Intergrowth structure of zeolite crystals as determined by optical and fluorescence microscopy of the template-removal process.

Zeolites and zeotype materials represent one of the most important types of heterogeneous catalysts widely used in the refining and petrochemical industry. Although industrial zeolite-based catalysts contain mostly nano-to-micrometersized crystallites, larger zeolite single crystals with dimensions of hundreds of micrometers have proven to be indispensable as model materials for diffusion and reactivity studies. It was established that these crystals often have complex structures consisting of several intergrown subunits. 11] The way in which the building blocks are interconnected may severely affect intracrystalline transport properties or even make certain parts of the zeolite crystal completely inaccessible for the reactant molecules. For example, the intergrowth structure of ZSM-5 crystals elucidated from electron diffraction, X-ray diffraction, and optical microscopy results has been discussed over the past 20 years. Recently, Lehmann et al. visualized subunits of AlPO4-5 crystals by probing intracrystalline concentrations of absorbed methanol by interference microscopy. The hydrothermal synthesis of zeolites calls for structuredirecting agents, such as alkyl amines, to ensure their longrange highly ordered porous structure. These template molecules have to be removed from the channels for the crystalline materials to be used as either adsorbents or shapeselective catalysts. To accomplish this, zeolites are calcined after synthesis to remove the template and allow reactant molecules to enter the channel system. Thermal decomposition of template molecules in different molecular sieves has been studied extensively mainly by thermoanalytical techniques, mass spectrometry, 19] and NMR spectroscopy. These methods are bulk techniques, which average information over the whole catalyst sample. In contrast, relatively few attempts have been made to investigate physicochemical processes that take place in a single zeolite particle in a spaceand time-resolved manner. The predominant method of choice to study calcination processes is IR microscopy, which has limited spatial resolution. Herein, a new method is presented for determining the intergrowth structure on the basis of in situ mapping of the template-removal process in individual zeolite crystals. The formation of light-absorbing and -emitting species during the heating process has been monitored by a combination of optical and confocal fluorescence microscopy. As the accessibility of the porous network in the subunits varies, the individual building blocks can be readily visualized by monitoring the template-removal process in time. This concept has been successfully applied to four different zeolite crystals: CrAPO-5 (AFI structure), SAPO-34 (CHA structure), SAPO-5 (AFI structure), and ZSM-5 (MFI structure). As a first example, we discuss the template-removal processes in CrAPO-5 materials as observed with in situ optical microscopy. The crystals (45 < 25< 22 mm) appear flawless under the bright-field illumination; however, microphotographs taken during the heating process show that the crystals darken as a result of coke formation. Surprisingly, this process resulted in a nonuniform color pattern (Figure 1a). First, darkening is observed on the hexagonal flat planes of the crystals at 640 K, and then on the side planes at 675 K. Further on the hexagonal faces clear up and only dark ring patterns remain. At the same time a darkening front on the side planes advances towards the central part as the temperature increases. As the heating continues, the crystals regain translucency, indicating the complete removal of the template molecules. Similar results were obtained when the crystals were calcined in an oxidative atmosphere, but clearly template decomposition took place in this case (see the Supporting Information). Together with these in situ optical microscopy measurements, we recorded Raman spectra, which showed a strong fluorescence background when the zeolite crystals were heated. These results suggest the formation of fluorescent intermediates, which could be further investigated with in situ fluorescence microscopy. The use of fluorescence microscopy in the field of heterogeneous catalysis has been pioneered by [*] L. Karwacki, Dr. E. Stavitski, M. H. F. Kox, Prof. Dr. ir. B. M. Weckhuysen Inorganic Chemistry and Catalysis Group Department of Chemistry Utrecht University Sorbonnelaan 16, 3584 CA Utrecht (The Netherlands) Fax: (+31)30-251-1027 E-mail: b.m. weckhuysen@uu.nl