Abstract: The coordination states of aluminum species in calcined HZSM-5 zeolites have been investigated using 29Si, 27Al MAS and 2D MQMAS NMR methods. A broad peak at δ 45 in presents in the 27Al MAS NMR spectra of calcined HZSM-5 zeolites. From the 27Al 3QMAS NMR spectrum, it is evident that the peak at δ 45 comes mainly from the distorted tetrahedral Al with a quadrupolar-coupling constant of 5.2 MHz. Simulation of the 27Al 1D MAS NMR spectra of samples calcined at 700 and 750 ℃ shows that a small peak at about δ 30 emerges, which is probably due to 5- coordinated non-framework Al. The existence of this Al species is also confirmed by 27Al 3QMAS NMR.HZSM-5 zeolite calcined at temperature higher than 700 ℃ suffers severe dealumination. Calcination causes part of the Al species “NMR invisible” in the corresponding 27Al MAS spectra.

Key words: HZSM-5, NMR, MQMAS, Dealumination, Calcination