Study Onthe Preparation And Regulation Of Nano ZSM-5 Zeolite Aggregateswith Hierarchical Porosity And Their Catalytic Performancesfor The Cracking Of N-heptane

Catalytic cracking of hydrocarbon is an effective process for producing ethylene,propylene and simultaneously obtaining light aromatics.The study and development of new and efficient catalysts has been attracting many attentionsfrom the researchersin this field.ZSM-5 zeolite exhibits the outstandingcatalytic performances for catalytic cracking because of its excellent shape selectivity,suitable acidity and goodhydrothermal stability and it has become the most extensively studied catalysts for the catalytic cracking of hydrocarbon.Conventional ZSM-5 zeolite contains only microporous channels,which leads to the difficulty problems including the restrained mass transfer capacity,the limited accessibility of acid sites in zeolites and weak anti-coking stability.The facile fabricationof nanoZSM-5 zeolite aggregates with tunable hierarchical porosity and crystal morphology hasbecome the focus of attention in this field.Amongthe nanoZSM-5 zeolite aggregates,the nanosheet ZSM-5 zeolite is expected to exhibit excellent catalytic performances in catalytic cracking reaction due to its special lamellar morphology,excellent mass diffusion ability,highly accessible acid sites and unique hierarchical structure.This dissertation workis mainly devoted tostudying and developinga simple and economical versatile strategy for the preparation of hierarchical porosity nano-ZSM-5 zeolite aggregates,especially the nanosheet ZSM-5 zeolite,and to further investigating their catalytic performances for the catalytic cracking of n-heptane reactions.The main research contents and conclusions are as follows:?1?Nanosheet ZSM-5 zeolite was prepared and its physicochemical properties were studied by means ofXRD,SEM,TEM,N₂ adsorption,NH₃-TPD and Py-IR.The results show that the nanosheet ZSM-5 zeolite has a large number of external surface acid sites,highly exposed?010?crystal planes,fully accessible channel intersection acid sites,and hierarchical porosity caused by the unique two dimensional lamellar morphology.The nanosheet ZSM-5 zeolite showed excellent catalytic performance for the catalytic cracking of n-heptane.When the reaction temperature was 550 ^oC,the conversion of n-heptane was 85.8%,which is 32.8%higher than that on conventional ZSM-5.When the reaction temperature was 600 ^oC,the conversion of n-heptane and the yield of propylene reached 99.6%and 34.9%,respectively,which were 13.9%and 3.5%higher than those of conventional ZSM-5.During 100 h of continuous reaction,nanosheet ZSM-5 kept a high initial activity and propylene yield,the conversion of n-heptane decreased from 99.6%to 89.6%?the conversion of n-heptane on commercial ZSM-5 decreased from 85.7%to 57.3%under the same conditions?,and the yield of propylene maintained about 35%.These results demonstrate that the nanosheet ZSM-5zeolite possesses high catalytic activity and stability simultaneously,and it is a kind of new and efficient catalyst for the catalytic cracking of hydrocarbon.?2?Rapid synthesis of nanosheet ZSM-5 zeolite was succeeded by seed-assisted method.The physicochemical properties of the as-prepared nanosheet ZSM-5 zeolite were studied by means of XRD,SEM,TEM,N₂ adsorption,NH₃-TPD and Py-IR.The results showed that a small amount of seed could promote nucleation and significantly shorten the induction period,and the crystallization time was reduced from 11¹3 d to3⁵ d,which greatly improved the synthesis efficiency.The desired nanosheet zeolite with typical"house-card structure"and regular three-dimensional flower-like microspheres morphology could be obtained via the optimization of the crystallization temperature,Si/Al,H₂O/SiO₂,Na₂O/SiO₂,aluminum source and silicon source species and other synthetic parameters.In addition,ZSM-5 zeolite with different morphologies,such as narrow nanosheets,nanostrips,nano hexagonal prisms and nanosheet were synthesized by using fumed silica,solid silica gel,silica sol and tetraethyl orthosilicate as silicon source,respectively.The effect of silicon source on the zeolite acidity was not significant,but it has obvious effect on the zeolite crystal morphology and texture properties.The hydrolysis rate and the distribution of polysilicate ions in different silicon sources are different,so it resuls in a variety of zeolite morphologies.In the reaction for the catalytic cracking of n-heptane,the nanosheet ZSM-5 zeolite showed good catalytic performance.When the reaction temperature was 600 ^oC,the conversion of n-heptane,the yields of light olefines and BTX were 95.8%,64.6%and 4.5%,respectively,which were 14.0%,8.8%and 0.6%higher than those on the hexagonal prismatic ZSM-5 zeolite under the same conditions.The outstanding catalytic performance of nanosheet ZSM-5 zeolite catalyst is attributed to the combined effect of high