Structural adjustment during intercalation of macrocyclic crown ether into LDH via swelling/restoration reaction: staging formation and mechanism insights
作者:Shulan Ma、Li Du、Juan Wang、Nankai Chu、Yahong Sun、Genban Sun、Xiaojing Yang、Kenta Ooi
DOI:10.1039/c1dt10982k
日期:——
The structural control involving staging formation was studied in the nanocomposites of macrocyclic tetraazacrown ether carboxylic acid derivative (TECA) and layered double hydroxide (LDH) obtained by an osmotic swelling/restoration process. After NO3-type MgAl–LDH was osmotically swollen in formamide, TECA was added, leading to a restoration of the LDH sheets and formation of TECA–LDH nanocomposites. In the wet state, the structure of the composites was homogenous, and the basal spacings of ∼2.0 or 1.8 nm were not changed by the water-washing process that removed formamide and caused the replacement of NO3− by CO32−. However, in the drying process, both the orientation of TECA in the interlayer and the formed staging structure varied with the TECA content. The TECA orientation changed from horizontal to tilted/twisted and finally to vertical with interlayer TECA density. The staging structure occurred for samples with both TECA and small inorganic guests coexisting in the interlayer. Third-staging, second-staging, and homogenous structures were observed at TECA/LDH weight ratios of low (0.125 and 0.25), medium (0.5) and large (1 and 2) values, respectively. The decrease of negative charge of TECA ions from −4 to −1 led to the co-existence of small ions (NO3−) with TECA in the gallery and the formation of a NO3−-containing staging structure, even at a high TECA/LDH ratio of 2. The study of the influence of CO32− on the composite structure indicates that a TECA/CO32− ion exchange progressed reversibly and the staging structure formed is thermodynamically stable, depending mainly on the chemical composition of the interlayer guests. The staging formation could be explained by the Daumas–Hérold model, which differs markedly from the Rüdorff model previously proposed for the LDH system. The treatment in formamide of the composite such as T2L+Na2CO3 with staging structure shows that the transformation process is reversible, and these kinds of composites may be used as an adsorbent for some harmful organic solvents.
研究了大环四氮唑冠醚羧酸衍生物(TECA)和层状双氢氧化物(LDH)通过渗透溶胀/复原过程获得的纳米复合材料中分期形成的结构控制。在甲酰胺中对 NO3 型 MgAl-LDH 进行渗透溶胀后,加入 TECA,LDH 片状结构得到恢复,并形成 TECA-LDH 纳米复合材料。在湿态下,复合材料的结构是均匀的,基底间距为 ∼ 2.0 或 1.8 nm,在水洗过程中甲酰胺被除去,NO3- 被 CO32- 取代,复合材料的基底间距没有发生变化。然而,在干燥过程中,TECA 在夹层中的取向和形成的分级结构都随 TECA 含量的变化而变化。随着层间 TECA 密度的增加,TECA 的取向从水平变为倾斜/扭曲,最后变为垂直。TECA 和小型无机客体共存于夹层中的样品会出现分级结构。在 TECA/LDH 重量比分别为低值(0.125 和 0.25)、中值(0.5)和大值(1 和 2)时,可观察到第三分期、第二分期和均质结构。TECA 离子的负电荷从-4 减少到-1,导致小离子(NO3-)与 TECA 在管廊中共存,并形成了含 NO3-的分级结构,即使在 TECA/LDH 的高重量比为 2 时也是如此。 CO32- 对复合结构影响的研究表明,TECA/CO32- 离子交换是可逆的,所形成的分级结构在热力学上是稳定的,主要取决于层间客体的化学成分。分级结构的形成可以用 Daumas-Hérold 模型来解释,该模型与之前针对 LDH 系统提出的 Rüdorff 模型有明显不同。在甲酰胺中处理具有分级结构的 T2L+Na2CO3 等复合材料表明,转化过程是可逆的,这类复合材料可用作某些有害有机溶剂的吸附剂。