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Article

Optimizing the Microscopic Structure of MIL-68(Al) by Co-Doping for Pollutant Removal and Mechanism

by
Wenju Peng
1,2,
Wenjie Yang
1,
Meng Wang
1,
Lin Zhang
1,
Xianxiang Liu
3,* and
Yaoyao Zhang
1,*
1
School of Chemistry and Materials Science, Hubei Engineering University, Xiaogan 432000, China
2
School of Civil Engineering, Hubei Engineering University, Xiaogan 432000, China
3
National & Local Joint Engineering Laboratory for New Petro-Chemical Materials and Fine Utilization of Resources, Hunan Normal University, Changsha 410081, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(9), 900; https://doi.org/10.3390/catal15090900
Submission received: 14 August 2025 / Revised: 14 September 2025 / Accepted: 15 September 2025 / Published: 17 September 2025
(This article belongs to the Special Issue TiO2 Photocatalysts: Design, Optimization and Application)
Browse Figures
Versions Notes

Abstract

Four different MIL-68(Al) catalysts were synthesized and characterized by XPS, SEM, TEM, XRD, DLS, Nitrogen adsorption removal, and other methods. An aluminum-based MOF (Metal Organic Framework) (MIL-68(Al))/graphite oxide (GO) composite with TiO2 showed the largest BET specific area with best adsorption performance. Representation demonstrated that MIL-68(Al) and TiO2 nanoparticles are uniformly dispersed on the surface of the GO lamellar, and a tight heterojunction structure is formed between them. The MIL-68(Al)/GO/TiO2 exhibits good pore characteristics, structural morphology, and catalytic performance. Adsorption experiments of methyl orange can reach 99.7% with the effect of MIL-68(Al)/GO/TiO2 in water for 20 min. Moreover, the pH range can be applied to 1–13 and a high concentration of 200 mg/L methyl orange solution also worked well. In addition, this kind of catalyst can also be used for rhodamine B, methylene blue, congo red, and tetracycline in 20 min with good adsorption. Meanwhile, simple filtration can quickly recover MIL-68(Al)/GO/TiO2 and effectively reuse it. Free radical capture experiments showed a large number of •OH radicals during the adsorption of MO (Methyl Orange) solution by MIL-68(Al)/GO/TiO2. Meanwhile, the electrostatic interaction, π-π packing and hydrogen bonding make MIL-68(Al)/GO/TiO2 have a higher adsorption capacity for MO. Therefore, co-doping optimized the structure of MIL-68(Al), enhancing its stability in strong acids and bases while improving adsorption performance across a broader pH range than previously reported. This work addresses the instability of MIL-68(Al) under extreme conditions, demonstrating its significant potential for wastewater treatment applications.

1. Introduction

With the progress of society, textile, paper, plastics, rubber, medicine, printing and dye manufacturing, and other industries have developed rapidly. Still, the ensuing environmental pollution problems are becoming increasingly severe [1,2,3,4]. Organic wastewater containing organic dyes and antibiotics produced by the industries impacts the survival and reproduction of microorganisms and fish, thus endangering human life and health. Therefore, addressing the problem of industrial wastewater pollution has been a significant challenge [5,6,7,8]. Currently, the commonly used methods include precipitation or flocculation, membrane separation, chemical oxidation, biological treatment, photocatalytic adsorption, solid phase adsorption, etc. [9,10,11]. Among them, solid phase adsorption, as a convenient method, has been widely studied and can effectively remove pollutants in water [12,13].
MOFs (Metal Organic Frameworks) are a new type of solid-phase adsorbent material with an infinite structure. They comprise metal ions and organic ligands connected by coordination bonds [12,13]. Many metal organic frame materials have many advantages, such as high specific surface area, regular pores, adjustable pore size and structure, and variable structure. They can be combined or doped with other materials [14,15,16,17,18,19]. MOF materials show potential application prospects in gas storage [20,21], gas separation [22,23], water purification [24,25,26], catalytic adsorption [27,28,29,30,31], etc. [32,33]. However, due to their fragile structure, they are easily affected by environmental factors such as temperature, humidity, and pressure, resulting in performance adsorption or even failure in the application process. MIL (Material Institute Lavoisier) is a kind of MOF material containing a Lavoisier skeleton structure, which is composed of trivalent metal cations (Al3+, Fe3+, Cr3+, In3+, etc.) and carboxylic acid groups [32,33,34,35,36,37], with large specific surface area and good skeleton stability structure. Especially in underwater conditions, the stability is still excellent. MIL-68(Al) shows good stability under aqueous and weakly alkaline conditions and is suitable for treating pollutants in aqueous solutions. However, it is sensitive to certain chemicals, which limits its range of applications. Therefore, in practical applications, appropriate measures need to be taken to improve the application range of MIL-68(Al) materials. Among them, the preparation of composite materials combining MIL-68(Al) with other functional materials is an effective means of improving its separation and adsorption technology [38,39]. In addition, the adsorption rate of dyes is affected by the initial concentration of dyes, the pH value of the solution, and the catalytic dose or loading active site. MIL-68(Al) has an unstable structure in strong acids and strong bases. If left in water for a long time, this will cause the structure to collapse. The preparation of new doped MIL-68(Al) is expected to improve the adsorption rate of materials and also enhance the structural stability of the materials.
Graphene oxide (GO) is rich in oxygen-containing functional groups (carboxyl, hydroxyl, epoxide), and its unique layered structure and hydrophilic functional groups make it well dispersed in water [40,41,42]. GO also has many benzene rings or conjugated benzene rings that are not oxidized, and these groups are hydrophobic. By studying the properties of GO at the interface of each phase, scientists have proved that GO is amphiphilic. Therefore, GO can be considered a surfactant and can reduce the energy between the two-phase interfaces [43,44,45]. Using this characteristic of GO is expected to solve the problem of difficult mass transfer between solid catalysts and water phase adsorption. It has been reported in the literature that crystals of MOF-based materials can grow on the GO layer, thus generating new channels at the interface between the frame and GO and enhancing the adsorption capacity of the material [46]. TiO2 is an ideal semiconductor photocatalyst material; however, it is easy to cake, reducing its specific surface area, which limits its practical application in catalysis [47,48,49,50]. Therefore, doping can make TiO2 produce some surface defects, slow down the recombination of electron-hole pairs, and make TiO2 produce more active free radicals. Moreover, doping also prevents TiO2 from caking, thus improving catalytic activity [51]. Therefore, we use the surfactant properties of graphene oxide to prepare doped MIL-68(Al) composites together with TiO2, hoping to solve the problem of the narrow application range of MIL-68(Al). The heterojunction structure has many advantages; inspired by this, GO and TiO2 heterojunction composites can be prepared to produce materials with excellent properties [52,53].
Herein, to address the issue of environmental pollution, an aluminum-based MOF (MIL-68(Al))/graphite oxide (GO) composite with TiO2 was synthesized using the hydrothermal reaction method. Four different MIL-68(Al) catalysts were characterized, and it was demonstrated that the MIL-68(Al)/GO/TiO2 exhibits good pore characteristics with structural morphology. Furthermore, all MIL-68(Al)-based catalysts were applied to the adsorption of MO, and the catalytic performance, kinetic reaction performance, pH range, and recovery performance were investigated. MIL-68(Al)/GO/TiO2 was also used for rhodamine B (RB), methylene blue (MB), congo red (CR), and tetracycline (TC) adsorption. Meanwhile, simple filtration can quickly recover MIL-68(Al)/GO/TiO2 and effectively reuse it. Additionally, the mechanism of the catalyst system was explored through free radical experiments. This study provides a new idea for developing efficient and stable catalytic materials. It can be applied in harsh environments with strong acids and strong bases, thus providing a new idea for the treatment of pollutants.

