Identifying Order and Disorder in Double Four-Membered Rings via Raman Spectroscopy during Crystallization of LTA Zeolite
Identifying Order and Disorder in Double Four-Membered Rings via Raman Spectroscopy during Crystallization of LTA Zeolite
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DOI：
10.1021/acs.chemmater.1c01420
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发表时间：
2021-08-17
影响因子：
8.6
通讯作者：
Fan, Wei
中科院分区：
文献类型：
--
作者：
Luo, Song;Wang, Tongkun;Fan, Wei
氟化物（F-）是合成低缺陷硅质沸石所必需的。已经假设F-平衡来自有机结构导向剂（OSDA）的正电荷并在沸石结晶期间稳定关键结构单元，例如双四元环（D4 R）。然而，由于缺乏研究中程结构的表征技术，包括沸石结晶过程中形成的环和笼，F-在稳定结构单元和保持局部电荷平衡的沸石组装过程中的作用尚未完全理解。本文采用拉曼光谱和周期密度泛函理论（DFT）计算研究了F-存在下全硅Linde A型（LTA）沸石的晶化过程。发现F-填充的双四元环（F-/D_4R）和空的D_4R具有明显的拉曼特征。以1，2-二甲基-3-（4-甲基苄基）咪唑鎓盐为OSDA，在F-存在下合成的LTA沸石中形成了F-/D4 R和空D4 R。观察到的F-/D4 R和空D4 R的拉曼谱带，沿着LTA超晶胞上的预测DFT计算，揭示了这两种D4 R单元在最终制成的LTA沸石中的有序分布。F-/D4 R和空D4 R单元的这些不同的拉曼特征的发现为研究沸石形成期间D4 R中的缺陷打开了一个有趣的新窗口。特别是，我们已经观察到在LTA结晶过程中F-/D4 R和空D4 R带的拉曼强度的变化;周期性DFT计算表明，所观察到的拉曼行为与包含被Q（3）Si包围的一个或两个Si空位的空D4 R单元一致，也与固态核磁共振测量一致。这些缺陷似乎在进一步结晶过程中愈合，导致形成无缺陷的LTA沸石晶体。总体而言，我们的研究结果提供了更深入的了解的作用，F-在电荷平衡和稳定完整的D4 R单位在沸石形成过程中。
Fluoride (F-) has been essential for the synthesis of low-defect siliceous zeolites. It has been hypothesized that F- balances the positive charges from organic structure- directing agents (OSDAs) and stabilizes key building units during zeolite crystallization such as the double four-membered ring (D4R). However, due to the lack of characterization techniques for investigating medium-range structures, including rings and cages formed during zeolite crystallization, the roles of F- in stabilizing building units and maintaining local charge balance during zeolite assembly are not yet fully understood. Here, the crystallization of siliceous Linde type A (LTA) zeolite in the presence of F- was investigated using Raman spectroscopy and periodic density functional theory (DFT) calculations. We have discovered that the F--filled double four-membered ring (F-/D4R) and the empty D4R exhibit rather distinct Raman features. Both the F-/D4R and empty D4R are formed in the LTA zeolite synthesized in the presence of F- using 1,2-dimethyl-3-(4-methylbenzyl) imidazolium as the OSDA. Observed Raman bands of the F-/D4R and empty D4R, along with predictive DFT calculations on LTA supercells, reveal an ordered distribution of these two D4R units in the final as-made LTA zeolite. The discovery of these distinct Raman signatures of F-/D4R and empty D4R units opens an interesting new window for studying defects in the D4R during zeolite formation. In particular, we have observed variation in Raman intensities of F-/D4R and empty D4R bands during LTA crystallization; periodic DFT calculations indicate that the observed Raman behavior is consistent with empty D4R units containing one or two Si vacancies surrounded by Q(3) Si-consistent also with solid-state nuclear magnetic resonance measurement. These defects appear to heal during further crystallization, leading to the formation of defect-free LTA zeolite crystals. Overall, our results provide deeper understanding on the roles of F- in charge balancing and stabilizing intact D4R units during zeolite formation.