Experimental investigation on the water stability of amino-modified indium metal–organic frameworks

Abstract

On the stability issue of MIL-68(In)–NH₂ in water, water-resistance, acid-tolerance and hydrothermal tests on MIL-68(In)–NH₂ were performed. The results demonstrated that MIL-68(In)–NH₂ remained good structural stability in mild conditions; whereas a partial or complete hydrolysis could occur to it under harsh conditions. Furthermore, MIL-68(In)–NH₂ exhibited much better water stability when compared with MIL-68(In). This study first provides a crucial reference for the applicable conditions of MIL-68(In)–NH₂ applied in water treatment.

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Metal–organic frameworks (MOFs), a new class of porous crystalline materials, have attracted tremendous attention considering their broad application prospect.^1–4 Recently, the MOFs has become a hot topic in adsorbing toxic metals,^5–7 pharmaceuticals^8,9 and other hazardous organics^10–12 from the aqueous medium. However, the liquid phase separations have very stringent stability requirements and many MOFs are suffering from stability issues. For instance, zinc-based MOFs are moisture-sensitive, losing their high porosity and surface area when exposed to humid air.¹³ Molecular dynamics simulations on the interaction between MOF-5 and water molecules indicated that distortions in the framework structure occurred even at rather low water content.¹⁴ Zhang et al.'s study¹⁵ proved that ZIF-8 crystals can degrade in water and the extent of the dissolution depends on the crystals to water mass ratio. The above studies strongly implied that the water molecules played a crucial role in the structural integrity and crystal morphology of the MOF materials. Therefore, it is essential to evaluate the stability of the MOFs in water before their practical application in aqueous environments.

Farrusseng's group has shown a kind of amino-modified MOF material, MIL-68(In)–NH₂, synthesized with metal salt In(NO₃)₃ and 2-aminoterephthalic acid (BDC-NH₂) by a solvothermal method.¹⁶ MIL-68(In)–NH₂ has shown remarkable potential in gas adsorption and photocatalysis.^17,18 Previous reports indicated that MIL-68(In)–NH₂ is acid-sensitive and easily hydrolyses in water.^19,20 In contrary, Liang et al.¹⁸ reported that the optimum photocatalytic performance of MIL-68(In)–NH₂ was achieved under strongly acidic condition (pH = 2). The stability of MIL-68(In)–NH₂ reflected from these studies seems contradictory and thus needs more study to elucidate it. To our knowledge, there has been no report on the stability of MIL-68(In)–NH₂ in water so far. Hence, in this study, the water stability of MIL-68(In)–NH₂ was evaluated with MIL-68(In) as a comparison. The research content can be summarized as follows: (1) effect of water molecules on the synthesis of MIL-68(In)–NH₂; (2) acid tolerance and (3) hydrothermal stability of MIL-68(In)–NH₂ and MIL-68(In); (4) stability study for MIL-68(In)–NH₂ and MIL-68(In) after a long time of exposure to water.

The effect of water molecules on the synthesis of MIL-68(In)–NH₂ was investigated (Fig. 1). Fig. 1a shows the XRD pattern of MIL-68(In)–NH₂ with the characteristic peaks at 2θ ∼9, 13 and 15°. For the sample synthesized at 12.9% water content (hereafter denoted 1), the major diffraction peaks of MIL-68(In)–NH₂ was preserved; this indicates that the MIL-68(In)–NH₂ framework structure was remained despite the appearance of new phases (2θ ∼12, 22 and 31°). At higher water content of 18.2%, the MIL-68(In)–NH₂ structure completely disappeared as a result of the missing characteristic peaks, accompanying with the formation of some new unknown phases. Consistent with the XRD results, as shown in Fig. 1b, the BET surface area severely decreased from 679 to 78 m² g⁻¹ when the water content was up to 12.9%. The reason may be greatly related to the water molecule, which was involved in the coordination of MIL-68(In)–NH₂ by replacing one of the coordinating MOF O atoms or forming hydrogen bonding between the water H and the MOF O.¹⁴ As the coordinated water molecules increased, the framework eventually collapsed. In addition, 1 also showed a large adsorption hysteresis loop (Fig. 1b inset), indicating a change in pore structure as comparison with the micropore structure of MIL-68(In)–NH₂.¹⁷ Fig. 1c–f show the SEM images of MIL-68(In)–NH₂ and 1. It is observed that only few crystals were formed at the water content of 12.9% (Fig. 1e). Furthermore, the surface of the crystal (Fig. 1f) became less smooth than that of MIL-68(In)–NH₂ (Fig. 1d), particularly the appearance of surface cracks and defects in the local areas.

