Microwave-Assisted Synthesis of DUT-52 and Investigation of Its Photoluminescent Properties

A zirconium metal-organic framework (MOF) of DUT-52 (DUT: Dresden University of Technology) was synthesized herein by reacting zirconium tetrachloride (ZrCl4) and 2,6-naphthalenedicarboxylic acid (H2NDC) in DMF under microwave heating at 115 oC for 25 min. This synthetic procedure was more efficient than a solvothermal method, by which a long thermal exposure (24 h) of 100-150 oC was required to produce the same MOF. The MOF has a thermal stability of 560 °C, prior to partial loss of interconnected 2,6-naphthalenedicarboxylate (NDC) linkers at some structure building units (SBU). Crystallinity of this DUT-52 was ca. 77 %, which was the same as one synthesized solvothermally. Diffuse Reflectance UV-Vis spectra revealed an absorption at λex of 287 nm, which was equivalent to a bandgap energy of 4.32 eV. Electron excitations of this DUT-52 at 275 and 300 nm gave emission wavelength of 433 nm (a purple region), indicating a prospective use of DUT-52 as a photoluminescent material. Copyright © 2018 BCREC Group. All rights reserved


Introduction
Physical properties of MOFs, such as surface areas, pore sizes and topologies, may be affected by types of metal clusters, organic linkers and syntheses methods [1].An appropriate synthesis method may result in MOFs with solid structures, high crystallinity, and high thermal stability.MOFs have been widely used in het-erogeneous catalyses [2][3][4][5], drug deliveries [6,7], gas storages [8,9], photocatalysis [10][11][12], gas sensors [13,14], and separations [15].An improvement in synthetic procedures is of highly importance in producing MOFs with high energy efficiency.Microwave-assisted syntheses of MOFs recently are used because polar rotations of organic linkers and solvents may heat reaction systems evenly and rapidly [16].A microwave heating may also result in homogeneous nucleations and thus, reduces crystallization time compared to a solvothermal method [17].
MOF with SBUs of oxohydroxozirconium(IV), [Zr6O4(OH)4] 12+ , and linkers of NDC was first synthesized by Zhong and co-workers [18] using a solvothermal method in DMF in the presence of HCl.The group studied the MOF for sensing small molecules [18].Kaskel and coworkers synthesized the same MOF and other Zr (or Hf)-NDC MOFs solvothermally in the same year, with a slight modification, i.e. employment of acetic acid with different concentrations as a structural modulator and named them DUT-52 (Zr), DUT-53 (Hf) and DUT-84 (Zr) [19].The synthesis was conducted by heating reaction mixtures in an oven at 120 o C for 24 h (72 h for DUT-53) [18,19].An SBU in DUT-52 is connected to 12 NDC ligands, which is isoreticular to UiO-66 [19,20], a Zr MOF with 1,4-benzenedicarboxylate (BDC) linkers.The reported DUT-52 was thermally stable up to 500 o C, with BET surface area (SBET) of 1399 m 2 .g - [19].This SBET was greater than that of UiO-66 [21].A direct interaction of microwave radiation and polar reactants is possible to occur in a microwave-assisted synthesis.Thus, it offers high heating rates and possible homogeneous heating throughout the sample to feasibly complete the reaction in a very short time [1].An example of a microwave-assisted MOF synthesis was the synthesis of MOF-5 (Zn).The MOF was synthesized using microwave irradiation for 10 to 60 min with a power up to 1 kW at 105 o C [16,17,22].Another example was Co-MOF-74, which was synthesized under microwave heating at 130 o C and the power of 300 W for 1 h [9].The use of microwave heating in Zr-MOF synthesis was recently reported by Reinsch group.They reported the use of 2,5-furandicarboxylate (FDC) as linkers to generate Zr-CAU-28 (CAU: Christian-Albrechts-University), at which the SBU was connected to 8 FDC linkers [26].We reported herein the first example of a microwave-assisted synthesis of DUT-52 in a relatively short time (25 min) and described its photoluminescent properties.Photoluminescent properties of MOFs may have applications in biomedical imaging, displays, electroluminescent devices, fluorescent sensors, nonlinear optics, and photocatalyses [14].An example of MOF employment in photocatalysis was MOF-5, as reported by Llabrѐs et al.The MOF was used in phenol degradation using UV irradiation [11].This MOF-5 was synthesized solvothermally from zinc salt and terephthalic acid as linkers.A photocatalyst material from Zr MOF of UiO-66 was reported by Shen et al. [23].The MOF was used in Cr(VI) reduction under simulated sunlight with a reduction activity up to 35 % after 3 hourirradiation.UiO-66 has an absorption band in a UV region (320 nm) which was correlated to a bandgap energy of 3.9 eV [23].Other NDC MOFs, such as DUT-53 and DUT-84 [19] were also synthesized with a long heating solvothermally.Therefore, an investigation of a more efficient synthesis method of DUT-52 and the study of its photoluminescent properties are important to evaluate the MOF as future photoluminescent materials.

