Synthesis Of HZSM-5/SAPO-11 Core-shell Molecular Sieves And Their Catalytic Behaviors In Isobutane Aromatization

Due to the excess of low carbon hydrocarbons and the demand of aromatic hydrocarbons continued to grow, low carbon hydrocarbon aromatization technology has attracted considerable attention. As the most commonly used catalyst in industry, HZSM-5 possesses many advantages like good catalytic activity, hydrothermal stability, excellent acid catalytic properties and shape selective catalytic etc. But coke deposition short the life of catalyst and limit its application in industry. Ga and Zn modified method on HZSM-5 were used to enhance synergistic catalytic action between strong and weak acid, but the introduction of metal will restrict the diffusion of reactants and products, which easily lead to inactivation, researchers also used acid and alkali treatment method to modulate the acidity and pore structure of HZSM-5 to improve the catalyst-life, but its activity and selectivity decreased. In this paper, a weakly acidic shell SAPO-11 was covered on the surface of HZSM-5, catalyst acidity and pore structure were by adjusted by changing Si content of SAPO-11 to enhance synergistic catalytic action between strong and weak acid and improve the selectivity of the target product.In this paper, two-stage crystallization method is used, the new method involves aging pretreatment step followed by crystallization process of a SAPO-11 precursor gel in a low temperature, and the SAPO-11 microcrystal in aging products were adsorbed by pretreated HZSM-5, then HZSM-5/SAPO-11 core-shell composite molecular sieve obtained by crystallization process of the mixture in a high temperature. HZSM-5/SAPO-11 core-shell composites possessing single-crystal HZSM-5 as core and polycrystalline SAPO-11 as shell, this molecular sieve have pore gradient and acidic gradient, the weak acid surface of HZSM-5/SAPO-11 reduces the cracking reactions, which can effectively reduce the catalyst carbon deposition, prolong the catalytic life and enhance its stability. In this article, the synthesis ratio of HZSM-5/SAPO-11 was optimized firstly, then the crystallization time of low temperature crystallization stages of the synthesis of core-shell molecular sieve was studie. Based on the research above, a series of different synthesis ratio of solid to liquid and silicon content of core-shell molecular sieves were prepared, and HZSM-5&SAPO-11 mechanical mixing molecular sieve was prepared to compared with HZSM-5/SAPO-11, all the molecular sieves were characterized by the XRD, FT-IR, SEM, TEM, TG, N₂-adsorption desorption, NH₃-TPD and Py-IR, and the effects of the isobutane aromation were investigated. The conclusions were showed as following:1) The synthesis ratio of SAPO-11 in HZSM-5/SAPO-11 were optimized: Al₂O₃ : 0.9 P₂O₅ : 0.6 SiO₂ : 1.3 DPA : 49 H₂O, Synthesis conditions: stirring temperature is 30℃, low temperature crystallization temperature is 150℃, 2⁴ h, high temperature crystallization temperature is 190℃, 24 h. When the low temperature crystallization time was prolonged to 210 min, the obtained HZSM-5/SAPO-11 composite molecular sieve had the characteristics of obvious core-shell structure and high coverage rate.2) HZSM-5 and SAPO-11 in HZSM-5/SAPO-11 core shell molecular sieve combined tighter than them in the mixture. The surface strong acid sites of HZSM-5 molecular sieve were covered with SAPO-11, then the acidity and acid strength in HZSM-5/SAPO-11 core-shell are weaker than them in mechanical mixture. Compare with mechanical mixing molecular sieve, the specific surface area and mesopore pore volume in HZSM-5/SAPO-11 were increased, which can increase the carbon capacity of the catalyst, the catalytic life improved. All the property of HZSM-5/SAPO-11 enhanced the selectivity of aromatics and improved the yield of liquid.3) The different proportion of SAPO-11 in core-shell molecular sieve has a great influence on the morphology and properties of HZSM-5/SAPO-11, With the increase of synthetic ratio of solid to liquid, the concentration of B acid in molecular sieve increased, the concentration of L acid remained constant essentially, the B/L increased, and the strong acid and total acid content also increased. The HZSM-5 phase molecular sieve is still the main catalytic activity center in the reaction, when the ratio of solid to liquid of HZSM-5/SAPO-11 reach 3/35, the catalytic performance is good.4) Silicon content of SAPO-11 which covered on HZSM-5 will influence the morphology of itself. With the increase of silicon content, thickness and mesopores of the shell are increasing, acid amount and acid strength of molecular sieve first increased and then decreased. Adjust silicon content of core-shell molecular sieve can optimize the L and B and B/L, and optimize the acidity and acid strength, which can enhance synergistic catalytic effect of aromatization to improve the selectivity of aromatic molecules, the conversion of iso-butane and the liquid yield. Meanwhile with the increase of silicon content, the average pore size molecular sieve increased, which can improve the stability of catalyst. when the silicon content reached 0.6, the solid acid property and pore structure of molecular sieve are more favorable to the reaction.