Coal fly ash to Y zeolite of great purity and crystallinity: A new and green activation method of combined in situ microwave and ultrasound

Highly crystalline and pure Y (FAU) zeolite were synthesized from coal fly ash (CFA) using a combined in situ microwave and ultrasonic irradiation-assisted method, improving the extraction efficiency of silicon and aluminum from coal fly ash with limited energy usage relative to conventional microwave or ultrasound methods. Significant improvement in the combined method was evident following activation in 4 mol/L sodium hydroxide solution for 1 h at 100°C whereby 39.80 wt% conversion of CFA was attained along with 70.26 mol.% conversion of Si and 68.03 mol.% conversion of Al. Solid-state 29Si and 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were used to investigate the framework chemical coordination environment, while the temperature-programmed desorption of NH3 (NH3-TPD) and pyridine-Fourier transform infrared (Pyridine-FTIR) spectra were employed to quantitatively detect the acidity and types of acid sites of the sample. The Si to Al ratio (SAR) of the as-obtained CFA-based Y zeolite was 4.65 with a specific surface area of 916.10 m2/g, leading towards excellent CO2 adsorption (107.14 cm3/g at 0 °C). Furthermore, the thermocatalytic performance of the CFA-Y sample was analyzed in the conversion of cumene, with high silica commercial ultra-stable Y (USY) zeolite as a reference.