Aerosol-assisted Synthesis Of Hierarchical High-silica Beta Zeolite

Beta zeolite with unique three-dimensional pore structure,adjustable acidity and high hydrothermal stability has been widely used in petrochemical industry.In particular,high-silica Beta zeolite exhibits unique properties of low acidity density and strong hydrophobicity,which has drawn considerable attention.However,there are still many challenges in the synthesis of high-silica Beta zeolite.In alkaline system,pure silica Beta has been synthesized using complex template,which makes the preparation complicated and the cost increased.The product is highly hydrophilic.In near-neutral systems,the zeolite can also been synthesized in HF media.The product has fewer defects and shows highly hydrophobic,but the crystallization time is longer and the crystal size is larger.In this paper,the aerosol-assisted hydrothermal method was used to synthesize high-silica Beta zeolite.It is expected that the amounts of template agent and mineralization agent can be reduced and the crystallization time can be shortened in high concentration environment.NaF agent instead of normal HF and NH₄F agents was used to increase the alkalinity of system and reduce crystal size.Then the synthesis conditions were optimized and the crystallization mechanism was studied.The catalytic performance was evaluated by the cracking of 1,3,5-TIPB.Beta zeolite was synthesized by aerosol-assisted hydrothermal method in NaF media.When SiO2/Al2O3 ratio is 50,synthesis conditions were systematic optimized.The effects of crystallization mode and crystallization temperature,template and mineralization amounts,crystallization time were studied.The optimal synthesis conditions are:TEAOH/SiO2=0.15,NaF/SiO₂=0.15,120°C pre-crystallized for 24 h then 150°C crystallized for 24 h.The template and mineralization amounts were reduced,and the crystallization time was shortened.Crystallization process followed liquid phase transformation.Beta zeolite synthesized in traditional fluoride system?NH4F?is the large crystal with micron size.However Beta zeolite synthesized in NaF system shows nano-aggregates,which has larger mesoporous volume and external surface area.Secondly,aboved optimal synthesis method was expanded to the synthesis of high-silica Beta zeolite.By reducing crystallization temperature,increasing crystallization time and adding seed,the hierarchical high-silica Beta zeolite was successfully synthesized without the second template.As SiO₂/Al₂O₃ ratio increases,the morphology of the Beta zeolite changes from spherical nano-aggregates to plate-like crystals.The amounts of the Br?nsted acid sites decrease,and the mesoporous volume increases gradually.BS-2 sample was synthesized from pure silica precursor.The synthesis conditions are:TEAOH/SiO₂=0.2,NaF/SiO₂=0.2,seed/Si=5 wt%,120°C pre-crystallized for 24 h then 130°C crystallized for 96 h.Defect free BS-2 sample shows hierarchical plate-like structure with a size of 100×500×500 nm and the SiO₂/Al₂O₃ ratio of 392.This study provides a new idea for the synthesis of hierarchical high-silica Beta zeolite.