2. Results

2.1. FT-IR Spectrum of MIL-68(Al)-Based Materials

As can be seen from Figure 1, the FT-IR spectra of MIL-68(Al) (a), MIL-68(Al)/GO (b), MIL-68(Al)/TiO2 (c), and MIL-68(Al)/GO/TiO2 (d) all showed similar characteristic peaks. Among them, the characteristic peaks at 1608 cm−1 belong to the -O-C=O- group, and 1091 cm−1 could be associated with the C-O-C group [54]. For the MIL-68(Al) (a) and MIL-68(Al)/GO (b) powder, the bands at 994 cm−1 were attributed to O-H from μ2-hydroxo groups of AlO4(OH)2 in MIL-68(Al) [55]. After doping with TiO2 in MIL-68(Al)/TiO2 (c), MIL-68(Al)/GO/TiO2 (d), the characteristic peak of 994 cm−1 becomes weaker. It must be emphasized that other characteristic peaks and peak intensities remain unchanged, indicating no effect on MIL-68(Al) with the addition of GO and TiO2. The skeleton structure of MIL-68(Al) remains intact.

2.2. XRD of MIL-68(Al)-Based Materials

XRD pattern of MIL-68(Al) (a), MIL-68(Al)/GO (b), MIL-68(Al)/TiO2 (c), and MIL-68(Al)/GO/TiO2 (d) were examined. MIL-68(Al) appears at 2θ 9.19°, 10.10°, 15.42°, 18.32°, 21.06°, 25.46°, 27.75°, and 32.61°, respectively [55,56]. It could be seen from the results that TiO2 peaks appear at 2θ 25.56°, 37.94°, 47.98°, 53.61°, 55.13°, 62.74°, 68.67°, 70.34° and 74.91°, respectively.
The diffraction peaks correspond to (101), (112), (200), (105), (211), (204), (116), (220), and (215) crystal faces of anatase (Figure 2c,d). As for GO, a separate diffraction peak (002) at 10.50° was reported in the literature [46]. Meanwhile, the peak of 10.50° is covered by the characteristic peak of 10.10° in MIL-68(Al), which is the high dispersion of GO in DMF upon sonication. As a result, the four different MIL-68(Al)-based catalytic materials showed similar characteristic peaks of MIL-68(Al) crystal faces (Figure 2a–d). This phenomenon proves that the addition of GO and TiO2 does not affect the basic skeleton structure of the MIL-68(Al).

