Metal–organic frameworks (MOFs) are many times lighter than proteins, while still capable of achieving similar local structure. A protein that absorbs one uranium atom extracts less than one-tenth of one percent of its final mass. A MOF cage offers similar three-dimensional connectivity as the protein, but weighs around 100 times less and may have multiple binding sites. MOFs absorbed slightly more than 20 percent of their mass in uranium. The 2013 research won an award.
Enzymes and proteins can have an unusual affinity for specific molecules. The researchers suspected that they could use the three-dimensional structure of the metal-organic frameworks to produce a binding pocket similar to those of the enzymes or proteins. They could then create a more efficient, lightweight version of a molecule that mimics the structure and function of the protein or enzyme.
Three metal–organic frameworks (MOFs) of the UiO-68 network topology were prepared using the amino-TPDC or TPDC bridging ligands containing orthogonal phosphorylurea groups (TPDC is p,p′-terphenyldicarboxylic acid), and investigated for sorption of uranium from water and artificial seawater. The stable and porous phosphorylurea-derived MOFs were shown to be highly efficient in sorbing uranyl ions, with saturation sorption capacities as high as 217 mg U g−1 which is equivalent to binding one uranyl ion for every two sorbent groups. Coordination modes between uranyl groups and simplified phosphorylurea motifs were investigated by DFT calculations, revealing a thermodynamically favorable monodentate binding of two phosphorylurea ligands to one uranyl ion. Convergent orientation of phosphorylurea groups at appropriate distances inside the MOF cavities is believed to facilitate their cooperative binding with uranyl ions. This work represents the first application of MOFs as novel sorbents to extract actinide elements from aqueous media.
Metal–organic frameworks are“very promising,” says Schneider, simply because it performed better than the best available materials have done under similar conditions.
Uranium obtained using the traditional process today would cost between $1,000 and $2,000 per kilogram—about 10 to 20 times the current market price, says Schneider. (The price of uranium did rise to around $300 per kilogram as recently as 2007, however.) The new process could cut that cost significantly.
Lin thinks it may eventually be possible to develop a metal-organic framework that is at least several times better than today’s system. He is confident that his lab can exploit the “tunability” of these hybrid materials to improve their affinity for uranium-containing ions and to address weaknesses that further testing may expose.
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