Among the more remarkable of these crystalline materials to be synthesized in the last decade are called "metal-organic frameworks," or MOFs for short, products of the mind and the laboratory of University of Michigan chemist Omar M. Yaghi. Yaghi’s pioneering work in the creation of microporous crystalline and solid-state materials has placed him among the 50 most-cited scientists in chemistry in the last decade, according to Thomson Scientific’s Essential Science Indicators. Since 1994 Yaghi has racked up over 20 papers with more than 100 citations each, and his seminal 1998 review article "Synthetic strategies, structure patterns, and emerging properties in the chemistry of modular porous solids," published in Accounts of Chemical Research, has itself tallied more than 600 citations (see table below, paper #1). Yaghi’s papers on MOFs have regularly figured in the Chemistry Top Ten in recent years—discussed, for example, in the issues for May-June 2001 and January-February 2004. A 2002 paper by Yaghi and colleagues, in fact, can be found in the Chemistry's Top Ten in this very issue (paper #5).

Yaghi, 39, earned his bachelor’s degree in chemistry from the State University of New York-Albany in 1984 and his doctorate in 1990 from the University of Illinois at Urbana-Champaign, where he worked with Prof. Walter G. Klemperer. He spent the next three years as a National Science Foundation postdoctoral fellow at Harvard, with Prof. Richard H. Holm, before starting his professorial career at Arizona State University. In 1999, he moved to the University of Michigan, where’s he’s now Robert W. Parry Collegiate Professor in the chemistry department.

  What got you interested in the idea of designing and building porous materials?

When I started out in chemistry, it seemed there was no logic to the synthesis of materials. This was 1986, the beginning of my graduate school work at the University of Illinois. A method of assembling materials from molecular building blocks in a logical way was not yet possible. I thought that it would be reasonable to expect that if one could prepare tailor-made materials then it would be possible to achieve highly selective and specific binding properties or tailored properties.

  What constituted the state of the art in 1986?

Well, attempts to link molecular building blocks into extended structures, under mild conditions that wouldn’t decompose those building blocks, often resulted in poorly crystalline or amorphous solids. This precluded characterization of their atomic structure and thus precise knowledge of their atomic connectivity, thereby impeding progress towards understanding how such chemical structures can be designed. And so the challenge was to make these as crystalline materials, so that they could be characterized by x-ray single-crystal diffraction methods, which are known to provide definitive data on location of the atoms and the connectivity of the atoms in the structure. Our original contribution was to show that such crystals can be made, and furthermore we proved their porosity for the first time. This opened up the field.

  What are the advantages of a crystalline structure?

In addition to the characterization issue that I already mentioned, having a crystalline structure means that the pores, or whatever structural attributes a material has, are periodic or homogenous across the crystals. This permits the improvement of the selectivity properties of the material—what’s often referred to as "shape-selective" separation or binding. In other words, those molecules having a shape that fits in the pore can go in, and you can work with those; the molecules whose shape and size don’t fit in the pores get rejected. In all, the homogeneity afforded by the crystalline structure is important for repeatable behavior by the material and, as we showed later, important for highly selective binding of chemicals and gases.

  Was it possible to do any of this before you started working out the techniques?

Before we started there were many porous materials, such as zeolites and porous carbon. However, these were not constructed by the building-block approach and were also not amenable to functionalization. In the materials we started making, the backbone is made of metal and organic units, and the advantage here is that these organic units can be easily functionalized. It’s more of a modular approach. We called these materials "metal organic-frameworks," or MOFs, and one can pick the building blocks that would impart a particular property into the framework. In this way, we’ve been able to make frameworks that take up a lot of methane, for instance, or a lot of hydrogen. So we have already found applications for the storage of fuel.

  When did you first figure out how to pull this off?

High-Impact Papers by Omar M. Yaghi et al., Published Since 1995 (Ranked by total citations) Rank Paper Citations 1 O.M. Yaghi, et al., "Synthetic strategies, structure patterns, and emerging properties in the chemistry of modular porous solids," Acc. Chem. Res., 31(8): 474-84, 1998. 632 2 H. Li, et al., "Design and synthesis of an exceptionally stable and highly porous metal-organic framework," Nature, 402(6759): 276-9, 1999. 483 3 M. Eddaoudi, et al., "Modular chemistry: Secondary building units as a basis for the design of highly porous and robust metal-organic carboxylate frameworks," Acc. Chem. Res., 34(4): 319-30, 2001. 475 4 O.M. Yaghi, G.M. Li, H.L. Li, "Selective binding and removal of guests in a microporous metal-organic framework," Nature, 378(6558): 703-6, 1995. 402 5 O.M. Yaghi, H.L. Li, "T-shaped molecular building units in the porous structure of Ag(4,4’-bpy) center dot NO3." J. Amer. Chem. Soc., 118(1): 295-6, 1996. 268 SOURCE: Thomson Scientific Web of Science

