Acidity of Bronsted acids

A first principle methodology was further rrsed in order to analyze the natrrre of irrtrirrsic acidity of Bronsted acid sites [lOSl]. Periodic models of morderrite with Si/Al = 5 were frrlly optitrrized by rising a plane-wave approach. It has been found that a property related to the electric field at the proton site correlates with intrinsic acidity. The parameter was the gradient of the electrostatic potential at the component of the electric field at the proton site in the direction of the OH bond. For the electronic stracture of mordenite, see [8811]. 

Because of the mentioned leveling effect of the solvent (or excess acid itself acting as such) the acidity cannot exceed that of its conjugate acid. In the case of water the limiting acidity is that of HsO. Proton-ated water, H30 (hydronium ion), was first postulated in 1907, and its preeminent role in acid-catalyzed reactions in aqueous media was first realized in the acid-base theory of Bronsted and Lowry. Direct experimental evidence for the hydronium ion in solution and in the... 

Catalyst acid properties depend on several parameters, including method of preparation, dehydration temperature, silica-to-alumina ratio, and the ratio of Bronsted to Lewis acid sites. 

From this work the relative efficiencies of Bronsted acids, in the presence of stannic chloride (0.166 M) in promoting hydrogen exchange at 25 °C can be ascertained from the rate coefficients (106Art) as follows HC1 (44) H20 (27) AcOH (1.6) CF3COOH (very small). Thus the ability of the dual acid systems to transfer protons is not simply related to the conventional acid strength of the BrQnsted component. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923 Lowry, 1923a,b), the solvent system concept (Day Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 

The theory of Bronsted (1923) and Lowry (1923a, b) is of more general applicability to AB cements. Their definition of an acid as a substance that gives up a proton differs little from that of Arrhenius. However, the same is not true of their definition of a base as a substance capable of accepting protons which is far wider than that of Arrhenius, which is limited to hydroxides yielding hydroxide ions in aqueous solution. These concepts of Bronsted and Lowry can be defined by the simple equation (Finston Rychtman, 1982) ... 

Comparison of Bronsted reaction 4.48 with Lewis reaction 4.49 shows that the Lewis theory is more generally applicable, but its interpretation is different in terms of the definition of acids and complexes. In fact, the Lewis theory is valid for all acid-base reactions (cf., eqns. 4.39 and 4.40). 

In the above titrations of Lewis acids or with the use of the highly reactive titrant LiAlH4, one needs inert solvents of Bronsted class 8, having a low dielectric constant (e.g., at 20° C = 2.0 for cyclohexane or 4.3 for diethyl... 

Instead of Bronsted acids, lanthanide triflates can be used to catalyze the reaction of indole with benzaldehyde (Eq. 7.7). The use of an ethanol/water system was found to be the best in terms of both yield and product isolation. The use of organic solvent such as chloroform resulted in oxidized byproducts.17... 

Scheme 11 Plausible role of Bronsted acid co-catalyst as supported by computational studies...

DRIFT spectroscopy was used to determine Av0h shifts, induced by adsorption of N2 and hexane for zeolite H-ZSM-5 (ZSM-a and ZSM-b, Si/Al=15.5 and 26), H-mordenite (Mor-a and Mor-b, Si/AI— 6.8 and 10) and H-Y (Y-a and Y-b, Si/Al=2.5 and 10.4) samples. Catalysts were activated in 02 flow at 773 K in situ in the DRIFTS cell and contacted than with N2 at pressures up to 9 bar at 298 K or with 6.1% hexane/He mixture at 553 K, i.e., under reaction conditions. Catalytic activities of the solids were measured in a flow-through microreactor and kapp was obtained as slope of -ln(l-X0) vs. W/F plots. The concentration of Bronsted acid sites was determined by measuring the NH4+ ion-exchange capacity of the zeolite. The site specific apparent rate constant, TOFBapp, was obtained as the ratio of kapp and the concentration of Bronsted acid sites. 

NH4 ion-exchange capacity, taken as the concentration of Bronsted acid sites... 

Apparent site-specific bimolecular rate constant, obtained by relating the kB app iimol-gcat."1 s"1) to the concentration of Bronsted acid sites (pmol-gc., 1). 

In spite of the nominal absence of Bronsted acid sites, it is possible to introduce copper species in the S-l matrix by following an ion-exchange procedure. The unit cell of the S-l can be written as follow [1] ... 

Figure 4 presents correlation between the basic and acid activities obtained with the model reaction and the surface area of 1230 cm 1 band of adsorbed CO2 species after evacuation at RT under vacuum (Figure 4a) and the quantity of Bronsted acid sites able to retain DMP at 150°C respectively (Figure 4b). 

Figure 4b shows a quasi linear relation between the activity in alcohol dehydration and the quantity of Bronsted acid sites obtained by DMP adsorption. However a low amount of sites is active in dehydration (on theta alumina) but practically no Bronsted acid sites are detected by DMP adsorption because of their low number and strength. 

Fig. 2 Free energy reaction coordinate profiles for the stepwise acid-catalyzed hydration of an alkene through a carbocation intermediate (Scheme 5). (a) Reaction profile for the case where alkene protonation is rate determining (ks kp). This profile shows a change in rate-determining step as a result of Bronsted catalysis of protonation of the alkene. (b) Reaction profile for the case where addition of solvent to the carbocation is rate determining (ks fcp). This profile shows a change in rate-determining step as a result of trapping of the carbocation by an added nucleophilic reagent.

Although the discussion to this point has been concerned with the explanation of the behavior of Bronsted acids as catalysts, there is an enormous range of reactions in which catalysis by acids and bases occurs. Many of the important types of organic reactions involve catalysis by acids or bases. In this section, several reactions will be mentioned, but the mechanistic details will not be presented in this book on inorganic chemistry. The discussion is intended to show the scope of catalysis by acids and bases. 

A very convenient method to quantitatively determined the number of Bronsted add sites in the often used photochemical nano-vessels, zeolites X and Y, is available.28 This method take advantage of indicator/probe molecules which undergo an intense color change upon protonation within the zeolite pore network. The amount of a base necessary to quench the color change gives a direct measure of the concentration of acidic sites. The base used to titrate the Bronsted sites must be more basic than the probe molecule and sufficiently basic to be completely protonated. 

The decrease in acidity is due to the selective elimination of Bronsted acid sited characterized by the band at 3600 cm in the infrared spectrum. The authors claim that at the same time new, strongly acidic sites are formed under these conditions. 

In their study of Bronsted acid induced cleveage of a [with axial C(l)-OMe] and (3 [with equatorial C(l)-OMe] glycopyranosides, Fraser-Reid et al.46 demonstrated that the (3/a rate ratios for hydrolysis of methyl pyranosides (Table 5) can be explained by the different intermediates and transition-state structures through which proceed the heterolysis of a and [3 isomers (Fig. 18). 

Adsorption of water is thought to occur mainly at steps and defects and is very common on polycrystalline surfaces, and hence the metal oxides are frequently covered with hydroxyl groups. On prolonged exposure, hydroxide formation may proceed into the bulk of the solid in certain cases as with very basic oxides such as BaO. The adsorption of water may either be a dissociative or nondissociative process and has been investigated on surfaces such as MgO, CaO, TiOz, and SrTi03.16 These studies illustrate the fact that water molecules react dissociatively with defect sites at very low water-vapor pressures (< 10 9 torr) and then with terrace sites at water-vapor pressures that exceed a threshold pressure. Hydroxyl groups will be further discussed in the context of Bronsted acids and Lewis bases. 

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