Role of oxyanions as promoter for enhancing nucleation and crystallization in the synthesis of MFI-type microporous materials

The addition of a small amount of certain oxyanions of Group VA and VIIA (such as perchlorate, phosphate, arsenate, chlorate and bromate) to the synthesis mixture of zeolites significantly enhances the nucleation and crystallization process of silica-based molecular sieves. Studies on MFI-type materials like ZSM-5 (Al-MFI), TS-1 (Ti-MFI) and Silicalite-1 (Si-MFI) are reported. Various promoters reduce the synthesis time of ZSM-5 to a varying degree. For example, the time taken to obtain fully crystalline ZSM-5 with different promoters followed the order: ClO^-₄ (7 h)≤PO^3-₄ (8 h)<AsO^3-₄ (14 h)none (48 h). A direct correlation is observed between the charge/radius ratio (Z/r) of the central cation of the promoter and the synthesis time of ZSM-5 prepared with that promoter. As the polarizing ability (Z/r of the central cation) of the promoter increases, the crystallization time decreases. The concept is applicable not only to aluminosilicate (ZSM-5) but also to metallosilicate (TS-1) and Silicalite-1. Detailed studies were carried out using phosphate as promoter. The quality of the material (crystallinity and adsorption/catalytic properties) synthesized in the presence of promoter was comparable with, if not better than, that prepared in the absence of promoter. Generally, the crystallite size of ZSM-5 samples prepared in the presence of phosphate as promoter was smaller with a narrower particle-size distribution than that of corresponding samples prepared in the absence of promoter. Although in the case of Silicalite-1 and TS-1 prepared by using tetraethyl orthosilicate as silica source, the particle size was quite small (0.1-0.2 μm) in the presence as well as absence (0.2 μm) of promoter, the solid yield of crystalline material was higher for the sample synthesized in the presence of promoter (82% versus 71% on the basis of SiO₂+TiO₂). For studying the role of promoter, the Silicalite-1/PO^3-₄ system was chosen because of its simplicity and low-temperature (85°C) synthesis in just 12 h. ³¹P nuclear magnetic resonance data of the synthesis mixture of Silicalite-1 using NaH₂PO₄ indicated a catalytic role of the promoter in enhancing the rate of nucleation and crystallization.