ACIDIC PROPERTIES OF H-BETA ZEOLITE AS PROBED BY BASES WITH PROTON AFFINITY IN THE 118-204 KCAL MOL(-1) RANGE - A FTIR INVESTIGATION

The interaction of Bronsted sites of H-beta (AH) with bases B with pro ton affinity (PA) ranging in the 118 (N-2) to 204 (NH3) kcal mol(-1) i nterval leads to the formation of 1:1 adducts. The vibrational manifes tations of these adducts change considerably as a function of the prot on potential. In 1:1 neutral adducts characterized by weak-medium hydr ogen bonds, the shift of the upsilon(AH) mode (Delta<(upsilon)over bar >) is proportional to the shift caused by the same bases on the Si-OH (weak) Bronsted acid used as a standard. As far as the 1:1 adducts for med with ethers and tetrahydrofuran (THF; PA = 196 kcal mol(-1)), the situation is more complex owing to high ionicity acquired by the hydro gen bond. When stronger bases are used (pyridine, Py), the true proton transfer is observed. The 2,(AH B) modes of the medium-strong 1:1 add ucts are modulated by Fermi resonances with 2 delta and 2 gamma overto nes and with internal modes of A and B moieties. A general review of t he phenomenon is given. Also the spectra of the ionic 1:1 adducts form ed with Py and NH3 are modulated by complex Fermi resonance effects. T he situation occurring at higher dosages of B is also examined: it is concluded that in the presence of an excess of H2O (and CH3OH) the ori ginally formed neutral 1:1 adducts are transformed into protonated clu sters.