COLLOIDAL MOLECULAR SIEVES: MODEL SYSTEM FOR KINETIC STUDY OF CRYSTAL GROWTH PROCESS

摘要

A clear TPAOH-Al_2O_3-SiO_2-H_2O solution was used as a model system for a kinetic study of crystal growth of colloidal zeolite ZSM-5 with MFI type structure by dynamic light scattering (DLS), X-ray diffraction (XRD), Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM) and nitrogen sorption measurements. The Raman scattering and nitrogen sorption results show that the initial hydrothermal treatment leads to a condensation of nanoparticles with porous amorphous matrix composed mainly of small-sized rings of TO_4 tetrahedra. HRTEM images reveal the establishment of long range order in few-nanometer-sized spatial regions inside the amorphous matrix under heating at elevated temperature. Prolonged hydrothermal treatment leads to the formation of a concentration of chain-like entities of five-membered rings typical of MFI structure in the disordered T-O network, that enable the subsequent transformation from a disordered to an ordered microporous system detectable with X-ray diffraction. Both Raman spectroscopic and X-ray diffraction data show that the amorphous precursor colloids are completely transformed into crystalline ZSM-5 zeolite under heating at 90℃ for 18 h. According to DLS observations the mean hydrodynamic radius of the crystalline nanoparticles corresponds to the average size of the XRD-amorphous particles formed immediately after preparation of the precursor aluminosilicate mixture.