Charge, adsorption, water stability and bandgap tuning of an anionic Cd(ii) porphyrinic metal-organic framework.

Due to the designability of metal-organic frameworks (MOFs), semiconductor MOFs have become the focus of research as photocatalysts of useful chemical processes utilizing clean solar energy. In this work, we developed a method of tuning the framework charge of MOF materials and determined how the framework charge can affect the band edge positions and bandgaps of the novel anionic Cd(ii) porphyrinic metal-organic framework (PMOF) 1 ([Cd3.2(H2TCPP)2][(CH3)2NH2]1.6). It was constructed from H2TCPP4- (H6TCPP = tetrakis(4-carboxyphenyl)-porphyrin) and Cd(ii), forming a tube-like structure, and shown to have a negatively charged framework with a 60% occupancy of one type of Cd(ii) ion. By increasing the reaction time and the amount of Cd(ii) ions in the reactants, the nearly neutral counterpart of PMOF 1 was also obtained. The [(CH3)2NH2]+ counterions of PMOF 1 were also exchanged with Li+. Although the surface area of PMOF 1 and its derived PMOFs were only 407-672 m2 g-1, the CO2 and CH4 uptakes reached, respectively, 44-65 ml g-1 (8.7-12.7%) and 22-26 ml g-1 (1.6-1.8%) each at 1.0 atom and 273 K; at 9.0 atm these values nearly tripled. Li+-exchanged 1 favoured N2, CO2 and CH4 adsorption, especially at 9 atm and a relatively low temperature (273 K). PMOF 1 subjected to a solvent exchange process showed an unstable structure in water, while PMOF 1 not subjected to this process was found to be stable in water. Thus, a method for making water-stable divalent-metal carboxylate MOFs was proposed. The counter ion type showed little effect on the band-edge positions and bandgaps, but the framework charge did show effects. Under visible light and with tris(2,2'-bipyridine)dichlororuthenium(ii) (Ru(bpy)3Cl2) as the co-catalyst and triethylamine (TEA) as the sacrificial agent, the efficiency of CO production resulting from CO2 reduction using 1-DMF reached 56 μmol g-1 h-1, about 5 times greater than that for the system without using Ru(bpy)3Cl2.