For this study, high-purity chemical compounds essential for the synthesis of Cerium Metal-Organic Framework (Ce-5AIA) and subsequent experiments were sourced. Cerium (III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O), with a purity of 99.9%, was procured from Central Drug House (P) Ltd in India. This compound serves as the primary source of cerium in our framework synthesis. 5-aminoisophthalic acid (H₂-5AIA) with 94% purity was obtained from Sigma Aldrich in China. This organic ligand plays a vital role in coordinating with the cerium ions, aiding in the formation of a stable and functional MOF structure. N, N-dimethylformamide (DMF), with a 99% purity level, was sourced from Daejung Chemicals and Metals Co., Ltd. in Korea. DMF serves as the solvent in our solvothermal synthesis process, effectively dissolving both the metal nitrate and organic ligand to ensure a uniform distribution of reactants, which is essential for optimal crystal growth during synthesis. Remazol Brilliant Violet 5R (RBV-5R), purchased from Sigma-Aldrich (Merck), has a purity of 95.0%. This compound was used as a model pollutant to assess the photoadsorptive degradation capabilities of the synthesized Cerium Metal-Organic Framework (Ce-5AIA). Its complex structure and resistance to biodegradation make it an ideal candidate for testing the effectiveness of our materials in degrading water pollutants.

A combination of 0.866 g (2 mmol) of cerium nitrate hexahydrate and 0.181 g (1 mmol) of 5-aminoisophthalic acid was dissolved in 40 ml of N, N-Dimethylformamide (DMF) and placed in a 100 ml Teflon-lined autoclave [18][19]. The autoclave was then heated in an oven at a constant temperature of 140˚C for 24 hours. After the heating period, the autoclave was allowed to cool to room temperature. The resulting product was filtered, washed three times with DMF, and then dried in the oven at 120˚C for 2 hours, yielding a marble white powder.

Several tests were conducted to characterize the synthesized Cerium Metal-Organic Framework and the organic linker. Functional groups were identified through Fourier transform infrared (FTIR) analysis with a Bruker Alpha Spectrometer. Crystallinity was assessed using an Empyrean X-ray Diffractometer. Surface morphology and elemental composition were analyzed using a ZEISS EVO MA 15 SEM-EDS. The thermal stability of the MOF was evaluated with the SDT Q600 V20.9 System in a nitrogen atmosphere.

The optical activity of the synthesized Cerium MOF was studied on Remazol Brilliant Violet 5R (RBV-5R). For each activity, a 100 mL beaker containing 50 mL of sample pollutant solution and Ce-5AIA as catalyst was stirred in the dark for 30 minutes to achieve adsorption/desorption equilibrium and then under solar light at 20,000 lux intensity for increasing time at 20 min interval to 120 min. The solar light intensity was measured using the YF-170 Digital Light Meter. A 5 mL aliquot was drawn at each stipulated time to observe the photocatalytic degradation of RBV-5R by the Cerium MOF using a JENWAY 7315 Spectrophotometer. The RBV-5R photocatalytic degradation efficiency was calculated using Equation (1).

where C_i is the initial concentration of the dye at time (t) = 0 (g/L), and C_t is the concentration at a given time (g/L).

The intermediates during the photocatalytic degradation were identified to confirm the removal of RBV-5R from water using SHIMADZU GC-MS QP 2020 with all the necessary operational specifications adopted.

Figure 1 shows the synthesized cerium-5-aminoisophthalic acid metal-organic framework (Ce-5AIA) before and after activation. A linen white precipitate (a) yielded a marble white coloured powder (b) after activation, having a percentage mass yield of 43.17%.

3.2.1. Functional Group Characterization

A comparative analysis of the FTIR spectrum of 5-aminoisophthalic acid (H₂-5AIA) and that of the cerium (Ce) metal-organic frameworks (Ce-5AIA) formed using H₂-5AIA as an organic linker is shown in Figure 2. The broad band from 3500 cm⁻¹ to 2220 cm⁻¹ for H₂-5AIA indicates the presence of the hydroxyl group

Figure 1. Images of synthesized Ce-5AIA (a) before activation, (b) after activation.

Figure 2. FTIR comparison of (a) 5-aminoisophthalic acid (H₂-5AIA) and (b) Synthesized Ce-MOF (Ce-5AIA).

