Clays, acid strength

Circulation flow system, measurement of reaction rate, 28 175-178 Clausius-Clapeyron equation, 38 171 Clay see also specific types color tests, 27 101 compensation behavior, 26 304-307 minerals, ship-in-bottle synthesis, metal clusters, 38 368-379 organic syntheses on, 38 264-279 active sites on montmorillonite for aldol reaction, 38 268-269 aldol condensation of enolsilanes with aldehydes and acetals, 38 265-273 Al-Mont acid strength, 38 270-271, 273 comparison of catalysis between Al-Mont and trifluorometfaanesulfonic acid, 38 269-270... 

Fig. 4. Acid strength distribution for the clays pillared with oxide sols heated at 500°C O, TiOo F3, Si02 TiO A. Si02-Feo03 pillared clays.

Transesterification Reactions. The heterogeneous acid-catalyzed transesterification of TGs has not been investigated as much as its counterpart, the base-catalyzed reaction. Various solids are available with sufficient acid strength to be effective catalysts for the named reaction. Among the solid acids available are functionalized polymers, such as the acid forms of some resins, as well as inorganic materials, such as zeolites, modified oxides, clays, and others. Some of these solids have already been found to be effective in transesterification reactions of simple esters and (3-ketoesters. 

The role of the clay is to provide a constant hydrated environment of its acidic sites (regulation of acid strength) and to avoid overcondensation reactions, which could lead to polyaromatic compounds containing more than two aromatic nuclei. 

Figure 1. Butylamine titration vs. acid strength for clays dried at 120°C. Key -----, attapulgite — —, montmorillonite -----, kaolinite and vertical lines denote titer uncertainties. (Reproduced from Ref. 10. Copyright 1957, American...

Walling (II) and Benesi (I) suggested a method of measuring the acid strength of a solid clay surface. They utilized a basic dye which adsorbed on the acid sites. By studying the color of several dyes of different basicity, they could ascertain the acid strength. 

Fowkes et ah (3) brought more physical chemistry into this type of investigation. They measured acid strengths and reaction rates by differential thermal analysis. They found that the chlorinated hydrocarbons —dieldrin and endrin—when mixed with clays, decomposed at a fixed rate independent of concentration. Thus, a zero-order reaction is indicated. 

The distributions of acid strengths of various pillared clays obtained by calorimetry of NH3 adsorption showed that Al-PILC displays high acidity, with a small number of sites as strong as those measured on zeolites. Acidity should be directly related to the environment of the A1 ions in the pillars, which could be modified by the preparation. The adsorption of NH3 on pillared beidellite showed the presence of many strong acid sites (160 kJ mol" ) [108]. 

A natural clay has been pillared with mixed solutions containing both A1 and Fe, Ti or Cr. The intercalation-generated solids distribution of acid strengths measured by calorimetric adsorption of ammonia is comparable to that of zeoUtes. The surfaces appear as heterogeneous and show initial adsorption heats close to 150-160 kJ moT if one excludes the first point of the differential heat versus coverage curves, which is much higher (=190kJ mol" ) [109]. 

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. 

As clay surfaces become drier, the reaction is driven further to the right and the protons are concentrated in a smaller volume of water so that the acidity becomes more extreme. The ranking of potential Bronsted acid strength for common exchangeable cations follows the order of polarizing power ... 

Clays have both Brpnsted and Lewis acid sites, the amount and strength of which can be substantially modified by acid treatments and/or ion exchange [31-34], The strength of the Brpnsted sites can be determined in different ways Hammett indicators, butylamine titration, IR spectroscopy using probe molecules, microcalorimetric or temperature programmed desorption methods [7,9,34-39], There is a direct correlation between acid strength and composition (Table 2),... 

Table 2. Acid strength and acidity (determined either by Hammett indicators in an aprotic solvent or by butylamine titration) of some clays [7,9,37],...

Since metal sulfates do not change the color of the basic indicators having pK = —5.6 or —8.2, their acid strengths are relatively low compared to those of silica-alumina pK — 8.2), natural clays... 

Weil-Malherbe and Weis (13) have used a colorimetric method for demonstrating the high acid strength of uncalcined clays (Table II). By the use of the indicator dicinnamylideneacetone, which changes color only in acids of high strength, these authors were able also to correlate the acid condition of the clay with its chemical pretreatment. Treatment with stannous chloride solution left the clay as an acidic substance... 

In the AD expanded saponite, acidity increases, Hgure 5-9C. As expected, the amount of pyridine retained on the clay acid centers monotonically decreases with temperature. If AD is replaced by SCD, Bronsted acidity is enhanced. Figure 5-9D. The steam-aged clays retain their sorption capacity for pyridine. However, their acid site strength is greatly reduced, and after degassing in vacuo for 2h at 200 C, pyridine is essentially removed from the steam-aged expanded clay samples. 

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