Bronsted relationship, between catalytic

Radioactive tracer experiments reported by Lombardo and Hall (4) showed that each butene isomer can be directly interconverted into the other two. These results are consistent with a common intermediate being in operation in this reaction. In Figure 3 the linear relationship between catalytic activity and percentage of Na+ replaced by H+ strongly favors a Bronsted acid catalyzed mechanism in which the common intermediate could be a secondary carbonium ion. This conclusion is also supported by the tracer experiments. 

Hydroisomerization/ hydrocracking of n-decane A1 Ga-(Be, M) impr. The order of activity found was USY >A1-Be > Ga-M > Al-M. A direct relationship between catalytic activity and Bronsted acidity associated with the Al and Si in the tetrahedral layers of Be. 52... 

FIG. 4 Relationship between catalytic activity (isomerization of 1-butene to 2-cis- and 2-rra ,s butenes) on several mixed oxide catalysts and their surface acidity on the aqueous scale, (a) Linear dependence between specific reaction rates and the density of active sites from PAD data (b) linear Bronsted relationship between the reaction rate of 2-cw-butene formation and the acid strength of corresponding active sites on different mixed oxides (c) same relationship for formation of 2-/ra 5-butene. 

Bruice and Schmir (3) have shown that for a series of imidazole derivatives, klm depends on the base strength of the catalyst and since pKA is an approximate measure of base strength, the value of klm should increase with increase in pKA. Table I shows that this is indeed the case. Imidazole, pKA = 7.08, has a catalytic constant eight times larger than that of benzimidazole, pKA = 5.53. Bronsted and Guggenheim (2) have obtained a linear relationship between log k/ and pKA for a series of carboxylic acids in the pKA range of 2 to 5, where kB is the carboxvlate anion basic catalytic constant for the mutarotation of glucose and Ka is the acid dissociation constant of the acid. Our results for imidazole and benzimidazole fit fairly well into the Bronsted plot. 

If a proton-transfer reaction is visualized as a three-body process (Bell, 1959b), a linear free energy relationship is predicted between the acid dissociation constant, Aha, and the catalytic coefficient for the proton-transfer reaction, HA. Figure I shows the relationships between ground-state energies and transition-state energies. This is a particular case of the Bronsted Catalysis Law (Bronsted and Pedersen, 1924) shown in equation (9). The quantities p and q are, respectively, the number of... 

The view of generalized acid-base catalysis as a prototropic shift assisted by acids and bases raises, quite naturally, the question of the relationship between the catalytic power of the acid or base and its own ionization constant. It had early been recognized that there is a correlation between the two constants. Taylor proposed the first quantitative relation, that the acid-catalytic constant of an acid knA was proportional to XjaA, the square root of its ionization constant. For generalized acid-base catalysis, the Bronsted equation, proposed later,"" has gained wide empirical use ... 

The relationship between the ability of a buffer component to catalyse hydrolysis, denoted by the catalytic coefficient, k, and its dissociation constant, K, may be expressed by the Bronsted catalysis law as... 

In this chapter, the use of solid acids as heterogeneous catalysts for the Friedel-Crafts acylahon reaction is described. Our review is split up into seven sechons, describing the application of zeolites, clays, metal oxides, sulfated zirconia, heteropoly acids. Nation, and other less-utilized solid catalysts (i.e., graphite). When possible, the relationship between the acid properhes of the solids (namely, Bronsted and Lewis types) and the catalytic efficiency is shown, as well as the role of the active site location on the catalyst surface. ... 

The relationship between thermodynamics and kinetics in chemical reactions is usually expressed by the Bronsted equation (eq. 3.52 in chapter 3.4) k = gKa, where k is the rate constant, K is the equilibrium constant of the elementary stage, and g and a (Polanyi parameter) are constant values for a serious of reactions. These constants are determined by parameters characterizing the elementary mechanism (composition and structure of the activated complexes, etc.) thus allowing for the existence of an optimum catalyst, on which the rate of catalytic reaction per unit of surface has a maximum value. Equations of the type (3.52) were used for the explanation of "volcano-curves", when catalytic activity as a function of thermodynamic characteristics follows a curve with a maximum. An example for a volcano curve in methanation of CO is given in Figure 7.6. 

Discussions of OH groups in the context of catalysis normally focus on their role as active centers in a number of reactions. The work by Haag et al. (94) constitutes a classic example the authors estahhshed a linear relationship between the concentration of aluminum in HZSM-5 (which imphes an equal concentration of bridging hydroxyls) and the activity for cracking of -hexane. It was concluded that aU protonic acid sites in the zeohte are characterized by the same turnover frequency. Many other correlations between catalytic properties of materials and the strength and/or density of their Bronsted acid sites are well estabHshed. We will not discuss this aspect in detail and recommend instead a number of recently pubhshed reviews (59,60,87). Two more points are worth mentioning. One point is that the cooperative action of Bronsted and Lewis acid sites has been demonstrated. The second is that, of course, OH groups must not necessarily be involved in a catalytic conversion in fact, they can even block the catalyt-icaUy active sites. 

Catalysis by solid acids is of paramount importance in industrial chmnistry, namely due to its application in catalytic cracking, one of the most important processes in the world. However, despite its enormous importance, only recently have practical and quantitative relationship between the acidity of the catalyst and its catalytic activity began to appear, unlike homogeneous acid catalysis, which has made use of the Brdnsted relations for many years. The difficulties to be overcome are of various nature but it was found by some of the authors that Bronsted-like relationships also apply to solid acid catalysts [1]. 

For many reactions which show general acid catalysis, the catalytic constant, k, is related to the strength of the catalyzing acid the stronger the acid, the better it is as a catalyst. Such a qualitative connection is inherently reasonable, but in many cases a precise quantitative relationship between K and K, the dissociation constant of the acid, is well obeyed. Following Bronsted and Pedersen (1924), this relationship may be written in the form of equation (24),... 

Bronsted sites was only 10-15% of the original value for a 90QOC pretreated zeolite sample (Table 1), the conversion of ethylene to higher hydrocarbons was still observed and the activity of HZSM-5 was even higher than the activity of a 300°C pretreated sample (Fig.2). This indicated that no direct relationship existed between the catalytic activity and the concentration of the hydroxyl groups in a zeolite san jle. 

OH groups, so-called Bronsted add sites, were regarded. This interest was particularly stimulated by the close relationship which was suspected to exist between (Bronsted and/or Lewis) acidity and the catalytic behavior of zeolites. 

For each alumina a linear relationship was found between the logarithm of the rate constant k j ), and the acid strength,( yj, as shown in Fig. 3.37. That is, the Bronsted rule of catalysis holds for each alumina. The fact that three linear plots were obtained means that the acid site having the same acid strength had different catalytic activity from one alumina to another. They explained iurther this difference between the three catalysts by the difference in the basicity of the aluminas shown in Table 3.16. The rate constant for acid strength k(j) becomes greater for alumina with higher basicity-to-acidity ratio. This implies that the acid-base pair sites are the active sites. 

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