An alternative UiO-66 synthesis for HCl-sensitive nanoparticle encapsulation

Abstract

The use of zirconium propoxide (Zr(OnPr)) in place of zirconium chloride (ZrCl₄) leads to an alternative synthesis route for producing high-quality crystals of UiO-66 with no generation of hydrochloric acid. This new method enables the inclusion of HCl-sensitive gold nanoparticles into the mother solution for encapsulation by UiO-66, laying the groundwork for the development of new MOF composites.

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Metal and metal oxide nanomaterials are of increasing interest due to their unique optical, electrical, thermal, magnetic, and catalytic properties that make them advantageous in semiconductors, sensing, imaging, and catalysis.¹ Research has led to the tailoring of these properties via controlled synthesis of nanomaterials to manipulate size, shape, composition, and structure.² However, the high surface-area-to-volume ratio often makes nanomaterials such as metallic nanoparticles unstable, even under ambient conditions. To solve this issue, nanoparticles are often supported either by a polymer shell or on porous materials to retain their size and shape. Typical porous supports limit aggregation, but they do not protect the nanomaterials from poisons that will alter the material properties. For instance, organosulphur compounds strongly bind to many nanoparticle surfaces blocking active catalysis sites. Confinement within microporous supports filters out such poisons and protects the particles.³

Metal–organic frameworks (MOFs) are micro-to-mesoporous, crystalline materials consisting of metal or metal oxide clusters connected by organic linkers. Their high surface areas and pore volumes, uniform pore size distributions, and chemical tunability give them potential in applications such as gas storage and separation, drug delivery, biomedical imaging, air purification, and catalysis.⁴ Using MOFs as a support for nanoparticles, specifically confining the nanoparticles within the structure to create MOF composites, allows us to exploit the chemical and physical properties of the nanomaterials and the selectivity of the MOFs. There have been several MOF composites, created by either impregnation or encapsulation of nanoparticles, reported to date.⁵ Impregnation describes the production of the nanoparticles within the MOF pores, while encapsulation indicates that the MOF crystallizes around pre-formed nanoparticles. Lu et al.⁶ used encapsulation to successfully incorporate a wide range of nanoparticles in ZIF-8 and demonstrated good spatial control of the nanoparticles, improving the catalytic, magnetic, and photoluminescent properties of the parent structure.⁶ Synthetic control over the spatial location of the nanomaterials is paramount for extending the applicability of MOF composites for catalysis, sensing, photovoltaics, and microelectronics.⁷ The expansion of this controlled encapsulation technique to a wide variety of MOFs will open the door for the creation of designer-specific supports.

UiO-66 is a zirconium-based MOF composed of Zr₆O₄(OH)₄ clusters linked by terephthalic acid. It is thermally stable, mechanically and chemically resistant, and easily tuned.⁸ Unfortunately, the synthesis of UiO-66 produces HCl, which is problematic when encapsulating metal or metal oxide nanoparticles. For instance, gold nanoparticles are widely studied and have interesting optic and catalytic properties,⁹ but they easily dissolve in HCl. In fact, HCl either reacts with, or dissolves, numerous metal or metal oxide nanoparticles with favorable properties.¹⁰ Guillerm et al. have reported an alternative procedure using a zirconium methacrylate secondary building unit (SBU), yielding UiO-66 with reduced crystallinity and porosity.¹¹ However, the synthesis of the SBU precursor requires air sensitive materials and is time intensive.¹² Herein, we report an alternative procedure to synthesize UiO-66 with equivalent crystallinity, porosity, thermal stability, and chemical resistance without forming HCl. A demonstration of this new method is also presented for the encapsulation of HCl-sensitive gold nanoparticles.

