Synthesis And MTO Catalytic Performance Of SAPO-17/H-ZSM-22 Composite Molecular Sieves

The conversion of methanol to olefins?MTO?has become an important industrial process for the production of low-carbon olefins.Catalysts used in MTO process require to be provided with high activity,olefins selectivity and more propylene production.Since the SAPO-34 molecular sieve catalyst actually used in industry is deactivated quickly,the catalyst needs to be carried out in a fluidized bed with reaction-regeneration system.Composite molecular sieves composed of two or more zeolites are recombination crystals with different structural characteristics,which would exhibit synergistic effect and special catalytic performance in the process of catalytic reactions for their good shape selective catalytic properties and reasonable distribution of acidity.This paper choosed the microporous molecular sieve SAPO-17 and the mesoporous molecular sieve H-ZSM-22 as the research objects to synthesized the SAPO-17/H-ZSM-22 composite molecular sieve under hydrothermal conditions.The catalyst's physicochemical properties and the catalytic performance in MTO reaction were investigated.Firstly,SAPO-17 molecular sieves were prepared by hydrothermal method.The effects of crystallization time and various synthetic raw materials,such as phosphoric acid,water,cyclohexylamine?CHA?,HF,KCl and silica sol,on the synthesis of SAPO-17 molecular sieves were systematically investigated.The optimum raw material ratio for the synthesis of SAPO-17 molecular sieves was determined to be 1Al₂O₃:1P₂O₅:0.08SiO₂:0.9CHA:50H₂O:0.9HF:0.01KCl.It was found that the introduction of HF and KCl in the synthesis system could greatly improve the crystallization rate and introduce more silicon atoms into the framework.The synthesized samples were characterized by XRD,SEM,NH₃-TPD,FT-IR and BET.The results showed that SAPO-17 molecular sieves seemed to be rodlike crystals and the selectivity of light olefins?C2^=C4⁼?was 84.03%but the lifetime was only 3 hours in the catalytic reaction of MTO.The SAPO-17/H-ZSM-22 composite molecular sieves were synthesized by coating method.The effects of crystallization time,crystallization temperature,addition amount of H-ZSM-22 and different ratio of silica-alumina on the synthetic process of composite molecular sieves were investigated.The optimum synthesis conditions were crystallizing at 200?for 24h with the adding of H-ZSM-22 of 1/10.The morphology,pore structure and acidity of the SAPO-17,H-ZSM-22 and the SAPO-17/H-ZSM-22 composite molecular sieves obtained by hydrothermal method as well as mechanical mixing method were characterized by XRD,BET,SEM,FT-IR and NH₃-TPD.The results showed that the morphology of the composite molecular sieves synthesized by hydrothermal method presented to be large-grained SAPO-17molecular sieves adhesion of small-sized H-ZSM-22 molecular sieves on the surface,while the mechanically mixed ones seemed to be just the physical connection of the two zeolites.Meanwhile,the synthesized SAPO-17/H-ZSM-22 exhibited better performance than the mechanically mixed ones in MTO reaction.The catalytic performance in MTO reaction of SAPO-17/H-ZSM-22 composite molecular sieves prepared under different conditions were investigated.The results showed that the SAPO-17/H-ZSM-22 composite molecular sieves catalysts carried out both the advantages of the two kinds of molecular sieves.The catalysts exhibited longer catalytic lifetime and larger ratio of C3⁼/C2⁼compared with the SAPO-17 molecular sieve,while showed higher selectivity of light olefins than the H-ZSM-22 molecular sieve.Specially,when the optimized complex ratio was 1/10,the composite molecular sieve catalyst showed the longest lifetime reaching up to 14 hours and the highest selectivity of light olefins up to 74.83%at 11 h reaction time,while the ratio of C3⁼/C2⁼was 1.19.It was found that the acidity of the composite molecular sieve was partly responsible for the difference of catalytic lifetime and C3⁼/C2⁼.