Layered clay catalysts

Another important group of catalytic compounds contain the layered double hydroxides (sometimes called anionic clays or hydrotalcites ). Presently, many researchers study these materials because of their adsorptive and catalytic properties. 

Unlike thermal cracking, the use of catalysts requires less energy and forms valuable hydrocarbons in the gasoline and diesel ranges, thus eliminating the requirement for further processing. 

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M. Campanati, M. Casagrande, 1. Fagiolino, M. Lenarda, L. Storaro, M. Battagliarin, A. Vaccari. Mild Hydrogenation of Quinoline. 2. A Novel Rh-containing Pillared Layered Clay Catalyst. J. Mol. Catal. A Chemical in press. 

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 conversion of a iayered material like OL-1 into a 1-dimensional porous material like OMS-1 is also not surprising. It is well known that certain fluid cracking three dimensional zeolite catalysts can be grown from layered clays. In addition, it is noted that birnessite is the most common manganese mineral. Its abundance may be important in geological transformation of birnessite into todorokite.68.69... 

Pillared layered clays (PILC) containing different hydrated cations in the interlayer space are also selective catalysts. Pinacol and 2,3-diphenyl-2,3-butanediol have been studied in the presence of Al-PILC, Zr-PILC, and Cr-PILC [38], Zr-PILC, with the lowest acidity, gave rise to the largest amount of diene from pinacol (32 % and 22 %, depending on the method of preparation). The rearrangement was selective in the presence of a mixed-metal pillared layered clay (Fe Aln-PILC, sealed tube, 398 K, 1 h, catalyst/diol = 5) [40],... 

However, the activity of these metal-loaded pillared clays is more than 10 times lower that for carbon-supported catalysts (15) for both unsaturated nitriles. This loss of activity could be explained in two ways the first supposes a lower accessibility of the metallic surface in the case of the pillared clays catalyst as compared with carbon based catalysts. We may suppose some strong diffusional effect. This would indicate that, assuming a good repartition of the metal into the layers of the pillared clay, the most active accessible metallic sites would just be those near the outer edge of the clay. 

FIGURE 5.91 Schematic presentation of the process of anchoring the coordination catalytic complex, MAO/me-tallocene, at the surface in the interlayer of layered clay, followed by ethylene polymerization at the immobilized catalyst site. (After Koppl, A., Alt, H. G., and Phillips, M. D. 2001. /. Appl. Polym., 80 (3), 454. With permission.)... 

Microscopical examination should start in the quarry where samples of each of the varieties of limestone, sandstone, shale, etc., are collected, layer by layer, by a geologist or someone with an adequate knowledge of the quarry. An assumption of the mineralogy of most quarried materials, as well as many of the industrial byproducts, is commonly questionable. Representative portions of each rock variety are sent off, if necessary, for thin sectioning (see partial list of professional companies in Table 11-3). Some of these companies will also stain the thin sections as directed. Another portion of the rock is crushed in the plant laboratory with a mortar and pestle or other suitable crushing device, and sieved to produce a 45- to 75- im fraction for examination in a powder mount, using, at first, a liquid with a refractive index of approximately 1.542. Samples of nondeposit materials, such as slag, fly ash, bottom ash, rice husk ash, clay catalyst, etc., are examined similarly. 

The above-mentioned acidic clay catalysts, such as Ti -mont particles, can also be combined with basic, layered claylike HT particles for one-pot sequential reactions [135], Because the acid sites of Ti -mont are located within the narrow interlayer spacing, basic sites on the surface of micron-sized HT particles cannot contact these. Ti +-mont catalyzed the deprotection of acetals to produce carbonyls, with the HT subsequently promoting the aldol reaction of nitriles with such carbonyl compounds to afford the corresponding nitrile compounds (Scheme 6.23). Notably, the HT cannot work with p-Ts0H-H20, and the Ti -mont caimot work with piperidine, as shown in Table 6.1. Compound 1 was obtained only when Ti " -mont and HT were used together. 

In many cases, homogeneous catalysts are attached to organic or inorganic supports such as polymer, silica, and layered clay to form heterogeneous catalysts. The major advantages of heterogenized catalysts are practical and economical since they may be readily recovered from reaction mixtures and reused multiple times without loss of catalytic activity. 

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