through resolution, oil-water separation and rectification. The application has the advantages of high continuous degree, good oxidation selectivity, less by-product salicylic acid, easy separation of the catalyst, and recyclable solvent and unreacted raw materials.

Description

Technical Field

[0001] This invention relates to the field of organic synthesis technology, specifically to a continuous synthesis process for the directional oxidation of o-cresol to prepare salicylaldehyde. Background Technology

[0002] Salicylic aldehyde, also known as o-hydroxybenzaldehyde, is an important fine chemical intermediate. Its molecular structure contains both phenolic hydroxyl and aldehyde groups, exhibiting high reactivity. It can be used in fragrances, pharmaceuticals, pesticides, dyes, resin auxiliaries, and metal complexes. With the increasing demands for purity and stable supply of raw materials from downstream fine chemical industries, developing highly selective, continuous, and environmentally friendly salicylic aldehyde preparation processes is of great significance. Existing methods for preparing salicylaldehyde mainly include the phenol hydroxymethylation oxidation method, the o-cresol oxidation method, and other multi-step synthetic methods. Among these, the methyl group in the o-cresol molecule is located ortho to the phenolic hydroxyl group; theoretically, the side-chain methyl group can be selectively oxidized to an aldehyde group, thus directly yielding salicylaldehyde. This route has a wide range of raw material sources and relatively simple reaction steps, making it promising for industrial application. However, the oxidation of o-cresol is a typical selective oxidation reaction. During the reaction, it is necessary to activate the side-chain methyl group, prevent the generated salicylaldehyde from being further oxidized to salicylic acid, and simultaneously inhibit the phenolic hydroxyl group from participating in side reactions. Because the aldehyde group of salicylaldehyde is highly reactive, it is prone to deep oxidation in the presence of oxygen and a catalyst, leading to an increase in salicylic acid, by-condensates, and coking products, thus reducing the selectivity and purity of the target product. Traditional o-cresol oxidation processes often employ batch reactors, which suffer from low gas-liquid mass transfer efficiency, uneven local oxygen concentration, difficulty in timely removal of exothermic gases, and wide distribution of reaction residence time. When oxygen or oxygen-containing gases are involved in the reaction, improper oxygen supply control can easily lead to localized over-oxidation or safety risks; conversely, insufficient oxygen supply results in low o-cresol conversion, making it difficult to balance conversion rate and selectivity. Therefore, it is necessary to provide a continuous synthesis process for the directional oxidation of o-cresol to salicylaldehyde. This process combines reaction liquid state control, micro-dispersion and segmented oxygen supply, multi-temperature zone fixed-bed catalytic oxidation, and timely aldehyde group capture to improve the selectivity of o-cresol side-chain oxidation, reduce the formation of byproducts such as salicylic acid, and achieve the recycling of raw materials and solvents, thereby enhancing the industrial applicability and production stability of the process. Summary of the Invention

[0003] In order to overcome the shortcomings of the prior art, the present invention aims to provide a continuous synthesis process for the directional oxidation of o-cresol to prepare salicylaldehyde. This process combines weak alkaline buffer regulation, gas-liquid micro-dispersion oxygen supply, segmented oxygenation, multi-temperature zone fixed-bed catalytic oxidation, and online aldehyde group capture, so that the methyl side chain of o-cresol can be selectively converted into aldehyde group under continuous flow conditions, thereby improving the selectivity and continuous stability of salicylaldehyde process.

[0004] To achieve the above objectives, the present invention provides the following technical solution: a continuous synthesis process for the directional oxidation of o-cresol to prepare salicylaldehyde, comprising the following steps: S1. o-Cresol, reaction solvent and weak base buffer are continuously mixed to obtain o-Cresol reaction solution, wherein the weak base buffer is used to allow o-Cresol to enter the oxidation reaction in a partially phenolic state. S2. The o-cresol reaction solution is fed into the gas-liquid micro-dispersion unit, and oxygen is introduced into the gas-liquid micro-dispersion unit and the subsequent reaction section in a segmented manner, so that the o-cresol reaction solution and oxygen form a gas-liquid micro-dispersion flow. S3. The gas-liquid micro-dispersed flow is continuously passed through a multi-temperature zone fixed-bed reactor packed with a directional oxidation catalyst, so that the methyl side chain of o-cresol is selectively oxidized to an aldehyde group to obtain a reaction effluent containing salicylaldehyde; wherein, the directional oxidation catalyst is a copper-vanadium-cerium composite oxide catalyst supported on a silicon carbide honeycomb carrier. S4. The reaction effluent is cooled immediately after leaving the fixed-bed reactor and continuously enters the aldehyde capture unit to form a reversible adduct between salicylaldehyde and the aqueous solution of bisulfite, thereby inhibiting the further oxidation of salicylaldehyde to salicylic acid. S5. The aldehyde capturing solution is analyzed, separated into oil and water, and distilled to obtain salicylaldehyde product. Unreacted o-cresol and reaction solvent are dehydrated and returned to step S1 for recycling.

