Hammett indicator colors

Zeolites such as HZSM-5 were considered as superacids on the basis of the initial product distribution in accord with C-H and C-C bond protolysis when isoalkanes were reacted at 500°C (the Haag and Dessau mechanism).135 The reactivity was assigned to superacidic sites in the zeolite framework.136 The superacid character of other solid acids was claimed on the basis of Hammett indicator color change137,138 or on the basis of UV spectrophotometric measurements.139,140 In 2000, a special issue of Microporous and Mesoporous Materials141 was devoted to the superacid-type hydrocarbon chemistry taking place on solid acids as suggested by the late Werner Haag. 

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... 

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... 

Many reactions are catalyzed by acid sites on the surface of the catalyst. Isomerization, polymerization, aromatiza-tion, and cracking are catalyzed by Lewis and/or Bronsted acid sites. The precise nature of these sites is open to debate however, intuitively one can use an alkaline material to titrate acid sites and hence determine the number of such sites present. Beses, such as n-butylamine, with a series of Hammett indicators have been used for titrating acid sites. However, the system must be free from water contamination and the catalyst must be colorless to enable one to note indicator color changes. Diffusion of the indicators into the porous network can be very slow and require long equilibration times. 

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... 

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 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. 

In 1950, Walling (I) discovered that the solid surface of some metal sulfates changes the color of Hammett indicators, suggesting a new series of solid acids, but did not consider their usage as a possible catalyst. In 1957, Tanabe et al. 2,3) were able to affirm that the solid acidity of the metal sulfates is an intrinsic one, not arising firom any impurities, and they found that it increased remarkably on heat treatment, attained a maximum value, and then decreased at higher temperatures. Since then, many of the metal sulfates heat-treated at optimum temperatures have been used as solid catalysts for various acid-catalyzed reactions. Among those reactions are the depolymerization of paraldehyde, the polymerization of propylene and of aldehydes, the hydration of propylene, the formation of formaldehyde from methylene chloride,... 

Before we discuss characteristic features of a metal sulfate catalyst, it is to be noted that the model reaction should be one which has as straightforward a mechanism as possible, preferably in a homogeneously catalyzed reaction. This is the only way we can critically evaluate the efficiency of the present solid catalyst system. The depolymerization of paraldehyde was most extensively studied in view of the foregoing criterion. For the homogeneous acid catalysis of the depolymerization of paraldehyde, there are ample data given by Bell and his associates in nonaqueous solvent (by proton acid as well as Lewis acid) and also in aqueous solution (55,56). Since most Hammett indicators change their color when adsorbed on the surface acids of both Bronsted and Lewis type, it is fortunate that this depolymerization proceeds easily by acids of both types. Evidently the dotted line in Fig. 2 shows excellent... 

In the early work of Hammett and Deyrup8 the measurement of the ionization ratio was based on the color change of the indicator. The solutions containing the indicator were compared at 25°C in a colorimeter with a standard reference. This reference was water, when the indicator was colorless in its acid form, and 96% sulfuric acid (or 70% perchloric acid), when the indicator was colorless in the basic form. 

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