The conventional synthesis of UiO-66(ZrCl₄) is a solvothermal method whereby a mixture of zirconium chloride (ZrCl₄) and terephthalic acid in dimethylformamide (DMF) are heated at 120 °C for 24 hours. HCl is produced during the reaction. To avoid this HCl formation, we have developed an alternative synthesis using a mixture of zirconium propoxide (Zr(OnPr)), terephthalic acid, acetic acid, methanol, and DMF. A systematic study was performed to evaluate the impact of various methanol : DMF ratios, acetic acid : Zr(OnPr) ratios, synthesis temperatures (25 °C to 120 °C), and reaction times (24 h–72 h) on the synthesis of UiO-66 (Table S1 and S2‡). The results of these experiments are shown in Fig. S5–S8.‡ Fig. 1 shows the powder X-ray diffraction (PXRD) spectra of UiO-66(Zr(OnPr)) for the case of MeOH : DMF = 1.9 and acetic acid : Zr(OnPr) = 30 carried out at 120 °C for 24 h. The PXRD matches very well the simulated UiO-66 pattern. In addition, the BET surface area, shown in Table 1, is comparable to UiO-66(ZrCl₄). PXRD and BET analysis prove this alternative synthesis yields porous, high-quality UiO-66 crystals. Thermogravimetric analysis (TGA) confirms thermal stability of UiO-66(Zr(OnPr)) up to 510 °C (Fig. S3‡). Finally, the water resistance of UiO-66(Zr(OnPr)) is confirmed by soaking activated samples in water for 24 h. The combination of PXRD (Fig. S4‡) and BET surface area analysis (Table S3‡) shows that the crystal structure and porosity remain intact after water exposure. Therefore, this alternate synthesis using zirconium propoxide produces UiO-66 crystals with porosity, thermal stability, and water resistance that are comparable to the conventionally synthesized samples.

Fig. 1 PXRD spectra of simulated UiO-66 (black), UiO-66(ZrCl₄) (green), UiO-66(Zr(OnPr)) (red), and Au@UiO-66(Zr(OnPr)) (blue).

Table 1 BET surface area comparison^a

Sample	BET surface area (m^2 g^−1)	Total pore volume^a (cm^3 g^−1)
a Measured at P/P0 = 0.6.
UiO-66(ZrCl4)	1118	0.48
UiO-66(Zr(OnPr))	1155	0.56
Au@UiO-66(Zr(OnPr))	1061	0.47

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Using the MeOH : DMF ratio specified above at 120 °C for 24 h, a systematic study of the effect of acetic acid : Zr(OnPr) ratio on crystal formation and porosity was conducted, and acetic acid was found to be critical for UiO-66(Zr(OnPr)) crystallization (Fig. S8‡). An acetic acid : Zr(OnPr) ratio of at least 15 : 1 is necessary for UiO-66(Zr(OnPr)) crystal formation. Fig. 2 shows the relationship between the ratio of acetic acid : Zr(OnPr), pH, and BET surface area of UiO-66(Zr(OnPr)). At an acetic acid : Zr(OnPr) ratio of 15 : 1, UiO-66 crystals form as shown by PXRD, but the BET surface area is reduced (see Fig. S12‡ for low acetic acid : Zr(OnPr) ratio). As the ratio of acetic acid : Zr(OnPr) increases to 30 : 1, the BET surface area is comparable to UiO-66(ZrCl₄). There are three possible reasons acetic acid is necessary to drive UiO-66(Zr(OnPr)) crystallization: (1) it creates an acidic environment (pH), (2) it forms a zirconium acetate precursor, or (3) it acts as a modulator, slowing nucleation through competitive coordination and increasing crystal growth. In order to determine the most likely reason acetic acid is required, we performed the same experiments with nitric acid (HNO₃) or benzoic acid instead of acetic acid. The data in Fig. 2 show that a pH between 1.3 and 4 yield highly crystalline, high-surface area UiO-66. The substitution of HNO₃ yields a mother solution with a pH of 1.5, well within the successful pH range observed for the acetic acid case, but produces a non-crystalline material after reaction (Fig. S9‡). Therefore, crystallization is not purely dependent on the pH of the mother solution. On the other hand, exchanging acetic acid for benzoic acid, another known modulator,¹³ yields crystalline UiO-66 (Fig. S9‡) with high surface area (1307 m² g⁻¹, Table S5‡). This suggests that modulation is necessary for the particles to reach the critical size for measurable crystallization. Thus, we can infer that the key role of acetic acid is to modulate the growth. The acetic acid competitively coordinates to zirconium ions in solution, which slows nucleation and increases growth so UiO-66 particles reach a critical diameter with measurable periodicity and significant porosity.

