Silica alumina catalysts active centers

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable.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

A choice remained between Mechanisms A-l and A-2 which requires that the active centers be either Lewis or Bronsted acid, respectively. Since A-2 would lead to the formation of molecular hydrogen, an attempt was made to detect and measure any hydrogen evolution concomittant with the chemisorption of triphenylmethane. Triphenyl-methane was chemisorbed on 28 gm of Houdry S-65 synthetic silica-alumina catalyst in a sealed, evacuated apparatus. When the chemisorption was completed the gas phase, collected using a Sprengle pump,... 

Cumene (isopropylbenzene) cracking by porous silica-alumina catalyst has been studied extensively. This includes studies with respect to coke production (I, M), the maximum depth of active centers (3), kinetics 4), and the effect of diffusion transport phenomena on the kinetics (5). 

At the start of the catalytic decomposition of methanol, the active surface constitutes 25-28% of the total surface for both catalysts studied. This is the case although the total surface area of the alumina is only half that of the silica-alumina catalyst. This would appear to support the idea that the active centers on which the decomposition of methanol takes place are qualitatively identical for the two materials. A more detailed discussion of this hypothesis will be undertaken later, when we shall consider the kinetics of dehydration of alcohol and ether on aluminum oxide and silica-alumina. 

We present below the results of our investigation of the kinetics of transformation of ethanol and diethyl ether on pure aluminum oxide and on silica-alumina catalysts of various compositions. These established some important facts concerning the nature of active centers and of the mechanism of this reaction. 

This is the same case with which in Eqs. (2)-(4) we demonstrated the elimination of the time variable, and it may occur in practice when all the reactions of the system are taking place on the same number of identical active centers. Wei and Prater and their co-workers applied this method with success to the treatment of experimental data on the reversible isomerization reactions of n-butenes and xylenes on alumina or on silica-alumina, proceeding according to a triangular network (28, 31). The problems of more complicated catalytic kinetics were treated by Smith and Prater (32) who demonstrated the difficulties arising in an attempt at a complete solution of the kinetics of the cyclohexane-cyclohexene-benzene interconversion on Pt/Al203 catalyst, including adsorption-desorption steps. 

The main components of FCC catalysts are Zeolite Y, e.g., REY orUSY as the major active component (10 to 50%), and a binder that is typically an amorphous alumina, silica-alumina, or clay material. In addition to these main components, other zeolite components, e.g., ZSM-5, and other oxide or salt components are quite frequently used additives in the various FCC catalysts available on the market. The addition of 1 to 5% ZSM-5 increases the octane number of the gasoline. ZSM-5 eliminates feed compounds with low octane numbers because it preferentially center-cracks n-paraffins producing butene and propene [14], These short-chain olefins are then used as alkylation feedstocks... 

In all the isomerization reactions carried out in heterogeneous conditions, the nature of the products and product ratio depended largely on the type of catalyst employed, and, moreover, in most of the cases no selectivity was found. Papers have recently appeared concerning the transformation of styrene oxide into phenyl acetaldehyde catalyzed by a series of natural silicates and amorphous silica-alumina (ref. 15) and by pentasil type zeolites (ref. 16). It is said that, in both cases, isomerization occurs on the acidic sites (si lands) of the external surface, which act as active centers even under mild experimental conditions. 

The 27A1 and 29Si NMR measurements (7) showed that after treatment with 0.01 molar HC1 most of the amorphous silica-containing material is removed from the parent catalyst A. This can be understood easily since the maximum solubility of silica (16) is reached at pH = 2. Although the improved performance of the treated catalyst cannot be entirely explained by the removal of less active material, i.e. the increase of the number of Lewis acid sites per mass unit, it is believed that these silica species block most of the catalytically active centers, i.e. the highly dispersed Lewis acidic alumina sites in the micro- and mesopores of the parent US-Y zeolite. 

The different performances of catalysts (A) and (B) was elucidated by characterization of these materials [29]. Al and Si MAS NMR showed that after treatment with 0.01 M HCl most of the amorphous silica material is removed from the parent catalyst (A), leaving extra-framework aluminum species also created by the steaming procedure [29]. This can be readily understood, because the solubility of silica is maximum at pH 2 [34]. It is believed that the silica species blocked most of the catalytically active centers, i. e. the highly dispersed Lewis acidic, extra-framework alumina sites, which seem to be partly bonded to the zeo-litic framework of the starting material (A). The EFA species are not, therefore, leached out. 

Further evidence that the active centers on silica alumina-are Lewis rather than protonic acids was provided by the spectral response to catalyst pretreatment shown in Fig. 28. Here curves A, B, and C represent the spectra of p-phenylenediamine chemisorbed on silica-alumina samples which were heated at increasing temperatures. The regular increase in the intensity of the 4680 A band, due to the cation radical, was taken as spectroscopic evidence for an increasing number of Lewis-acid sites. Although these spectra are in qualitative agreement with the known effect of thermal treatment on the relative abundance of Lewis and Bronsted acidity, quantitative conclusions cannot be drawn since the concurrent increase in intensity at 3240 A indicates that these measurements were not made under conditions of constant surface coverage. 

That the observed spectrum was the result of a chemical reaction between the hydrocarbon and the catalytically active centers of the silica-alumina surface (chemisorption), and not due to a general sur-fatochromic spectral shift, was demonstrated from the spectrum of this compound adsorbed on a nonacidic or very weakly acidic silica gel (29). The spectrum (Fig. 30, Curve B) of silica gel exposed to triphenylmethane vapor for 1000 hours at 100°C was identical to the spectrum (Curve A) of an alcoholic solution of this hydrocarbon. The close agreement between these spectra suggested that on silica gel the triphenylmethane was physisorbed. This was further evidenced by the marked loss of spectral intensity (Curve C) attendant to a four hour evacuation at 100°C. In contrast, on silica-alumina where the hydrocarbon was chemisorbed as the carbonium ion, no decrease in absorbance was noted even after 48 hr evacuation at 275°C. These data constituted the first direct demonstration of the formation of carbonium ions as a consequence of chemisorption of a tertiary hydrocarbon on the surface of a cracking catalyst by a reaction involving the rupture of an aliphatic C-H bond. The generality of this process of carbonium ion... 

On the basis of the indicated data, the active centers on the silica-alumina cracking catalyst can be represented in varying degrees of hydration by... 

We have already mentioned the existence of a chemical interaction of alcohols with a catalyst and the actual possibility of the formation of an intermediate compound on the surface of oxide catalysts. Numerous investigations by Soviet Scientists leave no doubt that the adsorption of alcohol on aluminum oxide and silica-alumina is accompanied by the formation of a surface compound of the ether type. In both cases, due to the similarity of their active centers which are surface hydroxyl groups connected to aluminum atoms /Al-OH, such a surface compound will ob-... 

The nature of the active centers on alumina catalysts is still a subject of discussion although pertinent new experimental information is becoming available in the literature. Peri has reported from infrared studies that butene adsorbed on alumina (11) and silica-alumina (12) remains olefinic in character and neither hydroxyl groups on alumina... 

---