Brsnsted Acidity

From the spectra in Figure 14 it is shown that the component 2 corresponding to weak BrSnsted acid centers is dominant in all five spectra. It seems thus that XPS is specially sensitive to the presence of these sites compared to bulk techniques such as IR spectroscopy. This suggests that the weak Brdnsted sites may be surface species. One possibility [49] is that these OH s would be adjacent to a surface silanol ... 

The activity in consecutive cyclohexene reactions will now be compared. The data presented in Table 2 and in Figure 2 indicate that only strong Bronsted acid sites are active in this reaction. BOR 1, not containing strong sites, is inactive. The activity in consecutive reactions increases in the order BOR 1 < BOR 2 < H-ZSM-5, i.e., in the same order as the strong BrSnsted acid sites content. 

As mentioned above, Pt/HZSM-5 showed higher activity than Pt/NaZSM-5 in the hydrodesulfurization of thiophene. This indicates that the BrSnsted acid site of HZSM-5 in Pt/HZSM-5 catalyst plays an important role for the hydrodesulfurization of thiophene.lt can be assumed that the Brdnsted acid site in the Pt/HZSM-5 catalyst is one site active for the activation of thiophene, whereas R is an active site for the activation of hydrogen. In other words, Pt/HZSM-5 catalyst is a bifunctional catalyst for the hydrodesulfurization of thiophene, in which both Brdnsted acid and platinum metal sites are active sites. 

Definitions. Define and illustrate the following terms (a) BrSnsted acid,... 

Fig.l describes the evolution of the number and the strength of acidic sites on Al(Ga)PON catalysts with increasing nitrogen content. The total number of acid sites decreases when the nitrogen content increases for AlPON as well as for AlGaPON catalysts. Previous in-depth works from our laboratory [6, 7] showed that both BrSnsted and Lewis acid sites are present on the Al(Ga)PON surface. The presence of Bronsted acid sites is mainly due to the P-OH species. The presence of Lewis acid sites is probably due to the existence of coordinatively unsaturated tetrahedral Al or Ga atoms. 

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. 

Acid sites on sulfated zirconia. (a) BrSnsted, (b) Lewis and [Pg.8]

The question about the relation between the reactivity of reactants and their structure is one of the fundamental problems of chemistry. This problem as one of the main directions of chemical kinetics was formulated in the general form about 100 years ago by N.A. Menshutkin in his works on hydrolysis of esters. One of the most important directions in this area is correlation equations relating the reaction rate constant to thermodynamic and structural parameters of reactants. The first correlation was proposed by Ch. Taylor (1914) who noticed a proportionality between the ionization rate constant of the catalyst and rate constant of the catalyzed reaction. The systematic work on correlations in chemical kinetics started from the works of J. BrSnsted and K. Pedersen who, using the results of their study of the reactions catalyzed by acids ( ha) and bases ( a), proposed the equations relating the rate constants of catalytic reactions to the dissociation constants of acids ituA (1924)... 

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