HZSM-5分子筛焙烧脱铝的27Al MQMAS NMR研究

用29Si、27Al魔角旋转固体核磁共振(MAS NMR)结合二维多量子魔角旋转(2D MQMAS)技术对焙烧脱铝的HZSM-5分子筛中铝的配位状态进行了研究.结果表明,HZSM-5分子筛经焙烧后,在化学位移(δ)45处出现一宽峰信号,其主要来自扭曲四配位铝.通过二维三量子铝谱计算出扭曲四配位铝的四极作用常数约为5.2 MHz.对700和750 ℃焙烧样品的铝谱进行分峰拟合,发现在δ 30处又出现一个小峰,归属为非骨架五配位铝.同时,在750 ℃焙烧样品的二维多量子铝谱中直接观察到非骨架五配位铝的信号.焙烧温度低于700 ℃,脱铝不明显；高于700 ℃,引起分子筛骨架的显著脱铝.焙烧还造成部分骨架铝的信号变得“不可观测”.

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 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.