Effect of seed form on the structure and properties of as-synthesized SAPO-34 molecular sieves and their catalytic performance in the conversion of methanol to olefins
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摘要: 采用常规水热合成法、添加粉末晶种和添加含有分子筛前驱体的液相晶种等方法制备了低硅铝比的SAPO-34分子筛。采用XRD、SEM、FT-IR及TPD等表征手段研究了合成方法对晶体结构、形貌、晶粒粒径及酸性的影响。并在固定床上对比了其甲醇转化制烯烃的催化性能差异。结果表明,晶种存在形态对分子筛的结晶度影响不大,但影响了硅在骨架中的分布、晶粒粒径和酸性等。与添加粉末晶种相比,采用含有分子筛前驱体的液相晶种法更有利于减小分子筛晶粒粒径,降低分子筛酸性,提高低碳烯烃选择性。Abstract: A series of SAPO-34 molecular sieves with low Si/Al ratio were synthesized by conventional hydrothermal method as well as adding powder crystal seed and liquid crystal seed containing SAPO-34 precursor. These SAPO-34 molecular sieves were characterized by XRD, SEM, FT-IR and NH3-TPD and the effects of synthesis method on the structure, morphology, crystal size and acidity of the resultant SAPO-34 molecular sieves as well as their catalytic performance in the conversion of methanol to olefins (MTO) were investigated. The results indicate that although the seed form has little influence on the crystallinity, the silicon distribution in SAPO-34 framework, crystal size, and acidic properties are strongly related to the seed form. In comparison with the SAPO-34 sample synthesized with powder form seed, the SAPO-34 molecular sieves prepared using liquid crystal seed containing SAPO-34 precursor exhibit smaller crystal size, weaker acidic strength, and higher selectivity to light olefins in MTO.
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Key words:
- low silicon/aluminium ratio /
- SAPO-34 /
- molecular sieves /
- methanol-to-olefin /
- solid seed method /
- liquid seed method
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表 1 不同方法合成的SAPO-34分子筛的孔结构
Table 1 Textural properties of SAPO-34 molecular sieves synthesized by different methods
Sample Surface area A/(m2·g-1) vmicro/(cm3·g-1) Average pore diameter d/nm total micro external Direct method 488.8 486.37 2.43 0.26 1.93 Solid seed method 492.9 491.59 1.31 0.27 2.11 Liquid seed method 496 492.5 3.49 0.26 2.21 表 2 不同方法合成的SAPO-34分子筛的NH3-TPD表征
Table 2 NH3-TPD results of SAPO-34 molecular sieves synthesized by different methods
Sample Acid density /(mmol·g-1) weak strong total a 0.23 0.20 0.43 b 0.20 0.10 0.30 c 0.15 0.11 0.26 表 3 不同方法合成分子筛的烯烃收率
Table 3 Yields of olefins for MTO over various SAPO-34 molecular sieves synthesized by different methods
Molecular sieves Yields w/% direct method solid seed method liquid seed method CH4 1.61 1.07 1.29 C2H6 1.61 0.89 1.28 C2H4 30.74 30.12 34.29 C3H8 4.50 3.21 3.91 C3H6 29.05 32.11 34.49 C4H10 0.45 0.45 0.48 C4H8 6.02 7.73 8.69 ∑C2=+C3= 59.79 62.23 68.78 Conversion x/% 89.14 95.66 98.58 remarks: the sampling behavior is taken after reaction 10 min -
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