Benzene and Phenol as Probes for Acid Sites

In the literature, various other probe molecules were proposed to characterize the acidity of zeoHtes by IR inspection of their adsorption. Among them were, besides those already mentioned, benzene, (cf. Ref. [669]). The wavenumber shift of the band due to Bronsted acid OH groups, which occurs on benzene adsorption, is easily determined and provides, in a manner similar to CO, an approximate measure of the acid strength. An example is illustrated in Fig. 49. 

Benzene adsorption into molecular sieves was extensively studied via IR spectroscopy by several workers. Karge et al. [791 ] used the prominent band at 1478 cm to identify benzene adsorbed in zeoUtes. De Mallmann and Barthomeuf [792,793] apphed FTIR spectroscopy of benzene adsorbed on various faujasite-type zeohtes (Na-X,Rb-X,Cs,Na-XNa-Y,Rb-Y,dealuminatedY) to determine the sites of adsorption and showed that the site preference depended on the balance between cation acidity and oxygen basicity. For a quantitative evaluation of the 

In related studies, Norberg et al. [722] investigated the location and motion of benzene molecules in Na-EMT and K-L by in-situ FTIR spectroscopy and concluded that almost all benzene molecules were facially coordinated to cations residing in the large cages of the adsorbents. 

Jacobs et al. [724] introduced the FTIR measurement of the shift of the OH stretching band upon benzene adsorption as a measure of the acid strength of the hydroxy groups. Jacobs [725] also provided a theoretical reasoning for this effect. Similarly, O Malley [794] developed an electrostatic model for predicting the shift of typical IR bands upon adsorption of aromatics on the respective zeolitic OH groups. 

In view of alkylation of benzene hypropene, Flego et al. [798] investigated by UV-Vis and IR spectroscopy the adsorption and co-adsorption of these two hydrocarbons on H-Beta, which proved to be an efficient alkylation catalyst. It was claimed by the authors that isopropyl cations, which formed on the Bronsted acid sites were the key intermediates of the reaction (cf. also Janin et al. [671 ] and Macedo et al. [672]). 

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