(725e) Computational Studies of the Enhanced Acidity of Defect MOF 808: The Effect of Activation Process

The hexanuclear zirconium metal-organic frameworks are nowadays considered as promising materials exhibiting different structures, excellent stability, and interesting textural properties useful for adsorption, gas separation and catalysis. Among these materials, the tricarboxylic acid based material named as MOF-808 a has emerged as a promising MOF for catalysis applications like esterification and dehydration. Moreover, the pore size of MOF-808 can even be increased by the creation of missing linker defects, which would enhance catalytic performance. The partial substitution of organic ligands by modulator agents like monocarboxylic acids during synthesis allows the engineering of the MOF-808 structure towards better accessibility/availability of zirconium centers. Activation treatment implies the use of higher temperatures than the one used during synthesis, allowing the node’s dehydroxylation and the modulator removal, which involves proton mobility into the zirconium node. In this regard, a comprehensive understanding of the activation process is needed to describe the resulting defective structure, which would allow the correlation between the missing linker defects and the MOF catalytic activity.

In this work, we performed state-of-the-art density functional theory (DFT) and molecular dynamics (MD) calculations for studying the activation processes of the MOF-808. By starting from a defect-free structure, we added different modulators (formic acid, acetic acid) and evaluated the effect of its addition with adsorption calculations of probe molecules (carbon monoxide, pyridine). For the acid sites created, before and after adsorption of probe molecules, we performed vibrational frequency calculations and charge distribution calculations to elucidate their acidic nature and quantify their acidic strength. It is demonstrated that the creation of defect centers might help to increase the acidity of the MOF-808. We also found out that the removal of labile coordinated species did not lead to open metal sites as expected; instead, it was the redistribution of charges which induced the acidity. This theoretical rationalization of the activation process can serve as a basis for engineering of defects in MOF-808 materials.