Drying and calcination

The drying and calcination steps, where the metal salts are dehydrated and/or converted to oxides, are very critical. The optimum conditions are required 

When the calcination step is not well controlled, or when it is not desirable to remove the organic additives from the catalyst before the sulfidation, it may be possible to dry the catalyst only at low temperature and to skip the calcination step. High activity can be obtained, for example, with dried-only Co/Ni-Mo/W hydrotreating catalysts containing particular carboxylic acids (e.g. glycolic acid or lactic acid) and particular organic S compounds 

Presulfiding as the Last Stage in Hydrotreating Catalyst Preparation 

It is well known that microwave (MW) drying of many solid materials is a very efficient and widely used process even on an industrial scale [3] it is also an attractive means of drying of heterogeneous catalysts. Microwave drying of catalysts and supported adsorbents has several advantages  

Microwave heating has been reported to produce materials with particular physical and chemical properties [4], Stable solid structures are formed at low reaction temperatures with unusually high surface areas, making them very useful as catalysts or catalyst supports. Calcination of solid precursors in a microwave field has significant advantages over conventional heating. The effective synthesis of the catalysts and supporting adsorbents has been reported for the examples below. 

The V205-Si02 catalyst for o-xylene oxidation prepared by wet impregnation under the action of microwave irradiation had several advantages [7] compared with that prepared by the conventional thermal method  

The more active cobalt catalyst for pyrolytic reactions was prepared by MW calcination of cobalt nitrate, which was converted to cobalt oxide by rapid microwave heating [8]. 

The catalysts were very active in NO decomposition, even at room temperature, 

---

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Transparent cobalt aluminum blue is prepared by the precipitation of diluted solutions of cobalt and aluminum with alkaHes. The formed precipitate of hydroxides is washed, filtered off, dried, and calcined at about 1000°C. 

The AIF. solution is filtered, AIF. precipitated by heating, flash dried, and calcined. 

Batch Furnaces This type of furnace is employed mainly for the heat treatment of metals and for the drying and calcination or ceramic articles. In the chemical process industry, batch furnaces may be used for the same purposes as batch-tray and truck dryers when the drying or process temperature exceeds 600 K (620°F). They are employed also for small-batch calcinations, thermal decompositions, and other chemical reactions which, on a larger scale, are performed in rotary Idlns, hearth furnaces, and shaft furnaces. 

The precipitated precursor can be dissolved and re-crystallized from fluorine-free solutions. This provides excellent conditions for deep purification of the material and reduction of problematic impurities such as titanium, fluorine, etc. Peroxometalates decompose at relatively low temperatures forming tantalum or niobium oxides containing small amount of absorbed water. The absorbed water separation is achieved by further thermal treatment - drying and calcination - of the product ... 

Use of flames as external heat source to induce quick drying and calcination One-step aerosol flame synthesis... 

An increasing intensity of the diffraction peaks of hematite is observed when comparing the dried and calcined catalyst as shown in Fig. 2(a), indicating that hematite forms at M er temperatures. No obvious diffraction peaks to lithium such as lithium iron oxide (LiFcsOg) could probably be ascribed to the small fraction of lithium or overlapped peaks betwem hematite and lithium iron oxide. The diffraction peak intensity of magnetite in tested catalysts increases significantly. 

Aluminas. Aluminas, porous AI2O3, are available in many forms. They constitute the most important carrier material in heterogeneous catalysis. Alumina is amphoteric and, as a con.sequence, soluble in both acidic and basic media. Precipitation can be performed from an acid solution by adding a base or from a basic solution by adding an acid, as schematically represented in Fig. 3.18. If, for example, at a pH of less than about 3 a base is added to an aqueous solution of aluminium sulphate, a precipitate is formed. If this material is filtered, dried and calcined, an amorphous porous AI2O3 is obtained. At other pH values different porous aluminas can be synthesized. 

The samples FI, F2, F3, F7 and F8 were prepared by wet impregnation using excess solutions as above the preparation of FI, F7 and F8 involved feeding the support with each solution in turn before finally drying and calcining (as for the samples of Table 1) whereas F2 and F3 involved drying and calcining before addition of the second component. In contrast, samples F4, F5 and F6... 

Torma, V., Peterlik, H., Bauer, U., Rupp, W., Husing, N., Bernstorff, S., Steinhart, M., Goerigk, G. and Schubert, U. (2005) Mixed silica titania materials prepared from a singlesource sol-gel precursor A time-resolved SAXS study of the gelation, aging, supercritical drying, and calcination processes. Chemistry of Materials, 17, 3146-3153. 

---