Controlling defects in zeolites is crucial for tuning their adsorption and catalytic properties. We have performed an integrated zeolite synthesis, spectroscopy, and density functional theory study to test the limit of F^– as a charge-balancing agent that mitigates defects in siliceous zeolites. We focused on the synthesis of siliceous zeolite LTA at 150 °C with 1,2-dimethyl-3-(4-methylbenzyl) imidazolium as the primary organic structure-directing agent (OSDA) and tetramethyl ammonium (TMA) as the secondary OSDA. By varying the amount of TMA in the synthesis gel, positive charges were titrated into the resulting as-made Si-LTA. Surprisingly, we found that greater TMA concentration does not induce more F^– to enter into the zeolite. ²⁹Si solid-state MAS NMR, Raman spectroscopy, and density functional theory suggest that this system has surpassed its capacity for F^– to balance OSDA charge, and additional positive charge is balanced by Si–O^– framework defects. The number of defects in the as-made Si-LTA can be precisely titrated by the amount of TMA in the zeolite structures. For the Si-LTA synthesized without TMA, framework defects formed in the early stage of crystal growth were found to heal during later crystallization, leading to defect-free Si-LTA. However, for the Si-LTA synthesized with TMA, the defects formed in early stages do not heal. A DFT thermodynamic analysis explains that crowding of Si-LTA pores by TMA impedes defect healing; this prediction is corroborated by synthesis experiments at an elevated temperature (170 °C). These results indicate that F^– can have a limited capacity to balance OSDA charge in zeolite synthesis, opening up a third route to zeolite synthesis intermediate between the fluoride and hydroxide routes.

中文翻译：

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使用多种有机结构导向剂将可控缺陷滴定到 Si-LTA 沸石晶体中

控制沸石中的缺陷对于调整其吸附和催化性能至关重要。我们进行了综合沸石合成、光谱学和密度泛函理论研究，以测试 F 的极限^——作为一种电荷平衡剂，可减轻硅质沸石中的缺陷。我们专注于在 150 °C 下合成硅质沸石 LTA，其中 1,2-二甲基-3-(4-甲基苄基) 咪唑作为主要有机结构导向剂 (OSDA)，四甲基铵 (TMA) 作为次要 OSDA。通过改变合成凝胶中 TMA 的量，将正电荷滴定到所得的原样 Si-LTA 中。令人惊讶的是，我们发现更高的 TMA 浓度不会诱导更多的 F ^-进入沸石。²⁹Si 固态 MAS NMR、拉曼光谱和密度泛函理论表明，该系统已经超过了 F-平衡 OSDA 电荷的能力^，并且额外的正电荷被 Si-O-平衡^-框架缺陷。制成的 Si-LTA 中的缺陷数量可以通过沸石结构中 TMA 的量精确滴定。对于在没有 TMA 的情况下合成的 Si-LTA，发现在晶体生长早期形成的框架缺陷在随后的结晶过程中会愈合，从而导致无缺陷的 Si-LTA。然而，对于用 TMA 合成的 Si-LTA，早期形成的缺陷不会愈合。DFT 热力学分析解释说，TMA 对 Si-LTA 孔的拥挤会阻碍缺陷愈合；这一预测得到了高温（170°C）合成实验的证实。这些结果表明，F ^-在沸石合成中平衡 OSDA 电荷的能力有限，为氟化物和氢氧化物路线之间的沸石合成中间体开辟了第三条路线。