Yuiko Tasaki-Handa‡
*,
Yukie Abe and
Kenta Ooi
Research Institute for Environmental Management Technology, National, Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki, Japan. E-mail: yuiko-tasaki@aist.go.jp; Fax: +81-29-861-8252; Tel: +81-29-861-8772
First published on 12th July 2016
Since coordination polymers (CPs) are generally obtained as fine powders, the fabrication of a CP is an issue that should be addressed. A monolithic CP was successfully prepared through the lyophilization of a cyclohexane organogel of [Sm(dehp)3] consisting of Sm3+ and di-(2-ethylhexyl)phosphoric acid. The crystalline domains of the CP possibly act as cross-linking points, and the crystallization of cyclohexane is likely to generate the void space in the resulting cryogels. The architecture of the material can be varied by changing the volume ratio of cyclohexane to [Sm(dehp)3]. While the morphology observed through the scanning electron microscope was completely different from the as-synthesized material, powder X-ray diffraction analysis revealed that the crystalline structure did not change. The distribution coefficient and kinetic constant of ion exchange in the lanthanide series were improved by cryogel formation.
We have found that [Ln(dehp)3] (Ln = lanthanide) shows selectivity in the lanthanide series.19–21 However, a handling difficulty resulting from its fineness was a drawback. Because it is known that [Ln(dehp)3] forms organogels by absorbing nonpolar organic solvents, such as decane,22 hexane,23,24 or toluene,25 we decided to prepare an organogel of [Ln(dehp)3] and tried to fabricate a monolithic [Ln(dehp)3] by cryogelation. In this study, [Sm(dehp)3] is employed as an example.
[Sm(dehp)3] was synthesized by mixing SmCl3 and Na-type dehp− in a 90:10 vol% methanol-water mixture at 20 °C (the details are described in the ESI†). The chemical composition was C, 49.52; H, 9.38; Ln, 15.06; P, 8.19% (calculated for [Sm(dehp)3]: C, 51.72; H, 9.22; Ln, 13.49; P, 8.34%). Approximately 0.1 g of the [Sm(dehp)3] was placed in (a) 0.50, (b) 0.75, (c) 1.5, (d) 3.0, or (e) 6.0 mL of cyclohexane at 20 °C. Cyclohexane was employed because it is applicable to cryogelation owing to its appropriate freezing point and sublimation heat, whereas cyclohexane, n-pentane, n-hexane, and n-heptane formed organogels (see the ESI†). [Sm(dehp)3] absorbed cyclohexane outward-in and turned totally transparent in three days. Fig. 1(i) shows [Sm(dehp)3] swelled by absorbing cyclohexane, where the volume ratio of cyclohexane to [Sm(dehp)3], R, is (a) 10, (b) 14, (c) 27, (d) 54, and (e) 110. The increase in R is accompanied by an increase in the bulkiness and softness of the gel. The gels a and b were hard and maintained their hardness for as much as several months, whereas the gels d and e were soft and their surfaces were flat. The maximum R value for [Sm(dehp)3] was ca. 450, which was obtained when [Sm(dehp)3] was placed in excess cyclohexane for seven days at room temperature. Although it was still a gel with high viscosity and a rough surface, it was hard to use it for the following measurements owing to its high volatility.
Fig. 1 Photos of the as-synthesized [Sm(dehp)3], the cyclohexane organogels (i), and the cryogels (ii). All the samples consist of 0.1 g of [Sm(dehp)3]. |
As shown in Fig. 2(i) and (ii), there is a peak at 2θ = 5.4° in the PXRD pattern of gel a, which is also observed in the pattern of the as-synthesized material. In addition, a broad peak at 2θ ≈ 23° in the PXRD pattern of the as-synthesized [Sm(dehp)3] (Fig. 2(i)) indicates the presence of a noncrystalline phase. These results indicate the coexistance of crystalline and noncrystalline domains. The X-ray absorption fine structure (XAFS) spectra of the gels are very similar to that of the as-synthesized [Sm(dehp)3] (Fig. 3), indicating that all the Sm3+ are six-coordinated with oxygen (see the ESI for more details regarding XAFS†). Although the crystallinity of [Sm(dehp)3] is too low to index a unit cell, one of the most feasible structures suggested is the one that comprises hexagonally packed linear chains with three O–P–O bridges between the lanthanide ions.26 Therefore, the noncrystalline phase may comprise a fibrous polymer of Sm3+–dehp− (Fig. 4(i)). Although the PXRD analysis showed no patterns for the gels b–e owing to their low densities (see the ESI†), we do not believe that the crystalline region disappeared because of gelation. As discussed later, this hypothesis is confirmed by the PXRD patterns of the material after lyophilization which show several peaks. As suggested for similar gelatinous systems reported in the literature,27 the crystalline domains may act as cross-linking points in the gels. Consequently, the noncrystalline part should absorb cyclohexane (Fig. 4(ii)). This hypothesis was supported by the gels of [Ln(dehp)3] (Ln = Ce, Sm, Dy, Yb) having different crystallinity. As shown in Fig. 5, R increases with a decrease in the crystallinity, indicating that the noncrystalline part of the [Ln(dehp)3] absorbs cyclohexane to form a gel (Fig. 4(i) and (ii)).
