Characterization of Acid-Base Sites in Zeolites

Abstract The review of the use of adsorption microcalorimetry for the characterization of acid-base properties of various types of zeolites is provided. Factors influencing these properties are introduced and explained. Furthermore, the relationship between the data obtained by this technique and catalytic activity of investigated materials is discussed. 

The linkage of tetrahedra within zeolites leads to open network structures the tetrahedra (primary building units) form rings of various sizes which are linked to form complex units (secondary building units).These secondary building units may be connected in many different ways to give a large number of different zeolite structure types. Thus formed network of interconnected tetrahedra results in the zeolite 

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Springer Series in Materials Science 154, DOI 10.1007/978-3-642-11954-5 9, Springer-Verlag Berlin Heidelberg 2013 

According to the pore size, zeolites are classified into small- (pore size up to 5 A), medium-(pore size 5-6 A), and large-pore types (pore size 7-8 A). Typical representatives of the different types are zeolite A, ZSM-5, and the faujasites (X and Y), respectively [5], According to the International Zeolite Association Website, about 190 different framework types of zeolites, zeolitic silicates and phosphates with precisely estimated structures are known to date (2010) [6], 

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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. 

An additional difficulty in the determination of actual TOF values for zeolite catalysed reactions deals with the accessibility by reactant molecules to the narrow micropores in which most of the potential active sites are located. The didactic presentation in Khabtou et al.[37] of the characterization of the protonic sites of FAU zeolites by pyridine adsorption followed by IR spectroscopy shows that the concentration of protonic sites located in the hexagonal prisms (not accessible to organic molecules) and in the supercages (accessible) can be estimated by this method. Base probe molecules with different sizes can also be used for estimating the concentrations of protonic sites located within the different types of micropores, which are presented by many zeolites (e.g. large channels and side pockets of mordenite1381). The concentration of acid sites located on the external surface of the... 

The number of studies in which adsorption microcalorimetry has been successfully applied to this end has increased in recent years, especially concerning the determination of the acidic function of molecular sieves, and extensive reviews of the systems investigated using this methodology have been published [1,5-14,19,78-81]. In particular, an extensive review [4] summarizes some of the most recently published results concerning applications of microcalorimetry to the study of the acid-base sites of zeolites and mesoporous materials. The efficiency of thermal analysis techniques for the characterization of the acid-base strength of zeolite materials is also discussed, as well as their ability to provide information consistent with catalytic data [4]. 

The alkylation of toluene with methanol is another test reaction to characterize acid and basic sites in zeolites. The acid-catalyzed route will lead to the formation of ring alkylated products, i.e., the xylenes. Through the base-catalyzed route, side chain alkylated products, i.e., ethylbenzene and styrene, will be formed (see Sect. 2.4.3, Scheme 10). Giordano et al. [213] showed that with increasing intermediate zeolite electronegativity, i.e., increasing acidity. 

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

A remarkable application of phosphines by Grey and coworkers for acid site characterization is the use of diphosphines with alkyl chain spacers of different length between the phosphine moieties. Based on careful NMR analysis and appropriate loading levels with diphosphines, the Al distribution can be determined [223, 224], The idea behind this tool is that the phosphine groups will be proto-nated, when they are close to an acid site in the zeolite structure. Protonation of both phosphine groups in one probe molecule will only occur, when the distance between the two acid sites is compatible with the molecular dimension of the diphosphine. 

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

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