Facile preparation of a hierarchically porous metal–organic nanocomposite with excellent catalytic performance

Abstract

Herein, we report a strategy to prepare a hierarchical micro–meso–macro porous metal–organic nanocomposite, hpCuL (L = 2,4,6-tris(3,5-dicarboxylatephenylamino)-1,3,5-triazine), using a facile gel-aging process. The effect of pH values and aging time on the growth morphologies was monitored. This 3-D material, comprised of continuous metal–organic framework nanocrystallines serving as pore walls, exhibits high catalytic activity and stability towards the reduction of 4-nitrophenol by NaBH₄ in water.

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Metal–organic frameworks (MOFs) are a class of crystalline porous materials that have been intensively investigated over the past decade as promising candidates for industrial-scale catalysis,^1–3 gas storage,^4,5 molecular recognition and separation.^6–8 They have been successfully prepared by linking different single metal ions or metal clusters with variable organic ligands.^9,10 In almost all studies to date, however, the reported MOFs are microporous (pore sizes <2 nm),^11,12 which limits their potential applications in regards to size exclusion and diffusion dynamics.¹³ Current efforts are devoted to using longer organic linkers to achieve larger pore sizes,¹⁴ but the synthetic pathways for these materials are typically time-consuming, costly, low-yielding, and complicated by interpenetrating structures or MOF collapses after removing guest molecules.¹⁵ Recently, a few metal–organic aerogels with micro/mesoporosities have been prepared, including the frameworks of Fe(III)-BTC (BTC = 1,3,5-benzenetricarboxylate) and Al(III)-carboxylate.^16,17 The preparation of these materials, however, requires supercritical drying with CO₂(I), a process that involves high pressure, sophisticated instrumentation and operation, and is relatively expensive.^18,19 These drawbacks hinder the application of these materials in large scale production. Further, a few mesoporous MOFs have been obtained through the cooperative template method containing surfactants,^20–22 but this method then requires the removal of the templates. Therefore, the development of other methodologies to produce hierarchically porous nanocrystalline MOFs is highly desirable, yet challenging.

Herein, we report the facile assembly of the hierarchical micro–meso–macro porous metal–organic nanocomposite hpCuL (L = 2,4,6-tris(3,5-dicarboxylatephenylamino)-1,3,5-triazine) via a gel-aging process. The pore walls of this material are the continuous nanocrystals of the MOF [Cu₃L(H₂O)₃]·10H₂O·5DMA (1).²³ The hpCuL exhibits high catalytic activity for the reduction of 4-nitrophenol by NaBH₄ in water. The present methodology highlights the concept of improving the capabilities of MOFs via controlling the crystal size and morphology of the aggregates, rather than altering the chemical components.^24,25

H₆L ligand was prepared in 94% yield by nucleophilic substitution between 5-aminoisophthalic acid and cyanuric chloride (ESI, Fig. S1†). A simple gel-aging process was used to prepare the hpCuL. First, H₆L (30 mg) was ultrasonically dissolved in 100 μL of dimethyl sulfoxide (DMSO), 95 μL of N,N-dimethylacetamide (DMA) and 10 μL of H₂O, the pH value of the solution was adjusted to 5–6 with 1.0 M NaOH (aq.). Cu(NO₃)₂ (165 mg) was dissolved in 75 μL of DMSO and 75 μL of DMA. The two solutions were mixed with vigorous agitation at room temperature. A homogeneous green gel was observed in less than 20 s, which was confirmed by the inverted test tube method (inset of Fig. 1). The prepared gel was then aged for 12 h at 100 °C and subsequently washed with DMA and ethanol to exchange the DMSO and DMA. The hpCuL product as a green powder was harvested by centrifugation followed by drying at 60 °C.

Fig. 1 (a) SEM image and (b) powder XRD patterns of the hpCuL (red) in comparison with as synthesized crystals (blue) and simulated patterns of single-crystal data of 1 (black); the hpCuL with micro–meso–macro ternary pores originating from the gel prepared at pH 5.5 and aged for 12 h at 100 °C; the insert in (a) shows a digital photograph of the hpCuL gel before the aging process.

The microstructural features of the hpCuL material were characterized by scanning electron microscopy (SEM) (Fig. 1a; ESI, Fig. S2†). These analyses show that hpCuL are composed of monolayers of interconnected nanoparticles (NPs) with an average diameter of approximately 40 nm.

Powder X-ray diffraction (XRD) was utilized to further investigate the phase purity of the hpCuL material. The XRD pattern of the hpCuL matches well with ones simulated single crystal structure and synthesized crystals of 1 (Fig. 1b). The weaker impure peaks of the hpCuL should be attributed to cross-linking and low crystallinity of the NPs. This indicates that the hpCuL material possesses the crystallinity (i.e., the ordered 3D framework) of 1, created from the three isophthalate moieties in the L ligand and copper paddlewheel units. There are three types of cages with diameters of 0.91, 1.2, and 1.72 nm in the frameworks.²³ These data suggest that the hpCuL has two levels of porosity: one from the MOF framework and another from the channel walls of interconnected nanocrystals.

