Effect of different lanthanide ions on the catalytic activation of peroxymonosulfate with lanthanide metal-organic frameworks (Ln-MOFs) with terephthalic acid

Ln-MOFs with four different Ln-ions (La, Ce, Gd, Tb) and terephthalic acid (H2BDC) were synthesised by solvothermal method. The crystals structure was confirmed by powder XRD and infrared spectroscopy (IR) corresponding to [Ln2(BDC)2(H2O)4]∞ and microstructural information of all samples was extracted by applying Rietveld refinement. By scanning electron microscopy (SEM) the morphology of the samples was revealed. First time applying Ln-MOFs for the catalytic activation of peroxymonosulfate (PMS) demonstrated their potential application as catalysts in a Fenton-like reaction for water purification from the antibiotic tetracycline hydrochloride (TCH). For the best performing Ce-MOF degradation of TCH reached 80% that’s why additional experiment for determination of activation energy and the mechanism of the catalytic reaction were performed. The stability of the catalyst was also confirmed by powder XRD after the reaction.


Introduction
The energy crisis combined with environmental pollution has been recognized as one of the most serious global concerns.Therefore, huge attempts have been devoted in recent decades to resolving these challenges by introducing more advanced materials with higher efficiency.Specially-designed catalysts are among the first candidates proposed to combat environmental contaminations [1].Recently, attention is attracted by the metal-organic frameworks (MOFs).These crystal hybrid materials are with constant and homogeneous content of the pores.Depending on the method of synthesis and combination of different metal centers as well as organic ligands, MOFs physical and chemical properties can be easily controlled [2].Combining these properties with the unique properties of the lanthanide ions emerge in very promising catalytic abilities of these materials.Ln-MOFs can possess Lewis acidic sites that are easily functionalized with Lewis basic and other groups within the pores.Furthermore, unlike many other metals, lanthanide ions can have wide variation and tunability in their coordination number and modes, producing novel catalytic centers.Most importantly, it is common to see many different Ln(III) ions forming a single isostructural series of frameworks, and variation of the Ln(III) ion used can often tune the catalytic activity.However, to function as catalysts, MOFs must be highly chemically and thermally stable and in this Ln-MOFs usually perform much better than transition metals MOFs [3][4][5][6].
In most cases MOFs are used as catalysts for heterogeneous catalytic reactions in organic synthesis [7,8] but rather rarely as heterogeneous catalysts for degradation of organic pollutants.Even when applied in photocatalytic experiments for degradation of organic pollutants, mostly transition metal based MOFs are studied [1,[9][10][11][12][13][14].Homogeneous transition metals catalysts require acidic conditions and are difficult to recycle, causing secondary pollution.Heterogeneous catalysts are more practical and intensively studied due to their advantages such as easily separating and recycling.
For the purpose of purifying waters from organic pollutants the reactive oxygen species (ROS) are known to play an essential role.Such ROS can be generated by catalytic activation of peroxymonosulfate, PMS [15][16][17].The sulfate radical (SO4* -) is one of those ROS, having high standard redox potential (2.5-3.1V)[18], capable of functioning in solutions with a wide range of pH, with long lifetimes in comparison with the hydroxyl radical [15].
Antibiotic tetracycline hydrochloride TCH is one of the well-known water pollutants.After being applied to animals, most antibiotics are discharged in the form of antibiotic active agents or metabolites through animal excretions [19].Because it is extremely difficult for microorganisms to degrade antibiotics, it causes antibiotic pollution in surface water, soil, sediment, and groundwater and influences the normal life activities of mankind, mammal, plants, microorganisms.This may cause the generation of antibiotic-resistant genes and antibiotic resistant bacteria and accelerate the spread of antibiotic resistance.It will not only lead to a vicious circle of antibiotic abuse, interfere with the distribution of bacterial communities, and ultimately threaten the safety of the ecosystem and endanger human life and health [20].Therefore, how to effectively degrade antibiotics in the environment, especially in water, has gradually gained global attention.Taking into account the considerations mentioned above, the antibiotic tetracycline hydrochloride, TCH, was used as a model solution in the current research.On the other side, Ln-MOFs have been tested only in photocatalytic reactions [21,22], furthermore Ln-MOFs with terephthalic acid have never been applied to catalytic reactions for degradation of organic pollutants.That's why the investigation on the catalytic activation of peroxymonosulfate by four Ln-MOFs and their potential application as a catalyst in a Fenton-like reaction for water purification from TCH was among the goals of our research.

