Photocatalytic degradation of methyl orange mediated by a silica coated nanomagnet porphyrin hybrid

The photocatalytic activity of a silica coated nanomagnet porphyrin hybrid ( NPH ) and of the corresponding porphyrin precursors ( H 2 P and ZnP ) was evaluated in the degradation of the methyl orange dye ( MO ) under visible light irradiation. The catalytic degradation of MO was performed under air, in the absence and in the presence of aqueous hydrogen peroxide. The results show that the hybrid NPH was the most effective photocatalyst causing the total degradation of MO after 270 min of irradiation in the presence of hydrogen peroxide. The remarkable photocatalytic activity of this NPH , associated with the possibility of reuse, makes this material a promising photocatalyst.


Introduction
Environmental pollution is one of the major and most urgent problems of the modern world requiring a constant and special attention from the scientific community.In particular, wastewater from dyeing industries has become exceptionally worrisome, since the presence of dyes can inhibit sunlight penetration and, consequently, reduce the photosynthetic process [1,2].Following these inputs, the scientific community is contributing with the development of novel and economically sustainable methodologies for the detoxification of textile wastewaters, aiming to respect the recommended quality criteria and thus close the water cycle [3,4].
The large-scale use of dyes results in the production of large volumes of pollutants, which are often discarded without any previous treatment.Therefore, the development of efficient methodologies in order to minimize the environmental impact caused by industrial effluents is of great interest [5].
Different methodologies have been developed for the degradation and/or scavenging of those dyes, especially azo dyes (the most common synthetic dyes, ca 70% wt.).These methodologies include adsorption, biological oxidation, membrane filtration, ozonation, oxidation using UV/H2O2, UV/TiO2 and UV or visible light and catalysis [3,[6][7][8][9].The approaches based on catalytic processes appear as important alternatives in the remediation of effluents.In fact, the fundamental pillar of green chemistry is catalysis, generally resulting in significant gains in terms of overall efficiency of a chemical reaction.Porphyrins and analogues are among the catalysts with recognized efficacy for azo dyes degradation [5,10].
The reaction of ROS with the target pollutants is an alternative method for removing organic pollutants [27,28].From the successful history of photocatalysts, the special attention given to porphyrins and analogues by the scientific community is due to their strong absorption bands in the visible region, versatile chemical structures, and facile tuning of the electronic properties.
Besides, the insertion of diverse metals into the core of the macrocycle can modulate the catalytic activity associated with the porphyrins' structure.
Additionally, the development of heterogeneous catalysts based on porphyrins represents an important challenge for a sustainable development.
Recently, magnetite nanoparticles (Fe3O4) have aroused great interest due to their physicochemical characteristics.Fe3O4 with different structures has been used in different areas, such as environmental technology, nanotechnology, medicine and catalysis [29][30][31][32][33].In addition to their exceptional variety of applications, magnetite nanoparticles have the advantage of reuse.For instance, Qin and co-workers studied Fe3O4/TiO2 magnetic nanoparticles for photocatalytic degradation of phenol [34].Kim and co-workers reported the synthesis and characterization of magnetic nanoparticles functionalized with [5,15-bis(phenyl)-10,20-bis(4-methoxycarbonylphenyl)porphyrin]platinum(II) and studied the photocatalytic activity of this material using 2,4,6-trichlorophenol as the target pollutant [35].More recently, magnetite nanoparticles with different morphologies (cubic-shaped and spherical) decorated with the 5,10,15,20-tetrakis(4carboxyphenyl)porphyrin showed to be able to generate 1 O2 under ultraviolet irradiation.Their study as photocatalysts towards bisphenol A (BPA) disclosed degradation values of 64% (cubic-shaped) and of 90% (spherical shape) in the presence and absence of hydrogen peroxide (H2O2) [36].Additionally, these materials retained their catalytic features for at least three catalytic cycles.
Following our interest in developing new materials for environmental remediation, in this communication we report the synthesis and characterization of core-shell magnetite-silica nanoparticles functionalized with the Zn(II) complex of a porphyrin obtained from the reaction of 5,10,15,20tetrakis(pentafluorophenyl)porphyrin with tosylethylenediamine. Furthermore, the efficacy of this material to act as photocatalyst was evaluated under visible white (380-800 nm) light irradiation using methyl orange dye (MO) as the target pollutant.The studies were performed under air in the absence and in the presence of aqueous H2O2.