The pores below 100 nm was confirmed by the N₂ sorption investigation as shown in Fig. 2a. The adsorption–desorption isotherm exhibits an type-IV curve characteristic with hysteresis loop, indicating that there are mesopores existing in this material. The specific surface area of the hpCuL is 1423 m² g⁻¹. The pore size distribution shows that there are two peaks of mesopores respectively centered at 2.9 nm and 10 nm, and the total pore volume is up to 0.20 cm³ g⁻¹. Meso–macropore sizes (2 nm to 100 μm) of the hpCuL is analysed using mercury intrusion porosimetry because the large pores are not reaching saturation in the relative N₂ pressure, so that condensation does not take place in these pores.²⁶ As shown in Fig. 2b, the pore width in the range from 2 nm to 1 μm indicates the presence of the meso–macropore with continuous size variation. The region centered at 4.3 μm at low pressure is assigned to the compaction of the powder and the filling of the interparticular porosity. From Fig. 2b, the specific surface area was 1576 m² g⁻¹, the pore volume of 1.6 cm³ g⁻¹ with the porosity of 76.33%. These results exhibit that the specific surface area was mainly contributed by small pores of size below 100 nm, and pore volume mainly by large pores of size above 100 nm. All these demonstrate that hpCuL is a typical material with micro–meso–macro ternary pores.

Fig. 2 (a) N₂ sorption isotherm and integrated pore volume and (b) pore size distribution of the hpCuL. Inset in (a) is the pore size distribution curve of the corresponding hpCuL.

To gain further insight into the growth process of the hpCuL material, we monitored the reaction products over time as the gel was aged. The images of the material obtained via centrifugation of the newly formed gel, reveals porous structures consisted of the NPs without clear boundary (Fig. 3a). In contrast, after the gel was aged for 12 h at 100 °C, the growth, cross-linking, aggregation or polymerization of the crystallites were significantly promoted, the NPs appeared, had organized into 3D cross-linked polymeric networks (Fig. 1; ESI, Fig. S2†). With increasing gel-aging to 24 h, the sizes of the particles remained nearly constant diameter of approximately 40 nm (Fig. 3b; ESI, Fig. S3†), which suggests that the growth of the particles have stopped after 12 h of aged time. These results show that the gel-aging process provides a sufficient driving force for metal–ligand coordination bonding and material organization via H-bonding and van der Waals interactions.

Fig. 3 SEM images of the products prepared at different pH values for various aging time (a) at pH 5.5 for 0 h; (b) at pH 5.5 for 24 h; (c) at pH 4.8 for 0 h, (d) at pH 4.8 for 12 h.

Exchanging the initial DMSO and DMA solvents with water, N,N-dimethyl formamide (DMF), ethanol or methanol yielded viscous precipitates, indicating that gel formation is dependent on the nature of the solvents. Controlling pH values of reactants is also very important for obtaining hpCuL. As a starting point for this work, we varied aging time, the concentration of Cu(NO₃)₂·3H₂O and H₆L ligand, but obtained frequently amorphous aggregation. In this case, we speculated that pH value should be the key factor that driving gelation growth of 1 surpasses the growth of crystal and precipitation. This is because that organic part (H₆L) of the gelator contains three imine groups, three pyridine nitrogen groups which are excellent pH-sensitive bonding sites acting as an H bond donor or an H bond acceptor. In addition, the coordination activity of complexible six carboxyls in H₆L is closely related to the pH values. Indeed, the pH of the mixture of Cu(NO₃)₂·3H₂O and H₆L ligand is difficult to measure, because the formation of gel or precipitation is often finished within a few seconds. Therefore, the pH described here was adjusted by adding a small amount of 1.0 M NaOH (aq.) into the solution of H₆L ligand. We found that an observable amount of aggregate precipitates would be immediately formed at higher pH (>6.5). A clear solution was observed at lower pH (<3.5). Interestingly, discrete spheres originating from centrifugation of the newly formed gel prepared at pH 4.8 could be obtained (Fig. 3c). Highly crystalline, phase-pure 1 with a diameter of approximately 1 μm was produced after the gel was aged at 100 °C for 12 h (Fig. 3d), which can be shown by XRD pattern of the product (ESI, Fig. S4†). Such morphological patterns are typical features of 1. Our target product, hpCuL was successfully obtained at pH 5–6.

An important challenge for industrial applications of MOFs is moisture/water resistance. To determine the property of the hpCuL, the material was exposed to air for 30 days at room temperature, which did not change its crystalline phases as seen by its PXRD pattern. More significantly, the hpCuL material retained its characteristic peaks after being immersed in water for 24 h (ESI, Fig. S5†).

