With a density of just 0.124 g/cm3, NU-1301 now holds the record for lowest density metal-organic framework, or MOF. Although NU-1301 is made up of just uranium oxide and tricarboxylate organic linker units, the MOF’s structure is incredibly complex: Its unit cell measures 173 Å across and is composed of 816 uranium nodes and 816 organic linkers (Science 2017, DOI: 10.1126/science.aam7851).
Omar K. Farha, a chemistry professor at Northwestern University who led the MOF-making effort, says his team didn’t predict they would get such an elaborate structure based on first principles. “This is the first MOF that has this kind of complexity,” he says.
Farha says his group knew the uranium oxide nodes and tricarboxylate bridging ligands would take the shape of a basic cuboctahedron building block, and that these could assemble into pentagons and hexagons. But from there, they weren’t certain what form the larger structure of the MOF would take.
They determined NU-1301’s structure using X-ray analysis and modeling—no easy feat because the uranium diffracts so strongly, it makes it impossible to observe the organic linkers using X-ray diffraction techniques. They found that the cuboctahedrons assemble into five types of larger cage structures that form the unit cell.
“A few people have been making MOFs with actinides, but it’s an area that has been not well understood,” Farha notes. “We were able to show that with an actinide building block, we could make a material that is complex, that is unusual, and that at the same time has the lowest density of any MOF that has been made so far.” He points out that most scientists don’t equate actinides, which are heavy elements that reside at the bottom of the periodic table, with low-density materials. He hoped that the size of the metal wouldn’t matter if the MOF’s structure was very porous and empty.
Not only is NU-1301 low density, he notes, it’s also high in surface area and high in pore volume. The MOF has other interesting properties too: It’s stable in water and can capture cations, which could make for some interesting applications in separation science, Farha notes.
Omar Yaghi, a MOF expert at the University of California, Berkeley, says the work from Farha’s group shows “how control of the angles between building units can have profound impact on the assembly of complex extended MOFs.” He adds, “It is clear the design of MOFs is taking another stride forward in its sophistication.”