Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts
Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts
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DOI：
10.1038/nature08288
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发表时间：
2009-09-10
期刊：
影响因子：
64.8
通讯作者：
Ryoo, Ryong
中科院分区：
文献类型：
--
作者：
Choi, Minkee;Na, Kyungsu;Ryoo, Ryong
沸石（微孔结晶铝硅酸盐）广泛用于石油化学和精细化工合成(1-3)，因为其均匀微孔内的强酸位点能够实现尺寸和形状选择性催化。但孔径直径低于 1 nm 的微孔的存在往往与扩散限制 (3-5) 密切相关，从而对催化活性产生不利影响。该问题可以通过减小沸石晶体的厚度来解决，从而缩短扩散路径长度，从而改善分子扩散(4,5)。这是通过合成沸石纳米晶体(6)、剥离层状沸石(7-9)以及通过模板策略(10-17)或脱金属工艺(18-22)在微孔材料中引入介孔来实现的。但除了剥离之外，这些策略都没有生产出厚度低于 5 nm 的“超薄”沸石。在这里，我们表明，适当设计的双功能表面活性剂可以同时引导中孔和微孔长度尺度上的沸石结构的形成，从而产生厚度仅为2 nm的MFI（ZSM-5，石化工业中最重要的催化剂之一）沸石纳米片，这对应于单个MFI晶胞的b轴尺寸。这些沸石外表面的大量酸性位点使其对大有机分子的催化转化具有很高的活性，并且晶体厚度的减小有利于扩散，从而显着抑制甲醇转化为汽油过程中因焦炭沉积而导致的催化剂失活。我们期望我们的合成方法可以应用于其他沸石，以提高它们在一系列重要催化应用中的性能。
Zeolites-microporous crystalline aluminosilicates-are widely used in petrochemistry and fine-chemical synthesis(1-3) because strong acid sites within their uniform micropores enable size- and shape-selective catalysis. But the very presence of the micropores, with aperture diameters below 1 nm, often goes hand-in-hand with diffusion limitations(3-5) that adversely affect catalytic activity. The problem can be overcome by reducing the thickness of the zeolite crystals, which reduces diffusion path lengths and thus improves molecular diffusion(4,5). This has been realized by synthesizing zeolite nanocrystals(6), by exfoliating layered zeolites(7-9), and by introducing mesopores in the microporous material through templating strategies(10-17) or demetallation processes(18-22). But except for the exfoliation, none of these strategies has produced 'ultrathin' zeolites with thicknesses below 5 nm. Here we show that appropriately designed bifunctional surfactants can direct the formation of zeolite structures on themesoporous and microporous length scales simultaneously and thus yield MFI (ZSM-5, one of the most important catalysts in the petrochemical industry) zeolite nanosheets that are only 2 nm thick, which corresponds to the b-axis dimension of a single MFI unit cell. The large number of acid sites on the external surface of these zeolites renders them highly active for the catalytic conversion of large organic molecules, and the reduced crystal thickness facilitates diffusion and thereby dramatically suppresses catalyst deactivation through coke deposition during methanol-to-gasoline conversion. We expect that our synthesis approach could be applied to other zeolites to improve their performance in a range of important catalytic applications.