(544as) Defect Engineering and Sulfation of MOF-808: Towards the Obtainment of Microporous-Mesoporous Structures with Strong Brønsted Sites for Catalysis Applications

Zirconium-based MOFs have been used as catalysts for Lewis catalyzed reactions such as the CO2 cycloaddition of styrene oxide, the cyclization of citronella to isopulegol, the Meerwein- Ponndorf- Verley (MPV) reduction and others. On the other hand, referred to Brønsted catalyzed reactions, they are just beginning to be explored, taking advantage of the chemical stability of these MOFs. As reported recently, among these materials, the MOF 808 arises as an attractive catalyst due to the presence of both Brønsted and Lewis sites into its structure. Since its structure is usually reported as a microporous one, the defect engineering strategy using modulator agents emerges as a promising method for the obtainment of MOFs with varied pore size distributions ranging from micropores to mesopores. Furthermore, the addition of sulfate groups through a post-synthetic process allows the formation of structures with strong Brønsted acidity.

In this work, we performed the solvothermal synthesis of the MOF 808 materials by using different modulator agents namely, formic acid, acetic acid and propionic acid. Through a post-synthesis process, sulfuric acid solutions were employed to attach sulfate groups to the zirconium clusters. The crystal structure, textural properties, surface properties, inclusion of defects, and acid properties of final materials were evaluated using XRD, Ar/N₂ adsorption-desorption isotherms, XPS, NMR-TGA, and CO-IR, respectively. The use of different modulator agents resulted in MOFs with different apparent surface areas and pore size distributions. The differences in the critical properties of the initial materials and the sulfated MOFs were discussed. The improvement of the catalytic activity due the sulfation process was tested through the esterification of different alcohols with acetic acid by microwave-assisted and conventional reflux reactions. Furthermore, the catalytic results were compared with known catalysts as amberlyst. The defect engineering and addition of strong Brønsted open doors for the applications of this kind of materials in acid catalyzed reactions in which their high and microporous – mesoporous surface area can be harnessed for the access of reagents to all acid sites incorporated into the framework.