Bronsted acidity acid strength variation

In summary, physisorbed nitrogen appears to offer several advantages as an infrared probe of acid sites in zeolites. It clearly distinguishes between Bronsted and Lewis acid sites without interference from gas phase species, it is small enough to probe sites in smaller pore zeolites, and its interaction with the zeolite is sufficiently weak and reversible to have negligible influence on the zeolite chemistry. It is not yet clear whether the method can probe variations in Bronsted acid strength. 

Together with the well-accepted threshold DPE value for superacidity (250 kcal/mol) [18], it is noteworthy that only marginal variation in was observed for the TMPH" " adduct as DPE decreases from ca. 320 (weak acidic strength) to 250 kcal/mol (super strong acidic strength) [18]. As such, it is obvious that the P-TMP NMR approach is inferior for differentiating Bronsted acidic strengths in soHd acid catalysts. 

We found the acidity of chemically dealuminated mordenites shows a large variation with aluminum content (Figure 9). The alpha values decrease over five orders of magnitude as the aluminum content decreases from four aluminum atoms per unit cell down to 0.8 Al/unit cell. If the Bronsted acid sites associated with boron are of equal strength as those associated with... 

Since 1924, the Bronsted relation has been applied to many general acid and base catalysed reactions, such as those discussed in Sect. 2.2, as well as to proton transfer equilibria like (43)—(45). Over limited ranges of acid strength and for variation within a similar catalyst type, G and a or j3 are constant and the relation holds well. Different catalyst types in a reaction often do not fit on a single Bronsted plot, but give different Bronsted lines. This was observed for the decomposition of nitramide [68]. It has also been observed in proton transfer from l,4-dicyano-2-butene(51)... 

Measurements of acid site strengths with Hr indicators showed that the Bronsted acid sites were stronger than 70% sulfuric acid. No variation with the degree of cation exchange was observed. 

Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. 

The chemical composition of the framework has a definite impact on the acid strength of zeolites and zeotypes. The total number of Bronsted acid sites present in a zeolite depends on the framework Si/Al ratio or in general on the cations. The variation of stretching frequency of bridging hydroxyls [94] and results from H NMR spectra [95] suggest that the strength of Bronsted sites in HY zeolites increases with decreasing Si/Al ratio. This has been rationalized with the concept of... 

Values of a or may be determined in different ways. Since proton transfer in the transition state is expected to vary with the strength of the Bronsted acid or base, a and p should in general represent tangents to a curved log /c-log K plot. For small variations in pK systematic deviations from linearity are usually too small to be observed and only an average value of a or can be obtained, but for larger changes the curvature may be sufficient to evaluate a quadratic term in the dependence (equation 33) ... 

The evidence would certainly support activity residing in Bronsted and/or Lewis-acid sites. However, subtle variation in activity can occur, effected by the strength of associated basic sites in dehydration catalysts. 

This problem is in principle the same as that of the structure of the substrate, but there are some differences in practice. In the first place, the acid-base strength of the catalyst is usually measurable by direct means, and a large part of the variations in reaction velocity merely parallel these variations in acid-base strength. In the second place, it is rarely practicable to vary the strength of the catalyst over a very wide range (because of the catalytic effect of the molecules or ions of the solvent). Moreover, most measurements on catalyzed reactions have been confined to a series of very similar catalysts, and little evidence is available as to how far catalysts of widely differing structure conform to a single Bronsted relation. 

Combined uses of infra red and microcalorimetry techniques may also be performed by contacting a zeolite sample with increasing successive doses of and analyzing the decrease in OH goup band, v OH = 3600 cm l the increase in NH4+ band = 1450 cm ) and the variations in the differential heat of NH3 adsorption. This is shown in fig. 7 taken from ref. 72. The combined use of both microcalorimetry and infra red is complementary since it allows to determine the strength amount and nature (Bronsted or Lewis) of the acidic sites. 

Since acid-base catalysis involves proton-transfer between the catalyst and the substrate, Bronsted-type relations should also apply to the variation of reaction velocity with the acid-base strength of the substrate, and the same statistical corrections should apply, as may be seen from the following general considerations. 

There are some obvious limitations to Marcus s treatment. It represents a quantitative formulation of Hammond s postulate and will not apply where this fails. Also, except where a homogeneous family of acids or bases are used, curvature in a Bronsted plot may be hard to separate from the natural dispersion of the points, and where a homogeneous family is used, the curvature may be too small to be detected within the range of pK values available. This point is illustrated by comparison of the Bronsted plot and the dependence of kn/ D upon base strength in the ionization of ethyl nitroacetate in Figures 8(a) and (b). The variation in would seem to imply... 

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