Clays catalytically active

There has been renewed interest in catalytically active clays since the report by Swift and Black ( 1) to the effect that replacement of octahedrally coordinated aluminium ions by nickel or cobalt in synthetic smectite clays, as done by Granquist ( ), results in a new type of catalyst, called nickel- (or cobalt-) substituted mica montmorillonite (Ni(Co)SMM), which is very active in the isomerization and cracking of hydrocarbons. 

Remarkably, seventy years after Houdry s utilization of the catalytic properties of activated clay and the subsequent development of ci ystalline aluminosilicate catalysts that arc a magnitude more catalytically active, the same fundamental principles remain the basis for the modern manufacture of gasoline, heating oils, and petrochemicals. 

The performance of clay materials (Halloysite, Pyrophyllite, Montmorillonite K-30) in the degradation of polystyrene (PS) was investigated in this study. The catalysts showed good catalytic activity for the degradation of PS with high selectivity to aromatics liquids. Styrene is the major product, and ethylbenzene is the second most abundant one in the liquid product. 

Vaccari (1983,1999) has given a state-of-the-art account of the preparation and catalytic properties of cationic and anionic clays. Some examples of industrial importance have also been reported. Clays exhibit many desirable features, such as low cost, wide range of preparation variables, ease of set-up and wOrk-up, high selectivity, and environmental friendliness. Cationic clays are widespread in nature, whereas anionic clays are rarely found in nature, but they can be synthesized cheaply. Cationic clays are prepared from the minerals but industrial anionic clays are generally synthetic. Smectite clays exhibit both Brpnsted and Lewis acid sites on the edges of the crystals. Hammet s acidity function values are as follows Na -montmorillonite (M), -3 to t- 1.5 NH4VM -3 to 1.5 H M -8.2 to -5.6 acid activated clay less than -8.2. Laporte also has a synthetic version of cationic clays, Laponite. The acid... 

X-Ray irradiation of quartz or silica particles induces an electron-trap lattice defect accompanied by a parallel increase in cytotoxicity (Davies, 1968). Aluminosilicate zeolites and clays (Laszlo, 1987) have been shown by electron spin resonance (e.s.r.) studies to involve free-radical intermediates in their catalytic activity. Generation of free radicals in solids may also occur by physical scission of chemical bonds and the consequent formation of dangling bonds , as exemplified by the freshly fractured theory of silicosis (Wright, 1950 Fubini et al., 1991). The entrapment of long-lived metastable free radicals has been shown to occur in the tar of cigarette smoke (Pryor, 1987). 

The intercalated catalysts can often be regarded as biomimetic oxidation catalysts. The intercalation of cationic metal complexes in the interlamellar space of clays often leads to increased catalytic activity and selectivity, due to the limited orientations by which the molecules are forced to accommodate themselves between sheets. The clays have electrostatic fields in their interlayer therefore, the intercalated metal complexes are more positively charged. Such complexes may show different behavior. For example, cationic Rh complexes catalyze the regioselective hydrogenation of carbonyl groups, whereas neutral complexes are not active.149 Cis-Alkenes are hydrogenated preferentially on bipyridyl-Pd(II) acetate intercalated in montmorillonite.150 The same catalyst was also used for the reduction of nitrobenzene.151... 

A combination of SIPS with the stabilising and synthesis-favouring properties of clay minerals was studied by Rode et al. (1999) in experiments involving dry/wet cycles. The simultaneous use of both SIPS and clay minerals as catalytically active surfaces led to peptides up to and including the hexamer (Gly)6. The question as to whether this technique fulfils prebiotic conditions can (within certain limitations) be answered positively, since periodic evaporation phases in limited areas (lagoons, ponds) are conceivable. The container material could have consisted of clay minerals. Further progress in the area of peptide synthesis under conditions which could have been present on the primeval Earth can be expected. 

In 1963, Armin Weiss (then at the University of Heidelberg, Germany) reported the intercalation of amino acids and proteins in mica sheet silicates (Weiss, 1963). Some years later, U. Hoffmann, also from Heidelberg, published an article titled Die Chemie der Tonmineralien (The Chemistry of Clay Minerals), in which he mentioned possible catalytic activity of clays in processes which could have led to the emergence of life (Hoffmann, 1968). 

B. Thomas, S. Prathapan, and S. Sugunan, Effect of pore size on the catalytic activities of K-10 clay and H-zeolites for the acetalization of ketones with methanol, Appl. Cat. A Gen., 277 (2004) 247-252. 

Y.L. Zhou, Z. Li, N.F. Hu, Y.H. Zeng, and J.F. Rusling, Layer-by-layer assembly of ultrathin films of haemoglobin and clay nanoparticles with electrochemical and catalytic activity. Langmuir 18, 8573-8579 (2002). 

Surface acidity and catalytic activity develop only after heat treatment of a coprecipitated mixture of amorphous silicon and aluminum oxides. Similar catalysts can be prepared by acid treatment of clay minerals, e.g., bentonite. The acidity is much stronger with silica-alumina than with either of the pure oxides. Maximum catalytic activity is usually observed after activation at 500-600°. At higher temperatures, the catalytic activity decreases again but can be restored by rehydration, as was shown by Holm et al. (347). The maximum of activity was repeatedly reported for compositions containing 20-40% of alumina. 

A recent investigation has demonstrated the usefulness of ultrasonic irradiation in the preparation of delaminated zeolites, which are a particular type of modified oxides - microporous crystalline aluminosilicates with three-dimensional structures - having a greater catalytic activity than the layered structures (clays) and mesoporous catalysts. In an attempt to increase the pore size of zeolites, a layered zeolite precursor was... 

Saltzman et al. (1974) compare the persistence of parathion on a glass surface and adsorbed on an oven-dried Ca " -kaolinite clay (Fig. 16.13). Parathion is relatively stable on a glass surface, but it breaks down partially in an aqueous solution with pH 8.5 and degrades much more when adsorbed on dry Ca -kaolinite. The differences in degradation of parathion in water and on the clay surface suggest a strong catalytic activity of the Ca " -kaolinite. 

Efforts should be made to stabilize an enzyme s activity. Certain agents (such as glycerol, ammonium ions, boric acid, polyethylene glycol, and even talcum powder or bentonite clay) have proven widely to be effective enzyme stabilizers. For multisubstrate enzymes, inclusion of one particular substrate with the enzyme (in the absence of other substrates or cofactors) often stabilizes an enzyme s catalytic activity. Such a substrate may also assist in unlocking the enzyme from a particularly inactive conformational form. In addition to substrates, other ligands and effectors (including reaction products. 

Figure 5. Catalytic activity in the optimized screening tests. (Cat. = 9 g of Mega Dry clay T = 290 °C P = 0.1 MPa Feed = 2.8 M DMED/CH3OH solution in N2 flow LHSV = 0.92 h" ).

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