Effect of Modification of Amorphous Silica with Ammonium Agents on the Physicochemical Properties and Hydrogenation Activity of Ir/SiO2 Catalysts

The modification of commercial silica with solutions of NH₄F or NH₄Cl salts, followed by thermal treatment, enabled generation of the acidic sites in SiO₂ and changed its textural properties. The use of ammonium salts solution also caused the generation of additional porosity. Using NH₄F solution caused significant decrease in the specific surface area and the increase in the average pore diameter. The number and strength of resulting acid sites depend on the nature of anion in the applied ammonium salt and the concentration of salt solution. It has been found that the sample treated with NH₄F presented higher total acidity (TPD–NH₃) and the amount as well as the strength of acid sites increased with the concentration of the used modifier. As modified amorphous SiO₂ materials used as a support for iridium (1 wt %, Ir(acac)₃) nanoparticles permitted to obtain highly active catalysts for toluene hydrogenation under atmospheric pressure. The highest activity (expressed as the apparent rate and TOF) was obtained for iridium catalysts supported on silica modified by NH₄F with the highest acidity. The modification of silica with NH₄F favors the generation of centers able to adsorb toluene, which results in higher activity of this catalyst.