Structure Control Of Zeolite Nanosheet And Its Catalytic Cracking Performance

ZSM-5 zeolite is widely used in catalytic researches due to its unique pore structure,adjustable acid properties,excellent hydrothermal stability,and crystallization properties.However,its microporous system(<2 nm)would seriously affect the mass transport of bulky molecule reactants and reduce the catalytic efficiency.Therefore,great efforts have been made to introduce secondary pores into the crystal and synthesis hierarchical zeolite.In this thesis,the structure control of zeolite nanosheet and its catalytic performance have been studied.By adjusting the synthetic gel components,and controlling the self-assembly method of bi-quaternary ammonium surfactant and its interaction with the microporous template,the thickness,accumulation method,interlamellar spacing,and pillar of zeolite nanosheets are precisely controlled.An ordered hierarchical pore system is gradually constructed.On this basis,zeolites with different structures are applied to the naphtha cracking reaction,and the influence of structure changes on the pore properties,acid properties and cracking performance are investigated.The structure-performance relationship of nanosheet zeolite in naphtha cracking reaction is established,which could help to better understand the reaction mechanism and design catalysts.Firstly,by changing the molar ratio of bi-quaternary ammonium surfactant to SiO2,the controllable modulation of nanosheet thickness was realized,and its effect on the pore structure,acid properties,and catalytic activity were investigated.The results showed that an appropriate amount of surfactant was beneficial to reduce the thickness of zeolite nanosheets(4.1 nm),and the mesoporous ratio of this sample was much higher than that of the sample with a thickness of 16.6 nm(6.682 vs 5.211).However,the catalytic results showed that the selectivity of ethylene and propylene of the sample with a thickness of 16.6 nm could reach 62.4%,and the yield reached 56.7%.While the selectivity of light olefins of the sample with 4.1 nm was only 52.3%.The external surface acid of zeolite nanosheets was further characterized and found that although the thinner nanosheet had shortened diffusion path,the ultra-high external surface area would expose more acid,increasing the probability of the product adsorbed on the acid sites and experienced secondary reactions.In addition to the micropores,the excessive hydroxyl groups on the external surface of the zeolite would also increase the mass transfer resistance in the catalytic reactions.To reduce the external acid ratio of zeolite nanosheets,the accumulation method of nanosheets was changed from the cross-type to parallel-type through the dual-templates method.The results showed that about 59.5%of the acid was distributed on the external surface of the crossed nanosheets,while the parallel-type zeolite had almost no external acid due to its structure characteristics and the acid content.The diffusion time constant of parallel-type zeolite was increased by 32%compared with the cross-type one by using nheptane as the probe molecule,and the acid utilization of parallel-type nanosheets in the cracking reaction was increased by 43%.Moreover,the selectivity of parallel zeolite nanosheets for light olefins was increased by 12.9%,the yield reached 56.6%,and the amount of coke deposits was greatly reduced.Accordingly,this thesis proposes the "acid wall barrier" of ZSM-5 zeolite in the catalytic cracking reaction.The hydroxyl groups on the external surface of the zeolite would restrict the diffusion of reactants into the internal acid sites and the diffusion of products to the gas phase,reducing the catalytic performance.The thickness of the mesoporous layer is also an important structural feature of zeolite nanosheets,and the controllable modulation of interlamellar spacing is achieved by precisely adjusting the dual templates ratio.The microporous template was responsible for introducing MFI pillars between nanosheets,alleviating the collapse of nanosheets after calcination,and initially constructing an ordered pore system.A detailed study of the crystal growth mechanism showed that the interlayer spacing was related to the self-assembly arrangement of amphiphilic surfactants,which was affected by the hydrophilic/phobic properties of the synthesis gel.The introduction of polar molecules such as microporous templates and ethanol could change the hydrophilic balance in the gel,therefore adjusting the assembly method of the amphiphilic templates,and affecting the structure of the crystal.The catalytic effect of MFI pillared zeolite with varying interlayer spacing could be divided into two categories,shape selectivity and acid.When the acid content of zeolite was sufficient,the interlayer spacing decreased from 6.3 to 5.9 and 5.5 nm,and the corresponding samples showed selectivity advantages in butene,propylene,and ethylene,respectively.The highest yield of ethylene and propylene was 53.0%.When the acid content was insufficient,the growth of MFI pillars improved the crystallinity and skeleton continuity of the nanosheets.More Al could exist as framework single Al at the intersection of pore channels.The conversion of n-heptane was increased by 25.8%compared with the sample without pillars,and the yield of ethylene and propylene reached 49.1%.However,the MFI pillar could not fully construct the ordered hierarchical pore system,so SiO2 pillar treatment was used to solve this problem.To analyze the structural function of the ordered pore system in the catalytic reaction,the disordered stacked zeolite was synthesized as a comparison.Pore structure,acid properties,and reaction performance of all hierarchical zeolite in this thesis and the research group were integrated,and the correlation coefficient was calculated by mathematical statistical methods.The results showed that the increase of mesoporous openness could increase the critical acid threshold of zeolite in the cracking reaction,that is,more acid could be the active sites for the production of light olefins without triggering excessive coke deposition reactions.The mesopores of stacked zeolite are disordered and the content is limited.The increase of acid amount would increase the mass transfer resistance of the product,and the critical acid threshold is low.In addition,the influence of mesoporous properties on the catalytic cracking gradually decreases with the enhancement of pore structure openness and order.The correlation coefficient between the mesoporous volume of the stacked zeolite and the selectivity of ethylene and propylene is 0.99,which is significantly correlated.When the mesopores gradually increase until an ultra-thin ordered pore structure is formed,the correlation coefficient between the mesoporous volume and the reaction performance is only 0.02.The highest yield of ethylene and propylene in ordered hierarchical zeolite was 60.0%,which reached the highest level in this thesis.