Bronsted acidity, correlation with

A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

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. 

Catalytic resnlts are well correlated with the acid strength of the active species irrespective of their natnre (Lewis or Bronsted). On the other hand, there is no clear correlation between the density of the active sites and the catalytic performances. While the FS03H/Si02 catalyst is very active (yields 99.5 -100%, Table 48.2), AICI3/MCM shows only moderate yields (14.3-20.1%) to N-acylsulfonamide, even if both samples exhibit a similar density (25 x lO , Table 48.1). 

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). 

Cp Zr(CH3)3]/Ti02.suifa,ed (<3h 1), and they are correlated with the surface Bronsted acidity [188]. 

The evaluation of the commercially and artificially deactivated samples was realized in the SR-SCT-MAT unit. The activity of the lab-deactivated samples with both protocols is in good correlation to the Bronsted acidity as presented in Figure 9.4. In detail, it is obvious that increment of the Bronsted acidity is reflected in higher activity of the catalytic sample, as the Cat/Oil ratio is decreasing for standard conversion level. 

Steric effects of other substituents such as Et, PhCH2, 2-pyridyl, C02Me, and MeCONH are larger than those five showing the Bronsted correlation. Consequently, they cause lower reactivity. For example, the reactivities of 2-acetylamino- and 2-(2-pyridyl)pyridine are about 10 and 25 times less, respectively, than that suggested by the rate-acidity correlation for the five substrates with an approximately uniform steric impediment.72... 

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

The catalytic activity of these supported BF3 samples was tested using the reaction of 1-octene with phenol (performed at 85°C using 0.05 M of each reactant, in 100 ml of 1,2 dichlorethane with lg of supported BF3 catalyst). Table 2 shows the phenol conversion and selectivities towards octyl-phenyl ether obtained after 23 hours reaction time. It is clear that the activity of the BF3(H20)2/SiC>2 catalyst prepared in ethanol is superior to the other samples. The activity can thus be correlated with the number and strength of Bronsted acid sites identified on these catalysts using TGIR. 

If HA is a stronger acid than the ammonium function of 2, the rate constant for proton transfer to 2, kuA, will be for diffusion and the observed rate constant will be independent of the acidity of HA. On the other hand, if HA is a weaker acid than the ammonium function of 2, the proton transfer from general acids, HA, to the nitrogen of 1 in Scheme 11.8 will be given by k K /K, where K A is the acid dissociation constant of HA and kA is the diffusion-controlled rate constant. The observed rate will nowbe dependent upon the acidity of the catalyst HA as described by a Bronsted correlation with slope equal to —1. 

Ionic liquids with discrete anions have a fixed anion structure but in the eutectic-based liquids at some composition point the Lewis or Bronsted acid will be in considerable excess and the system becomes a solution of salt in the acid. A similar scenario also exists with the incorporation of diluents or impurities and hence we need to define at what composition an ionic liquid is formed. Many ionic liquids with discrete anions are hydrophilic and the absorption of water is found sometimes to have a significant effect upon the viscosity and conductivity of the liquid [20-22], Two recent approaches to overcome this difficulty have been to classify ionic liquids in terms of their charge mobility characteristics [23] and the correlation between the molar conductivity and fluidity of the liquids [24], This latter approach is thought by some to be due to the validity of the Walden rule... 

For the esterification of terephthalic acid with ethylene glycol at 473 K, SO /TiCh calcined at 773 K is much more active than SiC -A Ch as shown in Fig. 6 [60]. The SO /TiCh showed a maximum activity when calcined at 573 K for the esterification of oleic acid with glycerol and of propionic acid with butanol at 403 K [61], where the active sites were attributed to Bronsted acid sites from a correlation between the activity and the Bronsted acidity. The esterification of phthalic anhydride with 2-ethylhexanol to form dioctyl phthalate is also efficiently catalyzed by solid super-acids, the selectivity being more than 90% [62]. The... 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

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