A novel nickel metal–organic framework with fluorite-like structure: gas adsorption properties and catalytic activity in Knoevenagel condensation

Abstract

A new non-interpenetrating 3D metal–organic framework {[Ni₄(μ₆-MTB)₂(μ₂-H₂O)₄(H₂O)₄]·10DMF·11H₂O}_n (DMF = N,N′-dimethylformamide) built from nickel(II) ions as connectors and methanetetrabenzoate ligands (MTB⁴⁻) as linkers has been synthesized and characterized. The single crystal X-ray diffraction showed that complex exhibits CaF₂-like fluorite structure topology and four types of 3D channels with sizes about 12.6 × 9.4 Ų, 9.4 × 8.0 Ų, 12.6 × 11.7 Ų and 14.9 × 14.9 Ų, which are filled with guest molecules. Conditions of the activation of the compound have been studied and optimized by powder X-ray diffraction during in situ heating, thermogravimetric analysis and infrared spectroscopy. Nitrogen and carbon dioxide adsorption showed that the activated sample exhibits a BET specific surface area of 700 m² g⁻¹ and a carbon dioxide uptake of 12.36 wt% at 0 °C, which are the highest values reported for the compounds of the MTB⁴⁻ series. The complex was tested in Knoevenagel condensation of aldehydes and active methylene compounds. Straightforward dependence of the substrate conversion on the size of used aldehyde was established. A possible mechanism of Knoevenagel condensation over a MTB⁴⁻ containing a metal–organic framework was proposed.