Modifying the Catalytic Activity of Ti-Zeolites by Isomorphic Substitution of Si by Ge Atoms. A Periodic Quantum-Chemical Study

The role of isomorphic substitution of Si atoms by Ge on the properties of Ti sites in Ti-zeolites has been studied in periodic models by means of the ab initio code CRYSTAL98. Several zeolitic models with different Ti, Si, and Ge contents have been considered for calculation, and differences in structural, energetic, and electronic features at the equilibrium geometry have been investigated. The behavior regarding the interaction with a given probe ligand (a water molecule) was also studied. It has been observed that introduction of Ge atoms close to the Ti centers results in the catalyst stabilization mainly due to increased flexibility. From the electronic point of view, the decrease in the framework electronegativity upon substitution of Si by Ge atoms increases the ionicity of the system as well as decreases the Lewis acidity. However, structural effects derived from framework flexibility can mask inductive effects on acidity, to the point that the expected trend can be reverted.