[0005] Further, in step S1, the reaction solvent is one of dimethyl carbonate, γ-butyrolactone, and propylene glycol methyl ether; the mass ratio of o-cresol to the reaction solvent is 1:(1.5-8.0).

[0006] Further, in step S1, the weak alkaline buffer is one of sodium carbonate, potassium carbonate, and sodium bicarbonate; the amount of the weak alkaline buffer added is 0.05 to 2.0% of the mass of o-cresol.

[0007] Furthermore, in step S2, the gas-liquid micro-dispersion unit is a microchannel mixer; the average diameter of the bubbles in the gas-liquid micro-dispersion flow is 50-100 μm, and the molar ratio of oxygen to o-cresol in the oxygen-containing gas is 0.6-1.6:1.

[0008] Further, in step S2, the oxygen is added in at least two streams. The first stream of oxygen-containing gas is added in the gas-liquid micro-dispersion unit, and the second stream of oxygen-containing gas is added in the middle or rear section of the fixed bed reactor. The first stream of oxygen-containing gas accounts for 35-75% of the total oxygen-containing gas, and the second stream of oxygen-containing gas accounts for 25-65% of the total oxygen-containing gas.

[0009] Further, in step S3, the multi-temperature zone fixed-bed reactor includes a preheating dispersion zone, a low-temperature induced oxidation zone, and a selective oxidation zone connected in sequence; the temperature of the low-temperature induced oxidation zone is 65–95°C, the temperature of the selective oxidation zone is 90–120°C, the reaction pressure is 0.5–1 MPa, and the liquid hourly space velocity is 0.2–0.9 h⁻¹. -1 .

[0010] Furthermore, in step S4, after the reaction effluent leaves the fixed-bed reactor, it is cooled to 20-30°C within 30-40 seconds and then continuously contacted with an aqueous solution of sodium bisulfite; the mass concentration of the aqueous solution of sodium bisulfite is 8-12%.

[0011] Furthermore, in step S5, the aldehyde capture solution releases salicylaldehyde after pH adjustment and analysis, and the resulting organic phase is subjected to vacuum distillation to obtain salicylaldehyde product; the mother liquor obtained by separation with o-cresol content higher than 30wt% is returned to step S1 after dehydration and removal of light components, and the water content in the circulating mother liquor is controlled below 0.2wt% and the salicylic acid content is controlled below 0.5wt%.

[0012] Compared with the prior art, the present invention has the following beneficial technical effects: (1) The present invention uses a weak alkaline buffer to make o-cresol in a partially phenolic state, thereby increasing the reactivity of the methyl side chain of o-cresol without using strong alkaline conditions. This is beneficial to promote the directional oxidation of methyl to aldehyde group, while reducing side reactions of phenolic hydroxyl group and polymerization side reactions. (2) The present invention adopts gas-liquid micro-dispersion and segmented oxygen supply method to make oxygen uniformly distributed in the liquid phase, reduce the risk of peroxidation caused by excessive local oxygen concentration, and improve the stability and safety of oxidation reaction in fixed bed. (3) The present invention uses a copper-vanadium-cerium composite oxide / silicon carbide honeycomb catalyst, which utilizes the redox synergy between copper, vanadium and cerium as well as the thermal conductivity and low pressure drop characteristics of silicon carbide honeycomb support to improve the selectivity of o-cresol side chain oxidation and reduce the difficulty of catalyst pulverization, loss and separation. (4) In this invention, the reaction effluent is rapidly cooled and aldehyde groups are captured after leaving the fixed bed, so that the generated salicylaldehyde is converted into a reversible adduct in a timely manner, reducing its residence time in the oxygen-containing high-temperature system, thereby inhibiting the further oxidation of salicylaldehyde to salicylic acid and improving the yield and purity of the target product. (5) The present invention recycles unreacted o-cresol and reaction solvent after dehydration, reducing the consumption of raw materials and solvents and reducing the amount of waste liquid discharged, which is suitable for continuous industrial production. Attached Figure Description

