Structural aspects and porosity features of nano-size high surface area alumina–silica mixed oxide catalyst generated through hybrid sol–gel route

Abstract

Alumina–silica mixed oxide nano-catalyst materials with compositions 83.6 wt.% Al 2O 3–16.4 wt.% SiO 2 (3Al 2O 3·1SiO 2), 71.82 wt.% Al 2O 3–28.18 wt.% SiO 2 (3Al 2O 3·2SiO 2), 62.84 wt.% Al 2O 3–37.16 wt.% SiO 2 (3Al 2O 3·3SiO 2) and 56.03 wt.% Al 2O 3–43.97 wt.% SiO 2 (3Al 2O 3·4SiO 2) have been prepared by a hybrid sol–gel technique using boehmite as the precursor for alumina and tetraethoxysilane as that for silica. The bonding characteristics and coordination features around Al and Si in the mixed oxide catalysts have been studied using FTIR and 27Al MAS NMR after calcination at 400 °C which is the temperature region where cross-condensation is seen to take place. A high BET specific surface area of 287 m 2 g −1 is obtained for 3Al 2O 3·1SiO 2 mixed oxide composition. The porosity features are further established by BET adsorption isotherms and pore size distribution analysis. The temperature-programmed desorption studies showed more surface active sites for the silica-rich composition, suggesting enhanced catalytic potential. The TEM features of the mixed oxides showed a homogeneous distribution of alumina and silica phases with particle sizes in the nano-range. The low silica-containing mixed oxide showed a needle-like morphology with a high aspect ratio of 1:50 and ∼10 nm particle size while the silica-rich composition had particle size in a wide range (∼20–75 nm).