Synthesis of microporous transition-metal-oxide molecular sieves by a supramolecular templating mechanism

Abstract

Mesoporous bulk^1,2,3,4 and thin-film^5,6,7 silicates with pore sizes of 20–100 Å can be synthesized by using micellar aggregates of long-chain organic surfactant molecules as templates to direct the structure of the silicate network. Because of the potential applications of these molecular-sieve materials as catalysts, separation membranes and components of sensors, it is desirable to extend the range of accessible pore sizes and material compositions. Mesoporous oxides in which transition metals partially⁸ and fully^{9,10,11,12,13} substitute for silicon have been made by similar means, in the latter case by ensuring strong interactions between the surfactants and the transition-metal alkoxide precursors. Templating with organic molecules has also been long used for the synthesis of microporous materials—synthetic zeolites—which have smaller pore sizes (4–15 Å), but here the organic molecules are shorter-chain amphiphiles which are too small to be considered true surfactants and so act as discrete entities around which the framework crystallizes^14,15,16. Here we show that even such short-chain molecules can aggregate into supramolecular templates when they form bonds with transition-metal (niobium) alkoxides, and that in this way they can direct the formation of transition-metal oxides with pore sizes of less than 20 Å. These pore sizes, which result from the smaller diameter of micellar structures of the short-chain amines relative to the longer-chain surfactants used for the synthesis of mesoporous materials, qualify the resulting molecular sieves as microporous, even though the supramolecular templating mechanism is similar to that used to make the mesoporous materials. Thus our approach extends the supramolecular templating method to afford microporous transition-metal oxides.