Efficient photocatalytic degradation of petroleum oil spills in seawater using a metal-organic framework (MOF)

Photocatalysis is a green approach that has appeared to be a viable option for the degradation of a variety of organic contaminants. This work outlines the process of preparing the titanium-based metal-organic framework (MIL-125) photocatalysts using a simple solvothermal method. Structural, morphological, and optical analysis of samples (MT18 and MT48) was carried out by XRD, FT-IR, Raman, SEM, TGA, BET, and UV–Vis. Results indicated that the sample prepared at 150 °C and reaction time of 48 h (MT48) has a low crystal size of 7 nm with an optical band gap of 3.2 eV and a surface area of 301 m2 g−1. Under UV–visible light irradiation, the as-prepared MOFs proved to upgrade photocatalytic activity in degrading crude oil spills in saltwater. Effects of catalyst dosage and exposure time on the degradation of an oil spill in seawater were studied and analyzed using UV–Vis spectrophotometry and gas chromatography (GC–MS) which emphasized that the use of 250 ppm of MT48 photocatalyst under UV–Vis irradiation can degrade about 99% of oil spills in water after 2 h of exposure. The study's data revealed that MIL-125 could be used to photocatalyzed the cleanup of crude oil spills.

bandgap, the MIL-125 can be stimulated by light to generate photo-generated electrons and holes 14,15 . Electrons are stimulated from the highest occupied molecular orbital (HOMO) of the organic linker to its lowest unoccupied molecular orbital (LUMO). The photoexcited electrons are then passed to the MOF nodes via a linkercluster charge transfer mechanism (LCCT), which activates the metal nodes and allows them to participate in the photooxidation reduction reaction 16,17 . As a result, boosting photocatalytic activity requires improvements in the quantity of generated electron-hole pairs, separation efficiency, and carrier utilization efficiency.
Direct synthesis of MIL-125 using traditional hydrothermal/solvothermal procedures is still the most widely utilized approach 18 . Solvents have a vital role in the crystallinity of finished goods, and the ideal solvent combination is often discovered by trial-and-error. Another important aspect that influences the shape of MOFs formed is the concentration of reactants 19 . Herein, we present a simple and efficient scheme to fabricate catalytic MOF by a solvothermal process. Furthermore, this study presents a novel and highly efficient photocatalytic degradation of crude oil spill using MIL-125.

Materials and methods
Materials. Terephthalic acid (H 2 BDC), HPLC methanol (MeOH), and anhydrous di-methyl formamide (DMF) were purchased from Fischer Scientific, UK. Titanium isopropoxide (TTIP) was produced in Sigma Aldrich, USA. Saline Water (TDS = 27,800 ppm) was taken directly from the Mediterranean Sea, Alexandria, and crude oil from the western desert, Egypt with the main characteristics shown in Table S1.

Methods. Preparation of MIL-125.
The preparation method is shown in Fig. 1; in which 6.0 mmol of terephthalic acid (H 2 BDC) was first dissolved into a mixed solvent of methanol (MeOH) and anhydrous dimethyl formamide (DMF) with a ratio 1:9. The solution was then treated with 1.9 mmol of titanium isopropoxide (TTIP) Ti(OCH(CH 3 ) 2 ) 4 for 1 h while stirring. The mixture was then sonicated for 25 min and transferred into a stainless-steel autoclave system (volume: 150 mL), reacting at 150 °C for different reaction times (18,48) h 9,20-22 . To eliminate other unreacted organic species in the mixture, the yellow solid result was washed twice with a mixed solvent of (DMF and methanol). Afterward, the solid product was isolated by centrifugation. Finally, the white powder was acquired after vacuum drying it overnight in an oven at 80 °C 23 . The prepared MIL-125 samples were coded (MT18, MT48).
Characterization of the prepared MOFs. Fourier transform-infrared (FT-IR). The functional groups of MIL-125 samples were investigated using a Fourier transform-IR spectrophotometer (FTIR-8400S, Shimadzu) with a resolution of 2 cm −1 . The powder samples were tested in the (4000-350) cm −1 wavenumber range.
