Synthesis Of Layered ZSM-5 Zeolite And Its Catalytic Properties In N-octane To Light Olefins And Aromatics

Light olefins（ethylene,propylene and butene）and BTX（benzene,toluene and xylene）are important basic raw materials for the petrochemical industry.The catalytic cracking of naphtha is one of the effective technologies to produce light olefins and by-product BTX.ZSM-5 zeolite has excellent catalytic performance due to its unique pore structure and shape selectivity.However,conventional ZSM-5 zeolite only has a single micropore,which has many disadvantages such as limited diffusion and mass transfer capacity,poor accessibility to acid sites,and easy carbon deposition and deactivation.The ZSM-5 zeolite nanosheets exhibits high activity and high stability in catalytic cracking and other reactions due to its short diffusion path along the b-axis,improved diffusion and mass transfer capacity and highly exposed acid sites.In view of the problems of diffusion limitation and deactivation of carbon deposition in the catalytic cracking and aromatization of alkanes,a small amount of Gemini quaternary ammonium salt surfactant[C₁₈H₃₇-N⁺（CH₃）₂-C₆H₁₂-N⁺（CH₃）₂-C₆H₁₃]Br₂(C_18-6-6Br₂)as a structure-directing agent and triethylamine（TEA）as a zeolite growth modifier,the layered ZSM-5 zeolite with tunable structure and suitable acidity was prepared to improve its acid site accessibility and diffusion mass transfer capacity.The main innovation results are as follows:（1）A series of layered ZSM-5 zeolite samples were prepared,and the influences of molar ratios of C_18-6-6Br₂/TEA on the as-prepared HZ5-2.5/x catalysts were investigated.The results show that by changing the content of triethylamine,the morphologies,texture properties and acid properties of the HZ5-2.5/x catalysts can be systematically regulated.Among them,the BET surface area(S_BET)of HZ5-2.5/10 sample reached 412 m~2/g,the amount of the total B acid reached163.2μmol/g and the HF’（Hierarchy factor）was 1.73.In the catalytic cracking of n-octane,the selectivity to light olefins was 70.7%over HZ5-2.5/10 catalyst.Moreover,the layered ZSM-5zeolite still has high catalytic activity and light olefins yield during 60 h,which may be attributed to the hierarchical porosity and more accessible acid sites of the layered ZSM-5 zeolite（2）By investigating the effects of different crystallization times on the crystallinity,morphology,acid properties and other physicochemical properties of the as-prepared ZSM-5samples,the growth process of layered ZSM-5 zeolite and the regulation effect of triethylamine on its acid properties were further investigated.It was found that the ZSM-5 zeolite entered the induced nucleation period after 4 h of crystallization,and the sample began to form a layered structure after 24 h of crystallization.With the prolongation of crystallization time,the crystallinity of ZSM-5 zeolite increased slightly.In addition,the introduction of triethylamine regulated the distribution of aluminum sites on the straight channels of the 10-membered ring of ZSM-5 zeolite,and then regulated the distribution of acid content.The Lewis acid content of the as-prepared MFI-T-S sample reached 39.1μmol/g.In the catalytic cracking of n-octane reaction,the yield of BTX（benzene,toluene,xylene）over the MFI-T-S catalyst was 29.3%.This result was mainly attributed to the increase of its weak acid content and L acid content,which promoted the hydrogen transfer reaction.（3）A series of Zn modified ZSM-5 zeolite with different Zn contents were prepared by equal volume impregnation method.The results show that Zn mainly exists in two forms,Zn O and Zn（OH）⁺.And the dehydrogenation active site was mainly Zn（OH）⁺,which was formed by the interaction of Zn species with the B acid sites.Therefore,with the increase of Zn content,the amount of B acid sites decreased gradually.In the n-octane aromatization reaction,the selectivity to BTX（benzene,toluene and xylene）over 2 wt%Zn/HZ catalyst was 83.2%,which was 14.7percentage points higher than that of the 2 wt%Zn/CZ catalyst（68.5%）under the same reaction conditions.