Structural Control Of Hierarchical Zeolite And Its Performance In Catalytic Cracking

Light olefins,such as ethylene and propylene,are among the most important feedstock in petrochemical industry.Currently,steam cracking of naphtha is the main process to produce ethylene and propylene.However,this method consumes much energy,produces greenhouse gases and has limitation in adjusting the production distribution.Accordingly,to crack the naphtha over porous catalysts has been developed recently,which has higher ethylene and propylene selectivity,lower reaction temperature and more adjustable product distribution.ZSM-5 zeolite,containing abundant protonic sites,has been widely applied to this process.However,the catalytic applications of ZSM-5 zeolites are limited by mass transport in the micropores when bulky molecules are involved in the reaction.In this regard,the introduction of mesopores into microporous zeolite to obtain the hierarchical zeolites,possessing the catalytic features of microporosity and the improved mass transport consequence of mesoporosity,would be an optimal choice for its better performance.Thus,in this disseration,hierarchical ZSM-5 zeolites with unchanged acidity and different micropore/mesopore ratios have been successfully synthesized by employing the organosilane as the template and adjusting TPAOH/SiO2 ratios.Through the n-heptane catalytic cracking over hierarchical zeolites with different micropore/mesopore ratios,the reaction model has been developed.The lower VMicro/VMeso ratio from 1.688 to 0.661 can enhance the mass transfer process more obviously and reduce the diffusion resistance within the catalyst,leading to improvement of ethylene and propylene selectivity from 55.3%to 64.7%.In addition,the effect of internal crystal size of hierarchical ZSM-5 zeolites on diffusion and catalytic performance has also been investigated.The reaction rate-limiting step in the zeolites changes as a function of internal crystal size.In the microporous ZSM-5 zeolite with a single crystal size of 300 to 500 nm,the n-heptane catalytic cracking proceeds under the transition-limiting condition,the selectivity of ethylene and propylene is relatively low.With the decrease of internal crystal size of hierarchical zeolites from 50 nm to 15 nm,the rate-limiting step turns to reaction control,leading to reduction of the diffusion resistance and rapid diffusion for products out of the channels.As a result,ethylene and propylene selectivity has been obviously improved from 51.2%to 62.2%,and coke deposition is accordingly decreased.Despite of the pore structure and crystal size,the acidity of hierarchical zeolite is another key factor to influence the catalytic cracking performance.The acid sites,distributed in the internal surface(micropore)or external environment(including external surface and mesopore wall),play different roles in chemical reaction.To explore the effect of internal/external acidity on the reaction,in this disseration,a series of hierarchical ZSM-5 zeolites with similar pore structures but different external acid properties have been synthesized.Tartaric acid was employed to poison the acidic sites in external environment selectively and extract A1 from the framework.By estimating the product distribution,the brief model of monomolecular/bimolecular reaction mechanism in micropore/external environment has been proposed.Selective deactivation of the external acid sites of the zeolite with lower Si/Al ratio can suppress undesirable bimolecular reactions more obviously,such as alkylation,hydrogen transfer and isomerization reactions.This leads to the external acid amount of hierarchical ZSM-5 zeolite with Si/Al ratio of 48 decreasing from 0.07 mmol·g-1 to 0.018 mmol·g-1 and the increase of the selectivity of light olefins by 8%,with which better resistance to coke formation is also obtained.Meanwhile,to further investigate whether all above reaction mechanisms can be also applied to the hierarchical zeolites with ordered mesopores,core-shell ZSM-5@MCM-41 hierarchical zeolite with order mesopore channels has been obtained.A series of ZSM-5@MCM-41 zeolites with different shell thickness have also been applied to the n-heptane catalytic cracking reaction.Through the comparison of pristine ZSM-5 and ZSM-5@MCM-41 zeolite with shell thickness of 60 nm,the latter exhibits the superiority of catalytic performance with enhanced n-heptane conversion from 61.8%to 69.5%and light olefins selectivity from 51.2%to 61.8%.These can be ascribed to the facilitated capture for large-molecule reactants in large channels,well-interconnection between the micropore and mesopore,and surface passivation by shell coverage.All these advantages are favorable for the reduction of diffusion resistance and suppression of bimolecular reactions.