Synthesis, characterization, and post-synthetic modification of a micro/mesoporous zirconium-tricarboxylate metal-organic framework: towards the addition of acid active sites

Zr-MOFs are characterized by their high thermal and chemical stability which may facilitate their application in heterogeneous catalysis. However, these applications could be restricted if large reactants exceed the MOF pore sizes leading to unavailable surface areas. In this work, we study the effect of acetic acid concentration, used as a modulator, on the formation of micro/mesoporous materials. This inclusion of a modulator during synthesis and its removal by an activation process generate materials with missing linker defects. We show that an increase in the concentration of modulator leads to an improvement of the calculated apparent surface area and to a modification of the MOF-808 pore structure by producing mesopores at the expense of micropores. Furthermore, we perform a post-synthetic modification (PSM) of MOF-808. We observe the expected sulfation of the zirconium oxo-cluster but also the sulfonation of the organic ligand. Also, we find that only the families of mesopores and the larger micropores are interconnected within the material, and the ultramicropores seem to be isolated from the porous structure. The PSM process led to the addition of Lewis and Bronsted acid sites to MOF-808. Experimental results are complemented by theoretical calculations using density functional theory and ab initio molecular dynamics simulations. The rationalization of the effect of synthesis conditions and the post-synthetic sulfation process on the final properties presented in this paper can serve as a basis for engineering of defects towards the synthesis of solid acid catalysts from MOF-808.