Role of ammonium fluoride in crystallization process of beta zeolite
Role of ammonium fluoride in crystallization process of beta zeolite
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DOI：
10.1016/j.jcrysgro.2007.05.055
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发表时间：
2007-09-01
影响因子：
1.8
通讯作者：
Sano, Tsuneji
中科院分区：
文献类型：
--
作者：
Jon, Hery;Oumi, Yasunorl;Sano, Tsuneji
在用NH4F制备的起始凝胶中加入Na+阳离子，显著延长了β沸石的结晶时间。然而，在纯二氧化硅沸石的情况下，添加更多的NH4F可使结晶时间缩短一半。从Si-29 MAS NMR测量中，发现在存在Na+阳离子的纯二氧化硅β沸石中，即使在低结晶度的固相中，归因于五配位硅的信号也比在没有Na+阳离子的情况下更为明显。结合理论计算结果，由于Na+[SiO4/2F]（-）种能量更稳定，可以推测纯硅β沸石中可能存在Na+[SiO4/2F]（-）种。此外，通过F-19 MAS NMR测量证实，在初始结晶阶段，四乙基氟化铵（TEAF）在固相中被完整地包合。这表明TEAF是β沸石骨架形成所必需的物质，即“SDA”，它决定了动力学有利相即β沸石的形成。此外，由于部分TEA（+）阳离子被钠离子取代，形成Na+[SiO4/2F]（-）种，使得TEA(+) [SiO4/2F]（-）种参与成核或晶体生长的数量减少，从而使结晶时间变长。(C) 2007 Elsevier b.v.。版权所有。
The addition of Na+ cations to the starting gel prepared using NH4F significantly prolonged the crystallization time of beta zeolite. However, in the case of pure silica beta zeolite an addition of more NH4F reduced the crystallization time by one-half. From Si-29 MAS NMR measurements, the signal attributed to pentacoordinated silicon was found to be more pronounced for pure silica beta zeolite in the presence of Na+ cations even in the solid phase with low crystallinity, as compared to that in the absence of Na+ cations. Considering the results of theoretical calculation, because of more energetically stable state of Na+[SiO4/2F](-) species, it could be presumed that Na+[SiO4/2F](-) species might exist in pure silica beta zeolite. Furthermore, as confirmed by F-19 MAS NMR measurements tetraethyl ammonium fluoride (TEAF) species were enclathrated intact in solid phase during the initial crystallization stage. This suggests that TEAF species are required for the formation of beta zeolite framework, in other words, as "SDA" which conditions the formation of kinetically favored phase, i.e., beta zeolite. Moreover, since part of TEA(+) cations is replaced by sodium cations, forming Na+[SiO4/2F](-) species, the amount of TEA(+) [SiO4/2F](-) species involved in either nucleation or crystal growth decreases and thus the crystallization time becomes longer. (C) 2007 Elsevier B.V.. All rights reserved.