Fig. 2 The PXRD patterns of the as-synthesized [Sm(dehp)3] (i), organogel a (ii), and cryogels A–E (iii). The irradiation area depended on the sample. |
Fig. 5 Relation between crystallinity of [Ln(dehp)3] and R. Crystallinity was estimated by the ratio of peak area in PXRD patterns (see the ESI for the details†). |
To fabricate a cryogel, the [Sm(dehp)3] organogels of different R were frozen in liquid nitrogen, followed by vacuum drying for 72 h (see the ESI†). Fig. 1(ii) shows the change in appearance after lyophilization. It is obvious from the scanning electron microscope (SEM) micrographs (Fig. 6) that the shape of the [Sm(dehp)3] organogels were different from that of the as-synthesized [Sm(dehp)3] and that this change is dependent on R. The as-synthesized [Sm(dehp)3] has a high-density structure of small particles, while the cryogels exhibit sponge-like architectures. The void volume increases and the thickness of the pore wall appears to decrease with increasing R. The PXRD patterns (Fig. 2(iii)) show that the cryogels have the same crystalline structure as the as-synthesized [Sm(dehp)3], although the peaks are slightly broader. In addition, the larger the R, the broader the peaks. This result indicates that large crystalline domains are partially broken by adsorbing cyclohexane. However, such a slight change in crystallinity does not explain the absorption of a large amount of cyclohexane; thus, it is expected that the void space shown in the SEM micrographs (Fig. 6) was formed because of frozen cyclohexane. A monolith [Sm(dehp)3] can be fabricated by lyophilization of a cyclohexane organogel, which is easier to handle than the powdery as-synthesized material.
Fig. 6 SEM micrographs of the as-synthesized [Sm(dehp)3] and its cryogels (A–E) with a ×400 magnification. The insets for A–C show the ×1500 magnification micrographs. |
The ion exchange reactivity of the material was evaluated because we have revealed that [Sm(dehp)3] can exchange its Sm3+ selectively with other Ln3+ ions.19–21 Cryogel D was used because its structure was homogeneously monolithic, while cryogel E was too light and flimsy. Fig. 7(a) and (b) compare the distribution coefficients (Kd/L g−1) of Ce3+, Nd3+, Eu3+, Dy3+, Er3+, and Yb3+ exchange reactions and the kinetic coefficients (k1/L mol−1) are roughly estimated using a pseudo-first-order kinetic reaction model for cryogel D with the as-synthesized [Sm(dehp)3],
(1) |
(2) |
Fig. 7 Distribution coefficient Kd (a) and the pseudo-first-order kinetic constant k1 (b) of ion exchange for the as-synthesized [Sm(dehp)3] and cryogel D. |
Monolithic CPs, i.e., [Sm(dehp)3] cryogels, were successfully fabricated via lyophilization of an organogel. The architecture of the cryogels could be altered by changing R, and the material was monolithic with sufficient void volume. The change in R with crystallinity and the XAFS analysis implied that the crystalline domains act as cross-linking points and that the noncrystalline parts absorb cyclohexane in a gel state. The SEM micrographs and the PXRD patterns for the cryogels and the Kd and k1 values of ion exchange indicated that the physical shape of the material was changed by gelation and lyophilization while the chemical properties remained the same. Because molecules with a rod-like structure are the most efficient gelators and are able to immobilize a large volume of organic solvent,27 other CPs with rod-like crystalline structures could be fabricated with monolithic structures if they can form an organogel with appropriate organic solvents such as cyclohexane. Cryogelation is one of the most feasible methods for fabricating monolithic structures as well as for the formation of aerogels; thus, it has potential in the fabrication of CPs and MOFs as separation and sensing materials.
Footnotes |
† Electronic supplementary information (ESI) available: Synthesis and preparation of [Sm(dehp)3], organogels, and cryogels; determination of R value; experimental procedure and results of elemental analysis, SEM, PXRD, XAFS, N2 absorption isotherm, and ion exchange. See DOI: 10.1039/c6ra12477a |
‡ Present address: Graduate School of Science and Technology, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama-shi, Japan. E-mail: E-mail: handa-ytasaki@apc.saitama-u.ac.jp; Tel: +81-48-858-3524. |
This journal is © The Royal Society of Chemistry 2016 |