Pyridine, Ammonia and Amines as Probes for Acid Sites

Pyridine, Ammonia and Amines as Probes for Acid Sites 

The bands of pyridine adsorbed on zeohtes are relatively narrow and, therefore, well-suited for a determination of the density of the sites, i.e., the amount of pyridinium ions generated through interaction with acid OH groups or the amount of pyridine molecules coordinatively bound to L- or C-sites. In fact, for a determination of the absolute concentrations of sites the knowledge of the extinction coefficients is required (cf. Sect. 3.2). 

Using the FTIR/pyridine technique, Janin et al. [680] also investigated silanol groups and extra-framework Al-containing debris in dealuminated H-Y zeohtes. A band at 3747-3749 cm was sensitive to pyridine adsorption and ascribed to 

A combination of X-ray photoelectron and infrared spectroscopy, where both techniques employed pyridine as a probe, was used by Borade and Clearfield [492] when they tested the acidity of a series of zeolites. All of their samples contained strong Bronsted sites, weak Bronsted sites and Lewis sites. With respect to the Bronsted acid strength of the hydrogen forms these authors arrived at the following sequence H-Omega H-Beta H-ZSM-22 H-mordenite H-Y. 

The effect of the synthesis medium (OH - or F -containing) on the acidity of [Si,Al]ZSM-5 was investigated by IR spectroscopy with and without pyridine as a probe in the work by Joly et al. [681 ]. These au ors found in the case of H-MFl synthesized in the presence of OH only Bronsted add sites, indicated by the OH band at 3600 and the pyridinium band at 1545 cm, whereas synthesis in F -con-taining media resulted in materials with more Lewis add sites (Py— L bands at 1622 and 1455 cm ), depending on the nsi/n i ratio. 

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