One-step adipic acid: zero greenhouse gas emissions

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Sinochem New Network News: Technical Highlights: Traditional processes involve high temperatures and pressures, low conversion rates, a two-step procedure, the release of NOx, and the production of succinic acid as a by-product. Green processes feature milder reaction conditions, higher conversion rates, a one-step procedure, no release of NOx; although succinic acid is produced as a by-product, it can be used as a raw material for catalysts. Green processes are truly promising. In its 2009 annual report, the Japanese company Daicel Chemical Industry stated that it had completed the design of a one-step process for producing adipic acid at a capacity of 150,000 tons per year, and it plans to form strategic alliances with manufacturers of adipic acid and its derivatives, as well as with engineering companies. The green production of adipic acid without nitrogen oxides (NOx) will soon become a reality. 　　As early as October 2007, when the Japanese company Daicel Chemical Industry announced that it was working together with Kansai University in Japan to accelerate the development of a one-step process for adipic acid that does not produce NOx, the industry was greatly excited by this development. 　　The N-hydroxyphthalimide (NHPI)-based oxidation catalyst developed by Professor Yasutaka Ishii of Kansai University, when combined with a small amount of metal salts (manganese or cobalt), can be used for the one-step conversion of cyclohexane into adipic acid. It is reported that adipic acid is prepared by the oxidation of cyclohexane in acetic acid solution using an NHPI catalyst; when the conversion rate of cyclohexane is 55%, the selectivity for adipic acid is 82%. To prevent corrosion of the reactor by acetic acid, the university developed a solvent-free cyclohexane oxidation process in which a fat-soluble NHPI catalyst with high solubility in cyclohexane is used; under the presence of small amounts of cobalt and manganese salts, cyclohexane is oxidized with air at 100°C and 1 MPa, achieving a selectivity of 85% for adipic acid. More importantly, this process does not produce nitrogen oxides. 　　Dai Nippon Ink Company developed a cost-effective production method based on Professor Ishii’s research, using succinic acid – a byproduct of adipic acid production – to manufacture N-hydroxyphthalimide, thereby reducing the amount of N-hydroxyphthalimide catalyst required for adipic acid production by two orders of magnitude. 　　This new aerobic oxidation process won Japan’s Third Green and Sustainable Chemistry Award. Traditional manufacturing processes face environmental challenges. Adipic acid ranks second in terms of production among dicarboxylic acids, and it serves as a raw material and intermediate for producing important chemical products such as nylon 6 and nylon 66. Traditional synthesis processes are classified by raw materials into the cyclohexane method, C4 olefin method, phenol method, and cyclohexene method. Among them, the cyclohexane process holds an absolute dominant position globally. The reaction conditions in the first step of this process are relatively harsh, making it difficult to control product selectivity, and the conversion rate of cyclohexane is low ； The second step generates large amounts of waste acid and releases significant quantities of NOx, causing severe environmental pollution; moreover, the by-products affect the properties of adipic acid. The phenol method was the earliest production process for adipic acid, but its raw material sources were limited. The cyclohexene process has high conversion and selectivity, but it generates large amounts of waste acid and releases significant amounts of NOx. The overall yield of adipic acid in the butadiene process can reach around 75%, but the process is complex, the reaction conditions are stringent, and there are many by-products. 　　The Japanese Ministry of Education, Culture, Sports, Science and Technology states that over 60% of reactions in the chemical industry are related to oxidation reactions. Over the past half century, no significant progress has been made in alkane oxidation reactions. It is now imperative to develop environmentally friendly oxidation methods, and such environmentally friendly technologies must meet three criteria: high atom economy, low environmental impact, and low environmental factors (referring to the amount of by-products generated per 1 kg of product produced). Industrialization facilities are on the horizon. The Japanese Ministry of Education, Culture, Sports, Science and Technology stated in its recognition document that conventional adipic acid production involves two steps, with a yield of only 3% to 7%, and it also generates large amounts of nitrous oxide (N2O), a gas with a high greenhouse effect. This innovative catalytic oxidation process for alkanes enables the oxidation of alkanes under mild conditions, results in few by-products, saves energy, reduces greenhouse gas emissions, and **improves the selectivity of the reaction as well as the yield of alkane oxidation. The application of this process in the one-step oxidation of cyclohexane is a successful example. Using the NHPI catalyst, at a conversion rate of 73%, its selectivity for adipic acid is as high as 73%. 　　Daicel Corporation states that the conventional process for adipic acid involves high-temperature and high-pressure reactions, which result in high energy consumption and low conversion rates. To increase the yield, nitric acid oxidation or halogen oxidation is often used as a supplementary method, which not only increases greenhouse gas emissions but also leads to an increase in harmful by-products. The greenhouse effect of the byproduct N2O is 310 times that of CO2. The NHPI process is a one-step process that can achieve high conversion rates at around 100°C; it is not only energy-efficient but also produces no by-products associated with the high-temperature greenhouse effect. 　　In its 2008 annual report, Daicel revealed that it had built an adipic acid production facility with an annual capacity of over 30 tons at its production site in Himeji, Hyogo Prefecture, Japan, thereby accelerating the development of N-hydroxyphthalimide catalysts for use in alkane oxidation under milder conditions. This one-step process without N2O will become a green model route for the future production of adipic acid. According to its 2009 annual report, the company is currently planning to build a plant with an annual capacity of 150,000 tons; the process package design has been completed, and testing and adjustments are underway. Once the technology is mature, adipic acid plants will also be built in regions with strong demand for nylon, such as China.