Theoretical Study On The Mechanism Of Oxidative-Extractive Desulfurization Process With The Assistance Of Imidazolium-based Ionic Liquids

Fuel desulfurization is one of the most important oil refining technologies because the presence of organosulfur compounds in petroleum not only has a negative effect on environment but also may induce the equipment corrosion.The oxidative-extractive desulfurization technology with the assistance of ionic liquids?ILs?has become the most promising emerging desulfurization method in recent years due to its mild reaction conditions,simple techniques and high desulfurization efficiency.In this work,the mechanism of oxidative-extractive process for the removal of thiophene?Th?with the assistance of imidazolium-based ionic liquid was theoretically investigated by carrying out density functional theory?DFT?calculations.We explored the catalytic role of ionic liquid in the oxidative reaction and studied the extraction mechanism of sulfur compounds before and after oxidation,furthermore,we clarified the reason why the desulfurization efficiency could be enhanced after oxidation from a theoretical point and finally the extractive selectivity was also inspected.Our contribution provided a theoretical basis for the practical application of IL-assisted oxidative-extractive process.Firstly,we calculated the oxidative process of Th to sulphone?SP?by H₂O₂ in the presence of imidazolium-based IL with different alkyl side chains and studied the influence of alkyl substituent in imidazolium ring on the oxidative-extractive process.Herein four imidazolium-based ILs with ethyl,butyl,amyl and hexyl side alkyl chain in the imidazolium ring were chosen in this calculation and they were named as [EMIM][PF6],[BMIM][PF6],[AMIM][PF6] and [HMIM][PF6],respectively.The result showed that the oxidation process from Th to SP experienced two steps and the reaction energy barriers could be reduced greatly when involved with IL.According to the energy statistics in each step,the energy barriers decreased from 336.8 and 285.3 kJ/mol?without IL?to 115.2 and 109 kJ/mol in the presence of [EMIM][PF6],respectively.It was also found that with increasing the alkyl chain length from two?ethyl?to six?hexyl?carbon atoms,the energy barriers in the two elementary oxidative steps are reduced by 23% and 29%,respectively,indicating that the longer the alkyl chain,the more easily the reaction takes place.This is mainly because IL can not only activate H₂O₂ as a catalyst,but also can affect the charge on S atom of Th.The charge on S atoms increased with the increase of the alkyl side chain length,therefore,the reaction was more likely to occur on S atom with higher electron density and accordingly lower reaction energy barrier was needed.On the basis of the oxidative process of Th to SP,the influence of alkyl side chain length in imidazolium ring on the extractive process was further inspected.It was found that whatever the cation,anion or entire ionic pairs exhibited stronger interactions with SP than those with TH,attributing to the larger dipole moment of SP and larger electronegativity of O atoms,and this was also the reason why the desulfurization efficiency can be greatly enhanced after the oxidation.Moreover,according to the NBO analysis,cation played the dominant role in the capture of S-containing compounds and the interaction of the cation with TH or SP was slightly decreased with the increase of the alkyl side chain length on the imidazole ring.For the extractive selectivity,the calculation results showed that all of benzene,methylbenzene,cyclohexene,hexane had a slightly weak interaction with IL and the nitrogen-containing compounds had a relatively strong interaction with IL which energy was calculated to be similar to that of sulfur-containing compounds,thus leading to a low extractive desulfurization efficiency and poor selectivity.But after the oxidation,the interaction between SP and IL was significantly increased and the interaction energy was much higher than those of IL with other competitive compounds.Therefore the IL assisted oxidative-extractive desulfurization technique can not only promote the desulfurization efficiency significantly,but also improve the desulfurization selectivity.