Grand Challenges and Future Opportunities for Metal-Organic Frameworks

Abstract

Metal-organic frameworks (MOFs) allow compositional and structural diversity beyond conventional solid-state materials. Continued interest in the field is justified by potential applications of exceptional breadth, ranging from gas storage and separation, which takes advantage of the inherent pores and their volume, to electronic applications, which requires precise control of electronic structure. In this Outlook we present some of the pertinent challenges that MOFs face in their conventional implementations, as well as opportunities in less traditional areas. Here the aim is to discuss select design concepts and future research goals that emphasize nuances relevant to this class of materials as a whole. Particular emphasis is placed on synthetic aspects, as they influence the potential for MOFs in gas separation, electrical conductivity, and catalytic applications.

Figure 1

Complexity of metal–organic frameworks arises from both structure and composition. Control of these parameters should provide access to a range of emerging applications that depend on pore structure. Depicted is Fe₂(BDP)₃, with the metal nodes shown as pink polyhedra. The void space of one of the pores is emphasized in the transparent blue triangle.

Figure 2

Permanent polarization (dipole moment, μ) of some familiar polar molecules (a) determines the interactions strength with the electric field produced by the framework. Most small gases feature no permanent dipole and their strength of interaction is determined by the magnitude of their polarizability (b).

Figure 3

Metal–organic frameworks feature band edges that are augmented representatives of their daughter components (a). Borrowing from the semiconductor field, the metal/ligand energy level alignments (b) can be thought of as Type I, II, or III offsets, and the resultant material features some orbital mixing (or band bending). Energy level matching is paramount for conductive applications, because the metal–organic–metal interface occurs periodically thereby exacerbating the energy mismatch at their interface (forming a rectifying heterojunction contact, c). Depending on the charge carrier (holes or electrons), the alignment of the ligand and metal orbitals can minimize the rectifying contact in the valence and/or conduction bands can yield and electrically conductive material (d) allowing for metal–ligand–metal– or spatial hopping conductive pathways (e).

Figure 4

Catalytic centers in MOFs introduced through (a) appendage (illustrated by metal anchoring to the inorganic node of defective NH₂–UiO-66) or (b) cation exchange provide site-isolation (shown schematically is cation-exchanged MFU-4l). Three representative catalytic transformations of interest are shown.^,, Metal nodes are depicted in blue, gray, and pink polyhedra.