Rapid collection and re-dispersion of MOF particles by a simple and versatile method using a thermo-responsive polymer

Abstract

MOF particles can be easily collected and re-dispersed in water with the help of a free thermo-responsive polymer. Compared to the polymer grafting approaches reported in the literature, this method requires no chemical reactions and it is highly energy-efficient and environmentally-friendly.

---

Metal–organic frameworks (MOFs) are a new class of porous materials, consisting of metal ions coordinated with organic ligands, forming two-dimensional or three-dimensional structures.^1–4 They are regarded as outstanding materials in separation,⁵ catalysis,⁶ sensors,⁷ and gas storage⁸ due to their large surface area, ultralow density, high thermal stability and uniform cavities. Although they have been prepared into membranes,^9–11 hollow spheres,¹² and many other architectures,^13–15 powder is the most studied MOF form. Many research studies focused on utilizing MOFs as sorbents for removal of contaminates, like dyes,^16–19 polycyclic aromatic hydrocarbons,^20,21 and metal ions from water.^22–24 It is also demonstrated that MOFs could enrich peptides and exclude proteins for peptidome analysis of complex biological fluids.²⁵ Moreover, MOFs are promising materials as novel catalysts or catalyst supports for various organic reactions.^26–29 However, it is always required to remove MOF particles after application. In an economic and environmental point of view, MOF materials are also expected to be regenerated and reused in such applications. Centrifugation is commonly used in literature to collect MOF particles, however, this procedure is usually energy consuming and labor intensive.

Surface modification was a popular method to induce special properties to MOF particles.^30–34 The recent advent of smart polymers provides an opportunity to easily collect MOF particles. Stimuli-responsive polymers are a type of smart macromolecules whose physical and chemical properties can be significantly changed by external stimuli, such as temperature, pH, light, redox, electrical or magnetic field, etc.^35–40 Due to their unique controllable properties, they have been applied in various fields like catalysis, smart surface, separation, drug delivery and controlled release.^41–44 Among them, modification of nanoparticles with smart polymers to introduce switching properties has drawn great interest.^45–47 Recently, we launched a research program aiming at improving MOF's processability by polymer approaches. In 2015, we used surface-initiated atom transfer radical polymerization (SI-ATRP) to graft the thermal-responsive copolymers of 2-(2-methoxyethoxy) ethyl methacrylate (MEO₂MA) and oligo (ethylene glycol) methacrylate (OEGMA) from an aluminium-based MOF, MIL-101(Al).⁴⁸ The polymer-modified MOF particles exhibited rapid coagulation and re-dispersion in water by increasing and decreasing temperature. Since then, Xie et al.⁴⁹ grafted poly(poly(ethylene glycol) methyl ether methacrylate) from UiO-66 using an activator regenerated by electron transfer-atom transfer radical polymerization (ARGET-ATRP) technique. The polymer-grafted MOF exhibited pH-sensitive water dispersivity and it was evaluated as a recyclable catalyst for reduction of 4-nitrophenol to 4-aminophenol. Polymer grafting from functional MOFs by ATRP technique has become a general method to obtain surface-modified MOFs.⁵⁰ In addition, there was another work on modification of MOF particles, which was via ‘grafting to’ method to introduce stimuli-responsive polymer onto MOFs.⁵¹ In summary, the conventional methods to introduce stimuli-responsive properties to MOFs were mainly based on grafting stimuli-responsive polymer from or to functional groups on MOF surface.

However, there are several drawbacks with surface modification of MOFs via polymer grafting. First, it involves complicated synthesis and purification procedures. An initiator must be immobilized on to MOF surface, followed by chain growth. Second, it can cause damages to the MOF structure and change its properties because of the chemical reactions. For example, initiator and monomer molecules can diffuse into porous MOF structure and polymerization occurs inside, blocking the pores. Third, it is limited to those MOFs with functional pendant on the ligand. In this work, we report a facile but effective method for rapid collection and dispersion of MOF particles. It is through simple addition of a thermal-responsive polymer, poly(N-isopropyl acrylamide), i.e. PNIPAM, into a MOF aqueous dispersion. PNIPAM induces ultrafast aggregation of MOF particles above its low critical solution temperature (LCST) (∼32 °C), while it acts as a stabilizing agent below its LCST. Compared with other reported methods, this approach avoids tedious chemical modification steps in introducing the thermal-responsive property to MOFs. The structures and properties of MOFs remain intact since no chemical reactions are involved.

