3D Nanoscale Chemical Imaging of the Distribution of Aluminum Coordination Environments in Zeolites with Soft X-Ray Microscopy
3D Nanoscale Chemical Imaging of the Distribution of Aluminum Coordination Environments in Zeolites with Soft X-Ray Microscopy
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DOI：
10.1002/cphc.201201015
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发表时间：
2013-02-25
期刊：
影响因子：
2.9
通讯作者：
Weckhuysen, Bert M.
中科院分区：
文献类型：
--
作者：
Aramburo, Luis R.;Liu, Yijin;Weckhuysen, Bert M.
沸石材料的催化和物理化学性质与铝含量及其在沸石骨架结构内的空间分布密切相关。[1-2]铝的分布和相关的铝分区可以发生在沸石骨架结构[3，4]以及沸石颗粒内的晶体T位点水平。[5，6]尽管这些是众所周知的现象，但需要关于单个沸石颗粒内铝分区的进一步信息。重要的是要记住，催化材料的性能与其活性位点的空间分布直接相关，[7-10]在沸石的情况下，活性位点是由铝和硅四面体的角共享形成的布朗斯台德酸位点。因此，这些布朗斯台德酸位点的强度与沸石颗粒中铝原子的数量和分布直接相关。[11此外，铝的空间分布也影响合成后的处理，如脱硅[13]或汽蒸[14]，这会改变沸石颗粒内的酸性和分子传输。[15-17]已经探索了许多表征方法来研究铝在大的微米尺寸沸石晶体和较小的多晶沸石聚集体中的空间分布。这些技术包括原子吸收光谱法（AAS），[18]电子探针分析（EPMA），[19] X射线荧光（XRF），[20]电感耦合等离子体原子发射光谱法（ICP-AES），[21]快速原子轰击质谱法（FABMS），X射线光电子能谱法（XPS），[22]能量色散X射线光谱（EDX）[23]以及质子诱导伽马射线发射（PIGE）。[24]不幸的是，这些方法通常需要溅射或研磨预处理以揭示来自沸石颗粒的更深部分的洞察。因此，有限数量的表征研究报告了铝在整个沸石颗粒体积中的分布，[25，26]而没有一个研究区分铝的不同配位环境。在这种情况下，它最近已被证明，3D扫描透射X射线显微镜（STXM）是非常适合于化学探测的内部微米大小的对象由于显微镜的焦点深度，穿透深度和空间分辨率的独特组合。[27在这里，我们提出了第一个纳米级化学成像研究，揭示了沸石材料中铝的量和配位环境的空间分布与三维扫描透射X射线显微镜（STXM）。为此，我们集中在两个展示样品涉及工业相关的沸石H-ZSM-5。第一种是煅烧的H-ZSM-5商业沸石粉末，标记为ZSM-5-C。第二种材料的样品名称为ZSM-5-S，是通过在700 ℃下将ZSM-5-C蒸汽处理3小时而获得的。关于这两种材料的制备和理化性质的更多详细信息，请参见最近一篇文章的支持信息。[29]为了深入了解ZSM-5-C和ZSM-5-S内铝量及其配位环境的3D分布，利用干涉控制的STXM仪器在光束线11.0处进行铝K-边缘处的化学绘图。2的劳伦斯伯克利国家实验室（LBNL，伯克利，美国）的先进光源（ALS）。[30]在61个不同的角度（θ 908，908）上获得2D STXM图像，每组图像之间的增量为38。为此目的，ZSM-5沸石聚集体被引入到玻璃毛细管中，并在毛细管中被分离。
The catalytic and physicochemical properties of zeolite materials are intimately connected with the aluminum content as well as its spatial distribution within the zeolite framework structure.[1–2] Distribution of aluminum and related aluminum zoning can occur at the level of the crystallographic T sites within the zeolite framework structure,[3, 4] as well as within a zeolite particle.[5, 6] Despite that these are well-known phenomena, further information on aluminum zoning within individual zeolite particles is needed. It is important to recall that the performance of a catalytic material is directly related to the spatial distribution of its active sites,[7–10] which in the case of zeolites, are the Brønsted acid sites, formed by the corner-sharing of aluminum and silicon tetrahedra. The strength of these Brønsted acid sites are therefore directly related to the number and distribution of aluminum atoms in the zeolite particle.[11, 12] Furthermore, spatial distribution of aluminum also impacts post-synthetic treatments, such as desilication [13] or steaming,[14] which alter the acidic properties and molecular transport within the zeolite particle.[15–17] Many characterization methods have been explored to investigate the spatial distribution of aluminum within large micron-sized zeolite crystals and smaller poly-crystalline zeolite aggregates. The techniques include atomic absorption spectrometry (AAS),[18] electron microprobe analysis (EPMA),[19] X-ray fluorescence (XRF),[20] inductively coupled plasma-atomic emission spectroscopy (ICP-AES),[21] fast-atom-bombardment mass spectrometry (FABMS), X-ray photoelectron spectroscopy (XPS),[22] energy-dispersive X-ray spectroscopy (EDX)[23] as well as proton-induced gamma-ray emission (PIGE).[24] Unfortunately, these methods often require sputter or milling pre-treatments to reveal insights from the deeper parts of the zeolite particle. As a consequence, a limited number of the characterization studies report on the distribution of aluminum throughout the entire zeolite particle volume,[25, 26] while none of them discriminate between the different coordination environments of aluminum. Within this context, it has been recently shown that 3D scanning transmission X-ray microscopy (STXM) is well suited to chemically probe the interior of micron-sized objects due to a unique combination of the microscope’s depth of focus, penetration depth and spatial resolution.[27, 28] Here, we present the first nanoscale chemical imaging study revealing the spatial distribution of the amount and coordination environment of aluminum in zeolite materials with 3D scanning transmission X-ray microscopy (STXM). For this purpose, we have focused on two showcase samples involving the industrially relevant zeolite H-ZSM-5. The first one is a calcined H-ZSM-5 commercial zeolite powder, labeled as ZSM-5-C. The second material with sample name ZSM-5-S has been obtained by steaming ZSM-5-C at 7008C for 3 h. Further details on the preparation and physicochemical properties of both materials can be found in the Supporting Information of a recent article.[29]To gain insight into the 3D distribution of the amount of aluminum and its coordination environments within ZSM-5-C and ZSM-5-S, chemical mapping at the aluminum K-edge was done making use of the interferometrically controlled STXM instrument at beamline 11.0. 2 of the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory (LBNL, Berkeley, USA).[30] 2D STXM images were obtained over 61 different angles (À908, 908) with an increment of 38 between each set of images. For this purpose, the zeolite ZSM-5 aggregates were introduced into a glass capillary and …