Photocatalytic degradation of ibuprofen using TiO₂ sensitized by Ru(II) polyaza complexes

Abstract

In this work, modification of TiO₂ was carried out by incorporation of two novel Ru(II) polyaza complexes. The N¹-(2-aminobenzyliden)-N²,N²-bis(2-(2-aminobenzyliden)aminoethyl)ethane-1,2-diaminoruthenium(II) and N¹,N²-bis(2-aminobenziliden)ethane-1,2-diaminoruthenium(II) complexes were synthesized via metal–ligand direct reaction. The complexes were characterized by UV-Vis, FTIR and fluorescence spectroscopy, and the chemical composition was obtained from elemental analysis by the combustion method; additionally, the sensitized TiO₂ catalysts were also characterized by XRD, SEM and diffuse reflectance techniques. The photocatalytic activity of the prepared catalysts was tested in a batch reactor under visible radiation for the degradation of ibuprofen in aqueous solution. The evolution of the drug degradation process was evaluated by high-performance liquid chromatography (HPLC), while the mineralization percentage was monitored by the determination of total organic carbon (TOC). The results indicated that the incorporation of these complexes improves the activation of TiO₂ under visible light, increasing the degradation and mineralization percentage of ibuprofen up to 35% compared to the unmodified material, thereby making it suitable for application in heterogeneous photocatalysis of the said pharmaceutical in aqueous media using visible light as the energy source.

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Introduction

Titanium dioxide (TiO₂) has been the most studied catalyst due to its potential application in the field of wastewater treatment.^1–3 However, the need for UV light to promote the electron transfer process arising from the wide TiO₂ band gap (ca. 3.2 eV) is a serious limitation for the practical applications of TiO₂ in heterogeneous photocatalysis. There has been increasing interest in discovering effective approaches for improving the photoresponse of TiO₂ particles under irradiation with low energy photons.^4–8 One of the most effective strategies is the sensitization of the semiconductor, by incorporation of coloured compounds, such as metal complexes that interact with the surface of a wide band gap semiconductor, e.g., TiO₂, facilitating electron transfer between the incorporated molecules and the host semiconductor. In this case, visible light excites the sensitizer organic molecules adsorbed onto the semiconductor, thereby enabling electron injection into the semiconductor conduction band. Surface photosensitization by coordination metal complexes via photo-induced sensitizer-to-TiO₂ charge transfer exhibits attractive features such as regenerative sensitization and the ability to mediate the degradation of non-visible absorbing substrates.^9–11

The difficulty of establishing stable surface anchorage of the charge-transfer photosensitizers is an important drawback of the semiconductor photosensitization approach. In this context, ruthenium(II) complexes have been preferred for use as charge transfer sensitizers owing to their wider absorption band (400–800 nm wavelength range) compared to the metal-free organic sensitizers. Ruthenium(II) complexes are widely used as sensitizers^12–17 in dye-sensitized solar cells and as catalysts or photocatalysts capable of performing water oxidation or oxidation of organic compounds; therefore, in this work, we proposed to apply two new polyaza Ru(II) complexes as TiO₂ sensitizers because of their particular properties and their ability to bind efficiently to various types of substrates.¹⁸

The Ru(II)-complexes, denominated N¹-(2-aminobenzyliden)-N²,N²-bis(2-(2-aminobenzyliden)aminoethyl)ethane-1,2-diaminoruthenium(II), and N¹,N²-bis(2-aminobenziliden)ethane-1,2-diaminoruthenium(II), labelled as RuL2 and RuL3, respectively (Fig. 1), were synthesized and incorporated onto the TiO₂ semiconductor in order to evaluate the effect of the sensitization on the photocatalytic activity of the modified TiO₂ catalysts under visible light irradiation. The nonsteroidal anti-inflammatory drug ibuprofen (C₁₃H₁₈O₂) in aqueous solution was used as a model compound to study the performance of the sensitized materials. Regarding ibuprofen degradation some studies reported the complete degradation of this drug using TiO₂ or ZnO irradiation with UV lamps, which implies a high energy cost. In another case Feng et al.¹⁹ reported the degradation of ibuprofen with TiO₂ under visible light, however they had to add H₂O₂ during the process in order to improve the degradation rate.^20–28 In this context, it is important to remark that no studies have been reported on the degradation of ibuprofen under visible light using TiO₂ sensitized with Ru(II) complexes in order to achieve an efficient process less dependent on UV energy.

Fig. 1 Structure of the polyaza ligands L2 and L3.

Experimental

Materials and reagents

RuCl₃, ethylenediamine, 2-aminoethylamine, ethanol, methanol, ethyleneglycol, dimethylsulfoxide and ibuprofen ((RS)-2-(4-(2-methylpropyl)phenyl)propanoic acid) were supplied by Sigma-Aldrich. Titanium dioxide P25 was supplied by Degussa, in the form of 30% rutile and 70% anatase phases, with an average particle size of approximately 21 nm and a surface area of 50 m² g⁻¹. All solvents were used without further purification.

Characterization

Elemental analysis of the synthesized products was conducted using a Perkin-Elmer 2400 Series II CHNS/O elemental analyser. FTIR spectroscopy analysis was carried out using a Perkin-Elmer FT-IR Spectrometer Paragon 1000 PC. The diffuse reflectance analysis was performed using a Thermo-Scientific spectrophotometer model 300 PC coupled to an integrating sphere. The powder XRD patterns of the photocatalysts were recorded on a Siemens D-5000 diffractometer. The Scherrer formula was used to estimate the average crystallite size of the prepared materials from the data obtained from their X-ray diffraction patterns. The micrographs of the catalyst surfaces were obtained using a JEOL JSM-6510 Scanning Electron Microscope.

Complex synthesis

All synthesis reactions were carried out under constant stirring provided by a Fisher Sonic Dismembrator model 500. The two polyaza macroligands were synthesized by linking 2-aminobenzaldehyde with 2-tris-(2-aminoethyl)amine and ethylenediamine, respectively. The precursor 2-aminobenzaldehyde was obtained by reaction of 2-nitrobenzaldehyde with Fe⁰ in hydrochloric acid.²⁹ The Ru(II) complexes were obtained at room temperature by direct reaction of the ligand and RuCl₃, both in ethyleneglycol in a 1 : 1 mol ratio (M : L). Each reaction mixture was stirred for 30 min, filtered, washed with ethanol, and finally dried in a vacuum desiccator. The complexes were isolated as dark red solids and were labelled as RuL2 and RuL3.

Semiconductor modification

For the modification of the semiconductor, a suspension of 1 g of TiO₂ in 1 L of water was prepared; separately a solution of each complex was prepared in 5 mL of dimethylsulfoxide. The incorporated complex amounts were 1 wt% and 1.5 wt% of both RuL2 and RuL3. The suspension and the complex solutions were sonicated for 30 min and then mixed and stirred for 3 h. Finally, the mixture was filtered and washed with 50 mL of acetone and 50 mL of water; the solids were recovered and dried prior to characterization by SEM, UV-Vis, FTIR and diffuse reflectance techniques.

Photocatalytic tests

Photocatalytic experiments were performed at room temperature. A quantity of 0.15 g of catalyst was added to a batch reactor containing 150 mL of 10 mg L⁻¹ ibuprofen solution. A visible light metal halide Philips lamp (25 W) emitting polychromatic radiation in the 400–700 nm spectral range (with the maximum in the 540–620 nm wavelength range) was used to irradiate the solution. The light intensity received by the vessel was 50 mW cm², as measured by using a HD 9221 radiometer with a spectral field from 450 to 950 nm. The photocatalytic reaction was performed under magnetic stirring at pH 6 and 8. Aliquots were taken every 30 min during the course of the degradation and filtered through a 0.45 μm nylon syringe filter, until the reaction time reached 300 min. The ibuprofen concentration decay was monitored by reversed-phase chromatography (Young-Lin model 9100) using an Ultra C18 Restek column (5 μm, 200 × 3 mm) and a photodiode array detector (Young-Lin 9160) at wavelength λ = 254 nm. The mobile phase was composed of methanol : 0.1 M acetic acid in water (70 : 30) using a 1 mL min⁻¹ flow rate. Mineralization was followed by TOC decay measurements performed using a Shimadzu VCSN analyser. Control adsorption experiments without irradiation and photolysis tests without catalyst were conducted in order to determine their contribution to the ibuprofen degradation. Additionally, reusing tests were carried out to establish the reusability cycles for the modified catalysts. These experiments were performed by recovering the catalyst after each cycle of use. The material was then filtered and washed with water and acetone and finally dried in a vacuum desiccator. All the experiments were performed in triplicate.

To determine the possibility of the release of the Ru(II) complexes in water, stability tests were carried out by suspending 0.15 g of modified catalyst in 150 mL of nanopure water. The suspension was vigorously stirred under visible light over 5 h, recording the UV-Vis spectra of the final solution and comparing with that of a Ru(II) complex suspension [Fig. SM1 and SM2†].

Results and discussion

The synthesized Ru(II)-complexes (RuL2 and RuL3) were crystalline powders, coloured, non-hygroscopic, and non-soluble in water. Examination of the results of the elemental analysis of the synthesized complexes (Table 1) shows that these were obtained successfully and according to the suggested chemical formula and structure. The most important data from the IR spectra (not shown) indicate that the two ruthenium(II) complexes were obtained, and also show that there is a shift to lower wavenumbers (higher bonding energy) for each signal, indicating an interaction between the N-heteroatoms and the Ru(II) in the coordination sphere.^14,30,31

Table 1 Chemical properties of the synthesized Ru(II) complexes

Complex	Empirical formula	%C found (required)	%H found (required)	%N found (required)
RuL2	C27H33N7Cl2Ru	47.1 (47.5)	4.3 (4.6)	18.0 (18.3)
RuL3	C16H18N4Cl2Ru	38.8 (39.1)	3.5 (3.7)	16.7 (17.1)

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The X-ray diffraction patterns shown in Fig. 2 indicate that there is no significant change in the 2θ angle of the signals for the sensitized TiO₂ catalysts compared to the unmodified TiO₂ Degussa; however, there is an effect of peak widening mainly at 25.4°, 62.5° and 55° 2θ, indicating a difference in crystallite size for the modified materials. Therefore, the crystallite size of the catalyst was determined from these peaks using the Scherrer formula:

The results in Table 2 indicate a decrease in the crystallite size as the Ru(II)-complexes are incorporated on TiO₂. This same behaviour has been reported by Bahadur et al.³² and Behnajady et al.³³ when metallic ions were incorporated by impregnation on the surface of the catalyst as a result of the interaction between the incorporated ion and TiO₂.

Fig. 2 X-Ray diffraction patterns for each material.

Table 2 Average crystallite size and calculated E_g values for TiO₂ modified catalysts

Catalyst	E g (eV)	λ (nm)	Average crystallite Size (nm)
TiO2	3.2	387	25.03
TiO2-RuL2 1%	3.0	413	22.31
TiO2-RuL2 1.5%	2.9	427	21.15
TiO2-RuL3 1%	3.1	400	22.69
TiO2-RuL3 1.5%	3.1	400	21.94

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The SEM micrographs for TiO₂, TiO₂-RuL2 and TiO₂-RuL3 1.5% samples (Fig. 3) show a difference in the size of particles, ranging from 20–35 nm for pure TiO₂ to 12–20 nm for the modified semiconductor. This decrease of particle size is also observed in the SEM images in agreement with the results obtained from the Scherrer equation; therefore, these results can indicate that the presence of the complexes on the surface of titania particles can enhance the surface area of the catalyst.³⁴

Fig. 3 SEM images of (a) TiO₂, (b) TiO₂-RuL2 1.5% and (c) TiO₂-RuL3 1.5%.

Fig. 4 shows the plots of (F(R)hν)^1/2vs. hν of the modified photocatalysts and TiO₂ Degussa. Band gap values were calculated for every material using the Kubelka–Munk function:

The absorption edge can be observed at approximately 3.2 eV, corresponding to a charge transfer process from the valence band to the conduction band of the semiconductor. This edge exhibits a displacement to lower energy values in the modified TiO₂, suggesting a narrowing of the band gap as a consequence of the Ru(II)-complex incorporation.^35,36 The most noticeable displacement is found in the catalyst modified with RuL2 at 1.5 wt% that exhibits the lower E_g value (2.9 eV) corresponding to the energy of the visible region (427 nm).

Fig. 4 Diffuse reflectance spectra for TiO₂ and sensitized TiO₂ catalysts.

In Table 2, the E_g values and the crystallite size for each catalyst are reported. It is noticeable that the smaller crystallite size (21.15 nm) is also observed for TiO₂-RuL2 1.5%. The lower E_g along with the small crystallite size, suggests that the catalyst TiO₂-RuL2 1.5% can be an adequate material for the activation with visible light. These changes of the E_g value and the crystallite size are a direct consequence of the presence of the Ru(II) complexes on the surface of TiO₂, which can enhance the photocatalytic activity under visible light.^37–39

Photocatalytic experiments

The results shown in Fig. 5 indicate that a higher degradation percentage (∼80%) especially at pH 8 (Fig. 5b), was obtained using the sensitized materials with 1.5% of RuL2 and RuL3, surpassing the activity of unmodified TiO₂. The isoelectric point of TiO₂ (P25) is approximately 6.2,⁴⁰ therefore at pH 8 the catalyst surface is mainly covered with negative charges Ti–O⁻ which interact as an electron donor with O₂ molecules generating more oxygen and hydroxyl radicals to oxidize the organic compounds.^41–44 Also, at pH values higher than its pK_a (5.2),⁴⁵ the carboxylic group of ibuprofen reaches the ionized state which can form ion–dipole bonds with water and both its hydrophilicity and solubility are improved, hence, ibuprofen can easily reach the active sites of the catalyst, improving the degradation process.^46–49

Fig. 5 Photocatalytic activity of TiO₂ and modified materials on the degradation of ibuprofen with visible light, at (a) pH 6 and (b) pH 8.

On the other hand, preliminary tests showed a poor performance in the case of the 1% complex modified catalysts, so that hereafter we only used modification with 1.5% complexes.

The photocatalytic degradation rate of organic compounds is commonly described in terms of the Langmuir–Hinshelwood model. It has been demonstrated in many reports^50–53 that photocatalytic reactions can be adjusted to this pseudo-first order model in a low-concentration solution under direct irradiation of light. Most researchers continue using the L–H kinetic model for fitting the experimental results to a pseudo-first order kinetic expression for the condition kC ≪ 1 since it describes a simpler model for the representation of this process in engineering applications. Therefore, the photocatalytic degradation rate of organic compounds at low concentrations is commonly defined by the following equation:

where C is the concentration of the reactant, t is the irradiation time, and k is the rate constant of the reaction.

The half-life (t_1/2) for the decrease of half of the concentration of ibuprofen was calculated by:

The degradation rate constant (k) was obtained using the slope from the linear fit of ln(C₀/C) vs. t. The kinetic parameters for the degradation process are shown in Table 3. It is clear that the catalyst modified with 1.5% of the RuL2 complex exhibits better photocatalytic activity for the degradation of ibuprofen under visible light irradiation, achieving a higher rate constant (5.5 × 10⁻³ min⁻¹) and a lower half-life time (126 min) of the oxidation process. No significant difference in the degradation percentage was observed for both modified catalysts. However, a significant difference is observed for the mineralization process.

Table 3 Kinetic parameters for the photocatalytic degradation of ibuprofen

	pH 6		pH 8
Material	k app (min^−1)	t 1/2 (min)	k app (min^−1)	t 1/2 (min)
TiO2-RuL2 1.5%	3.9 × 10^−3	177	5.5 × 10^−3	126
TiO2-RuL3 1.5%	3.7 × 10^−3	187	3.8 × 10^−3	182

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Fig. 6 shows the TOC decay values for photocatalytic experiments, at the times of 0, 90, 180 and 300 min; it can be observed that a higher mineralization percentage (64%) is achieved with the RuL2 1.5% modified catalyst, in contrast with the mineralization percentage reached with RuL3 1.5% (45%). This difference can be attributed to the more flexible and complex structure of RuL2 that enables it to anchor to the TiO₂ surface better and more easily than RuL3.

Fig. 6 TOC decay on the degradation of ibuprofen with visible light at pH 8.

This higher mineralization percentage could indicate that, the route for the ibuprofen decomposition yields organic molecules with lower molecular weight and better solubility than ibuprofen, such as decarboxylated and hydroxylated metabolites, as well as organic acids.⁵⁴

The selected TiO₂-RuL2 material was recovered to study its ability to be reused in the degradation of ibuprofen through four degradation cycles. While the degradation capacity does not change greatly during the first and second cycle, after the third cycle this capacity decreases by 20%, probably because of saturation of the surface by the formed by-products during the photocatalytic process. On the other hand, the stability tests demonstrated that the prepared catalysts are stable since the UV-Vis absorption bands of the Ru(II)-complex were not detected in the spectra of the final solution of the mentioned tests. This result suggests that the Ru(II) complexes can be anchored to the TiO₂ surface, allowing electronic transfer from the visible-light excited state of the complex to the conduction band of TiO₂ resulting in effective electron injection and therefore, in the activation of the catalyst with low energy radiation.⁵⁵

Conclusions

The enhancement of the photocatalytic properties of the sensitized materials is attributed to the presence of the complexes on the semiconductor surface that improves the absorption of visible light.

These results indicate that both Ru(II)-complexes (RuL2 and RuL3) can act as sensitizers; more importantly, in the case of RuL2, the complex incorporated at 1.5%, as was revealed by the higher ibuprofen mineralization percentage.

These sensitizers promote the activation of TiO₂ under visible light irradiation by injecting electronic charge into the conduction band of the semiconductor and avoiding the electron–hole recombination.

Acknowledgements

The authors would like to acknowledge the Faculty of Chemistry of the Autonomous University of Nuevo León, Monterrey, México, and J. F. Góngora is grateful to the CONACYT for the scholarship granted for this project, and also to the CONACYT Project No. CB-181057.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6pp00222f

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