[0013] Figure 1 This is a flowchart of the continuous reaction apparatus and process flow for Example 1. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0015] The directional oxidation catalyst is a copper-vanadium-cerium composite oxide catalyst supported on a silicon carbide honeycomb support. In the copper-vanadium-cerium composite oxide catalyst, the copper content is 1.0–8.0 wt%, the vanadium content is 0.2–3.0 wt%, and the cerium content is 0.5–6.0 wt% by total catalyst mass, with the balance being the silicon carbide support and unavoidable impurities; the copper, vanadium, and cerium are fixed on the support surface in the form of oxides and / or oxygen-vacancy composite oxides. The method for preparing the directional oxidation catalyst includes: sequentially washing a silicon carbide honeycomb support with alkali, acid, and water, drying it, and then immersing it in a composite precursor solution containing copper salt, vanadium salt, and cerium salt. After drying, pre-calcining in an inert atmosphere, and activation calcining in an oxygen-containing atmosphere, a directional oxidation catalyst with a copper-vanadium-cerium composite oxide active layer on its surface is obtained. The inert atmosphere pre-calcination temperature is 250–380°C, and the oxygen-containing atmosphere activation calcination temperature is 320–520°C.

[0016] The directional oxidation catalysts used in the following examples are all copper-vanadium-cerium composite oxide / silicon carbide honeycomb catalysts, and their preparation methods are as follows: A silicon carbide honeycomb carrier with a pore density of 200 pores / square inch was selected. The carrier was placed in a 5% sodium hydroxide aqueous solution and treated at 60°C for 2 hours. After removal, it was rinsed with deionized water. Then, it was placed in a 5% nitric acid aqueous solution and treated at room temperature for 1 hour. It was then washed with deionized water until the washing solution was neutral and dried at 110°C for 8 hours to obtain the pretreated silicon carbide honeycomb carrier. Based on 1000g of pretreated silicon carbide honeycomb carrier, 165g of copper nitrate trihydrate and 101g of cerium nitrate hexahydrate were weighed and dissolved in 800mL of deionized water; separately, 30g of ammonium metavanadate and 30g of oxalic acid were weighed and added to 500mL of deionized water at 70℃, and stirred until a clear solution was formed. The two solutions were mixed and adjusted to a suitable volume for carrier impregnation with deionized water to obtain the composite precursor solution. The composite precursor solution was loaded onto a silicon carbide honeycomb support using an equal-volume stepwise impregnation method. After each impregnation, the substrate was dried at 80℃ for 2 hours, and the impregnation was repeated three times. After impregnation, the substrate was dried at 110℃ for 10 hours, then heated to 320℃ at a rate of 2℃ / min under a nitrogen atmosphere and held for 2 hours. Subsequently, the temperature was switched to an air atmosphere and heated to 430℃ at a rate of 2℃ / min, held for 4 hours, and then allowed to cool naturally to obtain the copper-vanadium-cerium composite oxide / silicon carbide honeycomb catalyst. Inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis revealed that the catalyst contained 3.9 wt% copper, 1.2 wt% vanadium, and 2.9 wt% cerium, with the remainder being silicon carbide support and unavoidable impurities. Anhydrous sodium carbonate, anhydrous potassium carbonate, or sodium bicarbonate should be pulverized to a particle size D90 of no more than 20 μm before use, and a dispersion slurry with a mass concentration of 10% should be prepared using the corresponding reaction solvent. This slurry should be continuously added through a diaphragm metering pump to ensure the uniformity of the feeding of the weakly alkaline buffer. Example 1