Synthesis and Activation
All reagents, i.e.H2NDC (Sigma Aldrich, 99 %), chloroform (Merck), HCl (Merck, 37 %), N,N-dimethylformamide (Merck, 99.8 %), and ZrCl4 (Merck, 99 %), were of analytical grade and used without further purification.The synthesis of DUT-52 was performed by dissolving ZrCl4 (175 mg, 0.75 mmol) and H2NDC; 162 mg, 0.75 mmol) in DMF (20 mL) at room temperature (28 °C).Hydrochloric acid of 0.5 mL (12 M) was added to the mixture to initiate the grow of crystal nucleus [18] and the mixture was sonicated for 60 min.The mixture was capped tightly and kept in a microwave (SINEO Microwave Synthesis Workstation MAS-II).The synthesis was carried out at 110 °C, power of 800 W for 25 min.The crude product was cooled to room temperature and kept for 10 min to give white precipitates.The white product was filtered off, and rinsed within 3 days with DMF (once) and chloroform (twice).The product was finally dried under vacuum at 120 °C for 8 h.

Characterization
Powder XRD analysis was carried out using D8 Advance Bruker diffractometer with Cu-Kα radiation (λ = 0.15406 nm).FTIR analysis was carried out using a Bruker Alpha instrument (ZnSe beamsplitter) by dispersing the sample into a potassium bromide pellet.The spectrum was collected from 16 scans with a spectrum resolution of 2 cm -1 .A thermogravimetric analyzer was used to get information of thermal stability of the material using NETZSCH TGA Thermoanalyzer STA 449 F3 Jupiter.Heating rate of the TGA was 40 °C/min, started at room temperature (28 °C) and increased up to 900 °C under argon atmosphere.Percentage of reflectance (%R) was taken using a diffuse reflectance spectrophotometer (DRS) of Thermo Scientific Evolution 220.Value of n equals 1/2 was selected in this measurement, because the nature of this sample transition was a direct allowed transition.The obtained diffuse reflectance spectra were converted to a Kubelka-Munk function.A vertical axis was converted to F(R∞) value, which was proportional to an absorption coefficient.The α in a Tauc equation [24] was substituted with F(R∞).The value of (hυF(R∞)) 2 was plotted against hυ (described in eV) according to a Kubelka-Munk equation [24].A tangent line was drawn to a point of inflection of the curve and the hυ.The bandgap energy (Eg) value was derived from a point of intersection between the tangent line and the horizontal line of hυ.Barium sulfate was used as a reflectance coating in our DRS measurement.Photoluminescent (PL) properties of DUT-52 were measured in a solid phase at room temperature using F-7000 fluorescence spectrometer, with a xenon lamp as an excitation source.

