Zeolites acid strength effects

Fig. 1. a) Standard protonation enthalpy in secondary carbenium ion formation on H-(US)Y-zeolites with a varying Si/Al ratio, b) Effect of the average acid strength for a series of H-(US)Y zeolites experimental (symbols) versus calculated results based on the parameter values obtained in [11] (lines) for n-nonane conversion as a function of the space time at 506 K, 0.45 MPa, Hj/HC = 13.13 (Si/Al-ratios 2.6, 18, 60)... 

Extensive studies of the acidity and basicity of zeolites by adsorption calorimetry have been carried out over the past decades, and many reviews have been published [62,64,103,118,120,121,145,146,153,154]. For a given zeolite, different factors can modify its acidity and acid strength the size and strength of the probe molecule, the adsorption temperature, the morphology and crystallinity, the synthesis mode, the effect of pretreatment, the effect of the proton exchange level, the Si/Al ratio and dealumination, the isomorphous substitution, chemical modifications, aging, and coke deposits. 

Based on the above results, we conclude that the diazomethane modification of zeolites is an effective method to change selectively the amount and strength of the surface Bronsted acidic sites. Therefore the method could be used to study the role of Bronsted and Lewis acidic sites preferably for low temperature 300 0 catalytic reactions. 

Transesterification Reactions. The heterogeneous acid-catalyzed transesterification of TGs has not been investigated as much as its counterpart, the base-catalyzed reaction. Various solids are available with sufficient acid strength to be effective catalysts for the named reaction. Among the solid acids available are functionalized polymers, such as the acid forms of some resins, as well as inorganic materials, such as zeolites, modified oxides, clays, and others. Some of these solids have already been found to be effective in transesterification reactions of simple esters and (3-ketoesters. 

Zeolites [63] are extensively used as shape-selective solid acid catalysts in many industrial processes [64]. Their acidic properties stem from the presence of trivalent elements, such as Al, in the zeolite framework. The strength of these acid sites is one of the main features that determine the catalytic properties of a zeolite catalyst. Substitution of the Al atoms by other trivalent elements, such as Ga, Fe, and B, alters the strength of these acid sites, and hence also the catalytic properties of a zeolite. The possible effect of the partial substitution of the tetravalent Si atoms (which, in principle, do not create acid sites in zeolites) by Ge atoms (also tetravalent) on the catalytic properties of zeolite ZSM-5 [65] is presented here. The idea is that the different electronic and geometric properties of Ge, compared with Si, may influence the acid sites related to the Al atoms, and thereby the catalytic properties of ZSM-5. 

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