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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图 2 未预处理ZSM-5分子筛(a) 及其二次生长样品(b) 的SEM照片
Figure 2 SEM images of (a) ZSM-5 sample, (b) ZSM-5 sample after secondary growth without pretreatment
图 3 TPAOH预处理后CZ样品的SEM照片和EDS谱图
Figure 3 SEM images ((a) and (c), with different magnifications) of CZ samples after TPAOH pretreatment and the corresponding EDS chemical analyses of zone 1 (b) and zone 2 (d)
图 4 CSZ分子筛的SEM、TEM、EDS和SAED照片
Figure 4 SEM images ((a) and (b), with different magnifications) of CSZ sample, TEM image of CSZ sample ((c) and (d)), the corresponding EDS chemical analysis of surface (inset), and SAED patterns of the shell layer (e) and the core zeolites (f)
图 5 分子筛的N2吸附-脱附等温线和孔径分布图
Figure 5 N2 adsorption-desorption isotherms and pore size distribution (inset) of CZ and CSZ samples
图 7 不同pH值条件下合成分子筛的XRD谱图
Figure 7 XRD patterns of the zeolites prepared under different pH values
图 8 不同水量条件下合成分子筛的XRD谱图
Figure 8 XRD patterns of the zeolites prepared with different contents of H2O
图 9 不同二次晶化时间下合成分子筛的XRD谱图
Figure 9 XRD patterns of the zeolites obtained with different crystallization times
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 
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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
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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
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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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