[0017] This embodiment provides a continuous synthesis process for the directional oxidation of o-cresol to prepare salicylaldehyde, specifically including the following steps: S1. o-Cresol and dimethyl carbonate are continuously fed into a feed line equipped with an online shear mixer via metering pumps, with a mass ratio of o-cresol to dimethyl carbonate of 1:3.0. Simultaneously, sodium bicarbonate dispersion is added, ensuring that the actual amount of sodium bicarbonate added is 0.50% of the mass of o-cresol. The mixing temperature is controlled at 25℃, and the mixing time is 3 minutes to obtain the o-cresol reaction solution. S2. The o-cresol reaction solution is fed into a microchannel mixer, while an oxygen-nitrogen mixture with a volume fraction of 30% is simultaneously introduced. The molar ratio of oxygen to o-cresol is 0.95:1. The oxygen-containing gas is added in two streams: the first stream, comprising 55% of the total oxygen-containing gas, is added through the inlet of the microchannel mixer; the second stream, comprising 45%, is added through the middle of the fixed-bed reactor. After dispersion in the microchannel mixer, the average bubble diameter is 70 μm. S3. The resulting gas-liquid micro-dispersed flow is continuously fed into a multi-temperature zone fixed-bed reactor loaded with a copper-vanadium-cerium composite oxide / silicon carbide honeycomb catalyst. The fixed-bed reactor is sequentially configured with a preheating dispersion zone, a low-temperature induced oxidation zone, and a selective oxidation zone. The temperature of the preheating dispersion zone is 60℃, the temperature of the low-temperature induced oxidation zone is 75℃, and the temperature of the selective oxidation zone is 100℃. The reaction pressure is controlled at 0.60 MPa, and the liquid hourly space velocity (LHSV) is 0.50 h⁻¹ based on the total volume of the o-cresol reaction solution. -1 . S4. The reaction effluent immediately enters a tubular heat exchanger after leaving the fixed-bed reactor, where it is cooled to 26°C within 35 seconds. It then continuously enters the aldehyde capture unit, where it contacts a 10% (w / w) sodium bisulfite aqueous solution. The molar ratio of sodium bisulfite to the theoretical amount of salicylaldehyde in the reaction effluent is 1.15:1, and the contact time is 20 minutes, causing salicylaldehyde to form a sodium bisulfite adduct. S5. Add a 10% sulfuric acid solution to the obtained aldehyde capture solution to adjust the pH to 2.0. Stir and decompose at 40℃ for 40 min to release salicylaldehyde from the adduct. After standing and separating the layers, collect the organic phase, wash with water and dry with anhydrous sodium sulfate, and then perform vacuum distillation at 8 kPa to collect the main fraction of salicylaldehyde. The mother liquor containing o-cresol after distillation is dehydrated and light components are removed by thin-film evaporation, reducing the water content to 0.17 wt%, the salicylic acid content to 0.38 wt%, and the o-cresol content to 38.5 wt%, and then returned to step S1 for recycling. In this embodiment, after 72 hours of continuous operation, the o-cresol conversion rate was 72.8%, the salicylaldehyde selectivity was 87.1%, the salicylaldehyde separation yield was 61.9%, the purity of the obtained salicylaldehyde product was 98.9%, the salicylic acid content was 0.34 wt%, and the aldehyde group capture rate was 96.8%. Example 2

[0018] This embodiment provides a continuous synthesis process for the directional oxidation of o-cresol to prepare salicylaldehyde, specifically including the following steps: S1. o-Cresol and γ-Butyrolactone are continuously fed into an online shear mixer at a mass ratio of 1:4.0, while anhydrous sodium carbonate dispersion is added simultaneously, so that the actual amount of sodium carbonate added is 0.80% of the mass of o-cresol. The mixing temperature is 25℃ and the mixing time is 3 min to obtain o-cresol reaction solution. S2. The o-cresol reaction solution is fed into a microchannel mixer, while an oxygen-nitrogen mixture with an oxygen volume fraction of 30% is simultaneously introduced. The molar ratio of oxygen to o-cresol is 1.05:1. The oxygen-containing gas is added in two streams: the first stream, accounting for 60% of the total oxygen-containing gas, is added through the inlet of the microchannel mixer; the second stream, accounting for 40%, is added through the middle of the fixed-bed reactor. The average bubble diameter in the gas-liquid micro-dispersion flow is 65 μm. S3. The gas-liquid micro-dispersion flow is continuously fed into a multi-temperature zone fixed-bed reactor. The preheating dispersion zone temperature is 65℃, the low-temperature induced oxidation zone temperature is 80℃, and the selective oxidation zone temperature is 105℃; the reaction pressure is controlled at 0.70 MPa, and the liquid hourly space velocity is 0.45 h⁻¹. -1 . S4. After leaving the fixed-bed reactor, the reaction effluent enters a high-efficiency tubular heat exchanger, where it is cooled to 24°C within 32 seconds. It is then continuously mixed with a 10% (w / w) sodium bisulfite aqueous solution. The theoretical molar ratio of sodium bisulfite to salicylaldehyde is 1.20:1, and the contact time is 20 minutes. S5. The pH of the aldehyde capture solution was adjusted to 2.0 using a 10% sulfuric acid solution, and the solution was analyzed at 40℃ for 40 min. After oil-water separation, the resulting organic phase was subjected to vacuum distillation at 10 kPa to obtain salicylaldehyde product. The mother liquor containing o-cresol was returned to step S1 after dehydration and removal of light components. Testing showed that the recycled mother liquor contained 36.9 wt% o-cresol, 0.15 wt% water, and 0.31 wt% salicylic acid, meeting the requirements for recycling. In this embodiment, after 72 hours of continuous operation, the o-cresol conversion rate was 75.6%, the salicylaldehyde selectivity was 88.4%, the salicylaldehyde separation yield was 64.7%, the salicylaldehyde product purity was 99.2%, the salicylic acid content was 0.28 wt%, and the aldehyde group capture rate was 97.5%. This embodiment showed a good match between the o-cresol conversion rate and the salicylaldehyde selectivity. Example 3

