Spying On Catalysts                                                               
Infrared method probes reactions as they occur in zeolites                                                               
Mitch Jacoby                                                                                       
For the first time, researchers have appliedinfrared microspectroscopy to monitor catalytic reactions as they occurwithin the pores of zeolite crystals (Angew. Chem. Int. Ed.,DOI: 10.1002/anie.200705562). The technique provides scientists with anew procedure for probing the detailed relationship between acatalyst's structure and its function. The method also offers a meansfor elucidating reaction pathways mediated by industrially relevantcatalysts such as zeolites.
                                                              
  Eli Stavitski/Utrecht University                              
In and OutA new method can probe molecules, such as the cationic fluorinatedstyrene dimer depicted, as they form in the channels of a zeolitecrystal.
                                
  
Like law enforcement agents who search for ways to spy on criminalsso they can catch the perpetrators "in the act," chemists try todevelop methods to monitor catalysts under typical conditions and catchthe catalysts promoting chemical reactions.
Developing analytical methods that are compatible with elevatedtemperatures and pressures and other standard catalysis reactionconditions is challenging. Yet a few in situ microscopy andspectroscopy methods that can scrutinize the internal surfaces ofporous catalyst materials during the course of a reaction have alreadybeen developed. Now, researchers at Utrecht University, in theNetherlands, have added the molecular-structure-resolving power ofvibrational spectroscopy to that small but growing collection of insitu analytical tools.
Demonstrating the technique, chemistry professor Bert M. Weckhuysen,postdoc Eli Stavitski, and their coworkers exposed micrometer-sizedcrystals of an acidic zeolite, H-ZSM-5, to 4-fluorostyrene. They heatedthe samples and then probed the styrene oligomerization process invarious ways with high-intensity synchrotron IR radiation. In one setof experiments, the group focused on a 5- × 5-μm region of a singlecrystal for a prolonged period to monitor the evolution of theoligomerization process over time in that spot. In other experiments,the researchers scrutinized larger areas by scanning individualcrystals under the microscope's field of view.
Among other outcomes, the team observed the principal reactionintermediate, a bisphenyl-ylium cationic dimer. They identified thatspecies by comparing calculated spectra to spectra measuredexperimentally. The group also deduced the dimeric cation's molecularorientation within the zeolite's channels and mapped its microscopicdistribution across the catalyst both spatially and temporally.
Matthew Neurock,a professor of chemical engineering at the University of Virginia,notes that the new IR method, used either by itself or in conjunctionwith fluorescence and UV-Vis techniques, "will provide unprecedentedresolution of the time and spatial mapping of reactant, intermediate,and product molecules in catalytically active microporous systems underactual catalytic working conditions." He adds that the method "willgreatly increase our understanding of molecular transformations thatfollow during the course of catalytic reactions."