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One of the processes for producing adipic acid: the C4 olefin method. The C4 olefin method uses butadiene as the main raw material, and it can be divided into the hydrocyanation method and the carbonylation method. The butadiene hydrocyanation method involves subjecting butadiene to hydrocyanation in the presence of a catalyst to produce 3-pentylcyano and 4-heptenecyano; subsequent catalytic carboxylation yields 5-cyanopentanoic acid, which is then hydrolyzed to produce adipic acid. Among them, the yield of the hydrocyanation products is greater than 90%, while the yields of the carboxylation and hydrolysis products range from 85% to 92%. The adipic acid yield of this process is greater than 80%, but it generates many by-products during the process, which are difficult to separate. Butadiene carbonylation transforms the traditional benzene hydrogenation process. By using inexpensive C4 as a raw material, it not only eliminates the environmental pollution caused by traditional processes but also significantly reduces production costs. Due to the different catalysts used in various processes, different production routes have been developed: The BASF process in Germany: In this process, butadiene obtained from the cracking of C4 hydrocarbons (without extraction) reacts with carbon monoxide in a methanol solvent through a carbonylation reaction; after the first carbonylation step, methyl 3-pentenoate is produced, and after a second carbonylation step, dimethyl adipate is formed, which is finally converted into adipic acid through hydrolysis. Shell process: This method uses palladium acetate, 1,4-bis(diphenylphosphino)butane, and 2,4,6-trimethylbenzoic acid as a catalytic system, and the reaction is carried out in the presence of ethanol. The reaction temperature is 150–155°C, and the pressure is 3–6 MPa; the conversion rate of butadiene is greater than 94%, while the selectivity for methyl valerate is 88%. Monsanto process: This process uses PdCl2 as a catalyst, with 1,4-dimethoxy-2-butene as the starting material. It operates at a pressure of 6.8 MPa and at a reaction temperature below 100°C, resulting in a relatively slow reaction rate. Second method for the production of adipic acid: biocatalytic method. In the 1990s, DuPont developed a biocatalytic process that utilizes Escherichia coli to convert D-glucose into maleic anhydride, which is then transformed into methylenepentanoic acid. Subsequently, an aerobic denitrifying strain was used to isolate a gene strain that codes for an enzyme capable of converting cyclohexanol into adipic acid. This enzyme selectively converts cyclohexanol into adipic acid under appropriate growth conditions, and the yield of adipic acid produced through this process can reach 97%. Since the raw materials used in biological methods are recyclable and biodegradable substances, no pollutants can participate in the reactions or be generated, thereby enabling green and environmentally friendly production. The drawback is that the production costs are very high, making it unsuitable for large-scale industrial production; it is still in the early stages of development, and the manufacturing process is not yet mature. In summary, both processes for producing adipic acid have their own advantages and disadvantages; however, biocatalytic methods are more environmentally friendly and efficient, and they are in line with the future development trends of the chemical industry. Therefore, they represent the future direction for the production of adipic acid.