2.3. SEM and SEM Mapping of the MIL-68(Al)-Based Materials

We investigated the effect of the GO and TiO2 on the morphology of MIL-68(Al). Figure 3a–d display the SEM images and photographs of the samples. As Figure 3a shows, MIL-68(Al) had a well-defined rendering cube structure. The addition of TiO2 does not affect the three-dimensional morphology. What is more, due to the tiny particles of powdered TiO2 to the formation of templates, the morphology of MIL-68(Al)/TiO2 is more regular (Figure 3b). After the addition of GO, MIL-68(Al) crystals were vertically embedded on the surface of GO layers with a good lamellar structure based on GO [57]. Therefore, MIL-68(Al)/GO (Figure 3c) and MIL-68(Al)/GO/TiO2 (Figure 3d) both show the three-dimensional morphology of the lamellar structure. To further prove this, GO and TiO2 were evenly embedded into the MIL-68(Al) structure and the MIL-68(Al)/GO/TiO2 was characterized by SEM mapping. As Figure 3 shows, C, O, Al, and Ti were evenly distributed in the MIL-68(Al)/GO/TiO2 by scanning the element content in the lamellar structure.

2.4. TEM of the MIL-68(Al)/GO/TiO2

As shown in Figure 4, TEM and HRTEM were further used to analyze the morphological features of MIL-68(Al)-based catalysts. As with SEM, TEM images also exhibit MIL-68(Al)/TiO2 and MIL-68(Al)/GO/TiO2, which have regular and uniform morphology. In the HRTEM characterization of Figure 4d, only one fringe structure with a (100) crystal plane was observed, which belongs to the lattice fringe of the MIL-68(Al) structure.
In Figure 4e of MIL-68(Al)/TiO2, the characteristic crystal faces attributed to TiO2 were (101) crystal faces and (002) crystal faces, respectively. In addition, MIL-68(Al)/GO/TiO2 also showed crystal faces of both MIL-68(Al) and TiO2. No crystal face of GO was found in all of the MIL-68(Al)-based catalysts because GO does not exist in a crystalline form.

2.5. XPS of the MIL-68(Al)/GO/TiO2

To further prove that GO and TiO2 were successfully doped into the MIL-68(Al) structure, the MIL-68(Al)/GO/TiO2 was characterized by XPS (Figure 5). Figure 5a shows the vast region of all elements for Al, C, Ti, and O in MIL-68(Al)/GO/TiO2. Figure 5b exhibits C1s split peaks of MIL-68(Al)/GO/TiO2 resolved into three types of carbon assigned to C–C (284.6 eV), C–O (286.6 eV) and O-C=O (288.8 eV) [58]. Figure 5c shows that the binding energy of Al in MIL-68(Al)/GO/TiO2 material was around 74.5 eV, and the high-resolution XPS spectra of Al 2p were perfectly fitted; therefore, Al in the sample exists in the form of Al3+. In the Ti 2p split peaks of MIL-68(Al)/GO/TiO2, Ti 2p3/2 appeared at ~458.5 eV and Ti 2p1/2 assigned to ~464.1 eV (Figure 5d), indicating that Ti exists in the form of TiO2 [58].

2.6. Particle Size Distribution and Zeta Potential of Materials

DLS was used to study the hydrophilicity of the material in an aqueous solution and to determine the Dh in water at a concentration of 0.5 mg mL−1 at room temperature (Figure 6a). As a result, all of the three kinds of MIL-68(Al)-based catalysts gave monomodal symmetrical distributions with a low polydispersity index (PDI) for the hydrodynamic diameter analysis. MIL-68(Al)/GO gave a PDI of 0.224; the Dh was the minimum at 219.9 nm. This phenomenon indicated that hydrophilic functional groups such as hydroxyl and carboxyl groups on the surface of GO were conducive to the dispersion of MIL-68(Al) in aqueous solution, thus accelerating the adsorption of MO and other dyes. While TiO2 replaced GO, the PDI and Dh of MIL-68(Al)/TiO2 were metropolis enlargement. TiO2 was less hydrophilic than GO, so MIL-68(Al)/TiO2 gave a PDI of 0.257, and the Dh was the maximum with 318.8 nm. After the addition of GO, MIL-68(Al)/GO/TiO2 showed a PDI of 0.236, and the Dh was moderate at 295.2 nm. This result proves that GO can improve the dispersion of MIL-68(Al) and TiO2 in the dye solution, which is conducive to sufficient contact between the material and the dye aqueous solution, thus improving the adsorption efficiency.
To further reveal the adsorption principle of the material, the zeta potential of the prepared MIL-68(Al)/GO/TiO2 was tested. Figure 6b shows that the zeta potential values of MIL-68(Al)/GO/TiO2 materials are all positive in the pH 1–11 range. MO usually exists in sulfate in an aqueous solution with a large negative charge on the surface. Therefore, the material MIL-68(Al)/GO/TiO2 with positive potential can act well on the MO solution, and the electrostatic adsorption effect can act well between the catalyst and MO.

2.7. BET and Pore Diameter of the MIL-68(Al)-Based Materials

Information regarding the quantity adsorbed and pore volumes of MIL-68(Al)/GO, MIL-68(Al)/TiO2 and MIL-68(Al)/GO/TiO2 was determined by N2 adsorption–desorption isotherms, as shown in Figure 7. A type I isotherm illustrated that the material channel was a microporous structure [37]. Surface areas and porosities are listed in Table S1. MIL-68(Al)/GO’s BET surface area was 65.30 m2/g, and MIL-68(Al)/TiO2 was calculated as 125.74 m2/g, respectively. MIL-68(Al)/GO/TiO2 showed the largest BET surface area of 575.76 m2/g, which was higher than the value reported for this material in the literature [34,37]. It has been proved that the addition of GO and TiO2 is beneficial in increasing the specific surface area of MIL-68(Al), thus enhancing the adsorption property of MIL-68(Al)/GO/TiO2. The crystals of MOF-based materials can grow on the GO layer, thus generating new channels at the interface between the frame and GO and enhancing the MIL-68(Al)/GO/TiO2 adsorption capacity. The pore diameter and the total pore volume of MIL-68(Al)/GO/TiO2 were determined using the BJH method as a result of 2.09 nm and 0.22 cm3/g, respectively. All pore sizes were below 50 nm, indicating that MIL-68(Al)-based materials have a typical mesoporous structure. See Table S1 for detailed data in SI.

