Materials and Methods
2.1. Preparation of the Catalysts.In the synthesis of
the catalysts, the following reagents were used: silica (SiO2),
zinc oxide (ZnO), tin oxide (SnO2), hydrochloric acid (HCl),
and dry sodium sulfate (Na2SO4) (>99%). All were supplied
by Fluka. The aluminum oxide (γ-Al2O3) and ammonium
metatungstate were obtained from Sigma-Aldrich. Catalysts,
based on tungsten impregnation in dry porous support material,
were prepared by pore filling in the presence of a solution excess
(wet impregnation). Simple wet impregnation was carried out
by dissolving a certain amount of ammonium metatungstate 25%
w/v in 20 mL distilled water in a round-bottomed flask, followed
by the addition of the porous support material (γ-Al2O3, SiO2,