Intermolecular Interactions Governing Extraction Efficiency of Polycyclic Thiophenes by Deep Eutectic Solvents

Deep eutectic solvents (DESs) are a promising low-cost, low-toxicity medium for extractive desulfurization of petroleum, but the relationship between extraction performance and solvent properties requires further investigation. In this study, polycyclic thiophenes were extracted from a petroleum fuel analog (n-heptane) using different DESs to probe the effect of intermolecular interactions in DES systems on the extraction efficiency. Six DESs were prepared by combining quaternary ammonium salts [tetrabutylammonium chloride (TBAC), tetrabutylammonium bromide (TBAB), choline chloride (ChCl), benzyltriethylammonium chloride (BTAC), and benzyltributylammonium chloride (BTBAC)] with hydrogen bond donors [ethylene glycol (EG) and glycerol (Gly)]. Fourier-transform infrared (FTIR) and 1H NMR analysis was carried out to investigate the hydrogen bonding interactions and molar ratios. Model sulfur-containing aromatic impurities benzothiophene, dibenzothiophene, and 4,6-dimethyldibenzothiophene were extracted from n-heptane using the prepared DESs, and the extraction performance was quantified by the partition coefficient, defined as the ratio of solute concentrations between the DES and organic phase. Interestingly, the highest partition coefficient was achieved with tetrabutylammonium rather than benzyltributylammonium or benzyltriethylammonium ions, suggesting that possible π–π interactions are not sufficient to increase the partition coefficient in these systems. The interactions between DES components and polycyclic thiophenes were further investigated by electronic structure calculations, which indicate a preference for CH−π rather than π–π stacking interactions arising from the highly polarized C–H bonds of the quaternary ammonium ions.

低共熔溶剂（Deep eutectic solvents, DESs）是一类极具应用前景的低成本、低毒性介质，可用于石油的萃取脱硫，但萃取性能与溶剂性质之间的关联仍有待进一步研究。本研究采用多种低共熔溶剂从石油燃料模拟物——正庚烷（n-heptane）中萃取多环噻吩（polycyclic thiophenes），以探究低共熔溶剂体系内的分子间相互作用对萃取效率的影响。本研究通过将季铵盐（quaternary ammonium salts）与氢键供体（hydrogen bond donors）复配制备了六种低共熔溶剂：季铵盐包括四丁基氯化铵（tetrabutylammonium chloride, TBAC）、四丁基溴化铵（tetrabutylammonium bromide, TBAB）、氯化胆碱（choline chloride, ChCl）、苄基三乙基氯化铵（benzyltriethylammonium chloride, BTAC）以及苄基三丁基氯化铵（benzyltributylammonium chloride, BTBAC）；氢键供体则包括乙二醇（ethylene glycol, EG）与甘油（glycerol, Gly）。本研究采用傅里叶变换红外光谱（Fourier-transform infrared, FTIR）与核磁共振氢谱（¹H NMR）对体系内的氢键相互作用以及物料摩尔比进行了表征分析。研究人员采用制备得到的低共熔溶剂从正庚烷中萃取典型含硫芳香杂质：苯并噻吩（benzothiophene）、二苯并噻吩（dibenzothiophene）以及4,6-二甲基二苯并噻吩（4,6-dimethyldibenzothiophene），并以分配系数（partition coefficient，即溶质在低共熔溶剂相与有机相中的浓度之比）对萃取性能进行定量表征。值得注意的是，四丁基铵阳离子体系的分配系数最高，而非苄基三丁基铵或苄基三乙基铵阳离子体系，这表明在本研究的体系中，潜在的π-π相互作用（π–π interactions）并不足以提升分配系数。研究人员进一步通过电子结构计算（electronic structure calculations）探究了低共熔溶剂组分与多环噻吩之间的相互作用，结果表明：由于季铵盐阳离子存在高度极化的C-H键，体系更倾向于形成CH-π相互作用（CH−π interactions）而非π-π堆积相互作用（π–π stacking interactions）。