Al 3Q MAS NMR spectroscopy has been applied to study small amount of pentacoordinated Al emerges. The majority of the signal, however, arises from distorted tetrahedral Al. the coordination state of the species giving the 30 ppm Al NMR signal in the 27 Al MAS NMR spectrum of activated A two-step calcination results in a significant contribution of the pentacoordinated Al to the signal at 30 ppm. From the mordenite materials. From the 27 Al 3Q MAS NMR measurements it is evident that the broad peak at 30 ppm in simulated line-shape, a quadrupole coupling constant of 6.2 MHz is obtained for the latter signal. These data show that the 27 Al NMR spectrum of the mordenite calcined at temperatures up to 600 Β°C comes mainly from the distorted during the calcination of the mordenite, the coordination environment at the Al centre gradually becomes distorted to four-coordinated Al. By simulation a quadrupolar coupling constant of 5.8 MHz was estimated for the distorted give rise to the shoulder at 30 ppm. With the increase of the calcination temperature, pentacoordinated Al species form tetrahedral Al. For samples calcined at 650 and 700 Β°C, a out the second-order quadrupolar interaction of the Al nu- [a] Centrum voor Oppervlaktechemie en Katalyse, Departement Interfasechemie, K.U. Leuven, (Figure 1a) is assigned to the framework tetrahedrally coor-Kardinaal Mercierlaan 92, B-3001 Heverlee (Leuven), Belgium dinated Al atoms, while that at 0 ppm results from extra-