Amine-free synthesis of fly ash based ZSM-5 via interzeolite transformation with related investigation of mechanism

Fly ash is a solid waste consisted of SiO2, Al2O3, CaO, Fe3O4 and trace amounts of heavy metal (Hg and Cd), therefore, their disposal could pose some threats to the environment. In this paper, the interzeolite transformation of NaA to ZSM-5 was explored as a low-cost and high value-added green process, where fly ash was resource utilized, and the conventionally high-cost silicalite-1 seeds and high-contamination organic amine were not required. Firstly, fly ash was activated with NaOH at high temperature, followed by the alkali-dissolving purification to remove the impurities. Then, purified fly ash was utilized to synthesize NaA at 110 °C for 12 h, and the transformations of NaA to ZSM-5 were conducted and the process parameters were optimized such as crystallization temperature, duration time, the content of additives such as ethanol or sodium chloride and the pretreatment of NaA, and so on. The results showed that the introduction of a trace amounts of NaCl or EtOH could greatly facilitate the growth of ZSM-5, and the high crystalline ZSM-5 molecular sieves were successfully obtained at 160 °C for 36 h with the comparable textual properties to the commercial ZSM-5. Finally, an assembly mechanism of MFI framework in the interzeolite transformation of NaA to ZSM-5 was proposed.