Natural Clay Catalysts

Degradation of polystyrene using natural clay catalysts... 

Both the natural clay and the synthetic types of catalyst undergo normal aging. Abnormal aging, due to sulfur compounds, has been found only with natural clay catalyst the synthetic types are stable under similar conditions. Natural clay catalysts can be protected against abnormal aging from sulfur compounds by hydrating with steam after regeneration. Certain types of iron-free clays and bentonitic clays treated to remove iron do not show sulfur deactivation. 

Natural clay catalysts were replaced by amorphous synthetic silica-alumina catalysts5,11 prepared by coprecipitation of orthosilicic acid and aluminum hydroxide. After calcining, the final active catalyst contained 10-15% alumina and 85-90% silica. Alumina content was later increased to 25%. Active catalysts are obtained only from the partially dehydrated mixtures of the hydroxides. Silica-magnesia was applied in industry, too. 

The early type of catalytic cracking units involved the use of a fixed-bed operation and this type of processing has been largely supplanted by the fluid- and moving-bed types of operation. The catalysts are used in the form of powder, microspheres, spheres, and other preformed shapes. The catalysts employed are either synthetic silica-alumina composites or natural aluminosilicates. Other catalysts, such as silica-magnesia, alumina-boria, silica-zirconia, and silica-alumina-zirconia have found limited commercial application and, at present, the synthetic silica-alumina and natural clay catalysts dominate the field. 

The first products resulting from this unit were in line with expectations based on the 100 b/d pilot plant operations. The reactor feed had a 31.3 API gravity South Louisiana reduced crude including a slurry oil recycle (17 API) rate of 4% on feed. The reactor temperature was 910° F. with the cat to oil ratio of 3.5. A 53.5 vol% conversion was obtained with Super Filtrol natural clay catalyst. The regenerator operated at 1052° F., the spent catalyst containing 1.6 wt% carbon while the regenerated catalyst was reduced to 0.5 wt%. Flue gas excess oxygen was 4.4 wt%. The observed yields for PCLA 1 are recorded in Table II. 

The market for natural clay catalysts began slowing down in the late 1950 s and was replaced by more sophisticated catalysts, using alumina as a basic ingredient. These very early catalysts were used in a wide range of catalytic processes including the fixed bed Houdry unit, TCC, and the FCC. 

The composition of acid-treated natural clay catalysts was more or less duplicated in the synthetic catalyst formulations. TTie main advantage of synthetic catalysts was a reproducible composition with few impurities known to cause deactivation. Both silica/alumina and silica/magnesia formulations were nsed in early tests but, despite good activity, silica/magnesia catalysts were nnstable and difficult to regenerate. 

Use of natural clays as catalyst greatly improved cracking efficiency. 

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

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. 

Fast deactivation rates due to coking and the limited hydrothermal stability of pillared clays have probably retarded the commercial development of these new type of catalysts and prevented (to date) their acceptance by chemical producers and refiners. However, there is a large economic incentive justifying efforts to convert inexpensive (i.e. 40-100/ton) smectites into commercially viable (pillared clay) catalysts (56). Therefore, it is believed that work on the chemical modification of natural (and synthetic) clays, and work on the preparation and characterization of new pillared clays with improved hydrothermal stability are, and will remain, areas of interest to the academic community, as well as to researchers in industrial laboratories (56). 

In conclusion, pillared clays catalysts are not as good as initially predicted for the cracking of heavy gas oils, mainly because of the iron contamination of natural clays. There is a probability that they could be applied for the conversion of hydrotreated gas oils, giving a slightly lower gasoline yield, but higher octane number than REY zeolites. 

The superiority of synthetic catalyst over the natural clay type for the production of aviation gasoline from a yield and octane standpoint is shown in the following comparisons ... 

Figure 9. Thermal Stability of Natural Clay and Synthetic Types of Catalyst...

A number of other catalysts of both synthetic and natural clay types have been developed. Some of these catalysts have shown improvement in product distribution, but none have given appreciable octane benefit. For this reason, they have not been used commercially in fixed- or moving-bed units. 

Most of the commercial Fluid plants have found it economical to continue operations on the silica-alumina catalyst which was used exclusively during the war to maximize raw material production for aviation gasoline and synthetic rubber. The lower cost of the natural clays, however, coupled with the different product distribution, has influenced a number of plants to switch to the use of this material. As the choice of catalyst is... 

Acidic mixed oxides, including alumina and silica, as well as natural clays, and natural or synthetic aluminosilicates, are sufficiently (although mildly) hydrated to be effective as solid protic acids for the alkylation of aromatic hydrocarbons with olefins. The most studied of these catalysts are zeolites that are used in industrial... 

Acidic clay catalysts can also be used in alkylation with alcohols 98 The main advantages of these catalysts are the reduced amount necessary to carry out alkylation compared with conventional Friedel-Crafts halides, possible regeneration, and good yields. Natural montmorillonite (K10 clay) doped with transition metal cations was shown to be an effective catalyst 200... 

The natural clay minerals are hydrous aluminum silicates with iron or magnesium replacing aluminum wholly or in part, and with alkali or alkaline earth metals present as essential constituents in some others. Their acidic properties and natural abundance have favored their use as catalysts for cracking of heavy petroleum fractions. With the exception of zeolites and some specially treated mixed oxides for which superacid properties have been claimed, the acidity as measured by the color changes of absorbed Hammett bases is generally far below the superacidity range. They are inactive for alkane isomerization and cracking below 100 °C and need co-acids to reach superacidity. 

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