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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2019-06, Vol.48 (24), p.8678-8692
• Main Authors: Li, Qi, Luo, Yanping, Ding, Yue, Wang, Yina, Wang, Yuxin, Du, Hongbin, Yuan, Rongxin, Bao, Jianchun, Fang, Min, Wu, Yong
• Format: Article
• Language: English
• Subjects: Adsorption; Cadmium; Carbon dioxide; Charge materials; Chemical reactions; Clean energy; Crystallography; Differential thermal analysis; Energy gap; Exchanging; Infrared spectroscopy; Metal-organic frameworks; Methane; Occupancy; Organic chemistry; Reaction time; Reflectance; Solar energy; Triethylamine; Tuning; Water stability
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c9dt00478e

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 ([Cd 3.2 (H 2 TCPP) 2 ][(CH 3 ) 2 NH 2 ] 1.6 ). It was constructed from H 2 TCPP 4− (H 6 TCPP = 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 [(CH 3 ) 2 NH 2 ] + counterions of PMOF 1 were also exchanged with Li + . Although the surface area of PMOF 1 and its derived PMOFs were only 407-672 m 2 g −1 , the CO 2 and CH 4 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 N 2 , CO 2 and CH 4 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) 3 Cl 2 ) as the co-catalyst and triethylamine (TEA) as the sacrificial agent, the efficiency of CO production resulting from CO 2 reduction using 1-DMF reached 56 μmol g −1 h −1 , about 5 times greater than that for the system without using Ru(bpy) 3 Cl 2 . By changing the occupancies of the metal ions and counterions, the tuning of the framework charge, band-edge position and bandgap of a novel Cd( ii ) porphyrinic MOF 1 was achieved.