Issue Date: August 20, 2007
Coming Together On Catalysis
"HOMOGENEOUS, heterogeneous, and enzyme catalysis are three totally separate areas of chemistry," proclaimed Gabor A. Somorjai last month at the opening of the 13th International Symposium on the Relations between Homogeneous & Heterogeneous Catalysis. "Comparing one field to another has traditionally been like comparing apples to oranges." It shouldn't be this way, Somorjai said, "because the molecular fundamentals of all three fields are identical."
As he sees it, catalysis research ought to be unified, and he suggested a way to do that: Investigate all catalysts on the molecular scale and under the same types of reaction conditions. By doing so, Somorjai continued, "we can uncover molecular ingredients in one type of catalysis and use them to help us understand another type."
With those words, Somorjai, a longtime leader in the arena of surface science, a field closely related to catalysis, urged attendees to think about problems in catalysis science broadly and in nontraditional ways. Held last month on the tree-lined and scenic campus of the University of California, Berkeley, where Somorjai is a chemistry professor, the meeting drew more than 300 people from around the globe, including some who traveled from Inner Mongolia and elsewhere in Asia. The turnout was twice as large as Somorjai had initially planned for, a result he was pleased to attribute, in part, to the strong interest of young scientists in applying catalysis in the red-hot areas of green chemistry and nanotechnology.
The three principal areas of catalysis evolved independently, mainly because of the distinct reaction conditions under which various types of catalysts are used. Heterogeneous catalysts tend to be used to facilitate high-temperature gas-phase reactions. Homogeneous catalysts are generally used in organic solvents at moderate temperatures. And enzymes typically work in water near room temperature. As a result of such practical differences, devotees of each of the three subdisciplines developed unique ways of describing, characterizing, and thinking about their respective classes of catalysts.
TO STIMULATE creative and broad thinking that encompasses all areas of catalysis, Somorjai invited a number of internationally recognized experts in each of the three areas to address the entire conference. In that way, the meeting provided participants with opportunities to learn about major themes in catalysis that lie far from their own areas of expertise and to attend presentations on topics that they typically would not attend.
By design, the subjects of the plenary lectures varied widely. All of the presentations focused on research in catalytic chemical transformations, yet the systems studied, the methods of investigation, and the character of the presentations were derived from laboratory cultures as disparate as biology and physics.
One of the most biological lectures was given by Berkeley's Carolyn Bertozzi, a chemistry and biology professor. Bertozzi marveled at the "exquisite selectivity in biological catalysis," noting, for example, that enzymes active in cellular processes manage to target specific reactants from among many thousands of similar molecules. Scientists have long attributed a good part of that specificity to the unique three-dimensional structure and chemical composition of an enzyme's active site, which is tailored for molecular recognition.
Another differentiating feature of biological catalysis derives from the role of cellular compartmentalization. As Bertozzi explained, certain types of cells are composed of membrane-enclosed organelles that carry out enzymatic transformations on molecules passing through them. One example under study in Bertozzi's lab involves proteins that travel from the endoplasmic reticulum (the tube-like organelles that host most protein synthesis) through the various compartments within the Golgi apparatus (a layered organelle where certain proteins are packaged). As the proteins make this journey, they are altered sequentially by glycotransferases and sulfotransferases, which modify the proteins by adding sugar and sulfate units, respectively. Those modifications are believed to play important roles in cellular signaling processes central to inflammatory diseases such as arthritis and asthma.
At the other end of the catalysis spectrum in the meeting's agenda were physics-centered presentations such as the ones led by Jens K. Nørskov and Hans-Joachim (Hajo) Freund on heterogeneous (solid-state) catalysis. Nørskov, a physics professor at the Technical University of Denmark, Lyngby, reported on state-of-the-art quantum mechanical calculations that probe the origins of differences in chemical reactivity of various transition-metal and nanoparticle surfaces.
By carrying out computer "experiments" that compare the properties of various crystal faces of certain metals and the effects of sulfur or the presence of other foreign atoms that can inactivate or otherwise "poison" catalysts, Nørskov and coworkers have developed tools to distinguish between geometrical (structural) and electronic factors that control the chemical reactivity of solid surfaces. Such studies are essential for interpreting experimental results, designing future experiments, and developing new and improved types of catalysts.
On the experimental side of surface catalysis, Freund, a professor and director of the Fritz Haber Institute of the Max Planck Society, in Berlin, reviewed studies of model catalyst systems that were designed, in his words, "to catch some of the complexity of real catalysts." The surfaces of real catalysts are often dotted with crystal defects, which can serve as active sites that mediate reactions. For example, oxygen vacancies in metal oxides, which can be occupied by either one or two electrons, have been suspected of playing such a role, but the defects have been tough to characterize precisely. On the basis of spectroscopy and microscopy studies, Freund's team has shown that it can prepare model MgO films in which the two types of defects are readily identified and distinguished (J. Phys. Chem. B 2006, 110, 46).
Other speakers at the symposium drew upon investigations that straddled at least two of the principal areas of catalysis in one way or another. For example, chemist Jan-Erling Bäckvall of Stockholm University, in Sweden, described a process based on an enzyme and a homogeneous catalyst that work in concert in an organic solvent to yield highly enantiopure products (C&EN, Aug. 14, 2006, page 29).
Known as dynamic kinetic resolution, Bäckvall's procedure, which initially was applied to chiral secondary alcohols and has been commercialized by DSM, in the Netherlands, for production of intermediates, employs a ruthenium-based racemization catalyst and an enzyme (a lipase) to transform the alcohols into esters. As enzyme-catalyzed acylation selectively converts one of the alcohol enantiomers to the product, the ruthenium catalyst interconverts the nonreacting enantiomer to the reactive form. In that way, the technique can convert more than 90% of the starting material with nearly 100% enantioselectivity. Bäckvall pointed out that his research group has developed similar procedures for resolving enantiomers of amines, diols, and other types of racemic mixtures.
Meanwhile, Bruno Chaudret, who leads a group at the National Center for Scientific Research (CNRS) in Toulouse, France, takes advantage of solution-phase methods and organometallic complexes to synthesize nanostructured materials—often heterogeneous catalysts—in a variety of well-defined sizes and shapes. Typically, the group decomposes organometallic precursors in solution to form metal nanoparticles and then treats the resulting particles with various ligands to control the morphology of the products. Using thiols, amines, and other types of ligands to control product stability, the researchers have synthesized nanorods, nanocubes, and other nanoscale structures.
In one application of these synthesis methods, the Toulouse group examined the propensity of palladium nanoparticles coordinated to chiral diphosphite ligands to catalyze transformations of racemic mixtures. Their tests showed that the materials prepared in that way perform as fairly selective catalysts for C-C coupling reactions—specifically, asymmetric allylic alkylations.
A couple of days' worth of talks on a broad range of catalysis topics is not enough to change the way mature scientists think about their research. Nonetheless, for some leading chemists, the benefits of such a gathering are tangible. "I find it stimulating and scientifically productive to bring people working in these diverse disciplines together," commented Northwestern University chemist Tobin J. Marks.
Bringing together such a diversity of expertise, Marks noted, "provides an opportunity to bounce your ideas off people from different disciplines and sometimes to obtain novel feedback you wouldn't get from your usual audience."
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