Formation of a mixed-valence Cu(i)/Cu(ii) metal–organic framework with the full light spectrum and high selectivity of CO2 photoreduction into CH4

Abstract

Based upon the hetero-N,O ligand of pyrimidine-5-carboxylic acid (Hpmc), a new semiconductive Cu(I)/Cu(II) mixed-valence MOF with the full light spectrum and a novel topology of {4³·6¹²·8⁶}₂{4³·6³}₂{6³}₆{6⁴·8²}₃, {(Cu₄I₄)_2.5[Cu₃(μ₄-O) (μ₃-I) (pmc)₃(Dabco)₃]·2.5DMF·2MeCN}_∞ (NJU-Bai61, NJU-Bai for Nanjing University Bai group; Dabco = 1,4-diazabicyclo [2.2.2] octane), was synthesized stepwise. NJU-Bai61 exhibits good water/pH stabilities and a relatively large CO₂ adsorption capacity (29.82 cm³ g⁻¹ at 1 atm, 273 K) and could photocatalyze the reduction of CO₂ into CH₄ without additional photosensitizers and cocatalysts and with a high CH₄ production rate (15.75 μmol g⁻¹ h⁻¹) and a CH₄ selectivity of 72.8%. The CH₄ selectivity is the highest among the reported MOFs in aqueous solution. Experimental data and theoretical calculations further revealed that the Cu₄I₄ cluster may adsorb light to generate photoelectrons and transfer them to its Cu₃OI(CO₂)₃ cluster, and the Cu₃OI(CO₂)₃ cluster could provide active sites to adsorb and reduce CO₂ and deliver sufficient electrons for CO₂ to produce CH₄. This is the first time that the old Cu(I)_xX_yL_z coordination polymers' application has been extended for the photoreduction of CO₂ to CH₄ and this opens up a new platform for the effective photoreduction of CO₂ to CH₄.