Introduction

Azo dyes are currently one of the families of organic compounds with the greatest environmental impact [1, 2]. They represent more than 60% of the commercial dyes used in the textile, food, pharmaceutical, paper and ink industries, which are generally discharged to the environment in wastewater from industrial processes [3, 4]. The biological effects of this wastewater are associated with a low biological oxygen demand (BOD) and a high chemical oxygen demand (COD) which leads to serious damage to aquatic life (fauna and flora) [5, 6]. To reduce these negative agents, different removal or decomposition treatment methodologies have been proposed, such as the use of ultrasonic processing [7], the treatment of activated sludges [8], catalytic oxidation [9], or simple physisorption [10, 11]. Although some of them have been proven to be successful and reduce concentration, the challenge is currently focused on designing new solid materials than can be recovered and reused for better removal or disposal, and working under green chemistry conditions, using alternative energy sources such as sunlight [12, 13].

In this way, recently metal–organic frameworks (MOFs) have emerged as a new class of porous solid materials. In these solids, the metal centers mainly correspond to divalent and trivalent ions of 3d transition metals (Zn, Cu, Fe, Ni, etc.), 3p or lanthanoid metals bound to organic molecules through –COOH groups in particular, obtaining solid structures with stable and specific topologies characterized by crystalline structures, large specific surface area, high pore volume and structural adaptivity [14,15,16].

MOFs have potential applications in gas storage, separation and adsorption of molecules, catalysis, thin films, magnetism, or the administration of drugs in living organisms [17]. For applications in catalysis, MOFs have some advantages compared to zeolites, because they do not require activation or regeneration at high temperatures [18, 19]. In addition, MOFs behave as semiconductors when exposed to light and can undergo charge separation upon light absorption [20, 21], thus finding applications in different fields of interest such as solar-fuels production [22, 23], photovoltaic and light-emitting devices [24, 25], photocatalytic transformations such as CO2 reduction [26] and photocatalysis for pollutant degradation [27,28,29].

In this context, the photocatalytic process initiates when the semiconductor surface is irradiated with photons whose energy is equal to or greater than the energy gap between its bands, energy-rich electron–hole pairs are formed on its surface (reaction 1). Photogenerated holes (\(\text{h}^{ + }_{\text{VB}}\)) react with water (reaction 2) and electrons (\({\text{e}}^{ - }_{\text{CB}}\)) with oxygen (reaction 3) to form reactive oxygen species (ROS) such as hydroxyl radicals (·OH) and superoxide radical anions (\({\text{O}}_{2}^{ \cdot - }\)). Specifically, ·OH radicals are powerful oxidizing species in aqueous media with notable reactivity toward a wide variety of aromatic pollutants to achieve mineralization.

$${\text{Ti}} - {\text{MOFs}} + h\nu \to \text{h}^{ + }_{VB} + {\text{e}}^{ - }_{\text{CB}}$$
(1)
$$\text{h}^{ + } + {\text{H}}_{2} {\text{O}} \to {^{\cdot}{\text{OH}}} + {\text{H}}^{ + }$$
(2)
$${\text{e}}^{ - } + {\text{O}}_{2} \to {\text{O}}_{2}^{ \cdot - }$$
(3)

Although some examples of photocatalytic degradation of pollutants using MOFs have been reported in the literature, species that participate in the degradation process are not usually identified. For example, photodegradation of diclofenac and methylene blue has been performed in the presence of MIL-53 [30]. MOF-5 has been used as a microporous semiconductor for carrying out a study of the photocatalytic degradation of phenol in aqueous solutions [21]. Aqueous Cr(VI) photocatalytic reduction was performed in the presence of NNU-36 [31]. The photocatalytic behavior of MIL-100(Fe) was evaluated in the degradation of photocatalytic methyl orange in water [32].

