Ge-ZSM-5: The Simultaneous Incorporation of Ge and Al into ZSM-5 Using a Parallel Synthesis Approach

A series of Ge-ZSM-5 zeolites has been synthesized as the first part of a study on the effect of Ge incorporation on catalytically relevant properties of ZSM-5, such as framework acidity, framework structure, and crystal morphology. Products have been characterized by XRD, SEM, NMR, nitrogen adsorption, NHa-TPD, and infrared (FT-IR) spectroscopy. The simultaneous incorporation of Al and Ge has been proven by elemental analysis, ²⁷Al MAS NMR, FT-IR, and XRD. Ge incorporation changes the crystallization behavior in such a way that, in a controlled and reproducible manner, spherical aggregates of Ge-ZSM-5 crystallites could be prepared. Nitrogen physisorption measurements reveal that Ge-ZSM-5 contains more meso- and macroporosity than ZSM-5, which may be related to the high number of interfaces between the small crystallite particles of which these spherical aggregates are built up. Furthermore, Ge incorporation leads to a change in unit-cell symmetry of the zeolites (NH₄ ⁺ form) from monoclinic to orthorhombic. NH₃-TPD suggests that Ge incorporation does not lead to a different framework acidity. FT-IR experiments on H-ZSM-5 and H-Ge-ZSM-5, in contact with chemisorbed CO molecules, indicate that the strength of the strong Brønsted acid sites does not change upon Ge incorporation, as a similar frequency shift in the hydroxyl stretch vibration region has been observed upon interaction with CO. Upon Ge incorporation, a broadening of double five-ring lattice vibrations at around 550 cm^-1 is observed. Lattice local modes at around 950 cm ^-1, due to asymmetric stretching vibrations of Si-O-Ge groups, are found when Ge is incorporated into the MFI framework. The incorporation of Ge in the MFI framework causes an increased number of local distortions and defects leading to a greater variety of hydroxyl groups and significant changes in crystal morphology. The former can be concluded from an increase of the infrared absorption band related to Si-OH groups (3746 cm^-1) and the latter from SEM images and nitrogen physisorption isotherms.