Dissolution Characteristics Of Phenolic Compounds In Low Temperature Coal Tar And Their Extraction By Deep Eutectic Solvents

A large number of phenolic compounds in coal tar derived from low–temperature pyrolysis of low rank coal not only decreases the stability of coal tar but also increases the hydrogen consumption in hydrogenation.However,the phenolic compound,as an indispensable raw material,has been used widely in chemical industry.Thus,the separation and purification of phenolic compounds from coal tar are both technical benefit and economic benefit.Extraction has obtained a lot of attraction owing to the advantages of the lower energy consumption and the larger production capacity compared to other separation methods.Deep eutectic solvent,as a new type of ionic liquid,demonstrates the benefits of low-price and easy-synthesis.However,the method that a single solvent was employed widely to form deep eutectic solvents with phenolic compounds in coal tar for their separation has been reported mostly in previous work.The shortcomings of the single solvent method are narrow ligand selection range and small ratio change as well as inflexible regulation of extraction performance.In addition,the most of single solvent contains halogen atoms which corrodes steel extraction equipment gradually.The composition of coal tar is extremely complex,and the interactions between different compositions have a considerable effect on the dissolution of phenolic compounds in coal tar as well as the subsequent extraction process.The study of the solubility characteristics of phenolic compounds in low temperature coal tar has not been attracted enough attention so far.Typical phenolic compounds of low temperature coal tar were selected as an object in this work.The influence of model compounds with different structural characteristics on the dissolution behavior and mechanism of phenolic compounds were investigated by means of molecular simulation.Hypoeutectic solvent was synthesized by hydrogen bond donor and acceptor without halogen atoms.Through selecting ligands and changing their ratios,the extraction performance was flexibly regulated.The hypoeutectic solvent for extraction and separation of phenolic compounds was designed,synthesized and optimized,and its basic properties were characterized and its formation mechanism was studied.On this basis,the extraction mechanism of phenolic compounds in model oil extracted with low eutectic solvent was studied,and the extraction performance of real coal tar was verified under the optimized process parameters,and the influence of process parameters on mass transfer and the control mechanism of mass transfer process were studied.The main results obtained from the study:1.The dissolution behavior of phenolic compounds in coal tar is affected by the structural characteristics of each composition.The solubility of phenolic compounds with aromatic hydrocarbons,aliphatic hydrocarbons,and sulfur-containing compounds increases with the increasing of the carbon number of alkyl substituents,the number of alkyl substituents,and the number of hydroxyl substituents of phenolic compounds.The increase of carbon number and quantity of alkyl substituents of aromatic hydrocarbons makes the solubility of aromatic hydrocarbons and phenolic compounds weaker.The association of straight-chain alkanes with phenolic compounds becomes weaker as the carbon chain of alkanes grows,the solubility decreases.Compared with indole and carbazole,pyridine and quinoline of the nitrogen-containing compounds have greater solubility with phenolic compounds.Under the influence of structural characteristics and spatial site resistance effect,phenolic compounds showed good phase solubility with benzothiophene in sulfur-containing compounds.In addition,the solubility of phenolic compounds with aromatics,alkanes,sulfur-containing compounds in coal tar increased with the increase of temperature,while the phase solubility between phenolic compounds and nitrogen-containing compounds weakened.2.The association between the components of coal tar and the spatial site resistance effect is the key to influence the solubility characteristics of phenolic compounds.The non-bonding force between the molecules of each component forms the association interaction.The source of the association of phenol with benzene and benzothiophene isπ-πstacking.The association of phenol and benzene which is contributed by"face-to-face"π-πstacking is stronger than that of T-typeπ-πstacking interaction between phenol and benzothiophene.The CH-πinteraction and N-H???O hydrogen bonding are the association forces of phenol with n-hexane and indole.Therefore,the interaction between phenolic compounds and aromatics is the key to the separation of phenolic compounds,and the presence of nitrogen-containing compounds and their hydrogen bonding interactions with phenolic compounds are also non-negligible factors affecting the quality of phenolic compounds separated from tar.3.The betaine/glycerol（molar ratio of 1:2）extractant was preferably obtained by simulation and experimental studies.The density and viscosity of betaine/glycerol tended to decrease when temperature was increased,and the presence of water in the deep eutectic solvent significantly reduced its density and viscosity.In addition,compared with imidazole and pyridine based traditional ionic liquids,betaine/glycerol has a smaller thermal expansion coefficient(4.87×10^-4 K^-1)and good thermal stability.These characteristics are more conducive to storage,transportation and use.Deep eutectic solvents are formed by betaine and glycerol with the interaction of hydrogen bonding.A stable C-O---H-O-C-C-C-O-H-O-C seven-membered ring hydrogen bonding structure is formed between the molecules,and this structure makes the low eutectic solvent stable in nature.4.The betaine/glycerol deep eutectic solvent has high extraction rate,less neutral oil entrainment on the optimized process conditions.An extraction rate of 95.3%and 5.97%neutral oil entrainment could be achieved for phenolic compounds from the model oil when molar ratio of betaine/glycerol is 1:2,mass ratio of extractant to the initial phenol content is 4:1 and temperature is 298 K.An extraction rate of 93.87%was also achieved for the phenolic compounds from the actual coal tar,and the solvent recovered by the reverse extraction was basically indistinguishable from the new deep eutectic solvent and could be recycled.In addition,the separation of phenolic compounds is influenced by their own structure and the aromatic structure.The complex structure of phenolic compounds weakens their interaction with the deep eutectic solvents and reduces the extraction rate,thus increasing their interaction with aromatic in the model oil and reduces the neutral oil entrainment.The more complex structure,the larger molecular weight and size of the aromatic in the model oil,the more difficult it is for the aromatics to enter the extracted phase during extraction,which results in the reduction of neutral oil entrainment.The interaction between the extractant and phenolic components is weakened because of the presence of water in the deep eutectic solvents.It shows a decrease in extraction rate and neutral oil entrainment.Alkanes in the modle oil make it more difficult for aromatics in the oil to be entrained in the extraction and phenolic compounds were more easily extracted into the extractant phase.There is O???H-O hydrogen bonding interaction between the betaine/glycerol deep eutectic solvents and phenolic compounds,which is stronger than theπ-πstacking interaction between aromatics and phenolic compounds.This makes the separation of phenolic compounds in modle oil by deep eutectic solvents.5.The mass transfer kinetics study showed that the mass transfer during the extraction of phenolic compounds by betaine/glycerol was controlled by diffusion.The mass transfer rate and mass transfer coefficient increased with the increase of stirring intensity,and the extraction efficiency of phenolic compounds increase,which facilitated the mass transfer process.The change of specific boundary area had almost no effect on the mass transfer coefficient,while the extraction rate was positively correlated with the specific boundary area.The increase of temperature and water content of deep eutectic solvents can accelerate the extraction rate and increase the mass transfer coefficient,but also adversely affect the extraction efficiency of phenolic compounds.Therefore,the actual industrial extraction needs to consider the balance of extraction performance and extraction efficiency.The extractant,i.e.,the deep eutectic solvents,is the main determining phase of mass transfer resistance.Phenolic compounds from the coal tar cross the two-phase interface by diffusion and enter the extraction phase.In the extraction phase,the phenolic compounds and the deep eutectic solvents form a state of association with the action of hydrogen bonding and the whole mass transfer during extraction complete.