科研之友
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摘要
The location and coordination of nickel cations inside the hexagonal prism of dehydrated sodium faujasites have been investigated as a function of the framework Al content and distribution by density functional theory (DFT) calculations. Three different Si/Al ratios have been considered in order to represent the Na-LSX (Si/Al = 1), Na-X (Si/Al = 1.4), and Na-Y (Si/Al = 2.0) faujasites that have the same framework topology but different basic properties. For each system, the most representative Al repartitions among the framework tetrahedral sites have been taken into account and their related preferred Ni(2+) configurations have been established. Upon decrease of the Al content, the Ni(2+) coordination changes from a perfect octahedral environment with Ni(2+) located in the center of the hexagonal prisms (Ni/Na-LSX) to five 4-fold and four 3-fold coordinations in Na-X and Na-Y faujasites, respectively. The preferred locations, inside the hexagonal prism, are those where Ni(2+) can bind the highest number (highest coordination) of basic framework oxygen atoms even though the related Ni-O distances simultaneously increase. All configurations are associated to a significant distortion of the hexagonal prism as compared to the host Na-faujasite cluster which illustrates the zeolite flexibility. While 4-fold coordinated nickel weakly interacts with the framework Y-faujasite, it induces a stronger framework distortion. The monitoring effect of lattice oxygen atoms toward Ni-framework interaction and framework deformation as well as Ni(2+) coordination and location inside zeolites are rationalized.
- 出版日期2011-4-7