2.8. Adsorption Performance of MIL-68(Al)-Based Material

Figure 8 shows the effect of MIL-68(Al) material and modified MIL-68(Al)-based material on the adsorption of MO solution. It could be seen that the adsorption rate of MIL-68(Al) material without GO and TiO2 doping was 85.8% after reaction for 20 min (Figure 8b). Pure TiO2 and GO showed low adsorption efficiency with 12.9% (TiO2) and 17.5% (GO); pure TiO2 and GO have poor adsorption performance.
When doped with GO or TiO2, the adsorption rate could be improved. Especially using MIL-68(Al)/GO/TiO2 as the catalyst, the adsorption efficiency was up to 99.7% (Figure 8b), almost achieving the complete removal of MO in water. The heterojunction structure formed between MIL-68(Al) and TiO2 in MIL-68(Al)/GO/TiO2 effectively promoted the separation of photogenerated electron-hole pairs and improved the quantum efficiency. The adsorption process was linearly fitted further to study the kinetic characteristics of the MO adsorption process. The adsorption process was found to conform to the first reaction rate equation, and the linear correlation is good. The variance value was basically above 0.95 (Figure 8c). By calculating the reaction rate constant, the adsorption rate constant of MIL-68(Al)/GO/TiO2 was the highest at 0.255 (Figure 8d), which was higher than that of other MIL-68(Al)-based materials. It can be seen that, when doped with GO or TiO2, the specific surface area of MIL-68(Al)/GO/TiO2 could be significantly increased. The surface of MIL-68(Al)/GO/TiO2 was also modified with more hydrophilic functional groups of graphene oxide. The addition of GO and TiO2 is beneficial in enhancing the adsorption property of MIL-68(Al)/GO/TiO2. The crystals of MOF-based materials can grow on the GO layer, thus generating new channels at the interface between the frame and GO and enhancing the MIL-68(Al)/GO/TiO2 adsorption capacity. The above adsorption data and kinetic results showed that the co-doping method of GO combined with TiO2 can enhance the adsorption of MIL-68(Al).

2.9. Effect of pH on MO Adsorption Efficiency

To study the application range of the modified material, we investigated the adsorption efficiency of MIL-68(Al)-based material on MO at different pH conditions (Figure 9).
The experimental results showed that the pure MIL-68(Al) adsorption rate was only 31.6% under strong acid conditions of pH = 1. The individual MIL-68(Al) has an unstable structure in extreme environments such as strong acids and strong bases. Due to the short reaction time, it still has a certain removal rate. However, when doped by GO or TiO2, the adsorption rate of MIL-68(Al)-based materials could be significantly improved. When pH = 1, the adsorption rate of MIL-68(Al)/GO reached 54.6%, and the adsorption rate of MIL-68(Al)/TiO2 reached 62.1%. In addition, when GO was co-doped with TiO2, the adsorption rate of MIL-68(Al)/GO/TiO2 was the highest, reaching 76.7%. Even under strong alkaline conditions of pH = 13, the adsorption rate was 78.0%. MO usually exists in the form of sulfate with a lot of negative charge on the surface. According to zeta potential (Figure 6b), it can be seen that the surface of MIL-68(Al)/GO/TiO2 was positively charged when the pH value was between 1 and 11; therefore, the two are prone to electrostatic attraction in aqueous solution. Therefore, in this work, the MIL-68(Al)/GO/TiO2 material obtained by co-doping GO with TiO2 can significantly improve the pH applicability of the material, broaden the scope of the material in practical process applications, and be suitable for the requirements of various industrial wastewater treatments. Co-doping optimized the structure of MIL-68(Al), enhancing its stability in strong acids and bases while improving adsorption performance across a broader pH range.

2.10. Influence of Catalyst Amount and Initial Concentration of MO

We investigated the MIL-68(Al)/GO/TiO2 concentration and the initial MO concentration for adsorption efficiency (Figure 10).
When 0.05 g/L of the material was used, the adsorption rate was 58.7%. When the dosage was increased to 0.10 g/L, the adsorption rate could be increased to 99.7%. The adsorption rate was maintained at 99.7% by increasing the catalyst concentration. Regarding economic efficiency, the catalyst concentration of 0.10 g/L is the most appropriate. Different initial concentration results showed that with MO concentrations of 20 mg/L, 50 mg/L, 100 mg/L, 150 mg/L, and 200 mg/L, the adsorption rate can reach more than 80%. Therefore, the prepared MIL-68(Al)/GO/TiO2 material has an extensive concentration range and can be applied to various factory wastewater treatments. The result demonstrated that the prepared MIL-68(Al)/GO/TiO2 exhibits satisfactory universality. At the same time, this work’s adsorption efficiency and kinetic slope were also significantly higher than the general MO concentration reported in the existing literature [59,60].