Reagents and Equipment
All chemicals used in this study were purchased from Sigma-Aldrich or Merck and were of analytical grade.
ii) The tri-substituted free-base porphyrin (H2P) was obtained by structural modification of [H2(TPFPP)] in the presence of the nucleophile Ntosylethylenediamine, as described in the literature [38].
iii) Zinc(II) acetate (42.1 mg, 0.32 mmol) was added to a solution of H2P (50.0 mg) in dichloromethane/methanol (2:1, 15 mL) and the resulting mixture was refluxed at 60 ºC for 1 h (Scheme 1).After cooling to room temperature, the reaction mixture was washed with distilled water.The organic phase was dried (Na2SO4) and the solvent was evaporated under reduced pressure.

Preparation of the Nanomagnet Porphyrin Hybrid (NPH) material
The Nanomagnet Porphyrin Hybrid (NPH) material was prepared according to the procedure described in the literature [39].Briefly, the magnetic core (magnetite) was synthesized by the co-precipitation method under basic conditions using ammonium hydroxide.In the second step, the magnetite was coated with silica using the silicic acid method.In the third step, the nanoparticle was maintained under stirring for 24 h in the presence of (3-aminopropyl)triethoxysilane (APTS) in order to obtain the nanoparticle functionalized with aminopropyl chains.The magnetic aminopropyl silica nanoparticles (Si-NP) were isolated and then washed several times with ethanol, followed by purification by magnetic decantation.Subsequently, the immobilization of the porphyrin ZnP into Si-NP was performed.For this, a previously prepared ethanol suspension of the Si-NP [39] (13.

Photocatalytic activity
The photocatalytic activity of H2P, ZnP and NPH was evaluated in aqueous solutions using methyl orange (MO) as model substrate and under visible light irradiation, which was performed with a halogen 500 W lamp at an irradiance of 150 mW cm -2 .This light source was positioned 10 cm away from the batch reactor and the temperature was kept at ca 20 ºC.The reactions were performed under

Synthesis and characterization of the photocatalysts
The synthetic methodology used in the preparation of the homogeneous and the heterogeneous photocatalysts is summarized in Scheme 1.The porphyrin derivative H2P was prepared by a controlled nucleophilic substitution of three of the p-fluorine atoms in H2(TPFPP) with N-tosylethylenediamine, according to the procedure reported previously by our investigation group [38].
The subsequent metalation with zinc(II) ions of the inner core of H2P was performed in dichloromethane:methanol at 60 ºC using zinc(II) acetate (Scheme 1).The structural confirmation of H2P and ZnP was performed by 1 H NMR, mass spectrometry (Figs.S1-S4 in the Supporting information) and UV-Vis spectroscopy.
The covalent immobilization of ZnP onto the core-shell magnetite silica nanoparticles (Si-NP) functionalized with amino groups occurred via nucleophilic substitution of the p-fluorine atom at the C6F5 group.The functionalized nanoparticles Si-NP were obtained by treating the magnetic core (obtained by the co-precipitation approach) covered with silica with APTS [39,40].The immobilization of the ZnP was performed in DMSO at 160 ºC for 24 h.After this period, a violet insoluble nanoparticles was isolated and washed several times with appropriate solvents to afford the desired NPH.The rinsing solutions resulting from the washing process were collected and analyzed by UV-Vis in order to quantify the loading of ZnP in the solid material nanoparticles (NPH).
Then, NPH was re-suspended in DMSO and was also characterized by UV-Vis and fluorescence spectroscopy [39,40].