Synthesis
All four Ln-MOFs were synthesized under the same solvothermal conditions.Corresponding LnCl3*nH2O (1.03mmol) and terephthalic acid (1.55mmol) were dissolved in 16ml DMF (0.21mol).Then the solution was placed in 20ml Teflon-lined stainless-steel reactor and held at 150 o C for 36h.After cooling the precipitate was washed twice with DMF, three times with water and dried at 80 o C. Considering the experimental conditions, there was only one exception, namely the solvothermal synthesis, performed with the Ce(III) ions at different temperatures such as 110, 130, 150 and 180 o C. The samples obtained are mentioned in the text as Ce-MOF-T, where T is the symbol for the temperatures tested.

Methods for Characterization
The crystal structure and phase analysis were evaluated by powder X-ray diffraction (Malvern PANalytical Empyrean, Almelo, Netherlands) using Cu-Kα radiation (λ=1.5418Å).Scanning electron microscopy (Hitachi TM4000, Krefeld, Germany) was used to observe the morphology of the obtained materials.The infrared spectral analysis was carried out on a FT-IR Nicolet 6700-Thermo Scientific in KBr pellets.

Tetracycline Hydrochloride (TCH) Degradation
The catalytic activity of the prepared Ln-MOFs was evaluated towards activation of peroxymonosulfate for the degradation of tetracycline hydrochloride.The experiments were carried out in a catalytic batch slurry reactor (Lenz Laborglas, model LF100, 500 mL).The reaction temperature was kept constant at 25 o C by circulating bath (ArgoLab CB5-10).In a typical experiment 50 mg of the prepared Ln-MOF was mixed with 200 ml TCH solution (20 ppm) placed under constant stirring.After 30 mins of magnetic stirring to achieve sorption-desorption equilibrium 50 mg of PMS was added to start the degradation experiments.On a certain time interval, 2 ml of the slurry was extracted and 100 µl 0.01M Na2S2O3 was immediately injected to quench the reaction.The catalyst was separated by filtration through 0.22 µm membrane syringe filter and the concentration of the TCH was evaluated using UV-visible spectrophotometer (Thermo Scientific, Evolution 300) at maximum absorbance wavelength of 354 nm.The degradation efficiency was calculated using the following formula: % = The reaction kinetics was evaluated by pseudo-first and pseudo-second kinetic models using the following formulas: ln (    0 ) = − 1  for the pseudo-first kinetic model (2) where Ct and C0 are the moment and initial concentration of the TCH, while k1 and k2 are the pseudofirst and pseudo-second-rate constants, respectively.