[0019] This embodiment provides a continuous synthesis process for the directional oxidation of o-cresol to prepare salicylaldehyde, specifically including the following steps: S1. o-Cresol and propylene glycol methyl ether are continuously fed into an online shear mixer at a mass ratio of 1:5.0, while anhydrous potassium carbonate dispersion is added simultaneously, so that the actual amount of potassium carbonate added is 1.00% of the mass of o-cresol. The mixing temperature is 30℃ and the mixing time is 3 min to obtain o-cresol reaction solution. S2. The o-cresol reaction solution is fed into a microchannel mixer, while an oxygen-nitrogen mixture with an oxygen volume fraction of 30% is simultaneously introduced. The molar ratio of oxygen to o-cresol is 1.15:1. The oxygen-containing gas is added in two streams: the first stream, accounting for 65% of the total oxygen-containing gas, is added through the inlet of the microchannel mixer; the second stream, accounting for 35%, is added through the latter half of the fixed-bed reactor. The average bubble diameter in the gas-liquid micro-dispersion flow is 60 μm. S3. The gas-liquid micro-dispersion flow is continuously fed into a multi-temperature zone fixed-bed reactor. The preheating dispersion zone temperature is 65℃, the low-temperature induced oxidation zone temperature is 85℃, and the selective oxidation zone temperature is 110℃; the reaction pressure is controlled at 0.80 MPa, and the liquid hourly space velocity is 0.40 h⁻¹. -1 . S4. The effluent from the reaction is cooled to 23°C within 30 seconds, and then continuously contacted with a 12% sodium bisulfite aqueous solution. The theoretical molar ratio of sodium bisulfite to salicylaldehyde is 1.20:1, and the contact time is 18 minutes. S5. The pH of the aldehyde capture solution was adjusted to 1.8 using a 10% sulfuric acid solution. The solution was then analyzed at 40°C for 40 min. After oil-water separation, the resulting organic phase was subjected to vacuum distillation at 10 kPa to obtain salicylaldehyde. The mother liquor containing o-cresol was dehydrated and treated to remove light components before being returned to step S1. The test results showed that the content of o-cresol in the circulating mother liquor was 35.8 wt%, the water content was 0.16 wt%, and the salicylic acid content was 0.35 wt%, which met the requirements for recycling. In this embodiment, after 72 hours of continuous operation, the o-cresol conversion rate was 77.3%, the salicylaldehyde selectivity was 87.6%, the salicylaldehyde separation yield was 65.5%, the salicylaldehyde product purity was 99.1%, the salicylic acid content was 0.32 wt%, and the aldehyde group capture rate was 97.2%. Comparative Example 1 The only difference between this comparative example and Example 2 is that: in step S1, anhydrous sodium carbonate or other weakly alkaline buffers are not added, and o-cresol and γ-butyrolactone are directly and continuously mixed at a mass ratio of 1:4.0; the remaining gas-liquid micro-dispersion method, segmented oxygen supply ratio, reaction temperature, reaction pressure, liquid hourly space velocity, rapid cooling, aldehyde group capture and desorption distillation conditions are the same as in Example 2. Because no weak base buffer was added, o-cresol could not form a moderately phenolic state before entering the oxidation reaction, and the activation degree of the side chain methyl group was reduced. This comparative example was run continuously for 72 hours. The o-cresol conversion rate was 67.2%, the salicylaldehyde selectivity was 83.6%, the salicylaldehyde separation yield was 54.9%, the product purity was 98.1%, the salicylic acid content was 0.55 wt%, and the aldehyde group capture rate was 95.8%. The treated circulating mother liquor contained 42.7 wt% o-cresol, 0.18 wt% water, and 0.54 wt% salicylic acid. Since the salicylic acid content in the circulating mother liquor exceeded 0.5 wt%, it did not meet the requirements for direct recycling. Comparative Example 2 The only difference between this comparative example and Example 2 is that the microchannel mixer is removed in step S2, and a perforated gas distribution tube is used for conventional bubbling oxygen supply. Specifically, a first stream of oxygen-containing gas, comprising 60% of the total oxygen content, is introduced into the o-cresol reaction solution through a perforated distribution pipe located at the bottom of the gas-liquid premixing tank. A second stream of oxygen-containing gas, comprising 40% of the total oxygen content, is added from the center of the fixed-bed reactor. The average diameter of the bubbles formed by conventional bubbling is 0.8–1.5 mm, and the molar ratio of oxygen to o-cresol remains 1.05:1. All other reaction and post-treatment conditions are the same as in Example 2. Because conventional bubbling produces larger bubbles, the gas-liquid contact area is reduced, resulting in decreased uniformity of oxygen dispersion and mass transfer efficiency in the reaction liquid. Consequently, localized fluctuations in oxygen concentration are likely to occur in the fixed bed. Comparative Example 3 The only difference between this comparative example and Example 2 is that step S3 uses a single-temperature zone fixed-bed reactor, without setting up a low-temperature induced oxidation zone and a selective oxidation zone. Specifically, except for the preheated dispersion zone, the temperature of all catalyst beds was uniformly controlled at 105℃, the reaction pressure at 0.70 MPa, and the liquid hourly space velocity at 0.45 h⁻¹. -1 The molar ratio of oxygen to o-cresol, the average bubble diameter, the segmented oxygen supply ratio, the rapid cooling conditions, the aldehyde group capture conditions, and the analytical distillation conditions were all the same as in Example 2. Because the catalyst bed uses a single temperature control, the activation of the o-cresol side chain methyl group and the generation of the aldehyde group cannot be carried out in suitable temperature ranges. The temperature at the front end of the reactor is too high, causing some of the newly generated salicylaldehyde to be further oxidized to salicylic acid in an oxygen-containing catalytic environment.