Singlet oxygen generation
The ability of photocatalysts to generate ROS, namely singlet oxygen ( 1 O2), superoxide anion radical (O2 •-), hydrogen peroxide (H2O2) and hydroxyl radical ( • OH) is a crucial parameter to be considered with regard to the efficiency of the photocatalyst.[41] In general, when porphyrin derivatives are used as photocatalysts, 1 O2 is the major ROS involved.[25] The production of 1 O2 was assessed by an indirect chemical method using 1,3-diphenylisobenzofuran (DPiBF) as a probe.This compound, as other furans, is able to react as a diene in a [4+2] process with 1 O2 as the dienophile, thus affording the colorless odibenzoylbenzene; with this quencher, exclusively 1 O2 is detected [42].So, the photodegradation of DPiBF mediated by H2P, ZnP, NPH and also by H2TPP (used as reference) was qualitatively assessed by monitoring its absorbance decay at 415 nm as a function of the irradiation time with red light (654 ± 20 nm).The results in Fig. 3 show that the absorbance of DPiBF at 415 nm decreased in the presence of all porphyrin derivatives under irradiation as a function of time through a 1 st order kinetic.Comparing the photodecomposition slope promoted by each porphyrin is observed that H2P was the best 1 O2 generator, being even better than the reference (H2TPP), followed by ZnP and NPH.For NPH, a significant increase of 1 O2 production was observed when its concentration was increased from 0.5 to 5 M.This observation was important in order to establish the amount of material for the photocatalytic reactions.This reduction in 1 O2 production is in accordance with other studies involving silica nanoparticles or others nanoplatforms where the 1 O2 production by the porphyrin immobilized is significantly reduced [43].and ZnP (green shape); 5.0 µM of porphyrin immobilized in NPH (purple shape) and in the absence of catalyst (Control, blue shape).The absorption of MO was monitored at 464 nm.
The catalysts H2P, ZnP and NPH showed significant differences (p < 0.05) in terms of dye degradation efficiency when the irradiations were performed without H2O2 (Figure 4).NPH was the most efficient photocatalyst causing a reduction of 54% in MO dye concentration after 240 min of irradiation when compared with H2P and ZnP which attained a maximum of ca 12% under the same irradiation period.
The results obtained when the experiments were repeated in the presence of H2O2 are summarized in Figures 5 and 6, in the dark and after being irradiated, respectively.The results in Figure 5 show that in the absence of light (dark conditions) and after 90 min of reaction in the presence of any of the catalysts and H2O2 only 7% of MO was oxidized.After 270 min of reaction this value reached 13% for ZnP and NPH while for H2P the value remain almost constant (ca 8%) and was similar to that observed for the control assay (reaction performed in the presence of H2O2 only).These results indicate that there is no remarkable difference between the different catalysts when dye degradation is performed in the presence of H2O2 but in the absence of light.The results in Figure 6 show that the profile of these reactions in the presence of aqueous H2O2 are totally different when carried out under light irradiation.In fact, the action of light was particularly relevant for improving the rate of oxidation mediated by NPH (100% after 270 min) and ZnP (75% after 270 min).When compared with the photoreactions performed under light irradiation but in absence of H2O2 (Figure 3), an increment of ca 46% in catalytic activity was achieved for NPH, 56% for ZnP and 12.5% for H2P.So, the beneficial effect of H2O2 in MO photodegradation is obvious when the results are compared with those obtained in the absence of H2O2 (Figures 4 and 6).The photodegradation of MO in the presence of ZnP, NPH and H2O2 showed to be time-dependent.This time dependence was also observed for NPH in the absence of H2O2.In fact, a longer contact time with the photocatalyst can facilitate the interaction of MO with the oxidation promoting species.The pathway responsible for MO photodegration after photocatalyst activation by white light in the presence of molecular oxygen (O2) can involve ROS such as hydrogen peroxide, superoxide and hydroxyl radicals (type I photochemical pathway) and/or singlet oxygen (photochemical pathway type II).Additionally, the decomposition of H2O2 through a Fenton-like reaction can be facilitated in the presence of NPH affording hydroxyl radicals [44].
The results show that the photocatalytic activity of H2P, ZnP and NPH cannot be justified only by their efficiency to produce 1 O2 (H2P > ZnP > NPH) and probably other highly reactive species such as hydroxyl radical ( • OH) are also involved.In order to verify if the production of • OH is also involved in the photocatalytic degradation of MO, the reactions in the presence of H2O2 were carried out in the presence of mannitol, an effective radical scavenger for • OH [45] (Figure 7).photodegradation (Figure 7).Similar results were obtained for reactions performed with hydrogen peroxide:mannitol (Figure 7) and without hydrogen peroxide (Figure 4).Terephthalic acid (TA) has been used to indirectly measure the production of • OH by fluorescence.The • OH radical species react with terephthalate to yield an intensely fluorescent mono-hydroxylated derivative (HTA).shown that the TA probe have strong affinity for iron oxide surfaces resulting in lower concentration of the highly fluorescent HTA (Figure 8D).The fluorescence pattern observed for NPH in the presence of TA (Figure 8C) is due to the interaction between the nanoparticles' surface and TA.It is worth to mention that in the absence of hydrogen peroxide, the HTA signal is not observed.

