Hammett indicator

The probe molecules of greatest historical interest in catalysis are the Hammett indicators [13]. The difficulty of making reliable visual or spectrophotometric observations of the state of protonation of these species on solids is well known. We have recently carried out the first NMR studies of Hanunett indicators on solid acids [ 14]. This was also the occasion of the first detailed collaboration between the authors of this article, and theoretical methods proved to strongly compliment the NMR experiments. The Hanunett story is told after first reviewing the application of theoretical chemistry to such problems. Central to the application of any physical method in chemistry is the process of modeling the relationship between the observables and molecular structure. However often one does this, it is rarely an exact process. One can rationalize almost any trend in isotropic chemical shift as a function of some variation in molecular structure - after the fact, but the quantitative prediction of such trends in advance defies intuition in most nontrivial cases. Even though the NMR spectrum is a function... 

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. 

Figure 2. The Hammett indicator p-fluoronitrobenzene on the Br0nsted site of zeolites. Left -Theoretical structure calculated with DFT at the BLYP/DNP level. Right - Experimental MAS NMR spectra on HY (top) and HZSM-5 (bottom).

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. 

The acidity of the sol oxide pillared clays was also studied by a titration method with Hammett indicators [16], The acidity distributions of the three kinds of sol oxide pillared clays are shown in Fig. 4. The Ti02 pillared clay... 

Comparing the ionization ratio, %, of any weak base (B) to that, li , of a Hammett indicator (In), we have... 

A more flexible approach is due to Bunnett and Olsen (66CJC1899). The general difference between the activity terms for most solutes bases (B), Io8(Vb/Vbh+)> and for Hammett indicators (In), log(v,n/) i H ) is recognized and the simple cancellation of these terms (assumption 2) is replaced by proportionality ... 

Pernicone et al. [253,254] bring forward some evidence that surface acidity also plays a role with iron molybdate catalysts. Hammett indicators adsorbed over the molybdate assume the acid colour. Pyridine poisons the oxidation of methanol to formaldehyde. A correlation is reported between acidity and activity [253]. The authors agree with Ai that the acid sites are connected with Mo6+ ions. 

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). 

H0 is known as the Hammett acidity function, and the series of substituted anilines used to establish the scale are called Hammett indicators. 

Colors of adsorbed Hammett indicators can be used to bracket the H0 of a solid surface in the same way that the colors of more conventional acid-base indicators are used to bracket the pH of an aqueous solution. Thus, when an acid color is observed for the adsorbed indicator, the H of the solid surface is equal to or lower than the pKa of the indicator. As an example, a solid surface that has an H0 that lies between + 1.5 and -3.0 gives an acid color with benzeneazodiphenylamine (with a pKa of + 1.5) and a basic color with dicinnamalacetone (pKa of - 3.0). Color tests are made by adding 3-5 ml of dry solvent (e.g., benzene) to roughly 0.1 g of dried, powdered solid in a test tube, adding a few drops of a 0.1% solution of indicator in benzene, mixing the resulting suspension, and noting the color formed on the powdered solid. 

The indicator method is by far the easiest and quickest way of screening surface acidities of solid catalysts, but it has at least two drawbacks. First of all, the number of suitable indicators is limited because of the visual requirement that the color of the acid form mask that of the basic form. Second, the acid color of many of the Hammett indicators can be produced by processes other than simple proton addition. The first of these drawbacks can be overcome by using absorption spectroscopy to measure... 

Hammett Indicators Used for Visual Measurements of Acid Strengthtt... 

Hammett Indicators for Spectrophotometric Studies of Acid Strengtha... 

Fig. 2. Surface acidity of silica-alumina visual and spectrophotometric titrations (21, 24, 26). , A, Hammett indicators O, Fluorescent indicators , arylcarbinol indicators.

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... 

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