Reaction liquid, method and device for preparing m-toluic acid from m-xylene

Abstract

The invention provides a reaction liquid, a method and a device for preparing m-toluic acid from m-xylene, and belongs to the field of fine chemical engineering. The reaction liquid for preparing the m-toluic acid is prepared from the following components in parts by weight: 100 parts of mixture containing m-xylene, 0.015 to 0.08 part of organic transition metal salt, 0.03 to 0.1 part of organic bromide ligand and 50 to 70 parts of reaction solvent. According to the method, a mixture containing m-xylene is taken as a raw material, a homogeneous metal catalyst compounded by organic transition metal salt and organic bromide ligand is taken as a catalyst system, and liquid-phase oxygen selective oxidation is carried out to prepare m-toluic acid; a micro-nano bubble reactor is adopted. The method has the advantages of short reaction time, high selectivity of m-toluic acid, low energy consumption, high synthesis efficiency, environmental protection and the like.

Description

A reaction solution, method, and apparatus for preparing m-methylbenzoic acid from m-xylene. Technical Field

[0001] This invention belongs to the field of fine chemicals, specifically relating to a reaction solution, method and apparatus for preparing m-methylbenzoic acid from m-xylene. Background Technology

[0002] m-Toluic acid, as an important organic synthesis intermediate, holds an irreplaceable position in the fields of chemical engineering, pesticides, pharmaceuticals, and materials. In the pesticide industry, m-Toluic acid is a key precursor for the synthesis of highly effective herbicides (such as rice weed control agent) and insect repellents (such as DEET). DEET, in particular, is internationally recognized as a standard mosquito repellent due to its low toxicity to humans and animals, environmental friendliness, and long-lasting mosquito-repellent effect, resulting in continuously growing market demand. In organic synthesis, it is an important raw material for the preparation of m-toluenenitrile, toluenediethylamine, and m-toluyl chloride, which are further used in the production of pharmaceuticals, dyes, fragrances, and functional materials. Furthermore, m-Toluic acid is also used as a color film developer and a plastic / rubber additive, demonstrating its wide range of applications.

[0003] Currently, the mainstream industrial process for producing m-methylbenzoic acid uses m-xylene as a raw material and achieves the conversion of methyl to carboxyl groups through liquid-phase catalytic oxidation. Depending on the oxidant and process conditions, it can be mainly divided into air oxidation, nitric acid oxidation, and cobalt-bromine catalytic oxidation methods.

[0004] Air oxidation, as the most widely used process, typically employs soluble cobalt salts (such as cobalt acetate) as catalysts, introducing air to carry out the oxidation reaction under conditions of 0.2-0.4 MPa pressure and 110-150℃ temperature. While this method offers advantages such as low oxidant cost and simple operation, it has significant drawbacks. The oxidation reaction typically requires 5-16 hours, and to prevent over-oxidation, the single-pass conversion rate of m-xylene is only 10-15%, resulting in low reaction efficiency. This low conversion rate necessitates the recycling of large quantities of raw materials, increasing energy and material consumption. Although the reaction improves the selectivity of m-methylbenzoic acid by reducing the conversion rate, the formation of over-oxidation products such as m-carboxybenzaldehyde and / or isophthalic acid is still unavoidable, leading to a decrease in product yield and reduced m-methylbenzoic acid selectivity. Furthermore, the oxidation reaction is a free radical chain reaction; in a batch bubble column, uneven gas-liquid mixing easily leads to localized hot spots and material accumulation, posing a risk of reaction runaway and significant safety hazards.

[0005] Nitric acid oxidation typically uses 35-40% nitric acid as the oxidant, with the oxidation reaction occurring at high temperatures of 140-260℃ and high pressures of 0.51-7.09 MPa. However, this method still has drawbacks. Concentrated nitric acid is highly corrosive, and the high temperature and pressure conditions place extremely high demands on the materials used in the equipment, requiring the use of special alloy materials, which significantly increases equipment investment and maintenance costs. Furthermore, it carries a potential explosion risk, imposing extremely high requirements on production management and operational standards. Under these violent reaction conditions, complex side reactions can occur, leading to an increase in nitration byproducts and the generation of large amounts of nitrogen oxide waste gas (such as NO2), necessitating a complex tail gas treatment system, increasing environmental pressure and treatment costs.

[0006] The cobalt-bromine catalytic oxidation method requires a cobalt-bromine synergistic catalytic system (such as a combination of cobalt acetate and sodium bromide) to oxidize m-xylene using acetic acid as a solvent. Although this system can increase the reaction rate and shorten the reaction time to 3-4 hours, it also faces the problem of over-oxidation, resulting in poor selectivity for m-methylbenzoic acid.