Fig. 2 Relationship between acetic acid : Zr(OnPr) ratio, pH, and BET surface area. The closed points represent crystalline materials and the open points non-crystalline materials.

To test the capability of this synthesis with HCl-sensitive materials, we encapsulated gold nanoparticles (AuNPs) in UiO-66(Zr(OnPr)). The encapsulation procedure entails growing the MOF from pre-formed functionalized AuNPs. First, 3.1 ± 0.6 nm AuNPs are synthesized as previously reported.¹⁴ They are stabilized in solution using a mixed monolayer consisting of mercaptoundecanoic acid and dodecanethiol in a 2 : 1 ratio. These particles are then added to the UiO-66(Zr(OnPr)) mother solution as a precursor and the mixture is heated to 120 °C for 24 hours. Fig. 1 shows that the PXRD spectrum of Au@UiO-66(Zr(OnPr)) matches the simulated spectrum for UiO-66. Therefore, adding AuNPs to the mother solution does not significantly affect the structure. The BET surface area for the composite, Table 1, is also comparable to the parent UiO-66. The slight loss in specific internal surface area is expected with the addition of dense, non-porous materials. Normalizing the surface area by UiO-66 (rather than the composite) yields a BET surface area similar to the parent. This suggests that the reduced surface area of the composite is due to the increased material density rather than pore blockage by the AuNPs. Additionally, the composite is thermally stable up to 500 °C (Fig. S3‡) and retains its structure and porosity after water exposure (Fig. S4, Table S3‡). PXRD, N₂ sorption, and TGA analysis show that incorporating AuNPs in the synthesis does not affect UiO-66(Zr(OnPr)) structure, porosity, thermal stability, or water resistance.

Transmission electron microscopy (TEM) and scanning transmission electron microscopy, coupled with energy dispersive spectroscopy (STEM-EDS) are used to analyze AuNP size, distribution, location, and composition (see Table S6‡ for EDS analysis at the red dot in Fig. 3b). Fig. 3 shows that there are 16.2 ± 4.6 nm AuNPs scattered non-uniformly throughout the sample, suggesting that the AuNPs grow significantly during the synthesis. Also, there are several particles that are clearly anchored to the surface of the support (resting on both the UiO-66 particle and the TEM grid). The significant AuNP growth, surface-attached AuNPs, and non-uniform AuNP dispersion suggests a need for an optimized encapsulation procedure, but these experiments show the potential for the incorporation of AuNPs into UiO-66(Zr(OnPr)).

Fig. 3 (a) TEM and (b) STEM-EDS images of Au@UiO-66(Zr(OnPr)).

Conclusions

In summary, we have demonstrated an alternative UiO-66 synthesis procedure for HCl-sensitive materials for the encapsulation of AuNPs in UiO-66. This alternative procedure yields porous crystals with comparable properties to the conventional synthesis procedure, specifically, porosity, thermal stability and water resistance. We have demonstrated that UiO-66(Zr(OnPr)) crystallization is dependent on the addition of a modulator such as acetic acid. This dependence suggests that crystallization of UiO-66(Zr(OnPr)) is limited by crystal growth yielding insight for further crystallization control. The alternative procedure also allows AuNPs to be included in the mother solution in order to encapsulate them within the framework. This method lays the groundwork for the controlled synthesis of HCl-sensitive UiO-66 composites with potential for catalysis, sensing, gas storage and separation, photovoltaics, and microelectronics.

