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

Monika Kot,Ewa Janiszewska,Robert Wojcieszak,Mariusz Pietrowski,Michał Zieliński

doi:10.3390/ma14040968

Abstract

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.

Highlights

Hydrogenation reactions have become industrially important since 1897, when Sabatier found out that the application of nickel as a catalyst facilitates the addition of hydrogen to molecules of gaseous hydrocarbons [1]
The modified silica materials obtained in the work are new systems that have never been used as supports in catalysis
Summarizing the results of the research on the modification of amorphous silica with the use of ammonium salts of various concentrations, it can be concluded that the modification with NH4 Cl and NH4 F solutions with concentrations of 0.1 M allows to obtain mesoporous supports with a surface area comparable to the original silica, and their modification leads to the generation on their surface of the Lewis type acid centers of weak and medium strength

Summary

Introduction

Hydrogenation reactions have become industrially important since 1897, when Sabatier found out that the application of nickel as a catalyst facilitates the addition of hydrogen to molecules of gaseous hydrocarbons [1]. The catalytic activity in the processes of hydrogenation depends on both the type of active phase and properties of the support used. The surface properties of support, its crystalline and porous structure affect the dispersion of active phase and determine its reducibility. The fundamental task of heterogeneous catalysis is to find the most beneficial combination of metal active phase and support. The choice of catalyst requires many variables to be taken into account, whereas the type of catalytic process seems to be key factor

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