Silica-alumina catalyst structure

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable.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. 

Zeolitic Catalyst—Since the early 1960s. modern cracking catalysts contain a silica-alumina crystalline structured material called zeolite. This zeolite is commonly called a molecular sieve. The admixture of a molecular sieve in with the base clay matrix imparts desirable cracking selectivities. 

The activation energies for coke gasification on the three substrates shown in Figure 4 were 33 Kcal/mole for the three alumina-based materials and 54 Kcal/mole, for the silica-alumina catalysts. The increased activity and lower activation energy for the coke deposited on the aluminas (compared to that on the silica-aluminas) cannot be due to a direct catalytic effect of alumina on the gasification reaction, but rather to an indirect effect of the alumina that controls the nature and structure (surface area and structural disorder) of the coke during its deposition. 

Bronsted or the Lewis type, owe their existence in silica-alumina catalysts to an isomorphous substitution of trivalent aluminum for tetravalent silicon in the silica lattice (Hansford, 42 Thomas, 18 Tamele, 35). Such an isomorphous substitution would lead to a structure somewhat as follows ... 

According to Milliken et al., the aluminum ions at the interface of 7-alumina and silica micelles are believed to be in a three-coordinated structure corresponding to the anhydride of the acid, HA102. An anhydride of this structure is a potential acid of the Lewis type (see above example of Lewis acid structure), and this is believed to be the only type of acid which is present in the silica-alumina catalyst at cracking temperatures (500°C.). 

Plank (46) has suggested an alternative explanation for the acidity of silica-alumina catalysts. From a study of the differences between silica and silica-alumina gels, he concluded that alumina always becomes a terminal group in the micelle structure, and that therefore isomorphous substitution of silicon with aluminum does not occur. According to his hypothesis, the aluminum ions in the terminal alumina groups are coordinated with hydroxyl and water in such a way as to retain their normal octahedral coordination ... 

The area-temperature curves and the isotherms make clear the accelerated sintering produced by steam and the profound reorganization of pore structure effected in the case of silica-alumina catalysts. This reorganization or intimate interaction between steam or adsorbed water and the catalyst is not too surprising when it is considered that the catalysts are prepared in aqueous media, that surface water is necessary for activity (Hansford, 26) and that steam deactivates catalysts. This... 

I have not the time to say here what I learned and will limit myself to telling you a conclusion I reached, which is another foundation stone of my conception of catalysis. The thought occurred to all of us that we should endeavor to learn the structure of the silica-alumina catalysts we were using, and a project was started to this end. After many years of study by the methods we thought were the best, I recommended that further expenditures be abandoned until someone could provide an apparatus which would enable us to see not only the catalyst molecules but the way the atoms were arranged in the molecules, for I thought that catalytic reactions were made between atoms of the catalyst and the reactants. 

Synthetic silica-alumina catalysts containing 25% alumina are converted to gamma-alumina and mullite when thermally treated at 700-1260°. The phase transformation is accompanied by loss of catalytic activity and by collapse of the porous structure of the catalyst. The gamma-alumina phase is formed apparently by crystallization of the amorphous alumina in the catalyst, while the mullite formation apparently results from a combined silica-alumina amorphous phase. At sufficiently high temperature all alumina is converted to mullite. Silica-alumina catalysts made from more stable silicas have a greater tendency to form gamma-alumina. Such catalysts have lower initial catalytic activity and maintain relatively high catalytic activity after steam deactivation. 

Several investigators ) have shown that the thermal deactivation of synthetic silica-alumina catalysts is accompanied by a collapse of the porous structure. Previous x-ray diffraction studies of catalysts containing about 13 % alumina have shown that crystalline phases appear at temperatures of 1150° or above 2). Only in the case of catalysts containing 60 % or more alumina have crystalline phases been reported in the 800-1100° temperature range, where the thermal collapse and loss of catalytic activity occur... 

R. C. Hansford Union Oil Co. of California, Brea, Calif.) Professor Danforth s model for the structure of silica-alumina catalysts is certainly an interesting and novel one. The chemistry from which it is derived is equally interesting and novel. He is to be congratulated for this beautiful work, which has added to our understanding of these important catalysts. 

Rase and Kirk 24) have compared the adsorption coefficients of a series of alkylbenzenes calculated for cracking of these hydrocarbons on a silica-alumina catalyst from Eq. (1) with the bond strength of structurally related alkanes and have obtained a linear relation. The correlation coefficient is again high, 0.98. As has been shown in Table I, the corresponding rate constants can be correlated by the Taft equation. However, the plot of log Kj vs a gives a curve. 

On reviewing these data it is apparent that the most probable structure of calcined silica-alumina catalysts is a mixture of silica and alumina particles with the silicon and aluminum ions sharing oxygen ions at the points of contact. If this structure is actually present, the chemical properties of alumina in its various crystal forms will be the main controlling factor of the behavior of the mixed oxide system. The crystal habits of silica can be expected to be of merely secondary importance in determining the nature of the catalyst. 

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