Schmithorst Michael B, Prasad Subramanian, Moini Ahmad, Chmelka Bradley F
Department of Chemical Engineering, University of California, Santa Barbara, California 93106, United States.
BASF Corporation, Iselin, New Jersey 08830, United States.
J Am Chem Soc. 2023 Aug 23;145(33):18215-18220. doi: 10.1021/jacs.3c05708. Epub 2023 Aug 8.
The distributions of heteroatoms within zeolite frameworks have important influences on the locations of exchangeable cations, which account for the diverse adsorption and reaction properties of zeolite catalysts. In particular for aluminosilicate zeolites, paired configurations of aluminum atoms separated by one or two tetrahedrally coordinated silicon atoms are important for charge-balancing pairs of H cations, which are active for methanol dehydration, or divalent metal cations, such as Cu, which selectively catalyze the reduction of NO, both technologically important reactions. Such paired heteroatom configurations, however, are challenging to detect and probe, due to the typically nonstoichiometric compositions and nonperiodic distributions of aluminum atoms within aluminosilicate zeolite frameworks. Nevertheless, distinct configurations of paired framework aluminum atoms are unambiguously detected and resolved in solid-state 2D Al-Si and Si-Si NMR spectra, which are sensitive to the local environments of covalently bonded Al-O-Si and Si-O-Si moieties, respectively. Specifically, two H-chabazite zeolites with the same bulk framework aluminum contents are shown to have different types and populations of closely paired aluminum species, which correlate with higher activity for methanol dehydration. The methodologies and insights are expected to be broadly applicable to analyses of heteroatom sites, their distributions, and adsorption and reaction properties in other zeolite framework types.
杂原子在沸石骨架中的分布对可交换阳离子的位置有重要影响，而可交换阳离子决定了沸石催化剂多样的吸附和反应性能。特别是对于铝硅酸盐沸石，被一个或两个四面体配位硅原子隔开的铝原子对构型，对于作为甲醇脱水活性位点的H阳离子电荷平衡对，或对于选择性催化NO还原的二价金属阳离子（如Cu）而言很重要，这两个都是具有重要工业价值的反应。然而，由于铝硅酸盐沸石骨架中铝原子通常的非化学计量组成和非周期性分布，这种成对的杂原子构型难以检测和探究。尽管如此，在固态二维Al-Si和Si-Si NMR谱中能明确检测和分辨出成对骨架铝原子的不同构型，这两种谱分别对共价键合的Al-O-Si和Si-O-Si基团的局部环境敏感。具体而言，两种具有相同骨架铝含量的H-菱沸石被证明具有不同类型和数量的紧密成对铝物种，这与更高的甲醇脱水活性相关。这些方法和见解有望广泛应用于分析其他沸石骨架类型中的杂原子位点、它们的分布以及吸附和反应性能。
