Precursor Aluminum Effect in the Synthesis of Sol−Gel Si−Al Catalysts: FTIR and NMR Characterization

Abstract

Aluminosilicates were prepared by the sol−gel method using tetraethoxysilane as silica source and aluminum ethoxide, aluminum iso-propoxide, or aluminum sec-butoxide as alumina sources. An important effect of the alkylaluminum substitutes is observed in the BET specific area, ethoxide > isopropoxide > sec-butoxide. The 27Al NMR spectra of the solids showed that the relative concentrations of tetrahedral, octahedral, or pentacoordinated aluminum strongly depend on the aluminum alkoxide source. It is shown by 29Si NMR spectra that the formation of pentacoordinated aluminum is related to the odd silicon substitution, Si(1Al, 3Si) and Si(3Al, 1Si). The odd Si substitution was confirmed by FTIR spectroscopy following the Si−O−Si vibration band. The shift of the 1092 cm-1 band to 1029 cm-1 agrees well with the 29Si NMR odd substitution. In general the aluminum coordination as well as the odd Si substitution is largely affected when the hydrolysis water concentration is varied. High activity is shown by the aluminosilicates during the 2-propanol dehydration reaction.