Solid acid catalysts Bronsted acidic strengths

As can be seen in Fig. 5, N conversion using H-ZSM-11 zeolite seems to be correlated with the number of Bronsted sites on the external surface (deduced from measurements of methylene blue adsorption capacity) and not with the total niunber of Bronsted sites (determined by the total pyridine adsorbed on Bronsted sites and desorbed at 150°C by FT-IR spectroscopy), using the literature data on the integrated molar extinction coefficients [17], (for infrared absorption bands of pyridine adsorbed on solids acid catalyst [17], providing no dependence of the integrated coefficients on the catalyst or strength of the sites). 

The polymerization of aldehydes to high polymer is usually catalyzed not only by bases, but also by both Bronsted and Lewis acids. Takida and Noro (14) found that some metal sulfates do catalyze the polymerization. Measurements of the polymerization activity and the acidic property of solid sulfates of Fe(III), Cr, Zn, Ni, Mn, Mg, Cu, Fe(II), and Ca which were heat-treated at various temperatures revealed that the catalysts having acid sites of pK -f3.3 are effective for the polymerization. Actually, it is seen in Fig. 8 that there exists a fairly good parallelism between the catalytic activity and the acidity at acid strengths pK +3.3. On the other hand, the relation of the number of activated molecules (polymerization rate W divided by averaged... 

The acid forms of aluminosilicate zeolites have found wider use as acid catalysts than any other materials. Their outstanding utility derives from their relatively high acid strength, their high hydrothermal stability, their ability to impart shape selectivity to product distributions and the reproducibility with which they can be synthesised and modified. Each of these advantages stems directly from their crystalline structure. The two basic types of acid site types in microporous solids are Bronsted, which are protons located at bridging sites (Si-O-Al in zeolites, M-O-P in aluminophosphates) and Lewis, usually incompletely coordinated metal cations (especially aluminium in zeolites) in... 

Benesi titration [126] is one of the oldest methods used for counting the acid sites of solid catalysts in diff erent ranges of acid "strength. "The use of quotation marks is necessary because the method is based on the convention that the pA scale in aqueous media can be extrapolated to very strong acid media by use of an appropriate series of color indicators. Thus, when a basic indicator (B) reacts with a surface Bronsted acid site (HA) to form a surface base (A ) and the conjugated acid form of the indicator (HB" "), the acid strength is equated to the Hammett acidity function Hq in the following way ... 

The fact that a relationship similar to Eq. (8) was experimentally found for solid acid catalysts has both practical and theoretical significance. From a practical point of view, it demonstrates that at least one experimental method exists, namely the PAD method, that affords a direct and quantitative determination of both the number and strength of Bronsted acid sites on surfaces of porous oxide catalysts. 

In the light of these remarks, it becomes quite clear that quantitative spectroscopic methods for determining the Bronsted acidity are a necessary prerequisite for the characterization and understanding of solid catalysts. Quantitative determination of Bronsted acidity, however, means the measurement of (i) the concentration, (ii) the strength of acidity, and (iii) the accessibility of the Bronsted acid sites. 

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