Evolution of Framework Al Arrangements in CHA Zeolites during Crystallization in the Presence of Organic and Inorganic Structure-Directing Agents
Evolution of Framework Al Arrangements in CHA Zeolites during Crystallization in the Presence of Organic and Inorganic Structure-Directing Agents
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DOI：
10.1021/acs.cgd.2c00856
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发表时间：
2022-09-19
影响因子：
3.8
通讯作者：
Gounder，Rajamani
中科院分区：
文献类型：
--
作者：
Lee，Songhyun;Nimlos，Claire T.;Gounder，Rajamani
沸石骨架中Al杂原子的排列影响Brønsted酸催化反应的周转率和交换的金属阳离子和配合物的形成，这些金属阳离子和配合物是氧化还原催化的活性位点。沸石框架中Al取代Si产生阴离子晶格电荷，因此受到引导沸石结晶的阳离子结构定向剂（SDAs）的结构和电荷密度的影响。在这里，我们研究了在有机和无机sda存在下，无定形Al和Si前驱体转化为部分和完全结晶的铝硅酸盐相时，CHA沸石中的框架Al结构和排列是如何随着水热处理时间的变化而演变的。当N，N，N-三甲基-1- adamantylamium （TMAda+）作为唯一的SDA时，最初形成一个含有大量近端Al位点的无定形铝硅酸盐网络，通过Co2+滴定定量，该网络演变成CHA晶体，在六元环中含有大量近端Al位点（6-MR）。在体结晶完成后（433 K, 36 h），继续水热处理使框架Al的重排变得更加位隔离，最终导致CHA晶体中6-MR配对Al位的数量无法检测（433 K, 144 h）。框架Al排列的这些时间变化表明Si-O-Al键保持不稳定，并在水热条件下在晶体域内进行重组，允许原子重排向热力学上首选的框架Al分布（例如，6-MR仅在TMAda+存在下分离Al位）。从头算分子动力学（AIMD）模拟表明，在TMAda+场中，分离的Al构型的能量低于6-MR Al对构型，这支持了当TMAda+是唯一的SDA时Al位点分离的热力学驱动力。相比之下，在CHA结晶完成后（433 K, 96 h），同时使用Na+和TMAda+作为无机和有机co-SDAs并没有导致框架Al排列的进一步变化，这表明Na+和TMAda+的共遮挡抑制了框架Si-O-Al键的稳定性和Al在晶相中的重排。AIMD模拟报告，当Na+和TMAda+电荷平衡时，6-MR配对的Al位的能量比孤立的Al构型低。这些发现表明，在非晶态Si和Al前体完成体结晶后，沸石框架中的Al排列可以经过长时间的水热处理而演变，并且这种演变受热力学因素的影响，扩展了先前在沸石间转化路线中发生的这种现象的报道。
The arrangement of Al heteroatoms in zeolite frameworks influences turnover rates of Brønsted acid-catalyzed reactions and the speciation of exchanged metal cations and complexes that are active sites for redox catalysis. The substitution of Al for Si in zeolite frameworks generates anionic lattice charges and is thus influenced by the structure and charge density of the cationic structure directing agents (SDAs) that guide zeolite crystallization. Here, we investigate how framework Al structure and arrangements in chabazite (CHA) zeolites evolve as a function of hydrothermal treatment time as amorphous Al and Si precursors convert into partially and fully crystalline aluminosilicate phases in the presence of organic and inorganic SDAs. WithN,N,N-trimethyl-1-adamantylammonium (TMAda+) as the sole SDA, an amorphous aluminosilicate network initially forms that contains a large fraction of proximal Al sites, as quantified by Co2+titration, which evolves into CHA crystallites that contain a high fraction of proximal Al sites in six-membered rings (6-MR). After bulk crystallization has been completed (433 K, 36 h), continued hydrothermal treatment causes rearrangement of framework Al to become more site-isolated, eventually resulting in CHA crystallites with undetectable numbers of 6-MR paired Al sites (433 K, 144 h). These temporal changes in framework Al arrangement indicate that Si–O–Al linkages remain labile and undergo restructuring within crystalline domains under hydrothermal conditions, allowing for atomic rearrangement toward thermodynamically preferred framework Al distributions (e.g., 6-MR isolated Al sites in the presence of TMAda+only). Ab initio molecular dynamics (AIMD) simulations report isolated Al configurations to be lower in energy than 6-MR Al pair configurations in a field of TMAda+, supporting a thermodynamic driving force for Al sites to isolate when TMAda+is the sole SDA. In contrast, using both Na+and TMAda+as inorganic and organic co-SDAs did not result in any further changes to the framework Al arrangement after CHA crystallization was complete (433 K, 96 h), suggesting that the co-occlusion of Na+and TMAda+suppresses the lability of framework Si–O–Al bonds and Al rearrangement in crystalline phases. AIMD simulations report 6-MR paired Al sites to become lower in energy than isolated Al configurations when charge-balanced by both Na+and TMAda+. These findings indicate that Al arrangements in zeolite frameworks can evolve upon extended hydrothermal treatment after bulk crystallization has been completed from amorphous Si and Al precursors and that such evolution is influenced by thermodynamic factors, extending prior reports of such phenomena occurring during interzeolite conversion routes.