Heterogenization of molecular cobalt catalysts in robust metal–organic frameworks for efficient photocatalytic CO2 reduction

Abstract

Cobalt quaterpyridine (Coqpy) and cobalt tris(2-pyridylmethyl)amine (CoTPA) complexes, known for their ability to reduce CO₂ to CO under visible-light irradiation in homogeneous conditions, were immobilized using the impregnation method in two different Zr-based metal–organic frameworks, UiO-67-bpydc and PCN-777, respectively. The successful synthesis of Coqpy@UiO-67-bpydc shows that linker deficiencies can allow catalytic complexes (Cat) to enter into the cavities of the Zr-based MOF while they would not in the defect-free MOF. The composites were characterized through solid state characterization techniques complemented by density functional theory calculations in order to locate the catalysts into the MOF's pores and estimate the host/guest interactions involved. The two noble-metal free Cat@MOF composites reduce CO₂ to CO under visible-light irradiation in CH₃CN/H₂O (5 : 1) solutions, in the presence of [Ru(phen)₃]²⁺ as solution-dissolved external photosensitizer and dimethylphenylbenzimidazoline (BIH) as electron and proton donor. CO yields, TON and selectivity reached 619 μmol g⁻¹ h⁻¹, 268 and 93% respectively for Coqpy@UiO-67-bpydc and 368 μmol g⁻¹ h⁻¹, 482 and 94% for CoTPA@PCN-777 after 24 h solar simulated illumination. These photocatalytic studies evidenced that the composite materials are efficient, selective and recyclable and that the MOF scaffold offers a favorable environment for a constant CO production rate over a long time period.