Collect. Czech. Chem. Commun. 1995, 60, 393-402

Pyridinium Ions - New Probe for Basic Sites of Solid Acids

Josef Kotrlaa, Jan Floriánb, Ludmila Kubelkováa and Jacques Fraissardc

a J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 182 23 Prague 8, Czech Republic
b Institute of Physics, Charles University, 121 16 Prague 2, Czech Republic
c Laboratoire de Chimie des Surfaces, Université P. et M. Curie, CNRS-URA 1428, 4 Place Jussieu, 75252 Paris Cedex 05, France


The hydrogen bonding interactions of the pyridinium ion (PyH+) with the deprotonated skeleton of acid zeolites were studied using Fourier transform infrared (FTIR) spectroscopy and quantum chemical ab initio calculations. It was found that the frequency of the NH stretching vibration, n(NH), of the newly formed H-bond increases in the zeolite sequence HNaX < HY < HM ~ HZSM-5. This is related to decreasing basicity of the skeletal oxygens (basic sites conjugated to bridging hydroxyls). Therefore, pyridine can serve as a probe for the basic sites of acid zeolites. In this way it can complement the pyrrole probe, which is established for basicity monitoring of alkali-metal exchanged zeolites.