Catalytic behavior of metal–organic frameworks in the Knoevenagel condensation reaction

• MIL-100(Al), Cu3(BTC)2·(H2O)3, UiO-66 were studied in Knoevenagel condensation reaction.
• Activity of metal carboxylates depends on the “M+n–O2− Lewis acid–base” pair.
• The strength of interaction between metal and oxygen was estimated from ionic covalent parameter.
• Amount of NH2 groups in UiO-66-NH2 affects the basicity and catalytic properties.

Effect of basicity of porous metal-carboxylates, such as MIL-100(Al), Cu3(BTC)2·(H2O)3, UiO-66 and amino-modified UiO-66 materials (UiO-66-NH2) on their catalytic performance, was studied in the Knoevenagel condensation of benzaldehyde with malononitrile to 2-benzylidenemalononitrile. According to physicochemical studies, the strength of the basic sites for MIL-100(Al), Cu3(BTC)2·(H2O)3, UiO-66 was in the range of 830–839 kJ/mol, while the amount of basic sites decreased in the order MIL-100(Al) > Cu3(BTC)2·(H2O)3 > UiO-66. The insertion of NH2 groups into the UiO-66 framework led to an increase in the strength of basic sites from 839 to 867 kJ/mol. The catalytic activity of metal carboxylates correlated with the amount of basic sites and the strength of the interaction between metal and oxygen in the “M+n–O2− Lewis acid–base” pair, which was estimated from ionic covalent parameters. The activity of the UiO-66-NH2 materials was higher than that of UiO-66.

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