2.11. Adsorption Effects of Different Pollutants

In addition, to be suitable for various wastewater treatments, we also investigated the adsorption efficiency of different pollutants (Figure 11). The adsorption effects of RB (Figure 11a), CR (Figure 11b), MB (Figure 11c), and TC (Figure 11d) were investigated.
An initial solution with a concentration of 20 mg/L was prepared for adsorption using MIL-68(Al)/GO/TiO2. The results showed that the adsorption rate of RB and CR could reach up to 90% in 20 min. MB only takes 2 min to achieve a adsorption rate of 96.6%. MIL-68(Al)/GO/TiO2 is suitable for decomposing various pollutants and effective in decomposing antibiotics such as tetracycline.

2.12. Reuse of MIL-68(Al)/GO/TiO2 for MO

After the MO adsorption was completed, the reaction solution was centrifuged at 8000 r/min for 5 min, and the obtained solid powder MIL-68(Al)/GO/TiO2 was washed with deionized water several times and dried for the next cycle.
The experimental results showed that the recovered MIL-68(Al)/GO/TiO2 could be effectively recycled more than five times. The adsorption efficiency was not significantly reduced, as shown in Figure 12. The adsorption rate of the fifth cycle was still maintained at 97%, which proves that the structure of the MIL-68(Al)/GO/TiO2 material was stable (Figure 12). No apparent structural damage occurs during the cycle (Figure S1). Therefore, the GO and TiO2 doped MIL-68(Al)/GO/TiO2 material prepared in this work has several advantages in wastewater treatment, such as a wide pH range, wide adsorption range, stable material properties, and suitable industrial application prospects.

2.13. Adsorption Mechanism of MIL-68(Al)/GO/TiO2 for MO

Two kinds of free radical trapping agents were selected to explore the adsorption mechanism during the experiment. When isopropyl alcohol (IPA) was added to the MO solution, the adsorption efficiency of the catalyst decreased (Figure 13a: IPA).
With the increase in IPA, the adsorption inhibition increased, and IPA mainly captured •OH free radicals in the MO solution. When 8 mL IPA was added to the blank control, the concentration of MO solution decreased, and the absorbance value decreased. After EDTA was added to the MO solution, the adsorption rate changed slightly with the reduction (Figure 13b: EDTA). EDTA mainly captures h+ in the solution. Free radical capture experiments showed a large number of •OH radicals during the adsorption of MO solution by MIL-68(Al)/GO/TiO2. The valence band of MIL-68(Al)/GO/TiO2 produced an excited e− due to the absorption of UV energy, which transitioned to the MIL-68(Al)/GO/TiO2 conduction band and left an h+ in the valence band. e− and h+ were transferred to the surface of MIL-68(Al)/GO/TiO2 by excitation and reacted with O2, H2O and OH− in solution, respectively. •O2− and •OH radicals were then generated. Taking MO adsorption as an example, the adsorption of MIL-68(Al)/GO/TiO2 by electrostatic action, hydrogen bonding and π-π stacking all exist, and the inferred mechanism is shown in Figure 13. On the one hand, MO usually exists in the form of sulfate in aqueous solution, with a lot of negative charge on the surface. For MIL-68(Al)/GO/TiO2, according to its zeta potential (Figure 6b), it can be seen that the surface of MIL-68(Al)/GO/TiO2 was positively charged when the pH value is between 1 and 11; the two are therefore prone to electrostatic attraction in aqueous solution. On the other hand, MO is a two-dimensional planar molecule with a benzene ring structure. At the same time, MIL-68(Al)/GO/TiO2 composite contains many hexagonal carbon atomic planes, so the two can undergo π-π stacking [61]. In addition, the Al-O-Al structural unit of MIL-68(Al) contains μ2-OH, which can form hydrogen bonds with nitrogen and oxygen atoms in MO molecules [62]. Therefore, the electrostatic interaction, π-π packing and hydrogen bonding make MIL-68(Al)/GO/TiO2 have a higher adsorption capacity for MO.

3. Discussion

3.1. Comparison with Previously Reported Materials

The adsorption efficiencies of various correlative material species with regard to MO dye under various conditions are summarized in Table 1.
Generally, the efficiency of TiO2, MOF, or GO substances assisted MO adsorption, which could be effectively improved by dopant. Even though the MO concentration used in our work was up 200 mg/L, the adsorption efficiency with regard to MO dye was still very high in 20 min when compared with the results reported in the literature. So, the addition of GO and TiO2 is beneficial in enhancing the MIL-68(Al)/GO/TiO2 adsorption capacity.

3.2. Future Research

From the above characteristics and experimental results, it can be concluded that MIL-68(Al)/GO/TiO2 can be applied in harsh environments with strong acids and strong bases, thus providing a new idea for the treatment of pollutants. In future work, we will focus on optimizing the material and improving its performance to enhance its industrial applicability.

4. Materials and Methods

4.1. Materials

Terephthalic acid (H2BDC, >98%), aluminium chloride hexahydrate (AlCl3·6H2O, 99%), and N,N’-dimethylformamide were purchased from Energy Chemical. Methyl orange (MO), rhodamine B (RB, AR, >99%), methylene blue (MB, IND, >70%), congo red (CR AR, >99%), and tetracycline (TC AR, >98%) were purchased from Leyan. Other commercially available chemicals were laboratory-grade reagents from local suppliers. All solvents and reagents were purified by standard procedure.

