Fabrication of a monolithic cryogel from the cyclohexane organogel of a coordination polymer based on a phosphoester

Abstract

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.

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Since the vast majority of coordination polymers (CPs) and metal–organic frameworks (MOFs) synthesized through conventional methods (i.e., precipitation and solvothermal reactions) are fine powders, their fabrication is quite challenging. Shape controlled synthesis,^1,2 mechanical compression into pellets,^3,4 surfactant templating,^5,6 and incorporation into electrospun nanofibers^7,8 have been employed for some systems. One of the most favorable shapes is monolithic, which is suitable for many applications and easy to handle. Mechanical compression,⁹ extrusion,^10,11 and immobilization in monolithic polymers^11,12 have been demonstrated. However, these methods require binders or substrates, which result in a decrease in the material content. In this regard, the formation of an aerogel or xerogel is advantageous because the final product comprises pure CPs or MOFs. Lohe et al.¹³ and Li et al.¹⁴ reported the synthesis of MOF aerogels by drying ethanolic gels with liquid CO₂. They also demonstrated that their method enabled regulation of the void level. An analogous approach is cryogelation: the solvent crystals formed by freezing act as porogens.¹⁵ Typical porogens comprise water, whereas some solvents with high melting temperature such as cyclohexane,¹⁶ benzene,¹⁷ and acetic acid¹⁸ are also used. In this study, we report the fabrication of monolith coordination polymers based on samarium ions (Sm³⁺) and di-(2-ethylhexyl)phosphoric acid ((C₈H₁₇O)₂PO(OH), HDEHP), [Ln(dehp)₃], which have been studied as ion exchange materials. To the best of our knowledge, CP or MOF gels formed by cryogelation have not been reported, despite the fact that this could be a powerful tool to fabricate monolithic CPs or MOFs.

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. 3 XAFS spectra of gels b and c, and the as-synthesized [Sm(dehp)₃].

Fig. 4 Schematic of the formation of organogel and cryogel: (i) as-synthesized material, (ii) cyclohexane organo gel, (iii) frozen organo gel, and (iv) cryogel. Blue rectangle, crystalline domain of [Sm(dehp)₃]; light yellow region in (ii), liquid cyclohexane; and thick yellow polygon in (iii), cyclohexane crystals.

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 (K_d/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],

where Q, c(Ln³⁺), and cⁱ(Ln³⁺) are the concentration of Ln³⁺ in the solid phase, solution phase, and the initial concentration of Ln³⁺, respectively. For both materials, the Ln³⁺ ions with smaller ionic radii have larger K_d values, which is consistent with the results obtained with solutions of low Ln³⁺ concentrations in the previous study. Such a selectivity should result from the ability of Ln³⁺ to coordinate HDEHP (or dehp⁻). While the trend in k₁ values of cryogel D against the lanthanide series is also the same as that of the as-synthesized [Sm(dehp)₃], both the K_d and k₁ of cryogel D are slightly larger than those of the as-synthesized [Sm(dehp)₃]. An increase in surface area can explain this phenomenon; however, nitrogen adsorption measurements revealed that the Brunauer–Emmett–Teller (BET) surface area did not change after lyophilization (see the ESI†). We assume that the presence of large void spaces in gel D resulted in an increase in the effective surface area that is in contact with an aqueous solution. Since HDEHP is comparatively hydrophobic owing to two ethyl–hexyl chains, the solid/liquid interfacial tension should be large. For the as-synthesized [Sm(dehp)₃], the BHJ analysis indicated that there were pores of >10 nm (see the ESI†). The narrow spaces may prevent an aqueous solution from entering, resulting in a decrease in the effective surface area. In contrast, the voids inside the monolithic materials are sufficiently large to allow aqueous solutions to enter the interior. We believe that the ion exchange reactivity is basically the same as the result of maintaining the microstructure even when the macroscopic architecture is altered.

Fig. 7 Distribution coefficient K_d (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 K_d 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.

Acknowledgements

The synchrotron radiation experiments were performed at the BL11XU of SPring-8 with the approval of the Japan Atomic Energy Agency (JAEA) (Proposal no. 2015A-E06). This work was supported by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (KAKENHI) Grant Number 26810080.