Conclusions
Core-shell magnetite-silica nanoparticles decorated with a porphyrin bearing meso-aryl groups with N-tosylethylenediamine residues can be considered for application as excellent photocatalyst for dyes degradation.The The efficacy of this new material in the photodegradation of other dyes will be evaluated in future work, together with additional studies for the identification of the reactive species involved in the photocatalysis.
5 mL, corresponding to 250 mg of Si-NP) were filtered through a polyamide membrane, washed several times with DMSO and re-suspended in DMSO(6 mL).A solution of ZnP (20.0 mg, 12.3 µmol) in DMSO (2 mL) was added to the previous Si-NP suspension and the resulting mixture was stirred for 24 h at 160 °C (Scheme 1).The immobilization of ZnP was monitored by thin-layer chromatography: the spot corresponding to ZnP decreases progressively while the spot corresponding to the NPH material (at the application point) increases.The insoluble material with a violet color was washed several times with appropriate solvents: firstly dichloromethane and then a mixture of dichloromethane/methanol (90:10) until the Soret band of the ZnP was no longer detected through UV-Vis in the rinsing solvent.The quantification of the ZnP present in the washing solvent allowed to calculate the ZnP loading in the material (based on the ε value of the Soret band of ZnP).In the washing process, the hybrid material NPH was firstly decanted on a magnet field and then filtered under vacuum, using a polyamide membrane on a Büchner funnel.The NPH material was re-suspended and kept in dry DMSO (25 mL), making a stock solution of the NPH photocatalyst for the photocatalytic activity assays.NPH was characterized by UV-Vis and fluorescence spectroscopy.Singlet oxygen generationStock solutions of each porphyrin (H2P and ZnP) and of 1,3-diphenylisobenzofuran (DPiBF) in DMF at 0.1 mM and 10 mM, respectively, were prepared.The reaction mixture containing DPiBF (50 µM) and a solution of each photocatalyst (0.5 µM) in DMF was irradiated in a quartz cell (3 mL), under magnetic stirring at an irradiance of 10 mW.cm -2 with a homemade LEDs array.The LEDs array is composed of a matrix of 5 x 5 LEDs making a total of 25 light sources with an emission peak centered at 654 nm and a bandwidth at half maximum of ± 20 nm.During the irradiation period the solutions were stirred at ambient temperature.The DPiBF degradation was monitored by measuring the absorbance decrease at 415 nm at irradiation intervals of 1 min.The percentage of decay of DPiBF absorption is related with the production of singlet oxygen.The quantification was achieved by the difference between the initial and the final absorbance at 415 nm over a given irradiation time.The same strategy was adopted for NPH, but the PS concentrations used were 0.5, 1.0, 2.0, and 5.0 µM.The results obtained were compared with those obtained in the presence of H2TPP(5,10,15,20-tetraphenylporphyrin)  and in the absence of any porphyrin (negative control) under similar irradiation conditions.
photocatalyst (NPH), the solution was stirred in the dark (30 min) before irradiation in order to obtain an equilibrium point of initial physical adsorption of MO over the surface of the photocatalyst.All photocatalytic experiments were accomplished under similar conditions.The photocatalytic performance was monitored indirectly by relating the decrease in the absorbance of MO at 464 nm in solution with its degradation.The photocatalytic reactions in the presence of hydrogen peroxide were also performed in the presence of mannitol.
CH 2 Cl 2 /CH 3 OH (2:1), 60 ºC, 1 h ii) DMSO, 160 ˚C, nm.These features validated the success of the immobilization and the UV-Vis spectrum profile is similar to that obtained for the non-immobilized ZnP in solution.The UV-Vis spectra of ZnP and NPH in the solid-state show the typical absorption Soret band at 430 nm (Supporting Information).Additionally, the UV-Vis spectra were also acquired in a mixture of DMSO:H2O and are shown in the SI (FigureSXX).The spectrum of the silica nanoparticles (Si-NP) did not present bands in the region of 400 nm (FigureS4).