ZIF-67 was chosen as a model system to study the thermal-responsive collection and re-dispersion behaviour of MOF particles in PNIPAM solution. ZIF-67 was synthesized according to the literature.⁵² 2 mg ZIF-67 was dispersed in 4 ml pure water (sample 1-1) and 4 ml PNIPAM (0.02 mg ml⁻¹) aqueous solution (sample 1-2), respectively. PNIPAM is a well-known thermal-responsive polymer with a LCST of about 32 °C. The stability of ZIF-67 in water was compared to that in PNIPAM solution, after heated to 40 °C for 2 minutes (Fig. 1a). Sample 1-1 remained stable after 1 h, while large agglomerates were immediately observed in the PNIPAM solution sample. The ZIF-67 particles were easily collected without centrifugation.

Fig. 1 (a) The effect of PNIPAM concentration on collectability of ZIF-67 after heating at 40 °C for 2 minutes: 1-1 (0 mg ml⁻¹), 1-2 (0.02 mg ml⁻¹), 1-3 (0.04 mg ml⁻¹), 1-4 (0.06 mg ml⁻¹), 1-5 (0.08 mg ml⁻¹), 1-6 (0.10 mg ml⁻¹). (b) Comparison of the stability of ZIF-67 (0.5 mg ml⁻¹) dispersions in water (1-1) and PNIPAM solution (1-2) at room temperature.

The aggregation of PNIPAM-grafted particles was because the grafted polymer chains became hydrophobic above LCST and the particles aggregated by hydrophobic interaction.^53–55

However, another possible explanation put forward very recently was through hydrophobic interactions between free and grafted PNIPAM chains after inadequate purification of particles.⁵⁶ In our case, there were no grafted chains. It was thus confirmed that in the absence of PNIPAM grafting onto MOF particles, free PNIPAM chains were able to realize the temperature-induced MOF particle aggregation. Our hypothesis was that there were two populations of PNIPAM chains in the MOF-PNIPAM suspension. One was free chains, which were well dissolved in water at room temperature. The other was confined chains, which were covered on the surface of MOF particles by physical adsorption. Above LCST, free chains interacted with confined chains by hydrophobic interactions, leading to the aggregation of MOF particles. We then investigated the effect of PNIPAM concentration on the collectability of MOF particles (Fig. 1a). The PNIPAM concentrations in samples 1-1, 1-2, 1-3, 1-4, 1-5, 1-6 were 0, 0.02, 0.04, 0.06, 0.08, 0.10 mg ml⁻¹, respectively. The higher the PNIPAM concentration, the larger the ZIF-67 aggregates, and the faster the sedimentation. We took sample 1-2 as an example to study the recovery ratio of ZIF-67 by this method. The concentration of cobalt in the water before and after collection was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Table S1†), it showed that the recovery of ZIF-67 was about 99.2%.

PNIPAM is hydrophilic below its LCST. It is anticipated that it could stabilize ZIF-67 in water at room temperature. The stability of ZIF-67 in water and that in PNIPAM solution at room temperature were thus investigated and compared. Photographs of ZIF-67 dispersions recorded at different times after preparation, are shown in Fig. 1b. ZIF-67 particles in sample 1-1 started to precipitate out from water after standing still for 1.5 h, while ZIF-67-PNIPAM suspension became unstable after about 3.5 h. It is evident that PNIPAM interacted with ZIF-67 and helped the dispersion of the particles.

For further elucidation of the aggregation mechanism, pristine ZIF-67 (Fig. 2a) and ZIF-67 collected by PNIPAM (Fig. 2b) were characterized by scanning electron microscopy (SEM). Compared with pristine ZIF-67 particle, the rhombic dodecahedral shape of ZIF-67 was maintained, indicating that ZIF-67 was not destroyed in PNIPAM solution. Powder X-ray diffraction (PXRD) analysis also confirmed the preservation of MOF crystallinity after precipitation process induced by heating (Fig. S1†). Fig. 2b showed ZIF-67 particles were embedded into the collapsed PNIPAM matrix when the temperature was elevated above LCST, resulting in larger size MOF aggregates, which facilitated easy sedimentation or filtration.

Fig. 2 SEM images of (a) pristine ZIF-67 and (b) ZIF-67 collected by PNIPAM.

Our objective is to develop a very simply method for easy collection and re-dispersion of MOF particles by temperature variation. This reversibility is of great importance in MOF processing for saving energy in separation and eliminating wastes. Fig. 3 shows the reversible dispersion of ZIF-67 particles in the PNIAPM solution by cooling and collection by heating. Below LCST, ZIF-67 was well dispersed in the PNIPAM solution, while precipitation occurred quickly above LCST. This process was repeatable and it is very easy to operate.