[0020] This comparative example was run continuously for 72 hours. The o-cresol conversion rate was 73.8%, the salicylaldehyde selectivity was 82.7%, the salicylaldehyde separation yield was 59.3%, the product purity was 98.2%, the salicylic acid content was 0.66 wt%, and the aldehyde group capture rate was 96.0%. The treated circulating mother liquor contained 37.2 wt% o-cresol, 0.18 wt% water, and 0.63 wt% salicylic acid. Since the salicylic acid content in the circulating mother liquor exceeded 0.5 wt%, it did not meet the requirements for direct recycling.

[0021] Test methods and test data To verify the technical effectiveness of the continuous synthesis process for preparing salicylaldehyde by the directional oxidation of o-cresol in this invention, the reaction solutions and salicylaldehyde products obtained in Examples 1-3 and Comparative Examples 1-3 were tested as follows. I. Testing Methods 1. Tests on o-cresol conversion, salicylaldehyde selectivity, and salicylaldehyde separation yield. After the reaction temperature, pressure, feed flow rate and oxygen flow rate of each embodiment and comparative example were stabilized, they were run continuously for 4 hours as a stabilization stage. After stabilization, a sample of the reaction effluent was taken every 1 hour for a total of 3 times. The samples were mixed evenly and used as test samples. Take 1.00 g of the reaction effluent sample, add ethyl acetate to a final volume of 25 mL, and add ethyl benzoate as an internal standard. After thorough shaking and mixing, filter through a 0.22 μm organic filter membrane. Gas chromatography was used to determine the contents of o-cresol, salicylaldehyde, and major organic byproducts. The gas chromatography conditions were as follows: a non-polar quartz capillary column, an FID detector, an injection port temperature of 250 °C, a detector temperature of 280 °C, and a column temperature program of initial temperature 60 °C held for 2 min, then ramping to 220 °C at a rate of 10 °C / min and holding for 5 min.