[0007] These traditional processes each have their own characteristics, but they generally suffer from low reaction efficiency and poor selectivity, making it difficult to meet the green and efficient development needs of modern chemical industry. To overcome the shortcomings of traditional processes, improvements have been made to varying degrees in recent years, but each still has its limitations, and the technical challenge of efficient synthesis of m-methylbenzoic acid has not yet been fully solved. Patent CN106831392B discloses a method for the co-production of benzoic acid, p-methylbenzoic acid, and m-methylbenzoic acid. This method uses a mixed xylene containing ethylbenzene, p-xylene, and m-xylene, with one or more of transition metal salts or oxides, N-hydroxyphthalimide compounds, metal phthalocyanines, and metal porphyrins as catalysts. An oxygen-containing gas with an oxygen mass percentage of not less than 15% is introduced, the reaction temperature is 115-190℃, the pressure is 0.2-3 MPa, and the average residence time in the liquid phase is 0.3-5 h. This method aims to solve the problems of limited raw material sources, high production costs, and large differences in oxidation rates among the mixed xylenes in the original technology, and is significantly different from this application. Patent CN109999914A discloses a catalyst for preparing m-methylbenzoic acid, its preparation method and application, which uses metal porphyrin sulfonate as a catalyst to prepare m-methylbenzoic acid under air oxidation conditions. However, the metal porphyrin synthesis process in this method is complex, the catalyst is easily deactivated during the reaction, and recycling and reuse are difficult and costly, making it difficult to meet the economic requirements of large-scale industrial production. Patent CN107903165A discloses a method for the continuous preparation of m-methylbenzoic acid using a tubular reactor. Hydrogen peroxide is used as the oxidant, and cobalt acetate-sodium bromide is used as the catalyst system. The substrate and oxidant are injected separately into the reactor via metering pumps. After preheating and mixing, the reaction occurs. The reaction solution is quenched and distilled to obtain the final product. This method solves the mass and heat transfer problems of traditional batch reactors and improves process safety. However, the use of hydrogen peroxide as the oxidant significantly increases raw material costs, making it less economical than the air oxidation method. Furthermore, the process has unsatisfactory selectivity, with a m-xylene conversion rate of only 50% and a m-methylbenzoic acid selectivity of 77.5%, resulting in product yields fluctuating between 60-90% and insufficient stability. Summary of the Invention

[0008] To address the problems existing in the prior art, this invention provides a reaction solution, method, and apparatus for the selective oxidation of m-xylene to prepare m-methylbenzoic acid. One objective of this invention is to utilize a liquid-phase reaction feedstock containing a mixture of m-xylene, with a homogeneous metal catalyst system composed of two or more organic transition metal salts and at least one organic bromide ligand as the catalyst. By combining the catalytic system with a micro / nano bubble reactor, this invention solves the problems of small mass transfer contact area between organic matter and oxygen, poor gas-phase mass transfer efficiency, poor reaction system homogeneity, susceptibility to local side reactions, poor selective oxidation of m-xylene, and over-oxidation in traditional liquid-phase oxidation reactors. A further objective of this invention is to address the problems of byproducts reducing the purity of m-methylbenzoic acid and causing byproduct resource loss by incorporating the byproducts methylbenzaldehyde and m-methylbenzyl alcohol generated during the reaction into the mixture containing m-xylene.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: On one hand, the present invention provides a reaction solution for the selective oxidation of m-xylene to prepare m-methylbenzoic acid, comprising the following components in parts by weight: 100 parts of a mixture containing m-xylene, 0.015-0.08 parts of an organic transition metal salt, 0.03-0.1 parts of an organic bromide ligand, and 50-70 parts of a reaction solvent.

[0010] In the reaction solution of this invention, a mixture containing m-xylene is used as the liquid-phase reaction raw material. The amounts of the organic transition metal salt, organic bromide ligand, and reaction solvent are controlled to be 150-800 ppm, 300-1000 ppm, and 50-70% of the weight of the liquid-phase reaction raw material, respectively. A homogeneous metal catalyst composed of organic transition metal salt and organic bromide ligand is used as the catalyst system. Through synergistic effects, the catalyst is stabilized and its activity is restored, thereby increasing the oxidation reaction rate. During the reaction, the organic bromide ligand generates bromine free radicals, which activate the hydrogen on the methyl group of m-xylene, initiating a free radical chain reaction. The metal ions of the organic transition metal salt undergo a valence state change during the reaction, and undergo free radical transformation during single-electron transfer, accelerating the chain reaction, oxidizing the methyl group on m-xylene, and then reducing the metal ions to form a catalytic cycle. The reaction solvent provides a homogeneous liquid-phase environment for the entire reaction process, promoting the mass transfer efficiency between the gas and liquid phases during the oxidation reaction.

[0011] Preferably, a reaction solution for the selective oxidation of m-xylene to prepare m-methylbenzoic acid comprises the following components in parts by weight: 100 parts of a mixture containing m-xylene, 0.02-0.065 parts of an organic transition metal salt, 0.03-0.05 parts of an organic bromide ligand, and 55-68 parts of a reaction solvent; wherein the mixture containing m-xylene comprises one or more of m-xylene, m-methylbenzaldehyde, and m-methylbenzyl alcohol.

[0012] In the selective oxidation of m-xylene to m-methylbenzoic acid, the generation of intermediate byproducts cannot be avoided due to the chain reaction. However, this invention uses the intermediate byproducts m-methylbenzaldehyde and m-methylbenzyl alcohol generated during the oxidation reaction as components of the liquid-phase reaction raw materials, which can make resource utilization of byproducts, realize continuous production, reduce the impact of byproducts on m-methylbenzoic acid, and improve the utilization rate of m-xylene and the selectivity of m-methylbenzoic acid.

