Mixing It Up

By taking a synergistic approach, Adam J. Matzger and coworkers at the University of Michigan have combined zinc with two common organic linker groups—instead of the usual one—to construct a new type of metal-organic framework (MOF) with exceptional structural properties (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200705020). Aside from the porous material being of interest for gas storage, catalysis, and chemical separation applications, the synthetic strategy opens a new door for using combinatorial methods to prepare novel types of MOFs.

MOFs are crystalline coordination polymers with a network of pores made by linking metal atoms or clusters with organic groups. One of the most studied versions, MOF-5, is made from zinc and terephthalic acid (a dicarboxylate). Another version is MOF-177, which is made from zinc and tris(4-carboxyphenyl)benzene (a tricarboxylate). Because the linkers have different structures, the resulting MOFs have distinctive pore structures.

In making the new material, Matzger's group tried something different: combining the two functionally similar linkers into one MOF. Coordination copolymers derived from linkers with different functionality are already known. But rather than forming segregated crystals of MOF-5 and MOF-177 or a random copolymer, as might be expected, the outcome was a new, uniformly structured type of MOF with a porosity and surface area that exceeds that of either MOF-5 or MOF-177. In fact, the calculated pore surface area of University of Michigan Crystalline Material-1 (UMCM-1), as it is called, is 4,730 m²/g, the highest of any mesoporous material reported to date, according to the researchers.

UMCM-1's structure consists of six microporous cages with pores about 1.4 nm in diameter surrounding a mesoporous hexagonal channel about 3.2 nm in width. The channel "is like a highway connecting the microporous cages," Matzger says, a feature that could expedite filling the pores with hydrogen or methane, for example.

There are many new types of MOFs reported each year, Matzger adds, but they come about by tinkering with the metal, the synthetic conditions, or the organic linker. The concept of making coordination copolymers like UMCM-1 by mixing and matching organic linkers together "could change the thinking about the most efficient way to discover new porous solids," he says.