Influence of As-Formed Metal-Oxide in Non-Activated Water-Unstable Organometallic Framework Pores as Hydrolysis Delay Agent: Interplay Between Experiments and DFT Modeling

In this work, a new strategy to increase the stability of humid-unstable MOFs in humid environment has been investigated. Among many known humid-unstable MOFs, MOF-5 has been selected due to its unique properties. The present work quantifies the influence of exposure to humid environment on the properties of MOF-5 as a function of relative humidity, exposure time and activation process. The activated and non-activated MOF-5s are directly compared considering their humid stability for the first time. The properties examined include PXRD, TGA, SEM, and BET surface area. Moreover, the water adsorption isotherms are obtained using a gravimetric method at 22 degrees C using RH up to 87% and both A-MOF-5 and NA-MOF-5 showed type V isotherm. According to the results obtained, water vapor adsorption on NA-MOF-5 starts faster than that of A-MOF-5. Also, the uptake saturation of water vapor on NA-MOF-5 is about 80% higher than that of A-MOF-5. According to results obtained, in contrast to A-MOF-5, the water adsorbed in NA-MOF-5 will not lead to hydrolysis. Prior to exposure, the calculated BET surface area for A-MOF-5 and NA-MOF-5 was 2460 and 1134m(2)/g, respectively. For A-MOF-5, after exposure times of up to 72h, the BET surface area in RH=45% reduced to about 1220m(2)/g, and in RH=65%, because of the destruction of the crystal structure, surprisingly it reduced to 36m(2)/g. Also, for NA-MOF-5, relatively small changes in BET surface area were observed for exposure times of up to 72h (RH=45 and 65%). According to experimental results, deformation of A-MOF-5 after 72h exposure to RH=65% is quite obvious and the initial structure of A-MOF-5 is not recovered even by re-activation process. The results of BET surface area for activated NA-MOF-5 indicate that the high quality of MOF-5 can be obtained by activation of final humidity exposed NA-MOF-5. According to DFT-based computational results, the as-formed ZnO will act as a delayed species for hydrolysis and destruction of the NA-MOF-5 structure.