A Bright Way To Track Reactions

Adding a new method to the list of single-molecule analytical techniques, researchers in Belgium have demonstrated that fluorescence microscopy can be used to monitor individual catalytic reactions that occur on the surfaces of solid catalysts submerged in reagent solution (Nature 2006, 439, 572).

The technique, which was used to prepare still micrographs and microscopy videos, provides scientists with a new real-time and high-resolution method for probing molecular events that occur only on certain portions of catalyst surfaces. The work may broaden understanding of catalytic processes and lead to the design of new catalysts.

One feature common to many of the laboratory techniques that fill the toolbox of modern catalysis researchers is the need for specialized experimental conditions to carry out the measurements. Those conditions often differ in pressure, temperature, and chemical environment from the conditions typically encountered in industrial catalytic processes. For example, many analytical procedures require that a catalyst be maintained under high vacuum or in a pristine atmosphere. Other methods are applicable only to model reactions and single-crystal specimens.

The new technique, which was developed by chemists Maarten B. J. Roeffaers, Dirk E. De Vos, Bert F. Sels, Johan Hofkens, and their coworkers at Catholic University of Leuven, was applied to solution-phase reagents undergoing reactions on mineral-type catalysts. Specifically, the team used a specially configured microscope setup to probe the catalytic transformation of a nonfluorescent "reporter" compound (5-carboxyfluorescein diacetate, C-FDA) to a fluorescent product. The reactions were conducted by exposing reagent solutions to a lithium-aluminum gibbsite-type material known as a layered double-hydroxide catalyst.

The researchers explain that C-FDA becomes emissive upon catalytic hydrolysis in aqueous media or upon transesterification with an alcohol such as 1-butanol. They note that their strategy was to use a field of view wide enough to monitor all emission signals emanating from an entire catalyst particle. By conducting the experiments in that way, every time a fluorescent product was formed via a catalytic reaction (a "turnover" in catalysis parlance), the group counted the event and determined where on the catalyst surface it took place.

One of the study's key findings is that transesterification occurs over the entire outer surface of the catalyst crystal. In contrast, the catalyst is far more finicky about where it performs ester hydrolysis. According to the researchers, that reaction takes place only on select crystal faces.

Because of the technique's high sensitivity, the group was able to "beautifully map the spatial distribution of active sites over a single catalytic crystal," says Bert M. Weckhuysen, a chemistry professor at Utrecht University, in the Netherlands.

Weckhuysen, whose remarks appear in an accompanying commentary in Nature, notes that other crystal-face-dependent catalytic reactions have been identified previously. But the Leuven study is the first of its kind to focus on catalytic solids used in liquid-phase applications, he points out.

In most cases, "rational design of catalysts remains a pipe dream," Weckhuysen says, because the experimental tools available for monitoring catalysts in action are still, by and large, too rudimentary. The goal has not yet been met, he stresses. But the innovation demonstrated by the Leuven group moves catalysis research "a step further along the road to rational design."