Synthesis, characterization, and application of metal-free acidic ionic liquids as catalysts for oligomerization of isobutene

Butene oligomerization followed by hydrogenation is regarded as a promising way to produce alkylate oil, and the challenge of this process is the oriented conversion of butene to octene, which closely relates to the alkylate quality. Ionic liquids (ILs) are a kind of promising catalyst for olefin oligomerization. In this study, a series of functional metal-free acidic ILs (11 in total) were synthesized, characterized, and used for the oligomerization of isobutene. It was found that among the tested neat ILs, sulfobutyltriethylammonium triflate ([TENBs][CF3SO3]) exhibited the best performance. This IL catalyst could be reused for 25 times, without obvious loss in catalytic activity. To improve catalytic activity of the neat IL, composite ILs formed by mixing [TENBs][CF3SO3] with a second functional IL were developed. In the best composite system with tunable acidity, isobutene conversion could increase by similar to 88% compared with the neat IL, and meantime the dimer selectivity could increase to 79%. The conversion could increase from 25% to over 80% in the best composite system with enhanced mass transfer capability. Spectroscopic characterization and Gaussian simulation revealed the different catalytic behaviors of these ILs, which demonstrated that ILs with lower bond dissociation enthalpy (< 521 KJ.mol(-1)) and proper binding energy with isobutene (27 to 20 KJ.mol(-1)) could efficiently catalyze isobutene oligomerization, the acidic active sites were from the sulfonic acid group in the cation and the acidic anion, and that deactivation of the IL catalyst after being used for 29 times could be attributed to the loss of the acid sites in both the cation and anion.