Applications of ionic liquids and deep eutectic solvents for the extraction of phenolic compounds from coal-based crude oils

Phenolic compounds are unstable factors in coal-based crude oils with more than 10 wt%, but they are valuable raw materials and intermediates in the chemical industry. It leads to a goal for investigating different phenols extraction technologies. Due to the green and efficient properties, ionic liquids (ILs) and deep eutectic solvents (DESs) are receiving increasing attention in this separation field. This literature review aims to present a critical and integrative evaluation of the separation of phenolic compounds from coal-based crude oils using ILs and DESs. The types of ILs and DESs used for extracting phenols are categorized. The influence of key process parameters including extraction temperature, extraction time, solvent dosage, and initial phenolic content are compared, and the general trends or inconsistencies are identified. The relationships and differences of ILs/DESs regeneration methods are critically discussed, and the separation mechanism is organized and synthesized. The ultimate goal is to apply the new method to the industrial separation of phenols in real coal-based crude oil. Therefore, it is necessary to present the separation of phenols from real oils using ILs and DESs and conduct comprehensive reviews of their inconsistency with phenolic extraction from model oils. Process simulation of using ILs or DESs for phenolic compounds extraction is discussed. In addition, the article briefly introduces and summarizes the sources, uses and detection techniques of phenolic compounds, the properties of ILs and DESs, and the comparison of other extraction technologies. Finally, a comprehensive discussion is conducted on the critical remarks and future prospects.