In this context, the aim of this work was to study the catalytic activity of two solid titanium MOFs in photodegradation reactions of pollutants in aqueous phase at room temperature and atmospheric pressure. MIL-125 and MIL-125-NH2 were studied due to their promising electronic and photocatalytic properties, high thermal and chemical stability and unique structural characteristics [33]. Congo red dye was used as a model molecule to explore the photocatalytic properties of these solids [34, 35], and reactive oxygen species \({\text{O}}_{2}^{ \cdot - }\) and ·OH were identified by measurements of luminol chemiluminescence reactions [36]. Finally, the titanium MOF catalysts were recycled and reused without significant loss of activity. The development and knowledge of a stable photoactive system for degradation of emerging contaminants contribute to the development of sustainable technologies based on the use of solar energy to activate the catalyst, and recoverable and reusable solid materials topics.

Experimental

All chemicals were purchased from Sigma-Aldrich or Alfa Aesar and used without further purification. Commercial grade solvents were dried and deoxygenated by refluxing for at least 12 h over appropriate drying agents under argon atmosphere and were freshly distilled prior to use. X-ray powder diffraction (XRPD) analyses of solid titanium MOFs were performed in an X’Pert Pro MPD PANalytical instrument with a Cu anode (Cu Kα radiation, λ = 1.54056 Å) using a Bragg–Brentano configuration. Diffractograms of the powder samples were recorded at room temperature, with step size of 0.02 2θ and 10 s step time. The FT-IR ATR spectra were obtained with a Shimadzu IR prestige 21 spectrophotometer (Columbia, MD, USA). Nitrogen adsorption–desorption measurements were taken at − 196 °C on a Micromeritics ASAP 2010 adsorption analyzer to determine the Langmuir surface area. Before measurement, samples were evacuated for 12 h at 110 °C and 1 µtorr [37]. Pore volume was determined by applying the Dubinin–Astakhov (D–A) method, and the pore size was calculated using the Horvath–Kavazoe (HK) method [38, 39]. Diffuse reflectance UV–Vis spectra of solid samples were recorded on a Hitachi U-3000 UV–Vis spectrophotometer. The irradiation intensity of the halogen lamp used in photochemical experiments was measured by a Hand-Held Optical Power Meter Model 840-C, and the photon flux emission corresponded to 85 mW cm−2 at 390 nm. Band gap energy (Eg) of the photocatalysts was calculated from their diffuse reflectance UV–Vis spectra according to the Kubelka–Munk theory [40]. The band gap was obtained from a plot of [F(R∞)·E]1/2 versus energy of the exciting light (E) assuming that titanium MOFs are indirect crystalline semiconductors [41].

Synthesis and characterization of MOFs

Synthesis of MIL-125 (1)

MIL-125 or Ti8O8(OH)4[O2C–C6H4–CO2]6 was synthesized following a previously published procedure [42, 43] using terephthalic acid (TA) (0.5 g, 3 mmol) and titanium isopropoxide Ti(OiPr)4 (0.6 mL, 1.5 mmol) in a solution of 9 mL of N,N-dimethylformamide (DMF) and 0.5 mL of dry methanol. The mixture was stirred gently for 5 min at room temperature and then placed within a 25-mL Teflon liner and heated at 150 °C for 16 h. After reaction, the resultant suspension was filtered, washed with DMF and methanol, respectively, and dried at 150 °C under vacuum overnight to obtain a white solid product. FT-IR ATR (cm−1): 1535, 1512 (–COO asymmetrical vibration); 1390 (–COO symmetrical vibration).

Synthesis of MIL-125-NH2 (2)

MIL-125-NH2 was synthesized by using a solvothermal method that is based on the preparation of MIL-125 previously reported by Dan-Hardi [43, 44], except that TA was replaced by 2-aminoterephthalic acid (NH2–TA) and the solvothermal time was 60 h. FT-IR ATR (cm−1): 1571, 1496 (–COO asymmetrical vibration); 1386 (–COO symmetrical vibration); 1531 (N–H deformations); 1421 (CN vibrations).

