Acidic Properties of H−β Zeolite As Probed by Bases with Proton Affinity in the 118−204 kcal mol-1 Range:  A FTIR Investigation

The interaction of Brønsted sites of H−β (AH) with bases B with proton affinity (PA) ranging in the 118 (N2) to 204 (NH3) kcal mol-1 interval leads to the formation of 1:1 adducts. The vibrational manifestations of these adducts change considerably as a function of the proton potential. In 1:1 neutral adducts characterized by weak-medium hydrogen bonds, the shift of the ν(AH) mode (Δν̄) is proportional to the shift caused by the same bases on the Si−OH (weak) Brønsted acid used as a standard. As far as the 1:1 adducts formed with ethers and tetrahydrofuran (THF; PA = 196 kcal mol-1), the situation is more complex owing to high ionicity acquired by the hydrogen bond. When stronger bases are used (pyridine, Py), the true proton transfer is observed. The ν(AH···B) modes of the medium-strong 1:1 adducts are modulated by Fermi resonances with 2δ and 2γ overtones and with internal modes of A and B moieties. A general review of the phenomenon is given. Also the spectra of the ionic 1:1 adducts formed with Py and NH3 are modulated by complex Fermi resonance effects. The 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 originally formed neutral 1:1 adducts are transformed into protonated clusters.