Synthesis of core-shell structured MFI-type composite zeolites by isomorphous epitaxial growth
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摘要: 采用二次热液结晶法,以四丙基氢氧化铵水溶液预处理过的低硅ZSM-5分子筛为晶核,通过调控pH值、水量和晶化时间等二次结晶条件,在晶核上外延生长了高硅ZSM-5壳,制备了MFI/MFI核壳型复合分子筛。通过X射线衍射、扫描电镜、能量色散谱仪、透射电子显微镜、N2吸附-脱附和NH3-程序升温脱附等手段表征了所合成的核壳分子筛的晶体结构、表面形态及核/壳界面,并对它们的结构参数以及酸性进行了初步评估。结果表明,核壳复合分子筛的壳层由多层200 nm的MFI沸石晶粒组成;高硅ZSM-5分子筛壳层的生成,引入了介孔结构,显著增大了外比表面积;同时,核壳结构的形成降低了复合分子筛酸性和外表面的酸密度,但增加了弱酸量。当二次晶化母液pH值为8.5,H2O/SO2物质的量比为30,晶化时间为24 h时,高硅分子筛壳层更易可控生长。Abstract: MFI/MFI core-shell composite zeolites with a low-silica ZSM-5 core and a high-silica shell were successfully synthesized by secondary hydrothermal crystallization on the low-silica ZSM-5 cores that was pretreated with a basic TPAOH aqueous solution; the preparation parameters for shell growth including the pH value, water amount, and crystallization time were well considered. The crystal structure, surface morphology, core/shell interface, textural properties and surface acidity of the resultant core-shell zeolites were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometer, transmission electron microscopy, N2 physisorption and NH3 temperature programmed desorption. The results indicated that in the core-shell composite zeolites, high-silica ZSM-5 shell with a particle size of 200 nm is well developed on the surface of core crystal. The growth of ZSM-5 shell results in an increase of external surface area, decrease of external acid density and increase of weak acid sites without influencing pore structure. The isomorphous epitaxial growth of high-silica ZSM-5 shells can be effectively controlled under the conditions of a pH value of 8.5, H2O/SiO2 mol ratio of 30 and crystallization for 24 h during the secondary crystallization.
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Key words:
- ZSM-5 /
- secondary crystallization /
- core/shell structure /
- composite zeolite /
- surface acidity
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表 1 分子筛的织构性质
Table 1 Textural properties of the CZ and CSZ samples
Sample ABET /(m2·g-1) Amicroa /(m2·g-1) Aextermalb /(m2·g-1) vtotalc /(cm3·g-1) vmicrod /(cm3·g-1) CZ 249 208 41.0 0.173 0.095 9 CSZ 353 198 155.0 0.192 0.089 7 a, b, d: calculated by t-plot method; c: calculated at p/p0=0.995 表 2 不同pH值条件下合成分子筛的晶胞参数
Table 2 Unit cell parameters of the zeolites prepared under different pH values
pH value Unit cell parameters /nm Volume
V/nm3a b c 5.1 2.016 2.017 1.349 5.486 6.7 2.019 2.026 1.358 5.556 8.5 2.014 2.013 1.349 5.471 12.5 2.020 2.017 1.353 5.511 表 3 不同水量条件下合成分子筛的晶胞参数
Table 3 Unit cell parameters of the zeolites prepared with different contents of H2O
H2O/SO2
(mol ratio)Unit cell parameters /nm Volume
V/nm3a b c 30 2.015 2.017 1.359 5.525 80 2.020 2.026 1.353 5.536 150 2.015 2.022 1.359 5.537 250 2.019 2.026 1.358 5.556 表 4 不同二次晶化时间下合成分子筛的晶胞参数
Table 4 Unit cell parameters the zeolites obtained with different crystallization times
Time t/h Unit cell parameters /nm Volume
V/nm3a b c 12 2.016 2.017 1.354 5.550 24 2.012 2.013 1.350 5.468 36 2.017 2.031 1.359 5.566 48 2.015 2.017 1.359 5.525 -
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