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

Abstract

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 2δ and 2γ levels with the continuum distribution of levels associated with the ν(OH···B) mode coupled with the low-frequency ν(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.

Mechanism of NO-SCR by methane over Co,H-ZSM-5 and Co,H-mordenite catalysts

“…8B, spectrum (a)). The adsorbed H 2 O also generated three broad bands, the so called ABC triad, around 2860, 2380, and 1700 cm -1 (only the latter two are discernible in the shown frequency range), which are due to H-bonding between adsorbed water and acidic OH-groups [16,40]. The H-mordenite has two kinds of bridged hydroxyl groups: one, located in main channels and another in the side pockets, giving ν OH bands at -1 , respectively (vide infra).…”

Quantum chemical calculation of infrared spectra of acidic groups in chabazite in the presence of water

“…During the last decade, adsorption of water on zeolites has been extensively studied both experimentally [1][2][3][4][5][6][7][8] and theoretically [9][10][11][12][13][14][15][16][17][18][19][20] in order to understand the nature of the interaction of basic molecules with the Brøn-sted sites in the zeolites. The reaction can be studied by infrared spectroscopy because the frequencies of the vibrations of the zeolite hydroxyl group will change upon interaction with the water molecule.…”

“…The reaction can be studied by infrared spectroscopy because the frequencies of the vibrations of the zeolite hydroxyl group will change upon interaction with the water molecule. When water is adsorbed on H-ZSM-5, [1][2][3][4][5][6]21 HY, 7 H-MOR, 4 or chabazite 22 a triplet of bands appears at 2872-3000, 2403-2515, and 1700 cm Ϫ1 , known also as an ABC triplet. This triplet is accompanied by two peaks at 3700 and 3550 cm Ϫ1 .…”

“…The frequency of the A band that we have calculated is lower, 60 cm Ϫ1 . Zecchina et al 4 have reported frequencies for the zeolite in-plane bending of 1352 cm Ϫ1 and for the out-of-plane bending of 870 cm Ϫ1 . The theoretically calculated frequencies are again in a good agreement with the experiment.…”

Probing the Accessible Sites for n-Butene Skeletal Isomerization over Aged and Selective H-Ferrierite with d3-Acetonitrile

“…In recent years CD 3 CN has been widely employed in IR spectroscopic studies (21)(22)(23)(24)(25)(26)(27)(28) because of its intermediate proton affinity (29) and the specific and high sensitivity of its ν(CN) stretching mode for interactions with different acid sites present in zeolites. Besides the ability to probe Brønsted and Lewis acid sites, Bystrov (21) and Jolly et al (23) reported that CD 3 CN could be used to probe secondary and tertiary carbenium ions.…”