Smectites surface acidity

Mortland, M.M. and K.V. Raman (1968). Surface acidities of smectites in relation to hydration, exchangeable-cation and structure. Clays Clay Min., 16 393-398. 

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. 

Another important chemical property of adsorbed water on vermiculite and smectite surfaces is its Br nsted acidity. This property should refer principally to the acidity of the solvated exchangeable cations, as described by the reaction... 

M. M. Mortland and K. V. Raman, Surface acidity of smectites in relation to hydration, exchangeable cation, and structure, Clays and Clay Minerals 16 393 (1968). See also J. D. Russell, Infrared study of the reactions of ammonia with montmorillonite and saponite, Trans. Faraday Soc. 61 2284 (1965), andM. M. Mortland, Protonation of compounds at clay mineral surfaces, Trans. 9th Int. Cong. Soil Sci. (Adelaide) 1 691 (1968). 

Trombetta, M., Busca, G., Lenarda, M., Storaro, L., Ganzerla, R., Piovesan, L., Jimenez Lopez, A., Alcantara-Rodriguez, M., and Rodriguez-Castelldn, E. Solid acid catalysts from clays. Evaluation of surface acidity of mono- and bi-pillared smectites by FT-IR spectroscopy measurements, NH3-TPD and catalytic tests. Hpp/. Catal A Gen. 2000,193, 55-69. 

Anionic surfactants are adsorbed by smectites in acidic medium and, at pH near neutral, when inert salts (e.g., NaCl) are added to compress the diffuse ionic layers [53]. They can also be attached to the mineral surface by calcium bridges. 

Another type of reaction that responds to WD cycles is the fixation of K and NH4 ions by smectite (3-7). The fixation of K in smectite has been studied extensively by soil scientists because of its effect on the availability of plant nutrients. The reaction also decreases smectite s ability to swell, decreases its cation exchange capacity (CEC), and modifies its BrjSnsted acidity. Therefore, an understanding of this phenomenon is applicable to many fields of study that are concerned with swelling clays, fields such as soil fertility, soil mechanics, waste disposal, clay catalysis, and the geochemistry of ground and surface waters. 

These examples illustrate that biomolecules may act as catalysts in soils to alter the structure of organic contaminants. The exact nature of the reaction may be modified by interaction of the biocatalyst with soil colloids. It is also possible that the catalytic reaction requires a specific mineral-biomolecule combination. Mortland (1984) demonstrated that py ridoxal-5 -phosphate (PLP) catalyzes glutamic acid deamination at 20 °C in the presence of copper-substituted smectite. The proposed pathway for deamination involved formation ofa Schiff base between PLP and glutamic acid, followed by complexation with Cu2+ on the clay surface. Substituted Cu2+ stabilized the Schiff base by chelation of the carboxylate, imine nitrogen, and the phenolic oxygen. In this case, catalysis required combination of the biomolecule with a specific metal-substituted clay. 

Pyridine adsorption experiments have showed that the nickel containing smectites have Lewis acid sites and do not have Bronsted acid sites [9]. The Ni2+-substituted smectite catalysts have large surface areas even after 873 K treatment because many small fragments with the same smectite structure are intercalated in the interlayer region. The activities of the Ni2+ substituted catalysts are derived from Ni2+ Lewis acid sites located on the edge framework. 

In natural soils which commonly contain illite and smectite, there can be a significant charge imbalance between Si4+ and Al3+ in the structures of these clay minerals. This results in a net negative charge on the clay mineral surfaces, resulting in more adsorption of mobile cations. When an acid front encounters these adsorbed mobile cations, they are very easily displaced by the H ion, which by virtue of its small size is strongly adsorbed to the clay surface. As a result, the measured CEC of such clay-bearing soils is predicted to increase, as we have observed in our experiments. 

ATR-FTIR and the HF/3-21G(d,p) level of theory [103], it was predicted that salicylic acid is adsorbed strongly onto illite. Salicylate forms surface complexes predominantly with the Al3+ octahedra located on the edges of the illite grains. The authors of DFT studies [104, 105] show that the ordering for the best sorption of dioxine and furane on the surface of smectites follows the order Mg2+ > Fe2+ > Fe3+ > Li+, which was obtained also in the case of adsorption of nitrogen heterocyclics on smectites [106]. 

In the early history of acid catalysis, clays had a significant utility. The acid leaching of natural smectite clays produced amorphous silica-alumina [Si-Al] materials with large surface areas and active acid sites, which were used as acid catalysts [31],... 

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