Clay minerals Lewis acidity

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

Another major cause of waste is the use of mineral acids (H2SO4, H3PO4, etc.) and Lewis acids (AICI3, ZnCL), often in stoichiometric amounts, which cannot be recovered and recycled. A typical example is the HNO3/H2SO4 mixture used in aromatic nitrations. Consequently, there is a discernible trend towards the use of solid, recyclable Brpnsted and Lewis acids, e.g. zeolites, acidic clays, etc. (see later) as alternatives to conventional mineral and Lewis acids. 

Many standard reactions that are widely applied in the production of fine chemicals employ. strong mineral or Lewis acids, such as sulphuric acid and aluminium chloride, often in stoichiometric quantities. This generates waste streams containing large amounts of spent acid, which cannot easily be recovered and recycled. Replacement of these soluble mineral and Lewis acids by recyclable. solid acids, such as zeolites, acid clays, and related materials, would represent a major breakthrough, especially if they functioned in truly catalytic quantities. Consequently, the application of solid acids in fine chemicals synthesis is currently the focus of much attention (Downing et al., 1997). 

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... 

The practical applications of NaBH4 reductions on mineral surfaces for in situ generated SchifFs bases have been successfully demonstrated. The solid-state reductive amination of carbonyl compounds on various inorganic solid supports such as alumina, clay, silica etc. and especially on K 10 clay surface rapidly afford secondary and tertiary amines [126]. Clay behaves as a Lewis acid and also provides water from its interlayers thus enhancing the reducing ability of NaBH4 [22],... 

The surfaces of clay minerals can catalyze the polymerization of organic compounds through a free radical-cationic initiation process. This type of reaction is believed to be initiated by the abstraction of an electron by Lewis acid sites on mineral surfaces however, Bronsted acidity has also been shown to be important in certain cases (see Chapter 22). 

Reactions Promoted by the Lewis and BrRested Acidity of Clay Minerals... 

Clay Minerals as Lewis Acids. Lewis acid sites in a clay mineral are exchangeable (2) or structural ( 0) transition metal cations in the higher valence state, such as Fe + and Cu +, and octahedrally coordinated aluminum exposed at the crystal edges (38). Reduction of both exchanged and structural (octahedral) transition metal cations in the upper oxidation state is a reversible process (12,... 

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 Friedel-Crafts acylation of aromatic compounds is an important synthesis route to aromatic ketones in the production of fine and specialty chemicals. Industrially this is performed by reaction of an aromatic compound with a carboxylic acid or derivative e.g. acid anhydride in the presence of an acid catalyst. Commonly, either Lewis acids e.g. AICI3, strong mineral acids or solid acids e.g. zeolites, clays are used as catalysts however, in many cases this gives rise to substantial waste and corrosion difficulties. High reaction temperatures are often required which may lead to diminished product yields as a result of byproduct formation. Several studies detail the use of zeolites for this reaction (1). 

It is important to distinguish these novel reactions, which take place between the individual sheet silicates and involve the intercalation of one or more of the reactants, from those catalysed by clay mineral surfaces. While details of the mechanism of reactions involving intercalation in sheet silicates are not fully understood, it is generally known that acid sites present in sheet silicates (Bronsted, as well as Lewis) are involved in the reactions. Protonated intermediates such as carbonium or oxonium ions play a crucial role in such reactions. 

Mesoporous solids including silicas and acid-treated clays can be functionalised at their surfaces so as to provide high local concentrations of active sites. These sites can be introduced by post-modification or via sol-gel preparations. In this way a range of novel materials with useful catalytic and other properties can be prepared. One of the most valuable applications for these materials is as replacements for environmentally hazardous reagents including corrosive mineral and Lewis acids, caustic bases and toxic metallic compounds. 

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 ( )). 

Variations on this overview conceptualization occur because of differing isomorphic substitution patterns in 2 1 clay minerals. For example, both Li- and Na-vermiculite can form mono- and bilayer hydrates, whereas K-, Rb-, and Cs-vermiculite cannot. For the latter, inner-sphere surface complexes are stable against solvation of the cations because of the softer Lewis acid character of the latter and a favorable siloxane-surface stereochemistry [23]. 

The above sequence has been observed in studies of alkaline earth adsorption on y-Al203 (Huang Stumm, 1973). The trend is also consistent with the expectation based on the expected preference of harder Lewis acids for hard Lewis bases like surface hydroxyls. Limited spectroscopic evidence is available for sorption of alkaline earth metals because many of these metals do not exhibit sufficiently high K-shell fluorescence energies to be studied in the presence of corundum and water using current EXAFS methods. Chen and Hayes (1999) have shown that Sr(II) sorbs to montmorillonite, illite, and hectorite primarily as a weakly associated outer-sphere complex. Similar findings have been reported for sorption of Sr(II) to clay minerals (Parkman et al., 1998 O Day et al., 2000 Sahai et ah, 2000). 

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. 

Catalysis by sohd acids and bases is another area of current interest for the industrial manufacture of fine chemicals. In many cases strong mineral or Lewis acids (i. e. H2SO4 or AICI3) which are often used in stochiometric quantities, can be replaced by recyclable solid acids such as zeolites and clays. 

Etherification catalysts include mineral acids such as sulfuric and perchloric acid, but these tend to provide double the amounts of dehydration products compared with other catalysts. Lewis acids such as boron trifluoride and iron (Ill)-exchanged montmorillonite clays are also effective at catalyzing these reactions. Boron trifluoride provides the most consistent yields increases in catalyst concentration generally increase ether yields and/or reduce reaction times. Iron (III) clays provided high... 

---