[0013] Preferably, in the mixture containing m-xylene, the weight ratio of m-xylene to m-methylbenzaldehyde and / or m-methylbenzyl alcohol is (5-20):1.

[0014] Using m-xylene mixed with m-tolualdehyde and / or m-toluene as a liquid-phase reaction feedstock can increase the starting point of the oxidation reaction. Furthermore, there are numerous free radical transfer reactions between m-tolualdehyde and m-toluene, as well as between m-xylene and m-tolualdehyde and / or m-toluene, which enhances the efficiency of the oxidation reaction. Mixing m-xylene with intermediate by-products can further reduce local over-oxidation, help maintain a relatively stable level of intermediate by-products generated in the reaction system, and further improve selective oxidation.

[0015] As a further preferred embodiment, in the mixture containing m-xylene, the weight ratio of m-xylene to m-methylbenzaldehyde and / or m-methylbenzyl alcohol is (5-16):1.

[0016] Preferably, the organic transition metal salt in the component is at least two of the salicylates, maleates, or propions of titanium, nickel, chromium, palladium, cobalt, and manganese; the weight ratio of any two organic transition metal salts is (2-4):1. The metal ions in the organic transition metal salt and the bromine radicals in the organic bromide ligand can combine to form a multi-component synergistic catalytic system. Through valence state transitions, single electron transfers, and radical transfer, the efficiency of the oxidation reaction is promoted, the oxidation selectivity is improved, and the over-oxidation of m-methylbenzoic acid is avoided, as well as the ring opening and mineralization of the benzene ring.

[0017] In this invention, the organic transition metal salts not only play a role in the catalytic oxidation process due to the metal ions, but also allow different carboxylic acid ligands to fine-tune the formation and transformation of reaction intermediates through electronic or steric effects, thereby enhancing oxidation selectivity. For example, the unsaturated double bond of the maleate group can change the electron density of the metal center by accepting electrons, thus affecting the catalytic activity; the salicylate group itself can consume free radicals, which helps to inhibit over-oxidation and improve product selectivity; and the propionate group, due to its longer carbon chain, can improve its compatibility in the organic phase, making the distribution of active sites more uniform.

[0018] As a further preferred embodiment, the weight ratio of any two of the organic transition metal salts is (2.2-3.6):1.

[0019] Preferably, the organic bromide ligand in the component is one or more of tetraethylammonium bromide, tetrapropylammonium bromide, and tetramethylammonium bromide. By providing sufficient bromine radical activation and maintaining the efficient progress of the oxidation chain reaction, the concentration of bromine radicals at the reaction interface is guaranteed. Compared with sodium bromide, potassium bromide, or hydrobromic acid, the bromine source is more uniformly distributed, and the release of bromine radicals can be more stable and controllable. This helps to inhibit deep oxidation and improve the selectivity and yield of the target product, m-methylbenzoic acid.

[0020] Preferably, the reaction solvent in the components is one or more of acetic acid, propionic acid, and maleic acid. This provides a uniform liquid-phase environment for the reaction system, maintains the stability of the liquid-phase reaction raw materials and catalytic system, avoids excessively vigorous reactions that make it difficult to control the reaction temperature, and prevents the intensification of free radical movement, which could lead to excessive oxidation or coupling side reactions. It also prevents a decrease in oxidation selectivity, which could affect the purity of the target product, m-methylbenzoic acid.

[0021] On the other hand, the present invention provides a method for selectively oxidizing m-xylene to prepare m-methylbenzoic acid, wherein a homogeneous metal catalyst composed of a mixture of m-xylene, an organic transition metal salt and an organic bromide ligand is passed into a micro / nano bubble reactor for oxidation reaction, and then separated by distillation to obtain m-methylbenzoic acid.

[0022] In this invention, micro- and nanobubbles are introduced during the oxidation reaction process, increasing the efficiency of oxygen transfer to the liquid-phase reaction system, prolonging the contact time between oxygen and m-xylene, or m-xylene and m-methylbenzaldehyde and / or m-methylbenzyl alcohol, as well as organic transition metal salts and organic bromide ligands, thereby improving oxygen utilization. An oxygen-concentrated region is formed at the interface between the micro- and nanobubbles and the liquid phase, further promoting the oxidation reaction. The presence of the reaction solvent stabilizes the homogeneous environment of the reaction system, helps improve the uniformity of oxygen distribution, avoids local oxygen deficiency or excess, and effectively suppresses the occurrence of local side reactions, enabling efficient and highly selective conversion into the target product, m-methylbenzoic acid. When the micro- and nanobubbles rupture during the reaction, they also accelerate the movement of free radicals such as hydroxyl groups in the liquid phase, synergistically promoting a free radical chain reaction in the catalytic system and accelerating oxidation.

[0023] Preferably, in the oxidation reaction, the gas space velocity relative to the liquid phase volume is 200-600 h⁻¹, calculated for pure oxygen. -1 The oxygen concentration is 21-60%.

[0024] As a further preferred embodiment, in the oxidation reaction, the gas space velocity relative to the liquid phase volume is 250-450 h⁻¹, calculated using pure oxygen. -1 The oxygen concentration is 35-54%.

[0025] Preferably, in the oxidation reaction, the reaction temperature is 140-190℃, the reaction pressure is 0.9-1.7MPa, and the reaction space velocity (LHSV) is 0.08-0.3h. -1 .

