Cobalt(II) Bipyrazolate Metal–Organic Frameworks as Heterogeneous Catalysts in Cumene Aerobic Oxidation: A Tag-Dependent Selectivity

Abstract

Three metal-organic frameworks with the general formula <b>Co(BPZX)</b> (BPZX^2- = 3-X-4,4'-bipyrazolate, X = H, NH₂, NO₂) constructed with ligands having different functional groups on the same skeleton have been employed as heterogeneous catalysts for aerobic liquid-phase oxidation of cumene with O₂ as oxidant. O₂ adsorption isotherms collected at <i>p</i>_O2 = 1 atm and <i>T</i> = 195 and 273 K have cast light on the relative affinity of these catalysts for dioxygen. The highest gas uptake at 195 K is found for <b>Co(BPZ)</b> (3.2 mmol/g (10.1 wt % O₂)), in line with its highest BET specific surface area (926 m²/g) in comparison with those of <b>Co(BPZNH</b>_<b>2</b><b>)</b> (317 m²/g) and <b>Co(BPZNO</b>_<b>2</b><b>)</b> (645 m²/g). The O₂ isosteric heat of adsorption (<i>Q</i>_st) trend follows the order <b>Co(BPZ)</b> > <b>Co(BPZNH</b>_<b>2</b><b>)</b> > <b>Co(BPZNO</b>_<b>2</b><b>)</b>. Interestingly, the selectivity in the cumene oxidation products was found to be dependent on the tag present in the catalyst linker: while cumene hydroperoxide (CHP) is the main product obtained with <b>Co(BPZ)</b> (84% selectivity to CHP after 7 h, <i>p</i>_O2 = 4 bar, and <i>T</i> = 363 K), further oxidation to 2-phenyl-2-propanol (PP) is observed in the presence of <b>Co(BPZNH</b>_<b>2</b><b>)</b> as the catalyst (69% selectivity to PP under the same experimental conditions).