Brdnsted acids strength

Xhe Brdnsted acid strengths decrease from top to bottom. P.A Proton affinity (kJ/mol) ... 

Rates of model reactions are more commonly used to determine relative rather than absolute surface acidities and a variety of acid-catalyzed reactions have been used for this purpose (1-3). Xylene isomerization is a particularly well-substantiated model reaction, thanks to work by Ward and Hansford (43). They demonstrated that the conversion of o-xylene to p- and /n-xylenes over a series of synthetic silica-alumina catalysts increases as the alumina content is increased from 1 to 7%. The number of strong Brdnsted acids in each member of the catalyst series was measured by means of infrared spectroscopy. Since conversion of o-xylene was found to be a straight-line function of the number of Br0nsted acids (see Fig. 9), rate of xylene isomerization appears to be a valid index of the amount of surface acidity for this catalyst series. This correlation also indicates that the acid strengths of these silica-alumina preparations are roughly equivalent. 

The FR results are not correlated with the OH bands observed in the FTIR spectra of the numerous zeolites studied (second column in Fig. 1). For exanq>le X- and Y-faujasites have the same structure and OH-bands, but their FR spectra are quite strikingly different. The FR method seems to be more sensitive to distinguidiing Brdnsted acid sites with different strengths especially when the dependence on temperatme of the FR spectra is taken into consideration. 

Progressive CO adsorption has been studied by FTIR spectroscopy on two series of acid-leached steamed HEMT and HY zeolites with various Si/Alp ratios. Acidity of structural OH groups of steamed hexagonal faujasites was determined and compared with the results already obtained with the cubic series. Moreover, quantitative estimation of Brdnsted acidity of extraframework phase (numbers and strength) was carried out and allowed to complete the comparison between the two structural varieties. Only minor differences were detectable. 

Although there may be acids of different acid strengths, the rate constant of the Brdnsted conjugated acid form SH+ should have a BrOnsted acid ... 

Acid strength determination.- Acid centres of both Brbnsted and Lewis types can exist on solid surfaces. The BreSnsted sites are groups able to lose protons, while Lewis centres are surface sites which can accept electron pairs from adsorbing molecules. Acidic properties of the centre are characterized by the value of acid strength. For the Brdnsted acid sites, acid strength is defined by acidity functions as for acid solutions. The most... 

These findings allowed him to ascribe the observed catalytic activity differences of many catalysts to the different acid site concentrations, not to the presence of the active centres of different acid strength. The relative concentration of Brdnsted and Lewis sites, which are able to initiate cumene cracking, present on different catalysts are shown in Table 5. 

The unique catalytic properties of zeolites are mainly attributed to their acidic properties. An important characteristic of zeolites, and other acidic molecular sieves, is that each material contains a well-defined, discrete number of acid sites. However, the acidity of zeolites is difficult to characterize because these materials can contain both Lewis and Brdnsted acid sites and may exhibit a heterogeneous distribution of acid-site strengths. 

Results obtained by adsorption microcalorimetry are not sufficient to determine the nature of adsorbed species, or even to distinguish between different kinds of adsorbed species. When examination of catalyst surface is done, that possesses both Lewis and Bronsted acidity, by adsorption of base probe molecule such as ammonia, it is difficult to discriminate strong Lewis from strong Bronsted acid sites solely by adsorption microcalorimetry. This is due to the fact that differential heats of NH3 interaction with strong Lewis and Brdnsted acid sites are relatively close to each other. For this reason complementary informations from suitable IR, MAS NMR and XPS investigations are necessary to identify these sites [45]. However, because of the complex nature of the active site strength distribution it is not possible to make a detailed correlation between sites of different nature and their strength. 

Influence of structure on the number and strength of available Brdnsted acid sites was investigated on a series of dealuminated zeolites (HY, mordenite, ZSM-5 and... 

Brdnsted-Lowry theory, 194 contrast definitions, 194 indicators, 190 reactions, 188 titrations, 188 Acids, 183 aqueous, 179 carboxylic, 334 derivatives of organic, 337 equilibrium calculations, 192 experimental introduction, 183 names of common, 183 naming of organic, 339 properties of, 183 relative strengths, 192, 451 strength of, 190 summary, 185 weak, 190, 193 Actinides, 414 Actinium... 

However, for liquid phase reactions, it is very difficult to correlate the results wiHi BrOnsted or Lewis acidity as the reaction conditions used are different from those used for characterization. When the Diels-Alder reaction is conducted in a solvent, it appears that the maximum for the activity of HY zeolites is obtained for a Si/Al ratio of 15. This maximum was also observed for esterification of carboiQ lic acids (14), methylthiolation of phenol with dimethyldisulfide (15), acylation of toluene with benzoic acids (15) or dehydration of fhictose (15), and in solvents such as alcohols, water or hydrocarbons. If we assume that Lewis species are transformed to Brdnsted ones in the presence of water as solvent, this would thus mean that the Diels-Alder reaction is preferentially catalyzed by BrSnsted species, the maximum observed at Si/Al=15 for HY zeolites being a good balance between the niunber 6ind the strength of the protonic species. 

Recall that Eqs. 8.48 and 8.50 are called BrDnsted linear free energy relationships. If an acid or base is involved in the rate-determining step of a reaction, the rate of that reaction should depend upon the strength of the acid or base. Hence, a Bronsted correlation is often found. Eqs. 8.51 and 8.52 relate the rate constants for an acid- or base-catalyzed reaction, respectively, to the pfC, of the acid or conjugate acid of the base. The sensitivity of an acid-catalyzed reaction to the strength of the acid is a, whereas the sensitivity of a base-catalyzed reaction to the strength of the base is p. The a and p reaction constants indicate the extent of proton transfer in the transition state. In Chapter 9 we explore the use of these two equations in much more detail, and we apply them in Chapters 10 and 11. 

The Brdnsted definition of acidity implies that the acidity of the hydroxyl group is proportional to the strength of the O-H bond which through the force constant is related to the stretching frequency, cafO-H). Hence, by measuring this stretching frequency of the acidic proton in various zeolites it is possible to... 

The acid properties of A1-, Ga-, and Fe-substituted MCM-41-type mesoporous silicates have been probed using ammonia adsorption at 423 K [285]. Substitution led to the formation of Bronsted and Lewis acid sites of different type and strength. Initial heats of ammonia chemisorption decreased in the same order (185 (Al) > 162 (Ga) > 144 (Fe) kJ mol ) as the degree of isomorphous substitution of framework silicon by trivalent atoms. The ratio of strong to medium-strong Brdnsted sites fell in the same order. 

IR pyridine adsorption studies show that the AL-PILC sample Vc calcined at 680°C (Vc-680) has a strong Lewis acidity only (Fig.4). On this sample steamed at 550°C (Fig.5) a strong Bronsted acidity is observed still detectable after evacuation at 480°C. Steaming at 650°C reduces the strength of both Brdnsted and Lewis acid sites,no more detectable after degassing at 300°C. 

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