Spying On Catalysts

For the first time, researchers have applied infrared microspectroscopy to monitor catalytic reactions as they occur within the pores of zeolite crystals (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200705562). The technique provides scientists with a new procedure for probing the detailed relationship between a catalyst's structure and its function. The method also offers a means for elucidating reaction pathways mediated by industrially relevant catalysts such as zeolites.

Like law enforcement agents who search for ways to spy on criminals so they can catch the perpetrators "in the act," chemists try to develop methods to monitor catalysts under typical conditions and catch the catalysts promoting chemical reactions.

Developing analytical methods that are compatible with elevated temperatures and pressures and other standard catalysis reaction conditions is challenging. Yet a few in situ microscopy and spectroscopy methods that can scrutinize the internal surfaces of porous catalyst materials during the course of a reaction have already been developed. Now, researchers at Utrecht University, in the Netherlands, have added the molecular-structure-resolving power of vibrational spectroscopy to that small but growing collection of in situ analytical tools.

Demonstrating the technique, chemistry professor Bert M. Weckhuysen, postdoc Eli Stavitski, and their coworkers exposed micrometer-sized crystals of an acidic zeolite, H-ZSM-5, to 4-fluorostyrene. They heated the samples and then probed the styrene oligomerization process in various ways with high-intensity synchrotron IR radiation. In one set of experiments, the group focused on a 5- × 5-μm region of a single crystal for a prolonged period to monitor the evolution of the oligomerization process over time in that spot. In other experiments, the researchers scrutinized larger areas by scanning individual crystals under the microscope's field of view.

Among other outcomes, the team observed the principal reaction intermediate, a bisphenyl-ylium cationic dimer. They identified that species by comparing calculated spectra to spectra measured experimentally. The group also deduced the dimeric cation's molecular orientation within the zeolite's channels and mapped its microscopic distribution 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 conjunction with fluorescence and UV-Vis techniques, "will provide unprecedented resolution of the time and spatial mapping of reactant, intermediate, and product molecules in catalytically active microporous systems under actual catalytic working conditions." He adds that the method "will greatly increase our understanding of molecular transformations that follow during the course of catalytic reactions."