Fig. 3 The reversible dispersibility of ZIF-67 particles (0.5 mg ml⁻¹) in PNIPAM solution. For the first cycle, the well-dispersed ZIF-67-PNIPAM suspension (Left) was heated to 40 °C for 2 minute to form aggregation (Right). Then the vial was cooled and shaken at room temperature for 1 minute to re-disperse ZIF-67 again for the second cycle.

This method is versatile and it applies to different types of MOF particles. In addition to ZIF-67, we also tested MIL-53-NH₂(Al), MIL-100(Fe), MIL-125-NH₂(Ti), and UiO-66. The MOF materials were synthesized according to the literature.^57–60 All of these MOFs could be easily collected by heating after adding small amount PNIPAM (0.04 mg ml⁻¹) to water (Fig. 4). As shown in Fig. 4, various MOF particles were able to be collected immediately by adding PNIPAM and heated to 40 °C for 2 minutes. We also confirmed the preservation of crystallinity of these MOFs by PXRD. The PXRD patterns where the Bragg diffraction angles between samples and relevant simulated results were essentially identical (Fig. S2–S5†). The result confirmed that this method was widely applicable for different MOFs and it can be potentially used in various applications.

Fig. 4 Comparison of collectability of MOFs with/without PNIPAM (M_w = 39 000, PDI = 1.45). Samples of 1-1, 2-1, 3-1, 4-1, 5-1 were MOF suspensions in water, while 1-3, 2-2, 3-2, 4-2, 5-2 were relevant MOF suspensions in PNIPAM solution after heated to 40 °C for 2 minutes. The MOF and PNIPAM concentrations were 0.5 mg ml⁻¹ and 0.04 mg ml⁻¹, respectively. The photos were taken 2 minutes after heating except sample 2 and sample 5. Because these samples were light colored with low contrast to the background, they could not be shown clearly in 2 minutes, and thus they were recorded after 15 minutes.

A model application of this collectable and re-dispersible method was demonstrated by the photo-degradation of methylene blue (MB) with MIL-100(Fe). H₂O₂ was added as electron acceptors to enhance the photocatalytic performance of MIL-100(Fe). The initial MB concentration was 200 ppm. As shown in Fig. 5a, the dark blue water turned orange after 1 h photocatalysis by MIL-100(Fe). MIL-100(Fe) was easily separated from water by heating after adding PNIPAM into the reaction mixture.

Fig. 5 (a) The photo-decolorization of MB by MIL-100(Fe). The initial concentration of MB in water was 200 ppm and the concentration of MIL-100(Fe), H₂O₂ and PNIPAM was 0.5, 1.5 and 0.04 mg ml⁻¹, respectively. (b) The UV-vis absorption spectrum of MB in water before and after photocatalysis by MIL-100(Fe).

After the sedimentation of MIL-100(Fe), the supernatant became transparent and colorless. Ultraviolet-visible (UV-vis) adsorption spectra of the samples before and after MB photocatalysis showed significant difference. Fig. 5b shows that there was no peak observed at 664 nm, which is the maximum absorbance wavelength of MB, suggesting complete degradation of MB by MIL-100(Fe).

The reusability of MIL-100(Fe) was investigated next. After the supernatant decanted, fresh MB solution (200 ppm) and H₂O₂ were added and the same procedure was performed. The two additional cycles, and no obvious catalytic reactivity loss was observed, which was confirmed by UV-vis absorption (Fig. S6 and S7†).

In conclusions, an easy but versatile method for fast collection and re-dispersion of MOF particles in water has been developed and demonstrated. It is through simple addition of small amount thermal-responsive poly(N-isopropylacrylamide), that is, PNIPAM, to MOF particle suspension. Some added PNIPAM chains are adsorbed onto the surface of MOF particles, while other chains are free in the solution. The adsorbed chains improve the stability of MOF particles in water below LCST. The free chains interact with adsorbed chains when the temperature is elevated above LCST, leading to the formation of large MOF-PNIPAM aggregates, which can be easily collected. Compared to grafting smart polymers, this method does not involve any chemical reactions. It imposes little risk to damaging MOF structures and properties. It can also be applied to various MOF–water systems. A model application was performed using MIL-100(Fe) as photocatalyst to decolorize MB in water. The MOF catalyst can be quickly collected by adding free PNIPAM at 40 °C and reclaimed and reused for three times without any catalytic activity loss. This method paves a way for fast dispersion and recovery of nano to micro-sized particulate materials, which is very useful in catalyst and separation applications.

Acknowledgements

The authors thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for supporting the work. YYL would also like to thank Zhejiang University for supporting his stay at McMaster.

