On the Synthesis and Characterization of Lanthanide Metal-Organic Frameworks

In this study, lanthanide metal-organic frameworks Ln(BTC)(DMF)2(H2O) (LnMOFs) are synthesized using the metal nitrates as lanthanide (Ln = La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu) source and 1,3,5-benzenetricarboxylic acid (BTC) as a coordination ligand. X-ray diffraction (XRD) analysis, Fourier-transform infrared spectroscopy (FTIR), thermogravimetric (TG/DTG) analysis fluorescence spectroscopy (FLS), and scanning electron microscopy (SEM) are employed to characterize the newly synthesized LnMOFs.


Introduction
Metal-organic frameworks (MOFs), having zeolite-like network structure, are synthesized by self-assembly of polydentatic organic ligands and metal ions [1,2].MOFs are characterized as materials having large surface area, highly porous structures, and large (tunable) pore volume.In today's world, MOF are used in aspects of gas storage, gas and liquid adsorption and separation, luminescence, catalysis, and in electrochemical biosensors for ultrasensitive detection [1][2][3][4][5].
The metal ion center has a key role for metal-organic frameworks.Lanthanides possess a unique electronic shell structure because their 4f electron shells are not completely filled, and the number of electrons in 4f shells varies, in this case, lanthanides have relatively high coordination numbers, which allows us to synthesize MOF with desired structures.The synthesis of lanthanide metal-organic framework is dependent on a variety of internal and external parameters, such as ionic radius, reaction temperature, atmosphere, coordinating solvents, and the nature of counter anions.Lanthanide metal-organic frameworks having high porosity, specific pore size, and 2D and 3D coordination networks could be used as heterogeneous catalysts in organic synthesis and solvent-free reactions [3,4,6,7].General metal-organic framework synthesis types include solvothermal, hydrothermal, and solvent-free methods [8][9][10][11].In this study, the solvothermal synthesis method was chosen, aiming for good crystallinity of synthesised MOFs [3,4].

Experimental
Lanthanide nitrates, 1,3,5-benzentricarboxylic acid (BTC), N,N-dimethylformamide, and methanol were purchased from Aldrich and used directly without further purification.The syntheses of MOFs were performed using the following procedures:  O (0.322 g, 0.75 mmol) was dissolved in N,N-dimethylformide/water (3:1 v/v) mixture (20 mL).(0.116 g, 0.55 mmol) BTC was added and mixture was stirred for 30 min at room temperature.After complete dissolution the mixture was kept in the furnace at 65 • C for 24 h.The formed transparent rod-like crystals were filtered on a dense paper filter, washed with methanol (15) mL three times.Crystals were punt into chloroform for solvent exchange (removal of DMF) for 24 h, afterwards dried in air, which resulted in 0.195 g LaMOF, yield 45%.
The preparation procedures for other lanthanide MOFs were analogous for LaMOF and were performed using different starting materials.
Ce 3+ (BTC)(DMF) XRD data were collected at room temperature on a Rigaku Miniflex II system with a graphite monochromator, using Cu Kα1 radiation (speed 1 • /min).FTIR analysis of compounds was conducted using a Bruker Alpha FTIR spectrometer with Platinum ATR single reflection diamond module.Thermal analyses were conducted from room temperature to 600 • C under air atmosphere using Perkin Elmer Pyris 1 TGA thermal analyser and Pyris software.The heating rate was 5 • C/min.Excitation and emission measurements were acquired using Edinburgh Instruments FLS980 fluorescence spectrometer.
The emission spectra were conducted in the solid state.Scanning electron microscope (SEM) Hitachi TM3000 was used to study the main morphological features of obtained crystals.

XRD Analysis
The synthesised fourteen lanthanide metal-organic frameworks were found to be isomorphous having very similar structures.However, only the structure of LaMOF is described here in detail (Figure 1).
LaMOF is a three-dimensional open framework.Each asymmetric unit contains one eight-coordinated La 3+ ion, one BTC ligand, two coordinated DMF molecules, with eight oxygen atoms from four BTC ligands through two chelating carboxylate groups (O1-O4), two carboxylate groups (O5 and O6), and two terminal DMF molecules (O7 and O8).In the LaMOF molecule, one La 3+ ion is linked with four phenyl groups through two chelating bidentate carboxylate groups and two monodentate carboxylate groups.

