Basic site strength

Control of the basic sites strength by adjusting the active species dispersion... 

Barthomeuff has explored the basic site strength in seolites X,Y and Z, mordenite and ZSM-5 using pyrrole adsorption. The results indicate that cationic faujasites have coupled acid-basic sites. In these oxides the strength of acid and basic sites depends on the nature of the cation and upon... 

Molecular sieve materials appear to be of prime and fascinating interest because they are crystallogaphically and usually well defmed and because they can be relatively easily modified. Modifications include chemical changes as acidic or basic sites strength and site density and physical changes as additional constraints to diffusion of reactant and product molecules. 

The distribution of basic strength of reference and mixed oxides was different, as experimentally determined in [4], While in the reference oxides the majority of basic sites was strong, in the case of mixed oxides (Mg/Al/O and Mg/Fe/O) the main fraction of basic sites had medium strength. Acid sites of Mg/Fe/O sample were exclusively medium-strength Lewis-type [4],... 

Probing the strength, concentration and environment of basic sites in zeolites by IR spectroscopy... 

This study permits to discuss the efficiency of new probe molecules for the characterization of basic sites in zeolites. For MBOH and for methylacetylene, the observed frequencies shift account for the variations in the basic strength of the zeolitic framework oxygen atoms. Interestingly, methylacetylene also informs on the environment of the basic sites and H2S dissociation brings information on the amount of strong basic sites. These results show the high potential and the complementarity of these protic probes to describe the strength, concentration and environment of basic sites. 

The transformation of cyclopentanol/cyclohexanone mixture was also carried out at 200°C. The strength of the acid and basic sites was estimated from the activation energy (Ea) for the both reaction (Table 1). 

As stated above, when probes with specific adsorption characteristics are used, additional chemical information can be extracted from adsorption-desorption experiments. Temperature-programmed desorption (TPD) in particular is often employed to obtain information about specific sites in catalysts [55,56], The temperature at which desorption occurs indicates the strength of adsorption, whereas either the amount of gas consumed in the uptake or the amount of desorption upon heating attests to the concentration of the surface sites. The most common molecules used in TPD are NH3 and C02, which probe acidic and basic sites, respectively, but experiments with pyridine, Oz, H2, CO, H20, and other molecules are often performed as well [57-59], As an example, the ammonia... 

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

Pure alumina catalyst prepared either by hydrolysis of aluminum isopropoxide or by precipitation of aluminum nitrate with ammonia, and calcined at 600-800°, contains intrinsic acidic and basic sites, which participate in the dehydration of alcohols. The acidic sites are not of equal strength and the relatively strong sites can be neutralized by incorporating as little as 0.1 % by weight of sodium or potassium ions or by passing ammonia or organic bases, such as pyridine or piperidine, over the alumina. 

One of the parameters in the broad class of liquid adsorption mechanisms is the interaction between the acidic and basic sites of the adsorbent and the adsorbate. The acidity of zeolitic adsorbent is normally affected by the zeolite Si02/Al203 molar ratio, the ionic radii and the valence of the cations exchanged into the zeolite. In this contribution, Sanderson s model of intermediate electronegativity of zeolitic adsorbent acidity (SjJ can be calculated as a representation of the strength of the adsorbent acidity based on the following equation ... 

These expectations have been borne out. We have confirmed the anticipated correlations of experimental aqueous acid and base strengths with /s,min ° A base becomes stronger, and its p (b lower, as the /s,mm of the basic site decreases. An acid becomes stronger, and its pK lower, as the Is,mm of the conjugate base s basic site increases. To assess basicity and predict p Tb therefore, /s,mm should be computed for the basic site itself to assess acidity and predict pK, /s,mm must be obtained for the conjugate base, i.e. A for an acid HA. 

Concerning the nature of Lewis basic sites, little work has been done to establish general rules and models, except for alkaline earth metal oxides and zeolites. With respect to the former, i.e., the nature of oxygen Lewis basic sites on alkaline earth metal oxide catalysts, a charge-density model predicts that the strength of the sites decreases in the order > OH > H2O > H30. ... 

The strength of the basic sites can be expressed on a scale given by the H function defined by the equation... 

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

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