Locating Organic Guests in Inorganic Host Materials from X‑ray Powder Diffraction Data

Can the location of the organic structure-directing agent (SDA) inside the channel system of a zeolite be determined experimentally in a systematic manner? In an attempt to answer this question, we investigated six borosilicate zeolites of known framework structure (SSZ-53, SSZ-55, SSZ-56, SSZ-58, SSZ-59, and SSZ-60), where the location of the SDA had only been simulated using molecular modeling techniques in previous studies. From synchrotron powder diffraction data, we were able to retrieve reliable experimental positions for the SDA by using a combination of simulated annealing (global optimization) and Rietveld refinement. In this way, problems arising from data quality and only partially compatible framework and SDA symmetries, which can lead to indecipherable electron density maps, can be overcome. Rietveld refinement using geometric restraints were then performed to optimize the positions and conformations of the SDAs. With these improved models, it was possible to go on to determine the location of the B atoms in the framework structure. That is, two pieces of information that are key to the understanding of zeolite synthesisthe location of the organic SDA in the channel system and of the positions adopted by heteroatoms in the silicate frameworkcan be extracted from experimental data using a systematic strategy. In most cases, the locations of the SDAs determined experimentally compare well with those simulated with molecular modeling, but there are also some clear differences, and the reason for these differences can be understood. The approach is generally applicable, and has also been used to locate organic guests, linkers, and ligands in metal–organic compounds.沸石孔道体系内的有机结构导向剂 (organic structure-directing agent, SDA) 的位置能否通过系统化的实验方法予以确定？为解答这一问题，我们研究了六种具有已知骨架结构的硼硅酸盐沸石（SSZ-53、SSZ-55、SSZ-56、SSZ-58、SSZ-59及SSZ-60），此类沸石的有机结构导向剂位置在过往研究中仅通过分子模拟技术得到过预测。借助同步辐射粉末衍射数据，我们结合模拟退火（simulated annealing，全局优化）与里特维尔德精修（Rietveld refinement）方法，成功获取了该有机结构导向剂可靠的实验定位结果。通过该策略，我们得以克服因数据质量欠佳、骨架与有机结构导向剂对称性仅部分匹配所引发的难题——此类问题往往会导致电子密度图无法解析。随后，我们采用几何约束条件开展里特维尔德精修，以优化各有机结构导向剂的位置与构象。基于这些优化后的模型，我们得以进一步确定骨架结构中硼（B）原子的位置。换言之，理解沸石合成过程的两项关键信息——有机结构导向剂在孔道体系内的定位，以及杂原子在硅酸盐骨架中的占位——均可通过系统化策略从实验数据中提取得到。在多数情况下，实验测定的有机结构导向剂位置与分子模拟得到的结果吻合良好，但也存在部分显著差异，且此类差异的成因可得到阐释。该方法具备普适性，还可用于确定金属有机化合物中有机客体、连接基与配体的位置。