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.

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) x X y L 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.