Scanning electron microscope (SEM). Scanning electron microscopy (SEM) (JEOL JSM 6360LA, Japan) was used to investigate the microstructure and surface morphology of all prepared samples (MT18, MT48). Before SEM imaging, the powders were sputter-coated with gold using a sputter-coating unit.
Raman analysis. A SENTERRA Raman Microscope with laser selectors (532, 633, and 785) nm was used to collect Raman spectra; the wavelength and power of the laser employed as an excitation source were 514.5 nm and 20 mW, respectively. The incident light was focused on the prepared samples and the radiation scattered by the sample was collected using a 50 objective (back-scattered configuration). A single spectrum required 360 s to acquire.
x-ray diffraction (XRD). X-ray diffraction scans of samples were obtained using (X-ray 7000 Shimadzu-Japan) at room temperature. The degree of crystallinity was determined by applying Bragg's low where 2θ is in the range of 5°-80°, with the X-ray source being a Cu target produced at 30 kV and 30 mA and a scan speed of 4° per minute. The crystallite size is determined from the broadening of corresponding X-ray spectral peaks by the Scherrer formula 24 : www.nature.com/scientificreports/ where D is the crystallite size (dimension), λ is the wavelength of the X-ray radiation (Cu Kaα = 0.15418 nm), K is a constant (shape factor) usually taken as 0.89, θ is the angle between the incident and reflected rays and β is the line width at half-maximum height.
Thermogravimetric analysis (TGA). Thermogravimetric analysis is a sort of testing that is done on samples (5 mg) to assess weight loss variations as a result of temperature changes (using TGA-50 Shimadzu). This procedure was carried out at temperatures ranging from 50 to 800 °C, with a heating rate of 10 °C min −1 and a nitrogen flow rate of 20 ml min −1 . This technique is used to test the prepared MOFs' thermal stability.
Surface area Brunauer, Emmett and Teller (BET). The physical adsorption of an N 2 gas on the surface of the solid and the amount of adsorbate gas corresponding to a monomolecular layer on the surface are used to calculate the specific surface area of the prepared MOFs. Physical adsorption is caused by relatively modest interactions (van der Waals forces) between adsorbate gas molecules and the test samples' adsorbent surface area. A volumetric or continuous flow approach (Quantachrome Corporation Nova 1000, version 6.11 high speed, gas sorption analyzer) at 77 K can be used to determine the amount of gas adsorbed. Before the measurements, the samples were activated at 120 °C for 6 h 21,25 .
Optical properties. The samples' optical characteristics were recorded to figure out where in the electromagnetic spectrum they absorb light. The UV-Vis spectrum of prepared MIL-125 was recorded using a UV-Vis spectrophotometer (T60 UV-visible) at wavelength scan ranges from 270 to 650 nm.
Photocatalytic degradation test. The prepared MOFs catalysts (MT18, MT48) were tested for the photocatalytic degradation of oil spills using mixed light sources (UV, and visible), and the experimental conditions are shown in Table S2. Each experiment was designed by adding (40 µL) crude oil to (100 mL) seawater under continuous stirring 26 . Firstly, the solution was kept in the dark for 30 min under continuous stirring then irradiation with UV-Vis light (a tungsten lamp of 100-W intensity as a source of visible light and four UV lamps of 2-W each as a source of UV light) was performed and the samples were taken each 30 min and then centrifuged to remove the catalysts. Samples were mixed with an equal volume of toluene with vigorous stirring for 30 min. Leave samples in a separating funnel then measure the remaining hydrocarbons in an aqueous layer by using UV-spectrophotometer at λ = 420 nm and calculate removal efficiency (RE%) using the following equation 27,28 where TH Control is the oil concentration of the control and TH Treated is the oil concentration of the degraded sample.
