Bronsted acidity site interaction with probe

We have shown that the Fe or Fe-Co/Zeollte systems with a vs H/T type results provide a unique probe for obtaining particle sizes (3) of the metal clusters, and can be extended for computing a particle-size distribution. In addition, electronic interactions between the Bronsted acid sites and the Fe species can be elucidated. 

The strength of the interaction of a guest molecules (probe, reactant, product, adsorbate) with a Bronsted acid site is mainly determined by the acidity of the OH group and its accessibility. The acidity of zeolite OH groups can be affected by various factors, most of them studied both experimentally and computationally [126,127] (i) the aluminum content of the zeolite framework ... 

Most acidity studies have been made using basic molecules such as ammonia, pyridine, and piperidine as probes. These molecules have the property that their interaction with Bronsted acid sites, Lewis acid sites, and cations and their hydrogen-bonding interactions give rise to different species detectable by infrared spectroscopy. Thus, adsorption on Bronsted acid sites gives rise to ammonium, pyridinium, and piperidinium ions with characteristic absorption frequencies of 1475, 1545, and 1610 cm"1, respectively. Adsorption on Lewis acid sites—tricoordinated aluminum... 

Aliphatic and aromatic nitriles are another widely used class of compounds for testing accessibility. They are less strongly adsorbed on acid sites than pyri dines or amines. This class of probes includes the very small acetonitrile and propionitrile, isobutyronitrile, pivalonitrile, benzonitrile, ortho-tolunitrile, and 2,2-diphenylpropionitrile. Interaction with both Lewis and Bronsted acid sites typically occurs and can be monitored by IR spectroscopy. The nitriles are relatively weakly bound to acidic hydroxyls by... 

Interaction of Bronsted Acid Sites with Probe Molecules... 

It is important to emphasize that spectroscopic evidence shows that water transforms the Lewis acid sites of sulfated zirconia into Bronsted acid sites [80]. At the same time, water promotes isomerization reactions over sulfated zirconia for a moderate extent of catalyst dehydration. Similarities were reported between the effect of rehydration on the isomerization activity of sulfated zirconia [81] and on that of other oxide catalysts [49] that are consistent with the role of surface donor sites in hydrocarbon isomerization reactions. However, when spectroscopic methods using basic probes were used to compare sulfated zirconia and zeolites in terms of the strength of their acid sites, the results were inconsistent with all catalytic data. These findings illustrate the danger of comparing the acidity of catalyst systems that differ in structure and composition, such as zeolites and sulfated zirconia in these systems the "catalytic" and the "physicochemical" scales for the strength of acid-base interaction may contain significantly different parameters. 

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. 

Basic molecules such as pyridine and NH3 have been the popular choice as the basic probe molecules since they are stable and one can differentiate and quantify the Bronsted and Lewis sites. Their main drawback is that they are very strong bases and hence adsorb nonspecifically even on the weakest acid sites. Therefore, weaker bases such as CO, NO, and acetonitrile have been used as probe molecules for solid acid catalysts. Adsorption of CO at low temperatures (77 K) is commonly used because CO is a weak base, has a small molecular size, a very intense vc=0 band that is quite sensitive to perturbations, is unreactive at low temperature, and interacts specifically with hydroxyl groups and metal cationic Lewis acid sites.26... 

In summary, physisorbed nitrogen appears to offer several advantages as an infrared probe of acid sites in zeolites. It clearly distinguishes between Bronsted and Lewis acid sites without interference from gas phase species, it is small enough to probe sites in smaller pore zeolites, and its interaction with the zeolite is sufficiently weak and reversible to have negligible influence on the zeolite chemistry. It is not yet clear whether the method can probe variations in Bronsted acid strength. 

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