Applied Science and Convergence Technology 2023; 32(4): 97-99
Published online July 30, 2023
Copyright © The Korean Vacuum Society.
aInstitute of Chemical Technology, Vietnam Academy of Science and Technology, HoChi Minh City 70072, Vietnam
bDepartment of Chemistry, Pukyong National University, Busan 48513, Republic of Korea
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Ruthenium tris(bipyridine) [Ru(bpy)3] acts as a photocatalyst under visible-light irradiation. However, it undergoes dimerization and degradation and has a short photocatalytic life. In this study, we encapsulated Ru(bpy)3 as a guest molecule in a suitable host material, which is a zeolitic imidazolate framework-71 (ZIF-71), using a simple in situ synthesis method. This stabilized its photocatalytic activity. A series of Ru(bpy)3@ZIF-71 composites (𝑥 = Zn2+: Ru ratio = 1, 3, 5, 7, and 10) with different amounts of loaded Ru(bpy)3 were synthesized and characterized by X-ray diffraction, ultraviolet-visible (UV-vis) spectroscopy, and fluorescence spectroscopy. The crystal structure of ZIF-71 was maintained in the Ru(bpy)3@ZIF-71 samples, and their UV-vis absorbance and fluorescence were almost identical to those of pure Ru(bpy)3. Moreover, Ru(bpy)3@ZIF-71 exhibited a high conversion yield (95 %) for the photocatalytic conversion of 𝛼-bromoacetophenone to acetophenone and good recycling stability over three cycles. We believe this strategy can be applied to fabricate other similar host–guest photocatalytic systems.
Keywords: Host-guest composites, Photocatalyst, Ruthenium tris(bipyridine), Zeolitic imidazolate frameworks, ZIF-71
Encapsulation of guest molecules having different functionalities within molecular-sieve host materials, such as zeolites and metal-organic frameworks (MOFs), is a common strategy to develop novel functional materials [1–5]. Polypyridine metal compounds have been embedded in molecular-sieve materials to fabricate photocatalytic materials. For example, photo catalytically active ruthenium tris(bipyridine) [Ru(bpy)3] was embedded in zeolite Y to synthesize a new material with stable photochemical activity and electron transfer mechanism . Ru(bpy)3exhibits homogeneous photocatalysis under visiblelight irradiation, which induces charge transfer. However, free Ru(bpy)3 undergoes dimerization and degradation and exhibits a short photocatalytic life. Although encapsulating Ru(bpy)3in a zeolite solves the abovementioned issues, zeolites are chemically inert and have limited applications in photocatalysis.
MOFs are state-of-the-art nanopore host materials that are suitable alternatives to zeolites. Electron transfer in MOFs depends on the ligand and the metal ion type, facilitating diverse applications. In particular, zeolitic imidazolate frameworks (ZIFs) exhibit high-density porosity, chemical stability and the ability to capture simple hydrophobic molecules [7–11]. ZIF-71 has a rhombohedral structure with large cages (1.68 nm) interconnected through pore windows of 0.48 nm and is more hydrophobic than ZIF-8 .
In this study, we encapsulated Ru(bpy)3 in ZIF-71 using a simple
Zinc acetate dihydrate [Zn(CH3COO)2·2H2O, 98 %], a metal ion source, was purchased from Alfa Aesar. 4,5-Dichloroimidazole (C3-H3Cl2N2, 98 %) and tris(bipyridine)ruthenium(II) chloride [Ru(bpy)3- Cl2·6H2O, 99.95 %] were purchased from TCI. Triethanolamine [N-(CH2CH2OH)3, 99 %] and 2-bromoacetophenone (C6H5COCH2Br, 98 %) were purchased from Acros and used without additional purification.
To prepare the ligand solution, 4,5-dichloroimidazole (0.22 g, 1.6 mmol), the ligand of ZIF-71, and tris(bipyridine)ruthenium(II) chloride (0.01–0.05 mmol) were added to a 50 mL glass flask containing 15 mL of methanol. The solution was stirred at 25 °C. In a separate 50 mL glass flask, zinc acetate dihydrate (0.088 g, 0.4 mmol) and the metal ion source of ZIF-71 were dissolved in 15 mL of methanol. This solution was poured into the previously prepared ligand solution and stirred for 24 h at room temperature to initiate crystallization. The mixture was centrifuged, washed with methanol, and dried overnight in an oven at 60 °C to obtain Ru(bpy)3@ZIF-71.
Triethanolamine (200 µmol) and 2-bromoacetophenone (50 µmol) were added to a glass cell containing 5 mL acetonitrile. Ru(bpy)3@ZIF- 71 (3 mg) was added to the glass cell, which was bubbled with Ar gas for 5 min and sealed using a glass cap with grease. The closed glass cell was irradiated with a blue LED lamp (1 W, 445 nm) at room temperature for 4 h.
Ru(bpy)3 can be encapsulated in the nanopores of ZIF-71 during crystal growth as the diameter of Ru(bpy)3 is approximately 14 Å and the pore diameter in ZIF-71 is 16.8 Å, as illustrated in Fig. 1(a) [13,14]. We used an
Furthermore, the encapsulation of Ru(bpy)3 in ZIF-71 was validated using diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy, and the spectra of a series of Ru(bpy)3@ZIF-71 samples were compared with those of a Ru(bpy)3 methanol solution [Fig. 3(b)]. The series of Ru(bpy)3@ZIF-71 exhibited the characteristic metal-to-ligand charge-transfer (MLCT) band of Ru(bpy)3 at 500 nm, which was identical to that of pure Ru(bpy)3. In addition, the absorption intensity increased nearly linearly with an increase in the amount of encapsulated Ru(bpy)3. Moreover, the fluorescence spectra of the series of Ru(bpy)3@ZIF-71 were obtained at an excitation wavelength of 450 nm and compared with those of a pure Ru(bpy)3 methanol solution [Fig. 3(c)]. The fluorescence enhanced with the increasing concentrations of Ru(bpy)3, indicating that the optical properties of the Ru(bpy)3-@ZIF-71 series were well maintained. Moreover, a series of Ru(bpy)3-@ZIF-71 was dissolved in acetic acid to accurately quantify the concentration of Ru(bpy)3 in ZIF-71. A molar plot of the Ru(bpy)3@ZIF- 71 unit cell is shown in [Fig. 4(a)].
Next, we evaluated the photocatalytic performance of Ru(bpy)3@ZIF-71 during the photocatalytic debromination of α-bromoacetophenone, as illustrated in Fig. 1(b) [15,16]. The photocatalytic reaction was performed for 4 h in the presence of triehtnaolamine (TEOA), an electron donor, using a blue LED lamp (1 W, 445 nm) whose irradiation corresponded to the MLCT band of Ru(bpy)3. Pristine ZIF-71 was used as the control under identical conditions and did not undergo any photocatalytic reaction. However, the conversion yield of the series of Ru(bpy)3@ZIF-71 increased with an increment in the concentration of Ru(bpy)3, where Ru(bpy)3(0.05)@ZIF-71 exhibited a high conversion yield of approximately 95 %, which increased with the reaction time [Figs. 4(b) and 4(c)]. Leaching of the encapsulated Ru(bpy)3 was not observed during the reactions. Furthermore, Ru(bpy)3(0.05)@ZIF-71 maintained its conversion yield during four cycles during the reusability test, suggesting its excellent photocatalytic stability [Fig. 4(d)].
We demonstrated that ZIF-71 is an efficient host material for encapsulating Ru(bpy)3. A series of Ru(bpy)3(
This work was supported by a Research Grant from the Pukyong National University (2021).
The authors declare no conflicts of interest.