Catalytic improvement by open metal sites in a new mixed-ligand hetero topic metal–organic framework

A new porous MOF, [Zn2(BDC)1.5(L)(DMF)]·1.5DMF denoted as TMU-41 (L = pyridine 4-carboxylic acid and BDC = benzene-1,4-dicarboxylate), has been synthesized and fully characterized by single-crystal X-ray crystallography, thermogravimetric analysis (TGA), X-ray diffraction (XRD) and N2 adsorption–desorption isotherms. After removing the coordinated DMF molecules, the obtained surface area values are in the range of 740 m2/g. The standard procedure is to take the Zn2 group as a single secondary building unit (SBU). Each of these units is linked to five others to produce a 5-c high symmetry net with RCSR symbol bnn. Due to the presence of open metal sites (OMSs) in the metal–organic framework, it exhibits specific properties and it can act as a Lewis acid catalyst. Because of the special properties of unsaturated metal sites acting as catalysts, in this study it is tried to convert the structure with coordinated DMF molecules into open metal sites structure by removal of coordinated DMF under specific conditions. A metal–organic framework with open metal sites (TMU-41 (OMS)) was used as a catalyst for the knoevenagel condensation reaction and the catalytic activity of this MOF was investigated and compared to TMU-41. The results showed that in these reactions, (TMU-41 (OMS)) with a small amount of catalyst (0.008 g) was more efficient than TMU-41. This can be explained by the availability of open metal sites in this framework.