Encapsulation of ionic liquids into POMs-based metal–organic frameworks: screening of POMs-ILs@MOF catalysts for efficient cycloolefins epoxidation

Abstract

A series of ionic liquids (ILs, 1-octyl-3-methylimidazolium bromide)-functionalized polyoxometalates (POMs)-based metal–organic frameworks (MOFs) hybrids (i.e., POMs-ILs@MOFs) were facilely prepared via encapsulation strategy. The as-synthesized materials were well characterized via XRD, FT-IR, N2 sorption, TEM, SEM, UV–Vis DRS, TG, XPS, and ICP analysis to confirm that POMs was successfully immobilized in MOFs cages and the structure of MOFs were retained after immobilization. The obtained composites were screened for catalytic cycloolefins epoxidation with H2O2, and an enhanced catalytic activity was obtained via ILs modification. One of the POMs-ILs@MOFs hybrids, PMo10V2-ILs@MIL-100(Fe), when acted as the catalyst, exhibited superior catalytic properties (viz., activity and stability). Further investigation revealed that this excellent catalytic performance could be ascribed to synergistic influence of ILs, POMs, and MOFs: (1) mesoporous cages and large surface area of MIL-100(Fe) could provide suitable accommodation for reactants molecules and active species; (2) PMo10V2-ILs@MIL-100(Fe), where two Mo atoms were substituted via two V atoms, displayed stronger acidity than that of PMo12-ILs@MIL-100(Fe), and this was advantageous to cycloolefins oxidation; (3) Imidazolium cations originated from ILs were beneficial to activate POMs. Furthermore, PMo10V2-ILs@MIL-100(Fe) could be facilely recovered and maintained excellent catalytic activity over 12 cycles. Having a combination of superior catalytic activity, good recyclability, and eco-sustainability, PMo10V2-ILs@MIL-100(Fe) therefore provided a new perspective for cycloolefins epoxidation.