Aluminium coordinations in zeolite mordenite by Al-27 multiple quantum MASNMR spectroscopy

Al-27 3Q MAS NMR spectroscopy has been applied to study the coordination st ale of the species giving the 30 ppm Al NMR signal in the Al-27 MAS NMR spe ctrum of activated mordenite materials. From the Al-27 3Q MAS NMR measureme nts it is evident that the broad peak at 30 ppm in the Al-27 NMR spectrum o f the mordenite calcined at temperatures up to 600 degrees C comes mainly f rom the distorted four-coordinated Al. By simulation a quadrupolar coupling constant of 5.8 MHz was estimated for the distorted tetrahedral Al. For sa mples calcined at 650 and 700 degrees C, a small amount of pentacoordinated Al emerges. The majority of the signal, however, arises from distorted tet rahedral Al. A two-step calcination results in a significant contribution o f the pentacoordinated Al to the signal at 30 ppm. From the simulated line- shape, a quadrupole coupling constant of 6.2 MHz is obtained for the latter signal. These data show that during the calcination of the mordenite, the coordination environment at the Al centre gradually becomes distorted to gi ve rise to the shoulder at 30 ppm. With the increase of the calcination tem perature, pentacoordinated Al species form.