Hierarchical ZSM-5 based MCM-41 aluminosilicates: Ostwald ripening effect of 8 years old aged samples
B Rabindran Jermy and Vijaya Ravinayagam
MCM-41 with zeolitic nanocomposite has been reported to reduce diffusional restrictions, strengthen structural stability of mesophase and improve acidity. Here we study the natural growth process known as Ostwald ripening effect over composite zeolite particles with aging time. Three different MCM-41/ZSM-5 composites were studied along with conventional AlMCM-41 (AlM41) that are stored for 5 and 8 years. Textural features of MCM-41 with proto-zeolitic nanoclusters at pore walls (ZMST2), ZSM-5/MCM-41 composite with equiproportion mixture of meso and microphases (50 wt%) (ZM27), disordered hexagonal/ZSM-5 composite (ZM23), and AlMCM-41 were compared using x-ray diffraction (XRD), nitrogen adsorption isotherm, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), 27Al MAS NMR, scanning electron microscope (SEM) and transmission electron microscope (TEM). Calcined AlMCM-41 showed aging stability, while calcined ZMST2 showed high Ostwald ripening effect. The disordered hexagonal surface area and pore volume of ZMST2 increase to ~54% and ~68% compared to fresh ZMST2. Remarkably, aging in as-synthesised form (A-ZMST2) slowed down the Ostwald process and improved hexagonal thermal stability at 550 °C and 800 °C. ZM27 exhibited high aging stability even in calcined form. The surface area showed about 92% of retainment with an increase in the pore volume (~20%). ZM23 with disordered hexagonal structure showed that aging period of 5 years decreases meso and increases micropore characters of ZSM-5. Overall, the study shows Ostwald ripening effect dominates over calcined ZMST2, while A-ZMST2 and A-ZM27 in as-synthesised form are able to withstand the structural collapse during long shelf time. Prolonged aging rearranges aluminium coordination environment similar to micropore environment of ZSM-5. The study will further help to understand the atmospheric moisture interactive process with hierarchical zeolites and facilitate preventive measures for long-term industrial applications.