[0026] As a further preferred embodiment, the oxidation reaction is carried out at a temperature of 160-190℃, a pressure of 1-1.5 MPa, and a space velocity (LHSV) of 0.1-0.22 h⁻¹. -1 .

[0027] On the other hand, the present invention provides an apparatus for the selective oxidation of m-xylene to prepare m-methylbenzoic acid, wherein the micro-nano bubble reactor is a bubble column or a reaction vessel, and the micro-nano bubbles generated have a size of 1-50 μm; the bubble column is a cylindrical tower structure with a height-to-diameter ratio of (3-6):1, and is equipped with a circulating flow sleeve inside, through which the micro-nano bubbles are generated by a porous ceramic material aerator; the micro-nano bubbles in the reaction vessel are generated by a high-speed rotating cutting device.

[0028] As a further preferred embodiment, the height-to-diameter ratio of the bubble column is (3.5-5):1.

[0029] Compared with existing technologies, the beneficial effects of this invention are as follows: By combining a specific catalytic system with micro / nano bubble oxidation, this invention significantly improves the conversion rate of m-xylene and the selectivity of m-methylbenzoic acid compared to traditional cobalt-bromine synergistic oxidation or hydrogen peroxide methods, achieving a selectivity of greater than 92% for the product m-methylbenzoic acid. Furthermore, after distillation separation, this invention can achieve a purity of greater than 99.2% for the m-methylbenzoic acid product. Detailed Implementation

[0030] A general embodiment of the reaction solution for the selective oxidation of m-xylene to prepare m-methylbenzoic acid comprises the following components in parts by weight: 100 parts of a mixture containing m-xylene, 0.015-0.08 parts of an organic transition metal salt, 0.03-0.1 parts of an organic bromide ligand, and 50-70 parts of a reaction solvent. More preferably, the mixture contains 100 parts of a m-xylene-containing mixture, 0.02-0.065 parts of an organic transition metal salt, 0.03-0.05 parts of an organic bromide ligand, and 55-68 parts of a reaction solvent; the m-xylene-containing mixture includes one or more of m-xylene, m-methylbenzaldehyde, and m-methylbenzyl alcohol.

[0031] In some embodiments provided by the present invention, in the mixture containing m-xylene, the weight ratio of m-xylene to m-methylbenzaldehyde and / or m-methylbenzyl alcohol is (5-20):1, and more preferably, the weight ratio of m-xylene to m-methylbenzaldehyde and / or m-methylbenzyl alcohol is (5-16):1.

[0032] In some embodiments provided by the present invention, the organic transition metal salt is at least two of the salicylates, maleates, or propions of titanium, nickel, chromium, palladium, cobalt, and manganese; the weight ratio of any two organic transition metal salts is (2-4):1, and more preferably, the weight ratio of any two organic transition metal salts is (2.2-3.6):1.

[0033] In some embodiments provided by the present invention, the organic bromide ligand is one or more of tetraethylammonium bromide, tetrapropylammonium bromide, and tetramethylammonium bromide.

[0034] In some embodiments provided by the present invention, the reaction solvent is one or more of acetic acid, propionic acid, and maleic acid.

[0035] A method for selectively oxidizing m-xylene to prepare m-methylbenzoic acid involves passing a homogeneous metal catalyst composed of a mixture of m-xylene, an organic transition metal salt, and an organic bromide ligand into a micro / nano bubble reactor for oxidation, followed by distillation to obtain methylbenzoic acid. The intermediate byproducts m-methylbenzaldehyde and / or m-methylbenzyl alcohol obtained from the distillation separation can be used as components of the m-xylene-containing mixture and mixed with m-xylene to carry out the above-mentioned oxidation conversion reaction.

[0036] In some embodiments provided by this invention, in the oxidation reaction, the gas space velocity relative to the liquid phase volume is 200-600 h⁻¹, expressed as pure oxygen. -1 The oxygen concentration is 21-60%. More preferably, the gas space velocity relative to the liquid volume flow rate, calculated for pure oxygen, is 250-450 h⁻¹. -1 The oxygen concentration is 35-54%.

[0037] In some embodiments provided by this invention, the reaction temperature is 140-190℃, the reaction pressure is 0.9-1.7MPa, and the reaction space velocity (LHSV) is 0.08-0.3h. -1 More preferably, the reaction temperature is 160-190℃, the reaction pressure is 1-1.5MPa, and the reaction space velocity (LHSV) is 0.1-0.22h. -1 .

[0038] An apparatus for the selective oxidation of m-xylene to prepare m-methylbenzoic acid, wherein the micro-nano bubble reactor is a bubble column or a reaction vessel, and the generated micro-nano bubbles have a size of 1-50 μm; the bubble column is a cylindrical tower structure with a height-to-diameter ratio of (3-6):1, more preferably, the height-to-diameter ratio of the bubble column is (3.5-5):1, and the internal circulating flow sleeve is provided, through which the micro-nano bubbles are generated by a porous ceramic material aerator; the generation of micro-nano bubbles in the reaction vessel is achieved by a high-speed rotating cutting device.

[0039] After the reaction was completed, the product was analyzed by liquid chromatography. The selectivity of the product m-methylbenzoic acid was greater than 92%. After distillation, the purity of the m-methylbenzoic acid product was greater than 99.2%.

