Indirect observation of structured incipient zeolite nanoparticles in clear precursor solutions.
Indirect observation of structured incipient zeolite nanoparticles in clear precursor solutions.
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DOI：
10.1002/anie.200802941
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发表时间：
2008-10
影响因子：
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通讯作者：
L. Tosheva;B. Mihailova;Lik H. Wee;B. Gasharova;K. Garbev;A. Doyle
中科院分区：
文献类型：
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作者：
L. Tosheva;B. Mihailova;Lik H. Wee;B. Gasharova;K. Garbev;A. Doyle
沸石前驱体颗粒的结构是现代沸石科学中最具争议的问题之一，它存在于生成胶体硅-1 （mfi型结构的纯硅质沸石）的透明合成溶液中。由于合成的性质和不含Al，这些体系被广泛用作研究沸石结晶机理的模型。自从“纳米实验室”假说由马丁斯等人提出以来，它一直是许多批评的对象。主要的分歧是关于在最初的透明混合物中存在的亚胶体前体颗粒的结构。尽管Martens等人认为这些颗粒具有MFI结构特征，但在许多其他研究中也提出了颗粒的无定形性质。[2-8]在某些情况下，纳米颗粒是从合成溶液中提取出来的，人们担心提取过程可能会改变纳米颗粒的结构或产生干扰。[1,2,5]另一些作者基于29Si富集后的固体核磁共振分析结果，对前驱体颗粒的结晶性质提出了反对意见。[3,4]最近对硅酸盐水溶液的核磁共振研究表明存在典型的MFI结构单元其他补充技术，如动态光散射也已被用于进一步表征存在于前体溶液中的物质。[10,11]因此，尽管讨论硅石-1前体颗粒结构的论文已经发表了相当多，但讨论仍在进行中，新的出版物定期出现。红外光谱也被广泛用于研究硅石-1前驱体颗粒的结构。[1b, 5, 7, 12, 13]基于在550 cmÀ1附近存在一个被广泛认为是MFI结构特征的带，[14]可能是晶体[1b， 12]，也可能是前体颗粒的非晶态特征。[5,7,13]这些IR研究是在冻干样品，[12,13]提取前驱体颗粒，[1b， 5]和原始溶胶上进行的然而，迄今为止发表的红外光谱在1000-1300 cmÀ1光谱范围内显示出实质性的差异，这表明特定的采样影响了纳米颗粒的结构特征。近年来，同步辐射红外光谱首次应用于沸石晶体的原位催化反应研究基于对金表面上超薄沸石膜的同步加速器掠入射反射吸收红外（RAIR）光谱，我们给出了明确的证据，证明在透明前驱体溶液中存在初始的沸石纳米颗粒：这些亚胶体颗粒具有沸石框架的所有结构特征，但只有几个单位细胞的大小，不足以产生衍射图案。我们研究的新颖之处在于两个方面：样品制备和表征方法。首先，为了从溶液中分离亚纳米粒子，我们使用Langmuir-Blodgett （LB）方法将它们沉积在Au衬底上。LB方法允许亚胶体颗粒在空气/水界面处以漂浮膜的形式集中，并将这些膜转移到固体载体上，从而确保了原始颗粒结构的保存。其次，我们在Au表面制备LB膜，利用RAIR光谱对样品进行表征，这是超薄膜结构分析中最有效的光谱方法。振动光谱学在检测只有2-3个单元格大小的晶体颗粒方面比衍射方法有很大的优势，因为这种颗粒很大…
The structure of the zeolite precursor particles present in clear synthesis solutions yielding colloidal silicalite-1 (purely siliceous zeolite with MFI-type structure) is one of the most controversial issues in modern zeolite science. Owing to the nature of the synthesis and the absence of Al, these systems are widely used as models to study the mechanism of zeolite crystallization in general. Ever since the “nanoslab” hypothesis was proposed by Martens et al.[1] it has been the subject of a lot of criticism. The major disagreement is about the structure of the subcolloidal precursor particles present in the initial clear mixtures. Whereas Martens et al. suggest that these particles have MFI structural features, an amorphous nature of the particles has been proposed in numerous other studies.[2–8] In some cases, the nanoparticles were extracted from the synthesis solutions, and concerns have been raised about possible changes in their structure or interference from the extraction procedure.[1, 2, 5] Other authors base their arguments against the crystalline nature of the precursor particles on the results of solid-state NMR analysis after 29Si enrichment.[3, 4] A recent NMR study on aqueous silicate solutions revealed the presence of structural units typical of the MFI structure.[9] Other complementary techniques such as dynamic light scattering have also been used to further characterize the species present in the precursor solutions.[10, 11] Thus, despite the fact that a remarkable number of papers discussing the structure of the silicalite-1 precursor particles have already been published, the discussion is still ongoing and new publications regularly appear. Infrared spectroscopy has also extensively been used to study the structure of the silicalite-1 precursor particles.[1b, 5, 7, 12, 13] Based on the presence of a band near 550 cmÀ1, which is widely accepted to be characteristic of the MFI structure,[14] either crystalline [1b, 12] or amorphous character of the precursor particles has been suggested.[5, 7, 13] These IR studies were performed on freeze-dried samples,[12, 13] extracted precursor particles,[1b, 5] and original sols.[7] However, the IR spectra published so far showed substantial differences in the 1000–1300 cmÀ1 spectral range which indicate that specific sampling affects the structural features of the nanoparticles. Recently, IR spectroscopy with synchrotron radiation was used for the first time to study in situ catalytic reactions in zeolite crystals.[15] On the basis of synchrotron-based grazing-incidence reflection-absorption infrared (RAIR) spectra of ultrathin zeolite films on Au surfaces, we give unambiguous evidence for the existence of incipient zeolite nanoparticles in clear precursor solutions: these are subcolloidal particles having all the structural features of the zeolite framework but a size of only a few unit cells which is insufficient to generate a diffraction pattern. The novelty in our study is twofold: sample preparation and method of characterization. First, to separate the subnanoparticles from the solution, we used the Langmuir–Blodgett (LB) method to deposit them on Au substrates. The LB method allows concentration of the subcolloidal particles at the air/water interface as floating films and transfer of these films to the solid supports, which ensures preservation of the native particle structure. Second, we prepared the LB films on Au surfaces in order to characterize the samples by RAIR spectroscopy, which is the most efficient spectroscopic method for structural analysis of ultrathin films. Vibrational spectroscopy has great advantages over diffraction methods in detecting crystalline particles with a size of just 2–3 unit cells, because such particles are large …