All the photodegradation experiments were carried out in duplicate with the prepared photocatalyst. Gas Chromatography/Mass Spectrometry (GC/MS Shimazu QP2010 ultra) can help to make conclusive identification of compounds. These were the GC/MS operating conditions; the oven initial temperature was 80 °C for 0 min, raged at 10 °C min −1 to 240 °C, and then held for 2 min. The temperature was then ramped to 280 °C at 5 °C min −1 , and then held at 280 °C for 10 min. The injection temperature and volume were 250 °C and 1 µL, respectively. The split ratio was 20:1. The carrier gas used was He while the solvent delay time was 2.50 min. The transfer and source temperatures were 250 °C and 150 °C, respectively. Scanning was done from 50 to 500 Da and the column dimension was 27.0 m × 250 µm.

Results and discussion
The reaction of titanium isopropoxide as a metal precursor with terephthalic acid as an organic ligand in a proper percentage of solvent mixtures (N,N-dimethylformamide (DMF), and methanol) led to well-crystallized white powder of MIL-125. MIL-125 is composed of basic units of Ti 8 6 and is built up from cyclic octamers constructed from corner or edge-sharing octahedral titanium units, as shown in Fig. S1. These octamers are connected to 12 other cyclic octamers through H 2 BDC linkers, leading to a porous threedimensional quasi-cubic tetragonal structure having two types of cages, an octahedral (12.5 Å) and a tetrahedral (6 Å) cage, accessible through narrow triangular windows of ca. 6 Å 25 .
IR and Raman spectra in Fig. 2A,B were used to analyze the structure and morphology of the prepared samples (MT18, MT48). All IR spectra show characteristic vibrational bands in the region of (1400-1700) cm −1 for the carboxylic acid functional group of the Ti-coordinated MOF structure. Two absorption bands around (1640, and 1500) cm −1 can be assigned to carbonyl asymmetric stretching vibrations, whereas the bands at (1571, and 1628) cm −1 could be ascribed to the phenyl C=C ring stretching and the C=O stretching 27 . The band at 1260 cm −1 belongs to the C-H symmetric stretching vibrations of the benzene ring, which is assumed by the Raman band at 1611 cm −120 . The IR region of (400-800) cm − 12 octamers linked by terephthalic linkers to an edge-and-corner-sharing TiO 5 (OH) octahedron. The units are arranged in a centered cubic (cc) fashion, packing into the quasi-cubic tetragonal lattice, and each octamer has 12 units neighbors. The 3D structure has two types of cages corresponding to the octahedral and tetrahedral vacancies of cc packing, with effectively accessible diameters of 12.55 Å and 6.13 Å, respectively, and connected by triangular windows of 5-7 Å 18 . Because the photocatalytic activity is dependent on the active sites which are also influenced by the crystallite size, which is calculated using the Debye-Scherrer equation. It is found that the crystallite size of MT48 is 7 nm which is a little smaller than for MT18 as shown in Table 1.
The nitrogen adsorption-desorption isotherms, pore diameter, and pore volume of MIL-125 catalysts are shown in Fig. 3. Table 2 shows the pore size distribution displays pores with a diameter between (4.6 and 9.2) nm, for MT48 and MT18 respectively, which correspond to the microporous and mesoporous contributions, respectively, confirming the multimodal distribution. The MT48 catalyst's BET-specific surface area and total pore volume were 301 m 2 g −1 and 0.2444 cm 3 g −1 respectively, showing that it is a highly porous material. More active sites would be exposed if mesopores with a bigger surface area were formed, which would boost photocatalytic activity.
