Structural Features in Metal–Organic Nanotube Crystals That Influence Stability and Solvent Uptake

Abstract

Porous hybrid materials such as metal−organic nanotubes are of interest due to synthetic tunability, possibility for 1-D flow, and confinement of solvent molecules. In the current study, the stability and solvent selectivity of two hybrid materials with nanotubular arrays, (pip) 0.5 [(UO 2 )(HIDA)(H 2 IDA)]·2H 2 O (UIDA; IDA = iminodiacetate) and [(UO 2 )(PDC)(H 2 O)] (UPDC; PDC = pyridine dicarboxylate) were analyzed using X-ray diffraction, gas chromatography/ mass spectrometry, thermogravimetric analysis, and infrared spectroscopy. The fine details of the structural characterization, such as the presence of solvent molecules, were found to be important to the overall properties of the materials as evidenced by the increased stability of the UIDA compound when formed from a solvent mixture containing acetone. Careful analysis of the UPDC compound indicated that the ligated solvent molecule can be exchanged, which may impact the hydration state of the material. Overall, the UIDA compound displays complete selectivity to water, but the UPDC compound adsorbs THF, methanol, ethanol, and cyclohexane.

Exploring the unique water properties of metal-organic nanotubes

“…The UMON material can be synthesized using either methanol or acetone as the co-solvent and we have previously reported that when the compound is synthesized with methanol, it rapidly degrades in a high humidity environment. 123 A similar study was performed here for the Cu(II)-UMON (1:3 starting ratio) synthesized with methanol and acetone co-solvents by exposing the material to a saturated environment for a two month period and monitoring the solid state materials by powder X-ray diffraction over the course of the reaction period (Figure 4.9). For the pure UMON material synthesized with methanol, the material degraded within one week, 123 but the Cu(II)-UMON compound formed in the same co-solvent remained crystalline in a high humid environment for the entire period.…”

“…123 A similar study was performed here for the Cu(II)-UMON (1:3 starting ratio) synthesized with methanol and acetone co-solvents by exposing the material to a saturated environment for a two month period and monitoring the solid state materials by powder X-ray diffraction over the course of the reaction period (Figure 4.9). For the pure UMON material synthesized with methanol, the material degraded within one week, 123 but the Cu(II)-UMON compound formed in the same co-solvent remained crystalline in a high humid environment for the entire period. Both the pure and doped UMON material synthesized with acetone also retained crystallinity during the experiment.…”

“…Previous studies have also shown that small amounts of water will be incorporated into the sample as the material is being loaded into the instrument for analysis. 123 Only when the Cu(II)-UMON material was in contact with methanol did evidence for the presence of the solvent appear in the spectra, with bands located at 1027 cm -1 and 1061 cm -1 . This still suggested only minor incorporation…”

“…We also previously reported that the hydration state of the nanotubes could be tuned based on the relative humidity of the laboratory, with complete hydration achieved in the summer, but only partial uptake in the winter months. 123 Herein, we further explored the relationship between hydration state of the UMON material and relative humidity and determined uptake rates based upon these parameters. In addition, we extended our previous work with liquid solvents to test solvents in the vapor phases, specifically focusing a wide range of molecules with different polarities, shapes, and sizes to further investigate the selectivity of the UMON.…”

“…The nanoconfined solvent can be reversibly removed at 37 ˚C and is completely selective to water. 62,123 The observed properties could enhance dewatering and separations technologies, but creating similar materials with earth abundant elements is critical for utilizing these materials in a real-world application. To explore the role of…”