Interaction of Pyridine with Acidic (H-ZSM5, H-β, H-MORD Zeolites) and Superacidic (H-Nafion Membrane) Systems:  An IR Investigation

Abstract

The interaction of pyridine with acidic H-ZSM5, H-β, and H-MORD zeolites and a perfluorosulfonic superacidic membrane (H-Nafion) has allowed us to study the IR spectrum of PyH+ hydrogen bonded to the negative zeolite skeleton (Z-) or to CF2SO3 - groups of the membrane. In the presence of excess Py the formation of PyH+···Py is also detected. On H-MORD (a zeolite with higher Al content) the adsorption of Py at room temperature is limited to the external surfaces of the crystallite, owing to the blockage of channel mouths caused by the PyH+ formed in high concentration. The IR spectra of hydrogen-bonded PyH+ in the ν(NH···) stretching region are strongly influenced by Fermi resonance effects with the overtones and combinations of low-frequency modes. In the H-Nafion/Py system the modification induced by the proton transfer on the internal modes of the −SO3H groups (with formation of SO3 - species) gives additional information on the proton transfer process. To complete the assignment of all the observed surface species (including the weakly adsorbed ones) the interaction of Py with silicalite (a purely siliceous zeolite with a pentasilic structure) has also been investigated.

Vibrational Spectroscopy of Zeolites

“…This is, for instance, the case for NH 3 59 and Py, 30,60 which lead to the formation of Z Ð Ð ÐH-NH 3 C and Z Ð Ð ÐHPy C adducts. As Z is a weak base (the conjugated acid is strong) and NH 4 C and PyH C are also weak acids, the acid-base hydrogen-bonding interaction is weak and the shift N n of the n(NH) mode is consequently that typical of a weak interaction.…”

Lewis Acid Zeolites for Biomass Conversion: Perspectives and Challenges on Reactivity, Synthesis, and Stability

“…In addition, they can be used to compare acidity and reactivity among heteroatoms. Pyridine adsorption coupled with FTIR spectroscopy has been traditionally used to distinguish between Brønsted and Lewis acid sites in zeolites (95)(96)(97)(98). However, for most Lewis acid zeolites, bands associated with pyridine-bound Lewis acid centers are not resolved enough to accurately distinguish between framework, extraframework, and nonadsorbing sites.…”

Preparation and Characterization of Dealuminated Metakaolin and Its Use in the Transformation of Waste Plastics to Aromatic Hydrocarbons

“…1a) exhibited bands due to Brønsted bound pyridine (BPYR) at 1634 and 1541 cm −1 and for Lewis bound pyridine (LPYR) at 1616 cm −1 together with a shoulder on the higher wavenumber side of the 1445 cm −1 band. The 1445 cm −1 band, in combination with that at 1596 cm −1 , was assigned to hydrogen bonded pyridine, HPYR (40). The 1490 cm −1 band was assigned to pyridine bonded to both Brønsted and Lewis acid sites, although the Brønsted bound base is known to contribute a greater intensity at this frequency.…”