Two anionic Ni(II) porphyrinic metal−organic frameworks: Syntheses, flexibility and roles in visible-light photocatalytic CO2 reduction to CO in the Ru(bpy)3Cl2/TEA/CH3CN system

Abstract

A 3D, anionic novel porphyrinic metal-organic framework (PMOF) 1 having [NH2(CH3)2]+ as counterions and Li+ -exchanged version of 1 (Li+-1) were synthesized. A very interesting feature of 1 and Li+-1 is their frameworks can change shape without breaking bonds, causing substantial changes of their PXRD patterns depending on what and how much is in the channels. Li+-1 has much larger surface area (Langmuir: 640 ​m2/g), compared with the 339 ​m2/g of PMOF 1. The CO2 adsorption capacities of 1 and Li+-1 are 38 (7.5 ​wt%) and 45 ​mL/g (8.9 ​wt%), respectively at 1 ​atm and 298 ​K, and 165–167 ​mL/g (33%) at 9.9 ​atm. They are direct-bandgap semiconductors with bandgaps of 1.56 (for 1) and 1.62 (for Li+-1) eV. The conduction and valence band-edge positions were determined in non-aqueous solutions as −0.29 and 1.27 (1), −0.32 and 1.30 ​V (Li+-1) versus NHE. PMOF 1 or Li+-1/Ru(bpy)3Cl2/TEA/CH3CN (bpy ​= ​2,2-bipyridine, TEA ​= ​triethylamine) system photocatalytically reduce CO2 to CO under visible light. PMOFs act as cocatalysts, and the photocatalysts were found to be the triplet excited state of Ru(bpy)32+ (Ru(II)∗) and its photodecomposition products. The TOFs contributed from PMOFs after 2 ​h were 0.091 and 0.097 h−1 (224 and 241 ​μmol ​g−1 ​h−1), respectively for 1 and Li+-1, respectively. In addition, the Ru(bpy)3Cl2/TEA/CH3CN system were found to have an unprecedented high TOF of 0.31 h−1 (412 ​μmol ​g−1 ​h−1) when small amounts of H2O (200 ​μL ​H2O in 40 ​mL CH3CN and 10 ​mL TEA) was added.