Figure 4 .
Photocatalytic activityThe photocatalytic activity of H2P, ZnP and NPH was evaluated using MO as an azo dye model due to its resistance to environmental degradation.The experiments were performed under visible light irradiation (380-800 nm) under air, and in the absence or in the presence of aqueous H2O2 as oxidant.The degradation of MO in the presence of H2P, ZnP and NPH was monitored by measuring the decay of MO absorbance band at 464 nm.The photocatalytic efficiency of the different materials was expressed using the following equation:(A0-At)/A0, where A0 is the absorbance of MO in the reaction mixture at time zero and At is the MO absorbance at an established time (t).

Figure 5 .Figure 6 .
Figure 5. Degradation of methyl orange (MO) under dark conditions at different reaction times: in the presence of hydrogen peroxide and H2P (red bars); ZnP (green bars); NPH (purple bars) and with no catalyst (Control, blue bars).The MO absorbance was monitored at 464 nm.

Figure 7 .
Figure 7. Photocatalytic degradation of methyl orange (MO) in the presence of aqueous H2O2 and mannitol at different irradiation times (irradiation with white light at an irradiance of 150 mW cm -2 ) mediated by H2P (red bars), ZnP (green bars) and NPH (purple bars).The blue bars represent the control reaction (only hydrogen peroxide and mannitol).The band of MO was monitored at 464 nm.

Figure 8
photocatalysts are able to generate • OH radical in the presence H2O2 and under light, as indicated by the oxidation of the TA probe.Several studies[44] have

Figure 8 .D
Figure 8. Emission spectra of (A) H2P, (B) ZnP and (C) NPH solutions in PBS (pH = 7.0) and DMSO (1%) before and after irradiation times (irradiation with white light at an irradiance of 150 mW cm -2 ) in the presence of aqueous H2O2 (exc =315 nm) and (D)

Figure 9 .
Figure 9. UV-Vis spectrophotometric study of H2P, ZnP and NPH at a concentration of 5 µM in DMSO/water before and after white light irradiation at an irradiance of 150 mW cm -2 at different times (0 -240 min) using the same conditions of photocatalytic studies.

NPH
material turned out to be the most effective photocatalyst when compared with the porphyrin precursors in solution.Additionally, the photocatalytic activity can be improved with the addition of aqueous hydrogen peroxide.In this case, two mechanisms (type I and type II) can be involved in the MO photodegradation.