FTIR investigation of the formation of neutral and ionic hydrogen-bonded complexes by interaction of H-ZSM-5 and H-mordenite with CH3CN and H2O: Comparison with the H-NAFION superacidic system

The IR spectra of increasing doses of CH3CN interacting with H-ZSM-5, H-MOR, and H-NAFION are investigated and compared. In all cases the formation of neutral hydrogen-bonded adducts is observed and complete vibrational assignment is given. The basic IR spectroscopy of these complexes is discussed in the framework of the Evans approach on the Fermi resonance between the narrow 2 delta and 2 gamma levels with the continuum distribution of levels associated with the v(OH ... B) mode coupled with the low-frequency v(O ... B) vibrations and with the external (thermal) librations. At low dosages the interaction of H2O with H-ZSM-5 and H-MOR gives neutral hydrogen-bonded adducts. At higher dosages these species are transformed into H+(H2O)(n) (n(average)=3-5) ionic species. This process can be only partially reversed by decreasing the relative pressure of H2O. At low dosage the concentration of ionic species is higher on H-MOR than on H-ZSM-5. The interaction of H2O with the superacidic H-NAFION membrane readily gives directly the H+(H2O)(n) products without formation of the neutral intermediates. The decrease of the relative pressure of H2O is not accompanied by a back-transfer of the proton to the membrane. This differentiates the H-ZSM-5 and H-MOR solid acids from the superacidic H-NAFION membrane. The comparison of the IR spectra obtained on H-ZSM-5 and H-MOR on one side and on H-NAFION on the other side allows a detailed assignment of the spectroscopic manifestations of the neutral and protonated species.