Characterization of the Samples
The powder XRD patterns of the Ln-MOFs are presented at figure 1a.The patterns matches well with the crystal structure reported [23].The materials crystallize in the triclinic P-1 space group with the formula unit of [Ln2(BDC)2(H2O)4]∞.No significant difference in the phase composition of the samples was observed showing that all obtained MOFs are isostructural.The shifting towards higher angle of the XRD peak positions for the smaller Ln(III) ions is the most obvious difference among the samples (figure 1b).Additional microstructural information of all samples was extracted by applying Rietveld refinement as implemented in MAUD software [24] and the results obtained are presented in table 1.As already mentioned, the unit cell parameters significantly decrease from La-MOF to Tb-MOF which is in a good agreement with the difference of the ionic radius of the metal ions (rLa= 1.16 Å, rCe= 1.14 Å, rGd= 1.05 Å, rTb= 1.04 Å for eight coordination) [25].Along with the differences in the size of the unit cell, significant differences are observed at microstructural level.By changing the metal ion from La(III) to Tb(III) the size of the crystallites notably decrease from 124.4 nm for La-MOF to 96.6 nm for Tb-MOF.This is also followed by an increase of the microstrains value from La(III) to Ce(III) followed by much slower increase of their value from Ce-MOF to Tb-MOF.In general, the smaller the Ln(III) is, the more defective the obtained structure.For the XRD patterns of the obtained at different temperatures four samples of Ce-MOF no significant difference is observed at first glance (figure 2а).The closer inspection of the XRD pattern shows worthy of attention shift of the diffraction reflexes towards lower 2theta which is an indication of increased unit cell parameters of the samples (figure 2b).As expected, the unit cell parameters of the samples monotonously increase with the increased temperature, which is due to the positive thermal expansion of the obtained materials.Interestingly, the crystallites size significantly decrease which is unexpected considering that the higher temperature usually leads to larger crystallites.One possible explanation for this observation is the different mechanism of crystallization and subsequent crystal growth.Probably, at lower temperatures the crystallization is slower, involves nucleation and slow crystal growth, while at higher temperatures mass volume crystallization occurs leading to fast precipitation of the obtained materials.Furthermore, the microstrains slowly increase with the temperature.Microstrains can be considered as an indication for the defects in crystal structure i.e. the higher defects concentration leads to larger microstrains value.This can be considered as an additional indication that the crystallization is faster at high temperatures, developing simultaneously in the whole volume of the reaction mixture.Further information about the structure and complexation of the Ln-MOFs was obtained by FT-IR analysis with the IR spectra recorded (figure 3).The vibrations assignment of the free terephthalic acid was done according to [26].It is clearly seen that when complexation occurs all of the characteristic bands for the carboxylic group disappear which is an indication that the reaction is going on between the carboxylic group and the Ln(III) ion.Furthermore, a clear band around 3500-3600 cm -1 due to inclusion of water molecules in the crystal structure is detected.As expected, significant differences in the FT-IR spectra of the different Ln-MOFs are not observed, indicating additionally for their identical chemical composition and structure.The morphology of the samples was characterized by scanning electron microscopy and the obtained images are presented on figure 4. The morphology of samples can be described as lamellar-like structure in which the lamellae are grown together forming clusters resembling a micro peony flower like structure for the La-MOF and Ce-MOF.As the metal ion changes from Ce(III) to Tb(III) the hierarchical 7 structure is less visible.This is well seen on figure 4d where no obvious cluster assembly is observed for the Tb-MOF.