[0040] Example 1: A method for the selective oxidation of m-xylene to prepare m-methylbenzoic acid, comprising the following steps: 100 parts by weight of m-xylene (liquid-phase reaction raw material), 0.03 parts by weight of cobalt salicylate (organo-transition metal salt), 0.01 parts by weight of nickel salicylate, 0.04 parts by weight of tetrapropylammonium bromide (organo-bromine ligand), and 65 parts by weight of acetic acid (reaction solvent) are introduced into a micro / nano bubble reactor; wherein the catalytic system consists of nickel salicylate, cobalt salicylate, and tetrapropylammonium bromide. The oxidizing gas is 35% oxygen diluted with nitrogen, and the gas hourly space velocity (GHSV) relative to the liquid phase volume is 320 h⁻¹ (pure oxygen). -1 The reaction space velocity (LHSV) was 0.12 h⁻¹. -1 The reaction temperature was 180℃, the reaction pressure was 1.2MPa, and the reaction time was 2h. The micro-nano bubble reactor used in the reaction was a bubble column, which produced micro-nano bubbles with a size of 1-50μm. The bubble column was a straight cylindrical tower structure with a height-to-diameter ratio of 4:1. It was equipped with a circulating flow sleeve inside, and the micro-nano bubbles were generated through a porous ceramic material aerator.

[0041] Example 2: A method for the selective oxidation of m-xylene to prepare m-methylbenzoic acid, comprising the following steps: 100 parts by weight of a mixture of m-xylene-containing raw materials, 0.045 parts by weight of cobalt propionate and 0.024 parts by weight of manganese propionate (organic transition metal salts), 0.032 parts by weight of tetraethylammonium bromide (organic bromide ligand), and 60 parts by weight of acetic acid and propionic acid (reaction solvents) are introduced into a micro / nano bubble reactor. The mixture containing m-xylene consists of m-xylene, m-methylbenzaldehyde, and m-methylbenzyl alcohol. m-Methylbenzaldehyde and m-methylbenzyl alcohol are obtained by distillation and purification after the reaction in Example 1. The weight ratio of m-xylene, m-methylbenzaldehyde, and m-methylbenzyl alcohol is 12:1:1. The volume ratio of acetic acid to propionic acid is 9:1. The catalytic system consists of cobalt propionate, manganese propionate, and tetraethylammonium bromide. The oxidizing gas is 44% oxygen diluted with nitrogen, and the gas hourly space velocity (GHSV) relative to the liquid volume flow rate (based on pure oxygen) is 280 h⁻¹. -1 The reaction space velocity (LHSV) was 0.12 h⁻¹. -1 The reaction temperature was 185℃, the reaction pressure was 1.5MPa, and the reaction time was 1.5h. The reaction was carried out using a micro-nano bubble reactor, which produced micro-nano bubbles with a size of 1-50μm. The generation of micro-nano bubbles in the reactor was achieved using a high-speed rotary cutting device.

[0042] Example 3: A method for the selective oxidation of m-xylene to prepare m-methylbenzoic acid, comprising the following steps: 100 parts by weight of a mixture containing m-xylene as a liquid-phase reaction raw material, 0.05 parts by weight of the organic transition metal salts titanium maleate and manganese maleate, 0.025 parts by weight of the organic bromide ligand tetramethylammonium bromide, and 56 parts by weight of the reaction solvent acetic acid are introduced into a micro / nano bubble reactor. The mixture containing m-xylene is composed of m-xylene and m-methylbenzaldehyde, which is obtained by distillation and purification after the reaction in Example 1. The weight ratio of m-xylene to m-methylbenzaldehyde is 10:1. The catalytic system consists of titanium maleate, manganese maleate, and tetramethylammonium bromide. The oxidizing gas is 40% oxygen diluted with nitrogen, and the gas hourly space velocity (GHSV) relative to the liquid volume flow rate (based on pure oxygen) is 300 h⁻¹. -1 The reaction space velocity (LHSV) was 0.12 h⁻¹. -1 The reaction temperature was 180℃, the reaction pressure was 1MPa, and the reaction time was 2h. The reaction was carried out in a micro-nano bubble reactor, which produced micro-nano bubbles with a size of 1-50μm. The micro-nano bubbles in the reactor were generated by a high-speed rotary cutting device.

[0043] Comparative Example 1: This comparative example describes a method for preparing m-toluene from m-xylene via cobalt-bromine synergistic liquid-phase air oxidation, comprising the following steps: 100 parts by weight of m-xylene (a liquid-phase reactant), 0.02 parts by weight of cobalt acetate and manganese acetate (organotransition metal salts), 0.04 parts by weight of potassium bromide (a bromide ligand), and 50 parts by weight of acetic acid (a reaction solvent) are introduced into a high-speed paddle-stirred reactor. The catalytic system consists of cobalt acetate, manganese acetate, and potassium bromide. Air is used as the oxidizing gas, with a gas space velocity relative to the liquid volume flow rate (based on pure oxygen) of 300 h⁻¹. -1 The reaction temperature was 180℃, the reaction pressure was 1.5MPa, the stirring speed was 1000rpm, and the reaction time was 2h.

