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

Three metal–organic frameworks with the general formula Co(BPZX)(BPZX2–= 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 O2as oxidant. O2adsorption isotherms collected at pO2= 1 atm and T= 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 Co(BPZ)(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 Co(BPZNH2)(317 m2/g) and Co(BPZNO2)(645 m2/g). The O2isosteric heat of adsorption (Qst) trend follows the order Co(BPZ)> Co(BPZNH2)> Co(BPZNO2). 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 Co(BPZ)(84% selectivity to CHP after 7 h, pO2= 4 bar, and T= 363 K), further oxidation to 2-phenyl-2-propanol (PP) is observed in the presence of Co(BPZNH2)as the catalyst (69% selectivity to PP under the same experimental conditions).