Open Framework and Microporous Transition Metal Silicates
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Open Framework and Microporous Transition Metal Silicates Allan J. Jacobson, Xiqu Wang, Lumei Liu, and Jin Huang Department of Chemistry, University of Houston Houston, TX 77204, U.S.A. ABSTRACT A family of microporous and open-framework microporous transition metal silicates have been prepared by hydrothermal synthesis at T ≤ 240 ºC. The structures of the compound are based on a common principle in which anionic silicate components (layers, chains, or cluster anions) are connected by isolated transition metal polyhedra by sharing four corners in a square arrangement. Known examples of this class of compounds are reviewed and some recent progress on extending the syntheses to include organic templates are discussed. INTRODUCTION Porous crystalline materials that contain cavities and channels with dimensions on the nanometer scale have a wide range of important applications in molecular separations, as ionexchangers, and in heterogeneous catalysis. The aluminosilicates or zeolites that have structures based on frameworks of connected tetrahedral SiO4 and AlO4 building units are the most important class of porous crystals that have commercial applications. The aluminophosphates, substituted aluminophosphates, gallophosphates, and zinc and cobalt phosphates are other examples of ‘zeotypes’ based on tetrahedral building units that have been discovered in the last two decades in the search for new selective catalysts [1]. A second, though less explored, class of open-framework solids contains compounds with structures formed by linking tetrahedra with other types of metal ion centered polyhedra such as octahedra or square pyramids. A large number of mixed polyhedral frameworks containing phosphate groups have been synthesized but in contrast, relatively few silicates and germanates have been reported. though these are expected to have better stability over a wider range of catalytic conditions [2]. Several phases, for example the titanosilicate Na2TiSi5O13·H2O (ETS10), have wide channel systems and substantial micropore volume. We have recently focused on the synthesis of vanadium and other transition metal silicates because they are expected to show better thermal and hydrothermal stability than the corresponding phosphates. In contrast to the large family of vanadophosphates, only one synthetic large pore vanadosilicate (AM-6, isostructural with ETS-10) is known that contains silicate tetrahedra in combination with oxovanadium (IV) species that are an essential part of the framework [3]. We have obtained using mild hydrothermal synthesis conditions, a large family of open-framework and, in some cases microporous, transition metal silicates [4-9]. A new class of compounds (designated MSH-nA for Metal Silicate Houston-nA, where M is the specific transition metal, n is the framework type and A represents the non-framework cations) comparable to the zeolite family can be predicted based on the building units found in this series. EXPERIMENTAL The MSH-n compounds were synthesized by hydrothermal methods similar to
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