(O-H) stretch typical of carboxylic acids [20]. The revealing double peaks within the broad band at 2830 cm⁻¹ and 2600 cm⁻¹ indicate the N-H stretch characteristic of primary aromatic amine. A shift of a broad band between 3650 cm⁻¹ and 2862 cm⁻¹to the left, with the absence of double peaks observed for Ce-5AIA, depicts OH vibrational stretch due to water molecules adsorbed on the surface of Ce-5AIA from their metal hydrate salts, typical of most MOFs [21][22]. The absence of a double band indicates the presence of a tertiary amine, which signifies a possible deprotonation of the primary amine to enable nitrogen (N) to form bonds with the Cerium (Ce). However, the absence of CN at around at 1285 cm⁻¹ and 1258 cm⁻¹ could be due to the overlap of the broad intense symmetric vibrational bands of COO- found around the region [20][23]. The band at 1695 cm-1 for H₂-5AIA could be attributed to the presence of a C=O stretch carboxylic acid group [20][24]. The bands observed between 1540 cm⁻¹ and 1655 cm⁻¹ indicate C=C-C stretching present in the aromatic ring of both Ce-5AIA and H₂-5AIA [23]. The bands detected at 1452 cm⁻¹ and 1395 cm⁻¹ for H₂-5AIA depict the bending of C-O-H [20] while strong absorption bands between 1542 cm⁻¹ and 1372 cm⁻¹ in the Ce-5AIA could be related to the asymmetric and symmetric vibrational modes of carboxylate (COO-) ions [22][25]. The absence of COH in the synthesized MOFs indicates the deprotonation of COOH to yield COO⁻, which can bond to the metals. The bands between 1220 cm⁻¹ and 680 cm⁻¹ found in both Ce-5AIA and H₂-5AIA are associated with the bending vibrational modes in the benzene ring. The bands from 1126 cm⁻¹ to 930 cm⁻¹ could relate to the in-plane C–H vibrational mode, while bands from 900 cm⁻¹ to 680 cm⁻¹ are out-of-plane vibrational modes [23].

The low-intensity stretching vibrational bands of Ce−O could be seen at 673 cm⁻¹ and 432 cm⁻¹ [18] while the very weak vibrational stretch of Ce-N could be observed from 547 cm⁻¹ to 490 cm⁻¹. This is similar to metal-nitrogen (M-N) bond vibrations obtained in literature [26].

3.2.2. Crystallinity Characterization

The XRD pattern of the Ce-5AIA (b) in comparison with 5-Aminoisophthalic acid (H₂-5AIA) (a) is displayed in Figure 3. The diffractogram from 10˚ to 50˚ shows a 2θ peak pattern similar to the cerium-containing amine metal-organic frameworks reported [27].

The 2θ peaks at 16.13˚, 23.28˚, 28.56˚, 33.16˚, 41.07.44.59˚, and 47.79˚ were observed for Ce-5AIA (b), which were different peak patterns from those observed for H₂-5AIA. These peaks indicate that the Ce-5AIA is a crystalline compound and different from H₂-5AIA.

3.2.3. Morphology Characterization

The SEM-EDS image of 5-aminoisophtalic acid (H₂-5AIA) and synthesized Cerium metal-organic framework (Ce-5AIA) is shown in Figure 4. The SEM image of Ce-5AIA shows a rod-like rectangular prism shape morphology similar to other reported works related to cerium MOFs [27]-[29].

The EDS spectrum of 5-aminoisophtalic acid (H₂-5AIA) revealed the presence of carbon (C), oxygen (O), and nitrogen (N) with atomic percentages of 57.31% for carbon, 31.66% for oxygen, and 11.03% for nitrogen. The EDS spectrum of

Figure 3. XRD of (a) 5-aminoisophthalic (H₂-5AIA) and (b) synthesized Ce-5AIA compound.

Figure 4. SEM-EDS of (a) 5-aminoisophthalic acid and (b) Ce-5AIA.

Ce-5AIA revealed the presence of carbon (C), cerium (Ce), and oxygen (O) with atomic percentages of 67.01% for carbon, 23.76% for oxygen, and 7.22% for cerium (Ce). Nitrogen was not identified from the EDS shown for the Ce-5AIA. The absence of nitrogen is due to an overlap of C and O k-alpha peaks, which are heavier than nitrogen and fall close to it [30]-[32]. Nitrogen (Kα at 0.392 keV) is a light element with a low atomic number (Z = 7). Its characteristic peak is located in the low-energy region of the EDS spectrum, specifically between the Kα peaks of Carbon (C, 0.277 keV) and Oxygen (O, 0.525 keV). Because EDS detectors have limited energy resolution, if the sample has high concentrations of Carbon and Oxygen, their intense peaks can overwhelm or obscure the much weaker Nitrogen peak. In many organic or oxidized materials, the Nitrogen(N) peak is hidden by the close proximity or overlap of C and O peaks, making it difficult to distinguish, especially if the nitrogen concentration is low. It is also possible that the nitrogen in the samples was absorbed by the beryllium window that separated the sample chamber from the analyzer during the EDS analysis [33].

