Synthesis and Thio-Functionalization of Metal Organic Frameworks of Isonicotinate Ligands and their Application for Fluorescein Removal from Aqueous Solution

Mechanochemical syntheses of four (4) know metal organic frameworks (MOFs) were obtained by grinding stoichiometric amounts of metal salts [Zn (II) and Cu (II)] and corresponding carboxylate ligands [Fumaric and Isonicotinic acids] in a mortar with a pestle. Also, Solvent-based syntheses of MOFs were carried out with the reaction of metal salts and each carboxylate ligand by mixing in the presence of solvents. The compounds were characterized by the comparison of melting point, elemental analysis, FT-IR spectroscopy and XRPD results with those of the free ligands and literature. The analytical and spectroscopic data of the compounds prepared via the two different methods gave the expected product.

The presence of coordinatively unsaturated metal centers in the synthesized MOFs provides an accessible way to selectively functionalize them through coordination bonds. In this work, thiol-functionalization of MOFs were described by choosing six well known MOFs in which three (3) are Zn-based while the other three (3) are Cu-based MOF, by a facile coordination based postsynthetic strategy. The obtained thiol-functionalized MOFs were characterized by powder X-ray diffraction, CHN and infrared spectroscopy. The analytical and spectroscopic data of the unmodified and modified compound were different.

A series of [Cu (Ina)2] samples stoichiometrically decorated with thiol groups has been prepared through coordination bonding of coordinatively unsaturated metal centers with –SH group in thioglycolic acid. [Cu (Ina)2]. H2O and [Cu(Ina)2]-TH were investigated for the adsorptive removal of fluorescein from aqueous solution. The parameters that affect the dye sorption such as contact time, solution pH, initial fluorescein concentration and temperature, have been investigated and optimized conditions determined at PH of 7.8. Dubinin–Radushkevich Equilibrium isotherm studies were used to evaluate the maximum sorption capacity of MOFs and experimental results showed that the removal efficiency decreases in the order of [Cu(Ina)2]-TH > [Cu(Ina)2].H2O both in adsorption rate and adsorption capacity. The Langmuir, Freundlich, Temkin and Dubinin Radushkevich models have been applied and the data correlate well with Dubinin Radushkevich model. The adsorption mechanism may be explained with a simple electrostatic interaction and π- π interaction. Finally, it can be suggested that modification of MOFs can be use to improve their adsorption capacity and rate, making them a better adsorbent to remove emerging dyes contaminants from waste water.