Influence of crystal textures in metal-organic frameworks (MOFs) on catalysing heterogeneous Knoevenagel condensation reaction

Abstract

Metal-organic frameworks (MOFs) have received long-term research attention due to their exceptionally high specific surface areas and customizable active sites. These properties are typically critical indices for evaluating porous materials in applications such as adsorptive separation or heterogeneous catalysis. High crystallinity and specific surface area have been the focus of MOF synthesis. MOF crystals rich in small micropores may not be sufficient for effective heterogeneous catalysis, especially when the reactants are too large to access the micropores. This study examined ZIF-8 MOF (Zeolite Imidazole Frameworks, a representative and extensively studied MOF) crystal textures for their heterogeneous catalytic performance in the Knoevenagel condensation of benzaldehyde and malononitrile to form benzylidene malononitrile. The ZIF-8 samples were synthesized in aqueous media at a room temperature with or without the mediation of a small amount of ammonia. The obtained ZIF-8 samples were characterized by XRD, ATR, N2-adsorption, TGA and STEM. Although the ammonia-mediated ZIF-8-NH4OH samples exhibited superior crystallinity and high specific surface areas compared to the non-mediated ones, these are not beneficial for their catalytic kinetics in the Knoevenagel condensation of benzaldehyde and malononitrile. In contrast, the ZIF-8 crystals synthesized without ammonia meditation exhibited open mesoporous characteristics, which provided greater accessibility of the reactants to the catalytic active sites on the internal surface. This resulted in significantly faster catalytic conversion, comparable to that achieved in homogeneous catalysis. For the first time, this work systematically demonstrates the critical role of open mesopore formation within MOF crystal particles in promoting heterogeneous catalytic kinetics. The research findings open a new guideline for heterogeneous catalyst development from MOFs.