Adipic acid is one of the most important industrial chemicals used mainly for the production of Nylon 6,6 but also for making polyurethanes, plasticizers, and unsaturated polyester resins. The main route for adipic acid production utilizes benzene as feedstock and generates copious amounts of the greenhouse gas nitrous oxide (N2O). Hence, alternative clean production routes for adipic acid from renewable bio-based feedstock are drawing increasing attention.
Adipic acid production process
The conventional process for hexanedioic acid production involves three main steps: oxidation of cyclohexane to cyclohexanone and cyclohexanol (KA oil), oxidation of KA oil to adipic acid, and purification of adipic acid. The oxidation steps are carried out in the presence of nitric acid as an oxidant and a catalyst, which results in the formation of N2O as a byproduct. N2O has a 300-fold higher global warming potential than carbon dioxide, and an estimated 10% of its annual global emissions are a result of adipic acid production. Therefore, reducing or eliminating N2O emissions from hexanedioic acid production is a major environmental challenge.
Nitrogen oxide tail gas treatment
Several methods have been developed to treat the nitrogen oxide tail gas from hexanedioic acid production, such as catalytic decomposition, thermal decomposition, selective catalytic reduction, and biological reduction. Among these methods, catalytic decomposition is the most widely used one, as it can achieve high conversion rates of N2O without producing secondary pollutants. Catalytic decomposition involves passing the tail gas over a metal-based catalyst at high temperatures (400-600 °C) to decompose N2O into nitrogen and oxygen. The catalysts used for this process include noble metals (such as Pt, Pd, Rh), transition metals (such as Cu, Co, Fe), and metal oxides (such as MnO2, CeO2).
The performance of the catalysts depends on several factors, such as the composition, structure, surface area, acidity, and stability of the catalysts, as well as the operating conditions, such as temperature, pressure, gas flow rate, and space velocity. The main challenges for catalytic decomposition are the deactivation of the catalysts due to sintering, poisoning, or coking, and the optimization of the process parameters to achieve high N2O conversion and low energy consumption.
Table 1: Comparison of some catalysts for N2O decomposition from adipic acid tail gas
| Catalyst | Temperature (°C) | Space velocity (h-1) | N2O conversion (%) | Reference |
|---|---|---|---|---|
| Pt/Al2O3 | 450 | 30,000 | 98 | |
| Pd/Al2O3 | 450 | 30,000 | 95 | |
| Rh/Al2O3 | 450 | 30,000 | 99 | |
| Cu/ZSM-5 | 500 | 20,000 | 95 | |
| Co/ZSM-5 | 500 | 20,000 | 97 | |
| Fe/ZSM-5 | 500 | 20,000 | 98 | |
| MnOx/CeO2 | 400 | 50,000 | 96 |
Conclusion
Adipic acid is a valuable chemical that is mainly produced by a process that emits large amounts of N2O. To reduce the environmental impact of hexanedioic acid production, various methods have been developed to treat the nitrogen oxide tail gas. Among them, catalytic decomposition is the most promising one, as it can achieve high N2O conversion without producing secondary pollutants. However, there are still some challenges to overcome, such as improving the stability and activity of the catalysts and optimizing the process parameters. Future research should focus on developing novel catalysts with high performance and low cost, as well as exploring alternative routes for hexanedioic acid production from renewable feedstock.
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