References

1. M. Pang, A. J. Cairns, Y. Liu, Y. Belmabkhout, H. C. Zeng and M. Eddaoudi, J. Am. Chem. Soc., 2013, 135, 10234–10237.
2. X. Li, F. Cheng, S. Zhang and J. Chen, J. Power Sources, 2006, 160, 542–547.
3. O. Ardelean, G. Blanita, G. Borodi, M. D. Lazar, I. Misan, I. Coldea and D. Lupu, Int. J. Hydrogen Energy, 2013, 38, 7046–7055.
4. R. Zacharia, D. Cossement, L. Lafi and R. Chahine, J. Mater. Chem., 2010, 20, 2145–2151.
5. K. M. Choi, H. J. Jeon, J. K. Kang and O. M. Yaghi, J. Am. Chem. Soc., 2011, 133, 11920–11923.
6. L.-B. Sun, J.-R. Li, J. Park and H.-C. Zhou, J. Am. Chem. Soc., 2012, 134, 126–129.
7. J. Ren, N. M. Musyoka, P. Annamalai, H. W. Langmi, B. C. North and M. Mathe, Int. J. Hydrogen Energy, 2015, 40, 9382–9387.
8. R. Ostermann, J. Cravillon, C. Weidmann, M. Wiebcke and B. M. Smarsly, Chem. Commun., 2011, 47, 442–444.
9. A. Ahmed, M. Forster, R. Clowes, P. Myers and H. Zhang, Chem. Commun., 2014, 50, 14314–14316.
10. W. Y. Hong, S. P. Perera and A. D. Burrows, Microporous Mesoporous Mater., 2015, 214, 149–155.
11. P. Küsgens, A. Zgaverdea, H.-G. Fritz, S. Siegle and S. Kaskel, J. Am. Chem. Soc., 2010, 93, 2476–2479.
12. H.-Y. Huang, C.-L. Lin, C.-Y. Wu, Y.-J. Cheng and C.-H. Lin, Anal. Chim. Acta, 2013, 779, 96–103.
13. M. R. Lohe, M. Rose and S. Kaskel, Chem. Commun., 2009, 6056–6058.
14. L. Li, S. Xiang, S. Cao, J. Zhang, G. Ouyang, L. Chen and C.-Y. Su, Nat. Commun., 2013, 4, 1774.
15. S. Choudhury, D. Connolly and B. White, Anal. Methods, 2015, 7, 6967–6982.
16. D. C. Tuncaboylu and O. Okay, Langmuir, 2010, 26, 7574–7581.
17. I. Karakutuk and O. Okay, React. Funct. Polym., 2010, 70, 585–595.
18. X. Chen, W. Sui, D. Ren, Y. Ding, X. Zhu and Z. Chen, Macromol. Mater. Eng., 2016, 301, 659–664.
19. Y. Tasaki-Handa, Y. Abe, K. Ooi, M. Tanaka and A. Wakisaka, Dalton Trans., 2014, 43, 1791–1796.
20. K. Ooi, Y. Tasaki-Handa, Y. Abe and A. Wakisaka, Dalton Trans., 2014, 43, 4807–4812.
21. Y. Tasaki-Handa, Y. Abe, K. Ooi, M. Tanaka and A. Wakisaka, J. Colloid Interface Sci., 2014, 413, 65–70.
22. E. Yurtov and N. Murashova, Theor. Found. Chem. Eng., 2007, 41, 737–742.
23. R. D. Neuman, N.-F. Zhou, J. Wu, M. A. Jones, A. G. Gaonkar, S. J. Park and M. L. Agrawal, Sep. Sci. Technol., 1990, 25, 1655–1674.
24. C. Scharf, A. Ditze, K. Schwerdtfeger, D. Kaufmann, J. Namyslo, S. Fürmeier and T. Bruhn, Metall. Mater. Trans. B, 2005, 36, 429–436.
25. D. Peppard and J. Ferraro, J. Inorg. Nucl. Chem., 1959, 10, 275–288.
26. R. J. Ellis, T. Demars, G. Liu, J. Niklas, O. G. Poluektov and I. A. Shkrob, J. Phys. Chem. B, 2015, 119, 11910–11927.
27. P. Terech and R. G. Weiss, Chem. Rev, 1997, 97, 3133–3160.

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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.

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