Creating a Molecular Super Sponge, From the Ground Up

The Science

Mother Nature does it every day, but building complex materials from the bottom-up using only simple building blocks is incredibly difficult in the lab. Now, scientists created a structurally complex material using just two types of building blocks. Known as a metal-organic framework (MOF), the new porous, or cage-filled, superstructure called NU-1301 is quite large, with the largest unit cell of any non-biological material. In addition, it is the lowest density MOF reported to date. The low density means the MOF is a highly porous material, like a super sponge, which is great for storage applications including, but not limited to, storage of nuclear waste.

The Impact

This work demonstrates a set of design rules. These rules can now be used for the self-assembly of highly intricate, open structures using simple starting materials. For example, scientists can use the rules to design new materials that better filter pollutants and other toxic chemicals. In addition, this work highlights the potential role of uranium and other actinides for complex materials synthesis.

Summary

The structure of NU-1301 represents a new three-dimensional net, named Northwestern University Net (nun), which has been added to the reticular chemistry structure resource database. The net is highly complex with 17 topologically distinct nodal vertices and 18 discrete edges. Given the structural complexity of NU-1301, the single crystal x-ray structure of the resulting MOF could not be solved using traditional methods because only the position of the uranium atoms could be determined experimentally. As a result, the scientists simulated the organic components based on the uranium atom positions; then, they confirmed the final simulated structure using other techniques. NU-1301 has a high Brunauer-Emmett-Teller (BET) area (4750 m²/g) and pore volume (3.9 cm³/g) and is thermally stable to temperatures greater than 500°C (~900°F) under both nitrogen and air. The pores of NU-1301 are also accessible and anionic in nature, as shown by the encapsulation of positively charged small organic molecules and large biological molecules that can be separated from negatively charged molecules of similar sizes.

Funding

Supported by the U.S. Department of Energy (DOE) Office of Science, Basic Energy Sciences (grant DE-FG02-08ER155967) and Northwestern University for the synthesis of the linker, metal-organic framework (MOF), and MOF characterization (O.K.F.); the Colorado School of Mines Board of Trustees (D.A.G.G.); a Natural Sciences and Engineering Research Council of Canada postdoctoral fellowship (A.J.H.). The x-ray work made use of the Integrated Molecular Structure Education and Research Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental Resource (National Science Foundation NNCI-1542205), the State of Illinois, and the International Institute for Nanotechnology. Computational work made use of the BlueM supercomputer at the Colorado School of Mines. Scanning transmission electron microscopy (STEM) experiments were supported by the Chemical Imaging Initiative Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory (PNNL). The STEM work was performed using the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research and located at PNNL.

Publications

P. Li, N.A. Vermeulen, C.D. Malliakas, D.A. Gómez-Gualdrón, A.J. Howarth, B.L. Mehdi, A. Dohnalkova, N.D. Browning, M. O’Keeffe, and O.K. Farha, “Bottom-up construction of a superstructure in a porous uranium–organic crystal.” Science 356, 624-627 (2017). [DOI: 10.1126/science.aam7851]