Adsorption of pyridine and of bulkier bases, 2,6- and 2,4-dimethylquinoline (DMQ), was carried out over
five H-MCM-22 (MWW) samples differing by their Si/Al ratio (10, 17, 29, 50, 53) and their external surface
area (49, 114, 102, 55, 33 m2·g-1). The comparison of the IR spectra of 2,6- and 2,4-DMQ pure or adsorbed
on silica, alumina and large pore zeolites (H−BEA) shows that physisorbed and hydrogen bonded species
can be eliminated by evacuation above 473 K. Because of steric hindrance, DMQ molecules do not interact
with Lewis sites and hence can only be chemisorbed as dimethylquinolinium ions (DMQ+). Extinction
coefficients were estimated for characteristic bands at 1547 and 1649 cm-1 (2,6-DMQ+), at 1647 cm-1 (2,4-DMQ+) and at 3620 cm-1 (bridging hydroxyl groups). For adsorption temperatures of 473−573 K, 2,6-DMQ
molecules were protonated on most of the protonic sites of the H-MCM-22 samples. The bulkier 2,4-DMQ
molecules can be protonated only on part of the inner sites (most likely those located near the surface of
crystallite edges) and on the protonic sites of the external pockets. The concentration of the latter sites can
be estimated from the difference between the concentrations of 2,4-DMQ+ ions and of the inner bridging
hydroxyl groups (IR band at 3620 cm-1) which interact with 2,4-DMQ molecules. This concentration was
found to be close to the one of external pocket sites active in m-xylene transformation at 623 K. It is also
proportional to the external surface area. All that indicates that 2,4-DMQ chemisorption followed by IR
spectroscopy is a suitable method for characterizing the outer acidity of H-MCM-22 zeolites.
