Drying supported catalyst impregnation

H2PtClg solution of desired concentration, dried and calcined at 500°C in air for 2 h to get the PtO MoOj/yA Oj sample. For supported Pt catalysts, the granular y-A O support was impregnated with a H2PtClg solution, dried and calcined at 500°C in air for 5 h to obtain the Pt02/y-A12Oj sample. The pure Pt02 powder (40 mesh) was a commercial sample obtained from Alpha Chemicals, Ventron Corporation. 

In the preceding sections the use of catalysts in which vanadium oxides are supported on a more or less inert carrier has been mentioned quite often. Because of the importance of this type of catalyst they are discussed more extensively in this section. Often a distinction is made between the normal supported catalysts and so called monolayer catalysts. In the latter the vanadium oxide is supposed to be dispersed in a monomolecular layer on the support, which may be covered completely or only partly. The normal supported catalysts are usually made by impregnation, either wet or dry, of the porous carrier with an aqueous solution, often of NH4V03, sometimes with oxalate added.12 14,75,95,139,140... 

Supported catalysts can be prepared in several ways [12], but the simplest is that of impregnation. A support has a characteristic pore volume (e.g., 0.5 mL g-1) hence, adding this volume of a solution containing the appropriate amount of a convenient catalyst precursor (e.g., nickel nitrate in water to prepare a supported nickel catalyst) to the support will simply fill all the pores. However, by allowing the system to dry, and then heating it in air to decompose any undesired salts, the supported material will be converted to the oxidic form. Reduction in hydrogen then converts the oxidic precursor - at least partially - into a supported metal catalyst. 

Washing, drying, and frequent calcination occurs between and after impregnations [7,8] and in order to prepare metallic-supported catalysts, it is as well necessary to carry out a reduction process [9], The most common reducing agent is H2. 

This method of preparation of supported metal catalyst requires a closed reactor to perform the preparation in the absence of water, so both the organic solvent and the oxide support must be carefully dehydrated. The method is based on the following principle the metal is evaporated and co-condensed with the organic to 77 K on the walls of the reactor. Under dynamic vacuum, the co-condensate is then warmed up to 195 K, and melted. The oxide support is impregnated with the solvated metal atom (cluster) at the same temperature, After a given time of contact, the slurry is warmed up to ambient temperature, and the solvent is eliminated, after which the sample can be dried. 

The catalyst may be prepared by impregnating a silica-aluminum support with an aqueous solution of chromium trioxide. After drying, the catalyst is usually activated in a stream of dry air at temperatures ranging from ca. 400°-800°C. Under these conditions catalysts can be prepared with a major percentage of chromium in the form of chromium trioxide. Evidently, the support is not just an inert diluent for chromium oxide by itself is stable only as Cr203 at these temperatures. There must be interaction between chromium trioxide and the support which stabilizes the former. 

Samples of the supports were prepared according to described literature procedures [93,94,99-101]. Both MCM-41 and HMS silicas were preliminarily dehydrated at 400 °C at high vacuum. The supports were impregnated with MAO in toluene and, after washing with toluene and drying in vacuum at room temperature, the MAO/support solids were stirred with a certain amount of complex 4 in toluene at 50 °C for 3 h (Eq. 5). The solids were filtered, washed with toluene, and dried under vacuum at room temperature to obtain the supported catalysts 4/MAO/MCM-41 and 4/MAO/HMS. 

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