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Removing sulfur compounds from petroleum is essential in mitigating harmful emissions linked to fuel combustion. Problematically, thiophenic compounds that are readily present in fossil fuels resist conventional desulfurization methods. Extractive Desulfurization (EDS) provides an attractive alternative that can be selective for heterocyclic sulfur compounds, i.e., thiophene and its derivatives, through the choice of solvent employed. To this end, a burgeoning field has developed around the use of ionic liquids (ILs) given their ability to be fine-tuned with varying levels of polarity and solubility to suit the specific requirements of the desulfurization process. Hundreds of experimental studies featuring the use of IL technologies have provided encouraging trends for the design of more efficient extractants; however, conflicting data and a lack of a definitive understanding of important solute-ion interactions has presented challenges in advancing the field. More recently, computational investigations have been employed to unravel these key interactions and to inform design principles for future high-performance IL extractants. The myriad of intermolecular forces, e.g., coulombic, dispersive, and steric, and their subtle interplay present in IL-mediated EDS processes are prime for study using computational methodologies that include quantum mechanics (QM) at the ion-solute interaction level, molecular dynamics (MD) for the simulation of bulk-phase solvent properties, and the Conductor-Like Screening Model (COSMO) for high-throughput screening. This minireview summarizes computational advances and findings in the field of IL-mediated EDS in a format suitable for theoreticians and experimental chemists alike with discussions provided of future directions for the field. 

The conversion of biomass to 5-hydroxymethylfurfural (HMF) holds substantial promise as a renewable energy source. Notably, HMF can be transformed into 2,5-bis(hydroxymethyl)furan (BHMF), a crucial reactant in biofuel production, but requires harsh operating conditions, H2, and precious metal catalysts. A recently reported Cannizzaro reaction of HMF to BHMF, characterized by its efficiency, mild conditions, and eco-friendliness, instead employed ionic liquids (ILs) to achieve high yields. In this study, combined quantum mechanical and molecular mechanical (QM/MM) simulations in conjunction with Metropolis Monte Carlo statistical mechanics and free-energy perturbation theory utilized M06-2X/6-31+G(d), PDDG/PM3, and the OPLS-VSIL force field to uncover important solute–solvent interactions present in the HMF to BHMF reaction pathway. The Cannizzaro reaction was examined in water and in five ILs composed of the 1-butyl-3-methylimidazolium [BMIM] cation coupled to hexafluorophosphate, tetrafluoroborate, thiocyanate, chloride, and bromide. Energetic and structural analysis of the rate-determining hydride transfer between HMF and the hydride-donor anion HMFOH− attributed the enhanced reactivity to highly organized solvent interactions featuring (1) hydrogen bonding between the ring protons of [BMIM] and the negatively charged carbonyl oxygen atoms on the transition structure, (2) favorable electrostatic interactions between the IL anions and solute hydroxyl groups, and (3) beneficial π–π stacking interactions between [BMIM] and the two aromatic rings present in HMF and HMFOH−. 

Environmental regulatory agencies have implemented stringent restrictions on the permissible levels of sulfur compounds in fuel to reduce harmful emissions and improve air quality. Problematically, traditional desulfurization methods have shown low effectiveness in the removal of refractory sulfur compounds, e.g., thiophene (TS), dibenzothiophene (DBT), and 4-methyldibenzothiophene (MDBT). In this work, molecular dynamics (MD) simulations and free energy perturbation (FEP) have been applied to investigate the use of ionic liquids (ILs) and deep eutectic solvents (DESs) as efficient TS/DBT/MDBT extractants. For the IL simulations, the selected cation was 1-butyl-3-methylimidazolium [BMIM], and the anions included chloride [Cl], thiocyanate [SCN], tetrafluoroborate [BF4], hexafluorophosphate [PF6], and bis(trifluoromethylsulfonyl)amide [NTf2]. The DESs were composed of choline chloride with ethylene glycol (CCEtg) or with glycerol (CCGly). Calculation of excess chemical potentials predicted the ILs to be more promising extractants with energies lower by 1-3 kcal/mol compared to DESs. Increasing IL anion size was positively correlated to enhanced solvation of S-compounds, which was influenced by energetically dominant solute-anion interactions and favorable solute-[BMIM] pi-pi stacking. For the DESs, the solvent components offered a range of synergistic, yet comparatively weaker electrostatic interactions that included hydrogen bonding and cation-pi interactions. An in-depth analysis of the structure of IL and DES systems is presented, along with a discussion of the critical factors behind experimental trends of S-compound extraction efficiency. 

https://doi.org/10.1002/9781119625933.ch4 

Abstract Despite advances in creating dissipative materials with transient properties, such as hydrogels and active droplets, their application remains confined to temporal changes in structural properties. Developing out‐of‐equilibrium materials whose electronic functions are parameterized by a chemical reaction cycle is challenging. Yet, this class of materials is required to construct biomimetic materials. In contrast to traditional chemical reaction cycles that exploit molecularly dissolved building blocks at thermodynamic equilibrium, we show that fiber structures derived from reactive naphthalene diimide (NDI) building blocks can be used as resting states to form far‐from‐equilibrium conductive hydrogels after the addition of chemical fuels. Upon fueling the NDI‐derived fibers, a dual‐component activation and deactivation pathway is deduced by kinetic analysis and is absent when using a molecularly dissolved resting state. Investigating the solid‐state morphologies of the structures formed throughout the fuel‐driven reaction cycle using cryo‐EM reveals that the resting thermodynamic fibers evolve to transient thicker fibrils and layered superstructures. We show that the transient redox‐active hydrogels exhibit a nearly threefold increase in electrical conductivity upon fuel consumption before reverting to their original value over hours. These far‐from‐equilibrium materials are potential candidates in applications such as programmable biorobotics and chemical computing.