采用X射线粉末衍射(XRD)、魔角旋转固体核磁共振(MAS NMR)、程序升温脱附(TPD)以及吡啶-傅里叶变换红外(Py-FTIR)光谱等方法研究了稀土离子对Y型分子筛结构稳定性和酸性的影响.结果表明:稀土离子的引入不仅显著增强了Y型分子筛的结构稳定性,而且调变了分子筛酸性,使分子筛中等强度酸中心数量增多,强酸中心数量减少.结合密度泛函理论(DFT)计算结果,从理论上阐述了这种机制:进入分子筛β笼l′位的稀土离子与骨架O原子相互作用,增加了Al-O作用力,提高了分子筛的结构稳定性,从而避免了骨架Al的脱除和非骨架Al物种的生成,进而导致稀土氧Y(REHY)分子筛的Br(o)nsted (B)酸强度较USY分子筛的弱;但与HY分子筛相比,稀土离子的引入使骨架O1负电荷减弱,Al-O1键长变短,O1-H作用力减弱,导致REHY分子筛的B酸强度较HY分子筛的强;在酸中心数量方面,REHY分子筛的B酸中心数量比USY分子筛的多,但比HY分子筛的少.%The influences of rare earth (RE) cations on the stability and acidity of Y zeolites were studied by X-ray powder diffraction (XRD), solid-state magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR), temperature-programmed desorption (TPD) of ammonia, and Fourier transform infrared of pyridine (Py-FTIR). The results showed that the stability of Y zeolites was enhanced markedly and the medium acid amount increased, but the strong acid amount of Y zeolites decreased with the introduction of RE cations. Combined with the density functional theory (DFT) calculations, a comprehensive model was proposed to describe the mechanism of RE cations on the stability and acidity of Y zeolites. RE cations located at the sodalite p cage I' sites of Y zeolites bonded strongly with O atoms, which strengthened the interaction between framework Al and its neighbouring O atoms and then enhanced evidently the stability of Y zeolites. It was the enhanced stability of REHY zeolites that restrained the release of framework Al and the formation of extra-framework Al, hence led to weaker Bronsted acid strength of REHY than that of USY. Meanwhile, RE cations located at the sodalite p cage I1 sites of Y zeolites made the negative charge of 01 reduce and the bond length of Al-01 shorten, which resulted in stronger Bronsted acid strength of REHY than that of HY. Moreover, the Bronsted acid amount of REHY was more than that of USY, but less than that of HY.
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