These characteristics of the hpCuL material encouraged us to further investigate its application in catalytic reactions. Of the reported heterogeneous MOF catalysts, most are typically activated by prolonged vacuum heating to remove coordinated solvent(s) or other guest molecules occupied in the pores, which opens the unsaturated metal sites and functional organic sites on the pore surface.²⁷ The doping of MOFs with noble metal NPs is also a common method for improving their catalytic performance.^28–30

In the present case, the hpCuL material under neither desolvation nor loading with nobel metal NPs was used for the catalytic reduction of 4-nitrophenol with excess NaBH₄ in water as a proof of concept. As indicated in ESI, Fig. S6,† the absence of hpCuL did not produce the fading and bleaching of 4-nitrophenolate (4-NP). Within 40 min, only 4% 4-NP was adsorbed on the hpCuL (ESI, Fig. S7†). However, the addition of low concentrations of the hpCuL material to the 4-NP aqueous solution with NaBH₄ caused a rapid fading and bleaching of the 4-NP. The reduction reaction of 4-NP is completed within 120 s (Fig. 4a). This process was characterized with a gradual decrease in the absorption peak of 4-NP at 400 nm and simultaneously increase in the absorption peak of 4-aminophenolate (4-AP) at 300 nm with time.

Fig. 4 UV-vis absorption spectra and correlation of ln(A_t/A_o) versus reaction time for the reduction of 4-NP catalysed by hpCuL (a and b) and 1 crystals (c and d), respectively. A_t and A_o represent the absorption of 4-NP at 400 nm at time t and initially.

Since the concentration of BH₄⁻ used was in excess of 4-NP, the catalytic rate of the reaction is reasonably assumed to follow pseudo-first-order rate kinetics with regard to the 4-NP concentration.^31,32 Further, the ratio of A_t/A_o is directly proportional to the ratio of C_t/C_o, where A_t and C_t, A_o and C_o are the absorbance and concentrations of 4-NP at time t and initially, respectively. Therefore, the apparent pseudo-first-order reaction rate constant (k_a) of the reaction can be calculated from the linear correlation of ln(A_t/A_o) versus reaction time. Based on these, the rate constant of the reaction, 0.2565 s⁻¹, is obtained from Fig. 4(a and b). This value reveals that the catalytic capacity of the hpCuL material is significantly superior to those composites loaded with Au NPs and Au–Ag NPs.^33–37 For example, the catalytic capacity of hpCuL is approximately 50- and 20-times greater than those of the Au@Ag core–shell NPs immobilized on the representative MOF ZIF-8 (Zn(MeIM)₂, MeIM = 2-methylimidazole) and the Fe₃O₄/SiO₂/Au/por-SiO₂ composite,^38,39 respectively. Further, the concentration of the hpCuL catalyst used in these comparisons was approximately 2% of that used in the Au@Ag NPs and Au composite as described above.

In addition, we used crystals of 1 for the catalytic reduction of 4-NP in the presence of excess NaBH₄ for comparison; the crystals of approximately 30 μm diameter were synthesized by hydrothermal methods (ESI, Fig. S8†).²³ As shown in Fig. 4(c and d), the apparent rate constant of the reaction was determined to be 3.95 × 10⁻³ s⁻¹. The catalytic capacity of hpCuL is approximately 65-times higher than that of 1 crystals. These results demonstrate the important role of the hierarchically porous networks of hpCuL on catalytic reduction. The hpCuL material acts as an efficient electron relay system, accepting electrons from BH₄⁻ ions and relaying the electrons to the acceptor molecules of 4-NP,^40–42 like other MOFs.

The catalyst stability was another essential parameter. ESI, Fig. S8† revealed that there was no noticeable decrease in catalytic efficiency, morphology and size of the hpCuL after five cycles, which demonstrates that the hpCuL is stable during the catalytic reaction. The preliminary studies suggest the hpCuL also exhibits high catalytic activity towards the reduction of methyl orange, bromophenol blue and bromocresol green by NaBH₄ in water (ESI, Fig. S9†).

In conclusion, a facile synthesis of the hierarchically porous hpCuL material at room temperature is reported. The 3D networks of this material are comprised of monolayers of MOF NPs serving as pore walls. Aging the pre-cursor gelatinous material at 100 °C was shown to sufficiently promote growth and cross-linking of the NPs, with growth of the NPs concluding by 12 h of aged time. The hpCuL material under neither desolvation nor loading with noble metal NPs showed excellent catalytic activity towards the reduction of 4-NP in the presence of NaBH₄. The catalytic activity is attributed to the hierarchically porous microstructure. The present strategy may open a route for developing highly efficient hierarchical catalysts.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 20971080), and the Natural Science Foundation of Shandong Province (Grant No. ZR2013BM009) and the Project of Shandong Province Higher Educational Science and Technology Program (​Grant No. J13LD53).

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Footnote

† Electronic supplementary information (ESI) available: Full synthetic and experimental details. See DOI: 10.1039/c6ra22650g

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