Number and Strength of Basic Sites

Another frequently used reaction to estimate the number and strength of basic sites in zeolites is the Knoevenagel condensation. As an example, the Knoevenagel condensation of acetophenone and malononitrile is shown in Scheme 15. The reactions were studied, e.g., by Corma et al. [236-238] and Rodriguez et al. [239-241]. In this reaction, the basic sites in the zeolite abstract a proton from the methylenic compound, e.g., a nitrile or ester. The... 

Ammonia Chemisorption. Ammonia gas has been widely employed as a basic adsorbate to count the number and strength of acid sites on various solid surfaces (28,29). The nature and strength of these sites may relate the activity and selectivity character of the catalysts. Further, all acid sites on catalyst surface are easily accessible to the small molecules of NH (kinetic dla 0.26 nm) and... 

From these results, it can be concluded that the number and strength of acid sites on MgVAPO-5 and MgAPO-5 is higher than on VAPO-5 or VjOj/MgO samples. On the other hand, it has been proposed the presence of basic sites on V/MgO samples [1,2]. This basic sites are associated to vanadium-free magnesium sites on the catalyst surface. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

The most general methodology followed to prepare alkaline earth metal oxides as basic catalysts consists of the thermal decomposition of the corresponding hydroxides or carbonates in air or under vacuum. BaO and SrO are prepared from the corresponding carbonates as precursor salts, whereas decomposition of hydroxides is frequently used to prepare MgO and CaO. Preparation of alkaline earth metal oxides with high surface areas is especially important when the oxide will be used as a basic catalyst, because the catalytic activity will depend on the number and strength of the basic sites accessible to the reactant molecules, which is dependent on the accessible surface area. 

The basic strength of a solid surface is defined as the ability of the surface to convert an adsorbed electrically neutral acid to its conjugate base, i.e. the ability of the surface to donate an electron pair to an adsorbed acid. The amount of base (basic sites) on a solid is usually expressed as the number (or mmol) of basic sites per unit weight or per unit surface area of the solid. It is also sometimes more loosely called basicity. There are two main methods for the measurement of strength and amount of basic sites benzoic acid titration method using indicators and geseous acid adsorption method. 

In general, spectroscopic techniques and, in particular IR spectroscopy of adsorbed probe molecules such as the ones mentioned above, provide information about the nature of the basic sites on oxide surfaces. However, they do not give information about the number and strength distribution of the basic sites on a solid... 

Because a base-catalyzed reaction involves the abstraction of a proton by the catalyst, one approach to measurement of the total number of basic sites and also the base strength distribution is to use the reactions of molecules with various values (96-100). For instance, the basic site distribution in calcined MgAl hy-drotalcites was determined by Corma et al. (99), who used the Knoevenagel condensation (Scheme 7) between benzaldehyde and methylene active compounds with various pKa values, i.e., ethyl cyanoacetate (pKa = 9), diethyl malonate (pKa = 13.3), and ethyl bromoacetate (pKa = 16.5). The authors found that this material has basic sites with pKa values up to 16.5, although most of the basic sites... 

According to the Lewis theory, alkaline earth metal hydroxides are weaker bases than their oxides, the order of the strength of the basic sites being Ba(OH)2> SrO(OH)2 > Ca(OH)2 > Mg(OH)2. The hydroxides have been used recently as solid catalysts for organic transformations, such as the conjugate addition of methanol to a, S-unsaturated carbonyl compounds (12), cyanoethylation of alcohols (163,164), and transesterification reactions (166,167,171,172) which are described above. The extensive work of Sinisterra et al. (282) on the number and nature of sites and on the catalytic activity of the most basic alkali metal hydroxide, Ba(OH)2, is emphasized. It was found that commercial barium hydroxide octahydrate can be converted into... 

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