4.2. Analytical Methods

FT-IR spectra were used to test the functional groups of catalytic materials by using an AVATAR 370 Thermo Nicolet spectrophotometer (Shanghai, China). XRD patterns were recorded on a Bruker-D8 (Karlsruhe, Germany) diffractometer with Cu Kα radiation (λ = 0.15406 nm) to evaluate the crystal structure of MIL-68(Al)/GO/TiO2, MIL-68(Al)/GO, MIL-68(Al)/TiO2, and MIL-68(Al) samples. Nitrogen adsorption-desorption isotherms were obtained on a TriStar 3000-Micromeritics (Norcross, GA, USA) to obtain the materials’ specific surface area and aperture. DLS analysis was performed using a ZS90 Laser Particle Size Analyzer (Malvern, UK) to obtain Dh by preparing 0.5 mg mL−1 concentrations of materials and filtering through a 0.45 µm disposable polyamide membrane to free them from dust particles. The morphology of the MIL-68(Al)/GO/TiO2 was observed by SEM (Hitachi S3400N, Kunshan, China) and TEM (FEI Tecnai G20, Hillsboro, OR, USA). A Thermo Fisher ESCALAB250Xi spectrometer (Shanghai, China) was used for XPS analysis using monochromatic Al-Ka radiation at a detection angle of 30. Zeta potential (x-potential) was provided for the surface charge properties of the samples in solution by Malvern Zetasizer Nano ZS90 (Wellington, UK). UV-Vis Agilent 8453 was used to test the absorbance of the organic dyestuff solution. The concentrations of MO, RB, CR, MB and TC were evaluated by an UV-Vis spectrophotometer (Shimazu UV-2700, Shanghai, China) with absorption wavelengths of 464 nm, 554 nm, 497 nm, 664 nm and 358 nm, respectively.

4.3. Preparation of Different Kinds of Catalysts

MIL-68(Al)/GO/TiO2 Synthesis: Graphene oxide (GO) was prepared using the Hummer method. TiO2 was prepared by sol–gel method. After obtaining GO and TiO2, 0.17 g GO and 0.17 g TiO2 were added to 50 mL N,N’-dimethylformamide (DMF), and then ultrasonic dispersion was uniform to form DMF suspension of GO/TiO2. We added 10 mmol terephthalic acid (H2BDC) and 10.5 mmol aluminium chloride hexahydrate (AlCl3•6H2O) to the GO/TiO2 solution with stirring at room temperature for 1 h (Scheme 1). Then, the precursor solution was transferred into a 100 mL Teflon autoclave liner and maintained at 398 K for 18 h. After cooling to room temperature, the resulting solid was collected via centrifuge and washed several times with DMF and MeOH successively. Finally, the products were dried in an oven at 60 °C for 12 h to obtain the target product of MIL-68(Al)/GO/TiO2. FT-IR (KBr): γmax/cm−1 1676, 1606, 1507, 1446, 1412, 1294, 1249, 1137, 1091, 1019, 994, 890, 839, 753, 652, 609 cm−1.
The syntheses of MIL-68(Al)/GO, MIL-68(Al)/TiO2, MIL-68(Al) are shown in the Supporting Information (Text S1).

4.4. Catalytic Tests with Different MIL-68(Al)-Based Catalyst

The dye adsorption experiments of aqueous dye solutions were conducted under natural sunlight irradiation in July and August 2024 between 11:30 and 14:30. The MIL-68(Al)-based catalysts (10 mg) synthesized compounds were dispersed in 100 mL aqueous dye solution to measure the catalytic activities. Various dyes were used to test the catalytic properties of MIL-68(Al)-based catalysts, including methyl orange (MO), rhodamine B (RB) [67], methylene blue (MB), congo red (CR), and tetracycline (TC). Taking MO adsorption as an example, 20 mg/L MO solution was accurately prepared, and 10 mL of the above solution was retained to obtain initial absorbance by UV spectrophotometer with about 464 nm. An amount of 10 mg MIL-68(Al)-based catalyst was added to 100 mL of a 20 mg/L MO solution with vigorous stirring at room temperature under natural sunlight for the reaction. Then, 5 mL suspension was taken for centrifuging at 8000 r/min to obtain the supernatant. The absorbance of the original and MO pollutant aqueous solutions at different times was measured by a UV spectrophotometer at about 464 nm.
We calculated the adsorption rate of the MO solution of the catalytic material by using the following adsorption rate Formula (1).
D = (A0 − At)/A0 × 100%
In the formula, D is the adsorption rate; A0 is the initial absorbance value of MO solution; and At is the catalytic time absorbance value of MO solution [68].

4.5. Influence of Initial Concentration of MO

MO solutions of 20 mg/L, 50 mg/L, 100 mg/L, 150 mg/L, and 200 mg/L were prepared, and 10 mg of the MIL-68(Al)/GO/TiO2 powder was added to 100 mL of MO solution of different concentrations. The remaining steps were the same as the above adsorption with 20 mg/L concentration. After reacting for 20 min, the reaction solution was treated to detect the adsorption effect.

4.6. Adsorption of RB, CR, MB, and TC

The RB, CR, MB, and TC adsorptions are shown in the Supporting Information (Text S2).

