Relevance of the drying step in the preparation by impregnation of Zn/SiO2 supported catalysts

Zinc supported over silica materials, which are widely used in heterogeneous catalysis, are prepared in most cases by incipient wetness impregnation with solutions of zinc nitrate. After calcination, the formation of ZnO is usually assumed, although not always fully demonstrated. The goal of this paper is to clarify the nature of the zinc phase present in Zn/SiO2 samples not only after calcination, but also after drying. To achieve this goal, the supported zinc phase was characterized in such systems after drying (between 25 and 200 degreesC) and after calcination (450 degreesC). The use of four complementary techniques, UV-visible, diffuse reflectance infrared, and X-ray absorption spectroscopies and X-ray diffraction, allowed us to show that the ZnO phase supposed to be formed during calcination is a minor phase and that the main phase is a poorly crystallized zinc silicate Zn4Si2O7(OH)(2).H2O, also called hemimorphite. ZnO formation strongly depends on the drying temperature after impregnation: after drying at 25, 50, and 200 degreesC, ZnO does not form after calcination. After drying at 25 or 50 degreesC, the supported zinc phase is an amorphous nitrate, whereas after drying at 200 degreesC, it is almost already fully transformed into zinc silicate. At intermediate drying temperatures between 90 and 150 degreesC, part of the zinc forms crystallized zinc hydroxynitrates, which lead to the ZnO phase after calcination; almost all of the remaining zinc is a zinc silicate.