Photocatalytic degradation of Congo red

Photocatalytic degradation of Congo red (CR) was studied using a 100-ppm solution of CR in the presence of 150 mg of catalyst, maintaining a ratio of 1 mg of catalyst/mL of solution. The reaction was carried out under stirring and constant irradiation for 3 h using a batch photochemical reactor equipped with an immersion lamp (Phenix, 220 V, 150 W, λ > 390 nm). Follow-up of the degradation of azo dye pollutant was performed by calculating the concentration of Congo red by UV–visible spectroscopy (HP-8453) following the maximum absorption band at 498 nm. In a typical experiment, 1 mL reaction aliquots were taken every 30 min, which were centrifuged and their concentration compared to the calibration curve previously obtained with aqueous solutions of Congo red, using a concentration range of 100–2 ppm. In order to assess the role of dissolved O2, N2 was bubbled into catalyst aqueous suspensions for 1 h before irradiation and N2 atmosphere was maintained during photocatalytic reaction. Control experiments were performed in darkness conditions and in the absence of catalyst but under irradiation.

Degradation of luminol

To evidence the formation of ROS from catalyst suspensions, the degradation of luminol was carried out by adding 20 mg of catalyst to 20 mL of luminol aqueous solution (2.7 mmol, pH 7). The suspension was stirred in the dark for 1 h before irradiating, and O2 was bubbled through the suspension. The reaction mixture was irradiated with a Sylvania 125 W mercury lamp equipped with an optical filter S-8022 SCHOTT (λtransmitted > 360 nm). The incident light flux (Io) was measured by actinometry of potassium ferrioxalate, K3Fe(C2O4)3·3H2O, (Io = 1.5 × 10−5 mol L−1 s−1) [45, 46]. The reactions were performed at 25 °C. Aliquots of 0.2 mL were collected during irradiation, which were then centrifuged and quantified with luminol fluorescence at 431 nm. (Excitation wavelength was 405 nm.) Measurements of luminol fluorescence were taken with a Vernier SpectroVIS Plus spectrofluorometer. The concentration of luminol was compared from fluorescence intensity of a luminol solution of known concentration. Identification of reactive species was carried out in the presence of radical scavengers: Hydroxyl radicals (·OH) was verified by adding 100 µL of 0.1 M mannitol solution (a scavenger of ·OH [47, 48]) to the suspension before irradiation. Degradation of luminol was also carried out under visible irradiation by using a 50 W white LED (Philips, λ > 420 nm), and Io was 8.4 × 10−5 mol L−1 s−1 determined by Reinecke salt actinometry [49]. The experimental conditions under visible irradiation were the same as in UV irradiation experiments.

Results and discussion

Preparation of the catalyst

MIL-125 and MIL-125-NH2 solids have been synthesized by solvothermal method similar to a reported procedure [43]. This method employs anhydrous titanium isopropoxide, aromatic ligands substituted with carboxylic groups and a suitable solvent mixture (in this case, DMF and CH3OH) which favors the interaction between metal and the carboxylic groups of the ligand at 150 °C. Since the crystallinity, surface and structural characteristics of titanium MOFs are affected by the presence of traces of water, extra-dry solvents (previously distilled) and fully controlled reaction conditions must be employed to obtain solids with reproducible results [50].

Characterization of Ti-MOFs

Figure 1 shows the XRPD patterns of MIL-125 and MIL-125-NH2 solids prepared by solvothermal method. The strong and sharp diffraction peaks indicate good crystallinity of samples. These patterns are in accord with the University of Cambridge database: MIL-125 (cif-file CCDC 75115) and MIL-125-NH2 (cif-file COD 7211159) [43]. The indexing of the peaks was done considering a tetragonal unit cell with spatial group I4/MMM, and the cell parameters are presented in Table 1.

Fig. 1
figure 1

XRD patterns of MIL-125 and MIL-125-NH2 solids

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Table 1 Cell parameters obtained of MIL-125 and MIL-125 NH2
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No changes in the crystal lattice structure of MIL-125 were observed with the presence of –NH2 group of the organic linker.

Figure 2 shows the FT-IR ATR spectra of MIL-125 and MIL-125-NH2 in comparison with their respective dicarboxylic acid ligands. Typical signals for MIL-125 were exhibited at (in cm−1) 1535 (N–H deformation), 1512 (–COO asymmetrical vibration) and 1390 (–COO symmetrical vibration), whilst for MIL-125-NH2, the signals were observed at (in cm−1) 1531 (N–H deformations), 1421 (CN vibrations), 1571 and 1496 (–COO asymmetrical vibration) and 1386 (–COO symmetrical vibration) [51].

Fig. 2
figure 2

FT-IR spectra of a MIL-125 and b MIL-125-NH2 (solid line) in comparison with their respective ligands (dashed line)

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The R-COOH functional group bands of the starting ligands underwent major changes after complexation with Ti(IV) leading to the formation of MIL-125 and MIL-125-NH2 solids. In this case, the disappearance of the carbonyl group (C=O) band (at 1678 cm−1 for TA and 1689 cm−1 for NH2–TA) and the formation of two new bands at 1600–1500 cm−1 and 1450–1350 cm−1 could be appreciated, corresponding to asymmetrical and symmetrical stretching of the carboxylate group (COO−), respectively [50]. Thus, interaction between Ti(IV) and –COOH functional group of ligands could induce a resonance structure of carboxylate group (–COO). These results provide evidence for the successful complexation and formation of titanium MOFs.

Figure 3a compares the diffuse reflectance UV–Vis spectra of titanium MOFs with their respective starting ligands in solid state. The characteristic absorption bands of the ligands correspond to n → π* (246 nm for TA and 255 nm for NH2–TA) and π → π* (306 nm for TA, 402 nm for NH2–TA) electronic transition [36, 52]. Owing to their semiconducting properties, broader and less defined bands have been observed in the UV–Vis spectra for MIL-125 and MIL-125-NH2. MIL-125-NH2 exhibits an obvious redshift in the absorption bands and higher absorption above 350 nm than that of MIL-125. These changes indicate the influence of –NH2 groups on the electronic band structure of titanium MOFs [44].

Fig. 3
figure 3

a UV–Vis diffuse reflectance spectra of MIL-125 and MIL-125-NH2 (solid line) in comparison with their respective ligands (dashed line). b Band gap plot of MIL-125 and MIL-125-NH2 calculated from their diffuse reflectance UV–Vis spectra. A comparison with TiO2 P-25 is presented as reference material

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Band gap energies (Eg) of MIL-125 and MIL-125-NH2 were calculated from diffuse reflectance UV–Vis spectra according to the Kubelka–Munk theory [53] and were compared to that obtained for TiO2 P-25, which was used as the reference material (Fig. 3b) [54, 55]. The experimental value of Eg for TiO2 P-25 was 3.12 eV and matches well with the reported values in the literature [56]. The Eg values for MIL-125 and MIL-125-NH2 solids were 3.24 eV and 2.44 eV, respectively. According to their semiconducting properties, MIL-125-NH2 can be activated under visible light irradiating in contrast to TiO2 and MIL-125 which require of UVA light (< 390 nm) to initiate the photocatalytic process.

The surface features of the synthesized titanium MOFs were investigated by applying Langmuir model to the N2 adsorption/desorption isotherms measured at 77 K. The results are summarized in Table 2. The experimental sorption isotherms are shown in Fig. 4a. According to IUPAC classification, MIL-125 shows a type-Ia isotherm which is typically found for microporous structures. On the other hand, MIL-125-NH2 exhibits type-Ib. Type-Ib isotherms are characteristic of materials having pore size distributions in a wide range, ranging from wider micropores to narrow mesopores [37].

Table 2 N2 adsorption–desorption measurements for MIL-125 and MIL-125-NH2
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Fig. 4
figure 4

a Nitrogen adsorption–desorption isotherms and b corresponding pore size distribution curves of MIL-125 and MIL-125-NH2 solids

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The prepared titanium MOFs showed high surface areas. The values of surface areas correspond to 635 m2 g−1 for MIL-125 and 902 m2 g−1 for MIL-125-NH2, in contrast with the low area of TiO2 (50 m2 g−1). The pore volume was estimated using the Dubinin–Astakhov method, which is based on the linearization of the adsorption isotherms (Fig. 4b). Thus, pore volume was 0.35 cm3 g−1 for MIL-125 and 0.46 cm3 g−1 for MIL-125-NH2. A deviation from linearity was observed at both lower and higher pressures, which corroborates a microporous surface [57]. Likewise, when the pore size distribution was plotted by the Horvarth–Kawazoe (HK) method, which is the analytical method used for microporous structures [58], a monomodal distribution with diameters of 0.80 and 0.83 nm was obtained for MIL-125 and MIL-125-NH2, respectively.

Photocatalytic degradation of Congo red

Figure 5 compares the results obtained from the removal capacity of the Congo red dye by MIL-125 and MIL-125-NH2 by physical adsorption on the surface of each of the solids. The MIL-125 solid achieved 20% dye removal by adsorption, while a slight increase was obtained for MIL-125-NH2 with 24% adsorption. This greater increase in the removal is associated with the surface polarization of the solid induced by the –NH2 groups of the ligand in the MIL-125-NH2 catalyst [59]. In the case of TiO2 P-25, the adsorption capacity of the dye did not exceed 10%.

Fig. 5
figure 5

Comparison of the concentration change in the adsorption process by MIL-125, MIL-125-NH2 and TiO2 P-25

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Figure 6a shows the photocatalytic degradation of Congo red by MIL-125, MIL-125-NH2 and TiO2 P-25 (which was used as reference photocatalyst) in oxygenated aqueous suspensions and under UVA light irradiation (λ > 390 nm). Although all catalysts were able to achieve Congo red degradation, MIL-125-NH2 showed higher photoactivity and a substantial degradation (97%) after 3 h of irradiation. In contrast, the dye degradation was around 60% with MIL-125 and TiO2 P-25. Control experiments were also carried out, and the direct photolysis of the azo dye was negligible.

Fig. 6
figure 6

a Degradation of Congo red as a function of irradiation time for MIL-125 and MIL-125-NH2 and TiO2 P-25 catalysts. b Linearization of Congo red degradation (ln Ci/C0 vs. irradiation time)

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Photodegradation on a catalyst surface can be expressed by the Langmuir–Hinshelwood model and follows a pseudo-first-order kinetics with respect to dye concentration [60]:

$$- \ln ({\text{C}}_{i/} {\text{C}}_{0} ) = K_{\text{app}} t$$
(4)

where Ci and C0 are the dye concentration at time t = t and t = 0, respectively; Kapp is the apparent reaction rate constant; and t is time. Kapp was obtained from the slope of a ln(Ci/C0) versus t plot (Fig. 6b) and was higher for MIL-125-NH2 (Kapp = 0.0175 min−1) followed by MIL-125 (Kapp = 0.0062 min−1). These new photocatalysts showed faster kinetics than TiO2 P25 (Kapp = 0.0047 min−1) which has been typically used in degradation processes with important results. We highlight the fact that MIL-125-NH2 with higher surface area could have more catalytic surface sites to accelerate the reaction. Metal ions of the MOF can act as Lewis acid sites to coordinate Congo red that is then removed [61].

Figure 7 shows the effect of oxygen in the azo dye photodegradation by Ti-MOFs. In this case, N2 was bubbled through the suspension to remove dissolved oxygen from solution and Congo red degradation kinetics were investigated. In these conditions, degradation was 67% with MIL-125-NH2 and 42% with MIL-125, revealing that molecular O2 is required to achieve the advanced oxidation more quickly. Additionally, O2 is indispensable to trap \({\text{e}}^{ - }_{\text{CB}}\) generated after excitation of Ti–MOFs and also to avoid the undesirable recombination of \(\text{h}^{ + }_{\text{VB}} /{\text{e}}^{ - }_{\text{CB}}\) pairs. Additionally, oxygen could be considered as a green oxidizing agent since it is the predominant reactant able to generate ROS, such as \({\text{O}}_{2}^{ \cdot - }\) and ·OH, according to reactions 2 and 3. However, the radical \({\text{O}}_{2}^{ \cdot - }\) (pKa = 4.88) could undergo dismutation to generate hydrogen peroxide (reaction 5) which is finally reduced to ·OH, according to reactions 6–7 [62,63,64]. Therefore, ·OH could be considered as the main intermediate formed by MIL-125 and MIL-125-NH2 during the photocatalytic degradation process of Congo red.

$${\text{O}}_{2}^{ \cdot - } + {\text{HO}}_{2}^{ \cdot } + {\text{H}}_{2} {\text{O}} \to {\text{O}}_{2} + {\text{H}}_{2} {\text{O}}_{2} + {\text{OH}}^{ - }$$
(5)
$${\text{H}}_{2} {\text{O}}_{2} + {\text{O}}_{2}^{ \cdot - } \to^{ \cdot } {\text{OH}} +^{ - } {\text{OH}} + {\text{O}}_{2}$$
(6)
$${\text{H}}_{2} {\text{O}}_{2} + {\text{e}}^{ - }_{\text{CB}} \to^{ \cdot } {\text{OH}} +^{ - } {\text{OH}}$$
(7)
Fig. 7
figure 7

Analysis of O2 bubbling in the photocatalytic degradation of Congo red by comparison of change in concentration of CR as a function of irradiation time: a MIL-125 and b MIL-125-NH2

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Thus, with the aim of investigating the reactivity of titanium MOF materials and their ability to produce hydroxyl radicals, a luminol degradation test was carried out. Luminol could be selectively degraded by \({\text{O}}_{2}^{ \cdot - }\) and ·OH. The facile disproportionation reaction of superoxide in water (reactions 6–7) precludes the reaction between \({\text{O}}_{2}^{ \cdot - }\) and luminol. Thus, we assume that the ·OH radicals are mainly responsible for the luminol degradation [65].

Figure 8a shows luminol degradation by MIL-125 and MIL-125-NH2 in oxygenated aqueous suspensions, at room temperature, under UV (λ > 360 nm) irradiation. After irradiating for 1 h, luminol was degraded by up to 54% for MIL-125-NH2 and 34% for MIL-125. No degradation was observed when the experiment was performed in the dark as well as on the experiments realized without catalyst. Since luminol degradation could be mainly carried out by ·OH, the participation of these species was evidenced by the effect of mannitol, a known ·OH scavenger, into the titanium MOF dispersions. In these conditions, the degradation was only 8.3% for MIL-125 and 23% for MIL-125-NH2, confirming that ·OH radicals are formed by the titanium MOFs (Fig. 8a). The decay of luminol concentration could be fitted to the kinetic model of pseudo-first order (Eq. 1) to estimate Kapp for the photocatalysts (Table 3). The value of Kapp for MIL-125-NH2 was 0.053 min−1, while for MIL-125 it was 0.024 min−1. These values clearly indicate that the degradation rate depends on the type of material used.

Fig. 8
figure 8

Luminol degradation by MIL-125 and MIL-125-NH2 in oxygenated aqueous suspensions, at room temperature, under a UV irradiation (λ > 360 nm) and b visible irradiation (λ > 420 nm)

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Table 3 Kapp values obtained of MIL-125 and MIL-125 NH2 in luminol degradation
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The production of hydroxyl radicals as well as adsorption of Congo red on the catalyst surface plays an important role in the photocatalytic process. In turn, the adsorption can limit the rate and efficiency of the total reaction. In this case, we hypothesize that NH2-modified MOF has more active sites, which increases the adsorption capacity of organic molecules and accelerates the formation of ·OH radicals. In contrast, the surface of MIL-125 exhibits weak interaction with Congo red and slower kinetics of ·OH formation. The Congo red elimination depends principally on the production of ·OH radicals and/or adsorption on catalyst surface, as a prerequisite for its oxidation. The luminol degradation experiments demonstrated that these new photocatalysts, MIL-125-NH2 and MIL-125 MOF, in the presence of O2 and upon UV light irradiation, are capable of producing hydroxyl radicals, which are powerful oxidants.

On the other hand, we have found that the introduction of –NH2 groups into the MOFs induces changes in the band structure, producing shifts in band gap energy values at longer wavelengths. In this context, we have also evaluated the formation of ·OH under visible irradiation (λ > 420 nm) (Fig. 8b). The degradation of luminol could be carried out by MIL-125-NH2 with Kapp = 0.040 min−1 (see Table 3). In contrast, MIL-125 was not able to perform the luminol degradation, which indicates that the photocatalytic process was not initiated and no ·OH radicals could be formed. Since one of the most interesting features of photocatalysis is the possibility of using solar light which corresponds to 95% of visible radiation, MIL-125-NH2 is a new option to be used in environmental applications. This photocatalyst along with O2 and visible light would notably reduce the economic cost of the technique and make it more sustainable.

Recyclability of MIL-125 and MIL-125-NH2

In order to evaluate the stability of MIL-125 and MIL-125-NH2 during the photodegradation of Congo red, the solid catalysts were separated from the reaction medium by filtration after a catalytic run for 180 min, consequently washed with water and acetone, and finally dried in vacuum. These used solids were characterized by IR and UV–Vis DRS spectroscopy and TGA (see Fig. 9).

Fig. 9
figure 9

FT-IR spectra for as-made and used a MIL-125 and b MIL-125-NH2 catalysis texted in photocatalytic degradation of Congo red

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The IR spectra of the used solids in the range 800–1900 cm−1 exhibit bands due to stretching vibrations of the titanium-organic frameworks that are identical to those of the precursor MIL-125 and MIL-125-NH2 before catalysis. Furthermore, both thermogravimetric and XRPD analyses of the used MOF catalysts (Fig. 10) corroborate the structural integrity of the titanium MOFs. Thus, the structures of organic dicarboxylic acid ligands of the catalysts are not affected by the reactive oxygen species generated in the photocatalytic process.

Fig. 10
figure 10

TGA and XRPD analysis for as-made and used MIL-125 and MIL-125-NH2 catalysis

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In addition, to ascertain the reusability of the heterogeneous catalysts during the photodegradation reaction, each catalyst was tested over four additional catalytic runs.

Interestingly, as shown in Fig. 11, the catalytic activity is preserved, with a slight reduction in the photodegradation rate of Congo red from cycle to cycle. The results suggest that both MIL-125 and MIL-125-NH2 have good potential as catalysts to be used and recycled in photodegradation processes.

Fig. 11
figure 11

Percentage degradation of Congo red in different catalytic runs by MIL-125 and MIL-125-NH2

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Conclusions

In this paper, we have demonstrated that the MIL-125 and MIL-125-NH2 solids are photoactive catalysts that participate in the photoformation of reactive oxygen species. The Ti-MOF catalysts evaluated have shown the activation of molecular O2 in the decomposition processes of emerging pollutant molecules such as Congo red and have demonstrated their ability to be recycled and reused with only a small reduction in their photocatalytic activity. The proposed methodology has been proven to be a green alternative for the implementation of a photocatalytic process for water removal or decontamination, simply by the use of light and molecular O2.