Dicationic Ionic Liquid: A Novel Method for Improving the Isomerization Degree of n‑Pentane

A series of dicationic ionic liquids were successfully prepared, and they were first used to convert the n-pentane into iso-alkanes as an environmentally safe way to improve the octane number of gasoline. As a novel and green catalyst, the dicationic ionic liquid [tetramethylethylenediamine­(EtBr)2]-AlCl3 ([TMEDA­(EtBr)2]-AlCl3) exhibits higher catalytic performance in n-pentane isomerization than the traditional monocationic ionic liquid [1-butyl-3-methylimidazolium]­Cl-AlCl3 ([BMIM]­Cl-AlCl3). And for the n-pentane isomerization reaction catalyzed by [TMEDA­(EtBr)2]-AlCl3, the optimal reaction temperature, reaction time, and mass ratio of catalyst to oil were proven to be 100 °C, 3 h, and 1:1, respectively. For dicationic ionic liquid [TMEDA­(EtBr)2]-AlCl3, the acid strength of it increases steadily with the increase of AlCl3 mole fractions. And the initiator is conducive to increasing n-pentane conversion, improving the yield of i-C5 and i-C6, and inhibiting the C4 component. Additionally, as the length of substitute alkyl groups of cationic structures in dicationic ionic liquids increases, the catalytic conversion of n-pentane and the yield of iso-alkanes decrease constantly. For novel dicationic ILs, the moderate rising of temperature facilitates improving the catalytic performance of n-pentane isomerization, in particular, for the ionic liquids with higher melting points. Moreover, the mechanism of n-pentane isomerization catalyzed by dicationic ionic liquid was studied, which revealed the reason that [TMEDA­(EtBr)2]-AlCl3 IL has better catalytic performance than monocationic ionic liquid and other dicationic ILs.