Dealumination of HZSM-5 zeolite by calcination:: 27Al multiple quantum MAS NMR study

The coordination states of aluminum species in calcined HZSM-5 zeolites have been investigated using Si-29, Al-27 MAS and 2D MQM.AS NMR methods. A broad peak at delta 45 in presents in die Al-27 MAS NMR spectra of calcined HZSM-5 zeolites. From the Al-27 3QMAS NMR spectrum, it is evident that the peak at delta 45 comes mainly from the distorted tetrahedral Al with a quadrupolar-coupling constant of 5.2 MHz. Simulation of the Al-27 ID MAS NMR spectra of samples calcined at 700 and 750 degreesC shows that a small peak at about delta 30 emerges, which is probably due to 5- coordinated non-framework Al. The existence of this Al species is also confirmed by Al-27 3QMAS NMR. HZSM-5 zeolite calcined at temperature higher than 700 degreesC suffers severe dealumination. Calcination causes part of the Al species "NMR invisible" in the corresponding Al-27 MAS spectra.