Synthesis of hierarchical zeolite Beta with low organic template content via the steam-assisted conversion method
Synthesis of hierarchical zeolite Beta with low organic template content via the steam-assisted conversion method
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DOI：
10.1016/j.cej.2016.01.088
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发表时间：
2016-05
影响因子：
15.1
通讯作者：
Zhang Junliang;C. Peng;Huiyan Yan;Zhijie Wu;T. Dou
中科院分区：
文献类型：
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作者：
Zhang Junliang;C. Peng;Huiyan Yan;Zhijie Wu;T. Dou
采用水蒸气辅助转化法合成了低有机模板剂（四乙基氢氧化铵，TEAOH）含量的分级结构Beta沸石。既不需要沸石晶种，也不需要次级介孔剂。仅使用少量的有机模板剂（表示为TEAOH/SiO2 = 0.06）就可获得高度结晶的Beta沸石，其显著低于报道的最低值（0.10）。通过X射线衍射、扫描电子显微镜、透射电子显微镜、固体27 Al MAS NMR谱、N2吸附-脱附、X射线荧光、氨程序升温脱附和吡啶吸附红外光谱等表征手段对样品的理化性质进行了研究。这些表征结果表明，SAC方法通过改变合成凝胶的组成（Na 2 O/SiO2为0.12，H2O/干凝胶为0.20-0.30），可以得到SiO2/Al 2 O3为29-47的由20-40 nm纳米晶组装的分级结晶Beta沸石聚集体。这些沸石β聚集体具有大的外表面积（158-180 m2/g）和介孔体积（0.27-0.30 cm 3/g），这是由于形成晶间介孔和小的沸石晶体尺寸。在乙酰丙酸与乙醇的酯化反应中，所得到的分级结构Beta沸石的转化率为41.2%，四次催化反应的转化率均未明显下降，优于传统水热合成的Beta沸石。这是由于促进了反应物对分布在沸石纳米晶外表面上的酸中心的可及性以及在晶间介孔中的显著扩散。
We report the synthesis of hierarchical zeolite Beta with low organic template (tetraethylammonium hydroxide, TEAOH) content by the steam-assisted conversion (SAC) method. Neither zeolite seeds nor the secondary mesoporogen were required. Only a bit amount of organic template (denoted as TEAOH/SiO2ratio = 0.06) was used to obtain highly crystalline zeolite Beta, which was remarkably lower than the reported lowest value (0.10). Physicochemical properties of the resulting samples were investigated by comprehensive characterizations including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, solid state27Al MAS NMR spectra, nitrogen adsorption–desorption, X-ray fluorescence, temperature-programmed desorption of ammonia and infrared spectra of pyridine adsorption. These characterization results showed that the SAC method led to hierarchical crystalline zeolite Beta aggregates assembled by 20–40 nm nanosized crystals with controllable SiO2/Al2O3ratios among 29–47 by varying the compositions of synthetic gels with the Na2O/SiO2ratio of 0.12 and the H2O/dry gel ratios at 0.20–0.30. These zeolite Beta aggregates possessed large external surface area (158–180 m2/g) and mesoporous volume (0.27–0.30 cm3/g) resulted from the formation of intercrystalline mesopores and of the small zeolite crystal sizes. The obtained hierarchical zeolite Beta showed 41.2% conversion in the esterification reaction of levulinic acid with ethanol and four catalytic runs without obvious decrease in conversion, which were better than those on the zeolite Beta samples from the conventional hydrothermal synthesis. This was due to the promotion of the accessibility of reactants to the acid sites distributed on the external surface of zeolite nanocrystals and of the prominent diffusion in intercrystalline mesopores.