Quantum Chemical Evaluation of Deep Eutectic Solvents for the Extractive Desulfurization of Fuel

Sulfur compounds in fuels are converted to SOx during combustion, poisoning automotive catalytic converters and creating serious environmental concerns (e.g., acid rain). The efficient desulfurization of liquid fuel is thus a critical step toward minimizing SOx emissions and their associated environmental impact. To address this problem, governments worldwide have passed stringent legislation regulating the maximal sulfur levels allowable in fuels. In the petroleum refining industry, the conventional method for removing sulfur from fuel is catalytic hydrodesulfurization which, while highly efficient for removing mercaptans, thioethers, and disulfides, shows limited performance in removing aromatic organosulfur compounds exemplified by dibenzothiophene. To meet these strict environmental targets, innovative strategies beyond hydrodesulfurization for the deep desulfurization of fuel are sought. One key strategy entails the oxidation of refractory organosulfur compounds in liquid fuel, coupled with efficient liquid/liquid extraction of the oxidized sulfur compounds using an immiscible solvent phase (i.e., oxidative desulfurization). In this study, we employ computational chemistry to gain atomistic-level insight into the specific interactions responsible for the extraction of key organosulfur compounds and their oxidation products from fuel using deep eutectic solvents (DESs). Specifically, we perform quantum chemical calculations involving the well-studied DESs reline (1:2 choline chloride:urea) and ethaline (1:2 choline chloride:ethylene glycol) to characterize the intermolecular interactions, charge transfer behavior, and thermodynamics associated with their application for organosulfur extraction. We observe that the model aromatic sulfur compounds (ASCs) benzothiophene and dibenzothiophene interact with choline and the hydrogen bond donor (HBD; i.e., urea or ethylene glycol) of the DES via a plurality of weak noncovalent interactions. However, the chloride ion is essentially noninteractive with the ASC due to retention of the conventional hydrogen bond network existing within the initial DES. Oxidation of the model ASCs to their respective sulfoxide and sulfone products was shown to enhance interactions with the DES components, particularly the HBD species due to its propensity for forming multiple hydrogen bonds. We further demonstrate that, upon oxidation, the ASCs exhibit significant and favorable free energies of solvation, suggesting that oxidation will aid in the partition of these sulfur compounds from liquid fuel to a conventional DES phase.