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 Co(BPZX) (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 O-2 as oxidant. O-2 adsorption isotherms collected at p(O2) = 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 % O-2)), in line with its highest BET specific surface area (926 m(2)/g) in comparison with those of Co(BPZNH(2)) (317 m(2)/g) and Co(BPZNO(2)) (645 m(2)/g). The O-2 isosteric heat of adsorption (Q(2)) trend follows the order Co(BPZ) > Co(BPZNH(2)) > Co(BPZNO(2)). 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, p(O2) = 4 bar, and T = 363 K), further oxidation to 2-phenyl-2-propanol (PP) is observed in the presence of Co(BPZNH(2)) as the catalyst (69% selectivity to PP under the same experimental conditions).