Facile synthesis of mesoporous MOF/silica composites

Abstract

HKUST-1 MOF/silica composites with narrow mesopore size distribution were prepared by facile hydrothermal synthesis in the presence of cetyltrimethylammonium bromide (CTAB).

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Metal–organic frameworks (MOFs), also known as porous coordination polymers, are crystalline porous materials formed by bridging transition metal cations via organic spacers such as tri-carboxylic acids, have led to many potential applications because of their ultrahigh surface area and tunable porosity.^1,2 A great amount of effort has been made to build MOFs with new structures and explore their various applications.^3,4 However, there are two major drawbacks that limit the actual applications of this kind of materials, one is the low chemical stability, and the second is the poor mechanical strength.⁵

Recently, MOF composites have attracted a lot of attention because of their new physical and chemical properties and improved performances that cannot be achieved by the single phase and pure MOFs alone.⁶ Introducing silica on/in MOFs has been reported to strengthen the MOFs stability for biomedical imaging⁷ and improve its mechanical properties.⁸ However, most of the reported composites to date are analogous to microporous materials.^9,10 While high micropore volumes and large surface areas are desirable for many applications, such narrow pores do not allow for hosting large molecules and faster mass diffusion, thereby limiting their uses in separation, catalysis and adsorption.¹¹ Kondo¹² and Koodali et al.,¹³ prepared mesoporous MOF-5/SBA-15 and CuBTC/SBA-15 composites by using pre-synthesized SBA-15 as matrix, respectively. LeVan et al.,¹⁴ synthesized mesoporous CuBTC/MCM-41 composites through incorporation of CuBTC into MCM-4 support. However, these methods need two-step procedure, which is complex and time-consuming. Jaroniec et al.,¹⁵ reported one-step microwave hydrothermal synthesis of mesoporous MOF/silica composites with the help of P123 surfactant. However, no detailed information such as morphology, composition and mesopore size distribution etc. was previously reported.

Here we report a facile method to prepare mesoporous MOF/silica composites with narrow pore size distribution in the presence of cetyltrimethylammonium bromide (CTAB) under hydrothermal treatment. Briefly, 1.093 g of Cu(NO₃)₂ in 15 mL DI water was added into a solution of H₃BTC (0.525 g) in 15 mL ethanol. To the above solution, different amounts of TEOS (3 mmol, 5 mmol and 7 mmol) were added. The resulting solution was stirred at room temperature for 10 min, followed by the addition of 1.312 g of CTAB. Each sol was transferred to the Teflon lined autoclaves and hydrothermally treated at 120 °C for 24 h. All samples were filtered, thoroughly washed with ethanol and water several times and dried at 50 °C overnight. The composites were labeled according to the amount of TEOS added as MOF/Si-x, where x = 3, 5, 7. As a reference material, a sample of pure Cu₃(BTC)₂ (HKUST-1) was prepared according to the methods previously reported.^16,17 The resulting materials were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), in situ energy dispersive X-rays (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy, nitrogen adsorption at 77 K and thermogravimetric analysis (TG) (see ESI† for details).

Fig. 1 shows the powder XRD patterns of the pure HKUST-1 and the composites prepared. The well-resolved patterns are analogous to those for the HKUST-1 type framework, identified as Cu₃(BTC)₂(H₂O)₃ phase with cubic symmetry,¹⁶ which indicates that the formation of mesoporous composites does not affect the formation of perfect MOF crystals. Compared with the pure HKUST-1 sample, the intensity of the main diffraction peaks of the composites decreased and no other peaks could be observed in the patterns, which is consistent with the XPS results (Fig. S6b†), indicating that the amorphous SiO₂ formed in the composites. The small-angle X-ray diffraction patterns (Fig. S1†) of the composites are indistinguishable, suggesting a disordered mesostructure without long-range order in the arrangement of the mesopores.

Fig. 1 X-ray diffraction patterns of MOF/silica composites.

The SEM images of all samples show relatively regular diamond or cube shaped particles with sizes of ∼15um (Fig. 2). This morphology is similar to that of pure HKUST-1 crystals (Fig. S2†). Elemental mapping indicates a homogeneous distribution of Cu, C, O and Si in MOF-silica composite particles, which is also confirmed by the EDS line-scan (Fig. S3†), indicating that a homogeneously mixed phase formed in the composites.

Fig. 2 SEM images (a) MOF/Si-3; (b) MOF/Si-5; (c) MOF/Si-7 and (d) MOF/Si-5 and corresponding elemental mappings.

