Surface, solid, acid strength

Perfluorinated carboxylic acids are corrosive liquids or solids. The acids are completely ionized in water. The acids are of commercial significance because of their unusual acid strength, chemical stabiUty, high surface activity, and salt solubiUty characteristics. The perfluoroaLkyl acids with six carbons or less are hquids the higher analogues are soHds (Table 1). 

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

Following Walling (72), the acid strength of a solid surface can be defined as its proton-donating ability—quantitatively expressed by Hammett and Deyrup s H0 function (13, 14). H0 is defined by... 

Since solid acid catalysts are used extensively in chemical industry, particularly in the petroleum field, a reliable method for measuring the acidity of solids would be extremely useful. The main difficulty to start with is that the activity coefficients for solid species are unknown and thus no thermodynamic acidity function can be properly defined. On the other hand, because the solid by definition is heterogeneous, acidic and basic sites can coexist with variable strength. The surface area available for colorimetric determinations may have widely different acidic properties from the bulk material this is especially true for well-structured solids like zeolites. It is also not possible to establish a true acid-base equilibrium. 

The acid strength of a solid is defined as the ability of the surface to convert an adsorbed neutral base into its conjugate acid (2). If the reaction proceeds by means of proton transfer from the surface to the adsorbate, the acid strength is expressed by the Hammett acidity function Ho, Ho = pKa + log[B]/[BH + ], where [B] and [BH + ] are, respectively, the concentrations of the neutral base (basic indicator) and its conjugate acid, and pK.d is p BH +. If the reaction takes place by... 

Finally, the perturbation induced on the stretching mode of adsorbed CO (expressed as the difference AD between the stretching frequency of CO in the gas phase and CO in the adsorbed state) is proportional to the electric field E at the equilibrium distance from the adsorbing site (55,56). It will be shown that the AD (CO) a E relationship can be used to establish a spectroscopic scale of the polarizing tendencies of the positive centers at surfaces of ionic solids and, indirectly, to construct a resonable scale of their acidic strengths. The spectrum of CO adsorbed on NaCl films (Fig. 3), consisting of several bands, can be used to show that fourfold coordinated Na+ ions on steps have enhanced polarizing character toward CO. 

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. 

Perfluoroalkanedisulphonic acids (PFAS) are solid and possess strong acid properties both in solid state and in solution. To our knowledge, they have never been used in the alkylation of isobutane. They were obtained as dihydrate and as such were not acidic enough to be active in isobutane alkylation. Furthermore, they possessed low surface areas. The surface area can be increased by supporting PFAS on an amorphous solid, but it is critical that the solid does not attenuate the acid strength, We have found that a method for dehydrating PFAS and for supporting it on silica. The method allows to obtain an new catalyst, PFAS-Si(>2, which is active in the alkylation of isobutane. 

Bronsted acid sites can be directly probed through solid-state H NMR spectroscopy, as chemical shifts can be correlated with acid strength [195, 197, 198]. The precise chemical shift observed for any given Bronsted acid site is dependent on the material upon which it is located. For instance, on silica values of 1.6ppm are typically observed zirconia has two distinct OH sites, at 2.4 and 4.8ppm while on alumina a typical range may be -0.2 to 4.3 ppm. Early studies employing H NMR to study Bronsted acid sites focused on the characterization of the surface of amorphous silica-alumina materials [165, 199-201]. Extensive work, however. 

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]. 

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],... 

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