Synthesis And MTO Catalytic Performance Of SAPO-34/HZSM-22 Composite Molecular Sieves

Due to the shortage of petroleum resources in China,the traditional way of producing low carbon olefins from petroleum is limited.Therefore,Methanol to Olefins?MTO?is the most energy-saving method for the preparation of low carbon olefins.The core of MTO technology is that the catalyst should have high MTO activity,low carbon olefins selectivity and long life.SAPO-34 molecular sieve is the most commonly used MTO reaction catalyst in industry at present.It has high methanol conversion rate and low carbon olefins selectivity,but it also has the disadvantages of easy coking deactivation and short life.Composite molecular sieves is a crystal composed of two or more molecular sieves with different molecular sieve pore structures and acid properties by a certain method.Due to its unique crystal morphology,pore structure and rational distribution of acidity,it exhibits excellent synergistic and shape-selective properties in the catalytic reaction.In this work,firstly,the preparation conditions of the composite molecular sieves under hydrothermal conditions were optimized,a series of new SAPO-34/HZSM-22composite molecular sieve catalysts were prepared by changing the preparation method and mass ratio of the catalyst.Secondly,the HZSM-22 molecular sieve was modified and then combined with SAPO-34 to obtain the modified SAPO-34/HZSM-22.Thirdly,the structure,morphology,specific surface area and acidity of the composite molecular sieve catalysts were characterized by XRD,SEM,FT-IR,BET and NH₃-TPD.Finally,the MTO catalytic performance of the composite molecular sieve catalyst was investigated.The research contents and conclusions were shown as follows.?1?The SAPO-34/HZSM-22 composite molecular sieve was synthesized by hydrothermal method.The effects of crystallization temperature,crystallization time and mass ratio on the SAPO-34/HZSM-22 composite molecular sieve were systematically investigated,determine the optimal synthesis conditions as follows:crystallization temperature was 200°C,crystallization time was 24 h,the mass ratio of HZSM-22 and SAPO-34 was 2?HZSM-22/SAPO-34=2?,The SAPO-34/HZSM-22composite molecular sieve with better crystallinity could be prepared by adding HZSM-22 to the synthesis gel of SAPO-34?molar ratio,1Al₂O₃:1P₂O₅:0.4SiO₂:3TEA:50H₂O?.The mechanical preparation of SAPO-34/HZSM-22 composite molecular sieve was made by carefully grinding the mixture of HZSM-22 and SAPO-34 at a certain mass ratio.The composite molecular sieve catalysts were characterized by XRD,SEM,FT-IR,BET and NH₃-TPD,and their MTO properties were investigated.The results showed that some HZSM-22 crystals were agglomerated to form particles and attached to the outer surface of SAPO-34,while others were agglomerated to form larger HZSM-22 crystal particles.However,HZSM-22 crystal particles on hydrother-mal composite molecular sieve were larger.The acidity and MTO reaction performance of the two types of composite molecular sieves were different.The low olefins selectivity and propylene/ethylene ratio of the composite molecular sieve catalysts prepared by mechanical method were higher than that of the composite molecular sieve catalysts prepared by hydrothermal method.?2?HZSM-22 molecular sieve was modified by alkali treatment and alkali acid combination treatment respectively.The effects of alkali type,alkali treatment time,alkali solution solubility and hydrochloric acid pickling ratio on the physical properties of HZSM-22 were investigated.The HZSM-22 molecular sieve catalysts were characterized by XRD,SEM,FT-IR,BET and NH₃-TPD,and their MTO properties were investigated.The results showed that the crystallinity retention rate of the sample after alkali and acid treatment was higher,and it had higher specific surface area and micro-mesoporous property,while maintaining the framework structure of ZSM-22molecular sieve.Alkali-acid treatment could selectively remove some strong acid centers,and could significantly improve the catalyst life of MTO reaction under the condition of keeping the low carbon olefins selectivity unchanged.?3?The modified SAPO-34/HZSM-22 composite molecular sieves was obtained by mechanically compounding the optimized HZSM-22 with SAPO-34.After modification,the MTO reaction life of SAPO-34/HZSM-22 could be doubled.At 2 h of reaction,the selectivity of low carbon olefins was 75.18%,and the ratio of propylene/ethylene?C₃⁼/C₂⁼?is 1.12.With the increase of reaction time,the ratio of propylene/ethylene?C₃⁼/C₂⁼?increased rapidly.