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 well-known microporous materials could see restricted their applications in heterogeneous catalysis if large reactants exceed their pore sizes leading to unavailable surface areas. In this work, we studied the effect of acetic acid concentration, used as the modulator, on the formation of micro/mesoporous materials. This inclusion of a modulator during synthesis and its removal by activation process generate materials with missing linker defects. We showed that an increase in the concentration of modulator leads to an improvement of calculated apparent surface area and a modification of MOF-808 pore structure by producing mesopores at the expense of micropores. Furthermore, we performed a post-synthetic modification of the MOF-808. We observed the expected sulfation of the zirconium oxo-cluster but also the sulfonation of the organic ligand. Also, we found that only the families of mesopores and the larger micropores are interconnected within the material, and the ultramicropores seems to be isolated from the porous structure. The PSM process led to the addition of Lewis and Brønsted acid sites to the MOF-808. Experimental results were complemented by theoretical calculations using Density Functional Theory (DFT) and Ab Initio Molecular Dynamics (AIMD) simulations. The rationalization of the synthesis conditions effect and the post-synthetic sulfation process on 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.