Architecture-Dependent Distribution of Mesopores in Steamed Zeolite Crystals as Visualized by FIB-SEM Tomography
Architecture-Dependent Distribution of Mesopores in Steamed Zeolite Crystals as Visualized by FIB-SEM Tomography
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DOI：
10.1002/anie.201006031
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发表时间：
2011-01-01
影响因子：
16.6
通讯作者：
Weckhuysen, Bert M.
中科院分区：
文献类型：
--
作者：
Karwacki, Lukasz;de Winter, D. A. Matthijs;Weckhuysen, Bert M.
沸石是石化工业的主要支柱之一。它们精确定义的多孔体系、对恶劣反应条件的耐受性和优异的酸性使它们在许多催化过程中不可或缺。[1-6]一些工业上相关的沸石，包括Y沸石和ZSM-5，被广泛用于烃类裂解、异构化和烷基化等过程。[7-9]因此，由于沸石独特的催化性能在很大程度上依赖于分子扩散和酸性中心的可及性，因此大量的研究集中在这些性能的改善上。11]例如合成纳米晶体和剥离层状沸石。[12-14]虽然上述方法允许微调材料的可及性，但合成的复杂性和相关成本很可能超过工业应用材料的成本。相比之下，廉价高效的水蒸气脱铝和碱法脱硅已成为提高沸石分子吸附的有效方法。虽然这两种方法在促进分子扩散方面都相当简单，但到目前为止，人们对所获得的介孔的均匀性和尺寸变化知之甚少。主要的直接方法利用电子断层扫描(ET)，仍然局限于不超过几百纳米的材料，[19-21]而氮气物理吸附测量仅提供中孔体积的平均信息。[22]此外，从我们先前对ZSM-5分子筛中分子扩散势垒的研究得知，S晶体的形态和内部结构定义了直通道和正弦通道向表面开放的区域。因此，不同的沸石晶体在总体材料可及性和相关的催化行为上存在差异。[23-25]在这里，我们首次描述了聚焦离子束(FIB)和扫描电子显微镜(SEM)断层扫描的强大组合，以表征多孔固体，如沸石。如下所示，该方法导致了对蒸汽沸石中产生的中孔的类型(长度、宽度和形态)的新的定量洞察。为此，我们将注意力集中在尺寸为100 20 20μm~3的单个棺材形状的ZSM-5晶体上，其中的介孔是通过蒸汽引入的。
Zeolites are one of the major pillars of the petrochemical industry. Their precisely defined porous system, resistance towards harsh reaction conditions, and excellent acidic properties make them indispensible in many catalytic processes.[1–6] A handful of industrially relevant zeolites, including zeolite Y and ZSM-5, are extensively used in processes such as hydrocarbon cracking, isomerization, and alkylation.[7–9] Therefore, as the unique catalytic properties of zeolites rely to a great extent on molecular diffusion and accessibility of acid sites, a great number of studies has focused on the improvement of these properties.[10, 11] Examples include the synthesis of nanocrystals and exfoliating layered zeolites.[12–14] Although the above-mentioned methods allow the fine tuning of the material accessibility, the synthesis complexity and related costs most probably exceeds the costs of industrially applicable materials. In contrast, cheap and efficient dealumination by steaming and alkali-based desilication has become an efficient approach in boosting the molecular uptake of zeolites.[15–18] While both methods are fairly simple in terms of enhancing the molecular diffusion, until now not much is known about the uniformity and the size variations of the obtained mesopores. The main direct approach makes use of electron tomography (ET) and is still limited to materials not exceeding a few hundred nanometers,[19–21] while N2 physisorption measurements only provide average information on the mesopore volume.[22] Furthermore, as known from our previous study on the molecular diffusion barriers in ZSM-5 zeolites, the crystal s morphology and internal architecture define areas where straight and sinusoidal channels are open to the surface. Consequently, individual zeolite crystals differ in their overall material accessibility and related catalytic behavior.[23–25]Here, we describe for the first time the powerful combination of focused ion beam (FIB) and scanning electron microscopy (SEM) tomography to characterize porous solids, such as zeolites. As will be shown below, the approach leads to new quantitative insight into the type of mesopores (length, width, and morphology) generated in steamed zeolites. For this purpose, we have focused our attention on individual coffin-shaped ZSM-5 crystals with dimensions of 100 20 20 μm3, in which mesoporosity has been introduced by steaming.