稀土对SAPO-11分子筛结构与性能的影响

Modification of the Structure and Properties of SAPO-11 Using Rare Earths

分别采用浸渍法及离子交换法将稀土元素镧、铈引入SAPO-11分子筛,通过XRD、BET、孔分布、in-situ FTIR及NH3-TPD等表征手段考察了稀土对SAPO-11分子筛结构及表面性能的影响.结果表明,SAPO-11分子筛经稀土元素La、Ce改性后,分子筛摩尔组成改变、结晶度下降,其中La3+离子交换改性带来的变化最为明显.在分子筛表面聚集的稀土元素可作为Lewis酸中心,造成分子筛L酸含量增多,B/L比值下降;同时,由于三价稀土离子吸引电子的能力弱于Al3+,改性后催化剂的桥连羟基强度下降.经稀土改性后,催化剂总酸量下降.

Silicoaluminophosphate （SAPO）-11 molecular sieve was modified by rare earth La and Cc using impregnating and ion-exchanging methods. The RE-modified SAPO-11 samples were characterized by X-ray diffraction （XRD）, Brtmauer-Emmett-Teller （BET）, pore-size distribution （PSD）, in-situ Fourier transform infrared spectroscopy （FTIR）, and temperature-programmed desorption of ammonia （NH3-TPD）, and compared with unmodified SAPO-11 sample. The XRD patterns indicated that after RE modification, especially for the La-exchanged sample, the composition of molecular sieve was changed and the crystallinity decreased. The normalized surface area （NSA） of ion-exchanged samples proved that the unsubstituted metal oxides accumulated inside the SAPO-11 pores. Because of the RE cations that were loaded onto the surface of molecular sieve, which could be regarded as the Lewis acid sites, there was an increase in the amount of Lewis acid sites and decrease in the BrФnsted acid/Lewis acid （B/L） ratio. The ability of RE cations to acquire electrons was weaker than that of Al^3＋; therefore, the bond strength of bridged hydroxyl decreased after modification. The total acidity of SAPO-11 decreased after RE modification.