Abstract
A series of functionalized metal organic frameworks (MOFs) were synthesized by the post-synthetic modification (PSM) of Zr(iv)-containing UiO-66-NH2 MOFs using covalent grafting with various functional groups utilizing pendant –NH2 moieties. The tethering of amide (with/without pendant carboxylic acid), iminopyridine, phoshinic amide and sulphur-containing functionalities produced a library of eight different UiO-66-NH2 derivatives. The functionalized MOFs were characterized by FT-IR spectroscopy, NMR, PXRD, TGA, SEM-EDX and BET surface area analysis. Uranyl ion extraction with the functionalized MOFs was investigated in acidic/neutral/basic conditions (pH 1 to 9). This work presents a comprehensive study of different functionalized MOFs to investigate the effects of various analytical parameters, including pH, contact time, and desorption process. The MOFs as solid phase extractants (SPEs) provide a direct comparison of the sorption efficiencies of different functional groups on a common solid support. A phosphorous-functionalized material, UiO-66-PO-Ph, with enhanced thermal stability (∼500 °C) exhibits the best sorption capacity (∼96%) in an acidic medium (pH 3). The parent MOF UiO-66-NH2 (92%) and iminopyridine-functionalized UiO-66-IMP (90%) showed excellent sorption in neutral conditions (pH 7). Amide-containing MOFs UiO-66-AM1 (40%), UiO-66-AMMal (31%) and UiO-66-AMGlu (70%), sulfur-based MOFs UiO-66-SMA (65%) and UiO-66-SSA (27%), and phosphorous-functionalized UiO-66-PO-OPh (50%) displayed maximum sorption in basic conditions (pH 8). The kinetics studies revealed rapid uranium sorption in about 2 h due to the effective binding of uranyl ions with the anchored functional groups of MOFs; quantitative elution of uranyl ions from the MOF framework was carried out with 0.1/0.01 M HNO3. The MOFs also exhibit moderate recyclability for uranium sorption and can be regenerated by an acidic solution. The functionalized MOFs alter the stability in acidic/basic media; thus, UiO-66-NH2 is a versatile MOF material employed as an SPE for the extraction of radionuclides from aqueous media. This work also provides a platform for the development of new functionalized MOF materials for the efficient sorption of uranium as well as moderate recyclability for its removal, and the potential applications include the removal of uranium from aqueous waste streams.
Highlights
We report the synthesis and characterization of eight metal organic frameworks (MOFs) covalently gra ed via post-synthetic modi cation (PSM) and functionalized with different groups (Scheme 1), viz. amide, iminopyridine, phosphinic amide and sulphur-containing functionalities, to produce a library of eight different UiO-66-NH2 derivatives
The PSM of MOFs based on linker modi cation via covalent gra ing of new functional moieties allows the creation of new materials with different functionalizations and properties.[22,23,24,25,26]
This route proves to be advantageous because the requirement of speci c and convoluted linkers becomes dispensable for the construction of targeted MOFs
Summary
MOFs with terephthalate linkers (UiO-66 and UiO-66-NH2) composed of a Zr6(m3-O)4(m3-OH)[4] core possess strong Zr–O bonds, which contribute to the exceptional stability of these. The limited supply of non-renewable energy sources has created a surge in the development of alternative energy sources In this context, nuclear energy possesses several advantages in terms of the exclusion of combustion, which otherwise contributes to global warming.[27,28,29,30,31] Nuclear reactors use uranium (one of the isotopes) as fuel; the efficient recovery of uranium from different waste streams is essential. The functionalized MOFs were subsequently employed as new SPE materials for uranium(VI) extraction via batch processes, providing a direct comparison of the sorption efficiencies of different functional groups on a common solid support. The functionalized MOFs can be employed as SPEs in acidic/neutral/basic conditions; they are suitable materials for uranium(VI) sorption
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