Kaolinite, catalytic activity

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. 

Infrared spectra of pyridine adsorbed on kaolinite indicated that the dry clay (110°C) contained both Brtfnsted and Lewis acid sites (235). At 1% water content only protonic acid sites were observed. It was not possible to assign the polymerization activity to either type of acid site, since both were present on samples which were catalytically active. 

Ideal kaolinites are ineffective as solid acid catalysts even after thermal and acid activation. The effect of impregnating ZnCU, FeCb, MnCl2, SnCL and AICI3 on the catalytic activity of a natural kaolinite and its activated form is examined. The process leads to catalysts with improved activity, the maximum being associated with FeCla when employed in the Friedel-Crafts alkylation of benzene with benzyl chloride. 1 g of supported FeCL catalyst (1 mmol/g clay) gave 86 mole% conversion to diphenylmethane with 100% selectivity when 2 mole benzerie is alkylated with 0.1 mole benzyl chloride. Supported AICI3 catalysts proved to be the least effective for the alkylation studied. 

It is apparent that at low moisture content (<10% for the Na-saturated clay mineral and <5% for the Ca-, or Mg-saturated clay mineral), where water is not available for hydrolysis, hydrolysis does not occur. This low moisture content corresponds with the saturation of the cation s first hydration shell. As the moisture content is increased to the upper limit of bound water (50% moisture content), a significant enhancement of the hydrolysis of the epoxide is observed. When the moisture content exceeds the upper limit of bound water (>50%), the rate constant for the hydrolysis of the epoxide was reduced by a factor of 4. It was concluded that water in excess of sorbed water diminishes the catalytic activity of clay surfaces by reducing the concentration gradient across the double layer, effectively raising the surface pH closer to that of the bulk water. In similar studies with MTC, the addition of water to oven-dried Na-montmorillonite and Na-kaolinite retarded the hydrolysis rate of the carbamate. This observation is consistent with the fact that MTC exhibits only neutral base-catalyzed hydrolysis. 

KY zeolite induces mono-Y-alkylation of amines and amides by alkyl halides at 80°C with good yields and selectivity [49]. Sabu et al. investigated the acid properties and catalytic activities of natural kaolinitic clays containing transition metals for Friedel-Crafts alkylation [50]. High catalytic activity and selectivity for the reaction of benzyl chloride and benzene to diphenylmethane was reported. 

The catalytic activities of two types of sheet silicates, kaolinite and montmorillonite were examined for the ene reactions. Table 3.25 summarizes the results of the reac-... 

Clays and metal oxides and salts such as talc, kaolinite, mica, zinc oxide, titanium oxide, iron oxide, hydrated chromium oxide, cobalt blue, ultramarine blue, calcium carbonate, barium sulfate, etc. are widely used as pigments for cosmetics. Since these pigments possess acidic and basic surface properties, and hence catalytic activity, cos-... 

Alternatively, several workers have shown that not only is the soluble, zero-charged hydrolysis product considerably more surface active than the free (aquo) ion but also a polymeric charged or uncharged hydrolysis product may be formed at the solid-liquid interface at conditions well below saturation or precipitation in solution. Hall (5) has considered the coagulation of kaolinite by aluminum (III) and concluded that surface precipitates related to hydrated aluminum hydroxide control the adsorption-coagulation behavior. Similarly Healy and Jellett (6) have postulated that the polymeric, soluble, uncharged Zn(OH)2 polymer can be nucleated catalytically at ZnO-H20 interfaces and will flocculate the colloidal ZnO via a bridging mechanism. 

Ruggiero et al. (1989) investigated the ability of a natural silt loam soil and the clay minerals, montmorillonite (Mte) and kaolinite (Kte), to immobilize laccase. They compared the catalytic abilities of the soil-enzyme and clay-enzyme complexes to degrade 2,4-dichlorophenol. They found that the immobilized laccase remains active in removing the substrate even after 15 repeated cycles of substrate addition (Figure 2.24). However, Claus and Filip (1988) found that the activity of tyrosinase, laccase, and peroxidase is inhibited by immobilization on bentonite. The type of saturating cations on clay surfaces also substantially influences enzymatic activity (Claus and Filip, 1990). 

Gianfreda and Bollag (1994) investigated the behavior of laccase and peroxidase in the presence of a montmorillonite, a kaolinite, and a silt loam soil. They observed considerable variation in the retained activities of the two enzymes immobilized on the different supports as well as variation in the amount of each enzyme sorbed (Table 2.10). Interestingly enough, laccase immobilized on montmorillonite showed a higher specific activity (118%) than that of the free enzyme. This may be attributed to the steric modification of the immobilized enzyme or possibly due to the catalytic ability of montmorillonite itself. Their studies showed that the performance of these enzymes is significantly affected by soil mineral colloids. 

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