[0044] Comparative Example 2: This comparative example describes a method for preparing m-toluene from m-xylene via cobalt-bromine synergistic liquid-phase air oxidation, comprising the following steps: 100 parts by weight of the liquid-phase reaction raw material m-xylene, 0.03 parts by weight of the organic transition metal salt water cobalt salicylate, 0.01 parts by weight of nickel salicylate, and 0.04 parts by weight of potassium bromide, along with 65 parts by weight of the reaction solvent acetic acid, are introduced into a micro / nano bubble reactor; wherein the catalytic system consists of nickel salicylate, cobalt salicylate, and potassium bromide. The oxidizing gas is 35% oxygen diluted with nitrogen, and the gas space velocity relative to the liquid phase volume is 320 h⁻¹ (pure oxygen). -1 The reaction space velocity (LHSV) was 0.12 h⁻¹. -1The reaction temperature was 180℃, the reaction pressure was 1.2MPa, and the reaction time was 2h. The micro-nano bubble reactor used in the reaction was a bubble column, which produced micro-nano bubbles with a size of 1-50μm. The bubble column was a straight cylindrical tower structure with a height-to-diameter ratio of 4:1. It was equipped with a circulating flow sleeve inside, and the micro-nano bubbles were generated through a porous ceramic material aerator.

[0045] Comparative Example 3 describes a method for the selective oxidation of m-xylene to prepare m-methylbenzoic acid using hydrogen peroxide as an oxidant. The method includes the following steps: 100 parts by weight of m-xylene (liquid-phase reaction raw material), 1 part by weight of cobalt acetate (organo-transition metal salt), 0.04 parts by weight of sodium bromide (bromine ligand), and 100 parts by weight of acetic acid (reaction solvent) are introduced into a high-speed paddle-stirred reactor. The catalytic system consists of cobalt acetate and potassium bromide. Hydrogen peroxide is used as the oxidant, at a dosage twice the weight of m-xylene, and is added continuously, uniformly, and at a constant rate. The reaction temperature is 120°C, the reaction pressure is atmospheric pressure, the stirring speed is 1000 rpm, and the reaction time is 2 hours.

[0046] After the reaction in Examples 1-3 and Comparative Examples 1-2, the products were analyzed by liquid chromatography. The analytical results are shown in Table 1 below. As shown in Table 1, Examples 1-3 used m-xylene, or m-xylene with m-methylbenzyl alcohol and / or m-methylbenzaldehyde, in a bubble column or micro / nano bubble reactor. A homogeneous metal catalyst system containing two organic transition metal salts and organic bromide ligands was used as the catalyst, and oxygen-containing gas with a volume concentration of ≥21% was introduced for selective oxidation to prepare m-methylbenzoic acid. This achieved good results, with substrate conversion exceeding 99% and m-methylbenzoic acid selectivity exceeding 96%. Therefore, the technical solution provided by this invention has advantages such as short reaction time, high m-methylbenzoic acid selectivity, and low energy consumption.

[0047] In Example 1 of this invention, m-xylene is used as the liquid-phase reaction feedstock; the catalytic system comprises 300 ppm nickel salicylate, 100 ppm cobalt salicylate, and 400 ppm tetrapropylammonium bromide by weight relative to the liquid-phase reaction feedstock; acetic acid is used as the solvent, accounting for 65% of the weight of the liquid-phase reaction feedstock. Through a cobalt-nickel-bromine ternary synergistic catalysis, m-xylene is oxidized to m-methylbenzoic acid. Cobalt salicylate effectively initiates a free radical chain reaction. Under heating conditions and with the presence of oxygen, divalent cobalt ions are converted to trivalent cobalt ions. Due to their high activity, trivalent cobalt ions react with the methyl group on m-xylene to generate benzyl radicals. Simultaneously, trivalent cobalt ions are reduced back to divalent cobalt ions, forming a catalytic cycle. Divalent and trivalent nickel ions participate in the catalytic cycle during the reaction, assisting in the oxidation of byproducts and improving the oxidation selectivity of m-methylbenzoic acid. The bromine radicals present during the reaction promote the initiation and propagation of the free radical chain reaction, and, in conjunction with the oxidation of hydrogen in the methyl group of m-xylene by trivalent cobalt ions, further enhance the purity of the product m-methylbenzoic acid. The reaction is carried out in a homogeneous acetic acid solution medium, which promotes more complete contact and reaction between the reactants and facilitates the occurrence of free radical chain reactions.

[0048] In Example 2 of this invention, the liquid-phase reaction raw materials are m-xylene and m-tolualdehyde and m-toluene alcohol purified by distillation after the reaction in Example 1. The catalytic system consists of 240 ppm cobalt propionate, 75 ppm manganese propionate, and 320 ppm tetraethylammonium bromide by weight relative to the liquid-phase reaction raw materials. Acetic acid and propionic acid are used as solvents, accounting for 60% of the weight of the liquid-phase reaction raw materials. Mixing the separated and recovered intermediate byproducts m-tolualdehyde and m-toluene with m-xylene can accelerate the oxidation reaction process. In the cobalt-manganese-bromine ternary synergistic catalysis formed by the catalytic system, cobalt ions initiate the oxidation reaction through valence state transformation, and manganese ions further promote the decomposition of intermediate byproducts, thereby improving catalytic efficiency. Using a mixture of acetic acid and propionic acid as the reaction solvent can raise the boiling point of the solution, allowing the reaction to proceed smoothly under high-temperature conditions, and effectively increasing the reaction rate and shortening the reaction time.