Notes and references

1. S. L. James, Chem. Soc. Rev., 2003, 32, 276–288.
2. G. Férey, Chem. Soc. Rev., 2008, 37, 191–214.
3. S. T. Meek, J. A. Greathouse and M. D. Allendorf, Adv. Mater., 2011, 23, 249–267.
4. Y. Hu, H. Kazemian, S. Rohani, Y. Huang and Y. Song, Chem. Commun., 2011, 47, 12694–12696.
5. Z.-Y. Gu, C.-X. Yang, N. Chang and X.-P. Yan, Acc. Chem. Res., 2012, 45, 734–745.
6. J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450–1459.
7. L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. Van Duyne and J. T. Hupp, Chem. Rev., 2011, 112, 1105–1125.
8. R. Sabouni, H. Kazemian and S. Rohani, Environ. Sci. Pollut. Res., 2014, 21, 5427–5449.
9. H. Bux, C. Chmelik, J. M. van Baten, R. Krishna and J. Caro, Adv. Mater., 2010, 22, 4741–4743.
10. H. Zhu and S. Zhu, Can. J. Chem. Eng., 2015, 93, 63–67.
11. H. Zhu, H. Liu, I. Zhitomirsky and S. Zhu, Mater. Lett., 2015, 142, 19–22.
12. H. J. Lee, W. Cho and M. Oh, Chem. Commun., 2012, 48, 221–223.
13. H. Zhu, Q. Zhang and S. Zhu, Dalton Trans., 2015, 44, 16752–16757.
14. N. Yanai, M. Sindoro, J. Yan and S. Granick, J. Am. Chem. Soc., 2012, 135, 34–37.
15. J. Reboul, S. Furukawa, N. Horike, M. Tsotsalas, K. Hirai, H. Uehara, M. Kondo, N. Louvain, O. Sakata and S. Kitagawa, Nat. Mater., 2012, 11, 717–723.
16. Q. Zhang, J. Yu, J. Cai, R. Song, Y. Cui, Y. Yang, B. Chen and G. Qian, Chem. Commun., 2014, 50, 14455–14458.
17. X. Lian and B. Yan, RSC Adv., 2016, 6, 11570–11576.
18. Y.-F. Huang, Y.-Q. Wang, Q.-S. Zhao, Y. Li and J.-M. Zhang, RSC Adv., 2014, 4, 47921–47924.
19. M. Tong, D. Liu, Q. Yang, S. Devautour-Vinot, G. Maurin and C. Zhong, J. Mater. Chem. A, 2013, 1, 8534–8537.
20. S.-H. Huo, J. Yu, Y.-Y. Fu and P.-X. Zhou, RSC Adv., 2016, 6, 14042–14048.
21. S.-H. Huo and X.-P. Yan, Analyst, 2012, 137, 3445–3451.
22. G.-S. Yang, Z.-L. Lang, H.-Y. Zang, Y.-Q. Lan, W.-W. He, X.-L. Zhao, L.-K. Yan, X.-L. Wang and Z.-M. Su, Chem. Commun., 2013, 49, 1088–1090.
23. M. Babazadeh, R. Hosseinzadeh-Khanmiri, J. Abolhasani, E. Ghorbani-Kalhor and A. Hassanpour, RSC Adv., 2015, 5, 19884–19892.
24. Y. Xiong, F. Ye, C. Zhang, S. Shen, L. Su and S. Zhao, RSC Adv., 2015, 5, 5164–5172.
25. Z.-Y. Gu, Y.-J. Chen, J.-Q. Jiang and X.-P. Yan, Chem. Commun., 2011, 47, 4787–4789.
26. C.-D. Wu, A. Hu, L. Zhang and W. Lin, J. Am. Chem. Soc., 2005, 127, 8940–8941.
27. S.-H. Cho, B. Ma, S. T. Nguyen, J. T. Hupp and T. E. Albrecht-Schmitt, Chem. Commun., 2006, 2563–2565.
28. F. Vermoortele, R. Ameloot, A. Vimont, C. Serre and D. De Vos, Chem. Commun., 2011, 47, 1521–1523.
29. H. T. Nguyen, O. T. Nguyen, T. Truong and N. T. Phan, RSC Adv., 2016, 6, 36039–36049.
30. Z. Wang and S. M. Cohen, Chem. Soc. Rev., 2009, 38, 1315–1329.
31. S. M. Cohen, Chem. Sci., 2010, 1, 32–36.
32. T. Yamada and H. Kitagawa, J. Am. Chem. Soc., 2009, 131, 6312–6313.
