Methyltrioxorhenium-catalyzed epoxidation of olefins with hydrogen peroxide as an oxidant and pyridine N-oxide ionic liquids as additives

Four ionic liquids (ILs) with both a pyridine N-oxide moiety and an imidazolium moiety combined by an amide spacer were synthesized through a series of reactions including amidation, peroxidation, quaterization and anion exchange reaction. Their structures were fully characterized by 1H NMR, FT-IR, UV–vis and HR–MS. The ionic liquids were used respectively as additives in the methyltrioxorhenium (MTO) catalyzed epoxidation of olefins with 30% H2O2 as an oxidant. The catalytic results displayed that the ILs had excellent performances in suppressing epoxide ring-opening reaction, which led to the significant improvement of the selectivity of the MTO-catalyzed epoxidation with low loadings compared to other substances as additives. The coexistence of the pyridine N-oxide and imidazolium moieties in the structures of ILs is necessary in improving the MTO-catalyzed epoxidation reaction. It was also displayed that the improvement degree on the selectivity of epoxidation depended on the type of anion of the ILs, but not the position of the substituent with imidazolium moiety in the ring of pyridine N-oxide. Meanwhile, the results also showed that the introduction of the ILs caused the decrease of the epoxidation rate, but this side effect was small compared to those of other substances used as additives.

Graphical abstractMethyltrioxorhenium-catalyzed epoxidation of olefins with pyridine N-oxide ionic liquids as additives and 30% hydrogen peroxide as an oxidant.Figure optionsDownload full-size imageDownload high-quality image (97 K)Download as PowerPoint slideHighlights► Four novel ionic liquids (ILs) with pyridine N-oxide moiety were synthesized. ► The ILs as additives significantly increased the selectivity of MTO-catalyzed epoxidation. ► The ILs as additives improved the lifetime of MTO in the epoxidation. ► The anion type of the ILs is important in improving the selectivity of epoxidation.