5. Conclusions

Novel MIL-68(Al)-based catalytic materials were prepared, and FT-IR, XRD, SEM, TEM, XPS, and DLS sufficiently characterized four catalysts. The characterization results proved that MIL-68(Al)/GO/TiO2 has a uniform morphology, a large specific surface area, and a good distribution of elements. Remarkably, MIL-68(Al)/GO/TiO2 acquired the highest catalyst efficiency compared with MIL-68(Al)/GO and MIL-68(Al)/TiO2. The adsorption efficiency is much higher than that of various pure materials, GO and TiO2. The adsorption rate of MO can reach 99.7% in water for 20 min. Moreover, the MIL-68(Al)/GO/TiO2 could be easily recovered from the MO solution system for five cycles by simply centrifuge. The material of MIL-68(Al)/GO/TiO2 has excellent degradability, and the introduction of GO improves the specific surface area and adsorption properties of the composites, which is conducive to the enrichment of pollutants. The addition of GO and TiO2 is beneficial in increasing the specific surface area of MIL-68(Al), thus enhancing the adsorption property of MIL-68(Al)/GO/TiO2. The crystals of MOF-based materials can grow on the GO layer, thus generating new channels at the interface between the frame and GO and enhancing the MIL-68(Al)/GO/TiO2 adsorption capacity. In addition, MIL-68(Al)/GO/TiO2 composites also showed good cycle stability and reusability. This study provides a new idea for developing efficient and stable catalytic materials, which is expected to solve the problem of the instability of the MIL-68(Al) structure under extreme conditions.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal15090900/s1, Figure S1: FT-IR spectra with partial spectrogram from 2000 cm−1 to 400 cm−1 (A) and full spectrum from 4000 cm−1 to 400 cm−1 (B). MIL-68(Al) (a), MIL-68(Al)/GO (b), MIL-68(Al)/TiO2 (c), MIL-68(Al)/GO/TiO2 (d); Table S1 Summary of the FTIR spectrum of MIL-68(Al)-based materials; Figure S2 SEM of MIL-68(Al)/GO/TiO2 before and after used; Figure S3 EDS spectrogram of MIL-68(Al)/GO/TiO2; Table S2 Summary of the EDS elemental analysis of MIL-68(Al)-based materials; Table S3 elemental analysis of MIL-68(Al)-based materials in XPS; Table S4 BET surface area and pore properties of materials; Figure S4 The concentration of MIL-68(Al)/GO/TiO2 and pseudo-first-order kinetic model; Table S5 Pseudo-first-order kinetic model of MIL-68(Al)/GO/TiO2; Figure S5 Phenotype of representative mung beans treated by diffident solution [69,70,71,72,73].

Author Contributions

W.P.: data curation, formal analysis, methodology; writing—review and editing; W.Y. and M.W.: data curation, investigation, visualization; L.Z.: formal analysis, data curation; X.L.: resources, software; Y.Z.: conceptualization, funding acquisition, project administration, writing—original draft and review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (22278118, 22108065), the Natural Science Foundation project of Hubei (2024AFB967), The Youth Talent Cultivation Special Project of the Science and Technology Department of Hubei Province (2025DJA046), The Excellent Young and Middle-aged Scientific and Technological Innovation Team Project of Higher Education Institutions in Hubei Province (T2024024), Hubei Province Young Science and Technology Talent Chenguang Project, the opening Fund of Hubei Key Laboratory of Polymer Materials, and Hubei University.

Data Availability Statement

The original contributions presented in this study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author(s).

