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Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable.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

In contrast with these results, catalytic cracking yields a much higher percentage of branched hydrocarbons. For example, the catalytic cracking of cetane yields 50-60 mol of isobutane and isobutylene per 100 mol of paraffin cracked. Alkenes crack more easily in catalytic cracking than do saturated hydrocarbons. Saturated hydrocarbons tend to crack near the center of the chain. Rapid carbon-carbon double-bond migration, hydrogen transfer to trisubstituted olefinic bonds, and extensive isomerization are characteristic.52 These features are in accord with a carbo-cationic mechanism initiated by hydride abstraction.43,55-62 Hydride is abstracted by the acidic centers of the silica-alumina catalysts or by already formed carbocations ... 

In the above diagram, H-D refers to hydrogenation-dehydrogenation centers and A to acidic centers on the catalyst. The reaction sequence involves successive ring contraction and expansion steps, similar to the mechanism proposed by Pines and Shaw (P4) to account for transfer of tagged carbon from the side chain to the ring when ethylcyclohexane was contacted with a nickel-silica-alumina catalyst. 

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,... 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

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 question as to whether the acid properties of cracking catalysts are due to protons (Bronsted acids) or to electron-deficient atoms (Lewis acids) is somewhat more difficult to answer. Before considering this question, the origin of acid centers in silica-alumina compositions should be discussed. It is generally believed that acid centers, either the... 

It is now generally agreed, with some exceptions, that the cracking of hydrocarbons over silica-alumina and related catalysts involves a cationic mechanism in which the acidity of the catalyst plays a decisive role. There is some disagreement among workers in this field concerning the actual part played by the acid centers of the catalyst in the initiating step of the mechanism. 

The actual structure of the acid centers of silica-alumina and of related catalysts is still largely unknown and in a speculative state. Much work remains to be done to clarify this situation, and in particular the effect of temperature on the electron affinity or acid strength of these centers remains to be investigated. Needless to say, these are very difficult problems to attack, but until some reasonable solution to them is obtained there can be little further clarification of existing controversies. 

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... 

A 2000 study from Buffon [69] examined the reaction of Schrock-type, alkoxy-Mo-alkylidenes with the surface OH groups of silica, silica-alumina, and niobium oxide. The specific mode of attachment of the Mo-complex was found to be highly dependent upon the acid-base properties of the support. For silica, it appears to be a Lewis acid-base interaction between the Mo center and surface silanols, whereas in the case of silica-alumina, the attachment appears to occur via the addition of a surface OH group across the Mo-imido bond. In the case of niobium oxide, it is possible that the OH adds across the Mo-alkylidene, deactivating the complex, as the resulting material was completely inactive for metathesis. The activities of both immobilized catalysts were less than the parent homogeneous... 

Si02 —AI2O3 Silica-alumina is a representative acidic binary oxide which has been extensively studied. The concept of the solid acid catalyst was established through studies on silica-alumina. Cumulative studies have corroborated that the acidic centers on the solid surfaces act as catalytically active sites. Among the many reasons establishing the concept of solid acids, the following four are of primary importance. 

The choice of an appropriate support is of no less importance than that of active phase of a catalyst. We have focused our attention on the application of hydrophobic supports to prepare effective platinum catalysts for hydrosilylation since our preliminary experiments have shown that in a number of hydrosilylation reactions hydrophobic material-supported catalysts appeared to be superior to those based on hydrophilic supports such as alumina and silica. We have also aimed at selecting such supports which, in addition to their hydrophobicity, do not have acid centers on their surfaces, and due to this, they do not catalyze undesirable side reactions of isomerization. The supports selected for our study were styrene-divinylbenzene copolymer (SDB) and fluorinated carbon (FC), because nonfunctionalized SDB is free of acid sites and surface acidity of FC is extremely weak (H 9). The performance of SDB- and FC-supported platinum catalysts was studied in several reactions of hydrosilylation. 

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. 

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