Heterogeneous Catalysis DOI: 10.1002/ange.200904282 Label-Free Chemical Imaging of Catalytic Solids by Coherent Anti-Stokes Raman Scattering and Synchrotron-Based Infrared Microscopy** Marianne H. F. Kox, Katrin F. Domke, James P. R. Day, Gianluca Rago, Eli Stavitski, Mischa Bonn, and Bert M. Weckhuysen* Worldwide environmental restrictions on sulfur levels in gasoline have prompted the need for increasingly selective sulfur-removing catalysts. Zeolites have proven to be effective additives for the removal of sulfur-containing species during catalytic cracking processes. Whereas the structural building blocks of individual zeolite crystals are well understood and detailed reaction pathways on the molecular level have been proposed, the link between the macroscopic and molecular worlds, namely, the correlation of the crystals internal architecture and catalytic activity on the micrometer scale, has remained elusive. It is often assumed that a catalytic reaction occurs homogeneously throughout a zeolite crystal, not taking into consideration the role of the porous network, grain boundaries, or defect sites on the distribution of reactants and formation of reaction intermediates and products. A microscopic picture of what happens throughout an individual zeolite crystal is lacking. Three-dimensional, high-resolution chemical maps of the spatial distribution of catalytic species within crystal zeolites would provide unpre- cedented insight into the structure–reactivity relationship, which could lead to the design of improved catalysts. [1, 2] Herein, we propose a unique combination of multiplex coherent anti-Stokes Raman scattering (CARS) and synchro- tron-based infrared (IR) microspectroscopy experiments to achieve this goal. The latter, employing a light source 100– 1000 times brighter than conventional IR light, has been shown to be a powerful tool for obtaining two-dimensional (2D) chemical maps with micrometer spatial resolution of a solid acid in the course of a catalytic reaction. [3, 4] These efforts are complemented here with CARS microscopy, thus allow- ing z sectioning (3D imaging) at even better spatial resolu- tion. CARS is several orders of magnitude more sensitive than normal Raman spectroscopy and possesses an intrinsic sectioning capability of approximately 400 nm in the x and y directions and approximately 1 mm in the z direction, thus enabling 3D chemical mapping of small amounts of analytes in confined space with micrometer or sub-micrometer reso- lution. [5–9] In contrast to the more routinely employed (confocal) fluorescence microscopy, [10–15] CARS and IR vibra- tional signatures provide intrinsic chemical information about the species of interest and their interaction with the environ- ment, without the need for fluorescent or fluorescently labeled species. Details on the experimental setups can be found in the Supporting Information. We report the catalytic conversion of thiophene deriva- tives within individual H-ZSM-5 crystals in microscopic detail. We present multidimensional, label-free chemical maps of micrometer resolution of reactant as well as reaction intermediates and products that reveal an inhomogeneous distribution of active species throughout the zeolite crystals. The spectral changes that occur upon adsorption of the thiophene derivatives allow us to monitor in situ the interaction of the reactants with the zeolite and the consec- utive cation formation and ring-opening reactions. To visualize the distribution of thiophene reactants prior to catalytic conversion, coffin-shaped H-ZSM-5 crystals (100  20  20 mm 3 ) were exposed to differently substituted thiophene derivatives and left at room temperature for approximately 30 min to ensure all excess thiophene had evaporated. Subsequently, space-resolved CARS spectra were recorded on individual H-ZSM-5 crystals, which were analyzed using the maximum entropy method (MEM). [16] This method provides the quantitative Raman response, which will be referred to as “nonlinear Raman response”. Below, 2-chlorothiophene is used as an example. Results obtained for other thiophene derivatives and details on the MEM analysis procedure can be found in the Supporting Informa- tion. In Figure 1 a, the nonlinear Raman response of 2-chlorothiophene and an H-ZSM-5 crystal containing 2-chlorothiophene are presented for the C À H stretching region (3250–2950 cm À1 ). Examination of the response for the zeolite containing 2-chlorothiophene shows the presence of a band at 3078 cm À1 and a pronounced band at 3115 cm À1 with a [*] M. H. F. Kox, Dr. E. Stavitski, Prof. Dr. B. M. Weckhuysen Inorganic Chemistry and Catalysis Group Debye Institute for Nanomaterials Science, Utrecht University Sorbonnelaan 16, 3584 CA, Utrecht (The Netherlands) Fax: (+ 31) 30-251-1027 E-mail: b.m.weckhuysen@uu.nl Dr. K. F. Domke, Dr. J. P. R. Day, G. Rago, Prof. Dr. M. Bonn Biosurface Spectroscopy FOM Institute for Atomic and Molecular Physics Science Park 113, 1098 XG Amsterdam (The Netherlands) [**] We thank the Dutch National Science Foundation (NWO CW VICI, VENI and TOP Grant) and Research School Combination Catalysis (NRSC-C) for financial support. Dr. M. Mertens (ExxonMobil) is acknowledged for providing the H-ZSM-5 crystals; Dr. T. Visser, R. Smith (NSLS), and Dr. L. Miller (NSLS) are thanked for assistance with the IR spectroscopy measurements. The National Synchrotron Light Source (NSLS) is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract DE-AC02-98CH10886. K.F.D. thanks the Alexander von Humboldt Foundation (Germany) for a Feodor Lynen fellowship. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200904282. Zuschriften 9152  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2009, 121, 9152 –9156