[0022] The conversion rate of o-cresol is calculated using the following formula: X = (n0 - n1) / n0 × 100% In the formula, X is the conversion rate of o-cresol (%), n0 is the molar amount of o-cresol entering the reactor (mol), and n1 is the molar amount of unreacted o-cresol in the effluent (mol). Salicylaldehyde selectivity is calculated using the following formula: S = n² / (n₀ - n₁) × 100% In the formula, S represents the selectivity of salicylaldehyde (%), and n2 represents the molar amount of salicylaldehyde in the reaction effluent and the captured eluent (mol). The yield of salicylaldehyde separation is calculated using the following formula: Y = m1 / (n0 × M) × 100% In the formula, Y is the salicylaldehyde separation yield (%), m1 is the mass of salicylaldehyde product obtained after oil-water separation and vacuum distillation (g), and M is the molar mass of salicylaldehyde (122.12 g / mol). 2. Product purity test Take 0.50 g of salicylaldehyde product obtained by vacuum distillation, add ethyl acetate to make up to 25 mL, filter through a 0.22 μm filter membrane, and determine the product purity by gas chromatography area normalization method. The purity is determined by the percentage of the salicylaldehyde main peak area to the total organic peak area. Perform the test in triplicate and take the average value. 3. Salicylic acid byproduct content test Take 1.00 g of the reaction effluent or the aqueous phase sample after elution, dilute to 50 mL with a methanol-water mixture, sonicate for 10 min, filter, and determine the salicylic acid content using high-performance liquid chromatography (HPLC). The HPLC conditions were: C18 reversed-phase column; mobile phase: a mixture of methanol and 0.1% phosphoric acid aqueous solution (volume ratio 60:40); flow rate: 1.0 mL / min; column temperature: 30 °C; and UV detection wavelength: 230 nm. 4. Aldehyde group capture rate test The salicylaldehyde content in the reaction effluent before entering the aldehyde capture unit was measured, as well as the salicylaldehyde content released after sodium bisulfite capture, pH adjustment, and desorption. The aldehyde capture rate was calculated using the following formula: P = n³ / n⁴ × 100% In the formula, P is the aldehyde capture rate (%), n3 is the molar amount of salicylaldehyde captured and released (mol), and n4 is the theoretical molar amount of salicylaldehyde in the effluent before entering the capture unit (mol). 5. Continuous operation stability test The apparatuses used in the examples and comparative examples were run continuously for 72 hours under the same feed conditions, with samples taken every 8 hours to detect the o-cresol conversion and salicylaldehyde selectivity. The salicylaldehyde selectivity at the 8th hour of operation was used as the initial selectivity, and the ratio of the salicylaldehyde selectivity at the 72nd hour of operation to the initial selectivity was used as the selectivity retention rate to evaluate the stability of the continuous reaction. Selective retention rate is calculated using the following formula: R = S7² / S8 × 100% In the formula, R is the selectivity retention rate, %; S72 is the selectivity of salicylaldehyde after 72 hours of continuous operation, %; S8 is the selectivity of salicylaldehyde after 8 hours of continuous operation, %. 6. Circulating mother liquor quality test The circulating mother liquor after dehydration and removal of light components was tested. Water content was determined using the Karl Fischer titration method; salicylic acid content was determined using high-performance liquid chromatography (HPLC); and o-cresol content was determined using gas chromatography (GC). The water content in the circulating mother liquor was controlled below 0.2 wt%, the salicylic acid content was controlled below 0.5 wt%, and when the o-cresol content exceeded 30 wt%, it was returned to the front end to continue participating in the reaction.

[0023] Table 1: Reaction Performance Test

[0024] Table 2: Quality Test of Circulating Mother Liquor

[0025] As shown in Table 1, Examples 1-3 all achieved the directional oxidation of o-cresol to salicylaldehyde under continuous flow conditions. The only differences among the three examples were slight variations in buffer dosage, oxygen molar ratio, and reaction temperature. The o-cresol conversion rate, salicylaldehyde selectivity, and salicylaldehyde separation yield remained relatively stable, indicating that the process of this invention has good stability and repeatability within a certain parameter range. Example 2 showed a good match between conversion rate and selectivity, achieving a salicylaldehyde selectivity of 88.4%, a product purity of 99.2%, and a salicylic acid content of 0.28 wt%. In Comparative Example 1, no weakly alkaline buffer was added, preventing o-cresol from forming a suitable partially phenolic state and reducing the activation degree of the side-chain methyl groups. Therefore, both the o-cresol conversion rate and salicylaldehyde separation yield were lower than in Example 2. Comparative Example 2 used conventional bubbling oxygen supply, which reduced the uniformity of gas-liquid mass transfer and increased local oxygen concentration fluctuations, leading to decreased salicylaldehyde selectivity and increased salicylic acid content. Comparative Example 3 used a single-temperature zone fixed-bed reactor, which could not achieve segmented control of low-temperature induced oxidation and selective oxidation. This increased further oxidation of salicylaldehyde in the reaction bed, resulting in a higher salicylic acid content than in Example 2, and a decrease in both salicylaldehyde selectivity and separation yield. As shown in Table 2, the water content in the circulating mother liquor after dehydration and removal of light components in Examples 1-3 was all below 0.2 wt%, the salicylic acid content was all below 0.5 wt%, and the o-cresol content was all above 30 wt%, which meets the requirements for return to the front end for recycling. In Comparative Examples 1-3, due to insufficient reaction selectivity or byproduct control, the salicylic acid content in the circulating mother liquor was slightly above 0.5 wt%, which could easily lead to byproduct accumulation during long-term recycling, therefore it is not suitable for direct continuous reuse.

