Explanation of Preparation Knowledge of Adipic Acid

Adipic acid is an important organic compound with a molecular formula of C6H10O4. It is a white crystalline powder at room temperature, slightly soluble in water, and easily soluble in organic solvents such as ethanol and ether. As the main raw material for synthesizing nylon 66, it has a high industrial demand and is widely used in plasticizers, lubricants, food additives, and other fields. Laboratory preparation usually uses cyclohexanol or cyclohexanone oxidation method, while industrial production mostly uses cyclohexane air oxidation method.
The nitric acid oxidation of cyclohexanol method is a commonly used method in the laboratory, and special attention should be paid to this process: nitric acid and cyclohexanol release heat violently, and the dripping rate should be accurately controlled at 2-3 milliliters per minute. If the temperature exceeds 60 ℃, the dripping should be stopped immediately and continued after cooling. Measure 50 milliliters of 65% nitric acid, slowly drip cyclohexanol, and control the reaction temperature at 50-60 ℃. Place 20 milliliters of cyclohexanol in a 250 milliliter three necked flask, install a thermometer and a constant pressure dripping funnel, cool the reaction solution in an ice water bath to below 5 ℃, and the reaction solution gradually turns orange red, accompanied by the escape of nitrogen dioxide gas. It needs to be operated in a fume hood.
After the dropwise addition is completed, use a warm water bath to maintain a temperature of 55 ℃ for about 30 minutes. The color of the solution gradually becomes lighter. The yield of this method is about 60%, and the purity of the product can reach over 98%. Filter and collect white crystals, wash three times with 5 milliliters of ice water to remove residual nitric acid, and let them dry naturally in a surface dish. Stop heating and cool naturally. The ice water bath promotes crystal precipitation. Key control points include nitric acid concentration, reaction temperature gradient, and stirring speed. Temperature fluctuations exceeding ± 3 ℃ can lead to an increase in by-products.
Industrial scale production mainly adopts the two-step oxidation method of cyclohexane. In the second stage, nitric acid is used to oxidize the mixture to adipic acid at 60-80 ℃, which is then purified by activated carbon decolorization and recrystallization. In the first stage, cyclohexane is reacted with cobalt salt catalyst in air at 150 ℃ and 0.8MPa to produce a mixture of cyclohexanone and cyclohexanol. The yield of this process can reach 85%, and the consumption of cyclohexane per ton of product is about 1 2 tons, nitric acid 1 5 tons, the equipment needs to be made of titanium alloy material for corrosion resistance, and the exhaust gas treatment system needs to be equipped with an alkali absorption tower to treat nitrogen oxides.
The direct oxidation of cyclohexanone is a new process developed in recent years, but the cost of hydrogen peroxide is relatively high and is currently only used for the production of high value-added products. This method avoids the use of corrosive nitric acid and produces only water as a byproduct, which meets the requirements of green chemistry. Hydrogen peroxide is used as the oxidant and sodium tungstate is used as the catalyst. The reaction is carried out in a 70 ℃ aqueous phase for 8 hours, and the product is extracted and distilled to obtain a purity of 99% 5% adipic acid is obtained. Research data shows that the highest conversion rate is achieved when the catalyst dosage is 2% of the substrate mass, and excessive amount can lead to side reactions such as partial hydrolysis.
Microbial synthesis method is an emerging research direction. A laboratory in the United States has achieved a theoretical yield of 60%, but the fermentation cycle is as long as 72 hours, which is still far from industrialization. Glucose is metabolized by E. coli to produce viscous acid, which is then hydrogenated to prepare adipic acid. The coenzyme regeneration system of genetically engineered strains is currently a research hotspot. The latest literature reports that the introduction of exogenous dehydrogenases can increase the yield to 78%.
The experimental operation must strictly comply with safety regulations. The waste liquid treatment process consists of three steps: first, mix sodium carbonate to pH 6-8, add sodium sulfide to reduce residual nitrate ions, and finally precipitate and filter. The airtightness of the reaction device needs to be tested in advance to prevent nitrogen dioxide leakage. Concentrated nitric acid is highly corrosive and requires wearing acid and alkali resistant gloves and goggles. An accident case in a university laboratory showed that incomplete mixing of waste liquid resulted in gas generation during storage, causing the container to burst and causing nitric acid splashing injuries.
The product characterization requires melting point determination (152 ℃), infrared spectroscopy analysis (carboxylic acid characteristic peak 1700cm ⁻¹), and nuclear magnetic resonance hydrogen spectroscopy to verify the structure. When storing, it should be avoided to mix with strong oxidants, and the environmental humidity should be kept below 60%. Industrial grade product testing also includes heavy metal content (lead ≤ 5ppm), ash content (≤ 0 1%) and other indicators, attention should be paid to the possibility of crystal sublimation above 120 ℃ during transportation, and double-layer polyethylene bags should be used for packaging.
The future development trend focuses on catalyst innovation. Nano manganese oxide catalyst can increase the conversion rate of cyclohexanone to 92%, and the activity only decreases by 7% after 10 cycles of use. The application of ionic liquid medium shortens the reaction time by 40%, but the recovery cost is high. The microreactor technology disclosed in a certain patent improves the mass transfer efficiency by 5 times and is suitable for continuous production. These technological advances will promote the development of preparation processes towards high efficiency and environmental protection.