Montmorillonite Surface acidity

Taking this one step further, perhaps even an inorganic gene may have been provided by clay mineral sources. Earliest clay samples are of a mineral called montmorillonite that consists of sheets of aluminosilicates in which Fe2+, Fe3+ and Mg2+ are substituted for some of the Al3+, and Al3+ is substituted for Si4+. The oxygen content of the layers does not change and the alternative valencies allow the production of positive and negatively charged layers. Dramatically, Paecht-Horowitz and co-workers showed that the amino acid adenylate could be polymerised with up to 50 units on the montmorillonite surface in aqueous solution. Similar condensation reactions for carbohydrates on hydrotalcite surfaces have... 

The H+ and NH forms of homoionic montmorillonite promote the hydrolysis of chloro-s-triazines to the hydroxy analogs (hydroxy-s-triazines) (73). Apparently, the surface acidity of these clays was extremely high, since no degradation was observed in control experiments conducted at pH 3.5 in homogeneous aqueous solution. Russell et al. (73) suggested that the hydroxy-s-triazine products were stabilized in the protonated form at the silicate surface. The IR spectra of these surface complexes agreed with the spectra obtained in 6N HC1, and it was inferred that the pH at the clay surface was 3 to 4 units lower than that measured in suspension. 

Ethylene and dimethylamine would result successively from the formation of ylide by deprotonation, an intramolecular carbanionic attack, and finally reprotonation. These mechanisms in which the methyl derivatives have been used are different from those proposed for montmorillonite where the ethylammonium cations were mainly implied. The origin of these differences may be partially the reactivity of the hydrogen as well as the nature of the surface acid sites. These considerations prompted us to repeat our previous experiments (1) for ethylammonium-exchanged Y zeolites. 

The relationship of surface acidity of montmorillonites to the state of hydration and nature of the exchangeable cation was investigated by Mortland and Raman (243). By a combination of infrared spectroscopy and gravimetric and elemental analyses the direction of the equilibrium in the following reaction ... 

Recently, interest in clays as acidic catalysts has been quickened by the reported high catalytic activity of a synthetic mica-montmorillonite clay and its nickel-containing analogs. Wright et al. (247) have described the structure, thermal modification and surface acidity of the clay, which they designated SMM for synthetic mica-montmorillonite. 

When studying the surface acid-base properties of montmorillonite, it is essential to understand that hydrogen ions and cations of the support electrolyte can also participate in cation-exchange processes. The processes on the internal and external surfaces have to be taken into consideration simultaneously, and they both have to be included into the equilibrium thermodynamical models. 

For the interpretation of the results using the surface-complexation model, reactions 2.47-2.53 have to be taken into account. In addition, the surface acid-base properties and the neutralization reactions of the layer charge have to be included as in Section 2.4.2 the parameters determined there are treated as fixed, input data. In the case of copper- and zinc-montmorillonite, the copper and zinc concentration of the solution and solid also have to be determined, and these data have to be taken into consideration. That is, the quantity of the total sorbed valine and the copper or zinc ion concentrations versus pH function can be fitted, and KH2Valx, KAioH2Vai> and KSi0CuVal stability constants can be computed. The results of the parameter fit for copper- and zinc-montmorillonites as well as the obtained stability constants are shown in Figures 2.17 and 2.18, and in Table 2.12, respectively. 

Some characteristic properties of bentonites (CEC, sorption properties) are mainly governed by the montmorillonite content and the layer charge of montmorillonite. Other properties, however, depend on the circumstances under which the rock is formed. These are particle size distribution, external specific surface area, and surface acid-base properties. The quantity of the edge sites mainly depends on the specific surface area. The protonation and deprotonation reactions take place on the edge sites of other silicates and aluminosilicates present beside montmorillonite, so their effects manifest via surface reactions. Consequently, the origin of bentonite determines all properties that are related to external surfaces. 

From Table 5 and Table 6 it can be seen that all the samples of different calcination time and aging time possess nearly the same acidity at a particular calcination temperature, revealing the negligible effect of the calcination time and aging time on the surface total acidity. However, Table 7 shows that the total number of acid sites and the number of the BrOnsted acid sites on the catalysts decrease with an increase in calcination temperature. It is known that the surface acid sites of hydroxyl-Zr bentonite mainly stem from surface hydroxyl and exposed metal cation [6], and the Bronsted acid sites result from protons on the surface of the octahedral layers. When the calcination temperature increases, the migration of protons to the octahedral layers of montmorillonite will become easier, leading to the decrease of the number of BrGnsted acid sites. 

Both Lewis and Brdnsted acid sites exist on pillared clays. The acidity depends on the exchanged cations, the preparation method and the starting clay [8], It is known that surface acidity is important for SCR reaction of NO by NH3 [4, 5, 9]. Different pillared clays were synthesized and tested for their activities in the SCR NO [10, 11]. Research on titanium pillared clays was initiated by Sterte [12], who first reported the synthesis of titanium pillared montmorillonite using TiCU solution in hydrochloric acid. Bernier et al. 

Frenkel, M. 1974. Surface acidity of montmorillonites. Clays and Clay Minerals, 22 435-441. 

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). 

The probe molecule pyridine has often been used more recently in the study of clay surface acidity by IR methods. Figure 7 shows data of pyridine chemisorption on a synthetic mica-montmorillonite catalyst as an example (62,63) of the types of bands of interest. The spectra are interpreted as showing chemisorption of pyridine at both protic and aprotic sites. The clay was first heated for 15 h at 650°C under vacuum and then cooled and spectrum A taken. Note that there are no bands in the 1400-1700 cm region, the residual hydroxyl near 3450 cm S and edge silanol hydroxyl at 3747 cm . Pyridine vapor was then chemisorbed and spectrum B taken. The bands at 1456 cm and 1547 cm are assigned to Lewis and Bronsted sites, respectively. Since the 3747 cm edge silanol band decreases, it is assumed that these protons are involved in the mechanism. Lewis sites predominated under these particular conditions. 

---