Brdnsted acids characterization

M0O3) has been investigated for samples in both original and rehydroxylated form. The spectra are shown in Figure 3. It appears that Brdnsted acid sites, characterized by the 1636 and 1540 cm l bands are observed only, when the discs are rehydroxylated in wet air before the calcination under high vacuum in the IR cell takes place (Figure 3b). By consequence all spectra have been recorded for such rehydroxylated samples. Only one Lewis band is observed for the molybdenum-alumina sample, opposite to the observations of Kiviat and Petrakis (JL9), who have observed two Lewis bands for their samples. 

The molybdate surface layer in the molybdenum-alumina samples is characterized by the presence of BrGnsted acid sites ( 1545 cm- ) and one type of strong Lewis acid sites (1622 cm l). Cobalt or nickel ions are brought on this surface on impregnation of the promotor. The absence of BrtSnsted acid sites is observed for both cobalt and nickel impregnated catalysts, calcined at the lower temperatures (400-500°C). Also a second Lewis band is observed at 1612 cnrl.The reflection spectra of these catalysts indicate that no cobalt or nickel aluminate phase has been formed at these temperatures. This indicates that the cobalt and nickel ions are still present on the catalyst surface and neutralize the Brdnsted acid sites of the molybdate layer. These configurations will be called "cobalt molybdate" and "nickel molybdate" and are shown schematically in Figure 11a. 

Zeolites are widely used as solid acid catalysts for a number of organic transformations, such as the cracking of n-paraffins which are catalysed by Bronsted acid sites. " In the case of zeolites, the so-called bridging hydroxyl groups in the i-OH-Als configuration as shown in Eq. (1) are known to act as Brdnsted acid sites and they are responsible for the ability of zeolites to catalyse the reactions. Therefore, the characterization of acidic properties of solid acids is of great importance in discussing the catalytic properties of solid acids. 

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... 

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. 

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. 

The thermal stabilities of their corresponding pillar intmlayered montmorillonite clay minerals (PILCs) were characterized through the use of powder X-ray diffraction, differential thermal analyses and surface area measurements. These relative stabilities were also found to increase in the order Gaia-PILC < Alia-PILC < GaAl -PlLC. The relative Brdnsted and Lewis acidities of the montmorillonite itself, and of the clay mineral pillared with the different polyoxocations have been examined through infrared pyridine sorption studies. 

Isopropanol was used as a probe molecule to characterize the acidity of heteropolyacid compounds because the product s distribution upon reaction depends on the nature of the surface active sites. Strong Brdnsted (H ) and Lewis acid sites catalyze the dehydration of isopropanol to propylene (di-isopropyl ether over weak Lewis acid sites), and redox/basic sites lead to the dehydrogenation of the alcohol to acetone. 

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