Fig. 3 shows the N₂ adsorption–desorption isotherms and pore size distribution of MOF-silica composites, and Table 1 summarizes the textural properties of the samples prepared at different TEOS concentrations in this work. The isotherms of pure HKUST-1 sample are type I according to the IUPAC classification (Fig. S4†), which is characteristic of microporous materials. However, the composite samples are of type IV and shows steep hysteresis of type H1 over the range of 0.6 < P/P₀ < 0.9, corresponding to the mesostructure of the material. The BET surface area and the total pore volume of three samples are calculated to be in the range of 453–509 m² g⁻¹ and 0.36–0.42 cm³ g⁻¹, respectively. Note that, the BET surface area of pure microporous HKUST-1 is 1820 m² g⁻¹. With the addition of CTAB surfactant to the synthesis mixture, the hierarchically micro- and mesoporous HKUST-1 MOF could be obtained and it exhibited BET surface area of ∼1000 m² g⁻¹.¹⁸ Here, the BET surface area of samples prepared by adding CTAB and TEOS decreased to about 500 m² g⁻¹. This decreased surface area phenomenon could be explained as follows: first, the composites contain large amount of SiO₂ (the estimated weight of silica element in the MOF-Si-5 composites is 19 wt% by XPS) and mesoporous SiO₂ with occluded surfactant usually have a low surface area; second, the micropores of HKUST-1 were partially blocked, thus, the surface area of the composites decreased. When the amount of TEOS was increased, the surface area of resulting composites slightly decreased as expected because of dilution effect from silica. However, the general tendency of isotherms remained the same, and the mesopore diameter of all samples is about 7.6 nm with a narrow pore-size distribution (Fig. 2b). These results imply that this kind of mesoporous composite could be prepared using larger silica/MOF ratios. TEM image demonstrate that MOFs/composites have disordered wormhole mesopore structures (Fig. 4), which is consistent with the small-angle XRD characterization. The average pore diameters measured from the TEM images agree well with those derived from the N₂ adsorption–desorption isotherms. It was quite difficult to obtain high-quality TEM images for these materials, possibly because the samples are highly sensitive to the electron beam. The similar phenomenon was also reported by Qiu et al.¹⁸

Fig. 3 Nitrogen adsorption-desorption isotherms (a) and pore size distributions (b) for MOF/silica composites.

Table 1 Porosity properties of mesoporous MOF-silica composites

Samples	SBET m^2 g^−1	Slangmuir m^2 g^−1	Vt cm^3 g^−1	Vmeso cm^3 g^−1	Vmicro cm^3 g^−1
Pure HKUST-1	1820	2438	0.91	0.09	0.82
MOF/Si-3	509	533	0.39	0.29	0.10
MOF/Si-5	484	499	0.42	0.30	0.12
MOF/Si-7	453	475	0.36	0.23	0.13

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Fig. 4 TEM image of MOF/Si-5 sample.

The X-ray photoelectron spectroscopy (XPS) was performed to investigate the surface characteristic of the MOF/Si-5 composite (Fig. S5 and S6†). XPS scans in the carbon region (Fig. S6a†) show that carbon exists in two forms in Cu₃(BTC)₂. The peak at 284.8 eV represents carbons in the benzene ring of BTC, and the separate 1 s peak at 288.6 eV represents carbons in the carboxylic acid groups of BTC.¹⁹ In the copper region (Fig. S3c†), two peaks are observed at 935.0 eV (Cu 2p_3/2) and 955.2 eV (Cu 2p_1/2), respectively, which are characteristic of Cu(II). In addition, the presence of satellite peaks also confirmed this species,^20–22 this result means that copper in the composite material is bound to oxygen atoms from BTC in a similar way to the complexation of copper that occurs in CuBTC. However, it should be noted that two more peaks at 933.0 eV and 953.0 eV could be observed, this might be caused by a change of the coordination environment of Cu²⁺ center or partial Cu²⁺ reduction.^21,23–25 The thermal gravimetric analysis (TGA) of the as-synthesized samples performed under nitrogen flow is shown in Fig. S7.† The results showed that all samples exhibited good thermal stability up to ∼275 °C. With a further increase in temperature, a primarily weight loss appeared in the temperature range of 275–600 °C, which could be ascribed to the decomposition of BTC. Above 600 °C, no obvious mass loss could be found. Therefore, the estimated amount of HKUST-1 in the composite MOF/Si-3, MOF/Si-5 and MOF/Si-7 are 62 wt%, 55 wt% and 54 wt%, respectively.

In conclusion, mesostructured MOF/silica composites with a narrow pore size distribution were successfully prepared through a facile hydrothermal process. This is the first report where the MOF crystals were incorporated in situ into mesostructured silica support with the help of CTAB surfactant. The synthesis method allowed us to make mesoporous MOF/silica composites using larger silica/MOF ratios.

Acknowledgements

This work was supported by Fundamental Research Funds for the Central Universities (no. 2014QNA29), the Natural Science Foundation of Jiangsu Province (no. BK20140182) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09626f

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