Thermal Dehydrofluorination of GaPO-34 Revealed by NMR Crystallography
Thermal Dehydrofluorination of GaPO-34 Revealed by NMR Crystallography
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DOI：
10.1021/acs.jpcc.0c10871
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发表时间：
2021-01-25
影响因子：
3.7
通讯作者：
Ashbrook, Sharon E.
中科院分区：
文献类型：
--
作者：
Dawson, Daniel M.;Macfarlane, Laurie E.;Ashbrook, Sharon E.
结合固态核磁共振波谱、粉末 X 射线衍射 (pXRD)、热重分析和周期密度泛函理论 (DFT) 计算，我们研究了以 1-甲基咪唑 (mim) 或吡啶 (py) 作为结构导向剂 (SDA) 和氟化物作为电荷平衡阴离子制备的菱沸石型没食子磷酸盐 GaPO-34 的煅烧。我们证明，在 SDA 燃烧之前，mim 材料有一个不寻常的低温脱氟化氢步骤，温度约为 330 摄氏度，但 py 形式则不然。脱氟化氢中间体的 DFT 衍生结构包含五配位 Ga 物种，其中与 mim 氮原子具有 2.04 埃的 Ga-N 键，此外还有与相邻 PO4 四面体的四个 Ga-O 键。这一观察结果与 Ga-71 NMR 光谱一致，表明三分之一的 Ga 是五配位的，具有类似于 11 MHz 的大四极耦合常数。加热原位测量的粉末 X 射线衍射显示，在看到特征菱沸石结构之前，在 325 至 425 摄氏度之间短暂出现了明显的结晶相，这与有机 SDA 损失之前的脱氟化氢一致。 GaPO-34 的 py 形式没有观察到这种脱氟化氢中间体结构，这归因于吡啶鎓相对于 1-甲基咪唑鎓的 pK(a) 较低。
Using a combination of solid-state NMR spectroscopy, powder X-ray diffraction (pXRD), thermogravimetry, and periodic density functional theory (DFT) calculations, we investigate the calcination of the chabazite-type gallophosphate, GaPO-34, prepared with either 1-methylimidazole (mim) or pyridine (py) as the structure-directing agent (SDA) and fluoride as the charge-balancing anion. We demonstrate that, prior to SDA combustion, there is an unusual low-temperature dehydrofluorination step at similar to 330 degrees C for the mim material, but not for the py form. The DFT-derived structure for the dehydrofluorinated intermediate contains pentacoordinate Ga species with Ga-N bonds of 2.04 angstrom to the mim nitrogen atom, in addition to four Ga-O bonds to neighboring PO4 tetrahedra. This observation is consistent with Ga-71 NMR spectroscopy, which shows that one-third of the Ga is pentacoordinate with a large quadrupolar coupling constant of similar to 11 MHz. Powder X-ray diffraction measured in situ on heating shows the transient appearance of a distinct crystalline phase between 325 and 425 degrees C before the characteristic chabazite structure is seen, which is consistent with dehydrofluorination prior to loss of the organic SDA. No such dehydrofluorinated intermediate structure is observed for the py form of GaPO-34, which is ascribed to the lower pK(a) of pyridinium relative to 1-methylimidazolium.