As cracking catalysts

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... 

Over the years, thousands of compounds have been tried as cracking catalysts. These compounds fall into two general categories natural and synthetic. Natural catalyst, as the name denotes, is a naturally occurring clay that is given relatively mild treating and screening before use. The synthetic catalysts are of more importance because of their widespread use. Of the synthetic catalysts, two main types are amorphous and zeolitic. 

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. 

This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

The acidic properties of alumina pillared clays have been extensively studied from the interest in using the pillared clays as cracking catalysts [21-24]. Sakurai et al. [25] studied the acidic properties of the alumina pillared clays with different kinds of silicate layers and concluded that the alumina pillars between the silicate layers did not have any acidity and that the role played by the pillars was only to make the original acidity of the silicate interlayers more easily accessible through opening the interlayer spaces. 

Thus, we have seen that the surface of a solid such as silica-alumina has acidic properties. Similar considerations also apply to alumina, although alumina alone is appreciably less acidic than the silica-alumina compositions used as cracking catalysts. Treatment of alumina with... 

In the present work, samples of SAPO-37 with different silicon content have been synthesized, and studied from the point of view of their stability, physicochemical and texture characteristics. Finally their behavior as cracking catalysts has allowed us to formulate a hypothesis on the parameters which control activity and selectivity in SAPOS. 

Besides the Bronsted acidic sites created by the above explained methodology, in PILCs that are ion exchanged with multivalent cations and are partially dehydrated, Lewis acidic sites are produced [132], The main interest in developing acid PILCs was due to their potential applications as cracking catalysts. Certainly the prospect of making PILCs in which the big gaseous oil molecules can diffuse and meet the active acidic sites was a motivation for the development of these catalysts [122],... 

The catalytic activity of amorphous silica-alumina ([Si—Al]) in reactions via carbonium ions is due to the existence of Bronsted acid sites on their surface. Consequently, amorphous [Si-Al] acid catalysts provide acid sites and transport to the active sites easily. As a result, amorphous [Si-Al] acid catalysts have been widely operated as cracking catalysts. Acid zeolites have been successfully applied as cracking catalysts. However, in some industrial applications of acid catalysts, for example, in the cracking of hydrocarbons of high molecular weight, zeolites are not useful, since... 

Group A represents the best powders. They are fine and aeratable, such as cracking catalyst. They fluidize nicely, and expand particulately after reaching the point of incipient fluidization mf, until the first bubbles appear at a higher velocity, the minimum bubbling velocity umb. It is thus evident that the ratio of umJum is always greater than unity, and the greater is this value, the better the powder performs in fluidization. 

Probably the biggest use in terms of quantity is that of lanthanide-exchanged zeohtes as cracking catalysts in the petrochemical industry (mainly La and Ce), using about 5001 a day in the USA alone. 

At this point, the most promising types of pillared clays for use as cracking catalysts are the pillared rectorite developed by Jie et al. (3) and the large pore Ce/Al-pillared smectites developed by McCauley W. Further studies will have to be performed in order to determine the feasibility of these to types of materials for this application. 

Acid-treated clay minerals were employed as cracking catalysts in the first commercial process, the Houdry process, widely used in the early petroleum industries to produce high-octane gasoline. The Houdry process catalysts had been discussed extensively by many investigators (2) but were eventually completely replaced by synthetic silica-alumina or zeolite catalysts. Recently, the need for new catalytic materials has revived special interest in the layer lattice silicates because of their ion-exchange properties and their expandable layer structures. 

Clays have been used as cracking catalysts particularly for heavy feedstocks and have also been explored in the demetallization and upgrading of heavy crude oil. The results indicated that the prepared catalyst... 

Until 1960s, proton exchanged or acid treated clays were used as cracking catalysts. They have also proved to be good catalysts for many industrially important reactions like alkylation [1], dimerization and polymerization of unsaturated hydrocarbons [2], Diels Alder condensation [3] etc. Moreover acid treated clays are commonly used commercially for decolourising oils [4] and also in colour formation with leuco dyes in pressure sensitive recording paper [5]. 

The pillaring of clays has become an established technique of preparing a new class of porous materials. While their promise as cracking catalysts has not been fulfilled, never-the-less they exhibit interesting catalytic behavior. We have extended the technique to the pillaring of many other types of non-clay layered compoimds which include phosphates, oxides and layered double hydroxides. This extension broadens the field to the point where one can choose the degree of acidity or basicity and framework metals required for specific catalytic processes. 

The catalytic activity of certain activated clays toward hydrocarbons was observed as early as 1912, when Gurwitsch (5) in his studies of the adsorption of olefins on activated clays reported that polymerization occurred. Herbst (6) in 1926 observed that the decomposition of hydrocarbons is accelerated by kieselguhr at moderately elevated temperatures, and Kobayashi and Yamamota (7) obtained similar results with Japanese acid clays. Several patents (8) covering the use of floridin, pumice, and hydrosilicates of aluminum as cracking catalysts were issued in the period 1923-1932. 

For a variety of reasons, silica and alumina structures are important frameworks for the subject of heterogeneous catalysis 11-3]. Aluminas, and perhaps to a lesser extent silicas, are employed directly as cracking catalysts or as substrates for assorted catalytic systems (see Chapter 4). The silylation of silica surfaces also provides a strategy for immobilizing a catalytic center that has been found useful in the context of homogeneous catalysis [4], Furthermore, many heterogeneous catalytic systems based on zeolites, clays, or silica-aluminas have aluminosilicate frameworks for which silica and alumina structures serve as structural prototypes. 

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