Catalytic Decomposion of TCH
After the detailed physicochemical characterization of the materials, they were tested for catalytic degradation of tetracycline hydrochloride in water by peroxymonosulfate activation and the results are summarized in table 3.As it can be seen on figure 5 PMS alone has activity towards oxidation of TCH, but despite that it reaches maximum degradation efficiency of ~ 54%.The addition of Ln-MOF significantly increases the degradation efficiency and it reaches ~80% for Ce-MOF.
To exclude any interference of side reactions, additional experiment was performed with Ce-MOF and TCH without the presence of PMS.There was no degradation of TCH, confirming that Ce-MOF is acting only as a catalyst.The experiments clearly indicate the superior oxidation performance of the Ln-MOF/PMS systems compared to the PMS alone.The catalytic activity with the different Ln-MOFs follows the order Ce-MOF>La-MOF>Gd-MOF>Tb-MOF.Possible explanation for the better catalytic activity of Ce-MOF is the fact that Ce(III) can easily be oxidized by PMS to Ce(IV) which ensures better PMS activation during the catalytic reaction.For better understanding of the catalytic process the apparent rate constants were calculated using first the pseudo-first kinetic equation (equation ( 2)); the kinetic curves are presented on figure 6a.Even though pseudo-first kinetic order is widely used for the description of this type of catalytic reaction, we find hard to justify it in our case considering the low R 2 -values obtained for the linear fits.For this reason, the pseudo-second kinetic order was considered using the equation ( 3) and the results are presented on figure 6b.The obtained linear fits for the pseudo-second kinetic order show much higher R 2 -values (table 3) which clearly indicates that the PMS activation in our reaction obeys the pseudosecond kinetic law.The results show the superior catalytic activity of the Ce-MOF/PMS system as it possesses ~3.4 times higher pseudo-second order rate constant compared to the PMS alone.(4) where Kobs is the reaction rate constant at certain temperature, T is the reaction temperature in Kelvin, A is the pre-exponential factor, R is the molar gas constant, and Ea is the activation energy.The values for the calculated activation energy for the PMS and the Ce-MOF/PMS systems are presented on figure 7. The 2.6 times lower activation energy of the Ce-MOF/PMS proves the superior activity of this system compared to the thermal activation of PMS alone.Furthermore, the activation energy of 16.525 kJ/mol is higher than the typical value of diffusion-controlled reactions (10 -13 kJ/mol).The later indicates that the reaction rate is dominated by the rate of intrinsic chemical reactions on the Ce-MOF surface rather than the rate of mass transfer [27].The effect of the synthesis temperature of Ce-MOF on the catalytic activity of the obtained materials was also evaluated.Figure 8 shows the results of the catalytic activity of the four Ce-MOFs obtained at different temperatures.The rate of degradation slightly increases with the temperature from 73.2% to 81.2% for Ce-MOF-110 and Ce-MOF-180.The insignificant difference between Ce-MOF-150 and Ce-MOF-180 (79.5% and 81.2% respectively) can hardly justify the usage of higher temperatures and therefore the higher price for catalyst preparation.To identify the primary reactive radicals in the Ce-MOF/PMS system for TCH removal, free radical capture experiments were conducted.For this reason, p-benzoquinone (O2 •− quencher), methanol (•OH and SO4 •− quencher), NaN3 ( 1 O2 quencher) and tert-BuOH (•OH quencher) were used as scavengers.The obtained results (figure 9) show that the primary reactive oxygen species is the superoxide anion radical.The scavengers for singlet oxygen, hydroxyl and sulphate radicals visibly lower the rate constants but these radicals do not play any major role on the degradation rate of the TCH.The crystal structure stability of the catalyst during the catalytic reactions plays a crucial role in the long-term usage of any catalyst.In order to investigate the stability of the catalyst powder XRD analysis was performed on the Ce-MOF before and after the catalytic reaction.Figure 11 shows the obtained XRD patterns where no changes on the crystal structure or the phase composition is observed.This result indicates that the prepared Ce-MOF has a very good stability and recovery after the catalytic reaction and can further be used again.

Conclusions
The Ln-MOFs (Ln = La, Ce, Gd, Tb), based on terephthalic acid were synthesized by solvothermal method and the characterization by XRD and FT-IR proved isostructural samples obtained.The detailed investigation of the crystal parameters of the samples performed can be considered as an extension of the data about the crystal structure of the Ln-MOFs.The microstructural information received by Rietveld refinement displayed the influence of the ionic radius of Ln(III) both on the unit cell parameters and the crystallites size as well as on the defective structure.The research evidenced that Ln-MOFs activate successfully peroxymonosulfate and react as catalysts in a Fenton-like reaction for degradation of antibiotic tetracycline hydrochloride.The pseudo-second kinetic order of the catalytic reaction was suggested in the reaction for activation of peroxymonosulfate for a first time, according our knowledge.The kinetic order was well justified by the value of the correlation coefficients.By their decreasing catalytic activity, the Ln-MOFs studied can be arranged as Ce-MOF>La-MOF>Gd-MOF>Tb-MOF with the most active Ce-MOF among them.

Figure. 2 .
Figure. 2. (a) XRD patterns of the obtained at different temperatures Ce-MOF and (b) selected 2θ range 27 o -33 o

Figure 7 .
Figure 7. Calculated activation energy of (a) PMS only and (b) Ce-MOF/PMS system

Figure 10 .
Figure 10.Proposed reaction mechanism for TCH oxidation in Ce-MOF/PMS system

Figure 11 .
Figure 11.XRD patterns of Ce-MOF before and after catalytic reaction

Table 1 .
Unit cell parameters, crystallite size and microstrains of Ln-MOFs

Table 2 .
Unit cell parameters, crystallite size and microstrains of Ce-MOF synthesized at different temperatures

Table 3 .
Degradation efficiency, k1 and k2 of the TCH degradation experiments