Amino-functionalized basic catalysts with MIL-101 structure

Metal–organic framework (MOF) materials have only been scarcely explored for applications in heterogeneous catalysis in recent years. In addition to the use of MOFs as supports for the deposition of highly dispersed metal particles, the incorporation of active centers such as coordinatively unsaturated metal sites and the functionalization of the organic linkers with acidic or basic groups seem to be most promising. In our contribution, three different MOFs carrying amino groups at their organic linkers, namely Fe-MIL-101-NH2 (SBET = 3438 m2 g−1), Al-MIL-101-NH2 (SBET = 3099 m2 g−1) and CAU-1 (SBET = 1492 m2 g−1), were synthesized and tested in the Knoevenagel condensation of benzaldehyde with malononitrile and with ethyl cyanoacetate, respectively. It is shown that the expected products benzylidenemalononitrile (BzMN) and ethyl α-cyanocinnamate (EtCC) are formed with selectivities exceeding 99% and yields of 90 to 95% after 3 h (for BzMN). Due to the very small pore windows of CAU-1 (0.3 to 0.4 nm) the reaction proceeds much slower over this catalyst in comparison to the amino-MIL-101 derivatives, which possess open pore windows of up to 1.6 nm. Moreover, the catalyst Al-MIL-101-NH2 is recyclable without significant loss of activity.

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► We improved the synthesis of Al-MIL-101-NH2 by an innovative semi-batch approach.
► Amino-functionalized MIL-101 derivatives are highly active solid basic catalysts.
► The new MOF-catalysts show superior performance compared to inorganic basic solids.
► We proved the role of the NH2-moieties as the major active sites.
► Al-MIL-101-NH2 is a stable, recyclable and non-leaching basic material.