Clay minerals, catalytic activity

Figures 1, 2 and 3 summarize the results of the reactions of E2, E3 and E2M2 at 573 K, respectively. Yield was estimated after 60 min reaction. SA, SOa/ZrOz, alumina and modified clay minerals were active for the disproportionation. Other oxide catalysts such as TiOz, Zr02, Na-Y, MgO and niobic acid were inactive for the reaction, instead only a decomposition reaction took place. Si02 was totally inactive. Acidic catalysts showed good catalytic activity, while ones with weak or non-acidic character were inactive. Solid bases were inactive for the disproportionation reaction. Though W03/Ti02 and niobic acid have an acidic character and are excellent catalysts for the olefin isomerization [4] and the olefin-...

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). 

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. 

Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids (Sect. 6.3), accepting electron pairs. Catalytic reactions on clay surfaces involve surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which is affected by the type of clay and the clay-saturating cation involved in the reaction. Dissociation of water molecules coordinated to surface, clay-bound cations contributes to the formation active protons, which is expressed as a Bronsted acidity. This process is affected by the clay hydration status, the polarizing power of the surface bond, and structural cations on mineral colloids (Mortland 1970, 1986). On the other hand, ions such as A1 and Fe, which are exposed at the edge of mineral clay coUoids, induce the formation of Lewis acidity (McBride 1994). 

The active isomer of the Ru complex is believed to be cis despite the fact that it isomerizes to trans on adsorption on the clay minerals.283 It is believed that the isomerization does not occur with 100% yield and that the small amount of unisomerized cis-[Ru(bipy)2(HzO)2]2+ is responsible for the observed catalytic activity. This is supported by the observation that reactions starting from trans-[Ru(bipy)2(H20)2]2+ do not produce 02. 

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). 

The activity of clay minerals, proven in the reactivity of terrestrial (15-16), and postulated in Martian (j ) soils, is disproportionate to their quantity, relative to other minerals. This is the result of several factors small particle size, high specific surface area, Bronsted and Lewis acidity, redox and other potentially catalytically active sites common to clay minerals, and a limited capacity for size exclusion (which is influenced by the number and valence of exchangeable cations ( )). 

Reichle, W. T. (1985). Catalytic reactions by thermally activated, synthetic, anionic clay minerals. J. Catal. 94, 547. 

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. 

Bentonite can also be added to addle because it can sorb ammonia or ammonium ions. Moreover, clay minerals in bentonite can catalyze the transformation of organic compounds to humic substances, or the oxidation of ammonia to nitrite and nitrate. In this case, the surface properties, namely, the catalytic activity, play an important role. 

Bentonite, whose main ingredient is montmorillonite, is one kind of layer structure clay mineral. It is an ideal material for preparation of pillared clays. If metal cations with high catalytic activity were intercalated in the interlayers of montmorillonite, a new type of solid acid catalyst can be obtained. The catalytic activity of the catalyst is closely associated with its porosity, specific surface area and surface total acidity. One of the effective ways to enhance catalytic activity is to prepare catalyst with appropriate metal cations and structure. 

Montmorillonite is a layered smectite clay. Acid activation replaces the interlamellar cations with protons, leaches Al from octahedral layers resulting in increase of surface area, porosity and acidity. Clay is activated with a mineral acid for different time intervals. They are characterised by XRD, surface area and acidity by stepwise temperature desorption of ammonia Catalytic activity is studied on aniline alkylation reaction. 

The smectite clays do, however, have some important features which make them particularly attractive as catalyst supports. In addition to their high intrinsic surface area, their laminar structure may confer size and shape selectivity to the resultant catalysts. Another important feature is the negative charge on the silicate layers which may be able to polarise reactant molecules and enhance catalytic activity. Finally the intrinsic acidity of clay minerals provides the catalyst with bifunctionality. This may be useful for example in stabilising intermediate carbocations which would otherwise deprotonate. 

The enhancement in catalytic activity of cations such as Zn(II) which has been achieved both through ion exchange as well as deposition of Zn(II) salts onto clay surfaces led to studies of the acidity and catalytic activity of such ions when incorporated directly into the lattice sites of synthetic clay minerals. Luca et al. showed that Lewis acid sites are generated on Zn2+-substituted fluoro-hectorite.27 The Zn2+-substituted fluorohectorite was synthesised by a sol-gel route. The sol was allowed to crystallise in a Parr autoclave at 250 °C for 24 hours. The Lewis acid sites were identified as Zn2+ at the edges of the fluorohectorite crystallites and were active towards the Friedel-Crafts alkylation of benzene with benzyl chloride. 

Clay minerals occur abundantly in nature, and their high surface area, and sorptive and ion-exchange properties, have been exploited for cafa-lyfic applications fhrough decades. Solid clay materials have a broad range of functions in cafalysis, including the use as (1) catalytically active agents (usually as solid acids), (2) bifunctional or "inert" supports, and... 

Montmorillonite shows a high cation exchange capacity (CEC) that allows a wide variety of catalytically active forms of the clay mineral to be... 

W T Reichle. Catalytic reactions by thermaliy activated synthetic anionic clay minerals, J Catal 94 547-557, 1985. 

Z. Gabelica, B. Nagy, E. Derouane and J. Gilson, "The Use of Combined Thermal Analysis to Study Crystallization, Pore Structure, Catalytic Activity and Deactivation of Synthetic Zeolites", Clay Minerals. 1984,12, 803-824. 

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