[0049] In Example 3 of this invention, the liquid-phase reaction raw materials are m-xylene and m-methylbenzaldehyde; the catalytic system comprises 250 ppm titanium maleate, 80 ppm manganese maleate, and 350 ppm tetramethylammonium bromide by weight relative to the liquid-phase reaction raw materials; acetic acid is used as the solvent, accounting for 56% of the weight of the liquid-phase reaction raw materials. This embodiment of the catalytic system forms a titanium-manganese-bromine ternary synergistic catalysis. By controlling the proportions of each component, the generation of intermediate byproducts such as carboxylic acids during the oxidation reaction of maleate ions can be effectively avoided, while also preventing benzene ring opening and mineralization.

[0050] This invention employs micro-nano bubble technology for catalytic oxidation reactions. The generation of micro-nano bubbles is achieved through aeration or high-speed rotary cutting. Micro-nano bubbles significantly increase the contact area between oxygen and reaction components, improving the transfer efficiency of oxygen to the liquid-phase reaction system. The micro-nano bubbles can be effectively dispersed in the reaction liquid system, slowly rupture, and continuously supply oxygen, avoiding excessive local oxygen content differences, inhibiting the occurrence of side reactions, promoting efficient and highly selective catalytic oxidation to generate m-methylbenzoic acid, and improving the purity of the product m-methylbenzoic acid.

[0051] Comparative Example 1 used air as an oxidant for catalytic oxidation to prepare m-methylbenzoic acid. Product analysis showed a significant decrease in the conversion rate of m-xylene and the selectivity of m-methylbenzoic acid, while the proportion of intermediate byproducts m-carboxybenzaldehyde and isophthalic acid increased significantly, indicating that side reactions were exacerbated during the reaction. Introducing air into the reaction solution and using high-speed paddle stirring resulted in large bubbles with short residence times, making them prone to bursting and reducing oxygen utilization. This decreased the reaction rate and prolonged the reaction time. Large bubbles also caused localized excessively high or low oxygen concentrations, leading to significant differences in the degree of local oxidation, increasing the number of intermediate byproducts, and reducing the purity and selectivity of the target product, m-methylbenzoic acid.

[0052] Comparative Example 2 used potassium bromide instead of organic bromide salts as the bromine donor. Product analysis showed that, compared to Example 1, the conversion rate of m-xylene was similar, but the selectivity of m-methylbenzoic acid was significantly reduced, while the selectivity of the over-oxidation products m-carboxybenzaldehyde and isophthalic acid was significantly increased, indicating that the over-oxidation reaction was intensified during the reaction. The organic bromide ligand, with its more uniform bromine source distribution, can release bromine free radicals more stably and controllably, helping to suppress deep oxidation and improve the selectivity and yield of m-methylbenzoic acid.

[0053] Comparative Example 3 used hydrogen peroxide as an oxidant for catalytic oxidation to prepare m-methylbenzoic acid. Product analysis showed a significant decrease in the purity of m-methylbenzoic acid and a significant increase in the proportions of intermediate byproducts m-methylbenzyl alcohol, m-carboxybenzaldehyde, and isophthalic acid. The catalytic oxidation reaction for preparing m-methylbenzoic acid requires a relatively high temperature. High temperatures cause hydrogen peroxide to become unstable, easily decomposing and escaping from the reaction liquid phase during high-speed stirring. The hydroxyl radicals generated during decomposition are difficult to control and oxidize towards the target product. Furthermore, the addition of hydrogen peroxide during the reaction must be done dropwise, which inevitably leads to localized concentration changes and localized oxidation reactions, increasing the amount of intermediate byproducts. Insufficient oxidation also necessitates increasing the amount of hydrogen peroxide used, thus increasing operating costs.

[0054] In the reaction solution of this invention, m-xylene is mixed with m-methylbenzaldehyde and / or m-methylbenzyl alcohol as liquid-phase reaction raw materials. m-methylbenzaldehyde and / or m-methylbenzyl alcohol can be intermediate by-products generated by oxidation reaction, which not only effectively increases the starting point of oxidation reaction but also improves oxidation reaction efficiency. By controlling the ratio of m-xylene to intermediate by-products for mixing, local over-oxidation can be further reduced, which helps to maintain a relatively stable level of intermediate by-products generated in the reaction system and improves the selectivity of m-methylbenzoic acid. This not only achieves the resource utilization of by-products but also enables continuous production.

[0055] A homogeneous metal catalyst system composed of an organic transition metal salt and an organic bromide ligand is used. During the reaction, the organic bromide ligand generates bromine radicals, which activate the hydrogen on the methyl group of m-xylene, initiating a radical chain reaction. Its quaternary ammonium cation enhances phase transfer, promoting the transfer of bromide ions from the reaction solvent to react with m-xylene and intermediate byproducts. The metal ions of the transition metal salt undergo valence state changes during the reaction, involving single-electron transfer and radical propagation, accelerating the oxidation chain reaction. The metal ions can then complete reduction after oxidation, forming a catalytic cycle. Furthermore, it can combine with the organic bromide ligand to form a multi-component synergistic catalytic system, improving the efficiency and selectivity of the oxidation reaction, preventing over-oxidation of m-methylbenzoic acid, and avoiding ring-opening and mineralization of the benzene ring. The reaction is carried out in a homogeneous liquid phase environment. The reaction solvent maintains the stability of the liquid-phase reaction raw materials and catalytic system, effectively avoiding the uncontrollable intensification of the reaction at higher reaction temperatures. At the same time, it can avoid the occurrence of side reactions such as excessive oxidation or coupling caused by the intensification of free radical movement. It can also avoid the reduction of oxidation selectivity and improve the purity of m-methylbenzoic acid.

