Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204, United States.
Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16801, United States.
J Am Chem Soc. 2020 May 6;142(18):8211-8222. doi: 10.1021/jacs.9b13596. Epub 2020 Apr 27.
The synthesis of two-dimensional (2D) zeolites has garnered attention due to their superior properties for applications that span catalysis to selective separations. Prior studies of 2D zeolite catalysts demonstrated enhanced mass transport for improved catalyst lifetime and selectivity. Moreover, the significantly higher external surface area of 2D materials allows for reactions of bulky molecules too large to access interior pores. There are relatively few protocols for preparing 2D materials, owing to the difficultly of capping growth in one direction to only a few unit cells. To accomplish this, it is often necessary to employ complex, commercially unavailable organic structure-directing agents (OSDAs) prepared multistep synthesis. However, a small subset of zeolite structures exist as naturally layered materials where postsynthesis steps can be used to exfoliate samples and produce ultrathin 2D nanosheets. In this study, we selected a common layered zeolite, the MWW framework, to explore methods of preparing 2D nanosheets one-pot synthesis in the absence of complex organic templates. Using a combination of high-resolution microscopy and spectroscopy, we show that 2D MMW-type layers with an average thickness of 3.5 nm (ca. 1.5 unit cells) can be generated using the surfactant cetyltrimethylammonium (CTA), which operates as a dual OSDA and exfoliating agent to affect Al siting and to eliminate the need for postsynthesis exfoliation, respectively. We tested these 2D catalysts using a model reaction that assesses external (surface) Brønsted acid sites and observed a marked increase in the conversion relative to three-dimensional MWW (MCM-22) and 2D layers prepared from postsynthesis exfoliation (ITQ-2). Collectively, our findings identify a facile and effective route to directly synthesize 2D MWW-type materials, which may prove to be more broadly applicable to other layered zeolites.
二维（2D）沸石的合成因其在催化到选择性分离等应用中的优异性能而受到关注。先前的 2D 沸石催化剂研究表明，增强质量传递可以提高催化剂的寿命和选择性。此外，2D 材料的显著更高的外表面面积允许对太大而无法进入内部孔的大体积分子进行反应。由于难以仅在一个方向上将生长限制在几个单元晶胞内，因此用于制备 2D 材料的方法相对较少。为了实现这一点，通常需要使用复杂的、商业上不可用的有机结构导向剂（OSDA）进行多步合成。然而，一小部分沸石结构存在于天然层状材料中，其中后合成步骤可用于剥离样品并产生超薄的 2D 纳米片。在这项研究中，我们选择了常见的层状沸石，MWW 骨架，探索了在没有复杂有机模板的情况下一锅法制备 2D 纳米片的方法。使用高分辨率显微镜和光谱学的组合，我们表明可以使用表面活性剂十六烷基三甲基铵（CTA）生成具有平均厚度为 3.5nm（约 1.5 个单元晶胞）的 2D MMW 型层，CTA 分别作为双 OSDA 和剥离剂来影响 Al 位置并消除后合成剥离的需要。我们使用评估外部（表面）Brønsted 酸位的模型反应测试了这些 2D 催化剂，并观察到与三维 MWW（MCM-22）和通过后合成剥离制备的 2D 层（ITQ-2）相比，转化率显著提高。总的来说，我们的发现确定了一种直接合成 2D MWW 型材料的简便有效途径，这可能更广泛地适用于其他层状沸石。
