Nitriles as Probes for Acid Sites

The interaction of deuterated and chlorinated acetonitrile, CD3CN and CCI3CN, respectively, with Bronsted acid sites of H-ZSM-5 and H,D-ZSM-5 and the thus-induced changes in the IR spectra were interpreted by Pehnenschikov et al. [657] in the frame of the resonance theory of the A-B-C triplet, developed for molecular H-complexes. The approach was similar to that of water adsorption (vide supra). Kotrla and Kubelkova [729] discussed in great detail the spectral features observed on the adsorption of deuterated acetonitrile, designated as 

(or acetone, vide infra) on a series of Bronsted acid zeolites (H, Na-X, H, Na- 

H-ZSM-5) and correlated especially the shift, Av (OH), of the (high-frequency) OH stretching bands to the acid strength (Sanderson s charge on the H atom). The shifted OH bands appeared as broad signals between 3025 (AN H-X) and 2540 cm (AN - H-ZSM-5) for the neutral complexes. Also, the in-plane, 6(OH) and out-of-plane, y(OH), bending vibrations were measured and compared with the results of Fermi resonance calculations. Interaction of di-acetonitrile with NH4-, H, Na-, H-, and Co-Beta led to the detection of Brqnsted and Lewis sites through the appearance of bands at 2297 and 2325 cm , respectively. Na+ and Co + were identified by bands at 2284 and 2308 cm, respectively. 

In particular, the combined application of acetonitrile and the bulkier adamantanecarbonitrile (diameter greater than 0.6 mn) in FTIR experiments proved to be suitable for discriminating internal from external acid sites of medium pore zeolite crystallites such as ZSM-5, chabazite, erionite and ferrierite. For the same purpose, Trombetta et al. [695] employed 2,2-dimethylproprionitrile (pivalonitrile, PN) instead of adamantanecarbonitrile. They studied B- and L-sites on the (external) surface of ferrierite and the total surface of [Si]MCM-41 and [ Al] MCM-41. Upon adsorption of PN, the C = N band was shifted from 2236 to about 2250 cm. The weak B-sites of [Al]MCM-41 interacted with pivalonitrile and were also associated with an OH band at 3745 cm.  

Chloromethane was employed by Su and Jaiunain [768] as an FUR probe for Bronsted acidity and their relative strength in a number of hydrogen forms of 

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