Cracking catalysts alumina

This paper identifies alumina, rare earths, platinum, and magnesia as important SOx capture materials. Alumina is either incorporated directly into the matrix of a cracking catalyst or added as a separate particle. Cerium is shown to promote the capture of SO2 on high alumina cracking catalyst, alumina, and magnesia. Other rare earths are ranked by their effectiveness. The promotional effect of platinum is shown between 1200 and 1400 F for SO2 capture on alumina. Silica, from free silica or silica-alumina in the matrix of cracking catalyst, acts as a poison by migrating to the additive. 

Rare earths have also been included as desirable SOx catalyst components in early patents but the catalytic behavior of cerium, in particular, had not been clarified. This paper has presented evidence that cerium catalyzes the oxidative adsorption of SO2 on high alumina cracking catalyst, alumina, and magnesia. We also have shown the catalytic character of platinum. The details of the catalysis especially by cerium, however, remain unexplained. 

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... 

ALUMRJUMCOMPOUNDS - ALUMINIUMOXIDE(ALUMINA) - ACTIVATED] (Vol 2) Fluid cracking catalysts (FCC)... 

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. 

The first commercial fluidized cracking catalyst was acid-treated natural clay. Later, synthetic. silica-alumina materials containing 10 lo... 

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. 

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. 

This method has been applied (M5) for modeling the vapor-phase rate of dehydration of secondary butyl alcohol to the olefin over a commercial silica-alumina cracking catalyst. Integral reactor data are available at 400, 450, and 500°F. Two models considered for describing this reaction are the single site... 

The desire to have catalysts that were uniform in composition and catalytic performance led to the development of synthetic catalysts. The first synthetic cracking catalyst, consisting of 87% silica (Si02) and 13% alumina (AI2O3), was used in pellet form and used in fixed-bed units in 1940. Catalysts of this composition were ground and sized for use in fluid catalytic cracking units. In 1944, catalysts in the form of beads about 2.5 to 5.0 mm in diameter were introduced and comprised about 90% silica and 10% alumina and were extremely durable. One version of these catalysts contained a minor amount of chromia (Cr203) to act as an oxidation promoter. 

During the period 1940-1962, the cracking catalysts used most widely commercially were the aforementioned acid-leached clays and silica-alumina. The latter was made in two versions low alumina (about 13% AI2O3) and high alumina (about 25% AI2O3) contents. High-alumina-content catalysts showed a higher equilibrium activity level and surface area. 

A variety of material could be used as the basis for cracking catalyst, including synthetic silica-alumina, natural clay, or silica-magnesia. If these materials did not contain significant amounts of metals such as chromium or platinum that catalyzed the burning of carbon, the burning rate of the coke is independent of the base as shown in Fig. 7. 

Cracking catalysts include synthetic and natural sihca-alumina, treated bentonite clay, fuller s earth, aluminum hydrosUicates, and bauxite. These catalysts are in the form of beads, pellets, and powder, and are used in a fixed, moving, or fluidized bed. The catalyst is usually heated and hfted into the reactor area by the incoming oil feed which, in mrn, is immediately vaporized upon contact. Vapors from the reactors pass upward through a cyclone separator which removes most of the entrained catalyst. The vapors then enter the fractionator, where the desired products are removed and heavier fractions are recycled to the reactor. 

Cracking Catalyst Composition. Several workers (20-21) have reported differences among cracking catalysts to remove SOx which correlated qualitatively with alumina content. Our work confirmed these reports as shown on Figure 2. Plotted are %S02 removal... 

Rare Earths and Alumina. A much easier and cheaper way of getting the SO2 removal enhancement from rare earths that was observed with the well-exchanged rare earth Y zeolite was to add rare earths, especially cerium, by direct impregnation to high alumina cracking catalyst (24). 

Independence from cracking catalyst composition has been shown by adding cerium and alumina to cracking catalyst (25). The data on Table I demonstrate this. 

Table I. Effect of Cerium and Alumina Added to Cracking Catalyst...

