Zeolites base strength

The Lewis acid-base properties of zeolites received a growing interest recently. A series of works " suggested that the infrared and XPS spectra of adsorbed pyrrole can characterize both the Lewis basicity and the Lewis acidity of alkali-exchanged zeolites. The zeolite base strength decreases with an increase in Si/AI ratio of the framework, and increases when the electropositivity of the counter alkali cation is raised. The Lewis acid strength of counter cations increases with Si/AI ratio and decreases with electropositivity. In other words, the alkali-exchanged zeolites have both Lewis acid and Lewis base centres and provide an easy way to tune up the relative strengths of these two sites. 

I 6 Aspects of Mechanisms, Processes, and Requirements for Zeolite Separation Table 6.5 Base strengths of aromatic hydrocarbons relative to HF. 

Utilization of zeolite acidic strength in Cg-aromatics (xylens) systems is illustrated next. In the presence of strong acids, xylene isomers have varying basicity (Table 6.5), with m-xylene being the most basic and p-xylene the least basic among the Cg-aromatics [27]. Based on the basicity of the xylenes, the acidity of each zeolite can be properly adjusted to selectively adsorb m-xylene or p-xylene. As demonstrated in Figure 6.4, a more acidic zeolite such as NaY will selectively adsorb m-xylene from other Cg-aromatics [28, 29], In contrast. Figure 6.5 shows that a weaker acidic zeolite such as KY will selectively adsorb p-xylene from other Cg-aromatics [30, 31]. In both systems, toluene was used as the desorbent. 

In this chapter, a brief summary of studies that made use of calorimetry to characterize compounds comprising group IIIA elements (zeolites, nitrides, and oxides catalysts) was presented. It was demonstrated that adsorption microcalorimetry can be used as an efficient technique to characterize the acid-base strength of different types of materials and to provide information consistent with the catalytic data. 

Side-Chain Alkylation. There is continued interest in the alkylation of toluene with methanol because of the potential of the process in practical application to produce styrene.430 Basic catalysts, specifically, alkali cation-exchanged zeolites, were tested in the transformation. The alkali cation acts as weak Lewis acid site, and the basic sites are the framework oxygen atoms. The base strength and catalytic activity of these materials can be significantly increased by incorporating alkali metal or alkali metal oxide clusters in the zeolite supercages. Results up to 1995 are summarized in a review.430... 

However, the second step requires stronger basic sites than the first step. This demonstrates the importance of being able to tune the base strength. In this sense, zeolites form a valuable complement to other solid base catalysts, e.g. MgO, or mixed oxides derived from layered double hydroxides. 

Alternatively, quaternary ammonium residus can be immobilized on solid phases, and anion-exchanged with hydroxyl anions. These simple materials are highly similar to strong basic anion exchangers, but obviously, their basicity can never exceed that of OH anions (44). In summary, there is a large scale of zeolite-based base catalysts available, and the catalyst choice can be determined by the base strength desired. 

Cobalt, copper and nickel metal ions were deposited by two different methods, ionic exchange and impregnation, on an amorphous silica-alumina and a ZSM-5 zeolite. The adsorption properties towards NH3 and NO were determined at 353 and 313 K, respectively, by coupled calorimetric-volumetric measurements. The average acid strength of the catalysts supported on silica-alumina was stronger than that of the parent support, while the zeolite-based catalysts had (with the exception of the nickel sample) weaker acid sites than the parent ZSM-5. The oxide materials used as supports adsorbed NO in very small amounts only, and the presence of metal cations improved the NO adsorption [70]. 

We have recently studied systematically the Lewis basicity of zeolites using pyrrole as a probe molecule. Pyrrole has H-donor properties and adsorbs on basic zeolites forming NH—O bridges. The NH stretching vibration was shown by Barthomeuf [51-53] and others [54] to be a measure of the Lewis base strength of 0 in zeolites. 

However, it should be emphasized that the measurement and quantification of the acid/base strength of zeolites is complex and that it is difficult to directly compare the aciti base strength of a solid with that of a liquid. This results from the feet that the stabilization of carbocations and carbanions in a microporous solid differs from that in strongly polar acid and base solutions. For zeolites, it can be stated that the concentiatlbn of aluminum in the lattice is directly proportional to the concentration of acid sites and the polarity of the lattice and to a first approximation indirectly proportional to the strength of acid sites [40]. For a given chemical composition of the zeolite, the polarity of the lattice increases with decreasing framework density [41],... 

Owing to the emphasis on zeolite acid strength made in the literature, it is not surprising that reactivity of zeolites, when not shape selectivity phenomena are involved, has been mostly explained on the bases of the ionic terms of equation (1). 

Microheterogeneous electrocatalysis can be performed with zeolite-based catalysts using suspensions of such materials in organic solvents with low ionic strength by applying a DC voltage to suspensions of zeolite particles, as described by Rolison and Stemple (1993) for the Pd"-Cu"-NaY catalyzed oxidation of propene, but a wide variety of systems has been studied. 

Sometimes the choice of zeolite can determine which product is obtained. This is true in the alkylation of 4-methylimidazole with methanol (6.34) in the vapor phase.197 The product from a Meerwein-Ponnodorff-Ver-ley reduction (6.35) varied with the zeolite catalyst.198 High selectivity for acrolein, propylene, and allyl ether from allyl alcohol (6.36) can be obtained by using different zeolites.199 A combination of acid or base strength and cavity size may be responsible for these effects. 

It has been reported that zeolites can be used as base catalysts when exchanged with alkali metal ions. The base strength is inversely related to the charge to radius ratio of the compensating cation, i.e. the larger cation the stronger the basicity of the associated framework oxygen of the zeolite. 

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