Synthesis Of SAPO-34/ZSM-5/kaolin Composite Zeolite Based On MTA Reaction And Its Catalytic Performance

Methanol-to-aromatics（MTA）is a new way to synthesize high-quality aromatics at low cost by non-petroleum routes.The key to MTA technology is to develop new catalysts with high aromatic selectivity and carbon deposition resistance.In this paper,the CHA-MFI/kaolin based double-layer composite molecular sieve film material is designed according to the characteristics of the MTA catalytic reaction.The traditional one-step MTA reaction is divided into three stages:methanol-to-olefins,olefin-to-aromatics and aromatics rapid diffusion by the pore-limiting effect of the material.This improves the selectivity of aromatics,shortens the residence time of intermediate products in the catalyst,and suppresses the occurrence of deep side reactions.The new material will exhibit catalytic properties different from traditional zeolite in the field of MTA catalysis,and is expected to become a prospective catalytic material with research value and application prospects.SAPO-34 zeolite was prepared by hydrothermal synthesis method in this paper,and the effects of template type and dosage,crystallization time and silicon content on the crystal phase,crystal morphology,crystal size,structural properties and catalytic performance of SAPO-34 zeolite was studied,so as to optimize the best ratio of synthesis gel and synthesis conditions;secondly,the surface of ZSM-5/kaolin zeolite was modified by Tetrapropyl Ammonium Bromide（TPABr）modifier.The effects of modifier concentration and modification time on the equilibrium adsorption capacity were investigated,and the effects of surface charge and surface morphology on the micro environment of SAPO-34 zeolite were revealed;finally,the modified ZSM-5/kaolin is adsorbed to SAPO-34 grains under the action of electrostatic force and the SAPO-34/ZSM-5/kaolin composite zeolite film prepared by the secondary synthesis method,and the MTA reaction activity,BTX selectivity and carbon deposition resistance of the new composite material were explored by adjusting the Si/Al ratio of the composite molecular sieve.By investigating the preparation conditions of SAPO-34 zeolite by single factor,it is determined that the best molar ratio of the synthetic solution is n（Al₂O₃）:n（P₂O₅）:n（Mor）:n（Si O₂）:n（H₂O）=1:1:3:0.6:70,the crystallization time of 20h can produce SAPO-34 molecular sieve with a smooth crystal grain surface and a crystal size of 1.0μm.When the crystallization time exceeds 36 h,a small amount of needle-like SAPO-5 heterostructures will be produced.The silicon content of the synthetic gel has an important influence on the chemical environment of silicon atoms in the framework,namely the form and quantity of silicon islands.With the increase of silicon content,the amount of zeolite acid gradually decreases,and the acid strength is increased;when the silicon content is 0.6,The specific surface area and pore volume of the zeolite reached the maximum of 453.03 m²/g and 0.2462 cm³/g.At the same time,the initial methanol conversion rate can reach 100%,the olefin selectivity is as high as about 80%,the reaction life is as long as 6h.TPA-ZSM-5 molecular sieve was prepared by surface modification method,its isoelectric point p H₀ increased from 3.43 to 9.70,the negatively charged ZSM-5 zeolite was transformed into positively charged TPA-ZSM-5,the equilibrium adsorption capacity of TPABr modifier was 20000 mg/L,the equilibrium adsorption time of 5000mg/L,10000 mg/L,15000 mg/L and 20000 mg/L was 140 min,100 min,60 min and40 min respectively;on this basis,TPA-ZSM-5 molecular sieve was put into the suspension of SAPO-34 zeolite after pre crystallization to absorb SAPO-34 grains.It was concluded that the modified ZSM-5 zeolite was not conducive to surface loading of a large number of SAPO-34 zeolite crystal particles,on this basis,put it into the pre-crystallized SAPO-34 molecular sieve suspension to adsorb SAPO-34 crystal particles,it is concluded that the unmodified ZSM-5 molecular sieve is not conducive to the surface loading of a large number of SAPO-34 molecular sieve crystal particles,of which 15000 mg/L modified SAPO-34/ZSM-5/kaolin composite zeolite film has the most complete structure.The Si/Al ratio of the composite zeolite is adjusted,the results show that the surface area and pore volume of composite molecular sieves are lower than pure SAPO-34 and ZSM-5 molecular sieves,but higher than SAPO-34/kaolin zeolite film（201.34m²/g）,and the mesoporous/microporous volume is about one;The total acid content of the composite zeolite decreases with the increase of the Si/Al ratio,the strong acid/weak acid ratio increases from 1.046 to the maximum 4.938;SAPO-34 and ZSM-5 zeolite have different pore structure and acidic distribution,resulting in different carbon content and carbon deposition on the surface of the film.Compared with ZSM-5 single molecular sieve,the BTX selectivity of composite zeolite increased from 40.80%to 46.30%.