Study On Preparation Of Hierarchical Molecular Sieves And Their Catalytic Performance

Molecular sieve materials are widely used in adsorption separation,catalysis,ion exchange,etc.,due to their regular pore structure,large specific surface area,adjustable acidity and high thermal/hydrothermal stability.Zeolites with high crystallinity skeletons and orderly microporous/caged structures are extremely important catalysts and exhibit excellent catalytic properties in alkylation,isomerization,catalytic cracking and biomass conversion.However,traditional zeolite catalysts are limited by their microporous channels(<2 nm).The mass transfer process is dynamically affected,molecular diffusion is blocked,and byreactions and carbon accumulation are easily induced.Carbon accumulation can overcoat the active sites of catalysts and even block molecular sieve channels,leading to catalyst inactivation.Due to mass transfer constraints,many active sites within zeolites are not effectively utilized,and in the case of catalytic reactions,the external surface of zeolites is used only,which seriously affects catalytic effect.In order to improve the catalytic performance of traditional zeolites,the researchers put forward a number of strategies for the construction of high activity molecular sieves,mainly in two directions:1)to construct high activity catalytic centers to improve catalytic efficiency;2)to synthesize hierarchical molecular sieve catalysts to improve the mass transfer efficiency,so as to enhance catalytic effect.Through the structural adjustment of molecular sieve and metal oxide modification,the performance of catalyst is expected to be improved,which is very important for the effective utilization of resources and environmental protection.In this thesis,we focus on synthesizing highly active zeolite catalysts and their applications on organic reactions.Hierarchical molecular sieves with high catalytic activity were synthesized by post-treatment method and template method,and multiporous acid catalysts rich in active components of metal oxides were prepared by modification.The structure and catalytic mechanism of the catalyst were studied from the microscopic point of view,The alkylation properties of anthracene were studied for the SZ-DeAl-DFNS zeolite,the oxidation properties of anthracene were studied for the Mo1W2-MCM-41 zeolite,and the alkylation properties of naphthalene were studied for the PW-ZSM-5-Al zeolite.The main research contents and relevant conclusions are as follows:1.The Al-DFNS molecular sieve rich in microporous and mesoporous structures was synthesized by one-step hydrothermal method using surfactant hexadecetylpyridine bromide(CPB)as the template agent.By studying the effects of aluminum source,preparation method and cosurfactant on Al-DFNS,combined data analysis of SEM,TEEM,BET and XRD obtained the simultaneous presence of microporous and mesoporous structures at the beginning of the reaction.The prepared molecular sieve is a nanoflower structure with outward radiation from the center,the particle size is about 200～300 nm,and the molecular sieve contains rich microporous connected mesoporous structure.The active-component zirconium Zr and sulfate SO42were loaded on the carrier and the hierarchical SZ-DeAl-DFNS catalyst were prepared and applied to the alkylation reaction of anthracene with a conversion rate of more than 60%and a selectivity of more than 80%.2.Using the mesoporous MCM-41 molecular sieve as carrier,and introducing molybdenum Mo and tungsten W into the carrier by co-precipitation method,a series of hierarchical MoW-MCM-41 catalysts with different molybdenum-tungsten ratios were successfully constructed.The prepared Mo1W2-MCM-41 catalyst was characterized by XPS,H2-TPR and TPD,revealing the intrinsic relationship between catalyst performance and catalytic activity.Besides,the reaction of the catalyst oxidation from anthracene to anthraquinone under mild conditions was studied,and more kinetic studies and mechanism were explored.To date,the Mo1W2-MCM-41 catalyst has been successfully used for the oxidation of anthracene in more than 95%yield(laboratory trials),which provides a green synthesis method for the catalytic oxidation of PAHs.3.The microporous ZSM-5 molecular sieve was selected,and the hierarchical ZSM-5 molecular sieve carrier was prepared by post-treatment.The active components P and tungsten W were further loaded on the carrier surface by immersion,and the PWZSM-5-Al catalyst was prepared by calcination.With SEM and TEM analysis,the synthesized catalyst showed a pore-rich coffin shape.Combined with the BET data,the pore size distribution of PW-ZSM-5-Al is broader compared to the microporous ZSM5,rich in micropores(5 nm)and mesoporous(15 nm)structure.This catalyst facilitates the full contact of the reaction medium and the active components,and improves the catalytic efficiency of naphthalene and 2,6-diisopronaphthalene reactions.Meanwhile,an exploratory study on the alkylation of fluorene with this catalyst has been carried out and preliminary results have been obtained.The highest conversion rate is 28.6%and the selectivity of 2-tert-butylfluorene is 78.5%.