A series of 36 dual functionalized N-(poly)ethylen glycol N-allyl-/N-benzyl imidazolium and benzimidazolium ILs were synthesized for extractive desulfurization studies (EDS). The novel ILs were readily prepared from imidazole by a three-step dialkylation and anion exchange procedure and characterised (1H, 13C, 19F NMR; IR; HRMS positive/negative mode). The desulfurization studies were performed on a model oil with dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in n-dodecane. The N-allyl benzimidazolium based ILs exhibited higher extraction efficiency than corresponding imidazolium-based ILs (69 vs 59% DBT; 52 vs 29% 4,6-DMDBT). The EDS efficiency of the prepared ILs varied with the N-ethylen glycol chains length and by N-allyl or N-benzyl substitution on the imidazolium ILs. The results indicated that desulfurization efficiency is mainly affected by variation of the IL cation, as the IL anions (NTf2; N(CN)2-, SCN-, BF4-) only had minor impact on the extraction capacity of sulfur compounds.
1H NMR studies of IL-DBT solutions illustrated different EDS mechanisms, and specific interactions could explain the higher desulfurization capacity of dual functionalized benzimidazolium than the corresponding imidazolium ILs. 1H NMR confirmed that stronger π-π interactions between the planar aromatic electron system of benzimidazolium ILs and aromatic dibenzothiophenes play a major role for the high EDS extraction efficiency.
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