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 15 00900 g001
Figure 1. FT-IR spectra of MIL-68(Al) (a), MIL-68(Al)/GO (b), MIL-68(Al)/TiO2 (c), MIL-68(Al)/GO/TiO2 (d).
Figure 1. FT-IR spectra of MIL-68(Al) (a), MIL-68(Al)/GO (b), MIL-68(Al)/TiO2 (c), MIL-68(Al)/GO/TiO2 (d).
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Figure 2. XRD spectra of MIL-68(Al) (a), MIL-68(Al)/GO (b), MIL-68(Al)/TiO2 (c), MIL-68(Al)/GO/TiO2 (d).
Figure 2. XRD spectra of MIL-68(Al) (a), MIL-68(Al)/GO (b), MIL-68(Al)/TiO2 (c), MIL-68(Al)/GO/TiO2 (d).
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Catalysts 15 00900 g003
Figure 3. SEM of MIL-68(Al) (a), MIL-68(Al)/GO (b), MIL-68(Al)/TiO2 (c), MIL-68(Al)/GO/TiO2 (d), and SEM mapping of the serial element in MIL-68(Al)/GO/TiO2.
Figure 3. SEM of MIL-68(Al) (a), MIL-68(Al)/GO (b), MIL-68(Al)/TiO2 (c), MIL-68(Al)/GO/TiO2 (d), and SEM mapping of the serial element in MIL-68(Al)/GO/TiO2.
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Figure 4. TEM of MIL-68(Al)/GO (a), MIL-68(Al)/TiO2 (b), and MIL-68(Al)/GO/TiO2 (c), and HRTEM of MIL-68(Al)/GO (d), MIL-68(Al)/TiO2 (e), and MIL-68(Al)/GO/TiO2 (f).
Figure 4. TEM of MIL-68(Al)/GO (a), MIL-68(Al)/TiO2 (b), and MIL-68(Al)/GO/TiO2 (c), and HRTEM of MIL-68(Al)/GO (d), MIL-68(Al)/TiO2 (e), and MIL-68(Al)/GO/TiO2 (f).
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Figure 5. XPS spectra of the MIL-68(Al)/GO/TiO2 wide region (a); C 1s split spectra (b); Al 2p split spectra (c); Ti 2p split spectra (d).
Figure 5. XPS spectra of the MIL-68(Al)/GO/TiO2 wide region (a); C 1s split spectra (b); Al 2p split spectra (c); Ti 2p split spectra (d).
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Catalysts 15 00900 g006
Figure 6. Particle size distribution (a) and zeta potential of MIL-68(Al)/GO/TiO2 (b).
Figure 6. Particle size distribution (a) and zeta potential of MIL-68(Al)/GO/TiO2 (b).
Catalysts 15 00900 g006
Catalysts 15 00900 g007
Figure 7. BET (a) and pore diameter (b) of the MIL-68(Al)/GO, MIL-68(Al)/TiO2, MIL-68(Al)/GO/TiO2.
Figure 7. BET (a) and pore diameter (b) of the MIL-68(Al)/GO, MIL-68(Al)/TiO2, MIL-68(Al)/GO/TiO2.
Catalysts 15 00900 g007
Catalysts 15 00900 g008
Figure 8. MO adsorption (a), adsorption efficiency (%) (b), kinetics of catalytic adsorption (c), and comparison of rate constants for the different catalysts (d).
Figure 8. MO adsorption (a), adsorption efficiency (%) (b), kinetics of catalytic adsorption (c), and comparison of rate constants for the different catalysts (d).
Catalysts 15 00900 g008
Catalysts 15 00900 g009
Figure 9. The effect of pH on MO adsorption efficiency.
Figure 9. The effect of pH on MO adsorption efficiency.
Catalysts 15 00900 g009
Catalysts 15 00900 g010
Figure 10. The degradation rate (a) and kinetics (b) for concentration of MIL-68(Al)/GO/TiO2. and degradation rate (c) and kinetics (d) for initial concentration of MO for adsorption.
Figure 10. The degradation rate (a) and kinetics (b) for concentration of MIL-68(Al)/GO/TiO2. and degradation rate (c) and kinetics (d) for initial concentration of MO for adsorption.
Catalysts 15 00900 g010
Catalysts 15 00900 g011
Figure 11. Four kinds of pollutant adsorption RB (a), CR (b), MB (c), TC (d) in MIL-68(Al)/GO/TiO2 presence.
Figure 11. Four kinds of pollutant adsorption RB (a), CR (b), MB (c), TC (d) in MIL-68(Al)/GO/TiO2 presence.
Catalysts 15 00900 g011
Catalysts 15 00900 g012
Figure 12. The reuse of MIL-68(Al)/GO/TiO2 for MO adsorption.
Figure 12. The reuse of MIL-68(Al)/GO/TiO2 for MO adsorption.
Catalysts 15 00900 g012
Catalysts 15 00900 g013
Figure 13. Free radical capture experiment ((a): IPA, (b):EDTA) and proposed interaction mechanism for adsorption of MO on MIL-68(Al)/GO/TiO2.
Figure 13. Free radical capture experiment ((a): IPA, (b):EDTA) and proposed interaction mechanism for adsorption of MO on MIL-68(Al)/GO/TiO2.
Catalysts 15 00900 g013
Catalysts 15 00900 sch001
Scheme 1. Schematic representation for the synthesis of MIL-68(Al)/GO/TiO2.
Scheme 1. Schematic representation for the synthesis of MIL-68(Al)/GO/TiO2.
Catalysts 15 00900 sch001
Table 1. Comparison with previously reported works.
Table 1. Comparison with previously reported works.
CatalystMO (mg L−1)Time (min)Removal (%)Reference
Ag/TiO21055100 [53]
N-CDs/TiO2100 ppm2062.5 [48]
GO/TiO22024090.0 [63]
Salen-based MOF2025087.9 [64]
Activated carbon40024094.1 [61]
Zn-doped g-C3N42015090.0 [65]
CQD/CNTs104099.1 [66]
MIL-68(Al)/GO/TiO220–2002099.7This work
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Peng, W.; Yang, W.; Wang, M.; Zhang, L.; Liu, X.; Zhang, Y. Optimizing the Microscopic Structure of MIL-68(Al) by Co-Doping for Pollutant Removal and Mechanism. Catalysts 2025, 15, 900. https://doi.org/10.3390/catal15090900

AMA Style

Peng W, Yang W, Wang M, Zhang L, Liu X, Zhang Y. Optimizing the Microscopic Structure of MIL-68(Al) by Co-Doping for Pollutant Removal and Mechanism. Catalysts. 2025; 15(9):900. https://doi.org/10.3390/catal15090900

Chicago/Turabian Style

Peng, Wenju, Wenjie Yang, Meng Wang, Lin Zhang, Xianxiang Liu, and Yaoyao Zhang. 2025. "Optimizing the Microscopic Structure of MIL-68(Al) by Co-Doping for Pollutant Removal and Mechanism" Catalysts 15, no. 9: 900. https://doi.org/10.3390/catal15090900

APA Style

Peng, W., Yang, W., Wang, M., Zhang, L., Liu, X., & Zhang, Y. (2025). Optimizing the Microscopic Structure of MIL-68(Al) by Co-Doping for Pollutant Removal and Mechanism. Catalysts, 15(9), 900. https://doi.org/10.3390/catal15090900

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