Roomy Crystals

From catalysis and surface chemistry to gas storage and separation, materials with high porosity and surface area play key roles in industrial processes. Because of the importance of those properties to materials performance, researchers have developed techniques for preparing materials with very large surface areas and controlled pore dimensions. Yet no method has been established for determining the upper limit to a material's surface area.

Now, a team of chemists at the University of Michigan, Ann Arbor, and Arizona State University has developed a design strategy that maximizes a material's surface area by tailor-making its crystal structure. The researchers have demonstrated the strategy by synthesizing a specimen with record-breaking surface area.

Guided by geometry principles and the structure of high-surface-area materials such as graphite, Michigan chemists Adam J. Matzger, Diana Y. Siberio-Pérez, and Omar M. Yaghi and Arizona State's Michael O'Keeffe and their coworkers deduced that by maximizing the number of exposed faces and edges of six-membered-ring building blocks, a maximum-surface-area material can be constructed. Putting the idea to practical use, the team designed and prepared a new metal-organic framework material (MOF-177) that has a surface area of some 4,500 m² per g [Nature, 427, 523 (2004)].

Roughly equivalent to four football fields per gram of material, MOF-177's surface area shatters previous records held by other metal-organic framework compounds (about 3,000 m² per g) and disordered carbon (about 2,000 m² per g).

To build the porous crystals, the team linked triangular 1,3,5-benzenetribenzoate (BTB) units to octahedral zinc(II) carboxylate clusters, Zn₄O(CO₂)₆, such that each zinc cluster is attached to six BTB units. The material is constructed entirely of six-membered rings, and in each formula unit there are 84 exposed edges and only four fused edges.

The crystalline material is stable, robust, and highly porous, and its pores (roughly 11 Å across) readily accommodate molecules such as brominated naphthalene and anthracene, C₆₀, and large polycyclic organic dye molecules.

The new material "pushes the limits of specific surface area and pore volume to unexpected heights," remarks Krijn P. de Jong, a professor of inorganic chemistry and catalysis at Utrecht University, in the Netherlands. "It will be exciting to see whether hydrogen storage in this material correlates with the quoted surface area," he adds.

Jeffrey R. Long, an associate professor of chemistry at the University of California, Berkeley, comments that the new family of open-framework compounds is marked "by stability and porosity that few chemists would have imagined possible." The materials are "truly remarkable," Long adds, noting that the level of adjustability in the organic component of the framework gives rise to tremendous potential for new chemistry and future applications.