Cracking catalysts silica types

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... 

Villet and Wilhelm Ind. Eng. Chem., 53 (837), 1961] have studied the Knudsen diffusion of hydrogen in porous silica-alumina cracking catalyst pellets. They used apparatus of the type depicted in Figure 12P.1. 

Also other Type B and C series from Table II are consistent with the above elimination mechanisms. The dehydration rate of the alcohols ROH on an acid clay (series 16) increased with the calculated inductive effect of the group R. For the dehydrochlorination of polychloroethanes on basic catalysts (series 20), the rate could be correlated with a quantum-chemical reactivity index, namely the delocalizability of the hydrogen atoms by a nucleophilic attack similar indices for a radical or electrophilic attack on the chlorine atoms did not fit the data. The rates of alkylbenzene cracking on silica-alumina catalysts have been correlated with the enthalpies of formation of the corresponding alkylcarbonium ions (series 24). Similar correlations have been obtained for the dehydrosulfidation of alkanethiols and dialkyl sulfides on silica-alumina (series 36 and 37) in these cases, correlation by the Taft equation is also possible. The rate of cracking of 1,1-diarylethanes increased with the increasing basicity of the reactants (series 33). 

To explain the observed selectivity effect, consider the type of mechanism proposed by Matsumoto et al (9, 10) for reactions of o-ethyltoluene with H+A102 of amorphous silica-alumina cracking catalysts. 

The spectrum of ammonia chemisorbed on a silica-alumina cracking catalyst was studied to determine whether the acidity of these catalysts is due to a Lewis (nonprotonic) or a Bronsted type of acid (28, 29). This work was based on the premise that ammonia chemisorbed on Lewis sites would retain a NH3 configuration while ammonia chemisorbed on a Bronsted site would form NHt. The NH3 configuration was expected to have bands near 3.0 and 6.1 p and the NHt near 3.2 and 7.0 p. 

The catalyst used in these studies was a commercial type of bead silica-alumina cracking catalyst (Socony Mobil Oil Company, Inc.) containing 10% alumina by weight and having originally a bead diameter between 2 and 5 mm. This catalyst had a surface area of 350 m.2/g. and a CAT-A activity of 42 A.I. 13). 

The introduction of zeolites in cracking catalysts combined with various non-zeolite matrix types (a.o. higher stability silica-alumina types) certainly complicates the picture of FCC hydrothermal deactivation. Letzsch et al [7] have shown that like amorphous catalysts the zeolite is more strongly deactivated hydrothermally than purely thermally. 

Nickel and vanadium are contained within the crude oil as their respective porphyrins and napthenates (2). As these large molecules are cracked, the metals are deposited on the catalyst. Nickel which possesses a high intrinsic dehydrogenation and hydrogenolysis activity drastically increases the production of coke and dry gas (particularly H2) at the expense of gasoline. Vanadium on the other hand interacts with the zeolitic component of a cracking catalyst and leads to destruction of its crystallinity. This results in reduced activity as well as an increase in non-selective amorphous silica-alumina type cracking. Supported vanadium also has an intrinsic... 

A comparison of the effects of heat and steam on the activity and selectivity of three types of commercial cracking catalysts is illustrated in Table X. Silica-alumina is the most active initially and has the greatest heat stability in the absence of steam, but it is the most susceptible to... 

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. 

Silica gel cracking catalysts have also been studied. These materials are amorphous and yield no powder diagrams but they do give a very maiked small angle scatter. If the particle distributions obtained from small angle scatter are used to calculate surface areas and these areas are related to the activity, fairly reasonable correlations result. Certain complications concerning these relationships are introduced by various types of treatments applied to the catalysts and by the iron content of commercially deactivated samples. 

This type of analysis was employed by Weisz and Goodwin [P.B. Weisz and R.D Goodwin, J. Catal, 2, 397 (1963) 6, 425 (1966)] in the study of the regeneration of coked silica/alumina cracking catalysts. From equation (7-103) we can see that... 

R. H. Villet and R. H. Wilhelm [Ind. Eng. Chem., S3, 837 (1961)] studied the Knudsen diffusion of hydrogen in porous silica-alumina cracking catalyst pellets. They used an apparatus of the type depicted in Figure P12.2. The entire apparatus was immersed in a constant temperature bath at 25°C. The upstream hydrogen pressure was maintained constant at 770 torr. The pressure in the constant volume container (500 cm ) varied with time in the following manner ... 

This was soon supplanted by a more powerful spectroscopic method, wherein the IR behavior of chemisorbed amines onto acidic surfaces was found to distinguish the relative amounts of Lewis and Bronsted sites. The first report, for ammonia on a silica-alumina cracking catalyst, occurred in 1954 (61). Bands for NH3 and NH4+ were observed upon addition of water, the NH4 bands increased at the expense of NH3 bands. These results indicated two types of acid sites (1) NH3 chemisorbed by coordinate bond formation between the Lewis base (NH3) and a Lewis acid site, and (2) transfer of a proton from a Bronsted site to the base forming NH4" bound via coulombic forces. 

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