Aerosol-assisted Synthesis Of Sn-Beta Zeolite And Its Catalytic Performance In The Baeyer-Villiger Oxidation Reaction

Sn-Beta zeolites,which have unique three-dimensional 12-ring BEA topology and Lewis acidity,show excellent catalytic perfonnances in many catalytic reactions.However,the size mismatch between Sn and Si atoms brings many difficulties in the hydrothermal synthesis of Sn-Beta zeolite,including long crystallization time,low framework Sn content and large crystal size.In order to solve the problems,the aerosol-assisted hydrothermal method is proposed to synthesize Sn-Beta zeolite.The zeolite nucleation and growth can be regulated by the design of the zeolite precursor and the control of the synthesis conditions,to synthesize the hierarchical Sn-Beta zeolite with high Sn content and short crystallization time.The growth mechanism of the Sn-Beta zeolite was investigated by various characterization methods.The Baeyer-Villiger(B-V)oxidation reactions were applied to evaluate the catalytic performances of the Sn-Beta catalysts.The main contents and results are as follows:(1)The aerosol technique was applied to prepare the solid tin-silicon composite oxide microspheres,which were used as the zeolite precursors.The hydrothermal synthesis was performed in a super-dense crystallization system(H2O/SiO2=2.8-5.6).The crystallization process followed a liquid-phase mechanism.The peculiar features of the synthesis were investigated by optimizing the synthesis conditions and exploring the crystallization mechanism.As compared to the conventional hydrothermal synthesis method,this method can not only reduce the amounts of templating agent and fluoride mineralizer,but also significantly shorten the synthesis time,increase the Sn content and reduce crystal size.On this basis,the obtained Sn-Beta zeolite was further treated by weak base(ammonia)to prepare the hierarchical Sn-Beta zeolite.However,serious collapse of the zeolite structure and loss of Sn species were observed when the sample was treated with only ammonia.The addition of CTAB during the ammonia treatment can prevent the excessive etching of the zeolite crystals and the loss of the Sn species,and make the pore size distribution more uniform.The product yield(2h)using the post-treated Sn-Beta zeolite is 51%,which is 17 percentage points higher than that using the parent in the B-V oxidation reaction of 2-adamantanone.The post method can also be used to prepare the other hierarchical high-silica and transition metal-substituted zeolites.(2)To limit the growth of zeolite crystals the organosilane(PHAPTMS)was successfully doped into the tin silicon oxide precursor by the aerosol method.With the aid of tetraethylammonium hydroxide(40wt%),fluoride mineralizer and seeds,nano Sn-Beta aggregates were hydrothermally synthesized in a high concentration crystallization system(H2O/SiO2=2)for the first time.The morphology,the size of the nanocrystals and the pore structure can be adjusted by changing the amount of PHAPTMS.Due to its excellent diffusion performance,the nano Sn-Beta aggregates exhibit excellent catalytic performance in the B-V oxidation reaction of cyclohexanone and 2-adamantanone.In particular,the product yield(2h)is 56%in B-V oxidation reaction of 2-adamantinone.This work provides a new idea for the hydrothermal synthesis of the small-size zeolite in fluorine-containing system.(3)A Sn-Si composite oxide with hollow mesoporous structure was prepared by the aerosol method.By using this material as a precursor,hierarchical Sn-Beta zeolite can be hydrothermally synthesized in an extremely concentrated crystallization system(H2O/SiO2=1)without adding any mesoporogen.This strategy can greatly increase the Sn content(Si/Sn=38)of the Sn-Beta zeolite in a shorter crystallization time(4d).Moreover,the hierarchical structure can be obtained due to the voids between small grains in the zeolite.Due to its high Sn content and hierarchical structure,the catalyst exhibits excellent catalytic performance in the B-V reaction of 2-adamantanone.The yield reaches 80%after the reaction for 2h.The conversion and selectivity reach 99%when the reaction time is 4h.