Synthesis, structural properties, and catalytic behavior of Cu-BTC and mixed-linker Cu-BTC-PyDC in the oxidation of benzene derivatives

Mixed-linker metal-organic frameworks based on the Cu-BTC structure have been synthesized in which the benzene-1,3,5-tricarboxylate (BTC) linkers have been partially replaced by pyridine-3,5-dicarboxylate (PyDC). X-ray-based techniques (powder XRD and XAS), thermal analysis, and infrared spectroscopy proved that a desired amount of PyDC (up to 50%) can be incorporated without changing significantly the crystal structure. The pyridine unit can be seen as a defect site in the local coordination environment of the dimeric copper units, which is significantly altering their electronic structure and the catalytic properties. Both Cu-BTC and the mixed Cu-BTC-PyDCs catalyze the demanding direct hydroxylation of toluene both in acetonitrile and in neat substrate. Different selectivity toward the desired ortho- and para-cresol and other oxidation products (benzaldehyde, benzyl alcohol, methylbenzoquinone) was observed for Cu-BTC and the Cu-BTC-PyDCs, respectively. Leaching tests and comparison with homogeneously dissolved Cu catalysts indicate mainly a heterogeneous reaction pathway.