Ligand-Assisted Liquid Crystal Templating in Mesoporous Niobium Oxide Molecular Sieves.
A systematic study of the factors governing the formation of Nb-TMS1, a niobium-based mesoporous hexagonally-packed transition metal oxide molecular sieve, is reported. The synthesis of this material was achieved through a novel ligand-assisted liquid crystal templating mechanism in which a discrete covalent bond is used to direct the templating interaction between the organic and inorganic phases. In general, the synthesis of Nb-TMS1 is more strongly affected by starting conditions such as temperature, surfactant-to-metal ratio, pH, and solvent than by temperature and time of aging after the initial hydrolysis step. The results also show that Nb-TMS1 can be synthesized under conditions which strongly disfavor the formation of micelles. This suggests that Nb-TMS1 is formed via a mechanism involving self-assembly with concomitant condensation. It was found that with increasing surfactant-to-metal ratios, new hexagonal P63/mmc (Nb-TMS2) and layered (Nb-TMS4) phases could be formed, while increasing the surfactant chain length led to a new cubic phase (Nb-TMS3). Crystals of Nb-TMS1 of up to several mm in dimensions were also grown. These crystals are larger than the biggest mesoporous crystals reported by a factor of 3 orders of magnitude. These crystals retain their structure on micelle removal by acid treatment and are thus of great interest as hosts for quantum wires.
Antonelli
David M.
DM
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.
Nakahira
Atsushi
A
Ying
Jackie Y.
JY