[0056] This invention introduces micro-nano bubbles during the oxidation reaction, effectively solving the problem of low mass transfer efficiency in traditional air bubbling technology. It also addresses the difficulty in controlling the localized intensification of hydrogen peroxide oxidation, thus improving the selectivity of the target product, meta-methylbenzoic acid, while simultaneously enhancing the efficiency of the catalytic oxidation reaction. The slow rupture rate of the micro-nano bubbles effectively prolongs the contact time between oxygen and reactants, ensuring the stable progress of the catalytic oxidation reaction and suppressing significant local concentration changes that could lead to intensified side reactions.

[0057] This invention significantly improves the conversion rate of m-xylene and the selectivity of m-methylbenzoic acid by controlling the amount of each component in the reaction solution, achieving a selectivity of over 92% for the target product, m-methylbenzoic acid. After the catalytic oxidation reaction, the product is separated by distillation to obtain m-methylbenzoic acid with a purity exceeding 99.2%. This invention offers advantages such as short reaction time, high selectivity for m-methylbenzoic acid, low energy consumption, high synthesis efficiency, and environmental friendliness.

Claims

1. A reaction solution for the selective oxidation of m-xylene to prepare m-methylbenzoic acid, characterized in that, It comprises the following components in parts by weight: 100 parts of a mixture containing m-xylene, 0.015-0.08 parts of an organic transition metal salt, 0.03-0.1 parts of an organic bromide ligand, and 50-70 parts of a reaction solvent.

2. The reaction solution for the selective oxidation of m-xylene to prepare m-methylbenzoic acid according to claim 1, characterized in that, The mixture comprises the following components in parts by weight: 100 parts of a mixture containing m-xylene, 0.02-0.065 parts of an organic transition metal salt, 0.03-0.05 parts of an organic bromide ligand, and 55-68 parts of a reaction solvent; wherein the mixture containing m-xylene includes one or more of m-xylene, m-methylbenzaldehyde, and m-methylbenzyl alcohol.

3. The reaction solution for the selective oxidation of m-xylene to prepare m-methylbenzoic acid according to claim 1 or 2, characterized in that, In the mixture containing m-xylene, the weight ratio of m-xylene to m-methylbenzaldehyde and / or m-methylbenzyl alcohol is (5-20):

1.

4. The reaction solution for the selective oxidation of m-xylene to prepare m-methylbenzoic acid according to claim 1 or 2, characterized in that, The organic transition metal salt is at least two of the following: salicylates, maleates, or propions of titanium, nickel, chromium, palladium, cobalt, and manganese; the weight ratio of any two organic transition metal salts is (2-4):

1.

5. The reaction solution for the selective oxidation of m-xylene to prepare m-methylbenzoic acid according to claim 1 or 2, characterized in that, In the components, the organic bromide ligand is one or more of tetraethylammonium bromide, tetrapropylammonium bromide, and tetramethylammonium bromide.

6. The reaction solution for the selective oxidation of m-xylene to prepare m-methylbenzoic acid according to claim 1 or 2, characterized in that, The reaction solvent in the components is one or more of acetic acid, propionic acid, and maleic acid.

7. A method for the selective oxidation of m-xylene to prepare m-methylbenzoic acid, characterized in that, A homogeneous metal catalyst, composed of a mixture of m-xylene, an organic transition metal salt, and an organic bromide ligand, is passed into a micro / nano bubble reactor for oxidation, followed by distillation to obtain methylbenzoic acid.

8. The method for selective oxidation of m-xylene to prepare m-methylbenzoic acid according to claim 7, characterized in that, In the oxidation reaction, the gas space velocity relative to the liquid phase volume is 200-600 h⁻¹, calculated using pure oxygen. -1 The oxygen concentration is 21-60%.

9. A method for selectively oxidizing m-xylene to prepare m-methylbenzoic acid according to claim 7 or 8, characterized in that, In the oxidation reaction, the reaction temperature is 140-190℃, the reaction pressure is 0.9-1.7 MPa, and the reaction space velocity (LHSV) is 0.08-0.3 h⁻¹. -1 .

10. An apparatus for the selective oxidation of m-xylene to prepare m-methylbenzoic acid, characterized in that, The micro-nano bubble reactor is a bubble column or a reaction vessel, and the micro-nano bubbles produced have a size of 1-50μm. The bubble column is a straight cylindrical tower structure with a height-to-diameter ratio of (3-6):

1. It is equipped with a circulating flow sleeve inside and generates the micro-nano bubbles through a porous ceramic material aerator. The micro-nano bubbles in the reaction vessel are generated by a high-speed rotating cutting device.

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