XRD Analysis
The synthesised fourteen lanthanide metal-organic frameworks were found to be isomorphous having very similar structures.However, only the structure of LaMOF is described here in detail (Figure 1).
LaMOF is a three-dimensional open framework.Each asymmetric unit contains one eightcoordinated La 3+ ion, one BTC ligand, two coordinated DMF molecules, with eight oxygen atoms from four BTC ligands through two chelating carboxylate groups (O1-O4), two carboxylate groups (O5 and O6), and two terminal DMF molecules (O7 and O8).In the LaMOF molecule, one La 3+ ion is linked with four phenyl groups through two chelating bidentate carboxylate groups and two monodentate carboxylate groups.2) The crystallographic data of fabricated LnMOFs are summarized in Table 1.

Infrared (FTIR) Spectroscopy
All the FTIR spectra of synthesized lanthanide metal-organic frameworks were very similar (see Figure 3).Asymmetric and symmetric stretching vibrations of the BTC ligand carboxylate groups displayed bands at 1556 and 1382 cm −1 .The bands at 1621, 3078, 689, and 780 cm −1 are assigned to the vibration of aromatic skeleton of the benzene ring [12].The bands at 1678 and 2915 cm −1 are assigned to vCO and the asymmetric stretching vibration of the -CH3 group of the N,N-dimethylformamide molecules [12,13].The absence of FTIR bands at 2658, 2544 (OCOOH), and 1691 (CCOOH) cm −1 indicates that the BTC ligands were completely deprotonated after the reaction.The broad band at 3420 cm −1 is attributed to the hydrogen-bonded vOH groups from adsorbed (residual) water (Table 2).

Infrared (FTIR) Spectroscopy
All the FTIR spectra of synthesized lanthanide metal-organic frameworks were very similar (see Figure 3).Asymmetric and symmetric stretching vibrations of the BTC ligand carboxylate groups displayed bands at 1556 and 1382 cm −1 .The bands at 1621, 3078, 689, and 780 cm −1 are assigned to the vibration of aromatic skeleton of the benzene ring [12].The bands at 1678 and 2915 cm −1 are assigned to vCO and the asymmetric stretching vibration of the -CH 3 group of the N,N-dimethylformamide molecules [12,13].The absence of FTIR bands at 2658, 2544 (OCOOH), and 1691 (CCOOH) cm −1 indicates that the BTC ligands were completely deprotonated after the reaction.The broad band at 3420 cm −1 is attributed to the hydrogen-bonded vOH groups from adsorbed (residual) water (Table 2).

Thermal (TG/DTG) Analysis
The TG/DTG curves of synthesized lanthanide-containing MOFs are shown in Figure 4.As seen, all the MOF samples had similar thermal stability [14,15].The first mass loss in DTG curves was observed at about 120 °C (except for CeMOFs at ~200 °C).This first weight loss in the temperature range of 20-160 °C corresponds to the loss of water molecules and adsorbed moisture.The main mass loss, which continuously occurred up to 400-450 °C, is assigned to the decomposition of DMF.At higher temperatures (above 450 °C) the mass loss is associated with final decomposition of MOFs and formation of Ln2O2CO3, or Ln2(CO3)3 and Ln2O3 [16][17][18].

Thermal (TG/DTG) Analysis
The TG/DTG curves of synthesized lanthanide-containing MOFs are shown in Figure 4.As seen, all the MOF samples had similar thermal stability [14,15].The first mass loss in DTG curves was observed at about 120 • C (except for CeMOFs at ~200 • C).This first weight loss in the temperature range of 20-160 • C corresponds to the loss of water molecules and adsorbed moisture.The main mass loss, which continuously occurred up to 400-450 • C, is assigned to the decomposition of DMF.At higher temperatures (above 450 • C) the mass loss is associated with final decomposition of MOFs and formation of Ln 2 O 2 CO 3 , or Ln 2 (CO 3 ) 3 and Ln 2 O 3 [16][17][18].

