Synthesis, sulfonic acid functionalization and catalytic application of metal-organic frameworks

Abstract

Metal-organic framework is a class of porous materials comprised of infinite networks of metal centers or metal-organic clusters bridged by organic ligands through metal-ligand coordination bonds. Benefiting from the three-dimensional networks with channels and cavities, MOFs possess extremely high porosity. In spite of the presence of organic moieties, they have good thermal stability due to the strong covalent bonding between metal ions and multidentate ligands. In addition, the presence of both inorganic and organic components enables both the pore size and the chemical environment to be tailored to specific properties. With these appealing intrinsic properties that surpass those of other porous materials, MOFs give great potential applications in adsorption, separation, gas storage and catalysis. In this study, four MOFs, MIL-101(Cr), NH₂-MIL-53(Al), NH₂-MIL-101(Al) and NH₂-UiO-66(Zr) were synthesized through solvothermal self-assembly. Two functionalizations were developed to incorporate sulfonic acid groups into these four MOFs. One functionalization was through the formation of the coordination bond between taurine and Cr³⁺ in MIL-101(Cr). The other one was through the nucleophilic ring-open reaction between 1,3-propanesultone and amino groups in the rest three NH₂-MOFs mentioned above. XRD results showed that the structures of MOFs were well preserved during the functionalization. The successful incorporation of sulfonic acid groups was confirmed by FTIR. The quantities of incorporated sulfonic acid groups were measured by XRF. The porosity of MOFs was also preserved with a certain reduction of BET surface area and pore volume. The functionalized MOFs were stable up to 200 °C as found by TGA. The three MOFs functionalized by the second way were used to catalyze esterification between acetic acid and n-butanol. Kinetic study was done to acquire kinetic parameters of this reaction. All three MOF catalysts gave much higher conversions than that of the blank experiment. The activity of functionalized NH₂-MIL-101 was not only higher than that of all existing acid MOF catalysts, but also similar to two of the best commercial catalysts, Nafion NR50 and Amberlyst-15. The good performances reveal the unlimited potential of MOF in acid catalysis, as well as give insight into future development of other MOF catalysts.

Date of Award	2014
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology