Basic sites, zeolite structures

The foundation of equilibrium-selective adsorption is based on differences in the equilibrium selectivity of the various adsorbates with the adsorbent While all the adsorbates have access to the adsorbent sites, the specific adsorbate is selectively adsorbed based on differences in the adsorbate-adsorbent interaction. This in turn results in higher adsorbent selectivity for one component than the others. One important parameter that affects the equilibrium-selective adsorption mechanism is the interaction between the acidic sites of the zeolite and basic sites of the adsorbate. Specific physical properties of zeolites, such as framework structure, choice of exchanged metal cations, Si02/Al203 ratio and water content can be... 

The catalyst consists of basic and acid sites in a microporous structure provided by zeolite and microporous materials [58-62]. Basic sites are provided by framework oxygen and/or occluded CsO. Acid sites are provided by the Cs cation and, possibly, additives such as boric and phosphoric acids. The addition of Cu and Ag increased the activity [63, 64]. Incorporation of li, Ce, Cr and Ag also has been shown to increase the styrene to ethylbenzene product ratio [65]. The reactivity of catalysts is sensitive to the presence of occluded CsO, which is in turn influenced by the preparative technique as shown by Lacroix and co-authors [64] and pointed out by Lercher [61]. 

Zeolites may behave as Lewis acids at Al,+ sites, or as Br0nsted-Lowry acids by means of absorbed H+ ions. Because they have relatively open structures, a variety of small molecules may be accommodated within the —O—Al—O—Si— framework. These molecules may then be catalyzed to react by the acidic centers. Coordinatively, unsaturated oxide ions can act as basic sites, and in some catalytic reactions both types of centers are believed to be important. Catalysis by zeolites is discussed further in Chapter 15. 

Microcrystalline solids such as zeolites and zeolite like structures have shown the utility of those properties in the domain of acid catalysis. However, little is known on their possibilities as base catalysts. It has been shown [ref. 1,2] that zeolites have basic sites which are able to catalyze reactions needing weak and medium basic strengths. Moreover, a correlation between the basicity and the Sanderson s average electronegativity Df the framework has been observed [ref. 3], Then, their activity as base catalysts can be modified by changing the countercation [ref. 4], the framework Si/Al ratio, or by introducing atoms other than Si and Al in the framework [ref. 5],... 

Since solid acid catalysts are used extensively in chemical industry, particularly in the petroleum field, a reliable method for measuring the acidity of solids would be extremely useful. The main difficulty to start with is that the activity coefficients for solid species are unknown and thus no thermodynamic acidity function can be properly defined. On the other hand, because the solid by definition is heterogeneous, acidic and basic sites can coexist with variable strength. The surface area available for colorimetric determinations may have widely different acidic properties from the bulk material this is especially true for well-structured solids like zeolites. It is also not possible to establish a true acid-base equilibrium. 

The second aspect which makes the interest of the study of zeolites by XPS is that it allows to understand many unexpected aspects of this spectroscopy, in particular when applied to solids. Zeolites in many respects are materials which may be regarded as model solids and not only because of their regular pore structure. Indeed by varying the Si/Al ratio, the nature of counter cations, the ion exchange level, the occlusion of extraframework phases, such properties as the density and strength of acidic or basic sites may be varied in a predictable fashion. All these variations are reflected in the XPS spectra yielding data... 

Alkali metal-exchanged zeolites have been used to prepare activated alkenes of interest as prepolymers by Knoevenagel condensation of malononitrile with ketones having different positive charge density on the carbon of the carbonyl group benzophenone, cyclohexanone, and p-aminoacetophenone [85]. The reactivity depends both on ketone structure (the order of reactivity was benzophenone > cyclohexanone > p-aminoacetophenone) and on the catalyst used. For instance, when malononitrile is condensed with cyclohexanone in the presence of a CsY zeolite conversion was very low. CsX zeolite, however, which has a substantial number of basic sites with pX in the range 9 < P a < 10.7 and some with 10.7 < < 13.3 could be used to perform... 

In both instances the size of the alkali metal cation, the chemical composition, and the structure of the zeolite have an impact on the basic properties of the final catalysts. Hence, zeolites can be prepared with basic sites of different strength, enabling selection of the catalyst most suitable for a particular reaction. 

The effect of the content of alkali earth oxides MgO and BaO on the catalytic and basic properties of NaX zeolites have also been investigated by Kovacheva ct al. (32. 46]. For the MgO-modified NaX zeolites, the best catalytic properties were obtained by NaX zeolite impregnated with a maximum of 13 wt. % MgO. The significant improvement in the catalytic properties was attributed to the presence of a moderate amount of strong basic sites. The catalytic activity was found to depend on the temperature and the MgO content. It was demonstrated that the catalysts possess well preserved crystal structure and low amount of MgO crystal phase. It was reported that the amount of basic active sites but not the strength is dependent on the content of MgO in the zeolite. BaO/NaX was also found to be an effective catalyst for the oxidative methylation of toluene with methane. The catalytic properties of this zeolite was reported to be superior to those of the MgO/NaX zeolite. It was demonstrated that an increase in the barium content leads to an increase in the amount of basic sites and a corresponding increase in catalytic activity. The catalysts were found to possess a preserved crystal structure and the existence of differently dispersed BaO and formation of BaC03 were detected in the catalysts. The conversion of toluene, selectivity and the yield of ethylbenzene plus styrene were reported to depend on (i) the content of barium in the zeolite, (ii) the reaction temperature, (iii) the time of catalytic use, (v) the state of the zeolite crystal structure and (vi) the state of barium in the zeolite lattice. 

Spectroscopic methods have been devoted to the characterization of zeolite structure, of surface sites or species (hydroxyls, acid and basic sites, cations), of extraframework phases (oxides and hydroxides, supported metals), and of adsorbed phases (gaseous or liquids). The paper will consider examples of each type. 

Another way to prepare calcium aluminate supports and sorbents is based on the interaction of Ca(OH)2 with Al(OH)3 followed by heat treatment of the resulting Ca3[Al(OH)6]2- This last compound can be thermolyzed to yield Cai2Ali4033, in which the zeolite type structure was clearly established. IR spectroscopic investigation provided evidence for the bifunctional nature of the active sites on the surface of calcium aluminate based supports and sorbents. The presence of acidic and basic sites on the surface makes the use of these solids as catalysts for acid-base tranformations of organic substances promising. 

For instance, two series of alkali-metal ion-exchanged zeoUtes have been investigated in order to analyze the possible correlations between the acidity and basicity of the X and Y zeolite structures and their catalytic properties [137]. The catalytic results for the 4-methylpentan-2-ol conversion show that activity and selectivity are both affected to some extent by the acid-base character of the catalysts. The activity was found to increase from Cs to li for both X and Y zeoUtes. The dehydrogenation reaction occurred only on Cs-X+CS2O, which presents very strong basicity. The product selectivity of the reaction was found to depend on both Lewis acidity and basicity Lewis basic or acidic sites of zeoUtes can be considered as acid-base pairs, in which both basic framework oxygens and neighboring cations are important [ 137]. 

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