Luminescent Properties
It is well-known that lanthanides, especially Eu and Tb, can absorb ultraviolet radiation efficiently through an allowed electronic transition to convert to excited state 5 D 4 , and these excited states are deactivated to the multiplet 7 F J states by emitting visible light.Emission spectra of Pr, Sm, Eu, and Tb metal-organic frameworks are shown in Figure 5.The intensity of emission lines situated at 325, 416, and 443 nm (UV-blue region) of PrMOF (λ ex = 290 nm) were very weak.
The photoluminescence measurements showed that Ce and La MOFs with BTC ligands are optically inactive.This is in a good agreement with the literature data [16,17].The synthesized Nd, Gd, Dy, Ho, Er, Tm, Yb, Lu metal-organic frameworks, however, displayed very weak or even undetectable fluorescence at excitation wavelengths of 250-400 nm.

Luminescent Properties
It is well-known that lanthanides, especially Eu and Tb, can absorb ultraviolet radiation efficiently through an allowed electronic transition to convert to excited state 5 D4, and these excited states are deactivated to the multiplet 7 FJ states by emitting visible light.Emission spectra of Pr, Sm, Eu, and Tb metal-organic frameworks are shown in Figure 5.The intensity of emission lines situated at 325, 416, and 443 nm (UV-blue region) of PrMOF (λex = 290 nm) were very weak.
The photoluminescence measurements showed that Ce and La MOFs with BTC ligands are optically inactive.This is in a good agreement with the literature data [16,17].The synthesized Nd, Gd, Dy, Ho, Er, Tm, Yb, Lu metal-organic frameworks, however, displayed very weak or even undetectable fluorescence at excitation wavelengths of 250-400 nm.

Scanning Electron Microscopy
The representative SEM micrographs of synthesized lanthanide MOFs are shown in Figure 6.

Scanning Electron Microscopy
The representative SEM micrographs of synthesized lanthanide MOFs are shown in Figure 6.The SEM results revealed that all compounds consist of two types of particles.In most of the cases the formed rectangular plate-like crystallites of 25-70 µm in size were covered with nanosized differently shaped particles.We can conclude that nature of lanthanide does not influence significantly the surface morphology of fabricated lanthanide MOFs.

Conclusions
The lanthanide metal-organic frameworks Ln(BTC)(DMF)2(H2O) (LnMOFs) (Ln = La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu) were successfully synthesized using 1,3,5-benzenetricarboxylic acid (BTC) as a coordination ligand.These MOFs were obtained with very well-developed crystalline structure.All FTIR spectra of LnMOFs showed characteristic asymmetric and symmetric stretching vibration bands of the BTC ligand.The emission spectra of Pr, Sm, Eu, and Tb metal-organic frameworks were discussed in this study, however, the La, Ce, Nd, Gd, Dy, Ho, Er, Tm, Yb, and Lu metal-organic frameworks displayed very weak or even undetectable fluorescence at excitation wavelengths of 250-400 nm.The SEM micrographs of synthesized lanthanide MOFs showed the The SEM results revealed that all compounds consist of two types of particles.In most of the cases the formed rectangular plate-like crystallites of 25-70 µm in size were covered with nanosized differently shaped particles.We can conclude that nature of lanthanide does not influence significantly the surface morphology of fabricated lanthanide MOFs.

Conclusions
The lanthanide metal-organic frameworks Ln(BTC)(DMF) 2 (H 2 O) (LnMOFs) (Ln = La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb and Lu) were successfully synthesized using 1,3,5-benzenetricarboxylic acid (BTC) as a coordination ligand.These MOFs were obtained with very well-developed crystalline structure.All FTIR spectra of LnMOFs showed characteristic asymmetric and symmetric stretching vibration bands of the BTC ligand.The emission spectra of Pr, Sm, Eu, and Tb metal-organic frameworks were discussed in this study, however, the La, Ce, Nd, Gd, Dy, Ho, Er, Tm, Yb, and Lu metal-organic frameworks displayed very weak or even undetectable fluorescence at excitation wavelengths of 250-400 nm.The SEM micrographs of synthesized lanthanide MOFs showed the formation rectangular plate-like crystallites of 25-70 µm in size covered with nanosized differently shaped particles.

Figure 5 .
Figure 5. Emission spectra of Pr, Sm, Eu and Tb metal-organic frameworks.

Figure 5 .
Figure 5. Emission spectra of Pr, Sm, Eu and Tb metal-organic frameworks.