Fluorine-free Hydrothermal Synthesis Of Siliceous Zeolite Beta And Sn-Beta Zeolite

As important adsorbents and efficient catalysts,zeolites play an indispensible role in the field of chemical industry.It is meaningful to synthesize zeolites with new structures,to develop new synthetic methodology and to explore the new application fields of zeolite materials from the viewpoints of both scientific research and practical application.Pure silica Beta zeolite with a three-dimensional 12-ring channel system,large pore volume and high hydrophobicity shows excellent adsorption performance for volatile organic compounds(VOCs).At the same time,it can also be used as catalyst carrier for metal support,which has potential application values.The purpose of this work is to explore the efficient and green synthesis of pure silica Beta zeolite and achieve a breakthrough in the synthesis methodology.Fortunately,a novel fluorine-free hydrothermal synthesis strategy has been successfully developed for the synthesis of pure silicon Beta and Sn-Beta.And detailed characterization and physicochemical property research have been carried out on the synthesized samples.The work mainly includes into two parts:The first part:the siliceous zeolite Beta was synthesized with TEA+as template under the assistance of seeds and inorganic salt.It was found that various inorganic cations with small size including Na+,Li+,Mg2+,etc.,could effectively promote the synthesis of pure siliceous Beta zeolite,whereas anions have weak effect on the product phase and yield.The crystallization process of siliceous zeolite Beta was studied systematically.The intrinsic effect of inorganic cations on the improvement of solid yield is revealed to be owing to their promoting effect on the polymerization of SiO2 species.In the early stage of crystallization of siliceous Beta,the formation of pure silicon Beta follows the solid-solid hydrogel hydrogel rearrangement mechanism assisted by seeds.Subsequently,solution-mediated pathway contributes to the further crystal growth.In addition,it is found that higher calcination temperature can help heal the silanol defects and enhance the hydrophobicity of the materials.The obtained siliceous Beta has few framework defects,high hydrophobicity and large adsorption capacity for toluene,which implies its promising application for toluene adsorption.This strategy offers a feasible way to achieve the industrial synthesis of siliceous zeolite Beta with excellent performance.The second part:based on the synthesis strategy assisted by inorganic cations and seed crystals,Sn-Beta zeolite with a higher Sn content(3 wt%)and good crystallinity has been successfully synthesized through the adjustment of gel compositions and crystallization conditions.The micropore specific surface area and micropore volume of the synthetic material are lower than those of the reference sample synthesized by fluoride-assisted method,but their total specific surface areas are close and the former has slightly higher total pore volume.The characterization results revealed that the Sn atoms in the obtained samples mainly existed as framework tetra-coordinated Sn species and the framework has lower amount of Si-OH defects.