Results, again, show net gain over the same cracking catalyst used previously for cerium/alumina case. Cerium, moreover, seems to act as a promotor for other rare earths as could be implied from the synergistic effect observed between cerium and lanthanum (27). Our conclusions about the catalytic effect of cerium have been confirmed recently by others (28). 

These materials were made to contain 10 wt% oxides on gamma alumina. The percentage of SO2 removed after 50 minutes was measured, at 1250°F, for these additives at the 1 wt% level mixed with cracking catalysts. They were then ranked by the ratio of the % removed to that removed by cerium on alumina. 

Source of Silica. Silica can migrate either from free silica present in the cracking catalyst or from the silica alumina matrix but not as readily from the zeolite. Figure 11 shows SEM-EDAX silicon scans of cerium/alumina steamed in the presence of these three sources of silica. Again, the bright dots represent silicon. Qualitatively the sample steamed with pure silica contains more silicon than the sample steamed with silica-alumina. The sample steamed with zeolite shows silicon at the surface of the cross-sectioned particle but little in the interior. The surface silicon comes from dusting of the particle with very finely divided zeolite. 

The Mobility of Silica in Steam. The reactivity of silica and silica-containing materials to steam has been assumed in the literature to explain several phenomena, a few of which are the sintering of silica (35), the aging of amorphous silica alumina cracking catalysts (36) and the formation of ultrastable molecular sieves (37). The basis of all these explanations is the interaction of siliceous materials with water to form mobile, low molecular weight silicon compounds by hydrolysis (38) such as ... 

The rates of hydrolysis of siliceous materials will be affected by several factors. For instance, the rate will be directly related to surface area, explaining the low rates observed for silica deposition from the Vycor apparatus. Also, the composition of the siliceous material will Influence the rate of hydrolysis, explaining the differing amounts of silica transferred from pure silica, silica alumina, zeolite, and the high alumina cracking catalyst. 

How to Solve the Deactivation Problem. Solutions to the deactivation problem are difficult. The patent literature (42) has claims that either sodium, manganese or phosphorous added to alumina prevents deactivation by silica. In addition, removal of matrix silica from cracking catalyst formulations should prevent further deactivation because zeolitic silica, as we have shown, migrates more slowly. There is at least one patent relating to very high alumina matrix cracking catalysts (43). Another solution is to use more active SOx catalysts such as magnesia-based materials. 

Steam Stability. Steam stability of SOx removal agents is strongly affected by temperature. We have seen previously that at 1350 F deactivation of cerium/alumina additive, caused by silica poisoning, was influenced by how long the additive was steamed and whether the additive was steamed in the presence or absence of cracking catalyst. These results were extended to other temperatures. 

Two sets of experiments were made to show the effect of steaming temperature on stability. In the first set, steaming was done non-interactively. Cerium/alumina additive was steamed (100% steam, 1 atm) for 5 hours in a fixed bed from 1200 to 1450 F. SO2 removal ability was then measured on these steamed samples diluted with cracking catalyst. The data in Figure 14 show that, for steamings done separate from cracking catalyst, losses of SO2 removal ability are small but become more pronounced above 1350 F. 

Losses incurred in the non-interactive steamings, however, were lower than those found in the second set of experiments where the cerium/alumina additive was steamed together with a low alumina cracking catalyst at various temperatures. The results from this second set of experiments, shown in Figure 14, indicate that losses are important at temperatures above 1200 F. It should be noted that SO2 removal ability was measured under the same conditions in both sets of experiments. Also, these fixed bed steaming seem to be harsher than fluidized bed steamings because the losses incurred are greater. 

Oxidative Adsorption of Sp2 Oxidative adsorption of SO2 is also a strong function of temperature as shown on Figure 15. Plotted is the amount of SO2 removed after 92 minutes from room temperature to 1500 F. The material used for these experiments was a rare earth stabilized Rhone-Poulenc alumina which was tested without dilution with cracking catalyst. A fresh charge of alumina was used at each temperature. 

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