[0026] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0027] 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 the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

[0028] Those skilled in the art should understand that the above descriptions are merely several specific embodiments of the present invention, and not all embodiments.

Claims

1. A continuous synthesis process for the directional oxidation of o-cresol to prepare salicylaldehyde, characterized in that, Includes the following steps: S1. o-Cresol, reaction solvent, and weakly basic buffer are continuously mixed to obtain o-Cresol reaction solution. The weakly basic buffer is used to allow o-Cresol to enter the oxidation reaction in a partially phenolic state. The reaction solvent is one of dimethyl carbonate, γ-butyrolactone, and propylene glycol methyl ether. The weakly basic buffer is one of sodium carbonate, potassium carbonate, and sodium bicarbonate. The mass ratio of o-cresol to the reaction solvent is 1:(1.5-8.0). The amount of the weakly basic buffer added is 0.05–2.0% of the mass of o-cresol; S2. The o-cresol reaction solution is fed into the gas-liquid micro-dispersion unit, and oxygen is introduced into the gas-liquid micro-dispersion unit and the subsequent reaction section in a segmented manner, so that the o-cresol reaction solution and oxygen form a gas-liquid micro-dispersion flow. S3. The gas-liquid micro-dispersed flow is continuously passed through a multi-temperature zone fixed-bed reactor packed with a directional oxidation catalyst, so that the methyl side chain of o-cresol is selectively oxidized to an aldehyde group to obtain a reaction effluent containing salicylaldehyde; wherein, the directional oxidation catalyst is a copper-vanadium-cerium composite oxide catalyst supported on a silicon carbide honeycomb carrier. S4. The reaction effluent is cooled immediately after leaving the fixed-bed reactor and continuously enters the aldehyde capture unit to form a reversible adduct between salicylaldehyde and the aqueous solution of bisulfite, thereby inhibiting the further oxidation of salicylaldehyde to salicylic acid. S5. The aldehyde capturing solution is analyzed, separated into oil and water, and distilled to obtain salicylaldehyde product. Unreacted o-cresol and reaction solvent are dehydrated and returned to step S1 for recycling.

2. The continuous synthesis process for preparing salicylaldehyde by directional oxidation of o-cresol according to claim 1, characterized in that, In step S2, the gas-liquid micro-dispersion unit is a microchannel mixer; the average diameter of the bubbles in the gas-liquid micro-dispersion flow is 50-100 μm, and the molar ratio of oxygen to o-cresol in the oxygen-containing gas is 0.6-1.6:

1.

3. The continuous synthesis process for preparing salicylaldehyde by directional oxidation of o-cresol according to claim 1, characterized in that, In step S2, the oxygen is added in at least two streams. The first stream of oxygen-containing gas is added in the gas-liquid micro-dispersion unit, and the second stream of oxygen-containing gas is added in the middle or rear section of the fixed bed reactor. The first stream of oxygen-containing gas accounts for 35-75% of the total oxygen-containing gas, and the second stream of oxygen-containing gas accounts for 25-65% of the total oxygen-containing gas.

4. The continuous synthesis process for preparing salicylaldehyde by directional oxidation of o-cresol according to claim 1, characterized in that, In step S3, the multi-temperature zone fixed-bed reactor includes a preheating dispersion zone, a low-temperature induced oxidation zone, and a selective oxidation zone connected in sequence; the temperature of the low-temperature induced oxidation zone is 65–95°C, the temperature of the selective oxidation zone is 90–120°C, the reaction pressure is 0.5–1 MPa, and the liquid hourly space velocity is 0.2–0.9 h⁻¹. -1 .

5. The continuous synthesis process for preparing salicylaldehyde by directional oxidation of o-cresol according to claim 1, characterized in that, In step S4, after the reaction effluent leaves the fixed-bed reactor, it is cooled to 20-30°C within 30-40 seconds and then continuously contacted with an aqueous solution of sodium bisulfite; the mass concentration of the aqueous solution of sodium bisulfite is 8-12%.

6. The continuous synthesis process for preparing salicylaldehyde by directional oxidation of o-cresol according to claim 1, characterized in that, The aldehyde capture solution is pH adjusted and analyzed to release salicylaldehyde. The resulting organic phase is subjected to vacuum distillation to obtain salicylaldehyde product. The mother liquor with o-cresol content higher than 30 wt% is dehydrated and light component removed and returned to step S1. The water content in the circulating mother liquor is controlled below 0.2 wt%, and the salicylic acid content is controlled below 0.5 wt%.

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