SYNTHESIS OF MICROPOROUS TRANSITION-METAL-OXIDE MOLECULAR-SIEVES BY ASUPRAMOLECULAR TEMPLATING MECHANISM

Mesoporous bulk(1-4) and thin-film(5-7) silicates with pore sizes of 2 0-100 Angstrom can be synthesized by using micellar aggregates of long -chain organic surfactant molecules as templates to direct the structu re of the silicate network. Because of the potential applications of t hese molecular-sieve materials as catalysts, separation membranes and components of sensors, it is desirable to extend the range of accessib le pore sizes and material compositions. Mesoporous oxides in which tr ansition metals partially(8) and fully(9-13) substitute for silicon ha ve been made by similar means, in the latter case by ensuring strong i nteractions 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 ha ve smaller pore sizes (4-15 Angstrom), but here the organic molecules are shorter-chain amphiphiles which are too small to be considered tru e surfactants and so act as discrete entities around which the framewo rk crystallizes(14-16). Here we show that even such short-chain molecu les can aggregate into supramolecular templates when they form bonds w ith transition-metal (niobium) alkoxides, and that in this way they ca n direct the formation of transition-metal oxides with pore sizes of l ess than 20 Angstrom. 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 mate rials, qualify the resulting molecular sieves as microporous, even tho ugh the supramolecular templating mechanism is similar to that used to make the mesoporous materials. Thus our approach extends the supramol ecular templating method to afford microporous transition-metal oxides .