Introduction to special issue

Speeding Chemistry Along

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Science  14 Mar 2003:
Vol. 299, Issue 5613, pp. 1683
DOI: 10.1126/science.299.5613.1683

Catalytic processes abound in nature. From enzymes to mineral surfaces, catalysts increase the rate of a given reaction, often by reducing the activation energy that the reactants must overcome before they go on to form products. Catalysts have been developed for a wide spectrum of reactions, with perhaps the most familiar example being the converter in cars used to reduce toxic emissions. This special issue of Science on catalysis focuses on efforts to understand and improve industrial catalysis.

Most catalysts can be described as either homogeneous or heterogeneous. Homogeneous catalysts are molecularly dispersed with the reactants in the same phase, which provides easy access to the catalytic site but can make the separation of catalyst and products difficult. Heterogeneous catalysts—usually solids—are in a different phase from the reactants, which reduces separation problems but provides more limited access to the catalytic site. Cole-Hamilton (p. 1702) reviews recent efforts to facilitate the separation of homogeneous catalysts. Approaches include anchoring the catalyst to a soluble or insoluble support, which effectively “heterogenizes” the catalyst, and designing the catalyst so that it is soluble in a solvent that, under some conditions, does not mix with the reaction product.

The classic heterogeneous catalyst contains nanoscale metal particles dispersed on a solid support such as aluminum oxide. Bell (p. 1688) discusses how catalyst nanoparticles are synthesized and how particle size and shape affect reactivity. Cho's News story (p. 1684) describes how ordinarily inert gold becomes a useful catalyst when shrunk to nanoparticle size. Many solid-phase catalysts, such as highly crystalline zeolites, owe their high reactivity in part to their high surface area created by angstrom-scale microporosity. Rolison (p. 1698) examines newer examples of microporous catalysts and argues that porosity itself is far more important than crystallinity.

Enzymes have very high selectivities, which should allow the production of pure products, but several issues have limited their use in industrial synthesis. Schoemaker et al. (p. 1694) show how recent advances in directed evolution, high-throughput screening, metabolic engineering, and genomics are producing enzymes that can operate under the desired conditions and catalyze reactions that pose a challenge to synthetic homogeneous catalysts. Yoon and Jacobsen (p. 1691) highlight a particular class of synthetic homogeneous catalyst. “Privileged” chiral catalysts can catalyze asymmetric reactions that proceed via different mechanisms and produce very different types of products. With two examples (salen complexes and cinchona alkaloids), they illustrate the diversity of products that are accessible with these catalysts. Alper's News story (p. 1686) describes efforts to develop inorganic catalysts from hydrogenases, which are enzymes with which living organisms make and unmake molecular hydrogen—reactions with huge potential for future energy production.

Industrial catalysis will continue to require chemical engineers to take the work of chemists, and increasingly biologists, and run it efficiently on a grand scale. How well this interplay works in the United Kingdom as compared with how it works in the United States is discussed in an Editorial by Higgins (p. 1625).

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