Preparation of hierarchically structured Y zeolite with low Si/Al ratio and its applications in acetalization reactions

Abstract

Hierarchically structured Y zeolites were prepared by a post-synthetic strategy, where the as-made NaY zeolite was sequentially treated by a lactic acid solution and an alkaline solution containing cetyltrimethyl ammonium bromide (CTAB). Many techniques, including X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), N₂ adsorption–desorption, Fourier-transform infrared spectroscopy (FT-IR), Thermogravimetric analysis (TG-DTG) and NH₃ Temperature Programmed Desorption (NH₃-TPD), were applied to characterize the chemical and textural properties of the samples. The results show that lactic acid pre-modification of NaY zeolite may cause on the one hand Na⁺ cation removal by proton exchange and on the other hand the dealumination of the zeolite framework with the formation of amorphous silicon-rich species offering nutrients for the fabrication of mesoporosity. After alkaline treatment in the presence of surfactant CTAB, mesoporosity can be successfully introduced into the NaHY zeolites with the microporous structures well preserved. Furthermore, the hierarchically structured Y zeolites exhibit much better performances in the acetalization reactions with large sized molecules involved. This could be attributed to the enhanced diffusion ability of large sized guest molecules through the combination of mesoporosity and microstructures compared with pure microporous Y zeolites.