PRECURSOR ALUMINUM EFFECT IN THE SYNTHESIS OF SOL-GEL SI-AL CATALYSTS- FTIR AND NMR CHARACTERIZATION

Aluminosilicates were prepared by the sol-gel method using tetraethoxy silane as silica and aluminum ethoxide, aluminum iso-propoxide, or alu minum sec-butoxide as alumina sources. An important effect of the alky laluminum substitutes is observed in the BET specific area, ethoxide > isopropoxide > sec-butoxide. The Al-27 NMR spectra of the solids show ed that the relative concentrations of tetrahedral, octahedral, or pen tacoordinated aluminum strongly depend on the aluminum alkoxide source . It is shown by Si-29 NMR spectra that the formation of pentacoordina ted aluminum is related to the odd silicon substitution, Si(1Al, 3Si) and Si(3Al, 1Si). The odd Si substitution was confirmed by FTIR spectr oscopy following the Si-O-Si vibration band. The shift of the 1092 cm( -1) band to 1029 cm(-1) agrees well with the Si-29 NMR odd substitutio n. In general the aluminum coordination as well as the odd Si substitu tion is largely affected when the hydrolysis water concentration is va ried. High activity is shown by the aluminosilicates during the 2-prop anol dehydration reaction.