Embedding of SO3H-functionalized ionic liquids in mesoporous MIL-101 (Cr) through polyoxometalate bridging: A robust heterogeneous catalyst for biodiesel production

A robust heterogeneous catalyst SIL-PW/MIL-101(Cr) for biodiesel production was fabricated via a bottom-up strategy using phosphotungstic acid (HPW) as a bridge to integrate SO3H-functionalized ionic liquid (SIL) with MIL-101(Cr). The characterization results indicated that HPW was well dispersed in the cavities of MIL-101 (Cr). The subsequently introduced ionic liquid reacted with HPW to form heteropolyanion-based ionic liquid, which could be embedded in the mesoporous cages. These newly-developed SIL-PW/MIL-101(Cr) exhibited considerable catalytic activity, with 94.3 % conversion of oleic acid. The comparison experiments manifested the enhanced catalytic performance was attributed to the Bronsted acid sites originating from-SO3H groups together with the Lewis acid sites. Moreover, the mesoporous structure of MIL-101(Cr) remained intact during immobilization process, which was beneficial for diffusion of large-sized reactants. Due to the bridging function of HPW, no obvious leaching of active component during recycling was observed and the conversion maintained 91.8 % after reused for five times, displaying satisfactory reusability. This study could open new avenues for designing heterogeneous catalysts with multiple active sites through bridging strategy.