Silica-alumina cracking catalyst, structure

C. Vasile, P. Onu, V. Barboiu, M. Sabliovschi, G. Moroi, Catalytic decomposition of polyolefins. II. Considerations about the composition and the structure of reaction products and the reaction mechanism on silica-alumina cracking catalyst. Acta Polym. 36, 543 (1985). 

The somewhat random distribution of atoms required in the amorphous structure of the silica-alumina cracking catalyst makes it unreasonable to suggest a single exact distance between the aluminum atoms in the active sites along the edges of the catalyst ribbon. Furthermore, catalysts formed by impregnation of silica gel with alumina may be represented as forming by the condensation of aluminum hydroxyls with hydroxyls on adjacent... 

The uptake of ammonia by chemisorption has been reported by Tamele (9b) to differentiate between catalytically active and inactive oxides of similar physical structure. Figure 6 shows that the active silica-alumina cracking catalyst retained a larger amount of ammonia than silica gel. 

Schlaffer WG, Morgan CZ, Wilson JN Aging of silica-alumina cracking catalyst. I. Kinetics of structural changes by heat and steam,/ Phys Chem 61(6) 714—722, 1957. 

A preliminary overall picture of cracking catalyst structures is available in the first three horizontal rows of the composite plot of Fig. 2 and the corresponding data of Table I. Isotherms presented in the lowest row are discussed in Sec. IV. Only the general features of these representative types of cracking catalysts are indicated here, since the detailed plots of individual isotherms will be considered in subsequent sections on sintering. Cracking catalysts of principal interest are represented by three types silica-magnesia silica-alumina and activated clay. 

Even when it does not contain any RE ions, the Y zeolite is always responsible for a drop in octane number compared to the old amorphous silica-alumina-based catalysts. In order to gain a few points in the octane number, many refiners add to the principal catalyst a small percentage of a ZSM-5-based additive that has pores 0.55 nm in diameter that can only be penetrated by linear aliphatic structures and, to a lesser degree, by monobranched aliphatic structures (Tables 1 and 2). These hydrocarbons, which are those with the lowest octane number, are mainly cracked to olefin-rich LPG, obviously at the expense of a few percentage points in gasoline yield. 

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. 

Kaolin had little or no cracking activity, and catalyst activity as tested in the laboratory was directly related to silica-alumina gel content. However, the catalyst performed much better in commercial tests than anticipated from laboratory testing. Undoubtedly, this open structure encountered much less severe conditions at the outer surface of the microsphere during regenerations and made internal catalytic surfaces more readily available. This first of the so-called "semisynthetics" was called Nalco 783, and the matrix is still used in many forms some 28 years later.(7,13) Today it is estimated that some 200,000 tons/yr. of kaolin clay is used for cracking catalyst manufacture as reported by Georgia Kaolin Corporation.(24) Figure 10 shows the pore volume distribution for Nalco 783 and two other commercial semisynthetics from that period. 

The activity advantage of zeolite catalysts over amorphous silica-alumina has well been documented, Weisz and his associates [1] reported that faujasite Y zeolite showed 10 to 10 times greater activity for the cracking of n-hexane than silica-alumina. Wang and Lunsford et al. [2] also noted that acidic Y zeolites were active for the disproportionation of toluene while silica-alumina was inactive. The activity difference between zeolite and silica-alumina has been attributed to their acidic properties. It is, however, difficult to explain the superactivity of zeolite relative to silica-alumina on the basis of acidity, since the number of acid sites of Y-type zeolite is only about 10 times larger than that of silica-alumina. To account for it, Wang et al. [2] proposed that the microporous structure of zeolite enhanced the concentration of reactant molecules at the acid sites. The purpose of the present work is to show that such a microporous effect is valid for pillared clay catalysts. 

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. 

Zeolites are crystalline aluminosilicates that have exhibited catalytic activities ranging from one to four orders of magnitude greater than amorphous aluminosilicates for reactions involving carbonium ion mechanisms such as catalytic cracking (144). As a result extensive efforts have been undertaken to understand the nature of the catalytic sites that are responsible for the observed high activity. The crystalline nature of zeolites permits more definite characterization of the catalyst than is possible for amorphous acidic supports such as alumina and silica-alumina. Spectral techniques, in conjunction with structural information derived from X-ray diffraction studies, have led to at least a partial understanding of the nature of the acidic sites in the zeolite framework. 

Oborin (270,271) examined the suitability of alumina-containing catalysts including silica-alumina for the reactions involved in cracking of hydrocarbons and found that the activity of the catalyst could be explained on the basis of structure analysis evidence obtained by him which supported the view that a mixture of silica and alumina rather than an aluminum silicate was present. He also found that the distance between two aluminum atoms was approximately 2.56 A., which is close to the distance between alternate carbon atoms in a hydrocarbon, thus... 

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