Characterization of the internal and external acidity of H-MCM-22 zeolites

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 m(2.)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) (2,4DMQ(+)) 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.