Synthesis Of Hierarchically Structured ZSM-5 Zeolites And Their Catalytic Performance For Methanol-to-propylene

As an important chemical raw material,light olefins such as propylene and ethylene are widely used in the production of industrial products such as synthetic plastics,fibers and rubbers.However,the irreproducible oil resources,coupled with the shortage of oil resources and the richness of coal resources in China,make the traditional thermal cracking process difficult to meet the market demand for light olefins.Methanol-to-propylene（MTP）is one of the most promising non-petroleum routes because methanol can be produced cheaply and efficiently from coal,natural gas,biomass,and even CO₂.Numerous studies have shown that ZSM-5 zeolite is the preferred catalyst for MTP reactions.The acidic sites in ZSM-5 zeolites are the main active sites for methanol conversion.However,excessive acidic sites lead to side reactions such as hydrogen transfer and aromatization.Therefore,high-silica ZSM-5 zeolites are more suitable for MTP reaction.In addition,the sole microporous system in conventional ZSM-5 zeolite hinders the diffusion of reactants and products.In contrast,the mesoporous structures in hierarchically structured ZSM-5 zeolites（ZSM-5 HSZs）provide large specific surface area,shorten the diffusion path,and inhibit side reactions.Unfortunately,the introduction of mesoporous structures by post-etching or template methods is associated with the destruction of the zeolite framework,the high cost of mesoporous templates,and the removal of mesoporous templates that consume large amounts of energy and emit greenhouse gases.Very recently,our group has synthesized ZSM-5 HSZs without mesoporous agents based on the steam-assisted crystallization（SAC）process,but their Si/Al ratios are between 30 and 150,and MTP catalytic performances remain unclear.As a result,the following research works are carried out in this thesis:1.A series of high-silica（Si/Al=200）ZSM-5 HSZs were synthesized by mesoporogen-free SAC strategy.The effects of SAC duration on the structure,morphology,acidity and MTP catalytic performance were investigated.The characterization results show that the ZSM-5 HSZs achieve high crystallinity with interconnected micro/mesoporous structures,large specific surface area,total pore volume and uniform particle size when the SAC duration extended to 5 h.In addition,with the extension of SAC duration,some of framework Al species would be removed from the zeolite frameworks and finally re-incorporated again.Therefore,the relative concentrations of Br?nsted acid sites and Lewis acid sites of ZSM-5 HSZs change.The initial propylene selectivity of ZSM-5 HSZs after crystallization for 10 h was as high as 54.4%and the catalyst lifetime was as long as 175 h under the weight hourly space velocity of 1.0 g g^-1 h^-1 and reaction temperature of 460^°C.Interestingly,during the synthesis of ZSM-5 HSZs,the crystallized materials undergo a transition from“dry powders”to“damp powders”and finally to“precipitates”as the SAC duration extended.Based on the above results,we proposed a new“in situ alkali etching-hydrothermal restoring”mechanism.2.ZSM-5 HSZs with variable Si/Al ratio from 100 to 400 were synthesized by mesoporogen-free SAC strategy.The effects of different Si/Al ratios on the structure,morphology,acidity and MTP catalytic performance were investigated.The characterization results show that the aluminum content of samples have no significant effect on their micro/mesoporous structures and particle morphologies,but the acid densities decrease with the decrease of aluminum content.Further,the catalytic results showed that ZSM-5 HSZs with a Si/Al ratio of about 200 exhibited the longest catalyst lifetime,which was mainly due to the auxiliary mesoporous structure and appropriate acid density.In addition,the excessive or insufficient acid densities in ZSM-5 HSZs are not conducive to prolonging catalyst lifetime.3.ZSM-5 HSZs were synthesized by mesoporogen-free SAC strategy,and the concentration of structure-directing agent（tetrapropylammonium hydroxide,TPAOH）was controlled to adjust the zeolite particle size.Further,the effects of zeolite particle size on the MTP catalytic performance were investigated.The results showed that when the molar ratio of TPAOH to Si in precursor gel is increased from 0.07 to 0.13,the zeolite particle size decreased from 1μm to 200 nm,the initial propylene selectivity of the catalyst increased from 54.1%to 55.9%,and the catalyst lifetime increased from 91h to 220 h.Obviously,the smaller zeolite particles can effectively shorten the molecular diffusion paths,reduce diffusion barriers,and thus shorten the contact time between reactants and acidic sites,inhibit the occurrence of side reactions.