Microwave Assisted-synthesis of DUT-52
We reported herein a facile and fast synthesis of DUT-52 using a microwave method.The reaction was completed in 25 min, significantly shorter than the one with a solvothermal method (24 h) [18,19].PXRD patterns of the synthesized DUT-52 (Figure 1) revealed characteristic diffraction patterns of DUT-52 [18,19] at 2θ of 6.47°, corresponding to a d111 reflection and at 7.45°, corresponding to a d200 reflection [19].Coordinated NDC ligands to Zr in [Zr6O4(OH)4] 12+ clusters were observed by a slight shift of carbonyl groups at NDC from 1687 cm -1 in a free H2NDC ligand to 1651 cm -1 in the MOF (Figure 2), describing a weakened carbonyl vibration after metalation.
Average crystallites size of this DUT-52 was 42 nm, which was smaller than that of UiO-66 (150-200 nm) [21].In contrast to a long crystal growing of a solvothermal method, the short heating by microwave was able to generate a relatively good crystal quality of DUT-52.In fact, the crystallinity of this MOF was relatively the same as a solvothermally prepared DUT-52 (70 %) [27].
Thermogravimetric analysis of assynthesized DUT-52 showed three weight-loss steps between 25 and 600 o C (Figure 3).The first weight-loss (25 to 120 °C) of 13 % was attributed to the water removal.The second loss (150 to 400 °C) of 16.5 % was the removal of solvents (DMF).The framework started decomposing (22.5 %) in third weight-loss at 560 °C, attributed to the loss of NDC partially.This is likely the maximum thermal stability of DUT-52 before partial loss of the linker.DUT-52 was reported to have 12 interconnected-NDC at its SBU [19].Partial loss of NDC might give a reduced number of interconnected-NDC at some SBUs.Kaskel has reported a reduction of SBU connectivity from 12-NDC (DUT-52) to 8-NDC (DUT-53) and to 6-NDC (DUT-84) by increasing concentrations of acetic acid in the reaction [19].Here, the MOF thermally unchanged from 600 °C up to our final TGA observation of 900 °C (Figure 3).Zhong [18] and Kaskel [19] reported a slightly lower thermal stability of their solvothermally synthesized DUT-52, indicated by the earlier framework decomposition at ca. 500 °C and significantly continued afterwards.Thus, we observed a relatively higher thermal stability of DUT-52 which was prepared by a microwave heating, compared to the one prepared solvothermally.

Luminescence Character
Optical absorption of this DUT-52 was described in diffuse reflectance UV-Vis spectra (Figure 4).The percentage of reflectance (%R) was used to calculate the bandgap energy of the MOF using a Kubelka-Munk equation and Tauc Plot [24].The bandgap energy (Eg) of assynthesized DUT-52 was 4.32 eV, which might be classified as a semiconductor material [25].The presence of aromatics groups in DUT-52 linkers might affect optical properties of the material.The main absorption band of DUT-52 was at 287 nm.This likely described an electron transition from the linker to Zr(IV).The MOF thus might demonstrate a photoluminescence under a light exposure with a wavelength higher than 287 nm.
To observe the luminescence behavior, this DUT-52 was analyzed by a solid-state photoluminescence (PL) spectroscopy.PL analysis described an emission wavelength of this MOF at 433 nm in response to its electrons excitations at 275 and 300 nm (Figure 5).Zhong reported that a free NDC ligand gave a lower emission wavelength (356 nm) [18], indicating a redshift behavior of DUT-52 crystals to a visible region.The emission intensity was reported to be 5 times higher in the MOF [18], and thus confirmed the benefit of a MOF structure in the luminescence.It was also observed that higher energy given for an excitation (λex of 275 nm) resulted in higher intensity of the resulted emmision (Figure 5).This might describe a way to tune brightness of the luminescence.The luminescence itself could be rationalized by a strong electronic coupling between NDC linkers through [Zr6O4(OH)4] 12+ cluster and π-π* transitions of the linker [18].

Conclusions
A good crystalline DUT-52 with MOF crystallinity up to 77 % was successfully synthesized by a microwave heating in only 25 min.The MOF has a relatively higher thermal stability compared to the one prepared solvothermally.Diffuse reflectance UV-Vis spectra revealed an absorption at λex 287 nm which were identical to a bandgap energy of 4.32 eV.Electrons excitations at 275 and 300 nm gave an emission at 433 nm and thus indicated a possible application of this MOF to work as a photoluminescence material in a visible region.