Synthesis of cobalt-, nickel-, copper-, and zinc-based, water-stable, pillared metal-organic frameworks.

Abstract

The performance of metal-organic frameworks (MOFs) in humid or aqueous environments is a topic of great significance for a variety of applications ranging from adsorption separations to gas storage. While a number of water-stable MOFs have emerged recently in the literature, the majority of MOFs are known to have poor water stability compared to zeolites and activated carbons, and there is therefore a critical need to perform systematic water-stability studies and characterize MOFs comprehensively after water exposure. Using these studies we can isolate the specific factors governing the structural stability of MOFs and direct the future synthesis efforts toward the construction of new, water-stable MOFs. In this work, we have extended our previous work on the systematic water-stability studies of MOFs and synthesized new, cobalt-, nickel-, copper-, and zinc-based, water-stable, pillared MOFs by incorporating structural factors such as ligand sterics and catenation into the framework. Stability is assessed by using water vapor adsorption isotherms along with powder X-ray diffraction patterns and results from BET modeling of N2 adsorption isotherms before and after water exposure. As expected, our study demonstrates that unlike the parent DMOF structures (based on Co, Ni, Cu, and Zn metals), which all collapse under 60% relative humidity (RH), their corresponding tetramethyl-functionalized variations (DMOF-TM) are remarkably stable, even when adsorbing more than 20 mmol of H2O/g of MOF at 80% RH. This behavior is due to steric factors provided by the methyl groups grafted on the BDC (benzenedicarboxylic acid) ligand, as shown previously for the Zn-based DMOF-TM. Moreover, 4,4',4″,4‴-benzene-1,2,4,5-tetrayltetrabenzoic acid based, pillared MOFs (based on Co and Zn metals) are also found to be stable after 90% RH exposure, even when the basicity of the bipyridyl-based pillar ligand is low. This is due to the presence of catenation in their frameworks, similar to MOF-508 (Zn-BDC-BPY), which has also been reported to be stable after exposure to 90% RH.