1.1 Development of synthetic methods of m-methylbenzoic acid
In the early stage, for the synthesis of m-methylbenzoic acid, since the early petroleum industry was not developed, the source of meta-xylene was mainly obtained by rectification from coal tar, so the yield of meta-xylene was low and the price was high. Some researchers have tried to synthesize m-methylbenzoic acid from m-chlorotoluene, m-methylbenzaldehyde, o-methylphenol, etc.
However, since the sources of the raw materials in the above three routes are scarce and the reagents are rare, it is not economical to synthesize m-methylbenzoic acid, and therefore it is impossible to carry out industrialization.
With the development of the modern petroleum industry, the development of light oil catalytic reforming technology has led to a rapid increase in meta-xylene, which completely replaces the distillation of aromatic xylene from coal tar. Therefore, the price of m-xylene is correspondingly reduced, which makes the preparation of m-methylbenzoic acid from between toluene as raw materials has practical economic significance. The researchers proposed the preparation of m-methylbenzoic acid by nitric acid oxidation, potassium iodide/oxygen oxidation, and cobalt-catalyzed oxidation.
The first synthesis method uses traditional nitric acid and potassium permanganate strong oxidant as catalysts, and the reaction is very intense, and the reaction product is easily changed into carbon dioxide, isophthalic acid, m-benzyl benzoic acid, and other active benzyl alcohol esters. product. Therefore, the yield is not high and it cannot be industrialized. The oxidizing property of the catalyst in the second and third synthetic methods is weaker than that of the first method, so that the oxidizing ability is weak, the reaction time is long, and the industrial production efficiency is not high.
According to the above comparison, it is not difficult to see that the factors affecting the conversion rate, economic benefit, and synthetic process conditions in the synthesis of m-benzoic acid as a raw material are mainly the selection of catalysts and catalytic conditions, and finding one is beneficial to both. Catalysts and catalytic conditions that are conducive to ensuring conversion and economic benefits are critical. It is also the goal and direction of the researchers. The use of a variable metal salt as a catalyst, a bromide as a promoter, and acetic acid as a medium for the preparation of m-methylbenzoic acid has both advantages and disadvantages. The advantage is that the reaction rate is faster and the conversion rate is higher than other methods. However, this liquid phase oxidation reaction requires high equipment. It is generally used more in countries with more developed industries. Another type of use in foreign countries is to use a single variable-valence metal salt as a catalyst to prepare m-methylbenzoic acid by solvent-free oxidation of m-xylene. In China, m-benzoic acid is generally prepared by using cobalt naphthenate as a catalyst. For example, Luzhou Ruierfeng Chemical Co., Ltd. uses cyclohexane as a catalyst to oxidize m-xylene with air, wherein the reaction temperature is 125-135 ° C and the pressure is about 0.25 MPa. The reaction conditions of the method are mild and easy to produce, but the oxidation reaction rate is slow due to the low activity of the catalyst, about 16 hours, and the radical methyl chain reaction is obtained when m-benzoic acid is obtained by using m-xylene. There are many types of products and they will accumulate, and the raw materials are consumed. The product obtained by this method has a conversion of less than 50%.
1.2 The best synthesis method and process flow and evaluation
In order to obtain higher conversion rates and benefits, researchers have used liquid gas oxidation in a solvent-free condition using a single variable metal salt as a catalyst. This method is analyzed from a green chemical point of view. This method uses gas phase oxidation. One of the outstanding advantages is that it does not require other solvents and is relatively environmentally friendly.
The m-xylene having a purity of 99% is fed to the kettle together with a catalyst solution prepared in a predetermined manner, heated and stirred, and the bottom of the kettle and the wall of the vessel are continuously heated to maintain a suitable oxidation temperature as a catalytic condition. When the liquidus temperature is maintained at a certain temperature, it is oxidized by air. After the oxidation reaction is completed, the pressure should be released and the temperature should be lowered to 150-160 ° C to obtain the m-methylbenzoic acid product and some by-products. The crude product was subjected to rectification to separate unreacted meta-xylene and water, and again subjected to vacuum distillation to obtain m-methylbenzoic acid having a certain purity.
After several experiments with the above process to obtain the best production conditions, the final conclusion is: m-xylene: catalyst = 1:1.7 ‰, oxidation initiation temperature of 180 ° C, oxidation time of 120 min, oxidation reaction temperature of 190-200 ° C, Oxidation pressure ≥0.8MPa, air flow rate (STP) 1.2m3/h.kg, stirring rotation number: 500-600 r/min. With the above process conditions, the conversion can reach about 60%. From the economic efficiency analysis of the above process, I understand that the current price of meta-xylene on the market is 8,400 yuan / ton, the price of m-methyl benzoic acid is 33,000 yuan / ton, calculated according to the conversion rate of 60%, each ton Xylene can produce about 0.77 tons of m-methylbenzoic acid, so that the gross profit of producing m-benzoic acid is 17,000 yuan/ton (m-methylbenzoic acid).