Ce-MIL-140: expanding the synthesis routes for cerium(iv) metal–organic frameworks

Abstract

A microwave-assisted synthesis method for Ce(IV)-based MOFs crystallizing in the MIL-140 structure has been developed. Three different linker molecules, i.e. terephthalic acid (H₂BDC), 2-chloroterephthalic acid (H₂BDC-Cl) and 2,6-naphtalenedicarboxylic acid (H₂NDC) that have previously been used for the synthesis of Ce-UiO-66 which contains hexanuclear Ce-O clusters as the inorganic building unit (IBU), were employed. Under solvothermal reaction conditions (140 °C) with acetonitrile as the solvent the compounds Ce-MIL-140-BDC, -BDC-Cl and -NDC, with the general composition [CeO(linker)] were obtained as microcrystalline products. For all three MOFs an extended purification process had to be carried out. The MOFs were fully characterized and the structure of Ce-MIL-140-BDC was refined against PXRD data using the Rietveld method. In contrast to Zr-MIL-140-BDC a symmetry reduction to the space group P is observed. The MIL-140 structure type is built up by infinite CeO₇ polyhedra that are interconnected by dicarboxylate ions to generate 1D pores. For Ce-MIL-140-BDC the highest specific surface area of a_sBET = 222 m² g⁻¹ is observed and the MOF is thermally stable up to 370 °C. This new synthetic route to Ce(IV)-MOFs avoids the formation of the previously extremely dominant hexanuclear IBU, and paves the way for higher IBU diversity in Ce(IV)-MOFs.