Rational design, synthesis, purification, and activation of metal-organic framework materials

Abstract

The emergence of metal-organic frameworks (MOFs) as functional ultrahigh surface area materials is one of the most exciting recent developments in solid-state chemistry. Now constituting thousands of distinct examples, MOFs are an intriguing class of hybrid materials that exist as infinite crystalline lattices with inorganic vertices and molecular-scale organic connectors. Useful properties such as large internal surface areas, ultralow densities, and the availability of uniformly structured cavities and portals of molecular dimensions characterize functional MOFs. Researchers have effectively exploited these unusual properties in applications such as hydrogen and methane storage, chemical separations, and selective chemical catalysis. In principle, one of the most attractive features of MOFs is the simplicity of their synthesis. Typically they are obtained via one-pot solvothermal preparations. However, with the simplicity come challenges. In particular, MOF materials, especially more complex ones, can be difficult to obtain in pure form and with the optimal degree of catenation, the interpenetration or interweaving of identical independent networks. Once these two issues are satisfied, the removal of the guest molecules (solvent from synthesis) without damaging the structural integrity of the material is often an additional challenge. In this Account, we review recent advances in the synthetic design, purification, and activation of metal-organic framework materials. We describe the rational design of a series of organic struts to limit framework catenation and thereby produce large pores. In addition, we demonstrate the rapid separation of desired MOFs from crystalline and amorphous contaminants cogenerated during synthesis based on their different densities. Finally, we discuss the mild and efficient activation of initially solvent-filled pores with supercritical carbon dioxide, yielding usable channels and high internal surface areas. We expect that the advances in the synthesis, separation, and activation of metal-organic frameworks could lead to MOFs with new structures and functions, better and faster separation and purification of these materials, and processing methods that avoid pore blockage and pore collapse.