Surface properties calcination temperature

Preparation of MCM-41 materials [1] usually includes a high temperature calcination step that in fact opens mesopores by removing structure directing molecules (template) as well as results in the structure shrinkage due to high temperature condensation of silanol groups [2], Functionalization of mesoporous materials with organosilanes is often employed to synthesize materials of desired surface properties for advanced adsorption and catalytic applications [3] as well as to improve their stability [4]. 

Silica and aluminum phosphate have much in common. They are isoelec-tronic and isostructural, the phase diagrams being nearly identical even down to the transition temperatures. Therefore, aluminum phosphate can replace silica as a support to form an active polymerization catalyst (79,80). However, their catalytic properties are quite different, because on the surface the two supports exhibit quite different chemistries. Hydroxyl groups on A1P04 are more varied (P—OH and A1—OH) and more acidic, and of course the P=0 species has no equivalent on silica. The presence of this third species seems to reduce the hydroxyl population, as can be seen in Fig. 21, so that Cr/AP04 is somewhat more active than Cr/silica at the low calcining temperatures, and it is considerably more active than Cr/alumina. 

Figure 3.5 shows the effect of calcination on the physical properties of HyCOM TiOz s. The crystallite size of anatase and the BET surface area of as-prepared sample were 11 nm and 140 m2g-1, respectively. Upon elevating the calcination temperature, the crystallite size was increased and the surface area was decreased, reflecting crystal growth and sintering of the anatase crystallites upon calcination. It should be noted that even after calcination at 973 K the sample remained in the anatase phase and had a large surface area of 34 m2g-1. The factor of adsorptivity, [Ag+]ads, was also reduced by the calcination (Fig. 3.6) and almost proportional to the BET surface area (Fig. 3.7). This shows that the density (ca-...

Figure 14 gives the Njj lines obtained with five different samples of H-ZSM-5 zeolites [43]. For their preparation, the sodium precursors were first calcined at S00°C in air for 10 hours. Then the calcined samples were converted to ammonium form by repeated ion exchange with 1 M ammonium nitrate solutions. The protonic form was obtained by air calcination at 500°C for 10 hours and the residual sodium content measured by atomic absorption was less than 0.02%. Sample B was produced by acid leaching sample A with a 0.1 N solution of HCl at room temperature. Table 4 gives some of the bulk and surface properties of these samples. 

In this paper, we present as part of a continuing investigation [19,20] a study of the effect of parameters such as initial concentration of the reactants, drying and calcination temperatures, preparation method, as well as doping levels, on the surface properties of ceria doped with 1+ (alkali) or 2+ (alkaline earths) metallic cations. 

The properties of calcined AI2O3 (particle size and shape, specific surface area, reactivity) also depend to a certain degree, particularly at the lower calcination temperatures (up to about 1400 °C), on the type of the initial hydroxide and on the sequence of the structural transformations. Apart from the gel-like form arising by precipitation of aluminium salts, the following four crystalline hydroxides are known ... 

The interaction between titanium sulfate and zirconia influenced the physicochemical properties of prepared catalysts with calcination temperature. The presence of titanium sulfate delays the phase transitions of Zr02 from amorphous to tetragonal and from tetragonal to monoclinic. The specific surface area and acidity of catalysts increase in proportion to the titanium sulfate content up to 5 wt% of Ti(S04)2. The correlation between catalytic activity and acidity holds for both reactions, cumene dealkylation and 2-propanol dehydration, although the acid strength required to catalyze acid reaction is different depending on the type of reactions. 

Table 40 shows the physical properties of Zr/silica as a function of the calcination temperature. Also shown for comparison are values characterizing plain silica, titanated silica, and silica treated with two other metal ions. These samples were made by first drying the silica base, then treating it with a metal alkoxide or chloride, followed by calcination in dry air at the specified temperature. It is evident from both the surface areas and the pore volumes of these samples that silica-zirconia made in this way is quite stable, and more stable than silica-titania. 

A comparative review on the evaluation of the catalysts Al, A2, B1 and B2 reveals that activity and dispersity are two closely related parameters which is influenced by the mode of preparation especially with respect to removal of chloride or to the final calcination temperature. Earlier works had identified benzene hydrogenation on supported platinum catalysts as a facile or structure insensitive reaction(5). A facile reaction may be defined as one for which the specific activity is practically independent of its mode of preparation(6). In other words all surface atoms are believed to be the active sites in a facile reaction without any dependence on the coordination number of site or on the collective properties of the crystallite. 

In some very recent work, Deren et al. (35) studied the surface properties of a series of chromic oxide-impregnated alumina samples as a function of the calcination temperature. They discussed their results in terms of the progressive conversion of Cr + to Cr " " with increasing calcination temperature. 

---