exposure?010?crystal planes,fully accessible channel intersections acid sites and high accessibility of active sites.?3?A strategy for the synergistic regulation of nano ZSM-5 zeolite aggregates was developed by using growth modifier and the small amount of Gemini quaternary ammonium surfactant.Through employing inexpensive,commercially available alcohols chemicals as zeolite growth modifier,the growth of zeolite crystals was selectively controlled,and nano ZSM-5 zeolite aggregates with unique crystal morphology,acidity and texture properties were obtained.The physicochemical properties of as-prepared ZSM-5 zeolite were characterized by means of XRD,²⁷Al MAS NMR,SEM,TEM,N₂ adsorption,NH₃-TPD and Py-IR.The results showed that the present method not only could tune the morphology of zeolite,but also regulate the acidity and texture properties.With the addition of alcoholic growth modifier,the crystallinity of zeolite slightly increased,and the specific surface area,pore volume decreased slightly.The total acid amount decreased,and the B/L value increased gradually.The ability and specificity of the growth modifier to control the crystal morphology is determined by its own physicochemical properties.The alcohols chemicals as zeolite growth modifier specifically bind to zeolite crystal?010?crystal surface through hydrogen bond interaction,thus it inhibits the crystal growth along b-axis direction,while the selective controlling crystal growth in the a-c plane ultimately leads to the formation of nanosheets zeolite.In the reaction for catalytic cracking of n-heptane,the ZSM-5 zeolite synthesized by addition of ZGM-1 showed good catalytic performances.When the reaction temperature was 550 ^oC,the conversion of n-heptane,the yields of propylene and light olefines reached 94.9%,33.5%and62.0%,respectively,which were 3.7%,2.4%and 3.7%higher than those without adding growth modifier.The good catalytic performance is mainly due to the synergistic effectof suitable acidity and rich intercrystal mesopores.?4?The nanorods and nanosheets ZSM-5 zeolite aggregates were successfully synthesized by the combinational using of fatty amine as zeolite growth modifier and a small amount of Gemini quaternary ammonium surfactant as structure-directing agent.The physicochemical properties of the as-prepared ZSM-5 zeolite were characterized by means of XRD,²⁷Al MAS NMR,SEM,TEM,N₂ adsorption,NH₃-TPD and Py-IR.It was found that the fatty zeolite growth modifier simultaneously tuned zeolite crystal morphology,acidity and texture properties.The fatty amine growth modifier can tune and control the anisotropic growth rate of crystal via the specificity for binding to crystallographic surfaces through the electrostatic interaction with the negative charge on the crystal surface.With the addition of fatty amine growth modifier,the sample began to appear dual mesoporosity distribution.The total acid content and strong acid content of the samples increased slightly,and the B/L value increased to some extent.In the reaction for catalytic cracking of n-heptane,the nanosheet ZSM-5 zeolite synthesized by adding ZGM-5 growth modifier exhibited higher catalytic activity,especially at low reaction temperatures.When the reaction temperature was 400 ^oC,the n-heptane conversion and the light olefines yield were as high as 54.8%and 15.7%,respectively,which were 29.6%and 8.1%higher than those without adding growth modifier under the same reaction conditions,the synergistic effects of the specific layer morphology,higher acid density and B/L value result in the outstanding catalytic performances.?5?Nano rod-shaped ZSM-5 zeolite aggregates with hierarchical porosity were synthesized via hydrothermal crystallization by using a series of alcohol amines with both amine and hydroxyl chemical properties as zeolite growth modifier,and a small amount of Gemini quaternary ammonium surfactant as structure-directing agent.The physicochemical properties of the as-synthesized ZSM-5 zeolite were characterized by means of XRD,²⁷Al MAS NMR,SEM,TEM,N₂ adsorption,NH₃-TPD and Py-IR.It was found that the diameter of nanorod can be flexibly tuned from 5 nm to 40 nm by using the different alcohol amine growth modifiers.In addition,with the addition of alcohol amine growth modifier,the total acid content and B/L value of the as-prepared zeolite samples increased,and the zeolite samples possess intercrystal and intracrystal mesopores simultaneously.In the reaction for catalytic cracking of n-heptane,the nanorod ZSM-5 zeolite aggregates synthesized with ZGM-8 growth modifier exhibited good catalytic performances due to its suitable acid density and mesopores.When the reaction temperature was 500 ^oC,the yields of ethylene,propylene and light olefins were 15.6%,26.3%and 51.2%,respectively,which were 6.8%,3.1%and 8.8%,respectively,higher than those samples synthesized in the absence of zeolite growth modifier under the same reaction conditions.