SEM images Fig. 4A,B reveal that both (MT18, MT48) samples are made up of particles with typical circular plate-like morphology 9 with particle sizes (258 ± 89, 234 ± 65) nm, respectively. The thermogravimetric analysis (TGA), in Fig. 4C, indicates a steady mass loss of the prepared MOFs before 800 °C. The first weight loss  Optical characterization. The optical properties of prepared MOFs (MT18, MT48) were investigated using the UV-VIS spectrum which is shown in Fig. 5A. Due to the octahedral coordination in titanium species, the two prepared samples (MT18, MT48) possess a broad absorption edge at wave length (310 nm), MT18, MT48 absorb only at wavelength below 310 nm, inducing excitation from HOMO, made of π states in the ligand and O(2p) in the metal-oxo cluster, into the LUMO defined by Ti(3d), leaving behind holes near the Ti-oxo cluster 9,34,35 . The optical bandgap (Eg) demonstrates a relationship between the absorption coefficient and the incident photon energy and can be calculated by using the following Tauc's equation 35,36 .  www.nature.com/scientificreports/ where α is the coefficient of the absorption, hυ is the incident photon energy, A is a constant, Eg represents the bandgap, and n is determined based on the type of optical transition of prepared MOFs. These materials have an indirect optical transition, thus n = 2. The optical energy gap (Eg) can be deduced from plotting the relationship between (αhʋ) 1/2 and photon energy (hʋ) for MT18 and MT48 as shown in Fig. 5B.  Photocatalytic degradation of crude oil spills. Figure 6 shows the photo-degradation study of (crude oil/saline water) samples by using different MOF catalysts and it clearly shows that the disappearance of oil spills after photocatalytic reaction using MT18 and MT48. The concentration of crude oil before and after photodegradation can be determined using UV-Vis spectrophotometry and GC/MS technique. Figure 5 shows the UV-Vis spectrophotometry of degraded crude oil with respect to using the light as controlled value (absorbance = 3.344). Generally, the two prepared MIL-125 samples display a high efficiency toward the degradation of crude oil. Figure 7A shows that the use of different doses of MT18 as photocatalyst (125, 250, and 500) ppm in the degradation of crude oil spill revealed high degradation efficiencies reached 98.4% after 2 h of irradiation. On the other hand, Fig. 7B shows that using 250 ppm of MT48 as photocatalyst can degrade about 99% of crude oil spills after 2 h of irradiation. The MT48 has little more degradation efficiency under (UV-Vis light) this is due to the lower optical band gap (3.2 eV) and higher surface area (301.1 m 2 g −1 ) 37 . The trend observed in the UV-Vis spectrophotometry analysis is agreed with the optical properties and morphological characterization results of prepared photocatalyst. The GC/MS chromatograms of the analyzing wastewater samples before and after photodegradation are presented in Fig. 8. This analysis shows that water contaminated with crude oil spills (Control) contains different compounds such as Pentane-2-methyl, Tetrahydrofuran (THF), Cyclohexane, 6-Bromo-4-(2-choro-phenyl)-2piperan, Benzene, (1-butylhexadecl), Benzene, (1-methylnonyl), Benzene, (1-buylheptyl), Benzene, (1-propyloctyl), Octadecane and Benzene, (1-pentylheptyl). The obtained chromatograms after photocatalytic degradation using MT18 and MT48 cleared that all mentioned compounds are degraded with high efficiencies and their concentrations become nil 38 . Kinetics of the reaction rate. Under various experimental conditions, our findings indicated that MIL-125 could be used to degrade marine oil spills. The photoreaction process can be divided into two stages based on the kinetic reaction curves in Fig. 9. First, the concentration decreased rapidly at the start of the irradiation,  www.nature.com/scientificreports/ and the photoreaction confirmed a first-order kinetic pattern. Secondly, the concentration was nearly stabilized.
According to the first-order kinetic reaction: where C o is the initial reaction concentration of oil; C t is the concentration of oil measured at a fixed time; t is irradiation time, and K is is the reaction rate determined by the linear plot of In (C o /C t ) versus the time. Table 3 summarizes the kinetic data from this study's photo-degradation of oil under various experimental conditions. According to our results, photochemical degradation is an effective method for removing petroleum hydrocarbons from seawater.

Conclusions
From this research, it is possible to conclude that, efficient MIL-125 photocatalysts have been successfully prepared via a facile solvothermal method. The sample prepared at 150 °C and a reaction time of 48 h (MT48) has a low crystal size of 7 nm and a particle size of 234 nm with an optical bandgap of 3.2 eV and a surface area of 301 m 2 g −1 . Based on GC/MS and UV/Vis photodegradation results, the two prepared samples show high efficiency toward the degradation of crude oil. And the use of 250 ppm of MT48 photocatalyst under UV-Vis irradiation can degrade about 99% of oil spills in water. This study presents a novel and highly efficient photocatalytic degradation of crude oil spills using MIL-125.