3.2.4. Thermal Stability Characterization

The thermogram for the decomposition of Ce-5AIA is shown in Figure 5. Three major weight losses were observed from 50˚C to 800˚C. The first and second weight loss percentages of 6.88% and 13.74%, with corresponding exothermic rates of change of weight losses of 0.12%/min and 0.09%/min observed between 50˚C and 350˚C indicates the removal of the absorbed water molecules and organic solvents present in the Ce-5AIA [34]. A large weight loss percentage of 42.69% with a corresponding broad exothermic rate of change of weight loss peak at 0.13%/min observed between 350˚C and 750˚C indicates the decomposition of the Ce-5AIA [35].

Figure 5. TGA-DSC of synthesized Ce-5AIA.

The decomposition could be due to the elimination of ligand molecules (-5AIA) as reported for other Ce MOF [19]. A slight weight loss from 750˚C to 800˚C confirms the formation of cerium oxide after the decomposition of the Ce-5AIA relative to literature [19][36]. The decomposition of Ce-5AIA around 650˚C indicates Ce-5AIA can withstand excessive heat and will only disintegrate above 600˚C.

The photocatalytic degradation ability of Ce-5AIA on Remazol Brilliant Violet 5R (RBV-5R) is shown in Figure 6. Over 90% RBV-5R removal by Ce-5AIA was observed under solar irradiation after 120 min. Unappreciable degradation of RBV-5R was observed for both activity in the dark, with Ce-5AIA as a control experiment, and in the solar, without Ce-5AIA. A little over 5% result was observed throughout the experiment in the dark. This percentage is largely due to the adsorption/desorption activity of RBV-5R onto the surfaces of Ce-5AIA. A slightly lower percentage, marginally higher than 1%, was observed for the experiment under solar conditions without Ce-5AIA.

Figure 6. Photocatalytic activity of Ce-5AIA (Optimum parameters: [RBV-5R] = 0.30 g/L; Ce-5AIA Loading = 0.15 g/L; Solution pH = 6.2).

The UV-visible absorption spectrum of RBV-5R confirms a gradual reduction of peaks at 20 min time intervals over the 120 min, indicating RBV-5R's gradual removal from the sample solution.

Figure 7 indicates the variation of Remazol Brilliant Violet 5R (RBV-5R) concentration with constant Ce-5AIA load to observe an optimum concentration for the photocatalytic degradation activity. An increase in photocatalytic degradation activity was observed with decreasing concentration of the RBV-5R.

Figure 7. Effect of RBV-5R concentration variation on Ce-5AIA (Catalyst loading = 0.20 g/L; Solution pH = 6.2).

This decrease in photocatalytic degradation activity with an increase in concentration may be a result of the insufficient generation of photocatalytic radicals, such as ∙OH, due to excess RBV-5R in solution, which may cover the active sites on MOFs, limiting the surface generation of ∙OH radicals during photolysis. High RBV-5R concentration may also prevent a sufficient amount of light radiation from reaching the surfaces of the Ce MOF by absorbing a substantial amount of the radiation [10][37]. This may reduce the photo-oxidation process. A concentration of 0.10 g/L RBV-5R was chosen as the optimum concentration for the photoadsorptive degradation, with over 80% degradation of RBV-5R reached after 120 minutes.

Figure 8 shows the variation in the amount of Ce-5AIA with constant RBV-5R concentration to investigate their effect on the removal of RBV-5R. Lower RBV-5R removal was generally observed when the RBV-5R amount was decreased from 0.2 g/L to 0.05 g/L. The high pollutant removal efficiency for higher RBV-5R amount may be due to an increase in available active sites to enhance the removal of RBV-5R [11][38].

Figure 8. Effect of Ce-5AIA amount variation on the removal of Remazol Brilliant Violet 5R (RBV-5R) ([RBV-5R] = 0.10 g/L; Solution pH = 6.2).

Slow removal of the pollutants was observed when the amount of the Ce-5AIA was, however, observed between 100 min and 120 min for 0.20 g/L Ce-5AIA. The slow removal may be a result of the turbidity generated due to excess suspended MOF particles, which may limit the penetration of light radiation. An amount of the surface area of the Ce-5AIA may also be lost as a result of agglomeration [34][35]. The 0.20 g/L Ce-5AIA amount was chosen as the optimum catalyst loading for the photocatalytic degradation of RBV-5R since over 90% of degradation was observed at 120 minutes.

