Novel preparation method for porous materials based on a Si-N network in ionic liquids

Trabajo presentado en la 5th Czech-Italian-Spanish conference on Molecular Sieves and Catalysis, celebrada en Segovia (España) del 16 al 19 de junio de 2013.
Much less developments have been achieved in the solid base materials as compared with the broad number of known solids acid catalysts. It is therefore highly necessary to develop new materials with accessible and active basic sites. Silicon diimide (Si(NH)2) gel has been used as a precursor for microporous and mesoporous silicon imidonitride materials which can be used as solid base catalysts in reactions such as alkene isomerisation or the Knoevenagel reaction. Here we report a novel preparation method of silicon-nitrogen network-based materials with a significant amount of active NH groups from a silicon diimide gel obtained by the ammonolysis of silicon tetrachloride using an ionic liquid ([Emim][Tf2N]) as solvent. The gel resulting from the reaction was heated statically at 180 ºC for 43 hours. In several experiments pyrrolidine was added to the mixture. In order to remove the organic content, selected samples were heated under flowing ammonia at 600 ºC for 2 h. The FTIR spectra of the as-prepared samples show the bands corresponding to the anion ([Tf2N]-) of the ionic liquid showing that this compound is present in all the materials. The heat treatment under flowing ammonia leads to high surface area mesoporous materials (477- 629 m2/g). The TEM image (Fig 1) shows the mesopores of the materials. These results suggest that the generation of mesopores is due to the presence of the ionic liquid, which means that the ionic liquid species are occluded in the pores acting as a template. Analysis of the FTIR spectra of the ammonia-calcined samples (Fig 1) show the bands corresponding to various skeletal ν(Si-N) modes of the Si-N groups of the network and the band corresponding to a δ(NH) mode. The high intensity of this last band, indicate a high number of strong basic Si-NH sites in the materials, which are essential for catalysis.