Silica-alumina catalysts, active sites

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Zeolites as cracking catalysts are characterized hy higher activity and better selectivity toward middle distillates than amorphous silica-alumina catalysts. This is attrihuted to a greater acid sites density and a higher adsorption power for the reactants on the catalyst surface. 

The rate of reaction of propylene over the MeReOs/HMDS/silica-alumina catalyst (1.4 wt% Re) is shown in Figure 2b. The profile is similar to that of the Sn-promoted perrhenate catalyst, with kobs = (1-78 + 0.09) x 10" s, and the activity responds similarly to subsequent additions of propylene. In fact, the pseudo-first-order rate constant for the organometallic catalyst lies on the same line as the rate constants for the Sn-promoted perrhenate catalyst. Figure 3. Therefore we infer that the same active site is generated in both organometallic and promoted inorganic catalyst systems. 

Aluminosilicates are the active components of amorphous silica—alumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica—alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according to their strength have been found, e.g. by titration with f-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the H0 scale [38],... 

To study the interaction of adsorbed molecules with active sites in decationized zeolites we used optical electronic spectroscopy, which was successful (17-19) with silica-alumina catalysts. The results (17-19) were then extrapolated to zeolites 20-21). 

Andreu et ah (11) explained the increased activity (with increasing alumina content of amorphous silica-alumina catalysts) for cracking of sec-butylbenzene by the greater density of acid sites in the high-alumina-content catalysts. Adams et ah (12) proposed that the interaction of several active sites with reactant molecules in mordenite catalysts was partly responsible for the rapid rate of activity loss. 

The isomerization of n-butenes was used as a test reaction to follow the development of catalytic activity in Na-Y zeolite. The acidity was varied by the substitution of some of the Na " (0.3 to 5.7%) with Ca and by creating a Na deficiency (up to 0.94%) both series of catalysts were studied with and without water added as cocatalyst. A pure Na-Y zeolite containing no decationated sites was found cat-alytically inactive for this reaction. In contrast with silica-alumina catalysts, carbonaceous residues did not appear to play a role in the formation of the catalytic sites as long as H2O was used as cocatalyst. 

The carbonaceous material which was retained by the catalysts after evacuation was held in 2 forms 1 could be recovered as butene molecules (12) by exchange with isotopically labelled 1-butene the other could be removed only by combustion to CO2. With silica-alumina catalysts, the latter (residues) is thought to form the active sites for car-bonium ion activity (3,8,9). The present results showed that the activity correlated with the degree of decationation or the Ca content of the catalyst. Moreover, the amount of residue retained by the catalyst (nonexchangeable ) was about 2 orders of magnitude smaller than the number of decationated sites of the sample (12). It seems probable, therefore, that residues do not play an important role in the development of catalytic activity of these materials in the presence of H2O however, residues may supply the necessary protons for catalytic activity in its absence. 

In the silica s, the silica layers are not expected to exhibit any significant catalytic acid activity except for weak activity due to the presence of silanol groups. Pillaring with the Keggin aluminum hydroxyoxide species is expected to induce not only expansion of the gallery height but also allow for the evaluation of the catalytic activity inherent in this propant. A conq>arison between these porous silica-alumina catalysts and the crystalline zeohtes indicates inq)rovements are yet to be made in the acid site/aluminum ratios. At the present time these porous sihca/alumina s exhibit similar acitivy ratios of acid sites/aluminum as found in their amoiphous counterpart. The catalytic results are summarized in Figure 5. 

Nature of the Active Sites on Silica-Alumina Catalysts... 

There are other types of active sites. In 1955 Kobozev (17) pointed out that sometimes—especially with certain metals—the catalytic properties could be due to the properties of atoms, but that in other cases—as with some metal oxides—the catalytic property might be associated with the entire crystal We showed later (j ) that in at least one case, silica-alumina catalyst, it is very easy to err concerning the nature of the site Thus, evidence for Bronsted acidity could be interpreted as evidence for the presence of aluminum which can be ion-exchanged when the catalyst is placed in salt solution Looking at all these examples, we conclude that that there are many different sources of activity on the solid catalyst surface. 

Acidity has been ascribed to various combinations of Bronsted, Lewis or defect-type sites. Some people emphasize ordering as a critical property, but on the other hand, a good case can also be made for the importance of lattice defects in providing acidic hydroxyl groups. It can be argued that silica-alumina was active because of partial ordering of a disordered structure, while zeolite catalysts were active because of partial disordering of an ordered structure 41). 

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