Base strength distribution

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... 

In order to know the effect of the catalyst-support interaction on the surface basicity and base strength distribution, the chemisorption of CO2 at 100 C and TPD of CO2 from 50 to 900°C for the unsupported La-CaO cmd supported La-CaO ( with or without precoating the support by MgO or La203) catalysts have been measured. The data on the surface basicity of the catalysts is included in Table 3. 

Microcdlorimetry. Differential heats of adsorption of probe molecules can be measured with high accuracy by heat-flow calorimetry and DSC. These data provide information on the acid (or base) strength distribution. Ammonia and other amines have been used as probes for acid sites on oxides (75) and in H forms of zeolite (76), and carbon dioxide and sulfur dioxide were adsorbed as acidic probes on several oxides (75). 

As seen in Fig. 2.11, the acid - base strength distribution curves intersect at a point on the abscissa where acidity = basicity - 0. Hence, the strongest Ho value of the acid sites is equal to the strongest Ho value of the basic sites. Ho.max is defined as the Ho value at a point of intersection, which expresses the equal strongest Ho value of both acidic and basic sites. Each Ho.max value, which was determined from a point of intersection of each acid-base strength distribution curve and the abscissa, is given in Table 2.4. A unique Ho.max is found for every solid. The Ho.vax value changes on calcination. 

Here/(q) is the dipole oscillator strength distribution at q and e is the base of natural logarithm. The lowest excitation potential may be taken for qmin, whereas qmax = (E + EB)/2 with EB a defined mean electron binding energy (Mozumder and La Verne, 1984). 

Ammonia TPD is very simple and versatile. The use of propylamine as a probe molecule is starting to gain some popularity since it decomposes at the acid site to form ammonia and propene directly. This eliminates issues with surface adsorption observed with ammonia. The conversion of the TPD data into acid strength distribution can be influenced by the heating rate and can be subjective based on the selection of desorption temperatures for categorizing acid strength. Since basic molecules can adsorb on both Bronsted and Lewis acid sites, the TPD data may not necessarily be relevant for the specific catalytic reaction of interest because of the inability to distinguish between Bronsted and Lewis acid sites. 

When two immiscible solvents are placed in contact with each other and a non-ionizable compound is dissolved in one of the solvents, the compound distributes itself between the two solvents. This distribution is referred to as partitioning. The ratio of the concentrations of the compound in each phase is a constant for a specific set of solvents, pH, buffers, buffer concentrations, ionic strength and temperature. This ratio is referred to as a partition coefficient or distribution coefficient and is equal to the ratio of the solubilities in the two solvents. When the compound is a weak acid or base, the distribution of the compound can be shown to be given by the following equation for a monoprotic compound ... 

In the case of the parent molecules aziridine (117) and azetidine (142), polymerisations are complicated because the protom of the active species tend to distribute themselves among the nitrogen atoms of different types present in the reaction mixture, according to their base strengths. 

---