Designed Synthesis And Acidity Regulation Of Hierarchical ZSM-5/SAPO-34 Zeolitic Catalysts And Their Catalytic Performances Of MTO

Light olefins play an important role in industrial production,among which ethylene and propylene as vital basic organic raw materials are widely used in petrochemical and fine chemical fields.In the traditional preparation process,ethylene and propylene are mainly obtained by high-temperature pyrolysis of naphtha.Nowadays,with the increasing consumption of petroleum resources,the production of light olefins from petroleum can not meet the increasing demand of the market.Methanol to olefins（MTO）reaction as a key technology has been proven to be the most successful non-oil route to produce ethylene and propylene,effectively alleviating the problem of energy deficiency.In recent decades,the design and development of highly effective MTO catalysts have become a hot topic in this field.Zeolites are often used as solid acid catalysts for acid catalytic reactions because of their unique microporous structure,high thermal/hydrothermal stability,chemical stability,and strong acidity.Among these zeolite materials,ZSM-5（MFI）and SAPO-34（CHA）molecular sieves exhibit the most promising industrial catalysts in MTO reaction.However,in MTO reaction,these two catalysts still have some shortcomings such as large diffusion resistance,narrow acid regulation range,and high synthesis cost.Based on this,this paper focuses on seeking a simple,low-cost,one-step construction of hierarchical ZSM-5 and SAPO-34zeolitic materials,which effectively overcomes the problems of complex synthesis procedures and high costs in the preparation of hierarchical zeolite using secondary templates or alkaline treatment.Secondly,the acid density and strength of hierarchical SAPO-34 zeolite are effectively regulated by in-situ NH₃ poisoning strategy to prepare an efficient MTO catalyst with pore structure and acid energy matching.At last,combining the characteristics of ZSM-5with long lifetime and SAPO-34 with high olefins selectivity,a core-shell SAPO-34@ZSM-5was therefore prepared,which is expected to provide new ideas for the design and development of MTO catalyst through the above research.The specific contents are as follows:1.Using the traditional hydrothermal synthesis method,tetrapropylammonium hydroxide（TPAOH）as a single microporous template,hierarchically porous ZSM-5 zeolite was prepared by adjusting TPAOH/Si in the gel precursor solution.TPAOH can act as both a structure-directing agent and p H regulator in the synthesis of ZSM-5 zeolite,so the nucleation and growth rate of zeolite can be controlled by changing its addition amount,which is the decisive factor for the formation of hierarchical pore structure.XRD,SEM,TEM,NH₃-TPD,Py-IR,and N₂ adsorption-desorption isotherms were taken to characterize the morphology,pore structure,and acid characteristics of the samples in detail.The research shows that when TPAOH/Si is precisely adjusted to 0.13,hierarchical ZSM-5 zeolite with abundant intercrystalline voids formed by the loose accumulation of nanocrystals is successfully synthesized,and large surface area（87 m²/g）and mesoporous pore volume（0.14 cm³/g）are obtained.The prepared hierarchical ZSM-5 zeolite catalyst shows excellent catalytic stability in the MTO reaction.Under the same reaction conditions,the catalytic lifetime of ZSM-5-0.13 is 5 times longer than that of traditional microporous ZSM-5-com.By further analyzing the reaction products,it was found that the selectivity of ethylene and propylene is strongly affected by the amount of strong acid in zeolite;The larger the amount of strong acid sites is the easier the hydrogen transfer reaction of ethylene and propylene will occur,so it is not conducive to the generation of target products ethylene and propylene.However,for relatively large butene molecules,the diffusion ability is the main factor affecting their selectivity.Therefore,the hierarchical ZSM-5-0.13 catalyst has a large amount of acid site,thus showing relatively low selectivity for ethylene and propylene in MTO catalytic reaction,but the selectivity for butene is the highest.By systematically studying the structure-activity relationship between ethylene,propylene,and butylene and the acid content and pore structure of zeolite,theoretical support is provided for further optimization design of hierarchically porous zeolite.2.Using the traditional hydrothermal crystallization route,without adding any secondary templates,using tetraethyl orthosilicate（TEOS）with high solubility and high reactivity as the organic silicon source and regulating the morphology,crystal size,and pore structure parameters of SAPO-34 zeolite by changing the Si/Al molar ratio in the gel,the hierarchical SAPO-34 zeolite with adjustable mesoporous pore