Hammett titrations

The chemical reagents used for the preparation of stock solutions were reagent grade and were used without further purifieation. The chemicals were in the form of the nitrate salts and were obtained eommercially from Aldrich or Fluka. Pure ceria samples were prepared by precipitation of ceria from aqueous solutions containing O.OIM Ce by adding 1 M NH3 solution. The precipitate was dried at 523K. The surface acid sites concentration was measured using Hammett titrations, with phenolphthalein as indicator, and a O.IM NaOH solution as base. 

The measurement of the acidity of solid acid surfaces has been the focus of a vast number of studies. The most commonly used techniques are Hammett titrations, chemisorption of bases and TPD. Extensive discussions of diese methods and their shortcomings are available in the literature [4], The use of adsorption calorimetry makes it possible to determine quantitatively the surface acidity and the acid-strength distribution of solid acids. Surface acid-base properties of catalytic solids can also be studied by base desorption using TG [71]. 

Soybean 91.7 TMG immobilized on mesoporous silica XRD, FT-IR, SEM, TEM, Hammett titration 83... 

The single-electron reduction and oxidation of Co(salen) is solvent dependent as a result of the available coordination sites perpendicular to the CoN202 plane.1220 Furthermore, substituents on the phenyl rings modulate the observed redox potentials and subsequently the 02 binding constants. Hammett correlations are obtained.1221 Potentiometric titrations were performed to determine the 02 binding constants and species distribution as a function of pH for a variety of Schiff base Co complexes.1222... 

Acid properties. The acid properties of zeolites, including those of aluminum-deficient zeolites, have been described in several reviews (e.g. 33-35). The methods used to study the acidity of aluminum-deficient Y zeolites include infrared spectroscopy (primarily pyridine and ammonia sorption studies), n-butylamine titrations in the presence of Hammett or arylmethanol indicators, and to a lesser extent potentiometric titrations and calorimetric measurements. 

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. 

Titration method (consisting in the stndy of the interaction of indicator dyes with solids from solntions) is a techniqne for both qnalitative and qnantitative characterization of solid snrfaces. If a basic indicator B is nsed, the proton acidity of the snrface is expressed by the Hammett acidity fnnction. Similarly, the basicity can be defined when an acid is converted by its conjngated base. This allows defining acidity and basicity in the same scale. 

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... 

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],... 

In fact p for the acid-catalyzed hydrolysis and formation of benzoate esters is small, and since the fit to the Hammett equation is characteristically rather poor for these reactions, p is often taken as zero. Some typical values are listed in Table 16. Too much weight should not be attached to the absolute values of the reaction constants given in the table. The data do not fit the Hammett equation accurately, and it is known that much of the earlier work was subject to considerable error, because no allowance was made for the slow reaction of the catalyst with the solvent. This could affect the observed rate coefficients in three ways—by reducing the catalyst concentration, by decreasing the acid content as measured by titration, and by producing water, e.g. 

The pKa values of a series of conjugated acids of the arsonium ylides have been determined by potentiometric titration (73). A decrease in the basicity of the ylides with an increase in substituent electronegativity was observed. The effect of the substituents agrees well with the Hammett equation. Arsonium ylides are more basic (200-230 times) than the corresponding phosphonium compounds. 

Results of three independent titration studies of silica-alumina cracking catalyst are compared in Fig. 2. Benesi (26) and Hirschler (24) used n-butylamine as the basic reagent and detected endpoints visually by means of Hammett and arylcarbinol indicators, respectively. Drushel and Sommers (21) used diethylamine as the basic reagent and measured end-... 

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. 

There are numerous ways to determine experimentally pK values of chemical compounds (205). Classical methods are potentiometric titration and ultraviolet (UV) spectroscopy, among others. These techniques have been widely applied for nucleobases and also for metal-nucleobase complexes. For the extremes such as negative pK values (pK < —2) of singly or multiply protonated nucleobases, or very high pK values (pK >15) for deprotonation of exocyclic amino groups of nucleobases (C, G, A), modifications have to be employed. These include the consideration of the Hammett acidity function in superacidic solvents or solvent mixtures (206), as well as extrapolative techniques according to Bunnett-Olsen and Marziano-Cimino-Passerini to be applied in polar, aprotic solvents (45, 207). 

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. 

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