Amine-free synthesis of high-silica ZSM-5 assisted with calcined silicalite-1 and ethanol with the investigation of mechanism

Amine-free synthesis of high silica ZSM-5 with high crystallinity and well-defined morphology was challenging. In this work, ZSM-5 with the fed silica-alumina ratio (SAR) of 100 was rapidly synthesized with the assistance of the calcined silicalite-1 seed and ethanol, and their crystal structures, morphology and microporous properties were characterized by XRD, SEM, FT-IR, ICP and N2 adsorption-desorption and so on. Firstly, the crystallization temperature was optimized between 140-180 °C at a certain alkalinity and ethanol content, and 160°C was selected to not only provide high crystallization rate but also ensure the stability of high-silica skeleton. Secondly, the influences of the EtOH/SiO2 ratio, Na2O/SiO2 ratio and crystallization time on the crystallization of high-silica ZSM-5 were investigated at 160 °C. The results showed well-crystalline high-silica ZSM-5 could be synthesized within a wide range of the EtOH/SiO2 ratios (0.5-2.0) and the increase of ethanol content slightly inhibited the growth of ZSM-5 along b-axial of hexagonal prisms due to the preferential adsorption of excess ethanol on the {010} crystal plane and the formation of Si-O-C2H5; high alkalinity was favorable for the preparation of a small-sized and well-crystalline ZSM-5 due to the rapid depolymerization of calcined seed and the contribution of mass nucleation sites; meanwhile, excellent micropore characteristic of high-silica ZSM-5 further proved that the synthetic strategy of seed+ethanol was feasible. Finally, the synergistic action mechanism of calcined seed and ethanol for the synthesis of high-silica ZSM-5 was revealed that silicalite-1 with MFI structure unit combined with Na+(H2O/EtOH) with larger radius were very important for rapidly assembly of a large number of silica species into MFI framework.