Metatungstate and tungstoniobate-containing LDHs: Preparation, characterisation and activity in epoxidation of cyclooctene

摘要

Polyoxometalates (POMs) H2W12O406- and W4Nb2O194- have been intercalated between the brucite-like layers of Mg, Al and Zn, Al hydrotalcites by anion exchange, starting from the corresponding nitrate precursors. The solids have been characterised by Powder X-ray Diffraction (PXRD), Fourier Transform infrared (FT-IR) spectroscopy, N-2 adsorption-desorption at -196 degrees C and thermogravimetric (TG) and differential thermal analyses (DTA), and have been tested in the epoxidation of cyclooctene using H2O2 or t-BuOOH as oxidants. The results show that both anions are effectively located in the interlayer space maintaining their pristine structures without depolymerisation. Upon intercalation of such large anions microporosity is developed and subsequently an increase in the specific surface 06areas is also observed. In general, the prepared materials possess catalase and epoxidation activity, with ZnAl-intercalated H2W12O406- giving the best results in terms of epoxide yield (17% at 24 h). Product selectivity is different for the intercalated and free POMs, the latter yielding 1,2-cyclooctanediol as the only product, whereas the former produces only the epoxide. The epoxidation reaction seems to be catalysed in homogeneous phase by the POM. (c) 2007 Elsevier Ltd. All rights reserved.