Solid acid catalysts acidic strength characterization

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. 

The concept of acid site is based on the idea that protons are fixed at definite position. Thus, the measures of the acid strength, which are described so far, are basically based on the static properties of OH groups. However, the solid acid catalysed reactions are often carried out at higher temperatures than room temperature. In general, the catalysts undergo structural and chemical change under reaction conditions. Therefore, the characterization of properties of zeolites at high temperatures is more desirable. 

Microcalorimetric NH3 adsorption is one of the powerful techniques for energetic characterization of solid surfaces and provides a direct and accurate method for the quantitative determination of the number of acid sites of different strengths. Microcalorimetry invplves the measurement of differential heats evolved updn adsorption of smeill quantities (micromoles) of basic probe molecules on to the catalysts. Such measurement yields information about the acid strength distribution i.e., the number of sites having the particular heat of adsorption for the basic probe molecule. 

The alkylation of phenol with propylene over several solid acid catalysts such as HZSM-5 with different silica to alumina ratios, H-Beta, H-USY and Y-AI2O3 has been studied. It has been found that zeolite structure has great influence on product distribution. Apart from shape selectivity taking effect in phenol alkylation with propylene over HZSM-5 zeolites, acidic properties (i.e. acid strength and acid density) also influence product distribution. It has been found that H-ZSM-5 exchanged with different alkali metal ions, such as Na and Cs could apparently enhance the selectivity for para-iso-propylphenol due to the change of acidic properties. The acidic properties of the zeolites were characterized by NH3-TPD. 

Recent developments on acidity characterization of solid acid catalysts, specifically those invoking P solid-state nuclear magnetic resonance (SSNMR) spectroscopy using phosphorus-containing molecules as probes, have been summarized. In particular, various P SSNMR approaches using trimethylphosphine, diphosphines, and trialkylphosphine oxides (R3PO) will be Introduced, and their practical applications for the characterization of important qualitative and quantitative features, namely, type, distribution, accessibility (location/proximity), concentration (amount), and strength of acid sites in various solid acids, will be illustrated. 

Apart from discernment of acid type, qualitative and quantitative measures of acidic strengths in solid acids are also crucial for the detailed understanding of the mechanism and performance of the catalyst during catalytic reactions. Assorted illustrations for characterization of acidic strength in various solid acid catalysts invoking the aforementioned P SSNMR approaches wiU be discussed in this section. 

Besides the above reactions, any kind of acid-catalyzed reactions such as cracking of cumene, alkylation of benzene with propene, hydration of olefins, isomerization of cyclopropane, esterification of acetic acid with ethanol, etc. can be used for the estimation of the acidic property of solid acids. Skeletal isomerization of li-butane to -butane is used to check whether a solid acid has superacidity, since the isomerization is known not to be catalyzed even by 100% sulfuric acid. However, it should be noticed that the differentiation between acid strength and acid amount is not easy from the measurement of catalytic activity for an acid-catalyzed reaction. Characterization of acid catalysts by use of model reactions has been reviewed recently by Guisnet. ... 

Physicochemical methods, i.e. adsorption of probe molecules followed by varied analytical techniques (gravimetry, chromatography, calorimetry, spectroscopic techniques, etc.) are currently used for estimating more precisely the concentration of the potential active sites.[34 36] However, very few methods are well adapted for this purpose most of the methods employed for the characterization of the acidity of solid catalysts lead to values of the total concentrations of the acid sites (Brpnsted + Lewis) and to relative data on their strength, whereas few of them discriminate between Lewis and Brpnsted acid sites. It is however the case for base adsorption (often pyridine) followed by IR spectroscopy, from which the concentrations of Brpnsted and Lewis sites can be estimated from the absorbance of IR bands specific for adsorbed molecules on Brpnsted or Lewis sites. 

The above characterization techniques, however, do not indicate the presence of active sites able to promote catalytic reactions. Depending of the nature of active site general techniques widely applied for solid catalysts could also be in principle applied to G-based catalyst. For instance, acidity and basicity are two of the most important properties from the point of view of catalysis. Many different reaction types including additions, substitutions, condensations, rearrangements, etc. can be catalyzed by these types of sites. Also in G catalysts, acid or basic sites can be present and can participate in the reaction mechanism. Therefore, quantification of the density of these sites and a qualitatively measurement of their strength is very important. 

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. 

In heterogeneous catalysis, the catalytic activity (reaction rate) depends on the amount of active sites (e.g., of acidic or base sites having the appropriate strength) that are present on the catalyst as a whole. This means that the density of active sites (amount of sites per gram of the solid or per unit surface area) is an important parameter. On solids, amount and strength of acidic or basic sites are quite independent parameters, so both must be analyzed independently for a complete characterization. Additionally, several different families of acidic or basic sites may occur in the same solid surface, so their distribution (density of sites of any site family) must be characterized. 

The characterization of active sites of solid catalysts includes the determination of active sites nature, the estimation of their density (or population, i.e. the number of active sites per unit of mass or per unit of surface area), their strength and strength distribution. Active sites can be acidic, basic and, in certain cases, amphoteric. All mentioned characteristics are very important for catalysts functionality therefore, many experimental techniques are invented and adapted for their investigation. Among others, mainly spectroscopic methods (like NMR, IR, XPS, XRF...), temperature-programmed desorption is particularly important because it can be useful in the characterization of all mentioned features. 

---