Knoevenagel condensation reaction catalysed by Al-MOFs with CAU-1 and CAU-10-type structures

Abstract

The metal–organic frameworks (MOFs) CAU-1-NH₂ ([Al₄(OH)₂(OCH₃)₄(p-BDC-NH₂)₃], (p-BDC-NH₂)²⁻ = 2-aminoterephthalate) and CAU-10-NH₂ ([Al(OH)(m-BDC-NH₂)], (m-BDC-NH₂)²⁻ = 5-aminoisophthalate) which possess two different pore sizes were studied for their catalytic activity as heterogeneous solid base catalysts in the Knoevenagel condensation reaction between benzaldehyde and malononitrile under mild reaction conditions (40 °C, 7 h, ethanol). For comparison, isoreticular MOFs containing a smaller amount of –NH₂ groups (CAU-1-NH₂/H) or no –NH₂ groups (CAU-10-H) were synthesized. A two-step synthesis route including the synthesis of CAU-1-NH₂ and its use as crystal seeds was developed to obtain the mixed-linker CAU-1-NH₂/H compound. Only for CAU-1-NH₂, up to 100% selectivity towards the desired Knoevenagel condensation product is observed. Hence, the catalytic activity of CAU-1-NH₂ was tested between various benzaldehydes containing different substituents with malononitrile. By employing benzaldehyde and malononitrile as the starting materials, it is found that the mixed-linker MOF CAU-1-NH₂/H catalyses the formation of the acetal (benzaldehyde diethyl acetal), while both CAU-10 materials lead to mixtures of the two products. Furthermore, the catalyst stability was also examined through reusability and leaching experiments and it is observed that the catalyst can be reused with no significant drop in its activity.