Catalytic effects of clays

As a result of the interfacial processes on rocks and soils, the structure and chemical bonds of the sorbed compounds can be changed. For this reason, different chemical reactions can be initiated in which the components of rocks or soils act as catalysts. The most important mineral catalysts are zeolites and clay minerals. Naturally, the different oxides also have catalytic effects, and nowadays some of them are being artificially produced for catalytic purposes such as framework silicates (zeolites), the most effective and selective catalysts in organic syntheses. The catalytic applications of zeolites are too wide to summarize in this book, so we deal with the catalytic effects of clay minerals. 

Clay minerals are effective catalysts of many organic reactions, frequently showing product, regio, or shape selectivity. The other important advantage of clay minerals as catalysts is that the reaction conditions are frequently milder than in traditional procedures. The catalyst can be eliminated by filtration, and the use of hazardous solvents can be avoided, which reduces the quantity of waste. Consequently, clay minerals are often employed as catalysts in the so-called green chemistry. 

Clay minerals or rocks are used as catalysts in both natural and chemically modified forms. Their very first application dates back to the 1960s when acid-treated clays were used for cracking in oil industry (Franz et al. 1959). In the last decades, several reviews have been published on the catalytic effects of clay minerals (Theng 1974, 1982 Solomon and Hawthorne 1983 Laszlo 1986a, 1986b Adams 1987 McCabe 1996 Adams 1987 Adams and McCabe 2008). 

As seen previously, the acidity depends on the interlayer cation, and so catalytic activity can be affected by cation-exchange processes. In addition, when the interlayer cations, or even the cations of the octahedral sheet (e.g., Fe2+ or Fe3+), have different oxidation states, clay minerals can catalyze redox reactions, too. 

Geometric factors play a role in the catalytic processes on clays. Smectites, for example, consist of regular, parallel sheets. The size and swelling properties of the interlayer space determine the size and shape of the molecules intercalating between the layers, which will have an effect on the selectivity of the reaction. 

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The catalytic effects of clays on hydrolysis processes is generally associated with the acidic pH values measured at clay mineral surfaces. Numerous studies have demonstrated that the surface pH of clay minerals can be as much as 2 to 3 units lower than the bulk solution (Mortland, 1970 Bailey et al., 1968 Frenkel, 1974 Karickhoff and Bailey, 1976). The Bronsted acidity of clays arises primarily from the dissociation of water coordinated to exchangeable cations (2.115). 

Clay size layer silicates also have the ability to catalyze the polycondensation of phenolic compounds and amino acids. Wang et al. (1985) examined the catalytic effect of Ca-illite on the formation of N-containing humic polymers in systems containing various phenolic compounds and amino acids. The yields and N contents... 

The results of metal-coated MMT confirmed the catalytic effect of Fe atoms on the clay surface, which accelerates the appearance of degradation products in the gas phase but also at later stages,... 

H30 per liter, but for a solid acid such as acid-activated clay a sharp distinction must be made between soluble acidity and local acid strength . The soluble acidity can be readily measured by convential techniques such as titration or gas volumeter analysis. As to titration, the clay can be dispersed in water, and any acidity thus liberated can be neutralized. On this basis, Thomas, Hickey, and Stecker [89] found that raw montmorillonite yielded 0.41 milliequivalents of acid per gram of dry clay, while after acid treatment (removal of half of the aluminum) this value rose to only 1 milliequivalent per gram. If the clay were a liquid with the density of water, these results would mean hydrogen ion concentrations of 0.41 x 10 and 1 X 10 mole per liter, which corresponds to pH values of 6.39 and 6.00, respectively. Thus, even for the acid-activated clay the soluble acidity is extremely small, and cannot possibly explain the proven catalytic effect of this material. It does, however, explain the fact that TONSIL can be swallowed without harm. 

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. 

The DSC curves for thermoset epoxy with different weight fraction of organo-modified nano-clay are shown in Figure 9.11. The onset temperature of the curing and the temperature of the exothermal peak for neat resin are 115°C and 155 C, respectively. The addition of 5 wt% nano-clay in epoxy matrix reduces the onset temperature to 84°C and peak exothermal temperature to 143°C. The catalytic effect of the nano-clay on the crosslinking reaction of epoxy resin is responsible for the reduction. 

Enhancement in nanophased composites properties can be attributed to the possible catalytic effect of nanoclay that accelerates the curing [32] and increase in the crosslinking between polymer chains, resulting in higher crosslinking density [33,61,62]. Fnrthermore, well-dispersed and exfoliated clay platelets of nanoclay may have provided the barrier to exposed conditions. Hence, due... 

The catalytic effect of the natural clay was more evident for the degradation of phospho-nium salt than for ammonium compounds [43], The difference in the onset decomposition temperature of the phosphonium halides and clays modified by these surfactant reached... 

Finally, it should be stressed that the high interface area in nanocomposite materials enhances the total catalytic activity of clays [92], Indeed, the higher the dispersion degree achieved, the stronger effects on thermal and photooxidative stability of nanocomposite material were observed [91],... 

In Section 3.2.2, data were presented which show that char formation, possibly from a catalytic reaction between the polymer (PS, PA6, or a polymer compati-bilizer, PP-g-MA) and the clay, is often present when low peak HRRs (or mass loss rates) are observed. However, data were also presented which show that in the absence of any substantial charring there can still be a 50 to 60% reduction in peak HRRs if synthetic mica is used in PP/PP-g-MA nanocomposites. It appears that at least two mechanisms may be important to the function of nanocomposites one involving char formation and a second involving the inorganic residue alone. This dual mechanism may explain why the effectiveness of clay nanocomposites varies from polymer to polymer. 

The presence of the organically modified MMT, typically with octadecylam-monium (ODA), yielded in an even more pronounced degradation due to the influence of the ammonium ion which becomes preponderant. It was assumed that may generate acidic sites in the clay layers and even the complex crystallographic structure of clay may result in some acidic sites after functionalization [116]. Associated to the catalytic effect of transition metal cations via the reversible photochemically initiated redox reactions, it induced the formation of free radicals and chain scission upon UV exposure. Therefore, the degradation of these nanocomposites is much faster than the ones with raw MMT. 

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. 

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