Clay type catalysts

The clay type catalysts such as TCC clay and Fluid Filtrol have a considerably wider pore size distribution which includes pores having radii much greater than those of the synthetic silica-alumina and silica-magnesia. There is appreciable adsorption in the high relative pressure region, and the hysteresis loops are broad (Oulton, 42 Ries, Johnson, Melik, and Kreger, 48 Ritter and Drake, 53). 

This discussion has focused on the basic developments leading to successful commercialization of the Fluidized Solids Technique however, many areas were important to success of the project and were pursued vigorously at the same time. Some of these are listed in Table 3. For example, the availability of special equipment had to be assured, including cyclones, slide valves and expansion joints. Also, metals and refractories had to be tested and methods of fabrication developed. A large supply of catalyst was needed. Fortunately, the natural clay type catalyst used initially was readily available, having been used for clay treating of lubricating oils, etc. It was soon found that a synthetic silica-alumina catalyst was much better, with the result that a whole new industry was started to supply it. Twenty years later the silica alumina catalyst was displaced by the more active zeolites. 

Many other authors have investigated these clay-type catalysts, as well as other catalysts (hydrotalcites, mixed oxides, micro- and meso-porous materials) in the WHPCO reaction, but similar results and no peculiar performances have... 

Bakke et al. (1982) have shown how montmorillonite catalyses chlorination and nitration of toluene nitration leads to 56 % para and 41 % ortho derivative compared to approximately 40 % para and 60 % ortho derivatives in the absence of the catalyst. Montmorillonite clays have an acidity comparable to nitric acid / sulphuric acid mixtures and the use of iron-exchanged material (Clayfen) gives a remarkable improvement in the para, ortho ratio in the nitration of phenols. The nitration of estrones, which is relevant in making various estrogenic drugs, can be improved in a remarkable way by using molecular engineered layer structures (MELS), while a reduction in the cost by a factor of six has been indicated. With a Clayfen type catalyst, it seems possible to manipulate the para, ortho ratio drastically for a variety of substrates and this should be useful in the manufacture of fine chemicals. In principle, such catalysts may approach biomimetic chemistry our ability to predict selectivity is very limited. 

Other metal oxide catalysts studied for the SCR-NH3 reaction include iron, copper, chromium and manganese oxides supported on various oxides, introduced into zeolite cavities or added to pillared-type clays. Copper catalysts and copper-nickel catalysts, in particular, show some advantages when NO—N02 mixtures are present in the feed and S02 is absent [31b], such as in the case of nitric acid plant tail emissions. The mechanism of NO reduction over copper- and manganese-based catalysts is different from that over vanadia—titania based catalysts. Scheme 1.1 reports the proposed mechanism of SCR-NH3 over Cu-alumina catalysts [31b],... 

The interactions of the acid sites on clay cracking catalysts with water can be studied readily by immersional calorimetry even if chemisorption occurs and yields information concerning the type and energy distribution of these sites as we shall see in Sec. VII,A. 

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 ... 

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. 

The inherent limitations of the use of zeolites as catalysts, i.e. their small pore sizes and long diffusion paths, have been addressed extensively. Corma reviewed the area of mesopore-containing microporous oxides,[67] with emphasis on extra-large pore zeolites and pillared-layered clay-type structures. Here we present a brief overview of different approaches to overcoming the limitations regarding the accessibility of catalytic sites in microporous oxide catalysts. In the first part, structures with hierarchical pore architectures, i.e. containing both microporous and mesoporous domains, are discussed. This is followed by a section on the modification of mesoporous host materials with nanometre-sized catalytically active metal oxide particles. 

The first successful catalytic cracking process was the Houdry process, announced in 1933 (132) and commercialized in 1936 (172). This was a fixed-bed process employing, at first, an activated bentonite clay as catalyst. It had been known previously that certain types of decolorizing clays catalyzed the decomposition of hydrocarbon oils (165,188), but a carbonaceous deposit rapidly accumulated on the clay and seriously impaired its activity. During his early work in France, between 1927 and 1930, Houdry found that catalyst activity could be maintained at a satisfactory level by carefully burning off the carbonaceous deposit, or coke, at frequent intervals before the concentration became high enough to interfere seriously with the desired catalytic reactions. 

Exploration of the pillar-clay sheet reactivity and connectivity also indicate the important role of the specific clay type. 27 1 and 29si-MASNMR experiments have shown distinctive differences between pillaring mechanisms in trioctahedral hectorite and dioctahedral montmorillonite. Whereas Plee et al. (22) concluded that chemical crosslinking may occur between the pillar and tetrahedral layer in a beidellite montmorillonite, Pinnavaia et al. (23) showed that it did not occur in a hectorite. These are the first observations of a complex process that may depend upon several structural and chemical factors, such as substitution of Al in the tetrahedral layer, or the need for vacancies in the octahedral layer to allow rotation of structural units or migration of reactant species to facilitate crosslinking. Ongoing research should further elucidate refinements on these mechanisms, and direct the technology towards more optimized catalysts - presumably those which form chemical bonds between the pillar and clay layer. 

Although the title of this book, Perspectives in Molecular Sieve Science, avoids the zeolite definition controversy, a large majority of the research reported here centers on traditional zeolites. Only three of the 39 chapters comprising the book deal with materials that are clearly nonzeolitic Two cover clay-type derivatives, and one deals with carbon molecular sieves. Not surprisingly, interest in these materials lies in their possible use as catalysts. Only four chapters present work on mineral zeolites and three on aluminum phosphate-type molecular sieves. Two of those chapters are by workers from Union Carbide, the laboratory that did the pioneering work in this field. It is surprising that other workers have not submitted papers on the aluminum phosphates, but perhaps this situation indicates that although much activity may be underway, laboratories hesitate to publish until patent positions are established in this potentially lucrative area. Union Carbide s synthetic faujasites (zeolites X and Y) and zeolite A receive the most attention, while ZSM-5-class materials are accorded more attention than zeolite A alone. This reflects the important roles that zeolites X and Y and ZSM-5 materials have already played as catalysts. 

Bentonite is a rock rich in montmorillonite that has usually resulted from the alteration of volcanic dust (ash) of the intermediate (latitic) siliceous types. In general, reUcts of partially unaltered feldspar, quartz, or volcanic glass shards offer evidence of the parent rock. Most adsorbent clays, bleaching clays, and many clay catalysts are smectites, although some are palygorskite [1337-76 ]. 

C-21 dicarboxyhc acids are produced by Westvaco Corporation in Charleston, South Carolina in multimillion kg quantities. The process involves reaction of tall oil fatty acids (TOFA) (containing about 50% oleic acid and 50% hnoleic acid) with acryhc acid [79-10-7] and iodine at 220—250°C for about 2 hours (90). A yield of C-21 as high as 42% was reported. The function of the iodine is apparendy to conjugate the double bond in linoleic acid, after which the acryhc acid adds via a Diels-Alder type reaction to form the cycHc reaction product. Other catalysts have been described and include clay (91), palladium, and sulfur dioxide (92). After the reaction is complete, the unreacted oleic acid is removed by distillation, and the cmde C-21 diacid can be further purified by thin film distillation or molecular distillation. 

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. 

In the manufacturing of USY catalyst, the zeolite, clay, and binder are slurried together. If the binder is not active, an alumina component having catalytic properties may also be added. The well-mixed slurry solution is then fed to a spray dryer. The function of a spray dryer is to form microspheres by evaporating the slurry solution, through the use of atomizers, in the presence of hot air. The type of spray dr er and the drying conditions determine the size and distribution of catalyst particles. 

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