Preparation of Zeolites via the Dry-Gel Synthesis Method

The principles and advantages of the dry-gel conversion (DGC) method are described for the synthesis of zeolites Beta and EU-1 with different aluminum and gallium contents in the framework. By modern analytical techniques, the processes occurring during the conversion of a dry gel into zeolite particles were investigated. In the case of the dry-gel synthesis of zeolite [Ga]Beta, XRD indicated a fast formation of the long-range order as a function of the DGC time while, as revealed by NMR spectroscopy, the rearrangement of the local structure occurs during a period of up to 65 h. This rearrangement of the local structure during the drygel conversion process consists of different concerted mechanisms: The dominating step at the beginning of the dry-gel conversion is a breakage of chemical bonds leading to a strong increase in the concentration of defect SiOH groups. As the DGC time increases, the number of defect SiOH groups and of Q, Q, and Q silicon species decreases significantly, indicating a condensation of bonds. In the synthesis of zeolite [Ga]EU-1 it was found that the most critical parameters are the amount of water present in the autoclave during the crystallization process and the contents of sodium cations and template molecules in the dry gel.