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

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.