We thought it out in 1994 and 1995, and in 1999 we published in Nature an important compound that was called MOF-5. [Editor’s note: see table, paper #2.] The material is made of very simple components. You have zinc oxide joints and terephthalate, both of which, by the way, are very cheap. Zinc oxide is in sunscreen, and terephthalate is in various plastics. You put them together in solution, and out come these beautiful crystals of framework material, MOF-5. This is the interesting part: here you’re using cheap materials in one step, and you make a porous material in which you have both organic and inorganic components, but you also have a surface area above and beyond any material ever made. The surface area is 2,900 meters squared per gram. That’s the equivalent of 12 or so tennis courts per gram of material. And this was not the best that could be done. We have recently reported in Nature a strategy to increase that even further (see H.K. Chae, et al., Nature, 427[6974]: 523-7, 5 February 2004). We now have 4,500 meters squared per gram. That’s in a material called MOF-177.

The surface area, and the potential application for storage, is related to the number of adsorption sites in the framework. These are sticky points onto which gasses and molecules are adsorbed. This aspect is important for hydrogen storage, as one of the problems with using hydrogen fuel is that it’s difficult to store in any practical volume, like an automobile gas tank. To store enough in such space, one must apply either very high pressure or cool down to liquid-nitrogen temperatures; both impractical. However, with a porous MOF material, one can forgo the use of such extreme conditions, since hydrogen can be concentrated in the pores by stacking the molecules next to each other onto the walls of the framework. To get the hydrogen back out, you just pull the vacuum or heat it slightly. The hydrogen is just weakly bound to the surface through physical and not chemical means.

  How accurately can you predict the properties of the MOFs before you make them?

We have a very good handle on building structures with very specific functionality and very specific metrics. However, we still have some more work to do to make this general to all solid-state materials.

  Tell us what you’re working on now and what applications are in the works.

In terms of basic-science-level questions, we’re trying to figure out what structures we’re going to make from various-shape molecules. We’re trying to develop the blueprint for design. It’s like drawing the blueprint for constructing a building. So we want to develop the blueprint for designing structures. That’s one important aspect. The other is, now that we have a way to build such chemical structures, we wish to extend these techniques to materials having properties that go beyond just improving established storage, separation, and catalysis. Could we combine porosity with magnetism, electronics and such hybrid properties? Still, the most obvious direction at the moment is fuel storage, which we have demonstrated for methane and hydrogen. The next direction would be catalysis and the functionalization of pores in such a way as to approach an enzyme activity. The other interesting application is in nanotechnology. I haven’t used the word "nano" since we started talking, but a lot of these structures have properties on the nano scale. We now have pores that can act like vessels to perform reactions that could not happen any other way. In the environment of these pores, one may isolate reactive intermediates, or synthesize molecules that otherwise could not be made. It’s the equivalent of a molecular pressure vessel, in which one could create unusual reactivity. Additional directions include applying this chemistry to linking proteins and peptides into scaffolds.

  What do you think is sufficiently near-term that we might actually see it in the marketplace in the next five years?

There are several areas to expect. First, we now know that we can store enough methane, natural gas, in a fuel tank to allow us to change the tank from metal to plastic—because of use of milder pressure—and also to increase the range of driving, maybe double it, without refueling. This is a very important application, and it’s being developed jointly with BASF, a chemical company. Another prospect is using cartridges filled with MOFs that are impregnated with hydrogen to fuel mobile electronic gadgets: cell phones and laptops, for instance. As for fueling automobiles with hydrogen, that’s a more challenging problem—specifically, the problem for a long time has been storage. We need to get to 9% hydrogen by weight to make it economical, and so far we’ve gotten 2%. But remember, we’ve just started working on this, and the amazing thing is that these materials do take up significant amounts of hydrogen. I think we could at least double that in a short time.

  Are you surprised by how far you’ve taken these materials in the past decade?

I’m surprised that it’s not been developed before, and also a little surprised at how skeptical others were when we proposed it. Critics said that we’d never be able to crystallize the materials, and once we had crystallized them, they’d say that the materials would not be porous. Thanks to my friends’ support and students’ capable hands and minds, we showed that, with some persistence and insight, a dream can become true in science.