采用密度泛函理论研究金属离子改性Y型分子筛的酸性

采用量子力学中的密度泛函理论（DFT）研究了各种金属离子改性对Y分子筛 B酸强度的影响，并进一步探讨了分子筛酸性与金属离子性质的关系。结果表明，以水合离子形式进入Y型分子筛β笼Ⅰ′位的金属离子与分子筛骨架 O2和 O3 原子相互作用，使 O1原子的负电荷减弱，Al-O1键长变短，O1-H 伸缩振动频率减小，导致Y分子筛的 B酸强度增大；按照金属离子对Y分子筛 B酸强度的影响，可将改性用的金属离子分为3类，Ⅰ类为 V³⁺ 、Ga³⁺ 、Cr ³⁺ 、In³⁺和 Fe³⁺ ，Ⅱ类为 Y³⁺、Ce³⁺和 La³⁺ ，Ⅲ类为 Ni²⁺ 、Zn²⁺ 、Mg²⁺ 、Cu²⁺和 Mn²⁺ ，则金属离子改性Y分子筛的 B酸性的相对强度由大到小的顺序为Ⅰ类金属离子改性、Ⅱ类金属离子改性、Ⅲ类金属离子改性Y分子筛。

Study on the Acidity of Metal Cation Modified Y Zeolites by Density Functional Theory

Density functional theory was applied to study the B acidity of Y zeolites modified by metal cations. Furthermore, a relationship between the B acidity of metal cation modified Y zeolites and the properties of metal cations was discussed. The results showed that the interaction between the metal cation located in the sodalite cage Ⅰ′ site of Y zeolites and the framework O3 and O2 atoms made the reduction of negative charge of O1, the shortening of Al—O1 bond length and the decrease of O1—H stretching vibration frequency, which resulted in the increase of the B acid strength of modified Y zeolites. Moreover, it was proposed that the metal cations for Y zeolite modification could be divided into three groups according to their different influences on the acidity of Y zeolites, involving Ⅰ group(V³⁺ ，Ga³⁺ ，Cr ³⁺ ，In³⁺， Fe³⁺ ），Ⅱ group(Y³⁺ ，Ce³⁺， La³⁺+) and Ⅲ group( Ni²⁺ ，Zn²⁺，Mg²⁺ ，Cu²⁺， Mn²⁺). The B acid strength of the Y zeolites modified by these metal cations decreased in the order of Ⅰ group metal cations, Ⅱ group metal cations and Ⅲ group metal cations modified Y zeolites.