Synthesis Of SAPO-34/ZSM-5 Composite Molecular Sieve By In-situ Two-step Crystallization Method And Its Effect In The Methanol To Light Olefins Reaction

The methanol to olefin?MTO?process has effectively alleviated the current contradiction between oil supply and demand.SAPO-34 and ZSM-5 molecular sieves are the most promising catalysts for MTO reaction system and also the core of this process route.Modification and preparation of extending the lifetime of SAPO-34 and improving the selectivity of ZSM-5 have always been a research hotspot.In connection with the lifetime extension of SAPO-34 and the light olefins selectivity improvement of ZSM-5 are not obvious after current modification of surface acidity and structural properties,the researchers compounded ZSM-5 and SAPO-34 with different topological structures and acid site distributions.The composited molecular sieves could form abundant active centers and hierarchical pore structure,as well as the unique synergistic effect of special composite phase interface,thus the catalytic life can be prolonged and the selectivity of light olefins can be improved.However,the two molecular sieves are difficult to co-crystallize in the same system due to their incompatible pH environments.Normally,the post treatment synthesis method was applied to prepare composite molecular sieves?SAPO-34 was synthesized in advance following the surface modification.Afterwards,the modified SAPO-34 was placed in ZSM-5strong alkaline reaction system to synthesize composite molecular sieve?.This method is complicated,time-consuming and costly.And in the synthesis process,SAPO-34 and ZSM-5cannot co-crystallize in the same system,which will hinder the phase-to-phase contact of the two molecular sieves.It is not easy to produce the interfacial effect of the composite molecular sieve,and it is difficult to regulate the acid strength and element distribution,thus affecting the catalytic performance of the composite molecular sieve.Therefore,based on the study of the hydrothermal synthesis behavior of SAPO-34 and ZSM-5 in different pH environments,the paper first proposed and used in situ two-step crystallization method to synthesize SAPO-34/ZSM-5 composite molecular sieve?SZ-TS?.The hydrothermal synthesis behavior of SAPO-34 and ZSM-5 in different pH environments was analyzed,the synthesis principle and crystallization process of the composite molecular sieve were revealed,the structure-activity relationship of the two-phase synergistic catalysis of the composite molecular sieve was studied,and the reaction behavior and carbon deactivation behavior of the composite molecular sieve in the MTO reaction system were explored.The hydrothermal synthesis of SAPO-34 and ZSM-5 molecular sieves under different p H environments was studied by means of XRD,FT-IR,SEM,N₂ adsorption-desorption and NH₃-TPD.The pH value of the molecular sieve crystallization process was analyzed to determine the pH value co-crystallization range of the two-phase molecular sieves.The results showed that during the crystallization process of SAPO-34,the pH of the system increased gradually,and then decreased and tended to be stable after reaching the peak value?10.23?.The ZSM-5 molecular sieve condensation and crystallization process system was in the range of 10-12.Accordingly,by adjusting the pre-crystallization time of SAPO-34 molecular sieve,the pH of the crystallization process can be adapted to the requirements of the condensation and crystallization process of ZSM-5 molecular sieve in the alkaline environment,so as to realize the secondary co-crystallization of two-phase molecular sieves in the same system.In other words,the SZ-TS composite molecular sieve was formed by in-situ two-step crystallization with tightly bonded two-phase structure.By means of XRD,XPS,XRF,FT-IR,SEM,TEM,N₂ adsorption-desorption and NH₃-TPD,the synthesis principle and crystallization process of the composite molecular sieve were revealed,and the structure-activity relationship of the synergistic catalysis of the two phases of the composite molecular sieve was studied.SZ-TS composite molecular sieve synthesized by in-situ two-step crystallization went through two stages.In the stage of pre-crystallization,SAPO-34 precursor liquid was pre-crystallized for 24 h and ZSM-5precursor liquid was pre-crystallized for 6 h,the two-phase pre-crystallization precursors were directly mixed in the same system for co-crystallization.The two molecular sieve precursor fluids formed part of the primary and secondary structural units in their respective crystallization fluids?SAPO-34 was cubic morphology and ZSM-5 was hexagonal morphology?.In the secondary crystallization stage,the highly alkaline environment of the secondary crystallization liquid has an etching effect on SAPO-34 skeleton structure.The primary and secondary structural units formed by pre-crystallization was mainly used as crystal nuclei,and co-crystallization occured to form tightly bound composite molecular sieves.The highly alkaline environment of the secondary crystallization solution produced obvious interfacial effects on the etch and dissociation of SAPO-34 skeleton.Thus regulating to a suitable acid strength,element distribution,particle size and hierarchical pore structure.As a consequence,the strength of weak and medium-strong acid was weakened,the particle size of two-phase molecular sieve was decreased,the hierarchical pore structure with abundant mesoporous was formed.By using the SZ-TS in MTO reaction,the methanol conversion and light olefins selectivity were 98.4%and 93.0%,respectively,and the catalytic life was 1400 min.In-situ IR,XRD,SEM,N₂ adsorption-desorption,GC-MS,FT-IR,TG-IR and TPO-MS were used to explore the reaction behavior and carbon deactivation of the composite molecular sieve SZ-TS in the MTO reaction system.The results showed that polymethylbenze-ne generated in the MTO reaction behaves as the active intermediate of MTO reaction.SZ-TS with tightly bonded two-phase structure and smaller particle size could alleviate the formation rate of methyl substituted benzene and polycyclic aromatic hydrocarbons,therefore to display the excellent resistance to the carbon deposition inactivation.The carbon species of SZ-TS were mainly methyl substituted benzenes,naphthalene and its alkyl substituents,anthracene,phenanthrene and pyrene.Through fitting,the activation energies of SZ-TS carbon pyrolysis to ethylene and propylene were 15.60 kJ/mol and 15.83 kJ/mol,respectively.The lower activation energies indicated that the temperature required for carbon stripping pyrolysis was lower,which also indicated good regeneration performance.