Notes and references

1. (a) M. A. El-Sayed, Acc. Chem. Res., 2001, 34, 257–264; (b) P. K. Jain, X. Huang, I. H. El-Sayed and M. A. El-Sayed, Acc. Chem. Res., 2008, 41, 1578–1586; (c) M. Haruta, CATTECH, 2002, 6, 102–115.
2. (a) G. Frens, Nature (London), Phys. Sci., 1973, 241, 20–22; (b) M. J. Hostetler, J. E. Wingate, C. J. Zhong, J. E. Harris, R. W. Vachet, M. R. Clark, J. D. Londono, S. J. Green, J. J. Stokes, G. D. Wignall, G. L. Glish, M. D. Porter, N. D. Evans and R. W. Murray, Langmuir, 1998, 14, 17–30; (c) Y. N. Xia, Y. J. Xiong, B. Lim and S. E. Skrabalak, Angew. Chem., Int. Ed., 2009, 48, 60–103.
3. S. Goel, Z. Wu, S. I. Zones and E. Iglesia, J. Am. Chem. Soc., 2012, 134, 17688–17695.
4. (a) S. Kitagawa, R. Kitaura and S. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334–2375; (b) Metal Organic Frameworks: Applications from Catalysis to Hydrogen Storage, ed., D. Farruseng, Wiley-VCH Verlag GmbH & Co., 1st edn, 2011.
5. (a) M. Meilikhov, K. Yusenko, D. Esken, S. Turner, G. Van Tendeloo and R. A. Fischer, Eur. J. Inorg. Chem., 2010, 3701–3714; (b) H. R. Moon, D.-W. Lim and M. P. Suh, Chem. Soc. Rev., 2013, 42, 1807–1824.
6. G. Lu, S. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Qi, Y. Wang, X. Wang, S. Han, X. Liu, J. S. DuChene, H. Zhang, Q. Zhang, X. Chen, J. Ma, S. C. J. Loo, W. D. Wei, Y. Yang, J. T. Hupp and F. Huo, Nat. Chem., 2012, 4, 310–316.
7. (a) H. F. Yin, Z. Ma, M. F. Chi and S. Dai, Catal. Today, 2011, 160, 87–95; (b) D. Buso, J. Jasieniak, M. D. H. Lay, P. Schiavuta, P. Scopece, J. Laird, H. Amenitsch, A. J. Hill and P. Falcaro, Small, 2012, 8, 80–88; (c) D. Y. Lee, C. Y. Shin, S. J. Yoon, H. Y. Lee, W. Lee, N. K. Shrestha, J. K. Lee and S. H. Han, Sci. Rep., 2014, 4, 3930; (d) L. J. Lauhon, M. S. Gudiksen, C. L. Wang and C. M. Lieber, Nature, 2002, 420, 57–61.
8. (a) J. H. Cavka, S. Jakobsen, U. Olsbye, N. Guillou, C. Lamberti, S. Bordiga and K. P. Lillerud, J. Am. Chem. Soc., 2008, 130, 13850–13851; (b) P. M. Schoenecker, C. G. Carson, H. Jasuja, C. J. J. Flemming and K. S. Walton, Ind. Eng. Chem. Res., 2012, 51, 6513–6519; (c) S. J. Garibay and S. M. Cohen, Chem. Commun., 2010, 46, 7700–7702; (d) S. Biswas and P. Van der Voort, Eur. J. Inorg. Chem., 2013, 2154–2160.
9. M. C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293–346.
10. (a) A. P. Abbott, G. Capper, D. L. Davies, K. J. McKenzie and S. U. Obi, J. Chem. Eng. Data, 2006, 51, 1280–1282; (b) L. Li and Y.-J. Zhu, J. Colloid Interface Sci., 2006, 303, 415–418; (c) S. Elzey and V. H. Grassian, Langmuir, 2010, 26, 12505–12508; (d) H. Shi, H. Bi, B. Yao and L. Zhang, Appl. Surf. Sci., 2000, 161, 276–278.
11. V. Guillerm, S. Gross, C. Serre, T. Devic, M. Bauer and G. Ferey, Chem. Commun., 2010, 46, 767–769.
12. G. Kickelbick and U. Schubert, Chem. Ber./Recl., 1997, 130, 473–477.
13. A. Schaate, P. Roy, A. Godt, J. Lippke, F. Waltz, M. Wiebcke and P. Behrens, Chem.–Eur. J., 2011, 17, 6643–6651.
14. (a) M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and R. Whyman, J. Chem. Soc., Chem. Commun., 1994, 801–802; (b) M. J. Hostetler, A. C. Templeton and R. W. Murray, Langmuir, 1999, 15, 3782–3789.

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Footnotes

† The TEM used in this work is supported by funding DMR 0922776. This material is based upon work supported by Army Research Office PECASE Award W911NF-1-10-0079.

‡ Electronic supplementary information (ESI) available: Experimental and analytical data. See DOI: 10.1039/c4ra08856e

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