Fig. 1 (a) XRD patterns of MIL-68(In)–NH₂ and the samples synthesized in the presence of 12.9% and 18.2% water content. (b) N₂ adsorption–desorption isotherms of MIL-68(In)–NH₂ and 1; the inset separately shows the N₂ adsorption–desorption isotherm of 1. SEM images of MIL-68(In)–NH₂ (c and d) and 1 (e and f).

Fig. 2a and b show the XRD patterns of MIL-68(In)–NH₂ and MIL-68(In) after acid treatment for 2 h. For MIL-68(In)–NH₂, at pH 5, the XRD pattern remained essentially unchanged. As the pH value declined to 3, the peak intensity was slightly diminished, indicating the decrease of the crystallinity of MIL-68(In)–NH₂. However, after treatment under strongly acidic conditions (pH = 2 and 1), the MIL-68(In)–NH₂ structure was completely destructed and the characteristic diffraction peaks almost entirely disappeared. The crystallinity of the samples decreased seriously as a result of the broken lattice structure. A similar behavior was also observed for MIL-68(In). Changes in BET surface area of the corresponding samples are consistent with the XRD patterns. As seen in Fig. 2c, compared to the BET surface area of the as-synthesized MIL-68(In)–NH₂ (679 m² g⁻¹), those of the MIL-68(In)–NH₂ samples treated in the relatively mild acid conditions (pH = 5 and 3) essentially stayed the same (672 and 658 m² g⁻¹); while exposure of MIL-68(In)–NH₂ to the highly acid solutions (pH = 2 and 1) resulted in a dramatic drop in BET surface area (57 and 10 m² g⁻¹). However, for MIL-68(In), a sharp decrease in BET surface area occurred to it, even under the mild acid conditions. It seems that MIL-68(In) is more sensitive to the changes in solution acidity than MIL-68(In)–NH₂. As for the distinct decrease in crystallinity and porosity of both MIL-68(In)–NH₂ and MIL-68(In) under strongly acid conditions, it may be attributed to an acid hydrolysis process, which induces the dissociation of the framework.¹³

Fig. 2 XRD patterns of MIL-68(In)–NH₂ (a) and MIL-68(In) (b) after being immersed in acidic solutions with different pH values for 2 h. (c) Changes in BET surface area of MIL-68(In)–NH₂ and MIL-68(In) after acidic treatment.

The hydrothermal stability of MIL-68(In)–NH₂ and MIL-68(In) was demonstrated by immersing the samples in hot water (80 °C) for 24 h. For MIL-68(In)–NH₂, as shown in Fig. 3a, the XRD pattern, measured after hydrothermal test for 12 h, shows a significant loss in crystallinity. When the heat time reached 24 h, the MIL-68(In)–NH₂ structure was thoroughly decomposed and evolved into a new nanophase determined by the broadening of the diffraction peaks at 2θ ∼30 and 31°. Moreover, a low BET surface area (66 m² g⁻¹) occurred to the MIL-68(In)–NH₂ sample collected at 24 h (Fig. S1, ESI†), which lost ca. 90% of the original BET surface area when compared with control (679 m² g⁻¹). However, for MIL-68(In), when treated for 12 h, the main diffraction peaks have disappeared; meanwhile, some new peaks (2θ ∼8, 9 and 21°) appeared. Fig. 3c–f present the SEM images of MIL-68(In)–NH₂ and MIL-68(In) after hydrothermal tests. From Fig. 3c and d, it can be seen that the morphology of the MIL-68(In)–NH₂ sample after treatment was distinctly different from that of the original crystal (see Fig. 1c). For MIL-68(In), a morphology transition from the rodlike structures to the plate ones was also observed (Fig. 3e and f). Combining the above analysis, both of the MIL-68(In)–NH₂ and MIL-68(In) crystals were completely destroyed and transformed into an unknown phase after hydrothermal treatment. Therefore, it can be concluded that the two crystals underwent hydrolysis under hydrothermal conditions.

