17 O Labeling Reveals Paired Active Sites in Zeolite Catalysts
17 O Labeling Reveals Paired Active Sites in Zeolite Catalysts
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17 O 标记揭示了沸石催化剂中的配对活性位点
DOI：
10.1021/jacs.2c05332
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发表时间：
2022
影响因子：
15
通讯作者：
White, Jeffery L.
中科院分区：
文献类型：
--
作者：
Chen, Kuizhi;Zornes, Anya;Nguyen, Vy;Wang, Bin;Gan, Zhehong;Crossley, Steven P.;White, Jeffery L.
目前需要将沸石催化剂扩展到传统的气相烃化学之外，需要详细表征活性位点结构、分布和水热影响。对具有同位素富集的17 O骨架的脱水H-ZSM-5催化剂的一系列均相和异相NMR相关实验表明，至少存在两种类型的成对活性中心，其数量取决于完全骨架配位的四面体Al（Al（IV）-1）和部分骨架配位的四面体Al（Al（IV）-2）中心的数量，它们都可以表示为（SiO）4-n-Al（OH）n。Al（IV）-1和Al（IV）-2位点的相对量以及随后的配对不能从催化剂Si/Al比推断，而是取决于合成和合成后修饰。相关实验表明，平均而言，来自Al（IV）-1/Al（IV）-2对的酸性羟基比来自Al（IV）-1/Al（IV）-1对的酸性羟基彼此更接近，如计算DFT计算所支持的。利用17 O核的π-键和空间极化转移实验揭示了不同Si/Al催化剂中的一些不同酸性羟基，其相对量在合成后修饰后发生变化。用室温同位素交换法测定了17 O均匀地掺入到沸石骨架中，而17 O → 27 Al极化转移实验表明，骨架外AlnOm物种不发生17 O掺入。来自暴露于受控水解的样品的数据表明，框架中最近邻的Al对更容易受到水解攻击。本文报道的数据表明，Al（IV）-1/Al（IV）-2配对位点是协同位点，导致低温和高温反应中的反应性增加。没有发现配对的框架/非框架网站的证据。
Current needs for extending zeolite catalysts beyond traditional gas-phase hydrocarbon chemistry demand detailed characterization of active site structures, distributions, and hydrothermal impacts. A broad suite of homonuclear and heteronuclear NMR correlation experiments on dehydrated H-ZSM-5 catalysts with isotopically enriched17O frameworks reveals that at least two types of paired active sites exist, the amount of which depends on the population of fully framework-coordinated tetrahedral Al (Al(IV)-1) and partially framework-coordinated tetrahedral Al (Al(IV)-2) sites, both of which can be denoted as (SiO)4–n–Al(OH)n. The relative amounts of Al(IV)-1 and Al(IV)-2 sites, and subsequent pairing, cannot be inferred from the catalyst Si/Al ratio, but depend on synthetic and postsynthetic modifications. Correlation experiments demonstrate that, on average, acidic hydroxyl groups from Al(IV)-1/Al(IV)-2 pairs are closer to one another than those from Al(IV)-1/Al(IV)-1 pairs, as supported by computational DFT calculations. Through-bond and through-space polarization transfer experiments exploiting17O nuclei reveal a number of different acidic hydroxyl groups in varying Si/Al catalysts, the relative amounts of which change following postsynthetic modifications. Using room-temperature isotopic exchange methods, it was determined that17O was homogeneously incorporated into the zeolite framework, while17O →27Al polarization transfer experiments demonstrated that17O incorporation does not occur for extra-framework AlnOmspecies. Data from samples exposed to controlled hydrolysis indicates that nearest neighbor Al pairs in the framework are more susceptible to hydrolytic attack. The data reported here suggest that Al(IV)-1/Al(IV)-2 paired sites are synergistic sites leading to increased reactivity in both low- and high-temperature reactions. No evidence was found for paired framework/nonframework sites.