Combination of several in-situ X-ray methods clarify how micropores are created in zeolites

Professor Weckhuysen (Utrecht University, Netherlands) and his colleagues have recently solved the molecular mechanism for the organic-base-mediated synthesis of zeolites. AlPO₄-5 is a typical zeolite, which can be constructed from aluminium-based tetrahedra (AlO₄) and phosphorus-based tetrahedra (PO₄). The research group compared the formation of the chargeless AlPO₄-5 framework with the negatively charged framework (known as ZnAPO-34) that is formed by replacing Al³⁺ in AlPO4-5 with Zn²⁺. The former contains one-dimensional channels, but the latter spherical cavities rather than channels. By employing not only small and wide angle X-ray scattering (SAXS and WAXS), but also X-ray absorption spectroscopy, it was possible to observe in real time both the structural changes in the aluminophosphate gel and the conformational features of the organic base (tetraethylammonium hydroxide) used as a template for the crystallization of zeolite. The tetraethylammonium ion was found to form a complex with developing zeolite subunits in the gel, adopting a molecular structure close to that found in the final crystal. This molecular recognition process determines which type of crystal lattice is formed. The principal point here is that molecular organization takes place before crystallization. The experiments were done at BM26A, ESRF (Grenoble, France). For more information, see the paper, "A Combined SAXS/WAXS/XAFS Setup Capable of Observing Concurrent Changes Across the Nano-to-Micrometer Size Range in Inorganic Solid Crystallization Processes", A. M. Beale et al., J. Am. Chem. Soc., 128, 12386 (2006). Another interesting account can also be found in "Physical chemistry: Porous solids get organized", R. A. van Santen1, Nature, 444, 46 (2006).