Influencing factors mechanisms and prospects of high-temperature coal tar denitration with deep eutectic solvents

High-temperature coal tar is a critical component of China’s energy system. However, the nitrogen-containing compounds present in high-temperature coal tar not only contribute to combustion pollution and catalyst poisoning but also pose serious hazards to the environment and refining processes. Nonetheless, if efficiently separated and recovered, these nitrogen-containing substances can serve as valuable raw materials in fields such as pharmaceuticals, agrochemicals, and functional materials. Therefore, the development of green and efficient denitrogenation technologies holds dual significance for both environmental protection and resource utilization.Current methods for separating nitrogen-containing compounds from high-temperature coal tar include solvent extraction, acid-base purification, and ionic liquid extraction, which suffer from drawbacks such as significant environmental impact and high production costs. Deep eutectic solvents (DESs), a new class of green solvents composed of hydrogen bond donors and acceptors, have gained increasing attention as promising agents for high-temperature coal tar denitrogenation due to their tunability, ease of synthesis, low cost, low toxicity, and biodegradability. The article provides a systematic review of recent years advances in the application of DESs for high-temperature coal tar denitrogenation. It emphasizes the influence patterns of key operational parameters, viscosity, and water content on separation efficiency of nitrogen-containing compounds:The removal efficiency of nitrogen-containing compounds decreases with increasing temperature; it increases with prolonged duration before stabilising, but declines if left for excessively long periods; it rises with higher agent-to-oil ratios. The acidity of DESs is a key factor determining selectivity: acidic DESs exhibit greater selectivity towards basic nitrogen compounds, while neutral or weakly basic DESs are more suitable for non-basic nitrogen compounds. Reducing DES viscosity enhances nitrogen compound removal efficiency; Introducing moderate moisture into DESs improves nitrogen compound removal rates. Following extraction completion, recovered DESs via counter-extraction, antisolvent precipitation, or distillation/evaporation retain satisfactory separation capabilities, significantly reducing operational costs. Subsequently,methods for optimising separation conditions were analysed for each of the influencing factors.The review further explores the underlying microscopic mechanisms, such as hydrogen bonding interactions and steric hindrance effects.It was pointed out that hydrogen bonding is the central driving force for the selective separation and resolution of nitrogen-containing compounds through deep eutectic solvents, and that spatial potential resistance also has a important influence on the efficiency of denitrogenation. Finally, based on current research bottlenecks, the article outlines persistent challenges in the highly selective separation of specific nitrogen compounds,the adaptability of DESs to complex real high-temperature coal tar systems and the process for scaling up from laboratory to industrial applications. Prospects for future research directions are also discussed,including targeted molecular design of DESs,pilot-scale experimental simulation,optimization of integrated processes and sustainability assessment over the full life cycle.