size was successfully prepared,and the different substitution mechanisms corresponding to the different content of Si atoms entering the zeolite framework are also clarified.Compared with the traditional microporous SAPO-34zeolite,the synthesized SAPO-34 zeolite with nanocrystals（～50 nm）has good catalytic performance and excellent regeneration ability.Its catalytic lifetime（methanol conversion≥99.0%）is 2.5 times that of the traditional SAPO-34 zeolite,and the selectivity of olefins（ethylene and propylene）is increased by 10.1%.The introduction of mesopores does not affect the microporous characteristics of zeolite itself,and effectively improves the mass transfer and diffusion limitation of reactants and products,thus improving the catalytic stability.Secondly,based on the principle that the strength of the chemical bond formed by the combination of acid sites with different strengths in the solid acid catalyst and the same basic molecule is different,it is proposed to use the basic molecule to poison some of the strong acid sites of the catalyst to effectively regulate the acid density and acid strength of SAPO-34 zeolite,so as to reduce the secondary reaction of olefins（side reactions such as polymerization,cyclization,and hydrogen transfer）and further improve the selectivity of low-carbon olefins.Under the same catalytic conditions,compared with the parent SAPO-34molecular sieve,the highest selectivity of olefins is increased by 2.8%,the average selectivity is increased by 6.9%,and the selectivity of by-product C₄⁺is decreased by 7.4%.According to the calculation of TON value,the utilization efficiency of single catalytic active site of the catalyst poisoned by NH₃ is increased by 6.5%.It is found that this treatment method can effectively inhibit the hydrogen transfer reaction of olefins and the generation of by-product C₄⁺.It is worth mentioning that this method is also applicable to hierarchical ZSM-5 zeolite,which shows that weakening the strong acid density and acid strength of zeolite through NH₃poisoning has certain universality for improving the olefins selectivity.This work shows that a novel,simple and low-cost method can effectively regulate the acid strength and acid density of zeolite to further improve the selectivity of olefins,which will have an important reference value for the industrial application of MTO.3.A new core-shell type was prepared by combining ZSM-5 and SAPO-34 with two different topologies and with controllable acid sites distribution of SAPO-34@ZSM-5 zeolite catalyst.In this experiment,a two-step synthetic procedure was adopted.First,a layer of mesoporous MCM-41 was deposited on the surface of the core SAPO-34.Then,under hydrothermal conditions,the MCM-41 shell on the surface of SAPO-34 was converted into ZSM-5 zeolite by in-situ solid-solid transformation with the help of MFI crystal seeds,thereby forming a core-shell structured composite with SAPO-34 microspheres（～2μm）as the core and ZSM-5（～150 nm）as the shell.The results show that in the alkaline gel precursor solution of ZSM-5,the shell of MCM-41 plays an important role in protecting the core SAPO-34,which can effectively avoid the problem that SAPO-34 zeolite crystals are easy to be damaged during the crystallization of the MCM-41 shell.Secondly,the formation of ZSM-5 shell by in-situ crystallization of shell MCM-41 also effectively avoids the problem of two-phase separation.In addition,further exploration of the growth mechanism of core-shell zeolite shows that the addition of ZSM-5 crystal seeds and the MCM-41 shell of the SAPO-34@MCM-41 become key factors of the composite zeolite formed.In addition,SAPO-34（core）and ZSM-5（shell）are connected by an amorphous matrix,which is also an important reason for forming a complete core-shell structure.In the MTO reaction,compared with the physically mixed SAPO-34+ZSM-5 and the single component SAPO-34 catalyst SAPO-34@ZSM-5.While obtaining high selectivity for light olefins,the lifetime of the catalyst is 2.6 times that of the mechanically mixed catalyst and 3 times that of the single component SAPO-34 catalyst.The main reason can be attributed to the rich catalytic active center,suitable acid content,multi-level composite pore structure,and the synergistic effect between the core and shell of the core-shell composite zeolite.This paper is based on the research of hierarchical ZSM-5,hierarchical SAPO-34,and core-shell structure SAPO-34@ZSM-5.Furthermore,the effects of different topological structures,mesoporous pore size,zeolite acidity,and the synergistic effect between the two zeolites on the MTO reaction performance were systematically studied,which provide a theoretical basis for the design and development of high-efficiency MTO catalyst that has guiding significance for industrial application.