Post-synthetic cation exchange in anionic metal–organic frameworks; a novel strategy for increasing the catalytic activity in solvent-free condensation reactions

Abstract

In recent years anionic MOFs (metal–organic frameworks) have only been scarcely explored for application in heterogeneous catalysis. In this work an anionic metal–organic framework, [HDMA]₂[Zn₂(BDC)₃(DMA)₂]·6DMF, has been prepared and its cations, HDMA⁺, were exchanged by Ni²⁺, Na⁺, Li⁺, tetraethylammonium (TEA⁺) and tetrapropylammonium (TPA⁺) by a post-synthetic process. By the strategy developed here, we have been able to prepare cation exchanged anionic MOFs as novel heterogeneous catalytic systems for solvent-free Knoevenagel condensation reaction between benzaldehyde and malononitrile. These catalysts were easily isolated from the reaction mixture and were reused several times without a significant degradation in activity.