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

A 3D, anionic novel porphyrinic metal-organic framework (PMOF) 1 having [NH 2 (CH 3 ) 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 ​m 2 /g), compared with the 339 ​m 2 /g of PMOF 1 . The CO 2 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) 3 Cl 2 /TEA/CH 3 CN (bpy ​= ​2,2-bipyridine, TEA ​= ​triethylamine) system photocatalytically reduce CO 2 to CO under visible light. PMOFs act as cocatalysts, and the photocatalysts were found to be the triplet excited state of Ru(bpy) 3 2+ ( 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) 3 Cl 2 /TEA/CH 3 CN system were found to have an unprecedented high TOF of 0.31 h −1 (412 ​μmol ​g −1  ​h −1 ) when small amounts of H 2 O (200 ​μL ​H 2 O in 40 ​mL CH 3 CN and 10 ​mL TEA) was added.