33. M. Kondo, S. Furukawa, K. Hirai and S. Kitagawa, Angew. Chem., Int. Ed., 2010, 49, 5327–5330.
34. M. Sindoro, N. Yanai, A.-Y. Jee and S. Granick, Acc. Chem. Res., 2013, 47, 459–469.
35. Q. Zhang, W.-J. Wang, Y. Lu, B.-G. Li and S. Zhu, Macromolecules, 2011, 44, 6539–6545.
36. Y. Lu, G. Yu, W.-J. Wang, Q. Ren, B.-G. Li and S. Zhu, Macromolecules, 2015, 48, 915–924.
37. K. C. Hribar, M. H. Lee, D. Lee and J. A. Burdick, ACS Nano, 2011, 5, 2948–2956.
38. M. A. C. Stuart, W. T. Huck, J. Genzer, M. Müller, C. Ober, M. Stamm, G. B. Sukhorukov, I. Szleifer, V. V. Tsukruk and M. Urban, Nat. Mater., 2010, 9, 101–113.
39. S.-k. Ahn, R. M. Kasi, S.-C. Kim, N. Sharma and Y. Zhou, Soft Matter, 2008, 4, 1151–1157.
40. J. Pinaud, E. Kowal, M. Cunningham and P. Jessop, ACS Macro Lett., 2012, 1, 1103–1107.
41. J. Zhang, D. Han, H. Zhang, M. Chaker, Y. Zhao and D. Ma, Chem. Commun., 2012, 48, 11510–11512.
42. A. Bajpai, S. K. Shukla, S. Bhanu and S. Kankane, Prog. Polym. Sci., 2008, 33, 1088–1118.
43. G. Yu, Y. Lu, X. Liu, W.-J. Wang, Q. Yang, H. Xing, Q. Ren, B.-G. Li and S. Zhu, Ind. Eng. Chem. Res., 2014, 53, 16025–16032.
44. N. Nath and A. Chilkoti, Adv. Mater., 2002, 14, 1243–1247.
45. D. Li, Y. Cui, K. Wang, Q. He, X. Yan and J. Li, Adv. Funct. Mater., 2007, 17, 3134–3140.
46. C.-Y. Hong, Y.-Z. You and C.-Y. Pan, Chem. Mater., 2005, 17, 2247–2254.
47. J. Shan and H. Tenhu, Chem. Commun., 2007, 4580–4598.
48. H. Liu, H. Zhu and S. Zhu, Macromol. Mater. Eng., 2015, 300, 191–197.
49. K. Xie, Q. Fu, Y. He, J. Kim, S. Goh, E. Nam, G. Qiao and P. Webley, Chem. Commun., 2015, 51, 15566–15569.
50. K. A. McDonald, J. I. Feldblyum, K. Koh, A. G. Wong-Foy and A. J. Matzger, Chem. Commun., 2015, 51, 11994–11996.
51. S. Nagata, K. Kokado and K. Sada, Chem. Commun., 2015, 51, 8614–8617.
52. Z. Jiang, Z. Li, Z. Qin, H. Sun, X. Jiao and D. Chen, Nanoscale, 2013, 5, 11770–11775.
53. M.-Q. Zhu, L.-Q. Wang, G. J. Exarhos and A. D. Li, J. Am. Chem. Soc., 2004, 126, 2656–2657.
54. S. Chakraborty, S. W. Bishnoi and V. c. H. Pérez-Luna, J. Phys. Chem. C, 2010, 114, 5947–5955.
55. H. G. Schild, Prog. Polym. Sci., 1992, 17, 163–249.
56. S. J. Barrow, S. L. Henderson, J. del Barrio and O. A. Scherman, ACS Nano, 2016, 10, 3158–3165.
57. T. Rodenas, M. van Dalen, E. García-Pérez, P. Serra-Crespo, B. Zornoza, F. Kapteijn and J. Gascon, Adv. Funct. Mater., 2014, 24, 249–256.
58. R. Canioni, C. Roch-Marchal, F. Sécheresse, P. Horcajada, C. Serre, M. Hardi-Dan, G. Férey, J.-M. Grenèche, F. Lefebvre and J.-S. Chang, J. Mater. Chem., 2011, 21, 1226–1233.
59. S. Hu, M. Liu, K. Li, Y. Zuo, A. Zhang, C. Song, G. Zhang and X. Guo, CrystEngComm, 2014, 16, 9645–9650.
60. G. Lu, C. Cui, W. Zhang, Y. Liu and F. Huo, Chem.–Asian J., 2013, 8, 69–72.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13938h

---