CHEMICAL & ENGINEERING
NEWS OF THE WEEK FEBRUARY 6, 2006 - EDITED BY WILLIAM G. SCHULZ & MELISSA KUHNELL
CATALYSIS
RESEARCH
A BRIGHT WAY TO TRACK REACTIONS Fluorescence method detects molecular events with high spatial resolution
A
DDING A NEW METHOD TO
t h e list of single-molecule analytical techniques, r e searchers i n Belgium have d e m onstrated t h a t fluorescence m i -
IN A FLASH A new fluorescence microscopy method can pinpoint where a reaction occurs on a micrometer-sized catalyst surface. Comparing a single 96-millisecond exposure time image (left) with a composite of 256 images of the same catalyst particle in the same orientation (arrows) reveals that hydrolysis reactions (bright spots or peaks) occur almost exclusively along certain crystal edges.
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C&EN / FEBRUARY
croscopy can be used t o monitor individual catalytic reactions that occur on the surfaces of solid catalysts submerged in reagent solution {Nature 2 0 0 6 , 4 3 9 , 572). T h e technique, which was used to prepare still micrographs a n d microscopy videos, provides scientists w i t h a n e w real-time and high-resolution m e t h o d for probing molecular events t h a t occur only on certain portions of catalyst surfaces. T h e work may broaden u n d e r s t a n d i n g of catalytic p r o cesses and lead t o t h e design of new catalysts. O n e feature c o m m o n to many of the laboratory techniques t h a t fill the toolbox of modern catalysis researchers is the need for specialized experimental conditions t o carry out t h e m e a s u r e m e n t s . T h o s e c o n d i t i o n s o f t e n differ 6, 2 0 0 6
in p r e s s u r e , t e m p e r a t u r e , a n d chemical environment from t h e conditions typically encountered in industrial catalytic processes. For example, many analytical procedures require that a catalyst b e maintained under high vacuum or i n a p r i s t i n e atmosphere. O t h e r m e t h o d s are applicable o n l y t o m o d e l reactions and single-crystal specimens. T h e new technique, which was developed by chemists Maarten B.J. Roeffaers, D i r k E. D e Vos, Bert F. Sels, J o h a n Hofkens, and their coworkers at Catholic University of Leuven, was applied t o solution-phase reagents undergoing reactions on mineral-type catalysts. Specifically, the team used a specially configured microscope setup t o probe the catalytic transformation of a nonfluorescent "reporter" compound (5-carboxyfluorescein diacetate, C-FDA) to a fluorescent product. T h e reactions were conducted by exposing reagent solutions to a lithium-aluminum gibbsite-type material known as a layered doublehydroxide catalyst. T h e researchers explain t h a t
MORE ONLINE Video clip at C&EN Online shows individual catalytic events.
C - F D A becomes emissive u p o n catalytic hydrolysis i n aqueous media or upon transesterification with an alcohol such as 1-butanol. T h e y note that their strategy was to use a field of view wide enough t o m o n i t o r all emission signals e m a n a t i n g from a n entire catalyst particle. By c o n d u c t i n g t h e e x p e r i m e n t s i n t h a t way, every t i m e a fluorescent p r o d u c t was formed via a catalytic reaction (a "turnover" in catalysis parlance), t h e group counted t h e event and determined where on the catalyst surface it took place. O n e of the study's key findings is t h a t transesterification occurs over the entire outer surface of the catalyst crystal. I n contrast, t h e 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, t h e g r o u p w a s able t o "beautifully m a p t h e spatial distribution of active sites over a single catalytic crystal," says Bert M . Weckhuysen, a chemistry professor at Utrecht University, in the Netherlands. W e c k h u y s e n , whose remarks appear in an accompanying commentary in Nature, notes that other crystal-face-dependent catalytic r e a c t i o n s have b e e n identified previously. But the Leuven study is t h e first of its kind t o focus o n catalytic solids used in liquid-phase applications, he points out. I n most cases, "rational design of catalysts remains a pipe dream," W e c k h u y s e n says, because t h e experimental tools available for monitoring catalysts in action are still, by and large, too rudimentary. T h e goal has not yet been met, h e stresses. But the innovation demonstrated by the Leuven group moves catalysis research "a step further along the road t o rational design."-MITCH JACOBY WWW. C E N - 0 N L I N E . O R G
