Calcination chemical change during

Magnesium aluminate, the preferred support of one catalyst supplier, has a larger specific surface area. But this material must be calcined to a higher temperature during manufacture of the support particles to ensure it contains no free magnesium oxide, which would be hydrated to hydroxide at temperatures below 300 °C. This chemical change results in a volume increase, which would destroy the structure and impair the mechanical stability of the catalyst. 

Besides the prediction of calcination temperatures during catalyst preparation, thermal analysis is also used to determine the composition of catalysts based on weight changes and thermal behavior during thermal decomposition and reduction, to characterize the aging and deactivation mechanisms of catalysts, and to investigate the acid-base properties of solid catalysts using probe molecules. However, these techniques lack chemical specificity, and require corroboration by other characterization methods. 

Alumina membranes. It has been established that several phases of alumina exist and a particular phase of alumina is determined not only by the temperature it has experienced but also by the chemical path it has taken. For commercial membrane applications, the alpha- and gamma-phases of alumina are the most common. Alpha-alumina membranes are well known for their thermal and hydrothermal stabilities beyond 1,000 C. In fact, other transitional forms of alumina will undergo transformation towards the thermodynamically stable alpha-alumina at elevated tcmjxratures beyond 900 C. On the contrary, commercial gamma-alumina membranes are typically calcined at 400-600 C during production and are, therefore, subject to potential structural changes beyond 600°C. Moreover, alumina chemistry reveals that phase transition also occurs beyond that temperature [Wefers and Misra, 1987]. 

Such systems and the mechanism of the adsorbent porous structure formation can be used for purposeful modification of the nature of their surfaces and the value of their sorption capacity. In this case, during calcinating of a sample, the modifying additive can interact with the prevailing component, forming a new crystalline compound with properties differing essentially from those of the initial compounds. This fact opens unrestricted possibility of changing the chemical nature of the solid surfaces and, consequently, their adsorption and catalytic properties. 

Unlike the catalytic reaction discussed above, gas-solid reactions involve the solid particle as well as the gas in the reaction. Typical examples of industrial applications include spent FCC catalyst regeneration, calcination, coal combustion, gasification, and silicon chlorination. Owing to the solid particle involvement in the reaction, significant changes in the chemical compositions and physical properties of the particles occur during the reaction. Particles reduce in size and/ or increase in porosity in some reactions like coal combustion, whereas particles increase in size and/or decrease in porosity in other reactions such as limestone sulfation. As a result, the particle properties vary unlike those particle properties in catalytic reactions. However, as with catalytic reactions, gas-solid reactions take place on the particle surface as gas reactant adsorbs to the surface. 

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