Fig. 3 XRD patterns of MIL-68(In)–NH₂ (a) and MIL-68(In) (b) after hydrothermal tests in 80 °C water for 12 h and 24 h. SEM images of the MIL-68(In)–NH₂ samples heated for 12 h (c) and 24 h (d), the as-synthesized MIL-68(In) (e) and its sample heated for 24 h (f).

Fig. 4a and b show the XRD patterns of the MIL-68(In)–NH₂ and MIL-68(In) samples after immersion in water for various durations. The peak intensities of MIL-68(In)–NH₂ gradually decreased as an increase of immersion time. The presence of the characteristic peaks implies that the framework structure of MIL-68(In)–NH₂ was retained in all the collected samples. However, the decline of the peak intensities of MIL-68(In) was serious, meaning a severe decrease in crystallinity. Moreover, the disappearance of some crystal faces of MIL-68(In) can be observed from the absence of some characteristic peaks (2θ ∼4, 9, 13 and 18°). The loss of BET surface area for MIL-68(In)–NH₂ and MIL-68(In) after exposure to water was also investigated. From Fig. 4c, it can be seen that the BET surface area of MIL-68(In)–NH₂ showed an initial reduction of about 4% after 4 h immersion. The BET surface area remained nearly constant in the next 20 h and then began a slow decline. For MIL-68(In), within the first 4 h immersion, it showed a sharp decline by approximately 20% in BET surface area faster than MIL-68(In)–NH₂. This is consistent with the change in the XRD patterns occurring after just 4 h immersion. Furthermore, as the immersion time extended, the BET surface area of MIL-68(In) dropped significantly and ultimately to only 40.5%. Based on these data, MIL-68(In)–NH₂ seems to have higher water stability than MIL-68(In) despite a partial hydrolysis could occur to it. Jasuja et al.'s findings²¹ have indicated that the better stability of MOFs was contributed to the higher basicity of the ligands. For MIL-68(In)–NH₂, it contains additional N-donor coordination functionality in comparison with MIL-68(In). The relative high basicity of MIL-68(In)–NH₂ arising from the ligands may be one of the reasons leading to better stability. Furthermore, structural stability of MOFs in water also depends on ligand sterics.²² MIL-68(In)–NH₂ is constructed from infinite chains of InO₄(OH)₂ octahedra bridging with the BDC-NH₂ ligand and thus generates two types of 1D channels, which is analogous to the structure of MIL-68(In) (Fig. S2, ESI†).¹⁷ However, the integration of the amino group into the framework of MIL-68(In)–NH₂ increases the steric hindrance around the ligands and metals; this prevents water molecules from clustering near the metal centers.

Fig. 4 XRD patterns of MIL-68(In)–NH₂ (a) and MIL-68(In) (b) after being immersed in water for various durations, T = 25 °C, pH = 7. (c) Plots of BET surface area reduction vs. immersion time for MIL-68(In)–NH₂ (black) and MIL-68(In) (red).

In summary, we performed the water-resistance, acid-tolerance and hydrothermal tests in order to evaluate the structure stability of MIL-68(In)–NH₂ in water. The results indicated that partial decomposition of the MIL-68(In)–NH₂ framework could occur in strongly acid solutions or after a long time of immersion and heating in pure water. Furthermore, the presence of water molecules had an adverse effect on the synthesis of MIL-68(In)–NH₂. Anyhow, MIL-68(In)–NH₂ exhibited much better water stability than MIL-68(In), which may be associated with the basicity and steric hindrance resulting from the BDC-NH₂ ligand. In this study we have detailedly explored the applicable conditions of MIL-68(In)–NH₂ in aqueous solution for the first time, which provides a significant reference for the potential application of MIL-68(In)–NH₂ in water treatment. In addition, in the future work, more attention should be paid to the post-enhancement for water stability of MIL-68(In) and MIL-68(In)–NH₂ by introducing hydrophobic groups into the framework^23–25 or other ways.

Acknowledgements

This work was supported by the NSF of Guangdong (U1401235).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Materials, synthetic procedures and characterization. See DOI: 10.1039/c6ra09021d

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