Hammett indicator strength

We used DFT to optimize the geometries of various Hammett bases on cluster models of zeolite Brpnsted sites. For p-fluoronitrobenzene and p-nitrotoluene, two indicators with strengths of ca. -12 for their conjugate acids, we saw no protonation in the energy minimized structures. Similar calculations using the much more strongly basic aniline andogs of these molecules demonstrated proton transfer from the zeolite cluster to the base. We carried out F and experimental NMR studies of these same Hammett indicators adsorbed into zeolites HY and HZSM-5. 

Chemical probes such as titrations using Hammett indicators [188,189] and test reactions [190] have been often employed as well. Given that each method has its own strengths and limitations, a rational combination of specific techniques is often the best approach to the study of a given catalytic system. 

Aluminosilicates are the active components of amorphous silica—alumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica—alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according to their strength have been found, e.g. by titration with f-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the H0 scale [38],... 

Table XIII shows the strengths measured by Hammett indicators with pK.d values ranging from —5.6 to — 14.5. As described above, dried H3PW 204o possesses superacidity (127). The order of the acid strengths agrees with that...

The presence of Lewis acidity [type (2)] or Bransted acidity [type (1)] is revealed by the IR spectrum of pyridine sorbed on AI1.5PW12O40 (138, 139). IR spectra of sorbed of NH3 show that Cui.5PW]204o has Lewis acidity as well as Bransted acidity (140). Ghosh and Moffat (141) measured the acidities of several salts of H3PW12O40 with Hammett indicators. The acid strength increases with an increase in the calculated charge on the peripheral oxygen atom of the polyanion Zr > Al > Zn > Mg > Ca > Na (141). 

Silica-alumina Amine titration with Hammett indicators extension of the acid strength scale 69... 

Walvekar and Halgeri 124) reported n-butylamine titration measurements with Hammett indicators for a variety of metal oxides supported on alumina. They observed that amine titers and acid strengths of such binary oxides decrease in the following order ... 

Take et al. (69) extended the acid strength range of the n-butylamine titration method. They employed indicators such as 4-nitrotoluene (pKa = - 10.5) and 2,4-dinitrotoluene (pKa = - 12.8), which are considerably less basic than the other Hammett indicators used to measure surface acidity. Endpoints were determined spectrophotometrically. These authors found that the acid sites on silica-alumina catalyst had an acid strength corresponding to an H0 between - 10.5 and - 12.8 a few sites had even higher acid strengths (H0 < 12.8). Strong acid sites were eliminated when silica-alumina was poisoned with sodium ions. 

The effect of water content on the acidity of kaolinite was examined by Solomon and co-workers (235, 236). Kaolinite dried at 110°C (0% water) had strongly acidic sites, comparable to 90% sulfuric acid (pKa < —8.2), as indicated by the Hammett indicator method. The strongest sites were readily poisoned by water. At 1% wt water content the strongest acid sites were equivalent in strength to 48% sulfuric acid (pKa = 3.0). With... 

Subsequently, the same authors138 described the preparation of a solid superacid catalyst with acid strength of H0 = —16 with a sulfuric acid-treated zirconium oxide. They exposed Zr(OH)4 to 1A sulfuric acid and calcined it in air at approximately 600°C. The obtained catalyst was able to isomerize (and crack) butane at room temperature. The acidity was examined by the color change method using Hammett indicators added to a powdered sample placed in sulfuryl chloride. The... 

Acid strength of the Hf02 catalysts could not be measured by the visual color change method of Hammett indicators because the materials change color (to yellow) after calcination. The maximum activity was observed with calcination at 700°C, and its catalytic activity for the reaction of butane was close to that of the Zr02-I (650°C) catalyst. Thus, the catalyst treated at 700°C is considered to hold the acid strength close to Ho = - 16 on the surface (138). The superacid of Fe203 is also colored (brown) it is... 

Titration with a variety of Hammett indicators is one of the most widely used techniques to determine the distribution of acid strengths on the solid surface [79]. However, many arguments have been raised in the past against the use of Hammett indicators for evaluation of solid acidity [80-83],... 

The acid strengths shown in Table 17.3 were examined by the visual color change method using the Hammett indicators shown in Table 17.1 [43, 48]. The indicator dissolved in solvent was added to the sample in powder form in a nonpolar solvent, sulfuryl chloride [38] or cyclohexane [40]. The strength of colored materials such as S04/Fe203 and Mo03/Zr02 was estimated from their catalytic activities in comparison with those of the catalysts determined by the Hammett-indicator method. 

Determination of the acid strength of solid catalysts using Hammett indicators, however, has been criticized frequently because of the heterogeneity of the solid surface [81, 104, 110, 114—116]. The principle of the Hammett acidity function is based on the equilibrium equation in a homogeneous solution, and its application to the heterogeneous condition is subject to severe criticism. In addition, the color change of the adsorbed indicators on solids as determined by the naked eye is subjective. The effects of interactions between the solvent and the solid surface has also been raised [9]. 

The acid strength of nonsupported zeolite which was dried at 4S0°C in air was measured by use of Hammett indicators in benzene. 

Other Techniques. - Numerous other techniques have been applied to the characterization of zeolite acidity. Umansky et al. developed a spectrophotometric method, using Hammett indicators, for measuring the surface acid strength of solid acids. A number... 

The most direct method is to adsorb an indicator on the catalyst in suspension with a nonpolar solvent. The indictor has a known pK for the acid-base color change. A base is then added until the acid-base end point is observed. The amount of base indicates the number of acid sites with strengths less than the pX of the indicator. By using a range of indicators with different pK s, the distribution of acid strengths is determined. End points are detected either by visible color changes or with spec trophotometry. For visible detection, the Hammett indicators used are given in Table 7.12. 

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