Strategic Approach to Synthesize Metal-Organic Frameworks for Application as Sensors and Proton Conductors

Abstract

Since the last two decades, metal-organic frameworks (MOFs) have emerged as one of most versatile and important self-assembled crystalline materials made from metal ions coordinated to different organic ligands. Owing to the ease of synthesis, versatility in choice of metal ions and organic ligands, variation in the structures along with modulation of pore size, crystallinity to study the interactions and the mechanism using crystal structures, continuous improvement in thermal and chemical stability and possibility of post-synthetic modification, makes them perfect candidate in the field of material chemistry. Till date MOFs have been drastically explored and known to be the choice of material in various fields such as gas adsorption/separation, catalysis, sensing, proton conductive materials. In the introduction part of the thesis, brief discussion about the MOFs has been introduced. The properties of MOFs along with their synthesis protocols and their applications has been explained. In the later part of the thesis, the synthesis of new indium-based MIL-53 MOF using 2,5-dihydroxyterepthalic acid as the ligand possessing coordinating carboxylate and free two hydroxy moiety. The synthesized MOF possessing free hydroxy group has been utilized for the sensing of nitroaromatic compounds (NACs) which are considered to be explosives and toxics. The sensing experiments were performed in both polar (water) and non-polar solvent (chloroform). Trinitrophenol amongst the NACs showed the highest fluorescence quenching performance when added in dispersed solution of MOF in both the solvents. The detailed study about the mechanism and the interactions causing the selectivity for NACs has been revealed utilizing single crystal X-ray diffraction technique, florescence experiments, UV-visible spectroscopy, time dependent fluorescence experiments and anisotropy fluorescence experiments. The last part of the thesis demonstrates the strategic approach to synthesis sulfamate-, sulfate- and hydroxy-exchanged MOF-808. Sulfamate-exchanged MOF-808 displays very high proton conduction properties as compared to sulfate- and hydroxy-exchanged MOF-808. The reason behind such high proton conduction was explored using water adsorption experiments, infrared (IR) spectroscopy, and proton conduction experiments. IR spectroscopy study suggests different binding modes for sulfamate ion attached to zirconium metal cluster depending on thermal treatment condition, which leads to different proton conductivities overcoming the proton conductivity of even sulfate-exchanged MOF-808.