A New Type Of Core-shell Structure Composite Molecular Sieves Synthesis And Catalytic Alkylation Reaction

SAPO-31molecular sieves with different SiO2/Al2O3were successfully synthesized by hydrothermal method. The structure and acid properties of the catalysts were characterized by XRD, N2physical adsorption and desorption, SEM and NH3-TPD. The sample with SiO2/Al2O3=0.5was chosen as catalyst and tested on fixed-bed microreactor to investigate the influence of reaction temperature, reaction space velocity, reaction pressure and reaction raw materials on its catalytic performance. The optimized technologic conditions of alkylation of naphthalene with methanol to produce2,6-dimethylnaphthalene (2,6-DMN) were as follows:reaction temperature was400℃, reaction space velocity was1.5h-1, reaction pressure was4MPa, molar ratio of reaction raw materials was naphthalene:methanol:1,2,4-trimethylbenzene=1:2:8. The catalytic performance of SAPO-31molecular sieves with different SiO2/Al2O3were also studied under optimized technologic conditions. The results showed that the catalytic performance of SAPO-31molecular sieves were markedly affected by their acidity, the catalytic activity of the samples enhanced with the increase of SiO2/Al2O3, but the selectivity of2,6-DMN and2,6-/2,7-DMN ratio decreased to some extent.ZSM-12/AlPO4-31composite with different composition and ZSM-12/SAPO-31composite with different Si content were successfully synthesized by embedding method. The structure and acid properties of the samples were characterized by XRD, N2physical adsorption and desorption, SEM, TEM, NH3-TPD and Py-IR. The catalytic performance of the samples in alkylation of naphthalene with methanol were also investigated. The results showed that the composite possessed a core-shell structure with ZSM-12core and SAPO-31(or AlPO4-31) shell, their catalytic activity and selectivity of2,6-DMN were lower than mechanical mixture. Combining the characterization and reaction results, we knew the proper composition of the composite was that the mass ratio of ZSM-12zeolite and aluminium isopropoxide in the synthesis gel was1:20, the proper SiO2/Al2O3of SAPO-31shell was0.1.Nanoscale ZSM-5/SAPO-31composite with different composition were successfully synthesized by embedding method, the proper composition of the composite was that the mass ratio of nanoscale ZSM-5zeolite and aluminium isopropoxide in the synthesis gel was1:5. Microscale ZSM-5/SAPO-31composite were synthesized with the above proper composition. Core microscale ZSM-5zeolite was pretreated by poly dimethyl diallyl ammonium chloride salt solution to make nanoscale SAPO-31seeds adsorb on the surface of microscale ZSM-5zeolite. In this way, SAPO-31molecular sieve could grow on the surface of microscale ZSM-5zeolite more easily during the second synthesis procedure, the formed core-shell structure of the composite would be perfect and the SAPO-31shell would be thin. The samples were characterized by XRD, N2physical adsorption and desorption, SEM, TEM, NH3-TPD and Py-IR. The catalytic performance of the samples in alkylation of naphthalene with methanol were also studied. The results showed that the composite possessed a core-shell structure with nanoscale ZSM-5(or microscale ZSM-5) core and SAPO-31shell, relatively high catalytic performance and2,6-/2,7-DMN ratio were also obtained over the composite.