Laboratory of Crystallography, ETH Zurich , Vladimir-Prelog-Weg 5, Zurich CH 8093, Switzerland.
Berzelii Center EXSELENT on Porous Materials, Department of Material and Environmental Chemistry, Stockholm University , SE-106 91 Stockholm, Sweden.
J Am Chem Soc. 2017 Nov 22;139(46):16803-16812. doi: 10.1021/jacs.7b08810. Epub 2017 Nov 7.
The structure of the calcined form of the high-silica zeolite SSZ-70 has been elucidated by combining synchrotron X-ray powder diffraction (XRPD), high-resolution transmission electron microscopy (HRTEM), and two-dimensional (2D) dynamic nuclear polarization (DNP)-enhanced NMR techniques. The framework structure of SSZ-70 is a polytype of MWW and can be viewed as a disordered ABC-type stacking of MWW-layers. HRTEM and XRPD simulations show that the stacking sequence is almost random, with each layer being shifted by ±1/3 along the ⟨110⟩ direction with respect to the previous one. However, a small preponderance of ABAB stacking could be discerned. DNP-enhanced 2D Si{Si} J-mediated NMR analyses of calcined Si-SSZ-70 at natural Si isotopic abundance (4.7%) establish the through-covalent-bond Si-O-Si connectivities of distinct Si sites in the framework. The DNP-NMR results corroborate the presence of MWW layers and, more importantly, identify two distinct types of Q silanol species at the surfaces of the interlayer regions. In the first, an isolated silanol group protrudes into the interlayer space pointing toward the pocket in the adjacent layer. In the second, the surrounding topology is the same, but the isolated -SiOH group is missing, leaving a nest of three Si-O-H groups in place of the three Si-O-Si linkages. The analyses clarify the structure of this complicated material, including features that do not exhibit long-range order. With these insights, the novel catalytic behavior of SSZ-70 can be better understood and opportunities for enhancement recognized.
通过结合同步加速器 X 射线粉末衍射 (XRPD)、高分辨率透射电子显微镜 (HRTEM) 和二维 (2D) 动态核极化 (DNP) 增强 NMR 技术，阐明了高硅沸石 SSZ-70 的煅烧形式的结构。SSZ-70 的骨架结构是 MWW 的多型体，可以看作是 MWW 层的无序 ABC 型堆叠。HRTEM 和 XRPD 模拟表明，堆叠序列几乎是随机的，每个层相对于前一个层沿 ⟨110⟩方向偏移 ±1/3。然而，可以发现 ABAB 堆叠有微弱的优势。在天然 Si 同位素丰度 (4.7%)下，对煅烧 Si-SSZ-70 进行的 DNP 增强 2D Si{Si} J 介导的 NMR 分析，建立了骨架中不同 Si 位之间的共价键 Si-O-Si 连接。DNP-NMR 结果证实了 MWW 层的存在，更重要的是，在层间区域的表面识别出两种不同类型的 Q 硅醇物种。在第一种情况下，孤立的硅醇基团突出到层间空间中，指向相邻层的口袋。在第二种情况下，周围的拓扑结构相同，但孤立的 -SiOH 基团缺失，留下三个 Si-O-H 基团的巢代替三个 Si-O-Si 键。这些分析阐明了这种复杂材料的结构，包括不表现出长程有序的特征。有了这些见解，可以更好地理解 SSZ-70 的新型催化行为，并认识到增强的机会。
