Solvents extraction of nitrogen / sulfur-containing aromatic separation

Efficient separation of sulfur/nitrogen-containing aromatic compounds has a direct impact on the quality and performance of coal tar processing process.Being green extraction solvents,the ionic liquids (ILs) and deep eutectic solvents (DESs) provide a novel route for the extraction and separation of sulfur/nitrogen-containing aromatic compounds from coal tar.Heterocyclic compounds are the primary existing form of sulfur/nitrogen-containing compounds in coal tar.Despite the lower contents of sulfur (0.1%~0.4%) and nitrogen (0.6%~1.1%) in coal tar,the structures of the sulfur/nitrogen-containing heterocyclic compounds are complex and typical.The separation of high purity chemical monomers (eg.,carbazole and thiophene) from coal tar could effectively improve their utilization value and the production efficiency of industries.For the efficient separation of sulfur/nitrogen-containing aromatic compounds of coal tar,the extraction of nitrogen/sulfur-containing aromatic compounds from coal tar using ILs/DESs is comprehensively summarized in this paper.The influential role of hydrogen bonding and π-π interaction during the extraction is reviewed.The hydrogen bonding is a key phenomenon for improving the separation of nitrogen/sulfur-containing aromatic compounds by ILs/DESs.This intriguing hydrogen bonding depends on various factors like the proton donor’s acidity,the proton acceptor’s electronegativity,the size of substitutional groups of extraction solvent,etc.Besides the hydrogen bonding,the π-π interaction generated between the extractant solvent and the aromatic heterocycles of coal tar,which possess sulfur/nitrogen in their complex structure,also plays a pivotal role during the extraction.The occurrence of π-π interaction is governed by several key factors including the conjugation of π-bonds in extraction solvent,the electron attracting or donating ability of substituents in extraction solvent,the solvation effect of extraction solvent,and the π-complexation of metal ions.In addition to hydrogen bonding and π-π interaction,the operating conditions remarkably influence the separation efficiency and the yield of chemical monomers.For the successful extraction process by ILs/DESs,a few key concepts should be understood,including ① the quantitative relationship connecting the operating temperature,with hydrogen bonding and π-π interaction;② the in-depth knowledge about the intermolecular forces between ILs/DESs and nitrogen/sulfur-containing aromatic compounds;③ the methods to reduce the influence of aromatics on the separation process;④ the knowledge about the complex mixture of real oil,which includes the aliphatic hydrocarbons,aromatic hydrocarbons,ethers,esters and diverse heterocycle aromatic compounds,and how to reduce the influence of this complex mixture of real oil on the efficiency of ILs/DESs during the separation of sulfur/nitrogen-containing aromatic compounds from real oil.Therefore,for the better understanding of above-mentioned key concepts,it seems very essential to screen out a reasonable solvent system by calculation and analysis using a mathematical model,which would show the ways to improve the extraction efficiency of sulfur/nitrogen-containing aromatic compounds in the coal tar processing technology.