Figure 9 shows the photocatalytic degradation rate of RBV-5R solution with and without scroungers such as isopropanol (∙OH scrounger), potassium iodide (h⁺ scrounger), p-benzoquinone (O₂∙ scrounger), and silver nitrate (e⁻ scrounger).

The addition of isopropanol (∙OH scrounger) showed a significant decrease in percentage degradation of 21.98% compared with potassium iodide (h⁺ scrounger), p-benzoquinone (O₂^∙ scrounger), and silver nitrate (e⁻ scrounger), which showed very high percentage degradation of RBV-5R. The observed results reveal that the hydroxyl radical (∙OH) is the primary operating radical during the photocatalytic degradation of RBV-5R.

Figure 10 shows a repetition of the photocatalytic degradation test for four series by reusing Ce-5AIA for the repeated test. The Ce-5AIA was relatively stable with

Figure 9. Photocatalytic degradation of Remazol Brilliant 5R (RBV-5R) with and without various scavengers under solar at120 min. ([RBV-5R] = 0.10 g/L, Ce-5AIA loading = 0.20 g/L; Solution pH = 6.2).

Figure 10.Reuse of Ce-5AIA for Repeated Photoadsorptive Degradation Test.

no considerable loss of activity for the repeated photocatalytic adsorptive activity on RBV-5R. This stability indicates its potential application for wastewater treatment.

The X-ray diffraction (XRD) result of Ce-5AIA before and after the cycle activity is shown in Figure 11. Similar diffraction peaks were observed after the cycle test, confirming the stability of RBV-5R.

Intermediates of RBV-5R during the photocatalytic degradation were monitored to understand the degradation profile of the process using the SHIMADZU GC-MS QP 2020. These Remazol Brilliant 5R (RBV-5R) with a molar mass of 734.58 g/mol were degraded into fragments shown in Figure 12. From Figure 12, higher and lower fragment intermediates were observed with strong peaks.

Intermediates observed are summarized in Table 1.

Figure 11. XRD of Ce-5AIA before (a) and after (b) Cycle photocatalytic degradation test.

Figure 12. Mass spectroscopy of Pollutant after 120 min degradation by Ce-5AIA.

Higher intermediate fragments observed include Tetracosane (m/z 338), 4-Amino-5-hydroxy-2, 7-naphthalenedisulfonic (m/z 323), N-Phenyl-1-naphthylamine structure (m/z 219), Tryptophan (m/z 205), Diphenylamine 3-(m-nitrophenyl) sydnone (m/z 177), Phthalic acid (m/z 163), Ethyl benzoate (m/z 149), and Tyramine (m/z 133). Lower fragment intermediates observed include nitrobenzene (m/z119), 2-Heptylamine (m/z 115), Oxalic acid (m/z 91), Aniline (m/z

Table 1. Summary of Intermediates after 120 min Photocatalytic degradation of RBV-5R.

91), Phenyl ion (m/z77), and Acetic acid (m/z 57).

Based on the GC-MS analysis, a proposed degradation pathway diagram linking the identified intermediates could be achieved. Figure 13 shows the possible degradation pathway. Under high pressure or thermal conditions, Remazol Brilliant 5R (RBV-5R) fragments into larger species such as 4-Amino-5-hydroxy-2,7-naphthalenedisulfonic (m/z 323), N-Phenyl-1-naphthylamine (m/z 219), Tryptophan (m/z 205), Diphenylamine 3-(m-nitrophenyl) sydnone (m/z 177), Phthalic acid (m/z 163), Ethyl benzoate (m/z 149), and Tyramine (m/z 133). From the high molecular weight intermediates, various degradation pathways lead to lower molecular weight fragments. 4-Amino-5-hydroxy-2,7-naphthalenedisulfonic (m/z 323) and N-Phenyl-1-naphthylamine (m/z 219) degrades to produce 2-Heptylamine (m/z 115). N-Phenyl-1-naphthylamine (m/z 219) and Tryptophan (m/z 205) degrade to produce Nitrobenzene (m/z 119). Nitrobenzene (m/z 119) degrades further to produce Phenyl ion (m/z 77), which degrades to yield Acetic acid (m/z 57). Diphenylamine 3-(m-nitrophenyl) sydnone (m/z 177) degrades to produce Analine (m/z 91), which degrades further into Phenyl ion (m/z 77), which produces a lower fragment, Acetic acid (m/z 57). Phthalic acid (m/z 163), Ethyl benzoate (m/z 149), and Tyramine (m/z 133) degrade to produce Oxalic

Figure 13. Proposed degradation mechanism for Remazol Brilliant 5R (RBV-5R).

acid (m/z 91), which further degrades to a lower fragment, Acetic acid (m/z 57).